Structures of mixed Langmuir-Blodgett films of tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) with cadmium arachidate: A grazing incidence synchrotron X-ray diffraction study

Thin films of metallo-porphyrins have interesting properties which appear t o be related to the film structure. Mixed Langmuir-Blodgett (LB) films of t etrakis(3,5-di-tert-butylphenyl)porphinatocopper(TDBPC) with cadmium arachi date have been studied by grazing incidence X-ray diffraction (GIXD) to inv estigate their structures and miscibilities. In the two-component films the re are two 2-dimensional structures present in the film plane, with geometr ies and correlation lengths that are independent of the molar ratio of the two compounds. One is from cadmium arachidate domains, and has a rectangula r symmetry (alpha = 7.44 Angstrom b = 4.95 Angstrom, and gamma = 90 degrees ). The other is from the porphyrin domains and has an oblique symmetry (alp ha = 15.2 Angstrom, b = 8.61 Angstrom, and gamma= 80.3 degrees). The molecu lar chains of cadmium arachidate and the macrocycles of the porphyrin are p erpendicular to the film plane. The area per porphyrin molecule is 124 Angs trom(2), consistent with the value obtained from surface pressure-area meas urements on the floating monolayers. The components are immiscible. There i s no structural correlation between the layers in the LB films, and only th ose films with a high cadmium arachidate content exhibit a clear lamellar s tructure. The presence of long-chain alkane molecules (so-called trigger mo lecules) in the films did not alter the molecular orientation in the porphy rin domains, but did disturb the domain orientation with respect to the sur face normal of both materials.