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 S 0 and S 1 states, leading to large conjugation of two benzimidazole (BI) rings. The geometry of BBI is ...