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

Abstract

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-anthryl-methylphosphonate) 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, ω≈ 3.5 × 10⁸ s^–1, agrees very well with that predicted. The distance 23.7 ± 2 Å between the anthracenes and the mutual angle of 131 ± 3° between their orientational distributions, obtained at different temperatures, are in excellent agreement with independently determined values.