Limiting Quantum Yield Research Articles

Activation of unsaturated alkyl chlorides by photogenerated [Pd(CNMe) 3] .+ radicals. Transient absorbance kinetics studies of the formation of η 3-allyl and “oxaallyl” palladium complexes

The reactions of photogenerated [Pd(CNMe) 3] .+ radicals with the unsaturated alkyl chlorides, RCl = allyl chloride and chloroacetone, have been investigated by laser flash photolysis. Irradiation (λ > 290 nm) of acetonitrile solutions of [Pd 2(CNMe) 6][PF 6] 2 ( 1), in the presence of allyl chloride yields a mixture of the η 3-allyl complex [Pd(C 3H 5)(CNMe) 2][PF 6] and [PdCl(CNMe) 3][PF 6]. In the presence of chloroacetone, Cl-atom abstraction by photogenerated [Pd(CNMe) 3] .+ radicals yields the “oxaallyl” complex [Pd(CH 2C(O)Me)(CNMe) 2][PF 6] and [PdCl (CNMe) 3][PF 6]. The disappearance of 1 in the presence of RCl occurs with a limiting quantum yield of 0.07 ± 0.01 (R  C 3H 5) and 0.05 ± 0.01 (R  MeC(O) CH 2) at 313 nm. The rate of disappearance of [Pd(CNMe) 3] .+, generated by laser flash photolysis (355 nm), depends on [RCl]. The reaction is first-order over the concentration range 0.062–1.23 M for allyl chloride and 0.63–1.89 M for chloroacetone. The rate constants for the bimolecular reactions with allyl chloride and chloroacetone are 500 and 200 M −1 s −1, respectively. Mechanisms for the reactions of photogenerated [Pd(CNMe) 3] .+ with allyl chloride and chloroacetone are interpreted in terms of rate limiting Cl-atom abstraction to form [PdCl(CNMe) 3][PF 6] followed by rapid combination of a second [Pd(CNMe) 3] .+ radical with the resulting allylic radical.

General base catalysis of photo‐smiles rearrangement of 1‐(4‐nitrophenoxy)‐2‐anilinoethane in aqueous solution. Identity of the proton‐donating intermediate

Abstract4‐O2NC6H4OCH2CH2NHPh undergoes clean base‐catalyzed Smiles photorearrangement to 4‐O2NC6H4N(Ph)CH2CH2OH in dimethyl sulfoxide–water (25:75). A linear plot of ϕ−1 vs [OH−]−1 indicated that the limiting quantum yield at high [OH−] is 0·12 and that uncatalyzed photorearrangement does not occur. Rate constants for the deprotonation step were determined for a variety of bases having conjugate acid pKa values in the range 5–10. When plotted according to the Brønsted Catalysis Law, these data gave a non‐linear plot approaching slopes of zero and unity above and below a pKa of 6–7. This indicates that the proton‐donating intermediate in this photo‐Smiles rearrangement is the zwitterion diradical (pKa ≈ 7) rather than the Meisenheimer complex (pKa ≈ 2).

Photofragmentation kinetics of some triruthenium carbonyl clusters

The photokinetics of fragmentation reactions of [Ru3(CO)12] with L = PPh3, P(OPh)3, AsPh3, CO, 1-octene, and methyl acrylate in a variety of solvents have been studied. Quantum yields increase to limiting values at high [L] but the limiting values vary significantly with the nature of L. The low efficiency of photochlorination in chlorocarbon solvents, the absence of inhibition by CO of reactions with L = PPh3, and the absence of appropriate effects of varying incident light intensity all suggest that the first kinetically significant product is a non-radical reactive isomer of [Ru3(CO)12]. This can revert to [Ru3(CO)12] or react with L to form [Ru3(CO)12L] which itself can revert to [Ru3(CO)12] or undergo fragmentation. The former choice governs the rate of increase with [L] to a limiting quantum yield whereas the latter choice governs the dependence of the values of the limiting quantum yield on the nature of L. This scheme is also applicable to photoreactions of [Os3(CO)12] and the quantitative behaviour of the two clusters is not significantly different. Only lower limits for quantum yields for formation of the reactive isomers can be deduced from the data and it remains possible that the primary photophysical process is the formation of a very short-lived diradical by homolysis of a metal-metal bond. Photokinetic studies of reactions of [Ru3(CO)9L3] with L (L = PPh3 or PBun3) are also reported.

Flavin-sensitized photo-oxidation of unsaturated fatty acids

Photo-oxidation of unsaturated fatty acids or esters (LH = oleic acid, linoleic acid, methyl linoleate, and linolenic acid) by oxygen proceeds efficiently in the presence of a riboflavin derivative, riboflavin-2′,3′,4′,5′-tetra-acetate (Fl), in acetonitrile by a type II (singlet oxygen) mechanism with a limiting quantum yield (φ= 0.65) which corresponds to the quantum yield of the intersystem crossing of the flavin. The addition of HClO4—by which Fl is protonated to FlH+—showed a remarkable inhibitory effect on the Fl-sensitized photo-oxidation of LH, since FlH+ is photo-reduced by LH to yield the dihydroflavin radical cation FlH2+˙ which has no ability to sensitize the energy transfer to oxygen in its ground state.

The search for new biochemical photoprobes. II. The nucleophilic photosubstitution of 2-fluoro-4-nitroanisole.

The photosubstitutions of 2-fluoro-4-nitroanisole with several amines are studied. The preparative results, the thermal stability of the photochemical substrate, and the limiting quantum yield values obtained from photoreactions with several nucleophiles suggest the possible usefulness of 2-fluoro-4-nitrophenyl ethers as biochemical photoprobes.

Photocatalytic dehydrogenation of liquid alcohols by platinized anatase

The photocatalytic dehydrogenation of liquid methanol, ethanol, propan-1-ol and propan-2-ol has been investigated using platinized anatase and 366 nm u.v. radiation over the range 278–303 K. Activities and activation energies for carbonyl-compound formation were effectively identical for the four alcohols on Pt(0.5)/TiO2 prepared by photodeposition. The activation energy of 20 kJ mol–1 is associated with photoelectron transport through the anatase to the Pt particles. With Pt(0.5)/TiO2 prepared by impregnation and H2reduction, identical activities for the four alcohols were achieved after O2 treatment. From the effect of u.v. intensity on the rate of propanone formation at 293 K, the limiting quantum yield was found to be 0.45 for photodeposited Pt(0.5)/TiO2 under N2 and 0.82 for support anatase under O2. Arrhenius plots for reaction at reduced u.v. intensities showed that the activation energy fell to zero when these quantum yields were achieved. Different mechanisms follow from the manner in which photoholes are surface trapped; two mechanisms for dehydrogenation with a limiting quantum yield of 0.5 are discussed.

Photochemical and thermal behaviour of isocyanide complexes: V. photolysis of Fe(CNCH3)4(CN)2 in acetonitrile

Direct irradiation of acetonitrile solutions of Fe(CNCH3)4(CN)2 in both CT and LF bands leads to gradual ligand-solvent substitution and the solvated intermediates Fe(CNCH3)3(NCCH3(CN)2 and Fe(CNCH3)2(NCCH3(CN)2 are formed in two subsequent steps. The quantum yield of the first step of photosolvation (ϕ = 0.5) is independent of the wavelength of irradiation. Benzophenone is a high energy triplet donor for this compound, and sensitizes the photosolvation with a limit quantum yield of 0.56. The results point to the same reactive triplet state for the direct and the sensitized reaction.

Metalloporphyrin-sensitized photoredox reactions: mechanistic studies on the role of axial ligands on photoreactivity

An investigation of the tin(IV) porphyrin-sensitized photoredox reactions between triphenylphosphine (Ph 3P) as an electron donor and methyl viologen (MV 2+) as an electron acceptor is reported. The reaction is induced only in the presence of water molecules as axial ligands. A number of different axially ligated porphyrin complexes sensitize this reaction with quantum efficiencies that depend strongly on the ligand, pH and the concentrations of the various reagents involved. The reactivities of the sensitizers are in the order SnTPP(OH) 2SnTPP(OPPh 3) 2SnTPP(H 2O) 2SnTPP(H 2O)(Ph 3P) ⪢ SnTPP(Ph 3P) 2 (TPP  tetraphenylporphyrin). The limiting quantum yield of MV + production with SnTPP(OH) 2 is 0.3, while that with SnTPP(Ph 3P) 2 is almost zero. Kinetic and flash photolysis investigations indicate that an oxidative quenching of the triplet tin porphyrin by MV 2+ initiates the photoredox reactions. The key to obtaining net redox conversion in each case is the dark oxidation of a partially (one-electron) oxidized Ph 3P—water (or Ph 3P—hydroxide adduct to Ph 3PO by MV 2+. The photoreactivity is strongly affected by the rates of coordination of potential ligands in the reaction system to an unstable metalloporphyrin with a vacant coordination site which is generated in the course of the reaction. Hydroxide ion can reproduce the reactive SnTPP(OH) 2 by effective coordination to the vacant species which maintains a constant build-up of MV + .

Kinetic studies of the oxidative photosubstitution reaction in the charge transfer photochemistry of Os(OEP)[P(OMe) 3] 2 (OEP ≡ octaethylporphine) in hexane-halocarbon mixtures

The osmochrome Os II(OEP)[P(OMe) 3] 2, where OEP is octaethylporphine, undergoes an oxidative photosubstitution reaction in hexane in the presence of chlorinated hydrocarbons (CH 2Cl 2, CHCl 3, CCl 4, CHCl 2CH 3 and CH 2ClCH 2Cl) under steady state photolysis. The product has been identified as Os IV(OEP)Cl 2. Both initial rates of reaction and quantum yields are, within experimental error, independent of whether the hexane-chlorinated hydrocarbon solutions are argon purged, air equilibrated, N 2O purged or oxygen purged. The quantum yields increase with increase in CR 2Cl 2 (R  H or Cl) concentration; limiting quantum yields are 0.0017 ± 0.0001 (CH 2Cl 2), 0.25 ± 0.08 (CHCl 3) and 0.93 ± 0.10 (CCl 4). The reactivity of the chlorinated reactants is CCl 4 (280) > CHCl 3 (35) & >; CH 2ClCH 2Cl (1.4) ≈ CHCl 2CH 3 (1.1) ≈ CH 2Cl 2 (1.0) under the experimental conditions used; this order accords with expectations based on the CCl bond strengths. A mechanism is postulated in which the three essential steps are (L  P(OMe) 3) (i) Os(OEP)LL ⇌ Os(OEP)L + L (ii) Os(OEP)L + CR 2Cl 2 → products and (iii) Os(OEP)LL + CR 2Cl 2 → products The data are treated using both the conventional kinetic theory and the model of Noyes for the scavenging of photoproduced radical pairs in solution.

STRUCTURE‐REACTIVITY RELATIONSHIPS IN REDOX‐PHOTOSENSITIZED SPLITTING OF PYRIMIDINE DIMERS AND UNUSUAL ENHANCING EFFECT OF MOLECULAR OXYGEN *

Abstract Redox photosensitization using the phenanthrene‐p‐dicyanobenzene pair in acetonitrile has been applied to the respective four isomeric dimers of N.N′‐dimethylthymine (DMT) and N,N′‐dimethyluracil (DMU) as well as to several related cyclobutane compounds. The head‐to‐head (syn) dimers of both DMT and DMU can undergo photosensitized splitting in the following order of efficiency: cis, syn dimer of DMT > cis, syn dimer of DMU > trans, syn dimer of DMT. On the other hand, the head‐to‐tail (anti) dimers are totally unreactive and have higher oxidation potentials than the corresponding syn dimers. It is suggested that the key mechanistic pathway is the formation of π complexes between the dimers and the photo‐generated cation radical of phenanthrene by way of which splitting of the cyclobutane ring catalytically occurs without the formation of the discrete cation radical of the dimers. Structure‐reactivity relationships are interpreted in terms of through‐bond interactions between the n orbitals of N(l) and N(l′) involving the C(6)‐C(6′) bond, as well as in terms of steric repulsion. It was found that aeration of solution greatly enhances the quantum yields of photosensitized splitting; the limiting quantum yield for splitting of the cis, syn dimer of DMT is 100.

Fluorescence probes for polymerization reactions: Bulk polymerization of styrene, n‐butyl methacrylate, ethyl methacrylate, and ethyl acrylate

AbstractJulolidine malononitrile 3 was used as a fluorescent probe for high‐conversion (free‐radical) bulk polymerization of methyl methacrylate, ethyl methacrylate, n‐butyl methacrylate, ethyl acrylate, styrene, and the copolymerization of styrene/n‐butyl methacrylate. The fluorescence of the probe increased gradually as polymer conversion increased. This was followed by an abrupt rise in fluorescence intensities by a factor of 3 to 40 depending on the polymer formed. Finally the fluorescence intensities leveled off as the polymer limiting conversion was reached. The polymerization region in which fluorescence intensity increases sharply seems to correspond to the increase of the rigidity of the medium at the glass transition. A correlation between the limiting quantum yield of fluorescence of the dye and the polymer glass transition Tg and expansion coefficient α was found. These results were interpreted in terms of rotation‐dependent nonradiative decay which links the excited‐state conformation to the rigidity of the medium.

Doublet versus quartet reactivity in the photoanation of hexammine chromium(III) ion

The photoanation of [Cr(NH 3) 6] 3+ by Cl − and Br − was studied in acid solution. No difference in quantum yields was found for both anions. The photoanation by Cl − was further studied in alkaline solution and in acid solution containing Co(II). In those solutions the 2E g state of Cr(III) is quenched. The limiting quantum yield for complete doublet quenching (unquenchable part of the photoreaction) is identical in acid and alkaline solution and has the same value as for the aquation (φ nq = 0.14). The quenchable part of the photoreaction depends on the Co(II) concentration the same way as the phosporescence. The probable photochemical pathways are discussed.

Reactions in microemulsions. 5. Effect of surface charge and reaction products on a chlorin-sensitized photoreaction

The kinetics of the chlorin-(chlorophyll a and pheophytin a) sensitized photoreduction of methyl red by ascorbate has been examined in oil-in-water microemulsions of three surfactant charge types: anionic, neutral, and cationic. In all cases, the reaction remains zero order in oxidant and exhibits saturation (limiting quantum yield) at high reductant concentration. When a pH 7 buffer is employed as the aqueous phase, the limiting quantum yield decreases in the order anionic > neutral > cationic. However, the reverse order obtains for the initial increase of quantum yield with ascorbate concentration. In both cases, the results are ascribed primarily to the effect of surface charge on local concentrations of ionic species (hydrogen ion and ascorbate monoanion), although there does appear to be some contribution from nonelectrostatic effects. One of the products of the chemical reduction of methyl red, N,N-dimethyl-p-phenylenediamine, has been found to increase quantum yield two- to threefold.

Vacuum–ultraviolet (147 nm) photodecomposition of 1,1,2-trichloro-2,2-difluoroethane

The 147 nm photolysis of CF2ClCHCl2 has been investigated at 25 °C as a function of reactant pressure, conversion, and nitric oxide as additive. In the absence of NO the observed reaction products are CF2CHCl, CF2CCl2, and the diastereomers of (CF2ClCHCl)2. At constant reactant pressure the quantum yields of the olefin decrease with increasing conversion and there is a corresponding increase in the quantum yield of the C4 product. For fixed values of conversion the olefin quantum yields decrease with increasing reactant pressure and approach limiting values at ∼100 Torr. The addition of NO completely suppresses the formation of the chlorofluorobutanes, while it enhances the olefin quantum yields at higher conversion. These observations are interpreted in terms of reactions of chlorine atoms which result either directly (by near simultaneous expulsion of two Cl atoms), or via the dissociation of an excited Cl2* molecule produced by molecular elimination in the primary process. Chlorine atoms abstract hydrogen from the parent or add to the product olefins. These processes provide the principal source of halo–ethyl radicals in the system. The addition reaction leads to chemically activated radicals with a mean lifetime of τ?0.8×10−8 sec which is commensurate with RRKM-theory predictions. The addition of nitric oxide provides a competing channel for chlorine atom removal by way of their NO-catalyzed recombination. The functional dependence of the olefin quantum yields with conversion in the absence and presence of NO suggests that the major fraction of the principal product, CF2CHCl, derives directly from a primary process, while CF2CCl2 is formed via both, the molecular elimination of HCl and from radical precursors. The limiting quantum yields of CF2CHCl and CF2CCl2 are found to be φ0?0.68 and φ0′?0.19, in the absence of NO, respectively, and φ0,NO?0.56 and φ′0,NO?0.087 in the presence of NO. The extinction coefficient for CF2ClCHCl2 at 147 nm and 25 °C has been determined: ε= (1/PL) ln(I0/It) =404±40 atm−1 cm−1.

Photochemical reactions of amines on nitrophenazines. Part 2. Photoreduction versus photosubstitution

The (n–π*) singlet excited state of 2-nitrophenazine reacts efficiently with amines. With tertiary amines extraction of a hydrogen atom from the α-carbon of the amine takes place to yield the 2-nitrophenazinyl radical (limiting quantum yield 0.64). The reaction occurs through the intermediate formation of a non-emitting exciplex, while a ground state complex formed only at high amine concentration is unreactive. The triplet excited state is similarly reduced by triethylamine. On the other hand, primary amines do not reduce but add to the triplet excited state of 2-nitrophenazine yielding addition products which are then dehydrogenated by oxygen and/or other ground state molecules of 2-nitrophenazine to give alkylaminonitrophenazines.

Photochemical cycloaddition of an aromatic ketone to a 1‐alkynyl ether. Kinetics and mechanism

AbstractIn this paper kinetics and mechanism of the photoaddition of 10,11‐dihydro‐5H‐dibenzo‐[a,d]cyclohepten‐5‐one (dibenzosuberone, 4) to 1‐methoxy‐1‐propyne (5) with the formation of the unsaturated ester 8, are studied. In benzene at 25.0°C a limiting quantum yield Φ = 0.04 was found. The rate constant k2 of the addition of triplet ketone 4 to ground state 5 amounted to 1.9 · 1077 l.mol−1,s−1. The unexpected regiospecificity is not caused by attack of triplet alkyne on ground state ketone. The low quantum yield is mainly caused by the high revertibility of the addition reaction. The intermediacy of a triplet exciplex is discussed.

THE REDUCTION AND QUENCHING OF PHOTOEXCITED FLAVINS BY EDTA

Abstract—Riboflavin was irradiated anaerobically in aqueous EDTA solutions over the pH range 2.5–10. In other dye systems (Bonneau and Pereyre, 1975), only the trivalent anion of EDTA was found to have significant reactivity for photoreduction. For riboflavin, the reactivity begins with monoanionic EDTA, and the reactivity is markedly increased as the charge increases. This suggests that the charge on the reductant is more important to the electron transfer process for riboflavin than the formation of a nonhydrogen bonded nitrogen site on EDTA. At high concentrations of EDTA in the pH range 4–8, quenching of the photoreduction occurs, which can be explained by an energy transfer between the excited singlet state of riboflavin and trianionic EDTA, possibly as an association complex. The rate constants for the photoreduction of riboflavin by the monovalent, divalent, and trivalent anions of EDTA are 1.0 times 107M‐1 s‐l, 4.8 times 10′M‐1 s‐l, and 2.0 times 108M‐1s‐1, respectively. The rate constant for the singlet state quenching by trianionic EDTA is 3 times 109M‐l s‐1, and the limiting quantum yield for intersystem crossing for riboflavin in aqueous solution is 0.50 ± 0.05.

Redox-photosensitised cleavage of indene dimers using aromatic hydrocarbon–dicyanobenzene systems; catalysis by the cation radical of aromatic hydrocarbons

Selective photoexcitation of phenanthrene in the phenanthrene–dicyanobenzene–indene dimer–acetonitrile system resulted in the cleavage of indene dimer to give indene in a limiting quantum yield of 8·2; a mechanism is suggested involving catalysis by the cation radical of phenanthrene.

REACTION OF O(1D2) WITH ETHANE IN LIQUID AND HIGH-PRESSURE SF6

Abstract Sulfur hexafluoride has been found to be completely inert toward O(1D2) even in the liquid state, thus being quite suitable as a solvent for reactions of the O(1D2) atom. The reaction between O(1D2) and C2H6 was studied in high-pressure as well as liquid SF6. The limiting quantum yield of C2H5OH was found to be 0.67 in the high pressure limit and as large as 0.88 in the liquid state.

The photo-induced pyrolysis of ethylene in a cw CO 2 laser beam

Ethylene gas was irradiated using a 660 W/cm 2 cw CO 2 laser emitting at 10.6 μm. When the pressure increases, both the light absorption (α = 0.016 torr −1 cm −1) and the chemical decay rate reach a limiting value. Poor experimental agreement with Landau—Teller or SSH theory is observed when T changes, and the limiting reaction rate coincides with a limiting quantum yield, from which a corresponding activation energy of 192.4 kJ/mole is deduced. The qualitative mechanism of decomposition can be explained, assuming a heat cooling control of the output molecular energy deposited through the translational degrees of freedom, which is similar to the control by heat transport of the fluorescence slow decay rate.