Effect of bulky substituents in the donor and acceptor terminal groups on solvatochromism of Brooker's merocyanine

Stilbazolium merocyanine dyes with tert-butyl substituents in the donor terminal group were synthesized for the first time. Their absorption and fluorescence spectra in a series of solvents of various polarities were compared with those of Brooker's merocyanine and its known derivative with the same bulky substituents in the acceptor terminal group. Hereby, the effects of nucleophilic and electrophilic solvation on their solvatochromism have been discriminated, revealing that the former, although it has lesser impact in this instance, can be an essential factor in such solvents as pyridine and DMF. It has also been found that fluorescence of the studied dyes increases in protic methanol. However, when the electrophilic solvation of the carbonyl is hindered by tert-butyl groups, the fluorescence of stilbazolium merocyanines decreases and is maximal in more viscous solvents instead of methanol. A hypothesis about the participation of the n*-states in their fluorescence quenching has been verified by the (PCM)/TD-B3LYP calculations.