Polarized Total-Reflection X-ray Absorption Fine Structure of Zinc(II) Porphyrin at the Heptane−Water Interface

Abstract

The polarized total-reflection X-ray absorption fine structure (TR-XAFS) method was applied in situ to the liquid−liquid interface. The TR-XAFS spectrum at the Zn K-edge for 5,10,15,20-tetrakis(4-carboxypheny)porphyrinato zinc(II) (ZnTPPC4−) adsorbed at the heptane−water interface exhibited a spectral difference depending on the linear polarization of the incident X-ray. The 1s−4pz transition response for a square planar zinc(II) complex was observed in the pre-edge region (9662 eV) only with a vertically polarized X-ray. The appearance of the transition response indicates that the axial coordination sites of the zinc center tend to be partially dehydrated at the heptane−water interface. The polarization dependence of the TR-XAFS spectrum demonstrated that the porphyrin macrocycle orients almost in the interfacial plane similar to ZnTPPC at the air−water interface. Polarized TR-XAFS measurement was also carried out in the presence of a cationic surfactant, hexyltrimethylammonium bromide (C6TAB), in the aqueous subphase. The porphyrin molecules stand up slightly at the heptane−water interface by the ion association with C6TA+.