Detection of a single dendrimer macromolecule with a fluorescent dihydropyrrolopurroledione (DPP) core embedded in a thin polystyrene polymer film

Three generations of dendrimers with a fluorescent core were synthesized by attaching dendrons of the Frechet type to a dihydropyrrolopyrroledione (DPP) molecule. Samples were prepared by spin-coating a toluene solution containing 2 x 10(-9) M of the dendrimers and 3 mg/mL polystyrene (MW 45000) on a glass cover slip. By exciting near the absorption maximum of the chromophore in the dendrimer, single dendrimer molecules could be imaged via a confocal microscope. Using an electrooptical modulator (EOM) in the excitation beam path, linear polarizer light with a slowly rotating polarization direction (1 Hz modulation frequency) was produced. Two APD's (avalanche photodiodes) and a polarizing beam splitter were placed in the detection path. In this way we were able to distinguish single molecules from small clusters. Furthermore, from variations of the two detected intensities and from the phase shift of the signal with respect to the modulation, it was shown that the orientation of the absorption transition dipole of single dendrimer molecules in the polymer film changes in a time window of seconds.