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Phenol is a key intermediate for many chemicals and industrial products,and plays an important role in synthesis of drugs,dyes,and functional polymers.The electron-transfer oxidation of benzene is expected to an efficient way because of its no low yield,no poor selectivity,and no requirement of high temperature,as well as no further oxygenation of phenol to hydroquinone and COx.The triplet-excited DDQ has a very strong oxidizing ability which can overcome the one-electron oxidation potential of benzene to capture an electron from benzene.In the present work,we studied the visible light induce DDQ oxygenation of benzene to phenol in homogeneous media with complete active space self-consistent field(CASSCF)and density functional theory(DFT).It was found that irradiation at~400 nm results in DDQ in the S1(1ππ*)state,which is followed by a rapid relaxation and efficient intersystem crossing to the T1(3ππ*)state via the singlet-triplet crossing S1/T2 and the conical intersection T2/T1.The photocatalytic reaction is initiated by photoinduced electron transfer from benzene to the triplet excited state of DDQ,followed by the nucleophilic addition of water to benzene radical cation and hydrogen abstraction reaction of OH-adduct radical and finally produces the triplet phenol.