Porphyrin Self-Metalation on Rutile TIO2(110)
Floreano1, G. Lovat2, M. Dominguez2, M. Abadia3, C. Rogero3, D. Forrer4 and A. Vittadini4
1.CNR-IOM – Trieste, Italy
2.Graduate School of Nanotechnology, University of Trieste, Trieste, Italy
3.Centro de Fisica de Materiales, CSIC-UPV – San Sebastian, Spain
4.CNR-IENI and Consorzio INSTM, Padova, Padova, Italy
Among dye-sensitized solar cells, porphyrin molecules and titania surfaces are much studied because of the possibility to tune the optical window by a suitable choice of the porphyrin central metal, which can be incorporated post-growth either by metal deposition atop the molecular overlayer  or by temperature induced extraction from a metallic substrate.
Recently, the incorporation of deposited Ni atoms in the macrocycle of tetra-phenyl porphyrins (TPP) has been reported also on the TiO2(110) surface. In addition, the large anisotropy of the rutile TiO2(110) surface has been shown to drive the ordering of Zn-TPP into compact overlayer domains. In both cases, the optimal metalation and molecular ordering were achieved upon mild annealing to 550 and 420 K, respectively.
Here, we report a combined Synchrotron spectroscopy (XPS, NEXAFS), STM topography and DFT simulation study of the self-metalation of porphyrin molecules deposited on the rutile TiO2(110) surface. We have recently found that metal-free porphyrins are very reactive on this substrates, so that hydrogen capture by the iminic nitrogen atoms takes place already at room temperature. The incorporation of Ti substrate atoms into TPP is found to start at a substrate temperature as low as ~350 K. The interfacial molecules are found to be fully metalated at 400-450 K, irrespective of the molecular coverage (in the monolayer range). Interestingly, the Ti incorporation drives both conformational and orientational changes in isolated molecules, while no phase symmetry changes are observed for compact domains, likely due to steric limitation. We have observed equivalent annealing temperatures also for the self-metalation of differently functionalized porphyrins, namely octa-ethyl (OEP) and tert-butyl-tetra-phenyl (TBTPP) porphyrins.
In conclusion, the mild annealing, as routinely employed for improving the molecular ordering, makes the Ti self-metalation a process competitive with metalation by deposition, and makes potentially active the metal exchange reaction at the interfacial prorphyrin layer.
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