Frequency dependence of vibrational energy relaxation and spectral diffusion of the N-H stretching band of pyrrole-base complexes in solution.

A study on the vibrational dynamics of the NH stretching mode of pyrrole-base complexes in carbon tetrachloride, using subpicosecond infrared pump-probe (PP) spectroscopy, is reported. The time evolution of the PP signal of the NH stretching mode for all the complexes was frequency-dependent; the signal decay time increased with the frequency. This frequency dependence was thought to originate from the relationship between vibrational energy relaxation (VER) and spectral diffusion. For hydrogen-bonded systems, spectral diffusion corresponds to the reorganization of the solvent environment. Qualitative analysis of the frequency dependence of the PP signal decay time indicated that a simple energy gap law could not be applied to all the pyrrole-base complexes. This conclusion was supported by spectral simulation of the PP signal using the modified Smoluchowski equation to clarify the frequency dependence of the VER and the spectral diffusion.