Heavy atom-free Keto-di-coumarin as earth-abundant strong visible light-harvesting photosensitizer for efficient photocatalytic hydrogen evolution

Abstract

Constructing low-cost and environmentally benign catalytic systems for visible light-driven water splitting into hydrogen represents a promising strategy for the conversion of solar energy into clean fuel. In this work, coupling two coumarin molecules into a dimeric Keto-di-coumarin (K-2) molecule affords a long-lived triplet excited state (30.6 μs), which was used as earth-abundant and heavy atom-free photosensitizers (PSs) for visible-light-driven hydrogen evolution. Its photocatalytic performance was dramatically improved to 61 times higher than that of Keto-mono-coumarin (K-1)-containing system. Both K-1 and K-2 show a strong visible light-absorbing ability with the molar extinction coefficient of 44000 M−1·cm−1 at 470 nm and 71000 M−1·cm−1 at 477 nm, respectively. Detail investigation of transient spectra confirmed that the much enhanced photocatalytic activity of K-2-containing system can be mainly ascribed to existence of long-lived triplet excited state of the dimer under visible light excitation, while K-1 just gave a short-lived singlet excited state (0.87 ns). Photocatalytic processes of K-2 were systematically investigated by transient absorption spectra and electrochemical methods, indicating that reductive quenching mechanism is the dominated process during the photocatalytic reactions. This work not only provide a low-cost and environment-friendly photocatalytic system with the earth-abundant pure organic PSs, but also open up an avenue to design heavy atom-free PSs with strong visible light harvesting ability and long-lived excited state for efficient hydrogen evolution.