Femtosecond relaxation of photoexcited para-nitroaniline: solvation, charge transfer, internal conversion and cooling

The ultrafast relaxation of p-nitroaniline (PNA) in water and acetonitrile is studied experimentally and theoretically. Transient absorption spectra are measured by the pump–supercontinuum probe technique (PSCP) after 50 fs excitation at 400 nm. The relaxation includes several stages with distinct time scales: solvation, intramolecular charge transfer (CT), internal conversion and cooling. The spectral evolution before 100 fs reflects mainly solvation with dynamic Stokes shift of 3500 cm−1 in acetonitrile and 4000 cm−1 in water. CT and internal conversion are governed by twisting of the –NO2 group and proceed in water with 120 and 250 fs, respectively. A hot ground state upon internal conversion is characterized by an initial temperature of 1400 K. The subsequent solute–solvent energy transfer is characterized by exponential behavior between 1 and 3 ps and by a nonexponential decay at longer delays, the solute cooling time lies in the range 0.85–1.3 ps.