Theoretical and Experimental Characterization of Small Band Gap Poly(3,4-ethylenedioxythiophene methine)s

Abstract

Theoretical and experimental characterization of relatively small band gap methine-bridged poly(3,4-ethylenedioxythiophene)(PEDOT) derivatives is reported. AM1 and the modified extended Hückel theory were used to explore the ground-state geometric and electronic structures of poly[(3,4-ethylenedioxythiophene-2,5-diyl) methine] (PEDOT-M). The bond length alteration in PEDOT-M was found to be smaller than that in the reported methine-bridged polythiophene, and this resulted in a significantly lower band gap of 0.48 eV in PEDOT-M. The structure and properties of the methine-bridged PEDOT were further verified by synthesis of poly[(3,4-ethylenedioxythiophene-2,5-diyl)-benzylidene] (PEDOT-B) and poly[(3,4-ethylenedioxythiophene-2,5-diyl)-(p-methoxybenzylidene)] (PEDOT-MB). These polymers were found to be highly dehydrogenated and to contain a conjugated backbone of alternating aromatic and quinoid ethylenedioxythiophene segments. The optical and electrochemical band gaps of PEDOT-B are 0.87 and 1.05 eV, respectively, while those of PEDOT-MB are 0.86 and 1.01 eV. The methoxy substitution of PEDOT-MB results in a smaller ionization potential and electron affinity than those of PEDOT-B. The theoretical and experimental results show that methine-bridged PEDOT is a small band gap polymer.