1 Scientific RepoRts | 6:37352 | DOI: 10.1038/srep37352 www.nature.com/scientificreports tuning the electrical conductance of metalloporphyrin supramolecular wires Mohammed Noori 1,2 , Albert C. Aragonès 3,4,5 , Giuseppe Di palma 6 , Nadim Darwish 3,4 , steven W. D. Bailey 1 , Qusiy Al-Galiby 1,7 , Iain Grace 1 , David B. Amabilino 8 , Arántzazu González-Campo 6 , Ismael Díez-pérez 3,4,5 & Colin J. Lambert 1 In contrast with conventional single-molecule junctions, in which the current fows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-Dpp) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current fows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (Cpp) junctions have three-orders-of-magnitude higher electrical conductances than their current- in-plane (CIP) counterparts, ranging from 2.10 -2 G 0 for Ni-DPP up to 8.10 -2 G 0 for Zn-Dpp. the metal ion in the center of the Dpp skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We fnd that the binding energies of Zn-DPP and Co-DPP are signifcantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-Dpp as the favoured candidate for high- conductance Cpp single-molecule devices. Porphyrins ofer a variety of desirable features as building blocks for future molecular-scale devices including their highly-conjugated structure, rigid planar geometry, high chemical stability and their ability to form metal- loporphyrins by coordinating metal ions in the center of their macrocyclic and aromatic skeleton 1–5 . Following early work, which established their chemical and biological properties 6–9 , porphyrins have become a focus of interest both for experimental and theoretical investigations of molecular electronics 10–12 and for the design of complexes using supramolecular chemistry, leading to a diverse array of structures available for nano-scale build- ing blocks 13 . Tis unique combination of properties is exploited in nature, where for example metalloporphyrins acts as charge carriers in naturally occurring processes such as photosynthesis 14–17 and in the respiratory chain 18,19 . In many of these processes, the plane of the porphyrin skeleton is stacked perpendicular to the direction of charge transport, whereas previous studies 10–12 address conductance with the plane of the porphyrin skeleton aligned parallel to the direction of charge transport. In the latter “current in plane” (CIP) up-right confguration (Fig. 1a), the porphyrin skeleton was contacted to gold electrodes via thiol or pyridyl anchor groups and the electrical con- ductance was found to be low 10,20 (of order nanosiemens). For the purpose of developing future single-molecule electronics and thermoelectrics, it is highly desirable to increase the electrical conductance, since this can lead to higher switching speeds and reduce the relative efect of parasitic phonons in thermoelectric devices. In what follows we develop a strategy for increasing the electrical conductance of porphyrin-based single-molecule wires by investigating their conductance with the current perpendicular to the plane (CPP) (Fig. 1b). We report a joint experimental and theoretical study of CPP conductance trends and binding confgurations across a family of 5,15-diphenylporphyrins (DPPs), with a centrally-coordinated divalent metal ion of either Co(II), Ni, Cu or Zn and demonstrate that their conductance and stability can be tuned through the choice of metal atom. Tis is an 1 Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK. 2 Department of Physics, collage of Science, Thi-Qar University, Iraq. 3 Department of Physical Chemistry, University of Barcelona, Diagonal 645, Spain. 4 institute for Bioengineering of Catalonia (IBEC) Baldiri Reixac 15-21, 08028 Barcelona, Catalonia, Spain. 5 centro investigación Biomédica en Red (CIBER-BBN). Campus Río Ebro-Edifcio I+D, Poeta Mariano Esquillor s/n, 50018 Zaragoza, Spain. 6 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus Universitari, 08193 Bellaterra, Catalonia, Spain. 7 Physics Department, College of Education, Al-Qadisiyah University, Iraq. 8 School of chemistry, the University of Nottingham, University Park, Nottingham, NG7 2RD, UK. Correspondence and requests for materials should be addressed to C.J.L. (email: c.lambert@lancaster.ac.uk) Received: 27 June 2016 accepted: 21 October 2016 Published: 21 November 2016 opeN