Mixed-valence Radical Cations Research Articles

DFT investigations of phenyldithiafulvene dimers at different oxidation states.

Oxidative dimerization of aryl-substituted dithiafulvenes (Ar-DTFs) presents an efficient C-C bond forming method for the preparation of diverse redox-active π-conjugated molecules and conductive polymers. Previous experimental data indicated a reaction pathway in which direct combination of two Ar-DTF radical cations is a key step. However, mechanistic details about how Ar-DTF dimers are formed under different oxidation states have not yet been clearly established prior to this work. The assembly of two Ar-DTF molecules generates a vast conformational and configurational landscape, which is quite complex but fundamentally important for understanding the dimerization mechanism. To cast a deep insight into this aspect, we have performed density functional theory (DFT) calculations at the M06-2X/Def2-SVP level of theory to thoroughly investigate the potential energy surfaces (PESs) of various dimers of a phenyl-substituted dithiafulvene (Ph-DTF) in the mixed-valence radical cation and dication states. Key stationary points in these PESs, including minimum-energy conformers (π-dimers and σ-dimers) as well as the transition states connected to them, were examined and compared. We have also calculated the binding energies of these dimers to evaluate the energetic driving forces for their formation. Based on our computational results, the roles that various Ph-DTF dimers play in different pathways of oxidative dimerization have been clarified.

Combining Chirality and Hydrogen Bonding in Methylated Ethylenedithio-Tetrathiafulvalene Primary Diamide Precursors and Radical Cation Salts

Methyl-and dimethyl-ethylenedithio-tetrathiafulvalene ortho-diamide donors Me-EDT-TTF(CONH2)2 (1a) and DM-EDT-TTF(CONH2)2 (1b) have been prepared by the direct reaction of the corresponding diester precursors with aqueous ammonia solutions. The neutral (rac)-1a, (S)-1a and (S,S)-1b donors have been characterized by single crystal X-ray diffraction. In the three compounds, which crystallized in the non-centrosymmetric monoclinic space-group P21, the amide groups are disordered, yet they form the classical intramolecular hydrogen bond for such ortho-diamide motif. Electrocrystallization experiments afforded the mixed valence radical cation salts [(S,S)-1b]2XO4 and [(R,R)-1b]2XO4 (X = Cl, Re) containing four independent donors in the asymmetric unit, with the positive charge localized essentially on two donors while the two others are neutral. The topology of the organic layer is of '-type. Single crystal resistivity measurements show semiconducting behavior for [(S,S)-1b]2ClO4 and [(R,R)-1b]2ReO4, with a room temperature conductivity of 5 10-5 S cm-1 and activation energies Ea ≈ 3000 K. Tight-binding band structure calculations of extended Huckel type in combination with DFT calculations are in agreement with the semiconducting behavior and suggest a localized Mott type semiconductor rather than a band gap semiconductor.

Magnetic Molecular Conductors Based on Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the Tris(chlorocyananilato)ferrate(III) Complex.

Electrocrystallization of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) organic donor in the presence of the [Fe(ClCNAn)3]3- tris(chlorocyananilato)ferrate(III) paramagnetic anion in different stoichiometric ratios and solvent mixtures afforded two different hybrid systems formulated as [BEDT-TTF]4[Fe(ClCNAn)3]·3H2O (1) and [BEDT-TTF]5[Fe(ClCNAn)3]2·2CH3CN (2) (An = anilato). Compounds 1 and 2 present unusual structures without the typical segregated organic and inorganic layers, where layers of 1 are formed by Λ and Δ enantiomers of the anionic paramagnetic complex together with mixed-valence BEDT-TTF tetramers, while layers of 2 are formed by Λ and Δ enantiomers of the paramagnetic complex together with dicationic BEDT-TTF dimers and monomers. Compounds 1 and 2 show semiconducting behaviors with room-temperature conductivities of ca. 6 × 10-3 S cm-1 (ambient pressure) and 1 × 10-3 S cm-1 (under applied pressure of 12.1 GPa), respectively, due to strong dimerization between the donors. Magnetic measurements performed on compound 1 indicate weak antiferromagnetic coupling between high-spin FeIII (SFe = 5/2) and mixed-valence radical cation diyads (BEDT-TTF)2+ (Srad = 1/2) mediated by the anilate ligands, together with an important Pauli paramagnetism typical for conducting systems.

Perylene π-Bridges Equally Delocalize Anions and Cations: Proportioned Quinoidal and Aromatic Content.

A complete experimental and theoretical study has been carried out for aromatic and quinoidal perylene-based bridges substituted with bis(diarylamine) and bis(arylimine) groups respectively. The through-bridge inter-redox site electronic couplings (VAB ) have been calculated for their respective mixed-valence radical cation and radical anion species. The unusual similitudes of the resulting VAB values for the given structures reveal the intervention of molecular shapes with balanced semi-quinoidal/semi-aromatic structures in the charge delocalization. An identical molecular object equally responding to the injection of either positive or negative charges is rare in the field of organic π-conjugated molecules. However, once probed herein for perylene-based systems, it can be extrapolated to other π-conjugated bridges. As a result, this work opens the door to the rational design of true ambipolar bulk and molecular conductors.

Charge-Separated Mixed Valency in an Unsymmetrical Acceptor-Donor-Donor Triad Based on Diarylboryl and Triarylamine Units.

In this study, we report the generation of new mixed-valence (MV) subspecies with charge-separated (CS) characters from an unsymmetrical acceptor-donor-donor (A-D-D) triad. The triad was synthesized by attaching a dimesitylboryl group (A) to a D-D conjugate that consisted of triarylamine (NAr3) units. The MV radical cation, obtained by chemical oxidation of the triad, exhibited a strong intervalence charge transfer (IVCT) absorption derived from the bis(NAr3)•+ moiety in the near-IR region. The charge-separated MV (CSMV) state, obtained by photoexcitation of the triad, caused a blue shift in IVCT energy in the femtosecond transient absorption spectra, reflecting a bias of positive charge distributions to the D end site. This resulted from increased electron density at the A site and restructuring of the central D site from NAr3 to NAr2 sites. Interestingly, any shift in the IVCT energy that was caused by the polarity of the solvent was minimal, reflecting the unique characteristics of the CSMV state. These findings represent the first detailed analysis of the CSMV state, including a comparison with conventional MV states. Therefore, this work provides new insights into counterion-free MV systems and their applications in molecular devices.

Mixed-Valence Cations of Di(carbazol-9-yl) Biphenyl, Tetrahydropyrene, and Pyrene Derivatives

Although bis(diarylamino) mixed-valence radical cations have been quite extensively studied, their bis(carbazolyl) analogues have not, even though the hole-transporting properties of species such as of 4,4′-bis(9H-carbazol-9-yl)-1,1′-biphenyl, CBP, are widely exploited in organic light-emitting diodes. This work reports the generation by chemical oxidation of the radical cations of 4,4′-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,1′-biphenyl (a model for the unstable radical cation of CBP), 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-4,5,9,10-tetrahydropyrene, and 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyrene. The visible and near-IR spectra of these cations have been compared to those of the corresponding dication spectra, to the spectrum of the 3,6-di-tert-butyl-9-(4-(tert-butyl)phenyl)-9H-carbazole radical cation, and to the results of time-dependent density-functional calculations. The biphenyl- and pyrene-bridged species are found to be localized (class-II) mixed-valence compounds, whereas stronger...

Influence of P-Bonded Bulky Substituents on Electronic Interactions in Ferrocenyl-Substituted Phospholes.

2,5-Diferrocenyl-1-Ar-1H-phospholes 3 a-e (Ar=phenyl (a), ferrocenyl (b), mesityl (c), 2,4,6-triphenylphenyl (d), and 2,4,6-tri-tert-butylphenyl (e)) have been prepared by reactions of ArPH2 (1 a-e) with 1,4-diferrocenyl butadiyne. Compounds 3 b-e have been structurally characterized by single-crystal XRD analysis. Application of the sterically demanding 2,4,6-tri-tert-butylphenyl group led to an increased flattening of the pyramidal phosphorus environment. The ferrocenyl units could be oxidized separately, with redox separations of 265 (3 b), 295 (3 c), 340 (3 d), and 315 mV (3 e) in [NnBu4 ][B(C6 F5 )4]; these values indicate substantial thermodynamic stability of the mixed-valence radical cations. Monocationic [3 b](+)-[3 e](+) show intervalence charge-transfer absorptions between 4650 and 5050 cm(-1) of moderate intensity and half-height bandwidth. Compounds 3 c-e with bulky, electron-rich substituents reveal a significant increase in electronic interactions compared with less demanding groups in 3 a and 3 b.

Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes

Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on π-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that π-extended TTFs based on the indenofluorene core, prepared by Horner–Wadsworth–Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral·cation]) and π-dimer ([cation·cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin–Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6−, BF4−, and TaF6−) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed – as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.

Electroactive tetrathiafulvalene based pyridine-mono and -bis(1,2,3-triazoles) click ligands: synthesis, crystal structures and coordination chemistry

Electroactive ligands picoline-1,2,3-triazole-5-DM-TTF and lutidine-bis(1,2,3-triazole-5-DM-TTF) are synthesized by click chemistry following the ruthenium catalyzed azide–alkyne cycloaddition strategy. In the solid state structure of the bis(triazole) derivative, a tweezer conformation is observed, with the two TTF units arranged in a parallel manner. Cyclic voltammetry measurements show a splitting of the first oxidation wave, demonstrating sequential oxidation of the TTF donors in the intramolecular in through space mixed valence radical cation and then dication, suggesting that the tweezer conformation is maintained also in solution. A cobalt(II) complex based on the chelating picoline-triazole ligand is described. The lutidine-bis-triazole ligand shows no chelating behavior in the two structurally reported cadmium(II) complexes. In one of the complexes, the Cd(II) center is coordinated by four triazole rings from four different ligands, while the other four triazole units establish hydrogen bonds with one axially coordinated water molecule. The second complex is a one-dimensional coordination polymer, containing the CdCl2 fragment, in which the repeating motif is a pentanuclear “Cd5Cl10” unit decorated with two TTF ligands coordinated in a ditopical mode through the triazole units. The TTF donors establish intra- and intermolecular S⋯S contacts.

Charge transfer complexes and radical cation salts of chiral methylated organosulfur donors

The single crystal X-ray structure of the all-axial conformer of the (R,R,R,R) enantiomer of the chiral donor tetramethyl-BEDT-TTF (TM-BEDT-TTF) was described and compared to the all-equatorial conformer. (S,S,S,S)-Tetramethyl-BEDT-TTF formed crystalline 1 : 1 complexes with TCNQ and TCNQ-F4, as well as a THF solvate of the TCNQ complex. Donors bis((2S,4S)-pentane-2,4-dithio)tetrathiafulvalene and (ethylenedithio)((2S,4S)-pentane-2,4-dithio)tetrathiafulvalene, which contain seven-membered rings bearing chirally oriented methyl groups, only formed complexes with TCNQ-F4. The TCNQ-F4 complexes contain planar organosulfur systems, in contrast to the TCNQ complexes in which there is minimal charge transfer. A variety of crystal packing modes were observed. Electrocrystallization experiments with both enantiomers and the racemic form of tetramethyl-BEDT-TTF afforded mixed valence radical cation salts with the AsF6 and SbF6 anions formulated as (TM-BEDT-TTF)2XF6 (X = As, Sb). Electrical conductivity was only found in one charge transfer complex, while the radical cation salts are all semiconducting.

Mixed Valency in Hydrogen Bonded ‘Dimers of Dimers’

Dimolybdenum quadruply bonded compounds containing a pendant lactam functional group form self-complementary hydrogen bonded 'dimers of dimers' in the solid-state and CH(2)Cl(2) solutions. Electrochemical studies in CH(2)Cl(2) show two consecutive one-electron redox processes corresponding to oxidation of the Mo(2)(4+) cores. Spectroelectrochemical studies on the 'dimers of dimers' show no evidence of intervalence charge transfer bands in the mixed valence radical cations formed by one-electron oxidation, indicating that they are examples of proton-coupled mixed valency.

Extent of M2δ to Ligand π-Conjugation in Neutral and Mixed Valence States of Bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) Dimetal Complexes (MM), Where M = Mo or W, and Their Adducts with Tris(pentafluorophenyl)boron

The reaction between W(2)(T(i)PB)(4), where T(i)PB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W(2)(T(i)PB)(2)(nic)(2), 2, and T(i)PBH. Compound 2 is related to the previously reported molybdenum analog, Mo(2)(T(i)PB)(2)(nic)(2), 1. Compounds 1 and 2 react with 2 equiv of B(C(6)F(5))(3) in THF to form the adducts M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2), 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M(2)(T(i)PB)(2)(nic-B(C(6)F(5))(3))(2)·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M(2) δ to π* MLCT transitions, and upon complexation with B(C(5)F(5))(3) to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE(1/2) = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE(1/2) values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M(2) δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C(5)Me(5))(2)Co leads to the anions 1B(-) and 2B(-), which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV-vis-NIR absorption. The EPR spectra of 1B(-) and 2B(-) are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B(-)) and 4500 cm(-1) (2B(-)), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations [(Bu(t)CO(2))(3)M(2)](2)-μ(2)-(O(2)C-CO(2))(+), to which they are remarkably similar, as well as with other organic-based mixed valence systems.

Electron Transfer within Charge-Localized Arylhydrazine-Centered Mixed Valence Radical Cations Having Larger Bridges

Kinetics for intramolecular charge transfer between two diarylhydrazine units, measured by ESR, are reported for six charge-localized mixed valence compounds having 9, 11, 13, and 16 bonds between the nitrogen atoms. A 17-bond bridged compound had too slow electron transfer to measure the rate constant by ESR. The optical spectra of these radical cations are compared with tert-butyl,aryl-substituted hydrazines, and rate constants calculated using parameters derived from the optical spectra are compared with the experimental values where possible. The charge-transfer band overlapped too badly with bridge-centered absorption for the 16-bond bridged compound to allow the comparison to be made. The 13-bond bridged compound gave worse agreement than the other compounds. Its optical rate constant was about 5.4 times the ESR rate constant at a temperature between the ranges in which the data were collected.

Charge Distribution in Arylhydrazine-Centered Mixed Valence Compounds with Smaller Bridges (Five to Nine Bonds between Closest Nitrogens)

Charge distribution in six aromatic-bridged, aryldialkylhydrazine-centered mixed valence radical cations is discussed through consideration of their optical spectra. The compounds considered have two 2-phenyl-2,3-diazabicyclo-[2.2.2]octane-3-yl (HyPh) charge-bearing units linked by a 1,4-phenylene bridge and its p-methoxyphenyl (HyAn) analogue, as well as the (HyPh)(2)-substituted 1,4-naphthalene, 2,6-naphthalene, 9,10-anthracene, and 4,4'-biphenyl compounds in methylene chloride and acetonitrile. Consideration of band shape and position leads us to assign the 1,4-phenylene- and 2,6-naphthalene-bridged compounds as charge-delocalized (class III) in both solvents, but the 1,4-naphthalene-bridged one lies closer to the borderline, and appears to be charge-localized (class II) in acetonitrile. The 4,4'-biphenyl-bridged compound is clearly class II in acetonitrile, and possibly also in methylene chloride. The lowest energy absorption band for the 9,10-anthracene-bridged compound is assigned as a bridge-to-HyPh band, and its charge distribution is not clear. Problems with the often-used relationship that the electronic coupling is half the transition energy for the lowest energy band of class III mixed valence compounds are discussed, as is interpretation of the vertical reorganization energy near the class II, class III borderline.

Electron Paramagnetic Resonance Spectroscopy of Bis(triarylamine) Paracyclophanes as Model Compounds for the Intermolecular Charge-Transfer in Solid State Materials for Optoelectronic Applications

A set of seven bis(triarylamine) mixed-valence radical cations with different bridging moieties were investigated by temperature-dependent electron paramagnetic resonance (EPR) spectroscopy in methylene chloride and ortho-dichlorobenzene to evaluate the thermal electron transfer rate constants. The bridges used comprise [2,2]paracyclophane and [3,3]paracyclophane as well as fully conjugated phenylene spacers. The cyclophanes serve as model structures for studying the intermolecular electron transfer in solid state materials. The activation barriers derived by EPR measurements are compared with those estimated by the two-state Marcus−Hush analysis as well as by its extension to three states. Both methods gave good agreement with the EPR data, the three-state method being slightly better than the two-state method. On the basis of the choice of the different bridging groups, our study shows that the bridge can have a significant influence on the internal reorganization energy. [2,2]Paracyclophane and [3,3]paracyclophane bridge units show practically the same electronic coupling and thermal barrier. Conjugated bridges have thermal rates about 1 order of magnitude larger than the radical cations with broken conjugation. These two aspects show that in solid state materials triarylamines drawn close to their van der Waals radii may exhibit efficient coupling and rate constants by only 1 order of magnitude smaller than fully conjugated materials.

O⋯S vs. N⋯S intramolecular nonbonded interactions in neutral and radical cation salts of TTF-oxazoline derivatives: synthesis, theoretical investigations, crystalline structures, and physical properties

Racemic and enantiopure ethylenedithio-tetrathiafulvalene-thiomethyl-oxazoline (EDT-TTF-(SMe)-OX) derivatives have been synthesized. Single-crystal X-ray diffraction analyses reveal the establishment of O⋯S nonbonded interactions, unprecedented in the TTF series, characterized by short S⋯O distances and linear O⋯SMe motifs. Theoretical calculations at the DFT/B3LYP level on a model molecule demonstrate that both possible planar s-trans (O⋯S interaction) and s-cis (N⋯S interaction) conformations are energy minima, separated by an extremely weak energy gap. The energy barrier corresponding to the equilibrium between the two forms has been also estimated from DFT calculations, as well as their relative stability in the radical cation state of the TTF. According to the latter, there is a slight tendency towards the enhancement of the N⋯S interactions. Following the predictions of the theoretical calculations, the coexistence of both O⋯S and N⋯S nonbonded interactions is observed in two mixed valence radical cation salts of the racemic TTF with the dianionic cluster Mo6Cl14, prepared upon electrocrystallization. Interestingly, the ratio TTF : dianion is finely tuned by the choice of the electrocrystallization solvent. Physical measurements such as electrical conductivity and thermoelectric power on single crystals combined with magnetic susceptibility data in one case and extended Huckel tight-binding calculations demonstrate and rationalize the semiconducting behavior of both mixed valence salts. This study demonstrates that intramolecular N⋯S and O⋯S interactions can efficiently modulate and direct in the TTF series the occurrence of original solid-state structures provided with physical properties.

Fluorine Segregation in Crystalline Materials: Structural Control and Solid‐State [2+2] Cycloaddition in CF3‐Substituted Tetrathiafulvalene Derivatives

The well-known influence of long perfluorinated chains on the structures and stability of amphiphilic molecules in liquid crystalline mesophases or mesoscopic micellar arrangements is evaluated here in the realm of crystalline materials based on rigid aromatic molecules bearing only a limited number of CF(3) moieties. Tetrathiafulvalene (TTF) derivatives bearing one or two CF(3) groups, that is, (Z)- and (E)-(CF(3))(2)TTF ((Z)-1, (E)-1), EDT-TTF-CF(3) (2), and EDT-TTF(CF(3))(2) (3) (EDT=ethylenedithio) are prepared from the 1,3-dipolar reaction of methyl 4,4,4-trifluorotetrolate with ethylenetrithiocarbonate. The structures of neutral (Z)-1, (E)-1, 2, and 3 as indicated by single-crystal X-ray diffraction measurements reveal the recurrent formation of layered structures with a strong segregation of the fluorinated moieties and formation of fluorous bilayers, attributed to the amphiphilic character of those TTF derivatives upon CF(3) functionalization, and without need for longer C(n)F(2n+1) (n>1) perfluorinated chains. The short intermolecular distance between outer C==C double bonds observed in the layered structure of (E)-1 allows a solid-state [2+2] photocyclization with formation of chiral dyads incorporating the characteristic cyclobutane ring. These dyads containing two dihydrotetrathiafulvalene moieties facing each other exhibit reversible oxidation to the mixed-valence radical cation state and organize in the solid-state into the same layered structures with fluorous bilayers.

Electronically‐coupled MM quadruply‐bonded complexes of molybdenum and tungsten†

The reaction of M2(O2CBu(t))4 (M = Mo, W) with a dicarboxylic acid in toluene yields compounds of general formula [M2]-O2C-X-CO2-[M2] ([M2] = M2(O2CBu(t))3; X = conjugated spacer). The M2 units are electronically coupled via interactions between the M2 delta and dicarboxylate pi* orbitals, and the magnitude of this coupling is revealed by electronic structure calculations and spectroscopic data. These compounds show intense metal to ligand charge transfer (MLCT) absorptions in the visible region of the electronic spectrum that are temperature and solvent dependent. Evidence of electronic coupling is seen in their cyclic voltammograms, which show two successive one-electron oxidations. The extent of electronic coupling in the mixed valence radical cations [M2]-O2C-X-CO2-[M2]+, generated by oxidation with one equivalent of AgPF6 or FeCp2PF6, is evaluated by EPR and UV-vis-NIR spectroscopic data, and delocalized behavior is observed in compounds with W2 units separated by up to 13.6 angstroms. The simplicity of the frontier M2 orbital interactions with the bridge pi orbitals provides a convenient system with which to study electron transfer in mixed valence systems, as compared to the extensively studied, but more complicated, dinuclear t(2g)6/t(2g)5 mixed valence compounds. Oligomeric and polymeric compounds incorporating M2 units have also been synthesized, having general formula [M2(O2CR)2(O2C-Thio-CO2)]n (Thio = n-hexyl substituted ter- and quinque-thiophenes). They can be deposited as thin films by spin coating, and show photoluminescence and electroluminescence. These metallo-polythiophenes show potential for application in electronic materials. (

Highly substituted azulene dyes as multifunctional NLO and electron-transfer compounds.

Two highly substituted azulene derivatives were synthesised by Pd-mediated dimerisation from the corresponding tolan species. One azulene derivative (2) has donor functionalities (dianisylaminophenyl and dianisylamino) in the 1-, 2-, 3- and 6-positions, while the other (1) has donors (dianisylaminophenyl) in the 2- and 6-positions and acceptors (nitrophenyl) in the 1- and 3-positions. Each azulene derivative shows strong bond length alternation in the solid state, determined by X-ray crystal analysis, and an intense CT band around 450-500 nm in its UV/Vis spectrum. The first-order hyperpolarisability of 1 and of 2 was measured by hyper-Rayleigh scattering and is about that of disperse red DR1. Both azulene derivatives show multiple oxidation processes. The intramolecular adiabatic ET behaviour of the mixed valence radical cations of 1 and of 2 was investigated by UV/Vis/NIR spectroelectrochemistry. The intervalence-CT band of 1(+) could be analysed by the Generalised Mulliken-Hush theory, which yields an electronic coupling V=1140 cm(-1) for the optically induced adiabatic hole transfer.

The First Mixed Valence Radical Cation Salts of BEDT-TTF with the Photochromic Metal Mononitrosyl Complexes [RuNO X5] 2− ( X=Br, Cl) as Counterions

New bis(ethylenedithio)tetrathiafulvalene BEDT-TTF radical cation salts with the [RuNOX 5] 2− (X=Br, Cl) anions have been synthesized. The crystal structures of κ-(BEDT-TTF) 4[RuNOBr 5]·C 6H 5CN (1): a=8.747(2), b=12.088(1), c=17.300(2) Å, α=95.63(1), β=91.93(2), γ=94.79(2)°, V=1812.3(5) Å 3, P 1 , Z=1 and δ-(BEDT-TTF) 4[RuNOCl 5] 1.33 (2): a=6.721(1), b=15.016(2), c=35.386(3) Å, α=92.804(8), V=3567.0(8) Å 3, I2/ c, Z=2 have been determined. Their electronic band structures and transport properties have been studied. The radical cation salts κ-(BEDT-TTF) 4[RuNOCl 5]·C 6H 5CN (3) and δ-(BEDT-TTF) 4[RuNOBr 5] (4) have also been found to exist and are isostructural to 1 and 2, respectively. Evidence for commensurate and incommensurate structural modulations was found for 2 and 4, respectively. The crystals of 1 and 3 are semiconductors while those of 2 and 4 exhibit an M–I transition at 50 and 80 K, respectively.