Leary, Edmund and Limburg, Bart and Alanazy, Asma and Sangtarash, Sara and Grace, Iain and Swada, Katsutoshi and Esdaile, Louisa J. and Noori, Mohammed and González, M. Teresa and Rubio-bollinger, Gabino and Sadeghi, Hatef and Hodgson, Andrew and Agraït, Nícolas and Higgins, Simon J. and Lambert, Colin J. and Anderson, Harry L. and Nichols, Richard J. (2018) Bias-driven conductance increase with length in porphyrin tapes. Journal of the American Chemical Society. ISSN 0002-7863
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Abstract
A key goal in molecular electronics has been to find molecules that facilitate efficient charge transport over long distances. Normally molecular wires become less conductive with increasing length. Here we report a series of fused porphyrin oligomers for which the conductance increases substantially with length by > 10-fold at a bias of 0.7 V. This exceptional behavior can be attributed to the rapid decrease of the HOMO-LUMO gap with the length of fused porphyrins. In contrast, for butadiyne-linked porphyrin oligomers with moderate inter-ring coupling, a normal conductance decrease with length is found for all bias voltages explored (± 1 V), although the attenuation factor (β) decreases from ca. 2 nm-1 at low bias to < 1 nm-1 at 0.9 V, highlighting that β is not an intrinsic molecular property. Further theoretical analysis using density functional theory underlines the role of inter-site coupling and indicates that this large increase in conductance with length at increasing voltages can be generalized to other molecular oligomers.