Bimetallic Nanoarchitectonics: Generation of Pt/Co-Based Metallo-Macrocycles for Full-Spectrum Black Electrochromism in Smart Window Technologies

Modern smart windows that can efficiently regulate solar light and heat in the visible-to-NIR region are essential to reduce the indoor cooling cost for energy-efficient buildings. In this work, we developed a square-shaped bimetallic Pt(II)-Co(II) containing metallo-macrocycle (Pt-Co-TPYN) using a pyridine-terpyridine-based heteroditopic ligand (TPYN) via a stepwise complexation reaction. Structural analysis of Pt-Co-TPYN revealed that π-π stacking and the Pt···Pt interaction promoted self-assembled 2D nanosheets with high surface area and mesoporosity. The solution-processable electrochrome exhibited a reversible color change from transparent orange to deep black with an optical contrast of 85.2% within a voltage window of -2.0 to +0.5 V, driven by the Co(II)/Co(I) redox couple. The Pt-Co-TPYN film disclosed excellent electrochromic performance, including fast response time (8.6 and 12.1 s for bleaching and coloring, respectively), high coloration efficiency of 309.8 cm²C^-1, stable cycling over 800 cycles, and a long optical memory. Notably, the reduced black state blocked more than 99.97% of total solar irradiance. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations supported the origin of the broad vis-NIR absorption in the reduced state. A prototype smart window demonstrated a temperature drop of ∼11 °C between two redox states and nearly 50 °C compared to normal glass, highlighting the promising energy-saving smart building applications of the Pt-Co-TPYN electrochrome.