Competitive Electron Transfer from the S ₂ and S ₁ Excited States of Zinc meso-Tetraphenylporphyrin to a Covalently Bound Pyromellitimide: Dependence on Donor-Acceptor Structure and Solvent

Zinc meso-tetraphenylporphyrin (ZnTPP) is known to have a relatively slow S ₂ → S ₁ internal conversion rate, (1-3 ps) ^-1, so that electron transfer from S ₂ 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 S ₂ (excitation at 420 nm) and S ₁ 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 S ₂ 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 S ₁ 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 S ₂- 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.