Photophysical and photochemical processes in fullerenes under high-intensity illumination

Abstract The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.