42 43 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Bio-inspiredmolecules have great nanotechnology potential Related publication : Knecht, P., Reichert, J., Deimel, P. S., Feulner, P., Haag, F., Allegretti, F., Garnica, M., Schwarz, M., Auwärter,W., Ryan, P. T. P., Lee, T.L., Duncan, D. A., Seitsonen, A. P., Barth, J.V., & Papageorgiou, A. C. Conformational control of chemical reactivity for surface-confined Ru-porphyrins. Angewandte Chemie International Edition 60 , 16561–16567 (2021). DOI: 10.1002/anie.202104075 Publication keywords: Ab-initio calculations; CO ligands; Metalloporphyrins; Scanning probe microscopy; X-ray spectroscopy N ature is a great source of inspiration for researchers involved in developing new materials. For example, porphyrins are a class of pigment molecules (including haem and chlorophyll) that have a flat ring shape, some with a metal atom at the centre. The past decades have seen intense interest in using porphyrins as functional building blocks, and they have great potential for use in nanotechnology. One particularly desirable application is in single-atom catalysis on solid surfaces. The first step of catalysis is the binding of the reactant to the catalyst. A team of researchers from the Technical University of Munich investigated the reactivity of two similar Ru-porphyrins on a silver surface against carbonmonoxide (CO), a chemical of interest in industrial chemistry. To do so, they needed to characterise the relative reactivity of the Ru-porphyrins with submolecular resolution and quantify the binding strength of CO by temperature-programmed desorption. They used Diamond Light Source’s Surface and Interface Structural Analysis beamline (I09) because it offers a suitable experimental method for highly accurate determination of adsorption height, i.e. , how close the catalytically-active atom is to the surface before and after binding. This out-of-plane information cannot be derived accurately from microscopy measurements. Their results emphasise the crucial role of the flexibility of the Ru-porphyrin in the ligation process and the related ease of decoupling the ruthenium centre from the silver surface. For the creation of novel materials and devices, inspiration is frequently sought in nature. Porphyrins are naturally occurring cyclic molecules that can bindmetal atoms into their centre.They arewidespread throughout biology, and most famously comprise haem, found in haemoglobin, that transports oxygen around the body and is a catalyst of both reductive and oxidative reactions. The flexibility, yet robustness of these cyclic molecules (Fig. 1a black structures), combined with the ability to chemically tailor their periphery (Fig. 1a grey structures) offers a unique playground to construct artificial nanosystems that exploit interactions between molecules to self-assemble, notably on surfaces 1 . Such“self-assembled monolayers”are a cornerstone of nanotechnology. Single layer films of self-assembled metalloporphyrins, i.e. porphyrins with a metal atom in their centre, present well-defined and regular coordinatively unsaturated metal centres. These are provided by the generally favoured adsorption geometries with the macrocycle residing parallel to the substrate lattice. Such metal atoms can provide catalytically active sites on surfaces, suitable for heterogeneous catalysis with the inherent advantages of processing. The reactivity of individual metal atoms on surfaces is a topical issue in single- atom catalysis. Here porphyrins stabilising Ru in the catalytically active oxidation state +2 (Fig. 1) were investigated on the atomically planar, close-packed silver surface. To elucidate the molecular events associated with the surface reactivity, an extensive array of characterisation techniques was used, including real space single-molecule imaging, diffraction, photoelectron spectroscopy, temperature programmed desorption and computational modelling. Experiments were performed under ultra-high vacuum, so as to prepare and examine samples free from any possible contaminant and solvent effects. For a correlation of structure and function, the key Normal Incidence X-ray StandingWaves (NIXSW) measurements were performed at the I09 beamline of Diamond Light Source. The tailored functional groups, at the periphery of the porphyrin molecule, are responsible for the lateral arrangement and the conformation of the cyclic interior of the porphyrin molecule. On the Ag(111), the inspected macrocycles adopt a saddle shape (Fig. 2a) or a more planar, inverted bowl conformation (Fig. 2b). Both Ru porphyrins bind to the silver surface covalently at the Ru atom. The structural measurements at the beamline showed that the more planar molecule allowed the Ru atom to approach the surface ever so slightly more, just 0.1 Å 2 . Crucial differences were discovered in how CO binds to the Ru atoms in the porphyrin molecule, depending on the conformation of the macrocycle. Scanning Tunnelling Microscopy (STM) showed additional protrusions associated with CO, appearing solely on top of the Ru atoms hosted in the saddle-shape porphyrins (Fig. 1b-f): If the central cyclic part of themolecule had a saddle-shape, COwould bind; if, instead, the central cyclic part of themolecule was in a more planar / bowl-shape, CO would not bind. Thus, the tiny difference of the initial position of the Ru atom results in a completely different ability of the molecule to interact with the world around it. With the CO binding to the porphyrin, the porphyrin changes its binding to the surface. The whole molecule is displaced further from the surface. NIXSW analysis determined that the Ru atoms in particular are 3.18 Å away from the Ag(111) surface (Fig. 2d): the Ru bond to the surface is elongated by 0.7 Å and significantly weakened. X-ray and ultraviolet photoelectron spectroscopies showeda significantly alteredelectron charge transfer between theRuatomand the Ag surface after ligation. These can be described as a structural trans -effect, with both the carbonyl and the Ag surface considered as trans ligands to the Ru atom. In line with other observations on surface supported metalloporphyrins, the metal atom is electronically and physically decoupled from the substrate upon the inclusion of an axial ligand 3 . In addition to imaging the individual molecules on the surface, the scanning tunnelling microscope was also used to manipulate the molecules bound to the Ru atoms of the porphyrins. The tip of a scanning tunnellingmicroscope could be used like a scalpel to remove individual COmolecules with exquisite control (Fig. 3). Because of the binding strength these Ru pedestals exhibit (~ 0.8 eV), this operation could be performed at significantly higher temperatures than earlier studies 4 (150 K vs. 5 K). The insight gained in axial binding and manipulation of these ligands paved the way for making use of these porphyrin templated surfaces as regular pedestals for out-of-plane functional nanoarchitectures, as was indeed feasible for an organic ligand featuring a carbene anchor 5 . Futurework focuses on screening substituents and surface supports to finely control the coordination of various ligands. Here again, NIXSW will provide the structural measurements necessary to reliably compare this set of systems. These results will enable the bio-inspired precise engineering of the catalytic function of metalloporphyrins, akin to the optimisation of biological catalytic processes exerted by nature through Darwinian evolution. References: 1. Auwärter,W. et al . Porphyrins at interfaces. Nature Chemistry 7 , 105–120 (2015). DOI: 10.1038/nchem.2159 2. Knecht, P. et al. Tunable interface of ruthenium porphyrins and silver. The Journal of Physical Chemistry C 125 , 3215–3224 (2021). DOI: 10.1021/acs. jpcc.0c10418 3. Flechtner, K. et al. NO-induced reversible switching of the electronic interaction between a porphyrin-coordinated cobalt ion and a silver surface. Journal of the American Chemical Society 129 , 12110–12111 (2007). DOI: 10.1021/ja0756725 4. Omiya, T. et al. Desorption of CO from individual ruthenium porphyrin molecules on a copper surface via an inelastic tunnelling process. Chemical Communications 53 , 6148–6151 (2017). DOI: 10.1039/C7CC01310H 5. Knecht, P. et al. Assembly and manipulation of a prototypical N-heterocyclic carbene with a metalloporphyrin pedestal on a solid surface. Journal of the American Chemical Society 143 , 4433–4439 (2021). DOI: 10.1021/ jacs.1c01229 Funding acknowledgement: German Research Foundation (DFG): priority programme 1928 COORNETs, Excellence Cluster e-conversion, GSC 81 International Graduate School of Science and Engineering (IGSSE) at TUM, Heisenberg professorship. Engineering and Physical Sciences Research Council (EPSRC): Centre for Doctoral Training in Advanced Characterisation of Materials (grant number EP/L015277/1). European Research Council: Consolidator Grant NanoSurfs, no. 615233. European Union H2020-MSCA-IF-2014 programme: 2DNano, no. 658070. Diamond Light Source for the award of beam time under proposal SI24320-1 and related financial support. The computing resources at the Centro Svizzero di Calcolo Scientifico (CSCS), Lugano, Switzerland, under Project uzh11. Projekt DEAL for open access of the related publication. Corresponding author: Dr Anthoula C. Papageorgiou, Technical University of Munich,
[email protected] Structures and Surfaces Group Beamline I09 a c d e f g increasing CO exposure 10 Å b 10 Å +CO N N N N Ru N N N N Ru CO N N N N Ru +CO Figure 1: (a) Chemical structures of different Ru-porphyrins investigated on the Ag(111) surface. Substituents marked in grey; (b-g) STM images of progressive CO exposure in situ at 150 K of (b-c) a monolayer of saddle-shape Ru-porphyrins (outlined in raspberry) and of (e-g) a mixed layer of saddle-shape and planarised Ru-porphyrins (outlined in lila). Orange dots mark the capped molecules. CO is found only on the saddle-shape Ru-porphyrins and eventually populates all such porphyrins. -4 -2 0 2 4 E - E Bragg / eV 0 1 2 3 4 relative absortpion Ru 3d 5/2 f H = 0.83±0.11 P H = 0.35±0.05 a b + CO c d Figure 2: Ball-and-stick models of the (a) saddle-shape Ru porphyrin and (b) the bowl-shape (planarised) Ru porphyrin on Ag(111). The substituents are faded to highlight the macrocycle conformation differences 1 . (c) CO ligated saddle-shape Ru-porphyrin. Ru, C, N, O, H, and Ag in raspberry, grey, blue, red, white, and silver, respectively; (d) NIXSW photoelectron profiles and fits of the Ru 3d 5/2 region in (111) reflection for saddle-shape Ru(CO)-porphyrin on Ag(111). The Ru data points in purple, the reflection of the Ag(111) substrate in grey. The data was used to determine that the Ru atoms sit 3.18 Å away from the Ag(111) surface. a b 5 Å 0.8 0.6 0.4 0.2 0.0 1.3 1.5 1.7 1.9 2.1 tunnelling bias / V tip height / Å c Figure 3: Manipulation of a CO ligand from saddle-shape Ru porphyrin on Ag(111) with the STM tip; (a) While scanning the surface (from bottom), a voltage pulse was applied at the position marked by the orange cross; (b) The subsequent STM image shows an uncapped saddle-shape Ru porphyrin at the same position; (c) Tip height change during the voltage pulse from 1.28 V to 2.15 V, with a current of 50 pA. The abrupt approach of the tip at ~2.1 V indicates the removal of CO.