Intervalence charge transfer in mixed valence compound modified by the formation of a supramolecular complex

The electrochemical and spectroelectrochemical properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were investigated in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile and in the presence of trifluoroacetic acid (TFA) only for 18C6. In neutral acetonitrile, PDA undergoes two reversible oxidation processes, which lead first to the formation of the cation-radical considered as mixed valence (MV) compound, and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concentration, cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the second redox process only. This is attributed to a strong interaction between the PDA(+2) dication and two 18C6 molecules, leading to the formation of a supramolecular complex with an association constant value K(a) = 7.0 × 10(7) M(-2). The interaction of 18C6 with PDA(+2) dication has a direct effect on the PDA(+.) cation-radical corresponding to a decrease in the lifetime of the MV compound and of the intramolecular electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compound (k(th) = 1.35 × 10(10) s(-1) in 18C6-free neutral solution containing 5.0 × 10(-4) M PDA, and k(th) = 3.6 × 10(9) s(-1) in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation constant K(co) falls from 6.0 × 10(6) in 18C6-free solution to 1.6 × 10(3) at [18C6]/[PDA] = 20/1. In acidified acetonitrile and when TFA concentration is increased, PDA still shows the two successive and reversible oxidation processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidation waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, resulting in the formation of supramolecular complexes. In the presence of TFA, the value of K(co) is decreased to 4.3 × 10(4), but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compound. In this medium, IV-CT in the MV compound is greater with 18C6 (k(th) = 2.3 × 10(10) s(-1) for [18C6]/[PDA] = 20/1) than without (k(th) = 1.4 × 10(9) s(-1)), which indicates a more important IV-CT rate when 18C6 is present. The results show for the first time that is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compound is the most important. This control is not obtained as usual by chemical modification of the structure of the starting molecule, but by varying either the acidity or the 18C6 concentration as external stimuli, which lead to reversible formation/dissociation of a supramolecular complex species. Moreover, we also studied the electrochemical properties of PDA in the presence of wider crown ether such as DB24C8. We showed that PDA undergoes the same electrochemical behavior with DB24C8 than with 18C6 in neutral organic medium (K(a) = 2.9 × 10(3) M(-1)). This result suggests that the complexation between the electrogenerated PDA(+2) dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which is supposed to form inclusion complexes.     

Excited-state mixed valence in transition metal complexes

Mixed valence in the lowest-energy metal-to-ligand charge-transfer excited state of di-(4-acetylpyridine)tetraammineruthenium(II) complexes is defined and analyzed. The excited state has two interchangeably equivalent ligands with different oxidation states. The electronic absorption band energies, selection rules, and bandwidths are analyzed quantitatively in terms of the signs and orientations of the transition dipole moments, sign and magnitude of the coupling, and resonance Raman analysis of displaced normal modes.     

X-ray photoelectron study of mixed valence metatungstate anions

X-ray photoelectron spectra of metatungstate H₂W₁₂O^6?₄₀ and of reduced derivatives with 5, 12, and 24 electrons have been recorded. W(4f) signals are consistent with the presence of tetravalent tungsten in the reduced species. In particular the 12e^? derivative does not contain W^V but W^IV and W^VI in equal amounts. Valence band spectra show the W(5d) levels near 2 eV in reduced forms, this energy being appreciably lower than in W bronzes and WO₂. This can be correlated with the relative inertness of reduced metatungstates towards oxidation.     

Electrochemical and spectrophotometrical investigation of the electron-accepting strength of organic superelectrophiles: X-ray structure of their charge transfer complexes with tetrathiafulvalene

Nitro Benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles are ranked amongst the strongest electrophiles known to date. In the past twenty years, their propensity to act as electron organic acceptors has been less studied. In this paper, we report on the study of their electrochemical behavior and on the structural characterization of charge transfer complexes (CTC) deriving from their interaction with tetrathiafulvalene (TTF) derivatives, both in solution and in the solid state. The first half wave reduction potentials (E(1/2)(I)) associated with a reversible monoelectronic transfer process of a large set of nitro substituted benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles have been determined through a detailed electrochemical approach in acetonitrile with a microelectrode network using the ferrocene as an internal reference potential in this electrochemical study. Determination of the electron affinity (EA(CT)) of this series of substituted electrodeficient heteroaromatics as well as their LUMO energy was performed using the Charge Transfer Spectroscopic (CTS) method in solution and by DFT calculations, respectively. The use of the correlation EA(CT) versus the reversible half wave potential (E(1/2)(I)) appears to be a useful tool to estimate readily the E(1/2)(I) or EA(CT) values when they cannot be experimentally determined. The diffusion coefficient of these electrophiles has, for the first time, been determined in acetonitrile. These air stable electrodeficient heteroaromatics have been explored as potential new organic acceptors in the formation of charge transfer (CT) complexes with TTF derivatives. Crystallographic data of two CT complexes with TTF (especially the C-C and C-S bond lengths of the TTF moieties) indicate that these complexes exhibit weak electron delocalization and that both molecules remain neutral. Their resulting levels of charge transfer were probed using UV-visible, IR spectroscopy and by DFT calculations.     

Nonlinear X-ray diffraction. Determination of valence electron charge distributions

A new, nonlinear X-ray diffraction technique is described which permits the direct experimental determination of the valence electron charge density in a wide variety of covalently bonded materials.     

PCILO Calculations on charge transfer complexes

The equilibrium distances of the complex components and the stabilization energies were calculated for the molecular complexes ethylene–fluorine, ethylene–chlorine, tetracyanoethylene–benzene, tetracyanoethylene–durene, and quinone–hydroquinone using the PCILO method. The results are compared with the experimental values and the theoretical predictions of the CNDO /2 method.     

Conformationally controlled intramolecular charge transfer complexes

Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substituted homologues (2), have been prepared and are shown to form intramolecular charge-transfer (donor-acceptor) complexes. These weak complexes are turned on and off by the chair-chair interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the alpha-proton signals. The Gibbs free energies assigned to the donor-acceptor interactions range between 0 and -1 kcal mol(-1). A crystal structure of one of the complexes (1b) confirms the intramolecular donor-acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes were characterized by NOE interaction between the methyl and an alpha-proton cis to it.     

Alkyl halide charge transfer complexes with hard Lewis bases

The unusual charge transfer complexes formed between alkyl halide acceptors and hard Lewis base donors (amines and alcohols) in low dielectric solvent were examined using ultraviolet spectroscopy. The lambda(max) of the complex decreases with increasing ionization potential of the donor. The complex formation equilibria were probed by thermodynamic analysis and concentration variation. At ambient temperatures complex formation is generally slightly exergonic with a negative complexation entropy. The complex extinction coefficients are much lower (<10 l mol(-1) cm(-1)) than for typical charge transfer complexes. These complexes are extraordinary within a classical context since the halide acceptors have a negative electron affinity. They exhibited an atypical hypsochromic shift with increasing solvent dielectric constant.     

X-ray structure and DFT study of neutral mixed phosphine azoimine complexes of ruthenium

Geometry optimization for a cis-[Ru^II(dppe)LCl₂] (1-8) {L = C₆H₅NNC(COCH₃)NAr, Ar = 2,4,6-trimethylphenyl (L₁), 2,5-dimethylphenyl (L₂), 4-tolyl (L₃), phenyl (L₄), 4-methoxyphenyl (L₅), 4-chlorophenyl (L₆), 4-nitrophenyl (L₇), 2,5-dichlorophenyl (L₈); dppe = Ph₂P(CH₂)₂PPh₂} was effected using the gaussian 03 protocol at density functional theory (DFT) B3LYP level with 6-31G^∗/lanl2dz mixed basis. In addition, the complex cis-[Ru^II(dppe)L₃Cl₂] (3) has been further characterized by X-ray diffraction analysis. It was found that the optimized structure using 6-31G^∗/lanl2dz has a large agreement with the X-ray data. DFT calculations show that upon solvation both Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) molecular orbitals are stabilized and their energy gap is increased. TD-DFT calculations show that the intense broad band centered at λ_max ∼ 506 nm is assigned to “mixed metal-ligand-to-ligand charge-transfer” (MMLLCT) while the weak low energy band centered on ∼840 nm is assigned to the pure MLCT transition. The low intensity for the low energy MLCT transition can be explained by the large mixing between the azoimine (L) and (Ru(dπ)) orbital.     

芳基四硫富瓦烯与碘电荷转移复合物的合成及其晶体结构

采用缓慢挥发溶剂的方法合成了硫原子桥联芳基取代四硫富瓦烯（Ar-S-TTF）与碘的3种电荷转移复合物（1^+·）（I₃）·I₂、（2^+·）（I₅）·I₂和（3²⁺）（I₃）₂,采用单晶X射线衍射、紫外可见光谱、循环伏安对其进行了表征。复合物（1^+·）（I₃）·I₂为C2/c空间群,1^+·呈椅式构型。化合物1与碘之间在溶液中和复合物中电荷转移一致。复合物（2^+·）（I₅）·I₂为P1空间群,2^+·呈椅式构型。复合物（3²⁺）（I₃）₂为Pbca空间群,3²⁺呈独特的平面构型。化合物2和3与碘之间在溶液中和复合物中呈现不同的电荷转移。复合物中聚碘阴离子呈现不同的堆积结构：由I₃^-或I₅^-/I₂组成的一维链状和I₃^-/I₂组成的二维网格状。     

芳基四硫富瓦烯与碘电荷转移复合物的合成及其晶体结构

采用缓慢挥发溶剂的方法合成了硫原子桥联芳基取代四硫富瓦烯(Ar-S-TTF)与碘的3种电荷转移复合物(1^+·)(I₃)·I₂、(2^+·)(I₅)·I₂和(3²⁺)(I₃)₂,采用单晶X射线衍射、紫外可见光谱、循环伏安对其进行了表征。复合物(1^+·)(I₃)·I₂为C2/c空间群,1^+·呈椅式构型。化合物1与碘之间在溶液中和复合物中电荷转移一致。复合物(2^+·)(I₅)·I₂为P1空间群,2^+·呈椅式构型。复合物(3²⁺)(I₃)₂为Pbca空间群,3²⁺呈独特的平面构型。化合物2和3与碘之间在溶液中和复合物中呈现不同的电荷转移。复合物中聚碘阴离子呈现不同的堆积结构：由I₃^-或I₅^-/I₂组成的一维链状和I₃^-/I₂组成的二维网格状。     

Vibronic spectra of charge transfer excitons and of mixed charge transfer and Frenkel excitons

The linear absorption spectra in the excitonic and vibronic regions in the case of mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been theoretically studied for the exciton parameters of the crystals of MePTCDI and PTCDA. Two coupling parameters for the exciton–phonon coupling are introduced: the FE–phonon coupling and the CTE–phonon coupling. The main features of the vibronics in the absorption spectra are the following: (a) the existence of a doublet structure in the vibronic spectra of CTEs; (b) the vibronic levels of the FE at intermediate values of both coupling parameters are located in the continuum of the many-particle exciton–phonon states which makes its absorption line wide and flat; (c) in the case of strong coupling (coupling constants larger than 1) a doublet of bound states appears above this continuum; (d) in the case of vanishing CTE–phonon coupling the vibronics of the charge transfer excitons practically disappear in the absorption spectra.     

Origins of cooperative noncovalent host-guest chemistry in mixed valence complexes

The electronic effects resulting from noncovalent host-guest interactions between calix[6]arene and a ruthenium dimer, [Ru3O(OAc)6(CO)(ppy)]2-mu-pz (ppy=4-phenyl pyridine, pz=pyrazine), are presented. The noncovalent interaction is between the calix[6]arene and the ppy ligands of the dimer. The dimer can bind 2 equiv of calix[6]arene. The complex [Ru3O(OAc)6(CO)(ppy)]2-mu-pz forms a highly stable mixed valence ion with strong electronic coupling between the two Ru3 clusters. The strength of the electronic interaction is found to be moderated by calix[6]arene binding. Addition of calix[6]arene to the mixed valence ion causes the electronic coupling to decrease. The binding of calix[6]arene is found to be cooperative. The origins of cooperative binding are developed in terms of the potential energy surfaces associated with the symmetric and asymmetric mixed valence ion. In particular, it is found that symmetry breaking (through the binding of a single calix[6]arene) destabilizes the mixed valence state. Restoration of symmetry (through the binding of a second calix[6]arene) increases the stability of the mixed valence ion and provides an additional driving force for the binding of the second calix[6]arene.     

Electrical properties of polyvinylcarbazole-tetracyanoquinodimethane charge transfer complexes

Electrical properties of polyvinylcarbazole-tetracyanoquinodimethane (PVK:TCNQ) charge transfer complexes were investigated as a function of the composition of the mixture, temperature, pressure and the method of sample preparation. It was shown that the electrical conductivity of the PVK:TCNQ complex (for TCNQ content <10% by weight) increased only slowly with increasing concentration of TCNQ and was only slightly higher than the conductivity of pure PVK. At higher TCNQ concentrations however an abrupt increase of the complex conductivity was observed. This effect may be attributed to the formation of semi-conductive tracks composed of uncomplexed neutral TCNQ molecules. The electrical conductivity of PVK:TCNQ complex was lower than the conductivity of TCNQ alone. The activation energy for electrical conductivity in the complex decreased with increasing TCNQ concentration from 0.99 eV for pure PVK to 0.37 eV for PVK:TCNQ (6:1) complex. The high field conductivity of the PVK:TCNQ complex could be explained by using the Poole-Frenkel model.     

The formation of bifurcated charge transfer complexes with molecular iodine

I₂ complexes with triptycene and several di- and triaryl derivatives of methane and ethane were studied. For these complexes the values of λ_CT are virtually identical to those reported for the complexes with the analogous monoaryl donors, while the values of λ for their blue shifted I₂ peaks are significantly lower than those for the monoaryl complexes. Both the equilibrium constants and - ΔH⁰ values for the formation of complexes from the components lead to the conclusion that the complexes with the di- and triaryl compounds are more stable than those with the monoaryl donors. For the diaryl donors, the ΔS⁰₂₉₈ values for complex formation are less favorable than those of the monoaryl donors. The dipole moment for I₂ in diphenylmethane is larger than the moment of I₂ in toluene. All of these observations can be explained by taking into account the transannular effect of one aromatic ring on another and viewing the complexes as bifurcated ones in which the I atom at one end of an I₂ molecule simultaneously interacts with two rings in the donor molecules.     

Photoluminescence of europium(III) dithiocarbamate complexes: electronic structure, charge transfer and energy transfer

For the first time, we observed photoluminescence in Eu(III) dithiocarbamate complexes at room temperature -- more specifically in [Eu(Et(2)NCS(2))(3)phen], [Eu(Et(2)NCS(2))(3)bpy] and the novel [Eu(Ph(2)NCS(2))(3)phen], where phen stands for 1,10-phenanthroline and bpy for 2,2'-bipyridine. Correlations between the electronic structure of the dithiocarbamate ligands on one hand, and covalency, intensity, and ligand field spectroscopic parameters on the other, could be established. Moreover, the relative values of the emission quantum efficiencies obtained for these complexes, as well as their dependence with temperature, could be satisfactorily described by a theoretical methodology recently developed.