Unveiling Charge Dependent Law to Enhance Birefringence in 2‐Aminopyrimidine Family

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.