Photophysics and photochemistry of poly (4′-ethoxyacrylophenone)

Abstract

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 2Cl 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.