Exploring the Fِrster limit in a small FRET pair

The accuracy of Förster theory for measuring short donor–acceptor distances in a small chromophore pair is examined. Small chromophores are much better suited for measuring short distances relative to large chormophores. Nonetheless, it is found that the Förster expression introduces errors for distances below 20 Å, overestimating for some orientations and underestimating for others. Fortuitous cancellation due to orientation disorder in experimental investigations may result in better agreement with Förster rate. Not only the size of the molecule, but more significantly, the spread of transition dipole density is found to be a limiting factor.