Electron-Transfer Boat-Vibration Mechanism for Superconductivity in Organic Molecules Based on BEDT-TTF

The highest T_c organic superconductors all involve the organic molecule bis(ethylenedithio)tetrathiafulvalene (denoted as BEDT-TTF or ET) coupled with an appropriate acceptor. This leads to ET, ET⁺, or (ET)₂⁺ species in the crystal. Using ab initio Hartree–Fock calculations (6-31G* basis set), we show that ET deforms to a boat structure with an energy 28 meV (0.65 kcal/mol) lower than that of planar ET (D₂ symmetry). On the other hand, ET⁺ is planar. Thus, conduction in this system leads to a coupling between charge transfer and the boat deformation vibrational modes at 20 cm^–1 (ET) and 28 cm^–1 (ET⁺). We suggest that this electron–phonon coupling is responsible for the superconductivity and predict the isotope shifts (δT_c) for experimental tests of the electron-transfer boatvibration (ET-BV) mechanism. The low frequency of this boat mode and its coupling to various lattice modes could explain the sensitivity of T_c to defects, impurities, and pressure. We suggest that new higher temperature organic donors can be sought by finding modifications that change the frequency and stability of this boat distortion mode.