Resonance Raman spectroscopy of rhodizonates: Evidence of low-energy transitions in the solid state

The optical and Raman spectra of several salts of rhodizonic acid were investigated, aimed at a better understanding of the dramatic dependence of their colours on the nature of the cation. The results indicate that in addition to the maxima observed in the optical spectra, another transition in the red edge must be present in the solid state, as indicated by the large number of overtones and combination bands in the Raman spectra excited in the red. It is suggested that such a low‐energy transition observed only in the solid state is of intermolecular origin, as observed previously for other related systems.