CHARGE-TRANSFER STATE PHOTOPHYSICS IN A RIGID MOLECULE - COMPETITION BETWEEN ELECTRON-TRANSFER PROCESSES IN THE MARCUS NORMAL AND INVERTED REGIONS

Fluorescence, photoacoustic calorimetry, and picosecond optical calori metry studies on the rigid donor-spacer-acceptor (DSA) molecule, 1, in alkane and ether solvents are reported. The charge transfer (CT) stat e of 1 is formed with unit quantum efficiency and decays by numerous p rocesses. Charge recombination from the CT state generates both the gr ound state and the triplet state of the naphthalene chromophore. The q uantum yield of triplet state formation decreases with an increase in the dielectric constant of the solvent. The CT state radiative, nonrad iative internal conversion (CT --> S0), and nonradiative intersystem c rossing (CT --> T1) rate constants are determined and are analyzed usi ng semiclassical, single quantized mode, electron transfer theory. Ind ependent estimates of Absolute value of V(CT-S0), the electronic coupl ing matrix element between the charge transfer state and the ground st ate, are obtained from the radiative and nonradiative rate constant da ta and are in reasonable agreement (1100-1400 cm-1). Absolute value of V(CT-T1) is estimated to be 1.8 cm-1. The solvent-dependent partition ing of the CT state between S0 and T1 is attributed to the large diffe rence in the respective reaction energetics.