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 A2g 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.