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(NH 2 ) 3 ] 3 C 3 N 3 S 3 (GU 3 TMT), consisting of π‐conjugated planar (C 3 N 3 S 3 ) 3− and triangular [C(NH 2 ) 3 ] + groups. Interestingly, GU 3 TMT exhibits a large NLO response (2.0×KH 2 PO 4 ) and moderate birefringence 0.067 at wavelength 550 nm, even though (C 3 N 3 S 3 ) 3− and [C(NH 2 ) 3 ] + do not exhibit the most favorable arrangement in the structure of GU 3 TMT. First‐principles calculations suggest that NLO properties mainly originate from the highly π‐conjugated (C 3 N 3 S 3 ) 3− rings, and the π‐conjugated [C(NH 2 ) 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.