New Insights into the Mechanism of Triplet Radical-Pair Combinations. The Persistent Radical Effect Masks the Distinction between In-Cage and Out-of-Cage Processes

Abstract

Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. Calculation of the fractions of in-cage combinations of the triplet benzylic radical-pair intermediates based on photoproduct yields, Fc, from ACOB16 are shown to be incorrect as a result of the kinetic consequences of drastically different diffusion coefficients for the benzyl and p-hexadecylbenzyl radicals. Careful analyses of the transient absorption traces, based upon a new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k-CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed. Values of the rate constant for decarbonylation of the initially formed arylacetyl radicals, k-CO, have been measured from the rise portions of the laser-flash transient absorption traces. They confirm the assertion from results in liquid alkane media that decarbonylation rates are independent of microviscosity. The data separate components of a reaction from an (in-cage) "cage effect" and an (out-of-cage) "persistent radical effect" that are responsible for formation of AB-type (i.e., decarbonylated) products. The effects here are a consequence of vastly different rates of diffusion for coreacting A. and B. benzylic radicals rather than segregation of the radicals in different parts of a hetereogeneous environment (which leads to an excess of AA and BB products). Heretofore, observation of exclusive formation of AB products has been attributed to in-cage combinations of geminate radical pairs. We show that not to be the case here and provide methodologies which may be used for testing the importance of the "persistent radical effect" component of reaction.

Photochemical and Photophysical Studies of and in Bulk Polymers

“…163 When ACOB 1 was irradiated near the glass transition temperature, F c was found to depend on the degree of conversion of ACOB 1 . 149 At these temperatures, the rotational and translational mobility of ACOB 1 is reduced due to the near cessation of the long segmental chain motions of the matrix. As a result, the molecules of ACOB 1 are unable to change their conformations rapidly, and different conformations and sites can allow differing amounts of cage escape, return to ACOB 1 , and in -cage reactivity.…”

“…149 Thus, higher than statistical amounts of cross -termination (AA and BB type) products are observed during the photolysis of dibenzyl ketones in sodium dodecyl sulfate micelles. 167 High mobility of one of the radicals can also enhance the AB -type product even when reaction occurs out -of -cage due to the persistent radical effect.…”

“…145,148 They are somewhat sensitive to solvent polarity but not to medium viscosity, even in polymers such as PE 149 The singlet radical pairs generated during photo -Fries rearrangements of 1 -naphthyl acetate recombine in less than 1 ns in acetonitrile solutions at room temperature. 140,150 In anisotropic, viscous media, recombinations are slowed by the cages in which the reactant molecules reside.…”

“…11.20 is no longer valid and a kinetic model, involving the transient absorption of radicals, has been developed to determine the true recombination factor, F cAB . 149 This model uses k -CO and the intensities of the transient absorptions from the benzylic radicals immediately after the fl ash and after the completion of the incage portion of the radical combination reactions to calculate F cAB .…”

“…149 This result was interpreted as a manifestation of the persistent radical effect because the rates of translational diffusion of benzyl (A • ) and the 4 -hexadecyltolyl (B • ) radicals are expected to be very different, especially in a polymer matrix. In essence, the probability of one B • fi nding another for self -termination is very small due to their low diffusivity in PE ; however, the high mobility of A • radicals allows them to diffuse to the sites occupied by B • radicals to form the AB product.…”