Rotational dynamics of Li⁺ ions encapsulated in C₆₀ cages at low temperatures

Li⁺ ions encapsulated in fullerene C₆₀ cages (Li⁺@C₆₀) are expected to be suitable as molecular switches that respond to local electric fields. In this study, the rotational dynamics of Li⁺ ions in C₆₀ cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li⁺@C₆₀](PF₆^-), the Li⁺ ion rotates in the carbon cage even at 150 K. The rotational mode gradually changes into a librational mode below 120 K, which is associated with the localization of Li⁺ ions due to the electrostatic interactions with its screening image charge on the C₆₀ cage as well as with the neighboring Li⁺@C₆₀ and PF₆^- ions. A simple rotational/librational energy scheme for the Li⁺ ions successfully explains the spectroscopic results, and the potential of Li⁺@C₆₀ as a molecular switch is discussed based on the energy scheme.