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Dual laser emissions were observed from fluorescein-Na and eosin-B in ethanolic solutions individually in the concentration range from 10−2 to 10−3 mol dm−3 under N2 laser excitation. The first compound was found to lase at two distinct regions with wavelength maxima around 540, 550 nm, while the second one around 558, 574 nm. Steady-state absorption, fluorescence excitation, fluorescence polarization, fluorescence emission and decays of the dyes in various solvents under varying conditions of excitation and detection systems were carried out to identify the nature of the emitting species responsible for laser emissions in two distinct regions. Both the dyes exhibited concentration and excitation wavelength dependence of fluorescence and the effects were found to be more pronounced in binary solution. The fluorescence decays of dyes were monoexponential in ethanol, while in some other solvents used, the decays showed biexponential behavior. The absorption and excitation studies using thin layers of solutions revealed the formation of dimers with the dye concentration around 1×10−3 mol dm−3. Fluorescence polarization and decay studies confirmed the presence of dimers. The two laser bands observed in the shorter and longer wavelengths were respectively ascribed to monomeric and dimeric species.

The fluorescence quenching of 5-methyl-3-phenyl-2-[4′-amino-s-triazolo-3′-yl] indole-5′-hydrazide (MPIH) by aniline quencher in cyclohexane, benzene, toluene and dioxane solvents has been carried out at room temperature with a view to understand the quenching mechanisms. The quenching is found to be appreciable and shows positive deviation in the Stern–Volmer plots. The various quenching parameters have been determined using modified Stern–Volmer equation, which suggests that the sphere of action static quenching model agrees very well with the experimental results and this positive deviation is attributed to the static and dynamic quenching. Further, with the use of the finite sink approximation model, it is concluded that these bimolecular quenching reactions are diffusion limited and the distance parameter R′ and mutual diffusion co-efficient D are estimated independently.