Linear, cyclic, and mobius strip polyacenes: The influence of the topologyon the size-dependent HOMO-LUMO energy gap

The electronic structure of finite and infinite linear, cyclic, and Mobius strip polyacenes has been investigated by adopting simple Huckel methodolog y without and with variable beta resonance integrals and the more involved MNDO scheme. Using the Huckel approach, it turns out that the linear polyac enes and the Mobius ladder molecules exhibit HOMO-LUMO gap which monotonous ly decrease with chain length while their cyclic analogs have a sawtooth be havior. The originality of the approach is to combine band structure and MO approaches to get analytical forms of the energy levels. These results are corroborated at the MNDO levels where the geometry relaxation effects are taken into account. The optimized MNDO structures show that the Mobius defe ct is localized and extends over approximately one-third of the ladder. Acc ounting for geometry distortion at the MNDO level results in an increase of the gap of the Mobius strip relative to the linear and cyclic finite size structures, but the similar results for the two methods confirm that the to pology is the main factor acting on HOMO-LUMO energy gaps. (C) 2001 John Wi ley & Sons, Inc.