Ab initio study of novel low-band-gap donor–acceptor polymers based on poly(cyclopentadienylene)

Abstract

Abinitio determinations of the band structures of two novel donor–acceptor polymers, PPDCN and PPDCO, based on poly(cyclopentadienylene) (PPD) are reported. The repeat unit of these polymers is a bicyclopentadienylene unit bridged by an electron-accepting group Y (Y = [double bond splayed left]C=O in PPDCO and [double bond splayed left]C=C(CN)₂ in PPDCN). Both PPDCO and PPDCN, on the basis of their geometries and π-bond orders, are found to have benzenoid-like structures, as do the corresponding thiophene analogues poly-4H-cyclopenta-dithiophene-4-one (PCDT) and polydicyanomethylene-cyclopenta-dithiophene (PCNTh). However, the band-gap values of PPDCO and PPDCN are smaller than those of the thiophene analogues owing to the weak electronic effects of the [double bond splayed left]CH₂ bridging group. The smaller band-gap values of PPDCO and PPDCN are rationalised on the basis of the patterns of their frontier orbitals.