DFT and TD DFT study of isomeric linear benzodifuranones, benzodipyrrolinones and their homologues

Ground state energy and geometry of till unknown 24 mutually isomeric and π-isoelectronic linear polyacenes with two furanone or pyrrolinone rings as the end groups were investigated by density functional theory (DFT). Their 16 symmetrical diphenyl derivatives, among which two were synthesized, were studied, too. Vertical excitation energies were computed by time dependent (TD) DFT and semiempirical ZINDO methods. No tendency to perimeter carbon–carbon bond unification was found, the central hydrocarbon moiety is of quinodimethane character. Both methods predict the similar energies of the lowest allowed spectral transition and its dependence on a size of a conjugated system, but strikingly differ in a prediction of the energies of symmetry forbidden transitions. The comparison with experimental room and low temperature absorption spectra favours the TD DFT results to the ZINDO ones.