Remarkable conformational control of photoinduced charge separation and recombination in a giant U-shaped tetrad

Intramolecular photoinduced electron transfer (ET) with rates greater than 2 x 10(9) s(-1) has been observed in polar solvents for the U-shaped, syn,s yn isomer (syn,syn-4) of a giant tetrad with porphyrin and methyl viologen termini (4). In contrast, no ET is observed in a sample containing the exte nded isomers anti,syn and anti,anti of 4. Photoexcitation of the syn,syn is omer and subsequent ET leads to the formation of the charge-separated state which has an unusually long lifetime of 420 ns in acetonitrile and 230 ns in benzonitrile. Temperature-dependent fluorescence lifetime studies of syn ,syn-4 revealed the presence of a small (0.09 eV) activation barrier for th e ET process and they indicate the presence of two ground-state conformers, one where the terminal porphyrin and methyl viologen chromophores are-loca ted close enough to facilitate ET and one where they are not. The experimen tal data and theoretical calculations support the proposal that ET occurs b y a direct through-space process and that the observed slow charge recombin ation arises from an increase in the separation of the terminal chromophore s due to electrostatic repulsion between the porphyrin and methyl viologen radical cations.