Broadband Visible‐Near‐Infrared Electrochromism in Fluorene‐Bridged Viologens for Energy‐Efficient Smart Windows

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ABSTRACT Broadband regulation of solar radiation in both visible and near‐infrared (VIS–NIR) regions is critical for reducing building energy consumption. However, achieving strong dual‐band absorption within a single electrochromic component remains challenging for conventional viologen systems. Herein, fluorene‐bridged viologen derivatives (FBV‐Et and FBV‐Hex) are developed to realize broadband black‐state electrochromism through enhanced intermolecular electronic coupling and controlled radical aggregation. The rigid π‐conjugated fluorene bridge promotes charge delocalization and facilitates intermolecular interactions, while alkyl chain modulation adjusts aggregation compactness and spectral evolution. Spectroelectrochemical analysis reveals complementary absorption contributions from monomeric radical species and coupled radical aggregates, collectively covering the VIS–NIR range. Integrated electrochromic devices exhibit optical contrast values up to 99.56% at 670 nm and 90.55% at 1425 nm, accompanied by a reversible color transition from colorless to red and ultimately to deep black. Building energy consumption simulations further demonstrate the potential of the developed system for solar heat gain regulation. This work provides a molecular design strategy for achieving dual‐band electrochromism in single‐component viologen systems toward energy‐efficient smart window applications.

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