Polarisation sensitive single molecule fluorescence detection with linear polarised excitation light and modulated polarisation direction applied to multichromophoric entities

Recently, investigations of the fluorescence properties of a multichromophoric dendritic entity at the single molecule level have revealed multiple fluorescence levels, collective off-states, variations of the polarisation, large shifts in the spectral position and changes in the fluorescence decay time (Gensch et al. (1999), Hofkens et al. (2000)). In order to further elucidate the multiple processes taking place in this entity, measurements were done in which the polarisation direction of the linear polarised excitation light was modulated. The detection was sensitive for the s- and p-components of the emitted light. The patterns of modulation and relative intensity in the acquired traces reflect the energy transfer processes occurring in this multichromophoric molecule. In-phase modulation and no modulation are the typical modulation patterns that were observed. Simulations involving several models for energy transfer between the chromophores have been carried out taking into account identical conditions as for the performed measurements. The comparison of the modulation patterns and polarisation histograms to the measured data rules out certain models and refines the photophysical model for the multichromophoric entity