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Summary: Low‐bandgap π‐conjugated polymers that consist of alkyl thiophene/alkoxy phenylene and 2,3‐diphenylthieno[3,4‐b]pyrazine units have been prepared in high yields by a Sonogashira polycondensation. The copolymers are characterized by NMR, IR, UV, GPC, and elemental analysis. Thin films of the polymers P1, P2, and P3 exhibit an optical bandgap of ≈1.57–1.60 eV. Under simulated AM 1.5 conditions P2/PCBM devices on polyester foil provide a short circuit current of ISC = 10.72 mA · cm−2, an open circuit voltage of Voc = 0.67 V, and a power conversion efficiency of 2.37%.

The Horner-Wadsworth-Emmons olefination reaction of luminophoric dialdehydes 1 and 2 and bisphosphonates 3 provide high-molecular-weight and thermostable PPV/PPE hybrid polymers 4 and 5 of well-defined general constitutional structure -(CHdCH-Ph-CHdCH-Ph-CtC-Ph-CtC-Ph-) n, which was confirmed by NMR, infrared and elemental analysis. Soluble and good film-forming materials were obtained after attaching long linear alkoxy, e.g., dodecyloxy, octadecyloxy, or branched alkoxy side chains, e.g., 2-ethylhexyloxy, on the conjugated backbone. Thermotropic and lyotropic liquid crystalline behavior was observed with polarized optical microscopy. The presence of triple bonds along the PPV backbone increases the electron affinity of these polymers, which is reflected by the comparatively (with MEH-PPV) higher oxidation potential of 1.30 V vs Ag/AgCl. Polymers 4 and 5 are good photoconducting and highly luminescent materials. While almost identical photophysical behaviors for all polymers of type 4 (λ max,abs ) 450 nm, λmax,em ) 490 nm) or 5 (λmax,abs ) 470 nm, λmax,em ) 553 nm) were obtained in dilute chloroform solution, resulting in fluorescence quantum yields between 70 and 80% of the yellowish-green emission, the solid-state properties (color, thermal behavior, photoconductivity, absorption and emission spectra, and photoluminescence quantum yields) are dependent on the size, geometry, number, and location of the grafted alkoxy side groups. Exchanging for example the position of the side chains from 4ac) leads not only to a change in color of the material from orange to yellow but also a dramatic change in the photophysical behavior. In general, octadecyloxy side chains in position R 4 are necessary to obtain narrow and structured emission curves, small Stokes shifts, less excimer formation, and higher fluorescence quantum yields (30-40%). The fully substituted, deep orange-red polymer 5a (R 2 ) R 4 ) O(CH2)17CH3), for example, behaves as if its conjugated backbone was dissolved in a hydrocarbon solvent. This is confirmed by its very high photocurrent of 1.1 × 10 -9 A (which is at least 2 orders of magnitude higher than that of all the other polymers), detected at the lowest threshold voltage of 10 V, and its highest φfl value of 54%. It can be assumed from these facts that photoconductivity is more an intramolecular phenomenon than an intermolecular one. Strong π-π interchain interactions not only lead to fluorescence quenching through excimer formation but also have a negative effect on photoconductivity.

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 1H NMR, 13C 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., 13 cc) side chains at position R2 (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 ≈ 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 ≈ 2.30 eV, from the absorption spectra.