Broadening Frontiers of Infrared Nonlinear Optical Materials with π-Conjugated Trigonal-Planar Groups

Trigonal-planar units with high physicochemical stability and large polarizability anisotropy are one kind of promising fundamental building block (FBB) for constructing novel nonlinear optical (NLO) materials. Though great achievements have been made in the ultraviolet/deep ultraviolet (UV/DUV) region with trigonal-planar units, little attention has been paid to them in the infrared region owing to the lack of enough representatives. In this work, [AgSe3] and [HgSe3] are rationally proposed as NLO-active FBBs. In addition, Ag6HgMSe6 (M = Si, Ge) have been screened out by combined density functional theory calculations and experiments as new types of IR NLO materials. Experiments demonstrate that both Ag6HgSiSe6 and Ag6HgGeSe6 show strong second harmonic generation (SHG) responses, valuable phase-matchable features, and congruent-melting thermal behaviors. Moreover, the great contributions of the trigonal-planar units are also discussed in detail.