Short-range spectroscopic ruler based on a single-molecule optical   switch
  We demonstrate a novel all-optical switch consisting of two molecules: a primary fluorophore that can be switched between a fluorescent and a dark state by light of different wavelengths, and a secondary chromophore that facilitates switching. The interaction between the two molecules exhibits a distance dependence much steeper than that of Forster resonance energy transfer. This enables the switch to act as a ruler with the capability to probe distances difficult to access by other spectroscopic methods, thus presenting a new tool for the study of biomolecules at the single-molecule level. 
Mark Bates, Timothy R. Blosser, Xiaowei Zhuang