Fractals and Excitation Transfer in Molecular Assemblies

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Electronic excitation transfer has been studied with several kinds of molecular assemblies such as vesicles and Langmuir-Blodgett films with a view to establishment of the fractal structure in distribution of guest molecules. Dynamics of excitation transfer was investigated by means of a picosecond time-resolved fluorescence spectrophoto-meter. It was found that the fractal behaviors are essential among molecular assemblies in spatialdistribution of chromophores or in siteーenergy distribution, and that the morphology of molecular assembly frameworks affects the dynamics of energy relaxation processes. The fluorescence decays can be analyzed by an equation including a stretched exponential function. The mechanisms of singlet excitation energy relaxation were interpreted in terms of(1) the energy trapping at acceptor sites under the fractal distribution with the fractal dimension of 1.3 -1.5, and(2) the energy migration among energetically disordered monomer sites represented by ultrametric space (hierarchical energy distribution).The diffusion length of the excitation energy transport was estimated from the dynamic stokes shift and the fluorescence decay curve.