Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru^II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru^II complexes containing 2 or 4 [Ru(bpy)₂] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy)₂(L)]²⁺ where L is N,N′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy)₃]²⁺, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states ³MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy)₃]²⁺ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants k_q for quenching by O₂ of the ³MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10⁹ dm³ mol⁻¹ s⁻¹. The efficiencies f of singlet‐oxygen formation during oxygen quenching of these ³MLCT states are relatively high, namely 0.53 – 0.89. The product of k_q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k_q−k equals k, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG^CT, the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.     

DETERMINATION OF THE ACRYLAMIDE QUENCHING CONSTANT FOR PROTEIN AND MODEL INDOLE TRIPLETS

Abstract— The decay of the indole triplet of single tryptophan-containing proteins and model compounds can be readily measured at room temperature in aqueous solution by monitoring the triplet-triplet absorption or phosphorescence emission following a 265 nm exciting laser pulse. The quenching action of acrylamide on the triplet excited state of indole side chains was studied in an analogous fashion to that previously done at the singlet level (Eftink and Ghiron, 1977). The acrylamide triplet quenching constant (^tk_q) ranged from a high of 7.8 times 10⁸M^-1 s^-1 for the exterior indole of corticotropin (ACTH) to a low of 2 times 10⁵ Af^-1 s^-1 for the interior indole of ribonuclease T, (RNase T,). The ratio (7) of these values with their respective acrylamide singlet quenching constants (^tk_q),(γ=^tk_q⁸K_q) ranged from a high of 0.22 for ACTH to a low of 0.001 for RNase T₁,. Acrylamide is also an inefficient quencher of model indoles in various solvents (i.e. it has a γ less than 1). The magnitude of γ varied from a high of 0.3 in H₂0 to a low of 0.02 in acetonitrile, but did not correlate with viscosity, dielectric constant or polarity. The lower efficiency observed for internal indole groups can not be explained by that class of models which predict the presence of static quenching at the triplet level, since none was observed. The present results confirm the observation of Calhoun et al. of a large discrepancy between acrylamide's singlet and triplet quenching constants for buried indole side chains, but suggest that it may be largely explained by the fact that acrylamide is an inefficient quencher of the indole triplet state (1983). The magnitude of this inefficiency is probably determined by specific microenvironmental factors. Thus, unlike ⁸K_q, the environmentally sensitive ^lk_H cannot be easily used to characterize the dynamics of proteins.     

Chlorothioxanthone-Onium salts: Efficient photoinitiators of cationic polymerization

The interaction of iodonium, sulphonium, phosphonium, arsonium, selenonium, and heterocyclic nitrogen salts with the excited triplet state of 2-chlorothioxanthone (CTX) has been studied through time-resolved laser spectroscopy. Bimolecular quenching rate constants (k_e) have been measured for the first time. Measuring of the rate constants (k_q) of the quenching reaction by monomers permits the calculation of charge transfer (ø_CT) quantum yields. © 1992 John Wiley & Sons, Inc.     

Electron Transfer Fluorescence Quenching of Blepharisma japonicum Photoreceptor Pigments

Abstract— The hypericin analogs blepharismin (BP), oxyblepharismin (OxyBP) and stentorin (ST), the photosensing chromophores responsible for photomotile reactions in the ciliates Blepharisma japonicum (red and blue cells) and Stentor coeruleus, represent a new class of photoreceptor pigments whose chemical structures have recently been determined. In the case of ST it has been shown that the first excited singlet state can be deactivated by donation of an electron to an appropriate acceptor molecule (e.g. a quinone molecule). This charge transfer can be considered a possible mechanism for the primary photoprocess for the photomotile responses in S. coeruleus. To determine whether an electron transfer process also occurs in the deactivation of excited blepharismin, we studied the fluorescence quenching of OxyBP in dimethyl-sulfoxide (DMSO) and in ethanol using electron acceptors with different reduction potentials. Under our experimental conditions ground state and excited state complexes (like fluorescent exciplexes) are not formed between the fluorophore and the quenchers. In DMSO the bimolecular quenching constant values (k_q) calculated on the basis of the best fitting procedures clearly show that the quenching efficiency decreases with the quencher negative reduction potential, E⁰. The k_q (M^-1 s^-1) and E⁰ (V) values are, respectively, 7.8 times 10⁹ and -0.134 for 1,4-benzoquinone, 8.9 times 10⁹ and -0.309 for 1,4-naphthoquinone, 2.4 times 10⁹ and -0.8 for nitrobenzene, 0.009 times 10⁹ and -1.022 for azobenzene and 0 and -1.448 for benzophenone. These findings point to the conclusion that upon formation of the encounter complex between OxyBP and the quencher, an electron is released from excited OxyBP to the quencher, similar to what happens in ST. It is suggested that in the pigment granules such a light-induced charge transfer from excited blepharismin to a suitable electron acceptor triggers sensory transduction processes in B. japonicum.     

Factors affecting the complexation of polyacrylic acid with uranyl ions in aqueous solutions: A luminescence study

A study was carried out in aqueous solutions using luminescence technique to investigate the effects of pH, salt concentration, and temperature on the polyacrylic acid/uranyl ion (PAA/UO) complex formation as well as competitive phenomena of enhancement and quenching effects on photoexcited state of uranyl ions. It was found that excess of H⁺ and OH^? is not favorable for complexation between uranyl ions and polymer. Added nitrate salts of Na⁺ and K⁺ had significant enhancement effect on emission spectra of PAA/UO complex. These results indicated that the metal ion/polymer chain complex collapsed by addition of salts and then complex became more compact with consequent phase separation. No significant effect of temperature on the PAA/UO complex stability has been observed between 25–50 °C. The quenching rate constants obtained from Stern–Volmer plots were found to be in the order of k_q(H⁺) >> k_q(K⁺) > k_q(Na⁺). © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2737–2744, 2005     

THE REACTIVITY OF CITRONELLOL and α-THUJENE WITH SINGLET OXYGEN. RATE CONSTANTS OF CHEMICAL REACTION and PHYSICAL QUENCHING*

The quantum yields of Rose Bengal sensitized photooxidation of citronellol and α-thujene have been determined as a function of added acceptor and compared with those of furfuryl alcohol as a standard. The results permitted the calculation of the corresponding rate constants of chemical reaction (k_T) and physical quenching (K_q) of singlet oxygen. The sum (k_T+ k_q) has been verified independently by a Stern-Volmer analysis of the singlet oxygen luminescence quenching. α-Thujene reacts faster with singlet oxygen than citronellol, physical quenching being negligible in both cases.     

Photocatalytic decomposition of dispiro(diadamantane-1,2-dioxetane) initiated by Ce(CIO4)3 in the excited state

Photocatalytic decomposition of dispiro(diadamantane-1,2-dioxetane) (1) to adamantanone (2) initiated by Ce(ClO₄)₃ in the excited state in the MeCN−CHCl₃ (2∶1) mixture was studied. The bimolecular rate constants of quenchingk _q were determined from the kinetics of quenching of Ce^3+* by dioxetane at different temperatures. The Arrhenius parameters of the quenching were calculated from the temperature dependence ofk _q:E _a=3.2±0.3 kcal mol⁻¹ and logA=11.6±6. The quantum yields of photolysis of 1 depending on its concentration and the rate constant of the chemical reaction of Ce^3+* with 1 were determined. The latter coincides withk _q:k _ch=(2.6±0.3)·10⁹ L mol⁻¹ s⁻¹ (T=298 K). The fact that the maximum quantum yield of decomposition of dioxetane is equal to 1 indicates the absence of physical quenching of Ce^3+* with 1. Nonradiative deactivation of Ce^3+* in solutions of MeCN and in MeCN−CHCl₃ mixtures was studied. It is caused by the replacement of H₂O molecules in the nearest coordination surroundings of Ce³⁺ by solvent molecules and reversible transfer of an electron to the ligand. The activation parameters of the nonradiative deactivation of Ce^+* were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 724–729, April, 1997.     

Selective quenching of tryptophanyl fluorescence in bovine serum albumin by the iodide ion

Modified Stern-Volmer equation is obeyed by bovine serum albumin (BSA)-iodide system showing selective quenching of tryptophanyl fluorescence of BSA. The fraction of accessible protein fluorescence is 0.56 and the effective Stern-Volmer constant is 290 M^-1 at pH 7.4 in 0.005 M phosphate buffer at 25°C. Collisional quenching is operative both in the BSA -I⁻¹ system and the model system, tryptophan-I⁻¹. It is supported by the observed relationship between the ratio of quenching rate constants (k _q ) and diffusion coefficients and alsok _q with bulk viscosity.     

Photophysical insights on quantum dots-zinc porphyrazine system studied in solution

The present study reports spontaneous interaction of a quantum dots, namely, CdS_xSe_1-x/ZnS (QD) with zinc porphyrazine (1) in toluene. It is observed from steady state fluorescence measurements that photoluminescence of QD suffers quenching by 1. Time resolved fluorescence measurements reveal small change in the lifetime of QD (16.10 ns) following it interaction with 1 (15.77 ns). The magnitude of k_q for QD-1 system, i.e., k_q ​= ​5.25 ​× ​10¹² ​L⋅mol⁻¹⋅sec⁻¹ (evaluated from the stern-volmer plot) establishes that photoexcited QD undergoes decay by 1 according to static quenching mechanism. The results emerging from above study confirm that QD-1 system may be judiciously applied as an energy storage material in near future.     

Preparation and photoluminescence characteristics of polysiloxane pendant tris(2,2′-bipyridine)ruthenium (II) complex

Polysiloxanes containing pendant tris(2,2′-bipyridine)ruthenium(II) complex (Ru(bpy)3²⁺) were prepared by reaction of polysiloxane-pendant 2,2′-bipyridine (PSiO-bpy) with cis-Ru(bpy)₂Cl₂. In methanol solution, the polymer pendant Ru(bpy)3²⁺ showed absorption maximum at 456nm and emission maximum at around 609nm, both of which are shifted to longer wavelength than the monomeric Ru(bpy)₃²⁺. The lifetime τ₀ of the excited polymer complex with low Ru(bpy)3²⁺ content was almost the same as that of the monomeric one in methanol (830ns), but τ₀ of the polymer with higher complex content was shorter because of a concentration quenching. In a solid state, τ₀ was much shorter (306–503ns) than that in a methanol solution contrary to the conventional polymeric system. Higher complex content in the polymer film caused higher glass transition temperature (Tg), but shorter τ₀. These results indicate concentration quenching in the polymer film. The excited polymer pendant Ru(bpy)3²⁺ was quenched by oxygen, and the relative emission intensity followed the Stern-Volmer equation. In a methanol solution the quenching rate constant (k_q) was the same order of magnitude as the monomeric complex, and independent of the complex content in the polymer. In a film, k_q was higher for the polymer with higher complex content.     

Quenching mechanism of the UO2 2+ excited state by phenols and related compounds in aqueous solutions

Quenching rate constants,k _q, of the uranyl excited state by phenolic compounds were determined in H₃PO₄ 1M by luminiscence techniques. A plot of logk _q vs. ΔG₂ for the photo-induced electron transfer reaction rendered a slope value of 4.0 eV⁻¹ which was much smaller than that predicted from the Rehm-Weller correlation (16.9 eV⁻¹). This value is similar to those found for aromatic quenchers that fail to interact with the UO₂ ²⁺ ground state, and quench the uranyl luminescence through a non-radiative donor-acceptor complex formation.     

Sensitized photo-redox reaction: VII. The Synthesis,FAB mass spectrum,stationary absorption,emission, transient absorption spectra,redox potential of dihydroxy-tin (IV)-mesoporphyrin dimethyl ester and its sensitized photo-reduction of methyl viologen

In DMSO/water (4:1), photolysis of the dihydroxy-Sn (IV)-mesoporphyrin dimethyl ester (SnP)/methyl viologen (MV²⁺)/ethylene diamine tetraacetic acid (EDTA) ternary system produces methyl viologen cation radical with a quantum yield of 0.67, much higher than that of systems with other metal complexes of mesoporphyrin dimethyl ester. Neither EDTA nor MV²⁺ quenches the stationary fluorescence of SnP, implying that the reaction does not take place at the singlet state. With flash photolysis we obtain the T-T absorption spectrum of SnP (λ_max 440 nm). By following the decay of this absorption, the triplet life time of SnP is estimated to be 41 μs. The life time is related to the concentration of either MV²⁺ or EDTA. Good linear relationships are obtained by plotting τ₀τ vs. the concentration of MV²⁺ or EDTA (Stern-Volmer plot), from which we determine the quenching constants: _kq(MV²⁺) =5.5 × 10⁷ mol^?1, s^?1; k_q (EDTA) =2.7 × 10⁷ mol^?1, s^?1. The data suggests that upon photolysis of the above ternary system, both oxidative quenching and reductive quenching of the triplet state of the sensitizer are occurring. From the measured phosphorescence spectrum (λ_max 704 nm) and the ground state redox, potentials (E^red_1/2?-0.84V, E^ox_1/2?+1.43 V, vs. Ag/AgCl, KCl (sat.)), we obtain the redox potential of triplet SnP to be E(P⁺/P*_T)?-0.33 V, E(P*_T+/P^?)?+0.92 V. Matching this data with the redox potential of MV²⁺ and EDTA, we establish the fact that during the photolysis of the SnP/MV²⁺/EDTA ternary system, both oxidative and reductive quenching are thermodynamically favorable processes. This is also the reason why the SnP sensitized reaction is much more efficient relative to other mesoporphyrin derivatives.     

Fluorescence Quenching of Tris(2,2′-bipyridine)Ruthenium(II) Dichloride by Certain Organic Dyes

Fluorescence quenching of [Ru(bpy)₃]²⁺ by a series of organic dyes has been investigated by using the steady state fluorescence technique in aqueous medium. The dyes used are anthraquinone dyes: uniblue, acid blue 129, alizarin, alizarin red S and the azo dyes: congo red, sunset yellow, methyl orange, tartrazine, acid orange 63, methyl red and erichrome black T. The quenching of [Ru(bpy)₃]²⁺ was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear indicating dynamic quenching. The quenching rate constants (k _q) were calculated from the fluorescence data. The mechanism of quenching was discussed on the basis of the quenching rate constants as well as the reduction potential of dyes. The electron transfer mechanism has been proved by the calculation of Gibbs energy changes (ΔG _et) by applying the Rehm-Weller equation.     

AMPHIPHILIC COPOLYMERS AS MEDIA FOR LIGHT-INDUCED ELECTRON TRANSFER–I. ELECTROSTATIC EFFECT ON THE FORWARD REACTION AS STUDIED BY FLUORESCENCE QUENCHING

Abstract— Fluorescence quenching of amphiphilic copolymers, poly(9-vinylphenanthrene-co-sodium 2-acrylamido-2-methylpropanesulfonate) (APh) and poly(9-vinylphenanthrene-co-3-methacrylamidopropyltrimethylammonium methyl sulfate) (QPh), in aqueous solution, was studied using methyl viologen (MV²⁺) or 4,4'-bipyridinium-1, 1'-bis(trimethylenesulfonate) (SPV) as oxidative quenchers. The fluorescence of the excited phenanthrene groups in APh was found to be efficiently quenched by MV²⁺. The apparent second-order rate constant for the quenching, k_q, ranged in the magnitude of 10¹¹ -10¹²M^-1 s^-1, which are well beyond the diffusion-controlled limit. This is presumably due to an increase of the effective concentration of MV²⁺ around the fluorophore in the copolymer resulting from electrostatic attraction between MV²⁺ and anionic segments of APh. This strong electrostatic interaction also favors the formation of ground-state EDA (electron donor acceptor) complex between the phenanthrene residue and MV²⁺. Such striking behaviors were not observed with the related model compound. Unexpectedly, the quenching with SPV, a zwitterionic quencher, was also enhanced in the polymer system (k_q= 2–6 × 10¹⁰M^-1 s^-1), suggesting the presence of some attractive interaction between APh and SPV. Contrary to the APh system, the fluorescence quenching of the corresponding cationic polymer (QPh) with MV²⁺ was strongly diminished (k_q= 5 × 10⁸M^-1 s^-1). This indicates that the polycation of QPh effectively prevents the access of MV²⁺ to the polymer.     

Solvent effect and temperature dependence in the interaction of singlet oxygen with α-phenyl-N-tert-butyl nitrone

The solvent effect on the quenching of singlet oxygen by -phenyl-N-tert-butyl-nitrone /PBN/ has been investigated by laser flash photolysis technique registrating luminescence kinetics of¹O₂. The values of the rate constant /k_q/ of the quenching were at 293 K: /9.0±0.4/×10⁶, /4.4±0.3/×10⁶ and /18.3±0.5/×10^{6 –}M^–1 s^–1 in toluene, chloroform and acetonitrile, respectively. The rate constant for the chemical interaction between¹O₂ and PBN, was k_r<1×10⁵ M^–1 s^–1k_q independently of the solvent. At temperatures between 223 and 293 K in toluene E_q=0.4±0.4 kJ mol^–1.     

Excited-state electron-transfer quenching by a series of water photoreduction mediators

Rate constants k_q for the quenching of the excited state of Ru(bipy)₃²⁺ by a series of viologen salts having different redox potential E_{$\text{1}\text{2}$} have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The k_q values are found to decrease with increasing —E_{$\text{1}\text{2}$} and to correlate with the reaction free-energy change ΔG. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions.     

1,4-Dihydropyridine derivatives as deactivators of singlet oxygen

The constants of deactivation (k_q) of ¹O₂ by 1,4-dihydropyridine (1,4-DHP) derivatives were determined by quenching of the luminescence of singlet oxygen (¹O₂). The k_q values for 1,4-DHP derivatives range from 10⁶ to 10⁷ litersmole^–1-sec^–1 and depend to a considerable extent on the nature of the substituents in the 1,4-DHP ring. The presence of a substituent in the 4 position decreases k_q, while conversion of the 1,4-DHP system to the corresponding pyridine system deprives the compound of its ability to deactivate ¹O₂. As a result of tests of 12 1,4-DHP derivatives it was found that 2,6-dimethyl-3,5-di (phenylcarbamoyl)-1,4-dihydropyridine deactivates ¹O₂ most effectively.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 924–926, July, 1981.     

Folding and thermodynamic studies of Trp-cage based on polarized force field

Two replica exchange molecular dynamics (REMD) simulations were carried out to study the thermodynamics of a 20-residue Trp-cage folding based on a newly developed polarized protein-specific charge (PPC). Starting from a fully extended conformation, Trp-cage native conformation was successfully sampled using REMD based on a 3-step PPC update. Next, the obtained Trp-cage folded conformation was then used to calculate the PPC in which another REMD was performed to explore the thermodynamic stability of Trp-cage. The theoretical melting temperature T _m of ≈325 K was found to be in close agreement with experimental melting temperature, T _m of 315 K. This indicates that the PPC was correctly predicting the temperature dependence. The current study provides a direct proof of how electrostatic polarization affects protein folding.