Unusually strong delayed fluorescence of C-70

C-70 (like other fullerenes) is known to have a very weak fluorescence (Phi(F) congruent to 5 x 10(-4)), owing to the high efficiency of triplet formation. In this work we show that, under appropriate conditions, the fluorescence quantum yield increases by one or two orders of magnitude (up to an estimated maximum value Phi(F) = 0.08), through the mechanism of delayed thermal fluorescence. We also report a new estimate of the singlet-triplet gap (26 +/- 2 kJ mol(-1)), obtained from the temperature dependence of the delayed thermal fluorescence. This value is shown to be in agreement with the phosphorescence spectrum recorded in the same temperature range. An accurate value of the quantum yield of triplet formation (0.994 +/- 0.001) is obtained from a new method of data analysis.