Mobius strip versus Linear and cyclic polyacenes:: a Huckel and semiempirical investigation

The electronic structure of finite and infinite linear, cyclic and Mobius strip polyacenes has been investigated by adopting Huckel and semiempirical schemes. Using the Huckel approach, it turns out that the Mobius belting process modifies the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital(LUMO) gap in such a way its evolution with chain length is similar to the linear polyacenes rather than their cyclic analogs. These results are corroborated at the Austin model 1 (AM1) level, where the geometry relaxation effects are taken into account. The optimized AM 1 structures show that the Mobius defect is localized and extends over a third of the ring. With respect to the Huckel approach, accounting for geometry distortion at the AM1 levels results in an increase in the HOMO-LUMO gap of the Mobius strip relative to the linear and cyclic finite-size structures. On the other hand. when including electron-hole correlation at the configuration interaction singles/Zerner's intermediate neglect of differential overlap level the behavior with system size of the first excitation energy of cyclic and Mobius polyacenes differs from their linear analogs and leads to smaller singlet excitation energies.