Reanalysis of Raman Spectra of Singly and Doubly Charged BEDT-TTF Dimers in the Solid State

Recent first principles simulation of the Raman spectral intensity of a bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF or ET) singly charged dimer suggested that the very important C=C stretching region of a κ-phase BEDT-TTF salt needs a reinterpretation. In fact, the possible presence of the out-of-phase coupled infrared active C=C anti-symmetric stretching b1uν27 did not receive the proper attention Here, the comparison of the calculated Raman spectra of ET+ cation with those of a doubly charged dimer, (ET)22+, shows that in a dimer, not only the C=C stretching modes but also anti-symmetric C-S stretching and ring breathing b1u modes show Raman intensity comparable to that of the corresponding totally symmetric phonons. The calculations are validated through the comparison with the experimental Raman spectra of two charge-transfer salts, (ET)2[Re2Cl8] and (ET)2[Re2Br6CH3COO]0.5(C2H3Cl3), in which the ET molecules form almost isolated centrosymmetric (ET)22+ dimers. We also present the Raman spectra of a well-known ET salt, κ-(ET)2Cu[N(CN)2]Br (κ-CuBr), the experiment being performed on the rarely investigated (101) crystal face. The comparison with the Raman calculations on a singly charged ET dimer oriented as in the κ-CuBr salt allows us to clearly identify the out-of-phase coupled b1u phonons, yielding a reliable interpretation of the most important (ET)2+ spectral regions, i.e., the C=C and C-S stretching, and the ring breathing ones. The present results constitute a reliable basis for the interpretation of the Raman spectra of ET crystals characterized by the presence of isolated or almost isolated singly or doubly charged ET dimers.