Environmental Effects on the Absorption and Fluorescence Spectral Characteristics of Benzimidazole-2-carboxylic Acid and Its Esters

Article abstract of DOI:10.1246/bcsj.62.2668

The absorption and fluorescence spectra of benzimidazole-2-carboxylic acid (BIA) and 5-chlorobenzimidazole-2-carboxylic acid (CBIA) have clearly indicated that benzimidazole (BI) ring and -COOH group are coplanar to each other and are held together in a rigid frame by intramolecular hydrogen-bonding.Dual fluorescence is observed in polar and hydrogen bonding solvents.The short wavelength fluorescence band is assigned to a structure where the -COOH group is perpendicular to the BI moiety and the long wavelength fluorescence band to the planar configuration.This is further manifested from the spectral characteristics of the ester of 5-chlorobenzimidazole-2-carboxylic acid (CBIM).Various prototropic reactions taking place in S₀ ans S₁ states for all the three molecules are studied in the H₀/pH/H_- range of -10 to 16.All the dissociation constants are determined, both in the S₀ and S₁ states and are discussed.