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

The orientation of a conjugated dimer-type copper(II) porphyrin without a long alkyl chain was controlled by adding a small amount of long chain n-alkane such as n-hexatriacontane (CH3(CH2)(34)CH3) in the mixed LB films with cadmium icosanoate. The orientation of the dimer-type porphyrin was investigated by using the electron spin resonance (ESR) spectroscopy for the mixed LB films with the molar mixing ratio porphyrin/ cadmium icosanoate/n-alkane = 1.5/10/x in the range 0 less than or equal to x less than or equal to 5. Without n-alkane, the dimer-type porphyrin had an orientation tilted with the most probable angle theta approximate to 60 degrees between the plane normal of the molecule and the film normal, while the macrocycle plane of the porphyrin was oriented more vertically (theta approximate to 75 degrees) with respect to the film surface for the LB film with the mixing ratio x = 2.0 of n-hexatriacontane. The orientation change depended on the chain length of n-alkane. When the amount of n-alkane was kept at 2.0, n-alkanes with chain length more than 28 acted effectively as triggers whereas n-octadecane could not cause an orientational change. This indicates that there is a certain lower limit to the length of n-alkane for the molecule to act as an effective trigger. Polarized UV/vis spectroscopic study of the mixed LB films, combined with that of the molecular orientation in the oriented liquid crystal, indicates that the PD molecule lies as ''side-on'' on the film surface in the n-hexatriacontane-containing LB film. The role of the trigger molecule was such that the angle between the shorter molecular axis of PD and the film surface is controlled by the trigger.