A Spectroscopic Study of Solvent Reorganization Energy:  Dependence on Temperature, Charge Transfer Distance, and the Type of Solute−Solvent Interactions

The dependence of the free energy gap, Δ G (S 0 → CT), and of the solvent reorganization energy, λ S, on solvent, donor/acceptor separation, and temperature are determined from analyses of the intramolecular charge transfer absorption and emission bands from 1 and 2 . The following trends are observed: (a) for either probe molecule, differences in the CT state energies among the various solvents are attended by nearly identical magnitude (but opposite sign) differences in the solvent reorganization energies. This correlation is observed for solvents in which the most significant electrostatic moment is a dipole or a quadrupole. (b) Solvents with nearly zero dipole moments but large quadrupole moments (8−11 D-Å) solvate the CT state as effectively as moderately dipolar solvents (μ ≈ 1−2 D). (c) Larger charge separation distances produce larger solvent reorganization energies in the nonalkane solvents. The ratios of the solvent reorganization energies λ S (2 )/λ S (1 ) are roughly the same in the dipolar and quadrupolar solvents. (d) Changes in both Δ G and λ S upon increasing the temperature are consistent with a decrease in the solvent polarity. The absolute values of the temperature derivatives lie between 0.5 and 2.0 meV/K. In contrast to the correlated variation of Δ G (S 0 → CT) and λ S from solvent to solvent (i.e., Δ G solvent A − Δ G solvent B ≈ −(λ S,solvent A − λ S,solvent B ), the ratio (∂λ S /∂ T )/(∂Δ G /∂ T ) ∼ −(0.7−0.9). A simple continuum model, using dielectric constant data, is unable to reproduce the solvent and temperature dependence of Δ G (S 0 → CT) and λ S . A more detailed molecular model produces reasonable estimates of these two quantities across a wide range of solvents at 300 K but overestimates their temperature variation.