# Halogen‐Bond Coupled Halogenated‐π‐Conjugation Enables Giant Birefringence in Hydrogen‐Bonded Organic Frameworks

## TL;DR

A new strategy using halogen bonds and π-conjugation creates materials with extremely high birefringence for advanced optical applications.

## Contribution

A novel halogen-bond coupled halogenated-π-conjugation strategy to achieve record-breaking birefringence in organic frameworks.

## Key findings

- XB-HOF-1 achieves a birefringence of Δn = 0.97 at 546 nm, over 80 times higher than MgF2.
- Collinear N···I-Cl halogen bonds and aligned π-conjugated planes maximize in-plane anisotropy and minimize out-of-plane polarizability.
- The strategy enables rational design of high-performance birefringent materials with potential in polarization control.

## Abstract

Birefringent crystals are essential optical materials for modulating and detecting the polarization state of light. However, achieving large optical anisotropy remains a significant challenge, mainly because it is difficult to rationally align anisotropic fundamental building blocks (FBBs) to maximize their collective contribution. Here, we propose a halogen‐bond (XB) coupled halogenated‐π‐conjugation strategy. Using hydrogen‐bonded pyridinedicarboxylic acid as a template, we synthesized XOF‐1, Hybrid‐XOF‐1, and XB‐HOF‐1 through decarboxylation‐halogenation reactions. All three compounds are built from 0D halogen‐bonded FBBs and extended by halogen and/or hydrogen bonds. Notably, all three compounds exhibit large birefringence (0.69–0.97), with XB‐HOF‐1 reaching Δn = 0.97 @ 546 nm, over 80 times that of the commercial benchmark MgF2 and the highest value reported for any pyridine‐containing system. Theoretical calculations and structural analyses reveal that this exceptional performance originates from the synergistic combination of maximized in‐plane anisotropy, induced by collinear arrangement of N···I‐Cl halogen bonds along one crystallographic axis, and minimized out‐of‐plane polarizability, induced by coplanar alignment of halogenated π‐conjugated planes perpendicular to another axis. Our work establishes an effective strategy for pushing the boundaries of optical anisotropy and offers a new blueprint for the rational design of high‐performance birefringent materials.

The perfect crystal packing achieved via the halogen‐bond (XB) coupled halogenated‐π‐conjugation strategy effectively induces an ultrahigh birefringence (Δn = 0.97), which exhibits promising potential in polarization control and phase modulation.