Intervalence Transition Research Articles

A Tetrathiafulvalene‐Functionalized Radiaannulene with Multiple Redox States

Multiple states: Fusing a central expanded radiaannulene (RA) core with two tetrathiafulvalene (TTF) units furnishes a molecule that can exist in several redox states with characteristic UV/Vis/IR absorptions. The radical cation was found to be of mixed valence (MV) character as reflected by a low-energy intervalence transition.

ABAB Homoleptic Bis(phthalocyaninato)lutetium(III) Complex: Toward the Real Octupolar Cube and Giant Quadratic Hyperpolarizability

The concept of octupolar molecules has considerably enlarged the engineering of second-order nonlinear optical materials by giving access to 2D and 3D architectures. However, if the archetype of octupolar symmetry is a cube with alternating donor and acceptor groups at the corners, no translation of this ideal structure into a real molecule has been realized to date. This may be achieved by designing a bis(phthalocyaninato)lutetium(III) double-decker complex with a crosswise ABAB phthalocyanine bearing alternating electron-donor and electron-acceptor groups. In this communication, we present the first step toward this goal with the synthesis, crystal structure determination, and measurement of the molecular first-order hyperpolarizability β by harmonic light diffusion, of an original lutetium(III) sandwich complex displaying the required ABAB-type alternation for one face of the cube. This structure is characterized by an intense absorption in the near-IR due to an intervalence transition and exhibits the highest quadratic hyperpolarizability ever reported for an octupolar molecule, √<β(2)(HLS)> = 5750 × 10(-30) esu.

Redox-responsive organometallic foldamers from ferrocene amino acid: Solid-phase synthesis, secondary structure and mixed-valence properties

Oligoferrocenes Fmoc-Fca(n)-OMe (n=3-5) are assembled in a stepwise precise manner from Fmoc-protected ferrocene amino acid Fmoc-Fca-OH (H-Fca-OH = 1-amino-1'-ferrocene carboxylic acid; Fmoc = 9-fluorenylmethyloxycarbonyl) via amide bonds on solid supports by sequential Fmoc deprotection, acid activation and coupling steps. The resulting well-defined oligomers form ordered zigzag structures in THF solution with characteristic hydrogen bonding patterns. Electrochemical experiments reveal sequential oxidations of the individual ferrocene units in these peptides giving mixed-valent cations. Optical intervalence electron transfer is detected by intervalence transitions in the near-IR.

ChemInform Abstract: Photofragmentation of Mass-Selected (C6H6)+ n Clusters: Measurement of Monomer-Cluster Binding Energy for n = 7-15.

Benzene cluster cations up to 15 molecular units in size, produced in the throat of a supersonic nozzle by laser‐induced plasma formation, are studied by mass‐selected photofragmentation spectroscopy. All of the clusters exhibit a strong, broad absorption which extends at least from 690 to 1064 nm, centered near 900 nm. This absorption is assigned to the dimer based intervalence transition observed in solid state spectra of the dimer ions. Excitation into this band produces prompt ejection of neutral benzene molecules from the cluster ion. At low fragmentation laser intensities, a single daughter ion species is observed. We observe a simple dependence of the number of neutral molecules ejected on the fragmentation laser wavelength, strongly suggesting a sequential evaporation process of neutral benzene molecules. For clusters larger than six, an average bond energy ranges from 0.37–0.34 eV/molecule, quite close to the heat of vaporization for liquid benzene. The 14 molecule cluster is found to have an anomalously high binding energy, providing strong evidence that it forms a stable icosahedral structure. The general trend of the monomer bond energy with cluster size is well accounted for by a simple model based on macroscopic constructs and liquid benzene dielectric constant, surface tension, and heat of vaporization values.

Intervalence Charge Transfer in Cationic Heterotrinuclear Fe(III)−Rh(I)−Cr(0) Triads of the Polyaromatic Cyclopentadienyl−Indenyl Ligand

The challenge to realize polymetallic assemblies of unambiguous structure and stereochemistry, in which the nature of the intervalence transition (IT) is rationalized, has been faced by investigating the syn and anti isomers of η6-Cr(CO)3{η5-[(2-ferrocenyl)indenyl]Rh(CO)2} and their mixed-valence cations. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes, evidencing a significant difference in geometrical distortions and frontier MO composition between syn and anti isomers. Mixed-valence cations are generated and monitored by low-temperature spectroelectrochemistry in the visible, IR, and near-IR regions. The IT bands in the near-IR spectra are rationalized in the framework of Marcus−Hush theory and at quantum chemistry level by density functional theory. Noteworthy, the results reported provide rare experimental evidence that the presence of a third metal center (Rh) increases the metal−metal (Fe−Cr) interaction with ...

Long-distance electronic interaction in a molecular wire consisting of a ferrocenyl–ethynyl unit bridging two [(η5-C5H5)(dppe)M] metal centers

Several multinuclear ferrocenyl–ethynyl complexes of formula [(η5-C5H5)(dppe)MII−CC–(fc)n–CC–MII(dppe)(η5-C5H5)] (fc = ferrocenyl; dppe = Ph2PCH2CH2PPh2; 1: MII = Ru2+, n = 1; 2: MII = Ru2+, n = 2; 3: MII = Ru2+, n = 3; 4: MII = Fe2+, n = 2; 5: MII = Fe2+, n = 3) were studied. Structural determinations of 2 and 4 confirm the ferrocenyl group directly linked to the ethynyl linkage which is linked to the pseudo-octahedral [(η5-C5H5)(dppe)M] metal center. Complexes of 1–5 undergo sequential reversible oxidation events from 0.0 V to 1.0 V referred to the Ag/AgCl electrode in anhydrous CH2Cl2 solution and the low-potential waves have been assigned to the end-capped metallic centers. The solid-state and solution-state electronic configurations in the resulting oxidation products of [1]+ and [2]2+ were characterized by IR, X-band EPR spectroscopy, and UV–Vis at room temperature and 77 K. In [1]+ and [2]2+, broad intervalence transition band near 1600 nm is assigned to the intervalence transition involving photo-induced electron transfer between the Ru3+ and Fe2+ metal centers, indicating the existence of strong metal-to-metal interaction. Application of Hush’s theoretical analysis of intervalence transition band to determine the nature and magnitude of the electronic coupling between the metal sites in complexes [1]+ and [2]2+ is also reported. Computational calculations reveal that the ferrocenyl–ethynyl-based orbitals do mix significantly with the (η5-C5H5)(dppe)Ru metallic orbitals. It clearly appears from this work that the ferrocenyl–ethynyl spacers strongly contribute in propagating electron delocalization.

Radiative conductivity in the Earth’s lower mantle

Iron in crustal and mantle minerals adopts several possible oxidation states: this has implications for biogeochemical processes, oxygenation of the atmosphere and the oxidation state of the mantle. In the deep Earth, iron in silicate perovskite, (Mg(0.9)Fe(0.1))SiO(3), and ferropericlase, (Mg(0.85)Fe(0.15))O, influences the thermal conductivity of the lower mantle and therefore heat flux from the core. Little is known, however, about the effect of iron oxidation states on transport properties. Here we show that the radiative component of thermal conductivity in the dominant silicate perovskite material of Earth's lower mantle is controlled by the amount of ferric iron, Fe(3+). We obtained the optical absorption spectra of silicate perovskite and ferropericlase at pressures up to 133 GPa, corresponding to pressures at the core-mantle boundary. Absorption spectra of ferropericlase up to 800 K and 60 GPa exhibit minimal temperature dependence. The results on silicate perovskite show that optical absorption in the visible and near-infrared spectral range is dominated by O-Fe(3+) charge transfer and Fe(3+)-Fe(2+) intervalence transitions, whereas a contribution from the Fe(2+) crystal-field transitions is substantially smaller. The estimated pressure-dependent radiative conductivity, k(rad), from these data is 2-5 times lower than previously inferred from model extrapolations, with implications for the evolution of the mantle, such as generation and stability of thermo-chemical plumes in the lower mantle.

Synthesis, spectroscopic and electrochemical characterization and molecular structure of polypyridyl ruthenium complexes containing 4,4′-azobis(pyridine)

New coordination compounds of formulae: [Ru(trpy)(bpy)(4,4′-azpy)] 2+, 1, [Ru(tpm)(bpy)(4,4′-azpy)] 2+, 2, [(trpy)(bpy)Ru(4,4′-azpy)Ru(NH 3) 5] 4+, 3, and [(trpy)(bpy)Ru(4,4′-azpy)Ru(bpy)(trpy)] 4+, 5, with trpy = 2,2′:6′,2″-terpyridine, tpm = tris(1-pyrazolyl)methane), bpy = 2,2′-bipyridine and 4,4′-azpy = 4,4′-azobis(pyridine), have been synthesized as PF 6 - salts and characterized by spectroscopic and electrochemical techniques. The structure of the cation [Ru(trpy)(bpy)(4,4′-azpy)] 2+ has been determined by X-ray diffraction analysis, which reveals a trans-configuration of coordinated 4,4′-azpy and a packing structure that is based on extensive π-interactions within neighboring molecules. Upon controlled potential electrolysis of the dinuclear [Ru II,Ru II] complex 3, a stable [Ru II, Ru III] mixed-valent species, 4, was detected. From spectral data and Gaussian deconvolution analysis of the intervalence transition in 4, a strong electronic interaction between both ruthenium centers is disclosed ( H AB = 550 cm −1), pointing to possible applications of 4,4′-azpy as a conducting bridge in “molecular wires”.

Fresh Look at Electron-Transfer Mechanisms via the Donor/Acceptor Bindings in the Critical Encounter Complex

Seminal insights provided by the iconic R. S. Mulliken and his "charge-transfer" theory, H. Taube and his "outer/inner-sphere" mechanisms, R. A. Marcus and his "two-state non-adiabatic" theory, and N. S. Hush and his "intervalence" theory are each separately woven into the rich panoramic tapestry constituting chemical research into electron-transfer dynamics, and its mechanistic dominance for the past half century and more. In this Account, we illustrate how the simultaneous melding of all four key concepts allows sharp focus on the charge-transfer character of the critical encounter complex to evoke the latent facet of traditional electron-transfer mechanisms. To this end, we exploit the intervalence (electronic) transition that invariably accompanies the diffusive encounter of electron-rich organic donors (D) with electron-poor acceptors (A) as the experimental harbinger of the collision complex, which is then actually isolated and X-ray crystallographically established as loosely bound pi-stacked pairs of various aromatic and olefinic donor/acceptor dyads with uniform interplanar separations of r(DA) = 3.1 +/- 0.2 A. These X-ray structures, together with the spectral measurements of their intervalence transitions, lead to the pair of important electron-transfer parameters, H(DA) (electronic coupling element) versus lambdaT (reorganization energy), the ratio of which generally defines the odd-electron mobility within such an encounter complex in terms of the resonance stabilization of the donor/acceptor assembly [D, A] as opposed to the reorganization-energy penalty required for its interconversion to the electron-transfer state [D(+*), A(-*)]. We recognize the resonance-stabilization energy relative to the intrinsic activation barrier as the mechanistic binding factor, Q = 2H(DA)/lambdaT, to represent the quantitative measure of the highly variable continuum of inner-sphere/outer-sphere interactions that are possible within various types of precursor complexes. First, Q << 1 identifies one extreme mechanism owing to slow electron-transfer rates that result from the dominance of the intrinsic activation barrier (lambdaT) between the encounter and successor complexes. At the other extreme of Q > or = 1, the overwhelming dominance of the resonance stabilization (H(DA)) predicts the odd-electron mobility between the donor and acceptor to occur without an activation barrier such that bimolecular electron transfer is coincident with their diffusional encounter. In between lies a potentially infinite set of states, 0 < Q < 1 with opposing attractive and destabilizing forces that determine the location of the bound transition states along the reaction coordinate. Three prototypical potential-energy surfaces evolve as a result of progressively increasing the donor/acceptor bindings (H(DA)) extant in the precursor complex (at constant lambdaT). In these cases, the "outer-sphere" mechanism is limited by the weak donor/acceptor coupling that characterizes the now classical Marcus outer-sphere mechanism. Next, the "inner-sphere" mechanism derives from moderate (localized) donor/acceptor bindings and includes the mechanistic concept of the bridged-activated complex introduced by Taube for a wide variety of ligand-based redox dyads. Finally, the "interior" mechanism is also another subclass of the Taube (inner-sphere) classification, and it lies at the other extreme of very fast electron-transfer rate processes (heretofore unrecognized), arising from the spontaneous annihilation of the donor/acceptor dyad to the delocalized (electron-transfer) complex as it descends barrierlessly into the chemical "black hole" that is rate-limited solely by diffusion.

Crystal growth and below-bandgap optical absorption studies in InAs for non-linear optic applications

Abstract Indium arsenide (InAs) is a direct bandgap semiconductor ( E g =0.36 eV) and is well suited for non-linear optic (NLO) applications. The NLO applications require high linear optical transmission for below-bandgap energy photons in the material. Due to this stringent criterion, there is a need for understanding the fundamental optical absorption mechanisms in the material as well as correlating them with the presence of native defects and impurities introduced during the crystal growth. In this paper, we will present the experimentally measured and theoretically analyzed infrared absorption in an undoped and Te-doped n-type InAs in the wavelength region of 4–20 μm for horizontal Bridgman grown crystals. Transmission measurements at 300 K were performed on samples of undoped and Te-doped n-type InAs with carrier concentration in the range of 2.6×10 16 –1.7×10 17 cm −3 . The dependences of various absorption mechanisms as a function of wavelength have been presented. Free carrier absorption by electrons is found to be the dominant absorption mechanism at longer wavelengths ( λ >9 μm) for all the samples. At shorter wavelengths ( λ n i =1×10 15 cm −3 ), the inter-valence band transitions by free holes contributes significantly to the total absorption. Unique fitting parameters have been deduced to theoretically explain the infrared absorption mechanism in InAs for the above-mentioned carrier concentration range.

Synthesis and Electrochemical Properties of Tetranuclear Di-μ-oxo-bis[di-μ-phenolatodiiron(III)] Complexes

Abstract A series of tetrairon(III) complexes of di-μ-oxo-bis[di-μ-phenolatodiiron(III)] core, [{Fe2(L)(L′)}2(μ-O)2](PF6)2 (1–11), where L2− is the phenol-based macrocyclic compartmental ligand and (L′)− is acetate, monochloroacetate, dichloroacetate, trifluoroacetate, pentafluoropropionate, benzoate, p-nitrobenzoate, pentafluorobenzoate, diethyl phosphate, diphenyl phosphate, or diethyldithiophosphate for 1–11, respectively, were synthesized. X-ray crystallographic studies for 4, 7, and 9 indicated that two {Fe2(L)(L′)} units were connected by two oxo bridges to afford a tetranuclear di-μ-oxo-bis[di-μ-phenolatodiiron(III)] core. Cyclic voltammograms of 1–11 in acetonitrile showed a quasi-reversible or irreversible reduction wave between −332 and −620 mV (vs. Ag/Ag+), which were attributed to a two-electron process involving the Fe4III,III,III,III/Fe4II,II,III,III. The Fe4III,III,III,III/Fe4II,II,III,III potential shifted positive with a decrease in the electron-donating ability of the end-cap ligand, and a good linear correlation was found between the potentials and the pKa values of HL′. The Fe4II,II,III,III species were generated in acetonitrile by electrochemical reduction and showed broad intervalence transition bands in the range of 900–1500 nm.

Probing Mixed Valence in a New tppz-Bridged Diruthenium(III,II) Complex {(μ-tppz)[Ru(bik)Cl]2}3+(tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine, bik = 2,2‘-Bis(1-methylimidazolyl)ketone): EPR Silence, Intervalence Absorption, and νCOLine Broadening

The complex dication of the diruthenium(II) compound {(mu-tppz)[Ru(bik)Cl]2}(ClO4)2 can be oxidized and reduced in two one-electron steps each. In CH3CN/0.1 M Bu4NPF6, the odd-electron intermediates{(mu-tppz)[Ru(bik)Cl]2}n+, n=1 and 3, have comproportionation constants of 7x10(8) and 1x10(5), respectively. Both exhibit near-infrared absorptions, in the case of n=3 the 1640 nm band (epsilon=1200 M-1 cm-1, Deltanu1/2=1560 cm-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n=3) is EPR silent even at 4 K, the n=1 form shows g(parallel) 2.005 and g( perpendicular) 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz*- radical anion. The variation of energy and intensity of nuCO and of the ring vibration band around 1590 cm-1 has been monitored not only for {(mu-tppz)[Ru(bik)Cl]2}n+, n=0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}n+, n=0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm-1 on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(mu-tppz)[Ru(bik)Cl]2}3+ shows a perceptibly broader nuCO band, suggesting incomplete valence averaging (partial localization).

Magnetic behavior of Fe 3O 4 nanostructure fabricated by template method

One dimensional nanostructured magnetic materials are interesting because of their enhanced magnetic properties and potential applications such as information storage and biosensor. To prepare uniform nanostructure with controlled diameter, we have utilized porous anodic alumina template (AAT). The nanowire arrays are prepared by incorporation of monosized 10 nm magnetite nanoparticles into AAT, accompanied by thermal annealing. Transmission electron microscope and field emission scanning electron microscope showed that the nanoparticles were changed to the polycrystalline nanowire phase in AAT. The annealing procedure, which improved crystallinity of the nanoparticles and caused clustering of nanoparticles into nanowires, had a dramatic effect on their magnetic properties. For example, coercivity measured at cryogenic temperature increased from 350 to 8000 Oe after annealing. Clustering of the nanoparticles had also significant effect on the Faraday rotation spectra of the material. For example, inter-valence charge transfer transition of isolated magnetite nanoparticles vanished in wire assembles. It might be due to significant improvement of crystallinity of the wires, which might consists of enlarged Fe 3O 4 nanoparticles lined up in quasi-1D wires.

Charge Delocalization in the Special-Pair Radical Cation of Mutant Reaction Centers of Rhodobacter sphaeroides from Stark Spectra and Nonadiabatic Spectral Simulations

Stark and absorption spectra for the hole-transfer band of the bacteriochlorophyll special pair in the wild-type and L131LH, M160LH, and L131LH/M160LH mutants of the bacterial reaction center of Rhodobacter sphaeroides are presented, along with extensive analyses based on nonadiabatic spectral simulations. Dramatic changes in the Stark spectra are induced by the mutations, changes that are readily interpreted in terms of the redox-energy asymmetry and degree of charge localization in the special-pair radical cation. The effect of mutagenesis on key properties such as the electronic coupling within the special pair and the reorganization energy associated with intervalence hole transfer are determined for the first time. Results for the L131LH and M160LH/L131LH mutants indicate that these species can be considered as influencing the special pair primarily through modulation of the redox asymmetry, as is usually conceptualized, but M160LH is shown to develop a wide range of effects that can be interpreted in terms of significant mutation-induced structural changes in and around the special pair. The nonadiabatic spectra simulations are performed using both a simple two-state 1-mode and an extensive four-state 70-mode model, which includes the descriptions of additional electronic states and explicitly treats the major vibrational modes involved. Excellent agreement between the two simulation approaches is obtained. The simple model is shown to reproduce key features of the Stark effect of the main intervalence transition, while the extensive model quantitatively reproduces most features of the observed spectra for both the electronic and the phase-phonon regions, thus giving a more comprehensive description of the effect of the mutations on the properties of the special-pair radical cation. These results for a series of closely related mixed-valence complexes show that the Stark spectra provide a sensitive indicator for the properties of the mixed-valence complexes and should serve as an instructive example on the application of nonadiabatic simulations to the study of mixed-valence complexes in general as well as other chemical systems akin to the photosynthetic special pair.

A Redox‐Active Tri‐Star Molecule: Merging of TTF and HAT Chemistry.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A Joint Experimental and Computational Study on the Electronic Communication in Diethynylaryl‐Bridged (η5‐C5H5)Fe(η2‐dppe) and (η5‐C5H5)Fe(CO)2 Units

AbstractA family of bimetallic complexes [Cp(CO)2Fe–C≡C–Ar–C≡C–Fe(CO)2Cp] {Cp = C5H5; 6a–g: Ar = C4H2S (a), 3‐(C4H9)‐C4HS (b), 3‐(C16H33)‐C4HS (c), C6H4 (d), 2,5‐bis(OC4H9)‐C6H2 (e), 2,5‐bis(OC8H17)‐C6H2 (f), (C6H4)2 (g)} was prepared by the three‐step Pd‐catalysed extended one‐pot (EOP) synthetic protocol from Bu3Sn–C≡CH, X–Ar–X (X = I, Br) and Cp(CO)2FeI. Complexes 6a,d,g were then exposed to ultraviolet irradiation in the presence of an equivalent amount of 1,2‐bis(diphenylphosphanyl)ethane (dppe) to form the corresponding bimetallic complexes [Cp(dppe)Fe–C≡C–Ar–C≡C–Fe(dppe)Cp] (7a,d,g). Compounds 6a–g and 7a,d,g were characterised by cyclic voltammetry (CV). The most significant electrochemical information comes from the oxidation of the dppe derivatives. The ΔE° separations between the subsequent reversible waves suggest that the efficiency of the metal–metal electronic coupling decreases in the order 7a &gt; 7d &gt; 7g. Complexes 7a and 7g were also chemically oxidised with [Fe(Cp*)]2[BF4] {Cp* = C5(CH3)5} and [Fe(Cp)]2[BF4] respectively, and the near infrared (NIR) spectra of the mixed‐valence species 7a+ and 7g+ were recorded. A strong intervalence transition (IT) band was observed only for the radical cation 7a+. While this finding confirms the existence of an electronic interaction between the two termini when a 2,5‐thiophene group is present in the spacer, the NIR spectrum of 7g+ reveals a reduced efficiency in conveying electrons when the C4H2S moiety is replaced by a 4,4′‐biphenyl. In order to rationalise and quantify the extent of electronic communication, ruled by geometrical and electronic factors, density functional computational results on selected [Cp (PH3)2Fe] and [Cp(CO)2Fe] binuclear model complexes are reported. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

A redox-active tri-star molecule: merging of TTF and HAT chemistry

A planar pi-conjugated heteroaromatic molecule 1 has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor-acceptor nature in its neutral state and an intervalence charge-transfer transition in its 1(2+) mixed-valence state.

Synthesis of triester-functionalized molecular motors incorporating bis-acetylide trans-platinum insulating fragments

Bis-ferrocene compounds linked either by two triple bonds (1,4-di(ferrocenyl)butadiyne 1), or by the triple bond–platinum–triple bond sequence (trans-bis(ferrocenylethynyl)bis(triethylphosphine)platinum(II), 2) have been synthesized. The electronic coupling between the ferrocene groups has been estimated from the intensity of the intervalence transition in the electrochemically generated mixed valence complexes. Upon insertion of a platinum fragment a weak attenuation was observed, with the Vab parameter decreasing from 0.036 eV for 1 to 0.025 eV for 2. A theoretical study has also been performed, using a combination of DFT for geometry optimization and Extended Hückel Theory for the estimation of the electronic coupling. It was found that the electronic coupling decreases from 0.090 eV for 1 to 0.022 eV for a model of 2. In a second part of this work, we describe the synthesis of two molecular motors incorporating the ligand hydrotris[6-(ethoxycarbonyl)indazol-1-yl]borate which exhibits three pendant ester groups dedicated to be anchored onto an oxide surface. This stator is connected through a ruthenium centre to a pentasubstituted cyclopentadienyl rotor bearing ferrocene terminal electroactive groups, linked either by a phenylethynyl spacer (complex 4) or a spacer containing bis-acetylide trans-platinum insulating fragments (complex 8).

Synthesis and Photochromism of Organosiloxane-Based Organic/Inorganic Hybrid Containing an Inorganic Component Derived from Tungstic Acid

The organosilxane-based organic/inorganic hybrid was synthesized by adding 3-glycidoxypropyltrimethoxysilane (GPTMS) to an aqueous solution of tungstic acid and gelling it at 70-150°C. The FT-Raman and 13 C NMR studies revealed that the alkoxy and epoxy groups of GPTMS were subject to hydrolysis and ring-opening reaction, respectively. The 29 Si NMR study revealed that the inorganic component derived from tungstic acid appeared to have Si-O-W bonds with the GPTMS-derived siloxane network. The GPTMS-derived organic/inorganic hybrid containing the inorganic component derived from tungstic acid was transparent and colorless. In addition, it showed the photochromism that resulted from the incorporation of the inorganic component derived from tungstic acid into the GPTMS-derived organic/inorganic hybrid. A broad peak around 880 nm with a shoulder peak around 650 nm was observed in the spectra after the UV-irradiation, meaning two kinds of d-d bond intervalence transition states exist in the organic/inorganic hybrid. Assuming that the photochromism was the first-order reaction, the reaction rate constant was estimated to be 1.9 × 10 -1 min -1 .

Intervalence Transitions in the Mixed-Valence Monocations of Bis(triarylamines) Linked with Vinylene and Phenylene−Vinylene Bridges

(E)-4,4'-Bis[bis(4-methoxyphenyl)amino]stilbene, 1, (E,E)-1,4-bis[4-[bis(4-methoxyphenyl)amino]styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4'-bis[4-[bis(4-methoxyphenyl)amino]styryl]stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-[bis(4-methoxyphenyl)amino]styryl]styryl)benzene, 4, have been oxidized to their mono- and dications using tris(4-bromophenyl)aminium hexachloroantimonate. The intervalence charge-transfer (IVCT) band of 1(+) is narrow and asymmetric and exhibits only weak solvatochromism. Analysis of this band indicates that 1(+) is a class-III or class-II/III borderline mixed-valence species. In contrast, a broad, strongly solvatochromic IVCT band is observed for 2(+), indicating that this species is a class-II mixed-valence species. The assignment of 1(+ ) and 2(+) as symmetric class-III and unsymmetric class-II species, respectively, is also supported by AM1 calculations. Hush analysis of the IVCT bands of both 1(+) and 2(+) gives larger electronic couplings, V, than for their analogues in which the double bonds are replaced with triple bonds. The diabatic electron-transfer distance, R, in 1(+) can be estimated by comparison of the V estimated by Hush analysis and from the IVCT maximum; it is considerably less than the geometric N-N separation, a result supported by quantum-chemical estimates of R for 1(+)-4(+). In 3(+) and 4(+), the IVCT is largely obscured by an intense absorption similar to a band seen in the corresponding dications and to that observed in the monocation of a model compound, (E,E,E)-1-[bis(4-methoxyphenyl)amino]-4-[4-[4-(4-tert-butylstyryl)styryl]styryl]benzene, 5, containing only one nitrogen redox center; we attribute this band to a bridge-to-N(+) transition. The corresponding dications 1(2+)-4(2+) show a complementary trend in the coupling between redox centers: the shortest species is diamagnetic, while the dication with the longest bridge behaves as two essentially noninteracting radical centers.