79802-46-3Relevant academic research and scientific papers
Interactions of NO2- and SO32- with Organic Triplets. Charge Transfer versus Energy Transfer: The Role of Reorganization Energy in Triplet-Anion Interactions and Spectroscopic Methods for Its Evaluation
Loeff, I.,Treinin, A.,Linschitz, H.
, p. 5264 - 5272 (1992)
Charge-transfer (CT) and energy-transfer (NT) interactions of simple anions with organic triplets are reviewed and discussed in connection with new quenching rate constant (kq) and radical yield measurements for SO32- and NO2-.In the latter case, both processes may occur at high organic triplet energies.Reorganization energies for one electron oxidations are obtained for several anions, using data on charge-transfer-to-solvent (CTTS) spectra and photoelectron emission thresholds, which, like the kinetic parameters of Marcus-Hush theory, also reflect Franck-Condon strains.These results, combined with thermodynamic free energies, give vertical redox potentials which correlate better than do equilbirium potentials with quenching rates.The theoretical basis for correlation between kq and HνCTTS is discussed in the framework of Marcus rate theory.Assigning the total reorganization energy in the CT quenching reaction to the small anion component of the D-A pair gives reasonable agreement with data on quenching of dye triplets but too slow rates for aryl carbonyl triplets where exciplex formation may possibly occur.The optical reorganization energy for NO2- leads to values of the thermal self-exchange rate agreeing with those computed from the Marcus-Hush cross-relations, which also neglect bonding effects.The mechanism of NO2- interaction with triplets is discussed in detail, including indirect kinetic evidence for quenching of a short-lived exciplex by NO2- without radical formation.The possibility of reduction by triplet NO2- formed by initial NT from the organic triplet is also considered.Finally, a scheme is presented involving an equilibrium between CT and NT states and relating the free energy difference between these states to radical yields.
Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution
Marciniak,Hug,Bobrowski,Kozubek
, p. 13560 - 13568 (2007/10/02)
The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) × 109 M-1 S-1 for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S.·.S)-bonded radical cations, and intramolecularly (S.·.N)-bonded radical cations derived from peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO2 formation were measured by steady-state photolysis. A detailed mechanism of the CB-sensitized photooxidation of methionine-containing peptides was discussed and compared to that for methionine.
