Quantum Chemical Study of Redox-Switchable Second-Order Nonlinear Optical Responses of D-π-A System BNbpy and Metal Pt(II) Chelate Complex

A second-order nonlinear optical (NLO) molecular switching with redox has been investigated in the present paper. The static first hyperpolarizabilities of 5-(BMes(2))-5'-(NPh2)-2,2'-bipyridine (BNbpy) containing three-coordinate organoboron, Pt(II) chelate complex Pt(BNbpy)Ph-2, and their reduced forms have been calculated by density functional theory (DFT) combined with the analytic derivatives method. There is an enhancement of static first hyperpolarizabilities in the reduced form according to the calculations. That is, the beta(vec) value of one-electron-reduced form is similar to 7 times as large as that of neutral form BNbpy; the beta(vec) values of one- and two-electron-reduced forms are similar to 3 and similar to 4 times as large as that of neutral form Pt(BNbpy)Ph-2, respectively. In particular, the beta(vec) value of two-electron-reduced form Pt-3(BNbpy)Ph-2(2-) is 1349 x 10(-30) esu, similar to 286 times larger than its neutral form. Moreover, the component beta(z) value of the metal chelate complex Pt(BNbpy)Ph-2 is 25 x 10(-30) esu, which is similar to 14 times as large as that of ligand BNbpy; the corresponding F-/CN- compounds show a decrease in beta(x) values compared with the case of the ligand and Pt(II) complex. Analyses of geometries, density of states (DOS), and time-dependent DFT (TDDFT) calculations reveal that the one-electron reduction promotes the molecular conjugation in the x-axis and intensifies the interaction between the metal Pt(II) and ligand and then results in an enhancement of the static first hyperpolarizability, whereas the binding of F-/CN- to the B atom turns off the p(pi)-pi* conjugation and has no effect on the conjugation of bipyridine, which leads to a decreasing beta value in the x-axis.