Sulfonated Module Aggregation for Ultrahigh Birefringence in Aqueous-Processable Crystals

Abstract

Birefringent crystals play a pivotal role in advanced optics owing to their indispensable function in modulating polarized light. Progress in this field has been driven by the evolution of “material genes”, from small π-conjugated monomers to π-oligomers and ultimately to expanded π-motifs. However, the poor aqueous processability inherent to the hydrophobic nature of extended carbon or C–N skeletons remains a significant hurdle for industrial applications. Herein, we propose a sulfonated module aggregation strategy to simultaneously optimize optical anisotropy and enable aqueous processability. Using a mild evaporation method, we synthesized five 1,3,6,8-pyreneterasulfonate (PTS) salts exhibiting large birefringence values ranging from 0.558 to 0.811 @ 546 nm. Notably, Na₂(4-HPyH)₂(PTS)·H₂O [(4-HPyH) = 4-hydroxypyridinium] combines a wide band gap (E_g = 3.05 eV), enhanced aqueous processability, and a record-high birefringence (Δn = 0.811 @ 546 nm) among all sulfate- and sulfonate-based compounds, establishing it as a highly promising birefringent material. Structurally, the incorporation of [4-HPyH] modules reduces the space available for coordinated and guest water molecules, thereby maximizing the packing density of birefringence-active modules while maintaining ideal alignment of both [4-HPyH] and [PTS] units within the lattice. High-accuracy quantum chemical calculations further reveal a compensation mechanism associated with the ring fusion process─the supralinear polarizability anisotropy effect─which produces a supralinear enhancement of polarizability anisotropy upon aggregation of planar π-modules. This work offers a new paradigm for designing aqueous-processable birefringent crystals and expands the theoretical framework for understanding supralinear enhancement of micro-optical properties via π-module polymerization.