Adsorption of Phosphonic-Acid-Functionalized Porphyrin Molecules on TiO₂(110)

Abstract

We studied the adsorption geometry and bonding of mono-and tetraphosphonic-acid-functionalized tetraphenylporphyrin molecules on rutile TiO 2 (110) surfaces. The investigation was carried out by means of X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure (NEXAFS) measurements. We found that the molecules bind covalently to the surface in a mixed monodentate and bidentate mode implying deprotonation of one or two phosphonate hydroxyl groups. Our NEXAFS data suggest that molecules containing one functional phosphonic acid group are on average tilted further away from the surface than molecules with four functional groups. The highest occupied electronic state of both molecules is in the band gap at 2.1 eV below the Fermi level. Our results demonstrate that the number of functional phosphonic acid groups determine the adsorption geometry of tetraphenylporphyrins.

Surface Reactions and Electronic Structure of Carboxylic Acid Porphyrins Adsorbed on TiO2(110)

“…In order to extract information regarding the adsorption geometry in twofold-symmetry substrates, it is necessary to measure the absorption intensity as a function of both the polar and azimuthal photon incidence angles. , Here, we measured the NEXAFS spectra as a function of the polar angle for one azimuthal orientation preventing us from extracting precise adsorption angles. However, we know that carboxylic acid and phosphonic acid mono-functionalized tetraphenylporphyrin molecules exhibit no azimuthal dependence when adsorbed on TiO 2 (110). , Therefore, if we assume that MCTPP molecules adsorbed on TiO 2 (110) are not aligned in any particular azimuthal direction, then we can speculate on the molecular orientation. Under this hypothesis, we note that in both the C and N K-edge NEXAFS spectra corresponding to 0.2 ML, the π* transitions into the macrocycle molecular orbitals are very intense at grazing angles (red spectra) and almost vanish at normal incidence (black spectra).…”

“…NEXAFS measurements suggest that Zn protoporphyrins adsorbed on TiO 2 (110) undergo a flat lying-to-tilted adsorption geometry transition as the molecular coverage increases . The number and position of phosphonic acid and carboxylic acid peripheral groups are also known to modify the adsorption geometry of porphyrin molecules on TiO 2 (110) surfaces. , Free-base porphyrins adsorbed on TiO 2 (110) and TiO 2 (110)-2 × 1 surfaces can undergo reactions with co-deposited Ni metal atoms to form metalloporphyrins. , In addition, the molecule can also experience different reactions with the TiO 2 (110) surface. The iminic nitrogen atoms of free-base porphyrins are known to protonate after adsorption at room temperature .…”

Self‐Metalation of Anchored Porphyrins on Atomically Defined Cobalt Oxide Surfaces: In situ Studies by Surface Vibrational Spectroscopy

“…reported a minimum tilting angle of 25° (±15°) at low coverage and a maximum tilting angle of 77° (±5°) at monolayer coverage. Similar values were also suggested for MCTPP on TiO 2 (110) . Here, Fernández et al.…”

“…Here, Fernández et al. determined a tilting angle of 66° for MCTPP by Near‐Edge X‐Ray‐Absorption Fine Structure (NEXAFS) measurements of TPP molecules, with different numbers of carboxylic anchor groups . In summary, the MCTPP tends to lie flat at low coverage and reorients to form a nearly upright standing layer at monolayer coverage.…”

“…The authors studied carboxylic acid and phosphonic acid functionalized TPP molecules. The porphyrins carrying only one or two functional groups in cis conformation are tilted further away from the surface, whereas porphyrins with four anchor groups or two in trans conformation bind in a more flat‐lying geometry …”