Temperature effects on excitation and deactivation processes of coumarin 102. A comparison with coumarin 153

• Absorption and emission of coumarin 102 in function of temperature are reported.
• Temperature effects on fluorescence kinetics of coumarin 102 are shown.
• Temperature dependence of Coumarins 102 and 153 fluorescence anisotropy is shown.
• Hydrogen bonding and temperature affect coumarin 102 photophysical properties.
• Temperature affects coumarin 102 properties differently than those of coumarin 153.

The temperature effects on coumarin 102 steady-state absorption and fluorescence, and on the decay of its excited state are presented. Our results have shown that similar to coumarin 153, hydrogen bonding between coumarin 102 and protic solvents has an important impact on the deactivation process of the excited state of this dye. But the nature of this hydrogen bonding and especially its temperature dependence is clearly different for these coumarins. This difference leads in consequence to totally different temperature dependencies of the fluorescence lifetime and fluorescence quantum yield of both solutes. Direct experimental evidence is presented that different hydrogen bonding abilities of structurally similar coumarins may lead to distinct photophysical properties both in the ground, excited Franck–Condon and relaxed states, as well as at various temperatures. To obtain a better understanding of the differences between hydrogen bonds formed by coumarin 102 and by coumarin 153 in the excited state, fluorescence anisotropy kinetic measurements as a function of temperature were performed for both dyes as well in one highly protic solvent. Fluorescence anisotropy decays provided experimental evidence that rotation of coumarin 102 is slower than that of coumarin 153 due to stronger hydrogen bonds formed by the former dye, and that for the same reason in coumarin 102 rotation slows down more quickly than in coumarin 153 with decreasing temperature.