Photochemical reactions of coenzyme PQQ (pyrroloquinolinequinone) and analogues with benzyl alcohol derivatives via photoinduced electron transfer

Article abstract of DOI:10.1021/ja001351g

Photochemical redox reactions of the trimethyl ester of coenzyme PQQ (PQQTME) with benzyl alcohol derivatives (ArCH₂OH), tetrahydrofuran, and 1,4-cyclohexadiene occur efficiently under visible light irradiation in MeCN to yield PQQTMEH₂ (reduced PQQTME in the quinol form) and the corresponding dehydrogenated products (ArCHO, furan, and benzene) quantitatively. A similar photochemical oxidation of benzyl alcohols also occurs with phenanthrolinequinone derivatives (PTQ), benzoquinolinequinone derivatives (BQQ), and phenanthrenequinone (PQ). PQQTME and the analogues are essentially nonfluorescent in MeCN, and the photochemical reaction of the o-quinones is retarded significantly in the presence of molecular oxygen. Transient absorption spectra of the triplet excited states of the o-quinones were detected in laser flash photolysis of the MeCN solutions. From the decay of T-T spectra were determined the lifetimes of the triplet excited states of the o-quinones. The quenching rate constants of the triplet excited states by benzyl alcohols agree with the observed rate constants of the photochemical reduction of the o-quinones with the same substrates determined from the saturated dependence of quantum yields on the benzyl alcohol concentrations. Such an agreement confirms that the photochemical reaction proceeds via the triplet excited state of the quinones. Dependence of the observed rate constants k_obs of the photochemical redox reaction on the one-electron oxidation potential E^o_ox of the substrates as well as the results of kinetic deuterium isotopic study indicates that the photochemical redox reactions between the o-quinones and the substrates proceed via photoinduced electron transfer from the substrate to the triplet excited state of the o-quinone, followed by proton and hydrogen atom transfer to yield the quinol and the corresponding oxidation products. The transient absorption spectra of the radical ion pair formed in the photoinduced electron transfer have been detected successfully in laser flash photolysis of the o-quinone - benzyl alcohol systems.

Full text of DOI:10.1021/ja001351g

J. Am. Chem. Soc. 2000, 122, 8435-8443
8435
Photochemical Reactions of Coenzyme PQQ
(Pyrroloquinolinequinone) and Analogues with Benzyl Alcohol
Derivatives via Photoinduced Electron Transfer
Shunichi Fukuzumi,*,† Shinobu Itoh,*,‡ Takashi Komori,† Tomoyoshi Suenobu,†
Akito Ishida,§ Mamoru Fujitsuka, and Osamu Ito
Contribution from the Department of Material and Life Science, Graduate School of Engineering, Osaka
UniVersity, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan,
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan, The Institute of Scientific and Industrial Research, Osaka
UniVersity, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan, and Institute for Chemical Reaction Science,
Tohoku UniVersity, Sendai, Miyagi 980-8577, Japan
ReceiVed April 18, 2000. ReVised Manuscript ReceiVed June 27, 2000
Abstract: Photochemical redox reactions of the trimethyl ester of coenzyme PQQ (PQQTME) with benzyl
alcohol derivatives (ArCH2OH), tetrahydrofuran, and 1,4-cyclohexadiene occur efficiently under visible light
irradiation in MeCN to yield PQQTMEH2 (reduced PQQTME in the quinol form) and the corresponding
dehydrogenated products (ArCHO, furan, and benzene) quantitatively. A similar photochemical oxidation of
benzyl alcohols also occurs with phenanthrolinequinone derivatives (PTQ), benzoquinolinequinone derivatives
(BQQ), and phenanthrenequinone (PQ). PQQTME and the analogues are essentially nonfluorescent in MeCN,
and the photochemical reaction of the o-quinones is retarded significantly in the presence of molecular oxygen.
Transient absorption spectra of the triplet excited states of the o-quinones were detected in laser flash photolysis
of the MeCN solutions. From the decay of T-T spectra were determined the lifetimes of the triplet excited
states of the o-quinones. The quenching rate constants of the triplet excited states by benzyl alcohols agree
with the observed rate constants of the photochemical reduction of the o-quinones with the same substrates
determined from the saturated dependence of quantum yields on the benzyl alcohol concentrations. Such an
agreement confirms that the photochemical reaction proceeds via the triplet excited state of the quinones.
Dependence of the observed rate constants kobs of the photochemical redox reaction on the one-electron oxidation
potential Eo of the substrates as well as the results of kinetic deuterium isotopic study indicates that the
ox
photochemical redox reactions between the o-quinones and the substrates proceed via photoinduced electron
transfer from the substrate to the triplet excited state of the o-quinone, followed by proton and hydrogen atom
transfer to yield the quinol and the corresponding oxidation products. The transient absorption spectra of the
radical ion pair formed in the photoinduced electron transfer have been detected successfully in laser flash
photolysis of the o-quinone-benzyl alcohol systems.
Introduction
activities,4-6 and the nutritional importance7 of PQQ itself have
also been revealed to indicate that PQQ serves versatile functions
in several living systems. Thus, recent attention has been focused
on the chemistry of coenzyme PQQ to evaluate its biological
PQQ (pyrroloquinolinequinone) is a versatile o-quinone
cofactor that was first isolated and identified from methanol
dehydrogenase of methylotrophic bacteria in 1979.1 Since then,
much effort has been devoted to finding several kinds of
quinone-containing enzymes (referred to as quinoproteins) from
a variety of organisms of both bacterial and mammalian origins.2
In addition to the enzymological importance, the growth
stimulating activity for microorganisms,3 the pharmaceutical
(3) (a) Ameyama, M.; Shinagawa, E.; Matsushita, K.; Adachi, O. Agric.
Biol. Chem. 1984, 48, 2909. (b) Shimao, M.; Yamamoto, H.; Ninomiya,
K.; Kato, N.; Adachi, O.; Ameyama, M.; Sakazawa, C. Agric. Biol. Chem.
1984, 48, 2873. (c) Ameyama, M.; Shinagawa, E.; Matsushita, K.; Adachi,
O. Agric. Biol. Chem. 1984, 48, 3099. (d) Ameyama, M.; Shinagawa, E.;
Matsushita, K.; Adachi, O. Agric. Biol. Chem. 1985, 49, 699. (e) Ameyama,
M.; Matsushita, K.; Shinagawa, E.; Hayashi, M.; Adachi, O. BioFactors
1988, 1, 51. (f) Adachi, O.; Okamoto, K.; Matsushita, K.; Shinagawa, E.;
Ameyama, M. Agric. Biol. Chem. 1990, 54, 2751.
(4) (a) Watanabe, A.; Hobara, N.; Tsuji, T. Curr. Ther. Res. 1988, 44,
896. (b) Watanabe, A.; Hobara, N.; Ohsawa, T.; Higashi, T.; Tsuji, T.;
Hiroshima, J. Med. Sci. 1989, 38, 49.
(5) Nishigori, H.; Yasunaga, M.; Mizumura, M.; Lee, J. W.; Iwatsuru,
M. Life Sci. 1989, 45, 593.
(6) Hamagishi, Y.; Murata, S.; Kamei, H.; Oki, T.; Adachi, O.; Ameyama,
M. J. Pharmacol. Exp. Ther. 1990, 255, 980.
(7) (a) Killgore, J.; Smidt, C.; Duich, L.; Romero-Chapman, N.; Tinker,
D.; Reiser, K.; Melko, M.; Hyde, D.; Rucker, R. B. Science 1989, 245,
850. (b) Smidt, C. R.; Steinberg, F. M.; Rucker, R. B. Proc. Soc. Exp.
Biol. Med. 1991, 197, 19.
† Osaka University and Japan Science and Technology Corporation.
‡ Osaka City University.
§ The Institute of Scientific and Industrial Research, Osaka University.
Institute for Chemical Reaction Science, Tohoku University.
(1) Salisbury, S. A.; Forrest, H. S.; Cruse, W. B. T.; Kennard, O. Nature
1979, 280, 843.
(2) The presence of PQQ itself in certain enzymes has been disproved,
but instead amino acid derived coenzymes such as 6-hydroxydopa (TOPA),
tryptophan tryptophylquinone (TTQ), and 2-alkylthiophenol derivative (Tyr-
Cys) were found from bovine serum amine oxidase, methylamine dehy-
drogenase, and galactose oxidase, respectively. See: Principles and
Applications of Quinoproteins; Davidson, V. L., Ed.; Marcel Dekker: New
York, 1993.
10.1021/ja001351g CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/17/2000

8436 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Fukuzumi et al.
physics and the photochemical reactions have merited increasing
attention.22,23 However, there has been no report on the photo-
physics and photochemistry of coenzyme PQQ which absorbs
light between 300 and 500 nm.
We report herein for the first time the photophysics and
photoredox reactions of coenzyme PQQ and analogues which
are found to be much stronger oxidants than flavins, providing
valuable insight into the viability as a photoreceptor.24 Extensive
comparison of the photochemical redox reaction of PQQ with
related heterocyclic o-quinones is also made to elucidate the
common reaction mechanism of the photoredox reactions of the
o-quinones.
functions at a molecular level. Thermal redox reactions of
coenzyme PQQ with several biologically important substances
such as alcohols,8 amines,9 amino acids,10 thiols,11 and glucose12
have been studied extensively to provide valuable insight into
the biological functions of quinoproteins. Structure-reactivity
relationships of coenzyme PQQ have also been investigated in
detail by using several PQQ model compounds to reveal the
structural uniqueness of coenzyme PQQ as compared to ordinary
o-quinone compounds.13,14
Experimental Section
Materials. Trimethyl 4,5-dihydro-4,5-dioxo-1H-pyrrolo[2,3-f]quino-
line-2,7,9-tricarboxylate (PQQTME), 1,7-phenanthroline-5,6-dione (1,7-
PTQ), 4,7-phenanthroline-5,6-dione (4,7-PTQ), 1,10-phenanthroline-
5,6-dione (1,10-PTQ), 1-aza-phenanthrene-5,6-dione (1-BQQ), and
4-aza-phenanthrene-5,6-dione (4-BQQ) were obtained from previous
studies.25 Spectrophotometric grade acetonitrile used as a solvent was
purchased from Nacalai Tesque. R,R-Dideuterio-p-methoxybenzyl
alcohol (p-MeOC6H4CD2OH) was prepared from 4-methoxybenzoic
acid by the reduction with LiAlD4 according to the general procedures.
Tris(2,2′-bipyridine)ruthenium dichloride hexahydrate, [Ru(bpy)3]Cl2‚
6H2O (bpy ) 2,2′-bipyridine), was obtained commercially from Aldrich.
The oxidation of [Ru(bpy)3]Cl2 with lead dioxide in aqueous H2SO4
gives [Ru(bpy)3]3+, which was isolated as the PF6 salt, [Ru(bpy)3]-
(PF6)3.26 All other chemicals used in this study were commercial
products of the highest available purity and were further purified by
the standard methods, if necessary.27
Product Analysis. Typically, a CD3CN solution (0.7 mL) containing
PQQTME (5.0 × 10-3 M) and benzyl alcohol (1.5 × 10-2 M) in an
NMR tube sealed tightly with a silicon rubber cap and Parafilm was
deaerated by bubbling argon gas through it using a Teflon tube for 15
min. The solution was irradiated with a xenon lamp with UV-31
TOSHIBA color filter for 24 h. Since PQQTMEH2 is hardly soluble
in CD3CN, it gradually precipitates with the progress of the reaction.
Thus, the resulting aldehyde in the supernatant of the CD3CN solution
can be analyzed by 1H NMR, and then the solvent was removed under
reduced pressure to obtain PQQTMEH2 as a brown solid. Identification
of PQQTMEH2 and the oxidation products (benzaldehyde derivatives)
was performed by comparing the 1H NMR spectra to those of the
authentic samples.28 Product analysis of the photooxidation of THF
and 1,4-cyclohexadiene by PQQTME was carried out in a similar
manner, and the oxidation products (furan and benzene) were detected
Photophysics and photochemistry of ubiquitous coenzymes
such as flavins and NAD(P)H have so far been studied
extensively.15 Although flavins have been believed to act as
near-UV/blue-light photoreceptors,16 many photophysiological
data of the pertinent blue-light photoreceptors cannot be
explained by the exclusive action of flavins.17 Numerous
processes in plants, fungi, and microorganisms are controlled
by near-UV/blue-light receptors which may include unknown
photoreceptors absorbing light between 300 and 500 nm.16,18
A new class of photopigments such as hypericin,19 ble-
pharismin,20 stentorin,19 and oxybelepharismin21 have recently
been identified as photosensing chromophores and the photo-
(8) (a) Itoh, S.; Kawakami, H.; Fukuzumi, S. J. Am. Chem. Soc. 1997,
119, 439. (b) Itoh, S.; Kawakami, H.; Fukuzumi, S. Biochemistry 1998,
37, 6562.
(9) (a) Ohshiro, Y.; Itoh, S.; Kurokawa, K.; Kato, J.; Hirao, T.; Agawa,
T. Tetrahedron Lett. 1983, 24, 3465. (b) Itoh, S.; Kitamura, Y.; Ohshiro,
Y.; Agawa, T. Bull. Chem. Soc. Jpn. 1986, 59, 1907. (c) Sleath, P. R.;
Noar, J. B.; Eberlein, G. A.; Bruice, T. C. J. Am. Chem. Soc. 1985, 107,
3328. (d) Rodriguez, E. J.; Bruice, T. C. J. Am. Chem. Soc. 1989, 111,
7947. (e) Mure, M.; Itoh, S.; Ohshiro, Y. Chem. Lett. 1989, 1491. (f) Mure,
M.; Itoh, S.; Ohshiro, Y. Tetrahedron Lett. 1989, 30, 6875. (g) Itoh, S.;
Mure, M.; Ogino, M.; Ohshiro, Y. J. Org. Chem. 1991, 56, 6857. (h)
Ohshiro, Y.; Itoh, S. Bioorg. Chem. 1991, 19, 169.
(10) (a) Itoh, S.; Kato, N.; Ohshiro, Y.; Agawa, T. Tetrahedron Lett.
1984, 25, 4753. (b) Mure, M.; Suzuki, A.; Itoh, S.; Ohshiro, Y. J. Chem.
Soc., Chem. Commun. 1990, 1608. (c) Itoh, S.; Mure, M.; Suzuki, A.; Murao,
H.; Ohshiro, Y. J. Chem. Soc., Perkin Trans. 2 1992, 1245.
(11) (a) Itoh, S.; Kato, N.; Ohshiro, Y.; Agawa, T. Chem. Lett. 1985,
135. (b) Itoh, S.; Kato, N.; Mure, M.; Ohshiro, Y. Bull. Chem. Soc. Jpn.
1987, 60, 420.
(12) Itoh, S.; Mure, M.; Ohshiro, Y. J. Chem. Soc., Chem. Commun.
1987, 1580.
(20) (a) Checcucci, G.; Shoemaker, R. S.; Bini, E.; Cerny, R.; Tao, N.;
Hyon, J.-S.; Gioffre, D.; Ghetti, F.; Lenci, F.; Song, P.-S. J. Am. Chem.
Soc. 1997, 119, 5762, 9588. (b) Maeda, M.; Naoki, H.; Matsuoka, T.; Kato,
Y.; Kotsuki, H.; Utsumi, K.; Tanaka, T. Tetrahedron Lett. 1997, 38, 7411.
(21) Spitzner, D.; Ho¨fle, G.; Klein, I.; Pohlan, S.; Ammermann, D.;
Jaenicke, L. Tetrahedron Lett. 1998, 39, 4003.
(22) (a) Song, P.-S.; Kim, I.-H.; Florell, S.; Tamai, N.; Yamazaki, T.;
Yamazaki, I. Biochim. Biophys. Acta 1990, 1040, 58. (b) Cubeddu, R.;
Ghetti, F.; Lenci, F.; Ramponi, R.; Taroni, P. Photochem. Photobiol. 1990,
52, 567. (c) Yamazaki, T.; Yamazaki, I.; Nishimura, Y.; Dai, R.; Song,
P.-S. Biochim. Biophys. Acta 1993, 1143, 319. (d) Ghetti, F.; Checcucci,
G.; Lenci, F.; Heelis, P. F. J. Photochem. Photobiol. B Biol. 1992, 13, 315.
(23) (a) Angelini, N.; Quaranta, A.; Checcucci, G.; Song, P.-S.; Lenci,
F. Photochem. Photobiol. 1998, 68, 864. (b) Xia, G.; He, X.; Zhou, Y.;
Zhang, M.; Shen, T. J. Photochem. Photobiol. A Chem. 1998, 114, 31. (c)
Wells, T. A.; Losi, A.; Dai, R.; Scott, P.; Park, S.-M.; Golbeck, J.; Song,
P.-S. J. Phys. Chem. 1997, 101, 366, 7460.
(13) (a) Itoh, S.; Kato, J.; Inoue, T.; Kitamura, Y.; Komatsu, M.; Ohshiro,
Y. Synthesis 1987, 1067. (b) Itoh, S.; Inoue, T.; Fukui, Y.; Huang, X.;
Komatsu, M.; Ohshiro, Y. Chem. Lett. 1990, 1675. (c) Itoh, S.; Fukui, Y.;
Ogino, M.; Haranou, S.; Komatsu, M.; Ohshiro, Y. J. Org. Chem. 1992,
57, 2788. (d) Itoh, S.; Fukui, Y.; Haranou, S.; Ogino, M.; Komatsu, M.;
Ohshiro, Y. J. Org. Chem. 1992, 57, 4452.
(14) (a) Noar, J. B.; Rodriguez, E. J.; Bruice, T. C. J. Am. Chem. Soc.
1985, 107, 7198. (b) Noar, J. B.; Bruice, T. C. J. Org. Chem. 1987, 52,
1942.
(15) (a) Heelis, P. F. Chem. Soc. ReV. 1982, 11, 15. (b) Fukuzumi, S.;
Tanaka, T. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M.,
Eds.; Elsevier: Amsterdam, 1988; Part C, pp 578-687. (c) Fukuzumi, S.
In AdVances in Electron-Transfer Chemistry; Mariano, P. S., Ed.; JAI
Press: Greenwich, CT, 1992; pp 67-175. (d) Fukuzumi, S.; Itoh, S. In
AdVances in Photochemistry; Neckers, D. C., Volman, D. H., von Bu¨nau,
G., Eds.; Wiley: New York, 1998; Vol. 25, pp 107-172.
(16) Galland, P.; Senger, H. J. Photochem. Photobiol. B 1988, 1, 277.
(17) (a) Galland, P.; Senger, H. Photochem. Photobiol. 1988, 48, 811.
(b) Galland, P.; Keiner, P.; Do¨rnemann, D.; Senger, H.; Brodhun, B.; Ha¨der,
D.-P. Photochem. Photobiol. 1990, 51, 675. (c) Galland, P.; Eslava, A. P.;
Alvarez, M. I. Photochem. Photobiol. 1997, 66, 879.
(24) A preliminary report on the photochemical reaction of PQQ has
appeared: Itoh, S.; Komori, T.; Chiba, Y.; Fukuzumi, S.; Ishida, A.;
Takamuku, S. J. Chem. Soc., Chem. Commun. 1996, 465.
(25) Itoh, S.; Maruta, J.; Fukuzumi, S. J. Chem. Soc., Perkin Trans. 2
1996, 1429.
(18) Lenci, F. In Handbook of Organic Photochemistry and Photobiology;
Horspool, W. M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, 1995;
pp 1445-1449.
(19) Tao, N.; Orlando, M.; Hyon, J.-S.; Gross, M.; Song, P.-S. J. Am.
Chem. Soc. 1993, 115, 2526.
(26) DeSimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(27) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Butterworth-Heinemann: Oxford, 1988.
(28) Itoh, S.; Ohshiro, Y.; Agawa, T. Bull. Chem. Soc. Jpn. 1986, 59,
1911.

Photochemical Reactions of Pyrroloquinolinequinone
J. Am. Chem. Soc., Vol. 122, No. 35, 2000 8437
by 1H NMR. The 1H NMR measurements were performed using a JEOL
JNM-GSX-400 (400 MHz) NMR spectrometer or a JEOL EX 270 (270
MHz) NMR spectrometer.
the probe beam is focused to a computer-controlled monochromator
(CVI Laser, Digikrom-240) by two lenses and four mirrors. The output
of monochromator is monitored by a PMT (photomultiplier tube;
Hamamatsu Photonix, R1417 or R2497). The signal from the PMT is
recorded on a transient digitizer (Tektronix, 7912AD with plug-ins,
7A19 and 7B92A). Total system control is carried out with a
microcomputer (Sharp, X-98000) which is connected to all measurement
components with a GP-IB interface. The data processing was carried
out by Igor Pro version 3.10 on a Macintosh personal computer. To
avoid photolysis of sample solution by the probe beam, a suitable cutoff
filter was used. An irradiation cell was placed in the beam of the linear
accelerator and in front of the beam port with about 15 cm distance.
For the quenching experiments of the triplet excited state of
PQQTME by 2,4-dimethoxybenzyl alcohol, the irradiation laser
wavelength of PQQTME was 355 nm which excites PQQTME
selectively. The solution was deoxygenated by bubbling argon gas
through it for 30 min prior to the measurements. Relative intensities
of the triplet-triplet absorption spectrum at maxima (420 nm) were
measured for MeCN solutions containing PQQTME (5.0 × 10-5 M)
and the alcohol at various concentrations. There was no change in the
shape but there was a change in the lifetime of the T-T absorption
spectrum by the addition of the alcohol. The Stern-Volmer relationship
(eq 1) was obtained for the triplet lifetimes in the absence (τ0) and
presence of the substrate (τ) and the concentrations of the substrate
[DH2]. The observed quenching rate constants kq were obtained from
the slope of a linear plot of τ0/τ and [DH2] according to eq 1.
PQQTMEH2 (in DMSO-d6): δ 3.94 (s, 3H, -CH3), 4.00 (s, 3H,
-CH3), 4.11 (s, 3H, -CH3), 7.44 (d, 1H, J ) 2.0 Hz, H-3), 8.47 (s,-
1H, -OH), 8.54 (s,1H, H-8), 10.29 (s, 1H, -OH), 12.17 (s, 1H, NH).
The photochemical oxidation of benzyl alcohols by other o-quinones
was carried out in the same procedure, and the reduced products from
the quinones were identified by comparing their 1H NMR spectra with
those of the authentic samples. The authentic sample of the quinol
derivative was generated by the reduction of the corresponding quinone
by methylhydrazine in CD3CN.28,29
1,7-PTQH2: δ 7.58 (dd, 1H, J ) 4.3, 8,1 Hz), 7.62 (dd, 1H, J )
4.1, 8.1 Hz), 8.60 (dd, 1H, J ) 1.6, 8.1 Hz), 8.87 (dd, 1H, J ) 1.6, 4.3
Hz), 8.93 (dd, 1H, J ) 1.8, 4.1 Hz), 9.39 (dd, 1H, J ) 1.6, 8.1 Hz).
4,7-PTQH2: δ 7.64 (dd, 2H, J ) 4.6, 8.2 Hz), 8.94 (dd, 2H, J )
1.6, 4.6 Hz), 9.02 (dd, 2H, J ) 1.6, 8.2 Hz).
1-BQQH2: δ 7.49-7.60 (m, 2H), 7.68 (t, 1H, J ) 6.9 Hz), 8.23 (d,
1H, J ) 7.8 Hz), 8.53 (d, 1H, J ) 8.1 Hz), 8.80 (m, 1H), 9.14 (d, 1H,
J ) 7.8 Hz).
4-BQQH2: δ 7.54 (dd, 1H, J ) 4.5, 8.5 Hz), 7.58-7.72 (m, 2H),
8.28 (dd, 1H, J ) 1.2, 7.8 Hz), 8.65 (dd, 1H, J ) 1,2, 7.8 Hz), 8.86
(dd, 1H, J ) 1.4, 4.5 Hz), 9.01 (dd, 1H, J ) 1.4, 8.5 Hz).
PQH2: δ 7.52 (dt, 2H, J ) 1.5, 7.5 Hz), 7.60 (dt, 2H, J ) 1.5, 7.5
Hz), 8.21 (dd, 2H, J ) 1.5, 7.5 Hz), 8.67 (dd, 2H, J ) 1,5, 7.5 Hz).
Quantum Yield Determinations. A standard actinometer (potassium
ferrioxalate)30 was used for the quantum yield determination of the
photooxidation of benzyl alcohol derivatives by the o-quinones.
Typically, an MeCN solution (3.0 mL) containing PQQTME (1.5 ×
10-4 M) and benzyl alcohol (9.7 × 10-3 to 2.9 × 10-1 M) was deaerated
by bubbling argon gas through it using a tefron tube for 15 min in a
10-mm-square quartz cuvette sealed tightly with a silicon rubber cap
and Parafilm. This solution was irradiated with monochromatized light
of λ ) 360 nm from a Shimadzu RF-5000 fluorescence spectropho-
tometer. Under the conditions of actinometry experiments, both the
actinometer and PQQTME absorbed essentially all the incident light
of λ ) 360 nm. The light intensity of monochromatized light of λ )
360 nm was determined as 2.49 × 10-8 einstein s-1 with the slit width
of 20 nm. The photochemical reaction was monitored by using a
Hewlett-Packard 8452A diode-array spectrophotometer. The quantum
yields were determined from the decrease in absorbance due to
PQQTME (λ ) 356 nm, ꢀ ) 1.47 × 104 M-1 cm-1). The quantum
yields of the photochemical reactions with other benzyl alcohol
derivatives, THF, and 1,4-cyclohexadiene by PQQTME were deter-
mined in a similar manner.
τ0/τ ) 1 + kqτ0[DH2]
(1)
For the quenching experiments of the triplet excited states of 1,7-
PTQ and 4,7-PTQ (3.8 × 10-5 M), a 266 nm Nd:YAG laser was used
to obtain the T-T absorption spectra and to perform the quenching
experiments of the triplet excited states of the quinones (440 nm) by
benzyl alcohol (9.7 × 10-4 to 4.9 × 10-3 M).
For the measurements of transient absorption spectra in the reactions
of the triplet excited states of PQQTME and 4,7-PTQ with 2,4-
dimethoxybenzyl alcohol (5.0 × 10-4 M) or benzyl alcohol (5.0 ×
10-3 M) in MeCN, the PQQTME and 4,7-PTQ solutions (5.0 × 10-5
M) were excited by a Nd:YAG laser (Quanta-Ray, GCR-130, 6 ns
fwhm) at 532 nm with the power of 7 mJ. Since PQQTME and 4,7-
PTQ in MeCN containing benzyl alcohols disappeared appreciably by
each laser shot (532 nm; 7 mJ), the transient spectra were recorded
using fresh solutions in each laser excitation. All experiments were
performed at 295 K.
Phosphorescence Experiments. The phosphorescence spectra of the
quinones (1.0 × 10-4 M) in 2-methyltetrahydrofuran were measured
by irradiating with a xenon lamp using a capillary cell on a Hitachi
850 fluorescence phosphorescence spectrophotometer at 77 K.
Stopped-Flow Measurements. The transient absorption spectra of
the radical cation of 2,4-dimethoxybenzyl alcohol were measured in
the electron-transfer oxidation of 2,4-dimethoxybenzyl alcohol with [Ru-
(bpy)3]3+ (bpy ) 2,2′-bipyridine) using a UNISOKU RSP-601 stopped-
flow rapid scan spectrophotometer equipped with the MOS-type high
sensitive photodiode array under deaerated conditions. Gate time for
the measurement was set at 5 ms. Typically, deaerated MeCN solutions
of [Ru(bpy)3]3+ and 2,4-(MeO)2C6H3CH2OH were transferred to the
spectrophotometric cell by means of a glass syringe which had earlier
been purged with a stream of argon.
Electrochemical Measurements. The second harmonic alternating
current voltammetry (SHACV) and the cyclic voltammetry measure-
ments of heterocyclic o-quinones and substrates were performed on a
BAS 100B electrochemical analyzer in deaerated MeCN containing
0.10 M NBu4ClO4 as a supporting electrolyte at 298 K. The gold work-
ing electrode (BAS) was polished with BAS polishing alumina sus-
pension and rinsed with acetone before use. The counter electrode was
a platinum wire. The measured potentials were recorded with respect
to the Ag/AgNO3 (0.01 M) reference electrode. The E0red and E0ox values
(vs Ag/Ag+) are converted to those vs SCE by adding 0.29 V.31
In the case of 1,7-PTQ, 4,7-PTQ, 1-BQQ, 4-BQQ, and PQ,
monochromatized light of λ ) 280 nm was used for the photochemical
redox reaction, and the light intensity of monochromatized light of λ
) 280 nm was determined as 3.38 × 10-9 einstein s-1 with the slit
width of 20 nm. The progress of the reaction was followed by
monitoring the formation of the quinol at its λmax
.
Laser Flash Photolysis. Typically, an MeCN solution containing
PQQTME (1.0 × 10-4 M) was irradiated with a laser flash at room
temperature. A flash at 355 nm was obtained by the third-harmonic
oscillation from a Nd:YAG laser (Quantel Model Brilliant) which was
operated by a large current pulse-power supply. The single laser flash
has a diameter of 0.5 cm, 5 ns duration, 5 mJ pulse-1, and incident
photon number 1.4 × 1018 pulse-1 cm-2 measured by a pyroelectric
power meter (Gentec ED-500). An almost collinear arrangement was
used, such that the narrow laser beam passed through the central part
of the irradiated volume. The probe beam was obtained from a 450 W
Xe-lamp (Osram, XBO-450) which was also operated by a large current
pulse-power supply. The probe beam was passed through an iris with
a diameter of 0.2 cm, and sent into the sample solution with
perpendicular intersection of the laser beam. In the meantime, the probe
beam was arranged closely to the entrance side of the laser beam. Then
(29) For the mechanism of methylhydrazine reduction of PQQ, see:
Mure, M.; Nii, K.; Inoue, T.; Itoh, S.; Ohshiro, Y. J. Chem. Soc., Perkin
Trans. 2 1990, 315.
(30) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518.
(31) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non-
aqueous Systems, Marcel Dekker: New York, 1990.

8438 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Fukuzumi et al.
Figure 2. T-T absorption spectra of PQQTME obtained in the laser
flash photolysis of a deaerated MeCN solution of PQQTME (5.0 ×
10-5 M) at 298 K. Inset: Kinetic trace for the PQQTME triplet state
decay at λ ) 390 and 570 nm at 200 ns, 2.0 µs, 6.0 µs, and 14.0 µs
after laser excitation in deaerated MeCN at 298 K.
Figure 1. Spectral change observed in the photooxidation of p-MeOC6H4-
CH2OH (3.2 × 10-2 M) by PQQTME (5.1 × 10-5 M) in deaerated
MeCN at 298 K under irradiation with a xenon lamp through a UV-31
TOSHIBA filter; 20 s time intervals.
Table 1. T-T Absorption Maxima (λmax) and Lifetimes (τ0) of the
Triplet Excited State of Heterocyclic o-Quinones (3.8 × 10-5 to 7.5
× 10-5 M) in Deaerated MeCN at 298 K
Results and Discussion
Photochemical Redox Reactions of Coenzyme PQQ and
the Analogues. Irradiation of a deaerated MeCN solution
containing PQQTME (the trimethyl ester of coenzyme PQQ)
and p-methoxybenzyl alcohol (ArCH2OH, Ar ) p-MeOC6H4)
with a xenon lamp at 298 K results in formation of PQQTMEH2
(reduced PQQTME in quinol form) and the corresponding
aldehyde (ArCHO) as shown in eq 2. Figure 1 shows the spectral
heterocyclic o-quinone
λ
max, nm
τ0, µs
PQQTME
1,7-PTQ
4,7-PTQ
1-BQQ
4-BQQ
PQ
390
440
440
440
440
470
6.6
4.3
4.9
2.5
1.4
2.2
no thermal reaction occurred between the o-quinones and
ArCH2OH in the dark.
Excited States of PQQTME and Analogues. The photo-
chemical reactions of the o-quinones are retarded significantly
in the presence of dioxygen, which is a typical triplet quencher.
The excitation of the absorption maximum band of an MeCN
solution of each o-quinone results in no fluorescence.33 These
results indicate that the photochemical reaction of the o-quinones
may proceed via the triplet excited state rather than the singlet
excited state. Then, the direct detection of the triplet excited
state of the o-quinones (3Q*) was carried out by laser flash
photolysis. When PQQTME alone in deaerated MeCN was
flashed with 355 nm laser light, a transient triplet-triplet (T-
T) absorption spectrum having λmax at around 400 and 570 nm
together with a broad band above 700 nm was observed as
shown in Figure 2. The decay of each absorption obeyed first-
order kinetics with an identical lifetime (τ0) of 6.0 µs (see the
inset in Figure 2). Such a first-order decay suggests that a T-T
annihilation process is negligible under the present experimental
conditions. The transient triplet-triplet (T-T) absorption spectra
of other quinones were also obtained by using a 266 nm laser
light. The λmax values and the lifetimes (τ0) obtained from the
decay of the T-T absorption for the o-quinones are listed in
Table 1. The λmax value of PQQ is 50 nm blue-shifted as
compared to the values of PTQs and BQQs, whereas the λmax
value of PQ is 30 nm red-shifted.34
change observed in the photochemical reaction of PQQTME
(5.1 × 10-5 M) with p-methoxybenzyl alcohol (3.2 × 10-2 M).
The decrease in absorbance at λmax ) 356 nm due to the quinone
is accompanied by the appearance of a new absorption band at
λmax ) 326 nm due to the quinol (PQQTMEH2) with a clean
isosbestic point at 342 nm.28 The quantitative formation of
PQQTMEH2 and p-methoxybenzaldehyde in a 1:1 ratio was
1
confirmed by the H NMR spectra of the products in CD3CN
and DMSO-d6 (see Experimental Section). The photochemical
redox reaction of PQQTME also occurred with other benzyl
alcohol derivatives (Ar ) p-ClC6H4, C6H5, p-MeC6H4, 2,4-
(MeO)2C6H3), THF,32 and 1,4-cyclohexadiene to produce the
corresponding oxidation products, ArCHO, furan, and benzene,
respectively. The photochemical redox reaction of other o-
quinones (PTQs, BQQs, and PQ) with ArCH2OH also ocurs
The triplet excitation energies (E0-0) of the o-quinones are
determined from the 0-0 band of phosphorescence of each
o-quinone. The phosphorescence was monitored in 2-methyltet-
rahydrofuran glass at 77 K. The phosphorescence spectra
efficiently to produce the corresponding quinol and ArCHO
quantitatively (see Experimental Section). It was confirmed that
(32) For the catalytic thermal oxidation of THF by oxygen, see: (a) Sen,
A.; Lin, M.; Kao, L.-C.; Hutson, A. C. J. Am. Chem. Soc. 1992, 114, 6385.
(b) Fazlur-Rahman, A. K.; Tsai, J.-C.; Nicholas, K. M. J. Chem. Soc., Chem.
Commun. 1992, 1334. (c) Aresta, M.; Fragale, C.; Quaranta, E.; Tommasi,
I. J. Chem. Soc., Chem. Commun. 1992, 315. (d) Hata, E.; Takai, T.;
Mukaiyama, T. Chem. Lett. 1993, 1513.
(33) Dekker: R. H.; Duine, J. A.; Frank, J.; Verwiel, P. E. J.; Westerling,
J. Eur. J. Biochem. 1982, 125, 69.
(34) The blue shift of λmax of PQQTME as compared to the values of
other o-quinones may be caused by the lower excitation energy of PQQTME
than other o-quinones (see Table 3).

Photochemical Reactions of Pyrroloquinolinequinone
J. Am. Chem. Soc., Vol. 122, No. 35, 2000 8439
Table 2. Absorption Bands of Phosphorescence Peaks (λ0-0),
Triplet Excitation Energies (∆E0-0), One-Electron Reduction
Potentials of the Ground States (E0red), and Triplet Excited States
(E0red*) of the Heterocyclic o-Quinones
heterocyclic
o-quinone
λ0-0
,
∆E0-0
,
E0red vs SCE,
E0red* vs SCE,
nm
eV
V
V
PQQTME
1,7-PTQ
4,7-PTQ
1-BQQ
4-BQQ
PQ
585
551
556
545
554
549
2.12
2.25
2.23
2.27
2.24
2.26
-0.53
-0.53
-0.52
-0.62
-0.62
-0.71
1.59
1.72
1.71
1.65
1.62
1.55
Figure 4. Plots of Φ-1 vs [PhCH2OH]-1 for photooxidation of PhCH2-
OH by heterocyclic o-quinones (2.2 × 10-4 to 3.5 × 10-4 M) (1,7-
PTQ (O), 4,7-PTQ (b), 1-BQQ (4), 4-BQQ (2), and PQ (0)) in
deaerated MeCN at 298 K.
Figure 3. Dependence of quantum yields (Φ) on [PhCH2OH] for
photooxidation of PhCH2OH by heterocyclic o-quinones (2.2 × 10-4
to 3.5 × 10-4 M) (1,7-PTQ (O), 4,7-PTQ (b), 1-BQQ (4), 4-BQQ
(2), and PQ (0)) in deaerated MeCN at 298 K; monitored at λ ) 326
(1,7-PTQ and 4,7-PTQ), 320 (4-BQQ), and 312 nm (1-BQQ and PQ).
exhibited vibrational progression (see Supporting Information)
and the E0-0 values obtained from the λ0-0 values are listed in
Table 2. The λ0-0 value of PQQTME (585 nm) is slightly blue-
shifted as compared to that of riboflavin (610 nm).35 The λ0-0
values of other o-quinones (549-556 nm) are further blue
shifted as shown in Table 2.
3
Figure 5. Stern-Volmer plot for quenching of PQQTME* by 2,4-
(MeO)2C6H3CH2OH in deaerated MeCN at 298 K.
ferrioxalate actinometer (see Experimental Section). The Φ
value increases with an increase in the substrate concentrations,
[DH2], to reach a limiting value (Φ∞). Figure 3 shows typical
examples for the photoreduction of o-quinones by benzyl alcohol
(Supporting Information for other examples). Such a saturated
dependence of Φ on [DH2] can be expressed by a double-
reciprocal plot of Φ-1 vs [DH2]-1 according to
The one-electron reduction potentials (E0red*) of triplet excited
states of the o-quinones (3Q*) can be obtained by adding the
triplet excitation energy E0-0 to the one-electron reduction
potentials (E0red) of the ground state o-quinones. The E0red value
of PQQTME (-0.53 V vs SCE) in MeCN has been reported
previously.36 The E0 values of other o-quinones used in this
red
study were determined by the cyclic voltammograms which give
two reversible one-electron redox peaks corresponding to the
Φ-1 ) Φ∞-1[1 + (Kobs[DH2])-1]
(3)
Q•-/Q and Q2-/Q•- couples, respectively. The E0red and E0
*
*
red
values thus determined are also listed in Table 2. The Eo
red
where Kobs is the observed quenching constant of the triplet
values of the triplet excited state of PQQTME (1.59 V vs SCE)
and other o-quinones (1.55-1.72 V) are higher than that of the
triplet excited state of riboflavin (1.41 V vs SCE).37 This
indicates that the triplet excited states of the o-quinones (3Q*)
act as stronger electron acceptors than that of riboflavin.
Quantum Yields. The quantum yields (Φ) for the photo-
chemical redox reactions of 3Q* with benzyl alcohol derivatives
were determined from a decrease in absorbance due to the
quinone or an increase in absorbance due to the quinol using a
excited state, Q*. From the linear plots of Φ-1 vs [DH2]-1 in
3
Figure 4 are obtained the Φ∞ and Kobs values (see Supporting
Information for other examples). By using the τ0 value of Q*
3
listed in Table 1, the observed rate constants (kobs) can be
obtained from the Kobs values using the following relation, Kobs
) kobsτ0. The Φ∞ and kobs values are listed in Table 3.
The lifetimes of the triplet excited state of the o-quinones
were shortened significantly by the presence of the benzyl
alcohol derivatives. The Stern-Volmer plot (eq 1) is shown in
Figure 5. From the slope is determined the quenching rate
constant (kq). The kq values for the PQQTME-2,4-(MeO)2C6H3-
CH2OH, 1,7-PTQ-PhCH2OH, and 4,7-PTQ-PhCH2OH sys-
tems are determined as listed in Table 3. The kq values
(35) Sun, M.; Moore, T. A.; Song, P.-S. J. Am. Chem. Soc. 1972, 94,
1730.
(36) Itoh, S.; Kawakami, H.; Fukuzumi, S. J. Am. Chem. Soc. 1998, 120,
7271.
(37) Seeley, G. R. Photochem. Photobiol. 1978, 27, 639.

8440 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Fukuzumi et al.
Table 3. Free Energy Change of Electron Transfer (∆G0et), Limiting Quantum Yields (Φ∞), and Observed Rate Constants (kobs) in the
Photochemical Reactions of the Heterocyclic o-Quinones with Substrates in MeCN at 298 K
o-quinone
substrate
∆G0et/F, eV
Φ∞
kobs, M-1 s-1
kq, M-1 s-1
2.0 × 109
PQQTME
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
1,4-cyclohexadiene
THF
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
2,4-(MeO)2C6H3CH2OH
p-MeOC6H4CH2OH
p-MeC6H4CH2OH
p-ClC6H4CH2OH
C6H5CH2OH
-0.60
0.11
0.25
0.29
0.35
0.30
0.54
-0.73
-0.15
0.12
0.16
0.22
-0.72
-0.14
0.13
0.17
0.23
-0.66
-0.08
0.19
0.23
0.29
-0.63
-0.05
0.22
0.26
0.32
-0.56
0.02
2.3 × 10-2
4.7 × 10-2
4.6 × 10-3
1.6 × 10-2
1.5 × 10-2
3.1 × 10-2
7.3 × 10-2
4.3 × 10-1
4.9 × 10-1
4.4 × 10-1
4.4 × 10-1
5.5 × 10-1
3.7 × 10-1
4.8 × 10-1
4.3 × 10-1
5.5 × 10-1
7.3 × 10-1
4.9 × 10-1
5.4 × 10-1
3.9 × 10-1
4.4 × 10-1
5.7 × 10-1
6.7 × 10-1
5.3 × 10-1
6.2 × 10-1
6.5 × 10-1
6.0 × 10-1
5.1 × 10-1
4.2 × 10-1
3.7 × 10-1
3.0 × 10-1
4.6 × 10-1
2.3 × 109
1.4 × 108
5.2 × 106
1.2 × 107
1.4 × 106
5.0 × 106
1.2 × 105
5.1 × 109
2.2 × 109
4.2 × 108
4.5 × 108
1.0 × 108
3.0 × 109
1.2 × 109
1.9 × 108
3.1 × 108
1.3 × 108
4.2 × 109
2.5 × 109
2.7 × 108
9.5 × 107
5.5 × 107
8.5 × 109
5.2 × 109
5.3 × 108
1.9 × 108
3.1 × 108
7.1 × 109
2.7 × 109
1.0 × 108
1.2 × 108
4.5 × 107
1,7-PTQ
4,7-PTQ
1-BQQ
4-BQQ
PQ
1.1 × 108
1.3 × 108
0.29
0.33
0.39
3Q* (Table 3), respectively. The log kobs values in Table 3
including six different o-quinones are plotted against the ∆G0
et
values in Figure 6, which exhibits a typical feature of the photo-
induced electron-transfer process; the log kobs value increases
with a decrease in the ∆G0 value to reach a plateau value
et
corresponding to the diffusion rate constant (1.0 × 1010 M-1
s-1) as the photoinduced electron transfer becomes energetically
more favorable (i.e., more exergonic).39,40 The dependence of
the activation free energy of photoinduced electron transfer ∆Gq
on the free energy change of electron transfer to produce the
radical ion pair (∆Get) has well been established as given by
the Rehm-Weller free energy relation39,40
∆Gq ) (∆Get/2) + [(∆Get/2)2 + (∆Gq )2]1/2
(5)
0
where ∆Gq0 is the intrinsic barrier that represents the activation
free energy when the driving force of electron transfer is zero,
Figure 6. Plots of log kobs vs ∆G0et/F for oxidation of substrates by
heterocyclic o-quinones (1,7-PTQ (O), 4,7-PTQ (b), 1-BQQ (4),
4-BQQ ([), PQ (0), and PQQTME (9)) in deaerated MeCN at 298 K.
i.e., ∆Gq ) ∆Gq at ∆Get ) 0. The difference between ∆Get
0
for formation of the radical ion pair and ∆G0et for formation of
free radical ions is given by the work term wp that is the energy
required to bring the electron-transfer products, DH2•+ and Q•-,
to the mean separation distance of the radical ion pair. The wp
value for the opposite charges is taken as -0.1 eV.41 The ∆Gq
values are related to the observed rate constant of electron
transfer (kobs) as given by
determined from the decay of the T-T absorption agree well
with the observed rate constants (kobs) determined from the
dependence of Φ-1 on [DH2]-1. Such an agreement between
3
kq and kobs strongly indicates the involvement of Q* as the
reactive species in the photochemical reaction of the o-quinones.
Photoinduced Electron-Transfer Mechanism. The free
energy change of photoinduced electron transfer from DH2 to
3Q* (∆G0et) is given by
∆Gq ) (2.3RT/F) log[Z(kobs-1 - kdiff-1)]
(6)
where Z is the collision frequency that is taken as 1 × 1011
(38) Fukuzumi, S.; Kuroda, S. Res. Chem. Intermed. 1999, 25, 789.
(39) (a) Rehm, A.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73,
834. (b) Rehm, A.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(40) Patz, M.; Mayr, H.; Maruta, J.; Fukuzumi, S. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1225.
∆G0et ) F(E0ox - E0red*)
(4)
where E0ox and E0red* are the one-electron oxidation potentials
of DH2 (Table 3)38 and the one-electron reduction potential of

Photochemical Reactions of Pyrroloquinolinequinone
J. Am. Chem. Soc., Vol. 122, No. 35, 2000 8441
Figure 7. Transient absorption spectra observed in the photoreduction
of PQQTME (5.0 × 10-5 M) with 2,4-(MeO)2C6H3CH2OH (5.0 × 10-4
M) at 250 ns (O) and 2.5 µs (b) after laser excitation in deaerated
MeCN at 298 K.
Figure 8. Transient absorption spectra observed in the electron-transfer
oxidation of 2,4-(MeO)2C6H3CH2OH (5.0 × 10-3 M) by [Ru(bpy)3]3+
(3.0 × 10-5 M) in MeCN at 298 K, 100 ms interval.
M-1 s-1, F is the Faraday constant, and kdiff is the diffusion
rate constant in MeCN (2.0 × 1010 M-1 s-1).39 The best fit line
for the plot of log kobs vs ∆G0 in MeCN shown by the solid
et
line in Figure 6 gives the values of ∆Gq /F (0.1 eV) and wp/F
0
(-0.1 eV). All the data (32 kobs values) in Table 3 fit the single
correlation for the photoinduced electron transfer.42
The occurrence of photoinduced electron transfer in the
photochemical reaction of PQQTME with 2,4-dimethoxybenzyl
alcohol has been confirmed by the laser flash photolysis study
as follows. The transient absorption spectra in the visible region
are observed by the laser flash photolysis of a deaerated MeCN
solution containing 2,4-dimethoxybenzyl alcohol and PQQTME
with 355 nm laser light as shown in Figure 7. The triplet-
3
triplet absorption band of PQQTME* at 780 nm appearing
Figure 9. Transient absorption spectra observed in the photoreduction
of 4,7-PTQ (5.0 × 10-5 M) with 2,4-(MeO)2C6H3CH2OH (5.0 × 10-4
M) at 250 ns (O) and 2.5 µs (b) after laser excitation in deaerated
MeCN at 298 K.
immediately after nanosecond laser exposure decays in 1 µs
when the transient absorption bands at 390 and 570 nm are
observed. These bands disappeared completely in 200 µs. The
absorption band at 570 nm in Figure 7 agrees with the reported
absorption maximum due to the semiquinone radical anion of
1-methylated PQQTME.36,43
To assign the absorption band at 390 nm, the transient absorp-
tion spectrum of the radical cation of 2,4-dimethoxybenzyl alco-
hol produced in the electron-transfer oxidation with [Ru(bpy)3]3+
(bpy ) 2,2′-bipyridine) was measured using a stopped-flow
technique (see Experimental Section). Upon mixing an MeCN
solution of [Ru(bpy)3]3+ with 2,4-dimethoxybenzyl alcohol, the
absorption bands at 450 and 390 nm appear instantly and only
the latter band decayed to leave the absorption band due to [Ru-
(bpy)3]2+. The difference spectrum between the initial and final
spectra, exhibiting the absorption band at 390 nm due to the
radical cation of 2,4-dimethoxybenzyl alcohol, is shown in Fig-
ure 8.44 Thus, the absorption band at 390 nm in Figure 7 can
be assigned to the radical cation of 2,4-dimethoxybenzyl alcohol.
the absorbance at 570 nm due to PQQTME•- (see Supporting
Information). The rate constant for the electron transfer from
2,4-dimethoxybenzyl alcohol to 3PQQTME* is determined from
the rise of the absorbance at 570 nm due to PQQTME•- as 1 ×
109 M-1 s-1 45
which agrees with the value determined from
,
the dependence of Φ on the alcohol concentration (Table 3).
The occurrence of photoinduced electron transfer was also
confirmed in the photochemical reaction of 4,7-PTQ with 2,4-
dimethoxybenzyl alcohol as shown in Figure 9, where the
transient absorption spectra due to the radical cation of 2,4-
dimethoxybenzyl alcohol (390 nm) and the semiquinone radical
anion, 4,7-PTQ•- (480 nm),46,47 appear, accompanied by disap-
pearance of the triplet-triplet absorption due to 34,7-PTQ* (540
nm). Formation of 4,7-PTQ•- was also confirmed in the
photochemical reaction of 4,7-PTQ with benzyl alcohol (see
Supporting Information).
The kinetic deuterium isotope effects on the quantum yields
for the photochemical reactions were examined by using
PQQTME, 4,7-PTQ, and 4-BQQ as the o-quinones, and benzyl
alcohol, and p-methoxybenzyl alcohol as the substrates. The
kobs and Φ∞ values determined from the dependence of Φ on
the substrate concentration are listed in Table 4. Essentially no
3
The decay of the absorbance at 780 nm due to PQQTME*
obeys pseudo-first-order kinetics, coinciding with the rise of
(41) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305.
(42) Although there are some scatters in the correlation between log kobs
vs ∆G0et/F, they may be attributed to the difference of the work term wp
being sensitive to the steric effects in the radical ion pair.
3
(43) The T-T absorption of PQQTME* at 570 nm shown in Figure 2
is overlapped with the absorption due to PQQTME•- in the 250 ns spectrum
in Figure 7.
(45) The same value was obtained from the decay of the absorbance
3
(44) The transient absorption spectrum of the radical cation of 3,4-
dimethoxybenzyl alcohol (λmax ) 430 nm) has been reported recently. The
absorption maximum (λmax ) 430 nm) is red-shifted as compared to that
of 2,4-dimethoxybenzyl alcohol radical cation. Bietti, M.; Baciocchi, E.;
Steenken, S. J. Phys. Chem. A 1998, 102, 7337.
due to PQQTME* at 780 nm.
(46) Eckert, T. S.; Bruice, T. C. J. Am. Chem. Soc. 1983, 105, 4431.
(47) The broad absorption at 480-550 nm is also observed in the transient
absorption spectra in the photoreduction of 4,7-PTQ with PhCH2OH (see
Supporting Information, S7) and it can be assigned to PTQ•-
.

8442 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Fukuzumi et al.
Table 4. Limiting Quantum Yields (Φ∞), Observed Rate Constants (kobs), and the Kinetic Isotope Effects (Φ∞H/Φ∞D, kobsH/kobsD) for the
Photochemical Reactions of the Heterocyclic o-Quinones with Benzyl Alcohols in MeCN at 298 K
D
D
o-quinone
benzyl alcohol
Φ∞
Φ∞H/Φ∞
kobs, M-1 s-1
kobsH/kobs
PQQTME
C6H5CH2OH
C6D5CD2OH
1.5 × 10-2
9.5 × 10-3
4.7 × 10-2
3.5 × 10-2
4.8 × 10-1
4.4 × 10-1
5.3 × 10-1
4.5 × 10-1
1.4 × 106
1.4 × 106
5.6 × 107
5.6 × 107
5.6 × 107
5.6 × 107
5.2 × 107
5.4 × 107
1.6
1.3
1.1
1.2
1.0 ( 0.1
1.0 ( 0.1
1.0 ( 0.1
1.0 ( 0.1
p-MeOC6H5CH2OH
p-MeOC6H5CD2OH
p-MeOC6H5CH2OH
p-MeOC6H5CD2OH
p-MeOC6H5CH2OH
p-MeOC6H5CD2OH
4,7-PTQ
4-BQQ
kinetic isotope effect is observed on the kobs value (1.0 ( 0.1).
However, the kinetic isotope effects (1.1-1.6) are observed on
Φ∞.
the back electron-transfer process (kb). When ArCH2OH and
THF in Table 3 is replaced by ArCD2OH and THF-d8,
respectively, the kinetic deuterium isotope effects are observed
on Φ∞ as shown in Table 4. The observed kinetic deuterium
isotope effects in Table 4 may be ascribed to those for proton
The dependence of log kobs on ∆G0 in Figure 6, direct
et
detection of radical ions produced in the photoinduced electron-
transfer reactions, and the absence of the kinetic isotope effect
on kobs strongly indicate that the reaction of 3Q* with the
substrates (DH2) proceeds via photoinduced electron transfer
•+
transfer from DH2 to Q•- (kp). The small kinetic deuterium
isotope effects indicate that the proton transfer is a fast process
with a small activation energy in each case. In contrast to this,
an extraordinarily large kinetic isotope effect (e.g., kH/kD ) 61.5)
has been reported for hydride or hydrogen transfer from benzyl
alcohol to cis-[RuIV(bpy)2(py)(O)]2+ (py ) pyridine) in MeCN.50
The Φ∞ values are rather constant irrespective of the
difference in the E0ox values of benzyl alcohol derivatives. Such
3
•+
from DH2 to Q* as shown in Scheme 1. The DH2 such as
Scheme 1
*
a constant dependence of Φ∞ on the E0 values suggests that
ox
the back electron-transfer process from Q•- to DH2 is not
•+
affected by the free energy change of electron transfer. Thus,
the rate-determining step of the back electron transfer may be
the intersystem crossing from the triplet radical ion pair formed
initially to the singlet radical ion pair in which the back electron
transfer occurs rapidly.
The coupling of electron transfer and proton transfer is a well-
established alternative for the formal hydrogen atom transfer
in an organic photochemical system,51 particularly in the case
of photoreduction of flavins.15,37 Such a proton-coupled electron
transfer has been suggested to play an important role in
biological electron transfer.52 It is often difficult to determine
whether the proton-coupled electron transfer is consecutive, i.e.,
electron transfer followed by proton transfer or vice versa, or a
concerted process in one quantum mechanical tunneling event.53
In the case of photooxidation of benzyl alcohol derivatives with
the heterocyclic o-quinones, the photoinduced electron transfer
process clearly precedes the proton-transfer step as demonstrated
in this study.
benzyl alcohol radical cations produced in the photoinduced
electron-transfer reactions are strong acids judging from the
negative pKa values,15,38 while phenanthrolinequinone radical
anions are known as strong bases (pKa ) 8.0).48 Thus, proton
•+
transfer from DH2 to Q•- (kp) may occur efficiently in
competition with the back electron transfer from Q•- to DH2
•+
(kb) as shown in Scheme 1.49 The subsequent facile hydrogen
(electron and proton) transfer yields D and QH2.
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research Priority Area (Nos.
11228205 and 11228206) from the Ministry of Education,
Science, Culture and Sports, Japan.
By applying the steady-state approximation to the reactive
3
intermediates, Q* and the radical ion pair in Scheme 1, the
dependence of quantum yields on the substrate concentration
[DH2] may be derived as shown in
Supporting Information Available: Phosphorescence spec-
trum of a heterocyclic o-quinone (1-BQQ) (S1), dependence of
the quantum yields (Φ) on [PhCH2OH] for photooxidation of
PhCH2OH by PQQTME (S2), plot of Φ-1 vs [PhCH2OH]-1
Φ ) [kp/(kp + kb)]ketτ0[DH2]/(1 + ketτ0[DH2])
(7)
where ket is the rate constant of electron transfer from DH2 to
3Q*, kb is the rate constant of the back electron transfer, and kp
(50) (a) Lebeau, E. L.; Meyer, T. J. Inorg. Chem. 1999, 38, 2174. (b)
Roecker, L.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 746.
•+
is that of proton transfer from DH2 to Q•-. The limiting
(51) (a) Lewis, F. D. Acc. Chem. Res. 1986, 19, 401. (b) Wagner, P. J.;
Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am. Chem. Soc. 1986, 108,
7727. (c) Wagner, P. J.; Park, B. S. Org. Photochem. 1991, 11, 227. (d)
Jones, G., II; Mouli, N. J. Phys. Chem. 1988, 92, 7174. (e) Cermenati, L.;
Freccero, M.; Venturello, P.; Albini, A. J. Am. Chem. Soc. 1995, 117, 7869.
(52) (a) Okamura, M. Y.; Feher, G. Annu. ReV. Biochem. 1992, 61, 861.
(b) Malmstro¨m, B. G. Acc. Chem. Res. 1993, 26, 332. (c) Ferguson-Miller,
S.; Babcock, G. T. Chem. ReV. 1996, 96, 2889. (d) Hoganson, C. W.;
Babcock, G. T. Science 1997, 277, 1953. (e) Cukier, R. I.; Nocera, D. G.
Annu. ReV. Phys. Chem. 1998, 49, 337.
quantum yield Φ∞ corresponds to kp/(kp + kb). Thus, the Φ∞
values being smaller than unity in Table 3 and Table 4 may be
ascribed to the competition of the proton-transfer step (kp) with
(48) Evans, D. H.; Griffith, D. A. J. Electroanal. Chem. 1982, 134, 301.
(49) The small Φ∞ values of PQQTME as compared with those of other
o-quinones (Table 3) may be ascribed to the deprotonation of the pyrrole
proton associated with the reduction of PQQTME,36 which decelerates the
•+
proton transfer from DH2
.
(53) Cukier, R. I. J. Phys. Chem. A 1999, 103, 5989.

Photochemical Reactions of Pyrroloquinolinequinone
J. Am. Chem. Soc., Vol. 122, No. 35, 2000 8443
(S3), dependence of the quantum yields on [p-MeC6H4CH2OH]
for photooxidation of p-MeC6H4CH2OH by different o-quinones
(S4), plots of Φ-1 vs [p-MeC6H3CH2OH]-1 (S5), decay of 34,7-
PTQ* and rise of 4,7-PTQ radical anion observed in the
photoreduction of 4,7-PTQ by PhCH2OH (S6), transient absorp-
tion spectra observed in the photoreduction of 4,7-PTQ with
PhCH2OH (S7) (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA001351G