CONTRASTING PHOTOINDUCED ELECTRON-TRANSFER PROPERTIES OF 2 CLOSELY-RELATED, RIGIDLY LINKED PORPHYRIN-QUINONE DYADS

Two closely related, rigidly linked porphyrin-naphthoquinone dyads hav e been prepared and studied usings time-resolved fluorescence and abso rption methods. Dyad 1, whose quinone carbonyl groups are relatively c lose to the porphyrin macrocycle, exhibits photoinduced electron-trans fer rate constants that are virtually independent of solvent dielectri c constant and temperature within the range 77-295 K. Dyad 2, which ha s a similar donor-acceptor linkage but whose quinone carbonyl groups a re similar to 2 Angstrom farther from the porphyrin, features photoind uced electron-transfer rate constants that decrease with decreasing so lvent dielectric constant. Electron transfer in this molecule ceases a t low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin-quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the po rphyrin radical cation and the quinone radical anion, which leads to n early barrierless electron transfer and makes transfer less susceptibl e to effects due to changes in solvent dielectric properties and tempe rature. Charge recombination rates in the dyads are substantially slow er than charge separation rates, unlike those of many porphyrin-quinon e systems. This suggests that these molecules might be useful as compo nents of more complex molecular devices.