On-surface design of nanomaterials based on π-conjugated backbones

Abstract

In this ever-changing world, where the global crisis seems to be shackled, but the request for advances in technology increases every day, it is mandatory to find new strategies to develop new optoelectronic materials that require low-cost synthesis and can satisfy such necessities. Precisely, due to their scientific and technological properties, as well as envisioned potential to solve societal challenges, organic nanomaterials have captivated the attention of the scientific community. However, the advance in the synthesis of disrupting organic nanomaterials is often hampered by concomitant limitations in wet chemistry. Recently, the emergence of on-surface synthesis has allowed the capabilities to engineer on surfaces unique nanoarchitectures with great potential in optoelectronics, nanomagnetism and quantum information. In addition, the inherent capabilities of surface science techniques have allowed the inspection of the structural, electronic, magnetic and optical properties of such materials with unprecedented spatial resolution, thus conquering the ultimate resolution scale. Particularly relevant are one-dimensional and two-dimensional nanomaterials exploiting π-conjugation, which have allowed to increase the electrical conductivity of organic matter by orders of magnitude, while envisioning prospects for expression complex quantum phases of matter. In this thesis, polymeric nanomaterials embedding π-conjugated backbones have been synthesized on coinage metals, and their properties characterized with scanning probe microscopies and X-ray photoelectron spectroscopies. Firstly, one-dimensional π-conjugated polymers based on prochiral indenofluorene monomers have been grown on three different substrates, Au(111), Ag(111) and Ag(100), and the capability to induce homochiral segments by the selection of the substrate has been assessed, revealing a tendency to increase such segments by moving from Au(111) to Ag(111) and from Ag(111) to Ag(100). Secondly, two-dimensional Co-directed metal-organic networks, employing π-conjugated linkers functionalized with hydroxyl groups to steer coordinative schemes upon proper deprotonation, have been designed on Au(111). On one hand, a Co-HOTP network was engineered including the feasibility to design antiferromagnetic organic nanomaterials with very narrow bandgap, while preserving a large unquenched orbital magnetic moment. On the other hand, an additional antiferromagnetic Co-HOB network was designed, affording a very narrow bandgap nanomaterial. Even though both species were equipped with the same functional group and, in principle, they could have given rise to the same architecture, each of them is unique and reveal the role of both the coordinative schemes and the adsorbate-substrate interactions Thirdly and last, our research concludes with the study of the supramolecular assemblies on Au(111). These have been achieved by exploiting two novel molecular species equipped with terminal carbonitrile functional groups. An unprecedented molecular acceptor based on a tetracyano quinoidal thiophene and tetracyano quinoidal bithiophene moieties was reported, revealing a change in the orientation of its dipole upon surface adsorption, due to a change in the molecular conformation. Unfortunately, due to its three-dimensional molecular shape, the linker did not display any coordinative capability towards cobalt. Next, we move to study the Co-DCAAQ metal-organic network designed on Au(111). Herein, thanks to the flexibility of the ligands, a unique self-assembly is found, based on two-fold Co coordinated rows, linked together by Van der Waals interactions through individual ligands, altogether affording a two-dimensional architecture. The inspection of the electronic structure is ongoing, but preliminary experimental results, confirmed by theory, reveal the synthesis of a metallic nanoarchitecture, the first of its kind. Altogether, this thesis contributes to the development of the synthesis of unique materials exploiting π-conjugation backbones and anticipates fascinating research lines for tailoring unprecedented physical properties on nanomaterials