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.