Infrared reflection measurements of the half-filled two-dimensional organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1−x were performed as a function of temperature (5 K < T < 300 K) and Br-substitution (x = 0%, 40%, 73%, 85%, and 90%) in order to study the metal-insulator transition. We can distinguish absorption processes due to itinerant and localized charge carriers. The broad mid-infrared absorption has two contributions: transitions between the two Hubbard bands and intradimer excitations from the charges localized on the (BEDT-TTF)2 dimer. Since the latter couple to intramolecular vibrations of BEDT-TTF, the analysis of both electronic and vibrational features provides a tool to disentangle these contributions and to follow their temperature and electronic-correlations dependence. Calculations based on the cluster model support our interpretation.
The paper presents data on the synthesis, IR, electronic, X-ray photoelectron, and ESR spectroscopies of fullerene C 60 molecular complexes with various types of donor compounds, namely, substituted tetrathiafulvalenes, aromatic hydrocarbons, diazodithiafulvalene, tetraphenyldipyranylidene, tetrasulfur tetranitride, saturated amines, some metallocenes, and Co(II) and Mn(II) tetraphenylporphyrins. Crystal structures of some of these complexes are discussed. The degree of charge transfer in the complexes is evaluated from the shift of the F 1u (4) C 60 mode in the IR spectra. The factors responsible for the appearance of the symmetry-forbidden C 60 vibrations and the splitting of the absorption bands of C 60 in the complexes are considered. The electronic absorption spectra of some complexes show the decrease of the intensity of electron transitions of C 60 at 420±530 nm and the appearance of new absorption bands attributed to charge transfer from the donor to C 60 . The dependences of the energy of charge transfer in the C 60 complexes on ionisation and redox potentials of substituted tetrathiafulvalenes are derived. Peculiarities of donor±acceptor interaction of donor molecules with the spherical C 60 are considered.{Characteristics of C 60 complexes are available as supplementary data.
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