We measured the end-to-end diffusion coefficient of an alkyl chain-linked donor–acceptor pair using the time-resolved frequency-domain decay of the donor. The donor was a rhenium metal–ligand complex with a mean decay time ranging from 2.1 to 7.9 μs in the absence of the Texas red acceptor. The decay time was used to measure the donor-to-acceptor distance distribution and the mutual diffusion coefficient. Using this long lifetime donor, it was easily possible to determine a diffusion coefficient near 2 × 10⁻⁸ cm²/s and diffusion coefficients as low as 1.3 × 10⁻⁹ cm²/s were measurable. Such long lifetime donors should be valuable for measuring the flexing of peptides on the microsecond timescale, domain motions of proteins and lateral diffusion in membranes. The availability of microsecond decay time luminophores now allows luminescence spectroscopy to be useful generally for studies of microsecond dynamics of biological macromolecules.