A molecular thermometer based on the delayed fluorescence of C70 dispersed in a polystyrene film

A new optical molecular thermometer, based on the thermally activated delayed fluorescence of C70 dispersed in a polystyrene film, was developed. In the presence of oxygen, the fluorescence intensity of the C70 film is essentially temperature independent in a wide range. In the absence of oxygen, however, the fluorescence intensity markedly increases with temperature. At room temperature (25◦C), and after degassing the sample, the fluorescence intensity of C70 increases 22 times, while at 100◦C the fluorescence intensity is increased by 70 times. With our system, the very weak fluorescence of C70 (F ∼ = 5 × 10−4, in toluene) can be increased up to 91 times (up to an estimated maximum value F = 0.046). The estimate value of the singlet-triplet gap (29 kJ mol−1) and the fluorescence lifetime (0.63 ns) of the C70 in film are in agreement with the values reported in the literature for C70 in solution. The values of the phosphorescence lifetime at room temperature (23 ms) and the quantum yield of triplet formation (0.989) were also determined. The system is completely reversible with respect to heating-cooling cycles. KEY WORDS: Molecular fluorescence thermometry; C70; thermally activated delayed fluorescence; polystyrene film