Sensitization of fluorescence of crystalline fluorene by anthracene

Sensitization of fluorene fluorescence by highly fluorescent anthracene has been studied at room temperature. The weak violet fluorescence of fluorene is found to be enhanced by anthracene impurity without any change in spectral energy distribution.The rate of sensitization as a linear function of anthracene impurity is observed in the 10−3–10−2 mol per mol fluorene range. The mechanism proposed to explain sensitization consists of an inductive resonant energy transfer between singlet-excited anthracene and ground-state fluorene to produce triplet-excited fluorene. The triplets of fluorene further undergo a biphotonic triplet–triplet annihilation, in which the first excited singlet formed by energetic interaction deactivates radiatively and enhances the fluorescence. From the fluorescence spectra, it is also noted that the mixed crystal of fluorene containing 8 × 10−1 mol anthracene shows a shift of 1408.4 cm−1 towards red.