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 (C4H7N3)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, (C4H7N3)2+, (C4H6N3)+, and (C4H5N3) functional units are introduced into the crystal structure, both solely and in combination, to analyze the alteration in birefringence. Remarkably, a birefringence rule with (C4H7N3)2+ < (C4H6N3)+ < (C4H5N3), 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 (C4H5N3) functional unit within the crystal structure along with coplanar arrangement is a promising approach to enhance birefringence. Furthermore, D‐2 [(C4H5N3)(H3C3N3O3)] realizes Δncalc = 0.412 maximum in the (2‐AP) family. Notably, both A‐1 [(C4H7N3)(CF3SO3)2] and B‐1 [(C4H6N3)2(IO3)2(H2O)] crystalize in the non‐centrosymmetric space groups, with B‐1 exhibiting SHG efficiency of ≈1.4 × KDP @ 1064 nm, with the shortest λPM 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.