Francis Wilkinson

Rate Constants for the Decay and Reactions of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and Revised Compilation

An expanded and revised compilation on the reactivity of singlet oxygen, the lowest electronically excited singlet state of molecular oxygen, 1O2*(1Δg), in fluid solution is presented, which supersedes the publication of Wilkinson and Brummer, J. Phys. Chem. Ref. Data 10, 809 (1981). Rate constants for the chemical reaction and physical deactivation of singlet oxygen available through 1993 have been critically compiled. Solvent deactivation rates (kd) are tabulated for 145 solvents or solvent mixtures and second‐order rate constants for interaction of singlet oxygen with 1915 compounds are reported.

Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution

Quenching of excited singlet and triplet states of many substances by ground state molecular oxygen produces singlet oxygen, the lowest electronically excited singlet state of molecular oxygen, O2(1Δg). The fractions of singlet and triplet states quenched which produce singlet oxygen and the quantum yields of formation of singlet oxgyen in fluid solutions have been critically compiled. Methods for determination yield parameters have been reviewed. Data have been compiled from the literature through 1991. Photosensitizers such as aromatic hydrocarbons, aromatic ketones and thiones, quinones, coumarins, fluoresceins, transition metal complexes, and heterocyclics are included in Table 1. Porphyrins and phthalocyanines are included in Table 2. Other materials which have been investigated for singlet oxygen production, such as dyes and drugs, are collected in Table 3 along with heterogeneous systems such as polymer‐bound photosensitizers.

Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution

The available rate data on the reactivity and physical deactivation of molecular oxygen in its first electronic excited state (1Δg) in liquid solution have been critically compiled. Where possible, relative rates reported in the literature have been normalized to standard values selected by a statistical analysis of the experimental data. Second order rate constants for the deactivation and chemical reaction of singlet oxygen are reported for 670 compounds. Additionally, psuedo first order rate constants (kd) for solvent deactivation of singlet oxygen are reported for 50 different solvents.

Photoluminescence of semiconducting zinc oxide containing rare earth ions as impurities

Abstract The photoluminescence of zinc oxide sintered at 1100°C and doped with rare earths (Tm3+, Ho3+, Nd3+ and Er3+) has been investigated under such excitation conditions (around 390 nm) where the light is principally absorbed by ZnO. Contrary to previous reports, no emission from the rare earth could be observed and only the luminescence of the semiconducting substrate in which the rare earth put the fingerprint of its absorption has been characterized. Such partial reabsorption of the green emission of ZnO by the rare earth has been verified by the diffuse reflectance spectra of the same samples which show the same typical fine structures. Only holmium presents an emission when excited in its absorption band at 450 nm; no light emitted above 900 nm could be characterized in the present study.

Detection of triplet—triplet absorption in microcrystalline benzophenone by diffuse-reflectance laser flash photolysis

An apparatus is described which can detect transient absorptions in the nanosecond—microsecond time domain in opaque scattering materials by analysing diffusely reflected monitoring light following nanosecond laser excitation. For microcrystalline benzophenone excited by a 20 ns pulse at 354 nm, a transient absorption is easily recorded, having a maximum at 540 nm. Its decay is a mixture of first-and sedond-order processes, with the initial and final slopes of the first-order kinetic analysis differing by a factor of 10. The second-order rate constant is estimated to be (8.5 ± 1.0) × 10 9 cm 3 mol −1 s −1 . Microcrystalline benzophenone emits a phosphorescence which, within experimental error, has an identical kinetic decay.

Photochemistry on surfaces : fluorescence emission quantum yield evaluation of dyes adsorbed on microcrystalline cellulose

A simple method to determine the fluorescence quantum yield (ϕF) of dyes adsorbed on microcrystalline cellulose is presented. The method is based on corrected fluorescence emission spectra and can easily be applied provided the energy profile of excitation is accurately determined. The quantitative determination of ϕF is based on the ratio of the slopes of curves which correlate the fluorescence intensity and the absorbed light for both standard and unknown samples. The evaluation of the absorbed light is done by determining the reflectance R through the use of an integrating sphere. The remission function can then be determined as a function of wavelength in the two cases.Rhodamine 101 (R101), a rigid molecule with unitary ϕF, was used as a standard compound to determine the ϕF values for rhodamine 6G (R6G) and auramine O (AURO), all dyes adsorbed on cellulose, and we obtained 1.02 ± 0.03 and 0.14 ± 0.01, respectively.Strong aggregation was detected for the two rhodamine dyes in the ground state, which we assign to dimer formation known to influence the fluorescence emission with respect to absorbed intensity. AURO does not aggregate when adsorbed on microcrystalline cellulose for loadings up to 10 µmol g–1. The intensity of the emission depends on concentration in all cases, but no emission was detected from the aggregated forms of the two rhodamines.The ground-state diffuse reflectance study for R101 and R6G enables us to determine the equilibrium constants for dimer formation of these two rhodamines adsorbed on cellulose which are KD= 0.4 × 106 and 1.3 × 106 mol g–1, respectively.The results obtained for ϕF of these dyes show that the method is sensitive, making it possible to determine emissions from samples ranging from 1 nmol of the dye per gram of cellulose up to 10 µmol g–1. The determination of quantum yields can be obtained with an accuracy >3%.

Photophysical properties of lumichromes in water.

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 micros have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 micros has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, phi(Delta), are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D(2)O.

Photochromism of spiro-naphthoxazines : molar absorption coefficients and quantum efficiencies

A method is presented, which allows, with a minimum of assumptions, the determination of the molar absorption coefficients of the coloured form of photochromic compounds and their forward and reverse quantum efficiencies. The approach used combines a modified Fischers method with an improved intensity variation technique. The efficacy and accuracy of the proposed method are demonstrated by measuring the photochromic parameters of several naphthoxazinespiroindolines. The forward quantum efficiencies were found to vary from 0.2 to 0.9 and the ratios of reverse to forward quantum efficiencies were shown to be uniformly low, in the range 0–0.07. The molar absorption coefficients of the coloured form at the peak maximum were in the range (3.1–5.1)× 104 dm3 mol–1 cm–1.

Diffuse reflectance flash photolysis

Examples are given demonstrating recent progress which allows flash photolysis investigations of opaque materials by using diffuse reflectance from analysing sources for transient detection. Experimental details of a nanosecond diffuse reflectance laser flash photolysis system are presented. The limitations associated with the use of integrating spheres for collecting the analysing light are discussed. The decay of transient absorption and time-resolved spectra are reported for triplet states in microcrystals (benzil), adsorbed as a fraction of a monolayer on powdered silica (acridine) and dyed on cotton fabric (aluminium sulphonated phthalocyanine). Photoinduced changes in diffuse reflectance of Co-doped ZnO are also reported and tentatively assigned. The relationship between the measured changes in diffuse reflectance and the distribution of transients below the irradiated surface is discussed and equations which can be applied in two limiting cases are given. Finally, the first observation of a transient absorption on the picosecond timescale from an opaque sample is presented. The potential of the technique for studying the mechanism of heterogeneous photoreactions is stressed.

The efficiency of singlet oxygen generation by substituted naphthalenes in benzene. Evidence for the participation of charge-transfer interactions

Abstract An inverse correlation between the rate constant for oxygen quenching of the triplet state ( k T q ) and the efficiency of singlet oxygen generation ( S Δ ) is established for a range of substituted naphthalenes in benzene. The participation of charge-transfer interactions are implicated on the basis that the only property which varies significantly with the substituent is the oxidation potential ( E ox M ) of the naphthalene derivative. Using published electrochemical data, correlations between S Δ , k T q and the free energy change for charge transfer (Δ G CT ) have been found with values of S Δ and k T q ranging from 0.34 to 0.75 and from 5.0 × 10 9 to 1.2 × 10 9 M −1 s −1 for 1-methoxynaphthalene and 1-cyanonaphthalene, respectively.