Quantum Yield Of Photoreaction Research Articles

Four-Electron Photochemistry of Dirhodium Fluorophosphine Compounds

The singly bonded dirhodium(II) complex, Rh2(dfpma)3Br4 (dfpma = bis(difluorophosphine)methylamine, CH3N(PF2)2), photoreacts when irradiated with UV or visible light. The mixed-valence LRh0RhΙΙX2 species, Rh2(dfpma)3Br2(L), is obtained quantitatively when THF solutions containing Rh2(dfpma)3Br4 and excess L = dfpma or PR3 are photolyzed (λexc > 436 nm). The photoreaction quantum yield is ∼10-2 over the near-UV absorption manifold, decreasing slightly as the excitation wavelength is extended into the visible region (φp313-405 = 0.022(3), φp436-468 = 0.012(1)). Continued irradiation with excitation wavelengths coincident with the absorption manifold of the LRh0RhΙΙX2 complex results in a second two-electron elimination reaction to give the LRh0Rh0L dimer, Rh2(dfpma)3L2, again in quantitative yield but with an attenuated quantum efficiency (φp313-365 = 0.0035(3), φp405-436 = 0.0017(3)). For photoreactions performed in THF, the bromine photoproduct is HBr, which may be trapped with 2,6-lutidine. NMR experiments reveal the production of 2 equiv of HBr for each two-electron transformation of the dirhodium photoreagent. The wavelength dependence of φp and the results of extended Hückel calculations are consistent with the photoreaction occurring from an excited state of dσ* parentage. The ability to preserve the same dσ* electronic structure across the four-electron series allows us to overcome the barriers traditionally associated with metal−halogen bond cleavage and consequently to design a four-electron photoreaction among discrete molecular species.

Zinc-tetrabenzoporphyrine-doped poly(methyl methacrylate):?a new photochromic recording medium

A photochromic reaction in Zn-tetrabenzoporphyrine-doped poly(methyl methacrylate) was investigated at room temperature. A one-quantum thermoreversible photoreaction could be initiated in the sample with 633-nm He-Ne laser irradiation. The photoreaction quantum yield depended strongly on the concentration of an additional electron acceptor. The changes of the refractive index were also measured.

Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo2Cl2(6-mhp)2(PR3)2 Complexes

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.

Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) Complexes.

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.

Performance of Heterogeneous Photocatalytic Systems: Influence of Operational Variables on Photoactivity of Aqueous Suspension of TiO 2

The influence of incident light intensity and of some physical and chemical parameters on the absorbed and backward reflected photon flow and on the reactivity of a heterogeneous photocatalytic system was investigated. The oxidation of phenol in aqueous solution in the presence of polycrystalline TiO 2 powder was used as a test reaction. Home-prepared TiO 2 (anatase) specimens, subjected to various thermal treatments, were used for the experimental runs performed in a batch photoreactor directly irradiated. The physical parameters investigated were the surface area and the size of particles, while the chemical parameters studied were the initial pH of the suspension and the presence in the reacting medium of additives affecting the photoreactivity such as Cl − and H 2O 2. It was found that the ratio between backward reflected and incident photon flow does not depend on the used operative variables, while the apparent Napierian extinctance coefficient, which is a parameter related to the photon absorption rate, depends only on the particle size and decreases as the particle size increases. The photoreaction quantum yield, defined as the ratio between the phenol reaction rate and the photon absorption rate, was determined for the various experimental conditions used in this work. The results showed that at equivalent conditions the quantum yield is an intrinsic feature of the semiconductor material; the quantum yield varies only by varying the reaction rate which, in our case, was affected only by the initial pH of the suspension and by the presence of Cl − and H 2O 2.

Photoinduced addition of dioxygen molecules in the unsaturated sites of the Pd3(dppm)3CO2+ catalyst

The photoinduced addition of O2 onto the unsaturated cluster Pd3(dppm)3CO2+ (as a CF3CO2− salt) in acetonitrile is reported. The final product Pd3(dppm)3(O2)22+(v(O2) = 838 and 866 cm−1) is formed in a multi-step fashion in which two photochemical intermediates are observed (presumably Pd3(dppm)3(O2)(CO)2+ and Pd3(dppm)3(O2)2+. No X-ray structure could be obtained, but numerous spectroscopic findings demonstrate that O2 binds the Pd3 center as a peroxo-O2, and acts as a two-electron donor that triply bridges the metal atoms (forming a 44-electron cluster). The very small excited state lifetimes (between 25 and 35 ± 10 ps) obtained by picosecond flash photolysis indicate that the primary photoreaction is unimolecular, and demonstrate that the first photochemically added O2 molecule must be preassembled in the excited state prior to any photoinduced transformation. This [Formula: see text] ground state complex is responsible for the photoinduced production of the bisdioxygen compound and can be observed by UV–visible spectroscopy. The low efficiency of the photoreaction (quantum yield (Φ) = 0.033 ± 0.004) is explained by the very short excited state lifetime at 298 K, and the competition of O2 with solvent molecules to occupy the unsaturated site of the empty cavity in Pd3(dppm)3CO2+ (i.e., ground state guest–host chemistry). The binding constant for O2 with Pd3(dppm)3CO2+ is roughly estimated to range between 1 and 730 M−1 in the ground state and is considered to be weak. Keywords: clusters, photochemistry, guest–host, oxidation, dioxygen.

Fulgenolides. Thermally Irreversible Photochromic Lactones with Large Quantum Yields of Photoreactions

Abstract Three new photochromic lactones closely related to an indolylfulgide were synthesized. While the lactones with the carbonyl group adjacent to the isopropylidene group have large quantum yields of photoreactions, the isomer with the carbonyl adjacent to the indolylethylidene group showed poor photochromic properties. The absorption maxima of the colored forms of the lactones are shorter than that of the indolylfulgide.

Photophysical properties and photodegradation mechanism of 2-(2′-furanyl)-1H-benzimidazole (Fuberidazole)

Abstract The photodegradation mechanism of 2-(2′-furanyl)-1H-benzimidazole (Fuberidazole) in aqueous solution was studied by coupling photophysical and photchemical data with orbital calculations. Several photophysical properties were studied, the excited states were characterized and the acid—base equilibrium constants (p K a1 =4.8, p K a2 = 11.5, p K * a1 = 6.7 and p K * a2 = 10.8) in both ground and excited states were determined; the photoreaction quantum yields of the acidic and neutral forms were obtained as a function of molecular oxygen concentration. The first singlet and triplet were identified as π-π * transitions for the acidic, neutral and basic forms. The experimental results are in agreement with a planar geometry for both the ground and first excited singlet states. The qualitative results of the photodegradation as a function of oxygen concentration, and the higher reactivity of the acidic form relative to the neutral form, are discussed on the basis of the formation of a cyclopropenyl ketone intermediate. According to molecular orbital calculations, the process of formation of this intermediate is energetically favourable through the triplet of the acidic form, but not through the triplet of the neutral form, explaining the observed reactivity.

Reactant preordering in solid photopolymers

Abstract

Mechanistic studies of the phototoxic potential of PD 117596, a quinolone antibacterial compound

PD 117596 is a novel quinolone compound that is being investigated for use as an antibacterial agent. Early investigations demonstrated a significant phototoxic liability associated with this compound. These studies were undertaken to investigate the mechanism of phototoxicity using an in vitro model. In the UVA region, PD 117596 was found to be a more efficient producer of singlet oxygen than rose bengal, ciprofloxacin, nalidixic acid, or PD 118879, another quinolone under investigation. The quantum yield of photoreaction for PD 117596 was relatively low (φ = 0.021); however, it was approximately 10-fold higher than other tested quinolones. In vitro studies using a mouse erythrocyte model were used to further investigate the mechanism of phototoxicity. PD 117596-induced photohemolysis was found to be oxygen dependent with a relatively rapid onset that progressed even after removal of light. Preirradiation of the compound prevented subsequent hemolytic or photohemolytic action. BHA, BHT, α-tocopherol, and the iron chelator DTPA were all found to be effective at ameliorating the photohemolytic response. The photohemolytic response was markedly enhanced when D 2O was substituted for H 2O in the incubation medium, indicating a singlet oxygen-mediated mechanism of action. A rise in thiobarbituric acid products was noted within 1 hr of irradiation and was maximal at the time of onset of overt photohemolysis. These data suggest that singlet oxygen production by irradiated PD 117596 is responsible for secondary changes in mouse red blood cells including lipid peroxidation and ultimately results in cellular lysis.

Absorption spectra and luminescence properties of a series of pyridine carboxaldehyde phenylhydrazone ligands and their palladium(II) complexes

A series of pyridine carboxaldehyde phenylhydrazone ligands and their palladium(II) complexes were synthesized and their absorption and emission properties were investigated. A relatively intense absorption band which is present in all compounds in the region 300 – 420 nm is assigned to a π—π* transition with partial charge transfer (intraligand charge transfer (ILCT)) from the iminio moiety to the phenyl ring. All the compounds are luminescent in acetonitrile solution at room temperature and in butyronitrile rigid matrix at 77 K. The emitting level is the lowest-lying singlet ILCT excited state. The values of the emission quantum yields in acetonitrile solution are in the range 10 −4 – 10 −5; they depend on the excitation wavelength due to competition between radiative decay and Z → E photoisomerization; higher luminescence quantum yields are found at lower excitation energies, where the photoreaction quantum yield is lower. The geometry of the ligand and the presence of an electron-withdrawing substituent on the (acceptor) phenyl ring strongly affect the absorption and photophysical properties.

Mechanistic studies on the [2 + 2] photocycloaddition reaction of khellin

The photocycloaddition reaction of khellin with dimethylfumarate (DF) was investigated. The singlet excited state of khellin is efficiently quenched by DF without giving any cycloadduct. Azulene quenching and the effect of olefin concentration on the photocycloaddition quantum yield indicate that the photoreaction proceeds through a triplet excited state only. These results are further confirmed by the triplet sensitization of the photoreaction and the external heavy atom effects on the photoreaction quantum yield.

ChemInform Abstract: Photochemistry of Co(EDTA)‐‐I‐ System in Aqueous Solutions

Abstract The Co(EDTA)− complex in aqueous solution gives rise to a specific interaction with I− ions as evidenced by a new, relatively intense band formed at 290–300 nm. This specific interaction is attributed to the formation of an ion-pair between Co(EDTA)− and I−, even though they are like charged ions. Irradiation of this ion-pair in air- equilibrated solutions with 313 nm light, causes the reduction of the Co(EDTA)− to Co(EDTA)2− and the oxidation of the I− ion to I3−. The results obtained are interpreted on the basis of a mechanism in which Co(EDTA)2− and I· are the primary photoproducts. The I· radical is then scavenged by I− to yield I2−, which subsequently disproportionates to I3− and I− and reoxidizes Co(EDTA)2− to Co(EDTA)−. At the beginning of the photoreaction, the I2− decay is equally distributed on the two reaction pathways. It was possible to determine a value of 0.2 ± 0.05 for the photoreaction quantum yield and an efficiency of the primary photochemical step almost unitary. A schematic representation of the energetics of the overall reaction is reported.

Photochemistry of Co(EDTA) −-I − System in Aqueous Solutions

The Co(EDTA) − complex in aqueous solution gives rise to a specific interaction with I − ions as evidenced by a new, relatively intense band formed at 290–300 nm. This specific interaction is attributed to the formation of an ion-pair between Co(EDTA) − and I −, even though they are like charged ions. Irradiation of this ion-pair in air- equilibrated solutions with 313 nm light, causes the reduction of the Co(EDTA) − to Co(EDTA) 2− and the oxidation of the I − ion to I 3 −. The results obtained are interpreted on the basis of a mechanism in which Co(EDTA) 2− and I · are the primary photoproducts. The I · radical is then scavenged by I − to yield I 2 −, which subsequently disproportionates to I 3 − and I − and reoxidizes Co(EDTA) 2− to Co(EDTA) −. At the beginning of the photoreaction, the I 2 − decay is equally distributed on the two reaction pathways. It was possible to determine a value of 0.2 ± 0.05 for the photoreaction quantum yield and an efficiency of the primary photochemical step almost unitary. A schematic representation of the energetics of the overall reaction is reported.

Definition of Photoreaction Quantum Yield at Two‐quantum Excitation of Molecules in Solution

A new approach to define the photoreaction quantum yield in case of two‐quantum excitation of molecules in solution is proposed.

Intramolecular photoredox reactions in iron(III) cytochrome c and its azide derivative

The irradiation of deaerated solutions of horse heart cytochrome c causes the reduction of Fe(III) to Fe(II). The dependence of the photoreaction quantum yield on pH shows that the photoreactive species is a form of cytochrome c which contains methionine-80 and histidine-18 as heme ligands. The primary photochemical event consists of an electron transfer from the sulphur of methionine- 80 to iron. The re-oxidation of the photochemically obtained Fe(II) protein gives a Fe(III) cytochrome which exhibits a typical low-spin absorption spectrum, lacking the 695-nm band and indicating that a strong field ligand, other than methionine-80, coordinates to the sixth binding site of the heme iron. Spectrophotometric titration of the photochemically modified Fe(III) cytochrome shows that histidine- 18 remains bound in the fifth position. The substitution of methionine-80 with the more oxidizable azide ligand increases the efficiency of the intramolecular electron transfer. Azide radicals, detected by spin-trapping ESR technique, are formed in the primary act. Visible-UV spectral data indicate that histidine-18 and methionine-80 occupy the fifth and sixth position, respectively, in the photoreaction product. All the results obtained correlate well with those previously obtained in investigations concerning the photoredox behavior of iron porphyrin complexes.

O‐nitrobenzyl photochemistry: Solution vs. solid‐state behavior

AbstractThe efficiency of various 2‐nitrobenzyl groups in the photogeneration of carboxylic acid from nitrobenzyl esters has been determined, and the solution vs. solid polymer matrix photochemistry is compared. Generally, the quantum yield for photoreaction is lower in the solid state, probably due to decreased conformational mobility. The substitution that affords the largest increase in quantum yield (ϕ) is at the α‐carbon atom. The value of ϕ for α‐methyl‐2‐nitrobenzyl trimethylacetate in solution is 0.65 vs. 0.13 for the parent 2‐nitrobenzyl ester.

Preparation and photochemistry of furoisoxazolines. An extraordinarily unambiguous photo-induced rearrangement

Preparation and photolysis of furoisoxazoline derivatives is described. The respective compounds IVa, II and VI were obtained by a 1,3-dipolar cycloaddition of benzenenitrile oxide to 5,6-dimethoxycarbonyl-7-oxabicyclo[2,2,1]-2-heptene, 2,5-dihydrofuran and furan; IVb was synthesized from benzoylnitrile oxide. The biradical primarily formed by photolysis was stabilized with respect to the arrangement of the heteroatom and the isoxazoline chromophore. The new 1,3-oxazine derivatives III and V were obtained in high yields as single products. The quantum yield of photoreaction of II, IVa and XXVII was found to be 0.04, 0.26 and 0.07, respectively. Were the stabilization of the radical by conjugation with the lone electron pair of oxygen impossible, oxazoline VII should be formed. Photolysis of the benzoyl derivative IVb had an anomalous course and yielded the β-aminochalcone Xb.

A study of the photochemical response of o‐nitrobenzyl cholate derivatives in P(MMA‐MAA) matrices

AbstractOrtho‐nitrobenzyl cholate esters have been successfully used as photosensitive components in solution inhibition deep UV photoresists. The photochemical behavior of a variety of substituted o‐nitrobenzyl cholate derivaties has been examined and resist sensitivity is related to the quantum yield of the reaction. The most efficient system examined is o, o′‐dinitrobenzyl cholate/P(MMA‐MMA), where the quantum yield of photoreaction is 0.2 and resist sensitivity is ca. 150 mJ/cm2 for a 1.5 μm film. The mode of resist behaviour on irradiation and development is also reported.

Photochemical Reactions of Aromatic Compounds. XXXVII. Solvent Effects on Fluorescence Quenching and Photoreactions of Naphthonitrile–Olefin and Furan Systems. Qualitative Consideration on Electronic Structures and Reactivities of Nonemissive Exciplexes

Abstract Solvent effects on the fluorescence-quenching rate constants (γkc) as well as on the quantum yields (φ∞−A) of photoreactions have been investigated for the nonemissive exciplex systems in which 1- or 2-naphthonitrile is the electron acceptor whereas the donors are some enol ethers, a few simple alkenes, furan, 2-methylfuran, and ethyl phenyl ether. On the basis of the assumption of the solvent as a continuous dielectric medium, the relationships between electronic structures and reactivities of nonemissive exciplexes have been qualitatively estimated from the plots of logγkc and log[φ∞−A⁄(1−φ∞−A)] vs. (ε−1)⁄(2ε+1). In nonpolar and moderately polar aprotic solvents, the nonemissive exciplexes have low charge-transfer contributions but relatively high reactivities in the cycloaddition while much larger charge-transfer contributions are induced in more polar alcoholic solvents; the reactivities in the protic solvents decrease with the increase of solvent polarity. For some exciplexes, the occurrence of complete electron transfer in very polar solvents such as ethanol, methanol, and acetonitrile is strongly suggested from the product distributions. The possible availability of ET(30) or Z value as a scale of solvent polarity has been tested for the solvent effects on the processes from the nonemissive exciplexes.