Charge fluctuations and electron–phonon coupling in organic charge-transfer salts with neutral–ionic and Peierls transitions

The first-order transition of the charge-transfer complex tetrathiafulvalene–chloranil (TTF–CA) is both a neutral–ionic (NIT) and a Peierls transition. In related organic charge-transfer complexes, cooling at ambient pressure increases the ionicity ρ in strikingly different ways, and is sometimes accompanied by a dielectric peak, that we relate to lattice stiffness, to structural and energetic disorder, and to the softening of the Peierls mode in the far-IR. The position operator P for systems with periodic boundary conditions makes possible a systematic treatment of electron–phonon (e–p) interactions in extended donor–acceptor stacks in terms of correlated Peierls–Hubbard models. The IR intensity of the Peierls mode peaks at the Peierls transition at small ρ<1/2 in soft lattices, where the dielectric constant also has a large peak. In dimerized stacks, the IR intensity of totally symmetric (ts), Raman active, molecular vibrations is related to charge fluctuations that modulate site energies. Combination bands of molecular and Peierls modes are identified in regular TTF–CA stacks above Tc. Energetic disorder can suppress the Peierls transition and rationalize a continuous crossover from small to large ρ. The TTF–CA scenario of a neutral-regular to ionic-dimerized transition must be broadened considerably in view of charge-transfer salts that dimerize on the neutral side, that become ionic without a structural change, or that show vibrational evidence for dimerization at constant ρ<1.