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

Resonance Raman intensities of p-nitroaniline, a prototypical "push-pull" c hromophore with a large first hyperpolarizability (beta), have been measure d in dilute solution in five solvents having a wide range of polarities (cy clohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at exc itation wavelengths spanning the strong near-ultraviolet charge-transfer ab sorption band. The absolute Raman excitation profiles and absorption spectr a are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principa l Raman-active vibrations as well as estimates of the solvent reorganizatio n energies. The total vibrational reorganization energy decreases and the s olvent 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 obta ined from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a grou nd-state density functional theory calculation. The geometry changes upon e lectronic excitation involve predominantly the C-phenyl-N-nitro, N-O, and p henyl C-2-C-3 bond lengths, with little involvement of the amino group. Non resonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acet one, acetonitrile, and methanol show only a very small solvent dependence o f the vibrational frequencies. This suggests that changing the solvent affe cts the excited state more than the ground state, calling into question two -state models that treat the ground and charge-transfer excited states as l inear combinations of neutral and zwitterionic basis states with solvent de pendent coefficients. (C) 2001 American Institute of Physics.