[Rotational Isomers of Diphenhydramine].

Diphenhydramine (DP), an antihistaminic agent, may become colored and daker or more fluorescent during storage. Herein, we spectroscopically examined the causes of this phenomenon under various DP storage conditions and durations. The infrared vibration-rotation spectrum shows multiple Gauche (G)-type conformers with different intramolecular n→π* interaction strengths. The splitting pattern of the dimethylamino group protons in the 1H-NMR spectrum indicates that DP is mainly in the G-type with a small portion in the Trans (T)-type. The correlation between the red-shifted peak intensity in the UV•VIS absorbance spectrum and the coloring progression indicates a decreased intramolecular n→π* interaction of the G-type under elevated temperature during storage. Enhanced fluorescence detected in the Excitation•Fluorescence spectrum demonstrates G-type (quenching) to T-type (fluorescent) conformation conversion, which is due to activated internal rotation of the dimethylamino group under elevated storage temperature and electronic excitation in the phenyl groups under light irradiation during storage. A signal detected in the ESR spectrum corresponds to the G-type charge transfer (CT) structure wherein part of the nonbonding electron pair on the N atom is intramolecularly redistributed to the phenyl groups. The CT structure presents the G-type quenching characteristics, whereas weak CT bonding corresponds to coloring. The results indicate that the quenching G-type is converted to T-type by heat or light to become color faded and bright with enhanced fluorescence and that T-type is reverted to G-type after storage under cool and dark conditions or by vacuum distillation to lose fluorescence.