SYNTHESES, REACTIVITY, AND PHYSICAL PROPERTIES OF …

SYNTHESES, REACTIVITY, AND PHYSICAL PROPERTIES OF SPIRO-

TRICYCLIC PORPHODIMETHENES AND PORPHYRINS WITH EXOCYCLIC RINGS

By

IVANA BOŽIDAREVIĆ

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2004

Copyright 2004

by

Ivana Božidarević

In everlasting memory of my Father, Dragan, and my Grandparents Zlata, Boško, Vuka

and Spasa.

With all my love to my families Ćirić and Božidarević

ACKNOWLEDGMENTS

For becoming the chemist I am I have to thank my teachers, mentors and

colleagues; for becoming the person I am, I have to thank my family and friends.

The one who got me involved in chemistry when I was thirteen was my seventh-

and eight-grade chem. Teacher – Vera Kujačić. Milka Dokić successfully took over

when she became my high school chemistry teacher. My first lab TA in college, Dr.

Tibor Sabo, became my BS Thesis mentor 4 years later, and I learned a lot from him. I

learned much more when I started taking graduate courses at UF, and for that I have to

thank to Dr. Richardson, Dr. Talham, Dr. Abboud and Dr. Scott who taught these classes.

Even though I joined the Scott group relatively early in the Fall 1999, I kept the

desk in the X-ray lab that was assigned tome in the summer, and I was ‘hiding ’ there

until I started doing research in May 2000. During that time, Dr. Khalil Abboud was the

person I could always count on if I needed help, advice, or if I just wanted to talk. He

played a great part in my relatively quick adjustment to the new country, people, and

customs.

The biggest thanks, of course, go to my advisor Prof. Michael Scott for his

guidance, help, understanding and infinite patience. Working with him has been a great,

rewarding experience I learned a lot from. During the work on my Ph.D. thesis, there

were a few people, other than Mike who had ideas, explanations, questions and

instruments that helped my research. For that, I would like to thank Prof. Lisa McElwee-

iv

White, Prof. Dan Talham, Prof Kirk Schanze, Prof. Mark Meisel and my dear friend Dr.

Ksenija Haskins-Glušac.

The person who set up basis for my dissertation and taught me a lot about

porphyrin and porphodimethene syntheses was Dr, Michael Harmjanz, who will make a

great professor at University of New Orleans starting this fall. My experience in the

Scott group would not be what it is, if there weren’t for the past and the present group

members, so I need to thank Dr. Andrew Cottone for helping me set up and start working

in the lab and Dr. Matt Peters for answering my questions about how things work for

solid two years. Dan and Jen were here when I came, and left shortly after, Cooper and

Eric were around for a couple of years, Dolores, Javier, Pieter and Hanna came and left,

but all of these people made the work experience more enjoyable one for me. For

making our labs a better place to be these days I have to thank Nela, Ranjan, Ozge, Eric,

Erik, Priya, Claudia, Flo, Isaac, Candace and Hubert. The last person on this list is

someone who deserves more acknowledgements than I can provide right now, so I’ll just

say that I cannot imagine getting through the past five years without a lab mate like that.

Special thanks for loving me unconditionally, letting me become who I am, and

making my childhood a happy one go to my parents, Andjelka and Dragan, my

grandparents Boško, Zlata, Vuka and Spasa, and my little brother Dejan who taught me

how to fight for what I want. During past seven years, another family became very

important in my life, and I would like to thank my in laws Nada and Vlada for their

kindness, love and support. Lastly, I have to thank my husband, Nebojša, for his love,

patience and support that helped me overcome the obstacles, and always managed to put

a smile on my face.

v

These five years in Gainesville brought me probably more friends than I can

account for right now, and I will try mentioning them all, but I hope I will be forgiven if I

forget someone. There is no way I could tell how much all of them mean to me and why,

so I will just list their names in the order of appearance and thank them all for being here

when I needed them: Tamara, Isa, Ana I., Janina, Iwona, Luk, Josef, Celeste, Corey,

Ljubisa, Ksenija, Aleksa J., Ilka, Elon, Ana M., Andy, Balsa, Milan, Aleksa O., Vesna,

Jamshid, Feruza…

vi

TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iv

LIST OF TABLES...............................................................................................................x

LIST OF FIGURES ........................................................................................................... xi

ABSTRACT..................................................................................................................... xiii

CHAPTER 1 INTRODUCTION TO PORPHODIMETHENES ......................................................1

Tetrapyrrolic Macrocycles............................................................................................1 Porphodimethene Syntheses .........................................................................................2 Solid-state and Solution Structures...............................................................................7 Electronic Properties...................................................................................................10 Electrochemistry .........................................................................................................13 Reactivity....................................................................................................................13

2 PORPHODIMETHENE SYNTHESES.....................................................................16

Introduction.................................................................................................................16 Results and Discussion ...............................................................................................19 Conclusions.................................................................................................................25 Experimental...............................................................................................................25

General Procedures..............................................................................................25 Chromatography ..................................................................................................26 Syntheses of 2-4 and 2-5 .....................................................................................26 Synthesis of 2-6 ...................................................................................................27 Syntheses of 2-7 and 2-8 .....................................................................................28 X-ray Crystallography .........................................................................................30

3 METALLATION AND RING-OPENING REACTIONS........................................31

Introduction.................................................................................................................31 Results and Discussion ...............................................................................................32

vii

Metallation of Porphodimethenes........................................................................32 Structure of Metalloporphodimethenes. ..............................................................34

Palladium anthracenone porphodimethene ..................................................34 Palladium pyrenone porphodimethene.........................................................35 Palladium and platinum phenanthrenone porphodimethenes.......................37 Copper phenanthrenone porphodimethenes .................................................39 Nickel phenanthrenone porphodimethene....................................................41 Summary of the Structural Data...................................................................41

Reactivity of Porphodimethenes..........................................................................42 Conclusions.................................................................................................................48 Experimental...............................................................................................................49

General Procedures..............................................................................................49 Chromatography ..................................................................................................49 Synthesis of 3-2 ...................................................................................................49 Synthesis of 3-4 ...................................................................................................50 Synthesis of 3-5 ...................................................................................................51 Synthesis of 3-8 ...................................................................................................52 Synthesis of 3-9 ...................................................................................................52 Synthesis of 3-11 .................................................................................................53 Synthesis of 3-12 .................................................................................................53 Synthesis of 3-13 .................................................................................................54 Synthesis of 3-14 and 3-15 ..................................................................................54 X-ray Crystallography .........................................................................................55

4 PHOTOPHYSISCAL PROPERTIES OF PORPHODIMETHENES .......................59

Introduction.................................................................................................................59 Fluorescence Spectroscopy .................................................................................64 Phosphorescence Emission..................................................................................65 Transient Absorption ...........................................................................................66

Conclusions.................................................................................................................68 Experimental...............................................................................................................69

5 SYNTHESES OF PORPHYRINS WITH EXOCYCLIC RING SYSTEMS............71

Introduction.................................................................................................................71 Results and Discussion ...............................................................................................71

Cyclooctanone Porphyrins...................................................................................73 Cyclohexannone Porphyrins................................................................................78

Conclusions.................................................................................................................81 Experimental...............................................................................................................82

General Procedures..............................................................................................82 Chromatography ..................................................................................................82 Synthesis of cis-5-5 and trans-5-5.......................................................................82 Synthesis of 5-8 ...................................................................................................83

viii

Synthesis of 5-9 ...................................................................................................84 Synthesis of 5-10 .................................................................................................84 Synthesis of cis-5-11 and trans-5-11...................................................................85 X-ray Crystallography .........................................................................................86

6 PHOTOPHYSICAL PROPERTIES OF PORPHYRINS WITH EXOCYCLIC RING SYSTEMS ..................................................................................................................89

Introduction.................................................................................................................89 Cyclooctanone Porphyrins...................................................................................93 Cyclohexanone Porphyrins..................................................................................97 Azulenone Porphyrins .........................................................................................98

Conclusions...............................................................................................................101 Experimental.............................................................................................................102

7 SUMMARY.............................................................................................................104

LIST OF REFERENCES.................................................................................................106

BIOGRAPHICAL SKETCH ...........................................................................................111

ix

LIST OF TABLES

Table page 2-1. Selected bond lengths and angles for 2-5, 2-6 and 2-8 ...........................................23

2-2. Crystallographic data...............................................................................................29

3-1. Selected parameters from the solid-state structures of metalloporphodimethenes. ..................................................................43

3-2. Crystallographic data for compounds 3-2, 3-4, 3-7 and 3-8 ...................................56

3-3. Crystallographic data for compounds 3-9, 3-11, 3-12 and 3-15 .............................57

4-1. Selected UV-Vis absorption data for the free-base and metalloporphodimethenes. The presence of metals in macrocyclic ring induces the absorption maximum to shift towards longer wavelengths. ............................................................................61

4-2. The values of fluorescence emission maxima and quantum yields for selected free-base porphodimethenes. The fluorescence is very weak.........................................65

5-1. Selected bond lengths for trans-5-5 ........................................................................77

5-2. Crystallographic data for trans-5-5 and 5-10..........................................................87

6-1. Summary of photophysical data..............................................................................94

x

LIST OF FIGURES

Figure page 1-1 Depiction of four examples of tetrapyrrolic macrocycles.. ........................................1

1-2 Illustration of redox relationships between tetrapyrrolic macrocycles.. ....................2

1-3 Schematic representation of a porphyrin spectrum . ................................................11

1-4 UV-vis spectra of a porphyrin (---) and a porphodimethene (—).. ..........................11

1-5 Nickel porphodomethene and porphyrin MO diagrams...........................................12

2-1 Diagram of the solid-state structure of 2-5...............................................................20



2-4 Diagrams of the porphodimethene cores of 2-5, 2-8 and 2-6...................................24

2-5 The highly symmetric nature of the 1H NMR spectrum of 2-5 illustrates the fast flexing of the molecule in solution at room temperature. ........................................24






3-6 Diagram of the solid-state structure of 3-11.............................................................40



4-1: Illustration of the UV-Vis absorption spectra of free base porphodimethenes.. ........62

xi

4-2 Illustration of the absorption spectra of metalloporphodimethenes.. .......................62

4-3 Diagram of solid-state structure of 4-1b. .................................................................64

4-4 Depiction of phosphorescence emission for 4-1 and 4-2. ........................................67

4-5 Depiction of transient absorption of 4-1 and 4-2.. ...................................................68

5-1 Illustration of the reaction progress for synthesis of 5-4..........................................74

5-2 Diagrams (side view on the bottom) of the solid-state structure of trans-5-5. .......76

5-3 Diagram of porphyrins with exocyclic rings synthesized in Callot’s lab. ...............78


6-1 Diagram of the porphyrins with exocyclic rings used for the photophysical measurements reported herein..................................................................................90

6-2 The cycloheptanone porphyrins exhibit red-shifts in the absorption spectra...........91

6-3 Depiction of the phosphorescence emission of cis-6-1 and trans-6-1 .....................92

6-4 Illustration of transient absorption of cis-6-1 and trans-6-1. ...................................94

6-5 Diagram of the absorption spectrum of the mixture of cis-6-2 and trans-6-2 highlights the coincidence of their Soret bands at 438 nm. .....................................95

6-6 Diagram of the phosphorescence emission of the mxture of cis-6-2 and trans-6-2. The emission is quenched by saturation with air. ...................................96

6-7 Depiction of transient absorption of cyclooctanone porphyrins.. ............................97

6-8 Diagram of the UV-Vis spectra of trans-6-3 and cis-6-3.. ......................................98

6-9 Illustration of the room temperature phosphorescence emission of trans-6-3.........99

6-10 Transient absorption of cis-6-3 and trans-6-3..........................................................99

6-11 Diagram of the electronic absorption of cis-6-4 and trans-6-4..............................100

xii

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

SYNTHESES, REACTIVITY, AND PHYSICAL PROPERTIES OF SPIRO-TRICYCLIC PORPHODIMETHENES AND PORPHYRINS WITH EXOCYCLIC

RINGS

By

Ivana Božidarević

August 2004

Chair: Michael J. Scott Major Department: Chemistry

The MacDonald [2+2] condensation under Lindsey reaction conditions was

successfully employed towards the syntheses of spiro-tricyclic meso-aryl substituted

porphodimethenes from different 5-aryldipyrromethanes and aromatic vicinal diketones.

Depending on the diketone used, porphodimethenes capable of or resistant to ring

opening at the spiro-lock were prepared. The reactivity of porphodimethenes susceptible

to ring opening was studied. The porphodimethenes were metallated using palladium,

platinum, copper, nickel and zinc salts. The metal complexes were characterized and

their solid-state structures compared and analyzed.

Spiro-tricyclic porphodimethenes were used to synthesize unprecedented palladium

porphyrins with exocyclic eight-membered rings, while the synthesis of related

porphyrins with six-membered rings was accomplished through somewhat modified

literature procedures. Palladium porphyrins with six, seven and eight membered rings

were used for photophysical studies. These molecules have interesting electronic

xiii

properties, resulting in red-shifted absorption maxima in UV-Vis spectra. The results of

photophysical measurements performed on both the porphodimethenes and the

porphyrins are presented, and the dependence of the porphyrin photophysical properties

on the exocyclic ring size is discussed. The measurements include steady state emission

at room temperature and low temperature, transient absorption and singlet oxygen

quantum yield.

xiv

CHAPTER 1 INTRODUCTION TO PORPHODIMETHENES

Tetrapyrrolic Macrocycles

Tetrapyrrolic macrocycles (Fig 1-1) play a number of critical biological roles and

their importance has inspired an intensive research effort concerning artificial systems

that model the natural counterparts. One of the most abundant tetrapyrrols found in

nature, porphyrins, are cross-conjugated, planar ligands that are ubiquitous in living

systems, facilitating electron transfer and photosynthesis.1 In these macrocycles, the

pyrrolic carbon atoms are defined as either α or β, where the α carbons make up a part of

macrocycle core. The remaining carbon atoms in the core form the bridges between the

pyrrolic groups and these are referred to as meso carbons (Fig 1-1).

N HN

NH N

NH N

N NH

N N

N NH H

H HN HN

NH N

Porphyrin Chlorin Porphodimethene Porphyrinogen

α

βmesoβ

Figure 1-1. Depiction of four examples of tetrapyrrolic macrocycles. Pyrrolic carbons are

defined as α or β while the bridging carbons are called meso in all the tetrapyrroles.

The porphyrins are cross-conjugated, aromatic molecules containing an 18-

anulene system. Porphodimethenes differ from porphyrins in as much as they have

saturated carbons at two non-adjacent meso positions. The two sp3 carbon atoms cause

the macrocycle to adopt a “roof-like” folded structure, breaking the aromaticity and

1

2

disrupting the electronic communication between the two dipyrromethene halves. The

two halves of the porphodimethene macrocycle are still conjugated and the interplanar

angle between them is called a roof-angle.

Porphodimethene Syntheses

Although porphodimethenes were long recognized to be intermediates in the

oxidation pathway from porphyrinogens to porphyrins (Fig 1-2), a synthetic scheme for

their production was only first reported in 1974 by Buchler and Puppe.2 Buchler

reasoned that alkylation of meso carbons in the aromatic porphyrin could produce

porphodimethenes, and indeed, the reductive alkylation of zinc octaethyl porphyrin 1-1

resulted in formation of zinc porphodimethene 1-2 (Scheme 1-1).

N N

NH HN

NH N

N NH

NH HN

NH HN

N HN

NH N

Porphyrin

Porphodimethene

Porphyrinogen

N HN

NH HN

N HN

NH HN

Phlorin Porphomethene

2H+ 2e-

2H+

2e-2H+

2e-

-2H+

-2e--2H+

-2e--2H+

-2e-

Figure 1-2. Illustration of redox relationships between tetrapyrrolic macrocycles.

Porphyrins represent the most oxidized form of tetrapyrroles.

The presence of methyl groups at positions 5 and 15 (saturated meso-carbons)

prevented rapid oxidation of the macrocycle, allowing for the isolation of the first air-

stable porphodimethenes.

3

N N

N N1. 2e-

2. R-XN N

N N

R H

R H

(R = CH3, X = Br, I)

Zn Zn

1-1 1-2 Scheme 1-1. Depiction of the first stable synthesis of a porphodimethene. Buchler and

coworkers applied reductive alkylation of a zinc porphyrin to obtain the porphodimethene.

Over the years, Buchler et al. expanded the scope of this reaction to various

metalloporphodimethenes bearing different alkyl substituents on the saturated meso

carbon atoms.3-7 A significant library of X-ray structural data was collected and

electrochemical properties of these compounds were studied, but owing to the difficulties

associated with their isolation and separation, the reactivity studies of the alkyl

metalloporphodimethenes were never reported.

An alternative porphyrin route to porphodimethenes was discovered by Fontecave

et al. in 1984.8 During the catalytic reduction of allyl bromide by sodium ascorbate or

sodium dithionate, the catalyst, tetraphenylporphyrinato iron(III)chloride (Fe(TPP)Cl),

underwent a slow transformation to a porphodimethene species. Based on this

observation, a larger scale reaction of Fe(TPP)Cl with allyl bromide and sodium

dithionate (Scheme 1-2) was undertaken, and after demetallation with TFA, a stable free

base porphodimethene was obtained in 80% yield, as a complex mixture of anti and syn

axial and equatorial isomers.

Even though this study gave some insight into the iron porphyrin catalyzed

reduction mechanism, the synthetic method for porphodimethene preparation was not

explored further, due to the difficulties presented by the formation of product mixtures.

4

N N

N N

Ph

Ph

Ph

Ph Fe

Cl

N HN

NH N

Ph

Ph

Ph

Ph

RR

1) CH2CHCH2Br

2) TFA

R=CH2CHCH2

1-3 Scheme 1-2. Representation of reductive alkylation of FeTPPCl. Allylbromide catalyzes

this porphodimethene forming reaction.

In 1999 and 2000, several reports of new procedures for porphodimethene

synthesis appeared in the literature. The Floriani lab employed reductive dealkylation of

tin porphyrinogen to obtain hexaalkyl tin porphodimethene (Scheme 1-4).9 Using a

MacDonald’s [2+2] type condensation between a dipyrromethene and acenaphthenone,

our group reported the first synthesis of spiro-tricyclic porphodimethenes (Scheme 1-3).10

Almost concurrently with our report, another method for the high yield of

porphodimethenes was published. The Sessler group condensed dipyrromethane with an

excess of acetone to form a mixture of pyrrolic macrocycles from which

porphodimethene could be isolated in high yield (Scheme 1-4).11

R

NH NH

O

O

NH NN HN

R

R

O

O NH NN HN

R

R

O O1. TFA

2

2. DDQ

2

+

Anti Syn1-51-4

Scheme 1-3. Illustration of condensation of mesityl dipyrromethane and

acethnaphthenequinone to form syn and anti porphodimethenes. The isomers are easily separated by column chromatography.

5

N N

N N SnCl4(THF)2

R=alkylSn

R R

R R

R

R R

R

THF

THFN N

N N

Sn

R R

R R

R R

Cl

Cl

Ar

NH HN NH N

N HN

Ar

Ar

+ 1. acid2. DDQ

O

40 fold excess

2

1-7

1-6

Scheme 1-4. Depiction of syntheses of alkyl-substituted porphodimethenes. Alkyl

substituents prevent oxidation at meso carbons

The porphodimethenes illustrated in Scheme 1-4 contain alkyl groups at saturated meso

carbons; hence, they are ill suited for further functionalization. Our group was interested

in studying porphodimethenes with aromatic substituents at sp3 carbons, and the synthetic

pathway outlined in Scheme 1-3 will be further elaborated in the following chapters.

Recent developments in porphodimethene syntheses include the reaction of meso or

β- substituted porphyrins with alkyl lithium and iodo alkyl reagents,12 as well as the

condensation of pyrroles or dipyrromethanes with bulky aldehydes (Scheme 1-5).13, 14

The first method, consisting of two consecutive alkylation steps was developed by Senge

and coworkers,12 and it allows for facile isolation of asymmetrically substituted

porphodimethenes (Scheme 1-5).

6

N N

N N

Ph

Ph

Ni

Et Et

Et

Et

EtEt

Et

Et

N N

N N

Ph

Ph

Ni

Et Et

Et

Et

EtEt

Et

Et1. n-BuLi2. n-C6H13I n-C6H13n-Bu

HH

1-8 Scheme 1-5. Illustration of alkylation of nickel porphyrin. The use of alkyllithium and

alkyliodide reagents enables the synthesis of asymmetrically substituted porphodimethenes

The same group employed a condensation reaction between pivaldehyde and

pyrrole to prepare t-butyl substituted compounds.12 The Kim group made an interesting

choice of bulky reagent using ferrocene aldehyde for the condensation reaction with

dipyrromethane (Scheme 1-6).14 The resulting porphodimethene was stable to light and

oxidants in the absence of acid, but it readily formed a porphyrin upon irradiation in

acidic solution under anaerobic conditions.

NH NH2 Fe

HO

2+

N HN

NH NFcFc

HH

H+

1-10

NH

+ PhCHO + t-BuCHO

N HN

NH Nt-But-Bu

HH

1-9

Ph

Ph

H+

Scheme 1-6.Representation of condensation of pyrrole and dipyrromethane with bulky aldehydes. Oxidation of porphodimethenes to porphyrins is prevented by steric hindrance at the meso positions.

7

Solid-state and Solution Structures

From a detailed examination of the available crystallographic data for

porphodimethenes, a variety of important structural parameters can be identified. In

1974, Buchler’s group reported the first porphodimethene crystal structure of nickel

dimethyl-octaethyl porphodimethene,15 and over the next several years, the group

reported structures of related porphodimethenes with different metals (Scheme 1-7).2, 3, 7,

16, 17 All of these metalloporphodimethenes adopt a roof-like folded structure and the

roof angles between the two dipyrromethane halves of the molecule range between 128º

and 146º.

N N

N N

R H

R H

M

1-11: M = Ni, R = Me1-12: M = TiO, R = Me1-13: M = FeCl, R = Me1-14: M = OsCO, R = Me1-15: M = MnN, R = Me

Scheme 1-7. Illustration of metalloporphodimethenes with determined solid-state

structures. All of the dimethyl-octaethylporphodimethenenes adopt a roof-like folded structure in the solid state.

The metal center adopts a square planar arrangement with the four pyrrolic nitrogens in

1-11, while the geometry about the metals in 1-12 through 1-15 is square pyramidal.

Metal-nitrogen bond lengths vary from 1.902(5) Å in nickel porphodimethene to 2.113(3)

Å in the titanium oxo species. In all of the structures, methyl substituents on sp3 meso

carbons are in syn-diaxial conformation, which is crucial for the porphodimethene

stability under oxidative conditions. Interestingly, only one free-base porphodimethene

has been reported previous to 1999, and it had isopropyl substituents on the meso

carbons.18 The Buchler group initially thought the compound was a mixture of syn-

8

diaxial (aa), syn-diequatorial (ee) and anti (ae) isomers (Scheme 1-8),19 but out of these

three isomers, the syn-diaxial(aa) was proven to be the most stable, since the presence of

the alkyl groups at equatorial position on meso carbons increases the steric hindrance at

the periphery of a porphodimethene. Careful column chromatography of the reaction

mixture on alumina allowed for separation of the minor isomer fraction (the primary

product of the reaction was, as expected, the aa isomer).18 In the solid-state this molecule

possesses a slightly different geometry with two meso-substituents oriented trans to each

other and locked in an intermediate conformation between axial and equatorial. With the

roof angle of 180º, this planar stereoisomer was named diagonal (dd).

RH

R

HH

RH

H

R

RR

R

H

H

H

R

ae ee aa dd Scheme 1-8. Diagram of possible porphodimethene stereoisomers. In most cases aa is the

single isomer isolated from the porphodimethene reaction

More recent examples of porphodimethene solid-states structures have included

macrocycles with six meso substituents coordinating different transition metals (Scheme

1-9)20 and metalloporphodimethenes with long alkyl substituents on sp3 carbons (Scheme

1-10).12

N N

N NEtEt

EtEt

Et

Et

M

N N

N NEtEt

EtEt

Et

Et

M

N N

N NEtEt

EtEt

Et

Et

M

L L

L

1-16: M = Fe1-17: M = Co1-18: M = Ni

1-19: M = Co, L = Py1-20: M = Mn, L = THF

1-21: M = Mn, L = THF1-22: M = Mn, L = Py1-23: M = Fe, L = THF1-24: M = Mn, L = Py

Scheme 1-9. Depiction of several structurally characterized metalloporphodimethenes.

9

1-25: R = n-Bu1-26: R = H

N N

N N

n-Bu H

n-Bu H

MR n-Bu

Scheme 1-10. Illustration of two meso-substituted octaethyl porphodimethenes

In the recently characterized examples of porphodimethenes the bond lengths and

roof angles are consistent with the values described by Buchler except for roof angles in

1-23 and 1-24 - 180º, 1-20- 168.8º, 1-16- 149.3º, and 1-18-116.9º. In the case of Ni(II),

the smaller ionic radius of the metal causes the macrocycle to adopt a ruffled, saddle

shaped structure. Solid-state structural parameters of spiro-tricyclic porphodimethenes

are in agreement with those previously listed, and they will be further discussed in

chapters 2 and 3.

Owing to the lack of aromaticity within the tetrapyrrolic ring, several resonances in

the 1H NMR spectra of porphodimethenes are significantly shifted in comparison to the

fully aromatic porphyrin counterparts. In β-substituted porphodimethenes, the signals

arising from the meso hydrogens can be found in the region around 6.5 ppm for protons

on sp2 carbons and between 3.5 and 5.5 ppm for protons on sp3 carbons.5, 18 In addition,

the resonances for the β-pyrrolic protons in meso-substituted porphodimethenes always

occur as doublets between 5.5 and 7.2 ppm with coupling constants of approximately 4.5

Hz, and their position is influenced by the nature of meso substituents.21 When the

metals are inserted into the macrocycle, the separation between the pyrrolic doublets

increases in comparison to the free- bases.21 These changes can be attributed to both the

electronic modification of the dipyrromethane halves and the altered configuration of the

10

molecule induced by metal binding. In comparison to meso-substituted porphyrins, the

resonances for these β-hydrogens are significantly shifted upfield. The ring current in

porphyrins also induces a large shift of the signals for the pyrrolic hydrogens to between

–4 and –2 ppm, but since they lack this influence, the analogues NH resonances in

porphodimethenes are shifted far downfield to between 12 and 14 ppm.

Oftentimes, porphodimethenes exhibit 1H NMR spectra that look far more

symmetrical than would be expected upon inspection of their solid-state structures,21,22

and the ability of the macrocycle to flex along the axis defined by the two saturated meso

carbons may contribute to this phenomenon. In most instances, the roof-like fold of the

porphodimethene is not detectable on the NMR time-scale and the 1H NMR spectra

rather resemble the more symmetrical structure. Nickel porphodimethene 1-18 and the

free-base analog, on the other hand, feature three distinct sets of signals for the six ethyl

substituents at room temperature, but at 310 K the three signals collapse into two

resonances, further supporting the flexing of the porphodimethenes in solution.23

Electronic Properties

From a detailed comparison of porphodimethene properties to the closely related,

well-studied porphyrins, a clearer, more complete picture of the features for the former

compounds can be assembled. The electronic spectra of porphyrins exhibit two

characteristic absorptions: a strong band around 400 nm (Soret or B band) and weak

absorption bands between 550 and 650 nm (Q-bands).24 Figure 1-3 illustrates a

molecular orbital diagram and a typical absorption spectrum of a porphyrin, exemplified

here by octaethylporphyrinatozinc(II).

11

N N

N NZn

Q

B

S2

S1

S0

eg y (LUMO) eg x (LUMO)

a1u (HOMO) a2u (HOMO - 1)

a1ua2u

eg

400 500 600 nm

Et

Et

Et

Et

Et

EtEt

Et

a b c d Figure 1-3. Schematic representation of a porphyrin spectrum:

a)Octaethylporphyrinatozinc(II); b)Molecular orbitals; c)States; d)Absorption spectrum (adapted from Anderson, H.L. Chem. Commun. 1999, 2323-2330).

These spectral features arise from π−π* transitions that mix together by

configurational interaction and the constructive interference of the two results in a strong

Soret or B band, while the destructive interference gives rise to weaker Q-bands.25 As

can be seen in Fig 1-4, a typical porphodimethene spectrum has a broad absorption at

about 440 nm instead of a sharp Soret feature. The porphodimethenes also lack the long

wavelength Q-bands characteristic of porphyrins.

350 450 550 650

Wavelength (nm) Figure 1-4. UV-vis spectra of a porphyrin (---) and a porphodimethene (—). Typical

porphodimethene absorption is red-shifted in comparison to a porphyrin and has a smaller extinction coefficient.

12

N

N

N

N

HH

H

HNi

H

H

N

N

N

NH HNi

H

H

b2a1a2

a1b1b2b1a2

a1

eg

b1g

a1gega1ua2u

a1gdx2-y2

dxy

dxzdyz

dz2

pp

pp

Figure 1-5. Nickel porphodomethene and porphyrin MO diagrams. (adapted from Re, N.;

Bonomo, L.; Da Silva, C.; Solari, E.; Scopelliti, R.; Floriani, C., Chemistry-a European Journal 2001, 7, (12), 2536-2546)

Based on the MO diagrams exemplified in Fig 1-5, the absorption spectra of nickel

porphyrin and porphodimethene differ primarily for two reasons:

• The energies of the metal orbitals (especially dxy) in the porphodimethene complex are higher

• The degenerate porphyrin eg (dxz, dyz) orbitals split into two inequivalent b1 and b2 orbitals in the porphodimethene.

The significant increase in the energy of dxy orbital can be attributed to the smaller

M-N core size in virtually all metalloporphodimethenes when compared to corresponding

porphyrins, while the loss of degeneracy of the porphyrin eg(π*) orbital is caused by

lowering the macrocycle symmetry. Since the main absorptions in both porphyrin and

porphodimethene spectra originate from π−π* transitions, the aforementioned changes in

the relative energies of these orbitals lead to the red shift of the main porphodimethene

absorption with respect to the porphyrin Soret band.20 The presence of the two saturated

carbon atoms in tetrapyrrolic macrocycle breaks the aromaticity and disrupts the

13

electronic communication between the two halves of the porphodimethene molecule, and

therefore the UV-Vis spectra of these compounds resemble the sum of the two

dipyrromethene absorptions rather than the porphyrin absorption spectra.26

Electrochemistry

Previous to 1999, the electrochemical properties of porphodimethenes were

relatively unexplored with the exception of several molecules synthesized in Buchler’s

group and the ferrocenyl porphodimethene 1-10. In Ni(OEPMe2) 1-11, two reversible

oxidations and one reduction were found with respective half-wave potentials of 1.01 V,

0.64 V, and -1.52 V (vs. SCE). Surprisingly, even under oxidative potentials, the

porphodimethene did not dehydrogenate to give a porphyrin.26

Unlike square planar metals, axially bound iron and cobalt porphodimethenes

exhibit only a single oxidation and reduction in the presence of excess axial ligand, and

the potentials for these events are highly dependent on the nature of both the central

metal and the axial ligand.27 When the metal ion is incorporated into the meso-

substituent as is the case in the ferrocenyl porphodimethene 1-10 shown in Scheme 1-6

the potential for the metal-based oxidation for the porphyrin is reduced to values close to

zero (0.02 and 0.23 V, vs. SCE), due to effective stabilization of the monocation through

an extended π-system.14

Reactivity

The identity of the central atom as well as the presence and type of the axial

ligands on the metal influence the reactivity of the alkyl substituted

metalloporphodimethenes. Starting from the tin hexaethylporphodimethene 1-6,

magnesium and dilithium derivatives can be obtained by transmetallation.

14

N N

N NRR

RR

R

R

Sn

Cl

Cl N N

N NRR

RR

R

R

N N

N NRR

RR

R

R

MLi Li LLLi(s) i)

1-29: M= 2H, i) Et2O, H2O1-30: M= Ni, i) NiCl2THF21-31: M= ZrCl2, i) ZrCl4THF2

N N

N NRR

RR

R

R

Mg

THF

THF

Mg(s)

L=THFR= Et

1-6

1-27

1-28

THF

THF

Scheme 1-11.Reaction diagrams of transmetallation and demetallation. Shown here are

effective methods for obtaining variety of metalloporphodimethenes.

N N

N NEtEt

EtEt

Et

Et

Li Li LL LiNMe2-LiNMe2-HNMe2 N N

N N EtEt

EtEt

C

Et

Li LiNH HN

NH HNEtEt

EtEt

O

O

MeH

LiNMe2

-LiNMe2-HNMe2 H2O

CMeH

CMe H

1-32 1-28 1-33

Scheme 1-12. Depiction of exocyclic double bond formation. Dilithium

porphodimethenes undergo exocyclic double bond formation in the presence of LiNMe2.

N N

N NEtEt

EtEt

Et

Et

Zr RR

N N

N NEt

Et

Et

Et

Zr CH2Ph Et

Et THF

CH2Ph

R= CH2Ph

1-351-34 Scheme 1-13. Reactivity of zirconium porphodimethenes. Rearrangement of the axial

ligand is achieved with zirconium as a central metal in the porophodimethene.

15

Many different metalloporphodimethenes are accessible upon further

transmetallation of the dilithium species. (Scheme 1-11).23, 28 If the addition-elimination

reaction sequence is performed on the dilithium porphodimethene 1-28, it gives rise to

tetrapyrrolic species containing one or two exocyclic double bonds on the meso carbon

atoms (Scheme 1-12).29

Furthermore, porphomethenes and porphyrinogens can be generated from

zirconium and nickel porphodimethenes. Zirconium porphodimethene 1-34 undergoes

reductive alkylation at one of the sp2 meso carbons to form porphomethene species 1-35

(Scheme 1-13),28 while the nickel porphodimethene 1-36 acts as an electrophile in

reactions with different nucleophiles to form the porphyrinogens as illustrated in Scheme

1-14.

N N

N NEtEt

EtEt

Et

Et

Ni

N N

N NEtEt

EtEt

Et

Et

Nii)

R

R

i) LiCH2CN, R=CH2CNi) BuLi, R= Bui) LiHBEt3, R= H

Li(THF)2

(THF)2Li1-36 1-37

Scheme 1-14. Illustration of porphyrinogen formation. These reactions result in porphyrinogens with substitution patterns slightly different than the ones used to make hexaethyl nickelporphodimethene.

Unlike their alkyl substituted counterparts, spiro-tricyclic porphodimethenes are

easily metallated in one step using zinc, nickel, copper, palladium or platinum salts, and

they offer the unique ability to open the spiro-locks affording functionalized

porphyrins.10, 21 The conditions and the products of the ring-opening reactions will be

discussed in detail in chapters 3 and 5.

CHAPTER 2 PORPHODIMETHENE SYNTHESES

Introduction

The condensation of 5-aryl dipyrromethanes with aromatic aldehydes followed by

oxidation with DDQ was developed in 1984 by Lindsey30 as an efficient method for

syntheses of trans-meso substituted porphyrins (Scheme 2-1). The first step of this

reaction generates a porphyrinogen species, which is then easily oxidized to a porphyrin.

NH HN

NH HNAr

Ar

R

R

H

H

H

H

3 DDQ3 DDQH2

NH N

N HNAr Ar

R

R

2 Ar-CHOAcid

rt

NH NH

HR

2

+

Scheme 2-1. Representation of Lindsey condensation reaction.

NHNH

OO

O

HN O

OMe

COOMe

COOMe+ 2

Scheme 2-2. Illustration of reactivity of vicinal diketones

If the porphyrinogen oxidation can somehow be prevented at the two meso

positions, the reaction should produce a porphodimethene instead of a porphyrin. With

this issue in mind and inspired by the observation that aromatic vicinal diketones react in

a manner similar to aldehydes in condensation with pyrroles31 (Scheme 2-2), a synthetic

method for making meso-aryl substituted porphodimethenes was developed in our lab by

Dr. Michael Harmjanz.32 The condensation of acenaphthenequinone with 5-

16

17

aryldipyrromethanes followed by oxidation with DDQ resulted in the formation of

porphodimethenes. These macrocycles could be further transformed to give porphyrins

bearing two 8-functionalized naphthalene spacers upon reaction with base or sodium

boron hydride. The porphyrins have shown unusual electrochemical properties33 and

have been found to be excellent building blocks for heterometallic, one-dimensional

arrays.10 Initially, meso-aryl substituted porphodimethenes were prepared using 5-

aryldipyrromethanes and acenaphthenequinone containing carbonyl groups on a five-

membered ring (Scheme 2-3).

Several synthetic pathways, including reductive alkylation of porphyrins2 and

oxidative dealkylation of porphirinogens,23 can be used to generate meso-alkyl

substituted porphodimethenes, but these methods have been restricted to

porphodimethenes containing aliphatic substituents on the sp3 carbons.

HNNH

O

O 2.DDQ1.TFA

O

O O

NH NN HN

R

R

2

O

NH NN HN

R

R2R

+

R =

Br COOMe

ClCl

OMeOMeMeO

F F

t-But-Bu

Scheme 2-3. Depiction of the first spiro-tricyclic porphodimethenes.

More recently, Harmjanz employed aceanthrenequinone and phenanthrenequinone

in condensation reactions with 5-mesityldipyrromethane to obtain novel meso-aryl

18

porphodimethenes,22 and the work presented here with pyrrene-4,5-dione complements

his efforts to expand the scope of this reaction (Scheme 2-3).

O

O2.DDQ1.TFA

O

OOO

NH NN HN

R

RNH NN HN

R

R

Anti (4%)Syn (1%)

HNNH 2.DDQ1.TFA O

O

Anti (14%)

NH NN HN

R

R

R=Mesityl

O

O

2.DDQ1.TFA

OO O

Syn (4%)

NH NN HN

R

R2

Anti (18%)

ONH NN HN

R

R

2-1

2-2

2-4 2-5

2-3

2

2

O

O2

Scheme 2-4. Diagram of condensation reactions of different vicinal diketones.

We have demonstrated the use of different polycyclic vicinal diketones, with

carbonyl groups on both five- and six-membered rings, for preparing novel spiro-tricyclic

meso-aryl substituted porphodimethenes. The porphodimethene products were

characterized (including solid-state structure and fluorescence measurements), and tested

for ring opening at the spiro-lock. Since 2-3 was chosen as a subject of further reactivity

studies, analogues of this porphodimethene bearing aromatic substituents different then

mesityl were also synthesized (Scheme 2-5), including 2-8 as the only isolated syn isomer

of a phenanthrenone porphodimethene.

19

HNNH

2.DDQ1.TFA

O

O

NH NN HN

R

R2

O

O2+

HNNH

ClCl2.DDQ1.TFA

O

O

NH NN HN

R

R2

O

O2+

t-But-BuR =

R =

t-But-Bu

ClCl

O

NH NN HN

R

R

O

2-6

2-7 2-8

Scheme 2-5. Illustration of novel phenanthrenone porphodimethenes.

Results and Discussion

As demonstrated by Harmjanz et al.,22 porphodimethenes 2-1, 2-2 and 2-3 (see

Scheme 2-4) can be synthesized by a [2+2] MacDonald type condensation reaction of

aceanthrenequinone and phenanthrenequinone with 5-mesityldipyrromethane in the

presence of TFA as a catalyst and DDQ as an oxidant. The work presented here adds

pyrenone-4,5-dione to the list of aromatic polycyclic vicinal diketones that can be used in

the synthesis of porphodimethenes. Unlike acenaphthenequinone, phenanthrenequinone

and pyrene-4,5-dione both contain carbonyl groups on six-membered rings, and these two

molecules were selected to examine the influence of six-membered rings of the vicinal

diketones on the chemistry of the porphodimethenes (Scheme 2-3).

Aceanthrenequinone22, acetnaphthenequinone21 and pyrene-4,5-dione react with 5-

mesityldipyrromethane to yield both the syn and the anti isomer. Surprisingly,

condensation of phenanthrenequinone with 5-mesityldipyrromethane or 5-(3,5-di-tert-

butyl-phenyl) dipyrromethane gives exclusively the anti isomer.

20

The solid-state structure of 2-5 is shown in Figure 2-1. The porphodimethene

skeleton shares meso-carbons 5 and 15 as spiro centers with the 4 position of pyrenone.

The polycyclic substituents are aligned trans and oriented anti to each other. The

presence of two saturated carbons (5 and 15) causes the molecule to fold and adopt a

roof-like structure with an inter-planar roof-angle of 138.0(1)°.

Figure 2-1. Diagram of the solid-state structure of 2-5 (40% probability; carbon atoms

depicted with arbitrary radii). Hydrogen atoms are omitted for clarity.

The two dipyrromethane halves deviate only slightly from planarity (0.04 Å and

0.10 Å), while the four pyrrole rings are completely planar (mean deviation from the

plane is less then 0.01 Å). Bond lengths between α and meso carbons range from

1.396(9) Å for the unsaturated meso carbon to 1.555(9) Å for the aliphatic carbons. The

meso substituents are nearly perpendicular to the porphodimethene core with interplanar

angles between pyrenone moieties and the macrocylic ring of 88.3(1)º and 89.6(1)º, while

21

the angles between mesityl groups and tetrapyrrols are 83.4(2)º and 78.9(2)º. Selected

bond lengths and angles are listed in Table 2-2.

The X-ray structure of 2-6 is illustrated in Figure 2-2. Even though the

phenanthrenone substituent is less rigid than the pyrenone, the polyaromatic backbone

still retains planarity and forms the angle of 83.9(0)º with the porphodimethene core. The

molecule is virtually flat, with the roof-angle stretched out to 180º, and the mean

deviation from the plane defined by 20 carbon and 4 nitrogen atoms of the core being

0.075 Å. Bond lengths between α and meso carbons range from 1.377(3) Å for the

unsaturated meso carbon to 1.516(2) Å for the aliphatic one. The angle between di-t-

butyl-phenyl substituents and the porphodimethene core is 70.5(0)º, showing that these

substituents are somewhat more rotated toward the plane of the macrocycle than the

mesityl moieties in 2-5. The higher degree of rotation is attributed to less steric

hindrance due to the presence of hydrogens in 2 and 6 positions, instead of methyl

groups, and this phenomenon has previously been noted in meso substituted porphyrins.25

The angles on saturated carbons within the porphodimethene ring are 118.4(1)°, and this

number tends to change dramatically upon insertion of the metal in the macrocycle (vide

infra). Selected bond lengths and angles are listed in Table 2-2.

As illustrated in Figure 2-3, compound 2-8 represents the only example of

phenathrenone substituted porphodimethene in which the two oxygens point in the same

direction, forming the syn isomer. In this species, the distance between the oxygens is

3.691 Å, which is comparable to the distances found in other syn spiro-tricyclic

porphodimethenes.

22

Figure 2-2. Diagram of the solid-state structure of 2-6. (40% probability; carbon atoms

depicted with arbitrary radii). Hydrogen atoms are omitted for clarity. Primed and non-primed atoms are related by center of inversion.

Figure 2-3. Diagram of the solid-state structure of 2-8. (40% probability; carbon atoms

depicted with arbitrary radii). Hydrogen atoms are omitted for clarity.

23

The roof angle in this structure is 137.4(1)º, with the dipyrromethene mean plane

deviations of 0.042 Å and 0.094 Å. The angles between the phenanthrenone substituents

and the porphodimethene core are 89.0(0)º and 82.4(1)º, proving that the orientation of

the substituents does not affect them significantly. The same angles in the related

compound 2-6 are 83.9(0)º. The angles between the substituents on the sp2 meso carbon

atoms and the macrocycle are 85.4(1)º and 85.9(1)º, closer to those in 2-5 then in 2-6.

These findings are in agreement with the previously discussed relationship between the

degree of rotation and the presence of the substituents in 2 and 6 positions of the phenyl

ring. If the meso substituents are disregarded, the tetrapyrrolic cores of the

porphodimethenes with six-membered rings on the spiro-locks have very similar

structural parameters for compounds 2-5 and 2-8, while the core of 2-6 appears to be

significantly flattened compared to the first two. This is discrepancy, caused by the

difference in crystal packing is illustrated in Figure 2-4 and quantified by selected

parameters in Table 2-1.

Table 2-1. Selected bond lengths and angles for 2-5, 2-6 and 2-8 2-5 2-6 2-8

N1-C1 1.409(8) 1.391(5) 1.317(2)

N1-C4 1.351(8) 1.337(5) 1.417(2)

C1-C20 1.416(9) 1.405(6) 1.377(3)

C4-C5 1.531(9) 1.512(5) 1.512(5)

O1-C33 1.194(5) 1.216(2)

C4-C5-C6 113.9(5) 111.9(3) 118.4(2)

C14-C15-C16 113.9(5) 115.1(3)

24

Figure 2-4. Diagrams of the porphodimethene cores of 2-5, 2-8 and 2-6 (40% probability; carbon atoms depicted with arbitrary radii). Primed and non-primed atoms are related by center of inversion

2-6

2-82-5

NH NN HN

R

R

O

O

2-5

R= a a

b

cc

a

b

c

Figure 2-5. The 1H NMR spectrum indicates highly symmetric nature of 2-5 illustrating the fast flexing of the molecule in solution at room temperature.

25

Based on an examination of the solid-state structure, the sets of aromatic

substituents in the anti-isomers (2-3, 2-5) should be asymmetrical due to the roof-like

fold in the molecule, but both compounds exhibit equivalent resonance in 1H NMR for

the meso-bound polyaromatic systems as well as the mesityl substituents (as illustrated in

Figure 2-5), consistent with the fast flexing of a porphodimethene macrocycle as

observed in 1H NMR described earlier for related acenaphthenone derivatives.10

Conclusions

The use of different vicinal aromatic diketones for the syntheses of the spiro-

tricyclic porphodimethenes has been demonstrated. The aromatic groups on the

porphodimethene sp2 meso carbons can be varied easily, by changing the 5-aryl

substituents on the dipyrromethane starting material. Other molecules used for

phenanthrenone porphodimethene synthesis, but not included in this thesis, are 4-t-

butylphenyl and 3,4,5-trimethoxyphenyl dipyrromethanes.

Even though the reaction conditions were the same for all the porphodimethenes

reported herein, different ratios of cis and trans isomers were obtained, depending on the

identity of the diketone and the aryl group of the dipyrromethane. The spiro-tricyclic

porphodimethenes were tested for metallation and the ring opening at sp3 meso carbons,

and the details of the reactivity studies are described in chapters 3 and 5.

Experimental

General Procedures.

NMR spectra were recorded on Varian Mercury or VXR 300 MHz

spectrometers. UV-Vis spectra were recorded with a Varian Cary 50 spectrophotometer.

High resolution mass spec analyses were performed by University of Florida Mass Spec

services using FAB or ESI as ionization method. The compounds 5-mesityl

26

dipyrromethane, pyrene-4,5-dione, 5-(2,6-dichlorophenyl)-dipyrromethane and 5-(3,5-di-

tert-butyl-phenyl) dipyrromethane were prepared following the literature procedures.34-36

All solvents were used as purchased, unless otherwise specified.

Chromatography

Absorption column chromatography was performed using neutral

alumina (Aldrich, Brockman I ~ 158 mesh, 58Ǻ) or chromatographic silica gel (Fisher,

200 – 425 mesh).

Syntheses of 2-4 and 2-5

A sample of 2.000 g (8.62 mmol) of pyrene-4,5-dione and 2.276 g (8.62 mmol) of

5-mesityldipyrromethane were dissolved in 930 ml of CH2Cl2. Trifluoroacetic acid (1.19

ml, 14.61 mmol) was added, and the reaction mixture was stirred for 50 minutes at room

temperature. A portion of 1.909 g (8.62 mmol) of DDQ was then added, and the mixture

was stirred for another 20 minutes. The volume was reduced by 90%, and the mixture

was loaded onto alumina column and eluted with CH2Cl2. The first orange fraction was

collected. The solvent was evaporated, and the residue redissolved in a minimal amount

of o-dichlorobenzene and placed on a silica column. Separation was achieved with

benzene as eluent. Compound 2-4 was isolated as the second orange fraction. Removal

of the solvent yielded 2-4 as an orange solid (0.041g, 1%). UV-Vis [o-dichlorobenzene,

λmax (log ε)] 442 nm (4.97). mp 240ºC (dec). 1H NMR (300 MHz, o-dichlorobenzene-d4):

δ = 13.71 (s, 2H), 8.01 (d, 2H J = 7.49 Hz), 7.71 (d, 2H J = 7.79 Hz) , 7.44 – 7.37 (m,

4H) 7.54 (d, 2H, J = 8.99 Hz), 7.43 (d, 2H J = 8.39 Hz), 7.38 (d , 2H J = 7.49 Hz), 7.26

(dd, 2H J1 = J2 = 7.49 Hz), 6.47 (s, 2H), 6.37 (s, 2H), 5.91 (bs, 4H), 5.39 (d, 4H J = 4.20

Hz), 1.93 (s, 6H), 1.65 (s, 6H), 1.46 (s, 6H). HRMS (FAB) calcd. for MH+ (C68H49O2N4):

953.3855. Found 953.3833.

27

The anti isomer 2-5 was collected as the first fraction from the silica column in the

reaction procedure described for 2-4. Yield: 0.178 g (4 %). UV-Vis [o-dichlorobenzene,

λmax (log ε)] 440 nm (4.94). mp 320ºC (dec). 1H NMR (300 MHz, o-dichlorobenzene-d4):

δ = 13.82 (s, 2H), 8.53 (d, 2H J = 7.49 Hz), 8.33 (d, 2H J =7.49 Hz), 7.76 (d, 2H J = 8.08

Hz), 7.66 (d, 2H J1 = J2 = 7.79 Hz), 7.55 (d, 2H, J = 7.79 Hz), 7.48 (d, 2H 8.99 Hz), 7.42

– 7.34 (m, 4H), 6.40 (s, 4H), 5.95 (d, 4H J = 3.90 Hz), 5.71 (d, 4H, J = 4.20 Hz), 1.91 (s,

6H), 1.54 (s, 12H). HRMS (FAB) calcd. for MH+ (C68H49O2N4): 953.3855. Found

953.3869. Single crystals were grown by slow evaporation of a saturated solution of 2-5

in o-dichlorobenzene.

Synthesis of 2-6

Compound 2-6 was prepared following the literature procedure for 2-3.

Phenanthrenequinone (1.142 g, 5.49 mmol) and 5-(3,5-di-tert-butyl-phenyl)

dipyrromethane (1.832 g, 5.49 mmol) were dissolved in 550 ml of methylene chloride. A

sample of 0.75 ml (9.21 mmol) of TFA was added dropwise. The solution was stirred at

room temperature for two hours prior to addition of 1.238 g (5.49 mol) of DDQ, and the

mixture was stirred for an additional hour. Volume was then reduced to 10%, and the

solution was filtered through an alumina column with methylene chloride. The solvent

was evaporated under vacuum to yield 0.473 g (16 %) of orange solid. Slow diffusion of

pentane in a saturated chloroform solution of 2-6 afforded small, single crystals. UV-Vis

[methylene chloride, λmax (log ε)] 438 nm (4.90). 1H NMR (300 MHz, CDCl3) δ =

13.41 (s, 2H), 8.39 (dd, 4H, J1 = J2 = 9.0 Hz ), 8.14 (d, 4H, J = 7.8 Hz), 7.76-7.68 (m,

4H), 7.57-7.54 (m,4H), 7.40 (s, 2H), 7.24 (s, 4H), 6.36 (d, 4H, J = 4.2 Hz), 5.85 (d, 4H, J

28

= 3.9 Hz), 1.26 (s, 36H). HRMS (FAB) calcd. for MH+ (C74H69O2N4): 1045.5420. Found

1045.5438.

Syntheses of 2-7 and 2-8

A 1.429 g (6.87 mmol) portion of phenanthrenequinone and 2.000 g (6.87 mmol)

of 5-(o-dichlorophenyl) dipyrromethane were dissolved in 400 ml of methylene chloride

and 0.78 ml (9.58 mmol) of TFA was added dropwise to the solution. The solution was

stirred for 90 minutes at the room temperature and DDQ (1.670 g, 7.35 mmol) was

added. The reaction mixture was stirred for an additional hour. Excess DDQ was filtered

off, and the solution volume was reduced to 10%, and filtered through neutral alumina.

The first, orange fraction was collected, and the solvent was removed. The solid residue

was washed with toluene and filtered. The filtrate was loaded onto silica column. The

first orange fraction that came of the column with toluene as an eluent was collected, and

the solvent was evaporated to yield orange-brown powder. Yield: 0.340 g (9 %). UV-

Vis [toluene, λmax (log ε)] 433 nm (5.04). 1H NMR (300 MHz, CDCl3) δ = 13.17 (s, 2H),

8.25 (dd, 2H J1 = 1.5 Hz, J2 = 7.8 Hz), 8.16-8.11 (m, 4H), 7.90 (dd, 2H, J1 = 7.5 Hz, J2 =

1.8 Hz), 7.74 (ddd 4H, J1 = 1.5 Hz, J2 = J3 = 7.5 Hz), 7.54 – 7.44 (m, 6H),7.35 –7.32 (m,

4H) 6.11 (d, 4H, J = 3.9 Hz), 5.72 (d, 4H, J = 4.2 Hz). HRMS (FAB) calcd. for

MH+(C58H33N4O2Cl4, monoisotopic peak) 957.1358. Found: 957.1358.

The compound 2-8 was collected as precipitate from the toluene washing and

filtration in the reaction procedure described for 2-7. Yield: 0.110 g (3 %). UV-Vis

[toluene, λmax (log ε)] 440 nm (4.89). 1H NMR (300 MHz, CDCl3) δ = 13.27 (s, 2H),

8.40 (dd, 2H J1 = 1.2 Hz, J2 = 7.8 Hz), 8.17 (dd, 4H, J1 = 10.8 Hz, J2 = 8.4 Hz),

7.71(ddd, 2H J1 = 1.5 Hz, J2 = J3 = 7.5 Hz ), 7.57 – 7.43 (m, 6H), 7.39 – 7.31 (m, 6H),

29

7.27 (s, 2H) 6.11 (bs, 4H), 5.61 (d, 4H, J = 3.9 Hz). HRMS (FAB) calcd. for

MH+(C58H33N4O2Cl4, monoisotopic peak) 957.1358. Found: 957.1131

Table 2-2. Crystallographic data

2-5·4C6H4Cl2 2-6·2CHCl3 2-8·CH2Cl2

Formula C92H64Cl8N4O2 C76H70O2N4Cl6 C59H34O2N4Cl6

Formula weight 1541.07 1284.53 1029.58

Crystal system Monoclinic Monoclinic Triclinic

Space group P21/n C2/c P1

Z 4 4 2

Temp, K 173(2) 173(2) 193(2)

Dcalc/ gcm-3 1.350 1.276 1.289

a, Å 20.934(6) 29.624(1) 13.009(1)

b, Å 16.421(4) 12.557(1) 13.350(1)

c, Å 22.497(8) 19.250(1) 17.885(1)

a, deg - - 73.142(1)

β, deg 101.358(15) 111.071(1) 69.193(1)

γ, deg - - 68.285(1)

V Å3 7582(4) 6682(2) 2652.2(2)

µ, mm-1 0.352 0.307 0.369

Uniq. data coll./obs. 11196/7181 5885/4458 12126/7652

R1 [I > 2σ(I)data]a 0.0966 0.0566 0.0856

wR2 [I > 2σ(I)data]b 0.2395 0.1331 0.2635 a R1 = Σ||Fo| - |Fc||/ Σ| Fo| bwR2 = { Σ[w (Fo

2 – Fc2)2/ Σ[w ( Fo

2)2}

30

X-ray Crystallography

Unit cell dimensions were obtained (Table 2-1) and intensity data collected by Prof.

Michael Scott on a Siemens CCD SMART diffractometer at low temperature, with

monochromatic Mo-Kα X-rays (λ = 0.71073 Å). The data collections nominally covered

over a hemisphere of reciprocal space, by a combination of three sets of exposures; each

set had a different φ angle for the crystal and each exposure covered 0.3° in ω. The

crystal to detector distance was 5.0 cm. The data sets were corrected empirically for

absorption using SADABS.37 The structures were solved using the Bruker SHELXTL

software package for the PC, by direct method option of SHELXS.

The space groups were determined from an examination of the systematic

absences in the data, and the successful solution and refinement of the structure

confirmed these assignments. All hydrogen atoms were assigned idealized locations and

were given a thermal parameter equivalent to 1.2 or 1.5 times the thermal parameter of

the carbon atom to which they were attached. For the methyl groups, where the location

of the hydrogen atoms was uncertain, the AFIX 137 card was used to allow the hydrogen

atoms to rotate to the maximum area of residual density, while fixing their geometry.

Relevant crystallographic data are listed in Table 2-1.

CHAPTER 3 METALLATION AND RING-OPENING REACTIONS

Introduction

The insertion of metals into porphyrin macrocycles has frequently been employed

in syntheses, and the reaction conditions to prepare these complexes range from mild for

high yielding syntheses of manganese and iron derivatives38 to very harsh for moderate

yielding synthesis of palladium and platinum porphyrins.39 The resulting

metalloporphyrins often have different properties and reactivities than the porphyrin

precursors.1 The number of reported free-base porphodimethenes is very limited18, 40, 41,

due to the fact that the most porphodimethenes synthesized thus far can be obtained from

reductive alkylation of metalloporphyrins19 or oxidative dealkylation of

metalloporphyrinogens.9 The macrocycles isolated from these reactions are inherently

metallated, and in order to change the metal center, transmetallation of the product9 or the

starting material18 must be employed. The 2+2 MacDonald’s type condensation of 5-aryl

dipyrromethanes and vicinal diketones provides a number of free-base porphodimethenes

that can be metallated in one step, and the presence of keto-groups on the spiro-locks

introduces a new mode of reactivity to this class of tetrapyrolles. We were interested in

comparing the properties and reactivities of metalloporphodimethenes and their free-base

precursors.

Other than our entry in the field of the porphodimethene syntheses, all

porphodimethenes prepared thus far feature alkyl substituents at the sp3 meso carbons,

making these ill-suited for porphyrin forming reactions. With an interest in producing

31

32

compounds that could be used as precursors to otherwise inaccessible porphyrins, we

prepared the spiro-tricyclic porphodimethenes introduced in Chapter 2. These synthons

were designed to be susceptible to ring opening and rearrangement reactions producing

porphyrins bearing pendant functional groups or porphyrin with fused exocyclic rings.


Metallation of Porphodimethenes

Free-base porphodimethenes depicted in Scheme 3-1 are easily metallated using

nickel, zinc, copper, palladium or platinum salts. With the two sp3 carbons incorporated

into the macrocyclic ring, porhodimethenes are inherently more flexible than the

porphyrins, and can easily accommodate metals of different sizes.6

The insertion of metals within the porphodimethene macrocycle is generally a high

yielding reaction which can be easily monitored by UV-Vis spectroscopy, since the

typical porphodimethene absorbtion shifts from 430-440 nm to longer wavelengths (470-

510 nm).21 In addition, many of the characteristic features in the 1HNMR spectra of the

porphodimethene change upon metallation. For instance, the separation between the two

doublets arising from the pyrrolic protons in the 1H NMR spectrum of 3-7 decreases in

comparison to the metal free porphodimethene (0.67 ppm versus 0.53 ppm). The

disappearance of the singlet for the NH protons in the free-base provides perhaps the

most diagnostic change on going from the free-base to metalloporphodimethene. Altered

electronic situation within the dipyrromethene halves as well as a modification of the

structural configuration of the macrocycle upon metallation, clearly evident in a

comparison of the solid-state structures of free-base and metalloporphodimethenes, are

likely responsible for the observed change in the spectroscopy

33

O

O

NH N

N HN

R

RO

O

N N

N N

R

R

M

3-6: R = Mesityl

3-11: M = Cu; i) Cu(OAc)2 in MeOH, CH2Cl2, rt3-12: M = Ni; i) Ni(OAc)2 in MeOH, CH2Cl2, rt

3-7: M = Pd; i) Pd(PhCN)2Cl2, CH2Cl2, rt

3-10: R = 3,5-(di-t-butyl)-phenyl

O

O

NH N

N HN

R

R O

O

N N

N N

R

R

M

3-3 3-4: M = Pd; i) Pd(PhCN)2Cl2, reflux in xylenes3-5: M = Zn; i) Zn(OAc)2in MeOH, reflux in CHCl3

i)

R = Mesityl

3-8: M = Pt; i) PtCl2, reflux in PhCN

O

O

O

O

3-1: R = Mesityl 3-2: M = Pd; i) Pd(PhCN)2Cl2, reflux in PhCN

NH N

N HN

R

R N N

N N

R

R

M

3-9: M = Cu; i) Cu(OAc)2 in MeOH, CH2Cl2, rt

3-13: M = Pd; i) Pd(PhCN)2Cl2, CH2Cl2, rt

i)

i)

Scheme 3-1. Illustration of porphodimethene metallation reactions.

As previously outlined in Chapter 1, the properties of porphyrin and

porphodimethene macrocycles are markedly different. The metallation of the macrocycle

with palladium is more facile for porphodimethenes in comparison to porphyrins, with

shorter reaction times, better yields and stoichiometric amounts of the metal, most likely

34

due to the increased flexibility of the porphodimethene macrocycle. Porphodimethenes

can accommodate metals of different sizes imposing less strain on the structure than the

corresponding porphyrins. Due to the presence of saturated carbon atoms in the

macrocyclic ring, the metal-nitrogen bonds in palladium porphodimethenes are somewhat

longer than in their porphyrin counterparts.20 Although platinum porphodimethene has

very similar structure to its palladium analog (vide infra), the insertion of platinum is

limited by the low solubility of Pt(II) salts in the common solvents and harsher conditions

(i.e. refluxing in benzonitrile for 7 days ) are required, resulting in reduced yield.

Structure of Metalloporphodimethenes.

Palladium anthracenone porphodimethene

The crystal structures of various metalloporphodimethenes best illustrate the

flexibility these macrocycles exhibit in accommodating different metal ions.

Figure 3-1. Diagram of the solid-state structure of 3-2 (40% probability; carbon atoms are

depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity.

35

The solid-state structure of 3-2 is shown in Figure 3-1. This molecule adopts a

roof-like folded structure, with the interplanar angle of 136.4 °. The bond angles on the

saturated carbons within the macrocycle are somewhat greater than for an ideal

tetrahedron, but they do not exceed 113.8(6)° due to the strain imposed by the presence

of five-membered rings at the spiro-locks and the rigidity of the anthracene backbone.

Palladium adopts a square-planar arrangement, with bond lengths ranging from 2.001(6)

Å to 2.007(6) Å, and bond angles between 88.7(2)° and 90.4(2)°. The 20 carbon atoms

and the four nitrogens within the tetrapyrrolic ring define the mean plane of the

porphodimethene core, and the average deviation of the 24 atoms from the plane is 0.339

Å, with a maximum deviation being 1.005(8) Å for the sp3 carbon in the

porphodimethene core. Palladium is situated 0.212(2) Å above the mean plane of the

porphodimethene core. The 1H NMR spectrum of this molecule is consistent with a fast

flexing of the molecule in solution, equilibrating between the two roof-like folded

structures. Structural parameters, such as roof angle, palladium –nitrogen bond lengths

and angles, and the displacement of palladium from the plane defined by four nitrogens

for the related palladium porphodimethene with naphthenone substituents at the spiro-

lock42 are coincident to the ones discussed here. The correlation of these parameters

suggests that the additional six-membered ring in the anthracenone substituent compared

to the naphthenone moiety does not have significant influence on the solid-state structure

of 3-2.

Palladium pyrenone porphodimethene

Palladium porphodimethene 3-4 has a six-membered ring with the rigid pyrenone

backbone at the spiro-lock. The pyrenone substituents are orthogonal to the plane of the

36

porphodimethene core, while the mesityl groups form angles of 87.6(1) ° and 77.2(1) °

with this plane.

Figure 3-2. Diagram of the solid-state structure of 3-4 (40% probability; carbon atoms are depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity.

The interplanar angles between meso substituents and the tetrapyrrolic macrocycle

in 3-4 are almost identical to the corresponding angles in the free-base analog 3-3,

indicating that the insertion of palladium in 3-3 does not significantly disrupt the solid-

state structure of this porphodimethene. The mean plane deviation of the 24 core atoms

is 0.448 Å, with the maximum deviation being 0.985(4) Å for the sp3 carbon in the

macrocyclic ring, much like in 3-2. Unlike 3-2, the palladium atom in 3-4 is situated

only 0.099 Å below the mean plane, but it is still in a square planar arrangement.

37

Palladium and platinum phenanthrenone porphodimethenes

The increased flexibility of phenanthrenone substituents at the spiro-locks

compared to the pyrenone or anthracenone allows the porphodimethene core in 3-7 to

flatten out in the presence of palladium forcing angles of nearly 118° on the saturated

carbons within the ring


depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity. Primed and non-primed atoms are related by crystalographically imposed center of inversion.

. As can be seen from the figure 3-3, palladium is in a square planar conformation,

with angles of 90.2(1) ° and 89.8(1) ° between the neighboring nitrogens. The palladium-

nitrogen bonds average 2.037(6) Å, while the average deviation from the mean plane of

the 24 core atoms is only 0.052 Å. The angles between porphodimethene core and the

38

substituents are 88.1(0) º for the phenanthrenone and 71.1(0) ° for the mesityl moieties.

The 1H NMR spectrum is in good agreement with the solid-state structure.


depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity. Primed and non-primed atoms are related by crystallographically imposed center of inversion.

The structure of the platinum porphodimethene 3-8 (Figure 3-3) is, as expected,

very similar to the palladium analog, 3-7. The palladium nitrogen bond lengths in 3-7 are

the same as the platinum nitrogen bond lengths in 3-8 within the estimated standard

deviation. The porphodimethene core is virtually flat with the average mean plane

deviation of only 0.038 Å for the 24 core atoms and the platinum is situated within the

mean plane. The bond angles on the sp3 meso carbons within the porphodimethene ring

39

once again extend to almost 118 °. The angles between the porphodimethene core and

the mesityl substituents in 3-8 are slightly greater than in 3-7 (75.8(1)° vs. 71.1(0) °).

Copper phenanthrenone porphodimethenes

Even though the smaller size of copper compared to palladium and platinum can be

expected to induce roof-like folding in the structure of 3-9, the core of this

porphodimethene is also flat with the average mean plane deviation even smaller than in

3-7 or 3-8 (0.038 Å). The copper adopts a square planar geometry and is situated 0.001

Å above the mean plane of the core. Metal-nitrogen bonds in the copper

porphodimethene are somewhat shorter (2.025 Å) than in platinum and palladium

analogs. The meso substituents form angles of 76.8 ° (mesityl) and 87.0 °

(phenanthrenones) with the porphodimethene ring.

The solid-state structure of 3-11 is illustrated in Figure 3-5. The smaller size of

copper does, in this case, induce a roof-like folding of the macrocycle resulting in an.

angle of 131.7(8)° between the two dipyrromethene halves of the molecule (Figure 3-6).

Metal ligand bonds in 3-11 are shorter (1.966(3) Å to 1.992(3) Å) than in previously

discussed palladium, platinum and copper species, causing the saddle shape of the

molecule. Crystal packing forces can account for different geometries of the two copper

porphodimethenes. Phenathrenone substituents in 3-11 are almost perpendicular to the

porphodimethene core, while the di-t-butyl substituents are somewhat tilted forming the

angles of 75.1(1)° and 56.7(1)° with the core

40

Figure 3-5. Diagram of the solid-state structure of 3-9 (40% probability; carbon atoms are depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity. Primed and non-primed atoms are related by crystallographically imposed center of inversion

Figure 3-6. Diagram of the solid-state structure of 3-11 (40% probability; carbon atoms are depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity.

41

Nickel phenanthrenone porphodimethene

The small size of nickel(II) causes a severe ruffling of the porphodimethene core

upon metallation (Figure 3-5). In the solid-state structure, nickel maintains the square-

planar configuration with metal-nitrogen bond lengths ranging from 1.884(5) to 1.910(5)

Å and angles of 89.2(2)° to 90.9(2), but it is displaced from the porphodimethene core

mean plane by 0.134 Å. The roof-angle in this structure is smaller than in other

porphodimethenes reported here (124.9(1)°). Normally, the presence of nickel in

porphodimethenes locks the roof-like conformation in the solution, and the 1H NMR

spectra of the compounds are more complex.20 Compound 3-12 exhibited a similar

tendency for broad spectral features in 1H NMR spectrum at room temperature, and sharp

resonances were only evident upon heating to 105°. While nickel porphodimethenes with

five membered rings on the spiro-locks do not flex at all at room temperature21, the six-

membered ring of the phenanthrenone in 3-12 is less restraining and at elevated

temperatures this molecule flexes fast enough to display a less complex 1H NMR

spectrum, indicative of a symmetrical molecule - similar to its palladium analog 3-13.

Summary of the Structural Data

Selected bond lengths and angles as well as the interplanar angles and mean plane

deviations listed in Table 3-1 illustrate some of the trends observed in the solid state

structures of the spiro-tricyclic metalloporphodimethenes. The decrease in the metal-

nitrogen bond lengths is accompanied by the decrease in the roof angle. The short nickel-

nitrogen bonds induce a ruffling of the macrocycle that results in a roof angle of only

125°. The metal-nitrogen bonds are the longest in virtually flat palladium

phenanthrenone porphodimethenes (2.037 Å), and they become shorter when the roof-

like folding of the porphodimethene is imposed by more rigid anthracenone and pyrenone

42

substituents. The length of the copper-nitrogen bonds also varies considerably depending

on the conformation of the macrocycle. In the flat copper porphodimethene 3-9 metal-

nitrogen bonds are 0.044 Å longer then in the roof-like folded molecule 3-11. The

central metal does not have much influence on the angles of the substituents at C10 and

C20. These angles seem to be more dependant on the nature of the substituents

themselves. The mesityl groups form angles of 72.0 ° to 87.6 ° with the

porphodimethene core in all the structures, while the angles of di-t-butyl-phenyls are

more flexible and range from 56.9 ° to 80.2 °.


are depicted with arbitrary radii). Hydrogen atoms and the disordered t-butyl groups have been omitted for clarity.

Reactivity of Porphodimethenes

Apart from chelating metals, spiro-tricyclic porphodimethenes exhibit reactivity

towards ring-opening, resulting in functionalized porphyrins.21, 32 A related reaction was

described in 1986 by Chang and Kondylis31 for ring-opening of an acetnatphthenone

Table 3-1. Selected parameters from the solid-state structures of metalloporphodimethenes.

43

3-2 3-4 3-7 3-8 3-9 3-11 3-12

M-N (avg.) 2.005 (6) Å 2.004(4) Å 2.037(6) Å 2.037(3) 2.025(2) 1.981(3) Å 1.900(3) Å

N-M-N (avg.) 89.9(2) ° 90.0(2) ° 90.0(1) ° 90.0(1) ° 90.0(1 ) ° 90.0(1) ° 90.0(1) °

C4-C5-C6 113.3(8) ° 112.8(4) ° 117.7(1) ° 117.4(3) ° 117.4(3) ° 114.0(3) ° 110.3(3) º

C14-C15-C16 113.8(6) ° 113.2(4) ° - - - 111.3(3) ° 109.1(3) º

Roof angle 135.6(2) ° 136.8(1) ° 180.0 ° 180.0 ° 179.8 ° 131.7(8)° 124.9(1) º

Average mean plane deviation

0.339 Å 0.448 Å 0.052 Å 0.038 Å 0.012 Å 0.344 Å 0.536 Å

Maximum mean plane deviation

1.005(8) Å 0.985(4) Å 0.140 Å 0.121 Å 0.031 Å 0.881(4)Å 1.133(4) Å

M-mean plane 0.212(2) Å - 0.099 (1) Å 0.000 Å 0.000 Å 0.014 Å 0.134(1)Å 0.140(1)

44

dipyrromethane in refluxing 30% potassium hydroxide. Following this approach,

treatment of porphodimethenes with strong bases such as KOH in the presence of

dioxygen afforded porphyrins with two carboxylic functionalities.21 If sodium

borohydride is used instead of KOH, the ring-opening reaction yields porphyrin with two

benzyl alcohol groups (Scheme 3-2). In view of the ease at which functionalized

porphyrins can be isolated from spiro-ticyclic porphodimethenes, we undertook further

studies to establish the influence of the ring-size at the spiro-lock as well as the identity

of the groups on the meso positions on the reactivity of the macrocycle. The ring-

opening reactions were first tested on the naphthenone porphodimethenes with five-

membered rings on the spiro-locks,21 and the same conditions were successfully

employed to convert phenanthrenone porphodimethene with six-membered rings at the

spiro-locks to corresponding bis-functionalized porphyrins (Scheme 3-3).

Metalloporphodimethenes react in the same fashion in the presence of base or NaBH4 and

oxygen to form bisfunctionalized metalloporphyrins.21

Ring opening reactions of dispiro porphodimethenes appear to be driven by the

desire of the macrocycle to achieve fully aromatic 18-annulene ring system and we have

utilized that driving force to explore the reactivity of porphodimethenes with six-

membered ring on the spiro-lock. Both the pyrenone derivative 3-3 and the

phenanthrenone derivative 3-6 have a six-membered ring at the spiro-lock, but their

reactivities are considerably different.

45

HN

N

R

N

HN

R

1. NaOMe, THF, rt

2. O2

O

ONH

N

R

N

HN

R

1. NaBH4, THF, rt

2. O2, HCl

R = Mesityl

MeO

OMe

O

O

HO

OH

NH N

N HN

R

R

O

O NH N

N HN

R

R

Scheme 3-2. Diagram of the ring-opening of the acenaphthenone porphodimethene. Both

the dialcohol and the diester formation are almost quantitative.

HN

N

R

N

HN

R

1. NaOMe, THF, rt

2. O2

1. NaBH4, THF, rt

2. O2, HCl

R = Mesityl

CO2Me

MeO2C

HN

N

R

N

HN

R

CH2OH

HOH2C

O

O

NH N

N HN

R

R

O

O

NH N

N HN

R

R

3-14

Scheme 3-3. Diagram of ring-opening of the phenanthrenone porphodimethene.

Resulting α,β porphyrins interconvert to corresponding α,α isomers upon heating in toluene.

46

In addition to previously demonstrated ring-opening reaction of 3-6 to form

biphenyl- porphyrin dialcohol (the first example of biphenyl substituted porphyrin),22 we

have been able to obtain corresponding porphyrindiester derivatives shown in Scheme 3-

4. Much like the porphyrin dialcohol, the diester 3-14, which is the only isomer formed

in the ring-opening reaction, interconverts to α,α atropoisomer upon heating in toluene.

The interconversion reaches equilibrium at the isomer ratio 42: 58 (α,α : α,β).

The solid state structure of porphyrin 3-14 (Figure 3-8) shows that the biphenyl

moieties are twisted for 82.9(1) ° with respect to the porphyrin ring and the phenyl

groups within the biphenyls are rotated 51.6(1) ° relative to each other. This

conformation places the two carboxy carbons 3.104(3) Å above and below the porphyrin

plane, reducing unfavorable interactions of the methoxy groups with the electron rich

macrocycle.


are depicted with arbitrary radii). Hydrogen atoms have been omitted for clarity. Primed and non-primed atoms are related by center of inversion

47

The rigidity of the pyrene backbone, on the other hand, caused the compound 3-3

to be stable under harsh conditions (KOH in refluxing THF, or H2SO4 in refluxing o-

dichlorobenzene) and resistant to ring opening and porphyrin formation in the presence of

NaBH4 or KMnO4. If the six-membered rings were to open, the pyrenones would be

converted into 10-functionalized phenanthrene groups, and the carboxylates (or hydoxy

groups) would be forced to point directly at the ring of the resulting porphyrin, as

illustrated in Scheme 3-4. These unfavorable interactions appear to preclude the ring

opening at the spiro-lock of compound 3-3.

Another type of ring-opening reaction specific to metal containing spiro-tricyclic

porphodimethenes is a radical initiated rearrangement of carbon-carbon bonds to yield

porphyrins with exocyclic keto-rings.

1. NaBH4, THF, rt2. O2, HCl

O

O

NH N

N HN

R

R

HO

NH N

R

N HN

OH

R

3-3 Scheme 3-4. Illustration of the ring-opening reaction of 3-3. The resulting porphyrin

would have two functional groups pointing towards the macrocyclic ring.

The reaction conditions for this transformation are mild for naphthenone

porphodimethenes and very harsh for phenanthrenone porphodimethenes (Scheme 3-5).

Syntheses of porphyrins with exocyclic rings will be discussed in detail in Chapter 5.

48

Conclusions

Porphodimethenes with different aromatic meso substituents were successfully

metallated using various metal salts. The resulting metalloporphodimethenes readily

formed single crystals suitable for X-ray diffraction.

N N

N N

R

R

O

Pd

O

∆, C6H5CN

Pd(C6H5CN)2Cl2

N

N

RO

N

N

O

R

N

N

RO

N

N

OR

Pd

Pd

R = Mesityl

O

O

R1

R1

R1

R1

N N

N N

R

R

Pd

N

N

N

N

Pd

OO

R

R

R1

R1

R1

R1

N

N

N

N

Pd

O

O

R

RR1

R1R1

R1

hν, DDQ

CH2Cl2

R = Mesityl, R1 = t-Bu

Scheme 3-5. Illustration of heptanone and octanone porphyrin formation. These ring-

opening reactions are specific to metalloporphodimethenes.

49

The dissimilarities in the solid-state structures of the metalloporphodimethenes

arise from the differences in the central metal atom as well as the nature of the

substituents at saturated meso carbons and the size of the ring at the spiro-lock.

The nature of the substituents at spiro-locks also determines the reactivity of the

porphodimethenes. The presence of phenanthrenones at saturated meso carbons is

suitable for ring-opening reactions resulting in bis functionalized porphyrins.

Porphodimethenes with pyrenone substituents at the spiro-locks, on the other hand, are

unreactive under the ring-opening reaction conditions. In view of their unique stability,

these macrocycles should be suitable for further functionalization of carbonyl groups

without altering the porphodimethene backbone.

Experimental

General Procedures.




services using FAB or ESI as ionization method. All solvents were used as purchased,

unless otherwise specified.

Chromatography

Absorption column chromatography was performed using neutral alumina

(Aldrich, Brockman I ~ 158 mesh, 58 Ǻ) or chromatographic silica gel (Fisher, 200 – 425

mesh).

Synthesis of 3-2

A portion of 0.024g (0.02 mmol) of 3-1 was dissolved in 20 ml of xylenes and

0.015 g (0.04 mmol) of Pd(C6H5CN)2Cl2 in methanol was added. The mixture was

50

refluxed for 3 days, and the solvent was evaporated under vacuum. The solid residue was

redissolved in a minimal amount of methylene chloride and purified through a silica gel

column. The first bright orange fraction was collected, and the solvent was evaporated to

give 0.022g (83%) of dark orange solid. 1H NMR (300 MHz, CDCl3) : δ = 8 .70 (d, 2H,

J = 7.5 Hz), 8.63 (d, 2H, J = 6.9 Hz), 8.24 (J = 6.6 Hz), 7.98 – 7.79 (m, 10H), 6.80 (s,

4H), 6.27 (d, 4H, J = 4.5 Hz), 5.70 (d, 4H, J = 4.5 Hz), 2.26 (s, 6H), 2.06 (s, 12H). UV-

Vis [methylene chloride, λmax(logε) ] 491 nm (4.51). Anal.Calcd. for

C68H46N4O2Pd·2CH2Cl2: C, 68.62; H, 4.12; N, 4.58. Found: C, 66.88; H, 3.86; N, 4.48 MS

(MALDI-DIOS) calcd. for M+ (C68H46O2N4Pd): 1056. Found 1056. Experimental

isotope pattern matched the theoretical isotope pattern. Slow diffusion of pentane into a

saturated chloroform solution of 3-2 yielded single crystals suitable for diffraction

studies.

Synthesis of 3-4

A sample of 0.155 (0.16 mmol) g of 3-3 and 0.070 g (0.25 mmol)of

Pd(C6H5CN)Cl2 was dissolved in 50 ml of benzonitrile, and the mixture was refluxed for

90 minutes. The solvent was removed under vacuum, and the solid was washed with

methylene chloride – toluene (5-1) and filtered. The filtrate was preadsorbed on silica gel

and purified through a silica column with methylene chloride. The leading orange band

was collected, and the solvent was evaporated to give dark orange solid. Yield 87 %.

Slow evaporation of toluene solution of 3-4 afforded large single crystals. UV-Vis

[toluene, λmax (log ε)] 501nm (4.86). 1HNMR (300 MHz, CDCl3): 9.36 (d, 2H, J = 8.1

Hz), 8.96 (d, 2H, J = 6.6 Hz), 8.77 (s, 2H), 8.19 (d, 2H, J = 8.4 Hz), 8.08 (s, 1H), 8.05 (s,

51

1H), 7.90 (dd, 2H, J1 = 6.6 Hz J2 = 11.7 Hz), 7.84- 7.79 (m, 2H), 7.67-7.61 (m, 2H), 6.83

(s, 4H), 6.36 (d, 4H, J = 4.5 Hz), 6.03 (d, 4H, J= 4.2 Hz), 2.28 (s, 6H), 2.07 (s, 12H).

Synthesis of 3-5

A sample of 0.065g (0.07 mmol) of 3-3 was dissolved in 60 ml of chloroform, and

5 ml of a saturated methanolic solution of Zn(OAc)2 was added. The reaction mixture

was refluxed overnight, cooled to the room temperature, washed with water and dried

over Na2SO4. The solvent was evaporated to yield 0.065 g (94 %) of green solid. UV-Vis

[methylene chloride, λmax (log ε)] 474 nm (4.99). 1H NMR (300 MHz, o-C6D4Cl2): 8.59

(d, 2H, J = 7.5 Hz), 8.38 (d, 2H, J = 7.5 Hz ), 8.18 (d, 2H, J = 6.9 Hz), 7.93 – 7.71 (m, 10

H), 6.85 (s, 4H), 6.50 (d, 4H, J = 3.9 Hz), 6.02 (d, 4H, J = 4.2 Hz), 2.33 (s, 6H), 2.13 (s,

12H). HRMS (FAB) calcd. for MH+ (C68H47O2N4Zn) 1015.2990. Found: 1015.3021.

Synthesis of 3-7.

A sample of 0.010 g (0.03 mmol) of Pd(C6H5CN)2Cl2 was heated in 50 ml of

refluxing dry toluene until it completely dissolved. After cooling to 70 ºC, 0.025 g (0.03

mmol) of phenanthrenone porphodimethene 3-6 was added to the solution, and the

reaction mixture was heated to 90 ºC and kept at this temperature for 2.5 hours. The

solution was filtered through a plug of silica, and the solvent was evaporated under

vacuum yielding 0.027 g (97 %) of dark orange powder. The product was recrystallized

from chloroform/pentane. UV-Vis [methylene chloride, λmax (log ε)] 489 nm (5.03) .

Anal.Calcd. for C64H46N4O2Pd·2CHCl3: C, 63.51; H, 3.88; N, 4.49. Found: C,63.22; H,

3.43; N, 4.49. 1H NMR (300 MHz, CDCl3) δ = 8.28 (dd, 4H, J1 = J2 = 7.2 Hz), 8.14 (dd,

4H, J1 = 8.1 Hz, J2 = 11.1 Hz), 7.74 (dd, 2H, J1 = J2 = 7.2 Hz), 7.54 – 7.43 (m, 6H), 6.82

(s, 4H), 6.29 (d, 4H, J = 4.5 Hz), 5.74 (d, 4H, J = 4.5 Hz), 2.28 (s, 6H), 2.06 (s, 12H).

52

HRMS (ESI-FT-ICR) calcd. for M+ (C64H46O2N4Pd): 1008.2672. Found 1008.2665.

Slow diffusion of pentane in a concentrated acetonitrile solution of 3-7 produced large

single crystals.

Synthesis of 3-8

A sample of 0.050 g (0.05 mmol) of 3-6 was dissolved in 20 ml of benzonitrile

and 0.046 mg (0.16 mmol) of PtCl2 was added. The solution was heated to reflux, and

after 7 days of refluxing, the solvent was removed under vacuum. The solid residue was

redissolved in a minimal amount of methylene chloride. The solution was columned over

silica with methylene chloride : toluene 1: 1. The second, orange band was collected, and

the solvent was evaporated. The residue was recrystalized from methylene chloride -

hexanes. Slow diffusion of pentane into a saturated methylene chloride solution of 3-8

afforded crystals suitable for X-ray diffraction. Yield 0.010 g (16 %). UV-Vis [toluene,

λmax (log ε)] 496 nm (5.01). 1HNMR (300 MHz, CDCl3) δ = 8.26 (d, 2H, J = 7.50 Hz ),

8.18 – 8.11 (m, 6H), 7.75 (dd, 2H, J1 = J2 = 7.20 Hz), 6.83 (s, 4H), 6.36 (d, 2H, J = 4.50

Hz), 5.82 (d, 4H, J = 4.50 Hz), 2.29 (s, 6H), 2.07 (s, 12H). HRMS (FAB) calcd. for MH+

(C64H47O2N4Pt): 1098.3347. Found 1098.3352.

Synthesis of 3-9

A portion of 0.025 g (0.03 mmol) of 3-6 was dissolved in 20 ml of methylene

chloride and 10 ml of a saturated methanolic solution of Cu(OAc)2 was added. The

reaction mixture was stirred at room temperature for 40 minutes. The solution was then

filtered through a silica gel plug. Evaporation of the solvent resulted in the isolation of

0.027g (99 %) of red-orange powder. Slow diffusion of pentane into a saturated

cloroform solution of 3-9 produced large single crystals. UV-Vis [methylene chloride,

53

λmax (log ε)] 482 nm (5.15). HRMS (FAB) calcd. for MH+ (C64H47O2N4Cu): 966.2995.

Found 966.2989.

Synthesis of 3-11

A portion of 0.050 g (0.05 mmol) of 3-10 was dissolved in 40 ml of methylene

chloride and 10 ml of saturated methanolic solution of Cu(OAc)2 was added. The

reaction mixture was stirred at room temperature for 40 minutes. The solution was then

filtered through a plug of silica. Evaporation of the solvent resulted in the isolation of

0.052 g (98 %) of red-orange powder. UV-Vis [methylene chloride λmax (log ε)] 480 nm

(4.79). HRMS (FAB) calcd. for MH+ (C74H67O2N4Cu): 1106.4560. Found 1107.4130

(isotope distribution corresponds to combination of M+ and MH+). Slow diffusion of

pentanes into a chloroform saturated solution of 3-11 afforded crystals suitable for

diffraction studies.

Synthesis of 3-12

A sample of 0.050 g (0.05 mmol) of 3-10 was dissolved in 30 ml of methylene

chloride and 8 ml of saturated solution of Ni(OAc)2 in methanol was added. The reaction

mixture was refluxed overnight, washed with water, and dried over Na2SO4. The solvent

was evaporated to yield 0.035 g (66 %) of dark orange solid. Slow diffusion of ether into

a chloroform solution saturated with 3-12 produced X-ray quality crystals. UV-Vis

[toluene, λmax(log ε)] 487 nm (4.40), 433 nm (4.33). 1H NMR (300 MHz, toluene-d8,

105°C) δ = 9.74 (bs, 2H), 8.39 (d, 2H, J = 7.5 Hz ), 7.79 – 7.67 (m, 6H), 7.47- 7.45 (s,

2H), 7.29 – 7.20 (m, 8H),7.03 (s, 2H, under the solvent peak) 6.48 (d, 4H, J = 4.5 Hz),

5.90 (d, 4H, J = 4.5 Hz), 1.19 (s, 36H). Anal.Calcd. for C74H66N4O2Ni·2CHCl3: C, 68.08;

54

H, 5.11; N, 4.18. Found: C,68.22; H, 4.73; N, 4.28. HRMS (FAB) calcd. for MH+

(C74H67O2N4Ni): 1101.4617. Found 1101.4526.

Synthesis of 3-13

A sample of 0.025 g (0.02 mmol) of 3-10 was dissolved in 15 ml of methylene

chloride and a solution of Pd(PhCN)2Cl2 (0.010 g in 2 ml of methanol) was added. The

reaction mixture was stirred at room temperature until the starting material was consumed

(48 h). The solution was then concentrated and purified over silica column with

methylene chloride. The first orange band was collected, and the solvent was evaporated.

Yield: 0.021 g (76 %). 1H NMR (300 MHz, CDCl3) δ = 8.47 (d, 2H, J = 7.8 Hz), 8.33

(d, 2H, J = 7.6 Hz ), 8.18 – 8.11 (m, 4H), 7.74 (dd, 2H, J1 = J2 = 7.5 Hz), 7.57 (dd, 2H,

J1 = J2 = 7.5 Hz), 7.50 - 7.44 (m, 4H), 7.39 (s, 2H), 7.25 (s, 2H), 7.18 (s, 2H), 6.45 (d,

4H, J = 4.5 Hz), 5.81 (d, 4H, J = 4.5 Hz), 1.26 (s, 36H). UV-Vis [methylene chloride,

λmax (log ε)] 487 nm (5.30). MS (MALDI-DIOS) calcd. for M+ (C74H66O2N4Pd): 1148.

Found 1148. Experimental isotope pattern matched the theoretical isotope pattern.

Synthesis of 3-14 and 3-15

A sample of 0.025 g (0.63 mmol) of sodium was dissolved in 5 ml of methanol and

10 ml of THF. Following the dissolution of sodium, 0.027 g (0.03 mmol) of 3-6 was

added, and the reaction mixture was stirred under argon. After two hours of stirrting,

oxygen was bubbled through the solution, followed after 5 minutes, with 15 ml of water

and 35 ml of methylene chloride. The organic layer was collected, washed with water,

and dried over Na2SO4. The solvent was removed under vacuum, and the solid residue

was redissolved in a minimal amount of methylene chloride and purified through a plug

of silica. The first, green-purple band was collected and the solvent evaporated under

vacuum. Yield: 0.025 g (94%). UV-Vis [methylene chloride, λmax (log ε)] 422 nm

55

(5.53), 553 nm (3.64), 594 nm (3.61), 649 nm (3.27) . Anal.Calcd. for C66H54N4O4: C,

81.96; H, 5.63; N, 5.79. Found: C,81.87; H, 5.48; N, 5.31. 1H NMR (300 MHz, CDCl3) δ

= 9.04 (bs, 2H ), 8.70 (bs, 2H ), 8.50 (bs, 4H), 8.01 (d, 2H, J = 7.5 Hz), 7.78 (dd, 2H, J1 =

J2 = 7.5 Hz), 7.66 - 7.61 (m, 4H), 7.28 – 7.25 (m, 8H ), 6.61 – 6.51 (m, 4H), 3.67 (s, 6H),

2.61 (s, 6H), 1.73 (s, 12H), -2.86 (s, 2H). HRMS (FAB) calcd. for M+ (C66H55N4O4)

966.4145. Found: 966.4170

Upon heating the solution of 3-14 in toluene, interconversion to α,β isomer 3-15

was observed. Interconversion was complete after 2 hours of reflux and the ratio of the

two isomers was estimated by integration of the NMR resonances to be approximately

42: 58 α,α : α,β.

An analyticaly pure sample of 3-15 was separated from 3-14 on a silica column

using toluene : methylene chloride 1:1 as eluent. Compound 3-15 was collected as the

second fraction. UV-Vis [methylene chloride, λmax(log ε)] 423 nm (5.67), 518 nm (3.56)

553 nm (3.50), 650 nm (2.85). 1H NMR (300 MHz, CDCl3) δ = 9.00 (bs, 2H ), 8.68 (bs,

2H ), 8.49 (bs, 4H), 8.16 (d, 2H, J = 7.2 Hz), 7.81 (dd, 2H, J1 = J2 = 7.2 Hz), 7.72 - 7.65

(m, 4H), 7.28 – 7.25 (m, 4H ), 7.05 (bs, 4H), 6.39 (d, 2H, J = 7.2 Hz), 3.57 (s, 6H), 2.61

(s, 6H), 1.73 (s, 12H), -2.93 (s, 2H) . HRMS (FAB) calcd. for MH+ (C66H55N4O4)

967.4223. Found: 967.4224. Slow diffusion of pentane in the methylene chloride

solution of 3-15 produced small single crystals.


Unit cell dimensions were obtained (Tables 3-2 and 3-3) and intensity data

collected by Prof. Michael Scott on a Siemens CCD SMART diffractometer at low

temperature, with monochromatic Mo-Kα X-rays (λ = 0.71073 Å).

56

Table 3-2. Crystallographic data for compounds 3-2, 3-4, 3-7 and 3-8 3-2· CHCl3 3-4· 2C7H8 3-7 3-8·2CH2Cl2

Formula C69H47Cl3N4O2Pd C82H62N4O2Pd C64H46N4O2Pd C66H50Cl4N4O2Pt

Formula weight 1176.86 1241.76 1009.45 1267.99

Crystal system Triclinic Monoclinic Monoclinic Monoclinic

Space group P1 P21/c P21/n P2/m

Z 2 4 2 2

Temp, K 173(2) 173(2) 193(2) 193(2)

Dcalc gcm-3 1.116 1.358 1.474 1.523

a Å 13.361(2) 15.0316(9) 11.003(7) 13.806(6)

b Å 16.996(3) 15.0316(9) 13.186(8) 14.359(6)

c Å 17.448(3) 14.5815(9) 15.691(10) 15.511(7)

α, deg 69.749(4) - - -

β, deg 87.155(4) 91.8500(10) 92.229(15) 115.974(10)

γ, deg 70.886(3) - - -

V Å3 3503.2(10) 6074.0(7) 2275(2) 2764(2)

µ, mm-1 0.420 0.361 0.463 0.278

Uniq. data coll./obs. 9644/7387 10030/5762 15066/5278 6266/4678

R1[I > 2σ(I)data]a 0.1050 0.0453 0.0322 0.0283

wR2[I > 2σ(I)data]b 0.2894 0.1076 0.0893 0.0762 a R1 = Σ||Fo| - |Fc||/ Σ| Fo| bwR2 = { Σ[w (Fo

2 – Fc2)2/ Σ[w ( Fo

2)2}

57

Table 3-3. Crystallographic data for compounds 3-9, 3-11, 3-12 and 3-15 3-9·2CHCl3 3-11· CHCl3 3-12· ½C4H10O 3-15

Formula C66H48Cl6N4O2Cu C75H67Cl3CuN4O2 C75H71N4O2.5Ni C66H54N4O4

Formula weight 1205.32 1226.22 1127.11 967.19

Crystal system Tetragonal Monoclinic Monoclinic Monoclinic

Space group I41/a P21/c P21/n P21/c

Z 8 4 4 2

Temp, K 173(2) 193(2) 193(2) 193(2)

Dcalc gcm-3 1.455 1.302 1.013 1.252

a Å 30.220(2) 24.305(1) 15.134(1) 14.152(6)

b Å 30.220(2) 13.659(1) 17.449(1) 12.700(6)

c Å 12.046(1) 19.031(1) 26.541(2) 14.344(6)

β, deg 90.0 98.022(1) 97.660(2) 95.542(1)

V Å3 11001(1) 6256(1) 6946(1) 2566(2)

µ, mm-1 0.740 0.528 0.321 0.078

Uniq. data coll./obs. 33993/6312 14762/8270 16351/ 17653/6008

R1[I > 2σ(I)data]a 0.0618 0.0715 0.0863 0.0503

wR2[I > 2σ(I)data]b 0.1653 0.1739 0.2262 0.1374 a R1 = Σ||Fo| - |Fc||/ Σ| Fo| bwR2 = { Σ[w (Fo

2 – Fc2)2/ Σ[w ( Fo

2)2}

58

The data collections nominally covered over a hemisphere of reciprocal space, by a

combination of three sets of exposures; each set had a different φ angle for the crystal and

each exposure covered 0.3° in ω. The crystal to detector distance was 5.0 cm. The data

sets were corrected empirically for absorption using SADABS.37 The structure was

solved using the Bruker SHELXTL software package for the PC, by direct method option

of SHELXS. The space group was determined from an examination of the systematic

absences in the data, and the successful solution and refinement of the structure

confirmed these assignments. All hydrogen atoms were assigned idealized locations and

were given a thermal parameter equivalent to 1.2 or 1.5 times the thermal parameter of

the carbon atom to which it were attached. For the methyl groups, where the location of

the hydrogen atoms was uncertain, the AFIX 137 card was used to allow the hydrogen

atoms to rotate to the maximum area of residual density, while fixing their geometry.

CHAPTER 4 PHOTOPHYSISCAL PROPERTIES OF PORPHODIMETHENES

Introduction

Although porphodimethenes have long been recognized to be intermediates in the

oxidation pathway of porphyrinogens to porphyrins, the first synthetic scheme for their

production was only reported in the 1974.2 Despite the success of Buchler, investigations

of the physical properties of porphodimethenes have been hampered by the difficulties

associated with the synthesis and separation of these partially reduced porphyrins. Since

new, simple procedures for the isolation of multigram quantities of porphodimethenes,

are now available, we undertook a detailed examination of their physical and chemical

properties. To date, structural,23 theoretical,40 magnetic,41 and electrochemical27 studies

have been conducted on metalloporphodimethenes, and herein we present the first

detailed study of the photophysical properties, including steady-state emission

measurements, transient absorption, and the quantum yield of singlet oxygen generation

of this unique class of tetrapyrrolic macrocycles. While only two porphodimethenes (4-2

and 4-14) were chosen for detailed photophysical examination, electronic absorption

spectra of all the previously reported metalloporphodimethenes prepared in our lab and

the fluorescence emission of several free-bases (Scheme 4-1) were also measured and

will be discussed in this chapter.

59

60

O

O

N N

N N

R

R

M

4-11: M = Pd; R = Mesityl4-12: M = Zn; R = Mesityl

4-10: M = 2H; R = Mesityl

O


O

N N

N N

R

R

M

O

O

4-1: M = Pd; R = Mesityl; R1 = t-butyl4-1a: M = Pd; R = p-Tolyl; R1 = H

N N

N N

R

R

M

O O

N N

N N

R

R

M


O

O

N N

N N

R

R

M

4-4: M = Pt; R = Mesityl4-5: M = Cu; R = Mesityl

4-7: M = Pd; R = 3,5-di-t-butyl-phenyl4-8: M = Cu; R = 3,5-di-t-butyl-phenyl4-9: M = Ni; R = 3,5-di-t-butyl-phenyl

4-3: M = 2H; R = Mesityl4-2: M = Pd; R = Mesityl

4-6: M = 2H; R = 3,5-di-t-butyl-phenyl

O

O

4-15: M = Pd; R = Mesityl4-14: M = 2H; R = Mesityl

N N

N N

R

R

M

R1

R1 R1

R1

Scheme 4-1 Depiction of free-base and metalloporphodimethenes with measured

electronic absorption spectra.

61

Table 4-1. Selected UV-Vis absorption data for the free-base and metalloporphodimethenes. The presence of metals in macrocyclic ring induces the absorption maximum to shift towards longer wavelengths.

λmax [nm] log ε

4-1 491a 4.94

4-2 489b 5.03

4-3 432a 4.89

4-4 482c 5.15

4-5 496b 5.01

4-6 438b 4.90

4-7 487b 5.30

4-8 480b 4.79

4-9 487, 433b 4.40, 4.33

4-10 440d 4.94

4-11 501c 4.86

4-12 474b 4.99

4-13 442d 4.97

4-14 448c 4.94

4-15 491b 4.51

4-16 452a 4.92 a chloroform, bmethylene chloride, ctoluene, do-di-chlorobenzene

Porphodimethenes tend to be bright orange materials since they still maintain a

Soret-like feature in the high-energy region of the visible spectrum, but the absorption is

batochromically shifted, considerably broadened and has a smaller logarithmic extinction

coefficient in comparison to porphyrins. Slight variations are observed in absorption

spectra of different free base porphodimethenes depending on the nature and orientation

of the meso substituents (Figure 4-1).

62

Wavelength / nm350 400 450 500 550 600 650

Abs

orba

nce 4-3

4-104-14

Figure 4-1: Illustration of the UV-Vis absorption spectra of free base porphodimethenes.

The meso substituents do not significantly influence spectral characteristics of the free-base spiro-tricyclic porphodimethenes.

Wavelength / nm

350 400 450 500 550 600 650

Abso

rban

ce

4-74-84-9

Figure 4-2. Illustration of the absorption spectra of metalloporphodimethenes. Spectral

properties depend on the nature of central atom.

63

Insertion of metals into the porphodimethene core causes the absorption maximum

to shift toward longer wavelengths, and the magnitude of this shift varies with the nature

of the metal (Figure 4-2).

With palladium incorporated into the macrocycle, in porphodimethenes 4-1 and 4-2

depicted in Scheme 4-1, main absorption band shifts towards longer wavelengths in

comparison to the free base macrocycles: from 432 nm to 489 nm and from437 nm to 491

nm, respectively for 4-1 and 4-2. Despite numerous crystallization attempts, compound

4-1 consistently formed small needle-like crystals which were unsuitable for diffraction

experiments, but a related porphodimethene 4-1b was structurally characterized.42 This

species differs from 4-1 in that it bears p-tolyl groups at two meso-positions rather than

mesityls and lacks the t-butyl groups on the naphthyl substituents. As can be seen in the

solid-state structure of 4-1b presented in Figure 4-3, the palladium resides in a square

planar arrangement, but the metal center is situated slightly above the mean plane of the

four pyrrole nitrogens by 0.068 Å. The two spiro-locks, defined by the five-membered

ring of the acenaphthenone, are trans to each other in an anti-configuration, and these

saturated carbon atoms force the tetrapyrrolic macrocycle to adopt a roof-like fold with

an interplanar roof angle of 135 °. In solution, however, the macrocycle flexes about the

saturated carbons, equilibrating the resonances in the 1H NMR of the two p-tolyl groups

and the two naphthyl moieties, and this tendency to flex about the sp3 carbons appears to

be general for most metalloporphodimethenes.

In contrast to the naphthenone substituted porphodimethene 4-1, the increased

flexibility afforded by the two phenanthrenone moieties and the six-membered rings at

the spiro-lock in 4-2 allow the angle between the two dipyrromethene units to flatten out

64

Figure 4-3.Diagram of solid-state structure of 4-1b (40% probability; carbon atoms are depicted with arbitrary radii). Hydrogen atoms are omitted for clarity.

in the solid-state with a concomitant increase in the angle intra-macrocycle angle at the

sp3 spiro-carbons to 117.8(2)°.

The crystallographically imposed C2 arrangement of the two halves of 4-2 allows

the metal center to adopt a rigorous planar geometry, and the flattening of the macrocycle

also induces a slight increase of the average metal-pyrrole distance in 4-2 in comparison

to 4-1b.

Fluorescence Spectroscopy

The fluorescence spectra of compounds 4-3, 4-10, 4-13, 4-14 and 4-16 were

measured through excitation at the wavelength corresponding to absorption maxima

found in UV-Vis spectra. All of the compounds exhibited weak fluorescence emission

65

between 592 nm and 688 nm (Table 4-2). Quantum yields range between 1.02x10-4 and

8.9x10-3 depending on the substituents and the isomer (syn or anti), which is low in

comparison to quantum yields of porphyrins.42 No trend was observed among different

macrocycles with regard to their fluorescence, although isomer pairs exhibited

comparable quantum yields.

Table 4-2: The values of fluorescence emission maxima and quantum yields for selected free-base porphodimethenes. The fluorescence is very weak.

Compound λexc [nm] λem [nm] Φf

4-3 433 592 7.94 x10-4

4-10 440 624 8.70 x10-3

4-13 442 624 1.41 x10-3

4-14 452 688 1.13 x10-4

4-16 448 615 1.02x10-4

Phosphorescence Emission

As described above, the absence of a porphyrin-like 18-annulene ring system in

porphodimethenes simplifies the absorption spectrum of the macrocycle, and given the

paucity of data concerning the photophysical characteristics of porphodimethenes, we

instigated a study of the emission properties of both 4-1 and 4-2. While free base

porphodimethenes do exhibit weak fluorescence emission, the insertion of a heavy atom

(palladium) should influence the spin-orbit coupling within the macrocycle and thus

enhance the singlet-triplet intersystem crossing at the expense of the fluorescence

emission from the singlet state. Indeed, although emission from the triplet-excited states

of 4-1 and 4-2 (phosphorescence) is not observed at room temperature, strong bands in

the visible region are evident upon cooling the samples. The highest intensity emission

maximum (at 703 nm) for compound 4-2 was observed at 100 K (the solvent glass

66

transition temperature), and as the temperature of the sample was increased; the maxima

of this feature in the spectra shifted towards longer wavelengths, and the signal

disappeared completely when the solution reached room temperature (Figure 4-4). The

identity of the substituents at the spiro-locks on the macrocycle (4-1-naphthenone, 4-2-

phenanthrenone) appears to have negligible influence on the energy of triplet state,

presumably since the groups are perpendicular to the macrocyclic ring. The emission

maxima of compound 4-1 (at 708 nm) also reached maximum at 100 K. Unlike in 4-2,

this feature was more sensitive to temperature changes and was completely quenched at

200 K (Figure 4-4). Given that 4-1 has been shown to undergo a light induced radical

rearrangement at room temperature (vide infra), the rapid decrease in the intensity of the

spectral feature is likely a result of activation of the macrocycle by the light source. The

respective triplet state energies for compounds 4-1 and 4-2 are 40 kcal/mol and 41

kcal/mol.

Transient Absorption

Transient absorption spectroscopy was used to probe the lifetime of the triplet

states, and after irradiation at 355 nm, both 4-1 and 4-2 exhibit sharp transients at 520 nm

(Figure 4-5). The disappearance of transient absorption for 4-1 was fit with a two-

component decay (lifetimes 2.2 µs and 360 ns), likely corresponding to the fading of the

porphodimethene triplet state absorbance in addition to the decay of a radical formed

upon laser irradiation of the porphodimethene. For compound 4-2, on the other hand, the

decay of the transient absorption could be fitted to single component pathway with a

triplet lifetime of 2.7 µs, as expected given that 4-2 is not nearly as sensitive to light as 4-

1.

67

4-1

Wavelength / nm

660 680 700 720 740 760 780 800

Rel

. I100 K140 K200 K

4-2

660 680 700 720 740 760 780 800

Rel

. I

100 K140 K180 K220 K298 K

Figure 4-4. Depiction of phosphorescence emission for 4-1 and 4-2. The triplet state

energies are similar, but the phosphorescence of light-sensitive 4-1 disappears at lower temperature than the phosphorescence of 4-2.

68

Wavelength / nm500 600 700 800

∆ A

0.0

0.1

0.2

0.3

0.4

4-24-14

Figure 4-5. Depiction of transient absorption of 4-1 and 4-2. Both molecules feature sharp transients at 520 nm.

Even though the lifetimes of their triplet states are short, both 4-1 and 4-2 exhibit

effective energy transfer to dioxygen molecules resulting in generation of singlet oxygen

with quantum yields of 0.85 and 0.87 (respectively for 4-1 and 4-2).

Conclusions

Insertion of metals in tetrapyrrolic ring induces red-shifts in the UV-Vis spectra of

both the porphodimethenes with the alkyl20 and the aryl substituents.21 The spectra

presented here are in good agreement with this trend, showing that the nature of the

substituents on macrocycle does not influence the spectral properties significantly.

The presence of the central metal atom, on the other hand, changes both the

absorption and the emission spectra of the porphodimethenes. While the free-base

porphodimethenes exhibit weak fluorescence, the first detailed investigation of the

photophysical properties of porphodimethenes revealed that the fluorescence emission in

palladium derivatives 4-1 and 4-2, and these compounds show low temperature

69

phosphorescence instead. Porphodimethenes 4-1 and 4-2 have short triplet state lifetimes

and sharp triplet absorptions and can generate singlet oxygen in high quantum yields.

Experimental

Porphodimethenes were synthesized as described in chapters 2 and 3. All solvents

were used as received from commercial sources, unless otherwise specified. UV-Vis

spectra were recorded with a Varian Cary 50 spectrophotometer. Spectroscopic

experiments were carried out either in tetrahydrofuran or in distilled 2-

methyltetrahydrofuran (2-MeTHF) for variable temperature emission. Spectroscopy

carried out at room temperature was performed using samples that were degassed by a 20

min argon purge. Low-temperature spectroscopic experiments were conducted on

samples that were degassed by three repeated freeze-pump thaw cycles on a high-vacuum

line. Steady-state photoluminescence spectroscopy and singlet oxygen quantum yield

measurements were carried out using a SPEX Fluorolog 2 instrument. For steady-state

spectroscopy measurements, the samples were irradiated at the absorption maxima and

emission response in the UV-Vis region was monitored. Sample concentrations were

sufficiently low such that the absorbance at all wavelengths was less than 0.2. Integrity

of the samples was confirmed by taking excitation spectra. Quantum yield of singlet

oxygen is defined as the ratio of the number of generated singlet oxygen molecules to the

number of light-excited photosensitizer molecules. The standard used for quantum yield

calculations was tetraphenylporphyrin (Φ = 0.68).43 Samples 4-1 and 4-2 were dissolved

in THF, and the concentration was adjusted so that their absorbance at 420 nm matched

that of a standard (0.2). The solutions were then irradiated at 420 nm, the emission

response at 1270 nm (emission of the singlet oxygen) was monitored, and the resulting

spectra integrated to obtain the quantum yields by comparison with the standard. The

70

instrument used for transient absorption spectroscopy has previously been described in

the literature.44 Samples were contained in a cell that holds a total volume of 10 ml, and

the contents were continuously recirculated through the pump-probe region of the cell.

Samples were degassed by argon purging for 30 min. Excitation was provided by the

third harmonic output of a Nd:YAG laser (355 nm, Spectra Physics, GCR-14). Typical

pulse energies were 5 mJ‚ which corresponded to irradiance in the pump-probe region of

20 mJ/cm2. The samples were dissolved in THF with absorbance of 0.6 at 355 nm.

Transient absorption decay lifetimes were determined from the multiwavelength

difference-absorption data by using the SPECFIT/32 factor analysis software.

CHAPTER 5 SYNTHESES OF PORPHYRINS WITH EXOCYCLIC RING SYSTEMS

Introduction

The spectroscopic properties of terapyrolic chromophores can be altered by

distortions from planarity or the presence of aromatic systems fused to the periphery of

the macrocycle. These modifications induce red-shifts in the porphyrin absorption

spectra, which is of particular interest for optical or therapeutic materials.48 The

placement of one meso substituent in the plane of the porphyrin by fusing it at the β

position causes a batochromic shift in the visible absorption by approximately 100 nm.49

The inclusion of additional aryl moiety on another pyrrole β position in a porphyrin

should cause further change in the optical properties, but only a few examples of

porphyrins with two meso, β fused conjugated rings had been reported50-52 (prior to the

first synthesis of porphyrins with exocyclic rings in our group53). Herein, we present a

synthetic pathway for the preparation of unprecedented palladium porphyrins with

exocyclic eight-membered keto systems, as well as the modification of existing

methodology54 resulting in novel palladium porphyrins with six-membered exocycling

rings.


In work with the metallated porphodimethenes such as 5-1, solutions of the

compound were found to be susceptible to a light induced oxidative rearrangement

presumably provoked by the their inclination to form the fully aromatic, 2 electron

oxidized porphyrin macrocycles. Through the use of a combination of light and an

71

72

excess of the oxidant DDQ under anhydrous conditions, a preparative method was

developed for the formation of the non-planar porphyrins depicted in Scheme 5-1, likely

via a Norrish Type I pathway53. The reaction produces two structural isomers, cis-5-3

and trans-5-3, and in each, the porphyrin contains two naphthyl groups fused to the

macrocycle meso- and β-positions by cyloheptanone moieties. The compounds are easily

separated by column chromatography and can be isolated in high combined yield as

crystalline solids.53

O

O

R

R

R

R

N N

N N

Ar

Ar

Pd

N

N

N

N

Pd

OO

Ar

ArR

R

R

R

N

N

N

N

Pd

O

O

Ar

ArR

RR

R

hν, DDQ

CH2Cl2

Ar = Mesityl, R = t-Bu

5-1

cis-5-2

trans-5-2

Scheme 5-1. Illustration of the cycloheptanone porphyrin synthesis. The reaction proceeds at room temperature in 90 minutes.

Due to a steric clash between the naphthyl hydrogens and the β hydrogens on the

porphyrin ring in cis 5-2 and trans 5-2, these macrocycles are easily oxidized to sheet-

like porphyrins with exocyclic azulenone moieties (Scheme 5-2).

73

N

N

N

NPd

OO

Ar

ArR

R

R

R

N

N

N

NPd

O

O

Ar

ArR

RR

R

cis-5-2

trans-5-2

N

N

N

NPd

OO

Ar

ArR

R

R

R

N

N

N

NPd

O

O

Ar

ArR

R R

R

cis-5-3

trans-5-3

Ar = Mesityl, R = t-Bu

FeCl3, DDQ

∆, CH2Cl2

FeCl3, DDQ

∆, CH2Cl2

Scheme 5-2. Depiction of oxidation of heptanone porphyrins to azulenone porphyrins.

Steric clash of naphthyl and β hydrogens favors this oxidative coupling.

Cyclooctanone Porphyrins

In view of the tendency of naphthenone porphodimethene 5-1 to form porphyrins,

phenanthrenone porphodimethene 5-4 was subjected to similar reaction conditions, but

the macrocycle was particularly robust. Even in refluxing benzonitrile with one

equivalent of Pd(PhCN)2Cl2, complete conversion of the porphodimethene took seven

days. As with 5-4, two porphyhrin isomers are formed in the reaction, cis-5-5 and trans-

5-5 (Scheme 5-2). The reaction was followed spectroscopically, by monitorig the

disappearance of the porphodimethene absorption at 489 nm, and the emergence of the

porhyrin Soret band at 438 nm, as illustrated in Figure 5-1.

Despite numerous attempts, all efforts to separate the isomers by column

chromatography have been unsuccessful, but the material can be obtained as analytically

pure mixture of isomers cis-5-4 and trans-5-4. The physical measurements have been

74

performed on the mixture, which, based on NMR integrations, contained approximately

40% of cis-5-5 and 60% of trans-5-5.


Abs

orba

nce

20'6 h6 days

Figure 5-1. Illustration of the reaction progress for synthesis of 5-4. The

porphodimethene absorption at 489 nm disappears as the porphyrin Soret band grows in.

N N

N N

Ar

Ar

O

Pd

O

∆, C6H5CN

Pd(C6H5CN)2Cl2

N

N

ArO

N

N

O

Ar

N

N

ArO

N

N

OAr

Pd

Pd

Ar = Mesityl5-4

cis 5-5

trans 5-5 Scheme 5-3. Diagram of octanone porphyrin formation. The reaction takes 7 days in

refluxing benzonitrile.

75

Apart from the different exocyclic ring size, both cis-5-5 and trans-5-5 contain

biphenyl moieties in place of the naphtyl groups found in cis-5-2 and trans-5-2. As a

consequence, the increased flexibility of exocyclic substituents in cis-5-5 and trans-5-5

compared to cis-5-2 and trans-5-2 may decrease the polarity difference between the

isomer pair, making separation of cis-5-5 and trans-5-5 difficult under different

chromatographic conditions. Due to our inability to resolve the two isomers, all the

photophysical measurements for 5-5 described in Chapter 6, were conducted on the

mixture of cis and trans porphyrins.

In an effort to improve the chromatographic properties of cis- 5-4 and trans- 5-4,

the various metals including platinum, copper and nickel were incorporated into the

macrocyclic ring, and the meso aryl subtituents were replaced with 3,5-di-t-butyl phenyl

groups. Unfortunately, all attempts to separate the isomers were still unsuccessful.

Nevertheless, single crystals of trans-5-5 can be obtained after diffusion of pentane

into a concentrated toluene solution containing the two isomers. This molecule

crystallizes in the point group C2 and adopts anti configuration with respect to the

carbonyl groups of cyclooctanone moieties. As can be seen from the solid-state structure

(Figure 5-2), the presence of an exocyclic octanone moiety allows for the phenyl

substituents to assume a nonplanar conformation with respect to the porphyrin ring. The

angle between phenyl groups adjacent to the porphyrin ring and the ring itself is 70.2(1)º,

while the angle between the two phenyls within biphenyl moiety is 60.6(1)º. The

flexibility of the octanone rings relieves the strain imposed on the porphyrin by the

presence of heptanone rings in cis-5-2 and trans-5-2, and the core of trans-5-5 is nearly

planar with mean plane deviation of 0.125 Å. Palladium adopts a square-planar geometry,

76

with N-Pd-N angles of 89.3(1)º and 90.8(1)º. Selected bond lengths are listed in Table 5-

1.

Figure 5-2. Diagrams (side view on the bottom) of the solid-state structure of trans-5-5

(40% probability; carbon atoms depicted with arbitrary radii). Hydrogen atoms omitted for clarity. Primed and non-primed atoms related by 2-fold symmetry.

Without the steric clash between pyrrolic hydrogens of the porphyrin ring and the

aromatic hydrogens on the substituents, that is highly pronounced in 5-2, porphyrins cis-

and trans-5-5 did not exhibit reactivity towards further oxidation that would result in

forming highly conjugated counterparts of cis-5-3 and trans-5-3.

77

Table 5-1. Selected bond lengths for trans-5-5

trans-5-5·C7H8

Pd1-N1 2.003(2)

Pd1-N1’ 2.003(2)

Pd1-N2 2.013(2)

Pd1-N2’ 2.013(2)

N1-C4 1.368(4)

N1-C1 1.382(4)

N2-C9 1.378(4)

N2-C6 1.380(4)

O1-C11 1.207(5)

C11-C7 1.489(5)

78

Cyclohexannone Porphyrins

NNAr

N N

O

ArPd

Ar

NNAr

O

N N

O

ArNiNN

Ar

O

N N

O

ArNi

5-6 cis-5-7 trans-5-7Ar =

t-But-Bu Figure 5-3. Diagram of porphyrins with exocyclic rings synthesized in Callot’s lab.

With palladium porphyrins containing seven and eight-membered rings in hand, we

were interested in synthesizing analogues macrocycles with six-membered exocyclic

rings in order to explore the photophysical properties of the whole series of the

compounds. Palladium porphyrin with one six-membered ring 5-6 and nickel porphyrins

with two six-membered rings 5-7 were reported in the literature (Figure 5-3)54, 55 and the

published synthetic procedures were modified to obtain cis and trans palladium

cyclohexanone porphyrins (Scheme 5-4). Callot and coworkers isolated α,α and α,β

porphyrins 5-9 from the condensation reaction of α-methoxybenzaldehyde with 3,5-di-t-

butylphenyl aldehyde and pyrrrole. In this work, we synthesized 2-methoxyphenyl

dipyrromethane and condensed it with 3,5-di-t-butylphenyl aldehyde under Lindsey

conditions to obtain α,α and α,β 5-9 in combined yield of 34%. Porphyrin 5-9 was

metallated with palladium, diesters hydrolyzed and transformed to acid chlorides that

79

HNNAr

MeO2C

NH N

CO2Me

Ar

Ar =

t-Bu t-Bu

NNAr

O

N N

O

ArPd

NH HN

CO2Me+

i

i) TFA, DDQ

ii) LiOH, dioxane, H2Oiii) oxallylchloride, SnCl4, benzene

2ArCHO2

NNAr

MeO2C

N N

CO2Me

ArPdNN

Ar

O

N N

O

ArPdii, iii

5-8 5-9

5-10 cis-5-11 trans-5-11

Scheme 5-4. Diagram of cyclohexanone porphyrin formation.

80

were then subjected to intramolecular Friedel-Craft’s acylation to form porphyrins cis-5-

11 and trans-5-11 with exocyclic six-membered rings.

The first step in the synthesis of porphyrins with exocyclic six-membered rings

designed by Callot and coworkers55 involves condensation of 3,5-di-t-butylphenyl

aldehyde with pyrrole and 2-methoxy-bezaldehyde, resulting in a statistical mixture of a

number of porphyrins. This approach, while useful for obtaining starting material for

different mono and bi exocyclic porphyrins, produces individual porphyrins in low to

moderate yields ( 2 – 10%). By using 2+2 condensation of 3,5-di-t-butylphenyl aldehyde

and 2-methoxy-phenyl dipyrromethene, we increased the yield of 5-9 to 34% (combined,

for α,α and α,β) and minimized the formation of porphyrin side products. Metallation of

5-9 with palladium in the second step and carrying on the reaction sequence with

palladium instead of nickel as a central atom eliminated the demetallation and

trasmetallation steps required for synthesis of 5-6.55

In addition to obtaining palladium cyclohexanoneporphyrins, this methodology

allowed for isolation and structural characterization of a new palladium porphyrin

bearing two methyl ester groups, as illustrated by the solid-state structure diagram in

Figure 5-4. The esters are on benzylic carbons α to the two trans meso positions of the

porphyrin ring, and they are oriented away from each other. Phenyl rings bearing

functional groups are rotated 85.3(2) and 86.7(2) degrees with respect to the mean plane

of the porphyrin ring defined by 20 carbon atoms and four nitrogens of the tetrapyrrolic

core. The average deviation of the 24 atoms from the mean plane is 0.033 Å, and

palladium is situated in a square planar arrangement 0.014(3)Å below the mean plane.

81

The angles between the 3,5-di-t-butylphenyl substituents and the porphyrin core are

73.3(2)° and 72.2(2)°.

Figure 5-4. Diagram of the solid-state structure of 5-10 (40% probability; carbon atoms are depicted with arbitrary radii). Hydrogen atoms are omitted for clarity.

Conclusions

A synthetic pathway for unprecedented palladium porphyrins contaning eight-

membered exocyclic keto-rings was devised. Even though the isomers could not be

resolved chromatographically, a single crystal of trans-5-5 was obtained and the

compound was structurally characterized. Biphenyl moeties in this structure are rotated

away from the porphyrin plane, and the flexibility of the eight-memberd rings allows the

aryl and β pyrrolic hydrogens to point away from each other. In agreement with the lack

of steric hindrance observed for compounds 5-4 compared to the cycloheptanone

porphyrins 5-2, in which naphthyl and β hydrogens are in intimate proximity, compounds

5-4 showed no reactivity in further oxidative coupling to form analogues of 5-3.

82

A literature procedure for synthesis of porphyrins with exocyclic six-memberd

rings54, 55 was modified to give cis-5-11 and trans-5-11 that have not been previously

reported. The porphyrins were synthesized in fewer steps and with higher overall yields

than the literature analogues 5-6 and 5-7. Cyclohexanone porphyrins 5-11 show

significant red-shifts in their UV-Vis absorption spectra in comaparison to typical

tetraarylporpyrins, due to the extended π-conjugation outside of the porphyrin ring.

Palladium porphyrins with six and eight-memebered exocyclic rings were used for

photophysical studies described in Chapter 6.

Experimental

General Procedures




services using FAB or ESI as ionization method. All solvents were used as purchased,

unless otherwise specified.

Chromatography

Absorption column chromatography was performed using neutral alumina

(Aldrich, Brockman I ~ 158 mesh, 58 Ǻ) or chromatographic silica gel (Fisher, 200 – 425

mesh). HPL chromatography was performed on Waters Breeze HPLC system equipped

with the dual wavelength detector, using Alltech Econosphere 10µm silica column (10

mm x 250 mm).

Synthesis of cis-5-5 and trans-5-5

A sample of 0.050 g (0.05 mmol) of 5-4 was dissolved in 30 ml of benzonitrile and

0.021 g (0.05 mmol) of Pd(PhCN)2Cl2 was added. The solution was heated to reflux with

83

stirring, and the progress of the reaction was monitored by UV-Vis spectroscopy. After 7

days, the reaction was complete and the solvent was removed under vacuum. The solid

residue was redissolved in a minimal amount of methylene chloride and purified by

column chromatography with a 1:1 mixture of methylene chloride/toluene as the eluant.

The second, purple fraction was collected, and the solvent evaporated to dryness to afford

0.024 g (48%) of product as a mixture of cis-5-5 and trans-5-5 isomers. HRMS (FAB)

calcd. for MH+ (C64H45O2N4Pd): 1006.2498. Found 1006.2518. Anal.Calcd. for

C64H44N4O2Pd: C, 76.30; H, 4.40; N, 5.56. Found: C, 76.36; H, 4.39; N, 5.63. 1H NMR

indicated the presence of both the cis and the trans isomer in a ratio of 2:3 respectively,

and to date, all efforts to separate the two isomers using either regular silica column or

HPL chromatography have been unsuccessful. X-ray quality crystals of trans-5-5 were

grown from pentane/toluene diffusion.

Synthesis of 5-8

Dipyromethene 5-8 was synthesized using standard literature procedure.34 A

sample of 5.490 g (33.5 mmol) of 2-formyl methylbezoate was dissolved in 100.0 ml of

freshly distilled pyrole and 1.26 ml of BF3·Et2O was added. The mixture was stirred for

30 minutes in the dark and diluted with 170 ml of methylene chloride. It was then

washed with 170 ml of 0.1 M NaOH and water, dried over Na2SO4 and methylene

chloride was removed on the rotary evaporator. Excess pyrolle was distilled off on the

vacuum line; solid residue was redissolved in minimal amount of methylene chloride and

passed through alumina column. The first yellowish fraction was collected, solvent

evaporated and solid recrystalized from hot hexanes to yield 2.556 g (25 %) of white

solid. 1H NMR (300 MHz, CDCl3) d= 8.49 bs 2H, 7.77 (dd, 1H, J1 =8.1 Hz, J2 =1.5 Hz),

84

7.45-7.24 (m, 3H), 6.70 (dd, 2H, J1 = 3.9 Hz, J2 = 2.4 Hz), 6.31 (s, 1H), 6.14 (dd 2H, J1 =

5.7 Hz, J2 = 3.0 Hz), 5.92-5.90 (m, 2H), 3.81 (s, 3H).

Synthesis of 5-9

A portion of 1.000 g (3.56 mmol) of 5-8 and 0.779 g (3.56 mmol) of 3,5-di-t-butyl

benzaldehyde were dissolved in 360 ml of methylene chloride and 0.48 ml of TFA was

added dropwise. The mixture was stirred at room temperature for 30 minutes and 0.811 g

(3.56 mmol) of DDQ was added and stirred for another hour. The excess DDQ was

filtered off, two drops of triethylamine added to the filtrate, and the solution volume was

reduced to 10 %. The mixture was purified over neutral alumina, and the porphyrin

products were separated on a silica column using toluene as eluent. The third, purple

band was collected, and the solvent was evaporated to yield 0.310 g (18 %) of purple α, β

isomer. The fifth, purple band from this column was collected, and the solvent was

evaporated to give 0.276 g (16 %) of the α, α isomer of 5-9. α, α :1H NMR (300 MHz,

CDCl3) d= 8.72 (d, 4H, J = 4.8 Hz), 8.62 ( J = 4.5 Hz), 8.40 (d, 1 H, J = 2.1), 8.38 (d, 1H,

J = 1.8 Hz ), 8.18-8.16 (m, 4H), 7.97 (dd, J1 = J2 = 1.5 Hz), 7.87 – 7.77 (m, 6H). 2.87 (s,

6H), 1.54 (s, 18H),1.49 (s, 18 H) - 2.55 (s, 2H).

α, β: 1H NMR (300 MHz, CDCl3) δ = 8.81 (d, 4H), 8.59 (d, 4H, J= 4.8 Hz), 8.39

(d, 1H, J= 1.8 Hz), 8.36 (d, 1H, J = 1.5 Hz), 8.12 (d, 1H, J = 1.2 Hz), 8.10 (d,1H, J = 2.1

Hz), 8.04 (d, 4H, J= 1.8 Hz), 7.88 – 7.76 (m, 6H), 2.70 (s, 6H), 1.51 (s, 36H), - 2.52 (s,

2H).

Synthesis of 5-10

A sample of 0.310 g (0.325 mmol) of 5-9 (α, β isomer) was dissolved in 100.0 ml

of toluene and 0.155 g (0.692 mmol) of Pd(OAc)2 was added. The solution was kept at

85

reflux overnight. The reaction mixture was purified through a short silica gel column and

the first bright orange fraction containing both was collected, and the solvent was

evaporated to yield 0.284 g (83 %)of 5-10. The product was obtained as a mixture of

α,α and α,β isomers due to interconversion of 2-methoxy phenyl substituents in refluxing

toluene. Analytical sample of α,β isomer was purified on a silica column with toluene.

1H NMR (300 MHz, CDCl3) δ= 8.81 (d, 4H, J = 4.8 Hz), 8.59 (d, 4H, J = 4.8 Hz) 8.39 (d,

1H, J = 2.1 Hz), 8.36 (d, 1.2 Hz), 8.12 (d, 1H, J = 1.2 Hz), 8.10 (d, 1H), 8.04 (d, 4H, J =

2.1 Hz), 7.87 – 7.76 (m, 6H), 2.80 (s, 6H), 1.51 (s, 36H). Slow diffusion of pentane into

the chloroform saturated solution of α,β isomer of 5-10 afforded single crystals suitable

for X-ray diffraction studies.

Synthesis of cis-5-11 and trans-5-11

A portion of 0.204 g (0.193 mmol) of 5-10 was dissolved in 210 ml of dioxane and

a solution of LiOH (6.000 g) in 25 ml of water was added. The solution was kept at

reflux for 48 hours and it was the filtered. The precipitate was washed with 5% acetic

acid in methylene chloride. The filtrate was washed with water, organic layer separated

and dried over sodium sulfate. The solvent was removed under vacuum to yield 0.170 g

of orange solid (prphyrin diacid), and the compound was used without further

purification for the subsequent reaction.

A sample of 0.170 (0.165 mmol) g of the diacid was dissolved in 210 ml of

benzene and 6.3 ml of oxallyl chloride was added. The reaction mixture was stirred at

room teperature for 4 hours. Distillation of 20 ml of solvent (to remove the excess oxalyl

chloride) was followed by the addition of 4.2 ml of SnCl4. The reaction mixture was

stirred at room temperature for 75 minutes, diluted with 200 ml of methylene chloride,

86

neutralized with aqueous sodium hydroxide, washed with water and dried over sodium

sulfate. The solvent was evaporated, and the solid was redissolved in methylene chloride

and percipitated by addition of methanol. Yield:0.088 g (55 % overall for the two steps)

of product as a mixture of cis and trans isomers. A sample of the solid was redissolved

in CHCl3 and the two isomers were separated on semi-preparative HPLC column using

toluene as an eluant.

The trans isomer eluted first with retention time of 23 minutes. 1H NMR (300

MHz, CDCl3) δ= 9.07 (d, 2H, J = 5.1 Hz ), 9.02 (s, 2H), 8.54 (d, 2H, J = 5.1 Hz ), 8.47

(d, 2H, J = 5.1 Hz ), 7.82-7.76 (m, 8H), 7.51 (dd, 2H, J1 = J2 = 3.9 Hz ), 1.52 (s, 36 H).

UV-Vis [methylene chloride, λmax (log ε)] 765 nm (3.69), 681 nm (3.83), 466 nm (4.55).

HRMS (FAB) calcd. for M+ (C62H56O2N4Pd) 994.3437. Found: 994.3444.

The cis isomer eluted second, after 40 minutes. 1H NMR (300 MHz, CDCl3) δ=

9.06 (d, 2H, J = 5.1 Hz), 9.00 (s, 2H), 8.54 (d, 2H, J = 5.1 Hz), 8.47 (d, 2H, J = 7.5 Hz),

8.22 (d, 2H, J = 8.1 Hz), 7.83 – 7.72 (m, 6H), 7.28 (s, 4H, under the solvent peak), 1.52

(s, 18H), 1.50 (s, 18H). UV-Vis [methylene chloride, λmax (log ε)] 718 nm (4.18), 547

nm (4.05), 484 nm (4.83), 411 nm (4.32).


Unit cell dimensions were obtained and intensity data collected by Prof. Michael

Scott on a Siemens CCD SMART diffractometer at low temperature, with

monochromatic Mo-Kα X-rays (λ = 0.71073 Å). The data collections nominally covered

over a hemisphere of reciprocal space, by a combination of three sets of exposures; each

set had a different φ angle for the crystal and each exposure covered 0.3° in ω. The

87

crystal to detector distance was 5.0 cm. The data sets were corrected empirically for

absorption using SADABS.37

Table 5-2. Crystallographic data for trans-5-5 and 5-10 trans-5-5·2C7H8 5-10

Formula C78H60N4 O2Pd C66H54N4O4Pd

Formula weight 1191.70 967.13

Crystal system Monoclinic Triclinic

Space group C2 P1

Z 2 2

Temp, K 173(2) 193(2)

Dcalc/ gcm-3 1.329 1.313

a Å 25.2799(12) 11.1639(5)

b Å 9.5071(4) 11.9487(6)

c Å 12.7357(6) 12.5807(6)

a, deg - 116.497(1)

β, deg 103.398(1) 106.317(1)

γ, deg - 101.036(1)

V Å3 2977.6(2) 1339.8(1)

µ, mm-1 0.37 0.399

Uniq. data coll./obs. 5537/9702 7039/7596

R1[I > 2σ(I)data]a 0.0434 0.0370

wR2[I > 2σ(I)data]b 0.1179 0.0929 a R1 = Σ||Fo| - |Fc||/ Σ| Fo| bwR2 = { Σ[w (Fo

2 – Fc2)2/ Σ[w ( Fo

2)2}

The structure was solved using the Bruker SHELXTL software package for the

PC, by direct method option of SHELXS. The space group was determined from an

examination of the systematic absences in the data, and the successful solution and

refinement of the structure confirmed these assignments. All hydrogen atoms were

88

assigned idealized locations and were given a thermal parameter equivalent to 1.2 or 1.5

times the thermal parameter of the carbon atom to which it were attached. For the methyl

groups, where the location of the hydrogen atoms was uncertain, the AFIX 137 card was

used to allow the hydrogen atoms to rotate to the maximum area of residual density,

while fixing their geometry. Relevant crystallographic data are listed in table 5-2.

CHAPTER 6 PHOTOPHYSICAL PROPERTIES OF PORPHYRINS WITH EXOCYCLIC RING

SYSTEMS

Introduction

Artificial porphyrins have potential use in diverse applications ranging from

chemotherapy to material science, since they often exhibit unusual photochemical,56

magnetic,57 and electronic33 properties. For example, enhanced intersystem crossing in

free base and metalloporphyrins can be exploited for the photo-induced generation of

singlet oxygen within cancer cells resulting in cell death.58 Moreover, porphyrin species

with highly absorbing transients represent good candidates for optical limiters.59 Finally,

phosphorescent porphyrins are used to improve the efficiency of light-emitting devices.59

The ability of porphyrins to trap and transfer energy when organized in the arrays is

utilized for energy-transporting antennae in e.g. dye-sensitized TiO2 solar cells.60

Regardless of what the new artificial porphyrins could be used for, understanding their

physical properties is essential for the potential application.

Herein, we report detailed photophysical studies of palladium porphyrins with

unprecedented cyclooctanone exocyclic rings and related porphyrins with exocyclic

cycloheptanone and cyclohexanone systems.

Both cis and trans 6-1 are bright green due to a significant red shift of the

porphyrin Soret and Q-bands (Figure 6-2). Palladium porphyrins typically exhibit Soret

absorptions between 380-412 nm,24 but cis-6-1 has an absorption maximum at 470 nm

(log ε = 5.4) while the analogous absorption for trans-6-1 occurs at 486 nm (log ε = 5.4).

89

90

Along with shifting to lower energy (652 nm for cis-6-1 and 674 nm for trans-6-1), the

Q-bands for these green porphyrins are unusually broad and intense (log ε = 4.6).

N

N

N

N

Pd

OO

Ar

ArR

R

R

R

N

N

N

N

Pd

O

O

Ar

ArR

R R

R

Ar = Mesityl, R = t-Bucis 6-1 trans 6-1

N

N

ArO

N

N

O

Ar

N

N

ArO

N

N

OAr

PdPd

cis 6-2 trans 6-2

N

N

N

N

Pd

OO

Ar

Ar

N

N

N

N

Pd

O

O

Ar

Ar

Ar = 3,5-di-t-butylphenylcis 6-3 trans 6-3

Ar = Mesityl

N

N

N

N

Pd

OO

Ar

ArR

R

R

R

N

N

N

N

Pd

O

O

Ar

ArR

RR

R

cis 6-4 trans 6-4 Ar = Mesityl, R = t-Bu Figure 6-1. Diagram of the porphyrins with exocyclic rings used for the photophysical

measurements reported herein.

91

Wavelength / nm400 500 600 700 800

Eps

ilon

/ M-1

cm-1

0.0

5.0e+4

1.0e+5

1.5e+5

2.0e+5

2.5e+5

cistrans

Figure 6-2. The cycloheptanone porphyrins exhibit red-shifts in the absorption spectra.

In the solid-state, trans-6-1 adopts an anti configuration with respect to the

carbonyl groups of the cycloheptanone moieties. Due to a steric clash between

hydrogens on the napthyl group and the β-pyrrolic positions, the macrocycle assumes a

classic, ruffled B1u deformation with the meso-carbon atoms displaced alternately above

and below the plane formed by the four pyrrole nitrogens. Both the non-planar

distortion61 and the delocalization of electron density through the fused

naphthocycloheptanone ring systems likely contribute to the bathochromic shifts of the

Soret bands.

The rigidity of the seven-membered keto ring enables the electronic delocalization

by forcing a 30.1° angle between the mean plane of the porphyrin (defined by 20 carbon

atoms and 4 nitrogens of the core) and the naphthyl substituents. Moreover, the naphthyl

groups are only displaced by 15.3° from the pyrrole rings to which they are connected via

the carbonyl carbon. In view of the long wavelength absorption of these porphyrins, the

emission would be expected to occur in the low-energy region of the spectrum, and

indeed, both cis-6-1 and trans-6-1 exhibited low temperature phosphorescence emissions

92

in the IR region that disappear rapidly with increasing temperature (Figure 6-2). Triplet

state energies for cis-6-1 and trans-6-1 are 30 kcal/mol and 28 kcal/mol, respectively.

Wavelength / nm900 1000 1100 1200

Rel

. I

cistrans

Figure 6-3. Depiction of the phosphorescence emission of cis-6-1 and trans-6-1. Both

isomers emit in the IR region.

The local symmetry of the two isomers in solution is entirely consistent with their

solid-state structures. As expected for C2 symmetry, trans-6-1 exhibits only two peaks

for the mesityl o-hydrogens and 3 peaks for the mesityl methyls in the 1H NMR

spectrum, while the four one-proton singlets corresponding to mesityl o-hydrogens and

six peaks for the mesityl methyls are evident in the spectrum of the Cs symmetric cis-

isomer. The cores of both cycloheptanone porphyrins are distorted from the mean plane

defined by the 20 carbon and 4 nitrogen atoms in the macrocycle, and the distortion is

somewhat more pronounced in trans-6-1, as compared to cis-6-1 (mean plane deviation

of 0.27 Å vs. 0.24 Å), possibly explaining the red shift of the lowest energy band in the

former.62

The low-energy absorption in 6-1 is accompanied by the low- energy emission, and

the phosphorescence feature of trans-6-1 at 1017 nm is, to the best of our knowledge, the

93

longest wavelength emission measured for the monomeric palladium porphyrin species.

The triplet states in cis-6-1 and trans-6-1 have the absorption maxima at 520 nm and the

lifetimes of 10 µs and 4.3 µs, respectively (Figure 6-4). The absorption peaks in TA

spectra of these porphyrins are very broad and extend into IR region. The shape of the

TA spectra, together with the low energies and short lifetimes of the triplet states

suggests a high degree of electronic delocalization in cis-6-1 and trans-6-1.63

In the triplet excited state the cycloheptanone porhyrins will induce the excitation

of dioxygen molecules into the singlet state via energy transfer, and both compounds cis-

6-1 and trans-6-1 generate singlet oxygen with quantum yields of 1. Quantitative singlet

oxygen production makes these molecules particularly interesting oxygen sensitizing.

The strong absorption of 6-1 in far red region of the visible spectrum along with the

ability to tune chemical properties of the macrocycle through changes in the meso-aryl

groups makes these molecules particularly interesting for applications in photodynamic

therapy. The cycloheptanone porphyrins also exhibit significant triplet-state absorption

in the low-energy region, where their ground states do not absorb, a key feature for

materials used as optical limiters.64

Cyclooctanone Porphyrins.

Much like solutions of cis-6-1 and trans-6-1, the Soret bands in the UV- Vis

spectrum of mixture of cis- and trans-6-2 are red-shifted with the respect to simple

palladium porphyrins and are coincident at 438 nm (Figure 6-5). The spectrum also

features two bands in the low energy region at 544 nm and 584 nm, but both the Q-band

and the Soret bands are blue-shifted in comparison to 6-1, presumably due to a smaller

degree of delocalization of the π-system over the two exocyclic octanone rings.

94

Wavelength / nm

500 550 600 650 700 750 800

∆A

-0.3

-0.2

-0.1

0.0

0.1

0.2 cistrans

Figure 6-4. Illustration of transient absorption of cis-6-1 and trans-6-1. Broad transients

indicate high delocalization of electron density.

Table 6-1. Summary of photophysical data

Compound Abs [nm], ε [M-1cm-1]

Phosphorescence [nm] (kcal/mol) Φ 1O2 τ [ µs ]

cis-6-1 486, 250000 949 (30) a 1.0 10.2

trans-6-1 470, 250000 1017 (28) a 1.0 4.3

cis-6-2, trans-6-2 438 820 (35) b 1.0 12 c, 43 d

cis-6-3 484, 68000 1015 (28) a 2.3

trans-6-3 466, 35000 1010 (28) b 0.9

cis-6-4 553, 79000 - 0 -

trans-6-4 567, 79000 - 0 - a Low temperature. b Room temperature. c τ1. d τ2.

95

The conjugation outside the porphyrin ring in 6-2 extends only to the carbonyl groups,

which are displaced from the plane of the core by 30.3º, and any further electronic

communication is lost due to the rotation of the phenyl groups.

Wavelength /nm350 400 450 500 550 600

Abs

orba

nce

Figure 6-5. Diagram of the absorption spectrum of the mixture of cis-6-2 and trans-6-2

highlights the coincidence of their Soret bands at 438 nm.

The rotation of the phenyl substituents out of the plane of the porphyrin, causes a

substantial difference between the electronic absorption spectra of 6-2 and 6-1, and can

account for different shapes of their transient absorptions.

In this study only cis-6-2 and trans-6-2 exhibited room temperature

phosphorescence, and the emission maximum occurred at 820 nm (Fig. 6-6). Saturation

of the solution with air quenched this emission due to energy transfer to dioxygen

molecules.

In comparison to palladium tetraphenylporphyrin (PdTPP), the triplet state energy

of 6-2 is somewhat lower (40 kcal/mol vs. 35 kcal/mol), perhaps due to a slightly

extended π-conjugation to exocyclic carbonyl groups in the cyclooctanone porphyrins.

96

Wavelength / nm600 650 700 750 800 850

Rel

. I Argon degassedOxygen saturated

Figure 6-6. Diagram of the phosphorescence emission of the mxture of cis-6-2 and trans-

6-2. The emission is quenched by saturation with air.

The mixture of cis-6-2 and trans-6-2 exhibits a strong transient around 480 nm

(Figure 6-7). When the TA spectrum of the isomer mixture was deconvoluted in

SPECFIT, two spectra of nearly identical shape were identified, but the signals had

different ∆A intensities. The isomer with more intense absorption exhibited λmax at 575

nm with the lifetime of 43 µs, while the λmax for the isomer with less intense absorption

occurred at 585 nm with the lifetime of 12 µs. If the extinction coefficients of cis-6-2

and trans-6-2 are identical, the transient at 475 nm would correspond to the trans isomer,

while the one at 485 nm can be assigned to cis, based on the isomer ratio calculated from

the NMR spectrum of the isomer mixture. The position of the transient absorption in cis-

6-2 and trans-6-2 are comparable to PdTPP (λmax = 475 nm)63, but the lifetimes of the

triplet states are significantly shorter than the 250 µs lifetime of PdTPP.46, 63, 65 The

broadening of the transient absorption of 6-2 compared to the PdTPP spectrum is

consistent with a higher degree of delocalization due to the presence of carbonyl groups,

and the triplet-state absorptions of cis-6-1 and trans-6-1 are much broader than the

97

signals in TA spectra of cis-6-2 and trans-6-2, providing further evidence for higher

electron delocalization in the former isomer pair compared to the later.


∆A

-0.1

0.0

0.1

0.2

Figure 6-7. Depiction of transient absorption of cyclooctanone porphyrins. The spectrum

can be deconvoluted into two spectra with transients at 475 nm and 485 nm.

Cyclohexanone Porphyrins

The UV-Vis spectra of cis-6-3 and trans-6-3 (Figure 6-8) exhibit red-shifts in both

the Soret and the Q-bands with respect to the regular meso aryl substituted porphyrins.

The Soret absorptions at 484 nm and 466 nm respectively for cis and trans , as well as the

Q-absorptions at 681 nm, 765 nm (cis) and 718 nm (trans) are indicative of significant

electronic delocalization outside the porphyrin ring. The steric clash between the aryl

and β hydrogens in 6-3 is smaller than in 6-1, due to the smaller exocyclic ring-size in the

former. This reduced steric hindrance can allow the phenyl substituents on the six-

membered keto-rings to get closer to the porphyrin plane, thus enabling a higher degree

of electronic delocalization.

98

Porphyrin trans-6-3 exhibits room temperature phosphorescence emission at 1010

nm (Figure 6-9), while the cis isomer has a very weak room temperature phosphoresce at

1015 nm.

Wavelength / nm400 500 600 700 800

Abs

orba

nce

transcis

Figure 6-8. Diagram of the UV-Vis spectra of trans-6-3 and cis-6-3. Both the Soret and

the Q absorptions are red-shifted in comparison to simple meso-substituted porphyrins.

The triplet state absorption reaches maximum at 520 nm for trans-6-3 and at 760

nm for cis-6-3. The triplet-state lifetimes are 0.9 µs and 2.4 µs for the cis and the trans

isomer, respectively. The short triplet-state lifetimes and the broad features in the

absorption spectra of 6-3 give further support for extended electronic delocalization in

porphyrins with exocyclic six-membered rings.

Azulenone Porphyrins

Oxidative dehydrogenation of compounds cis-6-1 and trans-6-1 results in highly

conjugated porphyrins cis-6-4 and trans-6-4, that absorb light throughout the whole

visible (and a part of the IR) spectrum (Figure 6-11). The Soret bands for cis-6-4 and

trans-6-4 occur at 553 nm and 567 nm, respectively, while the lowest energy absorptions

99

Wavelength / nm

900 1000 1100 1200 1300 1400 1500

Rel

. I trans

Figure 6-9. Illustration of the room temperature phosphorescence emission of trans-6-3.

The low-energy emission is due to electronic delocalization outside the porphyrin ring.

Wavelength / nm

500 550 600 650 700 750 800

∆A

-0.2

-0.1

0.0

0.1

0.2

0.3

cistrans

Figure 6-10. Transient absorption of cis-6-3 and trans-6-3. Triplet states absorb

throughout the visible region.

100

are at 850 nm for the cis isomer and at 1145 nm for the trans. The Q-band of trans-6-4 at

1145 nm is, to the best of our knowledge, the furthest red-shifted absorption observed for

any monomeric porphyrin reported in the literature. The extended conjugation and

higher degree of electron delocalization in these fully planar macrocycles induce an

extreme shift to lower energy in the absorption spectra of cis-6-4 and trans-6-4 compared

to cis-6-1 and trans-6-1.

Wavelength /nm400 600 800 1000 1200

Eps

ilon

/ M-1

cm-1

0.0

2.0e+4

4.0e+4

6.0e+4

8.0e+4

cis trans

Figure 6-11. Diagram of the electronic absorption of cis-6-4 and trans-6-4. The

absorption of the trans isomer extends to 1145 nm.

Porphyrins cis-6-4 and trans-6-4 exhibit no detectable low-temperature emission,

or transient absorption. The lack of phosphorescence in these molecules is in agreement

with the energy gap law,66-68 considering that both compounds absorb in the low energy

region of the spectrum (850 nm and 1145 nm for cis-6-4 and trans-6-4, respectively).

Azulenone porphyrins did not create singlet oxygen since the energies of the triplet states

in these molecules are lower than the energy needed for the excitation of oxygen into the

singlet state.

101

Conclusions

Both the size of and the nature of the substituents on exocyclic rings influence the

photophysical properties of porphyrins. The greater degree of electronic delocalization in

cyclohexanone and cycloheptanone porphyrins causes a decrease in the triplet state

energy, and the compounds only phosphoresce in the IR region, while the less

delocalized cyclooctanone porphyrins 6-2 exhibit phosphorescence at room temperature

in the visible region. Increasing the amount of aromaticty also induces shorter triplet

lifetimes and broader TA spectra for 6-1 and 6-3 in comparison to 6-2. Literature data on

photophysical properties of simple PdTPP46, 63, 65 are in agreement with the trends found

in this work. PdTPP has virtually no electronic delocaliztion outside of the porphyrin

ring, exhibits higher energy absorption and phosphorescence emission, displays a sharper

transient absorption spectrum,63 and has a longer triplet state lifetime than the porphyrins

reported herein.46, 69

Considering the difference between the ground state absorption and TA spectra for

both 6-1 and 6-3, these porphyrins could be tested as optical limiters. For applications

such as oxygen sensing, room temperature phosphorescence observed in 6-2 is

particularly usefull.66, 67 Porphyrins cis-6-1, trans-6-1, cis-6-2 and trans-6-2 generate

singlet oxygen with quantum yield of 1, which is higher than any palladium porphyrin

studied thus far. Quantitative singlet oxygen generation is promising for potential

application of these molecules as photosensitizers. The quantum yield of 1 for singlet

oxygen generation in 6-1 and 6-2 indicates that the triplet states are fully populated, due

to enhanced intersystem crossing induced by the presence of palladium within the

macrocycles.

102

Experimental

All solvents were used as received from commercial sources, unless otherwise

specified. UV-Vis spectra were recorded with a Varian Cary 50 spectrophotometer (for

compounds 6-1 through 6-3) or Varian Cary 500 spectrophotometer (for compounds 6-4).

Spectroscopic experiments were carried out either in tetrahydrofuran or in distilled 2-

methyltetrahydrofuran (2-MeTHF) for variable temperature emission. Spectroscopy

carried out at room temperature was performed using samples that were degassed by a 20

min argon purge. Low-temperature spectroscopic experiments were conducted on

samples that were degassed by three repeated freeze-pump thaw cycles on a high-vacuum

line. Steady-state photoluminescence spectroscopy and singlet oxygen quantum yield

measurements were carried using a SPEX Fluorolog 2 instrument. For steady-state

spectroscopy measurements, the samples were irradiated at the absorption maxima and

emission response in the UV-Vis and IR regions was monitored. Sample concentrations

were sufficiently low such that the absorbance at all wavelengths was less than 0.2.

Integrity of the samples was confirmed by taking excitation spectra. Quantum yield of

singlet oxygen is defined as the ratio of the number of generated singlet oxygen

molecules to the number of light-excited photosensitizer molecules. The standard used

for quantum yield calculations was tetraphenylporphyrin (Φ = 0.68).46 Samples 6-1 – 6-3

were dissolved in THF, so that their absorbance at 420 nm matched that of a standard

(0.2). The solutions were then irradiated at 420 nm, the emission response at 1270 nm

(emission of the singlet oxygen) was monitored, and the resulting spectra integrated to

obtain the quantum yields by comparison with the standard. The instrument used for

transient absorption spectroscopy has previously been described in the literature.47

Samples were contained in a cell that holds a total volume of 10 ml, and the contents

103

were continuously recirculated through the pump-probe region of the cell. Samples were

degassed by argon purging for 30 min. Excitation was provided by the third harmonic

output of a Nd:YAG laser (355 nm, Spectra Physics,GCR-14). Typical pulse energies

were 5 mJ‚ which corresponded to irradiance in the pump-probe region of 20 mJ/cm2.

The samples were dissolved in THF with absorbance of 0.6 at 355 nm. Transient

absorption decay lifetimes were determined from the multiwavelength difference-

absorption data by using the SPECFIT/32 factor analysis software.

CHAPTER 7 SUMMARY

This work demonstrates the utility of aromatic vicinal diketones containing both

five- and six-membered rings for the preparation of spiro-tricyclic porphodimethenes.

Depending on the diketone used, porphodimethenes capable of or resistant to ring

opening at the spiro-lock can be prepared. All porphodimethenes have been successfully

metallated, and the reactivity of compounds susceptible to ring opening was studied,

revealing a synthetic pathway for unprecedented bis-cyclooctanone porphyrins. The

solid state-structures of different metalloporphodimethenes illustrate the influence of the

metal and the meso substituents on the geometry of the macrocycle.

The first extensive studies of photophysical properties of porphodimethenes were

conducted, and low temperature phosphorescence emission in the UV-Vis region was

detected. The rate of disappearance of the phosphorescence with increasing temperature

in two different porphodimethenes is in agreement with different light reactivity of these

compounds. The triplet-state lifetimes and the singlet oxygen quantum yields were

determined for two palladium porphodimethenes.

In order to investigate the differences in photophysical properties of porphyrins

with various exocyclic ring systems, palladium porphyrins bearing 6 and 8-membered

exocyclic rings were prepared, and their spectroscopic features were compared to the

porphyrins bearing 7-membered rings whose synthesis had been previously reported.

We have demonstrated that the size of exocyclic keto-rings fused to the porphyrin

periphery has a profound impact on the overall geometrical features of the macrocycle.

104

105

Smaller exocyclic rings allow for a higher degree of coplanarity between the aryl

moieties and the porphyrin core, inducing a higher degree of electronic delocalization

outside of the core. In the series of porphyrins with cyclooctanone, cycloheptanone and

cyclohexanone keto-rings the increase in the electronic delocalization is accompanied by

decrease in the absorption and emission energies, decrease in the triplet-state lifetimes,

and broadening of the transient absorption spectra.

Oxidative biaryl coupling of cycloheptanone porphyrins flattens out the

macrocycle, which further increases the delocalization, and the resulting azulenone

porphyrins absorb light throughout the whole visible and a part of the IR region of the

spectrum. Due to the low-energy absorption, no emission features were observed for

these macrocycles.

The ease of syntheses and the photophysical properties of cycloheptanone and

cyclooctanone porphyrins suggest that these molecules could be further modified to find

application in areas of oxygen sensing, optical limiting and photosensitizing.

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BIOGRAPHICAL SKETCH

Ivana Božidarević was born in 1975 in Belgrade, Serbia, where she finished

elementary school and high school. She decided to study chemistry when she was

thirteen, because she had the best chemistry teacher in the world. During high school,

she won a couple of awards for young researchers and was highly ranked at all the

chemistry competitions on the federal level. She spent her summers doing research in the

labs of Petnica Science Center for high school students. Ivana entered The Faculty of

Chemistry at Belgrade University in 1994 and defended her BS Thesis in June 1999,

being the second t to graduate from her class of 150 students. She started her Ph.D.

studies at the University of Florida in 1999 by joining Prof. Scott’s group, and she will be

graduating in August 2004 ready for new challenges in the world of chemistry.

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