Charge disproportionation in (TMTTF)2SCN observed by 13C NMR

The results of 13C NMR spectra and nuclear spin-lattice relaxation rate 1/T_1 for the quasi-one-dimensional quarter-filled organic material (TMTTF)2SCN are presented. A new inhomogeneous line appears below the anion ordering temperature (T_AO), below which the intensity of the distinct line owing to the inner carbon site from the inversion center is halved. This indicates that the charge transfer (disproportionation) occurs from one inner carbon site to the other inner site exclusively within one unit cell triggered by anion ordering. The 1/T_1 for the charge-donating inner site shows activated temperature dependence below T_AO, although 1/T_1 for the outer carbon decreases much more moderately in lowering temperature as has been discussed in weakly interacting one dimensional conductors. Our observation of the local gap for the spin excitation generated by anion ordering is consistent with the simple model assuming only the electrostatic interaction between the inner carbon sites and ordered anions, which is known to be much closer to the inner sites than to the outer sites. Nevertheless, we reveal that only the electrostatic interaction between the anions and molecules is insufficient to reproduce the observed divergence of the frequency shift and the linewidth of the inhomogeneous line stemming from the charge-accepting inner site at a much lower temperature than T_AO.