Triplet Quenchers Research Articles

First Demonstration of Selective Triplet Quenching in the Radical−Triplet Pair Mechanism (RTPM) of Chemically Induced Dynamic Electron Polarization (CIDEP)

Emissive chemically induced dynamic electron polarization (CIDEP) signals produced through the interaction between the lowest excited triplet phenothiazine (3PTH) and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) radical were detected by the time-resolved electron spin resonance (TRESR) method. The emissive spin polarization can be explained by the radical−triplet pair mechanism (RTPM). The fluorescence quenching and photoacoustic calorimetric experiments demonstrate directly that the emissive CIDEP signal is generated by the quartet spin states of the radical−triplet pair surviving through spin selective triplet quenching in the doublet spin states of the radical−triplet pair.

Characterization and reactivities of transient species generated by laser flash photolysis of N-hydroxypyridine-4-thione

The reaction intermediates produced by the nanosecond-laser-flash photolysis of N-hydroxypyridine-4-thione (4-HOPyT) have been studied in aqueous and nonaqueous solutions. The pyridyl-4-thiyl radical (4-PyS˙ at 420 nm) and ˙OH are formed by the homolytic N–OH bond cleavage of 4-HOPyT. The 720-nm band was assigned to the pyridyl-N-oxyl radical (4-SPyO˙) generated by the photodissociation of the O–H bond and by H abstraction from 4-HOPyT through ˙OH. The triplet state of 4-HOPyT [3(4-HOPyT)*], confirmed by energy transfer with triplet quenchers, is also produced concomitantly with the radical species. Diffusion controlled electron-transfer reactions occur between 3(4-HOPyT)* and donors (or acceptors), which may be caused by the relatively high triplet-state energy (ET1 = 60.1 kcal mol–1) of 3(4-HOPyT)*. In aqueous alkaline solution, the solvated electron (eaq–) and the one-electron semioxidized radical of 4-HOPyT (4-SPyO˙) are produced by photoionization of the anionic form of 4-HOPyT. Thus, it is possible to control the generation of ˙OH and eaq– by changing the pH of the solution.

Triplet and reversed triplet mechanism CIDEP studied by quenching experiments

Quenching experiments are shown to provide a convenient tool to check for the presence of triplet mechanism (TM) spin polarisation in time-resolved EPR spectra following laser flash photolysis. The effect of the triplet quenchers, trans-1,3-pentadiene, fumaronitrile, azo-tert-butane and azo-n-butane upon the spectra following laser photolysis of acetone/propan-2-ol and benzophenone/propan-2-ol photosystems show that no TM polarisation is present in the former system but emissive TM is present in the latter. Use of 2,2′-azo-bis[isobutryronitrile] produces an anomalous emissive polarisation upon quenching, which is tentatively attributed to a reversed TM in the triplet sensitised azo-compound.

A Steady-State and Picosecond Pump-Probe Investigation of the Photophysics of an Acyl and a Bis(acyl)phosphine Oxide

The photophysics (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) have been investigated by fluorescence, phosphorescence, and low temperature time resolved electron spin resonance. Both 1 and 4 undergo α-cleavage to produce benzoyl and phosphorous centered radicals. The photochemistry of 1 and 4 has been investigated by nanosecond laser flash photolysis, picosecond pump-probe spectroscopy, and steady-state photolysis. The singlet states of 1 and 4 and the phosphorous centered radicals produced by α-cleavage were characterized directly by time resolved absorption spectroscopy. The triplet states of 1 and 4 were characterized indirectly by quenching with 1-phenylnaphthalene as a selective triplet quencher. The use of 1-phenylnaphthalene indicates that α-cleavage occurs mainly from the triplet states of 1 and 4. However, the observed rate of formation of phosphorous centered radicals derived from picosecond investigations is experimen...

Investigation of the photo-Fries rearrangement reactions of 1- and 2-naphthyl acetates

The investigation of the photo-Fries reaction of 1- and 2-naphthyl acetates (1- and 2-NAs) was performed using stationary photolysis, laser flash photolysis and steady state and time-resolved chemically induced dynamic nuclear polarization (CIDNP). The transient absorption spectra of the 1- and 2-napthoxyl radicals and the 1-NA and 2-NA triplet states were detected, and the quantum yields and absorption coefficients were obtained. The influence of a triplet quencher on the napthoxyl radical quantum yield was studied, and the singlet nature of the primary radical pair was confirmed for both initial compounds. On laser flash photolysis of 1-NA, the formation of the ortho product was directly detected, and the rate constant of the [1,3] hydrogen shift reaction was established. The signs of the CIDNP signals during the photolysis of 2-NA point to a singlet precursor of the radical pair giving rise to the rearrangement products, and to a triplet precursor for the disproportionation products of the parent radical pair. A comparison of the slopes of the Stern-Volmer plots for the rearrangement product quantum yield, for triplet CIDNP, and for the optically detected 2-NA triplet state confirms the previously reported assumption about the main role of the excited singlet state and the involvement of two different triplet states in the reaction. A general kinetic scheme for 1- and 2-NA photolysis is proposed.

Mechanism of Photodimerization of Acenaphthylene

The mechanism of photodimerization of acenaphthylene (ACN) has been investigated in order to elucidate the roles of the singlet and the triplet excited states of ACN in the formation of the Z- and E-dimers in several solvents. The quantum yields and the ratio of the produced Z- to E-dimer were determined under irradiation of ACN at 435.8 nm in several solvents (1,2-dichloroethane, acetonitrile, cyclohexane, benzene, methanol, and DMF) over a wide concentration range (2.0 × 10-4−2.0 M) in the absence of additives and in the presence of 1,2-dibromoethane, a heavy-atom-containing solvent; Eosin-Y, a triplet sensitizer (irradiated at 546.1 nm); and ferrocene, a triplet quencher. On irradiation of ACN in dilute solution, the initially generated S1 state crosses over to the T1 state, though with low quantum yield, before reaction with the ground state ACN can occur due to the very short lifetime of the S1 state, and the resultant T1 state reacts with ground state ACN to give a mixture of the Z- and E-dimers in a ratio that depends on the solvent employed. In more concentrated solution, the S1 state reacts with ACN before intersystem crossing to the T1 state can occur, affording exclusively the Z-dimer.

Resolving Component Overlap in Multiwavelength Kinetic Spectroscopy: Application to Raman Scattering from Intermediates in Triplet‐State Photoreactions

Abstract— Understanding of reaction mechanisms in photochemistry is advanced both by kinetic measurements to determine rates of reaction and by spectroscopic studies of the structures of precursors and photoproducts. When kinetics and spectroscopy are combined in a time‐resolved, multiwavelength spectroscopic measurement on a reacting system, spectra and structures of intermediates can also be determined. In this paper, the application of multidimensional least‐squares and factor analysis techniques for resolving overlapped spectra from intermediates in photochemical kinetics are discussed. The methods are employed specifically to resolve Raman spectra of intermediates in triplet‐state photoreactions. By varying excitation intensity, spectra of excited‐triplet states are resolved from ground states and solvent. By varying the concentration of a triplet quencher in the sample, the Raman spectrum of a sensitizer excited state (benzophe‐none) can be resolved from those of acceptor‐triplet states or radical photoproducts. Applications of these concepts in other areas of photochemical kinetics are addressed.

Positive exchange interaction in the radical ion pair of benzophenone anion and 1,4-diazabicyclo[2,2,2]octane cation radicals studied by FT-EPR spectroscopy

Electron spin polarization generated from the photoreduction of benzophenone (BP) and its derivatives in the presence of 1,4-diazabicyclo[2,2,2]octane (DABCO) was studied in various solvents. The DABCO cation radical obtained showed a CIDEP spectrum with A/E (absorption/emission) polarization by RPM, while other neutral radicals gaee an E/A pattern. Using triplet quenchers, it was confirmed that these RPM signals were generated through the triplet reaction process. The present results revealed that the radical ion pair including the BP anion and DABCO +. cation radicals has a positive J, while the neutral radical pairs generated under the same condition have a negative J. The sign of J is independent of the polarity of the organic solvents used.

Styrene triplets: Absorption spectra, lifetimes and quantum yields

The triplet states of styrene, p-methyl-, p-methoxy- and p-chlorostyrene produced by direct excitation (255 nm) or by sensitization with triplet benzophenone in cyclohexane solution, were identified and their absorption spectra and their reactivity with triplet quenchers were determined. The absorption spectra are characterized by a band at about 280 nm (ϵ þe range 14000–24000 M −1 cm −1) and a weaker band (or shoulder) at about 320 nm ( ϵ ≈ 5000 M −cm −). For direct excitation, the quantum yield for triplet formation are 0.2–0.5. The rate constants for reaction of the triplets with O 2, 1,3-cyclohexadiene or ferrocene are of the order or 10 9 M −1 s −1.

Laser flash photolysis of 1,8-bis(halomethyl)naphthalenes

Transient absorption spectra were observed by the laser flash photolysis of 1,8-bis(halomethyl)naphthalenes for Cl and Br substituents. In the case of 1,8-bis(chloromethyl)naphthalene, the main transient absorption bands at 340 and 365 nm were attributed to the carbon-centred radical which was formed by bond fission of one of two CCl bonds. In the case of the dibromo derivative, the transient bands appeared at 435 and 350 nm. The 350 nm band was assigned to the carbon-centred radical. Because of the low reactivity to O 2 and triplet quenchers, the main transient absorption band appearing at 435 nm was attributed to the rearranged radicals. The radical structures were based on molecular orbital calculation.

The mechanism of photochemical CO or CS bond cleavage in aryl (thio) ethers

By photochemically induced 1H nuclear polarization ( 1H-CIDNP) spectroscopy, CO or CS bond cleavage is confirmed as the primary step in the photolysis of cyanomethyl-1-naphthyletther ( 1Aa), cyanomethyl-2-naphthylether ( 1Ba), 2-methylallyl-1-naphthylether ( 1Ab), 1,1-dimethylallyl-2-naphthylthioether ( 1Ce), 1,1-dimethylallyl-2-pyrenylether ( 1Dc) and 1,1-dimethylallyl-3-fluoranthenylether ( 1Ec). The polarization observed in the diamagnetic products formed via the primary aryloxy or naphthylthiyl and cyanomethyl-or methyl-substituted allyl radicals respectively is used to determine the spin multiplicity of the excited precursor states. Both the lowest excited singlet and triplet states are found to be photoreactive. These findings are supported by experiments using radical scavengers, triplet sensitizers or triplet quenchers. The kinetic constants of deactivation of the excited states are derived from the quantum yields, fluorescence decay times and laser flash spectroscopy of the transient triplets and radicals. Except for the rate constant of bond dissociation, these are very similar to those of related photostable methylaryl(thio)ethers. We conclude that bond cleavage is caused by a thermally activated crossing from bonding S 1 (ππ ∗ and T 1 (ππ ∗ states into dissociative πσ ∗ states in full support of an earlier theoretically study. The experimental conditions determine whether the singlet or triplet channel dominates. The spin densities of the radicals involved and the CO bond dissociation enthalpies are computed at the semiempirical AM1 level and are correlated with the experimentally observed product distributions and disappearance quantum yields.

A Time-Resolved Stimulated Nuclear Polarization Study of Biradicals in Low Magnetic Field

Stimulated nuclear polarization (SNP) in acyl−alkyl biradicals of different size formed as a result of α-cleavage of cyclic ketones in the triplet state has been investigated in low magnetic field around 10 mT. Time resolution was achieved by pulsing the light and the rf source and monitoring the SNP signal as function of the rf pulse delay. The SNP amplitude reaches its maximum for a delay around 40 ns and decays exponentially for longer delays. The decay time decreases with increasing molecular chain length. The different influence of triplet quencher cis-piperylene and radical scavenger CBrCl3 shows that the decay is mainly determined by the lifetime of the photoexcited ketone triplet state. The SNP maximum position can be attributed to the competition between spin-dependent and spin-independent channels of intersystem crossing.

Effect of Mesophase Structure on the Intermolecular Triplet Quenching of Photoexcited ZnTPP and CuTPP Mixtures

Time-resolved EPR detection of photoexcited Zn− and Cu−tetraphenylporphyrin (TPP) mixtures oriented in several thermotropic liquid crystals (LCs) is reported. Both the transient triplet EPR spectra of ZnTPP and the doublet ground state CW spectra of CuTPP were recorded and quantitatively analyzed via line shape analyses over a wide temperature range (130−370 K). It is shown that the photoexcited triplet state of ZnTPP is entirely quenched by the photoexcited CuTPP species exclusively in the fluid smectic phase of a multidomain LC (ZLI-1167), whereas in the other phases of this LC, as well as in other LCs (E-7 and ZLI-4389) there is no indication of any interaction between the different chromophores. The results are interpreted in terms of a ZnTPP−CuTPP transient complex formation, which depends on a favorable mutual orientation of the interacting chromophores, specifically imposed by the layers and multidomain structure of the smectic phase of ZLI-1167.

Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

Contrary to a previous report, it is concluded that formation of methanol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protonation of the intermediate enone π,π* triplet excited state and not by protonation of a relatively long-lived ground state trans-cyclohexenone. A rate constant for protonation of the triplet state of 3-methyl-2-cyclohexenone by sulfuric acid of 1.7 × 109 M−1 s−1 was determined by laser flash photolysis in ethyl acetate. Based on quantum efficiencies of product formation, a rate constant of ca. 108 M−1 s−1 was estimated for protonation of the enone triplet by acetic acid, which is too small to cause measurable reduction in the triplet state lifetime in the mM concentration range used in the preparative studies. The intermediate carbocation can be trapped by methanol, or revert to starting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed process, there is an acid concentration at which the yields of products are optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation could be quenched using 1-methylnaphthalene and cyclopentene as triplet quenchers; in the latter case, formation of [2 + 2] photoadducts was observed to compete with formation of methanol adducts. Quenching rate constants were determined by laser flash studies. Keywords: laser flash photolysis, kinetic absorption spectroscopy (KAS), photoacoustic calorimetry (PAC), protonation of triplet states, trans-cyclohexenones.

Photophysical and photochemical processes in fullerenes under high-intensity illumination

The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.

Characterization of the reactive intermediates in laser flash photolysis of adenine, adenosine and dAMP using acetone as photosensitizer

Transient absorption spectra of adenine, adenosine and 2′-deoxyadenosine 5′-monophosphate (dAMP) arising from 248 nm laser flash photolysis using acetone as a photosensitizer have been observed. The intermediates recorded are assigned to the excited triplet states and dehydrogenated radicals of adenine and its nucleoside and nucleotide. The excited triplet states of adenine and its derivatives are produced via triplet-triplet excitation transfer and observed for the first time, while the dehydrogenated radicals stemming from the interaction of triplet acetone with adenine and its derivatives via electron transfer through a five-member-ring electron donor-acceptor intermediate. The site of dehydrogenation is suggested to be the hydrogen atom on C(8) of the adenine moiety. Moreover, three sets of kinetic parameters of the triplet decay have been determined. The rate constants of the unimolecular decay ( k 0), the triplet quenching by the ground state ( k sq) and by the triplet quencher Mn 2+ ( k q) are 1.1 × 10 5, 7.9 × 10 4, 2.7 × 10 4 s −1, 6.9 × 10 8, 8.3 × 10 8, 3.6 × 10 8 dm 3mol −1 s −1 and 4.2 × 10 8, 3.5 × 10 8, 6.0 × 10 8 dm 3 mol −1 s −1 respectively for adenine, adenosine and dAMP.

Influence of Bridging Ligand Unsaturation on Excited State Behavior in Mono- and Bimetallic Ruthenium(II) Diimine Complexes

The redox and photophysical properties of a series of bis(bipyridine)-bridged bimetallic complexes of the type [(dmb){sub 2}Ru(BL)](PF{sub 6}){sub 2} and ([(dmb){sub 2}Ru]{sub 2}(BL))(PF{sub 6}){sub 4} (dmb = 4,4{prime}-dimethyl-2,2{prime}-bipyridine; BL = 1,4-bis(4{prime}-methyl-2,2{prime}-bipyridin-4-yl)buta-1,3-diene (bbdb), 1,4-bis(4{prime}-methyl-2,2{prime}-bipyridin-4-yl)-2-cyclohexene -5,6-dicarboxylic acid diethyl ester (bchb), and 1,4-bis(r{prime}-methyl-2,2{prime}-bipyridin-4-yl)benzene (bphb)) are reported. Complexes having cyclohexenyl and phenyl bridging ligands have redox and photophysical properties similar to the parent complex of the series, [(dmb){sub 3} Ru](PF{sub 6}){sub 2}. The butadienyl complexes exhibit very weak luminescence and transient absorbance spectra which are indicative of the presence of another excited state. For [(dmb){sub 2}Ru-(bbdb)](PF{sub 6}){sub 2} the room temperature luminescence lifetime differs from the lifetime obtained by transient absorbance and the luminescence and transient absorbance are quenched with different rate constants by a series of triplet quenchers. The results suggest that the complexes of bbdb have a second excited state which is populated along with the {sup 3}MLCT state. A comparison of the luminescence behavior of several different bridged complexes with the triplet energies of related aromatic hydrocarbons suggests that the triplet energy of the bridging ligand is relatively unaffected by coordination.

Excited-State Intramolecular Proton Transfer in Polymers

ABSTRACTExcited state intramolecular proton transfer (ESIPT) has been demonstrated in new intramoleculary hydrogen bonded (IHB) polymers of interest as photostabilizers, triplet quenchers, photochromic materials, laser dyes and electroluminescent materials. The new IHB polymers containing the 2-(2-hydroxyphenyl)benzoxazole moiety in the main chain were used to explore the effects of polymer structure, extended conjugation and competition with excimer formation on the ESIPT process. It was found that polymer structure, and particularly extent of conjugation, affects an IHB polymer's ability to exhibit ESIPT.

Photochemical formation of 9-silaanthracenes in rigid glass

UV photolysis of 9,10-dihydro-9-silaanthracenes in 3-methylpentane glass at 77 K results in the formation of 9-silaanthracenes and diphenylmethyl-type radicals; by use of a triplet quencher, it was confirmed that these were produced via the lowest triplet state.

Environmental and laboratory studies of the photodegradation of the pesticide fenarimol

The photophysics and photochemistry of the pesticide fenarimol were studied. From the absorption and fluorescence spectra, quantum yields and lifetimes, it is suggested that the lowest excited singlet state has predominatly n,π * character, and is localized on the pyrimidine ring. Phosphorescence measurements suggest a small singlet—triplet splitting. Halide ions are found to quench fenarimol flourescence. Photodecomposition of fenarimol in a variety of solvents was studied by gas chromatography—mass spectrometry. Although the products have not yet been characterized, degradation of this compound appears to involve cleavage of bonds to the quaternary carbon without significant dechlorination. Preliminary studies of the kinetics and relative quantum yields of photodegration show that the reaction proceeds via a first-order process, which is independent of pH, but which involves a photoactive intermediate. Chloride and bromide ions inhibit the photolysis,whereas the triplet quencher sorbic acid has no effect.