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 C-70 dispersed in a polystyrene film, was developed. In the presence of oxygen, the fluorescence intensity of the C-70 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 degrees C), and after degassing the sample, the fluorescence intensity of C-70 increases 22 times, while at 100 degrees C the fluorescence intensity is increased by 70 times. With our system, the very weak fluorescence of C-70 (Phi(F) congruent to 5 x 10(-4), in toluene) can be increased up to 91 times ( up to an estimated maximum value Phi(F) = 0.046). The estimate value of the singlet-triplet gap (29 kJ mol(-1)) and the fluorescence lifetime (0.63 ns) of the C-70 in film are in agreement with the values reported in the literature for C-70 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.