Metalation of 2H-tetraphenyl-porphyrins by picking-up a substrate metal atom
|
The possibility to obtain systems where the magnetic or catalytic atoms are well separated and arranged in regular arrays is allowed at the nanoscale by using molecular self assembly of π-conjugated molecules containing metal ions. Porphyrins, ubiquitous in many aspects of nature, are a class of π-conjugated molecules with a planar macrocycle that can accept metal ions of different kind at the center (metalation), thus forming metallo-porphyrins. As such, synthetic and naturally occurring metallo-porphyrins have been used extensively due to their catalytic, photophysical and magnetic properties as biomimetic agents in several aspects of technology. Man-made metallo-porphyrin units self-assembled into complex and functional superstructures are, therefore, exploding in recent years. |
|
This method allows to obtain metallo-porphyrins that, typically, are not stable as single-molecules thanks to the action of the support surface, which stabilizes the valence state of the evaporated metal. A risk, however, is the formation of metal clusters in addition to metallo-porphyrins due to the reduced mobility of some metal adatoms or to an excessive metal ion deposition.
Following seminal studies on Cu surfaces, we generalize an alternative route to porphyrin metalation in UHV without the need of ion evaporation and post-growth annealing. Using synchrotron radiation spectroscopy we demonstrate that depositing at room temperature one monolayer of 2H-tetraphenyl-porphyrins (2H-TPP) on Fe and Ni substrates, a substrate adatom binds directly to the macrocycle and metalates the porphyrins. For these molecules core level photoemission is a very sensitive tool of metalation. Actually, in the free-base macrocycle two different nitrogen atoms are present, iminic nitrogen (-N-H) and pyrrolic nitrogen (=N), with a quite different N 1s binding energy. On the contrary, when a metal ion is inserted at the center of the macrocycle, a redox reaction eliminates the hydrogen atoms as H2, and all the N atoms become equivalent. Fig. 1 shows the N 1s core level photoemission spectra of 2H-TPP monolayers deposited at room temperature on Ni(111), Fe(110) and Ag(111) compared with the spectrum of a 2H-TPP multilayer.
|
It is evident that, while in the multilayer and monolayer on Ag(111) the N 1s spectrum is composed of two peaks, corresponding to non equivalent iminic (397.9 eV and 397.6 eV, respectively) and pyrrolic (399.9 eV and 399.6 eV, respectively) nitrogen atoms, both in the case of Ni(111) and Fe(110) substrates the spectra consist of one single peak (at about 398.5 eV). This is the clear confirmation that Fe-TPP and Ni-TPP are formed. Energetics obtained by Density Functional Theory calculations confirms that this redox reaction happens on Fe and Ni substrates at room temperature. In particular, for the most critical case, Ni(111), the calculations demonstrate that the redox metalation reaction would be exothermic with an energy gain of 0.89 eV upon embedding Ni adatom in the macrocycle (see Fig. 2). In the monolayer regime upon adsorption at room temperature, TPP exhibits a conformational adaptation with a considerable rotation of the phenyl substituents with respect to the macrocycle plane, mainly due to the steric effect of hydrogen atoms. This causes the macrocycle to be at a distance d between 2.5 Å < d < 5 Å depending on the possible distortion of the macrocycle ring. |
|
Our data indicate that, even considering the large macrocycle distance, the metalation may occur. Obviously, this does not happen on all substrates, as demonstrated by the Ag and even Au cases, although both Ag and Au form stable porphyrin complexes.
This research was conducted by the research team µ- and nano-carbon laboratory and the beamlines BACH and ALOISA at Elettra, in collaboration with researchers of Swiss Federal Laboratories for Matrials Science and Technology.
- Andrea Goldoni, Giovanni Di Santo, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy and INSTM sezione di Trieste, Trieste, Italy.
- Carla Castellarin-Cudia, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy and IOM-CNR, Lab. TASC, Trieste, Italy.
- Federica Bondino, Elena Magnano, Alberto Verdini, IOM-CNR TASC Laboratory and CENMAT, Trieste, Italy
- Carlo A. Pignedoli, Daniele Passerone, EMPA, Dübendorf, Switzerland.
Reference
Andrea Goldoni, Carlo A. Pignedoli, Giovanni Di Santo, Carla Castellarin-Cudia, Federica Bondino, Elena Magnano, Alberto Verdini, Daniele Passerone, "Room temperature metalation of 2H-TPP monolayer on Fe and Ni surfaces by picking-up substrate metal atoms",ACS Nano 6, 10800 (2012); DOI: 10.1021/nn304134q.
