Energy flow in push-pull chromophores:: Vibrational dynamics in para-nitroaniline

para-Nitroaniline (PNA) plays an essential role as the prototype model of push-pull chromophores. The nature and degree of participation of vibrational degrees of freedom in the charge-transfer and internal-conversion processes are current issues of great theoretical and practical importance. Ultrafast time-resolved anti-Stokes resonance Roman spectroscopy (TRARRS) experiments on PNA in dimethyl sulfoxide with three different excitation wavelengths were performed to probe these dynamical influences. The vibrational dynamics associated with S-0 were independent of incident wavelength, and this supports the picture that the S-1 dynamics are fast relative to the rate of intersystem crossing. The phenyl breathing mode nu(29) (860 cm(-1)) and the symmetric NO2 stretch nu(29) (1310 cm(-1)) exhibited vibrational lifetimes in S-0 of 8.1 and 5.2 ps, respectively. No evidence for inhomogeneous broadening of the charge-transfer band in the UV/Vis absorption spectrum was found.