Dependences of thermal behaviors on the number of layers in Langmuir-Blodgett (LB) films of 7,8, have been investigated for both morphological and molecular structural aspects by using ultraviolet-visible (UV-vis) and infrared (IR) spectroscopies. The temperature-dependent morphological changes have been monitored by the base-line shift in the UV-vis spectra of the LB films. The changes in the base-line in the 450-600 nm region suggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the domains in the seven-and 11-layer films are stable until 80°C and undergo rather abrupt collapse between 80 and 90°C. These differences in the thermal behaviors of the domain structure between the one-layer film and the seven-and 11-layer films may be attributed to the differences in the film thickness, the effect of the interaction between the first layer and a CaF 2 substrate, and the longitudinal interaction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order-disorder transition temperature of the one-layer LB film is lower by about 5°C than that of the 11-layer film, which is close to the chain melting point of the bulk material. Of particular interest is that the domain structure of the onelayer film shows the gradual temperature change but that the molecular orientation and structure in the domains experience the clear order-disorder transition.

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The annealing effects on Langmuir-Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (abbreviated as octadecyl-TCNQ) have been investigated by atomic force microscopy (AFM) and ultravioletvisible (UV-vis) and infrared (IR) spectroscopies. An AFM image of a one-layer LB film of octadecyl-TCNQ scanned at room temperature shows that the film consists of numerous platelike microcrystal domains. A periodic structure of octadecyl-TCNQ molecules with a period of 0.85 nm can be observed inside these domains. After the annealing, cooling down the film from 130 to 30°C, some of these domains lie on top of one another, keeping their original morphology. The rest seem to melt into a globular bulk sample. In the case of an 11-layer film, it can be seen from its AFM images that a number of domains with smaller size cover the substrate and that they completely melt after the annealing cycle. The UV-vis spectra of the oneand 11-layer films measured before and after the annealing show that the stacked structure of the TCNQ chromophore recovers partially for the one-layer film, but it recovers little for the 11-layer film. The IR measurements of the one-, three-, seven-, and 11-layer LB films before and after the annealing reveal that the subcell packing of the hydrocarbon chain changes due to the cyclic temperature treatment up to 130°C. The present study also demonstrates that the annealing effects on the LB films of octadecyl-TCNQ are largely different from those on LB films of simple amphiphilic compounds such as fatty acids.

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.

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The molecular orientation and aggregation behavior of 5-(4-N-octadecyl-pyridyl)-10,15,20-tri-p-tolylporphyrin (porphyrin 338a) in mono- and multilayer Langmuir−Blodgett (LB) films have been investigated by employing ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. It has been found that the porphyrin planes assume a nearly flat orientation with respect to the surface of the solid substrates in the LB films, irrespective of the number of layers. This orientation remains unaltered even after aging of the LB films for 1 month or heating up to 150 °C, indicating high chemical and thermal stabilities of molecular arrangement in the porphyrin LB films. On the other hand, the porphyrin cores are distorted to some extent, and the attached long hydrocarbon chains are loosely packed in the LB films, as deduced from a comparison of the IR spectra of the LB and cast films with that of the solution. The porphyrin molecules form head-to-tail type (J-type) aggregates in the mono- and multilayer LB films, evidenced by significant red shift of the Soret bands in the UV−vis spectra of the LB films, compared to solution. The comparative and complementary investigations by the UV−vis and IR techniques also suggest that the structural features of the porphyrin mono- and multilayer LB films on CaF2 plate are similar to each other.

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