Fractal Structures and Excitation Energy Relaxation in Molecular Assemblies

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Organic molecules in general form self-assembled organizates. Typical examples are vesicles, Langmuir-Blodgett (LB) monolayr films and liquid crystals, in which some structural orders appear with respect to molecular orientation and spatial distribution of guest molecules, and determine the macroscopic morphology of these assemblies. In this study, the structural orders have been investigated by means of the excitation energy transfer between molecules monitored with a picosecond time-resolved fluorescence spectroscopy. The spatial distribution of molecules, which can be expressed in terms of "fractal", was investigated for polymers and LB films. It was found that the fractal dimension depends on (d) types of molecules, molecular inteaction : d = 1.7 in a surface of silicagel microcrystals, d = 1.3 - 1.7 in LB films of cyanine and rhodamine dyes, and d = 1.0 - 1.4 in hydrophobic plymers. The dynamical properties of excitation energy relaxation are characterized by the energy hopping migration among sites of slightly different energies.