J. Kudnig

Structure, electrical conductivity and dielectric relaxation of the phenothiazine-tetracyanoethene 1:1 complex

Abstract Phenothiazine (PTZ) as a donor and tetracyanoethene (TCNE) as an acceptor form the dark blue charge-transfer complex PTZ-TCNE. In the solid state stacks are found in which the donor (D) and acceptor (A) molecules alternate according to the sequence –D—A– –D—A– –D—A–. Within the stacks the donor and acceptor molecules are arranged coplanarly to each other in such a way that an optimum overlap of the corresponding HOMOs and LUMOs is guaranteed, as is shown by MNDO calculations. PTZ-TCNE is a semiconductor with a gap energy of E g = 1.69 eV and a pre-exponential factor of σ o = 575 S cm −1 . Its dielectric relaxation is of the non-Debye type (non-symmetrical Cole-Cole plot, described by a Havriliak-Negami equation) showing a temperature dependence characterized by an Erying equation with an activation enthalpy of ΔH ν = 0.99 eV and entropy ΔS ν ≈ 10 −3 eV K −1 . The temperature dependence was also analysed in terms of an Arrhenius equation leading to E a = 1.05 eV.

Structure, electrical conductivity and dielectric relaxation of a 1,2-dimethoxybenzene-tetracyanoethene 1:1 complex

Abstract 1,2-Dimethoxybenzene (Ver) as a donor and tetracyanoethene (TCNE) as an acceptor from the dark blue charge-transfer complex Ver·TCNE. In the crystal stacks, in which the donor (D) and acceptor (A) molecules alternate according to the sequence …DAD…A…DAD…A…, centro-symmetric DAD units can be distinguished. In these units the molecules are arranged in such a way that an optimum overlap of the corresponding HOMOs and LUMOs is guaranteed, as is shown by MNDO and HAM 3 calculations. Ver·TCNE is a semiconductor with a gap energy of Eg = 0.85 eV and a pre-exponential factor of σo = 0.91 S/cm. The temperature dependence of dielectric relaxation is characterized by an Eyring equation and gives an activation enthalpy of ΔH ‡ = 0.76 eV and entropy ΔS ‡ = 0.95 × 10 −4 eV/K .

Structure, electrical conductivity and dielectric relaxation of the 1,2-bis(methylthio)benzene-2,3-dichloro-5,6-dicyano-1,4-benzoquinone 1 : 1 charge-transfer complex

1,2-Bis(methylthio)benzene forms a 1 : 1 charge-transfer (CT) complex with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). X-Ray analysis shows the compound to have a columnar structure with stacks in which the equidistantly and coplanarly arranged donor and accepter molecules alternate. By the relative orientations of the successive donor and accepter molecules in the stacks an optimum overlap of the relevant HOMOs and LUMOs, which are of is-type according to MNDO calculations, is guaranteed.The electrical conductivity of the CT complex is relatively low (as a consequence of the crystal structure), but shows a temperature dependence typical of semiconductors. The dielectric behaviour of the compound has also been studied. The results can be represented by an electrical model circuit of the ladder type, indicating that the following contributions are of similar importance: (i) hopping conduction into the bulk of the polycrystalline sample, (ii) interfacial impedance of the polycrystals, and (iii) electrode adsorption-diffusion phenomena.

Elementorganische Verbindungen mit o-Phenylenresten, XXVIII [1] Charge-transfer-Komplexe von 2,3,7,8-Tetraalkoxy-chalkogenanthrenen mit 7,7,8,8-Tetracyan-2,3,5,6-tetrafluor-chinodimethan / Organometalloidal Compounds with o-Phenylene Substituents, Part XXVIII [1] Charge-Transfer Complexes of 2,3,7,8-Tetraalkoxy-chalcogenanthrenes wjth 7,7,8,8-Tetracyano-2,3,5,6-tetrafluoro-quinodimethane

2,3,7,8-Tetramethoxythianthrene and -selenanthrene, as well as -tetraethoxythianthrene give isostructural 1:1 charge-transfer complexes with 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane. In the columnar crystal structures there are alternating donor and acceptor molecules. The chalcogenanthrene molecules which are folded at their E···E axes in the pure state, are planar in the complexes indicating a charge-transfer according to [donor]+ [acceptor]- . Consecutive molecules of the stacks are arranged in such a way that an optimum overlap of the HOMO of the donor and the LUMO of the acceptor, both of which are of π-type character according to MNDO calculations, is secured.

Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-tetramethoxychalcogenanthrene–2,3-dichloro-5,6-dicyano-l,4-benzoquinone 1 : 1 charge-transfer complexes

2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-chalcogena-cyclo-diveratrylenes, ‘Vn2E2’, E = S, Se) form isotypical 1 : 1 charge-transfer (CT) complexes with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). X-ray analysis of Vn2S2·DDQ shows the compound to have a columnar structure with segregated stacks of donors and acceptors. The donors are virtually planar in accordance with a formulation of [Vn2E2]+[DDQ]–. Donor cations and acceptor anions are equidistant in their respective stacks, but in each case they inclined to the stacking axis, nevertheless guaranteeing an optimum overlap of the half-filled frontier orbitals which are of π-type character according to MNDO calculations. Dielectric ac measurements of permittivity Iµ′ and loss factor Iµ″ clearly reveal two processes, a dielectric one at low temperatures and a conductive one at high temperatures. The dielectric process can be described by the Havriliak–Negami (HN) and the Kohlrausch–Williams–Watts (KWW) model, and the conductive process by a Debye-type plot. Using these methods, the relevant parameters are evaluated. The dc conductivities of polycrystalline samples moulded at 108 Pa show a temperature dependence in the plots of ln σ vs. T–1, which is typical of semiconductors. Two slopes are found; that in the low-temperature region (< 285 K) is explained by an easy-path model (intragrain conductivity with low activation energies), whereas in the high-temperature region conduction across the grain boundaries (with higher activation energies) is becoming predominant. The activation energies for the intrinsic conductivities obtained by the ac and dc measurements are similar. Despite the columnar structure with segregated stacks, due to stoichiometric oxidation states of the components, the absolute values of conductivity are low (ca. 10 –6 S cm–1 at 293 K), though higher (by a factor of ca. 103) than those of compounds like Vn2E22·TCNQ with stacks in which donor and acceptor molecules alternate.

Structure, electrical conductivity and dielectric relaxation of charge-transfer complexes with 2,4,6,8-tetramethoxydibenzoselenophene as a donor

2,4,6,8-Tetramethoxydibenzoselenophene (m-Se) forms 1 : 1 charge-transfer (CT) complexes with the acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyano-1,4-quinone (DCQ) and is oxidized by iodine to [m-Se]I3. X-Ray analyses of m-Se–TCNQ and m-Se–TCNE show both CT complexes to have columnar structures with stacks in which the coplanarly arranged donor and acceptor molecules alternate.The electrical conductivities of the CT complexes with TCNQ, TCNE and DCQ and of the compound [m-Se]I3 were relatively low (as a consequence of the crystal structures), however, and show the typical temperature dependences of semiconductors. The dielectric behaviour of the three CT complexes have also been studied, by conventional ac. measurements and by the thermostimulated depolarisation current (TSDC) technique. Relaxation peaks can be observed after a transformation of the complex permittivity Iµ* to the complex polarisability α*. The existence of these relaxations is confirmed by the TSDC measurements. By thermal sampling a nearly constant distribution of activation energies is obtained.