Modulation of the Second-Order Nonlinear Optical Properties of the Two-Dimensional Pincer Ru(II) Complexes: Substituent Effect and Proton Abstraction Switch

The static second-order nonlinear optical (NLO) properties on a series of the two-dimensional (2D) pincer Ru(II) complexes with the substituted Tpy and H2SCS tridentate ligands (Tpy = 2,2':6',2-terpyridyl and H2SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl) have been investigated by density functional theory (DFT). Introducing different donor/acceptor substituents to two ligands has an influence on the static first hyperpolarizabilities (beta(tot)) of the 2D systems. Compared to the reference system 1 [Ru-(HISCS)(Tpy)](+), introducing the branches with strong electron acceptor group (p-NO2-phenylethynyl) to the Tpy ligand or the branches with strong electron donor group (p-NH2-phenylethynyl) to the H2SCS ligand can effectively improve the beta(tot) values. Time-dependent DFT (TDDFT) calculations indicate that the enhanced beta(tot) values of the substituted systems are dominated by the intraligand charge transfer (ILCT), metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) transitions. Furthermore, the proton abstraction plays an important role in tuning the second-order NLO response. Particularly, for system 5 bearing the branches with NO2 groups on H2SCS ligand, there is a dramatic enhancement in the beta(tot) values for its deprotonated forms. The beta(tot) values of the monodeprotonated system 5-H and the dideprotonated system 5-2H (58.712 X 10(-30) and 761.803 X 10(-30) esu) are about 7.58 times and 36.4 times larger than their diprotonated system 5, respectively. The second-order NLO responses based on substituent effect and proton abstraction switch are two-dimensional in characteristic with the large off-diagonal tensor values.