Nanosecond Laser Flash Photolysis Studies Research Articles

Molecular Rubies in Photoredox Catalysis.

The molecular ruby [Cr(tpe) 2 ] 3+ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy) 3 ] 3+ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz) 3 ] 2+ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4′-dimethoxycarbonyl-2,2′-bipyridine, bpz = 2,2′-bipyrazine), while [Cr(ddpd) 2 ] 3+ serves as a control system (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the CrIII driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe) 2 ] 3+ outcompetes [Cr(dmcbpy) 3 ] 3+ and even [Ru(bpz) 3 ] 2+ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe) 2 ] 3+ in this photoredox catalytic system.

Visible-light activation of the bimetallic chromophore-catalyst dyad: analysis of transient intermediates and reactivity toward organic sulfides.

In order to develop a new photocatalytic system, we designed a new redox-active module (5) to hold both a photosensitizer part, [Ru(II)(terpy)(bpy)X](n+) (where terpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine), and a popular Jacobsen catalytic part, salen-Mn(III), covalently linked through a pyridine-based electron-relay moiety. On the basis of nanosecond laser flash photolysis studies, an intramolecular electron transfer mechanism from salen-Mn(III) to photooxidized Ru(III) chromophore yielding the catalytically active high-valent salen-Mn(IV) species was proposed. To examine the reactivity of such photogenerated salen-Mn(IV), we employed organic sulfide as substrate. Detection of the formation of a Mn(III)-phenoxyl radical and a sulfur radical cation during the course of reaction using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. This is the first report for the electron transfer reaction of organic sulfide with the photochemically generated salen-Mn(IV) catalytic center.

Photophysics and photochemistry of aflatoxins B1 and B2

Aflatoxins are mycotoxins produced by fungi of the genus Aspergillus, which is widely spread in the tropics and subtropics. To date, aflatoxin phototoxicity has been recognized, but the mechanism responsible for this phototoxicity has not been fully characterized. In the present paper, nanosecond laser flash photolysis studies allowed us to elucidate the photochemical processes undergone by two mycotoxins, namely aflatoxin B(1) and B(2), upon UV irradiation. In brief, photolysis (308 nm) of the aflatoxins leads to intersystem crossing, giving rise to their triplet excited state. The triplet state can readily be quenched by indole and hydroquinone, and also by molecular oxygen yielding singlet oxygen (singlet oxygen quantum yields of 0.51 and 0.59 were found for aflatoxin B(1) and B(2), respectively). In addition, our data indicate the ability of the two aflatoxins to photoionize upon 248 nm excitation. The photoionization quantum yield for aflatoxin B(1) and B(2) have been estimated to be 0.11 and 0.29, respectively.

Mechanistic Aspects of the Formation of Aldehydes and Nitriles in Photosensitized Reactions of Aldoxime Ethers

The photooxidation of a series of aldoxime ethers was studied by laser flash photolysis and steady-state (product studies) methods. Nanosecond laser flash photolysis studies have shown that chloranil (CA)-sensitized reactions of the O-methyl (1), O-ethyl (2), O-benzyl (3), and O-tert-butyl (4) benzaldehyde oximes result in the formation of the corresponding radical cations. In polar non-nucleophilic solvents such as acetonitrile, there are several follow-up pathways available depending on the structure of the aldoxime ether and the energetics of the reaction pathway. When the free energy of electron transfer (DeltaGET) becomes endothermic, syn-anti isomerization is the dominant pathway. This isomerization pathway is a result of triplet energy transfer from CA to the aldoxime ether. For substrates with alpha-protons (aldoxime ethers 1-3), the follow-up reactions involve deprotonation at the alpha-position followed by beta-scission to form the benziminyl radical (and an aldehyde). The benziminyl radical reacts to give benzaldehyde, the major product under these conditions. A small amount of benzonitrile is also observed. In the absence of alpha-hydrogens (aldoxime ether 4), the major product is benzonitrile, which is thought to occur via reaction of the excited (triplet) sensitizer with the aldoxime ether. Abstraction of the iminyl hydrogen yields an imidoyl radical, which undergoes a beta-scission to yield benzonitrile. An alternative pathway involving electron transfer followed by removal of the iminyl proton was not deemed viable based on charge densities obtained from DFT (B3LYP/6-31G*) calculations. Similarly, a rearrangement pathway involving an intramolecular hydrogen atom transfer process was ruled out through experiments with a deuterium-labeled benzaldehyde oxime ether. Studies involving nucleophilic solvents have shown that all aldoxime ethers reacted with MeOH by clean second-order kinetics with rate constants of 0.7 to 1.2 x 10(7) M(-1) s(-1), which suggests that there is only a small steric effect in these reactions. The steady-state experiments demonstrated that under these conditions no nitrile is formed. This is explained by a mechanistic scheme involving nucleophilic attack on the nitrogen of the aldoxime ether radical cation, followed by solvent-assisted [1,3]-proton transfer and elimination of an alcohol, similar to the results obtained for a series of acetophenone oxime ethers.

Effect of Structure in Benzaldehyde Oximes on the Formation of Aldehydes and Nitriles under Photoinduced Electron-Transfer Conditions

The mechanistic aspects of the photosensitized reactions of a series of benzaldehyde oximes (1a-o) were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that the reaction of the oxime with triplet chloranil (3CA) proceeds via an electron-transfer mechanism provided the free energy for electron transfer (DeltaG(ET)) is favorable; typically, the oxidation potential of the oxime should be below 2.0 V. Substituted benzaldehyde oximes with oxidation potentials greater than 2.0 V quench 3CA at rates that are independent of the substituent and the oxidation potential. The most likely mechanism under these conditions is a hydrogen atom transfer mechanism as this reaction should be dependent on the O-H bond strength only, which is virtually the same for all oximes. Product studies have shown that aldoximes react to give both the corresponding aldehyde and the nitrile. The important intermediate in the aldehyde pathway is the iminoxyl radical, which is formed via an electron transfer-proton transfer (ET-PT) sequence (for oximes with low oxidation potentials) or via a hydrogen atom transfer (HAT) pathway (for oximes with larger oxidation potentials). The nitriles are proposed to result from intermediate iminoyl radicals, which can be formed via direct hydrogen atom abstraction or via an electron-transfer-proton-transfer sequence. The experimental data seems to support the direct hydrogen atom abstraction as evidenced by the break in linearity in the plot of the quenching rates against the oxidation potential, which suggests a change in mechanism. The nitrile product is favored when electron-accepting substituents are present on the benzene ring of the benzaldehyde oximes or when the hydroxyl hydrogen atom is unavailable for abstraction. The latter is the case in pyridine-2-carboxaldoxime (2), where a strong intramolecular hydrogen bond is formed. Other molecules that form weaker intramolecular hydrogen bonds such as 2-furaldehyde oxime (3) and thiophene-2-carboxaldoxime (4) tend to yield increasing amounts of aldehyde.

Reactivity of 4-vinylphenol radical cations in solution: implications for the biosynthesis of lignans

Nanosecond laser flash photolysis studies of the radical cation of 4-hydroxy-3-methoxystyrene show that the radical cation reacts with neutral 4-hydroxy-3-methoxystyrene and non-phenolic styrenes with rate constants that range from 1 x 10(8) to 5 x 10(8) M(-1) s(-1). Similar 4-vinylphenol radical cations such as the radical cations of isoeugenol and coniferyl alcohol display reduced reactivity, presumably due to the presence of beta-alkyl substituents. Overall, the results show that the reactivity of 4-vinylphenol radical cations with neutral styrenes parallels the reactivity of non-phenolic styrene radical cations, which are known to undergo efficient radical cation mediated dimerization reactions to give lignan-like compounds. The possibility that the biosynthesis of some lignans may follow a radical cation mediated mechanism is discussed.

Excited‐State Behavior and Photoionization of 1,8‐Acridinedione Dyes in Micelles—Comparison with NADH Oxidation

The photophysics and photochemistry of 1,8-acridinedione dyes, which are analogues of reduced nicotinamide adenine dinucleotide (NADH), are studied in anionic and cationic micelles. Acridinedione dyes (ADDs) are solubilized in micelles at the micelle-water interface and are in equilibrium between the aqueous and micellar phase. The binding of the ADDs with micelles is attributed to hydrophobic interactions and the binding constants are determined with steady-state and time-resolved techniques. Nanosecond laser flash photolysis studies are carried out in aqueous, anionic, and cationic micellar solutions. The ADD undergoes photoionization in the excited state to give a solvated electron. The solvated electron reacts with the ADD to give an anion radical. In anionic micelles, the yield of the solvated electron increases because of the efficient separation of the cation radical and the electron. Cation radicals derived from the photooxidation of ADDs are involved in keto-enol tautomerization. Under acidic conditions, an enol radical cation of the acridinedione dye is formed from the keto form of the cation radical by intramolecular hydrogen atom transfer. In cationic micelles, due to electrostatic attraction, the electron cannot escape from the micelle and recombination of the cation radical and the electron results in the formation of a triplet state. For the first time, a solvated electron is observed in the laser flash photolysis of ADDs in anionic micelles. The photoionization of ADDs depends on the excitation wavelength and is biphotonic at 355 nm and monophotonic at 248 nm. From the results with this NADH model compound, the sequential electron-proton-electron transfer oxidation of NADH is confirmed and the nature of the intermediates involved in the oxidation is unraveled; these intermediates are found to depend on the pH value of the medium.

Photosensitized reactions of oxime ethers: a steady-state and laser flash photolysis study.

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that chloranil-sensitized reactions of the oxime ethers result in the formation of the corresponding radical cations. The radical cation species react with nucleophiles such as MeOH by clean second-order kinetics with rate constants of (0.7-1.4) x 10(6) M(-1) s(-1). Only a small steric effect is observed in these reactions, which is taken as an indication that the reaction center is not the O-alkyl moiety, but rather somewhere else in the molecule. Product studies in a polar nonnucleophilic solvent (MeCN) revealed that in order for the oxime ether radical cation to react more readily, alpha-protons must be available on the alkyl group. The O-methyl (1), O-ethyl (2), and O-benzyl (3) acetophenone oximes all reacted readily to give acetophenone oxime as the major product (as well as an aldehyde derived from the O-alkyl group), whereas O-tert-butyl acetophenone oxime (4) did not. The product formation can be explained by a mechanism that involves electron transfer followed by proton transfer (alpha to the oxygen) and subsequent beta-cleavage. When using 3 in MeOH, a change in the product formation is observed, the most important difference being the presence of benzyl alcohol rather than benzaldehyde as the major product. On the basis of the data from LFP and steady-state experiments, it is suggested that the competing mechanism under these conditions involves electron transfer, followed by a nucleophilic attack on the nitrogen, a MeOH-assisted [1,3]-proton transfer, and subsequent loss of benzyl alcohol. This mechanism is supported by DFT (B3LYP/6-31G) and AM1 calculations.

Photohydration and photosolvolysis of biphenyl alkenes and alcohols via biphenyl quinone methide-type intermediates and diarylmethyl carbocations.

Evidence is presented for the photochemical generation of novel biphenyl quinone methide (BQM)-type intermediates on photolysis of hydroxybiphenyl alkenes 7 and 8 and hydroxybiphenyl alcohols 9 and 10. Mechanistic investigations utilizing product, fluorescence, and nanosecond laser flash photolysis (LFP) studies indicate two distinct pathways for the formation of these BQMs depending upon the functional groups of the progenitor. Formal excited-state intramolecular proton transfer (ESIPT) between the phenol and the alkene led to BQMs upon irradiation of the hydroxybiphenyl alkenes 7 and 8, while excited-state proton transfer (ESPT) to solvent followed by dehydroxylation was responsible for BQM formation from the hydroxybiphenyl alcohols 9 and 10. Photolysis of 7 and 8 in aqueous CH(3)CN gave photohydration products via attack of water on the respective BQMs, while photolysis of the analogous methyl ethers (of the phenolic moiety) gave only carbocation intermediates. Hydroxybiphenyl alcohols 9 and 10 yielded the corresponding photomethanolysis products in aqueous methanol, through attack of CH(3)OH on the respective BQMs. Although no evidence was found for BQM formation in LFP studies of 8 and 10, due to its suspected short lifetime, the respective diaryl carbocation (lambda(max) 420 nm, tau = 8.5 micros) has been observed upon irradiation of 8 in 2,2,2-trifluoroethanol. A BQM (lambda(max) 580 nm) was observed for 9 but not for 10, the latter having more complex chemistry on laser excitation, resulting in a transient that appears to mask any BQM absorption. Significant quenching of fluorescence from the hydroxybiphenyl alkenes at low water content implies that H(2)O is directly involved in reaction from the singlet excited state. The decrease in fluorescence intensity of 8 was found to depend on [H(2)O](3); however, the distance required for ESIPT in these systems is too large to be bridged by a water trimer. The nonlinear quenching has been attributed to deprotonation of the phenol by two water molecules, with concerted protonation at the alkene by another molecule of water. Fluorescence quenching of the hydroxybiphenyl alcohols required much higher water content, implying a different mechanism of reaction, consistent with the proposal of ESPT (to solvent water) followed by dehydroxylation.

Photolysis of aqueous solutions of aniline-2,5-disulfonic acid: steady state and laser flash photolysis studies.

The photochemical behaviour of aniline-2,5-disulfonic acid (ADSA) in aqueous solutions upon excitation at 254 nm, 310 nm and 334 nm has been studied. A specific phototransformation into o-anilinesulfonic acid (o-AS) via a desulfonation process was observed. Such a reaction was efficiently inhibited by molecular oxygen. The quantum yield was found to be 0.46 and 9.0 x 10(-3) in deoxygenated and oxygenated solutions, respectively. This inhibition was shown to result from an energy transfer process to oxygen through the formation of the lowest triplet excited state as demonstrated by a nanosecond laser flash photolysis study. The bimolecular quenching rate constant of the triplet state by oxygen is found to be 2.5 x 10(9) mol(-1) L s(-1). A mechanism taking into account the experimental results is proposed for the photochemical desulfonation of aniline-2,5-disulfonic acid.

Formation and quenching of the photoexcited triplet state of tetrasulfonated zinc phthalocyanine, ZnPcS 44− by azaferrocene and ferrocene

A nanosecond laser flash photolysis study with excitation at 532 nm showed that azaferrocene (AF) quenches the S 1 state of ZnPcS 4 4− in DMSO solutions giving its T 1 state, which is then quenched itself by the same solute with the rate constant 2.4×10 8 M −1 s −1. Ferrocene (Fc) quenches only the phthalocyanine T 1 state (and not the S 1 state) with the rate constant 4.4×10 8 M −1 s −1. 2,5-DimethylAF does not quench the S 1 state of ZnPcS 4 4−. These differences cannot be accounted for by thermodynamic considerations, but are discussed in terms of the possible oxidation of AF at the nitrogen atom.

TRIPLET STATE PROPERTIES OF C60PMMA CO-POLYMERS IN ORGANIC SOLUTION AND PMMA FILM

Two star-like C60-polymethylmethacrylate (C60PMMA) copolymers, which are soluble in a wide range of organic solvents, have been synthesized by free-radical polymerization. The polymers can be described by the formula 1%C60PMMA and 10%C60PMMA. They consist of a C60 core with a maximum of 6 polymethylmethacrylate (PMMA) chains. The difference between these derivatives is the weight distribution of C60. Nanosecond pulse radiolysis and laser flash photolysis studies, UV/VIS spectroscopic and fluorescence measurements have been performed to investigate the photophysical properties of 1%C60PMMA and 10%C60PMMA. The absorption spectra of the C60PMMA co-polymers exhibit a band at 210 to 220nm for PMMA and at 330nm for C60 independent of the solvent polarity showing that they have a small dipole moment in the electronic ground state. The triplet-triplet (T-T) absorption spectra of 10%C60PMMA in toluene and chloroform peak at the same wavelength and are broader than those of C60, indicating that they are addition spectra of 10%C60PMMA and neat C60. Physical addition of PMMA increases the solubility of C60 in chloroform, but does not change the T-T absorption spectrum, unless PMMA is chemically bound to C60. Gaussian band analysis of the T-T absorption spectra of 10%C60PMMA results in an additional band for pure 10%C60PMMA at (626.8±15.7)nm in toluene and (652.4±1.9)nm in chloroform. The triplet state lifetime of 10%C60PMMA is smaller than that of C60 at 79μs. The quantum yields of singlet oxygen production (ΦΔ) decrease with the C60 content in the co-polymer. The values for 1%C60PMMA and 10%C60PMMA in polar chloroform are 4% and 26% respectively. The intersystem crossing from S1 to T1 as well as the triplet state lifetime decrease with increasing number and length of PMMA chains connected to C60. 1%C60PMMA also shows no T-T absorption suggesting a relative high photochemical stability for 1%C60PMMA.

Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C60 dyad molecules

Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C60 and dodecathiophene-C60 dyad molecules (8T-C60 and 12T-C60, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, 18T*-C60 and 112T*-C60 showed energy transfer to 1C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and 1C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds. In more polar solvents (dielectric constant (∈s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.

Fast Decarboxylation of Aliphatic Amino Acids Induced by 4-Carboxybenzophenone Triplets in Aqueous Solutions. A Nanosecond Laser Flash Photolysis Study

Quenching of the 4-carboxybenzophenone triplet (3CB*) by H2N−CH2−CO2−Et and by amino acid anions of the general formula NR2−CR2−CO2-, where R is H or Me, has been investigated in basic aqueous solution. Spectral analysis of the primary products on the microsecond time scale showed that the major quenching process was electron transfer to 3CB*, producing the CB•- radical anion and the R2N•+−CR2−CO2- zwitterion aminium radical. However, on a nanosecond time scale, a small amount of (CBH)• was also formed, and this was attributed to a rapid proton transfer from about 10% of the aminium radicals to the CB•- anion radicals within the primary solvent cage. The values of the overall primary quenching rate constants were (8.5 ± 0.9) × 108 M-1 s-1 for N,N-dimethylglycine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine, (1.5 ± 0.2) × 108 M-1 s-1 for alanine, (1.3 ± 0.1) × 108 M-1 s-1 for glycine ethyl ester, and (0.3 ± 0.03) × 108 M-1 s-1 for α-methylalanine. The introduction of methyl groups into the glycine structure resulted in a pattern of reactivity similar to that observed for amines. Except in the case of glycine ethyl ester, there were strong secondary growths of CB•-. This was attributed to the reduction of CB by the R2N−•CR2 species produced from the decarboxylation of the R2N•+−CR2−CO2- aminium species. The second-order rate constants for CB reduction by the aminoalkyl radicals are (3.2 ± 0.4) × 108 M-1 s-1 for H2N−•CH2, (1.7 ± 0.3) × 109 M-1 s-1 for H2N−•C(Me)H, and (1.8 ± 0.3) × 109 M-1 s-1 for H2N−•CMe2. The transfer of protons from aminium radicals within the solvent cage gives rise to •NR−CR2−CO2- aminyl radicals, and these are known to undergo β-elimination of CO2•-. There was evidence for the presence of aminyl radicals in the case of α-methylalanine, where a small tertiary growth of CB•-, due to the reduction of CB by CO2•-, was observed. The magnitude of this growth matched the yield of (CBH)• from the spectral analysis of the primary products. In further experiments, the R2N•+−CR2−CO2- zwitterion aminium radicals were deprotonated by bulk OH- when NaOH was added at concentrations in the range of 1−4 M. As expected, this produced a lowering of the CB•- yield from the R2N−•CR2 radicals and an increase from the CO2•- species. An analysis of this effect was made assuming a diffusion-controlled rate of 1 × 1010 M-1 s-1 for the transfer of the proton from R2N•+−CR2−CO2- to OH-. It indicated that the rate constant for transfer of the proton from R2N•+−CR2−CO2- to CB•- within the solvent cage was (6.9 ± 1.5) × 109 s-1. The rate constant for the decarboxylation of the aminium species was estimated to be (8.7 ± 0.5) × 1010 s-1. The latter rate is at least 1 order of magnitude above those observed for decarboxylations of aliphatic acyloxyl radicals in aqueous media.

Laser Flash Photolysis Studies of the Photo-Ring-Opening Reaction of Flav-3-en-2-ol

Abstract Electronic structure and photodynamics of flav-3-en-2-ol have been studied by steady-state and time-resolved absorption measurements, and by semi-empirical MO calculations. The photo-ring-opening reaction to produce 2-hydroxychalcone has been determined to be a monophotonic process with the quantum yield of 0.29. The precursor to 2-hydroxychalcone has been assigned to the ground-state enol-form by nanosecond laser flash photolysis study at room temperature and steady-state photolysis study at 77 K. The efficiency of the tautomerization from the ground-state enol-form to keto-form of 2-hydroxychalcone has been estimated to be unity. The enol-form is transformed into 2-hydroxychalcone with the rate constant of 3.2 × 104 s−1 in neat acetonitrile, and this process is accelerated by protic molecules. The PM3 calculation of flav-3-en-2-ol showed that the dissociative potential surface in the first excited singlet-state is responsible for the ring-opening reaction.

Nanosecond Laser Flash Photolysis and Steady-State Photolysis Studies of Benzyltrimethylsilane and Trimethylsilyldiphenylmethane

Photophysical and photochemical processes of benzyltrimethylsilane and trimethylsilyldiphenylmethane have been studied by fluorescence measurements and by steady-state and nanosecond laser flash photolyses. A remarkable solvent effect was observed for the laser flash photolysis of benzyltrimethylsilane; in methanol, absorption bands due to benzyl-type radicals and the triplet were observed, while in cyclohexane, only T−T absorption was observed. On the other hand, no solvent effect was observed for the laser flash photolysis of trimethylsilyldiphenylmethane; a transient absorption attributable to the 1,3-trimethylsilyl-shifted intermediate was observed with a lifetime of ca. 27 ms in both solvents, and no absorption band ascribable to diphenylmethyl radicals or the triplet was observed. GC-MS spectra of photoproducts resulting from trimethylsilyldiphenylmethane were consistent with the assignment of the transient absorption. These results were explained in terms of the calculated molecular structure for the excited singlet states of these molecules by the PM3 method.

Photoelectron transfer induced decarboxylation of substituted carboxylic acids across a liquid/liquid interface

Methylene blue photocatalyzed electron transfer reactions of some phenoxyacetic acid derivatives and N-phenylglycine across a liquid/liquid interface led to their efficient decarboxylation. Nanosecond laser flash photolysis studies of this process, in a reverse micellar system of Aerosol-OT (AOT)-benzene-water indicated that the liquid/liquid interface acts as a barrier in controlling the energy wasting back electron transfer. Intramolecular CC bond forming reactions of these radicals were utilized for the synthesis of various chroman derivatives.

Photochemistry and photoinitiation activity of radical polymerization of 2‐substituted anthraquinone derivatives. III. Nanosecond laser flash photolysis study

A nanosecond laser flash photolysis study was undertaken on a selected range of 2-substituted anthraquinone derivatives and the data are discussed in relation to the photoactivities in industrial photopolymerization. All the compounds give rise to a triplet-triplet absorption that is quenched by ground-state molecular oxygen. The transient half-lives range from 0.64 to 11.3 μs of which the 2-(1,1 -dibromomethyl) anthraquinone exhibits the longest life-time. In a reductive solvent, 2-propanol, hydrogen atom abstraction takes place with the compounds haveing the lowest-lying triplet 3nπ* state, whereas those with a low-lying triplet3ππ* state show mixed kinetics. In the latter case, a disproportionation reaction involving the semianthraquinone radical may be taking place, competing with the direct hydrogen atom abstraction reaction. In addition, investigation of the halogenated derivatives has indicated the possibility of the corresponding halo radicals being formed. In the presence of a tertiary amine, triethylamine, all anthraquinone derivatives show the formation of stable species related to either the exciplex or the radical ion pair. The extent of exciplex formation is more effective with compounds possessing a lowest-lying triplet 3ππ* excited state than those with a triplet 3nπ* excited state. The results from the nanosecond laser flash photolysis study show the differences in behavior toward hydrogen atom abstraction and electron transfer processes that is dependent on the nature of the low-lying triplet state and the type of substituent present, i.e., electron-donating or electron-withdrawing. © 1996 John Wiley & Sons, Inc.

Photochemistry and photoinitiation activity of radical polymerization of 2-substituted anthraquinone derivatives. III. Nanosecond laser flash photolysis study

Photochemistry and photoinitiation activity of radical polymerization of 2-substituted anthraquinone derivatives. III. Nanosecond laser flash photolysis study

A nanosecond laser flash photolysis study of aqueous 4-chlorophenol

The mechanism of the aqueous photoreaction of 4-chlorophenol was studied by nanosecond laser flash photolysis. The carbene 4-oxocyclohexa-2,5-dienylidene is formed from the chlorophenol by loss of HCl. This species reacts efficiently with oxygen to form 1,4-benzoquinone-O-oxide which subsequently yields 1,4-benzoquinone. The carbene also undergoes reaction with water and coupling reactions to produce 1,4-hydroquinone and polyhydroxybiphenyls respectively.