Rotational dynamics of Li + ions encapsulated in C 60 cages at low temperatures.

Li ⁺ ions encapsulated in fullerene C ₆₀ cages (Li ⁺ @C ₆₀ ) are expected to be suitable as molecular switches that respond to local electric fields. In this study, the rotational dynamics of Li ⁺ ions in C ₆₀ cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li ⁺ @C ₆₀ ](PF ₆ ^- ), 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 C ₆₀ cage as well as with the neighboring Li ⁺ @C ₆₀ and PF ₆ ^- ions. A simple rotational/librational energy scheme for the Li ⁺ ions successfully explains the spectroscopic results, and the potential of Li ⁺ @C ₆₀ as a molecular switch is discussed based on the energy scheme.