Solvent reorganization around a "giant dipole" molecule

The fluorescence risetimes of the ¹L_a and ¹L_b excited states of p -dimethylaminobenzonitrile (DMAB) in ethanol and methylcyclohexane following irradiation at room temperature by the fourth harmonic (2650 A) of a Nd^{3 +} glass laser were measured directly using picosecond spectroscopy. The onset of ^!L _b fluorescence, governed by prior vibronic relaxation in both polar and nonpolar solvents, occurs within 7-20 psec after the exciting pulse. The ¹L _a fluorescence, characterized by a noticeably longer risetime of 40 psec at room temperature, yields directly the solvent orientational relaxation time in the field of the '£ state's large dipole moment (23 D). This measured relaxation time is consistent with a modification of the Debye theory of viscosity-limited solvent reorganization.