Charge-transfer Crystals Research Articles

Laser-induced ferroelectric structural order in an organic charge-transfer crystal.

We report the direct observation by x-ray diffraction of a photoinduced paraelectric-to-ferroelectric structural phase transition using monochromatic 100-picosecond synchrotron pulses. It occurs in tetrathiafulvalene-p-chloranil, a charge-transfer molecular material in which electronic and structural changes are strongly coupled. An optical 300-femtosecond laser pulse switches the material from a neutral to an ionic state on a 500-picosecond time scale and, by virtue of intrinsic cooperativity, generates self-organized long-range structural order. The x-ray data indicate a macroscopic ferroelectric reorganization after the laser irradiation. Refinement of the structures before and after laser irradiation indicates structural changes at the molecular level.

Intercalation Effect of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimetane Having Strong Electron Affinity in Self-Assembled Monolayers Composed of Charge Transfer Complex Prepared by Coadsorption and Layer-by-Layer Adsorption Methods

Charge transfer (CT) complex self-assembled monolayers (SAMs) on a gold substrate are prepared using layer-by-layer adsorption and coadsorption methods with mercapto-methyl-tetrathiafulvalene (TTF-CH2SH) and strong acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), in comparison with TTF-CH2SH/TCNQ SAMs consisting of moderate acceptor TCNQ. The layer-by-layer method yields TTF-CH2SH/TCNQF4 SAMs with the coexistence of neutral TCNQF4 and anionic TCNQF4-, where the TCNQF4- anions are laid on the TTF layer with the TTF-CH2SH molecules standing perpendicular to the gold substrate. The neutral TCNQF4 molecules are stacked above the TCNQF4- anion layer. This is in contrast to the fact that TCNQ having intermediate strength of acceptor character does not yield any CT SAMs when using the layer-by-layer technique. In the coadsorption method, TTF-CH2SH/TCNQF4 SAMs are formed, where all the TCNQF4 molecules are completely reduced as TCNQF4- anions, similar to those in bulk TTF-TCNQF4 crystals. Taking into account that TTF-CH2SH/TCNQ SAMs have the same fractional value (0.6) of the degree of charge transfer to that of bulk TTF-TCNQ crystal, the coadsorption technique can reproduce the electronic structure of the bulk CT complex in the 2D SAMs. The coadsorbed SAMs have an intercalation structure, where acceptor molecules are intercalated into the interstitials of TTF-CH2SH/Au units with the molecular axes of both acceptor and donor molecules parallel to each other. Such a donor/acceptor molecular arrangement can provide a favorable situation in the charge transfer between the two ingredients, resulting in the similar electronic structure to that of bulk CT crystals.

Temperature and Pressure Effects on Intermolecular Interactions in Charge Transfer Crystals of 2-(4-Methoxyphenyl)-1,4-benzoquinone and Quinhydrone

Abstract Temperature and pressure effects on the Raman spectra due to the intramolecular vibrations of charge transfer crystals of 2-(4-methoxyphenyl)-1,4-benzoquinone and quinhydrone were studied. In quinhydrone crystals involving the intermolecular charge transfer and the intermolecular hydrogen bonding, the temperature- and pressure-induced frequency shifts of the intramolecular vibrations showed blue and red shifts with decreasing temperature and increasing pressure depending on the vibrational modes. In the yellow crystal of 2-(4-methoxyphenyl)-1,4-benzoquinone involving the intramolecular charge transfer, and in the red crystal involving both the intra- and intermolecular charge transfers, the frequencies of the intramolecular vibrations showed only blue shift with increasing pressure. This blue shift was larger in the yellow crystal than in the red crystal. It was also observed that phase transitions take place under 1.8 and 2.7 GPa in quinhydrone crystal and that the phase transitions take place under 1.5 and 2.5 GPa in the yellow and red crystals of 2-(4-methoxyphenyl)-1,4-benzoquinone, respectively.

A model of Photoinduced Regular-Dimerized Stack Transitions

We present a model of photoinduced regular-dimerized stack transitions and discuss theoretically the behavior of the organic molecular mixed-stack charge-transfer crystal, Tetrathiafulvalene-Chloranil (TTF-CA). We investigate both the photoinduced structural phase transitions and the thermal phase transition between the regular-stack neutral phase at high temperatures and the dimerized-stack ionic phase at low temperatures. Our model is based on a simple phenomenological insight that the molecules have three stable displacements in the crystal. The cooperative effect is accounted for by introducing harmonic coupling between the molecules. It is the first attempt to include not only the photo-excitation but also the temperature effect on an equal footing.

Pulsed-ESR Study of Spin Solitons as a Probe of Neutral-to-Ionic Phase Transition of TTF-CA Single Crystal

Ionic phase of charge-transfer crystal composed of tetrathiafulvalene (TTF) and p -chloranil (CA) has been studied by two-pulse electron-spin echo (ESE) at 4 K. By measuring the angular dependence of the field-swept ESE spectrum, at least three kinds of paramagnetic centers have been resolved, which are presumably ascribed to spin solitons formed at domain boundary. With varying the time separation between the microwave pulses, envelope modulations due to anisotropic hyperfine interaction have been detected. By analyzing the modulation, we have firstly obtained ENDOR spectrum of the spin solitons. Response of the ESE intensity for the optical excitation has been also examined.

Pressure-driven neutral-ionic transition in ClMePD-DMeDCNQI

Application of about 0.8 GPa pressure is sufficient to induce the neutral-ionic transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-$p$-phenylenediamine--2,5-dimethyl-dicyanoquinonediimine ({\CD}). The ionicity increases continuously from $\sim$ 0.35 at ambient conditions to $\sim$ 0.65 when the pressure is raised up to 2 GPa. Moreover, stack dimerization begins well before the crossing of the neutral-ionic interface. The evolution of the transition is similar to what observed in the temperature induced phase change in the same compound. (cond-mat/0101179) A distinguishing feature is represented by the simultaneous presence of domains of molecules with slightly different ionicities across the transition pressure. A comparison of the present example of pressure driven neutral-ionic transition with the well studied cases of tetrathiafulvalene--chloranil and of tetrathiafulvalene--2,5-dichloro-p-benzoquinone puts in evidence the remarkably different evolution of the three transitions.

Theory for photoinduced structural phase transitions

A general concept for photoinduced structural phase transitions is developed in terms of the hidden multistability of the ground state and the proliferations of optically excited states. Taking the ionic→neutral (I - N) phase transition in the organic charge transfer crystal, TTF—CA, as a typical example for this type of transition, we, at first, theoretically show an adiabatic path of this transition, which starts from a single charge transfer exciton in the ionic phase, but finally reaches a neutral domain with a macroscopic size. In connection with this I—N transition, the concept of the initial condition sensitivity is also developed so as to clarify experimentally observed nonlinear characteristics of this material. In the next, using a more simplified model for the many-exciton system, we theoretically study the early time quantum dynamics of the exciton proliferation, which finally results in the domain formation of a large number of ex-citons. For this purpose, we derive a stepwise iterative equation to describe the exciton proliferation, and clarify the origin of the initial condition sensitivity.

A new type of neutral–ionic interface in mixed stack organic charge transfer crystals: Temperature induced ionicity change in ClMePD–DMeDCNQI

Raman and polarized infrared spectra of the mixed stack charge transfer crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed spectral interpretation allows us to gain new insight into the temperature induced neutral-ionic transition in this compound. In particular, the crossing of the neutral-ionic borderline appears to be quite different from that of the few known temperature induced neutral-ionic phase transitions. First of all, the ionicity change is continuous. Furthermore, the onset of stack dimerization precedes, rather than accompanies, the neutral-ionic crossing. The (second order) phase transition is then driven by the dimerization, but the extent of dimerization is in turn affected by the ionicity change.

Preparation of a Mott insulator based on a BEDT-TTF charge transfer complex of hydrogen cyananilate: α′-(BEDT-TTF)2HCNAL

BEDT-TTF (ET) and cyananilic acid (H2CNAL) afforded charge transfer crystals by a diffusion method. The chemical formula of the complex was deduced based on all the information concerning the elemental, structural, electric, magnetic and optical analyses. The ET molecules form one-dimensional columnar stacks composed of twisted dimers with a face-to-face overlap (α′-type stack). The acid works as an oxidant and is deprotonated to form the monoanion, HCNAL1−, which forms ribbon-like aggregation by means of hydrogen-bonds. The ribbons form anion layers that sandwich the ET layer. The crystal is semiconductive with a room temperature conductivity of 0.20–0.83 Scm−1 and activation energy of 0.15 eV along the stacking direction, though the band calculation by the extended Huckel method suggests a metallic nature, indicating strong electron-correlation in this system. The complex is a Mott insulator and its magnetic susceptibility is described by the one-dimensional S = 1//2 antiferromagnetic Heisenberg chain model with J/kB = −52 ± 3 K.

Diels−Alder Topochemistry via Charge-Transfer Crystals: Novel (Thermal) Single-Crystal-to-Single-Crystal Transformations

The solid-state [4+2] cycloaddition of anthracene to bis(N-ethylimino)-1,4-dithiin occurs via a unique single-phase topochemical reaction in the intermolecular (1:1) charge-transfer crystal. The thermal heteromolecular solid-state condensation involves the entire crystal, and this rare crystalline event follows topochemical control during the entire cycloaddition. As a result, a new crystalline modification of the Diels-Alder product is formed with a crystal-packing similar to that of the starting charge-transfer crystal but very different from that of the (thermodynamically favored) product modification obtained from solution-phase crystallization. Such a single-phase transformation is readily monitored by X-ray crystallography at various conversion stages, and the temporal changes in crystallographic parameters are correlated with temperature-dependent (solid-state) kinetic data that are obtained by 1H NMR spectroscopy at various reaction times. Thus, an acceleration of the solid-state reaction over time is found which results from a progressive lowering of the activation barrier for cycloaddition in a single crystal as it slowly and homogeneously converts from the reactant to the product lattice.

Time-resolved electron spin resonance detection of the mobile triplet exciton in a charge-transfer crystal

The time-resolved electron spin resonance (TR-ESR) method has been applied to the mobile triplet excited state of anthracene-dimethylpyromellitimide (A/DMPT) charge-transfer crystals at room temperature. The spin Hamiltonian parameters have been determined to be D=0.0693 cm −1, E=0.0081 cm −1 and g=2.0023, respectively. The line-shape analysis in the single- crystal experiments shows Loretnzian line shape as a result of the motional narrowing (the narrow line-width of Δ H msl=0.3 mT), which means the observed triplet state originating from the mobile triplet exciton. The relative populations (polarization) of each M S sublevel in the triplet state have been determined to be P X =0.6, P Y =0.4, P Z =0.0 from the spectral simulation. The TR-ESR method detects the strongly polarized triplet state directly, giving the intrinsic nature of the triplet state as well as the time profiles and its spin dynamics.

水素結合 分子結晶における電子‐プロトン連動と機能性

The cooperation of electron and proton in organic systems has a large potentiality for the creation of new molecular functions in molecular crystals. In this article, the electron-proton-cooperation in charge transfer crystals is reviewed, particularly focusing on the it spectroscopy of the phase transition in quinhydrone charge-transfer crystals under high pressures. As an example of the electron-proton cooperation at ambient pressure, the metal-insulator phase transition observed in [Pd (H2-xEDAG) (HEDAG) ]TCNQ (EDAG= ethylenediamino-glyoxime, x = ca. 0. 7) is reported.

Electrophilic aromatic nitrosation. Isolation and X-ray crystallography of the metastable NO+ complex with nitrosoarene

Isolation of the unstable 1∶1 complex of 4-nitrosoanisole with NO+PF6− allows its precise X-ray structural characterization. The charge-transfer crystal is formed via strong N⋯N coordination [the distance of 1.938(5) Å corresponding to a σ-bond order of ≈0.2] in the mean plane of the planar 4-nitrosoanisole donor. Thorough analysis of its molecular geometry in terms of valence resonance and MO schemes reveals a strong charge polarization with a local negative charge localized on the nitroso group and a local positive charge distributed over the adjacent p-methoxybenzyl moiety. Such a charge distribution accommodates the well-known passivation of nitrosoarenes to multiple nitrosation and explains the ease of demethylation of the complex. Comparison of a variety of nitroso- and nitroarene structures has shown that the nitrosoarene experiences a much stronger quinoidal distortion of the aromatic ring as compared with the latter. This indicates a stronger electron-withdrawing effect of the nitroso group relative to that of the nitro group. The weakened aromatic resonance in the nitrosoarenes could be responsible for the observed slower rate and the measurable isotope effect in electrophilic nitrosation as opposed to nitration.

Picosecond fluorescence analysis of charge transfer microcrystals by near-field microspectroscopy

A picosecond near-field microspectroscopy system was developed and applied to fluorescence analysis of charge transfer (CT) microcrystals. Fluorescence spectra and decay curves of anthracene/tetrachlorophthalic anhydride/pyromellitic dianhydride (anthracene/TCPA/PMDA) mixed microcrystals were measured with a resolution of ∼140 nm. Sub-μm-sized domain structures were observed, and fluorescence spectra and decay curves were different in the domains. Some domains with a common fluorescence spectrum but with different decay curves were observed, indicating an inhomogeneous distribution of quenching sites. New features of inhomogeneity of CT crystals are presented and considered.

Photoreactivities of Donor-Acceptor Crystals between 3,5-Dinitrobenzoic Acid and N-Alkylcarbazoles

A variety of 1: 1 charge-transfer (CT) crystals were prepared by using mainly dinitrobenzoic acids (as the acceptor) and carbazoles (as the donor). Most of the CT crystals prepared from 3,5-dinitrobenzoic acid (3) and a series of N-alkylcarbazoles (1a−1h) underwent photoredox reactions initiated by the excited nitro group, leading to α-oxidation of N-alkyl groups (13−15). The reactions were much more efficient in the solid state than in solution. These solid-state photoreactivities could be, on the whole, correlated with the C···O distances between the carbazole N-α-carbon atom and the nitro oxygen atoms nearby.

CONDENSATION OF SELF-TRAPPED CHARGE-TRANSFER EXCITATIONS AND INTERPLAY BETWEEN QUANTUM AND THERMAL EFFECTS AT THE NEUTRAL-TO-IONIC TRANSITION

The cooperativity between self–trapped electronic excitations is carried to extreme in the case of the neutral–to–ionic phase transition which is observed in some organic mixed–stack charge–transfer crystals. This electronic–structural phase transition manifests itself by a change of the degree of charge–transfer and a dimerization distorsion along the stacking axis in the ionic phase. Thermal charge–transfer excitations associated with the formation of structurally relaxed ionic strings along neutral chains are at the heart of mechanism of this uncommon phase transition. Symmetry and thermodynamics analysis of the neutral–to–ionic transition in the prototype compound, tetrathiafulvalene–p–chloranil, in particular the recent determination of the pressure–temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by a phenomenological approach, taking into account the quasi–one–dimensional nature of the system and the interplay between quantum and thermal effects, the experimental results show that the neutral–to–ionic transition results from the condensation and the ordering (crystallization) of charge–transfer excitations, following a phase diagram analogous to the solid–liquid–gas one.

水素結合における構造物性と機能 水素結合と電子状態

In this report, the cooperation of electron and proton in organic-inorganic hybrid systems is discussed.Particularly, the experimental results of the X-ray diffraction measurements on the 1-D metallic charge-transfer crystal of [Pd (H2-xEDAG) (HEDAG) ] TCNQ (EDAG=ethylenediaminoglyoxime, x=ca. 0.7) were focused. Below the metal-insulator (M-I) phase transition (180 K), commensurate superlattice reflections were observed with a period of c*/2, c*/3, and c*/12 along the 1-D axis, c. The full X-ray diffraction analyses of temperature-dependent reflections ignoring the superlattice spots show remarkable anomalies of the temperature factors below 180 K for several atoms which associate with the intermolecular H-bonds between donor-molecules. This is a result of cooperation between the order-disorder transition of the 1-D H-bond intermolecular superlattice and the M-I transition of the 1-D electronic state in TCNQ acceptor stacks. These experimental results support a model of purely 1-D metallic hole-conduction introduced by partial deprotonation of H-bonds in the ligands of [Pd (H2-xEDAG) (HEDAG) ] .

Simple model of neutral-ionic transition in organic charge-transfer crystals

Electron-intramolecular vibration coupling is shown to be an important factor for understanding neutral-ionic transition in organic molecular crystals composed of mixed stacks of donor (D) and acceptor (A) molecules. Molecular dimer DA and trimer DAD are discussed as models of charge-transfer compounds with 1:1 and 2:1 compositions, respectively.

Temperature-induced neutral-ionic transition in 2-chloro-5-methyl-p-phenylenediamine-2,5-dimethyl-dicyanoquinonediimine

A mixed-stack organic charge-transfer crystal of 2-chloro-5-methyl- $p$-phenylenediamine-2,5-dimethyl dicyanoquinonediimine (ClMePD-DMeDCNQI), which undergoes a temperature-induced neutral-ionic transition (TINIT), is reported. From infrared spectroscopy, the molecular ionicity (\ensuremath{\rho}) of ClMePD-DMeDCNQI was found to change continuously from \ensuremath{\rho}=0.3 at 300 K to \ensuremath{\rho}=0.6 at 100 K. The neutral-ionic interface was crossed without a discontinuity in \ensuremath{\rho} at around 200 K, accompanied by stack dimerization. These features were quite different from the discontinuous change in \ensuremath{\rho} observed for the TINIT of tetrathiafulvalene-$p$-chloranil (TTF-CA) and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The transition characteristics in ClMePD-DMeDCNQI are discussed by drawing a comparison with those seen in TTF-CA and TMB-TCNQ.

Electronic states and anti-ferromagnetic order in mixed-stack charge-transfer compound (BEDT-TTF)(F 2TCNQ)

Crystal structure, optical and magnetic properties are reported for a unique class of donor-acceptor charge-transfer (CT) complex (BEDT-TTF)(F 2TCNQ). Charge-transfer from the donor to the acceptor is complete, forming an ionic mixed-stack CT crystal. “Side-by-side” interaction between BEDT-TTF molecules results in a one-dimensional (1D) correlated electronic system that is perpendicular to the donor-acceptor mixed-stack column. Consequently, F 2TCNQ radical spins are strongly isolated from neighboring molecules, showing Curie-like behavior in magnetic susceptibility, followed by an anti-ferromagnetic order at around T N = 30 K.