Unveiling Charge Dependent Law to Enhance Birefringence in 2‐Aminopyrimidine Family
Muhammad Arif, Xu Liu, Zi‐Jian Li, Hangwei Jia, Yansheng Jiang, Zhihua Yang, Xueling Hou, Shilie Pan
Abstract
Abstract Salts and co‐crystals composed of planar π ‐conjugated functional units represent a fascinating class of materials, featuring both enlarged birefringence and a strong SHG effect. Herein, the optical properties of π ‐conjugated (C 4 H 7 N 3 ) 2+ functional unit within 2‐aminopyrimidine (2‐AP) family are investigated for the first time. Furthermore, twelve novel compounds are successfully synthesized by applying four different strategies. In these strategies, (C 4 H 7 N 3 ) 2+ , (C 4 H 6 N 3 ) + , and (C 4 H 5 N 3 ) functional units are introduced into the crystal structure, both solely and in combination, to analyze the alteration in birefringence. Remarkably, a birefringence rule with (C 4 H 7 N 3 ) 2+ < (C 4 H 6 N 3 ) + < (C 4 H 5 N 3 ), stating that an increase in net charge over π ‐conjugated functional unit leads to a reduced birefringence, is discovered. Consequently, the inclusion of a charge‐neutral (C 4 H 5 N 3 ) functional unit within the crystal structure along with coplanar arrangement is a promising approach to enhance birefringence. Furthermore, D‐2 [(C 4 H 5 N 3 )(H 3 C 3 N 3 O 3 )] realizes Δn calc = 0.412 maximum in the (2‐AP) family. Notably, both A‐1 [(C 4 H 7 N 3 )(CF 3 SO 3 ) 2 ] and B‐1 [(C 4 H 6 N 3 ) 2 (IO 3 ) 2 (H 2 O)] crystalize in the non‐centrosymmetric space groups, with B‐1 exhibiting SHG efficiency of ≈1.4 × KDP @ 1064 nm, with the shortest λ PM at 384 nm. This work provides new insight into discovering novel birefringent and non‐linear optical materials with high performances in the wide chemical space.