Competitive Electron Transfer from the S2 and S1 Excited States of Zinc meso-Tetraphenylporphyrin to a Covalently Bound Pyromellitimide: Dependence on Donor−Acceptor Structure and Solvent

Abstract

Zinc meso-tetraphenylporphyrin (ZnTPP) is known to have a relatively slow S2 → S1 internal conversion rate, (1−3 ps)-1, so that electron transfer from S2 to a nearby acceptor is possible. We have synthesized two zinc porphyrins in which a pyromellitimide (PI) acceptor is attached to the para position of a meso-phenyl group of the porphyrin (ZnTPP−PI) or to that of a β-phenyl group of the porphyrin (ZnTPP−β-PhPI). We report here on competitive electron transfer from the S2 (excitation at 420 nm) and S1 states (excitation at 550 nm) of these zinc porphyrins to PI in 2-methyltetrahydrofuran (MTHF) and toluene. Our transient absorption studies show that efficient electron transfer occurs from S2 of ZnTPP to PI in these porphyrins despite the presence of the intervening phenyl between the zinc porphyrin macrocycle and the PI acceptor. Moreover, the results show that while charge separation from S1 is about 6 times faster for PI attached to the meso-phenyl than for PI attached to the β-phenyl, the opposite is observed for charge separation from S2. Subsequent charge recombination is 2.6−2.8 times faster for PI attached to the meso-phenyl relative to that of the β-phenyl. Comparisons of rates between MTHF and toluene show that the ordering of rates is the same for both solvents, although the rate ratios are larger in MTHF than in toluene.