Charge-Transfer and Non-Charge-Transfer Processes Competing in the Sensitization of Singlet Oxygen: Formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) during Oxygen Quenching of Triplet Excited Naphthalene Derivatives

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Mechanism of Oxygen Quenching of the Excited States of Heteroleptic Chromium(III) Phenanthroline Derivatives

In this study, we report the synthesis and characterization of some heteroleptic Cr(III) complexes of the form [Cr(Phen)2L](OTf)3, where Phen = 1,10-phenanthroline and L is either 2,2'-bipyridine (bpy) or its derivatives, such as 4,4'-dimethyl-2,2'-bipyridine (4,4'-DMB), 4,4'-dimethoxy-2,2'-bipyridine (4,4'-DMOB), 4,4'-ditert-butyl-2,2'-bipyridine (4,4'-dtbpy), 5,5'-dimethyl-2,2'-bipyridine (5,5'-DMB), 4,4'-dimethoxycarbonyl-2,2'-bipyridine (4,4'-dmcbpy) or 1,10-phenanthroline derivatives, such as 5-methyl-1,10-phenanthroline (5-Me-Phen) and 4,7-dimethyl-1,10-phenanthroline (4,7-DMP). Heteroleptic complexes were prepared in two stages via the intermediate [Cr(Phen)2(CF3SO3)2](CF3SO3) and five examples have been crystallographically characterized. Steady-state absorption and luminescence emission characteristics of these complexes were measured in 1 M HCl solutions. The luminescence quantum yield of these complexes was found to be the lowest for [Cr(Phen)2(4,4'-dmcbpy)](OTf)3 and the highest for [Cr(Phen)2(4,4'-DMB)](OTf)3 with values of 0.31 × 10-2 and 1.48 × 10-2, respectively. The calculated excited state energy, E0-0, was found to vary within the narrow range of 163.1-165.0 kJ mol-1 across the series. Transient absorption spectra in degassed, air-equilibrated, and oxygen-saturated 1 M HCl aqueous solutions were also measured at different time decays and demonstrated no significant differences, indicating the absence of any ion-separated species in the excited state. Excited-state decay traces at the wavelength of maximum absorption were used to calculate oxygen quenching rate constants, kq, which were found to be in the range 3.26-5.27 × 107 M-1 s-1. Singlet oxygen luminescence photosensitized by these complexes was observed in D2O, and its luminescence intensity at 1270 nm was used for the determination of singlet oxygen quantum yields for these complexes, which were in the range of 0.20-0.44, while the fraction of the excited 2E state quenched by oxygen was in the range of 0.22-0.68, and the efficiency of singlet oxygen production was in the range of 0.44-0.90. The mechanism by which the excited 2E state is quenched by oxygen is explained by a spin statistical model that predicts the balance between charge transfer and noncharge transfer deactivation pathways, which was represented by the parameter pCT that was found to vary from 0.35 to 0.68 for this series of Cr(III) complexes.

Fractional dependence of the free energy of activation on the driving force of charge transfer in the quenching of the excited states of substituted phenanthroline homoleptic ruthenium(ii) complexes in aqueous medium

The photophysical characteristics of some homoleptic ruthenium(ii) phenanthroline derivatives are investigated in aqueous medium. The lifetimes of the excited ³MLCT state of the studied complexes were found to be very sensitive to the type of the substituents on the phenanthroline ligand and were found to increase from about 0.96 μs in case of the parent [Ru(Phen)₃]²⁺ complex to 2.97 μs in case of [Ru(DPPhen)₃]²⁺. The transient absorption spectra of the current set of complexes were studied also in aqueous medium. Quenching of the excited ³MLCT states of the studied complexes by molecular oxygen were studied and quenching rate constants were found to be in the range 1.02-4.83 × 10⁹ M^-1 s^-1. Values of singlet oxygen quantum yields were found to be in the range 0.01 to 0.25, and the corresponding efficiencies of singlet oxygen thereby produced, f ^T _Δ, were in the range 0.03-0.52. The mechanism by which the excited ³MLCT state is quenched by oxygen is discussed in light of the spin statistical factor rate constants and the competition between charge transfer and non-charge transfer quenching pathways. The partial charge transfer parameters, p _CT, were obtained and found to be about 0.88 for all complexes except for complexes with f ^T _Δ values lower than 0.25. The correlation of the activation free energies ΔG ^≠ of the exciplexes formation with the driving force for charge transfer, ΔG _CET, gives a charge transfer character of the exciplexes of about 35.0%.

Charge-Transfer-Mediated Mechanism Dominates Oxygen Quenching of Ligand-Protected Noble-Metal Cluster Photoluminescence

Photoluminescence (PL) quenching of ligand-protected noble-metal clusters (NMCs) by molecular oxygen is often used to define whether the PL of NMC is fluorescent or phosphorescent, and only energy transfer has been always considered as the quenching mechanism. Herein, we performed the Rehm-Weller analysis of the O₂-induced PL quenching of 13 different NMCs and found that the charge-transfer (CT)-mediated mechanism dominates the quenching process. The quenching rate constant showed a clear dependence on the CT driving force, varied markedly from 10⁶ to 10⁹ M^-1s^-1. Transient absorption spectroscopy and photon upconversion measurements confirmed the triplet sensitization of aromatic molecules by NMCs regardless of the quenching degree by O₂, establishing that the PL of NMCs under investigation originates from the excited triplet state (i.e., phosphorescence). The results herein provide an essential indicator for correctly determining whether the PL of an NMC is fluorescent or phosphorescent.

The distinct O2 quenching mechanism between fluorescence and phosphorescence for dyes adsorbed on silica gel

We herein aim to probe the emission quenched by O2 on silica gel. Our special focus is on the O2 quenching of the fluorescence of a series of organic D-π-A phosphonium compounds 1-3. The results show that the O2 quenching rate constants for the fluorescence of 1-3 are on the order of 1010 M-1 s-1, which are nearly on the same order as those measured for 1-3 and common organic compounds in solution. In yet another approach, the study of O2 quenching of phosphorescence in the solid phase indicates that the O2 quenching rate constant for the triplet state, i.e., , is smaller than by two orders of magnitude. Detailed investigation indicates that this distinction stems from the intrinsic O2 quenching rate constants for the singlet and triplet states subsequent to the formation of collisional complexes. In the absence of the solvent cage effect, is greatly influenced by the formation energy of the O2-dye CT complex, whereas in the solid phase is a nearly diffusion-controlled rate. Due to the larger distinction between and in the solid phase, O2 quenching of fluorescence is efficient for dyes in the solid phase. This leads to a feasible application of sensing O2 with regular fluorescent dyes adsorbed on porous solid substrates.

Partial charge transfer contribution to the solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by substituted ruthenium(II) bipyridyl complexes in aqueous media

The efficiency of singlet oxygen photosensitized by some ruthenium(ii) bipyridyl complex ions in aqueous media is reported in this study. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope on the lifetime of (3)MLCT excited states of these complexes is studied in H2O and D2O. The deuterium isotope effect was discussed in terms of the vibronic coupling to the solvent in addition to the charge transfer to the solvent mechanism due to their dependence on the oxidation potential of the sensitizer. Quenching rate constants, kq, for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen were found to be in the range of (2.08-3.84) × 10(9) M(-1) s(-1) in H2O and (1.69-3.48) × 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Δg), production as a result of the (3)MLCT quenching by oxygen, f, is reported in D2O and found to be in the range 0.25-0.56. It has been found that the lifetime of the excited state is longer in D2O, τ, than in H2O, τ, which was related to partial charge transfer to the solvent in addition to the vibronic coupling mechanism. Mechanisms by which the excited states of these ruthenium complexes are quenched by molecular oxygen that shows the competition between charge transfer, non-charge transfer deactivation channels or energy transfer assisted charge transfer deactivation mechanisms are reported.

From elementary reactions to chemical relevance in the photodynamic therapy of cancer

Theories of radiationless conversions and of chemical processes were employed to design better photosensitizers for photodynamic therapy (PDT). In addition to photostability and intense absorption in the near infrared, these photosensitizers were required to generate high yields of long-lived triplet states that could efficiently transfer their energy, or an electron, to molecular oxygen. The guidance provided by the theories was combined with the ability to synthesize large quantities of pure photosensitizers and with the biological screening of graded hydrophilicities/lipophilicities. The theoretical prediction that halogenated sulfonamide tetraphenylbacteriochlorins could satisfy all the criteria for ideal PDT photosensitizers was verified experimentally.

Mechanisms of singlet-oxygen and superoxide-ion generation by porphyrins and bacteriochlorins and their implications in photodynamic therapy

New halogenated and sulfonated bacteriochlorins and their analogous porphyrins are employed as photosensitizers of singlet oxygen and the superoxide ion. The mechanisms of energy and electron transfer are clarified and the rates are measured. The intermediacy of a charge-transfer (CT) complex is proved for bacteriochlorins, but excluded for porphyrins. The energies of the intermediates and the rates of their interconversions are measured, and are used to obtain the efficiencies of all the processes. The mechanism of formation of the hydroxyl radical in the presence of bacteriochlorins is proposed to involve a photocatalytic step. The usefulness of these photosensitizers in the photodynamic therapy (PDT) of cancer is assessed, and the following recommendations are given for the design of more effective PDT protocols employing such photosensitizers: 1) light doses should be given over a more extended period of time when the photosensitizers form CT complexes with molecular oxygen, and 2) Fe(2+) may improve the efficiency of such photosensitizers if co-located in the same cell organelle assisting with an in vivo Fenton reaction.

Photophysics of squaraine dyes: role of charge-transfer in singlet oxygen production and removal

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region approximately 600-720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine-oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.

Mechanism of quenching by oxygen of the excited states of ruthenium(ii) complexes in aqueous media. Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by [Ru(diimine)(CN)4]2−complex ions

In this study we report on the photophysical properties of some [RuL(CN)4](2-) complex ions where L = 2,2'-bipyridine (bpy), 5,5'-dimethyl-2,2'-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2':6',2'''-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((3)MLCT) of these complexes. Quenching of the (3)MLCT states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (3)MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M(-1) s(-1) in H2O and (3.38 to 5.69) x 10(9) M(-1) s(-1) in D2O. The efficiency of singlet oxygen, O2((1)Delta(g)), production as a result of the (3)MLCT quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M(-1) s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T1(3)Sigma) and (3)(T1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O2((1)Delta(g)), and O2((3)Sigmag-). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of (3)MLCT states by oxygen.

Photosensitized generation of singlet oxygen from rhenium(i) and iridium(iii) complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of iridium(III) and rhenium(I) complexes. The nature of the lowest excited state of the complexes under investigation is either metal-to-ligand charge transfer ((3)MLCT) or a ligand centred ((3)LC) state. Rate constants, k(q), for quenching of the lowest excited states by molecular oxygen are in the range 1.5 x 10(8) to 1.4 x 10(10) M(-1) s(-1). Efficiency of singlet oxygen production, f(Delta)(T), following oxygen quenching of the lowest excited states of these complexes, are in the range of 0.27-1.00. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition between a non-charge transfer (nCT) and a CT deactivation channel. The balance between CT and nCT deactivation channels, which is described by the relative contribution p(CT) of CT induced deactivation, is discussed. The kinetic model is found to be successfully applied in the case of quenching of the excited triplet states of coordination compounds by oxygen in acetonitrile, as was proposed for the quenching of pi-pi* triplet states by oxygen.

The Effect of Solvent Polarity on the Balance between Charge Transfer and Non-Charge Transfer Pathways in the Sensitization of Singlet Oxygen by ππ* Triplet States

A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.

Quantitative Determination of 1Σg+ and 1Δg Singlet Oxygen in Solvents of Very Different Polarity. General Energy Gap Law for Rate Constants of Electronic Energy Transfer to and from O2 in the Absence of Charge Transfer Interactions

The quenching of excited triplet states of sufficient energy by O2 leads to O2(1sigma(g)+) and O2(1delta(g)) singlet oxygen and O2(3sigma(g)-) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O2 the rate constants of formation of the three different O2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O2(1sigma(g)+) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O2(1sigma(g)+), O2(1delta(g)), and O2(3sigma(g)-) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O2(1delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O2.

Photosensitized Generation of Singlet Oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.

Synthesis and Characterization of New Fluorene-Based Singlet Oxygen Sensitizers

The synthesis, photophysical characterization, and determination of singlet oxygen quantum yields (Phi(Delta)) for a class of fluorene derivatives with potential application in two-photon photodynamic therapy (PDT) is reported. It has been demonstrated that these compounds possess the ability to generate singlet oxygen (1O2) upon excitation. A photochemical method, using 1,3-diphenylisobenzofuran (DPBF) as 1O2 chemical quencher, was employed to determine the singlet oxygen quantum yields (Phi(Delta)) of the fluorene-based photosensitizers in ethanol. Phi(Delta) values ranged from 0.35 to 0.75. These derivatives may have potential application as two-photon photosensitizers when pumped via two-photon excitation in the near-IR spectral region.

The balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by ππ* triplet states

A charge transfer (CT) channel and a non-CT deactivation channel, both leading to formation of O(2)((1)Sigma (g)(+)), O(2)((1) Delta(g)) and O(2)((3)Sigma(g)(-)), compete in the quenching of triplet states by O(2). Recent studies by our group demonstrated that these channels are described by rather simple and general quantitative relations. In the present paper we use the detailed kinetic data on the quenching by O(2) of pi pi* triplet sensitizers of three homologous aromatic series in CCl(4) to derive a parameter, which describes the balance between CT and non-CT deactivation. This quantity, p(CT), is the relative contribution of CT mediated deactivation and is easily calculated for a sensitizer of known triplet energy from its quenching rate constant. The parameter p(CT) quantitatively describes the balance between both deactivation channels without requiring any knowledge of oxidation potentials. It is shown how the variation of p(CT) influences the efficiencies and the rate constants of O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)) and O(2)((3)Sigma(g)(-)) formation in the quenching process.

Polyoxometalate sensitization in mechanistic studies of photochemical reactions: the decatungstate anion as a reference sensitizer for photoinduced free radical oxygenations of organic compounds

The photosensitized oxygenation of organic molecules plays a key role in numerous processes of biological and industrial significance, such as, for example, photodynamic action and photodegradation of polymers. These reactions proceed either via quenching by the substrate of photophysically generated singlet oxygen, O2(1deltag), or via addition of ground state oxygen to photochemically generated radicals derived from the substrate, or via both pathways. The evaluation of the contributions of both mechanisms to the overall process requires reference sensitizers that exclusively induce one of the corresponding reactions. Some compounds are known to produce singlet oxygen with unit efficiency, but no references to sensitizers producing free radicals but no singlet oxygen have been found so far. In this work, we propose to use the decatungstate anion, W10O32(4-), as a first reference sensitizer for free radical oxygenations of organic molecules. A combination of time-resolved and steady-state studies has been performed to compare the photo-oxygenation of simple reference compounds, including 2-methyl-2-pentene and 2,3-dimethylbutene, by W10O32(4-) and by classical O2(1deltag) sensitizers, such as methylene blue and ruthenium complexes. It is demonstrated that W10O32(4-) sensitized oxygenation of organic compounds occurs exclusively by a free radical pathway, which differs clearly from both Type I and Type II oxygenations. Comparison with Type II reactions shows that: (i) in spite of their weaker reactivity, singlet oxygen mediated reactions are associated with larger photo-oxygenation yields than W10O32(4-) induced processes, due to the longer lifetime of the reactive species; and (ii) reaction of alkenes with both singlet oxygen and decatungstate features charge transfer interactions, whose magnitude is larger in the case of O2(1deltag).

Sensitization of singlet oxygen via encounter complexes and via exciplexes of pipi* triplet excited sensitizers and oxygen

Both excited singlet states, 1sigma(g)+ and 1delta(g), and the triplet ground state, 3sigma(g)-, of molecular oxygen are competitively formed during the quenching by O2 of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT(1sigma), kT(1delta), and kT(3sigma) as well as the T1 state energies E(T) and the oxidation potentials E(ox), have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT(1sigma), kT(1delta), and kT(3sigma) are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1sigma(g)-), O2(1delta(g)), and O2(3sigma(g)-). One proceeds via internal conversion (IC) of excited 1,3(T1 x 3sigma) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of 1,3(T1 x 3sigma) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1sigma(g)+), O2(1delta(g)). and O2(3sigma(g)-) depend on E(T) and E(ox). The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy deltaG(CET) for complete electron transfer from T1 excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on E(T) and E(ox). Particular emphasis is laid on the question of whether these relations could generally be valid for pipi* triplet sensitizers.