A Second-Order Nonlinear Optical Material Consisting of Two π-Conjugated Groups

Abstract

This work reports a new second-order nonlinear optical (NLO) material [C(NH₂ )₃ ]₃ C₃ N₃ S₃ (GU₃ TMT), consisting of π-conjugated planar (C₃ N₃ S₃ )^3- and triangular [C(NH₂ )₃ ]⁺ groups. Interestingly, GU₃ TMT exhibits a large NLO response (2.0×KH₂ PO₄ ) and moderate birefringence 0.067 at wavelength 550 nm, even though (C₃ N₃ S₃ )^3- and [C(NH₂ )₃ ]⁺ do not exhibit the most favorable arrangement in the structure of GU₃ TMT. First-principles calculations suggest that NLO properties mainly originate from the highly π-conjugated (C₃ N₃ S₃ )^3- rings, and the π-conjugated [C(NH₂ )₃ ]⁺ triangles contribute much less to the overall NLO response. This work will inspire new thoughts with in-depth on the role of π-conjugated groups in NLO crystals.

Keywords: first-principles calculations; nonlinear optics; optoelectronics; structure-property relationships; trithiocyanates.