Investigated Porphyrins

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

M

M = Au (III) or Zn (II)

How will the site of electron transfer be effected in the presence of Sc3+?

PQ QPQ

Electrochemistry: Titration with Sc(OTf)3

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

M = Au (III)+

PF6-

MM PP PP210 mV shift

New process

MM PPPP

Site of electron transfer: Spectroscopy

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

M = Au (III)+

PF6-

300 400 500 600 700 800 900Wavelength, nm

(PQ)AuPF6 in CH2Cl2

a) 1st reduction

Eapp = -0.70V

436

409

549 587

b) 2nd reduction

Eapp = -1.20V

439

409

422

549 587

Metal centered

No isobestic point, initially

porphyrin centered, then

go to quinoxaline

Ou et al., Inorg Chem. 2004,43,2078-2086

Resulting spectra resembles 2nd reduction of (PQ)AuPF6

Absorbance plots for spectroscopic titrations

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

M = Au (III)+

PF6-

Indicates it is a 1 electron reduction

Indicates 2 Sc3+ ions are complexed

ESR signal of PQAuPF6 in the presence of Sc3+

In the presence of excess scandium, the complex is reduced by 1 electron with the site of electron transfer to the quinoxaline group of the

porphyrin, which has an ESR signal of a radical anion.

Ou et al., Inorg Chem. 2004,43,2078-2086

300 G

g = 2.0030

2047

2048

M =Zn (II)

Electrochemistry of PQZn

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

Electrochemical titration of (PQ)Zn with Sc3+in PhCN, 0.1 M

TBAPF6

-1.27

-1.27

-0.17

-0.17

-1.27

[Sc3+]/[PQM]

0.0

0.4

1.1

PP

M =Zn (II)

Site of electron transfer: Spectroscopy

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

500 600 700 800

0.1

0.3

0.5

0.7

UV-visible spectral titration of 1.62 x 10-5 M (PQ)Zn with Me10Fc (6.58 x 10-4 M) in the presence of 8.02 x 10-4 M Sc3+

437

460

640

radical anion

M =Zn (II)

Stoichiometric Plots for PQZn

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

M

N

N

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.5 1 1.5 2 2.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6 7

Plot of 451 nm absorbance of neutral PQZn vs. [Sc3+]/[(PQ)Zn]

Plot of 437 nm absorbance of (PQ)Zn vs. [Me10Fc]/[(PQ)Zn] with excess Sc3+

[Sc3+]/[(PQ)Zn][Me10Fc]/[(PQ)Zn]

1 electron transfer

2 Sc3+ ions

1 or 2 Sc3+ ions? 1

[SC3+]/[(PQ)Zn] with excess Me10Fc

[SC3+]/[(PQ)Zn]

ESR of PQZn + Sc3+ + (BNA)2 in PhCN

300 G

g = 2.0036

2047

2048

Indicates a radical anion

M =Zn (II)

Electrochemistry of QPQZn

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

M-1.13

-1.46

-0.19

-0.15

-1.14-1.28

-1.11

-0.44-0.65

[Sc3+][Sc3+]/[PQZn]

PPPP

0.0

0.3

1.1

3 new processes

M =Zn (II)

Site of electron transfer: Spectroscopy

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

M

(PQ)Zn, 1st step under same conditions, 2 Sc3+ ions after both processes

3 or 4 Sc3+ ions

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5 3 3.5

M =Zn (II)

Stoichiometric Plots for QPQZn

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

M

Plot of 441 nm absorbance of (QPQ)Zn vs. [Me10Fc ]/[(QPQ)Zn in the presence of excess Sc3

400UV-visible titration of (QPQ)Zn with Me10Fc in the presence of excess Sc3+

4411st step

2st step

500 600 700 800

460

441

535511

460

543

2 electrons

ESR of QPQZn + Sc3+ + (BNA)2 in PhCN

300 G

g = 2.0036

2047

2048

Indicates a radical anion, like PQZn

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

-0.28 -0.79

0.31

0.31

0.31

0.31

-0.05

-0.05

-0.05

-0.77 -0.26

-0.56

-1.37

-1.62

-0.91

-1.40

-0.56

-0.95

-1.45

-0.99

-1.55

4.71

3.63

2.55

1.34

0.00

Figure. Electrochemical titration of 7.42 x 10-4 M (QPQ) AuPF6

with Sc3+ (2.85 x 10-3 M) 0.1 M TBAPF6, PhCN

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

MM = Au (III)+

PF6-

Electrochemistry of QPQAuPF6

[Sc3+][Sc3+]/[QPQAuPF6]

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

MM = Au (III)+

PF6-

Site of electron transfer: Spectroscopy

300 400 500 600 700 900 800

515

650

455

416

600

UV-visible spectral changes of (QPQ)Au upon adding Me10Fc with excess Sc3+

Indicates is a porphyrin radical-type reduction,

similar to PQAuPF6

Spectral changes upon the addition of Me10Fc in the presence of excess Sc3+, PhCn

600 400 500 700 800 900

515

439

418

595

N

N

N

N

t Bu

Bu t

Bu t

t Bu Bu t

t Bu

t Bu

Bu t

N

NN

N

MM = Au (III)+

PF6-

Stoichiometric plots for QPQAuPF6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3

Plot of 455 nm absorbance vs. [Me10Fc]/[QPQAuPF6] in excess Sc3+, PhCN

[Me10Fc]/[QPQAuPF6]

2 electrons

DFT calculation of QPQAuPF6

QuickTime™ and a decompressor

are needed to see this picture.

(QPQ)Au

LUMO OrbitalsQuickTime™ and a

decompressorare needed to see this picture.

(QPQ)Au + H+

Basis set: B3LYP/Lanl2dz

-0.004

-0.002

0

0.002

0.004

400 500 600 700 800

, Wavelength nm

N

N

N N

ArAr

Ar Ar

ZnN

N

N

N

NN

Ar Ar

ArAr

AuN

N

[ZnPQ-AuPQ+] = 1.0 x 10-5 M[Sc(OTf)3] = 5.0 x 10–3 Min PhCNexcitation: 430 nm

ZnPQ-AuPQ+

0

0.001

0.002

0.003

0.004

0 1 2 3 4Time, μs

CS Lifetime: 430 ns

In the absence of Sc3+: τcs = 250 ps

Ohkubo, K. et al. Chem. Phys. 2006, 326, 3

CS lifetime of 2 bound PQM:

SummaryZn and AuPF6 (PQ)-type compounds undergo 1-electron reduction at the porphyrin-quinoxaline π-conjugated system in the presence of Sc3+, where 2 Sc3+ ions are complexed to the compound.

Zn and AuPF6 (QPQ)-type compounds undergo 2-electron reduction at the porphyrin-quinoxaline π-conjugated system in the presence of Sc3+, where 3 or 4 Sc3+ ions are complexed in the case of (QPQ)Zn.

(QPQ)AuPF6 titration with Sc3+ in the presence of Me10Fc needs to be performed as well as Sc3+ ion reactivity studies toward the PQZn-PQAu complex