Orientation Change of Functional Molecules in Langmuir-Blodgett Films Caused by a Trigger Molecule: Infrared Spectroscopic Study on the Orientation of n-Alkane, Trigger Molecule

R. Azumi, M. Matsumoto, S. Kuroda, and M. J. Crossley
[Langmuir, Vol.11, pp.4495-4498, 1995]

We previously reported that the orientation of a functional molecule meso-tetrakis(3.5-di-tert -butylphenyl)-porphinato- copper(II), PM, was controlled by the addition of a small amount of the trigger molecule, n-hexatriacontane (CH₃(CH₂)34CH₃), in the mixed Langmuir-Blodgett (LB) films with cadmium icosanoate. In this paper we investigated the orientation of n-hexatriacontane, the trigger molecule, in the mixed LB film by comparing the infrared transmission and reflection-absorption spectra of the mixed LB films. Deuterated n-hexatriacontane (CD₃(CD₂)₃₄CD₃, HA-d) was used to separate the infrared absorption bands of the trigger molecule from those of cadmium icosanoate. In the mixed LB film with a mixing ratio PM/cadmium icosanoate/HA-d = 0/10/0.5, the tilt angles g of the alkyl chains for HA-d and cadmium icosanoate (C20) with respect to the film normal were estimated at 12º and 13º , respectively. The introduction of PM with the mixing ratio 1.5/10/0.5 changed g of HA-d to 37º , whereas that of C20 stayed unchanged. The existence of the anisotropy of CD₂ stretching bands around the molecular axis indicates that HA-d was oriented with its C-C-C plane almost perpendicular to the film surface.

Scheme 1.