COMPARATIVE-STUDY OF LANGMUIR-BLODGETT-FILMS OF 3 KINDS OF DYES CONTAINING A 2-[[4'-(MONOALKYLAMIMO OR DIALKYLAMINO)PHENYL OR NAPHTHYL]AZO]-N-METHYLBENZOTHIAZOLIUM CHROMOPHORE BY INFRARED, VISIBLE ABSORPTION, AND RESONANCE RAMAN SPECTROSCOPIES

Visible absorption and infrared (IR) transmission and reflection-absor ption (RA) spectra have been measured for Langmuir-Blodgett (LB) films of octadecylamino)phenyl]azo]-N-methylbenzothiazolium perchlorate (Az o III), octadecylamino)phenyl]azo]-N-methylbenzothiazolium perchlorate (Azo IV), and tadecylamino)naphthyl]azo]-N-methylbenzothiazolium perc hlorate (Azo V) and compared with those of octadecylamino)phenyl]azo]- N-methylbenzothiazolium perchlorate (Azo I) and ctadecylamino)-phenyl] azo]-N-methylbenzothiazolium perchlorate (Azo II) in order to investig ate the effects of modifications in the chromophoric structure upon th eir molecular aggregation, orientation, and structure in the LB films. The visible absorption spectral study shows that Azo IV forms mainly dimers in the LB film while Azo III and Azo V form H aggregates. The f requencies of the CH2 stretching bands indicate that, in contrast to A zo I, Azo III, Azo IV, and Azo V molecules have highly ordered (trans- zigzag) hydrocarbon chains in their LB films. It is also concluded fro m the shape of the CH2 stretching band that the hydrocarbon chain the LB films of Azo III and Azo IV is in an orthorhombic subcell packing w hile that in the LB film of Azo V crystallizes with a hexagonal or pse udohexagonal subcell packing. Comparison between the IR transmission a nd RA spectra for the LB films of Azo III and Azo IV suggests that the ir benzothiazolium rings are fairly perpendicular to the substrate sur face but their phenyl rings and hydrocarbon chains are tilted apprecia bly as in the cases of LB films of Azo I and Azo II. The molecular ori entation in the LB film of Azo V is clearly different from those of th e rest; not only the naphthalene ring but also the benzothiazolium rin g is inclined significantly with respect to the surface normal, and it s hydrocarbon chain is tilted largely with its molecular plane nearly parallel to the surface. Resonance Raman spectra have also been obtain ed for the LB films of Azo III and Azo IV. These spectra, which show a band due to a -N=N- stretching mode near 1465 cm-1 and are very close to the spectra of their solutions, suggest that the -N=N- bond moieti es of Azo III and Azo IV take a relaxed trans conformation in their LB films.