ORDER-DISORDER TRANSITION IN LANGMUIR-BLODGETT-FILMS OF 2-(4'-(DIOCTADECYLAMINO)PHENYLAZO)-N-METHYLBENZOTHIAZOLIUM PERCHLORATE STUDIED BY INFRARED, VISIBLE ABSORPTION, AND RESONANCE RAMAN SPECTROSCOPIES

Infrared transmission, visible absorption, and resonance Raman spectra of 15-monolayer Langmuir-Blodgett (LB) films of 2-(4'-(dioctadecylamino)phenylazo)-N-methylbenzothiazolium perchlorate (Azo II) have been measured at elevated temperatures, and the order-disorder transition of the films has been investigated. Upward shifts of CH2 antisymmetric and symmetric stretching bands of the hydrocarbon chains suggest that the order-disorder transition of the tail groups occurs around 70-degrees-C in the films. Prior to the transition, the two CH2 stretching bands show broadening (50 approximately 70-degrees-C), indicating that the alkyl chain mobility begins to increase before their conformational disorder occurs. The visible absorption and resonance Raman spectra of the LB films of Azo Il undergo marked changes in the same temperature range as the infrared bands due to the CH2 stretching modes show the broadening. The spectral changes in the visible absorption and resonance Raman spectra are indicative of the conversion from an H-aggregate to monomers in the chromophoric part of Azo II, and therefore, it seems likely that the conversion is linked with the increase in the mobility of the hydrocarbon chains. Cyclic thermal treatment experiments in the infrared spectra of the Azo II LB film show that not only bands due to the chromophoric part but also the CH2 stretching bands do not return to original spectral features after the temperature is cycled to 70-degrees-C. This result indicates that the orientation and/or aggregation in the head groups do not recover upon the cyclic thermal treatment to 70-degrees-C and that the remaining disorder in the head groups hinders reversible order-disorder transition in the alkyl tails.