Analysis of the exponential character of single molecule rotational correlation functions for large and small fluorescence collection angles.

Coarse-grained molecular dynamics simulations and single molecule fluorescence microscopy experiments have been performed in order to investigate the influence of the numerical aperture (NA) of the microscope objective on the exponential character of the rotational correlation functions of probes embedded in complex matrices. The results obtained by using either a dry lens (NA=0.95) or an oil objective (NA=1.4) show that, in the moderately (simulations) and deeply (experiment) supercooled melts, the rotational (linear dichroism) correlation functions of the single molecules (SMs) exhibit a nonexponential character. Furthermore, by fitting Kohlrausch-Williams-Watt functions to the correlation curves, the stretching parameters turn out to be very similar for both types of objectives. Our results demonstrate that the nonexponentiality is intrinsic to the complex rotational dynamics of the SM in the supercooled solid and point to the validity of the use of a high NA dry lens to perform such experiments.