Rb–TCNQ(II) is shown to be a realization of the BOW phase of half-filled extended Hubbard models. The BOW phase has a regular array of sites with inversion (Ci) symmetry, a finite magnetic gap Em and broken electronic Ci symmetry. The phase is conditionally stable against dimerization for linear electron–phonon (e–ph) coupling. At 100 K, Rb–TCNQ(II) crystals have regular TCNQ− stacks at Ci centers, negligible spin susceptibility that indicates finite Em, and infrared (ir) spectra that indicate broken Ci symmetry. The 100 and 295 K crystal structures rule out a dimerization transition around 220 K that had previously been inferred from magnetic and ir data.Structural, magnetic and infrared data show that Rb–TCNQ(II) is a realization of the bond order phase of extended Hubbard models.

The unusually different photoluminescence (PL) and electroluminescence (EL) of an organic film is related to disordered dipoles and charge–dipole interactions that reduce the transport gap of localized electrons and holes in thermal equilibrium. The assignment of EL as direct emission from ion-pairs in films follows the exciplex interpretation of chemiluminescence (CL) in solutions that contain tritolylamine donors.The unusually different photoluminescence (PL) and electroluminescence (EL) of an organic film is related to disordered dipoles and charge–dipole interactions that reduce the transport gap of localized electrons and holes in thermal equilibrium. The assignment of EL as direct emission from ion-pairs in films follows the exciplex interpretation of chemiluminescence (CL) in solutions that contain tritolylamine donors.

Electronic (valence) and structural (Peierls) instabilities occur either separately or together in organic charge-transfer (CT) crystals with mixed stacks of donors (D) and acceptors (A). A Peierls–Hubbard model, HCT, with site D, A energies and linear electron–phonon coupling in a harmonic lattice is shown to provide a unified microscopic description and to distinguish between thermal and quantum Peierls transitions. Vibrational, magnetic and structural data are interpreted as thermal Peierls transitions in two ionic CT salts and as dimerized ground states with spin solitons in other salts. Quantum Peierls transitions are identified in largely neutral salts, and a neutral–ionic crossover is discussed in a salt with dipolar disorder that suppresses the Peierls instability. The coincident Peierls and neutral–ionic transition of tetrathiafulvalene-chloranil (TTF-CA) is another special case of HCT.

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.

The electronic polarization energies, P=P++P−, of a perylenetetracarboxylic acid dianhydride (PTCDA) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50 Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules.