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J. Schneider, F. Kollhoff, T. Schindler, S. Bichlmaier, J. Bernardi, T. Unruh, J. Libuda, T. Berger, O. Diwald:
"Adsorption, Ordering, and Metalation of Porphyrins on MgO Nanocube Surfaces: The Directional Role of Carboxylic Anchoring Groups";
Journal of Physical Chemistry C, 120 (2016), 26879 - 26888.

The understanding of porphyrin adsorption on
oxide nanoparticles including knowledge about coverages and
adsorbate geometries is a prerequisite for the improvement
and optimization of hybrid materials. The combination of
molecular spectroscopies with small-angle X-ray scattering
provides molecular insights into porphyrin adsorption on
MgO nanocube dispersions in organic solvents. In particular,
we address the influence of terminal carboxyl groups on the
adsorption of free base porphyrins, on their chemical binding,
on the metalation reaction as well as on the coverage and
orientation of adsorbate molecules. We compare the free base
form 5,10,15,20-tetraphenyl-21,23H-porphyrin (2HTPP) with
the carboxyl-functionalized 5,10,15,20-tetrakis(4-carboxyphenyl)-21,23H-porphyrin (2HTCPP) and show that without
carboxylic anchoring groups the free base form metalates on the nanocube surface and adopts a flat-lying adsorbate geometry.
The saturation limit for flat-lying adsorption on nanocubes with an average edge length of 6 nm corresponds to 90 ± 14
molecules per particle. This limit is surpassed when 2HTCPP molecules attach via their terminal carboxyl groups to the surface.
The resulting upright adsorption geometry suppresses self-metalation, on the one hand, and allows for much higher porphyrin
coverages, on the other (at porphyrin concentrations in the stock solution of 2 × 10−2 mol·L−1). UV−vis diffuse reflectance
results are perfectly consistent with conclusions from SAXS data analysis. The experiments reveal concentration dependent
2HTCPP coverages in the range between 0.4 to 1.9 molecules nm−2 which correspond to the formation of a shell of upright
standing porphyrin molecules around the MgO nanocubes. In contrast, after adsorption and metalation of nonfunctionalized
2HTPP the resulting porphyrin shells are in the range of a tenth of a nanometer and thus too thin to be captured by SAXS
measurements. Related insights advance our opportunities to prepare well-defined nanohybrids containing highly organized
porphyrin films.

http://dx.doi.org/10.1021/acs.jpcc.6b08956