Mixed-stack Charge-transfer Crystals Research Articles

Relaxor ferroelectricity in the polar M2P-TCNQ charge-transfer crystal at the neutral-ionic interface

We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (M$_2$P-TCNQ), which is polar at room temperature and just at the neutral-to-ionic interface (ionicity $\rho \approx 0.5$). We detect the typical dielectric signature of a relaxor ferroelectric and an asymmetric positive-up-negative-down behavior. While relaxor ferroelectricity is usually ascribed to disorder in the crystal, we find no evidence for structural disorder in the investigated crystals. To elucidate the origin of M$_2$P-TCNQ's dielectric properties we perform parallel structural and spectroscopic measurements, associated with theoretical modeling and quantum-mechanical calculations. Our combined effort points to a highly polarizable electronic system that is strongly coupled to lattice vibrations. The found indications for polarization reversal imply flipping of the bent conformation of the M$_2$P molecule with an associated energy barrier of a few tens of an eV, broadly consistent with an Arrhenius fit of the dielectric relaxation times. While the polarization is mostly of electronic origin, its possible reversal implies slow collective motions that are affected by solid-state intermolecular interactions.

Erratum: Towards first-principles prediction of valence instabilities in mixed stack charge-transfer crystals [Phys. Rev. B 95 , 155125 (2017)

Received 21 October 2018DOI:https://doi.org/10.1103/PhysRevB.98.199901©2018 American Physical SocietyPhysics Subject Headings (PhySH)TechniquesDensity functional theoryHubbard modelCondensed Matter, Materials & Applied Physics

Carrier Charge Polarity in Mixed-Stack Charge-Transfer Crystals Containing Dithienobenzodithiophene

Dithieno[2,3- d;2'3'- d']benzo[1,2- b;4,5- b']dithiophene forms mixed-stack charge-transfer complexes with fluorinated tetracyanoquinodimethanes (F nTCNQs, n = 0, 2, and 4) and dimethyldicyanoquinonediimine (DMDCNQI). The single-crystal transistors of the F nTCNQ complexes exhibit electron transport, whereas the DMDCNQI complex shows hole transport as well. The dominance of electron transport is explained by the superexchange mechanism, where transfers corresponding to the acceptor-to-acceptor hopping ( teeff) are more than 10 times larger than the donor-to-donor hopping ( theff). This is because the donor orbital next to the highest occupied molecular orbital makes a large contribution to the electron transport owing to the symmetry matching. Like this, inherently asymmetrical electron and hole transport in alternating stacks is understood by analyzing bridge orbitals other than the transport orbitals.

Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

We report extensive structural and spectroscopic characterization of four mixed stack charge-transfer (ms-CT) crystals formed by the electron donor 3,${3}^{\ensuremath{'}},5,{5}^{\ensuremath{'}}$-tetramethylbenzidine (TMB) with Chloranil (CA), Bromanil (BA), 2,5-difluoro-tetracyanoquinodimethane $({\mathrm{TCNQF}}_{2})$, and tetrafluoro-tetracyanoquinodimethane $({\mathrm{TCNQF}}_{4})$. Together with the separately studied TMB-TCNQ [Phys. Rev. B 95, 024101 (2017)] the TMB-acceptor series spans a wide range of degree of CT, from about 0.14 to 0.91, crossing the neutral-ionic interface, yet retaining similar packing and donor-acceptor CT integrals. First principle calculations of key phenomenological parameters allow us to get insight into the factors determining the degree of CT and other relevant physical properties.

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is the most spectacular manifestation of electronic ferroelectricity, in connection with a high degree of covalency between alternating donor and acceptor molecules along stacks. In addition, a charge-transfer instability from a quasi-neutral to a quasi-ionic state takes place concomitantly with the stack dimerization, which breaks the inversion symmetry. Moreover, these systems exhibit exceptional one-dimensional fluctuations, with an enhancement of the effects of electron-lattice interaction. This may lead to original physical pictures for the dynamics of pre-transitional phenomena, as the possibility of a pronounced Peierls-type instability and/or the generation of unconventional non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linear responsiveness. These different aspects will be discussed through the structural and dynamical features of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and conclude by their interplay with the photo-induced dynamics.

Towards first-principles prediction of valence instabilities in mixed stack charge-transfer crystals

Strongly correlated electrons delocalized on one-dimensional (1D) soft stacks govern the complex physics of mixed stack charge-transfer crystals, a well-known family of materials composed of electron-donor (D) and acceptor (A) molecules alternating along the 1D chain. The complex physics of these systems is well captured by a modified Hubbard model that also accounts for the coupling of electrons to molecular and lattice vibrational modes and for three-dimensional electrostatic interactions. Here we study several experimental systems to estimate relevant model parameters via density-functional theory calculations on DA units and isolated molecules and ions. Electrostatic intermolecular interactions, an important quantity not just to define the degree of charge transfer of the ground state but also to predict the propensity of the system towards multistability and hence towards discontinuous phase transitions, are also addressed. Results compare favorably with experimental data.

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π -electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by this class of crystals, Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer along the stack. The analysis of such a complex yet fascinating phenomenon has required many years of investigation, and still presents some open questions and challenges. We present an updated and extensive summary of the phenomenology of the temperature induced NIT, with emphasis on the spectroscopic signatures of the transition. A much shorter summary is given for the NIT induced by pressure. Finally, we report on the exploration, by chemical substitution, of the phase space of ...DADAD... CT crystals, aimed at finding materials with important semiconducting or ferroelectric properties, and at understanding the subtle factors determining the crystal packing.

Temperature-induced valence instability in the charge-transfer crystal TMB-TCNQ

The occurrence of so-called temperature-induced neutral-ionic transitions (TINIT) in mixed-stack charge-transfer crystals is quite rare. Here we reinvestigate one of the crystals which has been claimed to undergo such a transition, $3,{3}^{\ensuremath{'}},5,{5}^{\ensuremath{'}}$-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). Extensive optical data allow us to conclude that the transition should be classified as a valence instability, and not as a ``true'' TINIT, as the $\ensuremath{\sim}0.5$ neutral-ionic borderline is not crossed. The ionicity $\ensuremath{\varrho}$, or average charge at the molecular sites, indeed changes very little at the transition, from about 0.3 to about 0.4, and is accompanied by stack dimerization. The transition is first order with large hysteresis, and the crystal may crack or break. For this reason we have been unable to collect x-ray structural data on the low-temperature phase, but with the help of semiempirical calculations we are able to assess a plausible scenario for this peculiar phase transition and its mechanism.

Combining intra- and intermolecular charge-transfer: a new strategy towards molecular ferromagnets and multiferroics.

Organic ferroelectric materials are currently a hot research topic, with mixed stack charge transfer crystals playing a prominent role with their large, electronic-in-origin polarization and the possibility to tune the transition temperature down to the quantum limit and/or to drive the ferroelectric transition via an optical stimulus. By contrast, and in spite of an impressive research effort, organic ferromagnets are rare and characterized by very low transition temperatures. Coexisting magnetic and electric orders in multiferroics offer the possibility to control magnetic (electric) properties by an applied electric (magnetic) field with impressive technological potential. Only few examples of multiferroics are known today, based on inorganics materials. Here we demonstrate that, by decorating mixed stack charge transfer crystals with organic radicals, a new family of robust molecular ferromagnets can be designed, stable up to ambient temperature, and with a clear tendency towards multiferroic behaviour.

Prediction of Remarkable Ambipolar Charge-Transport Characteristics in Organic Mixed-Stack Charge-Transfer Crystals

We have used density functional theory calculations and mixed quantum/classical dynamics simulations to study the electronic structure and charge-transport properties of three representative mixed-stack charge-transfer crystals, DBTTF-TCNQ, DMQtT-F(4)TCNQ, and STB-F(4)TCNQ. The compounds are characterized by very small effective masses and modest electron-phonon couplings for both holes and electrons. The hole and electron transport characteristics are found to be very similar along the stacking directions; for example, in the DMQtT-F(4)TCNQ crystal, the hole and electron effective masses are as small as 0.20 and 0.26 m(0), respectively. This similarity arises from the fact that the electronic couplings of both hole and electron are controlled by the same superexchange mechanism. Remarkable ambipolar charge-transport properties are predicted for all three crystals. Our calculations thus provide strong indications that mixed-stack donor-acceptor materials represent a class of systems with high potential in organic electronics.

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.

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.

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.

Partial ionic ground state in the self-dimer mixed stack charge transfer crystal of TTF and TCNE

The vibrational spectra of the charge transfer complex of TTF with TCNE are explored. It is found that a degree of charge transfer of 0.5, namely that one electron has migrated from a dimer of TTF to that of TCNE molecules, characterizes the ground state of the complex. A Holstein-Hubbard model for a symmetric tetramer with two donor and two acceptor molecules allows the understanding of the vibronic bands observed in the infrared and Raman spectra and of the electronic charge transfer excitations.

Singlet and Triplet Energy Transfer in Tetracyanobenzene-Biphenyl Crystals

Singlet and triplet excitonic energy migration in mixed-stack charge-transfer crystals of tetracyanobenzene-biphenyl (TCNB-B) doped with anthracene (10−4-10−2 M/M) have been studied within temperature range 5-300 K by using optical, spectroscopic and kinetic techniques. It has been found, that the motion of excitons is thermally activated with the activation energies of 450 ± 60 and 550 ± 60 cm−1 for the singlet and triplet excitons, respectively. The later value differs from the activation energy in undoped TCNB-B crystals (where it is 360 ± 60 cm−1) and suggests that doping with anthracene perturbs the host crystal lattice.

Electronic energy transfer in the charge-transfer crystal of 2,3-dimethylnaphthalene-tetracyanobenzene

Optical, spectroscopic and kinetic studies of mixed-stack charge-transfer crystals of 2,3-dimethylnaphthalene with tetracyanobenzene (DMN-TCNB), undoped and doped with anthracene (concentrations 10 −5 − 2 × 10 −3 M/M), were performed between 5 and 300 K. The kinetic analysis of fluorescence and delayed fluorescence confirmed that excitons are self-trapped at low temperatures and they become mobile at higher temperatures. The obtained activation energies of mobility of singlet and triplet excitons are (380 ± 50) and (165 ± 30) cm −1, respectively. Triplet exciton motion (50–80 K range) is well described by the one-dimensional hopping model. Above 80 K, the intersystem crossing transition, T 1 → S 1, with the activation energy of (1450 ± 100) cm −1, becomes an efficient deactivation channel for triplet excitons. The experiments point to a possibility of structural disorder in DMN-TCNB crystals.

Mixed-stack charge-transfer films prepared by Langmuir-Blodgett technique and donor doping

The formation of mixed-stack charge-transfer (CT) films was studied. Spectroscopic analysis suggested that donor doping to a 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) film prepared by the Langmuir-Blodgett technique resulted in the formation of the mixed-stack CT film when 3,3′,5,5′-tetramethylbenzidine (TMB) was used as a donor, while degradation of the CT complex was observed when N,N,N′,N′-tetramethyl-p-phenylenediamine was used. Although X-ray diffraction measurements suggested that the order of the layered structure and crystallinity of the film were decreased by the doping, it was proved that the molecular columns of TMB and C18TCNQ were preferentially oriented along a substrate from polarized electronic spectra and scanning electron microscope observation. Therefore, non-linear electrical conduction as observed in mixed-stack CT crystals was expected. In the current density-electric field characteristic for the TMB-C18TCNQ film, the non-linearity was weaker than that observed in a TMB-TCNQ crystal. Furthermore, a temperature-induced neutral-ionic (N-I) phase transition was not observed from 4.2 K to room temperature in a spectroscopic measurement.

Two-step neutral-ionic phase transition in organic charge-transfer compounds: Possible staging effect.

A two-step neutral-ionic transition has been discovered in a 1:1 mixed-stack charge-transfer crystal, (3,3',5,5') tetramethylbenzidine-(7,7,8,8) tetracyanoquinodimethane (TMB-TCNQ) under pressure. The fraction of ionized molecules changes discontinuously at two critical pressures, P 1 ∼6 and P 2 ∼20 kbar, demonstrating a characteristic feature of a staging phenomena. This result is interpreted in terms of a frustration effect of Coulomb interactions in the crystal, as predicted theoretically by Hubbard and Torrance

Valence-bond analysis of half-filled Hubbard chains with long-range interelectronic interactions and on-site energy alternation.

The extended Hubbard model with unscreened electron-electron interactions, modified to account for on-site energy alternation, is investigated through diagrammatic valence-bond technique. Exact results on finite-size systems are carefully extrapolated to the infinite-size system. The coupling of electrons to adiabatic phonons modulating on-site and on-bond energies is accounted for. The calculations give information on intrinsic site-charge-density-wave systems (e.g., mixed-stack charge-transfer crystals) but also allows one to investigate the site-charge-density-wave regime of an extended Hubbard chain without on-site energy alternation in the presence of electron-phonon coupling.

The structural phase transition in anthracene—TCNB

Abstract The problem of the orientational disorder in mixed stack charge transfer crystals has been addressed. X-ray diffraction data in the HT phase of A-TCNB are reported. They were analysed using conventional structure-refinement procedures and utilizing symmetry-adapted functions. For the HT phase, the orientational probability function allowed us to characterize as dynamic the disorder of the anthracene molecules. The soft mode was measured by Raman and coherent neutron scattering. Measurement by Brillouin scattering of the elastic constant C11 led to another determination of the soft mode. The behaviour of this soft mode allows one to conclude the existence of a mixed regime (between displacive and order-disorder) for the LT phase of A-TCNB.