MOLECULAR AGGREGATION, ORIENTATION, AND STRUCTURE IN LANGMUIR-BLODGETT-FILMS OF 2-(4'-(ETHYLOCTADECYLAMINO)PHENYLAZO)-N-METHYLBENZOTHIAZOLIUM PERCHLORATE AND 2-(4'-(DIOCTADECYLAMINO)PHENYLAZO)-N-METHYLBENZOTHIAZOLIUM PERCHLORATE STUDIES BY INFRARED, VISIBLE ABSORPTION, AND RESONANCE RAMAN SPECTROSCOPIES

Infrared transmission and reflection-absorption (RA) spectra have been measured for Langmuir-Blodgett (LB) films of 2-(4'-(ethyloctadecylamino)phenylazo)-N-methylbenzothiazolium perchlorate (Azo I) and 2-(4'-(dioctadecylamino)phenylazo)-N-methylbenzothiazolium perchlorate (Azo II) and visible absorption and resonance Raman spectra have been obtained for Azo I to compare their molecular aggregation, orientation, and structure in the LB films. The visible absorption spectrum of the LB film of Azo I shows that in contrast to Azo II previously investigated, Azo I does not form an H aggregate in the LB film. A comparison of the infrared transmission and reflection-absorption (RA) spectra indicates that the benzothiazolium rings of Azo I and Azo II are fairly perpendicular to the substrate surface but their benzene rings are tilted appreciably. As for the hydrocarbon chains, the infrared transmission, polarized transmission, and RA measurements suggest that the chains of both compounds are neither perpendicular nor parallel to the substrate surface, being tilted with respect to the surface normal, and they are in a hexagonal or pseudohexagonal subcell packing with a uniaxial orientation. Vibrational frequencies of infrared bands due to CH2 antisymmetric and symmetric stretching modes indicate that Azo II molecules in the LB films have highly ordered (trans-zigzag) hydrocarbon chains, but Azo I molecules in the films have slightly less ordered chains. Probably, Azo II with two hydrocarbon chains is more easily ordered in the films than Azo I with one hydrocarbon chain. Resonance Raman spectra of Azo I in the LB and cast films and solutions are very similar to each other in contrast to the previous result for Azo II, indicating that the conformation and structure of the chromophoric part of Azo I change little among the three states.