Photochemical hole burning of 1,4-dihydroxyanthraquinone in polymer glasses: Poly(ethylene), poly(methacrylic acid), and their copolymer systems

Photochemical hole spectra of 1,4-dihydroxyanthraquinone in polymer matrices at liquid helium temperatures were investigated by changing the main chain structure of polymer molecules systematically. Polymer molecules adopted here were poly(ethylene-co-methacrylic acid) (EMA) and its homopolymers poly (ethylene) (PE) and poly(methacrylic acid) (PMAA). Polarity of matrices or the number density of the electric dipoles mainly localized around the methacrylic acid units are decreased in the sequence of PMAA, EMA, and PE. Remarkable matrix effects on the hole-growth profiles were observed: The holewidth in the saturated regime decreased with decreasing the polarity of the polymer matrices from 1.4 cm^-1 in PMAA through 0.15 cm^-1 in EMA to 0.05 cm^-1 in PE. The model of the photoinduced hole broadening is proposed in relation to the nature of two-level systems of the polymer glasses which exert influence on the holewidth. Dipole-dipole coupling is possibly a dominant interaction in the present systems. The results on the annealing properties of holewidth are also discussed.