Coupling of molecular motion and electronic state in the organic molecular dimer Mott insulator β′ -(BEDT-TTF) 2ICl2

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We have performed $^{1}\mathrm{H}$ NMR and $^{13}\mathrm{C}$ NMR measurements to investigate the coupling between molecular dynamics and the electronic state of ${\ensuremath{\beta}}^{\ensuremath{'}}$-(BEDT-TTF)${}_{2}{\mathrm{ICl}}_{2}$. From the $^{1}\mathrm{H}$ NMR measurements, we observed a frequency-dependent anomaly in the nuclear spin-lattice relaxation rate $^{1}\mathrm{T}_{1}^{\ensuremath{-}1}$ that originates from the slowing down of the ethylene motion. In the $^{13}\mathrm{C}$ NMR measurements, we found an anomaly in the linewidth of the NMR spectra at around 150 K, which is attributed to a nuclear spin-spin relaxation rate $(^{13}\mathrm{T}_{2})$ anomaly. The magnitudes of the anomalies in the linewidth and in $^{13}\mathrm{T}_{2}^{\ensuremath{-}1}$ are related to the hyperfine coupling constant. These results suggest that the ethylene motion modulates the molecular orbital of the BEDT-TTF molecules and gives rise to a difference in the orbital energy between the ``staggered'' and ``eclipsed'' conformations. We propose that significant coupling exists between the ethylene motion and the electronic state of the molecular dimer and that the ethylene dynamics can trigger the emergence of charge degrees of freedom inside the dimers and cause the dielectric anomaly in ${\ensuremath{\beta}}^{\ensuremath{'}}$-(BEDT-TTF)${}_{2}{\mathrm{ICl}}_{2}$.