A computational study of the factors affecting the electrical conductance of long chain n-porphyrin di-thiols

There has been much interest in the study of quantum transport in molecular nanowires due to their possible importance in molecular electronic devices. As the devices become smaller, it is important to understand the factors that govern the electrical conductance across single molecules. At the current time it is not possible to carry out ab initio calculations on all of these molecular systems due to the heavy computational demands. In this paper, we show that a semi-empirical approach, the self-consistent extended Hückel method (SC-EHM), gives results for small molecular systems that compare favourably with computations based on density functional theory and so may be used in the study of larger systems.We have used the SC-EHM to investigate the effect of various factors on the conductance of the n-porphyrin molecules. In particular, we have studied how the conductance varies with the length of the n-porphyrin molecule, the metal atom in the porphyrin ring and the presence of explicit water molecules. We find that the water molecules induce a change in the work function at the gold electrodes causing a shift in the features in the electron density of states which in turn enhance the conductance. The attenuation of the conductance with length for this system is found to be in accordance with the results of other workers.