Photosynthetic Model Systems That Address the Role of Superexchange in Electron Transfer Reactions

Four fixed-distance porphyrin-quinone molecules, 1–syn, 1–anti, 2–syn, and 2–anti, were synthesized. These molecules possess a zinc 5–phenyl-10,15,20–tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1–syn: 8.2 × 109 s-1; 1–anti: 1.7 × 1010 s-1; 2–syn: 8.5 × 109 s-1; 2–anti: 1.9 × 1010 s-1. The corresponding rate constants for the porphyrin cation — naphthoquinone anion recombination reaction are 1–syn: 1.4 × 1010 s-1; 1–anti: 2.5 × 1010 s-1; 2–syn: 5.0 × 1010 s-1; 2–anti: 8.2 × 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3–4 × faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.