PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER - FREE-ENERGY AND RIGIDITY EFFECTS

The relative photoreactivities of several bridged-ring systems 2-4 hav e been studied. Compound 3 is photoreactive whereas 2 and 4 are photoi nert when irradiated with 254 nm light. The systems contain aromatic e lectron donors and halogen electron acceptors which are held at well-d efined geometries that vary in the degree of rigidity at the electron acceptor termini. The relative photoreactivities in the photoinduced i ntramolecular electron transfer processes cannot be explained by calcu lated relative free-energy or rate constant values for electron transf er. The discrepancies between the relative experimental photoreactivit ies and those predicted based on calculated values of free energy (Wel ler equation) are rationalized by a rigidity argument.