Photophysics of fullerene derivatives

Abstract

A study is presented of the photophysics of a variety of fullerene derivatives, including C60O, C70O, C120, 1,2-C70 H2, Kr C60, and four mono-substituted benzo-adducts of C70. Measured photophysical properties include UV-vis absorption, triplet triplet absorption, temperature dependent triplet decay kinetics, fluorescence and phosphorescence emission, and triplet quantum yields. The findings for these compounds are tabulated and compared to those of similar derivatives. Trends in the properties are identified where possible. Temperature dependent studies of C60O reveal two distinct regimes of triplet decay, with very different activation energies. Reversible opening of the C-C epoxide bond is suggested as the source of this unusual behavior. C60O is also found to efficiently quench C60 triplets, with a bimolecular quenching constant of 3.6 +/- 0.6 x 109 M--1 s--1. Preliminary data on C70O suggest that its kinetic behavior resembles that of C60O, while spectrally it resembles other C70 derivatives. C120, the dimer of C60, dissociates into C 60 upon application of sufficient heat or light. Isosbestic points are found in the UV-vis spectra of C120/C60 mixtures, allowing absorptivity calibration of the C120 spectrum. The photolysis of C120 is found to proceed via the triplet state, with an activation energy of 5,500 cm--1 and a dissociative quantum yield of 2 x 10--3 at 297 K. A method based on energy transfer kinetics is used to determine the T1 energy of C 120. Kr C60, an endohedral van der Waals complex, is studied using UV-vis absorption spectroscopy and triplet-triplet kinetic spectroscopy. The results show small but distinct couplings of the C60 pi-electrons with the krypton atom. The photophysics of 1,2-C70H2 is thoroughly explored and compared to C70 and other fullerene derivatives. At higher temperatures, thermally activated S1 → S0 internal conversion becomes a major unexpected channel for triplet decay. Finally, the effect of derivatization site is explored using four benzo-adducts of C70. Some photophysical properties, such as triplet lifetime and wavelength of maximum triplet absorption, vary greatly, and may correlate with addend distance from the molecule's pole. Other properties, such as singlet and triplet energies, seem not to show such a correlation.