Size-dependent fluorescence emission spectra and lifetimes of microcrystals of the dye N,N'-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis(dicarboximide) (DBPI) studied by confocal fluorescence microscopy

The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy has revealed the nonexponential character of the fluorescence decay (0.1-10 ns) for highly fluorescent single microcrystals of the dye N,N'-bis(2,5-di-tert-butylphenyl)-3,4:9 (DBPI) of a size as small as a few micrometers. Steady-state and time-resolved emission spectra of the microcrystals have also been measured with a fluorescence microscope. A size dependence is observed for both fluorescence emission spectra and lifetimes. The fluorescence decay profiles of the microcrystals, when observed with confocal optics, exhibit a dominant contribution of a short-lived (0.11-0.14 ns) component. From these results it has been concluded that the self-absorption effect is playing a major role in characterizing the fluorescence of microcrystals of DBPI with no contribution from any new emitting species.