On-Surface Synthesis of a Pure and Long-Range-Ordered Titanium(IV)-Porphyrin Contact Layer on Titanium Dioxide

In this work, we show the possibility of tailoring the molecular arrangement, as well as chemical and structural modifications, of porphyrins at monolayer saturation coverage on TiO2(110) by synchrotron photoemission spectroscopy, electron diffraction, STM topography, and DFT calculations. Free -base tetraphenylporphyrins (2H-TPP) adsorb on the oxygen rows, where they can spontaneously capture two additional hydrogen atoms at their iminic nitrogens (4H-TPP). Both 2H-TPP and 4H-TPP molecules aggregate into a commensurate phase at the saturation coverage of 1 ML. Upon sample heating, a self-metalation reaction sets in at 100 degrees C, yielding full metalation of the saturated monolayer at similar to 200 degrees C. The Ti atoms are extracted from the substrate and, by simultaneous dehydrogenation of the pyrrolic nitrogen atoms, incorporated into the porphyrin macrocycle, where they remain coordinated to two underlying oxygen atoms. Neither the adsorption geometry (on-bridge, atop the oxygen rows) nor the molecular arrangement changes across the self-metalation transition up to 300 degrees C. On one hand, the robustness of this saturation phase makes it a promising system for implementation in applications for photocatalysis and photovoltaic devices. On the other hand, the possible manifestation of metal exchange with the very reactive Ti atoms must be, taken into account when designing porphyrin-sensitized solar cells because the critical temperature for the onset of self-metalation is very close to the normal operating temperature of photovoltaic devices.