The electrical conductivity of mixed-valence [MoV2MoVI16O54(SO3)2]6− tetraalkylammonium salts was investigated through dependence on the inter-cluster distance that is controlled by tetraethylammonium, tetrapropylammonium, and tetrabutylammonium cations. The crystallographic analysis of single crystals revealed that the inter-cluster distances are dependent on the chain length of the alkyl groups on the counter cations. In addition, the electrical conductivities of the single crystals were found to be dependent on both temperature and chain length. Mixed-valence polyoxometalate (POM) clusters are considered to be a molecular particle of Mo bronze by which highly ordered networks will be developed using single crystals, where POMs are rather small and have a well-organized structure compared to colloidal nanostructures.

Hydrogen bonding was investigated in a newly prepared salt of pyridyl-substituted tetrathiafulvalene (TTFPy) that formed a dimer structure by hydrogen bonding and was salted with PF6− anions, namely, (TTFPy2H)PF6 (1). Structural analysis of the salt by single-crystal X-ray diffraction and temperature-dependent FTIR spectra and measurements of its electrical conductivity and dielectric properties were performed. Thermal fluctuations were investigated for protons between two TTFPy molecules. An anisotropic dielectric anomaly was observed owing to local proton transfer between two pyridyl moieties. This behavior was well explained by a model of Debye-type dipole relaxation. In addition, DFT calculations showed that the dipole originated from local proton transfer, but the dipole moment in a network of TTFPy⋯H+⋯TTFPy dimers was amplified by a factor of 2 owing to coupling with the electronic polarization of TTFPy moieties.

Benzimidazolium is revealed to form quasi-isostructural crystals of mixed-valence molecular metal oxides that consist of [PMoV2MoVI10O40]5− and fully oxidized [BWVI12O40]5−. The structure and dielectric behaviour were compared and a dielectric anomaly was evident due to the electric dipole relaxation in the mixed-valence system.

The reduction of solutions of acidified molybdate leads to the formation of a family of nanostructured molybdenum blue (MB) wheels which are linked together in a series of complex reaction networks. These networks are complex because the species which define the nodes are extremely labile, unstable, and common to many different networks. Herein, we combine gel electrophoresis and electrospray ionization mass spectrometry (ESI-MS) to investigate the effect of the pH and the ratio of reactants and reducing agents, R (R = [S2O42–]/[MoO42–]), on the complex underlying set of equilibria that make up MBs. By mapping the reaction parameter space given by experimental variables such as pH, R, solvent medium, and type of counterion, we show that the species present range from nanostructured MB wheels (comprising ca. 154 Mo atoms) to smaller molecular capsules, [(SO3)2MoV2MoVI16O54]6– ({S2Mo18}), and templated hexameric [(μ6-SO3)MoV6O15(μ2-SO3)3]8–({S4Mo6}) anions. The parallel effects of templation and reduction on the self-assembly process are discussed, taking into consideration the Lewis basicity of the template, the oxidation state of the Mo centers, and the polarity of the reaction medium. Finally, we report a new type of molecular cage (TBA)5[Na(SO3)2(PhPO3)4MoV4MoVI14O49]·nMeCN (1), templated by SO32– anions and decorated by organic ligands. This discovery results from the exploration of the cooperative effect of two anions possessing comparable Lewis basicity, and we believe this constitutes a new synthetic approach for the design of new nanostructured molecular metal oxides and will lead to a greater understanding of the complex reaction networks underpinning the assembly of this family of nanoclusters.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a promising conductive polymer for electrical applications, and increases in its conductivity, transparency, stability, stiffness and strength have been explored. Herein, we demonstrate a facile fabrication method for a PEDOT film that exhibits large anisotropic conduction along in-plane directions. An aqueous solution of PEDOT added with polystyrene sulfonate (PSS) was mixed with an insulating oxide nanosheet based on montmorillonite (MMT) dispersed in aqueous media. Structural studies demonstrated it to be a hybrid film with a lamellar-like assembly that was structured with mono-layered sheets. In addition, an inhomogeneous mixture of pristine MMT (insulating) and PEDOT-rich (conductive) lamellar grains was proposed. The ratio between these two grains was controlled by tuning the content of the PEDOT:PSS solution in the initial mixing process. Increases in the content of PEDOT increased the electrical conductivity. This dependence is explained by a percolation model with random arrangements of PEDOT-rich conductive and insulating grains. The conductivity showed large anisotropy between in-plane and out-of-plane measurements. The ratio reached almost 105 and remained the same over a wide range of temperatures. The lamellar structure of the PEDOT and nanosheets is ascribed to the large anisotropy.

Herein, we report the synthesis of 1,1′-bi(2-pyridylazulene) (1), in which pyridyl moieties were coupled to a biaryl framework for hydrogen bonding between the two aryl skeletons. The two 2-pyridylazulene moieties in 1 were linked through facile aryl–aryl coupling between the 1- and 1′-positions of the azulene skeletons, where 1-haloazulene was stabilized by electron withdrawing pyridyl substitution. In addition, single crystals of the mono-protonated species (1H+) were successfully obtained as the BF4− salt. X-ray diffraction analysis at 153 K revealed an intramolecular hydrogen bond between the two pyridyl moieties, giving a racemic mixture of axial chiral species. DFT calculations were performed to understand the hydrogen bonding structure and an almost single minimum potential for thermal proton motion was suggested. The acid–base characteristics were investigated in acetonitrile and 1 was revealed to exhibit two-step protonation of its pyridyl moieties. By comparison with monomeric 2-pyridylazulene, the stronger basic character of 1 was confirmed. This is ascribed to the macrocyclic effect of the two pyridyl moieties bridged by a proton, as seen in the single crystal of 1H+. Furthermore, two different energy shifts associated with intramolecular transition were observed under protonation; the first protonation produced a blue shift in the absorption maximum, whereas a red shift was observed for the second protonation. The unusual blue shift was explained by the stabilization of the HOMO owing to an extended electronic structure between the two azulene skeletons. This unique steric structure was achieved by proton bridging in the pyridyl-substituted biaryl compound.

Polyoxometalates (POMs) are a subset of metal oxides with unique physical and chemical properties, which can be reliably modified through various techniques and methods to develop sophisticated materials and devices. In parallel with the large number of new crystal structures reported in the literature, the application of these POMs towards multifunctional materials has attracted considerable attention. This critical review summarizes recent progress on POM-based molecular and composite materials, and particularly highlights the emerging areas that are closely related to surface, electronic, energy, environment, life science, etc. (171 references).

A new salt—[NiII(DMSO)5(TTFPy)]2[α-SiW12O40] (1)—based on polyoxometalates was prepared by coordinating a cationic electron donor of pyridyltetrathiafulvalene (TTFPy) with NiII. Although the TTFPy molecule did not form a salt with the anionic α-[SiWVI12O40]4− because of the weak charge-transfer (CT) interaction, the coordination of Ni with the pyridyl moiety permitted salt formation driven by electrostatic interaction, giving a single crystal of 1. Crystallographic analysis, UV-vis and IR spectroscopy and electrochemical characterization revealed that the fully oxidized α-[SiWVI12O40]4− was crystallized with the neutral TTFPy moiety from the acetonitrile solution because of the low electron-withdrawing ability of α-[SiWVI12O40]4−, forming a brown–orange crystal. The crystal colour quickly turned to black by immersing in methanol, due to CT from TTF moiety to α-[SiWVI12O40]4−, which was caused by the solvent effect. Increase in the solvent acceptor number from 18.9 for acetonitrile to 41.3 for methanol resulted in the enhancement of the electron withdrawing ability of α-[SiWVI12O40]4− by 0.317 V in methanol.

Polyoxometalates (POMs) are a subset of metal oxides with unique physical and chemical properties, which can be reliably modified through various techniques and methods to develop sophisticated materials and devices. In parallel with the large number of new crystal structures reported in the literature, the application of these POMs towards multifunctional materials has attracted considerable attention. This critical review summarizes recent progress on POM-based molecular and composite materials, and particularly highlights the emerging areas that are closely related to surface, electronic, energy, environment, life science, etc. (171 references).

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a promising conductive polymer for electrical applications, and increases in its conductivity, transparency, stability, stiffness and strength have been explored. Herein, we demonstrate a facile fabrication method for a PEDOT film that exhibits large anisotropic conduction along in-plane directions. An aqueous solution of PEDOT added with polystyrene sulfonate (PSS) was mixed with an insulating oxide nanosheet based on montmorillonite (MMT) dispersed in aqueous media. Structural studies demonstrated it to be a hybrid film with a lamellar-like assembly that was structured with mono-layered sheets. In addition, an inhomogeneous mixture of pristine MMT (insulating) and PEDOT-rich (conductive) lamellar grains was proposed. The ratio between these two grains was controlled by tuning the content of the PEDOT:PSS solution in the initial mixing process. Increases in the content of PEDOT increased the electrical conductivity. This dependence is explained by a percolation model with random arrangements of PEDOT-rich conductive and insulating grains. The conductivity showed large anisotropy between in-plane and out-of-plane measurements. The ratio reached almost 105 and remained the same over a wide range of temperatures. The lamellar structure of the PEDOT and nanosheets is ascribed to the large anisotropy.

Benzimidazolium is revealed to form quasi-isostructural crystals of mixed-valence molecular metal oxides that consist of [PMoV2MoVI10O40]5− and fully oxidized [BWVI12O40]5−. The structure and dielectric behaviour were compared and a dielectric anomaly was evident due to the electric dipole relaxation in the mixed-valence system.

A new salt—[NiII(DMSO)5(TTFPy)]2[α-SiW12O40] (1)—based on polyoxometalates was prepared by coordinating a cationic electron donor of pyridyltetrathiafulvalene (TTFPy) with NiII. Although the TTFPy molecule did not form a salt with the anionic α-[SiWVI12O40]4− because of the weak charge-transfer (CT) interaction, the coordination of Ni with the pyridyl moiety permitted salt formation driven by electrostatic interaction, giving a single crystal of 1. Crystallographic analysis, UV-vis and IR spectroscopy and electrochemical characterization revealed that the fully oxidized α-[SiWVI12O40]4− was crystallized with the neutral TTFPy moiety from the acetonitrile solution because of the low electron-withdrawing ability of α-[SiWVI12O40]4−, forming a brown–orange crystal. The crystal colour quickly turned to black by immersing in methanol, due to CT from TTF moiety to α-[SiWVI12O40]4−, which was caused by the solvent effect. Increase in the solvent acceptor number from 18.9 for acetonitrile to 41.3 for methanol resulted in the enhancement of the electron withdrawing ability of α-[SiWVI12O40]4− by 0.317 V in methanol.