Stepwise Sequential and Parallel Photoinduced Charge Separation in a Porphyrin-Triquinone Tetrad ² Joseph Springer, Gerdenis Kodis, Linda de la Garza, Ana L. Moore,* Thomas A. Moore,* and Devens Gust* Department of Chemistry and Biochemistry, Center for the Study of Early EVents in Photosynthesis, Arizona State UniVersity, Tempe, Arizona 85287 ReceiVed: NoVember 13, 2002; In Final Form: February 11, 2003 A molecular tetrad consisting of a free base porphyrin (P) linked to a triquinone moiety consisting of a naphthoquinone bearing two benzoquinones (NQ-BQ 2 ) has been synthesized and its photochemistry has been investigated using time-resolved spectroscopic techniques. Excitation of the porphyrin chromophore of the P-NQ-BQ 2 tetrad in benzonitrile solution is followed by photoinduced electron transfer with a rate constant of 8.3 × 10 10 s -1 to produce an initial P •+ -NQ •- -BQ 2 state with a quantum yield of unity. A charge shift reaction gives a final P •+ -NQ-(BQ •- BQ) charge-separated state, which decays to the ground state with a rate constant of 1.9 × 10 10 s -1 . The tetrad features both sequential and parallel multistep electron- transfer pathways. No evidence was found for significant involvement of direct electron transfer from the porphyrin first excited singlet state to a benzoquinone. Introduction A key feature of photosynthetic energy conversion is photo- induced electron transfer to generate an energetic, long-lived charge-separated state in high quantum yield. The long lifetime is achieved by production of a final charge-separated state in which the electronic coupling between the radical ions is weak. Formation of this state with a high quantum yield is ac- complished using a multistep pathway whereby electrons migrate from the excited primary donor to the final acceptor by sequential transfer via a number of intermediate donor/ acceptor moieties. The electronic coupling for each intermediate electron “hop” is relatively large, so that transfer is rapid and efficient. Many photosynthetic models using porphyrins or related cyclic tetrapyrroles as donor chromophores covalently linked to various organic species that act as electron acceptors and secondary donors have been reported. 1-10 Quinones are often used as electron acceptor species and indeed are among the acceptors found in natural photosynthetic electron transport. Although the majority of such models contain only a single quinone as an electron acceptor, some contain multiple quinones. For example, porphyrins linked to diquinones of sequentially increasing reduction potential demonstrate two sequential electron-transfer steps leading to an enhancement of the lifetime of charge separation. 3,11-13 On the other hand, a porphyrin- diquinone triad was recently reported in which both quinone moieties were of similar reduction potential. 14 The porphyrin was covalently linked to a benzoquinone derivative, which in turn bore a second benzoquinone. It was reported that the porphyrin first excited singlet state was quenched by direct photoinduced electron transfer both to the directly attached benzoquinone and to the additional benzoquinone moiety. Sequential electron transfer was not detected, even when such a pathway was thermodynamically feasible. Herein we report the synthesis and spectroscopic study of tetrad 1, in which a porphyrin (P) is joined to a naphthoquinone (NQ) via an amide linkage. The naphthoquinone in turn bears two equivalent benzoquinone moieties (BQ). This molecule has the potential for demonstrating sequential photoinduced electron transfer in addition to single-step transfer to the naphthoquinone and/or benzoquinones. In the first step, the porphyrin first excited singlet state, 1 P-NQ-BQ 2 , could in principle decay to initial charge-separated states P •+ -NQ •- -BQ 2 or P •+ -NQ- (BQ •- BQ). Because the benzoquinones are more easily reduced than the naphthoquinone, charge shift from P •+ -NQ •- -BQ 2 to yield P •+ -NQ-(BQ •- BQ) is a possibility. The resulting state could have a longer lifetime than P •+ -NQ •- -BQ 2 , due to reduced electronic coupling of the radical ions. In addition, the yield of P •+ -NQ-(BQ •- BQ) could be higher than that for a similar system having only one benzoquinone because two electron-transfer pathways acting in parallel would compete with decay of the precursor state. Finally, having two identical benzoquinones as the ultimate electron acceptors might permit construction of a donor-acceptor system capable of demonstrat- ing accumulation of multiple reducing and oxidizing equivalents following multiple excitations. Tetrad 1 was prepared to investigate these possibilities and to compare its behavior with the earlier diquinone systems mentioned above. Synthesis The synthetic route to 1 and related model compounds is shown in Scheme 1. The triptycene derivative 6, bearing the three quinones protected as dimethoxybenzene and dimethoxy- naphthalene moieties, was prepared using the Diels-Alder reaction of the appropriate naphthoquinone with 1,4,5,8- tetramethoxyanthracene. Molecule 6 was linked to amino- porphyrin 10 through amide formation to yield 7, and the protecting groups were removed to generate tetrad 1. Dyad 2, in which the two benzoquinone moieties have been reduced and protected as dimethoxybenzene derivatives, was prepared by a ² Part of the special issue “George S. Hammond & Michael Kasha Festschrift”. 3567 J. Phys. Chem. A 2003, 107, 3567-3575 10.1021/jp022424g CCC: $25.00 © 2003 American Chemical Society Published on Web 03/27/2003