Determination of the extent of charge transfer in partially oxidized derivatives of tetrathiafulvalene and tetracyanoquinodimethan by resonance Raman spectroscopy

A number of tetrathiafulvalene (TTF) compounds with formal oxidation states, ρ, of TTF less than one (TTF^ρ+) were studied by resonance Raman spectroscopy. The ν₃ frequencies are intermediate between those in TTF⁰ and the cation radical TTF⁺•. The ν₃ frequency of TTF, TTFBr_0.7, TTFI_0.7, TTF(SCN)_0.52, and TTF⁺ salts was used to demonstrate a linear relationship between ρ and ν₃. This was used to determine ρ for transition-metal salts of TTF. TTF^ρ+ compounds were found to undergo laser-induced decomposition to form species which appear to contain TTF⁺. For the acceptor molecule tetracyanoquinodimethan (TCNQ), a similar relationship obtains between ρ and ν₄ in derivatives containing TCNQ^-. The resonance Raman spectrum of the conducting complex (TTF)(TCNQ) shows only one band each for ν₃ and ν₄ of TTF and TCNQ, respectively, from which the degree of charge transfer for both donor and acceptor is 0.58 ± 0.06. The total charge for each species is uniformly delocalized over all molecules in the segregated stacks in the solid.