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The photophysics and the long wave ( λ ≥300 nm) photochemistry of poly (4′-ethoxyacrylophenone) were investigated. The quantum yield for fluorescence is low (10 −4 mol (einstein) −1 ), and the principal excited species is the carbonyl triplet. The quantum yield for phosphorescence is greater and the λ max value is 445 nm. The triplet lifetime at 77 K is 0.41 s, but 8.7 μs at 300 K. These observations are consistent with the formation of a low-lying triplet, which, in turn is the result of substitution of the aromatic ring with the electron donating ethoxy group. The principal photoprocess occurring in dilute solution (in CH 2 Cl 2 ) of the polymer is a Norrish Type II decomposition, which results in random chain scission and a rapid decrease in the molecular weight. The relatively low quantum yield (6×10 −2 mol (einstein) −1 ) for the scission process reflects the low reactivity of a triplet (which has some π, π ∗ character) in abstraction reactions. In the case of PEAP, H-abstraction is a prerequisite for biradical formation, which, in turn leads to chain scission. Chain scission was subject to inhibition by triplet quenchers, such as naphthalene and cis -1,3-pentadiene, and the data conformed to Stern–Volmer kinetics. However, the quenching rate constants, k Q were lower in magnitude than the corresponding diffusion controlled values, and it is proposed that this is principally due to a steric effect, the actual encounter of the quencher with the carbonyl triplet being subject to adverse steric interactions associated with the proximity of the relatively bulky polymer chain. |
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