Ultrafast dynamics of malachite green at the air/water interface studied by femtosecond time-resolved electronic sum frequency generation (TR-ESFG): an indicator for local viscosity

Abstract

The ultrafast excited-state dynamics of malachite green (MG) at the air/water interface was studied with a newly-developed interface-selective nonlinear optical technique: femtosecond time-resolved electronic sum frequency generation (TR-ESFG) spectroscopy. MG is known as a viscosity indicator because of its well-known viscosity-dependent dynamics in bulk solution. Femtosecond time-resolved electronic spectra of MG at the interface were obtained with nearly the same quality as that of conventional time-resolved absorption spectra in the bulk. The time constants of the ground-state recovery at the air/water interface were determined and compared with those in bulk glycerol–water mixtures. It was found that the ground-state recovery of MG at the air/water interface is much slower than that in bulk water and almost equivalent to that in the glycerol–water mixture containing glycerol volume fractions as high as 65–80%. This result indicates higher local viscosity at the interface than in bulk water. However, it is possible that changes of the molecular properties, e.g. a change of shape of the potential energy surface, of MG at the interface also contribute to the apparently very large viscosity values evaluated. On the basis of the present results, we discuss the applicability of probe molecules to evaluate the physicochemical properties of the liquid interface and note the importance of the extrinsic and intrinsic factors that make the chemical process at liquid interfaces different from that in bulk solution.