Single-molecule nanoprobes explore defects in spin-grown crystals

Thin, platelike single crystals of p-terphenyl (PT) doped with terrylene impurity molecules can be prepared by spin-coating from solution. Strikingly, individual terrylene molecules can be observed traveling inside the crystal over distances of several micrometers by using single-molecule fluorescence imaging at room temperature. Analysis of the motion by single-particle tracking and correlation methods indicates that the molecules act as nanoprobes by exploring long, thin crack-like defects with correlated orientations, defects that can be difficult to observe by other means. Apparently, the regions accessible to the moving molecules are in the interior of the crystal and hence are partially protected from oxidation. In addition to the traveling molecules, which photobleach in times on the order of 32 s under continuous irradiation at 2 kW/cm2, two other spatially fixed populations are observed: one with transition dipole oriented along the c-axis of the crystal with a characteristic photobleaching time greater than 32 h, and one with a characteristic photobleaching time of 18 min.