Orientation Change of Dimer-Type Porphyrin in Langmuir-Blodgett Films Caused by a Trigger Molecule

R. Azumi, M. Matsumoto, S. Kuroda, L. G. King, and M. J. Crossley
[Langmuir, Vol.11, pp.4056-4060, 1995]

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)₃₄CH₃) 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/chi in the range 0 leqchi leq 5. Without n-alkane, the dimer-type porphyrin had an orientation titled with the most probable angle q=60° between the plane normal of the molecule and the film normal, while the macrocycle plane of the porphyrin was oriented more vertically (theta=75°) with respect to the film surface for the LB film with the mixing ratio chi= 2.0 of n-hexatriacontane. The orientation change depended on the chain length of n-alkane. 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.

Scheme 1.