Triplet excited-state charge transfer induced in indolenine-based squaraine for enhanced visible-light-driven H2 evolution

Abstract

The quest for efficient, economical and stable non-precious metal photocatalysts in the realm of photocatalytic hydrogen evolution reactions (HER) remains a formidable task. In this work, a range of indolenine-based squaraine (ISQ) derivatives featuring diverse N-based side chains, as well as their monothio- and dithio-analogues, have been meticulously synthesized and immobilized on the surface of Ti₃C₂T_x MXene. These organic–inorganic hybrid photocatalysts serve as noble-metal-free photocatalysts for HER. Notably, among the nine distinct ISQ variants tested, the optimal rate of hydrogen evolution is attained using the dithiosquaraine ISQ-6 with N-methyl side chains. The higher photocatalytic ability of the ISQ-6@MXene composite is inextricably linked to the structural compatibility of the N-methyl ISQs on the hydrophilic surface of Ti₃C₂T_x MXene. Synergistically, the introduction of S atoms in ISQ-6 promotes exciton transformation from the singlet to the triplet excited state via intersystem crossing and improves the separation transfer and utilization of photogenerated charges. This work offers novel insights for exploring organic dyes suitable for the construction of MXene hybrid structures, enabling them to drive photocatalytic HER efficiently.