Structural properties of Langmuir–Blodgett films of charge transfer salts: Pristine and iodine doped conducting films of (N-docosyl-pyridinium)TCNQ

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Abstract
We have investigated the molecular constitution and ordering of Langmuir–Blodgett films prepared by deposition of N-docosylpyridinium–TCNQ charge transfer salt onto CaF2 substrates and by their subsequent doping with iodine vapor to yield reasonably good (∼10−1 Ω−1 cm−1) conducting films. Optical and electron scanning microscopy of the precursor film show that it is made up of a mosaic of highly crystalline platelets lying flat on the substrate though with random orientation. Structural information at the molecular level is gained by measuring and analyzing the ESR spectra and their angular and temperature dependence as well as the UV-visible and infrared spectra and their linear dichroism. A triplet fine structure is observed in the ESR spectra of the precursor films with zero field splitting parameters ‖D‖=7.48×10−3 cm−1 and ‖E‖=1.2×10−3 cm−1 and singlet–triplet activation energy J=0.20 eV, markedly different from those of the N-docosylpyridinium–TCNQ powder salt. The UV-visible and infrared spectra show features typical of a system containing weakly interacting (TCNQ−)2 dimers similar to those found in a water solution. The dichroic behavior of the observed infrared active modes of the TCNQ− anion and of N-docosylpyridinium as well as of the vibronically induced absorptions of the interacting TCNQ system are analyzed both for the precursor and for the iodine doped film. According to such an analysis and to that of the angular dependence of the ESR line shapes, the polar sheets in the precursor film consist of slipped (ring-bond overlap) (TCNQ−)2 dimers with their molecular planes almost parallel to the plane of the supporting slide. The spectroscopic and linear dichroism data allow us to monitor the changes induced by the iodination process and to show that, besides the partial oxidation of TCNQ− anions to TCNQ0, there is a complete change in the orientation of the TCNQ units. In many respects, the UV-visible and infrared spectra of the conducting film resemble those of mixed valence, intermediate conductivity salts like, e.g., (TEA) (TCNQ)2. The TCNQ molecules stand edge on with their molecular planes roughly perpendicular to the plane of the slide and with their long axes close to being normal to it.