Temperature dependence of the phosphorescence and of the thermally activated delayed fluorescence of 12 C 70 and 13 C 70 in amorphous polymer matrices. Is a second triplet involved?

The phosphorescence and thermally activated delayed fluorescence (TADF) lifetimes of ¹² C ₇₀ and ¹³ C ₇₀ in two different glassy hydrocarbon polymers, one aliphatic (cyclic polyolefin) and one aromatic (polystyrene), were measured between -200 and 100 ºC. The temperature dependence of the lifetimes is equally well described by a three-state mechanism (ground state, S ₀ , and two excited states in thermal equilibrium, T ₁ and S ₁ , the lifetime of T ₁ being temperature dependent) and by a four-state mechanism (ground state, S ₀ , and three excited states in thermal equilibrium, T ₁ , T ₂ and S ₁ , all with temperature-independent lifetimes). The estimated S ₁ -T ₁ and T ₂ -T ₁ energy gaps (four-state mechanism) are in good agreement with spectroscopic measurements. These and the determined quantum yield of triplet formation, 0.997 ± 0.001, are found to be essentially independent of the polymer matrix and of the isotopic composition of the fullerene. On the other hand, the lifetimes of both T ₁ and T ₂ (four-state mechanism) are weakly dependent on the polymer matrix but strongly vary with the fullerene isotopic composition, nearly doubling when going from ¹² C ₇₀ to ¹³ C ₇₀ . A parameter useful for the characterization of TADF, the on-set temperature T ₀ , is also introduced.