Kinetics and mechanism of phosphate photorelease from benzoin diethyl phosphate: Evidence for adiabatic fission to an alpha-keto cation in the triplet state

The photodissociation of benzoin diethyl phosphate (1, "caged" phosphate) w as 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-phenylbenzo furan (2) and diethyl phosphoric acid, heterolytic adiabatic dissociation o f (3)1 yields a triplet cation and diethyl phosphate anion in water, triflu oroethanol, and hexafluoro-2-propanol solutions. The triplet cation (570 nm , tau approximate to 430 ns in water) undergoes intersystem crossing to the singlet ground state before it reacts with the solvent molecules.