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

Abstract

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 azobenzenecontaining 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 H-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°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°C. The infrared study has also given unambiguous evidence that supports this conclusion. For example, the peak intensities of CH 2 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°C. Therefore, it seems that the tilt angles of both the alkyl chain and chromophobic part change largely concomitantly with the conversion from the H-aggregate to the J-aggregate. Cyclic thermal treatment experiments for the UVvis spectra of the LB films show that the conversion from the H-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.

Orientation and Organization of Langmuir−Blodgett Mixed Films Consisting of a Phospholipid and a Viologen Studied by Infrared Spectroscopy

“…Previous IR studies 1,2,4,[8][9][10][11][12][13][14][15][16][18][19][20][21][22] have shown that the location of ν as (CH 2 ) and ν s (CH 2 ) modes are sensitive indicators for the lateral interactions between alkyl chains. Thus, the band position for the ν as (CH 2 ) mode of a crystalline polymethylene chain (2920 cm -1 ) is 8 cm -1 lower than that for the liquid state (2928 cm -1 ), whereas for the ν s (CH 2 ) mode the band position is 6 cm -1 lower in the crystalline sample (2850 cm -1 ) than in the liquid (2856 cm -1 ).…”

Synthesis and crystal structures of two hetarylazo indole disperse dyes

“…Infrared (IR) spectra were recorded on a Mattson 1000 Fourier Transform-Infrared (FT-IR) spectrophotometer in KBr. Proton nuclear magnetic resonance ( 1 H-NMR) spectra were recorded on a Bruker-Spectrospin Avance DPX 400 MHz Ultra-Shield in dimethylsulfoxide (DMSO)-d 6 and CDCl 3 with TMS as internal reference. Chemical shifts were expressed in d units (ppm).…”

“…some 3-substituted indole derivatives have antiinflammatory activity [1][2][3][4]. Azo compounds are the most widely used class of dyes because of their versatile applications in various fields such as the dyeing of textile fibres, nonlinear optics, optical data storage as well as biologicalmedicinal studies [5,6]. Their optical properties depend not only on the spectroscopic properties of the molecules, but also their crystallographic arrangements [7,8].…”

Synthesis and X-ray analysis of a perfluoroalkyl-substituted azobenzene dye

“…In addition, their method of synthesis opens the door to a myriad of colorants that are customized to fit specific applications. For instance, azo compounds derived from judiciously selected donor and acceptor groups can be used in non-linear optics and optical data storage [1,2]. From such studies, it is known that the optical properties obtained depend not only on the spectroscopic properties of the constituent molecules, but also on their crystallographic properties [3e5].…”