Forster energy transfer in thin films of conjugated polymers and in solution

Fluorescence resonance energy transfer (FRET) is routinely used to study equilibrium and dynamical properties of polymers and biopolymers in the condensed phases. It is now being widely used in conjunction with single molecule spectroscopy where one usually employs the Forster expression which predicts (1/R-6) distance dependence of the energy transfer rate. However, critical analysis shows that this expression can be of rather limited validity in many cases. We demonstrate this by explicitly considering a donor-acceptor system, polyfluorene (PF6)-tetraphenylporphyrin (TPP), where their sizes are comparable to the distance separating them. In such cases, one may expect much weaker distance (as 1/R-2 or even weaker) dependence. We also discuss recent application of FRET to study biopolymer conformational dynamics. We show that the well-known Wilemski-Fixman theory needs to be generalized to treat FRET at short separations. The utility of FRET in studying protein folding is also discussed