Virtual Winter School on Computational Chemistry
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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.Since these topology/aromaticity switches can be induced by different external stimuli,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.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.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.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. Overall, our work demonstrates how the concept of aromaticity and molecular topology can be exploited to create a novel type of efficient switching devices.
Figure 1.A bottom-up quantum chemical approach to design efficient nanoscale devices.
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Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Bologna, Italy
Laboratoire de Chimie UMR 5182, École Normale Supérieure de Lyon, CNRS, UCBL, Lyon, France
Two-dimensional electronic spectroscopy (2DES) is a developing multidimensional technique based on ultrashort laser pulses used to track electronic transitions in complex systems with femtosecond spectral and time resolution. 2DES in the ultraviolet (2DUV) can be used to investigate structure, conformation dynamics, energy transfer, and chemical/photochemical reactivity in a wide range of systems in physical chemistry, energy sciences and biophysics. The interpretation of 2D electronic spectra is challenging and computational modeling is required to disentangle the congested information contained in the nonlinear optical response of the sample. In this presentation, the 2DES technique and its theoretical basis are introduced along with an illustration of the computational tools and protocols that we developed to perform first-principles simulations of 2DES spectra. The methodology has been so far applied to the study of structure and dynamics of various biological systems, including proteic systems , organic fluorescent probes  and DNA/RNA nucleobases. Wavefunctions methods have been used to reliably calculate the electronic properties of multichromophoric systems, and compared with time-dependent density functional theory methodologies, using hybrid QM/MM schemes and in conjunction with molecular dynamics techniques to assess environmental and conformational effects that shape the 2D electronic spectra .
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