The transition from single molecule to ensemble revealed by fluorescence polarization

Fluorescence polarization measurements in the condensed phase provide rich information on rotationaldynamics and interactions between macromolecules. An important parameter in these studies is thelimiting polarization or po which is the emission polarization in the absence of molecular rotation. Here weexplore how molecular number averaging affects the observed value of po. Using a simple mathematicalmodel we show that for a collection of fluorescent dipoles (1–50 molecules) the fluorescence polarization (p)increases with the number of molecules (N) due to the progressive onset of photo-selection with a relation ofthe form p5 po(1 2 N2b). This concept is demonstrated experimentally using single molecule polarizationmeasurements of perylene diimide dye molecules in a rigid polymer matrix where it is shown that theaverage emission polarization increases significantly when the number of molecules per averaging windowis increased from 1 to 10 molecules. These results suggest that the definition of limiting polarization needs tobe refined in the quasi-single molecule regime. Moreover, these results pave a new way for measuringclustering of molecules from single cluster polarization histograms.