| Autor: |
Fritton M; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany.; Deutsches Museum , Museumsinsel 1 , Munich 80538 , Germany., Duncan DA; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany.; Diamond Light Source , Harwell Science and Innovation Campus , Didcot OX11 0QX , United Kingdom., Deimel PS; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany., Rastgoo-Lahrood A; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany.; Deutsches Museum , Museumsinsel 1 , Munich 80538 , Germany., Allegretti F; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany., Barth JV; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany., Heckl WM; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany.; Deutsches Museum , Museumsinsel 1 , Munich 80538 , Germany., Björk J; Department of Physics Chemistry and Biology, IFM , Linköping University , Linköping 58183 , Sweden., Lackinger M; Department of Physics , Technical University of Munich , James-Franck-Strasse 1 , Garching 85748 , Germany.; Deutsches Museum , Museumsinsel 1 , Munich 80538 , Germany. |
| Abstrakt: |
Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. To shed light on individual reaction steps and their progression with temperature, temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5-tris(4-bromophenyl)benzene precursor is comparatively studied on Ag(111) versus Au(111), whereby intermolecular bonds and network topologies are additionally characterized by scanning tunneling microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and density functional theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long lifetime of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces. |
