Solvent effects on triplet state quenching by tetramethylpiperidcme-N-oxide

Measurements of the triplet state quenching of a series of aromatic hydrocarbons by tetramethylpiperidine-N-oxide in two solvents, acetonitrile and perfluoro-n-hexane, are reported. In both solvents the previously established trend of quenching rate constants with the triplet energy of the aromatic hydrocarbon being quenched was observed. No correlation of quenching efficiency with the charge transfer properties of the aromatic hydrocarbon—free radical collision complex was found.     

Effect of a magnetic field on the kinetics of the annihilation of triplet exciplexes

Conclusions The application of an external magnetic field leads to a lower rate of annihilation of triplet exciplexes in media with sufficiently high dielectric constant, in which the dissociation of the exciplexes to radical-ions is observed. The magnetic effect is explained in the framework of a hyperfine interaction mechanism for singlet-triplet transitions in radical pairs.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2611–2614, November, 1987.     

A theoretical study on the quenching mechanisms of triplet state riboflavin by tryptophan and tyrosine

The reactions of tryptophan (Trp) and tyrosine (Tyr) with endogenous photosensitizer riboflavin (RF) have gained much interest for their crucial roles in various photobiological processes. In this paper, the quenching mechanisms of triplet state RF by Trp and Tyr have been explored employing density functional theory calculations. It is revealed that the H-atom transfer reaction from Trp and Tyr to triplet state RF is more favorable on thermodynamic grounds compared with direct energy transfer or direct electron transfer pathways. During the photosensitization, RF can photogenerate various reactive oxygen species (ROS) as intermediates, while the present study provides some deeper insights into the photosensitizing behaviors of triplet state RF by reacting directly with Trp and Tyr.     

Temperature effects on the quenching of the triplet state 3Au of biacetyl

Rate data for the quenching of the triplet state ³A_u of biacetyl were determined for a number of collision partners in the gas phase as a function of temperature. Quenching by oxygen and nitric oxide resulted in negative temperature coefficients and the results were interpreted on the basis of the formation of collision complexes. The Arrhenius parameters determined for the reaction of triplet biacetyl molecules with acetaldehyde are an indication that the n,π* configuration of biacetyl behaves like a biradical in the gas phase. The results are consistent with a hydrogen atom abstraction reaction channel.     

Energy transfer in the course of triplet state quenching of aromatic hydrocarbons by nitroxyl radicals

Quenching of triplet states of aromatic hydrocarbons by nitroxyl radicals has been investigated by the flash photolysis method. There are two different mechanisms of triplet quenching: quenching occurs via enhanced intersystem crossing on exchange interaction with the radical for the triplet states of aromatic hydrocarbons which have low triplet energy (E_T < 14700 cm^?1); for very high triplet energies, energy transfer from the triplet molecule to the nitroxyl radical occurs. The energy of the excited nitroxyl radical was estimated to be 18000 cm^?1.     

Spin-selective depopulation of triplet sublevels in rapidly rotating triplet exciplexes detected by a heavy-atom-induced magnetic field effect

A mechanism is presented explaining a reported heavy-atom-induced magnetic field effect as a consequence of non-equilibrium triplet sublevel population in an intermediate exciplex. The triplet exciplex spin polarization is induced by sub-level-selective intersystem crossing from the exciplex triplet to its singlet ground state and is decreased by an external magnetic field. The theory accounts almost quantitatively for the observed influence of magnetic field strength and heavy-atom substituents.     

Formation and conversions of terplexes in quenching of triplet state of p-chloranil by durene

Interaction of the triplet state of chloranil with durene in dioxane leads to the formation of short-lived polar complexes with a 11 composition (exciplex) and a 12 composition (terplex). One of the channels for destruction of the terplex is proton transfer between its counterions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1295–1299, June, 1990.     

Charge transfer complexing in the course of triplet state quenching of carbocyanine dyes by nitroxyl radical

Quenching of triplet states of carbocyanine dyes by nitroxyl radical has been investigated by the flash photolysis method. Quenching of triplet state carbocyanine dyes with one polymethyne chain occurs via enhanced intersystem crossing on exchange interaction with the radical. Quenching of triplet state carbocyanine dyes with two polymethyne chains occurs via partial charge transfer in the collision complex with the radical. In the second case, an increase in the dielectric constant of the solvent leads to an increase of the rate of quenching. In high polarity solvents (propanol, methanol) complete electron transfer from dye triplet state to radical occurs. Kinetic and spectral characteristics of a new dye radical (Dye.⁺) are reported.     

Contributions of spin-spin interactions to the magnetic field dependence of the triplet quantum yield in photosynthetic reaction centers

Calculations are presented which show that dipolar coupling in the primary radical ion pair of quinone-depleted photosynthetic reaction centers substanually affects the magnetic field dependence of the triplet quantum yield, as does exchange coupling to the semiquinone-Fe(II) complex, when quinone is present. Inclusion of these interactions resolves significant discrepancies between theory and experiment.     

A flash photolysis study of the triplet state quenching of anthracene by oxygen in the presence of polystyrene

The rate constant for the quenching of triplet anthracene by molecular oxygen in the presence or absence of polystyrene was measured. The quenching ability is not affected by the presence of polymer.     

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Effect of a magnetic field on the kinetics of the geminal recombination of triplet radical pairs adsorbed on glass with small pores

Conclusions A kinetic study was carried out for the geminal recombination of triplet radical pairs adsorbed in glass with small pores. The application of an external magnetic field is accompanied by a significant decrease in the fraction of radicals, which recombine in the cage. The magnetic effects in microporous glass are comparable in magnitude with the analogous phenomena in micelles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1202–1205, May, 1989.     

Solvent and substitution effects upon the phosphorescent triplet state of carbonyl pyridines

The effects of perturbing nitrogen upon localized triplet excitations are observed for several carbonyl pyridines. Conventional and pulse optically detected magnetic resonance (ODMR) techniques are used to characterize the spin-relaxation processes in these aromatic carbonyl molecules.