Distance dependence of electron transfer in rigid, cofacially compressed, pi-stacked porphyrin-bridge-quinone systems.

The electron-transfer (ET) dynamics of a series of unusually rigid pi-stacked porphyrin-quinone (P-Q) systems, in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic bridge, and quinonyl components of these assemblies, are reported. The photoinduced charge separation (CS) and thermal charge recombination (CR) ET reactions of [5-[8'-(2'',5''-benzoquinonyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (1a-Zn), [5-[8'-(4''-[8'"-(2'"',5'"'-benzoquinonyl)-1'"-naphthyl]-1''-phenyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (2a-Zn), and [5-(8'-[4''-(8'"-[4'"'-(8'""-[2'""',5'""'-benzoquinonyl]-1'""-naphthyl)-1'"'-phenyl]-1'"-naphthyl)-1''-phenyl]-1'-naphthyl)-10,20-diphenylporphinato]zinc(II) (3a-Zn) in CH(2)Cl(2) were investigated by pump-probe transient absorption spectroscopy. Analyses of these data show that the phenomenological ET distance dependence (beta) for both the CS and CR reactions in these systems is soft (beta(CS) = 0.43 A(-1); beta(CR) = 0.35 +/- 0.16 A(-1)). This work demonstrates that simple aromatic building blocks such as benzene, which are characterized by highly stabilized filled molecular orbitals and large HOMO-LUMO gaps, can provide substantial D-A electronic coupling when organized within a pi-stacked structural motif that features a modest degree of arene-arene interplanar compression.