Complex sentence: Shpolskii (or Shpol'skii) systems are low-temperature host-guest system-predominantly fast frozen solutions of polycyclic aromatic hydrocarbons in normal alkanes.
Split into simpler sentences: Shpolskii (or Shpol'skii) systems are low-temperature host-guest systems. They are predominantly fast frozen solutions of polycyclic aromatic hydrocarbons in normal alkanes.