Electronic spectrum of bibenzimidazole homologue: effects of solvents and acid concentration

Abstract

The absorption and fluorescence spectra of bibenzimidazole (BBI), N, N′-dimethylbibenzimidazole (MBBI), and methylene, 2,2′-bibenzimidazole (MtBBI) have been recorded in six solvents of different characteristics and at various acid concentrations. This study indicates the presence of intramolecular hydrogen bonding in BBI in both S0 and S1 states, leading to large conjugation of two benzimidazole (BI) rings. The geometry of BBI is the same in the S0 and S1 states. The spectral characteristics of BBI are insensitive to the solvents. The interaction of solvent molecule attached to the nitrogen lone pair and methyl group rotates the benzimidazolyl ring in MBBI around the single bond. This leads to a blue shift in the long wavelength absorption band maximum and a decrease in the intensity of this band. The two MBI rings are coplanar in the S1 state. Presence of methylene group inhibits the direct interaction between the BI groups and these rings behave almost independently, i.e., the spectral characteristics of this molecule nearly resemble those of benzimidazole. Keywords: absorption spectrum, fluorescence spectrum, pKa, bibenzimidazoles, excited state pKa.