Small Reorganization Energy of Intramolecular Electron Transfer in Fullerene-Based Dyads with Short Linkage

A bacteriochlorin-C₆₀ dyad (H₂BCh-C₆₀) and a zinc chlorin dyad (ZnCh-C₆₀) with the same short spacer have been synthesized. The rate constants for the charge-separation (CS) processes in these dyads were determined by fluorescence lifetime measurements of the dyads. The charge-recombination (CR) rate constants of the dyads were determined using laser flash photolysis. The photoexcitation of the zinc chlorin-C₆₀ dyad results in formation of the long-lived radical ion pair, which has absorption maxima at 790 and 1000 nm due to the zinc chlorin radical cation and the C₆₀ radical anion, respectively. Photoexcitation of the free-base bacteriochlorin-C₆₀ dyad with the same short linkage leads to formation of the radical ion pair, which decays quickly to the triplet excited state of the bacteriochlorin moiety. The driving force dependence of the electron-transfer rate constants of these dyads with a short spacer affords a small reorganization energy (λ = 0.51 eV) as compared with the λ value (0.66 eV) of zinc porphyrin-C₆₀ dyads with a longer spacer.