Charge-transfer Crystals Research Articles

Excimer states and enhanced two-photon absorption in intramolecular charge-transfer crystals

We explain enhance two-photon absorption in certain centrosymmetric organic molecular crystals in which intramolecular charge transfer takes place. Specifically we suggest that charge transfer allows the interacting molecules in a unit cell to have non-zero dipole moments in the excited state of the molecules, contributing to a resonance in the two-photon absorption coefficient. Anti-parallel orientation of the molecular dipole moments in the unit cell allows for formation of excimer states and disallows the production of SHG in the same materials. We estimate the ratio of the two-photon to the one-photon absorption coefficient for some typical parameters.

Theoretical study of the charge transfer in the organic crystal of dimethylquaterthiophene and tetrafluoro-tetracyanoquinodimethane

The electronic and geometrical structures of the organic charge-transfer crystal of dimethylquaterthiophene (DMQtT) and tetrafluorotetracyanoquinodimethane (${\mathrm{TCNQF}}_{4}$) are calculated self-consistently from first principles using Car-Parrinello techniques. The calculated electronic charge transfer from the DMQtT molecule to the ${\mathrm{TCNQF}}_{4}$ molecule is 0.46e\ifmmode\pm\else\textpm\fi{}0.05e. This charge transfer is accompanied by a structural change in which the DMQtT molecule adopts a more quinoid geometry (compared to the aromatic structure of the neutral DMQtT molecule) and the ${\mathrm{TCNQF}}_{4}$ molecule becomes more aromatic. The electronic structure around the Fermi level is characterized by one-dimensional charge transfer states.

Electronic and vibronic optical excitations of a quarter-filled periodic octamer

Linear optical conductivity spectra of 1-D quarter-filled charge transfer crystals are interpreted on the basis of a periodic octamer and a Holstein-Hubbard model. It is shown how the low frequency spectral features are due to combined effects of electron correlations and of the dimerization of the system. Fermion-phonon interactions introduce other spectral features which produce strong changes in the spectra.

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.

Effects of Acceptor Modification on Charge Transfer in Crystals of Donor‐Acceptor Systems of TTF

AbstractThe degree of charge transfer has been determined in six TTF‐fluorene derivative charge transfer complexes, using IR and Raman vibrational spectroscopies. It has been found that changes of the intermolecular electronic distribution is strongly influenced by the presence of the dicyanomethylene group, by the increase of the nitro groups, as well as by the symmetry of the acceptor.

Electronic Structure and Dynamics of the Excited State in CT Microcrystals As Revealed by Femtosecond Diffuse Reflectance Spectroscopy

Transient absorption spectra of charge-transfer (CT) crystals between methyl- or methoxy-substituted benzene derivatives and pyromellitic dianhydride (PMDA) were measured by femtosecond diffuse reflectance spectroscopy, and the electronic structure and dynamics of the excited state depending on the nature of the electron donor (D) are discussed. For such weak CT complexes, it was confirmed that excitation energy is localized in one donor−acceptor pair and the mixing of CT and locally excited (LE) configurations is important in the excited CT singlet state. The CT degree of the excited state depends on the oxidation potential of D and on the mutual configuration of D and electron acceptor (A) molecules. The second-order decay constant of the excited state was observed under usual photolysis conditions, from which a motion-limited diffusion was considered. A first-order decay of the transient absorption was obtained, when the excitation intensity was weak, and ascribed to charge recombination to the ground state. The energy gap (−ΔG) dependence of the charge recombination rate constant (kCR) was confirmed to give a linear relationship between ln(kCR) and |−ΔG|.

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.

Excitonic energy transfer in mixed‐stack charge transfer crystals

Excitonic energy transfer in mixed-stack charge transfer (CT) crystals doped with anthracene was studied within the temperature range 5–300 K using optical, spectroscopic and kinetic techniques. Excitons were found to be self-trapped at low temperature and mobile at higher temperatures, with the activation energy of mobility being dependent on their CT character. A 1D hopping model describes the motion of triplet excitons, whereas a 3D model fits the migration of singlet excitons.

Localization of a charge transfer excited state in molecular crystals: a direct confirmation by femtosecond diffuse reflectance spectroscopy

Transient absorption spectra of the durene-pyromellitic dianhydride (PMDA) charge transfer (CT) crystal, PMDA-doped durene crystal and its molten state were measured by femtosecond diffuse reflectance spectroscopy, and electronic structure and dynamics of the excited state in the CT crystal are discussed. It is directly confirmed that the excited CT singlet state is almost localized (self-trapped) in one donor(D)-acceptor(A) unit within a few picoseconds after excitation. The electronic structure of the excited state is quite polar but it is not an electron transfer state from D to A.

Migration of Singlet and Triplet Excitons in Charge-Transfer Crystals

Abstract Single crystals of some charge-transfer (CT) complexes were studied by optical techniques, within the temperature range 4.2–300 K, to gather information relevant to the dimensionality and self-traping of singlet and triplet excitons. Mobile singlet excitons were observed above 100 K when the self-trapping energy was overcome. Activation energy of the mobility of triplet excitons was found to be dependent on their CT character. This dependence confirms the self-trapping origin of the low temperature traps. 1D hopping model describes motion of triplet excitons whereas 3D model referres to migration of singlet excitons. Mobility of singlet and triplet excitons is considerably slowed down in the TCNB-DMN crystal, most probably because of orientational disorder of the donor molecules which leads to a dispersive-like character of exciton motion.

Photoreactions of crystalline charge-transfer complexes

Abstract

Photoinduced Electron Transfer in Charge-Transfer Crystals by Diffuse-Reflectance (Picosecond) Time-Resolved Spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotoinduced Electron Transfer in Charge-Transfer Crystals by Diffuse-Reflectance (Picosecond) Time-Resolved SpectroscopyStephan M. Hubig and Jay K. KochiCite this: J. Phys. Chem. 1995, 99, 49, 17578–17585Publication Date (Print):December 1, 1995Publication History Published online1 May 2002Published inissue 1 December 1995https://doi.org/10.1021/j100049a018RIGHTS & PERMISSIONSArticle Views368Altmetric-Citations51LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (913 KB) Get e-Alerts

Efficient up-conversion fluorescence in charge transfer compound crystal

A series of new intramolecular charge transfer compounds with absorption bands in shorter wavelengths have been synthesized. Bright visible light emits from the crystals of these new compounds under infrared pulsed laser pumping. The spectral and pump density dependent properties demonstrate that some of the emissions are up-conversion fluorescence due to two photon absorption. The fluorescent yields of one- and two-photon absorption are in the same order at high pump density. This work demonstrates that these new intramolecular charge transfer crystals are good candidates for nonlinear optical materials.

Towards a Unified View of Electron-Phonon Coupling in 1D Solids

We analyze the effect of on-site and on-bond electron—phonon (e—ph) coupling in different classes of quasi one-dimensional (1D) solids in terms of a single model. The model was originally developed to account for e—ph coupling in charge transfer crystals and to interpret their vibrational spectra. The same model is extended to conjugated polymers, showing that important information about the electronic structure can be obtained through a careful analysis of vibrational data. Evidence of e—ph coupling in the spectra of halogen-bridged transition-metal complexes is also presented. The unified model thus applies to different classes of 1D systems and yields transferable e—ph coupling constants. PACS numbers: 71.38.-1-i, 71.27.+a, 71.45.Lr

Electronic Correlations and Electron-Molecular Vibration Coupling in Pentamerized Charge Transfer Crystals

A cluster approach is used for a theoretical description of the optical properties of pentamerized low-dimensional molecular crystals. Electronic correlations are taken into account explicitly in the Hubbard model for 1, 2, 3, 4 and 5 electrons in a linear pentamer, as well as coupling of electrons to the totally symmetric intramolecular vibrations. Calculated absorbance is compared to the measured one in pentamerized salt of TCNQ and TTF molecules.

Pressure-induced neutral-ionic transition of (TTF)2(iodanil) and nonsymmetrical charge distribution in the trimer unit

Abstract Neutral-ionic transition under pressure in a 2:1 charge transfer crystal of tetrathiafulvalene (TTF) and iodanil (IA) is disscussed using a model of mixed-valence trimer with explicit consideration of electron-intermolecular vibration coupling. Ground state charge distribution in the TTF-IA-TTF trimer is analysed as a function of small polaron binding energies and transfer integral.

Pressure-induced neutral-ionic transition in a charge transfer crystal of tetrathiafulvalene and dimethyldicyanoquinonediimine

Abstract Neutral-ionic (N-I) transition was found for a mixed-stack charge transfer crystal of tetrathiafulvalene (TTF) and dimethyldicyanoquinonediimine (DMDCNQI) at 0.9 GPa. At the transition pressure (P e ), large ionicity jump of ca. 0.8 was observed in IR spectra.

Migration of electronic excitation energy in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene

Singlet and triplet exciton energy migration in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene, undoped and doped with anthracene (10 −4−10 −2M/M), have been studied in the 5–300 K temperature range. At low temperatures singlet and triplet excitons are self-trapped. They become mobile with increasing temperature with activation energies of 380 ± 70 and 400 ± 80 cm −1 for singlet and triplet excitons, respectively. The motion of triplet excitons in the temperature range 50–100 K can be described by one-dimensional hopping within cages, limited by structural defects of the host crystal. Mobile singlet excitons are observed for temperatures above 100 K and their motion is well described by a time-independent excitation transfer rate constant.

Neutral-ionic transition in dimethyl-TTF-chloranil

Abstract We report on a new mixed-stack organic charge-transfer crystal of 2,6-dimethyltetrathiafulvalene- p -chloranil (DMTTF-CA), which undergoes a temperature-induced neutral-ionic transition (TINIT). The discontinuous change in molecular ionicity was found to occur at 65K. A most striking feature of the TINIT in DMTTF-CA was that a neutral-ionic coexisting phase appeared at lower temperatures, unlike the homogeneous ionic phase observed for the TINIT of TTF-CA. The pressure-induced transition in DMTTF-CA was found to occur at 1.2 GPa. The transition characteristics of DMTTF-CA are discussed in comparison with those of TTF-CA.