Brian Stevens

The photoperoxidation of unsaturated organic molecules: VIII. The effect of biacetyl

Abstract The increased rate of diphenylanthracene consumption in the presence of biacetyl in air-saturated benzene far exceeds that computed on the basis of a catalyzed intersystem-crossing process, and appears to lead to the production of a 9,10-dihydroanthracene derivative which is not the endoperoxide normally formed. The effect is interpreted in terms of a biacetyl-oxygen complex or biacetyl oxciplex intermediate formed by oxygen quenching of the biacetyl triplet state and which has an estimated lifetime of at least 6 μsec.

Electron-Transfer Sensitized Cycloreversion of Rubrene Endoperoxide

Abstract Rubrene endoperoxide (RO2) quenches the fluorescence of 9,10-dicyanoanthracene (DCNA) in benzonitrile and undergoes simultaneous cycloreversion with a limiting quantum yield of 0.07 ± 0.01 which increases by ∼50% in the presence of an external magnetic field of 600 gauss. No quenching or cycloreversion is observed with either anthracene as sensitizer or benzene as solvent. Chloranil (triplet) sensitized cycloreversion of RO2 has a limiting quantum yield of 0.62 ± 0.03. It is proposed that cycloreversion involves the state 3RO2(πoo*, [sgrave]co*) produced by geminate recombination of the triplet radical-ion pair, formed directly from the chloranil triplet state or by spin-rephasing of the singlet ion pair produced by RO2 quenching of the DCNA fluorescent state.

An examination of diffusion-influenced fluorescence quenching by nearest quenching neighbors in liquids

Abstract The exclusive quenching of a potentially fluorescent species in solution by its nearest quenching neighbor imposes a finite upper integration limit to the flux equation and provides a modified version, i.e. of the Smoluchowski-Collins-Kimball formulation of the Stern-Volmer equation. This is applied to data reported for the fluorescence quenching of two anthraquinone derivatives byN,N-dimethyl-p-toluidine in acetonitrile solutions of different (pressure-induced) viscosities. Values of the relative fluorescence quantum yields I0/I(c) calculated from the above equation, using recovered values of the encounter diameter ϱ, the intrinsic quenching rate constant k0 and the viscosity-dependent diffusion coefficient D, agree with experimental data with an overall average deviation (2.7% for 53 data points) well within a reported experimental uncertainty of ±8%.

The photoperoxidation of unsaturated organic molecules XXV: Modification of the cage displacement model for mixed solvent quenching of O2(1Δg)

Abstract The lifetime τΔ of O2(1Δg) has been measured directly in binary mixtures of methylene chloride with various alkanes and alcohols as cosolvents. The dependence of τΔ on solvent composition is not satisfactorily reproduced by the solvent cage displacement model but is better described by equations developed from a modification of this model in which the solvent cage of the small oxygen molecule is treated as a collection of atomic groups (e.g. CH3, CH2, OH) rather than complete molecules. This provides some kinetic insight into the group additivity parameter for O2(1Δg) quenching by pure solvents.

OXYGEN QUENCHING OF ELECTRONICALLY EXCITED SCINTILLATORS IN SOLUTION

Insofar as the photosensitised addition of molecular oxygen to a dissolved unsaturated hydrocarbon M involves an O21Δg intermediate, this reaction provides a convenient O21Δg counter. An analysis of the dependence of O21Δg formation efficiency on both the concentration of dissolved oxygen and the inhibitor azulene is consistent with Figure options Download full-size image Download as PowerPoint slide as the processes solely responsible for oxygen quenching of excited singlet and triplet states, S1 and T1, of the sensitiser. Oxygen quenching of the triplet state T1, appears to be limited by the spin statistical factor of 1/9 attending this process.