Activity: Talk or presentation › Talk or presentation at a conference
Aromaticity is a key concept in describing the structure, stability, magnetic properties, and chemical reactivity of a large variety of compounds. Recent attention has been devoted to unravel the relationship between conductance and electron transport properties of active molecules in the field of molecular electronics . Such structure-property relationships provide a fundamental knowledge for a rational design and further property control of molecular electronic devices. Very recently, we have investigated the correlation between conductance and aromaticity in single-molecule junctions containing expanded porphyrins. Nowadays, expanded porphyrins are recognized as the test bed in which the concept of aromaticity can be explored, since they provide congeneric macrocycles with [4n+2] and [4n] π-electrons that can be easily interconverted by two-electron redox reactions. In addition, expanded porphyrins can adopt different -conjugation topologies, namely Mobius, Hückel and twisted-Hückel, each with distinct electronic properties and aromaticity . However, the quantification of Hückel and Möbius aromaticity in expanded porphyrins is challenging and requires the use of indices based on different criteria. Accordingly, we have proposed an extensive set of descriptors based on the structural, energetic, magnetic, reactivity and electronic criteria to describe the aromaticity in porphyrinoids [2,3]. Besides aromaticity, the electron transport properties of expanded porphyrins with different π-conjugation topologies and aromaticity were carefully investigated with the non-equilibrium Green´s function formalism in combination with density functional theory for various configurations of the gold contacts. Our findings reveal that the negative relationship between conductance and molecular aromaticity or polarizability does not hold for most of the configurations of the molecular junctions based on expanded porphyrins. Indeed, we found that enhancing aromaticity of Hückel untwisted expanded porphyrins increases single-molecule junction conductance . Eventually, the transport properties of expanded porphyrins are successfully rationalized in terms of the orbital rule, demonstrating that the phase and amplitude of the HOMO and LUMO orbitals play a major role in determining the fundamental aspects of molecular conductance of -conjugated molecules. Our results highlight the importance of the connectivity on determining the shape of the transmission functions of aromatic and antiaromatic states and we recently provide a set of qualitative rules to predict the presence of a quantum interference around the Fermi level for Hückel and Möbius cyclic polyenes at the Hückel level of theory . Together, our work demonstrates how the concept of aromaticity can be exploited to create a novel type of single-molecule switches with high ON/OFF ratios based on aromaticity and topology changes in porphyrinoids.