J . Am. Chem. SOC. 1982, 104, 253-259
Quenching of the Singlet Excited States of Meso-Substituted Porphines by p-Benzoquinone under Unimolecular and Bimolecular Conditions: Evidence for Electron Transfer in Competition with Vibrational Relaxation M. A. Bergkamp,? J. Dalton,t and T. L. Netzel*? Contribution from the Chemistry Departments, Brookhaven National Laboratory, Upton, New York 11973, and City of London Polytechnic, 31 Jewry Street, London, England EC3N 2EY. Received April 24, 1981. Revised Manuscript Received August 7, 1981
Abstract: Picosecond absorption spectroscopy was used to investigate the singlet excited-state (SI) relaxations in two meso-substituted porphines, triphenylb-benzoquin0ne)porphine (P,QP) and tetrakisb-benzoquin0ne)porphine (Q4P), and in solutions of meso-tetraphenylporphine with added amounts of p-benzoquinone. P3QP and Q4P have observable S1 states with lifetimes of 5 ns) and So states. Since -90% of the T1 states decay to ground states before forming relaxed electron-transfer (ET) products, reverse ET from a vibrationally unrelaxed triplet biradical to So levels in competition with continued relaxation of the triplet biradical seems indicated. To the extent the rapid decay of the SI states results from ET to form a singlet biradical, the formation of T1 states is also likely to result from a reverse ET while the singlet biradical is vibrationally relaxing. A working model of the multiple ET steps involved is proposed to summarize the above results and to guide future studies of unimolecular ET processes.
I. Introduction Numerous flash kinetic studies have shown quinones to be facile electron acceptors.'-" In addition they are known to be intermediates in photosynthetic electron-transport hai ins.'^-'^ Recently researchers1s17 have linked quinone electron acceptors to porphyrin and chlorophyll electron donors to model the dynamics of photosynthetic electron transfers. The general conclusion of extensive studies of the quenching of excited states of porphyrins and chlorophylls by added quinones and nitroaromatic molecules is that ionic photoproducts can be observed when the quenching reaction proceeds from a triplet (TI) excited state, but not when it proceeds from a singlet excited state (SI). It has been proposed1~'*Jg that the singlet states are quenched by the production of singlet radical pairs, but that the spin-allowed recombination of the electrons and holes exceeds their rate of dissociation. Thus ionic photoproducts are not seen. Triplet states, however, are quenched by the formation of triplet radical pairs. Apparently their spin-forbidden electron-hole recombination rate is less than their dissociation rate and ionic photoproducts are observable. In striking contrast to this behavior, recent studiesZoJ of a cofacial diporphyrin comprised of Mg-substituted and freebase subunits (Mg-H2) have provided evidence of a 5 Q4Pin CH,CI, O.Sf 15 * 10 0.04 >15 in DMF TPP in iodobenzene 4 x 10-3 1.0d >15j 'Based on a reported value of 0.13 for @F of TPP in benzene (ref 24 and 30). Based on a molar absorptivity of 1 X lo4 M-' cm-' (+20%)for TPP' in the 670-nm region (ref 37). Emission decay measurement. @F + @T = 1 (ref 24, 30, 31, and 32). e A small number of the SI states (