Ultrafast solvation dynamics at silica/liquid interfaces probed by time-resolved second harmonic generation

The static absorption and ultrafast solvation dynamics of a tricarbocyanine dye IR144 at the silica/acetonitrile and the silica/butanol interfaces wer e investigated by using steady-state second harmonic generation (SHG) spect roscopy and pump-probe time-resolved SHG (TRSHG) experiments. At both inter faces, the peak wavelength for the S-0 --> S-1 transition of adsorbed IR144 is significantly red-shifted with respect to the maximum absorption wavele ngth in the corresponding bulk solvent. Because IR144 is negatively solvato chromic, this shift is indicative of a smaller polarity at the interface. A theoretical model of TRSHG is discussed which allows extraction of the sol vation dynamics correlation function from the time dependence of the TRSHG signal measured in the pump-probe experiment at a chosen probe wavelength. The time constant for the diffusive component of solvation at the silica/ac etonitrile interface was found to be 1.05 +/- 0.14 ps. It is somewhat short er, although of the same order of magnitude, than the time constant for the diffusive solvation in bulk acetonitrile, 2.23 ps. At the silica/butanol i nterface, a time constant of 1.20 +/- 0.15 ps was extracted, whereas in the bulk butanol, the corresponding diffusive solvation is biexponential with times of 3.5 and 33 ps.