Kinetics and Mechanism of Phosphate Photorelease from Benzoin Diethyl Phosphate:  Evidence for Adiabatic Fission to an α-Keto Cation in the Triplet State

The photodissociation of benzoin diethyl phosphate ( 1, “caged” phosphate) was studied by nanosecond and picosecond laser flash photolysis. The lowest triplet state of 1, 3 1, was identified as the reactive excited state with an absorption maximum around 340 nm and a lifetime of 10−25 ns, depending on the solvent. Two different reaction paths were identified, both of which involve the release of diethyl phosphoric acid from 3 1 within 25 ns. In addition to a fast cyclization−elimination process leading to 2-phenylbenzofuran ( 2 ) and diethyl phosphoric acid, heterolytic adiabatic dissociation of 3 1 yields a triplet cation and diethyl phosphate anion in water, trifluoroethanol, and hexafluoro-2-propanol solutions. The triplet cation (570 nm, τ ≈ 430 ns in water) undergoes intersystem crossing to the singlet ground state before it reacts with the solvent molecules.