Electrostatically driven geometry changes accompanying charge separation in supposedly rigid bichromophoric systems

Calculations on the structures of a variety of charge-separated (CS) bichro mophoric systems in the gas phase were carried out at the UHF, CIS, and UB3 LYP theoretical levels, using the 3-21G, 6-31G(d), and 6-31+G-(d basis sets . The results indicate that the relatively computationally inexpensive UHF/ 3-21G method gives a reliable description of the molecular structure and ch arge distribution in these states. It is predicted that the CS states of mo st of the bichromophoric systems studied will undergo strong structural dis tortions due to the Coulombic forces operating between the two charged chro mophores. These structural distortions are largely associated with changes in the geometries of the charged chromophores, although the putative "rigid " saturated hydrocarbon bridges connecting the chromophores can also underg o some distortion. In the case of 8b, the distorted CS geometry predicted b y these calculations is consistent with results obtained from recent experi mental charge recombination studies on the CS state of the similar system, 8a. Geometry changes accompanying charge separation or charge recombination processes, on the scale predicted here, might influence the interpretation of rate data based on semiclassical ET theories.