Crystals Of Charge-transfer Complexes Research Articles

Intercalation-Induced Ordered Nanostructure in the Interlayer Modified with Methylviologen by Molecular Dynamics Simulation.

Intercalation of phenol in montmorillonite, a representative layered material, has historically been investigated, and modification of the interlayer with methylviologen (Mont-MV) results in color change due to formation of a charge-transfer complex. Its detailed nanostructure, however, has yet been revealed owing to its small gallery height and poor crystallinity. In the present study, we performed molecular dynamics simulation to investigate structural changes in Mont-MV by the intercalation of phenol. The value of the basal spacing of Mont-MV was well consistent with that reported experimentally, and the MV cations were distributed horizontally. Successive intercalation of phenol revealed that the interlayer swelled nonlinearly and both the MV cations and phenol molecules were tilted, which were roughly parallel to each other. The obtained ordered nanostructure was similar to that reported in the charge-transfer complex crystal of the MV cation and the naphthol derivative. Thus, the parallel orientation in the interlayer was found to be the key for the color reaction. Combined with the fact that the phenol molecules interacted with the Mont layers, the role of the MV cation was found to be that of a pillar for providing sufficient gallery height, and the formation of the charge-transfer complex is the secondarily derived function.

Organic single crystals of charge-transfer complexes: model systems for the study of donor/acceptor interactions.

The charge-transfer (CT) state arising as a hybrid electronic state at the interface between charge donor and charge acceptor molecular units is important to a wide variety of physical processes in organic semiconductor devices. The exact nature of this state depends heavily on the nature and co-facial overlap between the donor and acceptor; however, altering this overlap is usually accompanied by extensive confounding variations in properties due to extrinsic factors, such as microstructure. As a consequence, establishing reliable relationships between donor/acceptor molecular structures, their molecular overlap, degree of charge transfer and physical properties, is challenging. Herein, we examine the electronic structure of a polymorphic system based on the donor dibenzotetrathiafulvalene (DBTTF) and the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) in the form of high-quality single crystals varying in the donor-acceptor overlap. Using angle-resolved photoemission spectroscopy, we resolve the highest occupied molecular orbital states of the CT crystals. Analysis based on field-effect transistors allows us to probe the sub-gap states impacting hole and electron transport. Our results expand the understanding on the impact of donor and acceptor interactions on electronic structure and charge transport.

Bundle assemblies formation of alternating copolymer: Alternating copolymerization of symmetrical substituted p-quinodimethanes with asymmetrical N,7,7-tricyanoquinone methide imine in solid state

Symmetrical 7,7,8,8-tetrakis(alkoxycarbonyl)-p-quinodimethanes with methoxy(1a(Me)) and ethoxy(1b(Et)) as alkoxy groups formed 1:1 charge-transfer complex crystals with an asymmetrical quinoid compound, N,7,7-tricyanoquinone methide imine (2). The 1:1 formation of the crystals was ascertained on the basis of 1H NMR spectroscopy and single crystal X-ray structure analysis. These 1:1 charge-transfer complex crystals (1a(Me)/2 and 1b(Et)/2 monomer cocrystals) underwent photochemical and thermal polymerizations via a radical mechanism with keeping initial crystal shapes in appearance to form stereoregular alternating copolymers in quantitative yields. The stereoregular alternating copolymers formed bundle assemblies in the less order state. The formation of stereoregular alternating copolymers was explained on the basis of the structures of the monomer cocrystals solved by X-ray crystallography. Less-ordered bundle assemblies are considered to be formed by findings that the gravity of each component is not aligned on a straight line within the one-dimensional column with alternating stacked arrangements of 1a(Me) or 1b(Et) and 2 in the crystals, which are caused by asymmetrical 2.

Photoconductive Properties of Dibenzotetrathiafulvalene-Tetracyanoquinodimethane (DBTTF-TCNQ) Nanorods Prepared by the Reprecipitation Method.

Charge-transfer complex crystals have been extensively studied because of their metallic conductivity, photoconductivity, ambipolar charge transport, and high career mobility. Numerous studies of their applications for organic electric devices such as organic field effect transistors and solar cells have reported. However, bulky single crystals of charge-transfer complexes are difficult to handle, specifically to be made into a form of a thin film. Recently, nano/micro crystallization of charge-transfer crystal is attracted to realize thin film applications. In this paper, charge transfer complex nanorods composed of dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) were prepared by the reprecipitation method. The as-formed nanorods possess a kinetically metastable crystal structure different from the thermodynamically stable bulk crystal prepared by slow evaporation of the solvent. From photoconductive measurement, nanorod stacks show a significant photosensitivity (354.57 μA/W) on par with bulk crystal (417.14 μA/W). These results suggest dibenzotetrathiafulvalene-tetracyanoquinodimethane (DBTTF-TCNQ) nanorods have a favorable crystal structure for carrier transport due to the difference of molecular stacking assembly.

Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order–Disorder Phase Transitions

Crystals of charge-transfer (CT) complexes were synthesized using the polar molecule tetrachlorophthalonitrile (TCPN) as the electron acceptor and nonpolar aromatic hydrocarbons, such as perylene, coronene, chrysene, and pyrene, as the electron donors. Variable-temperature X-ray crystal structure analyses revealed that the TCPN molecules in all the CT crystals exhibit orientational disorder at room temperature. Some of the CT crystals undergo an order–disorder-type phase transition upon cooling, where the orientation of the TCPN molecules becomes ordered at low temperature. The CT crystals show large dielectric constants at room temperature arising from in-plane reorientation of the polar TCPN molecules. The order–disorder phase transition results in a drastic reduction of the dielectric constant of the CT crystals upon cooling. This study demonstrates that the formation of CT crystals from polar molecules represents a promising method for the development of dielectric crystalline materials, whose propert...

Nanocrystallization Process, Structure, and Properties of Charge-Transfer Complex Prepared by the Reprecipitation Method

Charge-transfer (CT) complex crystals composed of organic electron donor (D) and acceptor (A) have been extensively studied because of their attractive properties such as metallic conductivity, photoconductivity, ambipolar charge transport, and high career mobility. Numerous research efforts have been made for their use in devices such as organic field effect transistors and organic photocells. However, bulky single crystals of CT complexes are difficult to handle, specifically to be made into a form of a thin film needed for solar cell applications. Nano/micro crystallization is a promising path to overcome the above mentioned problem. However, the growth of high-quality nanocrystals is imperative for photoconductivity but often hampered, since CT complexes are poorly soluble and vapor pressures of donor and acceptor molecules are imbalanced[1-2]. Here we presente the “reprecipitation method” as facile synthesis route to obtain high quality organic CT complex nanocrystals. The process of nanocrystallization can be explained in four steps[3]; 1) A small amount of a solution of the target compound in its good solvent is injected into a large amount of its poor solvent under vigorous stirring. 2) The tiny droplets formed by the injection diffuse into the poor solvent as the two solvents are chosen to be miscible. 3) Nucleation and crystal growth of the target compound proceed due to its supersaturation at the boundary of good/poor solvents. 4) As the good solvent is completely diffused, nanocrystals are stably dispersed in the poor solvents. All these events are expected to occur in a short time (within tens of milliseconds[4]) and thus nanocrystals with kinetically stable structures are often obtained, which are different from thermodynamically stable single bulk crystal structures[5]. In this work, we have successfully applied the reprecipitation method to dibenzotetrathiafulvalene (DBTTF)-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF)-TCNQ to obtain its nanocrystal. The DBTTF-TCNQ nanocrystals were prepared by typical reprecipitation method from D-A mixed solution. The nanocrystals prepared in such a way have been found to possess kinetically stable crystal structure, different from the bulk material. Moreover, a direct current device employing a polycrystalline thin film of the nanocrystals prepared by simple filtration an equivalent photosensitivity (354.57 μA/W) with that of a bulk crystal (417.14 μA/W) despite of the presence of many grain boundaries, indicating a favorable molecular stacking in the nanocrystals for career transport. In addition, the TTF-TCNQ nanocrystals were prepared by two-step reprecipitation method. TTF (TCNQ) solution was injected to TCNQ (TTF) nanoparticle water dispersion. The TTF-TCNQ nanocrystals were formed when the nanoparticles reacted and the dispersion color was changed immediately from yellow to black. Details of morphology, characteristic and crystallization process of the nanocrystals will be discussed in presentation. Reference [1] M. Hiraoka, T. Hasegawa, T. Yamada, Y. Takahashi, S. Horiuchi, and Y. Tokura, Adv. Mater. 19, 3248 (2007). [2] Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, and G. Saito, Appl. Phys. Lett. 88, 073504 (2006). [3] K. Shito, N. Ito, and A. Masuhara, Jpn. J. Appl. Phys. 54, 06FK05-1 (2015). [4] J. Mori, Y. Miyashita, D Oliveira, H. Kasai, H. Oikawa, and H. Nakanishi, J. Cryst. Growth 311, 553 (2009). [5] L. Huang, Q. Liao, H. Fu, J. Ma, and J. Yao, J. Mater. Chem. 20, 159 (2010).

A New Strategy to Construct Functional Porous Crystals by Mixed Crystallization through Charge-transfer Interactions

An organic salt composed of triphenylmethylamine (TPMA) and 2-(4-sulfophenyl)anthraquinone (2-SPAQ) provides a porous crystal with 1D channel void spaces surrounded by anthraquinone. We succeeded in introducing fluorescence property into the porous crystal by mixed crystallization with a different salt composed of TPMA and 2-(4-sulfophenyl)anthracene (2-SPA) through charge-transfer (CT) interactions. As a result, the porous CT complex crystals showed a unique vapochromism for various volatile solvents.

Development of a Control Method for Conduction and Magnetism in Molecular Crystals

AbstractThis study concerns development of a non-destructive method to control conduction and magnetism of molecular solids such as single crystals of charge-transfer complexes. The method is named “optical doping”, where appropriate irradiation is utilized under ambient conditions. Owing to this feature, it can be applied to a wide range of substances while measuring the properties during the control. In addition, the method adds unique conduction and magnetic properties to common insulators. Unlike other doping methods, optical doping only affects the properties and/or structures of the irradiated part of a sample while leaving the rest of the sample unchanged. There are two patterns in the optical doping. Irreversible optical doping produces junction-structures on the single molecular crystals, which exhibit characteristic behavior of semiconductor devices such as diodes and varistors. Reversible optical doping produces “giant photoconductors” and “photomagnetic conductors” by realizing unprecedented metallic photoconduction. In the latter case, localized spins are also excited to produce a Kondo system, where carriers and localized spins interact with each other. Not only the control of conduction and magnetism, the optical doping has realized the observation of physical properties in molecular crystals hardly observed under any thermodynamic condition.

Fabrication of Conducting Thin Films on the Surfaces of 7,7,8,8-Tetracyanoquinodimethane Single-Component and Charge-Transfer Complex Single Crystals: Nucleation, Crystal Growth, Morphology, and Charge Transport

Electrically conducting TTF–TCNQ thin films are fabricated on various molecular crystals containing 7,7,8,8-tetracyanoquinodimethane (TCNQ) by exposing a tetrathiafulvalene (TTF) vapor under ambient conditions. To systematically investigate the properties of the films, mixed-stack TCNQ charge-transfer (CT) complex crystals with nine kinds of donors have been prepared as the substrates, and the morphology change of the films on the surfaces at the initial stage of the TTF vapor contact has been observed. When the substrate is a TCNQ single-component crystal, randomly oriented TTF–TCNQ nanometer-size needle crystals are grown by the reaction with a TTF vapor. However, when the substrate is a TCNQ CT complex crystal, TTF–TCNQ crystals are grown with alignment of their needle axis along the mixed-stack direction of the substrate. The surface roughness, the size of the needle crystals, and the degree of the dense packing of the needles have been found to systematically depend on the strength of the CT interact...

Ionic liquid-assisted growth of DBTTF–TCNQ complex organic crystals by vacuum co-deposition

High-crystalline DBTTF–TCNQ charge-transfer complex crystals with larger grains visible even by an optical microscope have been successfully grown, assisted by ionic liquid (IL) in vacuum co-deposition. Although the charge transfer reaction between the DBTTF and TCNQ molecules was ready to occur to form the complex regardless of the presence or absence of the IL even at room temperature, the subsequent crystal growth of the DBTTF–TCNQ complexes was enhanced by the IL, especially much more at temperatures higher than room temperature, leading to a significant improvement in the crystallinity of the complexes. The crystal growth mechanism of the DBTTF–TCNQ complexes in the IL was discussed based on the results of in situ optical microscope observation during the deposition of the DBTTF and TCNQ molecules into the IL.

Quantum ferroelectricity in charge-transfer complex crystals.

Quantum phase transition achieved by fine tuning the continuous phase transition down to zero kelvin is a challenge for solid state science. Critical phenomena distinct from the effects of thermal fluctuations can materialize when the electronic, structural or magnetic long-range order is perturbed by quantum fluctuations between degenerate ground states. Here we have developed chemically pure tetrahalo-p-benzoquinones of n iodine and 4–n bromine substituents (QBr4–nIn, n=0–4) to search for ferroelectric charge-transfer complexes with tetrathiafulvalene (TTF). Among them, TTF–QBr2I2 exhibits a ferroelectric neutral–ionic phase transition, which is continuously controlled over a wide temperature range from near-zero kelvin to room temperature under hydrostatic pressure. Quantum critical behaviour is accompanied by a much larger permittivity than those of other neutral–ionic transition compounds, such as well-known ferroelectric complex of TTF–QCl4 and quantum antiferroelectric of dimethyl–TTF–QBr4. By contrast, TTF–QBr3I complex, another member of this compound family, shows complete suppression of the ferroelectric spin-Peierls-type phase transition.

Novel method for the fabrication of a charge-transfer complex crystal by photoirradiation.

A novel method for the fabrication of a charge-transfer complex crystal was developed. Photoirradiation of a solution of TPP[Co(tbp)(CN)(2)] and TPP[Co(Pc)(CN)(2)] (tbp=tetrabenzoporphyrin, Pc=phthalocyanine, TPP=tetraphenylphosphonium) gave a molecular conducting crystal of a charge-transfer complex TPP[Co(tbp)(CN)(2)](2), which was produced by the process in which the photoexcited electron in tbp was transferred from the LUMO of tbp to that of Pc.

Varied charge-transfer complex crystals formed between diols and benzoquinone in the solid and solution states

Co-grinding of crystals of diols (electron donor) and benzoquinone (BQ, electron acceptor), and in some cases with crystals of a third aromatic compound, produces charge transfer (CT) complex crystals in the solid state without going through an obvious amorphous state, followed by a change in colour and in powder X-ray diffraction patterns. A milling power above a threshold is required for the new CT complex crystal formation, however, in some cases simple mixing of pulverized crystals is good enough to initiate the transformation. On exposing the CT complex crystals to air, without any mechanochemical force, they revert to the starting diol crystals by losing sublimable BQ molecules, rearranging hydrogen bonds and establishing a three-dimensional periodicity. Remarkably, solid-state and conventional solution crystallization give different results, i.e., formation or non-formation of CT complex crystals, or formation of different crystals with different colours. The phenomenon is surprisingly frequent and all three two-component CT systems and six out of the nine three-component systems investigated, exhibit different crystallization behaviour. A quartet structure where two BQ molecules are sandwiched by two naphthols was formed in the crystal obtained from solid-state crystallization. A new triplet CT complex structure in which a BQ is sandwiched by a naphthol and a biphenyl is also reported.

Modulation of the Structure and Electronic Density of Molecular Chains on Organic Conductor Surfaces

We used scanning tunneling microscopy (STM) to observe the bc-surface of the triethylammonium-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)2)charge-transfer complex crystal, which is terminated with the arrays of TCNQ chains. Three kinds of distinct structures of the quasi-one-dimensional (1D) TCNQ chains were found on different surface terraces, that is, the TCNQ molecules within a chain exhibit the monomerization, tetramerization, and octamerization, respectively. Each structure corresponds to a respective electron density distribution along the chain direction. The latter two reflect the different characteristics of the bulk and surface energy-band filling, respectively. The monomer structure indicates that the single molecular orbital was probed by STM due to the weakening of the overlapping between TCNQ molecules induced by the enlarging of the intermolecular distance. The two-dimensionality of the surface exerts an apparent influence on the quasi-1D TCNQ chains, with the specific manifestation of enhancing the interchain coupling even at the cost of the weakening of the intrachain interaction.

Order−Disorder Transition of Nonplanar Molecules and Dielectric Anomaly in a Crystal of Charge-Transfer Complex

Order−Disorder Transition of Nonplanar Molecules and Dielectric Anomaly in a Crystal of Charge-Transfer Complex

The contributions of chalcogen to the peierls instability in model crystals of charge-transfer complexes

We have already proposed a convenient method to estimate the magnitude of the Peierls instability in molecular crystals. In the present paper to examine the reliability of our treatment, we apply it to the two types of model crystals composed of naphthalene cations and chalcogen substituted ones (planar-1,4,5,8-tetrathianaphthalene [TTN] +). The results show that the chalcogen atoms in TTN molecules play an important role in the electronic conductivity. It was demonstrated that this treatment could reproduce ab initio results and become a useful tool to investigate the relationship between the crystal structure of charge-transfer complexes and their electronic conductivity.

Molecular orbital approach to the Peierls instability in polyenes and its application to model crystals of charge-transfer complexes

Molecular orbital approach to the Peierls instability in polyenes and its application to model crystals of charge-transfer complexes

Molecular movement and chiral arrangement in the solid state

Abstract

Crystal Structures and Physical Properties of 1,6-Diaminopyrene-p-chloranil (DAP-CHL) Charge-Transfer Complex. Two Polymorphs and Their Unusual Electrical Properties

Abstract A combination of 1,6-diaminopyrene and p-chloranil gives two kinds of 1 : 1 charge-transfer complex crystals from a benzene solution. The α-form is triclinic, space group , a = 8.963(1), b = 9.112(2), c = 6.730(1) Å, α = 99.59(1), β = 99.65(1), γ = 114.43(1)°, V = 476.0(1) Å3, and Z = 1; the β-form is also triclinic, space group , a = 8.123(3), b = 9.708(3), c = 6.791(2) Å, α = 110.73(2), β = 108.89(2), γ = 79.11(3)°, V = 472.3(3) Å3, and Z = 1. In both crystals, the donors and acceptors are neutral, and stack alternately. The main difference between the α- and β-forms is in the overlapping mode. Both crystals are electrically insulating at the initial states. The α-form single crystal, however, undergoes and irreversible conductivity change upon mild heating (< 380 K) or fracture of the crystal into powder. In both cases, the conductivity change amounts to 106 to 108, and the resultant solids are highly conducting semiconductors. Structural, optical, magnetic, and solid-state NMR studies suggest that the low-resistance state mainly comprises neutral molecules, despite its high conductivity. The resistivity of β-DAP-CHL also decreases upon heating to above 400 K, while fracture of the crystal has no effects. The resistivity change in β-DAP-CHL is considered as simple ionization of a part of the components.

High-resistance phase and low-resistance phase in 1,6-diaminopyrene-p-chloranil (DAP-CHL)

Abstract A combination of DAP and CHL gives two kinds of 1:1 charge transfer complex crystals from the benzene solution. Both of them are electrically insulating at the initial states. The α-form single crystal, however, undergoes an irreversible conductivity change upon mild heating or fracture of the crystal. In both cases, the conductivity change amounts to 10 6 to 10 7 , and the resultant solids are a highly conducting semiconductor. The structural, optical, magnetic, and solid-state NMR studies suggest that the low-resistant phase is mainly composed of the neutral molecules despite its high conductivity.