Solid State Effects in C₆₀: Relationship Between Phase Transition and Optical Properties, Intermolecular Charge Transfer Excitation, and Luminescence Quenching by Oxygen

N. Minami, S. Kazaoui, and R. Ross
[Physics and Chemistry of Fullerenes and Derivatives, pp.238-241, 1995]

Solid state effects in C₆₀as revealed by absorption and luminescence measurements are summarized and discussed. An absorption peak at 450nm that is specific to solid state C₆₀is found to increase steeply below the phase transition temperature at 260K, while a peak at 350nm originating from molecular absorption undergoes no such a change. This result together with an increase in luminescence intensity below the same temperature demonstrates intermolecular electronic interactions coming into effect in the solid state. The second evidence of the solid state effects is the fact that the luminescence efficiency decreases above a certain excitation energy (2.3eV), while no such effect is reported for isolated C₆₀. This effect in combination with electric field induced luminescence quenching that occurs only above the same excitation energy lead us to invoke intermolecular charge transfer excitation above 2.3eV. Moreover, the use of a UHV in situ measurement system has revealed that oxygen, which has reportedly relatively small effects on molecular luminescent properties, induces a drastic decrease in the luminescence from thin films; another manifestation of a solid state effect in C₆₀.