2,2′‐Thienoviologens with Low Reduction Potential for Electrochromism and Visible‐Light‐Driven Hydrogen Evolution Using g‐C₃N₄‐Based Composites via Hydrogen Bonds

Abstract A series of sulfur‐bridged 2,2′‐viologens, 2,2′‐thienoviologens (2,2′‐SV²⁺) with propyl bridge and hydroxyl bridges, are synthesized for the first time. The 2,2′‐thienoviologens exhibited improved visible‐light absorption, narrow energy gap, more negative reduction potential (160 mV lower than 4,4′‐thienoviologens) and more stable free radical states compared with 4,4′‐thienoviologens and parent 2,2′‐viologens. The utilization of femtosecond transient absorption (fs‐TA) demonstrated that 2,2′‐thienoviologen can produce distinct charge‐separated states under visible light excitation. Due to their excellent photophysical and electrochemical properties, the 2,2′‐thienoviologens are used for electrochromic devices and combined with g‐C₃N₄ via hydrogen bonds for visible‐light catalytic production. Based on the advantageous electron‐donating properties of 2,2′‐SV²⁺, the hydrogen release efficiency of the 2,2′‐thienoviologens‐modified composites is 8542 µmol·h⁻¹·g⁻¹, a value that is the highest reported for hydrogen production from organic small‐molecule‐modified g‐C₃N₄ and 78 times higher than that of unmodified g‐C₃N₄. This study presents a concise method to convert solar energy and broaden the potential applications of 2,2′‐viologens in photocatalytic systems.