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 10⁹ s^-1 has been observed in polar solvents for the U-shaped, syn, syn 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 extended isomers anti,syn and anti,anti of 4. Photoexcitation of the syn,syn isomer 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 the ET process and they indicate the presence of two ground-state conformers, one where the terminal porphyrin and methyl viologen chromophores are located close enough to facilitate ET and one where they are not. The experimental data and theoretical calculations support the proposal that ET occurs by a direct through-space process and that the observed slow charge recombination arises from an increase in the separation of the terminal chromophores due to electrostatic repulsion between the porphyrin and methyl viologen radical cations.