Alan Cox

Absorption spectrum, lifetime and photoreactivity towards alcohols of the excited state of the aqueous uranyl ion (UO2+2)

Microsecond flash photolysis of solutions of uranyl salts in water, H3PO4, H2SO4 and other media yields an intense, but short lived transient absorption spectrum in the region of 570 nm which exhibits vibronic structure with an average band separation of 580 cm–1. The decay of the transient in water was determined by laser flash photolysis to be first order with k1= 8.02 × 105 s–1, but with a considerable isotope effect (k1(D2O)= 4.20 × 105 s–1). k1 accords with the lifetime of the luminescence of aqueous UO2+2 ion determined by single photon counting (k1= 3.85 × 105 s–1). Both the 570 nm absorption and the emission (as determined by both single photon counting and conventional fluorimetry) are quenched on addition of various alcohols, the Stern–Volmer quenching constants closely correlating with the absolute quenching constants for the transient determined by laser flash photolysis. Large isotope effects were found for quenching by deuterated methanol, isopropanol and cyclohexanol both of the transient absorption and the emission. We conclude that the same electronic state of UO2+2 is responsible for (i) emission to the ground state, (ii) the absorption at 570 nm to a higher excited state and (iii) the photo-oxidation of aliphatic alcohols, principally by a process of abstraction of an H atom bonded to carbon from a —CH2OH or CHOH group.

Concentration and temperature quenching of the excited state of the uranyl ion in aqueous solution by laser flash photolysis

Abstract In aqueous acidic solution the excited state of the uranyl ion is quenched to a degree by ground-state uranyl ion ( k q = (4.0 ± 0.3) × 10 5 dm 3 mol −1 s −1 ) and is much shorter-lived at higher temperatures ( E a = 41.2 ± 0.7 kJ mol −1 ).

The photochemistry of some transition metal carbonyl compounds in a 2-methyltetrahydrofuran matrix

Abstract Photolysis of some carbonyl halides of Mn and cyclopentadienylmetal carbonyl halide derivatives of Fe, Mo and W in a 2-methyltetrahydrofuran (MTHF) matrix at about 77 leads to extrusion of CO, which becomes matrix-trapped, and formation of an unsaturated derivative characterised by i.r. spectroscopy; at ca. 30 K the Mo and W compounds readily undergo a reaction not involving loss of CO, the precise nature of which is uncertain. Photolysis at 77 K of the dimers [(π-C 5 H 5 )M(CO) 3 ] 2 , M = Mo, W results in i.r. changes compatible with metal-metal bond homolysis, whilst that of Re(CO) 5 X (unlike Mn- (CO) 5 X X = Cl, Br) does not involve loss of CO.

Photochemistry of π-cyclopentadienylcarbonylmetal halides in dimethyl sulphoxide and related solvents

The principal product when solutions in dimethyl sulphoxide or pyridine of π-cyclopentadienylcarbonylmetal halides of Fe, Mo, and W are irradiated at long wavelengths (λ > 400 nm) is a dimer: this is [(π-C5H5)Fe(CO)2]2 in the case of Fe (indicating loss of halogen) but appears to be [(π-C5H5)M(CO)2Cl]2 for Mo and W; free cyclo-pentadiene and chloride ion are also produced in the case of Fe. The reactions do not proceed in non-polar solvents. At shorter wavelengths (λ > 280 nm) the principal product in all cases is a dimeric species which could not be isolated, but which spectral data suggest to correspond to displacement of one CO molecule by solvent from the dimers obtained at longer wavelengths; moreover i.r. and n.m.r. spectral profiles of the λ > 280 nm reaction indicate that the latter are essential intermediates in all three cases.

Photopolymerization of 1,2-epoxypropane and 1,2-epoxybutane by arenediazonium salts: evidence for anion dependence of the extent of polymerization

Abstract The rates and extents of polymerization of 1,2-epoxypropane and 1,2-epoxybutane, induced by photolysis of dilute concentrations of salts of 2,5-diethoxy-4-tolylthiobenzenediazonium ion with various Lewis acids (SbF−6, PF−6, BF−4 etc.) have been determined by viscosity, 1H n.m.r. and polymer isolation methods. A clear dependence exists of the degree of conversion on the anion: thus SbF−6 salt yields up to 100% high polymer, PF−6 salt yields c. 20% high polymer, while BF−4 salt yields small conversions (2%, but increasing to 15–18% at high initiator concentrations) which consists almost entirely of oligomers, mainly the cyclic tetramer.

Photochemistry of phosphorus(III) complexes of bromocarbonyl(π-cyclopentadienyl)iron(II) and dicarbonylchloro(π-cyclopentadienyl)-molybdenum(II) and -tungsten(II)

Photolysis with visible light in pyridine or dimethyl sulphoxide solutions of the complex [(cp)Fe(CO)LBr] yields [(cp)Fe(CO)2]2 when L = P(OPh)3 but [(cp)Fe(CO)2(PPh3)]+Br– when L = PPh3(cp =π-cyclopentadienyl). Solutions of he complexes [(cp)M(CO)2LCl][M = Mo or W; L = P(OPh)3, PPh3 or P(C6H11)3] yield [(cp)M(CO)2Cl]2 on irradiation under similar conditions. Possible mechanisms for these photoreactions are discussed.

Inorganic photophysics in solution. Part 5.—Bis(8-hydroxyquinolinato)platinum(II)—absorption, excited state absorption and emission spectra, kinetics and energetics

The spectroscopy and photophysics of bis(8-hydroxyquinolinato)platinum(II) have been examined from the following viewpoints: (i) the sharp dependence of the ground-state absorption peaks upon solvent (as indicated by a correlation of vmax with the parameter ET), (ii) the solvent-dependence of the luminescence lifetime (which is small), (iii) the temperature-dependence (and hence the energetics) of the non-radiative decay process from the luminescent state and (iv) the discovery of an intense excited-state absorption spectrum (λmax 565, 850 and 930 nm).

The Photolysis of Arenediazonium Salts in Solutions and Films: Quantum Efficiencies and Reactive Intermediates

AbstractQuantum efficiencies φdec for the photodecomposition by monochromatic light of a large number of both simple and commercially important arenediazonium salts have been determined at 293 K in 0.05 M aqueous sulphuric acid, 0.05 M acidic (HaSO,) ethanol and cellulose acetate film, enabling comparison (and discrimination) between results reported at various points in the literature for experiments performed in a single medium. The values of φdec derived for solutions yield good Hammett plots, indicating a sensitivity of φdec towards the electronic effect of substituent groups. Very little such elfect was, however, apparent for a series of arenediazonium salts in cellulose acetate film. Variation of the irradiation wavelength within the absorption band of the arenediazonium salts was found to produce no significant change in φdec in aqueous acid solution.E.s.r. signals due to the Ar+ species formed during the photoreaction have been observed on photoirradiation of cellulose acetate films of 4-dialkylam...

Inorganic photophysics in solution. Part 2.—Temperature activation of decay processes in the luminescence of uranyl[UO2+2] ion

The luminescent decay of excited uranyl ion has been studied over a wide temperature range in dilute aqueous acid, concentrated HClO4, H2SO4, H3PO4, a borosilicate glass, acetone, CH3CN, CF3CO2H, CH3OH, CD3OD and a polymeric film (of cellulose acetate). In those cases where it was possible to work at temperatures as low as 77 K, it proved possible to rationalise the temperature-dependence of the rate in terms of the equation k(T)=B+A exp (–Delta;E[graphic omitted]/RT). B, which is similar for all systems studied (within a factor of 3), is taken as reflecting a combination of radiative and non-radiative processes of a physical nature. The second term is characterised by high values of ΔE≠ and (like the first) the presence of kinetic isotope effects, and is taken to refer to chemical quenching, usually by a process of rapidly reversed hydrogen–atom abstraction, but (when this is impossible) by electron-abstraction.

Inorganic photophysics in solution. Part 4.—Deactivation mechanisms of the 2Eg state of CrIII complexes from lifetime studies

The temperature dependences of the luminescence lifetimes, τlum, of several CrIII complexes of particular photochemical interest have been determined over temperature ranges 77–370 K in a variety of media, especially 9 mol dm–3 LiCl + H2O and cellulose acetate film. In most cases, klum fits an expression kTl+ANR exp(–ΔE[graphic omitted]NR/RT) where the subscripts refer to temperature-independent and (temperature-dependent) non-radiative terms. The strong solvent-dependence of ΔE[graphic omitted]NR, which is always only a fraction of the 4T2g–2Eg splitting, suggests a chemical pathway for deactivation of the 2Eg state rather than a purely physical route such as back-intersystem crossing. For complexes which have been examined in a wide variety of media, values of ln ANR correlate fairly well with those of ΔE[graphic omitted]NR, implying a common overall mechanism.