Abstract
Studies of the dependence of methine dye absorption and fluorescence properties on the molecular composition of the dyes, on solvent polarity and on the temperature of the sample were made. The results have confirmed our recent suggestion that the emission band of methine dyes in polar solvents is composed of molecular and excimer fluorescence. The spectral properties of different types of molecular composition of methine dyes indicated the centre in the dye molecule involved in excimer formation. On the basis of a liquid nitrogen temperature spectrum and the lifetimes of fluorescence it is suggested that this type of methine dye which can form the excimers has a strong tendency to aggregate in the ground states. These aggregates are able to quench the molecular fluorescence. An increase of solvent polarity causes disaggregation of dye molecules giving an increase of molecular fluorescence and an increased probability of excimer formation. Thus, in polar solvents the long-wavelength emission band arises.
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