Dimerization of $C70$ under high pressure: Thermal dissociation and photophysical properties of the dimer $C140$

We have recently shown that solid $C70$ efficiently converts into a cap-to-cap dimer $C2hC140$ at pressure of ∼1 GPa and temperature of ∼200 °C [1]. Here we report on measurements of the thermal dissociation of $C140$ (purified by chromatography) at ambient pressure and its photophysical properties, in particular photogeneration of singlet oxygen $(1O2).$ The kinetics of dissociation of $C140$ in the solid state between 130 and 200 °C is well described by a simple first-order process, with an activation energy of $1.6±0.03 eV$ (compare to $1.75±0.05 eV$ for the $C60$ dimer $C120$ [2]). In contrast, the thermal dissociation of $C140$ dissolved in o-dichlorobenzene shows different, non-Arrhenius behavior, suggesting a strong influence of the molecular surroundings. The quantum efficiency of $1O2$ generation in solutions of $C140$ and $C120$ is close to unity, i.e. similar to that of $C70$ and $C60.$