A Dynamic Hydrogen‐Bonded Organic Framework With Giant OFF–ON Birefringence

Birefringence, the optical property enabling materials to split light based on polarization and propagation direction, is crucial for many technological fields, spanning from telecommunications to imaging and photonics. Particularly, controlling the birefringence is vital in high‐performance applications such as nonlinear optics, adaptable spectroscopy filters, adaptive imaging, for biomedical, and space applications. Most known switchable birefringent materials suffer from either low birefringence values or narrow tuneable values, thus hindering their use in dynamic optical systems. Here, a novel approach based on a dynamic hydrogen‐bonded organic framework (HOF) capable of reversible single crystal to single crystal transformation, during guest molecule inclusion is reported. Changes in the planar conformation of the π–π stacking between layers during guest inclusion drastically alters the birefringence off–on, from negligible values to 0.73, surpassing all known switchable materials. This approach represents a novel platform for the development of high‐performance optical devices.