Optical properties of the radical cation tetrathiafulvalenium (TTF+) in its mixed-valence and monovalence halide salts

Measurements are presented of the optical properties of the radical cation TT${\mathrm{F}}^{+}$ (tetrathiafulvalenium) in a variety of different conditions: as TT${\mathrm{F}}^{+}$ monomers in solution, as ${\left(\mathrm{T}\mathrm{T}{\mathrm{F}}^{+}\right)}_2$ dimers in solution, in the monovalence (TTF)${\mathrm{Br}}_{1.0}$ solid, and in the mixed-valence (TTF)${\mathrm{Br}}_{0.79}$ 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)${\mathrm{Br}}_{0.79}$ 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 are 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 $4t\sim 1.1$ eV, the largest yet reported for an organic metal. From the energy of the higher-frequency charge-transfer band, we infer that the effective Coulomb correlation energy is $U\sim 1\frac{1}{4}$ eV, comparable with other organic salts. Nevertheless, the larger bandwidth in (TTF)${\mathrm{Br}}_{0.79}$ 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 TCN${\mathrm{Q}}^{-}$, TMTT${\mathrm{F}}^{+}$ (tetramethyl-TT${\mathrm{F}}^{+}$), and TSe${\mathrm{F}}^{+}$ (tetraselenafulvalenium, the selenium analogue of TT${\mathrm{F}}^{+}$).