Optimizing optical anisotropy in low-dimensional structures via intralayer hydrogen bonding modulation and anionic substitution

Anisotropy is a fundamental prerequisite for achieving significant birefringence (Δn) in optical materials, yet optimizing it to surpass the ideal range (Δn > 0.3) remains a substantial hurdle. In the unabated quest for novel birefringent genes, we have figured out that π-conjugated aminopyrazine, [APZ], is capable of producing low-dimensional linear structures for achieving enhanced birefringence due to their structural diversity and inherent anisotropy. Herein, the systematic substitutions of non-π-conjugated [(H₂PO₄)^- and (BF₄)^-] with heteroatom-substituted tetrahedral anions [(CF₃SO₃)^-, (NH₂SO₃)^-, (CH₃SO₃)^-] and subsequently with the aliphatic [C₄H₆O₄] anion, while keeping the cationic end constant, yield a series of seven compounds with a significant boost in Δn_calc = (0.145-0.658@546 nm) which is optimal in their respective families. The substantial increase in birefringence is ascribed to dimensional transition and the propensity of [APZ] to form low-dimensional frameworks, modulated by hydrogen bonds. The intralayer [N-H⋯O], [O-H⋯N], and [N-H⋯F] interactions regulate the perfect coplanar arrangement (ϑ = 0°) of birefringent active units resulting in more pronounced in-plane anisotropy. Moreover, theoretical calculations corroborate that the sequential anion exchange brings variations in optical polarizability, leading to superior linear optical performance of birefringent materials. This work presents a novel birefringent gene, offering promising prospects for synthesizing compounds with exceptional birefringence within low-dimensional systems.