Density functional crystal orbital study of cyano-substituted poly(para-phenylene-vinylene) and poly(quinoxaline-vinylene)

We have calculated the optical and electronic properties of several conjugated organic polymers: poly(p-phenylene-vinylene) (ppv) and its derivatives. Cyano substitutions on the phenylene ring: poly(2,5-dicyano-p-phenylene-vinylene) (2,5-dcn-ppv) and on the vinylene linkage: poly(p-phenylene-7(,8)-(di)cyano-vinylene) are considered. In addition, poly(quinoxaline-vinylene) (pqv) is studied. The infinite isolated quasi-1d chains are treated with periodic boundary conditions, using atomic basis sets. In a comparative study of ppv, some issues regarding the selection of the functionals and basis sets are discussed and excitation energies derived from time-dependent and from ordinary methods are compared. It is concluded that for these polymers the calculations are informative at the b3lyp/6-31g** density functional theory (dft) level. The absolute values might change with improved methods, but the similarity of the polymers suggests that the relative characterization is adequate. Band structures are communicated along with characteristics of the highest occupied and the lowest unoccupied crystal orbitals (hoco and luco). Electron affinities, ionization potentials, valence and conduction bandwidths, and effective masses at the bandgap are given.