Zinc(II) Porphyrins at the Air−Water Interface As Studied by Polarized Total-Reflection X-ray Absorption Fine Structure

Abstract

The polarized total-reflection X-ray absorption fine structure method was applied to characterize zinc porphyrins at the air-water interface. The X-ray absorption near edge structure exhibited a significant difference depending on the polarization of the X-ray. A shoulder peak of the Zn K-edge corresponding to the 1s-4p(z) transition for a square planar metal complex without axial coordination(s) was observed at 9662 eV, which indicates that the axial coordination sites of zinc porphyrin molecules examined are not fully hydrated at the air-water interface. The molecular orientation of zinc porphyrins was determined by analyzing the polarization dependence of the transition peak intensity. The meso-substituted porphyrin derivative 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) orients rather parallel to the solution surface. In contrast to ZnTPP, the zinc(II) protoporphyrin IX (ZnPP) with hydrophilic carboxyl groups at one side of the molecule stands up with respect to the solution surface, and the average tilting angle of the porphyrin plane to the surface was evaluated to be between 57 degrees and 43 degrees. In addition, the axial coordination of ZnPP is modified depending on the surface concentration, in which the axial hydration to the zinc center is effectively inhibited in the compressed surface layer.

Extended X-ray Absorption Fine Structure of Copper(II) Complexes at the Air-Water Interface by a Polarized Total-Reflection X-ray Absorption Technique

“…Further details of an analogous optical setup are described elsewhere. 20 The XAFS spectra for the copper complexes dissolved in DMF and the solid powder sample pressed as boron nitride pellets containing 5 wt% copper complex were also measured in the fluorescence mode without controlling the polarization. The concentrations of the bulk solution samples were 9.3 ¥ 10 -4 mol dm -3 for CuPP, 1.0 ¥ 10 -3 mol dm -3 for CuChl, and 1.1 ¥ 10 -3 mol dm -3 for CuTPPC, respectively.…”

“…For both copper complexes, the smaller value of A PAE0 must be associated with a favorable standing-up orientation of the porphyrin plane to the air-water interface in the monolayer. 20 . In the CuTPPC system, a gradual increase in the surface pressure with a decrease in the surface area is possibly due to spontaneous multilayer formation in analogy with the symmetrically meso-substituted metalloporphyrin, ZnTPP.…”

“…13 For a solid surface, the polarization-dependent XAFS technique has been utilized to elucidate a well-organized structure such as thin-layer materials and coordination sites of adatoms on crystals. [14][15][16][17][18] Recent applications of the polarized TR-XAFS method have revealed the specific solvation structure of zinc(II) porphyrins at the air-water interface, 19,20 in which the X-ray absorption near edge structure (XANES) exhibited a significant difference depending on the polarization of X-rays. A pre-edge peak, which is known to correspond to the 1sAE4pz transition for a square-planar metal complex without axial coordination, was observed at the Zn K-edge, and it was concluded that the axial coordination sites of the zinc porphyrin molecules examined were not fully hydrated at the air-water interface.…”

“…The planar molecules of meso-substituted porphyrin derivatives, 5,10,15,20-tetraphenylporphyrinato zinc(II) (ZnTPP) and 5,10,15,20-tetrakis(4-carboxyphenyl)-porphyrinato zinc(II) (ZnTPPC) were found to orient roughly parallel to the solution surface. 19,20 On the other hand, zinc(II) protoporphyrin IX (ZnPP) with hydrophilic carboxyl groups at one side of the molecule was standing-up with respect to the solution surface. 20 The axial coordination behavior of the metalloporphyrin plays an important role in the molecular recognition properties in sensor applications, [21][22][23] the self-assembling of supramolecular structures [24][25][26] and monolayer formations at the air-water interface.…”

“…19,20 On the other hand, zinc(II) protoporphyrin IX (ZnPP) with hydrophilic carboxyl groups at one side of the molecule was standing-up with respect to the solution surface. 20 The axial coordination behavior of the metalloporphyrin plays an important role in the molecular recognition properties in sensor applications, [21][22][23] the self-assembling of supramolecular structures [24][25][26] and monolayer formations at the air-water interface. 27,28 The surface concentration of bulky metal complexes well-oriented at the air-water interface without selfaggregation or multilayer formation is generally much lower than the metal ions attracted to a condensed surfactant layer.…”