Synthesis and Properties of New Small Band Gap Conjugated Polymers: Methine Bridged Poly(3,4-ethylenedioxypyrrole)

Abstract

Synthesis and electronic properties of new small band gap conjugated polymers, methine-bridged poly(3,4-ethylenedioxypyrrole) (PEDOP) are reported. Density functional theory (B3LYP with 6-31G basis) are used to obtain the optimized ground-state geometries and electronic structures of PEDOP and poly [(3,4-ethylenedioxypyrrole-2,5-diyl) methine] (PEDOP-M). Theoretical bond length alternation of PEDOP is reduced by incorporating the methine bridge and leads to the band gap reduction from 2.44 to 0.68 eV. The small band gap characteristic of PEDOP-M is further verified by preparing two conjugated polymers, poly[(2,5-n-benzyl-3,4-ethylenedioxypyrrolediyl)-benzylidene-(2,5-n-benzyl-3,4-ethylenedioxypyrrole-quinodimethanediyl)] (PbEDOP-b) and poly[(2,5-n-benzyl-3,4-ethylenedioxypyrrolediyl)-(p-nitrobenzylidene)-(2,5-nbenzyl-3,4-ethylenedioxypyrrole-quinodimethanediyl)] (PbEDOP-nb). The optical band gap of PbEDOP-b and PbEDOPnb are 1.77 and 1.45 eV, respectively, while the electrochemical band gap of the former is 1.59 eV. Although the bulky side groups of these two polymers result in a larger band gap than that of PEDOT-M, it indicates the small band gaps of such polymers. The nitrobenzene group could extend the -conjugation and lead to the smaller band gap of PbEDOP-nb than that of PbEDOP-b. The present study suggests that methine-bridged poly(3,4-ethylenedioxypyrrole) is a class of small band gap polymers.