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Speaker: Dr Mercedes Alonso
Institute: VU Brussels
Country: Belgium
Speaker Link: https://we.vub.ac.be/~algc/algc_new/alonsomresearch.html

Mercedes Alonso

General Chemistry Department (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, Brussels, Belgium

Creating functional nanoscale devices using single molecules as active electronic components is the ultimate goal of the field of molecular electronics. Besides their potential to meet the growing demand for miniaturization of electronics, molecular electronics opens up the possibility of devices with novel, unforeseen functionalities beyond silicon-based technologies, such as molecular switches. Through a bottom-up quantum chemistry approach, we have shown that expanded porphyrins are flexible enough to switch between different π-conjugation topologies encoding distinct electronic properties and aromaticity.[1] Since these topology/aromaticity switches can be induced by different external stimuli,[2] these macrocycles represent a unique platform to develop molecular switches for different nanoelectronic applications.

The first application involves the conductance switching in molecular junctions through aromaticity and topology changes. In this regard, the electron transport properties of the different states of the switches were carefully investigated with the non-equilibrium Green´s function formalism in combination with density functional theory for various configurations of the gold contacts.[3] 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, so we devise new selection rules to predict the occurrence of quantum interference around the Fermi level for Hückel and Möbius systems.[4] A second application concerns the design of bithermoelectric switches, an entirely new class of switches that revert the direction of the heat and /or charge transport. Our in-house calculations reveal that the Hückel-Möbius topology switch in heptaphyrins causes the Seedbeck coefficient or thermopower to change considerably from +50 mV/K to -40 mV/K.[5] Finally, the mechanical activation of this novel type of switches is explored for the first time, leading to a straightforward approach based on distance matrices for the selection of pulling scenarios that promote either the Hückel or the Möbius topology.[6] Overall, our work demonstrates how the concept of aromaticity and molecular topology can be exploited to create a novel type of efficient switching devices.[7]


Figure 1. A bottom-up quantum chemical approach to design efficient nanoscale devices.


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[1] M. Alonso, P. Geerlings, F. De Proft, Chem. Eur. J. 2013, 19, 1617.

[2] M. Alonso, B. Pinter, P. Geerlings, F. De Proft, Chem. Eur. J 2015, 21, 17631; T. Woller, J. Contreras-García, P. Geerlings, F. De Proft, M. Alonso, Phys. Chem. Chem. Phys. 2016, 18, 11885;

[3] T. Stuyver, F. De Proft, P. Geerlings, M. Perrin, M. Alonso, J. Am. Chem. Soc. 2018, 140, 1313.

[4] T. Stuyver, S. Fias, P. Geerlings, F. De Proft, M. Alonso, J. Phys. Chem. C 2018, 122, 19482.

[5] T. Stuyver, P. Geerlings, F. De Proft, M. Alonso, J. Phys. Chem. C 2018, 122, 24436.

[6] T. Bettens, M. Hoffmann, M. Alonso, P. Geerlings, A. Dreuw, F. De Proft, Chem. Eur. J. 2021, ASAP.

[7] Woller, T.; Geerlings, P.; De Proft, F.; Champagne, B.; Alonso, M. J. Phys. Chem. C 2019, 123, 7318.

3 thoughts on “A bottom-up approach towards the design of molecular electronic devices”

  1. Monday, 15 February 2021 09:21

    Great talk Dr Alonso!  I'm interested in knowing more about quantification of aromaticity and would appreciate any references. Thanks in advance!

    1. Monday, 15 February 2021 10:03

      Dear Hugo

      To quantify Hückel and Möbius aromaticity, we proposed a novel multidimensional approach based on global and local descriptors, rooted on the energetic, magnetic, reactivity and electronic criteria. 

      As global descriptors, the isomerization method is applied for evaluating the aromatic stabilization energies, the magnetic susceptibility exaltation and the relative hardness of expanded porphyrins. The ISE method involves comparison of the energies/hardness/magnetic susceptibility of the methyl derivative of the system and a nonaromatic methylene isomer. For the application of the ISE method you can check our first works:

      M. Alonso, P. Geerlings, F. De Proft, Chem. Eur. J. 2012, 18, 10916

      M. Alonso, P. Geerlings, F. De Proft, Chem. Eur. J. 2013, 19, 1617

      T. Woller, J. Contreras-García, P. Geerlings, F. De Proft, M. Alonso, Phys. Chem. Chem. Phys. 2016, 18, 11885

      With these indices the characterization of the global aromaticity becomes very easy since their sign is expected to reverse for aromatic and antiaromatic.

      In addition, we have also applied the widely-used NICS-indices to characterize the aromaticity of these macrocycles. NICS has been measured at the ring center and at 1 A above and below the molecular plane and also the out-of-plane component since it is considered to better reflect the p-electron effects in organic compounds such as [n]annulenes. Figure-eight structures should be oriented in such a way that the 2D projection exhibits the largest macrocyclic area, leading to the topology of a ring and the external magnetic field was applied to such projection. For the latter point you can check:

      T. Stuyver, M. Perrin, P. Geerlings, F. De Proft, M. Alonso J. Am. Chem. Soc. 2018, 140, 1313 

      All these indices can be computed with any quantum chemical program. In particular, we used the Gaussian code.

      In addition, several structural descriptors were measured along the annulene-type conjugation pathway, including HOMA and BLA, although you these indices should be used with caution since  the differences in these descriptors between the aromatic and antiaromatic counterparts are very small, as compared to classical macrocycles. 

      So far, all these indices allow us to measure the global aromaticity of the macrocycles, but expanded porphyrins contain multiple p-conjugation pathways and, as such, one of the main issues to describe their aromaticity concerns the selection of the macrocyclic conjugation pathway. For these reasons, we have recently applied several electronic indices that can be applied to identify the main conjugation pathway in large macrocycles, both Hückel and Möbius. 

      These indices are basically the AV1245, developed by E. Matito, which consists on the average of the 4-center multicenter index (MCI) values along the ring that keeps a positional relationship of 1, 2, 4, 5. And the AVmin, which corresponds to the minimal absolute value of the aforementioned 4-center MCI values along the ring. Both AVmin and AV1245 show large numbers for aromatic molecules and small numbers for antiaromatic and non-aromatic systems. 

      The calculation of the electronic aromaticity indices (AV1245, AVmin,BOA and FLU) uses a QTAIM atomic partition performed by the AIMAll software.The atomic overlap matricesresulting from this partition and the molecular geometries are input in the in-house ESI-3D code, which provides AV1245, AVmin, BLA, BOA, DIs, FLU, HOMA and MCI values. The ESI-3D code is available upon request ().

      This research has been done in close collaboration with Dr. Eduard Matito y Dr. Miquel Torrent-Sucarrat and the results have been published here:

      I. Casademont-Reig, T. Woller, J. Contreras-García, M. Alonso, M. Torrent-Sucarrat, E. Matito, PCCP 2018,20, 2787

      An alternative method to identify the most aromatic pathway in expanded porphyrins is the gauge-including magnetically induced current approach, known as the GIMIC approach.  We have recently estimated the main aromatic pathway in [26] and [28]hexaphyrins by visualizing the current flow. More detailed picture can be obtained by numerically integrating the current strength for the different covalent bonds.

      These results are not published yet but there will be submitted shortly.

      You can check these review on ring current methods.

      WIREs Comput Mol Sci 2016, 6:639–678. doi: 10.1002/wcms.1270

      I hope that this information is helpful.

      Best regards



      1. Tuesday, 16 February 2021 10:00

        Thank you so much for such a nice and detailed answer.