Shift of the emission band upon excitation at the long wavelength absorption edge. II. Importance of the solute–solvent interaction and the solvent reorientation relaxation process

Abstract

Fluorescence and phosphorescence of quinine and its related compounds such as 6-methoxyquinoline are found to exhibit a red shift when excitation is achieved at the long wavelength edge of the first absorption band, and this anomalous shift is investigated in detail. The anomalous shift is observed not only in room temperature fluorescence, but also in low temperature fluorescence and phophorescence. The term edge excitation red shift (EERS) is introduced. After various plausible interpretations we have suggested a logical mechanism for EERS. In the suggested mechanism, EERS is observed if the solvent reorientation relaxation rate is small compared with the emission rate, and the Franck–Condon solvation energy is reasonably large with respect to solvent orientation. All the experimental observations are satisfactorily interpreted within the framework of the suggested mechanism.