IS SINGLET OXYGEN FORMATION DOMINANT IN TRIPLET QUENCHING PROCESSES? MEASUREMENT OF THE QUANTUM YIELD OF SINGLET OXYGEN FORMATION BY THE TIME-RESOLVED THERMAL LENS METHOD

The quantum yields of singlet oxygen formation (Ø_Δ) by the quenching of triplet states of organic sensitizers are measured at various concentrations of the sensitizers by using the time-resolved thermal lens method. Above a certain concentration, Ø_Δ is independent of the sensitizer concentration. Below the threshold, Ø_Δ gradually decreases as the concentration of the sensitizer decreases. The extrapolation of Ø_Δ to zero concentration indicates that singlet oxygen formation is not necessarily dominant in the quenching process even for the ³ππ* state in benzene.     

Solute re-encounter probabilities from dissociative oxciplex relaxation

Measurements of the quantum yield of self-sensitized 1,3-diphenylisobenzofuran peroxidation as a function of dissolved oxygen of added azulene concentrations indicate that oxygen quenching of the sensitizer singlet state produces both triplet and ground states of the sensitizer in addition to O₂(¹Δ_g) and O₂(³Σ^?_g). This partitioning of quenching products may be due to the competitive relaxation of the initially formed complex (oxciplex), or to sequential relaxation of different oxciplex states in which symmetry and spin barriers are negotiated by complex dissociation and re-encounter of the solute pair in the required configuration. The latter interpretation provides re-encounter probabilities for the processes M(T₁) + O₂(¹Δ_g) → M(T₁) + O₂(³Σ^?_g) and M(T₁) + O₂(³Σ^?_g) → M(S_o) + O₂(¹Δ_g) from which estimated rate constants are compatible with theoretical expectation.     

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

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

The Employment of a Removable Chitosan‐Derivatized Polymeric Sensitizer in the Photooxidation of Polyhydroxylated Water‐Pollutants

The known O₂(¹?_g)‐sensitizer system Chitosan bounded Rose Bengal (CH‐RB), with Rose Bengal (RB) immobilized by irreversible covalent bonding to the polymer Chitosan (CH), soluble in aquous acidic medium, was employed in the photodegradation of three tri‐hydroxy benzene water‐contaminants (THBs). The system sensitizes the O₂(¹?_g)‐mediated photodegradation of THBs by a process kinetically favored, as compared to that employing free RB dissolved in the same solvent. Additionally the free xanthene dye, degradable by O₂(¹?_g) through self‐sensitization upon prolonged light‐exposure, is considerably protected when bonded to CH‐polymer. The polymeric sensitizer, totally insoluble in neutral medium, can be removed from the solution after the photodegradative cycle by precipitation through a simple pH change. This fact constitutes an interesting aspect in the context of photoremediation of confined polluted waters. In other words, the sensitizing system could be useful for avoiding to dissolve dyestuffs in the polluted waters, in order to act as conventional sunlight‐absorbing dye‐sensitizers. In parallel the interaction CH ‐ O₂(¹?_g) in acidic solution was evaluated. The polymer quenches the oxidative species with a rate constant 2.4 × 10⁸ M^?1 s^?1 being the process mostly attributable to a physical interaction. This fact promotes the photoprotection of the bonded dye in the CH‐RB polymer.     

PHOTOPHYSICAL AND PHOTOSENSITIZING PROPERTIES OF BENZOPORPHYRIN DERIVATIVE MONOACID RING A (BPD-MA)*

The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O₂(¹Δ_g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (λ≈ 688 nm, ?≈ 33 000 M^?1 cm^?1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, E_s= 42.1 kcal mol^?1, lifetime, Φ_f= 5.2 ns and fluorescence quantum yield, Φ_f= 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, τ_f >. 25 ns, an energy level, E_T= 26.9 kcal mol^?1 and the molar absorption coefficient is ?_T= 26 650 M^?1 cm^?1 at 720 nm. A dramatic effect of oxygen on the fluorescence (φ_f) and intersystem crossing (φ_T) quantum yields has been observed. The BPD-MA presents rather high triplet (φ_T= 0.68 under N₂-saturated conditions) and singlet oxygen (φ_Δ= 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (φ_Pb) of which was found to be on the order of 5 × 10^?5 in organic solvents.     

Thermal-lensing and phosphorescence studies of the quantum yield and lifetime of singlet molecular oxygen (1 delta g) sensitized by hematoporphyrin and related porphyrins in deuterated and non-deuterated ethanols

Time-resolved thermal-lensing was used to measure the absolute quantum yield (φ_Δ) of singlet molecular oxygen, O₂(¹Δ_g), produced by hematoporphyrin photosensitization in ethanol. Deuteration of the solvent did not affect the value of φ_Δ. The value of φΔ= 0.53 was then used as reference to evaluate φΔ in O₂ (¹Δ_g) phosphorescence experiments with the related porphyrins, monohydroxyethylvinyl deuteroporphyrin and dihematoporphyrin ether. The φ_Δ values, in conjunction with the respective quantum yields of intersystem crossing (measured using a nanosecond laser flash photolysis technique) served to evaluate efficiencies, S_Δ, of O₂ (¹Δ_g) production from the porphyrin triplet states. The lifetime T_Δ in monodeuterated ethanol was measured as 29 ± 3 μs and 30 ± 1 (xs by time-resolved thermal lensing and phosphorescence detection, respectively. T_Δ in ethanol and fully deuterated ethanol were in good agreement with values reported in the literature.     

An Iridium Complex as an AIE-active Photosensitizer for Image-guided Photodynamic Therapy

Image-guided photodynamic therapy (PDT) has received growing attention due to its non-invasiveness and precise controllability. However, the PDT efficiency of most photosensitizers are decreased in living system due to the aggregation-caused singlet oxygen (¹O₂) generation decreasing. Herein, we present an Iridium (III) pyridylpyrrole complex (Ir-1) featuring of aggregation-induced emission (AIE) and ¹O₂ generation characteristics for image-guided PDT of cancer. Ir-1 aqueous solution exhibits bright red phosphorescence peaked at 630 nm, large Stokes shift of 227 nm, and good ¹O₂ generation ability. The ¹O₂ generating rate of Ir-1 in EtOH/water mixture solution is 2.3 times higher than that of Rose Bengal. In vitro experimental results revealed that Ir-1 has better biocompatibility and higher phototoxicity compared with clinically used photosensitizers (Rose Bengal and Ce6), suggesting that Ir-1 can serve as a photosensitizer for image-guided PDT of cancer.     

1H-Phenalen-1-one: Photophysical Properties and Singlet-Oxygen Production

The efficiency of aromatic ketones as singlet-oxygen (¹O₂(¹Δ_g)) sensitizers can vary considerably with the electronic configuration of their lowest triplet state and the solvent used. Near-infrared measurements of tie luminescence of singlet oxygen have shown that the quantum yield of singlet-oxygen production (Φ_Δ) by 1H-phenalen-1-one ( 1 ) is close to unity in both polar (Φ_Δ = 0.97±0.03 in methanol) and non-polar solvents (Φ_Δ = 0.93±0.04 in benzene). Analysis of the absorption spectra of the ground state and phosphorescence measurements show that the lowest singlet and triplet states have dominant π, π* electronic configurations. The quantum yield of intersystem crossing (Φ_ISC) of 1 , determined by laser flash photolysis (partial-saturation method), is equal to unity. In comparison with other aromatic ketones, these parameters are important for the discussion of the surprisingly high Φ_ISC of 1 and the efficient energy transfer from its triplet state to molecular oxygen. The 1H-phenalen-1-one ( 1 ), being one of the most efficient singlet-oxygen sensitizers in both polar and non-polar media, could be used as a reference sensitizer, in particular in the area of relatively high energies of excitation.     

Monocyclometalated (C N) Gold(III) Metallacycles: Tunable Emission and Singlet Oxygen (1O2) Generation Properties

The synthesis, characterization and photoluminescent properties of four cyclometalated (C N)-type gold(III) complexes bearing a bidentate diacetylide ligand, tolan-2,2’-diacetylide (tda), are reported. The complexes exhibit highly tunable excited state properties and show photoluminescence (PL) across the entire visible spectrum from sky-blue (λ_PL=493 nm) to red (λ_PL=675 nm) with absolute PL quantum yields (PLQY) of up to 75 % in solution, the highest PLQY found for any monocyclometalated Au(III) complex in solution. As a consequence of the use of the strongly rigidifying diacetylide bidentate ligand, a significant increase in the excited state lifetimes (τ₀=16–258 μs) was found in solution and in thin films. The complexes showed remarkable singlet oxygen generation in aerated solution with absolute singlet oxygen quantum yield (ϕ^1Δ) values reaching up to 7.5×10⁻⁵ and singlet oxygen lifetimes (τ₀^1Δ) in the range of 66–95 μs. Furthermore, the radiative and non-radiative rates of singlet oxygen were determined using the ϕ^1Δ and τ₀^1Δ values and correlations are drawn between the formation of singlet oxygen and its interaction with cyclometalated (C N) gold(III) complexes.     

Rose Bengal‐Sensitized Photooxidation of the Dipeptides l‐Tryptophyl‐l‐Phenylalanine,l‐Tryptophyl‐l‐Tyrosine and l‐Tryptophyl‐l‐Tryptophan: Kinetics,Mechanism and Photoproducts¶

The Rose Bengal‐sensitized photooxidations of the dipeptides l ‐tryptophyl‐l ‐phenylalanine (Trp‐Phe), l ‐tryptophyl‐l ‐tyrosine (Trp‐Tyr) and l ‐tryptophyl‐l ‐tryptophan (Trp‐Trp) have been studied in pH 7 water solution using static photolysis and time‐resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O₂(¹Δ_g)) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di‐ and polypeptides of photooxidizable amino acids. The ratio between chemical (k_r) and overall (k_t) rate constants for the interaction O₂(¹Δ_g)‐dipeptide indicates that Trp‐Phe and Trp‐Trp are good candidates to suffer photodynamic action, with k_rlk_t values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp‐Tyr, a lower k_rlk_t value (0.18) has been found, likely as a result of the high component of physical deactivation of O₂(¹Δ_g) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O₂(¹Δ_g) attack is the Trp group and suggests a much lower accumulation of kynurenine‐type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a‐oxidized hydrogenated pyrrolo[2,3‐b]indoles.     

Room-Temperature Near-Infrared Metastable Species Measured by Phase-Sensitive Lock-in and Fourier Transform Techniques

An ultrasensitive detecting system coupled with either a lock-in or Fourier transform technique has been used to detect near-infrared (1000-2500 nm) metastable species in room temperature solutions. These species include O₂(¹Δ_g), O₂(¹σ⁺_g), the solvent induced satellite peak of molecular singlet oxygen (¹Δ_g) emission, and the triplet state of bis(triisobutylsiloxy)silicon-2,-3-naphthalocyanine (SilNC). Using the O₂(¹Δ_g) emission in benzene as a standard, the quantum yield and radiative decay rate of SiINC triplet state have been determined. Depending on types of spectral acquisition, special techniques such as phase-distinguishing and step scan capabilities were utilized. Their advantages and disadvantages are discussed.     

THE PHOTO-OXIDATION OF N, N-DIETHYLHYDROXYLAMINE BY ROSE BENGAL IN ACETONITRILE AND WATER

The Rose Bengal photosensitized oxidation of N,N-diethylhydroxylamine has been investigated in water and acetonitrile using the techniques of oxygen uptake, singlet oxygen phosphorescence and electron spin resonance. In both solvents H₂O₂ is the major oxidation product and diethylnitroxide is an intermediate. In water, superoxide dismutase decreases oxygen uptake suggesting involvement of superoxide anions in the oxidation process. Results indicate that in water the photo-oxidation proceeds mainly by a Type I(electron transfer) mechanism, while in acetonitrile a Type II(energy transfer) mechanism has been confirmed (Encinas et al., 1987, J. Chem. Soc. Perkin Trans. II,1125–1127).     

Flow injection determination of hydrogen peroxide by means of a solid-state-peroxyoxalate chemiluminescence reactor

A solid-state reactor for detection of hydrogen peroxide in aqueous samples by peroxyoxalate chemiluminescence is described. Bis(2,4,6-trichlorophenyl)oxalate in solid form is packed into a bed reactor, which eliminates mixing problems and facilitates the instrumental development. Perylene is added as a sensitizer to a water/acetonitrile (20:80) carrier stream into which the samples (200–600 μl) are injected. Detection limits of 6 × 10^?9 M H₂O₂ (0.2 μg l^?1) are obtained with both a commercial and a home-made luminescence detector. Calibration graphs are linear up to 10^?5 M. The r.s.d. for 2 × 10^?7 M (6.7 μg^?1) hydrogen peroxide (n = 10) is 2.8%. Sample throughput is ca. 120 h^?1.     

THE EFFECTS OF PORPHYRIN STRUCTURE AND AGGREGATION STATE ON PHOTOSENSITIZED PROCESSES IN AQUEOUS AND MICELLAR MEDIA

The efficiency of several porphyrins at 10 μM and 83 μM as sensitizers of the photooxidation of 0.1 mM tryptophan and histidine via a singlet oxygen-mechanism was studied in pH 7.4-buffered aqueous solutions and in aqueous dispersions of Triton X-100 micelles. The porphyrins were either solubilized in the bulk aqueous medium or associated with the micellar phase, whereas the amino acids were always located in the aqueous phase. With those porphyrins, such as uroporphyrin I, meso-tetra (4-sulfonatophenyl)porphine, meso-tetra(4-carboxyphenyl)porphine and meso-tetra)N,N,N-trimethylanilinium)porphine, which are > 98% monomeric in both media, the efficiency of histidine photooxidation was independent of the site of O₂(¹Δ_g) generation, as shown by the closely similar values for the photooxidation rate constant and oxygen-consumption quantum yield in the presence and absence of Triton micelles; the same indications were provided by photokinetic experiments with tryptophan. Actually, laser flash photolysis studies showed that the micelle-incorporation of the above mentioned porphyrins brought about only minor changes in their photophysical properties, including the relative yield of O₂(¹Δ_g) generation. On the other hand, hematoporphyrin IX, its Zn²⁺-complex, and coproporphyrin III are largely aggregated in homogeneous aqueous solution; their incorporation into Triton micelles caused an increase of the triplet quantum yield and an enhancement of the oxygen-consumption quantum yield and photooxidation rate constant for both histidine and tryptophan. The lower photosensitizing efficiency of aggregated porphyrin species in comparison with the corresponding monomeric porphyrin was confirmed by measuring the initial rate and quantum yield of oxygen consumption upon irradiation of 1 mM histidine and tryptophan in the presence of different hematoporphyrin concentrations within the 0.3-100μM range.     

Fluoroquinolone Antimicrobials: Singlet Oxygen, Superoxide and Phototoxicity

The fluoroquinolone antibacterial agents possess photo-sensitizing properties that lead to phototoxic responses in both human and animal subjects. The phototoxicity order reported in humans is: fleroxacin > lomefloxacin, pefloxacin > ciprofloxacin ? enoxacin, norfloxacin and ofloxacin. Studies both in vivo and in vitro have related this phototoxicity to the generation of reactive oxygen species including hydrogen peroxide and the hydroxyl radical. We determined the quantum yields of singlet oxygen generation (φ_Δ,) by detection of the singlet oxygen (¹O₂) luminescence at 1270 tun for several fluoroquinolones, naphthyridines and other structurally related compounds. All the fluoroquinolones examined have low φ_Δ values ranging from 0.06 to 0.09 in phosphate buffer at pD 7.5. We also determined the ¹O₂ quenching constants for these compounds and their values were on the order of 10⁶M^?1 s¹, except for lomefloxacin whose rate constant was 1.8 × 10⁷M^?1 s^?1. The φ_Δ values were significantly decreased in a solvent of lower polarity such as methanol (0.007 ≤φ_Δ≤ 0.02). The production of ¹O₂ by these antibiotics did not correlate with the order reported for their phototoxicity. We also measured the photogeneration (λ > 300 nm) of superoxide by these antibacterials in dimethylsulfoxide using electron paramagnetic resonance and the spin trap 5,5-dimethyl-l-pyrroiine N-oxide. Although there is not a one-to-one correspondence between the relative rates of superoxide generation and the phototoxicity ranking of the fluoroquinolones, the more phototoxic compounds tended to produce superoxide at a faster rate. Nevertheless, the magnitudes of the observed differences do not appear sufficient to explain the range of fluoroquinolone phototoxicity potencies in human and animal subjects in general and the high activity of fleroxacin and lomefloxacin in particular. For these latter drugs the photoinduced loss of the F₈ atom as fluoride and the concomitant generation of a highly reactive carbene at C-8 provide a more plausible mechanism for their potent phototoxic and photocarcinogenic properties.     

Uric Acid: A Less‐than‐Perfect Probe for Singlet Oxygen

Uric acid and/or its monoanion has long been used as chemical‐trapping agents to demonstrate the presence of singlet oxygen, O₂(a¹Δ_g), in aqueous systems. “Oxidative bleaching” of uric acid, generally monitored through changes in the uric acid absorption spectrum, is often used in support of claims for the intermediacy of O₂(a¹Δ_g). The bleaching of uric acid has also been used to quantify photosensitized O₂(a¹Δ_g) yields in selected systems. Unfortunately, experiments performed to these ends often neglect processes and phenomena that can influence the results obtained. For the present study, we experimentally examined the behavior of uric acid under a variety of conditions relevant to the photoinitiated creation and subsequent removal of O₂(a¹Δ_g). Although the oxidative destruction of uric acid can indeed be a useful tool in some cases, we conclude that caution must be exercised such as not to incorrectly interpret the data obtained.     

Time-Resolved Study of the Sensitized Formation and Decay of 1O2(1δg) in the Presence of Cyclodextrins*

The effect of β-cyclodextrin and β-hydroxypropyl-cyclodextrin on some properties of the aromatic ketone 1-H-phenalen-1-one and its sulfonate derivative 1-H-phen-alen-1-one-2-sulfonic acid was measured in aqueous solution. From the changes in the UV-visible range of the absorption spectra, the association equilibrium constants for the formation of inclusion complexes were determined. Because these ketones are very efficient sensitizers for the generation of singlet oxygen, time-resolved infrared luminescence was used to measure the lifetime of singlet oxygen in D₂O. Cyclodextrins are weak deactivating agents of singlet oxygen; the upper limits for the bimolecular deactivation constants are 2 times 10⁵M^?1s^?1 and 1 times 10⁵M^?1s^?1 for β-cyclodextrin and β-hydroxypropyl-cyclodextrin, respectively. Besides, they do not affect noticeably the extent of formation of singlet oxygen; this result is explained in terms of relocation of the sensitizer (exit from the cyclodextrin cavity) in the triplet excited state.     

Deactivation of singlet oxygen O2*(1Δg) on gold surfaces

The deactivation of singlet oxygen O₂*(¹Δ_g) on a gold surface and its change due to temperature variation and water vapour addition have been investigated in a flow system.     

Mechanism of the quenching of O2(1Δ g ) by erbium tetra(4-tert-butyl)phthalocyanine

The quenching of singlet molecular oxygen (¹O₂, ¹Δ _g ) by Er³⁺ tetra(4-tert-butyl)phthalocyanine in benzene was studied by the luminescence method. The quenching was demonstrated to be controlled by a physical-chemical mixed mechanism, with donor-acceptor interactions being the main contributor to the physical quenching of ¹O₂.     

PHOTOSENSITIZATION OF MELANIN: AN ELECTRON SPIN RESONANCE STUDY OF SENSITIZED RADICAL PRODUCTION AND OXYGEN CONSUMPTION

Abstract Rose Bengal is shown to photosensitize free-radical production and oxygen consumption in solutions of melanin from autooxidation of 3,4-dihydroxyphenylalanine (DOPA). In anaerobic solutions the sensitizer enhances rates of free-radical production by up to a factor of 20. In aerobic solutions, rates of oxygen consumption can be increased by a factor of several hundred. The reactions appear to involve the triplet state of the sensitizer. The effect of the sensitizer in increasing oxygen consumption is quenched by low concentrations of azide and enhanced by D₂O, suggesting that a singlet oxygen mechanism is involved.