We propose to continue and extend an experimental and theoretical investigation on the mechanisms and dynamics of excitation migration among molecular aggretates. We will study multicomponent molecular systems in disordered organic crystals, films and polymeric matrices. Simple aromatics (e.g. naphthalene) and biological species (e.g. chlorophyll) will carry the excitation in these biomimetic samples. Methods will be developed for the careful production of these samples, for the characterization of their structural properties, site heterogeneity and surface roughness as well as for the study of their excitations and excitation propagation. Energy transport will be studied as a function of molecular composition, concentration, excitation time profile and spatial distribution, energy domain, excitation density, electromagnetic fields and temperature. Phosphorescence, fluorescence, delayed fluorescence and resonance Raman techniques will be utilized to monitor exciton migration, trapping, detrapping, fusion and fission, and in particular spatial and spectral diffusion, using tunable laser excitations with nano-second time resolution, milli-Angstrom spectral resolution and micron spatial resolution, down to 1.5K, in conjunction with optional externally produced magnetic fields and field gradients and mechanical stretching or compression of film substrates. Theoretical formalisms, incorporating efficient computer simulation studies, will be applied to the diffusion-limited migration kinetics of excitations, ions and molecules that """"""""percolate"""""""" thru membrane-like heterogeneous media, emphasizing critical (catastrophic) aspects. These studies should result in detailed information on the switching (""""""""on and off"""""""") of excitation transport in the prototype systems studied, as well as for in-vivo aggregates, under both normal and abnormal conditions. Short range applications include the primary physical processes of photosynthesis and neuron excitations. The long range goal is to provide a model for a """"""""computer"""""""" based on molecular excitons and its application to damaging processes affecting the brain and nervous system.
