Kinetics and Mechanism of Phosphate Photorelease from Benzoin Diethyl Phosphate: Evidence for Adiabatic Fission to an R-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, 31, 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 31 within 25 ns. In addition to a fast cyclization-elimination process leading to 2-phenylbenzofuran (2) and diethyl phosphoric acid, heterolytic adiabatic dissociation of 31 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.