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, Delta G(S-0 --> CT), and of the solv ent reorganization energy, lambda(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 en ergies among the various solvents are attended by nearly identical magnitud e (but opposite sign) differences in the solvent reorganization energies. T his correlation is observed for solvents in which the most significant elec trostatic moment is a dipole or a quadrupole. (b) Solvents with nearly zero dipole moments but large quadrupole moments (8-11 D-Angstrom) solvate the CT state as effectively as moderately dipolar solvents (mu approximate to 1 -2 D). (c) Larger charge separation distances produce larger solvent reorga nization energies in the nonalkane solvents. The ratios of the solvent reor ganization energies lambda(S)(2)/lambda(S)(1) are roughly the same in the d ipolar and quadrupolar solvents. (d) Changes in both Delta G and lambda(S) upon increasing the temperature are consistent with a decrease in the solve nt polarity. The absolute values of the temperature derivatives lie between 0.5 and 2.0 meV/K. In contrast to the correlated variation of Delta G(S-0 CT) and lambda(S) from solvent to solvent (i.e., Delta G(solvent A) - Delta G(solvent B) approximate to -(lambda(S,solvent A) - lambda(S,solvent B)), the ratio (partial derivative lambda(S)/partial derivative T)/(partial deri vative Delta G/partial derivative T) similar to -(0.7 - 0.9). A simple cont inuum model, using dielectric constant data, is unable to reproduce the sol vent and temperature dependence of Delta G(S-0 --> CT) and lambda(S). A mor e detailed molecular model produces reasonable estimates of these two quant ities across a wide range of solvents at 300 K but overestimates their temp erature variation.