Solvent effects on ground and excited electronic state structures of p-nitroaniline

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the ${\mathrm{C}}_{\mathrm{phenyl}}{\mathrm{–N}}_{\mathrm{nitro}},$ N–O, and phenyl ${\mathrm{C}}_{\mathrm{2}}{\mathrm{–C}}_{\mathrm{3}}$ bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients.