Weak Charge-transfer Complexes Research Articles

Raman studies of lattice vibrations in weak charge-transfer complexes: Naphthalene-tetrachlorophthalic anhydride

Low-frequency (15–150 cm −1) Raman spectra of naphthalene (N) : tetrachlorophthalic anhydride (TCPA) complex and its individual parent molecules were recorded at room temperature and at 77 K. The results suggest that the crystal structures for the complexes and for TCPA are in good agreement with those determined by X-ray studies. Within the temperature range studied, no orientational order-disorder phase transitions were observed for all the crystals investigated. The observed 26.7 cm −1 lattice mode in N : TCPA might be assigned as the interaction coordinate for complex formation.

Fermi resonance effect and vibrational intensity in the weak charge-transfer complexes

Infrared and Raman spectra of naphthalene, tetrachlorophthalic anhydride, tetrabromophthalic anhydride and their respective weak charge-transfer complexes were recorded in solids. The spectral intensities of both donor and acceptor molecules were found to alter drastically upon complexation. The observed intensity changes might be attributed to the removal and formation of the Fermi resonance effect in the complexes.

Depletion layer studies and carrier photogeneration in doped merocyanine photovoltaic cells

The capacitance and photocurrent of Al/Cl2-doped merocyanine/Au thin film photovoltaic cells have been studied as a function of applied voltage bias. The capacitance data indicated that a Schottky barrier had formed between the Al and organic dye. The barrier width, built-in potential, and carrier concentration were 23 nm, 1.05 V, and 1018 cm−3, respectively. Photocurrent measurements at different excitation wavelengths strongly suggest that carrier photogeneration is a bulk phenomenon. Combining these observations with a study of the effects of doping with molecular chlorine, we propose that carriers are generated by electron transfer from a thermalized exciton to a dopant site. The dopant is thought to be adsorbed in the dye layer as the acceptor half of a weak charge transfer complex. A quantum yield of 11.2% at 632.8 nm based on light absorbed by a 30-nm thick merocyanine film was measured for the chlorine-doped cell.

14N nuclear quadrupole resonance in weakly paramagnetic organic dye cations

14N NQR spectra at 77 K for seven organic dye cations and one free base are reported. The cations are weakly paramagnetic at 77 K, exhibiting broad and featureless EPR spectra characteristic of weak charge-transfer complexes. The positive charge in the cations is largely delocalized. The existence of a simple relationship between the 14N NQR data and optical properties for three cyanine laser dyes is discussed.

ChemInform Abstract: REFLECTION AND ABSORPTION SPECTRA OF SINGLET CHARGE TRANSFER EXCITONS IN ANTHRACENE‐PMDA CRYSTALS

The polarized reflection spectra of single crystals of the weak charge transfer (CT) complex anthracene–pyromellitic dianhydride (A–PMDA) have been measured at 2 K. The oscillator strengths and energies of the singlet charge transfer transitions have been determined from the frequency dependent dielectric function derived from the reflection spectrum by means of a Kramers–Kronig transformation. A theory of exciton–phonon coupling appropriate for CT transitions in a mixed stack system is briefly described and used to interpret the derived absorption spectrum. The absorption spectrum consists of a purely electronic zero phonon line (ZPL) and a set of zero lattice phonon lines (ZLPL) corresponding to the excitation of high frequency intramolecular modes of the ions. Each ZLPL, like the ZPL, acts as the origin for several librational modes and for a progression in a lattice mode of approximately 30 cm−1. Exciton–phonon coupling is weakest for the vibrationless ZPL and increases with the energy of the intramolecular vibrational mode of the corresponding ZLPL. Approximate values for the exciton–phonon coupling parameter S are found for the ZPL and each ZLPL using the local excited state model.

Reflection and absorption spectra of singlet charge transfer excitons in anthracene–PMDA crystals

The polarized reflection spectra of single crystals of the weak charge transfer (CT) complex anthracene–pyromellitic dianhydride (A–PMDA) have been measured at 2 K. The oscillator strengths and energies of the singlet charge transfer transitions have been determined from the frequency dependent dielectric function derived from the reflection spectrum by means of a Kramers–Kronig transformation. A theory of exciton–phonon coupling appropriate for CT transitions in a mixed stack system is briefly described and used to interpret the derived absorption spectrum. The absorption spectrum consists of a purely electronic zero phonon line (ZPL) and a set of zero lattice phonon lines (ZLPL) corresponding to the excitation of high frequency intramolecular modes of the ions. Each ZLPL, like the ZPL, acts as the origin for several librational modes and for a progression in a lattice mode of approximately 30 cm−1. Exciton–phonon coupling is weakest for the vibrationless ZPL and increases with the energy of the intramolecular vibrational mode of the corresponding ZLPL. Approximate values for the exciton–phonon coupling parameter S are found for the ZPL and each ZLPL using the local excited state model.

Charge-Transfer Excitons in Organic Solids

Abstract The polarized reflection spectra of single crystals of three weak charge transfer complexes have been measured at 2 K. The Kramers-Kronig transformation was used to calculate the complex frequency dependent dielectric functions from the reflection data. The oscillator strengths and the positions of the charge transfer exciton levels have been determined.

Isotope Effect on the Phase Transition Temperatures in Aromatic Hydrocarbon-Tetracyanobenzene Complexes

Abstract It is well established that electrical and magnetic properties of crystals are strongly influenced by structural phase transitions. The strong charge transfer complexes such as tetrathiofulvalene-tetracyanoquinodimethan (TTF-TCNQ) have been intensely studied(1) but much less is known concerning weak charge transfer complexes and the influence of solid-solid transitions on their properties. Typical examples of some weak charge transfer complexes are the 1:1 complexes formed between aromatic donors (e.g. naphthalene) and aromatic acceptors (e.g. 1,2,4,5 tetracyanobenzene, TCNB).

Exploratory studies of some drug charge transfer interactions

Conductivity and capacitance titrations yield minima for the chlorpromazine hydrochloride-heparin interaction, confirming clinical suspicions of its occurrence. The effective dosage of heparin thus is reduced if administered in conjunction with chlorpromazine. The interaction is interpreted as charge transfer complex formation, occurring as an (electrode) surface reaction. It is suggested that the charge transfer complexing capability of heparin preparations, as evidenced by conductance and/or capacitance changes, evaluated against a well defined donor such as chlorpromazine hydrochloride, may be adapted as a more precise method of measuring heparin activity than coagulation time determinations. Phenytoin and chlorpromazine likewise yield conductance and capacitance minima; voltammetry indicates new peaks at +250mV and −300mV vers.SCE supporting the suggestions that an uncharged 1∶1 complex is being formed, again in a type of surface reaction. Phenytoin and lignocain form a precipitate at 0.002 equimolar; in conductance and capacitance titrations phenytoin behaves as a weak electron donor against iodine though as a weak acceptor against lignocain. Lignocain and chlorpromazine conductance and capacitance titrations using gold electrodes fail to show any evidence for their previously reported interaction on Pt/Pt electrodes. Voltammetry on Pt/Pt electrodes indicates 2 new peaks at zero and at −750mV vers.SCE. It is thought that these two compounds interact only on catalytically highly active surfaces, where they form a weak surface charge transfer complex. Adrenalin, in conductance and capacitance titrations, behaves amphoteric, i.e. as an electron acceptor against the strong donor chlorpromazine and as a donor against the strong acceptor tetramethyl-p-phenylenediamine. Voltammograms of the above listed interactions are interpreted as of the ECE type exhibiting mainly irreversible behaviour.

Benzene-lecithin interaction in lecithin-benzene solutions part II: Optical studies and quantum mechanical calculations

UV absorption and CD spectra of dipalmitoyl- sn-glycerine phosphatidylcholine (DPPC)-benzene and DPPC-cyclohexane systems at different temperatures (in the micellar and gel state) as well as quantum chemical calculations of a benzene-trimethyl ammonium supermolecule demonstrate that in the gel of the DPPC-benzene system a molar fraction of benzene molecules forms a weak charge transfer complex with the -N + (CH 3) 3 moiety of DPPC. In the gel state there exists a second fraction of benzene molecules, the reorientation freedom of which is restricted by a liquid crystal-like structure.

Photoconductivity of the charge transfer complex crystals. Anthracene-Dimethylpyromellitimide

An investigation is made of the mechanism of photoconductivity of needle-shaped single crystals of a weak charge transfer complex formed by anthracene and dimethylpyromellitimide. In the crystals of the charge transfer (CT) complex, the annihilation of triplet excitons is shown to generate charge carriers. The contribution of this process to the photoconductivity is evaluated. The photoconductivity of the CT-complexes is found to exhibit a sensitivity toward weak external magnetic fields as a result of the variations in the rate constants of triplet-triplet annihilation and intersystem crossing in excited states of the CT-complex. A scheme is proposed to represent the processes involving excited and ionized states in the crystals of the anthracene-dimethylpyro-mellitimide CT-complex.

Charge-transfer contributions to solvent shifts in fluorine magnetic resonance spectra

Fluorine chemical shifts have been determined as a function of solvent composition for dilute solutions of 1,1,1,10,10,10-hexafluorodecane, hexafluorobenzene, and difluorodibromomethane in a number of mixed solvent systems each consisting of n-heptane and one of several electron-donor cosolvents. The results provide no evidence for the existence of an appreciable reaction-field contribution to the solvent shift, assuming that such a contribution would depend on the solute polarity and on the solvent dielectric constant as required by published theoretical treatments. Instead, it appears that for hexafluorobenzene and difluorodibromomethane charge-transfer interactions contribute upfield shifts ranging from 0.6 ppm to more than 5 ppm in the neat electron-donor solvents. Approximate equilibrium constants for the formation of weak charge-transfer complexes are calculated from the data. The shifts appear to be affected both by such complex formation and by interactions occurring during random encounters of donor and acceptor molecules. The present results together with others in the literature suggest that such effects are much more common than is generally supposed, especially when the fluorine atom is attached either to an aromatic ring or to a carbon atom which bears one or more chlorine, bromine, or iodine substituents.

Biphotonic process in ionic photodissociation of weak charge transfer complexes

Dependences of the photoinduced radical-anion formation of solute-solvent charge-transfer complexes upon the exciting light intensity have been studied spectroscopically in ether-isopentane glass as well as in 2-methyltetrahydrofuran (MTHF) glass at 77 K. Biophotonic ionic dissociations have been found for the following complexes: TCNB(tetracyanobenzene)-ether, PMDA(pyromellitic dianhydride)-ether, TCPA(tetrachlorophthalic anhydride)-ether and TCPA-MTHF.

Drift Mobilities of Electrons and Holes in Single Crystals of Anthracene-1,3,5-Trinitrobenzene Charge-Transfer Complex

Abstract In view of the basic difficulties encountered in growing pure crystals of organic molecular complexes the anthracene 1,3,5 -trinitrobenzene (AN-TNB) is taken as a model substance for studying electrical and photoelectrical properties of weak charge-transfer complexes. The monoclinic unit cell of the 1: 1 AN-TNB system1 comprises four molecules of the complex, the component molecules being stacked alternatively plane to plan forming infinite columns extended along the c-axis. The planes of the aromatic rings of the anthracene and trinitrobenzene molecules make an angle of 83° with the c-axis. The closest intermolecular contacts are formed by adjacent molecules.

Rate constants of triplet-state ionic photodissociation of weak charge-transfer complexes formed between pyromellitic dianhydride (PMDA) and naphthalenes

Rate constants of triplet-state ionic photodissociation of weak charge-transfer complexes formed between pyromellitic dianhydride (PMDA) and naphthalenes

Biphotonic ionic dissociation of weak charge-transfer complexes in glassy solutions at 77 K

The dependence of photoinduced radical anion formation upon exciting light intensity has been studied for glassy solutions of tetracyanobenzene (TCNB) in ether—isopentane and 2-methyl-tetrahydrofuran (MTHF) at 77 K. It has been found that in ether—isopentane glass the TCNB anion is formed biphotonically via the lowest excited triplet state of the 1:1 TCNB—ether CT complex, whereas in MTHF glass the ionic photodissociation of the TCNB—MTHF CT complex occurs monophotonically in its lowest excited singlet state.

X-ray structure determination of the 1:1 charge transfer complex of naphthalene and tetrachlorophthalic anhydride at -153.deg.

Abstract : The crystal structure of the weak charge transfer complex of 1:1 perdeuterated naphthalene and tetrachlorophthalic anhydride has been determined at 120K. The structure was solved with Patterson and Fourier techniques and refined by least squares. The component molecules are stacked alternately in infinite chains along the b axis. The accurate, low temperature, structural parameters obtained for naphthalene and tetrachlorophthalic anhydride in the complex are within experimental error identical to those observed in the pure materials. (Modified author abstract)

Photoemission studies of TCNQ compounds

The densities of occupied states of a series of TNCQ compounds have been determined by u.v. photoemission. A clear difference between TCNQ weak charge transfer complexes and anion radical salts including TTF-TCNQ is found. Due to strong electron-molecule coupling narrow delocalized electron bands are not observable.

Ionic photodissociation of weak charge-transfer complexes. Donor acetonitrile and acceptor tetracyanoethylene and iodine

Ionic photodissociation of weak charge-transfer complexes. Donor acetonitrile and acceptor tetracyanoethylene and iodine

Ionic Photodissociation of Some Weak Charge Transfer Complexes of Tetracyanoethylene with n-Type Donor Molecules

Ionic Photodissociation of Some Weak Charge Transfer Complexes of Tetracyanoethylene with n-Type Donor Molecules