Changes of the Molecule-Substrate Interaction upon Metal Inclusion into a Porphyrin (original) (raw)

2012, Chemistry - A European Journal

The self-assembly of molecular building blocks on crystalline surfaces into highly ordered nanoarchitectures is an interesting approach towards the creation of novel materials with outstanding properties. [1-3] In this respect, porphyrins, which represent a group of molecules with an unsaturated macrocyclic ring, are of great technological and fundamental interest due to their distinct chemical and physical properties. Monolayers assembled from these molecules can be applied as active elements in chemical sensors, [4, 5] solar cells [3] and towards creation of catalytically active porphyrin-functionalized metal surfaces. [6] The understanding of the molecule-substrate adsorption geometry together with the interaction between the first layer and the hosting clean surface is of fundamental importance for the design of porphyrinbased systems with technological impact. In this respect, the capability of this type of molecules to bind various metal ions to its center [7] opens the discussion for how the presence of the central metal modifies the aforementioned interaction. Typically the inclusion of metal atoms in porphyrin molecules is obtained in solution. Nevertheless efficient ultra-high vacuum (UHV) in situ metalation by Fe, Zn, Co, and Ce of free-base porphyrins has been documented. [8-12] Metals can efficiently be coordinated to adsorbed porphyrin and phthalocyanine layers by evaporation of metal atoms in situ, [13] by direct coordination to the substrate, [14, 15] or to a reactive surface adatom under strict UHV conditions. [8] The possibility to create metal porphyrin complexes through in situ metalation of free-base molecules has open the discussion on the model mechanism of the metalation process. Namely, for the TPP molecules, the phenyl groups and macrocycle orientation with respect to the surface give insight into the possible mechanism of complex formation as well as into the distance between the molecule and the metal substrate, which strongly affects the charge transfer properties of the system. Despite a large number of studies on this subject in the past years, the model of the molecular structural configuration and adaptation onto the substrate surface needs to be refined in particular by taking the role of the metal atom in interface formation into account. It has been observed that the presence or absence of the metal in the macrocycle does not affect the supramolecular order in the monolayer deposited at room temperature. [16] Recently we have observed that the phenyl rings of the hydrogenated form of tetraphenylporphyrin (2H-TPP) on AgA C H T U N G T R E N N U N G (111), which typically have a non-flat orientation with respect to the macrocycle plane due to the steric hydrogen repulsion, adapt to a flat conformation when annealed above 525 K. [17] The assumption of a dehydrogenation with a subsequent formation of four new intramolecular CÀC bonds leads, in this case, to a sensitive reduction of the molecule-substrate distance. The resulting molecular structure is very similar to that of a phthalocyanine for which the macrocycle is at distance shorter than 3 from the substrate plane. [18] At this point we may wonder whether the presence of the metal ion in the macrocycle influences the observed interaction of porphyrins with the substrate when annealed at high temperatures. Among all the possible metalloporphyrins we chose CoTPP as a case study, due to the fact that this molecule exists as a pure compound with no need of stabilizers (such as O or Cl) and that Co can efficiently metalate the freebase compound in vacuum with no annealing requirements (like Cu [14] and contrary to Zn [19]). In the experiment reported herein, we investigate the role of Co atoms in in situ metalated TPP compounds by metal atom evaporation (Co-TPP) and in the same compounds presynthesized in solution (CoTPP) and then evaporated in UHV, concerning the interaction between the first molecular layer and the underlying Ag substrate. The spectroscopic characterization was carried out on 2H-TPP, CoTPP, and Co-TPP monolayers, by exploring the annealing effect from the point of view of the molecular orientation. Our results show that the flat conformation is driven by the geometrical adaptation of the free-base molecule upon annealing, before metal coordination. On the contrary the presence of the metal ion, both in CoTPP and in the RTmetalated Co-TPP compound, seems to inhibit a more flat