Roman Świetlik

Charge‐Assisted Halogen Bonding: Donor–Acceptor Complexes with Variable Ionicity

Charge-assisted halogen bonding is unambiguously revealed from structural and electronic investigations of a series of isostructural charge-transfer complexes derived from iodinated tetrathiafulvalene and tetracyanoquinodimethane derivatives, (EDT-TTFI2)2(TCNQF(n)), n=0-2, which exhibit variable degrees of ionicity. The iodinated tetrathiafulvalene derivative, EDT-TTFI2, associates with tetracyanoquinodimethane (TCNQ) and its derivatives of increasing reduction potential (TCNQF, TCNQF2) through highly directional C-I⋅⋅⋅N≡C halogen-bond interactions. With the less oxidizing TCNQ acceptor, a neutral and insulating charge-transfer complex is isolated whereas with the more oxidizing TCNQF2 acceptor, an ionic, highly conducting charge-transfer salt is found, both of 2:1 stoichiometry and isostructural with the intermediate TCNQF complex, in which a neutral-ionic conversion takes place upon cooling. A correlation between the degree of charge transfer and the C-I⋅⋅⋅N≡C halogen-bond strength is established from the comparison of the structures of the three isostructural complexes at temperatures from 300 to 20 K, thus demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral-ionic conversion in (EDT-TTFI2)2(TCNQF) is further investigated through the temperature dependence of its magnetic susceptibility and the stretching modes of the C≡N groups.

Charge transfer coupling to B1u modes in BEDT-TTF salts

Abstract We have studied the infrared absorption spectra of BEDT-TTF [bis(ethylene-dithio)tetrathiafulvalene] charge-transfer crystals with different anions [I 3 , Cl0 4 , SbF 6 , Cu(NCS) 2 ] focusing on bands related to CC double bond stretching vibrations. The coupling of charge transfer to b 1u modes of BEDT-TTF molecule is considered. Similar investigations have been performed for some compounds based on related molecules [DIMET, BVDT-TTF and BDMVDT-TTF with Cl0 4 , AsF 6 and SbF 6 anions, respectively]. The influence of metal-insulator transitions on IR bands assigned to b 1u modes (CC stretching vibrations) was observed and discussed as a consequence of modifications of the charge distribution.

Electron correlations in an irradiated MTPP(TCNQ)2 salt studied by IR and visible spectroscopy

Abstract Infrared and optical absorption spectra of an electron-irradiated organic semiconductor MTPP(TCNQ)2 are reported. The appearance of distinct doublets of activated ag modes, characteristic of the interacting tetramer system, is noticed. The origin of both the narrow and wide components of the doublets is discussed.

Infrared spectra of conducting BEDSe-TTF charge-transfer salts with halide mercurate anions

Abstract We report on the studies of single crystal reflectance spectra (650–6500 cm −1 ) of four conducting radical cation salts formed by the organic donor, bis(ethylenediseleno) tetrathiafulvalene (BETSe-TTF) and mercury halide anions, Hg 3 Br 8 , HgBr 4 , Hg 2 I 6 and HgCl 4 . Additionally, infrared (IR) and Fourier transform near infrared (FT-NIR) Raman spectra of neutral BEDSe-TTF were measured. Electronic bands are analysed using a Drude–Lorentz dielectric function. Vibrational bands, mainly those related to the coupling of electrons with internal vibrations of BEDSe-TTF, are discussed. Considerable differences between IR spectra of the studied salts are mainly due to different charge of BEDSe-TTF cations.

Charge Transfer Salt with Substituted Lindquist Polyoxometalate: Synthesis, Crystal Structure, and Physical Properties of α-(BEDT-TTF)6(VW5O19)⋅DMF⋅2H2O

A new charge transfer salt based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the vanadium substituted Lindquist anion [VW5O19]3− was synthesized and characterized by X-ray and spectroscopic analysis. Monoclinic, C2, Fw=3789.2, a=41.5019(5), b=11.2600(2), c=12.7514(2) Å, β=101.877(5)°, Z=2, R=0.060, based on 12095 reflections with I>2σ(I). The structure consists of alternating organic and inorganic layers. The inorganic layer is generated by [VW5O19]3− anions, dimethylformamide (DMF) and water molecules. The organic layer, which is generated by three different BEDT-TTF units, belongs to the α-type of packing and consists of trimerized chains parallel to the c direction. The compound is a semi-conductor with σ300 K=1.1 S⋅cm−1 and Ea=0.1 eV. Polarized reflectance spectra of single crystals were measured over the spectral range from 650 to 25000 cm−1. FT-NIR Raman spectra of powdered crystals dispersed in KBr pellets were also recorded. Vibrational and electronic features are discussed.

Spectral studies of highly conducting organic composites (BEDO-TTF)x/iodine

Abstract Infrared and Raman spectral properties of the organic composites obtained by direct solid–solid charge-transfer (CT) reaction between bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) and iodine are investigated for the first time. The band attribution, an evaluation of the average charge on the donor molecule, changes of the BEDO-TTF spectra after composite formation and an analysis of the plasma-edge-like dispersion are presented and discussed.

Infrared spectra of tetramerized organic semiconductors PrPht(TCNQ)2 and PrQuin(TCNQ)2 — the problem of ag phonon mode splitting

Abstract Polarized infrared reflectance spectra from single crystals of two organic charge-transfer salts PrPht(TCNQ) 2 and PrQuin(TCNQ) 2 (where TCNQ = tetracyano- p quinodimethane, PrPht = N -propylphtalazinium, PrQuin = N -propylquinoxalinium) are studied over the frequency range 400–7000 cm −1 at room temperature (in both compounds the TCNQ molecules are arranged to form linear stacks with the tetramer repeat unit). Kramers-Kronig analysis of the reflectance is used to determine the optical properties. Some vibrational lines related to the coupling of a g phonon modes of the TCNQ molecule with electrons are split. The origin of a g mode splitting is discussed. Moreover, the electronic spectra (up to 20 000 cm −1 ) are measured at room temperature using the KBr pellet technique. The charge-transfer bands in tetramerized TCNQ salts are analysed.

Solid-state characterisation of the 7,7,8,8-tetracyano-p-quinodimethanide salt of the 5,5-dimethyldibenzophospholium cation

The electrical conductivity, magnetic susceptibility and i.r. and u.v. spectra of (5,5-dimethyldibenzophospholium)+(7,7,8,8-tetracyano-p-quinodimethanide)–2, DMDBP–TCNQ2, are reported. The conductivity is anisotropic, with σ300 K= 5 × 10–4, 2 × 10–4 and 3 × 10–3 S cm–1 along a, b and c, respectively, and in the TCNQ stacking direction the conductivity varies as σ=σ0 exp (–Ea/kT), where Ea= 0.20 eV. The paramagnetic susceptibility is activated with J= 0.007 eV. The infrared spectrum of DMDBP–TCNQ2 shows two main features: (i) a broad electronic absorption and (ii) a series of relatively narrow lines which may be regarded as a consequence of electron–phonon coupling. Two models are proposed which adequately describe the properties.

Infrared studies of VINφ2(TCNQ)2 salt with two different TCNQ stacks

Abstract VINφ2(TCNQ)2 salt with two different one-dimensional TCNQ columns is characterized by an electron and vibrational spectroscopy (IR absorption, polarized reflection and Raman scattering). Anisotropy of the optical conductivity, origin and nature of the electron-molecular vibration coupling and charge distribution on the TCNQ moieties of the salt are discussed.

The metal-insulator transition in the organic conductor β″-(BEDT-TTF)2Hg(SCN)2Cl

We explore the nature of the metal-insulator transition in the two-dimensional organic compound β″-(BEDT-TTF)2Hg(SCN)2Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction (a-b), setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of Δσ=47 meV is extracted. From the activated resistivity behavior below T = 55 K, a charge gap of Δρ=60 meV is derived. At TCO = 72 K, the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of 2δρ=0.34e. In addition to a slight structural dimerization, charge-order causes stripes most likely perpendicular to the stacks.