Synthesis, characterization, optical properties and theoretical calculations of blue light-emitting 1-alkyl-2-substituted benzimidazoles

A series of novel blue light-emitting 1-alkyl-2-substituted benzimidazoles (4a-4g) were synthesized by reactions of the 2-substituted benzimidazoles (3a-3e) with ethyl bromide or n-butyl bromide as electrophilic reagent. Compounds 3a, 3c, 3e were synthesized by the condensation of 1a-1c with 1,2-diamino benzene o-phenylene diamine using air as oxidizing agent and compounds 3b, 3d were synthesized by the condensation of 2d-2e with 1,2-diamino benzene o-phenylene diamine using phosphoric acid and polyphosphoric acid as catalyst. Their optical properties were investigated by UV-visible spectroscopy and photoluminescence techniques in solution. They emit violetblue light (lEm max = 380-440 nm) with fluorescence quantum yields of 0.43 to 0.92 while diluted in acetonitrile solution. In order to find the relationship between the structures and properties of compounds 4a-4g, theoretical calculations using density functional theory at the B3LYP/6-31g*level were also studied.