1) Design of photoactive 2D-COF model systems.
The ORCA quantum chemistry code has been set up to perform calculations of the geometry and electronic structure of donor and acceptor molecular building blocks. Potential pairs of molecular precursors have been identified, guiding the experimental synthesis.
2) Synthesis of photoactive 2D-COF model systems.
Two-dimensional covalent organic frameworks (2D-COFs) can be fabricated through on-surface synthesis, which makes use of atomically flat surfaces as templates to confine condensation reactions in two dimensions. However, despite on-surface synthesis in ultra-high vacuum (UHV) enables the growth of atomically thin COFs, the resulting degree of crystallinity is typically low. This issue is caused by the irreversibility of covalent bond formation, which limits the synthesis of ordered structures. The formation of crystalline structures requires a mechanism for error correction, where covalent bonds can be broken and reformed. Such approach, called dynamic covalent chemistry, is based by the addition of the reaction by-products (e.g. water), which enable the healing of defects in the frameworks, facilitating the formation of highly crystalline structures. As the water partial pressure is a key parameter to determine the chemical equilibrium of the system, systematic studies are required to identify the conditions which favour the formation of crystalline structures. As water pressures in the millibar range are typically required for dynamic covalent chemistry, conventional X-ray photoelectron spectroscopy (XPS) cannot be used, as this approach is limited to high vacuum conditions. Therefore, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments have been performed. As result, by tuning the sample temperature and the water pressure in the experimental chamber, controlled formation or dissolution of the framework has been observed by NAP-XPS. For the case of a 2D-boroxine framework, under typical temperatures used for 2D-COF growth (about 120 °C), a pressure of 0.5 mbar is needed to enable covalent bond breaking.
3) On-surface synthesis of two-dimensional frameworks with delocalized electronic states.
Two-dimensional metal-organic frameworks (2D-MOFs) stand out as a fascinating class of atomically thin materials, combining the structural flexibility observed in molecular systems with the ordered crystalline arrangement typical of solid materials. At the heart of their structure is the robust bonding between organic linkers and transition metal centres, a feature that is expected to give rise to delocalized electronic states. Despite such expectation, the precise mechanism through which the band structure in 2D-MOFs emerges from the coupling of electronic states within the building blocks remains a substantial scientific puzzle. We presented a breakthrough in the characterization of the electronic structure of π-conjugated 2D-MOFs. Through a combination of experimental and theoretical methodologies, we provided direct evidence for the emergence of band structure upon hierarchical assembly of the organic linkers and transition metal centres into the 2D-MOF lattice. Our results elucidate the role of intra-layer charge transfer in driving the formation of energy dispersive states.
4) Structural characterization of 2D photoactive frameworks and imaging of excitonic processes.
A low-temperature scanning tunnelling/atomic force microscope (LT-STM/AFM) has been successfully installed in January 2024. The microscope enables high-resolution imaging of two-dimensional organic structure on surfaces, being the core WEPOF. The setup has been also coupled with a tunable laser source, where the laser beam will be tightly focused on the tip-sample junction to enable photoexcitation experiments.
