New alternative donor-acceptor arranged poly(aryleneethynylene)s and their related compounds composed of five-membered electron-accepting 1,3,4-thiadiazole, 1,2,4-triazole, or 3,4-dinitrothiophene units: Synthesis, packing structure, and optical properties

A series of new poly(aryleneethynylene)-type π-conjugated copolymers, which consist of an electron-accepting 1,3,4-thiadiazole, 4-alkyl-1,2,4- triazole, or 3,4-dinitrothiophene unit and an electrondonating 1,4-didodecyloxybenzene or N-dodecylpyrrole unit, were prepared in 81-93% yields by palladium-catalyzed polycondensation. Their related model compounds (PhC≡C-Ar-C≡CPh; Ar = the electron-accepting unit) were also synthesized. GPC traces of the polymers gave number average molecular weights (M_n's) of 6900-29 700. The polymers formed a molecular assembly and a birefringent phase as revealed by powder X-ray diffraction (XRD) analysis and polarized light optical microscopy (POM). They formed an aligned structure on a platinum plate. The UV-vis peaks of the polymers appeared in the range of 385-522 nm in solutions, and the peak was shifted by 10-44 nm to a longer wavelength in films, according to intermolecular electronic interaction. The density functional theory (DFT) calculations supported the presence of strong intermolecular interaction between the polymer molecules. The polymers composed of a 1,3,4-thiadiazole or a 1,2,4-triazole unit exhibited photoluminescence with quantum yields of about 50% in chloroform. The polymers were electrochemically active and showed the reduction peak in a range of -1.1 through -2.2 V vs Ag⁺/Ag.