A study is presented of the orientation of single 2.3, 8.9-dibenzanthanthrene molecules in n-tetradecane as a function of their spatial position on a length scale of 2 mm down to a few hundreds of nanometers at 1.4 K, which allows us to obtain structural information about this host/guest system. In the confocal detection volume of ∼10 µm 3 the guest molecules show two preferential orientations. A broad distribution of orientations of about (17˚is observed around these two mean orientations. The mean orientations exhibit long-range order even over a distance of 2 mm with gradual variations on a micrometer scale. Over distances smaller than the lateral dimension of the confocal detection volume these variations disappear almost completely whereas the widths of the distributions remain qualitatively the same. The results do not point to the presence of a polycrystalline structure which contradicts the common picture of Shpol'skii systems.

SummaryOptical far-field imaging of single molecules in a frozen solution at 1.2 K with a lateral resolution of 3.4 nm is reported. The mechanical stability of the fluorescence microscope, especially of the low-temperature insert, allows for the localization of fluorescing molecules with a reproducibility of better than 5 nm within observation times up to 10 min. For observation times of 9 h the reproducibility of the lateral position is limited to about 20 nm due to mechanical drift. Lateral position and orientation of 314 single molecules, present within the confocal detection volume of ~10 µ m 3 , are obtained. The possibility to correct for mechanical drift by monitoring the position of a spatial reference in the sample is demonstrated.

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