Strategy for Enhancing Second-Order Nonlinear Optical Properties of the Pt(II) Dithienylethene Complexes: Substituent Effect, π-Conjugated Influence, and Photoisomerization Switch

The second-order nonlinear optical (NLO) properties of a series of Pt(II) dithienylethene (DTE) complexes possessing the reversible photochromic behavior have been investigated by density functional theory (DFT) combined with the analytic derivatives method. The results show that the calculated static first hyperpolarizabilities (beta(tot)) of the open-ring and closed-ring systems significantly increase in the range of 2.1-4.5 times through strengthening of the electron-withdrawing ability of the substituent R (R = H, CF3, NO2) and an increase of the number of thiophene rings. Moreover, there is a large enhancement of the beta(tot) values from the open-ring systems to the corresponding closed-ring systems. This efficient enhancement is attributed to the better delocalization of the pi-electron system, the more obvious degree of charge transfer, and the larger f(os)/E-gm(3) (f(os) is the oscillator strength, and E-gm is the transition energy between the ground and the excited states) values in the closed forms according to the bond length alternation (BLA) and time-dependent density functional theory (TDDFT) calculations. In addition, the dispersion has less influence on the frequency-dependent first hyperpolarizabilities (beta(tot)(omega)) of the studied systems at the low-frequency area omega (0.000-0.040 au). Our present work would be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes.