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Functional materials from on-surface linkage of molecular precursors

Periodic Reporting for period 4 - SURFINK (Functional materials from on-surface linkage of molecular precursors)

Reporting period: 2020-03-01 to 2020-08-31

"The project has aimed at boosting a newly developing type of chemistry typically termed ""on-surface synthesis"". It is closely related to heterogeneous catalysis, which addresses the favorable effect of particular supports in reaction processes. On-surface synthesis maintains the reaction support of heterogeneous catalysis, normally using atomically well defined surfaces. However, in contrast to the catalysis case, here the product structures remain on the surface. The main goal is complementing conventional organic chemistry by extending its capabilities thanks to the surface confinement of the reactants, since molecules behave and react differently if they float in three-dimensional space (as in the solutions of conventional wet chemistry) or if they are confined to a two dimensional plane. In fact, the dimensionality can be reduced even further to one- or zero-dimensional reaction space by the use of appropriately tailored substrates. Apart from the implicit surface support, the rest of the environment can be a solution, controlled atmospheres or vacuum. This project focuses on the latter. Altogether, the reduced dimensionality and completely disparate vacuum environment thus cause the molecules to behave and react in strikingly different ways from what is typically known in conventional organic chemistry. On-surface synthesis can thus be seen as a new methodology or branch within the wider field of organic chemistry.

Furthermore, also the characterization techniques used in on-surface synthesis are different from those commonly applied in wet-chemistry. One important difference is that the two-dimensional space allows for scanned probe techniques to be applied for characterization, which in turn offer sufficient resolution to perform single-molecule studies. Whereas a statistical analysis of many molecules allows comparison with conventional ensemble averaging techniques, the single-molecule information provides unique insight into reaction processes and allows for a better characterization of minority products. Finally, the vacuum environment also allows generating and characterizing molecular structures that would be unstable under conventional conditions.

Advancing this new type of chemistry has allowed the development of new materials not achievable by conventional means. On the longer term, this may have an important impact in society, given the irrevocable and continuously increasing presence that synthetic materials have gained in our daily lives. In fact, beyond conventional ""cheap"" plastic use in packaging or as purely structural materials, the variety of their applications is continuously growing, including highly refined functionalities as for example in optoelectronic devices, catalysts, filters or batteries. Molecular materials are even being considered as strong candidates to become an important technological platform for future quantum technologies.

Within this project, the ultimate goal has been to bring this new type of chemistry a big step further, closer to applications. Going beyond the state-of-the-art, we have actually used this new type of chemistry to synthesize functional materials like atomically precise graphene nanoribbons, porous networks or organic semiconductors with predefined n-type or p-type behaviors."
The work performed during this project has reached most of the goals outlined in the initial proposal. In general terms, the work has been devoted to bring on-surface synthesis approaches to higher levels of complexity and utilize them to create new materials with tailored and/or enhanced properties. Doing so, we have for example succeeded in the synthesis of graphene nanoribbons of different widths, chiralities, heteroatoms (B, N) and edge functionalizations (amino, keto and cyano groups), which translates into linear conjugated molecular structures whose bandgap can be modulated over a large range from metal to semiconducting behaviors with additionally tunable energy level alignments. As a result, the GNRs can be controllably synthesized as p- or n-type semiconductors, paving the way towards the synthesis p-n networks or heterojunctions. Further refining the organic nanoarchitechtures, we have combined GNRs with Fe porphyrines into heterostructures that correspond to model device structures in which the porphyrines provide the magnetic functionality and the GNRs act as leads. The functionality has been furthermore tested in model two-terminal devices in which the scanning probe and the metallic substrate act as the two mesoscopic leads and the functional molecular nanoarchitecture is spanned in between. Similar experiments have been performed for other graphene nanoribbons with intrinsic magnetism at its ends.

Along different lines, a variety of reactants and functional groups have been used to render different porous structures. Whereas in some of them the functional grops were keto and enol groups whose tautomerization reaction determines the molecular self-assembly , in others we used thiol groups. For the latter, the porous structures are held together by Au-thiolate links. We have studied the influence of those metal-organic linkers on surface-supported alkyne coupling reactions. The metal-organic centers have been shown to have a remarkable influence on the reactions, reducing the activation temperature and modifying its outcome . Furthermore, the attractive interactions between the metal centers and the alkyne coupling motifs allows for the selective decoration of the metal-organic structures by periodically arranged molecular systems.

During the project’s progress, several side topics more focused on a further development of the on-surface synthesis toolbox and its improved understanding than on the synthesis of particular functional structures have also been addressed. Examples thereof are the development of new chemical reaction schemes, chirality transfer along surface-supported reactions , a better understanding of hierarchic chemical processes , the assessment of the catalytic action of the substrates and their leveraging for aligning the molecular structures atop , or the performance of reactions processes on non-metallic surfaces .

Altogether, the project has generated to date 24 peer-reviewed publications in internationally renowned journals. Each of the involved team members has actively contributed to the dissemination of the results at national and international conferences that have been chosen to cover both a broader range of attendants as well as a highly specialized audience. For the latter we have furthermore contributed organizing and establishing the “On-surface synthesis international workshop” as the reference forum for dissemination of results, exchange of ideas and launching collaborations within this growing field.
This project has clearly gone beyond the state of the art in the field of on-surface synthesis. When the project started in 2015, the vast majority of the research efforts in this field were devoted to the demonstration of new chemical reactions. Since then, the state of the art has been continuously shifting towards the exploitation of on-surface synthesis as a tool to obtain molecular materials with diverse functionality, from electronic to optic or magnetic. In this respect, the project´s results have been an important contribution to this overall development.