Synthesis, properties, and photocatalytic hydrogen evolution of isoindigo-based conjugated molecules

Abstract

Conjugated molecules exhibit optical and electrochemical properties similar to those of conjugated polymers. In principle, they should be active in photocatalytic hydrogen evolution (HE). In this study, triarylamine serves as the electron-donating (D) unit and isoindigo as the electron-accepting (A) unit to construct two D–A–D conjugated molecules. Furthermore, a [2 + 2] cycloaddition–retroelectrocyclization reaction has been employed to introduce tetracyanoethylene moieties into these structures, generating two additional molecules. Their thermal, optical, electrochemical, scattering, photocurrent, photovoltage decay, charge transport, and DFT-calculated properties have been systematically characterized. Their HE performance has also been evaluated, achieving an average hydrogen evolution rate (HER) of up to 9.7 mmol g⁻¹ h⁻¹. A comparison between the HERs and various physical properties has been conducted to gain insight into their correlations, indicating that multiple factors including absorption capacity, exciton lifetime, photocurrent, charge transfer, reorganization energy, and Pt loading may play significant roles in determining the HER. Overall, this study demonstrates the potential of triarylamine, isoindigo, and cyano functional groups in constructing D–A–D conjugated systems for HE. These findings pave the way for the rational design of new conjugated molecules targeting HE.