Chiral Metallic DM-EDT-TTF Radical Cation Salts: Anion Size-Dependent Structural and Electronic Transitions, Charge...

Both enantiomers (S,S) and (R,R) of the chiral mixed-valence radical cation salts (DM-EDT-TTF)₂XO₄ (X = Cl or Re) have been prepared by electrocrystallization. Single-crystal high-quality synchrotron radiation data allowed for the very accurate determination of their 298 and 18 K structures. At room temperature, they crystallize in the enantiomorphic space groups P6₂22 and P6₄22 for the (S,S) and (R,R) enantiomers, respectively, while at 18 K, the structures have been solved in the P6₂ and P6₄ space groups, respectively. This symmetry reduction results in a quadrupled unit cell containing four independent donors disordered over two inequivalent positions. In the perrhenate salts, there is strong structural evidence of the occurrence of charge ordering at 18 K as suggested by the alternation of charge-rich and charge-poor donors within the stacks. The easier establishment of charge disproportionation is very likely related to the higher metal-to-insulator transition temperature for perrhenate compared to perchlorate, as observed in the electrical resistance measurements. Spin-polarized DFT band structure calculations support the magnetic ground state of the materials and the activated low-temperature conductivity as a consequence of gap opening. Single-crystal Raman spectroscopy measurements indicate a stronger charge ordering degree at 10 K for the perrhenate than for the perchlorate salts. For the first time in chiral TTF-based conductors, the chirality-induced spin selectivity (CISS) effect is demonstrated through magnetoresistance measurements at room temperature on thin crystals of (DM-EDT-TTF)₂ClO₄, with a value of up to 30% for CISS-induced magnetoresistance.