Hi all,
This came up and Eq. 8 in the attached paper is how to determine average separation of molecules when dissolved in solution.
Daniel S. Tyson, Ignacy Gryczynski, and Felix N. Castellano
Long-Range Resonance Energy Transfer to [Ru(bpy)3]2+
J. Phys. Chem. A 2000, 104, 2919-2924