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

This work reports a new second-order nonlinear optical (NLO) material [C(NH2)(3)](3)C3N3S3 (GU(3)TMT), consisting of pi-conjugated planar (C3N3S3)(3-) and triangular [C(NH2)(3)](+) groups. Interestingly, GU(3)TMT exhibits a large NLO response (2.0xKH(2)PO(4)) and moderate birefringence 0.067 at wavelength 550 nm, even though (C3N3S3)(3-) and [C(NH2)(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 pi-conjugated (C3N3S3)(3-) rings, and the pi-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 pi-conjugated groups in NLO crystals.