Compounds consisting of coplanar π-conjugated B 3 O 6 -typed structures: An emerging source of ultraviolet nonlinear optical materials

Nonlinear optical (NLO) materials as a crucial part of the laser science have attracted increasing attention from researchers because of their wide applications in information storages, modern science and technologies, et al. Based on the anionic group theory, the π-conjugated 6-membered rings (6-MRs), B 3 O 6 -typed structures have key contributions to the superior optical properties of β -Ba 2 B 2 O 4 ( β -BBO). In recent years, the organic coplanar π-conjugated B 3 O 6 -typed structures have caught researchers’ attention due to similar configurations to the (B 3 O 6 ) groups, which are the potential sources for expending the (B 3 O 6 ) groups with outstanding optical properties. Up to know, researchers have obtained many ultraviolet (UV) NLO crystals with excellent properties with π-conjugated B 3 O 6 -typed groups. Herein, these B 3 O 6 -typed groups could be divided into different categories according to the atoms constituting the 6-MRs: (H x C 3 N 3 O 3 ) x−3 ( x  ​= ​0–3) (cyanurate ion), (H x C 4 N 2 O 3 ) x−4 ( x  ​= ​2, 3) (barbiturate ion), (C 3 H 7 N 6 ) + (melamine ion), (C 5 H 6 ON) + (4-hydroxypyridine cation), and (C 4 H 6 N 3 ) + (2-aminopyrimidinium cation). In this review, we introduced the research advances of NLO materials with π-conjugated B 3 O 6 -typed groups, and summarized the crystal structures, synthetic methods, optical performances, as well as the relationships between structures and properties. This work provided a clear perspective for understanding the coplanar π-conjugated B 3 O 6 -typed groups with their optical functional properties and promote the searches to develop potential NLO functional crystals.