Turning instability into functionality: Stoichiometry-and moisture-tuned reversible multichromism in Ethylammonium perovskites

Organic–inorganic hybrid perovskites located at the boundaries of the Goldschmidt tolerance factor exhibit a structural softness that affords unique stimuli responsiveness but complicates their intrinsic phase stability. Ethylammonium (EA)-based perovskites, in particular, have long suffered from ambiguous and conflicting crystallographic reports, with their responses to moisture environments often misinterpreted as irreversible chemical degradation. Here, we resolve this long-standing structural ambiguity by identifying environmental humidity as a hidden variable whose impact is critically gated by precursor stoichiometry. We demonstrate that reaching a specific compositional threshold ([EAI]/[PbI ₂ ] ≥ 1.6) transforms the otherwise structurally rigid or degradation-prone perovskite lattice into a highly flexible mixed-dimensional landscape. Within this responsive regime, moisture triggers a rapid, synchronized, and fully reversible reconfiguration between 1D- and 2D-related structural motifs, that we call a dynamic lattice rearrangement, without chemical decomposition (PbI ₂ formation), which manifests as a repeatable multicolor hydrochromism. Practically leveraging this stoichiometry-gated dynamics, we successfully demonstrate zero-power, autonomous smart windows that adapt to external weather conditions, as well as dual-functional medical moisture indicators capable of both irreversible moisture history logging and real-time quantitative monitoring. This work establishes a new paradigm for boundary-region perovskites, turning their inherent instability into innovative functionality for active environmental sensing and smart optoelectronic devices.