PHOTOPHYSICAL AND PHOTOCHEMICAL PROCESSES IN FULLERENES UNDER HIGH-INTENSITY ILLUMINATION

The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid s tate indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconducti vity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fulleren e single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensi ty. This non-linear increase is associated with a dramatic shift to th e red of the emission maximum. Under similar conditions, the photocond uctive response of the fullerenes is also seen to increase non-linearl y with input intensity. Temperature-dependent measurements indicate th at the non-linear processes are associated with an insulator-metal pha se transition in the material. The transition is reversible and the ob served photophysical changes coincide with a reversible shifting of th e characteristic fullerene Raman lines to lower energies. At room temp erature, in many samples, the shifting becomes irreversible, and a hig h molecular weight, insoluble material is formed. The photochemical pr ocess is proposed to be a polymerisation-like reaction of the fulleren e molecules in the triplet excited state. This is supported by the obs ervation that the rate of the reaction is reduced greatly in the prese nce of oxygen, an efficient triplet quencher. In conclusion, the respo nse of Fullerene crystals to light is divided into three categories. A t low intensities the photophysical processes are characteristic of th ose of a molecular insulator, the electronic wavefunctions being molec ularly localised. At higher intensities, the material undergoes an opt ically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the ma terial is found to be photochemically unstable under some conditions b ut analysis of the temperature and intensity dependence of Raman spect roscopy shows that the photodegradation process can be predicted and t herefore controlled.