What are the requirements for FRET? I have a question about fluorescence resonance energy transfer (FRET), and the requirements needed for this technique.

Q: Which of the following need to be met for FRET?

A) Strong overlap between the donor emission spectrum and the acceptor absorption spectrum
B) Molecules must be $&lt;10\,\mathrm{nm}$ apart
C) Molecules must be $&gt;10\,\mathrm{nm}$ apart
D) Low quantum yield of the donor
E) Small extinction coefficient of the acceptor


I'm aware that A and B are true, and C is not a requirement. But what about a low quantum yield of the donor or a small extinction coefficient of the acceptor? Do the size of these numbers really matter?
A low quantum yield would indicate that there are efficient intra-molecular non-radiative decay pathways, likely internal conversion or spin-orbit coupling. These decay pathways may be faster that FRET, rendering FRET essentially a non-competetive process.

A small extinction coefficient would indicate a small transition dipole moment. If the transition dipole moment is small it is difficult for the FRET partner to transfer energy since it is a weak transition resulting in a small coupling  between the two chromophores.