We investigated electronic structures of four sets of monomers and polymers comprising of phenyl rings and five-membered hetero(aromatic) moieties connected with double-bond -X=X- linkages (X = CH, SiH, GeH, N, P, As) by density functional theory, time-dependent density functional theory, and periodic boundary condition calculations with B3LYP functional. Electronic structures of poly(p-phenylenevinylene) (PPV) analogues are primarily dominated by central double-bond moieties. The introduction of ethylene homologues with group 14 and 15 elements was demonstrated to be a promising approach to modify electronic structures of conjugated oligomers and polymers. Excitation energies of monomers with double-bond linkages were reduced by around 13-50% with respect to corresponding dimers of phenyl, thienyl, furyl, and pyrrolyl rings. Similarly, band gaps of poly(p-phenylene) and polythiophene were decreased by 0.3-0.9 eV upon the insertion of double-bond linkages. Furthermore, excitation energies of monomers presented decreasing trends when descending through groups 14 and 15. For group 14 ethylene homologues, the decreasing trend in the lowest excitation energies was rationalized by a progressively favoring of pi-sigma* interactions as descending X = CH, SiH, and GeH. Increasing p contents of central bonds along X = N, P, and As accounted for geometry features and the lowest excitation energies of group 15 species. A decrease in the extent of electronic communications between aromatic rings and -X=X- linkages within higher congeners was also revealed.