Efficient conducting channels formed by the ð-ð stacking in single [2,2]paracyclophane molecules

Abstract

The electronic transportproperties of single [2,2]paracyclophane molecules directly connected to gold and platinumelectrodes have been investigated both theoretically and experimentally by using first-principles quantum transport simulations and break-junction experiments. For comparison, investigations on [3,3]- and [4,4]-paracyclophanes have also been performed. Our calculations show that the strength of the ?-? interaction in paracyclophanes is critically dependent on the inter-ring distance. In contrast to [4,4]paracyclophane in which the ?-? interaction is very weak due to the large inter-ring distance, the ?-? interaction in [2,2]- and [3,3]-paracyclophanes is rather strong and dominates the electronic transportproperties. In particular, for the asymmetric Au-[2,2]paracyclophane-Au junction in which the [2,2]paracyclophane molecule is connected to each goldelectrode through a Au adatom and the two Au adatoms are attached in ?1-fashion to two carbon atoms in the benzene backbones connecting with different ethylene groups, the transmission coefficient at the Fermi level is calculated to be 1.0 ? 10?2, in excellent agreement with experiments. When the goldelectrodes are replaced by platinum, the calculated transmission coefficient at the Fermi level of the symmetric Pt-[2,2]paracyclophane-Pt junction with one Pt adatom used as the linker group is increased to 0.83, demonstrating that the ?-? stacking in [2,2]paracyclophane is efficient for electron transport when the molecule-electrode interfaces are electronically transparent. This is confirmed by our preliminary experimental studies on the Pt-[2,2]paracyclophane-Pt junctions, for which the low-bias junction conductance has reached 0.40 ? 0.02 G0 (G0 is the conductance quantum). These findings are helpful for the design of molecular electronic devices incorporating ?-? stacking molecular systems