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 Δn_calc = 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 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.