Electrical characterization of 7 nm long conjugated molecular wires: experimental and theoretical studies

Abstract

This article describes arylene–ethynylene molecular wires with 7 nm long backbones and thiolated termini. Cyclic voltammetric studies in solution reveal that the reduction waves of the fluorene, 9-[(4-pyridyl)methylene]fluorene and 9-[di(4-pyridyl)methylene]fluorene units which are embedded in the conjugated π-systems endow these wires with n-doping characteristics. An x-ray crystal structure investigation of 2,7-diiodo-9-[bis(4-pyridinium)methylene]fluorene bis(tetrafluoroborate) 8 established that protonation occurs on both nitrogens of this unit. Self-assembled monolayers of the 7 nm wire 2 on gold substrates exhibit symmetrical current–voltage (I–V) characteristics when contacted by a gold scanning transmission microscope (STM) tip. The dipyridyl functionality of 2 served to obtain a rectifying junction in which the diprotonated cationic wire is the electron accepting component in combination with an adjacent anionic phthalocyanine as the electron-donating layer. This ionic Au–2H22+[CuPc(SO3−)4(Na+)n]2/(4−n) bilayer assembly exhibits rectification with current ratios of 15–50 at ± 1 V. This dramatic change in I–V characteristics upon simple chemical manipulation proves that the conductivity is a property of the wire molecules 2 in the junction. Ab initio calculations suggest that the molecular wires possess useful structural features which allow the conductance of the molecule to be altered by changing the properties of the side groups attached to the fluorene units.