Heavy Atom-Free Keto-di-coumarin As Earth-Abundant Strong Visible Light-Harvesting Photosensitizer For Photocatalytic Hydrogen Evolution

Recent years,the exploitation and consumption of fossil fuels has brought about increasingly serious environmental problems,Conversion of solar light into clean fuel hydrogen via visible-light-driven water splitting represents one of the most promising methods to meet future humankind energy demand with an environment-friendly way.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?s?,which was firstly used as earth-abundant and heavy atom-free photosensitizers?PSs?for visible-light-driven hydrogen evolution.Both K-1 and K-2 show a strong visible light-absorbing ability with the molar extinction coefficient of 44000 M^-1·cm^-1 and 71000 M^-1·cm^-1,respectively.The photocatalytic performance of K-2 was dramatically improved to 61 times higher than that of Keto-mono-coumarin?K-1?-containing system.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.According to the above test results,the photocatalytic process is obtained:For K-2,its singlet excited state was populated upon light excitation and an efficient ISC process proceeded to yield a long-lived triplet state,which can accept an electron from DMT to produce the long-lived reduced state.The highly active reduced state of K-2 could consecutively transfer two electrons to C-1 to generate Co⁺,which can be used to reduce protons for hydrogen production.K-1 showed a similar photocatalytic mechanism with K-2,except that the photocatalytic cycle of K-1 was initialed by its short-lived singlet excited state,which may result in low efficient electron transfer from DMT to the excited K-1 and the subsequent photocatalytic hydrogen evolution.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.