Excited Ru Research Articles

Quenching of excited polyazaaromatic Ru(II) complexes by oxygen: evidence for an electron transfer process by photoelectrochemical study

The possible contribution of an electron transfer process to the quenching of excited Ru(II) polypyridyl complexes by oxygen has been the subject of controversy for many years; the existence of this process is re-examined in this work with new polyazaaromatic complexes and by a photoelectrochemical (PEC) method. The luminescence quenching rate constants k Q by oxygen in acetonitrile for a series of polyazaaromatic complexes with 1,4,5,8-tetraazaphenanthrene (tap), 1,4,5,8,9,12-hexaazatriphenylene (hat) and 2,2′-bipyridine (bpy) as ancillary ligand, can clearly be correlated with the oxidation potentials in the excited state. A PEC study of the quenching of Ru(bpy) 3 2+* by oxygen in acidic medium allows determination of the quenching rate constant via the measurements of cathodic photocurrents induced at a transparent SnO 2 electrode. These two investigations show that the electron transfer mechanism is certainly involved in the luminescence quenching of these Ru(II) complexes by oxygen and that the energy transfer contribution becomes rather weak for the more reducing excited complexes.

Mechanism and Kinetics of the Photocatalyzed Oxidation of p-Phenylenediamines by Peroxydisulfate in the Presence of Tri-2,2'-bipyridylylruthenium(II)

The autocatalytic oxidation of p-phenylenediamines with peroxydisulfate can be strongly enhanced by irradiation in the presence of tri-2,2'-bipyridylylruthenium(II). Ru(bpy) 3 2+ acts as photosensitizer and photocatalyst. First, Ru(bpy) 3 3+ is formed by quenching excited Ru * (bpy) 3 2+ with S 2 O 8 2+ . Then the ruthenium(III) complex oxidizes p-phenylenediamine. The resulting Ru(bpy) 3 2+ can start the cycle again. The photocatalytic oxidation of N,N-diethyl-p-phenylenediamine as well as the dark reactions were studied with stopped-flow techniques. The mechanism of this complex reaction is discussed in detail

Photoinduced intramolecular electron transfer reactions within donor-acceptor linked compounds in solution and within donor-acceptor ion-pairs in crystal

Intramolecular electron transfer (ET) processes within donor-acceptor linked compounds in solution and donor-acceptor ion-pairs in crystal have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electron-transfer to either ruthenium(III) ion, rhodium(III) ion or a cobalt(III) ion, followed by back ET to regenerate the original reactant. An Arrhenius plot of the ET rate gave a straight line with an intercept (frequency factor) and a slope (activation energy) for the photoinduced ET and the back ET. Mixed-valence isomer states produced via photoinitiated ET rapidly decayed via back ET. A common and large frequency factor observed for Ru(II)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. For the back ET in the Ru(II)-Co(III) compounds, the frequency factors are reduced because of negative entropy change. ET within donor-acceptor ion-pair of Ru(bpy)23 and Co(CN)36 in crystal took place very rapidly compared with in water.

The effect of pore size at the liquid/solid interface: the O2 quenching of excited Ru(bpy) 3 2+ on porous glass

The effect of pore size at the liquid/solid interface: the O2 quenching of excited Ru(bpy) 3 2+ on porous glass

Intracomplex electron transfer between ruthenium-cytochrome c derivatives and cytochrome c1.

The reactions of a beef heart cytochrome c1 preparation containing the hinge protein with horse cytochrome c derivatives labeled at specific lysine amino groups with (dicarboxybipyridine)(bisbipyridine)ruthenium(II) (Ru(II)) were studied by flash photolysis. All of the ruthenium-cytochrome c derivatives formed complexes with cytochrome c1 in low ionic strength buffer (5 mM sodium phosphate, pH 7). Excitation of Ru(II) to Ru(II*) with a 0.4-microseconds laser flash resulted in rapid electron transfer to the ferric heme group in cytochrome c, followed by electron transfer from the ferrous heme group of cytochrome c to the ferric heme group of cytochrome c1. The kinetic difference spectra displayed maxima at 546 nm and minima at 554 nm characteristic of electron transfer between the two cytochromes. The rate constants were independent of concentration at low ionic strength, indicating intracomplex electron transfer. The rate constants were 4,800, 6,800, 22,000, and 22,000 s-1 for cytochrome c derivatives modified at lysines 13, 27, 25, and 72, respectively. The observed rate constants were independent of ionic strength up to about 50 nM and then decreased progressively with further increases in ionic strength indicating dissociation of the complex. Second-order kinetics were observed at 310 mM ionic strength, with rate constants of 1.0 x 10(6), 1.6 x 10(7), 1.2 x 10(8), and 3.0 x 10(7) M-1 s-1 for the derivatives modified at lysines 13, 27, 25, and 72, respectively. The ionic strength dependence of the second-order rate constants is comparable to that involving native horse cytochrome c and is consistent with electron transfer reactions between oppositely charged proteins.

Photoinduced ET and back-ET in bimetallated compounds of Ru(II)-Rh(III) and Ru(II)-Co(III)

Intramolecular electron transfer processes in bimetallated donor-acceptor compounds have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electrontransfer to either a rhodium(III) ion or a cobalt(III) ion, followed by back-electron transfer, an Arrhenius plot of the electron-transfer-rate gave a straight line of intercept (frequency factor) and slope (activation energy) for the photoinduced electron transfers and the back electron transfers. A common and large frequency factor observed for Ru(H)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. The activation energy observed consists of outersphere rearrangement energy depending on the metal ion-metal ion distance. For the photoinduced electron transfers and subsequent back-electron transfers in the Ru(II)-Co(III) compounds, the electron-transfer-rates are reduced because of weak electronic coupling, large rearrangement energy and negative entropy change.

Intracomplex electron transfer between ruthenium-cytochrome c derivatives and cytochrome c oxidase

The reactions of bovine cytochrome c oxidase with horse cytochrome c derivatives labeled at specific lysine amino groups with (dicarboxybipyridine)bis(bipyridine)ruthenium (II) were studied by laser flash photolysis. All of the derivatives form complexes with cytochrome c oxidase at low ionic strength (5 mM sodium phosphate, pH 7). Excitation of Ru(II) to Ru(II*) with a short laser flash resulted in rapid electron transfer to the ferric heme group of cytochrome c, followed by electron transfer to cytochrome c oxidase. The photoreduced heme Fe(II) in the cytochrome c derivative modified at lysine 25 on the periphery of the heme crevice domain transferred an electron to CuA with a rate constant of 1.1 x 10(4) s-1. CuA then transferred an electron to cytochrome a with a rate constant of 2.3 x 10(4) s-1. The derivatives modified at lysines 7, 39, 55, and 60 remote from the heme crevice domain of cytochrome c have nearly the same kinetics. The rate constant for electron transfer from the cytochrome c heme to CuA is greater than 10(5) s-1, and the rate constant for electron transfer from CuA to cytochrome a is 2 x 10(4) s-1. The cytochrome c derivatives modified at lysines 13 and 27 in the heme crevice domain react much more slowly than the other derivatives, with intracomplex rate constants for oxidation of cytochrome c ranging from 1000 to 6000 s-1. The bulky ruthenium group at the heme crevice domain of these derivatives apparently alters the binding orientation, leading to smaller electron-transfer rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoresponsive bilayer membranes composed of Ru(bpy) 2+3 and methylviologen-containing layers sandwiched between ITO electrodes

A new sandwich type photoresponsive bilayer polymer device was fabricated which utilizes the photochemical reaction between Ru(bpy) 2+ 3 and methylviologen (MV 2+) confined in each Nafion layer. This photochemical diode, ITO/Nafion[Ru(bpy) 2+ 3]/Nafion[MV 2+]/ITO, generated (under air) a cathodic photocurrent with respect to the Ru complex-coated electrode. The action spectrum for the photocurrent showed that the photoexcitation of the Ru complex induced a photocurrent, indicating that electron transfer from the excited Ru complex to MV 2+ is the origin of the photoelectrochemical event. Higher membrane thickness induced higher photocurrent density within 1.6 μm of each layer. The photocurrent is discussed in relation to the partial mixing of the two reaction components in the membrane.

Photoresponsive electrode based on the reaction between oxygen and the excited Ru(bpy) 2+3 complex incorporated in a coated Nafion film

A photoresponsive electrode was prepared by coating a Nafion film on a indium tin oxide (ITO) electrode incorporating the Ru(bpy) 2+ 3 complex in the film. A photocurrent was induced at the coated electrode dipped in an aqueous electrolyte in the presence of oxygen. The action spectrum showed that the photocurrent is based on the reaction between O 2 and the excited Ru complex. Viologen molecules present as a solute or as the second film on the top of the Nafion[Ru(bpy) 2+ 3] film increase the induced photocurrent by about a factor of two by working as a mediator between the excited Ru(bpy) 2+ 3 and O 2.

Photoinduced electron transfer between cytochrome c peroxidase and horse cytochrome c labeled at specific lysines with (dicarboxybipyridine)(bisbipyridine)ruthenium(II)

The reactions of yeast cytochrome c peroxidase with horse cytochrome c derivatives labeled at specific lysine amino groups with (dicarboxybipyridine)(bisbipyridine)ruthenium(II) [Ru(II)] were studied by flash photolysis. All of the derivatives formed complexes with cytochrome c peroxidase compound I (CMPI) at low ionic strength (2 mM sodium phosphate, pH 7). Excitation of Ru(II) to Ru(II*) with a short laser flash resulted in electron transfer to the ferric heme group in cytochrome c, followed by electron transfer to the radical site in CMPI. This reaction was biphasic and the rate constants were independent of CMPI concentration, indicating that both phases represented intracomplex electron transfer from the cytochrome c heme to the radical site in CMPI. The rate constants of the fast phase were 5200, 19,000, 55,000, and 14,300 s-1 for the derivatives modified at lysines 13, 25, 27, and 72, respectively. The rate constants of the slow phase were 260, 520, 200, and 350 s-1 for the same derivatives. These results suggest that there are two binding orientations for cytochrome c on CMPI. The binding orientation responsible for the fast phase involves a geometry that supports rapid electron transfer, while that for the slow phase allows only slow electron transfer. Increasing the ionic strength up to 40 mM increased the rate constant of the slow phase and decreased that of the fast phase. A single intracomplex electron transfer phase with a rate constant of 2800 s-1 was observed for the lysine 72 derivative at this ionic strength. When a series of light flashes was used to titrate CMPI to CMPII, the reaction between the cytochrome c derivative and the Fe(IV) site in CMPII was observed. The rate constants for this reaction were 110, 250, 350, and 140 s-1 for the above derivatives measured in low ionic strength buffer.

Time-resolved luminescence studies of a ruthenium(II) complex on porous glass. Pore size dependence of photoinduced electron transfer

Photoinduced electron transfer occurs from Ru(bpy) 3 2+ (bpy, bipyridine) adsorbed on porous glass to aqueous titanium species in the pore. The lifetime of excited Ru(bpy) 3 2+ decreases with an increase in the pore size, indicating that the lifetime reflects the mobility of the molecules in the pore.

Chemically initiated electron-exchange luminescence of Ru(bpy)3Cl2 in catalytic reaction with 1,2-dioxetane

We have investigated chemiluminescence activated by Ru(bpy)3Cl2 in the reaction of decomposition of 1,2-dioxetane. Catalysis and activation of chemiluminescence of 1,2-dioxetanes by a ruthenium complex occur according to a mechanism of chemically initiated electron exchange, which leads to effective excitation of Ru(bpy)3Cl2 with yield\(\Phi _{Ru}^* \)=0.20±0.05.

Intramolecular Electron-Transfer of Covalently-Linked Polypyridine Ruthenium(II)/Rhodium(III) Binuclear Complexes in the Excited State. Observation of the Marcus Inverted Region

Abstract Ru(II)bpy2Mebpy-CH2CH(OH)CH2-MebpyRh(III)L2 (L = bpy. phen) (1) were newly synthesized. Intramolecular electron-transfer in excited 1 was studied with a time-correlated single photon counting method. In H2O, the excited Ru(II) complex exhibits a biexponential decay. The presence of a slow component suggests that the excited state can be repopulated by thermal activation from the Ru(III)–Rh(II) and the direct process to the ground state lies in the Marcus Inverted region.

Oxidative quenching of excited Ru(bpy) 32+ with S 2O 82− at various pH and external magnetic field values

The luminescence quenching of excited Ru(bpy) 3 2+ by S 2O 8 2− and the quantum yield of Ru(bpy) 3 3+ formation in aqueous solution in the pH range 1 – 10 and at various external magnetic fields were investigated by time-resolved luminescence and steady state photochemical methods. The maximum quenching efficiency was obtained at pH 3 and the largest overall quantum yield of Ru(bpy) 3 3+ formation was measured at pH 5. The results are consistent with a unit probability of the escape efficiency of the ·SO 4 radical from the cage [Ru(bpy) 3 3+ … S 2O 8 3−] and the pH dependence of both the rate coefficient of electron transfer quenching and the effect of thermal side reactions. An external magnetic field ranging from that of the Earth to a magnetic flux density of 1.5 T has no influence on the rate of the quenching reaction at pH 5.

Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate.

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.

The electron exchange regeneration of excitation in the reaction of Ru (II) with adamantylidenadamantane-1,2-dioxetane

We have reported the chemically-induced electron exchange luminescence (CIEEL) of Ru(bpy)sCl 2 in the decomposition of adamantylidenadamantane-l, 2-dioxetane (I) to give adamantone (II) catalyzed by Ru(ll) [i]. A new photocatalytic reaction of excited Ru(bpy)3Z+ with (I) was observed. Furthermore, we have found the regenerative excitation of Ru(ll) due to electron exchange. The catalysis of the decomposition of (I) by Ru(ll) in the ground state is a function of the ratio of the redox potentials E(I)I(I_) and ERu(III)/Ru(II) = 1.28 V and proceeds with a rate kRu 0~ mole/liter) and the maximum quantum yield ~ k*Ru/k q were obtained from independent measurements of the Ru(II) photoluminescence intensity in the presence of (I). Thus, the catalytic activity of Ru(II) upon excitation increases by more than a factor of l0 s . The photocatalysis is based on initiated electron exchange Ru(II) -5 hv ~ Ru*(I I )

Spin-orbit coupling and magnetic field effects in photoredox reactions of ruthenium(II) complexes

Spin-orbit coupling (SOC) and magnetic field effects (MFEs) on the efficiency {eta}{sub net} of net electron transfer in the reductive and oxidative quenching of excited Ru{sup II} trisbipyridyl complexes have been investigated by nanosecond laser flash spectroscopy and by a special continuous photolysis technique. The reductive process was studied with tris(4,4{prime}-bis(ethoxycarbonyl)-2,2{prime}-bipyridine)ruthenium(2+) quenched by anilines X-An (X = H, 4-Cl, 4-Br, 2-I, 4-I). Increasing SOC of the substituent X leads to a decrease of relative {eta}{sub net} values (1.0, 0.79, 0.45, 0.22, 0.12). With X = 4-I {eta}{sub net} is decreased by 12.5 {plus minus} 2.5% in a magnetic field of 1 T. These effects are interpreted as SOC effects of the X center on the spin-forbidden back electron transfer in triplet radical pairs. The oxidative process was studied with (Ru{sup II}(2,2{prime}-bipyridine){sub 3}){sup 2+} and methylviologen (MV{sup 2+}) as a quencher. Here a MFE of {minus}10% at 1 T was observed on the yield of radicals detected on the nanosecond time scale. A detailed MF dependence between 0 and 1 T of the yield of MV{sup {center dot}+} radicals was measured by continuous photolysis using EDTA as a sacrificial electron donor to reduce Ru{sup III}. The MFE shows a gradual onsetmore » followed by a linear increase above 50-100 mT. The onset field decreases and the (negative) slope of the MFE increases as the solvent viscosity is increased in water/ethylene glycol mixtures. On the basis of measurements of lifetime and radical yield at various quencher concentrations, it is shown that the magnetic field mainly affects {eta}{sub net} and not the rate of quenching k{sub q}, as recently asserted by other authors. The MFE is attributed to SOC effects in the Ru{sup III} complex constituent of the primary radical pair.« less

The reductive quenching of excited Ru(bpy) 32+ by aromatic amines: solvent and structural effects

The luminescence quenching of excited Ru(bpy) 3 2+ by 1-naphthylamine, 2-naphthylamine, N, N-dimethylaniline and diphenylamine was studied in methanol, ethanol, 2-propanol, acetonitrile, propionitrile and acetone. The bimolecular rate constants for the electron transfer process from the aromatic amines to the excited ruthenium complex were measured as a function of temperature. Activation enthalpies and entropies were determined for each amine in all the solvents. The results were analysed in the framework of Marcus theory for electron transfer reactions. In the alcoholic solvents the process seems to follow the predictions of the theory if an outer-sphere mechanism is assumed. However, for the aprotic solvents the observations are not in agreement with the expected behaviour. In these solvents the rate constants for a given free energy change are dependent on the structure of the aromatic moiety and are much lower for the naphthylamines.

ChemInform Abstract: Bell‐Shaped Temperature Dependence in Quenching of Excited Ru(bpy)2+ 3 by Organic Acceptor

ChemInform Abstract: Bell‐Shaped Temperature Dependence in Quenching of Excited Ru(bpy)2+ 3 by Organic Acceptor

Bell-shaped temperature dependence in quenching of excited Ru(bpy)32+ by an organic acceptor

Electron transfer quenching reactions of excited tris(2,2'-bi-pyridine)ruthenium(II), *Ru(bpy)/sub 3//sup 2 +/, have received special attention as tests of Marcus theory as well as in their application to redox photosensitization. Nevertheless, the mechanism of oxidative quenching of *Ru(bpy)/sub 3//sup 2 +/ in acetonitrile, exhibiting negative temperature dependence in endoergic and moderately exoergic regions, is not explicitly understood. They now report the origin of the negative temperature dependence and the reaction mechanism of oxidative quenching of *Ru(bpy)/sub 3//sup 2 +/. The quenching rate constant (k/sub q/) at a given temperature was obtained by conventional Stern-Volmer plots based on both the emission intensity and the lifetime of *Ru(bpy)/sub 3//sup 2 +/. The effect of diffusion on the observed quenching rate constant was corrected.