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Excitation energy transport through an ultrathin film has been investigated with a designed structure of copolymers containing carbazole and anthracene chromophores. Poly(isobutyl methacrylate) gives a stable monolayer at the air-water interface, and it is transferrable to a solid substrate. Various kinds of chromophores can be incorporated into this monolayer in the form of methacrylate esters. Using the Langmuir-Blodgett (LB) technique with these copolymers, artificial arrangements of the chromophoric units can be realized in a layered structure of the LB film. The carbazole chromophore exhibited simple monomer emission, and no excimer was detected. This spectroscopic property enabled us to carry out quantitative analyses of the interlayer energy-transfer process. Time-resolved fluorescence spectroscopy clearly showed efficient energy migration among carbazole chromophores. After several steps of interlayer migration, the excitation energy transfers to the anthracene layer coated on the carbazole layers. Although some unknown traps exist in this LB film, statistically random distribution of both the donor and acceptor moieties in each layer results in a high quenching yield of the carbazole excitons. Besides the uniform distribution, an advantage of polymer LB films is the thinness of each layer. These properties make possible the efficient energy transport through the layered structure.

Monoand multilayered films of poly(vinyl octal) containing pyrene chromophore have been prepared by the Langmuir-Blodgett (LB) technique. The photophysical processes, excimer formation and energy trapping of pyrene chromophore in the 2-dimensional and quasi 3-dimensional systems, have been investigated by photostationary and time-resolved fluorescence spectroscopy. Monolayered films of the polymer on pure water were transferred to a quartz plate at 19 °C as a Y-type film. UV absorbance and fluorescence intensity of the built-up films were proportional to the number of layers. The obtained films show little excimer emission. The ratio of excimer fluorescence to the monomer fluorescence is much less than that of mixed films fabricated from a conventional fatty acid with pyrene chromophore, at the same concentration of the chromophore in a plane. Fluorescence decay analysis by a 2-dimensional energy-transfer model gives the density of excimer-forming sites. The results indicate that the polymer LB films attain a statistically uniform distribution of pyrene moieties. In multilayered films, the decay curves did not fit to the Forster kinetics in which energy migration is not involved, and they were analyzed by the Yokota-Tanimoto Padé model involving migration of the excited state among pyrene moieties. The diffusion coefficient for the energy migration was found to be 1 X 10"8 cm2 s'1. The observed characteristics of the polymer LB films are attributed to the uniform distribution of pyrene moieties and the thin thickness of each layer.Recently, Yamazaki et al. performed a fine investigation on chromophore distributions in mixed LB films by means of picosecond fluorescence decay analysis.2'3 In their system, the amphiphilic pyrene moieties were dispersed in the LB film of long-chain fatty acids but the chromophore formed aggregates or ground-state dimers of pyrene units.3 They found uniform distribution of pyrene moieties in a plane of LB film difficult to obtain.Another disadvantage of the LB films of fatty acids is the thermal and mechanical instability. Some investigators made LB films stable by polymerization of a monolayer or multilayers containing a polymerizable group.4'13Recently some preformed polymers were found to form a stable monolayer at the air-water interface and to be transferable to solid substrates.13'25 One of these polymers is poly (vinyl alkanal), which has been reported by Ogata et al.25 This polymer is prepared by the acetalization of poly(vinyl alcohol) with alkylaldehydes. To this polymer can be easily introduced various kinds of functional groups by the coexistence of chromophoric aldehydes with alkylaldehyde in the acetalization.26 Excimer formation and energy trapping of chromophores in polymer solids depend on the chromophore distribution in the polymer matrices. Recently, we succeeded in preparing the polymer having a carbazole chromophore in which the moieties are uniformly

Molecular orientation of mono-and multilayered polyimide films has been studied by the fluorescence polarization method. The high sensitivity of the fluorescence method enabled us to measure the orientation of extremely thin films. A perylene pigment was synthesized as a fluorescence probe and was introduced into polyamic acid chains. Layered films of polyamic acid alkylamine salt were prepared by the Langmuir-Blodgett (LB) technique. Even after chemical conversion to polyimide, molecular axes of the polyimide chain retained a 2-dimensional distribution in the substrate plane. The perylene probes in the LB film showed fluorescence anisotropy, and their second moment of orientation, (cos2 w ) , was determined to be 0.54-0.58 by 2-dimensional analysis. The principal axis of the orientation was in agreement with the dipping direction. These measurements reveal the orientation of a "genuine surface" of molecular dimension. This LB polyimide surface aligns highly liquid-crystal molecules as a rubbed surface of polyimide film does. The ability of LB films to orient a liquid crystal correlated with the surface orientation measured by the fluorescence method.

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