20 22 24 26 28 30 32 34 36 38

500 450 400 350 300 270

} exc

=255nm; Troom

AN solution CH solution

red-shifted by 445cm-1; excitation at 0,0 band Supersonic Jet Neat Ar matrix

exc=275nm; T=25K

Wavelength (nm)

~/1000 cm-1

Flu

ores

cenc

e In

tens

ity

Charge-Transfer Fluorescence of Phenylpyrrole (PP) Charge-Transfer Fluorescence of Phenylpyrrole (PP) and Pyrrolobenzonitrile (PBN) in Cryogenic Matricesand Pyrrolobenzonitrile (PBN) in Cryogenic Matrices

20 22 24 26 28 30 32 34 36 38

500 450 400 350 300 270

0.7% AN; T=25KPBN in Ar matrix

exc

=285nm

exc=290nm

exc

=300nm

~/1000 cm-1

Flu

ores

cenc

e In

tens

ity

Wavelength (nm)

20 22 24 26 28 30 32 34 36 38

500 450 400 350 300 270

+1% AN; T=25K

~/1000 cm-1

Flu

ores

cenc

e In

tens

ity

PP in Ar matrix

exc=270nm

exc

=275nm

exc=278nm

exc

=284nm

Wavelength (nm)

En

ergy

Deformation0,

0 lin

e29

0nm 292

nmCTemission

20 22 24 26 28 30 32 34 36 38

500 450 400 350 300 275

~/1000 cm-1

T=25K; exc

=292nm

Neat Ar matrix

Flu

ores

cenc

e In

tens

ity

Wavelength (nm)

20 22 24 26 28 30 32 34 36 382

4

6

8

10

12

14

Lif

etim

e (n

s) LE

CT

20 22 24 26 28 30 32 34 36 38

500 450 400 350 300 270

} exc

=280nm; Troom

AN solution CH solution

exc

=266nm; T=25K

Supersonic Jet Neat Ar matrix

blue-shifted by 446cm-1; excitation at 0,0 band

/1000 cm-1~

Wavelength (nm)

Flu

ores

cenc

e In

tens

ity

PBN emission in neat Ar matrix is of LE bands (at high frequencies) superimposed on a broad CT background. The LE emission includes 2 vibronic progressions due to 2 different trapping sites. One progression is blue-shifted by 446 cm-1 relative to the jet whereas the other is blue-shifted by only 86 cm-1.The minimal frequency of the 0,0 transition in the matrix is therefore 34,510 cm-1, which corresponds to 290 nm.

Since fluorescence of PBN in neat Ar matrix is observed upon excitation at energies lower than the LE 0-0 band (at the absence of “hot lines”) it is concluded that the CT state can be populated directly by light absorption, and not only via the LE state.

Fluorescence spectra of PP (5) an PBN (6) in AN doped Ar matrix, in various excitation wavelengths (exc).

PPPBN2 bands: normal LE band and a shifted CT band.

A slight effect compared to other polar media.

Strong dependence of the intensities of the 2 bands

A Slight shift narrow distribution of sites.

DF is observed in both PP and PBN in matrices. CT is therefore possible in cryogenic temperatures and under the motional restrictions in this rigid environment.The different photo-physical behavior of PP and PBN in argon matrices was explained in terms of the “matrix wall” model (see D. Schweke poster). PBN emits in AN-doped Ar from a strained adduct, shifted to the blue with respect to its spectrum in fluid systems, in which large amplitude motions are allowed.The global minimum of the A state of PBN in Ar matrix is lower than the B state, and can be directly populated by light absorption. in addition to its population by a non-radiative process from the B state.

ConclusionsConclusions

8

4 53 6

9

Hagai Baumgarten, Danielle Schweke and Yehuda HaasHagai Baumgarten, Danielle Schweke and Yehuda HaasDepartment of Physical Chemistry and the Farkas center for Light induced Processes

The Hebrew University of Jerusalem, Jerusalem Israel

31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5

315 310 305 300 295 290 285

Neat Ar matrix

Supersonic Jet blue-shifted by 446cm-1

Supersonic Jet blue-shifted by 86cm-1

/1000 cm-1~

Wavelength (nm)

Flu

ores

cenc

e In

tens

ity

PBN in Ar matrix

LiteratureLiterature1. D. Schweke, Y. Haas. J. Phys. Chem. A. 107 (2003) 9554.2. D. Schweke, H. Baumgarten, Y. Haas, W. Rettig and B. Dick. J. Phys. Chem. A. 109 (2005) 5763. T. Yoshihara, V.A. Galiewsky, I.S. Druzhinin, S. Saha, K.A. Zachariasse. Photochem. Photobiol. Sci. 2 (2003) 342.4. L. Belau, Y. Haas and W. Rettig. J. Phys. Chem. A. 108, 3916-3925 (2004).5. S. Zilberg, Y. Haas.Phys. Chem. A.1 (2002) 106 .

Schematic energy level diagram of PP and PBN. The diagram describes the ICT process, which accounts for the appearance of dual emission.The A state surface includes two forms [5]: Quinoid and Anti-Quinoid.

Dopin

g ef

fect

exc

PPPBN

LE emissionBroad CT emissionLE vibronic bands

445 cm-1 Red-shifted (Batochromic effect) B>X.

Blue-shifted(Hypsochromic effect) B<X.

Emitt

ing

state

s

LE shift

relat

ive t

o jet

ResultsResults

ObjectivesObjectivesWe studied the photo-induced Intramolecular Charge Transfer (ICT) of PP (N-Phenylpyrrol) [1] and PBN (4-(1H-pyrrol-1-yl)benzonitrile) [2] in cryogenic matrices by spectroscopic research of the Dual Fluorescence (DF) phenomenon.We performed fluorescence spectra and time resolved measurements in both neat and AN-doped Ar matrices. Our results are compared to previous studies of DF and ICT in solutions [3] and in gas phase [4].The DF is due to two different excited states: LE (Locally Excited) labeled as the B state giving the normal emission and the CT (Charge Transfer) labeled as A state state giving an anomalous red-shifted emission. The ground state is labeled as X.PP and PBN belong to a family of para-substituted aromatic systems with Donor (D) and Acceptor (A) groups. Their fluorescence spectra exhibit a strong dependence on the environment polarity.The motivation to study the photo-physical properties of ICT in rigid matrices stems from their restrictions on the trapped molecules’ degrees of freedom: translation and rotation, including the torsion mode.Our goal was to find out whether ICT occurs in PP and PBN in the different matrices. The low temperature prevents the occurrence of barrier-dependent relaxation processes and the appearance of “hot” lines. Therefore the resulting spectra have a simple structure than in solutions. The low temperature also enables a long lifetime of the excited states.

Fluorescence spectra of PP (3) and PBN (4) in neat Ar matrix, Cyclohexane (CH), Acetonitrile (AN) and jet-cooled spectra of the bare molecules, which allows the assignment of the LE spectrum, and the location of the 0,0 band.To the right: life-times measurements of PBN, supporting the assignment of the emission to 2 different excited states: LE and CT.

ExperimentalExperimental

1PC

Photo-diode

Prism array

Sample

MonochromatorPMT

Excitation beam

Trigger Data

Signal

Fluorescence

Movementcontrol

LASER

Scope

Matrix deposition system: planned to enable a delicate flow of the gas mixture onto the window which is held under low temperatures (down to 14K) and pressure.

2

Signal collection setup

GasMix.

HostGas

Needle Valve

ANPBN

TurboPump

RotationPump

Dewar

MatrixDeposition

PBN Heater

HeCryostat

Temp.Control

Pressure gauge

Valve

Cold Window

Gas mixture

Guest Host

7

LE State

GroundState

CT State;Gas phase

CTFluorescence

LEFluorescenceAbsorption

QuinoidForm

Deformation (Torsion, Quinoidization)

En

ergy

60o 90o30o0o

CT; Polar environment

Anti-QuinoidForm

Curve Crossing

Ab

s.

LE

CT

We thank Prof. B. Dick, Prof. W. Rettig, Dr. W. Fuss and Dr. K. Zachariasse for enlightening discussions.

This research was supported by the Israel Science Foundation and by The Volkswagen-Stiftung (I/76 283). The Farkas Center for Light Induced Processes is supported by the Minerva Gesellschaft mbH.

AcknowledgementsAcknowledgements

PBNPBNPPPP