Triplet Quenchers Research Articles

Photophysics and photochemistry of poly (4′-ethoxyacrylophenone)

The photophysics and the long wave ( λ≥300 nm) photochemistry of poly (4′-ethoxyacrylophenone) were investigated. The quantum yield for fluorescence is low (10 −4 mol (einstein) −1), and the principal excited species is the carbonyl triplet. The quantum yield for phosphorescence is greater and the λ max value is 445 nm. The triplet lifetime at 77 K is 0.41 s, but 8.7 μs at 300 K. These observations are consistent with the formation of a low-lying triplet, which, in turn is the result of substitution of the aromatic ring with the electron donating ethoxy group. The principal photoprocess occurring in dilute solution (in CH 2Cl 2) of the polymer is a Norrish Type II decomposition, which results in random chain scission and a rapid decrease in the molecular weight. The relatively low quantum yield (6×10 −2 mol (einstein) −1) for the scission process reflects the low reactivity of a triplet (which has some π, π ∗ character) in abstraction reactions. In the case of PEAP, H-abstraction is a prerequisite for biradical formation, which, in turn leads to chain scission. Chain scission was subject to inhibition by triplet quenchers, such as naphthalene and cis-1,3-pentadiene, and the data conformed to Stern–Volmer kinetics. However, the quenching rate constants, k Q were lower in magnitude than the corresponding diffusion controlled values, and it is proposed that this is principally due to a steric effect, the actual encounter of the quencher with the carbonyl triplet being subject to adverse steric interactions associated with the proximity of the relatively bulky polymer chain.

Chemically induced dynamic electron polarization investigation of the triplet‐radical system in the solution of the triplet quencher

AbstractThe chemically induced dynamic electron polarization (CIDEP) of the triplet molecule/triplet quencher/2, 2, 6, 6‐tetramethyl‐1‐piperidinyloxyl (TEMPO) systems were measured using the high time‐resolved ESR spectrometer. The competition between the radical‐triplet pair mechanism (RTPM) and triplet mechanism (TM) or radical pair mechanism (KIM) polarization in the solution of the triplet quencher was investigated, and the relationships between reaction rate of the radical‐triplet pair and quenching rate of triplet was deduced.

Influence of quenchers on excited complexes upon triplet-triplet annihilation of tetrapyrrole molecules in liquid solutions

The influence of triplet quenchers on the kinetics of triplet-triplet annihilation (TTA) for chlorophylla and tetraphenylporphine was investigated. It was found that the rate constants for the quenching of triplet states by TTA increase with increasing the quencher concentration [Q]. The greatest values for the triplet deactivation parameter are proportional to [Q]. Experimental results are consistent with the rationalization of the triplet annihilation through the formation of complexes from the triplet molecules. The linear dependence of the degradation rate constant of the triplet-triplet complexes to the ground-state molecules on [Q]2 suggests that two quencher molecules are necessary for the quenching of one complex molecule. This means that two locally excited triplet states exist in the complex. It is likely the spin correlation time of the triplet states is longer than the lifetime of the complexes.

The photochemistry of 6‐phenanthridinecarbonitrile I. Product analysis

AbstractWhen 6‐phenanthridinecarbonitrile (3) is irradiated at 2537 Å in neutral 9:1 2‐propanol/water, three major products are formed. These are dimethyl‐(6‐phenanthridinyl)methanol (4), phenanthridine (5) and 6,6′‐biphenanthridine (6). When benzophenone is present in the reaction mixture, diphenyl‐(6‐phenanthridinyl)‐methanol is also formed. 6‐Phenanthridinyl radical which is common to the formation of all these products, is formed by a monophotonic process involving hydrogen atom abstraction from an alcohol molecule by an excited state of 3. Unlike what is generally found with other nitrogen‐heterocycles, the photochemistry of 3 appears to involve only a π,π* singlet state. The fluorescence of 3 is quenched with the triplet quencher cis/trans‐piperylene as a function of the concentration of the diene without the accompaniment of an exci‐plex emission.

Mechanism of formation of guanine as one of the major products in the 254 nm photolysis of guanine derivatives: concentration and pH effects.

The quantum yield of formation of guanine (Gua), one of the major products formed in the 254 nm steady-state photolysis of the following Gua derivatives (9-ethylguanine [9Et-Gua], deoxyguanosine [dGuo], guanosine [Guo], guanosine 5'-monophosphate [GMP]) was determined under different conditions. The formation yield increases in the following order: 9Et-Gua < dGuo approximately Guo < GMP. Electron scavengers or triplet quenchers were incorporated into the irradiated solutions with the purpose of reacting specifically with postulated or previously identified intermediates in the photolysis of Gua derivatives. A decrease in the yield of formation of Gua is observed with increasing electron scavenger concentration or with pH. These results suggest a major contribution from Gua-derivative electron adducts on the process of N(9)-R bond breakage. At an acidic pH a tautomer of the radical cation (E) of Guo is proposed as the precursor for Gua formation. The relative efficiency of the radical cation for initiating the release of free Gua depends on the pH of the solution, being less than 39% in neutral pH. Reactions from OH radicals may also result in base release as shown using N2O as additive. Finally, the formation of aggregates by the bases at concentrations used, plays an important role in the deactivation of the excited states and also in the probability of formation of the free base.

Triplet quenching and antioxidant effect of several carbon black grades in the photodegradation of LDPE doped with benzophenone as a photosensitiser

In a previous paper it was demonstrated for the first time, that carbon black pigments have the ability to operate as effective singlet and triplet quenchers. This work was undertaken directly in the polymer using the inherent long-lived luminescent species as triplet donor species. In order to ascertain the effectiveness of the quenching mechanism and its inter-relationship with carbon black parameters, a total of eight different carbon black pigments have been incorporated into low density polyethylene at different concentrations (0.1, 0.2, 0.4% w/w) and their effects determined on the quenching the triplet state of a known triplet photosensitiser, benzophenone (0.05% w/w). Using phosphorescence analysis at 77 K a large variability was observed in the benzophenone triplet lifetime quenching, the effect varying with the pigment type and the concentration of pigment. Photodegradation rates on the films were also performed in order to relate the quenching effect of the carbon black with films exposed to UVA light at 70°C. An unusual synergistic effect between the carbon black and the benzophenone sensitiser was observed compared with films containing only carbon black. The implications of carbon black pigments to operate as “super quenchers”, as well as antioxidants in this study and under practical concentration conditions is discussed. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. However, the presence of surface containing oxygen groups can be related to the synergistic effect reported, using experimental design statistical analysis.

Simultaneous photoconjugation of methylene blue and cis-Rh(phen)2Cl2+ to DNA via a synergistic effect.

Irradiation of the red-light absorbing dye, methylene blue (MB), in the presence of the metal complex, cis-Rh(phen)2Cl2+ (BISPHEN), leads to irreversible photobinding of both reagents to DNA. Evidence from absorption and emission spectroscopy indicates that the dye is strongly complexed to the DNA at the concentrations used in the experiments and that this complex is unaffected by the presence of BISPHEN. The level of covalent binding is proportional to the absorbed light dose, with the quantum efficiency for covalent binding of BISPHEN to the DNA with 633 nm light equal to 3.5 x 10(-4). Electrospray ionization mass spectrum of a mixture of DNA fragments created by enzymatic degradation of DNA isolated following irradiation indicates that purine adducts are formed with both BISPHEN and the dye. In addition, UV-Vis and high-performance liquid chromatography analyses of the irradiated MB/BISPHEN/DNA mixture and isolated adducts show extensive conversion of the dye and metal complex to the corresponding N-demethylated and aquated derivatives, respectively. Triplet quenchers for MB, for example oxygen and benzoquinone, inhibit both the photoconjugation and the photochemistry of BISPHEN. A mechanism for the synergistic interaction is proposed that involves photoconjugation of both partners to the DNA following oxidation and reduction via electron transfer between 1MB*/DNA and 3MB*/BISPHEN.

Behaviour of carbon black pigments as excited state quenchers in LDPE

A total of eight different carbon black pigments have been incorporated into low density polyethylene at 0.2% w/w concentration and their ability to operate as effective singlet and triplet quenchers examined using luminescence analysis. All pigments effectively reduced the fluorescence and phosphorescence emission intensities. A large variability was observed in the triplet lifetime quenching, the effect varying with the pigment type. Although not consistent throughout, there is some indication that low surface area carbon black pigments can be the most effective quenchers of active excited states. The implications of carbon black pigments to operate as “super quenchers” under practical concentration conditions is discussed.

Effects of a sulfur substituent on the behavior and generation mode of aroyloxyl radicals in the photolysis of diaroyl peroxides

Continuous light irradiation of bis( p-methylthiobenzoyl) peroxide ( p-MeSBPO) at 300 nm in acetonitrile gave only p-methylthiobenzoic acid as a product. On pulsed laser photolysis the absorption spectrum due to intermediate p-methylthiobenzoyloxyl radicals was observed in the region 400–800 nm, and the radical yield was significantly decreased in the presence of 1,3-pentadiene or oxygen as a triplet quencher. The results indicate that p-MeSBPO is decomposed from the excited triplet state and the radicals exclusively undergo hydrogen-atom abstraction from the solvent. The origin of the effect is discussed on the basis of experimental and theoretical results.

Photochemical behaviour of oxyfluorfen: a diphenyl-ether herbicide

The photochemical behaviour in different solvents of the herbicide oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene (CAS RN 42874–03–3)] was studied. Photochemical reactions were carried out by using a high pressure mercury arc and a solar simulator. Kinetic parameters and quantum yields were determined. Identification of the photoproducts was performed by GC-MS and the main compounds were confirmed by [ 1H] NMR. The photochemical reactions were also carried out in the presence of either a singlet or a triplet quencher, and in the presence of either a radical initiator or a radical inhibitor. Results indicate that the first excited singlet state can undergo both homolytic and heterolytic cleavage of the ethyl-oxygen bond in the side chain of oxyfluorfen. Moreover, the presence of reduction products in the reaction mixture is supposed to occur via a monoelectron transfer process with the formation of a transient exciplex during the reaction.

Singlet oxygen and dye-triplet-state quenching in solid-state dye lasers consisting of Pyrromethene 567–doped poly(methyl methacrylate)

Solid-state dye lasers based on poly(methyl methacrylate) (PMMA) doped with Pyrromethene 567 dye (P567) have been investigated. The preparation techniques employed provided high photostability and laser damage threshold for P567 in pure PMMA with 270,000 pulses emitted before the conversion efficiency fell to half its initial value for a pump fluence of 0.16 J cm(-2). When PMMA was modified with 1, 4-diazobicyclo [2, 2, 2] octane singlet oxygen quencher, the longevity increased to 550,000 pulses, corresponding to a normalized photostability of 270 GJ mol(-1). Modification of PMMA with a triplet quencher (perylene) yielded no improvement, but in ethanol solutions both additives enhanced photostability. It is possible that in PMMA, stabilization by means of triplet quenching that depends on dye diffusion is prevented but that stabilization by means of singlet oxygen quenching that depends on the faster oxygen diffusion rate will succeed.

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Laser excitation of 0.01 M solutions of 1-indanone (Ia), 1-tetralone (Ib), 1-benzosuberone (Ic), and their a,a -dimethyl derivatives IIa-c, respectively, in benzene, produced transients with maximum absorption at 425 nm, and lifetimes ranging from 62 ns (IIa) to 5.5ms (Ic). Quenching studies using well known triplet quenchers such as 1,3-cyclohexadiene and oxygen demonstrated the triplet nature of these transients. In the presence of hydrogen donors, such as 2-propanol, the triplet state decay of the ketones Ia-c leads to the formation of the corresponding ketyl radicals, i.e. IIIa-c, which show absorption spectra very similar to the parent ketone, with lmax at 430 nm and lifetime in excess of 20 ms. Steady state irradiations show that the a,a -dimethyl ketones IIa and IIc form ortho-alkyl benzaldehydes probably derived from an initial a-cleavage of the corresponding triplet excited states.

Mechanism of Photosolvolytic Rearrangement of p-Hydroxyphenacyl Esters: Evidence for Excited-State Intramolecular Proton Transfer as the Primary Photochemical Step

The photosolvolytic rearrangement of a variety of p-hydroxyphenacyl esters and related compounds 7−16 has been studied in solutions with up to 50% aqueous content, using product studies, triplet quenchers, and nanosecond laser flash photolysis. The p-hydroxyphenacyl moiety has recently been proposed as a new and efficient photoactivated protecting group in aqueous solution. Practical applications have been demonstrated, but much less is known about the mechanism of photoreaction. Our data support a novel mechanism in which the primary photochemical step from the singlet excited state is formal intramolecular proton transfer from the phenolic proton to the carbonyl oxygen of the distal ketone, mediated by solvent water, to generate the corresponding p-quinone methide phototautomer. This reactive intermediate (most likely in its excited state) subsequently expels the carboxylic acid with concerted rearrangement to a spiroketone intermediate, which subsequently leads to the final observed product, p-hydroxyphenylacetic acid. An alternative mechanism is deprotonation of the phenolic proton, loss of the carboxylate, and rearrangement to the spiroketone, all in one concerted primary photochemical step from S1.

Formation, excited state and electron transfer properties of the phenoxyl radical of 4,4′-thiodiphenol

The transient absorption bands ( λ max = 335, 500, 650–800 nm) formed on pulse radiolysis of N 2O-saturated basic aqueous solution of 4,4′-thiodiphenol (TDPH) are assigned to phenoxyl radical (TDP ⋅) formed on one-electron oxidation of phenoxide ion (TDP −) of TDPH. Specific one-electron oxidants (N ⋅ 3, I ⋅− 2, Br ⋅, CCl 3OO ⋅) also produced phenoxyl radicals with similar transient absorption spectrum. Phenoxyl radicals are able to undergo electron transfer reaction with N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD) with a bimolecular rate constant of 4.5 × 10 9 dm 3 mol −1 s −1. Pulse radiolysis of N 2-saturated TDPH in benzene showed absorption bands at 330, 500, 650–800 nm. The second-order decay of these bands was not affected by oxygen and anthracene, well-known triplet quenchers and the transient spectrum is assigned to phenoxyl radicals formed on fast decay of solute triplets with a G value of 0.29. Benzophenone triplet is able to undergo energy transfer with TDPH with a bimolecular rate constant of 6 × 10 9 dm 3 mol −1 s −1 and formed benzophenone ketyl radical ( λ = 545 nm) and phenoxyl radical (TDP ⋅, λ = 500 nm) on H ⋅ atom transfer from TDPH to benzophenone in an intermediate complex. The G value of phenoxyl radicals was determined to be 1.44. Picosecond laser flash photolysis of TDPH in acetonitrile failed to form any transient absorption in 450–900 nm region, supporting that only a small fraction of TDPH triplet decay to phenoxyls. Pulse radiolysis and picosecond laser flash photolysis studies indicate that the triplets of TDPH are highly unstable. The oxidation potential of TDP −/ TDP ⋅, H + couple is estimated to be between 0.265 and 1.03 V.

Time‐Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time‐resolved electron paramagnetic resonance (TREPR) at X‐band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin‐polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2‐bro‐mo‐2‐methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4‐hexadienoic acid). This channel is also found to be monophotonic.

Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.

Triplet Energy Transfer through the Walls of Hemicarcerands: Temperature Dependence and the Role of Internal Reorganization Energy

The dependence of the rate of electronic excitation transfer from a triplet donor (biacetyl) trapped inside a hemicarcerand cage to a range of triplet quenchers in free solution was studied as a function of the driving force and the internal reorganization energy of the acceptor, λacceptor. Acceptors with internal reorganization energies ranging from ∼0 to more than 1.1 eV were investigated. It was found that quenchers with nearly identical triplet energies can lead to transfer rates differing by almost 3 orders of magnitude as a result of large differences in their ineternal reorganization energies. The data were analyzed in terms of the semiclassical Marcus−Jortner theory. Variable-temperature measurements were performed in order to independently evaluate the activation energies and thus to unequivocally determine which acceptors belong to the “normal” and which to the “inverted” Marcus region. Four distinct groups of triplet acceptors emerged from the analysis: (a) rigid aromatics with small geometry changes and modest internal reorganization energies; (b) acyclic olefins exhibiting a large-amplitude internal relaxation and correspondingly large reorganization energies; (c) cyclic olefins with exceptionally large λυ values; and (d) molecular oxygen, O2, with negligibly small internal reorganization energy.

Photoreaction of Valerophenone in Aqueous Solution

Kinetics and products of the photoreaction of the phenyl ketone valerophenone were investigated as a function of temperature, pH, and wavelength in aqueous solution. Under these conditions (<10-4 M), the photoreactions are pseudo-first-order with respect to valerophenone concentration. Type II quantum yields for photoreaction were close to unity throughout the 290−330 nm spectral region and in the temperature range from 10 to 40 °C. The quantum yields for the photoproducts were 0.65 ± 0.04 for cleavage to acetophenone and propene and an overall yield of 0.32 ± 0.03 for cyclization to two cyclobutanols at 20 °C. A small amount of 1-phenylcyclopentanol (∼2% yield) also was formed. These photoreactions were quenchable by additions of the triplet quenchers sorbic alcohol or sorbic acid, and Stern−Volmer plots were linear up to at least 80% quenching of the photoreactions. On the basis of quenching studies with steady-state irradiations, the triplet lifetime of valerophenone at 20 °C was estimated to be 52 ns,...

Preparation and photochromism of sulfonated spiropyran-silica nanocomposites

Reversible and stable photochromic properties of sulfonated spiropyran (SP-SO 3 −) were achieved by trapping SP-SO 3 − in a silica matrix by the sol-gel route. The presence of organic anions was essential for the photochromic properties, indicating the importance of the coexistence of polar and non-polar regions. The half-life time ( t 1/2) of the photoinduced merocyanine (MC) trapped in the silica matrix in the presence of dodecylbenzenesulfonic acid (DBS) was larger than 30 days ( k<2.7 × 10 −7 s −1). The reversibility of SP-SO 3 − was improved by the addition of triplet quenchers such as benzophenone, naphthalene and anthracene.

Photochemical C−C Bond Cleavage of 1,2-Diarylcyclopropanes Bearing an Acetylphenyl Group. Generation and Observation of Triplet 1,3-Biradicals

The photochemical properties of 1,2-diarylcyclopropanes bearing an acetylphenyl group were studied by product analysis and laser flash photolysis. All cyclopropanes showed efficient cis−trans photoisomerization followed by inefficient isomerization to 1,3-diarylpropenes. All the products were unquenched by the addition of triplet quencher, 2-methyl-1,3-butadiene (ET ≃ 60 kcal mol-1), whereas molecular dioxygen gave oxygenated products. Triplet 1,3-biradicals generated from a short-lived acetophenone-like triplet (≲1 ns) were observed as intermediates in these reactions through nanosecond laser flash photolysis. Polar substituent effects on the lifetime of the 1,3-biradicals were small, except for a heavy atom effect in the case of Br. Spin−orbit coupling calculations on model 1,3-biradicals show a negligible effect on polar substituent and thus predict a negligible effect on the intersystem crossing rate. Conjugated biradicals in general now seem unlikely to show the “ionic character effect” on intersyste...