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

Abstract

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.

Cage Effect under Photolysis in Polymer Matrices

“…One approach is based on product analysis of photodissociation of organic compounds in polymers. It was successfully used by Weiss, Chesta, and their collaborators [8][9][10][11][12][13]. Norrish type I cleavage of 1-(4-alkylphenyl)-3-phenyl-2-propanone (ACOB) was used as a probe of polymer matrices:…”

Photochemical and Photophysical Studies of and in Bulk Polymers

“…The combination of benzylic radical pairs requires that they be within van der Waals contact distances, be oriented appropriately, and have singlet character. 163 The triplet benzylic radical pair, therefore, must undergo intersystem crossing before or during C -C bond formation. If the size of the sites occupied by a pair of benzylic radicals is very small, the triplet -singlet energy gap will be very high, and intersystem crossing (ISC) will be very slow.…”

“…164 In the case of ACOB 1 the excited -singlet state ( τ = 3.6 × 10 − 9 s at 295 K in benzene) undergoes ISC to its triplet state quantitatively ( ϕ ST ∼ 1; k ISC > 10 8 s − 1 at 295 K in benzene). The triplet state then undergoes α -cleavage very rapidly ( ∼ 10 − 10 s), followed by decarbonylation of the arylacetyl group ( k -CO > 10 6 s − 1 at 295 K in hexane 163,165 The reactivity and photoproduct distribution of dibenzyl ketones in PE fi lms depend mainly on the mean free volume. The free volume available in partially crystalline PE (113 Å 3 for PE with 68% crystallinity 163 ) is comparable to the calculated molecular volume of ACOB 1 .…”

“…The triplet state then undergoes α -cleavage very rapidly ( ∼ 10 − 10 s), followed by decarbonylation of the arylacetyl group ( k -CO > 10 6 s − 1 at 295 K in hexane 163,165 The reactivity and photoproduct distribution of dibenzyl ketones in PE fi lms depend mainly on the mean free volume. The free volume available in partially crystalline PE (113 Å 3 for PE with 68% crystallinity 163 ) is comparable to the calculated molecular volume of ACOB 1 . As a result, the " residence time " of the triplet radical pairs in their reaction cages is comparable to the spin evolution times, and high values of F c are found.…”

“…11.19 ). 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.…”