ELECTRONIC-STRUCTURES OF NEW PI-CONJUGATED CYCLIC POLYMERS WITH QUINOID STRUCTURES

Geometrical and electronic structures of new pi-conjugated five-membered ring polymers were theoretically investigated. These polymers are analogous to heterocyclic polymers, but adopt as bridging groups >CH2, >CF2, >SiH2, >SiF2, >C=CH2, >C=O and >C=S moieties instead of heteroatoms. The ground-state geometries of the polymers were predicted to be quinoid from semiempirical band calculations with AM1 Hamiltonian. The electronic properties of these systems were obtained using the modified extended Huckel method. The calculated band-gaps (E(g)) were analyzed in terms of geometrical relaxations and electronic effect of the bridging groups using the equation of E(g) = Delta E(delta r) Delta E(1-4) + Delta E(el). The effect of bond-length alternation (Delta E(delta r)) amounts to 1.1-1.4 eV for the aromatic forms and 1.8-1.9 eV for the quinoid forms of the polymers. The interactions (Delta E(1-4)) between C1 and C4 atoms of the cis-PA type backbone tend to decrease the band gaps of the aromatic forms and to increase the gaps of the quinoid forms as much as 0.2-0.5 eV, depending on the size of a bridging atom. It is found that the electronic effect (Delta E(el)) of these bridging groups is quite small compared to that found in heterocyclic polymers such as polythiophene, polypyrrole, and polyfuran. Delta E(el) of >CF2, >SiH2, and >SiF2 bridging groups are negligible and that of the other groups amounts to 0.3-1.0 eV. Therefore, the band gaps of these systems almost correspond to the Delta E(delta r) values which arise from the bond-length alternations, except the case of the polymers with >C=O and >C=S bridging groups whose pi* orbitals strongly interact with the pi system of the polymeric backbone. (C) 1995 American Institute of Physics.