Tuning Second-Order Optical Nonlinearities in Push-Pull Benzimidazoles

The synthesis and full characterization of new chromophores with second order optical nonlinearities containing the 2- phenyl-(5,6)-nitrobenzimidazole group is reported. Starting from 2-{4-[(4-N,N-dihydroxyethylamino)phenylazo]phenyl}- 5(6)-nitrobenzimidazole, a combined theoretical and experimental approach, including theoretical computations of second order nonlinear optical activity (MNDO/AM1), X-ray structural analysis and synthetic strategies, has led to a significant optimization (more than 50%) of the NLO activity of related chromophores. The results indicate that 6-nitro-substituted compounds are more active than 5-nitro-substitutedones and that a further increase of NLO activity can be achieved by insertion of a carbon−carbon double bond between the benzimidazole and 2-phenyl rings. Calculations also suggest that an additional improvement of the nonlinearity should be expected upon functionalization of the N1 atom of the 6-nitrobenzimidazole with electron-withdrawing groups. Experimental nonlinearities (EFISH technique, μβ/10−48 esu, λ = 1.907 μm, DMF solution) between 940 and 1550 were measured.