Saturation of Forster resonance energy transfer between two optically nonlinear cyanine dyes of small Stokes shift energies in polymer thin films

We report the saturation of Forster resonance energy-transfer (FRET) between two optically nonlinear cyanine dyes in polymer thin films, where the energy donor and acceptor have small Stokes shift energies. Our combinations of dyes show the energy transfer rate of inversely proportional to the square of the distance between the energy donor and acceptor dyes. This result quite differs from the normal FRET results. From speculation based on the number of excited acceptor dye, our FRETs are saturated even at such a long distance approximately 10 nm between the energy donor and acceptor because of the optical excitation of acceptor dyes in addition to FRET process. (C) 2011 American Institute of Physics. [doi:10.1063/1.3653228]