Photoionization of α-alkoxybenzyl radicals to yield α-alkoxybenzyl cations. Photochemistry of ω,ω-dimethoxy-ω- phenylacetophenone in polar solvents at high light intensities 1

Abstract

On photolysis of the photopolymerization initiator PhC(O)C(OMe)₂Ph in aqueous or alcoholic solution with 248 nm laser pulses or with 248 nm followed by 308 nm pulses (two-laser–two-colour technique), the short-lived species PhC^˙(OMe)₂, PhC⁺(OMe)₂ and e^-_solv were detected by their optical absorptions. The carbocation PhC⁺(OMe)₂ and e^-_solv are the products of the monophotonic ionization of the radical PhC^˙(OMe)₂ [ϕ(308 nm) = 0.2 in aqueous solution] generated from the parent, PhC(O)C(OMe)₂Ph, by α-cleavage [ϕ(248 nm) ca. 0.5]. PhC⁺(OMe)₂ reacts with water to produce the hemi-orthoester PhC(OMe)₂OH and H⁺ which was identified by time-resolved conductance. PhC(OMe)₂OH decomposes to yield PhC(O)OMe and MeOH (identified by GC). The α-alkoxybenzyl radicals PhCH^˙OR, produced by 248 nm photoinduced α-cleavage of the benzoin ethers PhC(O)CH(OR)Ph (R = Me, Et, Prⁱ, Bu^t), also undergo ionization upon 248 or 308 nm photolysis. The rate constants for reaction of PhCH^˙OR with the oxidants Fe(CN)₆^3-, Ir(Cl)₆^2-, O₂ and PhI²⁺ are on average 5.9 × 10⁹, 4.2 × 10⁹, 2.8 × 10⁹ and 1.7 × 10⁸ dm³ mol^-1 s^-1, respectively. From the facile photoionization of the α-alkoxybenzyl radicals it is concluded that this process may also take place under the typical high-intensity light conditions used in industrial photocuring, i.e. that the latter may involve cationic polymerization.