Substituent Effects on the Site of Electron Transfer during the First Reduction for Gold(III) Porphyrins

Article abstract of DOI:10.1021/ic035070w

Gold(III) porphyrins of the type (P-R)AuPF₆, where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO₂ (2), or NH₂ (3) which is substituted at one of the eight β-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin π-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF₆ (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO₂ group is introduced on the porphyrin ring to give (P-NO₂)AuPF₆ (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin π-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (λ) of the metal-centered electron transfer reduction for (P-H)AuPF₆ (1) in benzonitrile was determined as λ = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The λ value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than λ values of ligand-centered electron transfer reactions of metalloporphyrins.

Full text of DOI:10.1021/ic035070w

Inorg. Chem. 2004, 43, 2078−2086
Substituent Effects on the Site of Electron Transfer during the First
Reduction for Gold(III) Porphyrins
,†
Zhongping Ou,† Karl M. Kadish,* Wenbo E,† Jianguo Shao,† Paul J. Sintic,‡ Kei Ohkubo,§
,§
,‡
Shunichi Fukuzumi,* and Maxwell J. Crossley*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, School of
Chemistry, The UniVersity of Sydney, NSW 2006, Australia, and Department of Material and
Life Science, Graduate School of Engineering, Osaka UniVersity, CREST, Japan Science and
Technology Agency (JST), Suita, Osaka 565-0871, Japan
Received September 10, 2003
Gold(III) porphyrins of the type (P−R)AuPF , where P ) 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and
6
R is equal to H (1), NO (2), or NH (3) which is substituted at one of the eight â-pyrrolic positions of the macrocycle,
2
2
were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound
undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III)
porphyrin π-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered
reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF (4), where PQ ) 5,10,-
6
15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins
overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast,
when a NO group is introduced on the porphyrin ring to give (P−NO )AuPF (2), the site of electron transfer is
2
2
6
changed from the gold metal to the macrocycle to give a porphyrin π-anion radical in the first reduction step. This
change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV−vis
spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction.
The reorganization energy (λ) of the metal-centered electron transfer reduction for (P−H)AuPF (1) in benzonitrile
6
was determined as λ ) 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited
states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The λ value of the
metal-centered electron transfer of gold porphyrin (1) is significantly larger than λ values of ligand-centered electron
transfer reactions of metalloporphyrins.
Introduction
electron transfer reduction of metalloporphyrins can involve
either the central metal or the porphyrin ligand, both of which
are redox active.12 The site of electron transfer, whether it
is on the metal or on the ligand, is an important factor to
know or control, since the reorganization energy of electron
transfer, the magnitude of which determines the electron
Synthetic porphyrins have been used as components of
artificial photosynthetic systems because of their resemblance
to natural components.1-5 Gold(III) porphyrins have fre-
quently been used as electron acceptors in porphyrin dyads
and triads due to their ability to be easily reduced.5-11 The
(2) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K.
M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2000; Vol. 8, pp 153-190. (b) Gust, D.; Moore, T. A.; Moore,
A. L. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 3, pp 272-336. (c) Fukuzumi, S.;
Imahori, H. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 2, pp 927-975.
* To whom the correspondence should be addressed. E-mail: Kkadish@
uh.edu (K.M.K.); fukuzumi@chem.eng.osaka-u.ac.jp (S.F.); m.crossley@
chem.usyd.edu.au (M.J.C.).
† University of Houston.
‡ The University of Sydney.
§ Osaka University.
(1) (a) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox, M.
A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, pp 161-
206. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (c)
Wasielewski, M. R.; Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M.
P. Sol. Energy Mater. Solar Cells 1995, 38, 127.
(3) (a) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69,
797. (b) Maruyama, K.; Osuka, A.; Mataga, N. Pure Appl. Chem.
1994, 66, 867.
(4) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in Chemistry; Balzani,
V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 270-337.
2078 Inorganic Chemistry, Vol. 43, No. 6, 2004
10.1021/ic035070w CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/12/2004

Reduction of Au(III) Porphyrins
transfer reactivity, will be quite different depending on the
site of oxidation/reduction.12 Metal-centered electron transfers
normally afford a large reorganization energy whereas ligand-
centered electron transfers require minimal change in struc-
ture and solvation to give a small reorganization energy of
electron transfer.12
for photoinduced electron transfer,18 but the site of electron
transfer was not explored.
We report herein the electrochemistry and spectral char-
acterization (UV-vis and ESR) of neutral and reduced
hexafluorophosphate[5,10,15,20-tetrakis(3,5-di- tert-butyl-
phenyl)porphyrinato]gold(III), represented as (P-H)AuPF6
(1), and their derivatives, 2-4 (see structures shown here).16
The products of the first and second one-electron reduc-
tions of most metalloporphyrins are extremely stable in
nonaqueous media, but the singly reduced complexes can
be chemically converted to a chlorin after disproportionation
or further reduction in the presence of a proton source.13
Early electrochemical experiments on a gold(III) 5,10,-
15,20-tetraphenylporphyrin (TPP)Au[AuCl4] and two other
gold porphyrins in DMSO showed the compounds to undergo
three one-electron reductions,14 only two of which can
involve the porphyrin π-ring system, but the authors con-
cluded, on the basis of electrochemical criteria and spectro-
electrochemical data, that Au(III) was electrochemically inert
when incorporated into a porphyrin macrocycle. This was
also the conclusion based on early theoretical calculations
and electrochemical experiments involving the reduction of
(TPP)AuCl in dichloromethane,15 and until recently,16 it was
assumed to be true for all other subsequently investigated
gold(III) porphyrin complexes.17
The first reversible half-wave potential for reduction of
gold porphyrins in some functionalized multi-porphyrin
systems has been studied to determine a redox driving force
(5) (a) Chambron, J. C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.;
Sauvage, J. P. Pure Appl. Chem. 1993, 65, 2343. (b) Blanco, M.-J.;
Consuelo Jime´nez, M.; Chambron, J.-C.; Heitz, V.; Linke, M.;
Sauvage, J.-P. Chem. Soc. ReV. 1999, 28, 293. (c) Chambron, J.-C.;
Collin, J.-P.; Dalbavie, J.-O.; Dietrich-Buchecker, C. O.; Heitz, V.;
Odobel, F.; Solladie, N.; Sauvage, J.-P. Coord. Chem. ReV. 1998, 178-
180, 1299. (d) Harriman, A.; Sauvage, J.-P. Chem. Soc. ReV. 1996,
25, 41.
(6) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Dixon, I.
M.; Sauvage, J.-P.; Williams, J. A. G. Coord. Chem. ReV. 1999, 190-
192, 671.
(7) (a) Harriman, A.; Heitz, V.; Sauvage, J.-P. J. Phys. Chem. 1993, 97,
5940. (b) Harriman, A.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc.
1995, 117, 9461.
(8) (a) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Barigelletti, F.; Flamigni,
L. Angew. Chem., Int. Ed. 2000, 39, 1292. (b) Flamigni, L.; Dixon, I.
M.; Collin, J.-P.; Sauvage, J.-P. Chem. Commun. 2000, 2479. (c) Linke,
M.; Chambron, J.-C.; Heitz, V.; Sauvage, J.-P.; Encinas, S.; Bari-
gelletti, F.; Flamigni, L. J. Am. Chem. Soc. 2000, 122, 11834. (d)
Flamigni, L.; Marconi, G.; Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.
J. Phys. Chem. B 2002, 106, 6663.
(9) Kilså, K.; Kajanus, J.; Marcpherson, A. N.; Mårtensson, J.; Albinsson,
B. J. Am. Chem. Soc. 2001, 123, 3069.
(10) Flamigni, L.; Barigelletti, F.; Armaroli, N.; Collin, J.-P.; Sauvage, J.-
P.; Williams, J. A. G. Chem.sEur. J. 1998, 4, 1744.
(11) (a) Shimidzu, T.; Iyoda, T.; Segawa, H.; Honda, K. NouV. J. Chim.
1986, 10, 213. (b) Shimidzu, T.; Segawa, H.; Iyoda, T.; Honda, K. J.
Chem. Soc., Faraday Trans. 2 1987, 83, 2191.
(12) Fukuzumi, S. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds; Academic Press: San Diego, CA, 2000; Vol. 8,
pp 115-152.
(13) Abou-Gamra, Z.; Harriman, A.; Neta, P. J. Chem. Soc., Faraday Trans.
2 1986, 82, 2337.
(14) Jamin, M. E.; Iwamoto, R. T. Inorg. Chim. Acta 1978, 27, 135.
(15) Antipas, A.; Dolphin, D.; Gouterman, M.; Johnson, E. C. J. Am. Chem.
Soc. 1978, 100, 7705.
(16) Kadish, K. M.; E, W.; Ou, Z.; Shao, J.; Sintic, P. J.; Ohkubo, K.;
Fukuzumi, S.; Crossley, M. J. Chem. Commun. 2002, 356.
(17) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;
Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2000; Vol. 3, p 1.
Our studies were carried out in up to six different nonaqueous
solvents and show that gold(III) porphyrins are in most cases
not electrochemically inert as was previously claimed but
can, depending upon the macrocycle, be rapidly and revers-
ibly converted to a Au(II) form of the compound rather than
to a Au(III) porphyrin π-anion radical. Compound 1 was
previously characterized in pyridine, benzonitrile, and tet-
rahydrofuran, and this study has now been extended to six
different nonaqueous solvents in order to investigate the
effect of changing solvent properties on the site of electron
transfer. The reorganization energy of metal-centered electron
transfer reduction for the same gold porphyrin (1) was
determined by analyzing the rates of electron transfer in light
of the Marcus theory of electron transfer.19 The effect of
porphyrin ring substituents on modulating the site of electron
transfer is also reported.
Experimental Section
Chemicals. Phenanthrene and pyrene were purchased com-
mercially and purified by the standard methods.20 The sodium salts
(18) (a) Yeow, E. K. L.; Sintic, P. J.; Cabral, N. M.; Reek, J. N. H.;
Crossley, M. J.; Ghiggino, K. P. Phys. Chem. Chem. Phys. 2000, 2,
4281. (b) Fukuzumi, S.; Ohkubo, K.; E, W.; Ou, Z.; Shao, J.; Kadish,
K. M.; Hutchison, J. A.; Ghiggino, K. P.; Sintic, P. J.; Crossley, M.
J. J. Am. Chem. Soc. 2003, 125, 14984.
(19) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2079

Ou et al.
of the naphthalene radical anion (2.4 × 10-1 M in THF) were
prepared by reduction of naphthalene (5.5 mmol) with sodium (5.0
mmol) under deaerated conditions in distilled THF at 298 K.
Benzonitrile (PhCN) obtained from Fluka Chemika or Aldrich Co.
was distilled over phosphorus pentaoxide (P2O5) under vacuum prior
to use. Absolute dichloromethane (CH2Cl2), pyridine, tetrahydro-
furan (THF), N,N-dimethylformamide (DMF), and dimethyl sul-
foxide (DMSO) were received from Aldrich Co. and used as
received without further purification. Tetra-n-butylammonium per-
chlorate (TBAP) and tetra-n-hexylammonium perchlorate (THAP)
were purchased from Sigma Chemical or Fluka Chemika Co.,
recrystallized from ethyl alcohol, and dried under vacuum at
40 °C for at least one week prior to use.
tert-butylphenyl)porphyrinato]aurate(III) (1) (145 mg, 85%) as a
reddish-brown solid, mp > 300 °C. IR (CHCl3): 2966s, 2905m,
2870m, 1838w, 1792w, 1718w, 1593s, 1518w, 1477m, 1428w,
1395w, 1365s, 1316w, 1299w, 1268w, 1247m, 1230m, 1214w,
1082w cm-1. UV-vis (CHCl3): 345 (log ꢀ 4.07), 393sh (4.50),
416 (5.58), 486 (3.75), 524 (4.29), 561sh (3.51) nm. 1H NMR (400
MHz; CDCl3): δ 1.54 (72 H, s, tert-butyl H), 7.93 (4 H, t, J ) 1.8
Hz, aryl H), 8.09 (8 H, d, J ) 1.8 Hz, aryl H), 9.35 (8 H, s,
â-pyrrolic H). 31P NMR (162 MHz; CDCl3): δ -145.34 (1 P, h,
J ) 713 Hz, PF6). MS (MALDI-TOF): 1259.0 ([M - PF6]+
requires 1258.5). MS (HRFAB): m/z 1257.6988 [M - PF6]+, calcd
for C76H92N4Au: 1257.6987. Anal. Calcd for C76H92N4AuPF6: C,
65.0; H, 6.6; N, 4.0. Found: C, 65.1; H, 6.7; N, 4.1%.
General Procedures. Microanalyses were performed by the
Campbell Microanalytical Laboratory, University of Otago, New
Zealand. Infrared spectra were determined on a Perkin-Elmer model
1600 FT-IR spectrophotometer as solutions in the stated solvents.
UV-vis spectra were recorded on a Cary 5E UV-vis-NIR
spectrophotometer in chloroform that was deacidified by filtration
Hexafluorophosphate[2-nitro-5,10,15,20-tetrakis-(3,5-di-tert-
butylphenyl)porphyrinato]aurate(III) (2). 2-Nitro-5,10,15,20-
tetrakis(3,5-di-tert-butylphenyl)porphyrin (120 mg, 0.108 mmol),
potassium tetrachloroaurate(III) (111 mg, 0.294 mmol), and sodium
acetate (40 mg, 0.52 mmol) were dissolved in a solution of
chloroform (15 mL) and glacial acetic acid (18 M, 15 mL). The
mixture was heated at reflux for 22 h and diluted with dichlo-
romethane (100 mL). The mixture was then washed with water
(2 × 100 mL), sodium carbonate solution (2 × 100 mL), and water
(2 × 100 mL), dried over anhydrous sodium sulfate, and filtered.
The filtrate was evaporated to dryness and the residue dissolved in
chloroform (15 mL). The organic phase was then stirred with a
saturated solution of potassium hexafluorophosphate (2.31 g, 12.45
mmol) in water (10 mL) for 20 h. Dichloromethane (100 mL) was
then added and the mixture washed with water (4 × 100 mL), dried
over anhydrous sodium sulfate, and filtered. The filtrate was
evaporated to dryness and the residue purified by chromatography
over silica (dichloromethane-light petroleum; 1:1 gradually in-
creased to dichloromethane-methanol; 25:1). The first band was
collected and the solvent removed to give 2-nitro-porphyrin (6 mg).
The major polar band was collected and the solvent removed to
yield hexafluorophosphate[2-nitro-5,10,15,20-tetrakis(3,5-di-tert-
butylphenyl)porphyrinato]aurate(III) (2) (113 mg, 73%) as a
reddish-brown microcrystalline solid, mp > 300 °C. IR (CHCl3):
3693w, 3604w, 2967s, 2906m, 2870m, 1592s, 1535m, 1477m,
1428w, 1395w, 1365s, 1338w, 1298w, 1247m, 1232m, 1216w,
1199w, 1169w, 1123w, 1084w cm-1. UV-vis (CH2Cl2): 363 (log
ꢀ 4.22), 424 (5.40), 495 (3.83), 532 (4.21), 565 (3.90), 646 (2.97)
1
through an alumina column. H NMR spectra were recorded on a
Bruker DPX-400 (400 MHz) spectrometer, with tetramethylsilane
(Me4Si) as the internal standard. Signals are recorded in terms of
chemical shift (δ) in ppm from Me4Si, multiplicity, coupling
constants (in Hz), and assignments, in that order. 31P NMR spectra
were acquired on a Bruker DPX-400 (162 MHz) spectrometer. 31
P
NMR chemical shifts are referenced to external, neat trimethyl
phosphite taken to be 140.85 ppm at room temperature. Matrix
assisted laser desorption ionization time-of-flight (MALDI-TOF)
mass spectra without a matrix were recorded on a VG TofSpec
spectrometer. Fast atom bombardment (FAB) high-resolution mass
spectra were recorded on a VG ZAB-2SEQ instrument at the
Research School of Chemistry, Australian National University.
Column chromatography was carried out using flash chromatog-
raphy on Merck silica gel type 9385 (230-400 mesh). All solvents
used for chromatography and for recrystallizations were redistilled
before use. Light petroleum refers to the fraction of bp 60-80 °C.
Where solvent mixtures were used, the proportions are given by
volume.
Hexafluorophosphate[5,10,15,20-tetrakis(3,5-di-tert-butyl-
phenyl)porphyrinato]aurate(III) (1). 5,10,15,20-Tetrakis(3,5-di-
tert-butylphenyl)porphyrin (130 mg, 0.122 mmol), potassium
tetrachloroaurate(III) (108 mg, 0.286 mmol), and sodium acetate
(88 mg, 1.07 mmol) were dissolved in a solution of dichloromethane
(15 mL) and glacial acetic acid (18 M, 15 mL). The reaction mixture
was heated at reflux for 24 h and then allowed to cool, and fresh
potassium tetrachloroaurate(III) (80 mg, 0.212 mmol), sodium
acetate (60 mg, 0.731 mmol), and glacial acetic acid (18 M, 1 mL)
were added. The mixture was heated for a further 24 h, allowed to
cool, and then diluted with chloroform (100 mL). The mixture was
then washed with water (3 × 100 mL), sodium carbonate solution
(10%, 2 × 100 mL), and water (2 × 100 mL), dried over anhydrous
sodium sulfate, and filtered. The filtrate was evaporated to dryness
and the residue dissolved in dichloromethane (15 mL). The organic
phase was stirred with a saturated solution of potassium hexafluo-
rophosphate (2.10 g, 11.4 mmol) in water (10 mL) for 24 h. The
mixture was then diluted with dichloromethane (100 mL) and
washed with water (4 × 100 mL), dried over anhydrous sodium
sulfate, and filtered. The filtrate was removed under vacuum and
the residue purified by chromatography over silica (chloroform-
methanol; 50:1). The major polar band was collected and the solvent
removed to yield hexafluorophosphate[5,10,15,20-tetrakis(3,5-di-
1
nm. H NMR (400 MHz; CDCl3): δ 1.51 (18 H, s, tert-butyl H),
1.52 (27 H, s, tert-butyl H), 1.53 (18 H, s, tert-butyl H), 1.54 (9 H,
br s, tert-butyl H), 7.86 (1 H, t, J ) 1.8 Hz, aryl H), 7.90-7.91 (2
H, m, aryl H), 7.92 (1 H, t, J ) 1.8 Hz, aryl H), 8.05 (2 H, d, J )
1.8 Hz, aryl H), 8.07 (4 H, t, J 1.8 ) Hz, aryl H), 8.09 (2 H, d,
J ) 1.8 Hz, aryl H), 9.23-9.28 (5 H, m, â-pyrrolic H), 9.32 (1 H,
d, J ) 5.3 Hz, â-pyrrolic H), 9.49 (1 H, s, H-3). 31P NMR (162
MHz; CDCl3): δ -145.34 (1 P, h, J ) 713 Hz, PF6). MS (MALDI-
TOF): 1304.1 ([M - PF6]+ requires 1303.5). MS (HRFAB): m/z
1302.6838 [M - PF6]+, calcd for C76H91N5O2Au: 1302.6886. Anal.
Calcd for C76H91N5O2AuPF6: C, 63.0; H, 6.3; N, 4.8. Found: C,
63.4; H, 6.25; N, 5.1%.
Hexafluorophosphate[2-amino-5,10,15,20-tetrakis-(3,5-di-tert-
butylphenyl)porphyrinato]aurate(III) (3). Hexafluorophosphate-
[2-nitro-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato]-
aurate(III) (2) (50 mg, 0.0345 mmol) and tin(II) chloride dihydrate
(45 mg, 0.199 mmol) were dissolved in dichloromethane (10 mL).
Concentrated hydrochloric acid (10 M, 0.5 mL) was added and the
mixture stirred in the dark under nitrogen for 24 h. The mixture
was then diluted with dichloromethane (100 mL) and washed with
water (2 × 100 mL), sodium carbonate solution (2 × 100 mL),
and water (2 × 100 mL), dried over anhydrous sodium sulfate,
(20) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
2080 Inorganic Chemistry, Vol. 43, No. 6, 2004

Reduction of Au(III) Porphyrins
and filtered. The filtrate was evaporated to dryness. The resultant
residue was dissolved in dichloromethane (10 mL) which was then
stirred with a saturated solution of potassium hexafluorophosphate
(2.05 g, 11.06 mmol) in water (10 mL) for 20 h. Dichloromethane
(100 mL) was then added, the mixture washed with water (4 ×
100 mL), dried over anhydrous sodium sulfate, filtered and the
solvent removed under vacuum. The residue was purified by
chromatography over silica (dichloromethane-methanol; 50:1
increased to dichloromethane-methanol; 25:1), the major polar
band was collected and the solvent removed to yield hexafluoro-
phosphate[2-amino-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)por-
phyrinato]aurate(III) (3) (42 mg, 86%) as a reddish-brown micro-
crystalline solid, mp > 300 °C. IR (CHCl3): 3691w, 3602w, 3499w,
3394m, 2966s, 2905m, 2870m, 1618m, 1592s, 1572w, 1552w,
1522m, 1503w, 1477m, 1460m, 1428w, 1395w, 1365s, 1315w,
1299w, 1247m, 1236m, 1192w, 1083w, 1040m cm-1. UV-vis
(CHCl3): 317 (log ꢀ 4.08), 400 (5.14), 424 (4.93), 529 (3.99), 575
(3.95), 594 (3.98), 628sh (3.69) nm. 1H NMR (400 MHz; CDCl3):
δ 1.50 (18 H, s, tert-butyl H), 1.51-1.52 (54 H, m, tert-butyl H),
5.18 (2 H, s, NH2), 7.87-7.90 (5 H, m, aryl H), 7.94 (2 H, d, J )
1.8 Hz, aryl H), 7.98 (1 H, t, J ) 1.8 Hz, aryl H), 8.02 (2 H, d,
J ) 1.8 Hz, aryl H), 8.03 (2 H, d, J 1.8 ) Hz, aryl H), 8.19 (1 H,
s, H-3), 8.95 (1 H, d, J ) 5.2 Hz, â-pyrrolic H), 9.10 (1 H, d, J )
5.2 Hz, â-pyrrolic H), 9.17-9.21 (3 H, m, â-pyrrolic H), 9.24
(1 H, d, J ) 5.2 Hz, â-pyrrolic H). 31P NMR (400 MHz; CDCl3):
δ -145.34 (1 P, h, J ) 713 Hz, PF6). MS (MALDI-TOF): 1273.6
([M - PF6]+ requires 1273.6). MS (HRFAB): m/z 1272.7097
[M - PF6]+, calcd for C76H93N5O2Au: 1272.7078). Anal. Calcd
for C76H93N5AuPF6: C, 64.35; H, 6.6; N, 4.9. Found: C, 64.5; H,
6.6; N, 5.0%.
Electrochemical Measurements. Cyclic voltammetry was car-
ried out by using an EG&G Princeton Applied Research (PAR)
173 potentiostat/galvanostat. A homemade three-electrode cell was
used for cyclic voltammetric measurements and consisted of a
platinum button or glassy carbon working electrode, a platinum
counter electrode, and a homemade saturated calomel reference
electrode (SCE). The SCE was separated from the bulk of the
solution by a fritted glass bridge of low porosity that contained the
solvent/supporting electrolyte mixture. UV-vis spectroelectro-
chemical experiments were performed with a home-built thin-layer
cell21 that had a light transparent platinum net working electrode.
Potentials were applied and monitored with an EG&G PAR model
173 potentiostat. Time-resolved UV-vis spectra were recorded with
a Hewlett-Packard model 8453 diode array spectrophotometer.
ESR Measurements. ESR spectra of reduced gold porphyrins
were measured in frozen PhCN under nonsaturating microwave
power conditions with a JEOL X-band spectrometer (JES-RE1XE)
using an attached variable temperature apparatus. The magnitude
of modulation was chosen to optimize the resolution and the signal-
to-noise (S/N) ratio of the observed spectra when the maximum
slope line width (∆Hmsl) of the ESR signals was unchanged with
larger modulation. The g values were calibrated with a Mn2+
marker.
fresh deaerated solutions in each laser excitation. All experiments
were performed at 298 K.
Results and Discussion
Synthesis and Characterization of Neutral Gold Por-
phyrins. The gold porphyrins 1 and 2 were prepared in good
yields from the corresponding free-base porphyrins by
treatment with 3-4 equiv of potassium tetrachloroaurate(III)
and sodium acetate followed by workup involving solvent
extraction, ligand exchange, and chromatography steps. The
gold(III) 2-aminoporphyrin 3 was prepared by reduction of
the nitro group of 2 using tin(II) chloride and HCl. While it
is possible that the gold(III) was also reduced under these
conditions, the complex showed no sign of demetalation and
workup was carried out under aerobic conditions which
would have readily reoxidized any reduced gold species.
Ligand exchange and chromatography yielded 3 in 86%
overall yield. Each of the gold porphyrins 1-3 have
spectroscopic properties, including 1H and 31P NMR spectra,
elemental analysis, and high resolution mass spectra that are
fully consistent with the structures. The synthesis of hexaflu-
orophosphate[5,10,15,20-tetrakis(3,5-di-tert-butyl-phenyl)-
quinoxalino[2,3-b]porphyrinato]aurate(III), (PQ)AuPF6 (4) is
published elsewhere.22
Electrochemistry of Gold Porphyrins. The electrochemi-
cal behavior of the gold porphyrins 1-4 was investigated
by cyclic voltammetry in up to six different nonaqueous
solvents, and the measured half-wave potentials for the three
electrode reactions are summarized in Table 1. The difference
in half-wave potentials between the first and second revers-
ible reductions, ∆red.(1-2), and between the reversible
second and third reductions, ∆red.(2-3), are also included
in this table. Examples of the cyclic voltammograms of
compounds 1-3 obtained in pyridine containing 0.1 M
TBAP are illustrated in Figure 1. Cyclic voltammograms of
(PQ)AuPF6 (4) obtained in CH2Cl2 or pyridine containing
0.1 M TBAP and THF containing 0.4 M TBAP are illustrated
in Figure 2. As mentioned in the Introduction, the first
reduction of Au(III) porphyrins has long been thought to
occur on the macrocycle of the porphyrin because Au(III)
was described as “electrochemically inert” when inserted into
the cavity of a porphyrin.14 This view, however, was recently
overturned16 when we investigated the first reduction of
(P-H)AuPF6 (1) in three commonly used electrochemical
solvents (pyridine, THF, and PhCN) and saw in all cases a
metal-centered reduction, leading to formation of a Au(II)
porphyrin with an uncharged macrocycle. The second and
third reductions of the compound were then assigned to be
ring-centered by comparison with results for Cu(II) and Zn-
(II) porphyrins having the same macrocycle.16 In the present
study, three reversible reductions are observed under almost
all solvent conditions as in the case of pyridine (Figure 1),
with the only exceptions being compounds 1 and 3 in CH2Cl2
where the third reduction is located at E1/2 values beyond
the negative potential limit of the solvent although a further
Laser Flash Photolysis. Nanosecond transient absorption mea-
surements were carried out using a Nd:YAG laser (Continuum,
SLII-10, 4-6 ns fwhm) at 355 nm with the power of 10 mJ as an
excitation source, a continuous Xe-lamp (150 W) and an InGaAs-
PIN photodiode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a photomultiplier
tube was recorded with a digitizing oscilloscope (Tektronix,
TDS3032, 300 MHz). The transient spectra were recorded using
(22) Crossley, M. J.; Sintic, P. J.; Walton, R.; Reimers, J. R. Org. Biomol.
Chem. 2003, 1, 2777.
(21) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2081

Ou et al.
Table 1. Half-Wave Potentials (V vs SCE) of Au Porphyrins in Different Solvents Containing 0.1 M TBAP
reduction
compd
solvent
oxidation
first
∆red.(1-2)
second
∆red.(2-3)
third
(P-NH2)AuPF6 (3)
pyridine
CH2Cl2
THFc
DMF
pyridine
PhCN
CH2Cl2
DMSO
THFc
pyridine
CH2Cl2
THFc
a
-0.60
-0.69
-0.40
-0.49
-0.52
-0.56
-0.64
-0.54b
-0.24
-0.32
-0.38
-0.25
-0.35
-0.39
-0.47
-0.30
0.55
0.54
0.70
0.50
0.56
0.53
0.51
-1.15
-1.23
-1.10
-0.99
-1.08
-1.09
-1.15
-1.24b
-0.77
-0.72
-0.80
-0.87
-0.87
-0.88
-0.97
-0.91
0.68
-1.83
a
1.25b
(P-H)AuPF6 (1)
a
0.67
0.75
0.68
0.72
-1.77
-1.74
-1.76
-1.81
a
1.06b
a
1.66
1.59
a
a
a
a
a
a
a
1.54
a
-1.68b
-1.42
-1.40
-1.40
-1.67
-1.69
-1.81
-1.80b
-1.75
(P-NO2)AuPF6 (2)
(PQ)AuPF6 (4)
0.53
0.40
0.42
0.62
0.52
0.49
0.50
0.55
0.65
0.68
0.60
0.80
0.82
0.93
0.83
0.82
pyridine
PhCN
CH2Cl2
toluened
a Beyond potential limit of solvents. b Peak potential at a scan rate of 0.1V/s for irreversible reaction. c Solution containing 0.4 M TBAP. d Solution
containing 0.6 M THAP. Data obtained at 45 °C.
Figure 1. Cyclic voltammograms of (a) (P-NO2)AuPF6 (2), (b) (P-H)-
AuPF6 (1), and (c) (P-NH2)AuPF6 (3) in pyridine containing 0.1 M TBAP.
reduction can be seen when a solvent with a wider potential
Figure 2. Cyclic voltammograms of (PQ)AuPF6 (4) in (a) CH2Cl2, 0.1 M
window (such as THF) is utilized. In pyridine, the first
reduction potentials of these four porphyrins range from
-0.32 V versus SCE in the case of (P-NO2)AuPF6 (2) to
-0.60 V in the case of (P-NH2)AuPF6 (3) while values of
E1/2 for the second and third reductions range from -0.72
to -1.15 V and -1.40 to -1.83 V, respectively. As seen in
Table 1, the difference in E1/2 between the first two
reductions, ∆red.(1-2), are similar to each other for com-
pounds 1, 3, and 4 in all solvents except for THF. For
example, this value ranges from 0.50 to 0.56 V for
(P-H)AuPF6 (1), from 0.54 to 0.55 V for (P-NH2)AuPF6
(3), and from 0.49 to 0.55 V for (PQ)AuPF6 (4). A smaller
∆red.(1-2) value from 0.40 to 0.42 V is observed for
TBAP, (b) pyridine, 0.1 M TBAP, and (c) THF, 0.4 M TBAP.
(P-NO2)AuPF6 (2) in both pyridine and CH2Cl2. This
difference from the other compounds might be attributed to
the fact that the nitro group shows a strong electronic
interaction with the porphyrin macrocycle which thus affects
the energy level of the LUMO, or as discussed below, it
might be related to a change in electron transfer site.
Also, for each of the four investigated compounds, the
∆red.(1-2) value in THF is higher than what is seen for the
same porphyrins in the other nonaqueous solvents. For
example, the ∆red.(1-2) value is 100-200 mV higher in
THF than in the other utilized solvents.
2082 Inorganic Chemistry, Vol. 43, No. 6, 2004

Reduction of Au(III) Porphyrins
Table 2. Spectral Data (λmax/nm and log(ꢀ/M-1 cm-1)) of Neutral and of Singly Reduced Au(III) Complexes in Different Solvents Containing 0.2 M
TBAP
neutral
singly reduced
Soret band
410 (5.17)
compd
solvent
CH2Cl2
Soret band
visible band
visible band
538 (4.17)
(P-NH2)AuPF6 (3)
399 (5.19)
424 (4.96)
402 (5.26)
435 (4.84)
414 (5.54)
415 (5.69)
418 (5.49)
419 (5.44)
414 (5.10)
417 (5.38)
421 (5.41)
424 (5.38)
426 (5.34)
394 (4.41)
436 (5.05)
396 (4.54)
439 (5.17)
529 (4.04)
593 (4.02)
530 (4.03)
602 (4.07)
524 (4.26)
524 (4.47)
526 (4.26)
526 (4.20)
524 (3.80)
526 (4.11)
531 (4.21)
532 (4.25)
535 (4.19)
548 (4.01)
590 (3.90)
548 (4.03)
592 (3.98)
pyridine
410 (5.24)
540 (3.69)
(P-H)AuPF6 (1)
THF
414 (5.12), 439 (4.80)
420 (5.57)
423 (5.39)
423 (5.30)
421 (4.95)
CH2Cl2
pyridine
PhCN
DMF
DMSO
THF
CH2Cl2
pyridine
CH2Cl2
535 (4.50)
536 (4.23)
537 (4.14)
534 (3.39)
insoluble product
(P-NO2)AuPF6 (2)
(PQ)AuPF6 (4)
427 (4.71)
427 (5.05)
432 (4.97)
411 (4.94)
436 (5.03)
408 (4.82)
441 (4.98)
734 (3.82)
657 (3.73)
658 (3.87)
548 (4.01)
590 (3.85)
548 (4.25)
592 (4.20)
pyridine
The potential differences between the second and third
reductions, ∆red.(2-3), are also close to each other for
compounds 1-3 in THF, pyridine, and CH2Cl2 (see Table
1). These values range from 0.67 to 0.68 V for (P-H)AuPF6
(1) and from 0.60 to 0.68 V for (P-NO2)AuPF6 (2). The
separation for (PQ)AuPF6 (4) under similar solution condi-
tions ranges from 0.80 to 0.83 V which is much higher than
∆red.(2-3) seen for other complexes. This result may be
related to the presence of the quinoxaline (Q) which is fused
directly to the â-pyrrolic carbons of the porphyrin macro-
cycle. As compared to reductions, none or only one oxidation
process is observed for the gold complexes within the anodic
potential limit of the utilized solvents (see Table 1). When
observed, the oxidation should be macrocycle-centered since
Au(IV) would not be an expected central metal oxidation
state.
The electron-withdrawing property of the fused quinoxa-
line group in (PQ)AuPF6 (4) can be seen by comparing the
reduction potentials of compounds 4 and 1. The potentials
of all three reductions of (PQ)AuPF6 (4) shift anodically in
THF, pyridine, CH2Cl2, and PhCN except for the third
reduction in PhCN.
Substituent Effects on the Three Reduction Potentials.
Figure 1 illustrates how reduction potentials of compounds
1-3, which possess closely related structures, vary as a
function of the macrocyclic substituent. The three reduction
potentials for (P-NH2)AuPF6 (3) shift nearly the same
amount as the reactions of (P-H)AuPF6 (1) while the first
reduction of (P-NO2)AuPF6 (2) shifts 160 mV less than the
other two reductions.
Figure 3. UV-vis spectral changes of (P-H)AuPF6 (1) upon (a) first
electroreduction at an applied potential of -0.65 V, (b) second reduction
at -1.30 V, and (c) third reduction at -1.85 V in pyridine containing
0.2 M TBAP.
Spectroelectrochemical Characterization of Reduced
Gold Porphyrins. Thin-layer UV-vis spectra taken during
the electroreduction of (P-H)AuPF6 (1) in pyridine contain-
reduction of (P-H)AuPF6 (1) in pyridine is comparable to
what was observed in PhCN16 and corresponds to a metal-
centered reduction. Isosbestic points are seen, indicating the
lack of any spectrally detectable intermediates in the reduc-
tion. There is no evidence for coupled chemical reactions
before, during, or after the electron transfers. The Soret band
of (P-H)AuPF6 (1) at 423 nm continues to decrease in
intensity during the second reduction, indicating a ring-
ing 0.2 M TBAP are shown in Figure 3. Spectral data (λmax
,
nm and log(ꢀ, M-1 cm-1)) of the neutral and singly reduced
Au(III) complexes in solvents containing 0.2 M TBAP are
summarized in Table 2.
The intensity of the Soret band slightly decreases for
compound 1 (Figure 3), and a new Soret band at 423 nm
arises after a one-electron addition. The spectral change for
Inorganic Chemistry, Vol. 43, No. 6, 2004 2083

Ou et al.
Figure 4. UV-vis spectral changes upon the first electroreduction of
(a) (P-NH2)AuPF6 (3) at an applied potential of -0.80 V vs SCE, (b)
(P-H)AuPF6 (1) at -0.65 V, and (c) (P-NO2)AuPF6 (2) at -0.60 V in
pyridine containing 0.2 M TBAP.
Figure 5. (a) UV-vis spectral changes of (PQ)AuPF6 (4) upon the first
electroreduction at an applied potential of -0.60 V vs SCE in pyridine
containing 0.2 M TBAP, and UV-vis spectra of the neutral porphyrins (b)
(PQ)Cu and (c) (PQ)Pd in pyridine.
centered reaction which is consistent with what was reported
for the same complex in PhCN.16 The spectra obtained during
the third one-electron addition to (P-H)AuPF6 (1) involve
a decrease of the Soret band intensity and the appearance of
a new Soret band and a broad visible band indicating that
this reaction is macrocycle-centered. Spectral changes of
(P-H)AuPF6 (1) follow the same trends in other investigated
solvents except DMSO in which the singly reduced species
is not soluble in the solvent. This is consistent with the fact
that three reductions in DMSO are all irreversible.
assigned spectra of unreduced (PQ)Cu and (PQ)Pd taken in
the same solvent are also shown in this figure. A metal-
centered reduction is proposed for (PQ)AuPF6 (4), leading
to formation of a Au(II) porphyrin with an uncharged
macrocyclic ring. The Soret band at 439 nm for (PQ)AuPF6
(4) broadens as its intensity decreases, and a new Soret band
at 408 nm appears which is not seen in the case of (P-H)-
AuPF6 (1). The resulting spectrum is similar to the spectra
of neutral (PQ)Cu and (PQ)Pd which means that the singly
reduced species shows M(II) character. The splitting of the
Soret band into two transitions occurs due to good electronic
communication between the porphyrin and the quinoxaline
appendage for the singly reduced species.16 In contrast, the
spectrum obtained during the second reduction of (PQ)AuPF6
(4) is characterized by a decreased Soret band at 441 nm
and the appearance of a new Soret band at 424 nm just after
application of the reducing potential. Isosbestic points are
not obtained during this reaction, indicating that these spectral
changes do not correspond to a single one-electron transfer
process. On the basis of this fact, it is proposed that the
second electron addition to (PQ)AuPF6 (4) occurs in two
steps. The first step involves an electron addition to the
porphyrin macrocycle. The electrogenerated species is not
stable and transfers the extra negative charge to the qui-
noxaline group. The new band at 424 nm subsequently
decreases in intensity during the third reduction, which is
similar to what is seen in the case of (P-H)AuPF6 (1) (Figure
3). This electron transfer mechanism can also be used to
Spectral changes for (P-NH2)AuPF6 (3) during the three
reductions resemble those of (P-H)AuPF6 (1), and this
indicates the same electron transfer mechanisms for each
reduction. Comparison of spectral changes during the first
reduction is shown in Figure 4. Two different types of
spectral changes are seen for compounds 1-3 during the
first one-electron reduction. (P-NO2)AuPF6 (2) behaves in
a different style that is characteristic of reduction of the
porphyrin macrocycle. This indicates that the nitro group
changes the reaction site of electron transfer during the first
reduction. The nitro group, which is located at one of the
eight â-pyrrolic positions of the porphyrin macrocycle, is a
strong electron-withdrawing group, resulting in lowering the
one-electron reduction potential of the porphyrin macrocycle.
The change in the site of electron transfer depending on the
substituent of the â-position of the porphyrin macrocycle
can be further clarified by ESR measurements (vide infra).
The thin-layer UV-vis spectra taken during the first
electroreduction of (PQ)AuPF6 (4) in pyridine containing
0.2 M TBAP are shown in Figure 5. For comparison, the
2084 Inorganic Chemistry, Vol. 43, No. 6, 2004

Reduction of Au(III) Porphyrins
Figure 7. (a) T-T absorption spectrum of phenanthrene (1.0 × 10-4 M)
obtained by the laser flash photolysis in deaerated PhCN at 1.0 µs after
laser excitation (355 nm) at 298 K. (b) Decay time profile of T-T absorption
of phenanthrene at 490 nm in the absence and presence of (P-H)AuPF6
(1) in deaerated PhCN. (c) Dependence of decay rate constant (kobs) on
concentration (P-H)AuPF6 (1).
Figure 6. ESR spectra of singly reduced gold porphyrins (1.0 mM)
generated by reduction with 1 equiv of naphthalene radical anion (1.0 mM)
at -160 °C: (a) (P-NH2)AuPF6 (3) in DMF, (b) (P-H)AuPF6 (1) in
DMSO, (c) (P-NO2)AuPF6 (2) in DMF, and (d) (PQ)AuPF6 (4) in DMF.
porphyrin ligand due to the electron-withdrawing effect of
the NO2 group.
explain the extra large potential difference between the
second and third reductions, ∆red.(2-3), for (PQ)AuPF6 (4)
(see Table 1).
Reorganization Energy of Metal-Centered Electron
Transfer. Upon laser excitation at 355 nm of a PhCN
solution of phenanthrene (1.0 × 10-4 M), a transient triplet-
ESR Characterization of Singly Reduced Gold Por-
phyrins. The site of electron transfer is further confirmed
by ESR spectra obtained after chemical reduction of 1-4
with 1 equiv of naphthalene radical anion in DMF or DMSO.
These spectra are shown in Figure 6. The broad signal at
g ) 2.06 observed for the one-electron reduced species of
(P-NH2)AuPF6 (3), (P-H)AuPF6 (1), and (PQ)AuPF6 (4)
is quite different from the sharp signal at g ) 2.006 which
is assigned to the Au(III) porphyrin π-anion radical. The large
g-value (2.06), which is characteristic of the metal-centered
radical, agrees with the reported value (2.065 ( 0.005) of
Au(II) phthalocyanine.23 Thus, the broad signal is clearly
assigned to the Au(II) species. Although both the Au(II)
porphyrin and the Au(III) porphyrin π-anion radical are
evident in the frozen solution, the latter species is negligible
judging from the large difference in their line widths.24 In
the case of (P-NO2)AuPF6 (2), however, the observed ESR
spectrum in Figure 6c is quite different from those of the
other compounds (1 and 3) in Figure 6a,b. The absence of a
broad signal due to the Au(II) species clearly indicates that
the site of electron transfer is changed from the metal to the
triplet (T-T) absorption spectrum is observed with λmax
)
490 nm as shown Figure 7a. The T-T absorbance decays
obeying second-order kinetics due to T-T annihilation with
a diffusion-limited rate. In the presence of (P-H)AuPF6 (1),
the decay rate becomes faster and obeys pseudo-first-order
kinetics. The observed pseudo-first-order rate constant (kobs
)
increases linearly with increasing concentration of 1 (Figure
7b,c). The free energy change of electron transfer from the
triplet excited state of phenanthrene to 1 (∆G°et) is largely
negative (-0.55 eV) judging from the one-electron oxidation
potential of the triplet excited state of phenanthrene (E°ox
)
-1.11 V vs SCE)25 and the one-electron reduction potential
of 1 (-0.56 V vs SCE). Thus, the acceleration of the triplet
decay is ascribed to photoinduced electron transfer from the
triplet excited state of phenanthrene to 1. The observed
second-order rate constant of photoinduced electron transfer
(kobs) is determined as 1.4 × 109 M-1 s-1 from the linear
plot in Figure 7c. The triplet quenching measurements were
also carried out using pyrene instead of phenanthrene. The
∆G°et of photoinduced electron transfer from the triplet
excited state of pyrene to 1 (-0.33 eV) is less negative as
compared with the ∆G°et value of the triplet excited state of
phenanthrene because of the less negative E°ox value of the
(23) MacCragh, A.; Koski, W. S. J. Am. Chem. Soc. 1965, 87, 2496.
(24) In solution at 25 °C, only the ESR signal due to the π-anion radical
is observed because of the fast relaxation time of the Au(II) species.
The amount of the π-anion radical is negligible as compared to the
initial concentration.
(25) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
Inorganic Chemistry, Vol. 43, No. 6, 2004 2085

Ou et al.
triplet pyrene (-0.89 V vs SCE).25 The kobs value of
photoinduced electron transfer from the triplet excited state
of pyrene to 1 was determined as 1.6 × 108 M-1 s-1 which
is smaller than the value of triplet phenanthrene, in agreement
with the less negative ∆G°et value.
of pyrene to 1 from the ket and ∆G°et values using eq 3 which
is derived from eq 1. Virtually the same λ value (1.27 eV)
is obtained from ket and ∆G°et values of photoinduced
electron transfer from the triplet excited state of phenanthrene
to 1.
The dependence of the photoinduced electron transfer rate
constant (ket) on the free energy change of electron transfer
(∆G°et) for outer-sphere electron transfer has been well
established by Marcus as given by eq 1, where λ is the
reorganization energy of electron transfer and Z is the
collision frequency taken as 1 × 1011 M-1 s-1.19
λ ) -∆G°et - 2RT ln(ket/Z) +
[{∆G°et + 2RT ln(ket/Z)}2 - (∆G°et)2]1/2 (3)
Such an agreement confirms the validity of the analysis
to determine the λ value. The λ value obtained for electron
transfer reduction of 1 is significantly larger than the reported
λ values of ligand-centered electron transfer reactions of
metalloporphyrins (λ ) ca. 0.6 eV).12
ket ) Z exp[-(λ/4)(1 + ∆G°et/λ)2/RT]
(1)
The ket value can be obtained from the kobs value using eq
2, where the effect of diffusion is taken into account and
kdiff is the diffusion rate constant in PhCN (5.6 × 109 M-1
s-1).26
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) and the Texas Advanced
Research program to K.M.K. under Grant 003652-0018-2001
is gratefully acknowledged. This work was also partially
supported by a Grant-in-Aid for Scientific Research Priority
Area (13440216) from the Ministry of Education, Culture,
Sports and Science and Technology, Japan, and a Discovery
Research Grant (DP0208776) from the Australian Research
Council.
1/ket ) 1/kobs - 1/kdiff
(2)
Then, the λ value is determined as 1.23 eV for the
photoinduced electron transfer from the triplet excited state
(26) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.; Fujitsuka,
M.; Ito, M. J. Am. Chem. Soc. 1998, 120, 8060.
IC035070W
2086 Inorganic Chemistry, Vol. 43, No. 6, 2004