Titania surfaces, which are ubiquitously employed in dye-sensitized solar cells (DSSCs) and photocatalysis, have been hitherto considered to have little or no chemical interactions with the macrocycle of porphyrins. In our combined experimental/theoretical investigation, we show the conversion on rutile TiO2(110) of a long-range ordered monolayer phase of free-base porphyrin (2H-TPP) into a pure (100%) and robust phase of Titanium(IV)-TPP via uptake of Ti substrate atoms upon mild annealing. As deposited 2H-TPP molecules are found to adsorb in a bridge site atop the Obr rows through hydrogen bonding of the two pyrrolic groups to the Obr atoms underneath. Upon self-metalation, the macrocycle is fully de-hydrogenated and a Ti atom is incorporated in the porphyrin pocket preserving the same molecular adsorption site. The central Ti atom displays the same oxidation state of the substrate ones and maintains the original coordination to the two Obr atoms underneath (TiO2-TPP). In fact, the (2x4)-oblique phase formed at the monolayer coverage molecules is unchanged across the self-metalation reaction.
The onset of the self-metalation reaction is found at a temperature of 100˚C, which is very close to the normal working conditions of solar cells, and is completed at 150˚-200˚C.The (2x4)-oblique phase is stable up to 300˚C, whereas it irreversibly transforms into a (2x6)-rect phase beyond 350˚C, which is associated with a change of molecular conformation. The latter reaction is driven by partial cyclo-dehydrogenation and carbon-carbon rebonding, which flatten the molecule albeit preserving the same absorption site, chemical state of the macrocycle and aromaticity of the molecular orbitals (TiO2-TPP-deH). The (2x6) phase is stable up to 450˚C, a temperature where porphyrins are fully desorbed from unreactive metals or decomposed/polymerized on more reactive ones (e.g. copper). The exceptional thermal stability of the TiO2-TPP-deH phase is due to the combination of little interaction with the substrate of the peripheral aromatic rings and strong chemical bonding to the Obr rows of the inner macrocycle, since deposition at room temperature.
Our results indicate a new route to the synthesis of ultra-pure and ordered metal-porphyrin layers on the facets of titania grains. On one side, the low onset temperature for self-metalation asks for a general reconsideration of the nature of the interface in porphyrin/titania DSSCs and of the standard protocols of porphyrin ordering and desorption on TiO2(110). On the other side, the extremly high temperature stability of the titanium(IV)-TPP contact layer makes it technologically relevant for sensors and photocatalysis applications in harsh environment.