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

Abstract

The excited-state relaxation of malachite green and brilliant green in solvents of various viscosity has been investigated at liquid/liquid interfaces and in bulk solutions by surface second harmonic generation and transient absorption spectroscopy. Mixtures of water and glycerol in various proportions have been used as solvents of variable viscosity. Transient absorption measurements in bulk revealed that both dyes are suitable as a probe of local viscosity for water+glycerol mixtures and that two of three processes following the optical excitation exhibit the same power dependence on solvent viscosity. This observation leads to assignment of the processes to a twist and twist-back of the aromatic rings attached to the central carbon atom of the dye. Therefore, identification of the intermediate state observed in the radiationless deactivation pathway with the twisted form of the dye has been supported. The time profiles of the second harmonic signal recorded at water+glycerol/ dodecane interfaces have been found to be monoexponential at low dye concentrations (below 10 -5 M) and biexponential at higher concentrations, and therefore the origin of the slower component has been attributed to the relaxation of dye aggregates adsorbed at the interface. The decay times measured at interfaces increased with increasing amount of glycerol in the mixture, but the rise was slower than in bulk solution. Therefore, the viscosity at the interfacial region, higher than that of the bulk solution, is mainly determined by structural modification of the solvent resulting from interactions between the two liquids that constitute the interface and addition of glycerol affects viscosity, only to a lesser extent. We have also shown that if the viscosity of the upper layer is much higher (at least 1 order of magnitude) than that of water or short alkanes, a slowdown of the relaxation is observed. This contradicts earlier findings and means that large amplitude motion of all three rings is involved in the deactivation of the excited molecule, but the rotation of the phenyl ring, which is smaller than the alkyl-substituted aniline groups, becomes a bottleneck for the relaxation in very viscous environments.

Nonradiative Deactivation of Excited Hemicyanines Studied with Submolecular Spatial Resolution by Time-Resolved Surface Second Harmonic Generation at Liquid−Liquid Interfaces

“…Although its interfacial composition might differ from the bulk one, the glycerol/water mixture has already proven to be an efficient system for tuning interfacial friction. 24 For both dyes, the measured TRSSHG profiles were independent, within the experimental error, of the glycerol content of the mixture (Figure 2A). With DHASP, the long lifetime remains around 125 ps, whereas it is of the order of 240 ps with DBASP, and with both dyes its relative amplitude stays around 0.75.…”

Development of Electronic Sum Frequency Generation Spectroscopies and Their Application to Liquid Interfaces

“…53−70 It is also noteworthy that a simpler fitting analysis than the present analysis based on eqs 10 and 11 is often employed in time-resolved SHG studies. 58,65 Such a treatment is inadequate in the case that multiple transient species appear such as in the case of photodissociation of the R800 dimer. ESFG and SHG provide |χ (2) | 2 , not χ (2) itself, because the intensity of the second-order nonlinear signal light is detected.…”

Photoinduced Electron Transfer Reactions: From the Elucidation of Old Problems in Bulk Solutions Towards the Exploration of Interfaces

“…[37] The properties of liquid solutions, where and Malachite Green (MG). [42,43] The first singlet excited state (S 1 ) of these dyes decays mainly through a non-radiative channel involving a torsional motion of the phenyl rings. The S 1 lifetime depends strongly on the viscosity of the environment.…”

“…Similar experiments have also been performed at alkane-(water-glycerol) interfaces. [43,44] Information on other interfacial properties can be obtained by performing TRSSHG measurements with appropriate probe molecules. For example, the excitedstate lifetime of Eosin B has been found to strongly depend on its ability to form hydrogen bonds with surrounding solvent molecules.…”