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

Abstract

We previously reported that the orientation of functional molecule meso-tetrakis(3,5-di-tert-butylphenyl)-porphinatocopper(II), PM, was controlled by the addition of a small amount of the trigger molecule, n-hexatriacontane (CH 3 (CH 2 ) 34 CH 3 ), in the mixed Langmuir-Blodgett (LB) films with cadmium icosanate. 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 (CD 3 (CD 2 ) 34 CD 3 , HA-d) was used to separate the infrared absorption bands of the trigger molecule from those of cadmium icosanate. In the mixed LB film with a mixing ratio PM/cadmium icosanate/HA-d = 0/10/0.5, the tilt angles γ of the alkyl chains for HA-d and cadmium icosanate (C20) with respect to the film normal were estimated at 12° and 13°, respectively. The introduction of PM with the mixing ratio 1.5/10/0.5 changed γ of HA-d to 37°, whereas that ofC20 stayed unchanged. The existence of the anisotropy of CD 2 stretching bands around the molecular axis indicates that HA-d was oriented with its C-C-C plane almost perpendicular to the film surface. The orientation of HA-d was further studied with various mixing ratios of 1.5/10/x to investigate the mechanism of orientation control of PM by HA-d. A crossover point was clearly seen around the mixing value x = 0.5. With the mixing value x in the region 0 < x ≤ 0.5, the electron spin resonance spectra can be simulated satisfactorily by adding (0.5 - x)/0.5 times the spectrum with the mixing ratio 1.5/10/0 and x/0.5 times the spectrum with the mixing ratio 1.5/1/0.5, suggesting specific molecular interaction of the two components. At x = 0.1, the C-C-C plane of HA-d was distributed almost homogeneously around its own molecular axis ; whereas with increasing x in the region 0 < x ≤ 0.5, the C-C-C plane of HA-d was oriented gradually in an anisotropic manner or the fraction of the anisotropically oriented HA-d had increased. In the region x ≥ 0.5, the fraction of HA-d not participating in the orientation change of PM took an orientation similar to that of HA-d in the mixed LB film with C20 alone. In contrast to the case of HA-d, the alignment of C20 did not change by the introduction of HA-d, supporting the argument that the orientation change of PM occurred through the direct interaction between HA-d and PM.