SOLVATOCHROMISM OF A TYPICAL MEROCYANINE - STILBAZOLIUM BETAINE AND ITS 2,6-DI-TERT-BUTYL DERIVATIVE

The absorption and emission spectra of DTBSB (the 2,6-di-tert-butyl derivative of stilbazolium betaine (SB)), in its neutral and protonated forms, have been measured in 28 solvents. Those corresponding to the parent SB (4'-hydroxy-1-methylstilbazolium betaine or 4-[2-(1-methyl-1,4-dihydropyridinylidene)ethylidene]cyclohexa-2,5-dien-1-one)) have been recorded in the solvents necessary for comparison. The effect of the ortho bulky groups is to efface the effect of the solvent acidity. The remaining polarity effect on the solvatochromism is very weak but is nevertheless always negative, even in apolar solvents. In consequence, the behavior of SB is not in contradiction with Onsager's models of general solvent effects. Stilbazolium betaines cannot be used as probes of solvent polarity due both to their relative insensitivity and to the complexity of the first band of absorption which presents in some solvents a well-resolved coarse structure. This structure disappears by interaction with acid or polar solvents and by protonation on the carbonyl group.