#	Pagina
attuale pagina	/open-h2020/projects/200785/results.html

Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - AMSEL (Atomic Force Microscopy for Molecular Structure Elucidation)

Teaser

Summary

Identifying molecular structures is of great importance in synthetic chemistry, pharmacy, life sciences and environmental sciences. Atomic Force Microscopy (AFM) with functionalized tips, pioneered in our group, evolved as a novel tool for molecular structure elucidation, complementing conventional techniques such as nuclear magnetic resonance and mass spectrometry. In contrast to other techniques, AFM offers two unique advantages: AFM can identify the structure of an individual molecule and it can be combined with atom manipulation for on-surface synthesis. In addition to identification of molecules, AFM can elucidate properties of individual adsorbed molecules, such as conformation, adsorption geometry, adsorption site, bond-order relationships, charge state and charge distribution. This is further complemented using scanning tunneling microscopy (STM) for electronic characterization of the adsorbed molecules. For a recent review of the technique see: Atomic Force Microscopy for Molecular Structure Elucidation, L. Gross et al. Angew. Chem. Int. Ed. 2018, 57, 3888.
The three main objectives of the project AMSEL are:

1. Further improve AFM for molecular structure elucidation, increasing its applicability, sensitivity, speed, ease-of-use, and expanding the molecular properties that can be measured.

2. Apply AFM for molecular structure elucidation of novel and a priori unknown samples of increasing fragility, complexity, size, and three-dimensionality. We focus on samples that are challenging to characterize with conventional methods. Complex molecular mixtures with different applications are investigated molecule-by-molecule taking advantage of the single-molecule sensitivity. Unstable and highly reactive molecules that can be stabilized in our AFM, at low temperature on inert substrates, are investigated. The absolute stereochemistry of molecules is determined.

3. Employ atom manipulation for the creation of novel, elusive, custom-designed molecules and investigate them by AFM. We create radicals, diradicals, reaction intermediates, antiaromatic molecules, molecular wires and switches and study their properties by AFM and STM. We explore novel reaction schemes induced by atom manipulation and the operation of single molecular machines is being investigated.

Work performed

1. Improving AFM for molecular structure elucidation
- We measured for individual molecules the reorganization energy, a fundamental parameter for the description of electron transfer rates in molecular systems. For the first time we determined the reorganization energy for an individual molecule and on an insulator. To this end we used the AFM for tunnelling spectroscopy, detecting currents on the order of zeptoampere (Nat. Nano. 2018, 13, 376). See video: https://www.youtube.com/watch?v=R2JslFl1Syw
- To improve chemical sensitivity by AFM, we studied several model compounds in experiment and theory, facilitating their identification in complex molecular mixtures containing a priori unknown structures (Chem. Sci. 2017, 8, 2315).
- We also studied chiral molecules and resolved the chirality of a single molecule by AFM. We demonstrated reversible switching of its chirality by atom manipulation (Chem. Sci. 2017, 8, 2315).

2. Complex molecular mixtures
- We characterized molecules found in the early stages of soot formation. Our data shed light on one of the most complex and still debated aspects of soot formation, i.e., the nucleation process (Proc. Comb. Inst. 2018, DOI 10.1016/j.proci.2018.06.100, covered as a Nature Research Highlight).
- We investigated fuel pyrolysis products and developed a new method that integrates AFM with analytical tools such as high-performance liquid chromatography with diode array ultravioletâˆ’visible absorbance, and mass spectrometry along with synthetic chemistry. This interdisciplinary approach enables the detection, identification, and quantification of novel polycyclic aromatic hydrocarbons in complex molecular mixtures (J. Am. Chem. Soc. 2018, 140, 8156).
- We studied heavy oil related samples of different origin and after different processing steps applied and obtained a basis for modelling geochemical oil formation processes (Energy & Fuels 2017, 31, 6856).
- We studied marine dissolved organic carbon from different depth. The results indicate that structural recalcitrance is the reason for the old age of deep ocean dissolved organic carbon (Geophys. Res. Lett. 2018, 45, 5590).

3. Molecules generated by atom manipulation
- We generated unsubstituted triangulene, an elusive molecule with possible applications in molecular spintronics. We demonstrated the open-shell character of triangulene on Xe in agreement with its triplet ground state (Nat. Nano. 2017, 12, 308, including journal cover).
- We generated a meta-aryne on bilayer NaCl on Cu(111) by atom manipulation and we confirmed its diradical structure (ACS Nano 2017, 11, 10768).
- We created and studied an antiaromatic molecule, highlighting the importance of molecule-surface interactions on the aromaticity (Nat. Commun. 2018, 9, 1198).
- We generated and characterized polyynes, which constitute single-atom wide molecular wires. We demonstrated skeletal rearrangements by atom manipulation (see image). We generated linear atomic wires from six to sixteen carbon atoms in length, which we characterized electronically by measuring their transport gaps (Nat. Chem. 2018, 10, 853). See video: https://www.youtube.com/watch?v=PQtCSO5rOLQ
- We triggered reversible molecular reactions on insulators attaching/detaching single electrons and accessing multiple charge states. This marks an important step for the project, aiming to fabricate by atom manipulation custom-designed covalently bound nanostructures with applications as single-electron devices on insulators. (Phys. Rev. Lett. 2018, 121, 226101).

Final results

We apply AFM for molecular structure elucidation to contribute to solving ecologic and economic problems. For example, we work on resolving the structures initially formed in fuel combustion and elucidating the pathways for the formation of soot with impact on climate, environment and human health.

Working with molecules on insulating substrates we developed novel unconventional methodologies to perform tunneling spectroscopy and atom manipulation by electron attachment/detachment using the AFM instead of the STM, as the latter cannot be employed on insulators. We managed to do spectroscopy using the AFM detecting tunneling currents on the order of zeptoampere which we used to determine the reorganization energy of a molecule on an insulator (Nat. Nano. 2018, 13, 376).

Moreover, we found a way to trigger molecular reactions on insulators attaching/detaching single electrons and accessing multiple charge states. This marks an important step for the project, aiming to fabricate by atom manipulation custom-designed covalently bound nanostructures with applications as single-electron devices on insulators. We demonstrated a reversible dissociation upon the attachment of two electrons (Phys. Rev. Lett. 2018, 121, 226101).

We are setting up a combined system of electrospray ionization deposition combined with a low temperature atomic force microscope/scanning tunneling microscope for molecular structure elucidation. With this tool we expect to prepare in ultraclean conditions and resolve atomically by AFM large and complex molecules of sizes up to several thousand atomic units.