Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of the molecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has been previously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this paper show that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Having the advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtained from fluorescence decay measurements, making it feasible to probe systems with lower viscosity. 1. Introduction Fluorescent stilbenoid dyes are among the classes of chromophores most widely used in staining biological samples to be investigated by fluorescence microscopy methods [1–4]. Stilbenoid dyes show complex excited state dynamics, such as isomerisation processes, which are often accompanied by the emergence of charge transfer states [2, 5–9]. An important consequence of these dynamics is that their fluorescence properties are particularly sensitive to the local environment, which not only increases the contrast between sample features, but also delivers specific information on local properties. In the particular case of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide), solvent polarity as well as viscosity can influence the fluorescence properties of this dye. To a good approximation, the peak of the emission spectrum can be used as an indicator for the polarity of its surroundings, while the fluorescence lifetime varies with the viscosity of the local environment [8, 9]. It has been previously demonstrated that the fast molecular dynamics processes in DASPMI, such as the rearrangements of the dye upon transitions to intramolecular charge transfer states, are sensitive to the local viscosity [2, 10, 11]. In particular, the fluorescence lifetime of DASPMI is quite sensitive to the local viscosity, making fluorescence lifetime measurements a method of choice to probe the microheterogeneous environments in confining media such as sol-gel based or biological systems [10, 12, 13]. Previous studies have shown that when DASPMI is incorporated in sol-gel-derived media, the lifetime of the fastest decay component can increase up to a hundred fold while the lifetime of the longer decay components tend
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