Raman spectroscopic study of excited states and photo-polymerisation of C₆₀ from solution

The Raman scattering signal of C₆₀ in solution was monitored as a function of the excitation intensity at 514.5 nm. Whereas at low intensities the A_2g pentagonal pinch mode was positioned at 1469 cm^-1, at intensities where an inverse saturable absorption is observable, it was positioned at 1466 cm^-1. This mode positioning is ascribed to the molecular triplet state. When measured at the cuvette interface, the ground-state Raman mode, at 1469 cm^-1 in solution was seen to degrade quickly under illumination to a lower Raman frequency of 1458 cm^-1 where an insoluble deposit formed, which has been identified as a photochemical product of C₆₀ analogous to the solid-state C₆₀ polymer formed by a 2+2 cyclo-addition across π bonds of neighbouring molecules. The process is seen to be reversible under high-intensity illumination, via an intermediate state with mode positioning at 1463 cm^-1. This state is associated with a high-intensity excited state previously reported in the solid state of C₆₀.