The present invention relates to methods and compositions for detecting an interaction between a protein and a ligand, comprising: (i) binding at least one fluorescent molecular rotor to said ligand or protein; and (ii) detecting a change in fluorescence emitted by said fluorescent molecular rotor after contact of the bound fluorescent molecular rotor with the other of said ligand or protein, thereby detecting an interaction between the ligand and the protein, wherein the fluorescent molecular rotor comprises: a rotating ?-bond; an electron-donating moiety; an electron-accepting moiety; and a ?-conjugated linker.