Investigation of the photophysical and electrochemical properties of alkoxy-substituted arylene-ethynylene/arylene-vinylene hybrid polymers

High-molecular-weight, soluble and thermostable alkoxy-substituted arylene-ethynylene/ arylene-vinylene conjugated polymers, 13 and 14, have been successfully synthesized through the Horner-Wadsworth-Emmons olefination of luminophoric dialdehydes 7 and 9 and bisphosphonate 12 in very good yields. They were characterized through H-1 NMR, C-13 NMR, IR, and elemental analysis. The investigation of their photophysical and electrochemical properties has been carried out. Although almost identical absorption and emission spectra were obtained in dilute chloroform solution for all polymers 13, the full width at half-maximum (fwhm) value of the emission curves depends on the length of the attached side chains. The presence of anthracenylene units in 14 leads to a red shift of its absorption and emission spectra relative to 13. Strong self-reabsorption after excitation in solution was observed for this polymer. The solid-state photophysical properties of 13 and 14 (photoconductivity, absorption and emission spectra, fluorescence quantum yield, Stokes shift, and fwhm) greatly depend on the nature (linear or branched), length, and location of the grafted alkoxy side groups. Photoconductivity is easily detected in polymers having octadecyloxy chains (13aa, 13ab, 14). Long linear (octadecyl, i.e., 13aa) or short branched (2-ethylhexyl, i.e., 13cc) side chains at position R-2 (phenylene-vinylene segment) are necessary to obtain sharp and well-resolved emission spectra accompanied by high fluorescence quantum yields. The quasi-donor (phenylene-vinylene segment)-acceptor (arylene-ethynylene segment) nature of these polymers could explain the great discrepancy between the electrochemical band gap energy, E-g(ec) approximate to 1.60 eV, as obtained from the onset values of the redox potentials in cyclic voltammetry and in differential pulse polarography measurements, and the optical band gap energy, E-g(opt) approximate to 2.30 eV, from the absorption spectra.