Platinum(II)-Based Optical Probes for Imaging Quadruplex DNA Structures via Phosphorescence Lifetime Imaging Microscopy

G-quadruplex DNA is a non-canonical structure that forms in guanine-rich regions of the genome. There is increasing evidence showing that G-quadruplexes have important biological functions, and therefore molecular tools to visualise these structures are important. Herein we report on a series of new cyclometallated platinum(II) complexes which, upon binding to G-quadruplex DNA, display an increase in their phosphorescence, acting as switch-on probes. More importantly, upon binding to G-quadruplexes they display a selective and distinct lengthening of their emission lifetime. We show that this effect can be used to selectively visualise these structures in cells using Phosphorescence Lifetime Imaging Microscopy (PLIM).

Platinum(II)-Based Optical Probes for Imaging Quadruplex DNA Structures via Phosphorescence Lifetime Imaging Microscopy

G-quadruplex DNA is a non-canonical structure that forms in guanine-rich regions of the genome. There is increasing evidence showing that G-quadruplexes have important biological functions, and therefore molecular tools to visualise these structures are important. Herein we report on a series of new cyclometallated platinum(II) complexes which, upon binding to G-quadruplex DNA, display an increase in their phosphorescence, acting as switch-on probes. More importantly, upon binding to G-quadruplexes they display a selective and distinct lengthening of their emission lifetime. We show that this effect can be used to selectively visualise these structures in cells using Phosphorescence Lifetime Imaging Microscopy (PLIM).

Emerging properties from mechanical tethering within a post-synthetically functionalised catenane scaffold

Using a post-synthetic modification strategy we have prepared a series of functionalised [2]catenanes to study the impact of mechanically-enforced proximity on functional group properties, including emission, electrochemistry and photoreactivity.

Molecular rotors as reporters for viscosity of solutions of collagen like peptides

We have studied the suitability of using a molecular rotor-based steady-state fluorometric assay for evaluating changes in both the conformation and the viscosity of collagen-like peptide solutions. Our results indicate that a positive charge incorporated on the hydrophobic tail of the BODIPY molecular rotor favours the dye specificity as a reporter for viscosity of these solutions.

Visualising UV-A light-induced damage to plasma membranes of eye lens

An eye lens is constantly exposed to the solar UV radiation, which is considered the most important external source of age-related changes to eye lens constituents. The accumulation of modifications of proteins and lipids with age can eventually lead to the development of progressive lens opacifications, such as cataracts. Though the impact of solar UV radiation on the structure and function of proteins is actively studied, little is known about the effect of photodamage on plasma membranes of lens cells. In this work we exploit Fluorescence Lifetime Imaging Microscopy (FLIM), together with viscosity-sensitive fluorophores termed molecular rotors, to study the changes in viscosity of plasma membranes of porcine eye lens resulting from two different types of photodamage: Type I (electron transfer) and Type II (singlet oxygen) reactions. We demonstrate that these two types of photodamage result in clearly distinct changes in viscosity - a decrease in the case of Type I damage and an increase in the case of Type II processes. Finally, to simulate age-related changes that occur in vivo, we expose an intact eye lens to UV-A light under anaerobic conditions. The observed decrease in viscosity within plasma membranes is consistent with the ability of eye lens constituents to sensitize Type I photodamage under natural irradiation conditions. These changes are likely to alter the transport of metabolites and predispose the whole tissue to the development of pathological processes such as cataracts.

Kynurenic acid and its chromophoric core 4-hydroxyquinoline react with tryptophan via proton-coupled electron transfer, and with tyrosine via H-transfer

Kynurenic acid (KNA) and 4-hydroxyquinoline (4HQN) are photochemically active products of tryptophan catabolism that readily react with tryptophan (Trp) and tyrosine (Tyr) after optical excitation. Recently, transient absorption experiments have shown that at neutral pH Trp reacts with triplet KNA via proton-coupled electron transfer (PCET), and not via electron transfer (ET) as it was suggested before. PCET includes the stepwise transition of both electrons and protons from Trp to triplet KNA. In this work, we confirmed that PCET is the reaction mechanism by the alternative method of time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). Further studies by TR-CIDNP revealed hydrogen transfer as the mechanism of the reaction between triplet KNA and Tyr in neutral solutions and a transition of both PCET and H-transfer mechanisms to ET under acidic conditions. 4HQN, being the chromophoric core of KNA, exhibits different spectral and photophysical properties from KNA but employs the same mechanisms for the reactions of its triplet state with Trp and Tyr at neutral and acidic pH.

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part II. Amino acids within the protein globule of lysozyme

An acidosis, a decrease of pH within a living tissue, may alter yields of radical reactions if participating radicals undergo partial or complete protonation. One of photosensitizers found in the human eye lens, kynurenic acid (KNA^-), possesses pKa 5.5 for its radical form that is close to physiological pH 6.89 for a healthy lens. In this work we studied the influence of pH on mechanisms and products of photoinduced radical reactions between KNA^- and amino acids tryptophan (Trp) and tyrosine (Tyr) within a globule of model protein, Hen White Egg Lysozyme (HEWL). Our results show that the rate constant of back electron transfer from kynurenyl to HEWL^• radicals with the restoration of initial reagents - the major decay pathway for these radicals - does not change in the pH 3-7. The quantum yield of HEWL degradation is also pH independent, however a shift of pH from 7 to 5 completely changes the outcome of photoinduced damage to HEWL from intermolecular cross-linking to oxygenation. HPLC-MS analysis has shown that four of six Trp and all Tyr residues of HEWL are modified in different extents at all pH, but the lowering of pH from 7 to 5 significantly changes the direction of main photodamage from Trp⁶² to Trp¹⁰⁸ located at the entrance and bottom of enzymatic center, respectively. A decrease of intermolecular cross-links via Trp⁶² is followed by an increase in quantities of intramolecular cross-links Tyr²⁰-Tyr²³ and Tyr²³-Tyr⁵³. The obtained results point out the competence of cross-linking and oxygenation reactions for Trp and Tyr radicals within a protein globule and significant increase of oxygenation to the total damage of protein in the case of cross-linking deceleration by coulombic repulsion of positively charged protein globules.

Influence of pH on radical reactions between kynurenic acid and amino acids tryptophan and tyrosine. Part I. Amino acids in free state

In the human eye lens the endogenous chromophores of UV-A light (315-400 nm) are able to sensitize radical reactions leading to protein modifications during normal aging and the cataract progression. Kynurenic acid (KNA^-) is the most photochemically active dye of the human eye lens reported to date with pK_a(KNAH₂^•) 5.5 for its radical form. Cataract is thought to develop under oxidative stress which could be accompanied by acidosis, an acidification of the intracellular environment. Protonation of kynurenyl radicals at mildly acidic conditions may change the outcome of radical reactions leading to additional damage to proteins. In this work we investigated the influence of pH on the degradation of initial reagents and the formation of products in photoinduced radical reactions between KNA^- and amino acids tryptophan (Trp) and tyrosine (Tyr) in free states. Our results have shown that pH variation has minor influence on kinetics of reagent decay and accumulation of products in reactions between tyrosyl and kynurenic acid radicals. However in the case of Trp a two-fold decrease of the reagent degradation without visible changes in the composition of formed products was observed with pH decrease from 7 to 3. Time-resolved measurements have shown similar acidification-induced two-fold acceleration of decay of kynurenyl and tryptophanyl radicals via Back Electron Transfer (BET) with the restoration of initial reagents. Experiments with tryptophan derivatives with different pK_a values for their radical forms point out the protonation of tryptophanyl radical as the driving force for BET acceleration at low pH. Our results demonstrate that the protonation of kynurenyl radical does not change its reactivity towards amino acids radicals but the total yield of radical photodamage decreases with the protonation of tryptophanyl radicals. It could be expected that radical induced damage to proteins will depend on the pK_a of tryptophanyl radicals within a protein globule.

Cyclopropyl Substituents Transform the Viscosity-Sensitive BODIPY Molecular Rotor into a Temperature Sensor

A quantitative fluorescent probe that responds to changes in temperature is highly desirable for studies of biological environments, particularly in cellulo. Here, we report new cell-permeable fluorescence probes based on the BODIPY moiety that respond to environmental temperature. The new probes were developed on the basis of a well-established BODIPY-based viscosity probe by functionalization with cyclopropyl substituents at α and β positions of the BODIPY core. In contrast to the parent BODIPY fluorophore, α-cyclopropyl-substituted fluorophore displays temperature-dependent time-resolved fluorescence decays showing greatly diminished viscosity dependence, making it an attractive sensor to be used with fluorescence lifetime imaging microscopy (FLIM). We performed theoretical calculations that help rationalize the effect of the cyclopropyl substituents on the photophysical behavior of the new BODIPYs. In summary, we designed an attractive new quantitative FLIM-based temperature probe that can be used for temperature sensing in live cells.

UV-A induced damage to lysozyme via Type I photochemical reactions sensitized by kynurenic acid

In this work we studied the mechanisms of Type I photodamage to a model protein, hen egg white lysozyme (HEWL), sensitized by kynurenic acid (KNA) - one of the most efficient photosensitizers of the human eye lens present in trace amounts within tissue. The kynurenic acid radical, KNA^•-, formed in the quenching of triplet KNA by HEWL, can be readily oxidized by molecular oxygen with the formation of superoxide anion radical O₂^•-. This leads to two ways of damage to proteins: either via the direct reactions between KNA^•- and HEWL^• radicals (Type Ia) or via the reactions between superoxide anion O₂^•- and HEWL^• radicals (Type Ib). Our results demonstrate significant degradation of the protein during Type Ia photolysis with the formation of various oligomeric and oxygenated forms of HEWL and several deoxygenated products of KNA. Liquid chromatography-mass spectrometry analysis revealed the cross-linking of HEWL via tryptophan (Trp⁶²) and tyrosine (Tyr²³) residues and, for the first time, the covalent binding of KNA to protein via tryptophan (Trp⁶² and Trp¹²³) residues. It was found that Type Ib reactions lead to substantially smaller damage to HEWL; the degradation quantum yields (Φ_deg) of HEWL are 1.3 ± 0.3% and 0.12 ± 0.03% for Type Ia and Ib photolyses, respectively. Low Φ_deg values for both types of photolysis indicate the Back Electron Transfer (BET) with the restoration of initial reagents as the main radical decay path with significantly higher BET efficiency in the case of Type Ib reactions. Therefore, in essentially oxygen-free tissues like the eye lens, the direct radical reactions via Type Ia mechanism could induce significantly larger damage to proteins, leading to their cross-linking and oxidation. The accumulation of these modifications can cause the development of various diseases, in particular, cataracts in the eye lens.

Competition of singlet and triplet recombination of radical pairs in photoreactions of carboxy benzophenones and aromatic amino acids

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and transient absorption (TA) were applied to reveal the branching ratio of the singlet and triplet recombination channels in the reaction of short-lived radicals of carboxy benzophenones and the aromatic amino acids histidine, tryptophan, and tyrosine in neutral aqueous solution. It was established that the share of triplet recombination increases with increasing number of carboxylic groups: no triplet recombination was found for 4-carboxy benzophenone, whereas ∼13% of radicals of 4,4'-dicarboxy benzophenone (DCBP) and ∼27% of radicals of 3,3',4,4'-tetracarboxy benzophenone (TCBP) react with histidine radicals from the triplet state of radical pairs. The main idea is that the protonated (π,π*) triplet state of TCBP or DCBP is populated via back electron transfer from the ketyl radical of TCBP or DCBP to the radical of the amino acid. The protonated triplet state of the ketone decays with the formation of a metastable hydroxylated product, which is detected by TA. Taking into account triplet recombination provides excellent coincidence between experimental data and the simulated CIDNP kinetics.

Determination of Hyperfine Coupling Constants of Fluorinated Diphenylacetylene Radical Anions by Magnetic Field-Affected Reaction Yield Spectroscopy

Magnetic field-affected reaction yield (MARY) spectroscopy is a spin chemistry technique for detecting short-lived radical ions. Having sensitivity to transient species with lifetimes as short as nanoseconds, MARY spectroscopy usually does not provide detailed information on their magnetic resonance parameters, except for simple systems with equivalent magnetic nuclei. In this work, the radical anions of two fluorinated diphenylacetylene derivatives with nonequivalent magnetic nuclei and unknown hyperfine coupling constants ( A_HF) were investigated by MARY spectroscopy. The MARY spectra were found to be resolved and have resonance lines in nonzero magnetic fields, which are determined by the A_HF values. Simple relationships between the positions of resonance MARY lines and the A_HF values were established from the analysis of the different Hamiltonian block contributions to the MARY spectrum. The obtained experimental A_HF values are in agreement with the results of quantum chemical calculations at the density functional theory level.

The role of S-bond in tenoxicam keto–enolic tautomerization

A non-covalent interaction between the sulphur atom of thiophenyl moiety and oxygen of the carbonyl group (S-bond) plays a crucial role in keto–enol tautomerization of tenoxicam leading to the crystallization of latter only in zwitterionic (ZWC) and not in β-keto–enolic (BKE) form.

Crystal packing control of a trifluoromethyl-substituted furan/phenylene co-oligomer

Furan/phenylene co-oligomer single crystals are considered as future materials for organic optoelectronics. Here, the effects of trifluoromethyl substituents on the crystallization, structure and optical properties of furan/phenylene co-oligomer 1,4-bis{5-[4-(trifluoromethyl)phenyl]furan-2-yl}benzene are studied systematically. The solution growth methods and physical vapor transport result in the formation of three polymorphs depending on the growth method and the solvent. Single-crystal X-ray analysis reveals the crystal structures to correspond to H-, J- or mixed aggregates. All obtained crystals exhibit high photoluminescence efficiency and have optical properties which strongly depend on the crystal packing. Variable-temperature X-ray powder diffraction analysis shows the thermal transition of two forms (H- and J-aggregates) into a third one (mixed aggregate). Terminal trifluoromethyl groups induce weak intermolecular interactions which control the crystal packing and optical properties of co-oligomer single crystals.

Photooxidation of herbicide amitrole in the presence of fulvic acid

Fulvic acid (Henan ChangSheng Corporation) photoinduced degradation of non-UVA-absorbing herbicide amitrole (3-amino-1,2,4-triazole, AMT) as a way for its removal from polluted water was investigated in details. It was shown that the main primary species generated by fulvic acid under UVA radiation, triplet state and hydrated electron, are not directly involved in the herbicide degradation. AMT decays in reactions with secondary intermediates, reactive oxygen species, formed in reactions of the primary ones with dissolved oxygen. Singlet oxygen is responsible for 80% of herbicide oxidation, and ^•OH and O₂^-• radicals-for the remaining 20% of AMT. It was found that quantum yield of AMT photodegradation (ϕ ^365nm) decreases linearly from 2.2 × 10^-3 to 1.2 × 10^-3 with the increase of fulvic acid concentration from 1.1 to 30 mg L^-1. On the contrary, the increase of AMT concentration from 0.8 to 25 mg L^-1 leads to practically linear growth of ϕ ^365nm value from 1.8 × 10^-4 to 4 × 10^-3. Thus, the fulvic acid exhibits a good potential as UVA photooxidizer of organic pollutants sensitive to the singlet oxygen (ϕ ^532nm(¹O₂) = 0.025 at pH 6.5).

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Ultrafast excited state decay of natural UV filters: from intermolecular hydrogen bonds to a conical intersection

An unsaturated bond in the side chain leads to the ultrafast decay of the excited statesviaa conical intersection independent of solvent properties.

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315-400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA^•- and NTrp^• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA^•- to NTrp^• with the formation of oxidized NTrp species and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA^•- radicals with the rate constant of (2.0 ± 0.2) × 10⁹M^-1s^-1, leading to the restoration of KNA and the formation of superoxide radical O₂^•-. The latter reacts with NTrp^• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

Visualising the membrane viscosity of porcine eye lens cells using molecular rotors

The plasma membranes of cells within the eye lens play an important role in metabolite transport within the avascular tissue of the lens, maintaining its transparency over the entire lifespan of an individual. Here we use viscosity-sensitive 'molecular rotors' to map the microscopic viscosity within these unusual cell membranes, establishing that they are characterised by an unprecedentedly high degree of lipid organisation.

Spin-state-correlated optical properties of copper(ii)–nitroxide based molecular magnets

Pronounced thermochromism of copper(ii)–nitroxide based molecular magnets is explained.

Estimation of the Fraction of Spin-Correlated Radical Ion Pairs in Irradiated Alkanes using Magnetosensitive Recombination Luminescence from Exciplexes Generated upon Recombination of a Probe Pair

We suggest a convenient probe exciplex system for studies in radiation spin chemistry based on a novel acceptor-substituted diphenylacetylene, 1-(phenylethynyl)-4-(trifluoromethyl)benzene that has a very short fluorescence lifetime (<200 ps) and low quantum yield (0.01) of intrinsic emission, provides efficient electron capture in alkanes and efficient exciplex formation upon recombination in pair with DMA radical cation, while exhibiting a shifted to red exciplex emission band as compared to the parent system DMA – diphenylacetylene. After chemical, luminescent, radiation and spin-chemical characterization of the new system we used the magnitude of magnetic field effect in its exciplex emission band for experimental estimation of the fraction of spin-correlated radical ion pairs under X-irradiation with upper energy cutoff 40 keV in a set of 11 alkanes. For linear and branched alkanes magnetic field effects and the corresponding fractions are approximately 19–20% and 0.28, while for cyclic alkanes they are lower at 16–17% and 0.22, respectively.

Estimation of the fluorescence lifetime for optically inaccessible exciplexes in nonpolar solutions under ionizing irradiation

X-irradiation of nonpolar solutions likely provides a possibility to create exciplexes for any donor-acceptor pair that would energetically and sterically allow this. Thorough study and characterization of X-radiation generated exciplexes usually cannot be carried out with conventional methods because of the complex and non-exponential formation and decay dynamics of these species. In this paper, we present a simple and universal experimental approach for the estimation of fluorescence lifetimes (τF) of X-radiation generated exciplexes. The suggested procedure is based on the comparison of quenching of the exciplex emission band and the emission band from a standard luminophore with a known excited state lifetime by dissolved oxygen. Using this approach we report the τF values for two systems with optically inaccessible exciplexes, diphenylacetylene-N,N-dimethylaniline (DMA) and p-terphenyl-DMA, and for two typical exciplex forming systems, naphthalene-DMA and anthracene-DMA. All the found τF values for the X-radiation generated exciplexes lie in the range of 50-70 ns. The accuracy of this approach was checked by time-resolved measurements under X- or near-UV irradiation for those pairs, whose properties make this feasible. The proposed method gives a possibility to avoid a complex numerical evaluation of the non-exponential kinetics of recombination luminescence, and can be used to estimate the characteristic τF values for luminophores and excited complexes formed under X-radiation.

Photochemical properties of UV Filter molecules of the human eye

PURPOSE

To compare the photochemical properties of UV filter molecules present in the human lens (kynurenine, KN; 3-hydroxykynurenine, 3OHKN; 3-hydroxykynurenine O-β-D-glucoside, 3OHKG; 4-(2-aminophenyl)-4-oxobutanoic acid, AHA; and glutathionyl-kynurenine, GSH-KN) with the use of the following parameters: excited singlet lifetime τ(S), fluorescence quantum yield Φ(fl), triplet quantum yield Φ(T), and photodecomposition quantum yield Φ(dec).

METHODS

The excited singlet lifetimes were measured with the use of fluorescence upconversion (time resolution, 210 fs) and pump-probe transient absorption (time resolution, 200 fs) methods. The fluorescence quantum yields were determined relative to an aqueous solution of quinine bisulfate. The triplet quantum yields were measured with the use of nanosecond laser flash photolysis. The photodecomposition quantum yields were determined by steady state photolysis followed by the high-performance liquid chromatography analysis.

RESULTS

The secondary UV filters--AHA and GSH-KN are better photosensitizers than the primary ones--KN, 3OHKN and 3OHKG: the singlet state lifetimes of the secondary UV filters are longer, and the quantum yields of fluorescence and triplet state formation are higher.

CONCLUSIONS

With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract.

Photoinduced electron transfer reactions: from the elucidation of old problems in bulk solutions towards the exploration of interfaces

The activities of our research group in the field of photoinduced electron transfer reactions are discussed and illustrated by several examples.

Competition between ultrafast relaxation and photoionization in excited prefluorescent states of tryptophan and indole

The quantum yield of photoionization of TrpH and IndH from the nonrelaxed prefluorescent state S* increases with the temperature decrease. This effect is attributed to the competition between temperature independent ionization and ultrafast thermal relaxation S* --> S1. The rate constant of the relaxation does not depend on the solvent and on the presence of the amino acid side chain: the temperature dependences of photoionization quantum yield, obtained for TrpH and IndH in different solvents, practically coincide. The activation energy for the relaxation rate constant Er approximately 4.5 kJ/mol probably corresponds to intramolecular process or to the formation of the vibronically excited transient complex between photoexcited molecule and solvent molecules.

Photochemistry of Kynurenine, a Tryptophan Metabolite: Properties of the Triplet State

Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine TKN. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is PhiT = 0.018 +/- 0.004. The dissociation constant of TKN, which is attributed to deprotonation of the anilino group, has a pKa value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for TKN by oxygen is kq = 2 x 10(9) M(-1) s(-1).