Spectroscopic studies on phase transitions in Langmuir-Blodgett films of an azobenzene-containing long-chain fatty acid: Dependence of phase transitions on the number of monolayers and transition cycles among H-, J-, and J'-aggregates in multilayer films

Ultraviolet-visible (UV-vis), infrared transmission, reflection-absorption (RA), and fluorescence spectra have been measured for one-, three-, and nine-monolayer Langmuir-Blodgett (LB) films of an azobenzene-containing long-chain fatty acid (4-dodecyloxy-4'-(3-carboxytrimethyleneoxy)azobenzene, abbreviated 12A3H) at various temperatures to investigate dependences of phase transitions and annealing effects on the number of monolayers. The UV-vis spectra of the LB films have revealed that the fatty acid investigated forms I-I-aggregates in the LB films irrespective of the number of monolayers at room temperature. With a temperature increase the H-aggregate in the one-monolayer LB film gradually breaks into the monomers, while that in the three- and nine-monolayer LB films abruptly changes into the J-aggregate near 90 degrees C. These observations show that the one-monolayer LB film does not have a clear order-disorder transition, while the three-and nine-monolayer films have a phase transition near 90 degrees C. The infrared study has also given unambiguous evidence that supports this conclusion. For example, the peak intensities of CH2 antisymmetric and symmetric stretching bands at 2920 and 2850 cm(-1) in the transmission spectrum of the one-monolayer film gradually decrease with temperature, suggesting that the alkyl chain becomes tilted little by little with respect to the surface normal. In contrast to the one-monolayer film, the peak intensities of most of the infrared bands of the three- and nine-monolayer films undergo a marked change near 90 degrees C. Therefore, it seems that the tilt angles of both the alkyl chain and chromophobic part change largely concomitantly with the conversion from the I-I-aggregate to the J-aggregate. Cyclic thermal treatment experiments for the UV-vis spectra of the LB films show that the conversion from the I-I-aggregate to monomers in the one-monolayer film is nearly reversible, while annealing of the three- and nine-monolayer films causes a transition from the J-aggregate to another J-aggregate. The latter J-aggregate is further converted to the original H-aggregate by leaving the LB films in the atmosphere or irradiating them with UV laser light.