Search

Spectral and photochemical studies of ferrocene, ruthenocene, and several of their benzoyl-substituted derivatives have provided valuable information about the electronic structures and excited state reactivities of these group 8 metallocenes. The presence of a benzoyl group on one or both cyclopentadienyl rings of the metallocene strongly influences the electronic transitions observed in the UV–vis spectral region. While the parent complexes display low-intensity, solvent-insensitive ligand field absorption bands, the monobenzoyl and 1,1′-dibenzoyl derivatives exhibit bands that are much more intense and sensitive to the solution environment. This behavior results from the mixing of appreciable metal-to-ligand charge transfer (MLCT) character into the low-energy excited states of the benzoyl-substituted complexes. Resonance Raman data lend strong support to this MLCT assignment. Irradiation of a metallocene dissolved in ethyl 2-cyanoacrylate results in the anionic polymerization of the electrophilic monomer. Studies of the kinetics and mechanism of this photoinitiated polymerization process reveal that it occurs by two competing charge transfer-induced pathways, the relative importance of which depends upon the identity of the metal and the presence or absence of the benzoyl substituent.

The crystal structure, quantum size effect, surface chemistry, photoinduced redox processes and photocatalytic selective oxidation properties of quantum confined Q-TiO 2 nanoparticles produced by a Sol–Gel method were studied by TG–DSC, X-ray diffraction, diffuse reflectance spectra, X-ray photoelectron spectroscopy, EPR and selective oxidation photocatalytic measurements.

The structure, physical characteristics and photocatalytic selective oxidation properties of nanometer-size TiO2 particles produced by a sol–gel method were studied by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and photocatalytic selective oxidation measurements. Analysis of the XRD results shows that sol–gel-produced TiO2 nanoparticles have the anatase structure at annealing temperatures ≤973 K, that the rutile structure begins to emerge at annealing temperatures ≥973 K and the particles have the pure rutile structure at 1023 K. DRS indicates that the obtained TiO2 nanoparticles exhibit a blue shift with decreasing crystallite size. Analysis of the XPS results shows that the TiO2 nanoparticles have a lot of oxygen vacancies. The EPR spectrum of TiO2 at 77 K is composed of a strong isotropic EPR Surface-Ti3+ signal(I) at g = 1.926 and a weak broad Bulk-Ti3+ signal (II) at g = 1.987. Quantitative EPR indicates that both signals show a size and temperature dependence. Photocatalytic oxidation of cyclohexane into cyclohexanol with high selectivity and activity has been obtained by activation of molecular oxygen over sol–gel-produced TiO2 nanoparticles under mild conditions in dry solvent, which reveals that the quantum size effect and surface state effect of nanoparticles are key points for governing the selective photocatalytic reaction. The photocatalytic oxidation mechanism under dry solvent is different from that in aqueous solutions. Copyright © 2003 Society of Chemical Industry

The classical metallocenes, ferrocene and ruthenocene, dissolve readily in neat ethyl 2-cyanoacrylate (CA) monomer. The electronic spectra of the resulting solutions display a near-ultraviolet absorption band assigned as a charge-transfer-to-solvent (metallocene → CA) transition. Irradiation into this band causes the one-electron oxidation of the metallocene to the corresponding metallocenium cation accompanied by reduction of CA to its radical anion. Addition of the latter species to CA initiates the anionic polymerization of the electrophilic monomer. The progress of photoinitiated polymerization was monitored in real time by attenuated total reflectance infrared spectroscopy.

Photosensitive materials composed of a substrate (monomer, oligomer or polymer) and a photoinitiator play a central role in numerous commercial processes. Most photoinitiators in common use are organic compounds that, upon irradiation, produce radicals and/or acids. The desire to expand the scope of photoinitiated chemistry has led to the recent discovery of several classes of inorganic and organometallic photoinitiators that photochemically release neutral Lewis bases and/or anions. This article provides an overview of the author's research program to develop novel photobase and photoanion generators. Fundamental studies of these systems and examples of their potential applications are described.

Spectroscopic, photochemical, and photoinitiation studies are reported for a series of monobenzoylferrocenes and 1,1‘-dibenzoylferrocenes. These nonionic metallocene complexes absorb strongly in the ultraviolet and visible wavelength regions owing to electronic transitions of mixed ligand field/charge-transfer character. Irradiation of the complexes in methanol results in ring−metal cleavage to yield a benzoyl-substituted cyclopentadienide carbanion and the corresponding half-sandwich iron(II) complex. This process occurs with an appreciable quantum efficiency (>0.30 at 546 nm) for 1,1‘-dibenzoylferrocenes; in contrast, monobenzoylferrocenes are appreciably less photosensitive. When the solvent is neat ethyl α-cyanoacrylate, the photogenerated carbanion initiates rapid anionic polymerization of this electrophilic monomer. Several factors that can influence the performance of benzoyl-substituted ferrocenes as anionic photoinitiators are considered.

Ruthenium(II) sandwich complexes of the type Ru( η 6 -arene) 2 2+ (arene is benzene or 1,3,5-trimethylbenzene) undergo photosubstitution of one arene ring by solvent (S) to yield Ru( η 6 -arene)(S) 3 2+ . Reaction occurs from the lowest ligand field triplet state of the complex with a quantum efficiency that decreases with increasing methylation of the arene. Half-sandwich complexes of general formula Ru( η 6 -arene)(L) 3 2+ (L is CH 3 CN or NH 3 ) undergo competitive photosubstitution of arene and L by solvent. The relative importance of these pathways depends upon a number of factors, including excitation wavelength, solvent, and the nature of L. Mechanisms for these photoreactions are discussed and comparisons to earlier studies are presented.

Ultraviolet irradiation of Pt(acac)2 (acac- is the anion of acetylacetone) in nonaqueous solvents results in competing photochemical processes whose relative importance depends upon the solution en...

Irradiation of Ru( η 6 -mes) 2 2+ (mes is 1,3,5-trimethylbenzene, or mesitylene) in acetonitrile results in the substitution of one mesitylene ring by solvent to yield the half-sandwich complex, Ru( η 6 -mes)(CH 3 CN) 3 2+ . The quantum efficiency of this process exhibits little dependence upon the excitation wavelength or the presence of oxygen, but varies with solvent in the order acetonitrile > water > acetone. Photolysis of Ru( η 6 -mes)(CH 3 CN) 3 2+ in acetonitrile occurs via two competing pathways: substitution of mesitylene by solvent to produce Ru(CH 3 CN) 6 2+ , and exchange of coordinated CH 3 CN with bulk solvent. Quantum efficiencies for both pathways increase with decreasing excitation wavelength and, at each wavelength, CH 3 CN exchange is more efficient than mesitylene substitution. Photochemical mechanisms for these processes are discussed and comparisons with related complexes are presented.

Continuous photolysis, flash photolysis and photosensitization studies are reported for Fe(η6-mes)22+, where mes = 1,3,5-trimethylbenzene (mesitylene). The complex absorbs in the visible and ultraviolet spectral regions owing to the presence of ligand field, mes → Fe charge transfer, and intraligand π-π∗ absorption bands. Selective irradiation of the various bands causes substitution of both mesitylene ligands by solvent. The quantum efficiency of arene deligation ranges from 0.6 to 1.0 depending upon the excitation wavelength, but is independent of complex concentration, solvent, oxygen content and excess PF6− counterion. Reaction occurs from both ligand field and mes → Fe charge transfer excited states, with no evidence of communication between the states.

A series of aliphatic polycarbonates based on the condensation copolymerization of the bischloroformate of 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCBD) with itself and the diols of 3-cyclohexene-1,1-dimethanol, 1,4-cyclohexanediol (CHD), in various molar ratios, exo,exo-tricyclo[2.2.1.02,6]heptane-3,5-diol, and 4,8-bis(hydroxymethyl)tricyclo[5.2.102,6]decane (BTD) were synthesized. Incorporation of cyclic and tricyclic repeat units produced polycarbonates with high glass transition temperatures (Tg's), high molecular weights, and low refractive indexes. The crystallinity of the TMCBD homopolymer was minimized in the CHD/TMCBD copolymers with little effect on Tg but with decreased thermal stability. Thermal analysis yields decomposition temperatures ranging from 354 °C for the TMCBD homopolymer to 292−298 °C for the CHD/TMCBD copolymers; all polymers undergo complete thermal degradation. Refractive indexes range from 1.493 for the 1:1 copolymer of BTD and TMCBD to 1.481 for the CHD/TMCBD copolymers. Thi...

The photocatalytic destruction of 2,4,6-trinitrotoluene (TNT) in aqueous suspensions of titanium dioxide (TiO2, Degussa type P25) was investigated. Under aerobic conditions, irradiation with the Pyrex-filtered output of a 200-W high pressure mercury-arc lamp resulted in complete (95 ± 5%) mineralization of 50 ppm of TNT within a few hours. Rapid photodegradation of TNT also occurred under anaerobic conditions but without appreciable mineralization. In the absence of TiO2, direct photolysis of TNT yielded a complex mixture of colored organic products in both aerobic and anaerobic environments.

The cationic cobalt(III) am(m)ine complexes, Co(NH 3 ) 6 3+ , Co(en) 3 3+ (en is ethylenediamine) and Co(NH 3 ) 5 Br 2+ , form ion pairs with the tetraphenylborate anion in methanol and 1:9 methanol/dichloromethane solutions. These ion pairs display a tetraphenylborate-to-metal charge transfer absorption band at ∼ 320 nm that tails into the visible wavelength region. Irradiation into this band causes highly efficient redox decomposition of the ion pair owing to the rapid thermal reactions of the primary photoproducts: the corresponding cobalt(II) complex and the tetraphenylboranyl radical. The photoredox quantum yield depends upon complex concentration, excitation wavelength, solvent polarity and concentration of dissolved oxygen. Incorporation of cobalt(III) am(m)ine tetraphenylborate complexes into thin films of an epoxide-functionalized polymer affords photosensitive materials that undergo crosslinking upon irradiation and subsequent heating.

Photoinduced loss of one or both ligands from bis(arene) metal complexes provides an attractive route to cationic Lewis acids that can function as initiators for polymerization and crosslinking reactions. In this article, we examine the spectroscopy, photochemistry and photoinitiation activity of two such complexes: Fe(mes)22+ (mes is mesitylene) and Cr(tol)2+ (tol is toluene). Acetonitrile solutions of Fe(mes)22+ undergo photochemical loss of both mesitylene rings to yield solvated Fe2+. The quantum efficiency of deligation ranges from 0.6 to 1.0 depending upon excitation wavelength, but is independent of complex concentration, solvent, oxygen concentration and excess counter-ion. Reaction appears to occur from both ligand field and ligand-to-metal charge transfer excited states. Low thermal stability, however, limits the use of Fe(mes)22+ as a practical photoinitiator. Deligation of both arene rings also occurs upon 366 nm irradiation of Cr(tol)2+ in acetonitrile. The quantum efficiency for this process is 0.04 and is independent of oxygen concentration. Photoinitiated crosslinking and insolubilization occur in a thin film of COP (copolymer of glycidyl methacrylate and ethyl acrylate) containing Cr(tol)2+.

Electronic absorption and resonance Raman spectral studies of benzoylruthenocene (BRc) and 1,1'-dibenzoylruthenocene (DRc) indicate that the low-energy electronic excited states of these 4d(6) metallocenes possess metal-to-ligand charge transfer (MLCT) character. While this MLCT contribution should weaken the metal-ring bonding in the excited state, neither compound is photosensitive in nonhalogenated solvents such as methanol, acetonitrile, and cyclohexane. In contrast, irradiating BRc and DRc in the good electron-accepting solvent, carbon tetrachloride, results in ring loss via a pathway that appears to originate from a charge-transfer-to-solvent excited state. Both metallocenes function as photoinitiators for the anionic polymerization of ethyl 2-cyanoacrylate, and the kinetics and mechanism of this process have been investigated. Comparing the present results on BRc and DRc with those reported earlier for the corresponding benzoyl-substituted ferrocene compounds reveals some interesting commonalities and differences between the excited-state properties of these 3d and 4d metallocenes.

Electrospray ionization mass spectrometry is used as an on-line technique to investigate the solution photochemistry of [CpFebz]+ (Cp, eta5-cyclopentadienyl; bz, eta6-benzene). Direct irradiation of samples in the optically transparent tip of the electrospray source allows the detection of products with lifetimes down to the millisecond regime. Photolysis of [CpFebz]+ in acetonitrile (AN) yields the short-lived half-sandwich complex, [CpFe(AN)3]+. The suspected parent-offspring relationship between this species and the fully ring-deligated product, [Fe(AN)6]2+, has been confirmed. In solutions containing cyclohexene oxide, [CpFebz]+ generates several photoproducts capable of initiating the cationic polymerization of the epoxide monomer. The initiation process and the first few steps in the subsequent growth of the polymer chain have been observed.

Solutions of [CpFebz]PF6 (Cp = η5-cyclopentadienyl, bz = η6-benzene) were photolyzed in the tip of an electrospray ionization source and the photoproducts analyzed by mass spectrometry. Two main series of products were found in acetonitrile (AN): [CpFe(AN)1-3]+ and [Fe(AN)3-6]2+. In 1,2-dichloroethane solutions containing cyclohexene oxide (CHO), three main product series were detected: [(H2O)Fe(CHO)4-12]2+, [(H2O)CpFe(CHO)1-3]+, and [X(CHO)0-5]+, where X+ (m/z = 140) is a non-iron species. Involvement of these CHO-containing products in initiating epoxide polymerization is discussed.

Spectroscopic and photochemical studies of several benzoyl-functionalized ferrocene complexes in nonaqueous solvents are reported. Bands observed above 300 nm in the electronic absorption spectrum of the unsubstituted complex, Fe(eta(5)-C(5)H(5))(2), and assigned to ligand field transitions shift to longer wavelengths and intensify upon introduction of a benzoyl group into one or both cyclopentadienide rings. Such behavior suggests that these transitions have acquired some charge-transfer character. Visible-light (546 nm) irradiation of 1,1'-dibenzoylferrocene, III, dissolved in CH(3)CN, CH(3)OH, or ethyl alpha-cyanopropionate causes ring-metal cleavage to produce the benzoylcyclopentadienide ion, C(6)H(5)C(O)C(5)H(4)(-), and the corresponding half-sandwich cationic complex, Fe[(eta(5)-C(5)H(4))C(O)C(6)H(5)](S)(3)(+) (S is solvent). The disappearance quantum yield, phi(dis), for III is 0.45 in CH(3)OH and 0.28 in ethyl alpha-cyanopropionate and is unaffected by the presence of dissolved O(2), added H(2)O (10 000 ppm), or added methanesulfonic acid (30 ppm). 1,1'-Dibenzoylferrocenes containing substitutents on both phenyl rings undergo photoinduced ring-metal cleavage in CH(3)OH with phi(dis) values very similar to that of III, while monobenzoylferrocenes are appreciably less photoreactive. A mechanism that accommodates the photochemical behavior of benzoyl-functionalized ferrocene complexes is discussed. In addition, a previous suggestion concerning the role of III in the photoinitiated anionic polymerization of an alpha-cyanoacrylate monomer is reconsidered in light of the present study.

Photosensitive materials composed of a substrate (monomer, oligomer or polymer) and a photoinitiator play a central role in numerous commercial processes. Most photoinitiators in common use are organic compounds that, upon irradiation, produce radicals and/or acids. The desire to expand the scope of photoinitiated chemistry has led to the recent discovery of several classes of inorganic and organometallic photoinitiators that photochemically release neutral Lewis bases and/or anions. This article provides an overview of the author's research program to develop novel photobase and photoanion generators. Fundamental studies of these systems and examples of their potential applications are described.

Thin films composed of the copolymer of glycidyl methacrylate-ethyl acrylate and a cobalt(III)-am(m)ine tetraphenylborate salt undergo crosslinking upon deep-UV irradiation and subsequent heating. The mechanism of this process involves the efficient photoredox decomposition of the cobalt salt with the release of multiequivalents of Lewis base. In a thermally activated step, the base then crosslinks the copolymer via nucleophilic attack on the epoxide rings. Near-UV irradiation of Pt(acac)2 (acac is the acetylacetonate anion) and Ru(Cp)2 (Cp is the cyclopentadienide anion) dissolved in ethyl (alpha) -cyanoacrylate initiates rapid anionic polymerization of the acrylate monomer. The active initiating species is acac in the former system, while the radical anion of the monomer is a likely candidate in the latter system.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Photosensitive Metal-Organic Systems: An Overview Reactive Intermediates in the Carbonylation of Metal Alkyl Bonds Studied by Time-Resolved Infrared Spectral Techniques Photochemical Reactions of Organometallic Molecules on Surfaces: CO Substitution Chemistry of Surface-Confined Derivatives of (*h]5-C[5H[5) Mn(CO)[3 Photocatalytic Behavior of Tungsten, Iron, and Ruthenium Carbonyls on Porous Vycor Glass Metal-Organic Photochemistry in the Millisecond to Picosecond Time Domain: A View to the Formation and Dissociation of Cu-C Bonds Ru(II) Diimine Complex Photosensitized Reduction of Alkyl and Aryl Halides: Inner Sphere and Outer Sphere Approaches Photochemistry and Redox Catalysis Using Rhenium and Molybdenum Complexes Photoredox Chemistry of d]4 Bimetallic Systems Patterned Imaging of Palladium and Platinum Films. Electron Transfers to and from Photogenerated Organometallic Radicals Photoinduced Electron Transfer Reactions Between Excited Transition Metal Complexes and Redox Sites in Enzymes Luminescence Probes of DNA-Binding Interactions Involving Copper Complexes Molecular Models for Semiconductor Particles: Luminescence Studies of Several Inorganic Anionic Clusters Polyoxometalates in Catalytic Photochemical Hydrocarbon Functionalization, and Photomicrolithography. Excited State Lifetimes and Subsequent Thermal Processes involving W[1[0O[3[2]4]- Photoredox Chemistry of Metal Complexes in Microheterogeneous Media Heterogeneous Photocatalyzed Oxidations of Phenol, Cresols, and Fluorophenols in TiO[2 Aqueous Suspensions Homogeneous Metal Catalyzed Photochemistry in Organic Synthesis Photooxidation of Metal Carbynes Photocatalysis Induced by Light-sensitive Coordination Compounds Photoredox Reactivity of Copper Complexes and Photooxidations of Organic Substrates Light Sensitive Organometallic Compounds in Photopolymerization Photoinitiator Activity, Electrochemistry and Spectroscopy of Cationic Organometallic Compounds

Photoinitiated polymerization and cross-linking reactions form the basis of numerous applications in the coatings, reprographic, and microelectronics industries.1–5 Nearly all of the commercially significant photoinitiators are organic compounds that generate free radicals and/or strong acids upon exposure to light.5–8 Common examples include benzoin and its derivatives, benzil ketals, acetophenone derivatives, aromatic ketone/amine combinations, and onium salts belonging to the diaryliodonium, triarylsulfonium, and triarylseleonium families. Despite a substantial number of studies exploring the potential of inorganic and organometallic coordination complexes as photoinitiators, these systems have yet to gain widespread acceptance. Several factors contributing to this dominance of the photoinitiator market by organic compounds can be cited: (1) no compelling case had been made that inorganic and organometallic complexes that generate free radicals offer significant advantages over the traditional organic free radical sources, (2) the facile production of strong Bronsted or Lewis acids via the photodecomposition of onium salts had no counterpart among inorganic and organometallic complexes, and (3) electronic structure-reactivity relationships and other fundamental concepts needed to understand the primary photochemical processes in photoinitiator systems were more advanced for organic compounds.

Thin films composed of the copolymer of glycidyl methacrylate-ethyl acrylate and cobalt(III) ammine or alkylamine salts as photoinitiators undergo crosslinking upon deep-uv irradiation and subsequent heating. The mechanism of crosslinking involves nucleophilic attack by photoliberated am(m)ime base on the epoxide ring of the copolymer. The quantum efficiency for photodecomposition of the cobalt(III) salts is about 1% at 254 nm. Oxygen plasma etching studies suggest that the cobalt(III) salts are dispersed as submicron particles throughout the film.

The structure, physical characteristics and selective photocatalytic oxidation properties of quantum confined nanosize iron doped TiO2 (Q-TiO2/Fe3+) particles were studied by TG-DSC, XRD, DRS, EPR and Selective Oxidation Photocatalytic Measurements. It is shown that the solubility of iron in the obtained Q-TiO2/Fe3+ nanoparticles is 1.0 atom% and the iron doping level has a great influence on the transformation of anatase to rutile (A-R). The quantum confined effect was observed for Q-TiO2/Fe3+ nanoparticles. All of the samples have EPR Bulk-Fe3+ and Surf-Fe3+ signals, which are located in the bulk and surface of TiO2 nanoparticles respectively. Quantitative EPR results indicate that the relative EPR intensity of these paramagnetic centers shows regular change with varying corresponding iron modification level. In situ EPR indicates that the photo-generated charge carrier (h+, e−) could be trapped by different Fe3+ sites simultaneously, i.e., trapping of h+ is due to Surf-Fe3+ sites at g = 4.30, whereas that of e− is attributed to Bulk-Fe3+ sites at g = 1.99. Selective photocatalytic oxidation of cyclohexane into cyclohexanol with higher selectivity has been obtained by molecular oxygen activation over Q-TiO2/Fe3+ nanoparticles under mild conditions. It is thought that the optical surface state of Q-TiO2/Fe3+ nanoparticles play a key role in the selective photocatalytic oxidations.

The valence isomerization of norbornadiene to quadricyclene can be photosensitized with high quantum efficiency by certain copper(I)-phosphine compounds. Sensitization yields in the presence of Cu(PPh/sub 3/)/sub 2/BH/sub 4/ and Cu(PPh/sub 2/Me)/sub 3/BH/sub 4/ attain limiting values of 0.76 and 0.54, respectively, at high norbornadiene concentrations. In contrast, Cu(diphos)BH/sub 4/ (diphos is 1,2-bis(diphenylphosphino)ethane), which contains a bidentate phosphine ligand, is decidely less effective. The mechanism of sensitization by these copper(I)-phosphine compounds is fundamentally different from that exhibited by simple cuprous salts in that no ground-state coordination to the metal center occurs. A pathway involving the bimolecular interaction of the photoexcited Cu(I) compound with a ground-state norbornadiene molecule is better able to accommodate the available data. Electronic absorption and emission studies reveal some interesting differences between the spectral properties of effective vs. noneffective sensitizers.

A detailed study of the thermal and photochemical behavior of Cu(HB(pz)/sub 3/)(CO) in which (HB(pz)/sub 3//sup -/is hydrotris(1-pyrazolyl)borate) in the presence of norbornadiene (NBD) is reported. In solution NBD displaces CO from the coordination sphere of copper to produce a new complex formulated as Cu(HB(pz)/sub 3/)(NBD). This ground-state process is readily discernible by a shift in the absorption spectrum of the system to longer wavelengths. Irradiation of the complex with 313-nm light induces the valence isomerization of norbornadiene to quadricyclene with high quantum efficiency. The occurrence of this pathway is explicable in terms of the bonding changes that result from populating an excited state possessing Cu-NBD charge-transfer character.

A detailed investigation of the photoassisted valence isomerization of norbornadiene (NBD) to quadricyclene (Q) in the presence of copper(I) salts is reported. In the prototypal cuprous chloride-norbornadiene system, spectral evidence indicates that a 1 : 1 ClCu-NBD ..pi.. complex is present in solution and is, in fact, the photoactive species. Quantum yield data obtained in several solvents and at different initial NBD concentrations support this premise. The photoreaction is thought to originate via population of an olefin-metal charge-transfer state of the complex. Possible pathways by which this excited state yields Q are considered. The absence of NBD photodimerization products suggests that Cu(I) salts containing strongly binding anions have little tendency to coordinate more than one NBD molecule.

Low-temperature (77 K) luminescence spectra and lifetimes have been measured for the Cu(PPh/sub 3/)/sub 2/BH/sub 4/ (PPh/sub 3/ is triphenylphosphine) and Cu(prophos)BH/sub 4/ (prophos is 1,3-bis(diphenylphosphino)propane) complexes. Cu(PPh/sub 3/)/sub 2/BH/sub 4/ exhibits multiple emission from /sup 3/IIII* and /sup 3/(sigma-a/sub ..pi../) excited states in appropriate solvents, while Cu(prophos)BH/sub 4/ shows only a single /sup 3/(sigma-a/sub ..pi../) emission in most of the solvents chosen for this study. The electronic transition associated with the sigma-a/sub ..pi../ assignment is analogous to the l-a/sub ..pi../ transition for the free phosphine ligand. These unique emission results are discussed in terms of intramolecular and intermolecular (solvent-solute) effects. 23 references, 4 figures, 2 tables.

A photochemical study of trans-Rh(tfa)/sub 3/ (tfa is the anion of 1,1,1-trifluoro-2,4,-pentanedione) has revealed the occurrence of two photoinduced processes whose relative quantum efficiencies exhibit a striking dependence upon the solution environment. Thus trans to cis isomerization is the sole observable photoprocess in cyclohexane, whereas only decomposition of the complex occurs in ethanol and 2-propanol. The key role of solvent in regulating the photochemical behavior is particularly evident in mixed cyclohexane-2-propanol systems where both photoprocesses can be observed. Oxygen quenching and wavelength-dependence studies provide strong evidence for the existence of two photoactive excited states in trans-Rh(tfa)/sub 3/. The enhanced photoreactivity in the presence of hydrogen atom donors suggests that one or both of these photoactive states imparts considerable radical character to the complex. Flash photolysis results obtained under a variety of experimental conditions support this suggestion and allow a more precise delineation of several steps in the overall reaction sequence. 7 figures, 4 tables.

Irradiation of an acetonitrile solution containing tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)/sub 3//sup 2 +/), the methylviologen dication (MV/sup 2 +/), and quadricyclene (Q) with 436-nm light initiates a catalytic cycle for the conversion of Q to its more stable valence isomer norbornadiene (NBD). Quantum yields of NBD production above 1 and turnovers of Ru(bpy)/sub 3//sup 2 +/ greater than 10/sup 2/ have been measured. Spectral, continuous-photolysis, flash-photolysis results support a mechanism in which the primary photochemical step is oxidative quenching of the emissive metal-to-ligand charge-transfer excited state of Ru(bpy)/sub 3//sup 2 +/ by MV/sup 2 +/ to form the strong ground-state oxidant, Ru(bpy)/sub 3//sup 3 +/. Subsequent oxidation of Q by Ru(bpy)/sub 3//sup 3 +/ regenerates the original metal complex and produces the structurally labile cation radical, Q/sup .+/. Rearrangement of Q/sup .+/ to NBD/sup .+/ and oxidation of another Q molecule by NBD/sup .+/ constitute the key chain-carrying steps in the catalytic oxide. 25 references, 2 figures, 1 table.

Decomposition et isomerisation geometrique a 366 nm, 150°C du complexe cite, sous N 2 , sous vapeur de C 6 H 6 et sous vide. Mais inertie du meme complexe en solution benzenique ou mesitylenique dans les memes conditions. Rendements quantiques. Mecanismes. Cinetique du processus bimoleculaire. Spectre electronique UV

A solar energy storage system based upon the valence isomerization of norbornadiene to quadricyclene possesses several attractive features, including high specific energy storage capacity, kinetic stability of the energy rich photo-product in the absence of suitable catalysts, and relatively inexpensive reactants. An inherent difficulty with the system is the lack of absorption of solar energy by norbornadiene. Attempts to overcome this shortcoming have focused upon the use of transition metal compounds to sensitize the desired energy storage step. Results to date indicate that complexation of norbornadiene to the metal provides a potentially useful route to sensitization. Several copper(I) compounds have thus far been found to be effective. The facors which affect the ability of a transition metal compound to function as a sensitizer via a complexation mechanism are discussed.

Etude mecanistique du systeme ReBr−(CO) 3 (bpy)/TEOA/DMF/CO 2 . Extinction de la luminescence, photolyse continue et photolyse eclair. Rendement quantique de formation de CO

A study was made of the norbornadiene (NBD) to quadricyclene (Q) conversion in the presence of Cu(P(CH/sub 6/H/sub 5/)/sub 3/)/sub 2/BH/sub 4/, Cu(P(C/sub 6/H/sub 5/)/sub 2/(CH/sub 3/))/sub 3/BH/sub 4/, and Cu(diphos)BH/sub 4/ (diphos is 1,2-bis(diphenyl-phosphino)ethane). The first two compounds were approximately 60 to 90 times more effective than the latter compound in sensitizing the production of Q. Furthermore, the mechanism of sensitization appears to be fundamentally different from that involving CuX salts (where X is Cl, Br, I, or C/sub 2/H/sub 3/O/sub 2/). (JSR)

Electrochemical oxidation of quadricyclene results in its isomerization to norbornadiene by a redox chain mechanism.

Etude mecanistique de la formation de CO dans differents solvants. Influence du triethanolamine, du triethylamine, des ions bromures et du DMF en solution dans l'eau ou l'ethanol

Near-ultraviolet flash photolysis of trans-Co(tfa)3 (tfa is the anion of 1,1,1-trifluoro-2,4-pentanedione) in cyclohexane generates a long-lived (t 1 2 ∼ 3 sec) strongly-absorbing transient. The yield of this species decreases upon addition of good hydrogen-atom donors such as 2-propanol and tri(n-butyl)stannane. In pure 2-propanol the formation of the transient is completely suppressed, and only a bleaching of the absorbance, due to production of weakly-absorbing Co(II) products, is observed. Flashing a methanol solution of trans-Co(tfa)3 produces a transient similar (but not identical) to that observed in cyclohexane; this species subsequently decays to Co(II) products. Based upon their spectral characteristics and reactivities, the flash-induced transients are assigned as Co(II) complexes containing a carbon-bonded ligand. These species result from an intramolecular rearrangement of the reduced metal-oxidized ligand radical pair formed in the primary photochemical step. Good hydrogen-atom donors can intercept this radical pair and thus inhibit transient formation. Previous mechanistic proposals about the ultraviolet photochemistry of tris(β-diketonate)cobalt(III) complexes need to be modified in view of the present results.

Formation of a ground-state complex between CuBr(py)(PPh/sub 3/) (py is pyridine) and norbornadiene (NBD) occurs upon mixing the compounds in benzene. Irradiation of the complex with 313-nm light generates the valence isomer quadricyclene (Q) with moderate observed quantum efficiency. This efficiency varies with the nature and concentration of excess ligands present in solution. Spectral studies reveal that this behavior arises fom the creation of new Cu(I) species whose sensitization properties differ from those of CuBr(py)(PPh/sub 3/). Thus a sufficient excess of triphenylphosphine converts the parent compound to CuBr(PPh/sub 3/)/sub 3/. The latter species functions as an effective sensitizer for Q production but by a pathway involving bimolecular interaction of photoexcited CuBr(PPh/sub 3/)/sub 3/ with ground-state NBD. These results demonstrate that the mechanism of sensitization by Cu(I) can be controlled by the judicious addition of ligands to the system.

Isomerisation de valence du quadricyclane (Q) en norbornadiene acceleree par irradiation a 313 nm en presence du catalyseur complexe PdCl 2 (η 4 -norbornadiene). Le rendement quantique peut exceder 10 2 et depend de la concentration de Q, de l'intensite lumineuse et du solvant. Mecanisme de reaction en chaine redox