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

On photolysis of the photopolymerization initiator PhC(O)C(OMe)(2)Ph in aqueous or alcoholic solution with 248 nm laser pulses or with 248 mn 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) [phi(308 nm)= 0.2 in aqueous solution] generated from the parent, PhC(O)C(OMe)(2)Ph, by alpha-cleavage [phi(248 nm) ca. 0.5]. PhC+(OMe)(2) reacts with water to produce the hemi-orthoester PhC(OMe)(2)OH and H+ which was identified by time-resolved conductance. PhC(OMe)(2)OH decomposes to yield PhC(O)OMe and MeOH (identified by GC). The alpha-alkoxybenzyl radicals (PhCHOR)-O-., produced by 248 nm photoinduced alpha-cleavage of the benzoin ethers PhC(O)CH(OR)Ph (R = Me, Et, Pr-i, Bu-t), also undergo ionization upon 248 or 308 nm photolysis. The rate constants for reaction of (PhCHOR)-O-. with the oxidants Fe(CN)(6)(3-), Ir(Cl)(6)(2-), O-2 and PhI2+ are on average 5.9 x 10(9), 4.2 x 10(9), 2.8 x 10(9) and 1.7 x 10(8) dm(3) mol(-1) s(-1), respectively. From the facile photoionization of the alpha-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.