Effect of Salt on the Excited-State Dynamics of Malachite Green in Bulk Aqueous Solutions and at Air/Water Interfaces: a Femtosecond Transient Absorption and Surface Second Harmonic Generation Study

Abstract

The ultrafast excited-state dynamics of Malachite Green (MG) in bulk aqueous solutions and at air/water interfaces, in particular the effect of the presence of various sodium salts in the aqueous phase, has been investigated by transient absorption and surface second harmonic generation. In bulk solutions, a slowing down of the ground-state recovery that can be unambiguously ascribed to the formation of aggregates of various sizes is observed at high (>0.3 M) salt concentrations only, with the exception of NaSCN where an effect is already found at 0.05 M. At the interface, small amounts of salt result in two effects: 1) an increase of the stationary surface second harmonic signal and 2) a slowing down of the ground-state recovery of MG. These phenomena are explained by the formation of aggregates due to an increase of the interfacial MG concentration upon addition of salt. The dependencies of both effects on salt concentration are correlated and vary with the anion as SCNThis order is almost the opposite as that in the Hofmeister series for the salting-out strength.

Local Viscosity of Binary Water+Glycerol Mixtures at Liquid/Liquid Interfaces Probed by Time-Resolved Surface Second Harmonic Generation

“…Although in our previous work, 24 we suggested that some features of the DADS found in transient absorption spectra of MG are consistent with S x being a vibrationally hot ground state, we believe that the arguments based on the current results prevail.…”

“…In previous studies, both single 21 and biexponential 24,26,48 decays of the SSHG signal have been observed with MG at various interfaces. The faster component is always identified with the decay of the S 1 state of monomeric MG molecules.…”

“…24 They show a clear dependence of the SSHG signal kinetics on the dye concentration (Figure 8a), the decay being slower at higher concentrations.…”

“…This observation supports the identification of the slower component as the decay of the excited state of aggregates adsorbed at the surface and indicates that, above a certain dye concentration, a saturation regime is reached: the increase of the bulk concentration does not cause the increase of the relative concentration of aggregates at the interface. The concentration dependence was not revealed by the previous studies 24 of air/ water interfaces, either because this effect is intrinsic to liquid/ liquid interfaces, or because those measurements were carried out at concentrations above the saturation threshold. The interpretation of the slower decay time as the relaxation of excited aggregates means that the SSHG signals originating from monomeric molecules decay monoexponentially, whereas for the ground state recovery biexponential kinetics are expected (the first step of the excited state deactivation, corresponding to the shortest time constant τ 1 seen in the transient absorption measurements would not be resolved with the temporal resolution of our SSHG setup).…”

“…[19][20][21][22][23][24] In many of the reported time-resolved experiments, physical properties of the interfacial region were investigated by analyzing electronic relaxation of malachite green (MG, Figure 1a), a dye which belongs to the family of the triphenylmethane (TPM) dyes, 25 adsorbed at the interface. [19][20][21]24,26,27 The first singlet * To whom correspondence should be addressed. E-mail: Eric.Vauthey@ unige.ch.…”