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.     

Intra- and intermolecular quenching of carbazole photoluminescence by imidazolidine radicals

Mechanisms of carbazole photoluminescence quenching by the free and chemically bound nitroxyl radicals in the model bound system “carbazole (CBZ)—imidazolidine nitroxyl radical R^•” were investigated and the photophysical properties of the system were studied and compared with those of free CBZ and R^• in solution. The quantum yield and lifetime of fluorescence from the local singlet excited state of the carbazole moiety in the bound CBZ—R^• system is three orders of magnitude lower than in free CBZ. The lifetime of the local triplet excited state of the carbazole moiety in the bound system is shorter than 50 ns. The rate constants for intermolecular quenching of the singlet and triplet excited states of free CBZ by R^• in acetonitrile were found to be (1.4±0.1)·10¹⁰ and (1.5±0.2)·10⁹ L mol⁻¹ s⁻¹, respectively. The most plausible mechanisms of both free and covalently bound carbazole luminescence quenching by nitroxyl radicals are exchange energy transfer and acceleration of internal conversion due to electron exchange.     

Quenching dynamics study on photoinduced excited triplet duroquinone by TEMPO in 1,2-propandiol

Chemically induced dynamic electron polarization (CIDEP) spectrum and transient absorptive spectrum are recorded in photolysis of duroquinone (DQ) in 1,2-propanodiol (PG). Durosemiquinone neutral radical DQH^• and PG ketyl radical \textCH₃[(\textC)\dot]\textOHCH₂\textOH{\text{CH}}_{3}{{{\dot{\text{C}}}\text{OHCH}}}_{2}{\text{OH}} are produced through hydrogen transfer reaction from PG to ³DQ*. When stable radical TEMPO is added to DQ/PG solution, photolysis results in CIDEP on TEMPO, which can be interpreted as a quartet precursor radical-triplet pair mechanism (QP-RTPM). There is competition between PG and TEMPO to quench ³DQ*. The CIDEP intensity of DQH^• decreases with the increase of TEMPO concentration. The quenching dynamics in photolysis of DQ/TEMPO/PG system is analyzed in detail. Based on the dynamics analysis and the measurement of the lifetime of ³DQ* by its transient absorbance decay, the quenching rate constant of ³DQ* by TEMPO in PG is obtained as 1.34 × 10⁷ L mol⁻¹ s⁻¹. This quenching rate constant is closely diffusion-controlled.     

Chemically induced dynamic electron polarization investigation of the triplet-radical system in the solution of the triplet quencher

The chemically induced dynamic electron polarization (CIDEP) of the triplet molecule/triplet quencher/2,2,6,6-te-tramethyl-1-piperidinyloxyl (TEMPO) systems were measured using the high time-resolved ESR spectrometer.The competition between the radical-triplet pair mechanism (RTPM) and triplet mechanism (TM) or radical pair mechanism (RPM) polarization in the solution of the triplet quencher was investigated,and the relationship between reaction rate of the radical-triplet pair and quenching rate of triplet was deduced.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

Photoluminescence of Uranium(VI): Quenching Mechanism and Role of Uranium(V)

The photoluminescence of uranium(VI) is observed typically in the wavelength range 400–650 nm with the lifetime of several hundreds μs and is known to be quenched in the presence of various halide ions (case A) or alcohols (case B). Here, we show by density functional theory (DFT) calculations that the quenching involves an intermediate triplet excited state that exhibits uranium(V) character. The DFT results are consistent with previous experimental findings suggesting the presence of photoexcited uranium(V) radical pair during the quenching process. In the ground state of uranyl(VI) halides, the ligand contributions to the highest occupied molecular orbitals increase with the atomic number (Z) of halide ion allowing larger ligand‐to‐metal charge transfer (LMCT) between uranium and the halide ion. Consequently, a larger quenching effect is expected as Z increases. The quenching mechanism is essentially the same in cases A and B, and is driven by an electron transfer from the quencher to the UO₂²⁺ entity. The relative energetic stabilities of the triplet excited state define the “fate” of uranium, so that in case A uranium(V) is oxidized back to uranium(VI), while in case B uranium remains as pentavalent.     

Mutual Effect of Functional Groups and the Radical Center on the Reactivity of Nitroxyl Radicals

This review considers the correlation between the reactivity of nitroxyl radicals (piperidine, pyrroline, pyrrolidine, imidazoline, dihydroquinoline, tetrahydroquinoline, diphenyl nitroxide, etc.) and their chemical structure in terms of the rate constants of reactions between these radicals and hydrazobenzene. 4,4′-Di(tert-butyl)diphenyl nitroxyl has the highest reactivity, and the nitroxyl radical of benzoindolopyrrolidine is the least reactive (the difference is a factor of ∼10⁴). The effects of the metal atom in stable organometallic nitroxyl radicals and of the halogen atom in halogenated nitroxyl radicals on the reactivity of the nitroxyl center are considered. Data on the effect of the nitroxyl center on the reactivity of functional groups in the piperidine nitroxyl radical are generalized. Nitroxyl radicals with an activated double bond are shown by quantum chemical calculations to form cyclic transition complexes with amines, involving both the paramagnetic center and a double bond. This explains why the activated double bond in nitroxyl radicals is more reactive in nucleophilic additions of amines than the same bond in their diamagnetic analogues. The rate constants of nitroxyl reduction with hydrazobenzene and of nitroxyl oxidation with tetranitromethane are related to the σ_ESR constant derived from isotropic hyperfine coupling constants HFC_(aN), and their correlation with Hammett constants is demonstrated. The role of solvents in the reduction and oxidation of the nitroxyl radicals is considered. The influence of hydroxyl radical-polar solvent complexes and hydroxylamine-polar solvent H complexes on the course of reactions is considered for hydrogen atom transfer in systems of a sterically hindered nitroxyl radical and hydroxylamine.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 506–528.Original Russian Text Copyright © 2005 by Malievskii, Shapiro.     

Kinetics of fast reactions of triplet states and radicals under photolysis of 4,4′-dimethylbenzophenone in the presence of 4-halophenols in micellar solutions of sodium dodecyl sulfate in magnetic field

Quenching kinetics of the 4,4′-dimethylbenzophenone triplet state with para-substituted phenol derivatives RC₆H₄OH (R = H, F, Cl, Br, I) was studied by nanosecond laser photolysis in aqueous micellar solutions of sodium dodecyl sulfate. The kinetic data were processed in the framework of a model with the Poisson distribution of phenols between micelles. The partition constants of RC₆H₄OH between the aqueous and micellar phases and the rate constants of their escape from a micelle and quenching of the 4,4′-dimethylbenzophenone triplet state with phenols in micelles were obtained. The quenching proceeds with high rate constants through hydrogen atom transfer to form the ketyl and phenoxyl radicals (no radicals are formed in the case of 4-iodophenol), which then recombine in a micelle or escape into the outer aqueous volume. The application of an external magnetic field retards radical pair recombination in a micelle and increases the fraction of radicals escaped into the aqueous phase. The quantum yield of radical pairs decreases 2.5-fold, and the rate of their recombination in micelles increases 2.5-fold on going from 4-chloro- to 4-bromophenol. This is caused by the acceleration of triplet radical pair recombination in the solvent cage. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1391–1396, June, 2005.     

Photochemical stability of nitroxyl derivatives

Different classes of compounds with imidazoline radicals were studied by EPR spectroscopy. The effects of light and atmospheric oxygen on the stability of these compounds in alcoholic solutions were investigated. The study of the photochemical stability of rhodium complexes with imidazoline radicals in oxygen-containing and oxygen-free media demonstrated that the photolysis of these compounds in the absence of oxygen causes the disappearance of paramagnetism. The reaction is reversible, and the observed effects are due to the formation of hydroxylamine groups via the interaction between excited nitroxyl radicals and the solvent in the absence of oxygen. When present in this system, oxygen deexcites the nitroxyl groups. A similar effect of oxygen is observed for nitroxyl derivatives of the fullerenes C₆₀ and C₇₀. A quite different photolytic behavior is shown by copper complexes with bidentately bonded nitroxyl radicals. These compounds are stable to photolysis in both oxygen-containing and oxygen-free media. It was demonstrated using phenyl-tert-butylnitrone (PBN) as the spin trap that photolysis in the absence of the trap results in the decomposition of the copper complex to copper metal. It is assumed that PBN incorporates into the complex at free coordination sites and competes with the copper ion in its reaction with the earlier formed radical of the ligand.     

QUENCHING OF THE EXCITED STATES OF 2-PHENYLBENZOXAZOLE BY AZIDE ANION. FLUORESCENCE AND ESR STUDY

Electron spin resonance, spin-trapping and fluorescence techniques demonstrate that 2-phenylbenzoxazole (P) participates in photo-induced reactions with alcohols and electron donors like the azide ion. Irradiation of Pat 300 nm in deaerated ethanol produces ethoxyl and hydroxyethyl radicals which can be detected with the spin trap, 5,5-dimethyl-l-pyrroline-l-oxide (DMPO). However, irradiation of P in the presence of N^-₃ leads to the appearance of the azide radical, N^-₃, which also reacts with DMPO. Studies with the nitroxyl radical, 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO), suggest that electron transfer from the azide anion to an excited state of P yields the semi-reduced sensitizer, P^-, which in turn reacts with TEMPO. The effect of sodium azide upon the fluorescence intensity and lifetime of P in aqueous ethanol has also been studied.     

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.     

Simultaneous evaluation of one‐electron reducing systems and radical reactions in cells by nitroxyl biradical as probe

In the present study, a novel probe for the simultaneous evaluation of one‐electron reducing systems (electron transport chain) and one‐electron oxidizing systems (free radical reactions) in cells by electron chemical detection was developed. Six‐membered cyclic nitroxyl radicals (2,2,6,6‐tetramethylpiperidine‐1‐oxyl; TEMPO series) are sensitive to one‐electron redox systems, generating the hydroxylamine form [TEMPO(H)] via one‐electron reduction, and the secondary amine form [TEMPO(N)] via one‐electron oxidation in the presence of thiols. In contrast, the sensitivities of five‐membered cyclic nitroxyl radicals (2,2,5,5‐tetramethylpyrrolidine‐1‐oxyl; PROXYL series) to the one‐electron redox systems are comparatively low. The electron chemical detector can detect 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), TEMPO(H) and PROXYL but not TEMPO(N). Therefore, nitroxyl biradical, TEMPO‐PROXYL, as a probe for the evaluation of one‐electron redox systems was employed. TEMPO‐PROXYL was synthesized by the conjunction of 4‐amino‐TEMPO with 3‐carboxyl‐PROXYL via the conventional dicyclohexyl carbodiimide reaction. TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were simultaneously quantified by HPLC with Coularray detection. Calibration curves for the quantification of TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were linear in the range from 80 nm to 80 μm , and the lowest quantification limit of each molecule was estimated to be <80 nm . The relative standard deviations at 0.8 and 80 μm were within 10% (n = 5). Copyright © 2015 John Wiley & Sons, Ltd.     

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τ_T=0.1 μs), these new adducts show a longer-lived triplet excited state (τ_T=9.5 μs). Based on the singlet oxygen quantum yield (Φ_Δ=7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D₁↔Q₁) at ∼80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J>0, 0.05∼0.10 eV).     

Antioxidative properties of nitroxyl radicals and hydroxyamines in reactions with triplet and deaminated kynurenine

The reactions of triplet kynurenine and 4-(2-aminophenyl)-4-oxocrotonic acid, formed upon the thermal decomposition of kynurenine, with nitroxyl radicals and cyclic N-hydroxylamines were studied. Nitroxyl radicals were found to quench efficiently the triplet state of kynurenine (rate constant 3–6·.10⁸ L mol^-1 s^-1). The quenching proceeds via the spin-exchange mechanism and affords no new products. Neither nitroxyl radicals, nor hydroxyl-lamines react with 4-(2-aminophenyl)-4-oxocrotonic acid under conditions similar to physiological.     

Alkyl radical adducts of aromatic N-oxides as hydrogen-abstracting agents: The reactivity of phenazine-N,N′-dioxide-methyl radical adduct

An O-methylated analog of protonated phenazine-di-N-oxide radical anion abstracts hydrogen from primary and secondary alcohols in a slow (k ₁ < 500 M⁻¹ s⁻¹) bimolecular reaction. No kinetic evidence has been found for the unimolecular release of free methoxyl radicals through the homolytic N-OMe bond cleavage in these species. DFT calculations at the UB3LYP 6-31G(d) level indicate that protonated and O-alkylated radical anions of pyrazine, quinoxaline and phenazine di-N-oxides are close analogues of aromatic nitroxyl radicals with the highest spin density localized on the oxygen and nitrogen of the nitrone moiety.     

Metallophthalocyanines photosensitize the breakdown of (hydro)peroxides in solution to yield hydroxyl or alkoxyl and peroxyl free radicals via different interaction pathways

Interactions of organic peroxides (R'OOR) and hydroperoxides (R'OOH), including H2O2, with excited triplet and singlet state metallophthalocyanines (MPc, M = Zn, Al) have been studied by T-T absorption decay and fluorescence quenching. The ensuing photochemical processes result in decomposition of (hydro)peroxides as assessed by photo-EPR (electron paramagnetic resonance) and spin trapping. In argon-saturated apolar solutions and low MPc concentrations, alkoxyl free radicals (*OR) were identified as the primary products of (hydro)peroxide breakdown. Similarly, photosensitized decomposition of symmetric disulfides results in the formation of sulfur-centered radicals. In air-free aqueous solutions, ROOH photosensitization always gave rise to a mixture of hydroxyl and peroxyl radical (*OOR) adducts in varying molar ratios. At high MPc concentrations, both in polar and in apolar solutions, the most abundant products of ROOH decomposition were identified as *OOR. This indicates a change in the predominant interaction pathway, most likely mediated by MPc exciplexes and involving H-atom abstraction from ROOH by MPc-cation radicals. The prevalence of MPc singlet vs. triplet state interactions was confirmed by the much higher singlet quenching rate constants (log kq up to 9.5; vs. log kT < or = 4.5). In contrast to the triplet quenching, singlet quenching rates were found to depend on the (hydro)peroxide structure, following closely the trend of varying *OR yields for different substrates. Thermodynamic calculations were performed to correlate experimental results with models for electronic energy and charge transfer processes in agreement with the Marcus theory (Rhem and Weller approximation) and Savéant's model for a concerted dissociative electron transfer mechanism.     

Photoinduced bond homolysis of B12 coenzymes. An FT-EPR study

A Fourier Transform Electron Paramagnetic Resonance (FT-EPR) study was made of free radicals produced by photoinduced homolytic cleavage of the Co—C bond in methyl- and 5′-adenosylcobalamine (B₁₂ coenzymes) and R(4-t-butyl-pyridyl)cobaloximes, R = methyl or ethyl. Spectra of methyl and adenosyl free radicals generated by the cobalamines show Chemically Induced Dynamic Electron Polarization (CIDEP) produced in precursor radical pairs. The polarization pattern can be accounted for in terms of bond cleavage via a singlet excited state of the cobalamines. In the case of methylcobalamine the polarization pattern is wavelength dependent confirming earlier findings that bond cleavage occurs via two reaction channels. Spectra of the methyl and ethyl radicals given by the cobaloximes show a remarkably strong dependence on solvent and the identity of the axial ligand trans to the leaving alkyl group. This illustrates that the character of the excited state involved in the bond cleavage reaction is strongly dependent on axial ligation of the cobalt ion.     

Chemically induced dynamic electron polarization generated through the interaction between singlet molecular oxygen and nitroxide radicals

An absorptive chemically induced dynamic electron polarization (CIDEP) was generated by the quenching of singlet oxygen by nitroxide radicals (TEMPO derivatives). The spin polarization decay time of the nitroxide (measured by time-resolved EPR) correlates with the lifetime of singlet oxygen (measured by singlet oxygen phosphorescence spectroscopy). In addition, a deuterium isotope effect on the spin polarization decay time was observed, a signature of singlet oxygen involvement. With use of isotope labeled nitroxides (15N, 14N), the relative spin polarization efficiencies of TEMPO, 4-oxo-TEMPO, and 4-hydroxy-TEMPO by singlet oxygen were determined. The relative spin polarization efficiencies (per quenching event) decrease in the order 4-hydroxy-TEMPO > TEMPO > 4-oxo-TEMPO, whereas an opposite trend was observed for the total quenching rate constants of singlet oxygen by the nitroxides where the order is 4-hydroxy-TEMPO < TEMPO < 4-oxo-TEMPO.     

RESEARCH NOTE ON THE CHARGE-TRANSFER PATHWAY IN THE MEROCYANINE 540 TRIPLET STATE QUENCHING BY NITROXYL RADICAL

Abstract— Quenching of merocyanine 540 triplet state by nitroxyl radical has been investigated by flash photolysis. An increase of solvent polarity leads to an increase of the quenching rate. This polarity effect shows that merocyanine 540 triplet state is able to react with nitroxyl radical via a charge-transfer process. The data presented in this work are shown to be consistent with previous results obtained in micelles and liposomes.     

表面活性剂TX-100与SDBS胶束中光解苯甲醛自由基的化学诱导动态电子极化

利用时间分辨ESR波谱仪，研究了苯甲醛在乙二醇和表面活性剂SDBS，TX-100 的胶束溶液中的激光光解化学诱导动态电子极化（CIDEP）现象。苯甲醛在激光照 射下可以从体系和自身中得到氢生成α-羟基苄自由基和苯酰自由基，在SDBS胶束 中是自由基对机理RPM极化，而在TX-100胶束中是三重态机理TM极化。计算机模拟 谱图进一步证实了自由基的产生和极化机理。