Investigation of the relationship of the excited state geometry change and the solvent dependent fluorescence of 9-methyl anthroate

Due to a geometry difference, the polarity of the first excited singlet state of 9-methyl anthroate is shown to be greater than that of the ground state. This polarity difference results in solvent dependent fluorescence properties for this molecule. From the Stokes shift variations of 9-methyl anthroate fluorescence in polar aprotic solvents, the excited state dipole moment is 4.5 D larger than the ground state value. In protic solvents, the Stokes shifts are larger than in polar aprotic solvents indicating a large hydrogen bond contribution to excited state solvation in the former solvents. In non-polar and some polar aprotic solvents, the fluorescence quantum yield (Øf) of 9-methyl anthroate is greater than that of anthracene. It is suggested that the first excited singlet lies at an intermediate position between the first and second triplet states. Thus, the intersystem crossing process to either triplet states. Thus, the intersystem crossing process to either triplet is inhibited and the Øf increases. The Øf of 9-methyl anthroate decreases with increasing solvent polarity. This quenching effect is greatest in the protic solvents. Fluorescence lifetime and Øf data indicate that the quenching in protic solvents is the result of increased rates of radiationless decay. A strong hydrogen bond interaction between the solvent and the excited state inducing an increase in the rate of internal conversion is the likely mode for non-radiative decay in protic solvents