AB-INITIO STUDY OF NOVEL LOW-BAND-GAP DONOR-ACCEPTOR POLYMERS BASED ON POLY(CYCLOPENTADIENYLENE)

Ab initio determinations of the band structures of two novel donor-acc eptor polymers, PPDCN and PPDCO, based on poly(cyclopentadienylene) (P PD) are reported. The repeat unit of these polymers is a bicyclopentad ienylene unit bridged by an electron-accepting group Y (Y = drop C=O i n PPDCO and drop C=C(CN)(2) in PPDCN). Both PPDCO and PPDCN, on the ba sis of their geometries and pi-bond orders, are found to have benzenoi d-like structures, as do the corresponding thiophene analogues poly-4H -cyclopenta-ithiophene-4-one (PCDT) and polydicyanomethylene-cyclopent a-dithiophene (PCNTh). However, the band-gap values of PPDCO and PPDCN are smaller than those of the thiophene analogues owing to the weak e lectronic effects of the drop CH2 bridging group. The smaller band-gap values of PPDCO and PPDCN are rationalised on the basis of the patter ns of their frontier orbitals.