Relaxation of optically excited p-nitroaniline:: Semiempirical quantum-chemical calculations compared to femtosecond experimental results

p-Nitroaniline (pNB) was examined with the semiempirical SAM1 Hamiltonian in vacuo and in water. Geometry optimizations were performed in the ground and the lowest excited state along the -NH2 wagging and the -NO2 twisting coordinate. The latter is shown to play a key role in the spectroscopy and dynamics of pNA. The strong charge-transfer (CT) absorption band is conformationally broadened. Red-edge excitation prepares the CT state with a distribution of -NO2 conformations which is concentrated around the coplanar conformation with the benzene moiety. In water, C-amino and C-nitro stretching vibrations are also excited. The structural reorganization along those modes is assumed to occur on the same time scale as ultrafast polar solvation. In this case the relaxation dynamics in water after similar to 100 fs consists mainly in an evolution of the -NO2 twist distribution toward a deep minimum at the perpendicular conformation; the dipole moment change during that process has an upper limit of 2 D. Stimulated emission is observable for the first few hundred femtoseconds. Thereafter a multidimensional conical intersection with the,ground state is reached. The SI population decay can be monitored independent from the twisting process by observing excited-state absorption at 4 eV. All of these predictions are consistent with recent pump/supercontinuum probe measurements of PNA in water.