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Measurements are presented of the optical properties of the radical cation TTF+ (tetrathiafulvalenium) in a variety of difFerent conditions: as TTF+ monomers in solution, as (TTF+), dimers in solution, in the monovalenc'e (TTF)Br, s solid, and in the mixed-valence (TTF)Br&79 salt. From a comparison of these spectra and from polarized measurements on single crystals, the observed absorption peaks are unambiguously assigned as either intramolecular (excitons) or intermolecular (charge-transfer bands). It is shown that the organic metal (TTF)Br079 has two such charge-transfer bands, at 0.6 and 1.5 eV. The existence of the lowerenergy band is related to the high conductivity of this salt, and both of these @re shown to be due to the mixed-valence nature of this salt. From the oscillator strength (plasma frequency) of this band, we infer a bandwidth along the stacks of 4t -1.1 eV, the largest yet reported for an organic metal. From the energy of the higher-frequency charge-transfer band, we infer that thd effective Coulomb correlation energy is U -1' /4 eV, comparable with other organic salts. Nevertheless, the larger bandwidth in (TTF}Br079 causes the Coulomb correlations to be relatively less important than in TCNQ (tetracyano-p-quinodimethane) salts, for example. A comparison is also made of the corresponding spectra for TCNQ, TMTTF+ (tetramethyl-TTF+), and TSeF+ (tetraselenafulvalenium, the selenium analogue of TTF+).

A phase transition starting at 84 K and indicated by a marked color change of the sample has been found in the organic insulator tetrathiafulvalene-chloranil. Optical, infrared, and Raman measurements indicate that this is a reversible transition from a nominally neutral (N) solid to a nominally ionic (D salt. Surprisingly, the N-I transition is not first order, but occurs over a broad temperature region (~ 30 K), in which there is a coexistence of N and I molecules.