Solvent effects on the fluorescent emission of some new benzimidazole derivatives

The benzimidazole moiety is present in a great variety of pharmaceutical compounds with anthelmintic action and other applications. These compounds exhibit native fluorescence which often depends on the substituents on the benzimidazole ring and the solvents. The fluorescence emission also changes with pH. In this article the solvatochromic effect is described in different solvents (polar protic and aprotic, alkanes, pyridine and acetic acid) of a series of new benzimidazole derivatives, namely 1-methyl-6-nitro-benzimidazole (6-NBI), 1-methyl-6-aminobenzimidazole (6-ABI) and 1-methyl-6-(p-tolylamino)benzimidazole (6-TABI). The fluorescence intensity is weaker for 6-NBI than for the other derivatives in all solvents tested due to quenching of fluorescence emission by the nitro group. The excitation and emission wavelengths are red shifted for 6-TABI with respect to those observed for 6-NBI and 6-ABI, due to the presence of the tolyl group. The fluorescence emission maximum is shifted for 6-ABI from λem = 362 nm in formic acid to λem = 470 nm in pyridine; this behaviour is explained in terms of the acid-base characteristics of these compounds. A remarkable shift in the emission wave-length can be also observed for 6-ABI with changing solvent polarity from 354 nm in alcohols to 420 nm in dimethyl sulphoxide. This behaviour could be related to the different equilibria that take place in the excited state, i.e., acid-base equilibria and also charge transfer processes, depending on the solvents.