Semiconducting Single-Walled Carbon Nanotubes (SWCNT) have small bandgaps between the ground state and the lowest singlet excited state. Such a low singlet-state bandgap will also result in even a smaller triplet bandgap between the ground state and the lowest triplet state. Due to big variations in singlet bandgaps with the nanotube diameter, it is possible to expect that some small-diameter SWCNT species will possess triplet states above the singlet oxygen level, corresponding to ~1270 nm wavelength, and some larger-diameter SWCNTs will be below. Correspondingly, smaller-diameter SWCNT may generate excited singlet oxygen if their triplet state will be quenched by molecular oxygen, while other may be efficient quenchers of singlet oxygen. Both processes are highly important for different photochemical and photophysical effects.

In the presented work we have discovered and investigated fluorescence of SWCNTs resulting from interaction of their water-based and toluene-based dispersions with singlet oxygen generated by sensitizers. Applying chopper-offset and photon-counting time-resolved techniques has allowed us to obtain luminescence spectra and kinetic dependencies for SWCNT and also for singlet oxygen. Among other observations we have determined a cut-off for energy transfer from singlet oxygen to SWCNT triplet, corresponding to approximate equality between the triplet state energy in SWCNT species (7,6) and the singlet oxygen energy. Nanotubes of smaller diameters have triplet states above the singlet oxygen level, but those of larger diameters – below it, and, correspondingly, can generate or quench singlet oxygen. While a direct triplet state lifetime for SWCNT was not possible to resolve in these experiments because it was shorter that the singlet oxygen lifetime, the data indicate it is on the order of a microsecond and decreases with the SWCNT bandgap. For some SWCNT species, especially (8,6), the delayed luminescence after triplet formation is rather efficient and its intensity could be compared with prompt fluorescence with direct excitation.

Thermal activation is the main mechanism for the conversion to a SWCNT excited singlet state after formation of its triplet state by energy transfer from singlet. This is not a surprising result because of rather small (~ 1000 wavenumbers) singlet-triplet splitting between the lowest excited singlet and triplet states, as determined here. However, triplet-triplet annihilation and some other photophysical and photochemical processes also influenced the intensities and kinetic traces of the delayed fluorescence. No SWCNT phosphorescence (emission from the triplet states) was observed, as consistent with usual properties in other organic molecules.