Charge Fluctuations in the Dimer-Mott Insulating State of (rac-DM-EDT-TTF)₂PF₆

Low-dimensional molecular conductors are known to display various insulating ground states that result from significant electronic correlations, depending on the degree of dimerization. Of particular interest are intermediate systems where the interplay between dimer-Mott and charge-ordered phases is observed. Here we report an optical study of (rac-DM-EDT-TTF)2PF6, a weakly dimerized organic conductor that is based on the chiral DM-EDT-TTF donor molecule. Variable-temperature infrared reflectance spectra clearly demonstrate a quasi-one-dimensional response together with a metal–insulator phase transition at 110 K. Both the broadening of the charge-sensitive vibrational ν2 mode observed in Raman spectra and the strong EMV-activated ν3 mode in the reflectance spectra polarized in the stack direction suggest the charge fluctuations to develop in the insulating state. Significant modification of the hydrogen-bonding type interaction between the conducting DM-EDT-TTF layer and the PF6 anions is detected below 110 K. We suggest that the low-temperature insulating state in (rac-DM-EDT-TTF)2PF6 should be regarded as a coexistence of the dimer-Mott state and fluctuating charge-order.