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 tri plet formation. In this work we show that, under appropriate condition s, the fluorescence quantum yield increases by one or two orders of ma gnitude (up to an estimated maximum value Phi(F) = 0.08), through the mechanism of delayed thermal fluorescence. We also report a new estima te of the singlet-triplet gap (26 +/- 2 kJ mol(-1)), obtained from the temperature dependence of the delayed thermal fluorescence. This valu e is shown to be in agreement with the phosphorescence spectrum record ed in the same temperature range. An accurate value of the quantum yie ld of triplet formation (0.994 +/- 0.001) is obtained from a new metho d of data analysis.