Solvent reorganization around a ``giant dipole'' molecule

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Abstract
The fluorescence risetimes of the 1La and 1Lb excited states of p‐dimethylaminobenzonitrile (DMAB) in ethanol and methylcyclohexane following irradiation at room temperature by the fourth harmonic (2650 Å) of a Nd3+ glass laser were measured directly using picosecond spectroscopy. The onset of 1Lb fluorescence, governed by prior vibronic relaxation in both polar and nonpolar solvents, occurs within 7–20 psec after the exciting pulse. The 1La fluorescence, 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 the 1La state's large dipole moment ([inverted lazy s] 23 D). This measured relaxation time is consistent with a modification of the Debye theory of viscosity‐limited solvent reorganization.