Electron-molecular Vibration Coupling Research Articles

Electron–molecular vibration coupling in trimerized isostructural mixed-stack complexes (EDT-TTF-I2)2TCNQFn (n = 0, 1, 2)

Infrared and Raman spectra of three isostructural charge transfer complexes (EDT-TTF-I2)2TCNQFn (n = 0, 1, 2) are studied. The planar molecules in these complexes are arranged in one-dimensional stacks formed by donor–acceptor-donor (DAD) centrosymmetric trimeric units with a different degree of charge transfer between D and A. In the IR electronic spectra two bands attributed to D-D and D-A charge transfer transitions are distinguished. In the IR vibrational spectra, numerous bands related to coupling of D and A intramolecular vibrations with electrons, are seen. The electron–molecular vibration coupling is clearly seen for acceptor modes attributed to CN, CC stretching, and CH bending as well as for CC stretching donor modes. These modes are investigated as a function of temperature (T = 300–10 K). The temperature effect is especially significant for the n = 1 complex which undergoes a neutral-to-ionic phase transition. In our study we show that the electron–molecular vibration coupling has also an influence on Raman spectra.

Structure, optical and electro-physical properties of tetramerized anion-radical salt (N-Xy-Qn)(TCNQ)2

The (N-Xy-Qn)(TCNQ)2 anion-radical salt characterized by tetramerized stacks of the TCNQ acceptor molecules has been synthesized and characterized using vibrational spectroscopy and electrical resistivity measurements. The bond lengths analysis based on the crystal structure data, indicates that the TCNQ molecules are non-uniformly charged with −0.83 e localized on the inner B molecules and −0.33 e on the outer A molecules within ABBA tetramers. Both infrared and Raman spectra of (N-Xy-Qn)(TCNQ)2 are dominated by vibrational modes of TCNQ and display splitting related to the tetramerized structure. Many of these features are affected by the strong electron-molecular vibration (EMV) coupling. Other charge-sensitive modes allowed estimation of charge localized on TCNQ, with the results that confirm the charges estimated on basis of the crystal data. Electrical measurements revealed the low-conducting behavior with room temperature conductivity value of 2.6 mS cm−1 and temperature dependence of resistivity that can be explained within the band conduction model. The calculated activation energies range from 0.169 eV to 0.187 eV, depending on the crystallographic direction and thermal history of the sample.

Metal-insulator phase transition in the δ−(BEDT−TTF)4 [2,6-anthracene- bis(sulfonate)] ·4H2O studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\ensuremath{\delta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}$[2,6-anthracene-bis(sulfonate)]$\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at ${T}_{\mathrm{MI}}=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\ensuremath{\cdots}\mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\ensuremath{\cdots}\mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of \ensuremath{\sim}24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Investigation on Single-Molecule Junctions Based on Current⁻Voltage Characteristics.

The relationship between the current through an electronic device and the voltage across its terminals is a current–voltage characteristic (I–V) that determine basic device performance. Currently, I–V measurement on a single-molecule scale can be performed using break junction technique, where a single molecule junction can be prepared by trapping a single molecule into a nanogap between metal electrodes. The single-molecule I–Vs provide not only the device performance, but also reflect information on energy dispersion of the electronic state and the electron-molecular vibration coupling in the junction. This mini review focuses on recent representative studies on I–Vs of the single molecule junctions that cover investigation on the single-molecule diode property, the molecular vibration, and the electronic structure as a form of transmission probability, and electronic density of states, including the spin state of the single-molecule junctions. In addition, thermoelectronic measurements based on I–Vs and identification of the charged carriers (i.e., electrons or holes) are presented. The analysis in the single-molecule I–Vs provides fundamental and essential information for a better understanding of the single-molecule science, and puts the single molecule junction to more practical use in molecular devices.

Temperature-Induced Neutral-Ionic Phase Transition in the (EDT-TTF-I2)2TCNQF Mixed-Stack Charge-Transfer Salt

We report the infrared (IR) and Raman study of the 2:1 mixed-stack charge transfer salt (EDT-TTF-I2)2TCNQF, where EDT-TTF = ethylenedithiotetrathiafulvalene and TCNQ = tetracyanoquinodimethane, which undergoes a temperature-induced neutral-ionic phase transition. Polarized infrared (IR) and Raman spectra of single crystals were measured in the 8–293 K temperature range. Temperature variations of vibrational modes that are assigned to both donors and acceptors show that the average degree of charge-transfer is growing continuously from about zero at room-temperature to about 1e at T = 8 K; nevertheless, at about 100 K a regime change is observed. The coexistence of molecular species of different ionicity is detected in the whole temperature range. The IR vibrational features due to electron-molecular vibrational coupling increase their intensities considerably upon cooling giving an evidence of relatively small gradual distortions of crystal structure. Existence of ferroelectric domains is suggested. A scheme of the neutral-ionic phase transition in a 2:1 complex is proposed and discussed.

Electron-molecular vibration coupling in (DMtTTF)Br and (o-DMTTF)2[W6O19] salts studied by vibrational spectroscopy

A novel 1:1 salt encompassing radical cations of DMtTTF (DMtTTF=dimethyltrimethylene–tetrathiafulvalene) and the Br−anion has been synthesized. Close inspection of the salt's solid state structure revealed the presence of quasi-isolated dimers containing DMtTTF radical cations, a specific arrangement whereby the microscopic parameters of DMtTTF+ might be studied. Analysis of the corresponding single crystal IR and Raman spectra of (DMtTTF)Br allowed us to study the material's electronic and vibrational structure and to evaluate the electron-molecular coupling constants via the isolated dimer model. Additionally, using previously published IR data, analogous calculations were performed on the salt (o-DMTTF)2[W6O19] (o-DMTTF=o-3,4-dimethyltetrathiafulvalene), which also contains well isolated dimers of o-DMTTF radical cations. These calculations revealed that the coupling constants for the unsymmetrical donors studied herein are comparable to those for symmetric TTF derivatives.

Evidence of charge delocalization in Ce1−xFex2+(3+)O2−ynanocrystals(x=0, 0.06, 0.12)

We have measured Raman scattering spectra of pure and iron-doped Ce${}_{1\ensuremath{-}x}$Fe${}_{x}$${}^{2+(3+)}$O${}_{2\ensuremath{-}y}$ ($x=0$, 0.06, and 0.12) nanocrystals. According to the x-ray diffraction study, Fe doping produces contraction of the CeO${}_{2}$ unit cell, leading to the Raman mode hardening. Contrary to expectation, the ${F}_{2g}$ Raman mode exhibits softening and broadening by changing the valence state of Fe dopant, as a consequence of the electron-molecular vibration coupling. This finding supports the assumption that additional charge in highly oxygen-deficient pure and Fe-doped CeO${}_{2\ensuremath{-}y}$ samples are not only localized at Ce${}^{3+}$ ions but also delocalized onto Ce(Fe)-O(V${}_{O}$)-Ce(Fe) orbitals. Delocalization of electrons from Ce${}^{3+}$ ions causes insulator-to-metal transition in highly oxygen-deficient nanoceria. The far-infrared reflectivity spectrum of nanoceria shows metalliclike reflectivity, which in the low-frequency region is well fitted with the Hagen-Rubens approximation for metals. Photoluminescence measurements revealed the existence of a defect-related band in the energy gap of CeO${}_{2\ensuremath{-}y}$. The electron-molecular vibration (phonon) coupling constants $\ensuremath{\lambda}$ and density of electron states at the Fermi level per spin and molecule $N(0)$ were determined within the framework of Allen's theory. The proposal of an energy band structure of nanoceria is also presented.

Charge Sensitive Vibrations and Electron-Molecular Vibration Coupling in Bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF)

A complete analysis of the vibrational dynamics of the most important molecule in the field of organic superconductors, bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), has been performed through first-principles calculations. Different ionization states have been investigated, both as isolated BEDT-TTF unit, and as symmetric self-dimer. The ionization process affects the frequencies of some vibrations, the “charge sensitive modes”, but also their intensities. In particular, a 100-fold increase in the infrared intensity of the antisymmetric C═C stretching mode is predicted to occur upon removal of one electron. The discovery of this dramatic difference will help to interpret the spectral phenomena observed in correspondence to the charge-order processes undergone by some BEDT-TTF salts. The electron-molecular vibration (e-mv) coupling and its effects on the infrared spectra is also fully reanalysed by adopting the proper molecular symmetry of the BEDT-TTF+ monomer, and by investigating (BEDT-TTF)22+ and...

Vibronic activation of molecular vibrational overtones in the infrared spectra of charge-ordered organic conductors

A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated. The anomaly appears at approximately the same frequency (\ensuremath{\sim}2700 cm${}^{\ensuremath{-}1}$), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering. Isotope-shift measurements for \ensuremath{\theta}-(BEDT-TTF)${}_{2}$RbZn(SCN)${}_{4}$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] indicates a relationship between the overtone of a C=C stretching mode of the BEDT-TTF molecule and this anomalous signal. Calculations of electron-molecular vibration coupling based on a diatomic molecular dimer model reveals that the overtone is activated by an anharmonicity developed in the adiabatic potential in a charge-separated system. It is presented that numerical calculation based on the simple cluster model reproduces essential features of the experimentally obtained conductivity spectrum.

Experimental and Theoretical Investigation of Vibrational Spectra of Coordination Polymers Based on TCE‐TTF

The room-temperature infrared and Raman spectra of a series of four isostructural polymeric salts of 2,3,6,7-tetrakis(2-cyanoethylthio)-tetrathiafulvalene (TCE-TTF) with paramagnetic (Co(II), Mn(II)) and diamagnetic (Zn(II), Cd(II)) ions, together with BF(4)(-) or ClO(4)(-) anions are reported. Infrared and Raman-active modes are identified and assigned based on theoretical calculations for neutral and ionized TCE-TTF using density functional theory (DFT) methods. It is confirmed that the TCE-TTF molecules in all the materials investigated are fully ionized and interact in the crystal structure through cyanoethylthio groups. The vibrational modes related to the C=C stretching vibrations of TCE-TTF are analyzed assuming the occurrence of electron-molecular vibration coupling (EMV). The presence of the antisymmetric C=C dimeric mode provides evidence that charge transfer takes place between TCE-TTF molecules belonging to neighboring polymeric networks.

ChemInform Abstract: N,N′-Dicyanoquinonediimines as a Molecular Constituent of Organic Conductors: Vibrational Behavior and Electron-Molecular Vibration Coupling

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Valence state dependent room-temperature ferromagnetism in Fe-doped ceria nanocrystals

Room-temperature ferromagnetism was observed in undoped and Fe2+(3+)-doped CeO2 nanocrystals. In Fe-doped samples the enhancement of ferromagnetic ordering occurs by changing the valence state of Fe ions, whereas Raman spectra demonstrated strong electron-molecular vibrational coupling and increase in oxygen vacancy concentration. Air annealing showed degradation of ferromagnetic ordering and appearance of hematite phase in Fe3+-doped sample. The observed ferromagnetic coupling in Fe-doped samples, associated with the presence of magnetic ions mediated by single charged O2− vacancies, demonstrated that valence state of dopant has a strong influence on magnetic properties of CeO2 nanoparticles.

Electron-intramolecular vibration coupling in TTF-TCNQ systems

We report calculations of the linear electron molecular vibration coupling constants in TCNQ, TCNQ−, and TTF. A complete neglect of differential overlap (cndo) type parameterization is used for the electronic structure and a modified valence force field (mvff) approach for the determination of the vibrational normal modes. Direct experimental information on the magnitude of the coupling constants can be obtained in the gaseous phase from photoemission experiments and in the solid state from polarized optical reflectance measurements on organic linear chain conductors. Agreement between theory and experiment is good. The relevance of electron-intramolecular vibrational coupling constants for the stabilization of the charge density wave (cdw) state in TCNQ anion radical salts and in the determination of polaron binding energies is stressed.

Charge-transfer processes in radical ion molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x : The superconductor (x = 0.9) and the conductor with the metal-insulator transition (x = 0)

Optical spectral investigations of low-dimensional organic molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x with x = 0.9 (the superconductor with T c = 11.3 K) and x = 0 (the metal with the metal-insulator transition at T < 50 K) are performed in the range 50–6000 cm−1 (6 meV–0.74 eV) at temperatures from 300 to 20 K. The optical conductivity spectra are quantitatively analyzed in terms of the proposed model, according to which the charge transfer involves two types of charge carriers, i.e., electrons (holes) localized on clusters (dimers and tetramers formed by BEDT-TTF molecules) and quasi-free charge carriers, with the use of the tetramer “cluster“ model based on the Hubbard Hamiltonian for correlated electrons and the Drude model for quasi-free charge carriers. Physical parameters of the model, such as the energy of Coulomb repulsion between two electrons (holes) in one molecule, the transfer integrals between molecules inside the dimer and between dimers, and the electron-molecular vibration coupling constants, are determined. The anisotropy of the spectra in the conducting plane is explained. The inference is made that only electrons localized on clusters couple with intramolecular vibrations.

IR and Raman spectra of β″-(BEDT-TTF)2RCH2SO3 (R = SF5, CF3): dimerization related to hydrogen bonding

The beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3) organic conductor has been synthesized by electrocrystallization. The crystal structure was determined by single-crystal X-ray diffraction and electronic properties examined. The polarized IR reflectance of beta''-(BEDT-TTF)(2)SF(5)CH(2)SO(3) and beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3), as well as Raman spectra of beta''-(BEDT-TTF)(2)CF(3)CH(2)SO(3) have been measured as a function of temperature. Both materials display charge-nonequivalence within the whole temperature range and unusual activation of vibronic modes with lowering the temperature. In particular, electron-molecular vibration coupling is involved in the activation of the nu(27) mode. The effect is discussed in terms of lattice dimerization involving hydrogen bonding between the anion layer and the conducting BEDT-TTF layer.

Energy band and electron-vibration coupling in organic thin films: photoelectron spectroscopy as a powerful tool for studying the charge transport

This article describes the origins of the width of the highest occupied molecular orbital (HOMO) state observed in the ultraviolet photoemission spectra (UPS) of thin organic semiconductor films. Although much research has been performed on the electrical properties of organic devices, a lot of crucial problems still remain. Among these problems, the charge mobility in organic semiconductor systems is one the most important subjects to be elucidated. In order to discuss the mobility, it is essential to understand both the intermolecular interaction and the electron-molecular vibration coupling. Experimental measurements of the energy band dispersion give information about the intermolecular interaction, and experimental detection of the HOMO hole-vibration coupling is indispensable to comprehend impacts of the electron-vibration coupling on the hole transport. Since most of the information is hidden behind the finite bandwidth of the HOMO, only careful UPS measurements can provide information on these important phenomena related to charge carrier dynamics. In this article, we summarize our recent challenges on UPS measurements of organic thin films, which give the band dispersion of the HOMO and the HOMO hole-vibration coupling, and discuss the origins of the UPS bandwidth that relates to the charge carrier dynamics.

Organic Field-effect Transistor Based on a Thin Film of Polydiacetylene Prepared from 10,12-Pentacosadiynoic Acid

Abstract The organic field-effect transistors based on polydiacetylene as an active semiconductor have been made. The hole mobility and the on/off ratio have been determined to be 1.3 × 10−3 cm2/V s and 1.0 × 104, respectively, for the transistor based on the blue phase of polydiacetylene prepared from 10,12-pentacosadiynoic acid. The gate-voltage-induced infrared absorption spectrum has been attributed to the positive carriers injected by field effect. The spectrum shows a remarkable electron–molecular vibration coupling between vibration levels and an electronic transition.

Inelastic tunneling spectra of an alkyl self-assembled monolayer using a MOS tunnel junction as a test-bed

IETS (Inelastic Electron Tunnelling Spectroscopy) technique was used in Metal-Oxide-Silicon (MOS) tunnel junctions to provide information concerning the vibrational and excitational modes. In this work, we use the IETS method to investigate electron-molecular vibration coupling in SAMs (Self Assembled Monolayer) using the MOS tunnel junctions as a test-bed.

Optical characterization of2kFbond-charge-density wave in quasi-one-dimensional34-filled(EDO−TTF)2X(X=PF6andAsF6)

We present the electronic and vibrational spectra of quasi-one-dimensional $\frac{3}{4}$-filled ${(\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ ($\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF}=\text{ethylenedioxy-tetrathiafulvalene}$, $X={\mathrm{PF}}_{6}$ and ${\mathrm{AsF}}_{6}$) above and below the metal-insulator phase transition (${T}_{\mathrm{MI}}=280\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the ${\mathrm{PF}}_{6}$ salt and $268\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the ${\mathrm{AsF}}_{6}$ salt). For the low-temperature insulating phase, the pattern of both bond and charge order was identified. Almost all charge density is localized on the strongly bound central pair of 0110 tetramer giving rise to a characteristic spectrum of electronic excitations. Infrared spectra along the stacking axis show evidence of strong electron-molecular vibration coupling between the charge transfer band within the pair and some specific intramolecular vibrations. This charge order is assisted by a molecular deformation.

Electron-molecular vibration coupling effect on the Raman spectrum of organic charge transfer salts

Vibrational spectra of dimerized and tetramerized radical clusters have been calculated to understand the features of electron-molecular vibration (EMV) coupling effects for the charge ordered system (CO). The calculated spectra show that the totally in-phase Raman band, which is usually used as a measure of the molecular ionicity, approaches to the frequency corresponding to the average molecular ionicity in the cluster, with increasing the EMV coupling constant. On the other hand, when large charge disproportionation (CD) presents, vibronic bands show steep shift for small variation of the CD ratio. These results suggest that concerning to the normal modes with a large EMV coupling constant, we should focus on the frequency of the vibronic bands instead of the totally in-phase mode to discuss the charge distribution.