A Comparison of the Solvent and Temperature Dependence of the Fluorescence Quantum Yield of Methyl-1-, Methyl-2-, and Methyl-9-Anthroate

Two flouescence parameters, the room temperature quantum yield and the activation energy for nonradiative decay, were studied for the three possible monosbstituted methyl esters of anthroic acid in a number of aprotic, nonaromatic solvents. For the 1 and 2 substituted esters, the quantum yield increased and went through a maximum with decreasing S1 energy. In contrast, for methyl-9-anthroate, a general monotonic decrease in quantum yield with decreasing S1 energy was found. We assume, that intersystem crossing is the only nonradiative decay process that competes with flourescence for the esters. Since we find that the esters show temperature dependent flourescence, intersystem crossing must occur by thermal activation to a triplet enery level (Tx) that lies higher in enery than the first singlet excited state (S1)