Comparison of Diffusion of N,N-Dimethylaniline and N,N-Dioctadecylaniline in a Low-Density Polyethylene Film. Activation Energies and Detection of Two Diffusion Pathways

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

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.

Combinations of chiral and prochiral singlet radical-pairs in reaction cavities of polyethylene films. Control and analysis of radical tumbling and translation

The regio- and stereo-chemistries of combination products from chiral 1-naphthoxy/(R)-2-phenylpropanoyl and prochiral 1-naphthoxy/1-phenylethyl singlet radical-pairs (radical-pairs A and B, respectively) have been studied at different temperatures in polyethylene (PE) films with different crystallinities. The radical-pairs have been generated as intermediates along the photo-Fries reaction course of 1-naphthyl (R)-2-phenylpropanoate ((R)-1) and the photo-Claisen reaction course of 1-naphthyl (R)-1-phenylethyl ether ((R)-2). Radical-pair was produced directly upon lysis of the first excited singlet state of (R)-2 and indirectly after irradiation of (R)-1 and subsequent decarbonylation of the 2-phenylpropanoyl radical of radical-pair A. Comparison of the fates of the directly and indirectly formed radical-pairs provides detailed information about the nature of the reaction cavities within the polyethylene hosts and how the combinations of the radical-pairs are influenced by their initial locations within a cavity. The results, especially when taken with those from irradiations in n-alkanes, indicate that the cavities are "templated" by the (R)-1 and (R)-2 guest molecules and that the templated shapes are retained in some form for periods that are at least as long as the time required for decarbonylation of a 2-phenylpropanoyl radical. In addition, the enantiomeric excesses of the decarbonylated photoproducts from (R)-1(2, 2-(1-phenylethyl)-1-naphthol (2BN), and 4-(1-phenylethyl)-1-naphthol (4BN)) or 2BN and 4 BN from (R)-2 indicate different influences of temperature on translational and tumbling motions of the radicals of radical-pairs B within their polyethylene cages.

Temperature-dependent cage effects from triplet radical pairs generated upon irradiation of 1-(4-methylphenyl)-3-phenyl-2-propanone in polyethylene films

The photochemistry of 1-(4-methylphenyl)-3-phenyl-2-propanone (MeDBK) in polyethylene (PE) films of varying crystallinity and at different temperatures has been investigated. Specifically, the selectivity of combination of the initially formed triplet pairs of benzylic radicals derived from irradiation of MeDBK has been used to determine the natures of the PE reaction cavities and how they influence the motions of the radicals. The results indicate that in-cage combinations are favored at lower temperatures and in PE of higher crystallinity. At temperatures significantly above the glass transition of PE, the fraction of radical pairs combining in-cage (F(c)) is independent of the extent of conversion of MeDBK. The data are interpreted according to a structural model that considers the shape, free volume, wall stiffness, and permeability of the reaction cavities and a dynamic model that includes the competition between the rates of in-cage combination and cage escape by the radical pairs. Near the glass transition temperature, F(c) decreases as the temperature is reduced and the data are interpreted using a stochastic model that considers the slow relaxation of the walls of the reaction cavities. The results demonstrate that the cavities afforded by PE can be subtly 'tuned' by temperature changes and microscopic variations in the polymer structure and morphology.