Solvent reorganization around a ``giant dipole'' molecule

摘要

The fluorescence risetimes of the1Laand1Lbexcited states ofphyphen;dimethylaminobenzonitrile (DMAB) in ethanol and methylcyclohexane following irradiation at room temperature by the fourth harmonic (2650 Aring;) of a Nd3plus;glass laser were measured directly using picosecond spectroscopy. The onset of1Lbfluorescence, governed by prior vibronic relaxation in both polar and nonpolar solvents, occurs within 7ndash;20 psec after the exciting pulse. The1Lafluorescence, characterized by a noticeably longer risetime of inverted lazy s 40 psec at room temperature, yields directly the solvent orientational relaxation time in the field of the1Lastate's large dipole moment (inverted lazy s 23 D). This measured relaxation time is consistent with a modification of the Debye theory of viscosityhyphen;limited solvent reorganization.