Electron Transfer Quenching Research Articles

Investigating Design Rules for Photoinduced Electron Transfer Quenching in Triangulenium Probes.

Fluorescent probes based on photoinduced electron transfer (PET) quenching of long lifetime triangulenium fluorophores have found multiple applications. For such probes a successful design relies on the right balance between the rates of PET quenching and fluorescence. In a series of ADOTA (A) and DAOTA (D) triangulenium fluorophores appended with aniline-like quencher moieties, we have investigated the rates of quenching and its relation to thermodynamic driving force, distance, and conjugation within the quencher moiety. Three different quenchers, a short (1), a long (2), and a long twisted (3), 4-aminophenyl, 4'-aminobiphenyl, and 2,2'-dimethyl-4'-aminobiphenyl, respectively were investigated. Time-resolved fluorescence measurement was used to measure PET rate constantkPETranging from >4x1011s-1to 2x109s-1. Interestingly, PET quenching is equally efficient/fast from1and2, even with increase in distance between the donor and the acceptor. However, when twisting the biphenyl in3, a 20-fold decrease in quenching is found. Even with the decrease inkPETthe quenching in3A/Dis still highly efficient, with nearly 99% quenching. The study show that long lifetime fluorophores can be efficiently switched even by relative slow PET processes and that PET quencher moieties can be removed far from the fluorophore if conjugated linkers are applied.

Electron Transfer Quenching of Rhodamine 6G by N-Methylpyrrole Is an Unproductive Process in the Photocatalytic Heterobiaryl Cross-Coupling Reaction.

In the heterobiaryl cross-coupling reaction between aryl halides (Ar-X) and N-methylpyrrole (N-MP) catalyzed by rhodamine 6G (Rh6G+) under irradiation with visible light, a highly active and long-lived (millisecond time range) rhodamine 6G radical (Rh6G•) is formed upon electron transfer from N,N-diisopropylethylamine (DIPEA) to Rh6G+. In this study, we utilized steady-state and time-resolved spectroscopy techniques to demonstrate the existence of another electron-transfer process occurring from the relatively electron-rich N-MP to photoexcited Rh6G+ that was neglected in the previous reports. In this case, the radical Rh6G• formed is short-lived and undergoes rapid recombination (nanosecond time-range), rendering it ineffective in reducing Ar-X to aryl radicals Ar• that can subsequently be trapped by N-MP. This is further demonstrated via two model reactions involving 4'-bromoacetophenone and 1,3,5-tribromobenzene with insignificant product yields after visible-light irradiation in the absence of DIPEA. The unproductive quenching of photoexcited Rh6G+ by N-MP leads to a lower concentration of photocatalyst available for competitive charge transfer with DIPEA and hence decreases the efficiency of the cross-coupling reaction.

Large-scale direct pyrolysis synthesis of excitation-independent carbon dots and analysis of ferric (III) ion sensing mechanism

A one-step direct pyrolysis method without any post-treatment steps and a mass yield of up to 69% was realized for preparing N-doped carbon dots (NCDs). The NCDs exhibit excellent excitation-independent blue fluorescence with quantum yield (QY) of 65.5%, and their luminescence mechanism was explored based on optical properties and structural composition, which mainly originates from surface state photoluminescence (PL). The NCDs display highly selective and sensitive detection of Fe3+ in tris-HCl buffer system, a good linear relationship between PL intensity ratios and Fe3+concentrations was obtained in concentration range of 0 to 400 μM with a low LOD of 0.703 μM. The formation of ferric hydroxide colloid is the main reason for the specific recognition of Fe3+ by NCDs, and buffer solution plays a key role in the formation of ferric hydroxide colloid and selective detection of Fe3+. The sensing mechanism is a dynamic quenching electron transfer process in the action of hydrogen bonding and colloidal adsorption. Three real water samples were applied to evaluate actual application of NCDs probe, satisfactory recoveries (93.5%–105.6%) indicate that NCDs has great practical potential for detection of metal ions in environmental water assays.

Coordination Environment-Controlled Photoinduced Electron Transfer Quenching in Luminescent Europium Complexes.

The quenching of sensitized Eu(III) luminescence by photoinduced electron transfer from the excited light-harvesting antenna to Eu(III) was investigated. A series of complexes incorporating different metal binding sites and thus having varying Eu(III)/Eu(II) reduction potentials were prepared. The complexes were fully characterized using a combination of single-crystal X-ray crystallography and paramagnetic 1H NMR spectroscopy, the results of which support the structural similarity of the complexes. The redox and photophysical behavior of the Eu(III) center and the light-harvesting antenna were studied using cyclic voltammetry and steady-state and time-resolved emission spectroscopy on the nanosecond and millisecond time scales. The contribution of photoinduced electron transfer to the overall reduction of the Eu(III) luminescence quantum yield was found to be comparable and, in many cases, larger than the quenching caused by well-established processes such as coupling to X-H oscillators. These results suggest that the elimination or mitigation of photoinduced electron transfer could substantially improve the emissive properties of the widely used Eu(III)-based emitters.

Photoinduced Electron-Transfer Quenching of Luminescent Silicon Nanocrystals as a Way To Estimate the Position of the Conduction and Valence Bands by Marcus Theory

Photoluminescence of silicon nanocrystals (SiNCs) in the presence of a series of quinone electron acceptors and ferrocene electron donors is quenched by oxidative and reductive electron transfer dynamic processes, respectively. The rate of these processes is investigated as a function of (a) the thermodynamic driving force of the reaction, by changing the reduction potentials of the acceptor or donor molecules, (b) the dimension of SiNCs (diameter = 3.2 or 5.0 nm), (c) the surface capping layer on SiNCs (dodecyl or ethylbenzene groups), and (d) the solvent polarity (toluene vs dichloromethane). The results were interpreted within the classical Marcus theory, enabling us to estimate the position of the valence and conduction bands, as well as the reorganization energy (particularly small, as expected for quantum dots) and electronic transmission coefficients. The last parameter is in the range 10–5–10–6, demonstrating the nonadiabaticity of the process, and it decreases upon increasing the SiNC dimensions: this result is in line with a larger number of excitons generated in the inner silicon core for larger SiNCs and thus resulting in a lower electronic coupling with the quencher molecules.

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes.

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

Light Harvesting in Microscale Metal–Organic Frameworks by Energy Migration and Interfacial Electron Transfer Quenching

Microscale metal-organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers. Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N',N'-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching.

Reexamination of the Rehm–Weller Data Set Reveals Electron Transfer Quenching That Follows a Sandros–Boltzmann Dependence on Free Energy

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.

Bidirectional Photoinduced Electron-Transfer Quenching Is Observed in 4-Amino-1,8-naphthalimide-Based Fluorescent Anion Sensors

The synthesis and photophysical evaluations of two new fluorescent photoinduced electron-transfer (PET) anion sensors, 1 and 2, is described. These are based on 4-amino-1,8-naphthalimide fluorophores and diarylthiourea anion receptors, connected via a methylene spacer to the imide. The sensing of acetate, phosphate, and fluoride, on all occasions, gave rise to quenching in the fluorescence of 1 and 2, similar to that seen for the structural isomer 3. These results demonstrate that bidirectional PET sensing occurs in such naphthalimide-based anion sensors.

Photoinduced Electron Transfer in a Protein−Surfactant Complex: Probing the Interaction of SDS with BSA

Photoinduced fluorescence quenching electron transfer from N,N-dimethyl aniline to different 7-amino coumarin dyes has been investigated in sodium dodecyl sulfate (SDS) micelles and in bovine serum albumin (BSA)-SDS protein-surfactant complexes using steady state and picosecond time resolved fluorescence spectroscopy. The electron transfer rate has been found to be slower in BSA-SDS protein-surfactant complexes compared to that in SDS micelles. This observation has been explained with the help of the "necklace-and-bead" structure formed by the protein-surfactant complex due to coiling of protein molecules around the micelles. In the correlation of free energy change to the fluorescence quenching electron transfer rate, we have observed that coumarin 151 deviates from the normal Marcus region, showing retardation in the electron transfer rate at higher negative free energy region. We endeavored to establish that the retardation in the fluorescence quenching electron transfer rate for coumarin 151 at higher free energy region is a result of slower rotational relaxation and slower translational diffusion of coumarin 151 (C-151) compared to its analogues coumarin 152 and coumarin 481 in micelles and in protein-surfactant complexes. The slower rotational relaxation and translational diffusion of C-151 are supposed to be arising from the different location of coumarin 151 compared to coumarin 152 and coumarin 481.

Photoinduced electron transfer from N,N-dimethylaniline to 7-amino Coumarins in protein-surfactant complex: Slowing down of electron transfer dynamics compared to micelles

Photoinduced electron transfer from N,N-dimethylaniline to different Coumarin dyes has been investigated in dodecyl trimethyl ammonium bromide (DTAB) micelles and in Bovine serum albumin (BSA)-DTAB protein-surfactant complex using steady-state and picosecond time-resolved fluorescence spectroscopy. We observed a slower fluorescence quenching rate in the DTAB micelles and in the protein-surfactant complex as compared to that in pure acetonitrile solution. Moreover, the observed fluorescence quenching in BSA-DTAB complex was found to be slower than that in DTAB micelles. In the correlation of free-energy change with the fluorescence quenching constant we observed a deviation in the fluorescence quenching electron transfer rate for Coumarin 151 (C-151) from the normal Marcus curve. This observation is ascribed to the stronger interaction of C-151 with the surfactant molecules present in the micelles. This is evident from the slower translation diffusion (D(L)) of Coumarin 151 compared to other probe molecules.

Ultrafast Spectroscopic Investigation of the Charge Recombination Dynamics of Ion Pairs Formed upon Highly Exergonic Bimolecular Electron-Transfer Quenching: Looking for the Normal Region

The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔGCS= −2.2 eV), but charge recombination is weakly exergonic (ΔGCR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).

Study of the Mechanism of Electron-Transfer Quenching by Boron−Nitrogen Adducts in Fluorescent Sensors

The mechanism of the change in fluorescence quenching by the amine in boronic acid-based carbohydrate sensor molecules has been explored using density functional theory (DFT). The geometric constra...

Reductive Electron Transfer Quenching of MLCT Excited States Bound To Nanostructured Metal Oxide Thin Films

The ruthenium compounds Ru(deeb)(bpz)2(PF6)2, Ru(deeb)2(bpz)(PF6)2, and Ru(deeb)2(dpp)(PF6)2, where deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, bpz is 2,2‘-bipyrazine, and dpp is 2,3-bis(2-pyridyl)pyrazine, have been prepared, characterized, and anchored to mesoporous nanoparticle thin films comprised of the wide band gap semiconductor TiO2 or the insulator ZrO2. The metal-to-ligand charge-transfer (MLCT) excited states of these compounds are potent photooxidants (E°(RuII*/+) > +1.0 V vs SCE) with long lifetimes (τ > 1 μs) that efficiently oxidize iodide and phenothiazine with rate constants that approach the diffusion limit in acetonitrile. Photogalvanic cells based on the sensitized TiO2 materials yield photocurrent action spectra that agree well with the Ru(II) absorptance spectra. The photocurrent efficiency was very low, φ < 10-4. Transient absorption data show that neither the excited nor the reduced state of the ruthenium compounds efficiently inject electrons into the TiO2 particles. The cage escape yields following excited-state electron transfer are approximately 2/3 lower in the mesoporous thin films than in fluid solution. Intermolecular energy transfer across the nanoparticle surfaces is manifest in a second-order component to the excited-state relaxation kinetics.

Long Distance Photoinduced Electron Transfer in Solutions: A Mechanism for Producing Large Yields of Free Ions by Electron Transfer Quenching

Free ion and fluorescence quantum yields from the geminate ion pairs formed by electron-transfer quenching of the excited acceptor 9,10-dicyanoanthracene by aromatic electron-donors are measured in different solvents. Theeffects of solvent polarity, energetic driving force, and steric substitution on free ion yields are studied. From the comparison of the effect of driving force on free ion yields and fluorescence quantum yields in different solvents, especially those of moderate polarity, it is concluded that free ion yields are controlled by both recombination rate constants and the separation distance distribution of the initially formed geminate radical ion pairs. In dichloromethane (DCM), the recombination and free ion formation processes are directly observed by fluorescence lifetime and transient photocurrent measurements. The time-resolved results indicate that free ion formation is faster than the recombination process. This means that the geminate radical ion pairs that form free ions and those that are neutralized by electron-transfer recombination have different histories with different separation distances. From studies of steric effects on free ion yields in different solvents, it is concluded that, in polar solvents such as acetonitrile and butyronitrile, the main effect of steric hindrance is to decrease the recombination rate constant and increase the escape probability, whereas in moderately polar solvents such as tetrahydrofuran, DCM and 1,2-dichlorobenzene, the main effect of steric bulk is to change the initial separation distance distribution of the geminate radical ion pairs formed by electron-transfer quenching. As an example, we compare donors such as durene (DUR) with those of greater steric bulk like 1,2,4,5-tetra-iso-propylbenzene (TIPB), for which the driving force for electron transfer is similar. The free radical ion yield for TIPB is more than 40 times greater than it is for DUR in DCM. This is the first example from our work in which the infinite rate boundary condition for ion recombination used by Onsager is not adequate, because there is no perfect sink at the origin. The free ion yield data are analyzed by the theory of Hong and Noolandi under the Collins and Kimball boundary condition.

Charge Recombination Dynamics of Geminate Ion Pairs Formed by Electron Transfer Quenching of Molecules in an Upper Excited State

An investigation of the charge recombination (CR) dynamics of geminate ion pairs formed upon electron transfer quenching of azulene, benz[a]azulene, and xanthione in the second singlet excited state by several electron donors, using ultrafast time resolved spectroscopy and photoconductivity is reported. The ion pairs have two possible CR pathways: (i) a highly exergonic CR to the neutral ground state or (ii) a moderately exergonic CR leading to the formation of the neutral acceptor in the first singlet excited state. This investigation shows strong evidence of the predominance of the second pathway. CR in ion pairs formed with the azulenes is faster by a factor of more than 50 than in ion pairs having a similar energy but with the first CR pathway only. The electron transfer quenching of xanthione in the second singlet excited state by several weak donors does not lead to a significant reduction of the triplet yield of this molecule. The relevance of these results to explain the absence of the inverted region in highly exergonic bimolecular charge separation reactions is discussed.

Energy- and Electron-Transfer Quenching of Porphyrin Triplets by C60

Energy- and electron-transfer from porphyrins in the triplet excited state to C60 in toluene and benzonitrile has been studied with Fourier transform EPR (FT-EPR). Pulsed-laser excitation of the systems magnesium tetraphenylporphyrin (MgTPP)/C60 and free-base octaethylporphyrin (OEP)/C60 in toluene gives rise to a resonance peak from C60 triplets (3C60) generated primarily by the energy-transfer process 3MgTPP (3OEP) + C60 → MgTPP (OEP) + 3C60 as evident from the emissive spin polarization of the 3C60 signal observed at early times. No FT-EPR signals from redox products could be detected. In benzonitrile, triplet energy transfer from 3MgTPP or 3OEP to C60 is a minor process. In this polar solvent photoexcitation of MgTPP/C60 produces FT-EPR spectra with signal contributions from MgTPP+ and C60- in addition to the 3C60 resonance. From the spin polarization and time profile of the signal from the anion radical, it can be deduced that the primary route of electron transfer is oxidative quenching of the porphyrin triplets, 3MgTPP (3OEP) + C60 → MgTPP+ (OEP+) + C60-. The reduction in 3C60 lifetime indicates that C60- is generated as well by the reductive triplet quenching reaction 3C60 + MgTPP (OEP) → C60- + MgTPP+ (OEP+).

Effect of DNA Scaffolding on Intramolecular Electron Transfer Quenching of a Photoexcited Ruthenium(II) Polypyridine Naphthalene Diimide.

Intramolecular emission quenching of a photoexcited ruthenium(II) polypyridine by a covalently linked naphthalene diimide (NDI) has been measured in aqueous buffer both without and with calf thymus DNA. The complex consists of a Ru(2,2'-bipyridine)(2)(2,2'-bipyridine-5-carboxamide)(2+) electron donor covalently attached by way of a -CH(2)CH(2)CH(2)- linker to a 1,4,5,8-naphthalene diimide acceptor (Ru-NDI, 1). The NDI portion of the complex intercalates in calf thymus DNA, as indicated by the hypochromism of its optical absorbance bands and observation of an induced circular dichroism spectrum in the same region. Emission quenching in Ru-NDI has been measured relative to a Ru tris-bpy model lacking the NDI moiety by both lifetime and emission quantum yield techniques. Using lifetime averages, the relative emission quenching is, respectively, 99.1% and 97.9% in aqueous buffer solutions without and with DNA. The emission quenching is ascribed to intramolecular electron transfer within the Ru-NDI complex with an estimated driving force (-DeltaG degrees ) of 0.33 eV. In buffer, the emission decays of Ru-NDI alone are fit well with a triexponential model with lifetimes of 0.34 (0.88), 1.99 (0.11), and 12.6 (0.008) ns (relative amplitude). The emission decays of the DNA-intercalated Ru-NDI complex are also fit well with a triexponential model with lifetimes of 0.31 (0.79), 2.00 (0.13), and 11.8 (0.08) ns. Thus, the fractional amplitudes of the lifetimes change upon DNA intercalation of the complex, while the lifetimes themselves remain essentially the same. The average rates of electron transfer in aqueous buffer without and with DNA are, respectively, 1.6 x 10(9) and 6.8 x 10(8) s(-)(1). The striking result of this study is that the overall character of electron transfer quenching in Ru-NDI is very similar whether or not it is bound to DNA. Intercalation of the NDI in DNA apparently has negligible consequences for electron transfer, implying either that the activation energy and electronic coupling in Ru-NDI are largely unaffected by this, at first glance, seemingly significant environmental change or that changes in these parameters on DNA binding cancel fortuitously.

Sensing of Carbon Dioxide by a Decrease in Photoinduced Electron Transfer Quenching

We described a new approach to sensing of carbon dioxide based on photoinduced electron transfer (PET) quenching. Fluorophores like naphthalene and anthracene are known to be quenched by unprotonated amines by the PET mechanism. We examined the fluorescence spectral properties of two amine-containing fluorophores, 1-naphthylmetylamine (NMA) and 9-ethanolaminomethylanthracene (EAA). When dissolved in an organic solvent, both fluorophores displayed increased intensity when equilibrated with gaseous carbon dioxide. In the case of NMA, we found that the mean lifetime increased with increasing partial pressures of CO2. The intensity and lifetime changes of NMA are completely reversible when CO2 is removed by purging with argon. Our results are consistent with decreased quenching by the covalently linked amino groups when CO2 is dissolved in the solution. At present, we are not certain whether the increased intensity is due to protonation of the amino groups or to carbamate formation. In either event, these results suggest that CO2 can be detected directly using amine-containing fluorophores without the use of bicarbonate and a pH-sensitive fluorophore.

FT-EPR Study of Spin-correlated Radical Pairs Formed by Electron Transfer Quenching of Porphyrin Triplets in Micellar Solution

The spin-correlated radical pairs (SCRPs) formed by photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin ( ZnTPPS ) to quinones in micelles of the cationic surfactant cetyltrimethylammonium chloride ( CTAC ) were studied by means of Fourier transform EPR (FT-EPR). It is shown that variation of the power of the microwave pulse allows the separation of EPR signals arising from SCRPs and free radicals. The measured kinetics of radical formation can be accounted for in terms of a statistical model taking into account the non-uniform distribution of the solutes over the micelles. The rate constant of electron transfer quenching (kq) of the ZnTPPS triplet state by duroquinone (DQ) is found to be 1.05 × 106 s−1. The FT-EPR measurements gave information also on the kinetics of the homogeneous electron transfer reaction DQ − + DQ → DQ + DQ − in CTAC solution in which the DQ − anion radicals were generated by light-induced electron transfer from ZnTPPS . It is found that the dependence of the rate of this reaction on quinone concentration deviates from the linear relationship found in homogeneous solutions. A statistical model is proposed to account for the data. Based on this model, the rate constant of the self-exchange reaction (k ex ) is 4.1 × 106 s−1. From results obtained with duroquinone and benzoquinone as acceptors, it is concluded that ZnTPPS is located at the micelle/water interface.