Diyne-Containing PPVs:  Solid-State Properties and Comparison of Their Photophysical and Electrochemical Properties with Those of Their Yne-Containing Counterparts

Diyne-containing poly(p-phenylene−vinylene)s, 4a − d, of general chemical structure −(Ph−C⋮C−C⋮C−Ph−CHCH−Ph−CHCH−) n , obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)-buta-1,3-diyne (2) with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 3a − d, are the subject of this report. The polymers exhibit great disparity in their degree of polymerization, n, which might be ascribed to side-chain-related differences in reactivity of the reactive species during the polycondensation process and which led to n-dependent absorption (solution and solid state) and emission (solution) behaviors of the polymers. Polarizing optical microscopy and differential scanning calorimetry are employed to probe their thermal behavior. The structure is investigated by means of wide-angle X-ray diffraction for both isotropic and macroscopically oriented samples. Comparison of photophysical (experimental and theoretical) and electrochemical properties of the polymers with those of their yne-containing counterparts 6a − d [−(Ph−C⋮C−Ph−CHCH−Ph−CHCH−) n ] has been carried out. Similar photophysical behavior was observed for both types of polymers despite the difference in backbone conjugation pattern. The introduction of a second yne unit in 4 lowers the HOMO and LUMO levels, thereby enhancing the electron affinity of polymers 4 compared to polymers 6. The “wider opening” introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical processes leading to minimal discrepancy, ΔE g, between the optical and electrochemical band gap energies. Polymers 6, in contrast, show significant side-chain-dependent ΔE g values. Low turn-on voltages between 2 and 3 V and maximal luminous efficiencies between 0.32 and 1.25 cd/A were obtained from LED devices of configuration ITO/PEDOT:PSS/polymer 4/Ca/Al.