Charge order and metal-insulator transition in the chiral molecular conductors

Optical measurements reveal the charge-ordered nature of the insulating state in the chiral molecular conductors [(S,S/R,R)-DM-EDT-TTF]2⁢ClO4 (DM-Cl) and [(S,S/R,R)-DM-EDT-TTF]2⁢ReO4 (DM-Re). These enantiopure materials exhibit metallic conductivity at room temperature and a metal-insulator transition upon cooling below TMI=40 and 105 K, respectively. Optical conductivity spectra display a polarization-independent in-plane response, attributed to the hexagonal structure of the conducting DM-EDT-TTF layer, featuring broadband electronic excitations, molecular vibrational modes, and a Drude component in the metallic phase. The gap opening at 150 and 250 cm−1 for DM-Cl and DM-Re, respectively, signals the transition from a metallic to an insulating state upon cooling. Drude-Lorentz-Fano analysis of the optical spectra yields Hubbard parameters, the intersite Coulomb repulsion 𝑉 and the bandwidth 𝑊, supporting the presence of charge order in the insulating phase. Raman spectra of DM-Cl and DM-Re, focusing on charge-sensitive molecular vibrations, further confirm a charge-ordered insulating state with a charge disproportionation of 0.10⁢𝑒 and 0.13⁢𝑒, respectively. Strong DM-EDT-TTF molecular modes, activated in optical spectra through coupling with the electronic background, support the presence of lattice frustration.