Unusually Strong Delayed Fluorescence of C₇₀

Abstract

C70 (like other fullerenes) is known to have a very weak fluorescence (ΦF ≅ 5 × 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 ΦF = 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.

A Molecular Thermometer Based on the Delayed Fluorescence of C70 Dispersed in a Polystyrene Film

“…1 it is possible to observe the strong temperature dependence of C 70 fluorescence, the delayed fluorescence (DF) being much stronger than the PF. After degassing the sample at room temperature, the DF is 22 times stronger than the PF (20 times in C 70 in paraffin [17]). Table I summarizes the I DF /I PF values for C 70 -PS and compares them with the C 70 in paraffin [17] data.…”

“…After degassing the sample at room temperature, the DF is 22 times stronger than the PF (20 times in C 70 in paraffin [17]). Table I summarizes the I DF /I PF values for C 70 -PS and compares them with the C 70 in paraffin [17] data. The I DF /I PF ratio for C 70 -PS is always higher when compared with the values for C 70 in paraffin, it being possible to go to higher temperatures, and at 100 • C the DF is 70 times stronger than the PF.…”

“…The C 70 -PS system is completely reversible (cooling the sample from 100 • C to room temperature allows to recover the values measured The standard method of analysis of the delayed fluorescence, due to Parker [7], is to measure the steady-state DF and the steady-state phosphorescence (P) intensities, as a function of temperature, and then to determine the S 1 -T 1 energy gap ( E ST ) from a plot of ln(I DF /I P ) vs. 1/T. However, in our case, we use an alternative approach already applied to the C 70 -paraffin system [17], where the steady-state intensities ratio of DF to PF, given by…”

“…The unique photophysical properties of C 70 , namely the quantum yield of triplet formation close to one [14], the small S 1 -T 1 gap [15] and the long intrinsic phosphorescence lifetime [16], lead to the discovery of a strong TDF in fullerene C 70 [17]. Its dependence with temperature, in conjugation with a high thermal stability of C 70 , opens new perspectives in molecular thermometry.…”

“…Its dependence with temperature, in conjugation with a high thermal stability of C 70 , opens new perspectives in molecular thermometry. The initial studies [17] were made in paraffin solution and for temperatures below 70 • C. Also a C 70 derivative [18], fullerene C 60 [19] and some C 60 derivatives [20,21] exhibit TDF, but weaker than that of C 70 .…”