REPEAT UNIT SYMMETRY EFFECTS ON THE PHYSICAL AND ELECTRONIC-PROPERTIES OF PROCESSABLE, ELECTRICALLY CONDUCTING, SUBSTITUTED POLY(1,4-BIS(2-THIENYL)PHENYLENES)

The effect of repeat unit symmetry on the physical and electronic properties of a series of conjugated polymers composed of 2,5-disubstituted 1,4-bis(2-thienyl)phenylene repeat units, symmetrically and asymmetrically substituted with methoxy, heptoxy, dodecyloxy, hexadecyloxy, eicosinoxy, and hexyl groups has been studied. The polymers were prepared via an oxidative polymorization using ferric chloride and isolated after dedoping with ammonium hydroxide. The neutral polymers, soluble in common organic solvents including chloroform, methylene chloride, and tetrahydrofuran, are predominantly linked at the 5 and 5' positions of the thiophene rings to yield fully conjugated main chains that are stable to 360-degrees-C under nitrogen. Molecular weight analyses indicate weight-average degrees of polymerization (X(w)) of ca. 10-20, and, thus, a substantial fraction of chains in each sample contains 30-50 aromatic rings. Differential scanning calorimetry studies revealed that melt transitions, indicative of molecular ordering, are present in the symmetrically substituted polymers. The order found in these polymers was confirmed by X-ray diffraction and was found to persist at elevated temperatures, yielding birefringent melts, suggestive of liquid crystalline order. The asymmetrically substituted polymers were found to be amorphous. Upon oxidation of the insulating polymers, bipolaronic charge carriers were formed with the oxidized polymers exhibiting electrical conductivities up to 4 OMEGA-1 cm-1.