Electronic excitation energy can be transferred between chromophores separated by distances of the order of 30 Å. Förster proposed that the transfer occurs by a dipole-dipole resonance interaction which depends on certain spectroscopic and geometric properties of the donor-acceptor pair. His prediction that the rate of transfer depends on the inverse sixth power of the distance between the chromophores was verified previously. In this work, we tested a second prediction of Förster's theory, namely, that the transfer rate is proportional to J , the magnitude of the overlap between the emission spectrum of the energy donor and the absorption spectrum of the energy acceptor. The energy donor was an N-methylindole moiety, and the acceptor was a ketone. These chromophores were fused to a rigid steroid that separated them by 10.2 Å. Rate constants for singlet-singlet energy transfer in this system were obtained by nanosecond flash spectroscopy. J was varied over a 40-fold range simply by altering the solvent. We found that the transfer rate is proportional to J , as predicted by Förster's theory. The results bear on the potential use of this energy transfer process to measure distances in biological macromolecules. It is evident that the length of such a spectroscopic ruler can readily be controlled by varying the magnitude of the spectral overlap integral of the energy donor-acceptor pair.

Haugland, R. P., Yguerabide, J., & Stryer, L. (1969). DEPENDENCE OF THE KINETICS OF SINGLET-SINGLET ENERGY TRANSFER ON SPECTRAL OVERLAP. Proceedings of the National Academy of Sciences, 63(1), 23–30.