Effects of the substituting groups and proton abstraction on the nonlinear optical properties of heteroleptic bis-tridentate Ru(II) complexes

The second-order nonlinear optical (NLO) properties have been carried out on a series of Ru(II) complexes with different 4′-substituted terpyridine derivatives and the tridentate ligand 2,6-bis(benzimidazole-2-yl)pyridine by using density functional theory (DFT). The introduction of different substituents enhances the static first hyperpolarizabilities in various degrees. Time-dependent density functional theory (TD-DFT) calculations indicate that the additional metal-to-ligand charge transfer (MLCT) transition, which is vectorially opposite to the intraligand charge transfer (ILCT) transition, could contribute to the smaller βvec in species with electron-withdrawing groups compared to ones with electron-donating groups. The stepwise deprotonation brings about a change in electron density of the benzimidazole moiety and finally makes the moiety turn to be as donor, which subsequently leads to an efficient second-order NLO switching. For the species 3 with electron-donating group, the βvec value of the mono-deprotonated system is 49.9 and 11.1 times as small as that of its diprotonated and fully deprotonated ones.