Electronic-energy migration and molecular rotation within bichromophoric macromolecules .1. Test of a model using bis(9-anthrylmethylphosphonate) bisteroid

We report a model for determining the rate of energy migration, within pairs of donor (D) molecules from the fluorescence anisotropy. The D molecules within each pair reside in an anisotropic environment, and undergo rotational motions, similar to what could be the case in a protein molecule. To test the model experimentally, we have synthesized mono- and bis(9-anthrylmethylphosphonate) bisteroid molecules. A procedure is presented for extracting the rate of energy transfer, as well as the D-D distance from the fluorescence anisotropy. The rate of energy migration obtained from experiments, omega approximate to 3.5 x 10(8) s(-1), agrees very well with that predicted. The distance 23.7 +/- 2 Angstrom between the anthracenes and the mutual angle of 131 +/- 3 degrees between their orientational distributions, obtained at different temperatures, are in excellent agreement with independently determined values.