ORIENTATION CHANGE OF PORPHYRIN IN LANGMUIR-BLODGETT-FILMS CAUSED BY A TRIGGER MOLECULE

The orientation of the functional molecule PM, tetrakis(3,5-di-t-butylphenyl)porphinatocopper(II), was controlled by adding a small amount of the trigger molecule, n-hexatriacontane (CH3(CH2)34CH3) in the mixed Langmuir--Blodgett (LB) films with cadmium icosanoate. The orientation of PM was investigated by using the electron spin resonance (ESR) and UV/vis absorption spectroscopies for mixed LB films with the molar mixing ratio PM/cadmium icosanoate/n-hexatriacontane = 1.5/10/x in the range 0 less-than-or-equal-to x less-than-or-equal-to 5. The ESR spectra showed that there were two PM components, having different orientation, in each of the LB films. Without n-hexatriacontane, the major component had an orientation tilted with the most probable angle theta0 = 58-degrees between the plane normal of the molecule and the film surface, while the macrocycle plane of the major component was oriented almost vertically (theta0 = 80-degrees) with respect to the film surface for the LB film with the mixing ratio x = 0.5 of n-hexatriacontane. This mixing ratio means that the number of n-hexatriacontane molecules can be as small as one-third the number of PM molecules to control the orientation of PM. Polarized UV/vis absorption spectra indicated that doubly degenerated transition dipole moments of the Soret band of the porphyrin give rise to the two absorption bands due to the aggregation of molecules. They are J-like and H-like bands resulting from the interaction between the in-plane (parallel to the film surface) and out-of-plane (tilted with respect to the film surface) components of transition dipole moments, respectively. The dichroic ratio A(s)/A(p) of each band obtained by using UV/vis polarized light at the incident angle 45-degrees is well explained by taking in to account the molecular orientation determined with the ESR measurements. Study of the dependence of the mixing ratio x of n-hexatriacontane showed that there is a transition in the orientation of PM from the theta0 = 58-degrees to the theta0 = 80-degrees state in such a manner that the population of the former species decreases with increasing x and becomes indiscernible at x = 0.5. The X-ray diffraction analyses and IR absorption spectroscopy showed that the orientation of cadmium icosanoate is not affected significantly by the presence of n-hexatriacontane, suggesting that the orientation of PM is controlled through the molecular interaction with n-hexatriacontane. At x = 2, the orientation of PM is slightly different from the one at x = 0.5 probably due to the change in the structure of the LB film caused by the introduction of an excess amount of the nonamphiphilic molecule, n-hexatriacontane.