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Characterization of Individual Submicron Distyrylbenzene Aggregates Using Temperature-Dependent Picosecond Fluorescence and Atomic Force Microscopy

The time-resolved fluorescence of individual distyrylbenzene aggregates is studied in order to determine the effects of aggregate morphology on the emission properties. Using a confocal microscope and a photoncounting streak camera, we measure the full time and wavelength dependence of the fluorescence from individual submicron aggregates at the temperatures 290, 100, and 4 K. The morphology of these aggregates is characterized separately using tapping-mode atomic force microscopy. All aggregates show temperaturedependent fluorescence decays and spectral shifting, most likely due to activated energy transfer to lower energy sites which correspond to spectrally distinct species. We find significant aggregate-to-aggregate variations in the fluorescence spectra and decays, but the amount of variation depends on preparation conditions: solvent-annealed aggregates have a more uniform appearance and less variation in their spectral properties. To analyze the data, we model the dynamics in terms of two coupled states, which correspond to the intrinsic exciton in solid DSB and a lower energy defect. Even this simple model requires additional assumptions, but it allows us to extract spectra corresponding to both species, and generate histograms of the two different decay rates. Variations in the decay rates are ascribed to variations in the concentration of quenching defects between individual aggregates.