1. Shanghai Institute of Applied Physics,Chinese Academy of Sciences,Shanghai 201800,China 2. Graduate School of Chinese Academy of Sciences,Beijing 100049,China 3. The North-East Wales Institute,Plas Coch,Wrexham LL11 2AW,U.K.
Abstract：The representative of carotenoids,β-carotene,can scavenge reactive oxygen radicals like singlet molecular oxygen,nitrogen dioxide radical and peroxyl radical due to the effective antioxidative properties. In medicine,β-carotene is used to alleviate the disease erythropoietic protoporphyria (EPP),by intercepting the triplet state of protoporphyrin (a porphyrin lacking a central metalion,a precursor to haem) therefore preventing the formation of singlet oxygen. Epidemiological evidence has suggested that dietary β-carotene may inhibit certain types of cancer. Much of work has been carried out in benzene,toluene,or chloroform as most caroienoids are sufficiently soluble in these nonpolarity solvents. In the present paper,the generation and properties of triplet β-carotene in acetonitrile solution were investigated with 355 nm laser flash photolysis. 2-acetonaphthone was used as an excitation energy donor to sensitize the production of the triplet state of β-carotene. Excitation of the solution containing 2-acetonaphthone and β-carotene upon 355 nm laser flash produced the triplet of 2-acetonaphthone (420 nm) firstly. Subsequently,the excitation energy of triplet 2-acetonaphthone was transferred to β-carotene generating triplet β-carotene. Characteristic absorption spectra of triplet β-carotene (510 nm) were recorded. By means of transfer of excitation energy,the molar absorption coefficients of triplet β-carotene were determined to be 23 000 dm3 mol-1·cm-1 at 510 nm. The triplet lifetime for β-carotene in acetonitrile solution was observed to be 15.6 μs. The rate constant for the reaction of triplet energy transfer from triplet 2-acetonaphthone to β-carotene was calculated to be 1.5×1010 dm3·mol-1·s-1. Obviously,the triplet β-carotene has very low excitation energy. Taking the advantage of the photochemical properties of triplet β-carotene,β-Carotene has been widely used as energy acceptor to determine the excited state characteristic of other substance. This work extends the understanding of photochemical properties of β-carotene.
Key words：β-Carotene;Laser flash photolysis;Triplet states
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