Decoupling Electrochromism and Photoprotection via Side-Chain Engineering Enables Intrinsically UV-Stable Neutral Black Smart Windows.

Electrochromic smart windows offer a promising route for dynamic solar regulation and building energy savings, yet their practical deployment is hindered by the intrinsic instability of conjugated polymers under ultraviolet (UV) irradiation. In particular, neutral black electrochromic systems rely on highly delocalized donor-acceptor backbones for broadband light absorption, which simultaneously increases their susceptibility to photodegradation, resulting in a fundamental trade-off between optical performance and environmental stability. Here, we present a generalizable side-chain engineering strategy to decouple electrochromic functionality and photoprotection at the molecular level. By covalently incorporating benzotriazole units into the side chains, the resulting polymer enables intrinsic UV screening while preserving the conjugated backbone responsible for panchromatic visible-light modulation. This decoupled design maintains the neutral black electrochromic behavior without compromising optical performance. The corresponding electrochromic device exhibits fast switching (0.77/0.74 s), high coloration efficiency (956 cm2 C-1), and excellent cycling stability (>11,400 cycles). Notably, the device retains over 90% of its initial optical contrast after prolonged UV irradiation, demonstrating significantly enhanced photostability. Beyond material-level performance, thermal experiments and building energy simulations reveal effective suppression of solar heat gain, reduced indoor temperature rise, and decreased cooling demand. This work establishes a general design principle for integrating intrinsic photoprotection and electrochromic functionality, providing a viable pathway toward durable and energy-efficient smart-window technologies.