Dendrimer effects on intermolecular energy-transfer of photoexcited triplet states of dendritic multiporphyrin arrays and electron transfer vs fullerene[60]

Photochemical and photophysical properties of the triplet excited states of dendritic multiporphyrins arrays (nP_Zn, n = 1, 3, and 7) have been investigated by measuring the nanosecond transient absorption spectra in the visible and near-IR regions with changing the generation number. Intermolecular triplet-triplet annihilation rates decrease with the dendrimer generation, which was interpreted on a proposed kinetic model assuming that the excited triplet energy almost localizes in one P_Zn unit in nP_Zn. In the presence of C₆₀, intermolecular electron-transfer takes place via the excited triplet states of nP_Zn (³nP_Zn*), yielding the cation radical of nP_Zn (nP_Zn^.+) and the anion radical of C₆₀ (C₆₀^.-) in PhCN. Deceleration of the electron-transfer rate-constants from 1P_zn to 3P_Zn and the acceleration from 3P_Zn to 7P_Zn were observed, in which the latter tendency was interpreted by considering an increase in effective encounter radius for ³7P_Zn*. The observed small change of the rate constants for back electron transfer between the oppositely charged species with the dendrimer generation was also reasonably interpreted by taking a smaller effective radius due to electrostatic attraction into consideration. Dendrimer generation effect was also observed for the intermolecular hole-transfer process.