Li ions encapsulated in fullerene C cages (Li@C) 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 cages at low temperatures are experimentally revealed for the first time using terahertz absorption spectroscopy. In crystalline [Li@C](PF), 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 cage as well as with the neighboring Li@C and PF ions. A simple rotational/librational energy scheme for the Li ions successfully explains the spectroscopic results, and the potential of Li@C as a molecular switch is discussed based on the energy scheme.

As terahertz (THz) frequencies correspond to those of the intermolecular vibrational modes in a polymer, intense THz wave irradiation affects the macromolecular polymorph, which determines the polymer properties and functions. THz photon energy is quite low compared to the covalent bond energy; therefore, conformational changes can be induced “softly,” without damaging the chemical structures. Here, we irradiate a poly(3-hydroxybutylate) (PHB) / chloroform solution during solvent casting crystallization using a THz wave generated by a free electron laser (FEL). Morphological observation shows the formation of micrometer-sized crystals in response to the THz wave irradiation. Further, a 10−20% increase in crystallinity is observed through analysis of the infrared (IR) absorption spectra. The peak power density of the irradiating THz wave is 40 MW/cm2, which is significantly lower than the typical laser intensities used for material manipulation. We demonstrate for the first time that the THz wave effectively induces the intermolecular rearrangement of polymer macromolecules.

The  and  crystals of polyamide-6 were examined by terahertz absorption spectroscopy from 90 to 230 °C. For the  crystals, the absorption band at 6.5 THz, assigned as a lattice vibration perpendicular to the molecular chain, decreased in intensity with increasing temperature, ending near the Brill transition temperature (T Brill ≈ 160 °C). The band at 8.7 THz, assigned as the skeletal vibration along the molecular chain, shifted to higher frequency with increasing temperature. The rate of shift changed at the glass transition temperature (T g ≈ 50 °C), indicating coupling of the molecular vibration in the crystal with freezing/relaxing phenomena in the surrounding amorphous phase. A significant annealing effect was observed. On heating polyamide-6 films that had been annealed at a temperature (T ann) ≥ 80 °C, an increase in spectral intensity above 9 THz occurred at ~T ann , due to the melting of semi-crystals formed by recrystallization during annealing.

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