Peng Huo

A New Type of Charge-Transfer Salts Based on Tetrathiafulvalene–Tetracarboxylate Coordination Polymers and Methyl Viologen

Although charge-transfer compounds based on tetrathiafulvalene (TTF) derivatives have been intensively studied, {[cation](n+)·[TTFs](n-)} ion pair charge-transfer (IPCT) salts have not been reported. The aim of this research is to introduce functional organic cations, such as photoactive methyl viologen (MV(2+)), into the negatively charged TTF-metal coordination framework to obtain this new type of IPCT complex. X-ray structural analysis of the four compounds (MV)2[Li4(L)2(H2O)6] (1), {(MV)(L)[Na2(H2O)8]·4H2O}n (2), {(MV)[Mn(L)(H2O)2]·2H2O}n (3), and {(MV)[Mn(L)(H2O)2]}n (4), reveals that the electron donor (D) TTF moiety and the electron acceptor (A) MV(2+) form a regular mixed-stack arrangement in alternating DADA fashion. The TTF moiety and the MV(2+) cation are essentially parallel stacked to form the column structures. The strong electrostatic interaction is a main force to shorten the distance between the cation and anion planes. Optical diffuse-reflection spectra indicate that charge transfer occurs in these complexes. The ESR and magnetic measurements confirm that there is strong charge-transfer-induced partial electron transfer. Compounds 2, 3, and 4 show an effective and repeatable photocurrent response. The current intensities of 3 and 4 are higher than that of 2, which reflects that the coordination center of the Mn(II) ion has a great effect on the increasing photocurrent response.

A tetrathiafulvalene-grafted titanium-oxo-cluster material: self-catalyzed crystal exfoliation and photocurrent response properties

Many exfoliation strategies have been developed to prepare 2-D sheet materials from single crystals. However, most of the published research has used micro-scale crystals. The transformation of bulk crystals to micro- or nano-scale structures has rarely been achieved by chemical etching. We report here the preparation of a tetrathiafulvalene (TTF)-grafted titanium-oxo-cluster (TiO–TTF) that can be easily exfoliated at a millimeter scale. This is the first example of a titanium-oxo-cluster successfully combined with a TTF moiety. In an oxygen atmosphere, the red crystal of the TiO–TTF cluster changes to a black ‘crystal’ composed of regularly arranged floorboard-like micro-plates. The exfoliation properties of the material are a result of the easy oxidation of the TTF group and the catalytic properties of the TiO cluster. The TTF moiety in the cluster is not only an oxidation center, but also adds new photoelectroactive properties to the TiO material. An enhanced photocurrent response was observed for the TiO–TTF and TiO–TTF(+) materials.

Synthetic methods and structural study of coordination polymers of Cd(II) and Co(II) with tetrathiafulvalene–tetracarboxylate

A series of coordination polymers of cadmium(II) and cobalt(II) with TTF–tetracarboxylate ligand (L) were prepared using room temperature synthetic methods and solvothermal method. The room temperature methods are subdivided to controlled evaporation method and two layers diffusion method. The fundamental structures of these compounds are {[M(L)0.5(B)(S1)x]·yS2}n (1–6) (M = Cd(II) or Co(II), B (base) = 2,2′-bpy, phen or 4,4′-bpy, S1 (coordinated solvent) = C2H5OH or H2O, and S2 (co-crystallized solvent) = H2O or CH3OH, x = 1 or 2, and y = 0 to 3). Compound 7 is a known ionic pair n[Co(4,4′-)2(H2O)4]·[Co(L)(H2O)2]n. At room temperature, no matter what the metal ions are, Cd(II) or Co(II), and the synthetic methods are, controlled evaporation or two layers diffusion, the products 1–4 and 7 are 1-D coordination polymers. Both the 2-D Co(II) coordination polymer 5 and 3-D coordination polymer 6 were prepared by solvothermal method. A new type of coordination mode of the TTF–tetracarboxylate ligand is found in the structures of 2 and 3, in which two carboxyl groups have a μ2-η2:η1 bridging mode, and the other two carboxyl groups are not coordinated. Electrochemical properties of the solid-state compounds were also investigated by cyclic voltammetry using surface-modified electrodes.

Effect of conjugated structures of bipyridinium cations on ion assembly and charge-transfer of their tetrathiafulvalene-bicarboxylate salts

A series of tetrathiafulvalene (electron donor, D) charge-transfer salts with three methyl bipyridiniums (electron acceptor, A) in the basic formulas MV(HL)2 (1), MeBpe(HL)2 (2) and MeBpa(HL)2 (3) (L = dimethylthio-tetrathiafulvalene-bicarboxylate, MV = methyl viologen, MeBpe = 1,2-bi(4-pyridinium) ethene and MeBpa =1,2-bi(4-pyridinium) ethane) was synthesized and their structures were characterized by X-ray single crystal analysis. The three bipyridiniums allow the exploration of the effect of different conjugated systems on molecular packing and charge-transfer interactions. A type of –DDDD– stacking column with edge-to-edge short D⋯A⋯D contacts is found in MV compound 1, while in π-extended MeBpe compound 2, a –DADDAD– type of stacking column with face-to-face short D⋯D and A⋯D stacking is formed, which shows the most effective cation–anion interaction. No effective interaction is found for the non-conjugated MeBpa compound 3. The charge-transfer from an anion to a cation measured by UV-vis, CV and ESR is in the order of 2 > 1 ≫ 3, which is greatly in accordance with the order of the cations conjugated state and CT interaction. It is known that charge-transfer is a key factor in obtaining photoelectric materials. Photocurrent responses of these compounds are consequently studied and the results agree with the intensity of charge-transfer. This is a systematic study of the effect of different π-conjugated systems on charge-transfer properties in terms of crystal structures.

Effect of Metal Coordination on Photocurrent Response Properties of a Tetrathiafulvalene Organogel Film

Organic low molecular weight gelators with a tetrathiafulvalene (TTF) unit have received considerable attention because the formed gels usually exhibit redox active response and conducting or semiconducting properties. However, to our knowledge, metal coordination systems have not been reported for TTF-derived gels up to date. We have designed and synthesized a series of TTF derivatives with a diamide-diamino moiety that can coordinate to specific metal ions with square coordination geometry. Gelation properties and morphologies of the films prepared by the gelators in different hydrophobic solvents are characterized. The TTF derivative with a dodecyl group shows effective gelation properties, and electrodes with the organogel films are prepared. The effect of the Ni(II) and Cu(II) coordination on the photocurrent response property of the electrodes is examined. The metal square coordination significantly increases the photocurrent response. This gel system is the first metal coordination related TTF-gel-based photoelectric material. The mechanism of the metal coordination-improved photocurrent response property is discussed based on the crystal structural analysis and theoretical calculations.

Effects of alkyl chain length on film morphologies and photocurrent responses of tetrathiafulvalene-bipyridinium charge-transfer salts: a study in terms of structures

Viologen dication derivatives with a series of alkyl chains are used to assemble charge-transfer salts and films with dimethylthio-tetrathiafulvalene-bicarboxylate which aims to explore the relationship of the alkyl chain length to molecular structures and film morphologies which are important points for the design of molecular materials and manipulation of molecular devices. Four charge-transfer salts generally formulated as [(CnV)(HL)2] [n = 4(1), 8(2), 12(3) and 16(4)] are prepared and 1–3 are characterized by single-crystal X-ray structural analysis. In the short alkyl chain compound, TTF and C4V moieties are regularly arranged and effectively stacked with strong charge-transfer interactions. Lengthening the alkyl chain of the bipyridinium cation reduces the regular and close arrangement of the cations and anions in the crystals, which weakens the charge-transfer interaction while increasing the amphiphilic assembly of the films. Their film morphologies change from crystals to fused crystals, band-like structures and finally to smooth films with the increase of the alkyl chain length. The effects of alkyl chain length on photocurrent intensity are different between the crystal sample-modified electrodes (in the order of 1 > 2 > 3 = 4) and the film-modified electrodes (in the order of 2 > 1 = 3 > 4), because both inter-ion interactions and film-forming properties should be considered for the film electrodes. This is a systematic study of the effect of alkyl chain substitution on film morphologies in terms of crystal structures.

Photocurrent responsive films prepared from a nickel-dithiolate compound with directly bonded pyridyl groups

Because the pyridyl group can act as a proton acceptor and a coordination biting unit, electrodes with films of dithiolene complex [n-Bu4N][Ni(4-pedt)2] (1) (4-pedt = 1-(pyridine-4-yl) ethylene-1,2-dithiolate) are prepared to investigate the photocurrent response behaviours of a metal dithiolene compound upon protonation and transition metal coordination. The morphologies of film 1 are characterized by SEM analysis. Photoelectrochemical measurements are carried out with the electrodes of film 1. The results are as follows: (1) the stronger the medium acidity is, the weaker the photocurrent becomes. (2) Photocurrent intensities of Mn(II), Co(II) and Zn(II) treated film 1 are increased in comparison with that of the original film 1 due to the polymeric coordination. (3) When film 1 is treated by Fe(III) ions, it is oxidized to a neutral [Ni(4-pedt)2] film that shows weaker photocurrent response. In addition, a new ionic pair compound MV[Ni(4-pedt)2]2 (2) (MV2+ = methylviologen dication) synthesized by cation exchange shows good photocurrent response properties due to the improvement of electron transfer.