New Electronically Conducting Polymers: Effects of Molecular Structure on Intrinsic Electronic Properties

Abstract

Of prime importance and interest in the continuing search for new electronically conducting polymers is the achievement of small or vanishing values of the semiconductor band gap (E_g) which governs the intrinsic electronic properties of materials. This paper will report on experimental studies of the effects of molecular structure on the intrinsic electronic properties of polymers. First, the structure and band gap of hetero-aromatic semiconducting polymers, including polythiophenes and polypyrroles, are discussed. It is suggested that planarity of polymer backbone is essential to small band gaps. From the results on “model compounds” for poly(2,5-thiophenediyl) it is suggested that this polymer has a non-planar backbone and an S-cis chain structure which largely accounts for about half of its 2.2eV band gap. Novel polybithiophenes¹ and polyterthiophenes² which have band gaps as small as 1.20 eV are described. Second, the synthesis and intrinsic electronic properties of a novel class of conjugated polymers containing alternating aromatic and quinonoid sequences is described³. These polymers exhibit band gaps as small as 0.75 eV, the smallest known value of band gap for organic polymers⁴. The quinonoid character of the polymer chains which is related to molecular parameters amenable to synthetic manipulation only partially explains why the band gap of this class of polymers is generally small. It is suggested that quantum well and superlattice effects may exist in one-dimensional conjugated polymer chains containing “mixed polymer repeating units” having different band gaps similar to inorganic semiconductor superlattice and quantum well heterostructures^5–7.