Triplet reactivity of psoralens containing a benzo-fused furan ring: A laser flash photolysis study

• The triplet excited state reactivity of psoralens containing a benzo-fused furan ring was investigated.
• The triplet excited state of both psoralens was efficiently quenched by hydrogen donors.
• The triplet excited state of both psoralens was efficiently quenched by electron donors.
• Reaction constants from Hammett plots are consistent with the electrophilic character of the reactive carbonyl triplet states.

Laser flash photolysis (LFP) studies have been performed in order to investigate the triplet reactivity toward hydrogen and electron donors of the benzo-fused furan psoralens 3-ethoxycarbonyl-2H-benzofuro[2,3-e]-1-benzopyran-2-one (1) and 3-ethoxycarbonyl-2H-benzofuro[3,2-d]-1-benzopyran-2-one (2). Photolysis of the psoralens 1 and 2 results in the formation of the corresponding triplet excited state which is efficiently quenched by DABCO, triethylamine and phenols containing polar substituents. The reaction of the triplet state of 1 and 2 with electron donors (DABCO and triethylamine) leads to the formation of the corresponding anion radical whereas with phenols the corresponding aryloxyl radical was easily detected. Quenching rate constants for psoralen 2 are at least one order of magnitude greater than for 1. The Hammett plot for the reaction of 1 and 2 with phenols yielded a reaction constant ρ of −1.88 ± 0.29 and −1.60 ± 0.21, respectively, which reflects the high electrophilicity of the carbonyl group of both psoralens.

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