Halogen Bond‐Driven Reversible Thermochromism in a Two‐Dimensional Hybrid Perovskite

Metal halide perovskites (MHPs) represent a versatile platform capable of accommodating new chemical design schemes for the development of innovative stimuli-responsive materials. In this work, we highlight that halogen bond (XB) can be exploited as a supramolecular tool to reversibly promote a single-crystal phase transition, which regulates the properties of a two-dimensional MHP under a low-energy thermal stimulus. By employing 2-iodoethylammonium (2IEA+) as a telechelic hydrogen- and XB donor tecton, we demonstrate that modulating the strength of the XBs through an external stimulus induces reversible cation isomerization, which in turn drives a phase transition from a Dion–Jacobson-like to a Ruddlesden–Popper-like stacking motif. This XB-driven mechanism allows the controllable and reversible thermochromism as well as heat switchable birefringence in MHPs, opening new avenues to further broaden the application landscape of this exciting class of materials.