SMALL-BANDGAP CONDUCTING POLYMERS-BASED ON CONJUGATED POLY(HETEROARYLENE METHINES) .2. SYNTHESIS, STRUCTURE, AND PROPERTIES

A series of soluble conjugated poly(heteroarylene methines) with varyi ng number of 2,5-thienylene rings have been synthesized, characterized , and shown to have small intrinsic optical and electrochemical bandga ps. The conjugated polymers containing alternating aromatic and quinoi d heteroarylene moieties in the main chain were prepared by oxidative dehydrogenation of precursor poly-(heteroarylene methylenes) with 2,3- dicyano-5,6-dichloro-1,4-benzoquinone (DDQ). The optical and electroch emical properties of the conjugated poly(heteroarylene methines), such as bandgap, redox potentials, ionization potential, and electron affi nity, were found to be significantly modified by the size of the quino id moieties in the main chain and the type of donor or acceptor side g roup at the methine carbon. Some of the poly(heteroarylene methines) h ave intrinsic bandgaps (E(g)) that are significantly smaller than foun d for the parent polythiophene (E(g) similar to 2 eV): PBTHBQ (E(g) = 1.14 eV); PBTBQ (E(g) 1.27 eV); PBTTBQ (E(g) = 1.31 eV); and PBTNBQ (E (g) = 1.45 eV). These same four polymers have peak oxidation potential s in the range 0.29-0.54 V (eV) and peak reduction potentials in the r ange -1.82 to -1.49 V (SCE). The electrical conductivity of iodine-dop ed thin films of PBTBQ, PBTNBQ, and PBTHBQ was 6 X 10(-3), 2.2 X 10(-3 ), and 1.6 x 10(-1) S/cm, respectively, compared to a conductivity of less than 10(-7) S/cm for the undoped materials. These results confirm that conjugated poly(heteroarylene methines) are small-bandgap conduc ting polymers which can be prepared by oxidative dehydrogenation of pr ecursors.