Coupling of molecular motion and electronic state in the organic molecular dimer Mott insulator ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{ICl}}_2$

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 ${\beta }^{\prime }$-(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 ${}^1\mathrm{T}_1^{-1}$ 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 $\left({}^{13}\mathrm{T}_2\right)$ anomaly. The magnitudes of the anomalies in the linewidth and in ${}^{13}\mathrm{T}_2^{-1}$ 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 ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{ICl}}_2$.

Figure 1

(a) The crystal structure of ${\beta }^{\prime }$-(BEDT-TTF)${}_2{\mathrm{ICl}}_2$ seen from the $c$ axis. (b) The dimer arrangement viewed along the long axis of the BEDT-TTF molecule. The closed circles represent inversion centers. (c) Two types of conformation of the terminal ethylene groups in the BEDT-TTF molecule. (d) Definition of the “inner” and “outer” ${}^{13}\mathrm{C}$ sites in a BEDT-TTF dimer.

Figure 2

Temperature dependence of the proton nuclear spin-lattice relaxation rate ${}^1\mathrm{T}_1^{-1}$ at magnetic fields of 0.55 T (green), 1.14 T (red), and 2.00 T (blue). The solid lines represent fitted curves using the BPP model.

Figure 3

Temperature dependence of the inverse correlation time of the ethylene motion (time scale of the ethylene motion) determined by ${}^1\mathrm{H}$ NMR measurements.

Figure 4

Temperature dependence of the linewidths of the ${}^{13}\mathrm{C}$ NMR spectra. The inset shows the ${}^{13}\mathrm{C}$ NMR spectra between 80 and 250 K.

Figure 5

Temperature dependence of the nuclear spin-spin relaxation rate $\left({}^{13}\mathrm{T}_2^{-1}\right)$ measured by ${}^{13}\mathrm{C}$ NMR.

Figure 6

Schematic diagram of the electronic state in a dimer when both BEDT-TTF molecules are (a) eclipsed or (b) staggered. The electronic state when one BEDT-TTF is staggered and the other is eclipsed is shown in (c). Blue circles and squares represent a dimer and individual BEDT-TTF molecule, respectively. Yellow ellipses represent the charge distribution in a dimer. The shaded portion of the ellipse corresponds to the existence probability. In case (c), CD occurs in the dimer and an electric dipole is generated.