Using 4-methyl-4-phenyl-2-pentanone as a singlet probe of benzene,cyclohexane, tributylphosphate and its multiple scavenger effects

A special probe of excited singlet states was established. The Norrish type II elimination reaction of 4-methyl-4-phenyl-2-pentanone (MPP) induced by γ-rays was reinvestigated. By measuring the G value of the sensitization radiolytic product α-methylstyrene (α-MS), we obtained the G value of excited singlet states. It was found that the G value of benzene excited singlets is 1.4±0.2, the rate constant of excitation energy transfer from benzene excited singlets to MPP is (5.2±0.5)×10⁹dm³mol^-1s^-1; the G value of excited tributylphosphate singlets is 1.4±0.3, the lifetime of its excited singlets is (1.3±0.1)×10^-7s; the G value of excited cyclohexane singlets is 1.5±0.2, the rate constant of excitation energy transfer from cyclohexane excited singlets to MPP is (4.1±0.4)×10¹⁰dm³mol^-1s^-1. It was found that MPP has multiple effects in quenching the active species formed in the radiolysis of benzene and tributylphosphate, both excitation energy transfer and reaction with subexcitation electrons have occured. A yield of subexcitation electrons of benzene 4.0, that of tributylphosphate 5.0, was obtained. The kinetics of both processes were also discussed.     

Pico-second pulse radiolysis of cyclohexane solutions

A rapid (t < 500 psec) transfer of singlet energy is observed in the picosecond pulse radiolysis of dilute (10⁻³–10⁻² M) solutions of aromatic solutes in cyclohexane. This event is not associated with any ion neutralization reaction, but is a direct transfer of energy from an excited state of cyclohexane (t_e < 500 psec) to the solute.     

NANOSECOND IRRADIATION STUDIES OF BIOLOGICAL MOLECULES-I. COENZYME Q 6 (UBIQUINONE-30)

Abstract— The coenzyme ubiquinone, an isoprenoid benzoquinone present in the electron-transport chain of mitochondria, has been studied using nanosecond laser flash photolysis and pulse radiolysis. The hitherto undetected triplet excited state of the coenzyme has been identified and some of the physico-chemical properties determined. These measurements may assist the understanding in molecular terms of the degradative action of light upon biological materials, photophosphorylation and the possible initiation of biological electron transport via quinone light absorption. Laser photolysis of ubiquinone in cyclohexane and pulse radiolysis of ubiquinone in benzene results in the formation of a transient absorption with maximum around 440 nm and a half-life of 650 nsec in cyclohexane and 450 nsec in benzene. Energy transfer sensitisation of the β-carotene triplet absorption by ubiquinone in cyclohexane at a rate consistent with the life-time of the 440 nm transient absorption, yields strong evidence that this transient species is triplet ubiquinone. The triplet reacts with oxygen with a rate constant of 2 × 10^--⁹ mole^-1 sec^-1. Photolysis studies of ubiquinone in ethanol and isopropanol and addition of ethanol to ubiquinone in cyclohexane show that little ubisemiquinone is formed by reaction of the triplet with alcohols. Electron spin resonance studies support this conclusion, and also show that some ubisemiquinone is however formed on photolysis of solutions of ubiquinone in methylcyclohexane. Energy transfer experiments in the presence of various triplet energy donors and acceptors suggest that the triplet energy of ubiquinone lies between 176 and 123 W mole^-1, and that the triplet extinction coefficient at 440 nrn is 19 ,000 mole^-1 cm^-1 in cyclohexane and 13 ,000 mole^-1 cm^-1 in benzene (at 430 nm). The singlet to triplet crossover efficiency for ubiquinone in cyclohexane was estimated to be 0.04. The low triplet energy level, crossover efficiency and abnormal type of reaction with alcohols are reflections of the profound influence of the isoprenoid chain upon excited states of this quinone.     

The mechanism of singlet energy transfer from alkanes

By means of pulse radiolysis energy transfer from the first excited singlet of cyclohexane and C₇- to C₁₀-normal alkanes to benzene as solute was studied. For this solute the reaction probability of singlet energy transfer is expected to be one. If this condition is fulfilled, from the reaction radii measured conclusions can be drawn on the mechanism of singlet transfer. Using known diffusion coefficients reaction radii R were determined which are close to the sum of molecular radii. From a comparison with Foerster radii and the Stokes-Einstein relationship it is concluded that the energy transfer from the singlet states of the alkanes investigated to benzene as solute proceeds by collisional interaction.     

Picosecond time-resolved spectroscopy and the intersystem crossing rates of anthrone and fluorenone

The rise time of the T_n ← T₁ absorption of anthrone in benzene is determined to be 70 ps. The mechanism of efficient intersystem crossing of anthrone has been discussed and is compared with that of benzophenene. The rise time of fluorenone has been found to be sensitive to solvent, changing from 140 ps in cyclohexane to 12 ns in acetone. This tendency is explained in terms of the change in the character of the lowest excited singlet state of fluorenone from ππ^* in acetone to nπ^* in cyclohexane.     

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.     

Photophysical properties of gatifloxacin in aqueous solution by laser flash photolysis and pulse radiolysis

GFX in water, at pH 7.0, shows intense absorption bands with peaks at 284 and 333 nm, (ε=24,670 and 12,670 M⁻¹ cm⁻¹). Both the absorption and emission properties of GFX were pH-dependent; the pK_a values for the protonation equilibria of the ground state (5.7 and 8.9) and excited singlet state (3.6 and 7.5) of GFX were determined spectroscopically. GFX fluoresces weakly, with a maximum quantum yield for fluorescence emission (0.06) at pH 4.7. A series of experiments were performed to characterize the transient species of GFX in aqueous solution using laser flash photolysis and pulse radiolysis. GFX undergoes monophotonic photoionization with a quantum yield of 0.16 on a 355 nm laser excitation. This process leads to the formation of a long-lived cation radical with a maximum absorption at 380 nm. Triplet-triplet absorption had maximum absorption at 510 nm. The reaction of GFX with one-electron oxidant N₃• was investigated and the bimolecular rate constant was determined to be 3.1×10⁹ M⁻¹ s⁻¹.     

On optically detected ESR spectra from radical cations of liquid hydrocarbon solvents

The optically detected electron spin resonance (OD ESR) method has been employed to study the origin of radical-cation ESR signals in some saturated hydrocarbons with small amounts of 2.5-diphenyloxazol or p-terphenyl under radiolysis. In cyclohexane, the ESR, signal with resolved hyperfine structure was ascribed to c-C₆H₁₀⁺/PPO^? radical-ion pairs produced from primary c-C₆H₁₂⁺/PPO^? ones by monomolecular decay, of cyclohexane radical cations to cyclohexene radical cations. Cis- and trans-decalin under radiolysis accumulate 9,10-octalin which captures solvent holes and form 9,10-octalin radical cations giving a resolved OD ESR spectrum. 9,10-octalin is present in non-irradiated commercial decalin as an impurity. The OD ESR technique has been shown to be very sensitive to some impurities in hydrocarbon solvents.     

The scintillation process in three-component systems: The luminescence minimum revisited

The scintillation yield from the three-component system C + B + T, where T is the scintillation solute, is studied as a function of the electron fraction of component B for both β^--particle excitation and for optical excitation of C both below and above its ionization threshold. When C = cyclohexane and T = N,N,N',N'-tetramethylphenylenediamine, deep minima (20–40%) are observed when B = benzene under all excitation conditions, and much shallower minima (⋍5%) are observed when B = toluene and then only for β^--particle excitation and excitation above the ionization threshold of C. Analysis of these results indicates that the production of S₁ states of B (i.e. B¹) via either energy transfer from electronically excited C or via charge transfer from C⁺ followed by B⁺ + e^- → B¹ is intrinsically inefficient in dilute cyclohexane solutions. For B = benzene this inefficiency resides mainly in the electronic energy-transfer process and for toluene in the ion recombination process. When C = benzene (or toluene) and B = toluene (or p-xylene) with T = diphenyloxazole (10^-3 M), very shallow minima (<3%) are observed under β^--particle excitation. The analysis of these results illustrates the inadequacy of previous theories of the luminescence minima.     

Large reaction radii for the quenching of fluorescent excited states of liquid cis-decalin and cyclohexane

The large rates of quenching by different solutes of the fluorescent solvent excited state in liquid cis-decalin and cyclohexane as observed by pulse radiolysis are explained by large reaction radii. Values of 14, 13, and 15 Å are found for the quenching of the excited state in cis-decalin by CCl₄, in cyclohexane by CCl₄, and by O₂ respectively.     

Ergosterol (provitamin D2) triplet state: an efficient sensitiser of singlet oxygen, O2(1 delta g), formation

The triplet state of ergosterol (provitamin D2) has been produced in benzene by pulse radiolysis and characterised in terms of absorption spectrum, lifetime, self-quenching properties and relaxed triplet energy. The amount of singlet oxygen, O₂(¹Δ_g), produced as a consequence of the oxygen quenching of this species has been determined by kinetic infrared emission spectroscopy. Ergosterol is significantly more efficient as a singlet oxygen sensitiser in benzene than is naphthalene, the absolute standard employed in this work.     

Absolute rate constants for the gas-phase reaction of OH radicals with cyclohexane and ethane at 295 K

The absolute rate constants for the gas-phase H-atom abstraction by hydroxyl radicals from cyclohexane and ethane have been determined at room temperature. OH radicals were produced by pulse radiolysis of an H₂O-Ar mixture, and the decay of OH was followed by monitoring the transient light absorption around 309 nm. The rate constants were found to be k = (5.24±0.36) × 10⁻¹² and (2.98±0.21) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ for cyclohexane and ethane, res- pectively. These results are compared with literature data.     

Time-resolved spectroscopy of solid poly(1-vinyl naphthalene)following electron beam pulse radiolysis: pulse radiolytic studies on polymers

Transient emission studies following pulse radiolysis of solid poly(1-vinyl naphthalene) show existence of excited monomers and two excimers. Quenching experiments indicate that excimers are not formed directly by recombination of ions but probably by trapping of migrating monomeric excitation in performed traps whose density is approximately one in 10³.     

First observation of intra-5f fluorescence from an actinyl center: Np(VI) near-IR emission in Cs2U(Np)O2Cl4

Fluorescence from an excited 5f state of Np(VI) has been observed in the doped impurity system Cs₂U(Np)O₂Cl₄. This is the first intra-5f fluorescence transition that has been detected at room temperature in a condensed-phase system with an actinyl (An(VI)O₂²⁺) core, and it is a rare example of fluorescence of any kind from non-uranyl ions of this type. The emission originates from an excited state approximately 6890 cm⁻¹ above the ground state. Its emission spectrum and fluorescence lifetime at 295 K will be discussed. Vibronic structure in the emission spectrum is assigned based on comparison with the detailed analysis of the absorption spectra published by Denning et al.     

Evaluation of the ionic liquids 1-alkyl-3-methylimidazolium hexafluorophosphate as a solvent for the extraction of benzene from cyclohexane: (Liquid + liquid) equilibria

In the catalytic hydrogenation of benzene to cyclohexane, the separation of unreacted benzene from the product stream is inevitable and essential for an economically viable process. In order to evaluate the separation efficiency of ionic liquids (ILs) as a solvent in this extraction processes, the ternary (liquid + liquid) equilibrium of 1-alkyl-3-methylimidazolium hexafluorophosphate, [C_nmim][PF₆] (n = 4, 5, 6), with benzene and cyclohexane was studied at T = 298.15 K and atmospheric pressure. The reliability of the experimentally determined tie-line data was confirmed by applying the Othmer–Tobias equation. The solute distribution coefficient and solvent selectivity for the systems studied were calculated and compared with literature data for other ILs and sulfolane. It turns out that the benzene distribution coefficient increases and solvent selectivity decreases as the length of the cation alkyl chain grows, and the ionic liquids [C_nmim][PF₆] proved to be promising solvents for benzene–cyclohexane extractive separation. Finally, an NRTL model was applied to correlate and fit the experimental LLE data for the ternary systems studied.     

The effect of crystalline phase on the fluorescence characteristics of solid cyclohexane and bicyclohexyl

Emission from cyclohexane and bicyclohexyl solids has been found to critically depend on their crystalline structure. The emission spectrum of cyclohexane suddenly blue-shifts 2900 cm^?1 on cooling below its solid—solid phase transition temperature (?87.1°C). The bicyclohexyl emission intensity sharply decreases on freezing to solid and then again sharply recovers at a heretofore unreported phase transition temperature (≈ 0°C) with the spectrum 2500 cm^?1 blue-shifted from that of the liquid. The results are discussed utilizing the partial Rydberg nature of the excited cycloalkane states.     

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene-h₄ (and -d₄), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B?(π^?1) → X?(π^?) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-,o-,m-,p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B? states. Emission, with quantum yield > 10^?5, could not be detected with electronically excited radical cations of chloro-,o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B? states, which arise by σ^?1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B?(π^?1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B? → B? band systems of the three title cations.     

Solvent effects on relaxation of the 1B2u state of benzene

Values of triplet yields for dilute solutions of benzene in methylcylohexane, cyclohexane, methanol, ethanol, acetonitrile, perfluoro-n- hexane, and water, and also fluorescence yields and lifetimes in perfluoro-n-hexane are reported. The calculated rate constants for fluorescence and intersystem crossing are only slightly affected by solvent, except for water. The enhancement of “internal conversion” by increasing solvent polarity is interpreted as a result of an increase in the pre-exponential factor for a possible ¹(π,π^*) → ¹(σ,π^*) transition.     

60Co-v Radiolysis of a Mixture of Benzene and Cyclohexane in Presence of Diphenyl Mercury

In the radiolysis of cyclohexane in presence of 4×10^?3M diphenylmercury (Hg φ₃) three isomers of hexane, methylcyclopentane (G=0.018), benzene (G=0.42) and cyclohexene (G=0.047) were detected. Addition of benzene in the mixture of cyclohexane and Hg φ₃ formed two isomers of pentane, hexene and one isomer of hexane as additional products, while cyclohexene was eliminated completely. Normally, eight products were detected in presence of 10 to 50% benzene. Total radiolytic yield of products increased in presence of 15 to 25% benzene but in presence of 35 to 50% benzene G values became very low. Considerable amount of hexene is formed in a mixture of benzene and cyclohexane but neither benzene nor cyclohexane in presence of Hg φ₂ formed this compound. In the presence o. benzene and φ₂Hg hexane yield is very much reduced. Protection is observed in presence of 10% as well as 35 to 50% benzene in this system. The plot of benzene concentration in moles/litre versus methylcyclopentane is linear and from the slope of the straight line, the values of rate constants were found to be 2.65×10^?2 litre/mole sec., 5.25×10^?3 litre mole sec., 9×10^?7 litre/mole sec. for methylcyclopentane, cyclohexane and benzene respectively. A plot of G(–c-C₆H₁₂) versus 1/[C₆H₆] also gave a straight line which confirms the sponge type protection in this multicomponent system.     

A re-evaluation of the photophysical properties of 1,4-bis(phenylethynyl)benzene: a model for poly(phenyleneethynylene)

Photophysical measurements, recorded in aerated cyclohexane at 283 K, indicate that 1,4-bis(phenylethynyl)benzene behaves in a conventional manner, undergoing emission from the lowest vibrational level of the first excited singlet state; there is no evidence for aggregation of this material in cyclohexane solution in the concentration range (1-250) x 10(-6) mol dm(-3). However, in highly viscous, low-temperature glasses, the material does exhibit inhomogeneous fluorescence behavior, and wavelength-dependent excitation and emission spectra, indicative of a slow rate of relaxation of conformers of the excited states compared to the rate of fluorescence.