Electronic absorption and fluorescence of 9-anthraldehyde and its conjugate cation

The appearance or lack of vibrational structure in the (1)L(a) and (1)L(b) absorption bands of the neutral species and cation derived from 9-anthraldehyde is explained in terms of adherence to or violation of the Franck-Condon principle. The lack of vibrational structure is attributed to the motions of the aldehyde group during the course of electronic transition, a process which destroys vibration quantization in the Franck-Condon excited state. The appearance of well-defined vibrational structure in the (1)L(a) band of the cation establishes that intramolecular charge-transfer alone, without accompanying vibrational or solvent relaxation which is contemporaneous with electronic transition, is not responsible for the lack of structure in most intramolecular charge-transfer transitions. The previously reported fluorescence of 9-anthraldehyde in hydroxylic solvents is found to be in error and is likely to originate from an impurity produced by photo-reduction. The actual fluorescence of 9-anthraldehyde is at much lower frequency than previously reported.