Theoretical investigation on the surrounding effects of second-order nonlinear properties for (N^C^N)Pt(II)Cl complexes.

Pt(II) complexes are widely used as nonlinear optical (NLO) materials. The geometric and electronic structures, second-order NLO property and UV-Vis absorption spectra of (N^C^N)Pt(II)Cl complexes (1-4) N^C^N binding by central benzene and two lateral N-heterocycles) are evaluated by density functional theory (DFT) and time-dependent DFT calculations. The detailed environmental effect of total first hyperpolarizability (β) in the solution and crystal phases is simulated by polarized continuum model (PCM) and quantum mechanics/molecular mechanics (QM/MM) method, respectively. The results highlight that the complex 3 exhibits largest β value in the gas, solution and crystal phases which can be attributed to the higher electron π-delocalization of ligands. Further, an evident red shift towards longer wavelength is observed for the complex 3. The origin of larger β value can be reasonably interpreted by the two-level model. In addition, the surrounding exerts an important influence on modulating second-order NLO properties. The solvent effect results in the larger β value than that of gas phase. The intermolecular interaction plays an important role in crystal phase. The formation of dimer can reduce the β value in comparison with the β value of the monomer in the crystal phase, because the centrosymmetric configuration of dimer implies a decrease of dipole moment (μ) in contrast to the large μ value of monomer. It is expected that this work will provide some guidance for designing Pt(II) NLO materials.