writing class
TRANSCRIPT
Metal Ion Effects on the Redox Properties of
Porphyrins
Rachel Garcia
AGEP Writing Class
April 18,2007
Osaka, Japan
Electron Transfer in Porphyrins:Key Component in Photosynthesis
It is important to study the electron transfer, or redox properties of porphyrins
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
M = Au (III) or Zn (II)
How will the site of electron transfer be effected in the presence of Sc3+?
PQ
Cyclic Voltammetry:
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
Titration with Sc(OTf)3
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
Ou et al., Inorg Chem. 2004,43,2078-2086
Resulting spectra resembles 2nd reduction of (PQ)AuPF6
Porphyrin with reducing agent ( add electrons to compound) and Sc3+
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.
300 G
g = 2.0030
2047
2048
Metal Centered
Ou et al., Inorg Chem. 2004,43,2078-2086
Radical anion
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
Spectra when one electron is added
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
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
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
Predicts the first electron will go to the quinoxaline group
Another Key Component in Photosynthesis: The Electron Donor-Acceptor Compound
Another Key Component in Photosynthesis: The Electron Donor-Acceptor Compound
e-
The time of the charge separated state of donor-acceptor compounds is a key factor in determining electron donor acceptors
-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:
Summary
Zn and AuPF6 (PQ)-type compounds undergo 1-electron reduction at the quinoxaline in the presence of Sc3+
2 Sc3+ ions are complexed to one compound
The redox properties of Zn and AuPF6 (PQ)-type compounds were studied in the presence of Sc3+:
In the presence of Sc3+, the charged-separated lifetime of the conguated Zn(PQ)-AuPF6(PQ) compound has increased, which is ideal for photosynthetic systems.
Acknowledgements• AGEP• My boss, Dr. Kadish• Dr. Fukuzumi, Osaka University