PRIMARY INTERMEDIATES IN THE PULSED IRRADIATION OF RETINOIDS

Abstract Laser flash photolysis and pulse radiolysis have led to the characterisation of several shortlived intermediates formed after irradiation of retinoic acid and retinyl acetate in hexane or methanol. For retinoic acid, the triplet state, wavelength maximum 440 nm, extinction coefficient 7.3 × 10⁴ dm³ mol^?1 cm^?1, decay constant 6.2 × 10⁵ s^?1, is formed with a quantum yield of 0.012 for 347 nm excitation. The radical cation, absorption maximum 590 nm, extinction coefficient ～7 × 10⁴ dm³mol^?1 cm^?1, is formed in a biphotonic process. The radical anion, absorption maximum 510nm in hexane, 480 nm in methanol where its extinction coefficient is 1.2 × 10⁵ dm³mol^?1 cm^?1, appears to decay partially in methanol into another longer-lived neutral radical, wavelength maximum 420 nm, by loss of OH^?. For retinyl acetate, the triplet state, absorption maximum 395 nm, extinction coefficient 7.9 × 10⁴dm³mol^?1 cm^?1, decay constant 1.2 × 10⁶s^?1 is formed with a quantum yield of 0.025 for 347 nm excitation. Monophotonic photoelimination of OCOCH₃? in methanol produces the retinylic carbenium ion, wavelength maximum 590 nm, whose decay is enhanced by ammonia, k ～ 2 × 10⁶ dm³ mol^?1 s^?1 and retarded by water. The radical cation also has a wavelength maximum at 590 nm, its extinction coefficient being ～ 1.0 × 10⁵ dm³mol¹ cm^?1. The long-lived transient absorption with maximum at 385 nm, extinction coefficient 1.0 × 10⁵ dm³mol^?1 cm^?1, obtained from the reaction of the solvated electron with retinyl acetate in methanol may be due to either the radical anion itself or more likely the radical resulting from elimination of OCOCH₃? from this anion. These results suggest that skin photosensitivity caused by retinyl acetate might be greater than that due to retinoic acid.     

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.     

二氟沙星在中性水溶液中的光化学性质

对二氟沙星在中性水溶液中的光化学性质进行了研究. 在pH值为7.17 的水溶液中二氟沙星的紫外吸收峰位于273 nm (摩尔消光系数ε=33000 dm³·mol^-1·cm^-1), 323 nm (ε=15500 dm³·mol^-1·cm^-1), 335 nm (ε=15500 dm³·mol^-1·cm^-1)处. 荧光吸收和发射光谱均显示二氟沙星具有pH效应, 其pK_a(电离平衡常数)测定分别为5.9 和9.8. 二氟沙星的荧光量子产额较低, 在pH=3.00 时达到最大值, 为0.06. 同时对二氟沙星在中性水溶液中的激光光解和脉冲辐解进行详细研究. 激光光解研究发现在水溶液中二氟沙星的三重激发态位于620nm, 其摩尔消光系数为7900 dm³·mol^-1·cm^-1. 通过能量转移的方法得到其三重激发态的能量为263.5 kJ·mol^-1,三重激发态的量子产额为0.21. 在激光激发下, 二氟沙星进行单光子电离其量子产额为0.02. 脉冲辐解研究表明二氟沙星可以与水合电子(e_aq^-)及羟基自由基(·OH)快速反应, 其二级反应动力学常数分别为1.72×10¹⁰和1.0×10¹⁰ dm³·mol^-1·s^-1. 本文对二氟沙星光化学性质的研究有助于确定其光敏毒性的产生机理.     

The radicals formed by photoinduced electron transfer from hypocrellins to electron acceptors

The photoinduced electron transfer from excited singlet and triplet states of hypocrellins to three electron acceptors, namely, methyl viologen chloride (MV), tetrachloro-p-benzoquinone (TCQ) and 2,3-dichloro-5,6-dicyan-p-benzoquinone (DDQ), has been investigated by fluorescence and time-resolved transient absorption spectra. In acetonitrile solution, DDQ and TCQ quenched the fluorescence and T-T absorption of hypocrellins (HA and HB) efficiently, while neither fluorescence nor T-T absorption of them could be quenched by MV. The quenching resulted from the electron transfer between excited hypocrellins and the electron acceptors was controlled by diffusion. The rate constants of electron transfer from excited singlet and triplet of HA to DDQ are 9.20×10¹⁰ dm³ mol^?1 s^?1 and 1.28×10⁹ dm³ mol^?1 s^?1, respectively. The transient absorption spectra of the formed radical cations of hypocrellins are reported.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

THE EFFECT OF MERCURIC IONS ON THE EXCITED STATES OF DNA-INTERCALATED ETHIDIUM BROMIDE

The first excited triplet state of DNA-intercalated ethidium bromide is produced with a quantum yield of 0.010.002 on irradiation at 532 nm. A difference extinction coefficient of 1.50.210³ m² mol^?1 is measured for the triplet state at 380 nm. Mercuric ions quench the first excited singlet state of DNA-intercalated ethidium bromide via induced spin orbit coupling to give an increased yield of ethidium triplet states. The same mercuric ion that quenches the singlet state then quenches the triplet state, via the same mechanism, with a rate constant of ca 3.510³ s^?1. An upper limit for the rate of detachment of Hg²⁺ from its binding site in DNA may be fixed at ca 10³ s^?1.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

Photophysical Properties of Rufloxacin in Neutral Aqueous Solution

The photophysical properties of rufloxacin, 9-fluoro-2_r3-dihydro-10-(4-methyl-l-pyrazinyl)-7-oxo-7-H-pyri-do[l,2,3-de]-l,4-benzothiazin-6-carboxylic acid, a fluoroquinolone antibacterial drug exhibiting photosensitizing action toward biological substrates, were studied in aqueous solutions at neutral pH. The lowest excited electronic states of the zwitterion were characterized by both experimental techniques and theoretical methods. Steady-state and time-resolved emission, triplet-state absorption and singlet oxygen production were investigated. The results indicate that the lowest excited singlet is a fluorescent, relatively long-lived state (φ_r= 0.075, T_r? 4.5 ns) with an efficient intersystem crossing to the triplet manifold (φ_isc? 0-⁷)- The lowest triplet is a long-lived state (T_T? 10 μs at 295 K in 0.01 M phosphate buffer), with properties that make it a good candidate for being the precursor of the photodecarboxylation of the drug. It is quenched by oxygen at a rate of 1.7 times 10⁹M^-1 s-¹ and singlet oxygen is formed with a quantum yield of 0.32 in air-saturated solutions.     

Photoreactivity of biologically active compounds. XVI. Formation and reactivity of free radicals in mefloquine

The formation and reactivity of the triplet state and free radicals of mefloquine hydrochloride (MQ) have been investigated by pulse radiolysis and flash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet-triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M(-1) cm(-1) and the quantum yield for intersystem crossing was determined to be close to unity. Deactivation of the triplet, in the absence of oxygen, led to the formation of MQ cation and/or anion radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M(-1) cm(-1), whilst that for the anion radical was 2400 M(-1) cm(-1) at 620 nm and 3600 M(-1) cm(-1) at 350 nm. The molar absorption coefficients of the proposed neutral radical at 320 nm and 520 nm were 4000 M(-1) cm(-1) and 1300 M(-1) cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ triplet, was determined to be close to unity. Aqueous solutions of MQ were found to photoionize to yield hydrated electron and cation radical of MQ in a biphotonic process. The influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were evaluated. The reactions between MQ and solvated electrons and superoxide anion were also studied.     

Halogenated Squaraine Dyes as Potential Photochemotherapeutic Agents. Synthesis and Study of Photophysical Properties and Quantum Efficiencies of Singlet Oxygen Generation*

Abstract— We describe the synthesis and photophysical studies, including measurements of quantum yields of triplet excited states and singlet oxygen generation of bis(3,5-dibromo-2,4,6-trihydroxyphenyl)squaraine (2) and bis(3,5-diiodo-2,4,6-trihydroxyphenyl)squaraine (3). These dyes exist in solution in the protonated, neutral, single and double depro-tonated forms, depending on pH. The pK_a values of these dyes were found to be relatively lower than those of the parent bis(2,4,6-trihydroxyphenyl)squaraine (1). Only the single deprotonated forms (Sq) of 2 and 3 showed measurable fluorescence. In microheterogeneous media such as in the presence of β-cyclodextrin, cetyltrunethylammonium bromide and polyvinylpyrrolidone), bathochromic shifts in the absorption and emission spectra of Sq were observed with a substantial enhancement in their fluorescence yields. Triplet excited states are the main transient intermediates obtained upon 532 nm laser excitation of the various forms of 2 and 3 in methanol. These triplets have lifetimes in the range from 0.061 to 132 μs. The triplet quantum yields of double deprotonated forms are low (φT = <0.01), whereas the neutral and Sq^?forms of 2 (φr = 0.12 and 0.22) and 3 (φ_T= 0.24 and 0.5), respectively, exhibited significant triplet yields. Quantum yields of singlet oxygen generation by Sq^?forms of 2 and 3 were determined in methanol and were found to be 0.13 and 0.47, respectively, which are in good agreement with the triplet yields obtained in these systems.     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Riboflavin-sensitized photochemical radical ion chain reaction of sulfo-group substitution for halogen in halogenated hydroxynaphthalenes

It was shown that riboflavin (RF) is a sensitizer for the radical ion chain reaction of sulfo group substitution for halogen in halogenated hydroxynaphthalenes. The initaition mechanism involves the electron transfer reaction between the sulfite ion and excited riboflavin. The quantum yields of RF radical anions from the singlet and the triplet states upon excitation in an aqueous sodium sulfite solution are 0.01 and 0.15, respectively, as determined by means of flash photolysis. The principal decay reaction for RF radical anions is their recombination with sulfite radical anions in the bulk of solution at a rate constant of (3.8 ± 0.5) × 10⁹ dm³ mol^?1 s^?1. The quantum yields of the riboflavin-sensitized substitution reaction increases in the presence of electron scavengers (chloranil, dinitrobenzene) and inorganic salts in the system.     

Long-lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis-acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×10⁴ M⁻¹ cm⁻¹ at 570 nm), and the singlet oxygen quantum yield (Φ_Δ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S₁ and T₁ states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet-triplet energy transfer (TTET) and triplet-triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.     

Eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide

The mechanism of eosin-sensitized photoreduction of benzil with 1-benzyl-1,4-dihydronicotinamide — a model compound of NAD(P)H and the behavior of the excited states of eosin have been investigated. The effect of anthracene as a diffusion-controlled quencher of the photoreaction indicates that both excited triplet state and an unquenchable excited singlet state of eosin participated in the sensitized photoreaction. From the Stern-Volmer plot of quantum yield vs. anthracene concentration, the triplet reaction rate constant has been calculated to be 0.78 × 10⁸ L M^?1S^?1 while the singlet reaction rate constant determined from quenching of eosin fluorescence by benzil is equal to 7.2 × 10⁹ L M^?1S^?1. The singlet and triplet quantum yields are also determined to be 0.09 and 0.18 respectively. Since both the singlet and triplet energies of eosin are lower than that of benzil, energy transfer sensitization is not feasible. It is proposed that electron transfer from the excited eosin to benzil is responsible for the initiation.     

Rational Design of Emissive NIR‐Absorbing Chromophores: RhIII Porphyrin‐Aza‐BODIPY Conjugates with Orthogonal Metal–Carbon Bonds

The facile synthesis of Group 9 Rh^III porphyrin‐aza‐BODIPY conjugates that are linked through an orthogonal Rh?C(aryl) bond is reported. The conjugates combine the advantages of the near‐IR (NIR) absorption and intense fluorescence of aza‐BODIPY dyes with the long‐lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge‐transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh^III conjugates exhibit strong aza‐BODIPY‐centered fluorescence at around 720 nm (Φ_F=17–34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet‐oxygen quantum yield (Φ_Δ=19–27 %, λ_ex=690 nm) have been observed. Nanosecond pulsed time‐resolved absorption spectroscopy confirms that relatively long‐lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.     

Absolute fluorescence quantum yields for vapour-phase benzene and naphthalene,and comments on the non-radiative processes

Optic—acoustic measurements have been employed in the determination of absolute quantum yields for benzene and naphthalene. Heat yields are measured by a method using oxygen quenching of both triplet and singlet states. For vibrationally relaxed excited singlet states the fluorescence quantum yields, φ^B_f, are 0.16 ± 0.02 and 0.79 ± 0.02 for benzene and naphthalene respectively. For 0.07 torr naphthalene at room temperature with 248 nm excitation, φ_f = 0.35 ± 0.03 and the quantum yield of internal conversion is less than 0.05. The decay of the highly vibrationally excited triplet state is dominated by vibrational relaxation for 0.07 torr naphthalene, but for benzene, even at high pressures, strong competition comes from an indirect coupling process to the ground state.     

没食子酸的激光光解研究

The transient species of gallic acid(GA)have been studied by 266 nm nanosecond laser flash photolysis inaqueous solution and acetonitrile.The intermediate with absorption at 320 nm was identified as excited triplet state(~3GA~*),the decay rates of which were obtained in aqueous solution and acetonitrile respectively.Energy transferfrom ~3GA~* to β-carotene was observed and the energy transfer rate constant k_(ent)was determined to be 2.2×10~9mol~(-1)·L·s~(-1).GA underwent photoionization during photolysis and the quantum yield of photoionization was de-termined to be 0.12 at room temperature with KI as a reference.     

Broadband Visible‐Light‐Harvesting trans‐Bis(alkylphosphine) Platinum(II)‐Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

A heteroleptic bis(tributylphosphine) platinum(II)‐alkynyl complex ( Pt‐1 ) showing broadband visible‐light absorption was prepared. Two different visible‐light‐absorbing ligands, that is, ethynylated boron‐dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt‐2 and Pt‐3 , which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt‐1 , with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M ^?1 cm^?1. Long‐lived triplet excited states lifetimes were observed for Pt‐1 – Pt‐3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady‐state and time‐resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time‐resolved transient difference absorption spectra. A triplet‐state equilibrium was observed for Pt‐1 . The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt‐1 .     

Superposition of different photoionization mechanisms upon laser irradiation of pyrene in tetrahydrofuran solutions at room temperature

The photoconductivity signal consecutive to a light pulse from a 337.1 nm N₂ laser can be understood as the superposition of photocurrents stremming from two distinct photoionization mechanisms. Low light intensity (5 × 10²¹ quanta/cm²s) favors the triplet—triplet annihilation path where light absorption produces an initial population of excited triplets which subsequently undergo diffusion controlled collisional encounters resulting partly in the generation of charge carriers; high light intensity (10²³ quanta/cm²s) favors a biphotonic photoionization mechanism in which ionization occurs during the light pulse by consecutive absorption of two photons. Oxygen quenching does not affect the ion quantum yield in the biophotonic process. This fact and evidence derived from a kinetic simulation involving only the triplet and singlet states are discussed in terms of the possible intermediate state(s) necessary for consecutive biphotonic ionization.     

Laser flash photolysis of carboxymethylcellulose in an aqueous solution

Laser flash photolysis with excitation at 248 nm was used to study photochemically derived changes of carboxymethylcellulose (CMC) in aqueous solutions. Transient absorption spectra of solutions after photolysis revealed a broad band with a maximum of approximately 720 nm, which could be ascribed to the signal of the hydrated electron. The interaction of the hydrated electron with CMC was slow (<10⁷ dm³ mol^?1 s^?1), but the OH radical, formed by the decomposition of H₂O₂, reacted with CMC at a high rate constant (9.5–11.0 × 10⁸ dm³ mol^?1 s^?1). The rate constant of the reactions of CMC with hydroxyl radicals depended on the conformation of the macromolecules, which was determined by the pH of the solution. Transient absorption was recorded at a wavelength shorter than 370 nm for CMC solutions photolyzed in the presence of H₂O₂. As a result of OH attack, long‐lived radicals were formed on CMC. The recombination of macroradicals led to the formation of crosslinking bonds between side‐chain groups, and as a result of it an insoluble gel arose in low‐pH solutions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 505–518, 2005