Effects of diamine bridge length and substituents on the spectral properties of N,N′-bis(α-substituted salicylidene)diamines in solution

Absorption and fluorescence spectra of thirteen N,N'-bis(alpha-substituted salicylidene)diamines in solution were investigated with the intention of investigating the role of the substituent and diamine bridge length on their optical properties. The fluorescence efficiency was improved by an increase in the electron-donating property of the substituents on the azomethine carbon accompanied by an increase of the n-->pi* transition;absorption. However, the effect did not occur for the substituents on the azomethine nitrogen, in which no drastic changes in fluorescence efficiency could be observed. Through the investigation of the diamine bridge length effects, it was found that a diamine Schiff base seems to form neither an inter- nor an intramolecular dimer with any peculiar fluorescence in the solution even if it has a long methylene bridge. It was also suggested that the diamine Schiff base has a third fluorescence species in the excited state, which might be a pre-keto form, the existence of which is strongly affected by the hydrogen bond strength between the hydroxy and azomethine groups.