Photochemical and electrochemical properties of zinc chlorin-C₆₀ dyad as compared to corresponding free-base chlorin-C₆₀, free-base porphyrin-C₆₀, and zinc porphyrin-C₆₀ dyads

The photochemical and electrochemical properties of four chlorin-C₆₀ or porphyrin-C₆₀ dyads having the same short spacer between the macrocycle and the fullerene are examined. In contrast with all the previous results on porphyrin-fullerene dyads, the photoexcitation of a zinc chlorin-C₆₀ dyad results in an unusually long-lived radical ion pair which decays via first-order kinetics with a decay rate constant of 9.1 × 10³ s^-1. This value is 2-6 orders of magnitude smaller than values reported for all other porphyrin or chlorin donor-acceptor of the molecule dyad systems. The formation of radical cations of the donor part and the radical anion of the acceptor part was also confirmed by ESR measurements under photoirradiation at low temperature. The photoexcitation of other dyads (free-base chlorin-C₆₀, zinc porphyrin-C₆₀, and free-base porphyrin-C₆₀ dyads) results in formation of the ion pairs which decay quickly to the triplet excited states of the chlorin or porphyrin moiety via the higher lying radical ion pair states as is expected from the redox potentials.