Resonance energy transfer from the excited singlet state of dye molecules to a stable free radical

Fluorescence quenching of dye molecules such as coumarin 153, DCM and nile red by the stable 2,4,6-tri-tert-butylphenoxyl radical in an MTHF glass matrix at 77 K was studied by means of subnanosecond time-resolved fluorescence spectroscopy. The quenching process is ascribed to the dipole-dipole interaction amenable to Fِrster theory. The average critical energy transfer distances were determined to be 20, 27 and 31 إ for coumarin 153, DCM and nile red, respectively. These values are consistent with those evaluated from spectroscopic data.