P. Valat, V. Wintgens, J. Kossanyi
Sep 19, 2001
Citations
0
Influential Citations
9
Citations
Journal
Helvetica Chimica Acta
Abstract
The fluorescence behavior of 2-(3-fluorophenyl)-2,3-dihydro-1H-benzo[f]isoindole-1,3-dione (1) was studied in solvents of different polarity and viscosity. Dual luminescence is observed and the short-wavelength emission is found to increase considerably with the solvent polarity. The ratio of the fluorescence quantum yield of the two states emitting, the one (SW*) at short wavelength and the other (LW*) at long wavelength, shows a bell-shaped dependence on the reciprocal of the temperature in diethyl ether, butyronitrile, and propane-1,2,3-triol triacetate (glycerol triacetate; GTA). This has been interpreted as the result of a reversible interconversion between the two states. The enthalpy difference between the SW* and LW* excited states, as deduced from the slope of the ln (Φ/Φ) vs. 1/T curves in the high temperature range, is found to be solvent polarity and solvent viscosity independent as the same value (−7.3 kJ/mol) is obtained in the three above-mentioned solvents. The independence from polarity is the consequence of a similar difference in dipole moment between the ground-state and the SW* and LW* excited states (4.5 and 4.9 D, respectively, derived from solvatochromy). The activation energy of the SW*LW* step deduced from the low temperature measurements in the nonviscous solvents, increases with solvent polarity (11.6 and 17.5 kJ/mol for diethyl ether and butyronitrile, respectively); they are greater than the viscous-flow activation energy of the solvents indicating that the resolvation of the excited dipole controls the kinetics. In the nonviscous solvents, the LW* state originates from the SW* state, while in the viscous GTA solution, both states are formed simultaneously within the 1-ps laser pulse.