Gordon L. Hug

Triplet--Triplet Absorption Spectra of Organic Molecules in Condensed Phases

We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendati...

Rate Constants for the Quenching of Excited States of Metal Complexes in Fluid Solution

The rate constants for the quenching of the excited states of metal ions and complexes in homogeneous fluid solution are reported in this compilation. Values of kq for dynamic, collisional processes between excited species and quenchers have been critically evaluated, and are presented with the following information, among others, from the original publications, when available: description of the solution medium, temperature at which kq was determined, experimental method, range of quencher concentration used, lifetime of the excited state in the absence of quencher, activation parameters, quenching mechanism. Data collection is complete through the end of 1986, and covers the coordination compounds of 26 metals, including the ions and complexes of the inner‐ and outer‐transition metals, and porphyrin complexes of nontransition metals. Data for 261 excited states quenched by more than 400 inorganic quenchers and 600 organic quenchers have been extracted from almost 500 publications. The introduction to th...

Extinction Coefficients of Triplet–Triplet Absorption Spectra of Organic Molecules in Condensed Phases: A Least‐Squares Analysis

A global least‐squares technique is developed to assist in the critical evaluation of data consisting of large sets of measurements. The technique is particularly designed to handle sets of data where many of the measurements are relative measurements. A linearization procedure is used to reduce the inherently nonlinear problem to a traditional multivariate linear regression. The technique developed here is used to evaluate extinction coefficients, e’s, of triplet–triplet absorption (TTA) spectra of organic molecules in condensed phases. A previous assumption, that there are no solvent effects on the TTA spectra, is investigated and modified so that a group of compounds measured in benzene is treated separately. The set of 445 e’s obtained from the global least‐squares fit, including these solvent effects, is presented in the accompanying tables. How these least‐squares results can be used in a hierarchy of TTA e standards is discussed. Further solvent effects such as the separation of polar and nonpolar ...

Multiple time scales in pulse radiolysis. Application to bromide solutions and dipeptides

Abstract A description is given of how multiple time scales are incorporated into a new computer program for running pulse-radiolysis experiments on a linear accelerator. For any kinetic trace, the data can be presented on ten distinct time scales. The algorithms for doing this are presented. The utility of this procedure is illustrated with the radiolysis of bromide solutions. The kinetic traces are discussed in terms of formation of a BrOH − adduct and its approach to equilibrium with Br − 2 . A second example, with the radiolysis of a dipeptide Met–Met, illustrates the ease of simultaneously determining the yield and decay behavior.

Neighboring Amide Participation in Thioether Oxidation: Relevance to Biological Oxidation

To investigate neighboring amide participation in thioether oxidation, which may be relevant to brain oxidative stress accompanying beta-amyloid peptide aggregation, conformationally constrained methylthionorbornyl derivatives with amido moieties were synthesized and characterized, including an X-ray crystallographic study of one of them. Electrochemical oxidation of these compounds, studied by cyclic voltammetry, revealed that their oxidation peak potentials were less positive for those compounds in which neighboring group participation was geometrically possible. Pulse radiolysis studies provided evidence for bond formation between the amide moiety and sulfur on one-electron oxidation in cases where the moieties are juxtaposed. Furthermore, molecular constraints in spiro analogues revealed that S-O bonds are formed on one-electron oxidation. DFT calculations suggest that isomeric sigma*(SO) radicals are formed in these systems.

Sensitized photo-oxidation of sulfur-containing amino acids and peptides in aqueous solution

Abstract Qualitative and quantitative studies of the photo-oxidation of sulfur-containing amino acids and methionine-containing dipeptides and tripeptides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investigated using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB triplet by twelve sulfur-containing amino acids and six methionine-containing peptides were determined to be in the range 10 8 –10 9 M −1 s −1 for both neutral and alkaline solutions. The amino acids varied in structure, having different numbers of COCH and NH 2 terminal groups and their sulfur atom at different locations relative to the terminal groups. The methionine-containing peptides were MetGly, GlyMet, MetMet MetGlyGly, GlyMetGly and GlyGlyMet. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet states of CB, ketyl radicals of CB, radical anions of CB and radical anions of CB and radical cations derived from the amino acids and peptides. The radical cations identified were intermolecularly (S.·.S) + -bonded cations, intramolecularly (S.·.N) + -bonded cations and an intramolecularly (S.·.S) + -bonded radical cation that was observed in experiments with MetMet. The quantum yields of the transients and their kinetics of formation and decay were measured by flash photolysis. The quantum yields of CO 2 formation were determined by steady state photolysis. Electron transfer from the sulfur atom to the triplet state of the ketone was found to be a primary photochemical step. A detailed mechanism of the CB-sensitized photo-oxidation of sulfur-containing amino acids and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, contrasting behavior between the peptides and amino acids as quenchers is emphasized.

A reinvestigation of the mechanism of photoreduction of benzophenones by alkyl sulfides

Abstract The photoreduction of benzophenone and 4-carboxybenzophenone by dimethyl sulfide was examined in aqueous, mixed water—acetonitrile (1:1v/v) and acetonitrile solutions by the use of nanosecond laser photolysis. Bimolecular quenching rate constants were determined and were found to be in the range (1.5–4.6)×109 M−1s−1. Electron transfer from the sulfur atom to the triplet state of the benzophenones was found to be a primary photochemical step. This was established by the large values of quenching rate constants and by the observation of free radical ions, i.e. ketyl radical anions and (S∴S)+ radical cations of dimethyl sulfide in aqueous and mixed water-acetonitrile solutions. The overall quantum yields of photoproducts (ketyl radicals and ketyl radical anions) are low (Φtotalketyl=0.26 in aqueous solutions, are in the range 0.16–0.20 in mixed water-acetonitrile solutions, and decrease to less than or equal to 0.01 in pure acetonitrile), suggesting that back electron transfer within the charge-transfer complex to regenerate the reactants is the dominant process.

Quenching of the excited singlet state of acridine and 10-methylacridinium cation by thio-organic compounds in aqueous solution

In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH + ) and 10-methylacridinium (AcrMe + ) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component ( KS )t o the quenching. The experimental KS values were compared to theoretical KS values for outer-sphere complexes based on Debye–Huckel theory and the Fuoss equation. The general agreement between experimental and theoretical KS values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the ZAZQ =− 1 and −2 cases being solvent-separated ion pairs while those of the ZAZQ =− 3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe + with the anionic forms of the quenchers (having charges ZQ =− 1, −2 and −3). The rate constants (extrapolated to ionic strength, µ = 0) for the quenching processes were determined to be 0.3–5.3 × 10 10 M −1 s −1 depending on the ionic charge (ZQ) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.

Redox reactions of the urate radical/urate couple with the superoxide radical anion, the tryptophan neutral radical and selected flavonoids in neutral aqueous solutions

The kinetics of several processes involving the potential antioxidant role of urate in physiological systems have been investigated by pulse radiolysis. While the monoanionic urate radical, ·UH-, can be produced directly by oxidation with ·Br-2 or ·OH, it can also be generated by oxidation with the neutral tryptophan radical, ·Trp, with a rate constant of 2 × 107 M-1s-1. This radical, ·UH-, reacts with ·O-2 with a rate constant of 8 × 108 M-1s-1. Also, ·UH- is reduced by flavonoids, quercetin and rutin in CTAB micelles at rate constants of 6 × 106 M-1s-1 and 1 × 106 M-1s-1, respectively. These results can be of value by providing reference data useful in further investigation of the antioxidant character of urate in more complex biological systems.

Photooxidation of Methionine Derivatives by the 4-Carboxybenzophenone Triplet State in Aqueous Solution. Intracomplex Proton Transfer Involving the Amino Group

Abstract— Oxidation of the triplet state of 4‐carboxybenzophenone (CB) by a series of five substituted methionines and three methionine‐containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. Branching ratios were evaluated for the decay of the charge‐transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton‐transfer tendencies of the nominal sulfur‐radical‐cationic species. A systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton‐transfer yields. The enhanced propensity of the sulfur radical cations to depro‐tonate is due to deprotonation at the carbons adjacent to the sulfur‐cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron‐withdrawing abilities of the substituents, making the radical‐cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur‐containing substrates in pulse radiolysis experiments.