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

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.