Electrical rectification through cumulenic bridge: a computational study

Frontier molecular orbital analysis for the electrical rectification behavior of some donor-acceptor cumulenes has been carried out at B3LYP/6-31G** level of theory and the potential drop (PD) which is a measure of rectification behavior of the molecular system is estimated from the orbital energies of molecular orbitals responsible for the conduction channel. It is observed that for cumulenes with even numbers of double bonds as bridges, there is complete spatial localization of the frontier molecular orbitals, Highest occupied molecular orbital (HOMO) is localized on donor, Lowest unoccupied molecular orbital (LUMO) is localized on acceptor side and LUMO + 1 is localized again in the donor part, where as for molecules with odd number of double bonds, the frontier molecular orbitals (HOMO as well as LUMO) are delocalized all over the molecule. It is also observed that there is a decrease in PD (from 1.56-0.82 to 0.46 eV for molecules with two, four and six cumulenic double bonds) for increase in the number of double bonds and this is quite the opposite trend to that of the well studied D-σ-A systems where increase in the number of methylenic linkage increases the PD drastically. For the molecules with odd number of double bonds due to their low lying LUMO delocalized all over the molecule, it is suggested that they may find applications as molecular conductors. These donor-acceptor cumelenic compounds which are showing the opposite trend in rectifying behavior to that of the normal D-σ-A systems can be a useful alternate for sigma bonded molecular electronic systems.