Photodegradation of poly(3,5-dimethoxyacrylophenone) in the long-wave UV region

Abstract

The photophysics and long-wave (λ ≥ 300 nm) photochemistry of poly(3,5-dimethoxyacrylophenone) (P3,5DMAP) were investigated. The quantum yield for fluorescence is low (< 10 −4) and the principal excited species is the triplet carbonyl. Emission (phosphorescence) occurs at 450 nm, the triplet energy (i.e. E T (0-0)) is 270 kJ mol −1 and the phosphorescence lifetime is long (23 μs). These observations are consistent with the formation of a low-lying π,π ∗ triplet which, in turn, is the result of substitution of the aromatic ketone by electron donating methoxy groups. The principal photoprocess is a Norrish type II decomposition which results in random chain scission. The low value of the quantum yield, i.e. Φ cs = 10 4 , is further evidence for a π,π ∗ triplet which is typically unreactive in abstraction reactions. In the case of P3,5DMAP, H-abstraction is a prerequisite for biradical formation (and for chain scission). Inhibition of chain scission was investigated, using naphthalene and cis-1,3-pentadiene as quenchers, and the data could be represented by Stern-Volmer kinetics. However, the quenching rate constants ( k O ) were lower than the corresponding diffusion controlled values, and it was proposed that the actual encounter of triplet and quencher is subject to adverse steric interactions associated with the proximity of the polymer chains.