Fluorescence correlation spectroscopy as a tool to investigate single molecule probe dynamics in thin polymer films

Fluorescence correlation experiments were performed on rhodamine 6G in PDMS spin-coated films on glass surfaces. With polarised excitation, ensemble b leaching of the dye and single molecule intensity fluctuations were observe d. From the statistics of single molecule intensity data taken at different positions in the film, correlation functions were calculated. Two modes of motion with exponential decay shapes and correlation times of tau (c) = 0. 15 s and tau (c) = 0.7 s could be detected. Potential origins of intensity fluctuations are lateral diffusion, rotational diffusion or intramolecular fluctuations of dyes involving spectral diffusion or photoinduced processes . From the experimental results, lateral diffusion can be ruled out as a mo tional mode. Single molecule fluctuations are assigned to changes of the mo lecular configuration of the dyes, which are rigidly bound to the glass. To assess the environmental influence on such molecular motions, the bulk vis cosity of the PDMS was varied over two orders of magnitude, leading to chan ges of tau (c) of the slow mode by a factor of four. This result proves the sensitivity of the single molecule fluctuations to the molecular scale dyn amics of the surrounding polymer matrix and makes the correlation time a me asure of the local environment of dye probes.