Identifying surface reaction intermediates with photoemission tomography

Abstract

The determination of reaction pathways and the identification of reaction intermediates are key issues in chemistry. Surface reactions are particularly challenging, since many methods of analytical chemistry are inapplicable at surfaces. Recently, atomic force microscopy has been employed to identify surface reaction intermediates. While providing an excellent insight into the molecular backbone structure, atomic force microscopy is less conclusive about the molecular periphery, where adsorbates tend to react with the substrate. Here we show that photoemission tomography is extremely sensitive to the character of the frontier orbitals. Specifically, hydrogen abstraction at the molecular periphery is easily detected, and the precise nature of the reaction intermediates can be determined. This is illustrated with the thermally induced reaction of dibromo-bianthracene to graphene which is shown to proceed via a fully hydrogenated bisanthene intermediate. We anticipate that photoemission tomography will become a powerful companion to other techniques in the study of surface reaction pathways.

Molecular anchoring stabilizes low valence Ni(i)TPP on copper against thermally induced chemical changes

“…PT has already been proven to be a suitable tool for identifying possible intermediate reaction products, resulting from a stepwise dehydrogenation followed by a C-Cu bond formation, as demonstrated in the case of the thermally induced reaction of the transformation of dibromo-bianthracene into nano-graphene. 44 In our case, however, the one-to-one match between the momentum patterns of the as-deposited and the annealed molecular overlayers clearly shows that neither a cyclodehydrogenation at the macrocycle rim followed by a ring closure reaction nor a C-Cu bond formation takes place upon annealing, as otherwise the molecular symmetry would have been affected and the appearance in the reciprocal space of the corresponding molecular orbitals would have changed.…”

“…41,42 The simulated momentum maps of the gas-phase molecules were obtained as the Fourier transforms of the respective Kohn-Sham orbitals as described previously. 43,44 Further details about the DFT simulations and a comparison of the molecular orbitals of NiTPP and dh-NiTPP can be found in the ESI. †…”

Going beyond Pentacene: Photoemission Tomography of a Heptacene Monolayer on Ag(110)

“… 33 Thus, they allow not only the emissions to be assigned to particular orbitals but also the identification of molecules and their orientation on the surface. 34 , 100 Here, we have applied PT to confirm that indeed a well-oriented 7A monolayer can be grown on Ag(110) via thermal evaporation from D7A powder. This allows us to investigate the adsorption geometry of the prepared monolayer and orbital structure of 7A in great detail.…”

Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization

“…We argue that it can serve as an additional indicator of the type of aromatic bonding and give insight into the aromatic stabilization mechanism of a given molecule. By exploiting the capabilities of photoemission tomography to reveal an orbital-resolved spatial electron density, [22][23][24][25][26][27][28][29] we can unambiguously confirm Clar's model for the aromaticity of kekulene.…”