CHARGE DYNAMICS IN COMPLEX 2D HETEROSTRUCTURES

In recent years, a lot of effort has been spent on finding sustainable alternatives to classical semiconductor and conductor materials, whose supplies are limited. Organic-based conductor and semiconductor materials present several advantages compared to inorganic materials commonly used in electronics, including an easier bottom-up synthetic approach, renewability, higher sustainability, and the possibility for growth on non-conventional substrates, obtaining flexible and transparent devices. Among the materials that have been studied for this purpose, covalent organic frameworks (COFs), which are constituted from molecules linked together by covalent interactions, have demonstrated very promising results, and are the object of many studies, due to their modular synthesis. In particular, our research was focused on examining the electronic properties of boroxine-based frameworks, which are made from the self-condensation of three boronic acid molecules, that result in the creation of a central six-membered ring made of alternating B and O, the boroxine ring. This structure presents very high thermal and chemical stability, making it interesting for a series of applications. In the electronic field, an interesting discovery, it was also discovered that 2D boroxine films present remarkable charge transport properties towards metal surfaces, in the form of ultra-fast charge delocalization channels. Therefore, we focused our attention on the creation of boroxine-based structures starting from boronic acid derived from polycyclic aromatic hydrocarbons (PAH), intending to exploit the interesting optoelectronic properties of their π electrons. We focused our attention on three PAH boronic acids: the Pyrene 1-Boronic Acids (PyBA) and the two commercial congeners of the Anthracene boronic acid, the 2-Anthracene boronic acid (2-ABA) and the 9-Anthracene boronic (9-ABA). All the molecules were deposited on top of a Au(111) surface via evaporation in UHV conditions. By conducting an extensive investigation, taking advantage of the ANCHOR-SUNDYN endstation of the ALOISA beamline at ELETTRA Synchrotron radiation facility, we discovered evidences of ultrafast charge transfer (few fs) from the metal to the boroxine organic film at the boroxine-gold interface. The presence of interesting charge transfer, both inside the film and at the metal-organic interface, was further proven by a Time-resolved investigation, where we discovered that a series of dynamics in different timescales (s, μs, and ps) would take place in the film, indicating that the material presents interesting electronic properties. In the form of hole-transport capabilities, and the possibility to undergo singlet fission in its LUMO state. Alongside the UHV characterization, we also analyzed its in-solution behavior via UV-vis absorption, which allowed us to evaluate its HOMO-LUMO band gap, and to discover that by tweaking the solution pH, it is possible to trigger the PyBA condensation into trimers. The investigation of the 2-ABA allowed us to discover that this molecule, when deposited at higher substrate temperatures was able to create larger, more complex structures, created by the reaction between two unsaturated boroxine rings and active oxygen species present on the metal surface that derive water molecules created during the boronic condensation, resulting in boroxines linked together by B-O-B bridges. On the other hand, the 9-ABA would condense on the crucible and be evaporated on the gold substrate in the form of trimers, similarly to the PyBA, and no evidence of the formation of larger structures was to be found. We, therefore, speculate that the formation and absorption of water molecules on the surface, which are retained by the metal in the form of O atoms, is required to allow for the creation of the observed polymerized boroxine structures.