Hydrated Ionic Polymer with Optimized Halogen Ligand for Thermochromic Smart Windows: Enhanced Luminous Transmittance and Solar Modulation

Thermochromic smart windows dynamically adjust solar transmittance with temperature changes, offering a promising solution for energy-efficient buildings. Transition metal complexes are widely studied for their optical tunability and cost-effectiveness. However, the tetrahedral coordination configuration, typically a high-temperature absorber, has absorption peaks in the 500–700 nm range, reducing luminous transmittance and limiting applications requiring high optical clarity. This study proposes a hydrated ionic polymer ([(C₂H₅)₂NH₂]₂NiCl_4-xBr_x@PVP) that achieves thermochromism via hydration-driven coordination transitions. Both experimental and ab initio calculations confirm that substituting Cl⁻ with Br⁻ weakens the ligand field strength, causing a red shift in the absorption peak. As a result, the luminous transmittance (380−780 nm) in the hot state increases to 46.0%, while solar modulation (300−2500nm) remains high at 36.3%. Molecular dynamics simulations further reveal that Br⁻ alters hydrogen bonding, influencing hydration and dehydration processes. Experiments show that under ≈1000 W m⁻² irradiance, the film has solar transmittance comparable to standard glass at low temperatures, and darkens within 20–40 min, reducing transmitted irradiance to 550 W m⁻². Simulations indicate it could reduce annual energy consumption by ≈20 kWh m⁻² and raise indoor comfort probability from 52% to 69%. Overall, this study provides a new pathway for developing high-performance thermochromic materials for energy-efficient buildings.