Synthesis, characterization, and electrical properties of poly(1-alkyl-2, 5-pyrrylene vinylenes): New low band gap conducting polymers

A series of poly(1-alkyl-2,5-pyrrylene vinylenes) (alkyl = methyl(la), hexyl (Ib), dodecyl (Ic)) have been synthesized from the monomers, 1-alkyl-2,5-bis(thiophenylmethylene)pyrrole (2), by base-induced elimination and polymerization. Characterization of the polymers includes IR, H-1 and C-13 NMR, and UV-vis spectroscopy as well as TGA and molecular weight studies. Varying the nitrogen substituent on the pyrrole ring among methyl, hexyl, and dodecyl groups strongly affects their electrical properties. The resulting deep purple conjugated polymers (Ib, Ic) were soluble in a variety of organic solvents. The highest yield was obtained when the polymers were synthesized by refluxing in THF with a monomer/base mole ratio of 1:4. The band gaps of the undoped polymers were 1.82 eV (la), 1.69 eV (Ib), and 1.67 eV (Ic).