Cyanurates consisting of intrinsic planar π-conjugated 6-membered rings: An emerging source of optical functional materials

Optical crystalline materials, particularly for linear birefringent crystals and non-linear optical (NLO) crystal materials, play an incredible role in opto-electronics, laser industry and academic research fields. Cyanurates consisting of intrinsic coplanar π-conjugated (HxC3N3O3)x−3 (x = 0 ~ 3) 6-membered rings (6-MRs) exhibit same structural configuration, but stronger conjugacy than that of (B3O6)3− 6-MRs in borate analogues. Accordingly, cyanurates possess large linear anisotropic polarizability and microscopic second-order-generation (SHG) susceptibility, consequently resulting in great birefringence and strong SHG effects in the crystals. Moreover, they have wide band gaps and high transparency in the near infrared, visible and ultraviolet ranges. These eminent characteristics make cyanurates to be promising birefringent and NLO materials with outstanding optical properties critical for end use. In this review, we summarized the current status of crystal structures, synthetic methods, optical performances, and connections between structure and physical property of all the found cyanurates according to anion groups’ types. Then we compartmentalized the central and acentric cyanurates and discussed how to improve the probability of the acentric structures through the deprotonization and multiple cations engineering. Finally, we proposed the directions for the design and exploration of cyanurate functional materials in the future. This work presents a clear perspective to understand cyanurates with planar π-conjugated anionic groups, and speed up the exploration of unknown birefringent or NLO functional crystals for acute demands in optics technology and facilities.