  We show the possibility of tailoring the molecular arrangement, as well as
the chemical and structural modifications, of porphyrins at the monolayer
saturation coverage on TiO2(110) by Synchrotron photoemission, electron
diffraction, STM topography and DFT calculations. Free-base
tetra-phenyl-porphyrins (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 one monolayer. Upon sample heating, a
self-metalation reaction sets in at 100C, yielding full metalation of the
saturated monolayer at ~200C. 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 oxygen atoms underneath. Neither the adsorption geometry (on-bridge, atop
the oxygen rows) nor the molecular arrangement change across the
self-metalation transition up to 300C. On one side, the robustness of this
saturation phase makes it a promising system for its implementation into
applications for photocatalysis and photovoltaic devices. On the other side,
the possible manifestation of metal exchange with the very reactive Ti atoms
must be taken into account when designing porphyrin-sensitized solar cells
since the critical temperature for the onset of self-metalation is very close
to the normal operating temperature of photovoltaic devices.
