Contrasting photoinduced electron-transfer properties of two closely related, rigidly linked porphyrin-quinone dyads

Two closely related, rigidly linked porphyrin-naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77-295 K. Dyad 2, which has a similar donor-acceptor linkage but whose quinone carbonyl groups are ∼2 Å farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrinquinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, which leads to nearly barrierless electron transfer and makes transfer less susceptible to effects due to changes in solvent dielectric properties and temperature. Charge recombination rates in the dyads are substantially slower than charge separation rates, unlike those of many porphyrin-quinone systems. This suggests that these molecules might be useful as components of more complex molecular devices.