A Photoluminescent, Segmented OligoPolyphenylenevinylene Copolymer with Hydrogen-Bonding Pendant Chains

The syntheses and electronic spectroscopy of E,E-2,5bis[2-hydroxyethoxy]-1,4-bis[2-(3,4,5trimethoxyphenyl-1,2-ethenediyl]benzene, 3, and alternating block copolymer analogue alt-poly[1,8octanedioxy-2,6-dimethoxy-1,4-phenylene-1,2-(E)ethenediyl-2,5-(2-hydroxyethoxy)-1,4-phenylene-1,2(E)-ethenediyl-3,5-dimethoxy-1,4-phenylene], 2, are described. Polymer 2 has UV-vis absorbances at 320 nm (shoulder) and 389 nm (maximum), with fluorescence emission at 449 nm (maximum) and 480 nm (shoulder) upon excitation at various wavelengths. Model compound 3 shows virtually identical spectral behavior. Neat solid films of polymer 2 show UV-vis absorbance in the 400-450-nm region (broad) and fluorescence at about 490 nm. The 40-nm shift from solution to neat film emission for 2 is attributed to hydrogen-bond-assisted interactions between chromophores in the solid state. The fluorescence quantum yields in chloroform at room temperature (365-nm excitation) for 2 and 3 are 0.36 and 0.74, respectively, relative to quinine sulfate standards. There is no evidence of excimer formation, although crystallographic studies of model system 3 show hydrogen-bonded chains involving the hydroxyethoxy pendant chain. The model system also has a highly planar crystal structure, consistent with its high emission quantum yield; the planarity may be favored by interactions between the pendant hydroxyethoxy groups and the ethenediyl groups of the chromophore