Shpolskii systems are low-temperature host–guest systems – they are typically rapidly frozen solutions of polycyclic aromatic hydrocarbons in suitable low molecular weight normal alkanes. The emission and absorption spectra of lowest energy electronic transitions in the Shpolskii systems exhibit narrow lines instead of the inhomogeneously broadened features normally associated with spectra of chromophores in liquids and amorphous solids. The effect was first described by Eduard Shpolskii in the 1950s and 1960's in the journals Transactions of the U.S.S.R. Academy of Sciences and Soviet Physics Uspekhi.