Möbius strip versus linear and cyclic polyacenes: A Hückel and semiempirical investigation

The electronic structure of finite and infinite linear, cyclic and Möbius strip polyacenes has been investigated by adopting Hückel and semiempirical schemes. Using the Hückel approach, it turns out that the Möbius 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 (AM 1) level, where the geometry relaxation effects are taken into account. The optimized AM 1 structures show that the Möbius defect is localized and extends over a third of the ring. With respect to the Hückel approach, accounting for geometry distortion at the AM 1 levels results in an increase in the HOMO-LUMO gap of the Möbius 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 Möbius polyacenes differs from their linear analogs and leads to smaller singlet excitation energies.