Time sequencing of laser excitation can control the microscopic patterns of molecular excitations (excitons) in nanometer scale molecular ensembles. This is demonstrated for isolated guest chains in polymer blends (P1VN/PMMA) and for molecular clusters in molecularly doped polymers (naphthalene/PMMA). The subwavelength exciton patterns are monitored via time resolved luminescence and fusion kinetics. The heterogeneity exponent (h) is 0.5 for isolated P1VN chains, zero (classical) for pure P1VN and "fractal-like" throughout certain concentration regimes. Correlation is made with morphology changes (phase separation, filamentation). At concentrations below 0.01%, the excitons are constrained to a truly one-dimensional topology (wire). At higher concentrations there is a fractal-like topology which may be associated with spinoidal decomposition. Similar studies were conducted on naphthalene-doped PMMA (1-20 wt%). The lower concentration samples are neither segregated nor random solution phases. They probably consist of islands (dots) of naphthalene aggregates. The exciton distribution in such islands is non-Poissonian under steady-state excitation or under pulse excitation with delay times, resulting in anomalous exciton fusion kinetics.
Peer Reviewed
http://deepblue.lib.umich.edu/bitstream/2027.42/29559/1/0000647.pdf