Metal Complex Spread on Liquid Surface Studied by Polarization-dependent X-ray Absorption Spectroscopy Recent advances in a total-reflection X-ray absorption fine structure (TR-XAFS) method allow us to investigate the solvation structure of ions at the air- water interface [1-3]. There is very little information on this phenomenon due to the lack of a suitable experimental technique; however, such information is important for colloid and solution chemistry. Further progress in the TR-XAFS method with linearly polarized X-rays described here has been achieved by introducing an X-ray phase retarder to the undulator beamline [4]. Experiments were carried out at the undulator beamlines BL39XU and BL37XU . The linearly polarized X-ray on the horizontal plane from the undulator was converted to a vertically polarized one using a diamond crystal phase retarder. The degree of linear polarization, P L , is defined by the following equation : P L = ( I h – I v ) / ( I h + I v ), where I h and I v are the intensities of the polarized X-rays. P L = -1 corresponds to a perfect linear polarization on the vertical plane. I n the present setup, the P L value was estimated to be approximately – 0 . 9 for the vertically polarized X-ray. V arious metalloporphyrins have been applied to photoenergy conversion systems as well as to supramolecular structures involving self-assembly features that could further improve the functionality of porphyrin-containing thin films organized by interfacial processes. I n this work, we applied the polarized TR- XAFS method to a monolayer of p lanar zinc( II ) p o r p h y r i n , m e s o - t e t r a k i s ( 4 - c a r b o x y p h e n y l ) porphyrinato zinc( II ) ( Z nT PPC ), at the air-water interface in order to determine the coordination structure around zinc and the molecular orientation. U nder the present acidic condition, four carboxyl g r o u p s o f Z n T P P C a r e n o t d i s s o c i a t e d , a n d consequently, the neutral form of Z nT PPC does not dissolve into the aqueous subphase. The X-ray absorption near-edge structure (XA N ES) spectra at the Z n K -edge for Z nT PPC are displayed in Fig. 1. A XA N ES spectrum with a horizontally polarized X-ray (a) is different from that with a vertically polarized one (b); the most striking difference is the appearance of a very strong first peak at 9662 e V in Fig. 1(b), which must correspond to the shoulder structure of the powder sample (d) at the same energy, but is absent in the horizontal spectrum (a). The strong pre-edge or shoulder peak in the XA N ES spectrum at the Z n K -edge must be associated with the 1 s - 4 p z transition in the absence of the axial coordination for zinc atoms, since the assignment of such a peak has been well established for square planar complexes [ 5 ]. The polarization dependence of the XA N ES spectrum indicates that the plane of the Z nT PPC molecule is unambiguously oriented parallel to the air-water interface and there is no coordination to the axial sites of the zinc atoms. O n the other hand, the absence of any pre-edge peak in the spectrum for ethyl acetate solution in Fig. 1(c) suggests that the solvent molecules are coordinated to the axial sites of a zinc atom in ethyl acetate (Fig. 2 ). Energy (eV) Normalized ( μ t) 9660 9680 9700 9720 9740 (a) (b) (c) (d) horizontal polarization vertical polarization solution powder Fig. 1. XANES spectra at the Zn K -edge for ZnTPPC spread as a monolayer in the acidic aqueous solution taken with (a) horizontal and (b) vertical polarization by the TR-XAFS method, (c) in the ethyl acetate solution by the fluorescence mode, and (d) in the solid powder by the transmission mode. 71 Hirohisa Nagatani a , Hajime Tanida b and Iwao Watanabe c (a) Department of Natural Sciences, Hyogo University of Teacher Education (b) SPring-8 / JASRI (c) Faculty of Science, Osaka Women’s University E-mail: nagatani@sci.hyogo-u.ac.jp References [1] I. Watanabe: SPring-8 Research Frontiers 1999/2000, p. 55. [2] I. Watanabe et al. : Rev. Sci. Instrum. 68 (199 7 ) 33 0 7 . [ 3 ] I. Watanabe et al. : J. Am. C hem. Soc 119 (199 7 ) 12018. [ 4 ] H. Tanida, H. Nagatani and I. Watanabe: J. C hem. Phys. 118 (200 3 ) 10 36 9. [5] A.I. Frenkel et al. : J. C hem. Phys. 116 (2002) 9 44 9. Fig. 2. Schematic drawing of the predicted molecular orientation of ZnTPPC at the air-water interface and the solvation structure in the solution. X AFS is a powerful techni q ue for clarifying the coordination structure in a comple x around a metal center. The present results clearly demonstrate that the TR- X AFS method enable the determination of the orientation of a planar comple x at the air-water interface in a very simple manner by introducing polari z ed X -rays. Furthermore, E X AFS analysis would allow us to characteri z e the molecular structure of metal comple x at the interface within the hori z ontal or the vertical plane separately, e.g., solvation and coordination distances between metal and ligands involved in the self-assembled monolayer, or the L angmuir- B lodgett film formed or in process. X AFS is a powerful techni q ue for clarifying the coordination structure in a comple x around a metal center. The present results clearly demonstrate that the TR- X AFS method enable the determination of the orientation of a planar comple x at the air-water interface in a very simple manner by introducing polari z ed X -rays. Furthermore, E X AFS analysis would allow us to characteri z e the molecular structure of metal comple x at the interface within the hori z ontal or the vertical plane separately, e.g., solvation and coordination distances between metal and ligands involved in the self-assembled monolayer, or the L angmuir- B lodgett film formed or in process. N N N N In solution Solvent N N N N vertical horizontal At air-water interface Polarized X-ray Zn Zn 72
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