Role of excited states in radiolysis of cyclohexane

The mechanism of cyclohexane radiolysis in the condensed phase is discussed with particular attention paid to the cleavage of CC bonds. The contribution of ions and various neutral excitation states to the formation of radiolysis end-products is analysed. As follows from the experimental data shown, short-lived (≈10⁻¹²) superexcited states (S.E.S.) take part in the cyclohexane radiolysis reactions in the liquid phase. The basic process accompanying the decay of S.E.S. molecules is the formation of hydrogen atoms which may acquire excess kinetic energy (“hot atoms”). It is assumed that inspection of hot H atoms along the CC bond constitutes the main source of saturated products formed by cleavage of the cyclohexane ring (C.R.C.P.=cyclohexane ring cleavage products): CH₄, C₂H₆, C₆H₁₄ and so on. In condensed medium, the cleavage of two or more CC bonds in the same molecule, giving rise to unsaturated C.R.C.P. (C₂H₄, C₃H₆, etc.), originates mainly from S.E.S. A smaller amount of unsaturated C.R.C.P. is probably formed via the decay of excited molecular ions. Cleavage of a single CC bond with formation of hexene proceeds effectively from lower excited states which derive particularly from ion-recombination.Experimental data concerning the incidence of polymorphous transformation in cyclohexane on the C.R.C.P. yields are given. A general scheme for cyclohexane radiolysis, based on the cleavage of CH bonds, is postulated.     

Theoretical description of pressure shifts and quenching of excited atomic states in liquid helium

The pressure dependence of the linewidth and shift for the 2 ³S→2 ³P and 3 ³S→2 ³P transitions observed in electron-bombarded liquid helium is calculated by applying static line-broadening theory within the framework of a theoretical model. The calculated pressure shift, width and asymmetry are in good agreement with recent measurements over the pressure range of 1 to 25 atm. A new mechanism is proposed for the strong pressure quenching observed for several atomic and molecular emission lines.     

Evidence for static quenching of MLCT excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(PF(6))(2) [where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine] in acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K(sv) = 40,000 M(-)(1). Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I(2)(-)(*) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF(6))(2) in acetonitrile, similar behavior could be observed at iodide concentrations >100 times that required for dichloromethane.     

The yield of solvent excited states in the radiolysis of cyclohexane solutions

It is confirmed by pulse radiolysis that emitting solvent excited states are produced in the radiolysis of n-hexane, methylcyclohexane and cyclohexane. The emission is quenched by benzene and benzene emission appears. Applying stern - volmer kinetics to emissions from solvent, benzene and toluene in cyclohexane a very high energy transfer rate constant, viz., k = 2.8 × 10¹¹M⁻¹ sec⁻¹ is obtained. The yield of the excited state of cyclohexane is not greater than 0.3, and it is concluded that the major part of the excited states of other aromatics produced in cyclohexane solutions comes from ion neutralisation.     

The fate of oxygen in the quenching of excited singlet states

Measurement of the efficiency of rubrene-sensitizied photooxidation of 1,3-diphenylisobenzofuran imply that direct formation of singlet oxygen via oxygen quenching of excited rubrene singlets is inefficient. This contrasts with recent publications based upon studies of self-sensitized rubrene photooxidation. The observed inefficiency can be rationalized in terms of the spin-allowed decay of an initially formed ³(T₁ + ¹Δ) complex state to a lower energy ³(T₁ + ³Σ) state prior to complex dissociation.     

Photo-acoustic detection on electronic quenching rate constants of NO excited states

The electronic quenching rate constants of NO A(2)Σ (υ'=0, 1), E(2)Σ (υ'=2, 3, 4) and F(2)Δ (υ'=1, 2, 3) states by gas air are reported. The experiments were carried out by measuring the total fluorescence intensity of A(2)Σ (υ'=0, 1)→X(2)Π (υ″) transition at various air pressures. It gives the Stern-Volmer plots. The quenching rate constants of A(2)Σ (υ'=0, 1) states are obtained from the slope of Stern-Volmer plots and the known radiative lifetime. Based on the primary results of the work, we have measured the quenching rate constants of high excited E(2)Σ (υ'=2, 3, 4), F(2)Δ (υ'=1, 2, 3) states for the first time with the technique of photo-acoustic (PA) spectroscopy. It is shown that the electronic quenching rate constants of NO E (υ') and F (υ') states are in the order of 10(-10)cm(3)/molecules. They are much larger than those of A(2)Σ (υ') state, whose rate constants are in the order of 10(-13)cm(3)/molecules. For E (υ') and F (υ') states, it is also found that the quenching rate constants increase with the vibrational energy levels. Similar result has been reported also for A(2)Σ (υ'≥2) states in existing literatures. The agreement indicates the potential use of PA spectroscopy for measuring the electronic quenching rate.     

Photo-oxidation poly(butadiene); quenching of excited states by nitroxides

The phosphorescence from three model bicyclic enones and those formed in thermally oxidized poly(butadiene) has been shown to be quenched efficiently by a nitroxide [3,3,4,4-tetramethyldiazetine N,N′-dioxide] in CH₂Cl₂:tetrahydrofuran glass at 77K. The Perrin model for static quenching was applied, and the interchromophoric distance for half-quenching found to be ～ 12 A. At room temperature the nitroxide additive inhibited the formation of volatile products from thermally oxidized poly(butadiene) films: results also obeyed the Perrin relationship, pointing to a quenching mechanism of photostabilization. By contrast, a commercial hindered amine stabilizer was found not to act as a quencher of phosphorescence at 77K, and to have a concentration dependence for the reduction in volatile product formation at 298K in PBD films different from the nitroxide, indicating different mechanisms of action.     

Pulse radiolysis studies on the formation of singlet excited states of coumarin 153 in cyclohexane solutions

By using the technique of nanosecond pulse radiolysis, pulsed electron beam induced light emission from coumarin dyes in hydrocarbon solvents has been studied. The emission spectra so obtained were similar to the optically excited fluorescence spectra. The emission lifetimes were of the same order as the fluorescence lifetimes in the respective solvents, showing that the emitting species are the same in both the cases viz. singlet excited states of the dyes. In one system viz. C 153 in cyclohexane experiments were carried out in presence of electron and hole scavengers and also the concentration dependence of emission intensity studied over a wide range. From these it is concluded that the solute excited states are formed mainly by energy transfer from the solvent excited states, part of which may arise from excitation by cerenkov light generated in the medium.     

Transient absorption spectra and quenching of coumarin excited states by nucleic acid bases.

Triplet-triplet absorption spectra of coumarin show different profiles and maxima in ethanol from those in water, which are very similar to those reported in benzene. Long-lived transient species other than triplet states were generated as initial photoproducts between coumarins and nucleic acid bases. The excited singlet and triplet states of coumarins were quenched by nucleic acid bases. Adenine base quenched the excited singlet state of coumarins more efficiently than thymine base. However, photocycloadducts of furocoumarins are formed predominantly with thymine, and not with adenine. Moreover, it is reported that the poly[dA-dT].poly[dA-dT] sequence region is the most favourable site for the photocycloaddition reaction of furocoumarins. The results imply that adenine contributes to singlet-state photocycloaddition reaction of furocoumarins with thymine, probably through an adenine-furocoumarin-thymine termolecular interaction.     

Static and dynamic quenching of Ru(II) polypyridyl excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.     

Studies in mixed solvents: Comparison between solvated electron reactions and quenching of excited states

The rate constants for the reaction of electron pulse produced solvated electrons and a number of solutes in water-isopropanol mixtures have been measured. The quenching of the singlet excited state of naphthalene has also been studied in the same mixtures, using triethylamine and acrylamide as quenchers. The variation of the bimolecular solvated electron reaction rate constants with the composition of the solvent has been compared with the variation in the quenching constants with the composition of the solvent. Both these variations are surprisingly similar, with acrylamide behaving in a reverse manner (to the other solutes) in both the cases. It has been possible to quantitatively correlate both sets of data using dielectric constant (?) as a measure of polarity and the viscosity (η) as an index of the microstructure. The curves obtained provide insights with respect to the nature of charge transfer processes involved. © 1995 John Wiley & Sons, Inc.     

A ratiometric and non-enzymatic luminescence assay for uric acid: differential quenching of lanthanide excited states by anti-oxidants

The excited states of Tb and Eu complexes of a common macrocyclic ligand are quenched preferentially by electron transfer from the urate anion, allowing the creation of a new assay to measure uric acid in biological fluids.     

Influence of viscosity of medium on processes of oxygen quenching of excited states of mesoporphyrin

The oxygen quenching rate constants for singlet and triplet excited states of the dimethyl ester of mesoporphyrin IX increase with decreasing viscosity of the medium and reach a maximum at a viscosity of approximately 0.4 mPa·sec, after which the rate constants begin to decrease. The drop in rate constant with increasing fluidity of the medium may be related to a nonequilibrium character of the elementary act in the interaction of the reactants in the solvent cage. In viscous media such as isopropyl alcohol, isobutyl alcohol, and isoamyl alcohol, the reaction radius is greater than the sum of the radii of the reacting particles. The long-range character of these processes is due to the relatively weak dependence of the quenching probability on distance.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 161–167, March–April, 1989.     

Properties of excited singlet states of N-arylurethanes : Photo-fries reactions,fluorescence, quenching and sensitization

The deactivation of the first excited S(ππ^*) states of N-arylurethanes (produced upon irradiation with UV light) by emission (fluorescence), chemical reaction (photo-Fries rearrangement and fragmentation), energy transfer to quenchers, and radiationless transitions to ground and triplet states is investigated. Arylurethanes exhibit fluorescence (λ_f ≈ 295–350 nm, φ_f ≈ 10^?2, τ_f ≈ 1–6 ns) and phosphorescencs (λ_p ≈ 370–410 nm). The variations of the quantum yields of the fluorescence and of the photo-Fries rearrangement of N-arylurethanes by substituents and solvents are essentially due to variations of the rate constants for the radiationless processes. Fluorescence and photo-Fries reactions can be quenched by diffusion-controlled energy transfer to aliphatic ketones. Quenching is accompanied by sensitization of the ketone fluorescence. The urethane fluorescence and photo reactions may be sensitized by aromatic hydrocarbons. The results of all the quenching and sensitization experiments demonstrate that the photo-Fries reactions of N-arylurethanes proceed via the first excited singlet states of the urethanes.