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 were investigated by using steady-state second harmonic generation (SHG) spectroscopy and pump−probe time-resolved SHG (TRSHG) experiments. At both interfaces, the peak wavelength for the S 0 → S 1 transition of adsorbed IR144 is significantly red-shifted with respect to the maximum absorption wavelength in the corresponding bulk solvent. Because IR144 is negatively solvatochromic, this shift is indicative of a smaller polarity at the interface. A theoretical model of TRSHG is discussed which allows extraction of the solvation 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/acetonitrile interface was found to be 1.05 ± 0.14 ps. It is somewhat shorter, 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 interface, 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.