The interpretation of temperature effect on fluorescence emission is rather complicated and not well understood. Several established temperature influences are not the most likely contributors to the non-radiative decay process. In order to understand the effect of temperature in the excited state, the fluorescence emission spectra of 4-methyl-7-methoxy (I), 4-methyl-5,7-diethoxy (II), 4-methyl-5-ethoxy-7-methoxy (III) and 4-methyl-7,8-diethoxy (IV) coumarins in aqueous solution as well as in benzene solution have been studied at different temperatures. It is observed that the fluorescence intensity decreases with increase in temperature for the first three coumarins but remains constant in the case of 4-methyl-7,8-diethoxy coumarin (IV). The radiative life-time and the activation energy for the deactivation process have been estimated. It seems that intramolecular quenching by the substituent group plays an important role in the deactivation process. The possible physical interpretation of the quenching mechanism is discussed in terms of the position and the nature of the substituent groups which could twist about the connecting bonds leading to a non-planar non-fluorescent state of the molecule.
