J. Phys. Chem. 1991, 95, 1933-1941 polarization for success, may not be suitable,
Summary and Outlook A large amount of data has been collected and simulated for both types of biradicals. The time dependence of the signal intensities and shape of the spectra is well accounted for by using perturbation theory for the spin mechanics and a simple kinetic model that includes chemical reactions and spin relaxation. The spin-spin coupling J and the end-to-end contact rate k, can be extracted from the simulations. The chain length dependence of the exchange interaction will be examined in detail in a later paper. It should be noted that each of the 1764 nondegenerate states in the ensemble for the bis(alky1s) has different overall decay kinetics; therefore, it is perhaps better to assign the biradicals a half-life than an exponential decay rate. In the case of the bisalkyls this half-life is about 1.5 ps. For biradicals with Boltzmann populations, it is possible to measure J only when it is small relative to the hyperfine coupling constants. In this case 124 can be read directly from the spectrum as the splitting between the singlet-triplet and triplet-triplet transitions. The presence of spin polarization in the biradicals provides a clear advantage over other experimental methods for measuring the sign and the magnitude of J values. The intensities of the transitions are a function of the J coupling and can therefore
1933
be used to estimate its magnitude by spectral simulation. In addition, the directions of the transitions (emission or absorption) indicate the sign of J . Future experiments will attempt to investigate shorter chain lengths as well as more elaborate electronic and structural effects on the biradical EPR spectra. As has been demonstrated above, if the electronic spin-spin interaction is too large, very little spin polarization can be produced, and therefore small, rigid molecules are unlikely to render observable biradicals in liquid solution at room temperature. It will be interesting to repeat the experiment at different microwave frequencies, since the polarization patterns produced via state mixing depend greatly on the proximity of the triplet and singlet sublevels. Also, at higher microwave frequencies, the g-factor difference between the ends of the acyl biradial may have a dramatic effect on the shape of the spectra.
Acknowledgment. We are grateful to C. L.Braun for providing the Monte Carlo simulation program and to J. A. Naim for helpful discussions on the chain dynamics of n-alkanes. The work at The University of Chicago was supported by N S F Grant CHE8520326. Work at Argonne National Laboratory was supported by the Office of Basic Energy Sciences, Division of Chemical Science, U S . Department of Energy, under Contract W-3 1109-ENG-38.
Modeling Long-Range Photosynthetic Electron Transfer in Rigidly Bridged Porphyrin-Quinone Systems Michael Antolovich, Peter J. Keyte, Anna M. Oliver, Michael N. Paddon-Row,* Department of Chemistry, University of New South Wales, P.O. Box 1, Kensington N.S.W. 2033, Australia
Jan Kroon, Jan W. Verhoeven,* Laboratory of Organic Chemistry, University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands
Stephan A. Jonker, and John M. Warman* Interuniversity Reactorllnstitute, Mekelweg 15, 2629 JB Delft, The Netherlands (Received: June 5. 1990; In Final Form: September 17, 1990)
Photoinduced charge separation as well as subsequent charge recombination is studied in bichromophoric molecules containing a quinone unit (Q) and either a porphyrin (P) or a zinc porphyrin (ZnP) unit that are interconnected by a rigid saturated hydrocarbon bridge that unequivocally determines both the separation and the relative orientation of the chromophores. Across a bridge comprising a separation equivalent to two saturated carbon-carbon bonds (Le., in P[Z]Q), extensive direct overlap between the mystems of the chromophores is still possible and accordingly very fast photoinduced charge separation (typically on a 30-40-ps time scale) is observed. However, even across a bridge comprising an extended array of six saturated bonds, charge separation times in the order of 100 ps can still be realized if the driving force for this process is optimized by modification of the porphyrin and of the solxent (a minimum charge-separation time of 65 ps was observed for ZnP[6]Q in chloroform). This implies a rate of charge separation comparable to or somewhat higher than that of the charge transfer from pheophytin to quinone in natural photosynthesis in spite of the fact that the interchromophore distance in the latter process is slightly smaller. The time-resolved microwave conductivity method was employed to confirm the occurrence of photoinduced charge separation as well as to measure the rate of charge recombination in ZnP[6]Q. The latter was found to decrease dramatically at lower solvent polarity, thus indicating the importance of a relatively apolar environment for achieving long-lived charge separation and for storing as large a fraction as possible of the initial light energy used to initiate it.
1. Introduction The recent decade has brought remarkable parallel progress in the understanding of long-range photoinduced electron transfer and in the clarification of the structure of the photosynthetic unit where long-range electron transfer plays a pivotal role. Investigations of molecular systems consisting of relatively simple electron donor (D)and electron acceptor (A) moieties interconnected by conformationally well-defined, saturated hydrocarbon bridges (Le., D-bridge-A) have been particularly revealing with 0022-3654/91/2095-1933$02.50/0
regard to the distance and orientation dependence of electron transfer over distances far exceeding the sum of the van der Waals radii of the donor and acceptor species.’” Furthermore, such ( I ) Connolly, J. S.;Bolton, J. R. In Photoinduced Electron TramJeG Fox, M . A,, Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part D, p 303. (2) (a) Stein, C. A,; Lewis, N. A.; Seitz, G. J . Am. Chem. Soc. 1982,101, 2596. (b) Beratan, D.N.; Hopfield, J. J. J. Am. Chem. Soc. 1984,106. 1584. (3) Pasman, P.; Rob, F.; Verhoeven, J. W. J . Am. Chem. Soc. 1982, 101, 5127. (b
1991 American Chemical Society
1934 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 -14A
P R
\
?I
P
d
R,
R
M 5
B1-
H PtOlyl 2 e H z M e H 2bZnMeH 1
H2
Figure 1. Structure of bridged porphyrin-quinone systems described in the literature"*'5 with a center-to-center distance comparable to the pheophytin-quinone separation found in the photosynthetic unit.
studies have provided convincing e v i d e n ~ efor ~ ~the , ~ importance of through u-bond interactions in mediating long-range electron transfer, a mechanism now accepted as being of crucial importance for understanding many long-range electron transport phenomena in, e.g., redox protein^.^ It has been recognized widely that it would be desirable to extend studies on rigidly bridged systems to molecular assemblies which provide a closer mimic of the more complicated chromophores and the large distances present in the photosynthetic unit. The chromophores involved in the primary electron-transfer steps of photosynthesis* are a chlorophyll dimer (the special pair), whose electronically excited state transfers an electron to a pheophytin site in -3 ps (Le., a rate of -3 X 10" s-l), followed by a second step in which the charge is relayed from pheophytin to a quinone in -250 ps (i.e,, with a rate of -4 X lo9 s-l). Little effort seems to have been undertaken to model the first step by using a completely covalently bound and rigid model. The formidable but certainly worthwhile challenge that such an effort would present is complicated by the fact that this step appears to involve mediation by an additional chlorophyll unit that acts as a relay between the special pair and thC phe~phytin.~Much more attention has been paid to porphyrin-bridge-quinone molecules that may be considered to mimic the donor/acceptor pair involved in the second step. A plethora of such bridged porphyrin/quinone systems has been studiedlJOand such molecules have also been extended through inclusion of additional redox centers to provide fascinating intramolecular electron transport chains." However, only in a limited number of cases have both distance and relative orientation of the chromophores been achieved that approach the situation found in the photosynthetic unit. The latter is known from X-ray analysis for both Rhodopseudomonas viridis'* and Rhodobacter sphberoides" and in both cases the pheophytin and quinone *-systems are displaced sideways with a center to center distance of 13-14 A while the (4) Ocvcring, H.; Paddon-Row, M. N.; Heppcner, M.; Oliver, A. M.; Cotsaris, E.; Vcrhocven, J. W.; Hush, N . S. J . Am. Chem. Soc. 1987, 109, 3258. (5) Oliver, A. M.:Crain. M. _.D. - . C.: - . Paddon-Row. .- - - ... ... . . .. N.: . .. Krmn. . . ... .. J.: ...Verhaeven. . .... . ---, J. W. Chem. Phys. I r t r . 1988, 150, 366. (6) Johnson, M. Il; Miller, J. R.; Green,N . S.;Closs, G. L. J . Phys. Chem. 1989. 93. I173 (7) Gwler, E. E.; Raphael, Chemisrry; Lip1
.
~
therein. (10) Wasielewski, M. R. In Phoroinduced Electron Transfer, Fox, M. A,, Elsevier: Amsterdam, 1988; Part A, p 161. Chanon, M., Us.; ( 1 I ) Gust, D.; Moore, T.A. Scfence 1989, 244, 35, (12) Deirrenhofer, J.; Epp, 0.; Miki, K.; Huber, R.; Michel, H. J . Mol. Bfol. 1984, 180, 385. (13) Allen, J. P.; Feher, G.;Yeatea,T. 0.;Komiya, H.; Rees, D. C. Proc. Nail. Acad. Scl. U.S.A. 1981, 84, 5730.
Antolovich et al. normals to the *-planes make an angle of 35 f 6 O . A bridge that provides the correct center-to-center distance is that incorporated in systems 114 and 2a,bI5 (Figure 1). In 1, where a tetraaryl porphyrin was employed, the reported rate of photoinduced charge separation is very much smaller than that of the second step in photosynthesis, being a t most 1.5 X lo7s-l in a few solvents of intermediate polarity, and virtually undetectable in both very polar and very apolar solvent^.'^ It has been recognizedlO that the low rates in 1 may be due more to a lack of intrinsic driving force than to a lack of electronic coupling between the chromophores. Thus the compounds 2, which incorporate the more powerful octaalkylporphyrin donor site, and especially system 2b, where the driving force is further enhanced by both the lower oxidation potential and the higher excitation energy of the porphyrin-Zn complex, display much higher rates of charge separation in a variety of solvents, reaching a value'" as high as 1.25 X 1Olo s-l. The latter rate appears to represent that occurring under barrier-free conditions and may thus be directly compared to that (-4 X lo9s-l) of the second step in photosynthesis, which from its virtual temperature independence16J7 is also inferred to occur under optimally exothermic conditions, that is, where the driving force (AGO) equals the overall reorganization energy (A). It is therefore quite interesting to note that the rates of photoinduced, intramolecular electron transfer in the model compound 2b can surpass those of the (thermal) pheophytin to quinone transfer in the photosynthetic unit. This may be related to the fact that about 50% of the bridge length in 2 actually consists of a pphenylene *-system formally conjugated with the porphyrin. However, the extent of this conjugation is strongly offset by a severe twist around the bond interconnecting these moieties, making it difficult to define the effective edge-to-edge separation of the donor and acceptor systems. This situation is also to be found in several more recently reported systems which differ from 1 and 2 in that the bicyclooctane unit, having an effective length of five carbon-carbon bonds, is replaced by shorter bridges comprising of or fourI9 carbon-carbon bonds. Furthermore, an effort to increase both the edge-to-edge and the center-to-center distance in 2 by extending the bridge with a second bicyclooctane unit gave rise to systems that display essentially no photoinduced charge separation (rate < lo7 S-I).'~~,~ Over the past several years the UNSW group has convincingly demonstrated20 that rigidly fused polynorbornyl-bicyclo[2.2.0]hexyl systems offer a number of exceptional and, perhaps, unprecedented advantages Over other hydrocarbon bridges commonly employed: (1) The norbornyl spacer is an extremely efficient mediator of through-bond coupling, as demonstrated by numerous photoelectron spectroscopic20~2'a*and electron transmission (14) Bolton, J. R.; Ho, T. F.; Liauw, S.;Siemiarczuk, A,; Wan, C. S. K.; Wedon, A. C. J . Chem. Soc., Chem. Commun. 1985, 559. (15) (a) Joran, A. D.; Leland, E. A.; Gellcr, G. G.; Hopfield, J. J.; Dervan, P. E. J . Am. Chem. Soc. 1984, 106,6090. (b) Leland, E. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dcrvan, P. E. J . Phys. Chem. 1985,89, 5571. (c) Joran, A. D.; Leland, B. A.; Felker, P. M.; &wail, A. H.; Hopfield, J. J.; Dervan, P. E. Narure (London) 1987, 327, 508. (16) Schenck, C. C.; Parson, W. W.; Holton, D.; Windsor, M. W.; Sarai, A. Biophys. J . 1981, 36, 479. (17) Kirmaier, C.; Holton, D.; Parson, W. W. Biochim. Biophys. Acra 1985, 810, 33. (18) Wasielewski, M. R.; Nicmczyk, M. P. In Porphyrins-ExciredSrares and Dynamics;ACS Symposium Series No. 321; Goutcrman M.. Rentzepia, P. M., Straub, K. D., Eds.; American Chemical Society; Washington DC, 1986; p 154. (19) Sakata, Y.; Nakashima, S.; Goto, Y.; Tatemitsu, H.; Misumi, S.; Asahi, T.; Hagihara, M.; Nishikawa, S.; Okada, T.; Matap, N.J. Am. Chem. Soc. 1989, I I I 8979. (20) (a) Paddon-Row, M. N . Acc. Chem. Res. 1982, 15, 245. (b) Paddon-Row, M. N.; Jordan, K. D. In Molecular Srrucrure and Encrgerics; Liebman, J. F.,Greenberg, A., Eds; VCH Publishers: New York, 1988; Vol. 6, Chapter 3. (c) Paddon-Row, M. N.; Cotaaris, E.; Patney. H. K. Terrahedron 1986, 42, 1779. (21) (a) Paddon-Row, M. N.; Patney, H. K.; Brown, R. S.;Houk, K. N. J . Am. Chem. Soc. 1981,103,5575. (b) Paddon-Row, M. N.; J e ~ e n l e n F. , S.; Patney, H. K. J . Chem. Soc., Chem. Commun. 1983,573. (c) PaddonRow, M. N.; Patney, H. K.; Peel, J. E.; Willett, 0. D. J . Chem. Soc., Chem. Commun. 1984,564. (d) Balaji, V.; Ng, L.; Jordan, K. D.; Paddon-Row, M. N.: Patney, H. K. J . Am. Chem. Soc. 1987,109,6957. (e) Paddon-Row, M. N.; Wong, S.S.Chem. Phys. I r r r . 1990,167, 432.
E T in Bridged Porphyrin-Quinone Systems
=
300 OC: IH emission spectra for solutions with an absorbance 10.2 at the N M R (500 MHz, CDCI,) 6 -2.55 (br s, 2 H), 2.54 (d, J 8 Hz, excitation wavelength (550-600 nm for the free-base porphyrins, 1 H), 2.66 (d, J 8 Hz, 1 H), 4.53 (br s, D 2 0 exchange, 2 H), 4.72 ca. 530 nm for the zinc complexes). (s, 2 H), 6.36 (s, 2 H), 7.68 (s, 2 H), 7.74-7.79 (m, 10 H), 7.91 3.4. Fluorescence Lifetime Measurements. For the longer (t, 5 7 . 6 Hz, 2 H), 8.13 ( d , J 7 Hz, 4 H), 8.22 (d, J 7 Hz, 4 H), lifetimes (>2 ns) a straightforward pulse-probe technique was 8.71 (s, 2 H), 8.91 (half of AB q, J 5 Hz, 2 H), 8.93 (half of AB employed using a setup described extensively elsewhere.43 Esq, J 5 Hz, 2 H). sentially the sample is excited by the pulse (308 nm, fwhm 6 ns) P[6]QH2. The dimethoxybenzene compound P[6]QMe2 (1 50 of a Lambda-Physik EMG 101 XeCl excimer laser and the mg, 0.1 5 mmol) was cleaved under identical conditions with those fluorescence signal monitored at right-angle by a photomultiplier used above, to give P[6]QH2 (150 mg, 99%), mp > 300 "C: 'H wired for fast response via a Zeiss MQ I1 monochromator. The NMR (500 MHz, CDC13) 6 -2.53 (br s, 2 H), 1.08 (s, 6 H), 1.60 signal of the detector is fed into a Tektronix 11302 oscilloscope (d, J 7 Hz.1 H), 1.75 (br d, J 7 Hz, 2 H), 1.96 (br s, 2 H), 2.08 equipped with a digitizing camera system (Tektronix DCSO1). (br s, 3 H), 3.48 (s, 2 H), 3.53 (s, 2 H), 4.29 (br s, D 2 0 exchange, The digitized data are analyzed by a home-written deconvolution 2 H), 6.42 (s, 2 H), 7.52 (s, 2 H), 7.75-7.80 (m, 10 H), 7.91 (t, program. J7.6Hz,2H),8.14(d,J7Hz,4H),8.23(d,J7Hz,4H),8.71 For shorter decays the time correlated single photon counting (s, 2 H), 8.92 (half of AB q, J 5 Hz, 2 H), 8.94 (half of AB q, technique was employed using equipment described extensively J 5 Hz, 2 H). before! In the present experiments picosecond (-80 p)excitation General Synrheses of the Quinones P[21 Q and P[6] Q. PbOz pulses at 593 nm were produced by synchronously pumping a ( I 0 equiv) was added to a solution of the hydroquinone in CHCI, Rh6G dye laser (Coherent 490 with extended cavity) with a (0.4 mM) and the suspension shaken for 10 min. The reaction mode-locked Ar' laser (Coherent CR8, repetition rate 94.6 MHz). mixture was then filtered and concentrated. The product was The pulse repetition frequency sets an upper limit of about 4 ns purified by preparative TLC on silica (CHCI, eluting solvent) to the decay times that can be analyzed reliably, while the detector immediately prior to spectroscopic measurements. The formation response (Hamamatsu R1564U-01 microchannel plate photoof the quinones was confirmed by their characteristic absorbances multipier) and the pulse width set a lower limit of about 20 ps at 256 nm and about 1655 cm-I. in the present setup after deconvolution. It should be noted that ZnP[qQ. To a solution of P[6]Q (1.75 mg, 1.7 pmol) in CHC13 while the model systems P[2] and ZnP[2] as well as the por(7.5 mL) was added zinc acetate (1 mg, 4.5 pmol) in MeOH (2.5 phyrin-hydroquinones display monoexponential fluorescence decay mL). The reaction mixture was warmed to 30 'C and the progress characteristics the fluorescence decay of P[2]Q, P[6]Q, and of the reaction monitored by TLC. On completion of the reaction, ZnP[6]Q often contains, in addition to the main component given the product was purified as for the free-base porphyrin compounds. in Table I, a weak, longer lived component that increases in mp > 300 "C: 'HNMR (300 MHz, CDCI,) 6 1.05 (s, 6 H), 1.53 amplitude upon storage of the solution. This is a feature often (d, J 7 Hz,1 H),1.70 (d, J 7 Hz, 1 H), 1.82 (d, J 7 Hz, 1 H), reported'" for such systems and attributed to the presence of traces 1.91 ( s , 2 H ) , 2 . 0 ( d , J 7 H z , I H ) , 2 . 1 0 ( ~ , 1H ) , 3 . 4 7 ( s , 2 H ) , of the hydroquinone or formation of products in which the quinone 3.58 (8, 2 H), 6.53 (8, 2 H), 7.61 (s, 2 H), 7.74-7.78 (m, 10 H), function is degraded in another reaction that destroys its acceptor 7.89 (pert. t, 5 7 Hz, 2 H), 8.15 (m, 4 H), 8.21 (dd, J , 7.5 Hz, properties. ~~
(42) Cookson, R. C.; Hill, R, R.; Hudec, J . J . Chem. Soc. 1964, 3043.
(43) Van Ramesdonk, H. J.; Verhoeven, J. W. Tek lmogcr 1989, I , 2.
J . Phys. Chem. 1991,95, 1941-1944
1941
3.5. Time-Resolved Microwave Conductivity (TRMC). The TRMC measurements were carried out as described fully elsewhere.35 The solute was photoexcited by using the 308-nm XeCl line of a Lumonics HyperEX-420 laser with a 5-11s FWHM pulse of approximately 10 mJ/cm2 integrated intensity. The change in microwave power reflected by the cell containing the solution of interest was monitored by using a Tektronix 7912 transient digitizer. Signal noise was reduced by averaging up to 64 single-shot traces. The pulse shape was monitored by using a subnanosecond time response photodiode. The time response of the microwave cavity, which was the limiting factor in the overall time response and equal to approximately 6 ns, was determined accurately by measurement of the reflection characteristics of the cavity containing the solvent of interest. Data analysis involved numerically solving the appropriate rate equations by the Runge Kutta method including the known pulse shape and time response and correcting for the variation of light intensity with penetration as described p r e v i ~ u s l y . ~ ~ Figure 6. Fluorescence decay, as measured by picosecond time correlated single photon counting, for ZnP[2] and ZnP[6]Q in tetrahydrofuran. Excitation at 593 nm, detection at 680 nm, channel width 10 ps.
In Figure 6 we show the experimental fluorescence decay curves of ZnP[2] and ZnP[6]Q in tetrahydrofuran. While the former can be fitted perfectly with a single lifetime (1 260 ps), the fit for the latter is significantly improved by biexponential fitting with a main component (90%) of 285 ps and a minor component of -1100 ps.
Acknowledgment. We thank the Australian Research Council for support. The present investigationswere furthermore supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Research (NWO) and by the Netherlands Ministry of Economic Affairs Innovation-oriented Research Programme on Polymer Composites and Special Polymers (IOP-PCBP). The valuable assistance of Ing. D. Bebelaar in the realization of the picosecond measurements is gratefully acknowledged.
Non-Arrhenius Temperature Dependence of Electron-Transfer Rates M. Bixon* and Joshua Jortner* School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, 69978 Tel- Aviv, Israel (Received: June 7, 1990)
A surprisingly weak temperature dependence of electron-transfer (ET) rates in a dense medium may be induced by vibrational excitations of high-frequency quantum vibrational modes of the donor and acceptor centers, which accompany ET. Practically activationless ET prevails over a broad range of the free energy gap within the inverted region for moderate values of electron-vibration coupling. The effects of intramolecular vibrational excitations on the primary ET and recombination rates in the photosynthetic reaction center are elucidated.
Introduction Weller made central contributions to our understanding of intramolecular and intermolecular electron-transfer (ET) processes in solution. A fascinating observation reported by Rehm and Weller in 1970' pertained to the rates of highly exoergic ET reaction, which were found to be independent of the thermodynamic driving force, Le., the free energy gap (-AG). This observation was in variance with an important prediction of the Marcus theory2 that the rate of highly exoergic ET reactions slows down with increasing -AG in the so-called inverted region. The work of Rehm and Weller initiated the exploration of quantum effects on ET processes, elucidating the role of high-frequency intramolecular vibrational modes of the donor and acceptor centers on ET dynamics.) These quantum effects are expected to not only modify free energy relationship for ET',' but also exert a dramatic ( I ) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 259. (2) (a) Marcus, R . A. J . Chem. Phys. 1956,24966. (b) Marcus, R. A. J. Chem. Phys. 1957,26,867. (c) Marcus, R. A. Discuss.Faroday Soc. 1960, 29, 21.
0022-3654191/2095-1941$02.50/0
effect on the temperature dependence of ET rates. The Marcus theory2 of electron transfer (ET) in solution, which constitutes the first quantitative description of a chemical reaction in a dense medium, resulted in the Arrhenius-type temperature (3) (a) Efrima, S.;Bixon, M. Chem. Phys. Lrrr. 1974,25, 34. (b) Kertnor, N. R.; Logan, J.; Jortner, J. J. Chem. Phys. 1974,78,2148. (c) Van Duyne, R. P.; Fischer, S.F. Chem. Phys. 1974, 5, 183. (d) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975,63,4358. ( e ) Efrima, S.;Bixon, M. Chcm. Phys. 1976, 13,447. (f) Fischer, S.F.; Van Duyne, R.P.Chem. Phys. 1977,26,9. (8) Webman, I.; Kestner, N. R. J . Phys. Chrm. 1979,83,451. (h) Kestner, N, R . J . Phys. Chem. 1980,84,1270. (i) Marcus, R. A. J . Chcm. Phys. 1984, 81, 4494. (4) (a) Miller, J. R.; Calcaterra, L. R.; Close, 0. L.J . Am. Chrm. Soc. 1984, 106, 3047. (b) Miller, J. R.;Bcitz, J. V.; Huddlerton, R. K. J . Am. Chem. Soc. 1984, 106, 5057. (c) Closs, G. L.; Calceterra, L.T.; Own, N. J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986.90, 3673. (d) Irvine, M. P.;Harrison, R. J.; Beddard, G. S.;Leighton, P.;Sanden, J. K.J. Chew Phys. 1986, 104, 315. (e) Gould, 1. R.; Ege, D.; Mattas, S. L.;Farid, S.J . Am. Chem. Phys. 1987, 109, 3794. (f) Gould, 1. R.; Morer, J. E.; E 0. D.; Farid, S.J . Am. Chem. Phys. 1988, 110, 1991. ( ) Ohno, R.;Yorhfmura, A.; Shioyama, H.; Mataga, N.J . Phys. Chcm. lS!, 91,4365. (h) Mats a, N.; Asahi, T.;Kanda, Y.;Okada, T.; Kakitani, T. Chem. Phys. 1988, 197, 249. (i) Asahi, T.; Mataga, N. J . Phys. Chem. 1989, 93, 6575.
0 1991 American Chemical Society
