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 thedevelopment of innovative stimuli-responsive materials. In this work, we highlight that halogen bond (XB) can be exploited as asupramolecular tool to reversibly promote a single-crystal phase transition, which regulates the properties of a two-dimensionalMHP under a low-energy thermal stimulus. By employing 2-iodoethylammonium (2IEA+) as a telechelic hydrogen- and XBdonor tecton, we demonstrate that modulating the strength of the XBs through an external stimulus induces reversible cationisomerization, which in turn drives a phase transition from a Dion–Jacobson-like to a Ruddlesden–Popper-like stacking motif. ThisXB-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.