Temperature effect on fluorescence ability of cationic symmetric polymethine dyes

A temperature dependence of fluorescence of cationic polymethine dyes DODC and HIDC in methanol, water, toluene, and styrene is studied. An increase with temperature even within a narrow range (from 20 to 60 degrees C) is shown to significantly reduce the fluorescence intensity for all the dyes investigated. This reduction is the most prominent (threefold) in styrene and the least significant (1.8-fold), in methanol. The reason of this discrepancy is discussed. Based on quantum-mechanical calculations of the electronic structure of DODC and HIDC in the ground and excited states the observed temperature effect is accounted for by an increase in the probability of rotation about the polymethine chain bonds due to reduction of their order upon excitation.