Photolysis of dibenzyl ketones sorbed on MFI zeolites in the presence of spectator molecules: cage effects, kinetics, and external surface sites characterization

Over the past two decades, the photolytic reactions of dibenzyl ketones sorbed on zeolites have been investigated. The reported results are consistent with a supramolecular model that takes into account the physical and chemical nature of the structure of the zeolites and their effect on the reactive radical intermediates produced by photolysis of adsorbed molecules. The model incorporates various phenomena such as surface coverage, external and internal sorption, surface diffusion, radical sieving, and the resulting product distributions. This account reports direct evidence for the validation of the model through FT-IR spectroscopy and through a new method for "titrating" the binding sites via EPR spectroscopy. It is shown that it is possible to adjust and modulate the photolytic product distribution by varying the parameters of the system. The effects of co-adsorbed spectator molecules with different polarities, namely water, pyridine, and benzene, on the photolysis of o-methyldibenzyl ketone and dibenzyl ketone sorbed on MFI zeolites is examined. This study provides insights into a displacement mechanism caused by spectator molecules and further demonstrates how the product distribution of photolysis of sorbed ketones can be controlled. The kinetics of persistent radicals formed by photolysis of ketones sorbed on zeolites is directly monitored over time by EPR, providing a measure of the lifetime of these reactive organic intermediates. Finally, measurement of Langmuir isotherms was employed to provide classical evidence for the model.     

An EPR investigation of persistent radicals from the photolysis of p, p'-dialkyl-substituted phenyl benzyl ketones adsorbed on MFI zeolites

The photolysis of isomeric pairs of p,p'-dialkyl-substituted phenyl benzyl ketones adsorbed on MFI zeolites has been investigated by EPR spectroscopy. Photolysis produces persistent "benzoyl type" and "benzyl type" radicals. The dominant persistent radical produced by photolysis of any particular isomeric pair depends on the length and position of the p-alkyl chain. The results are attributed to supramolecular stereoisomers resulting from preferential adsorption of the longer alkyl chain into the pores of the zeolite.     

Supramolecular steric effects as the means of making reactive carbon radicals persistent. Quantitative characterization of the external surface of MFI zeolites through a persistent radical probe and a langmuir adsorption isotherm

The photochemistry of tetraphenylacetone (1) adsorbed on the external surface of a MFI zeolite (the sodium form of LZ-105) has been investigated in combination with computational chemistry, surface area measurements, EPR analysis, and classical adsorption isotherms. All of the methods are consistent with a supramolecular structural model in which 1 is first adsorbed strongly through intercalation of a single benzene ring into a hole on the LZ-105 external surface (site I) followed by a weaker binding to the external framework between the holes (site II) until a monolayer of 1 is formed. From both computational and surface area measurements, it is estimated that the site I holes on the external surface will be filled at ca. 0.3-0.5 wt %/wt loading of 1/LZ-105, which corresponds to 6.5 x 10(18) (ca. 10(-)(5) mol) of holes or molecules of 1 adsorbed in holes per gram of zeolite. The supramolecular composition of ca. 0.3-0.5% of 1 on LZ-105 characterizes a "break point" for the photochemistry and the EPR measurements, since it represents the value for saturation of the site I holes with 1. These conclusions are supported quantitatively by experimental isotherms of the adsorption of 1 on LZ-105. Photolysis of 1 intercalated in the site I holes causes fragmentation into two isomeric supramolecular diphenylmethyl (DPM) radicals, one (DMP)(in) which is adsorbed into the internal surface and becomes strongly persistent (half-life of many weeks) and the other (DMP)(ex) which diffuses on the external surface and rapidly dimerizes (less than a few minutes) to produce the radical-radical combination product tetraphenylethane (2). Photolysis of 1 adsorbed on the solid external surface produces two supramolecularly equivalent DPM radicals (DMP)(ex) that diffuse on the external surface and rapidly dimerize to produce 2, and do not produce persistent DPM radicals.     

Mechanism of action of sensors for reactive oxygen species based on fluorescein-phenol coupling: the case of 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid

We demonstrate the ability of a sensor containing a tethered fluorescein-phenol structure to react with peroxyl radicals and with an oxidizing agent such as potassium ferricyanide. This latter reaction yields the corresponding peroxyl radical as observed by EPR analysis. We propose that the reaction of the sensor with peroxyl and alkoxyl radicals is also initiated by the formation of the phenoxyl radicals, which is followed by radical-radical reactions and product hydrolysis responsible for the release of fluorescein. The proposed mechanism is based on results obtained by laser flash photolysis, HPLC and EPR studies of the reaction of peroxyl and alkoxyl radicals with 4-phenoxylphenol, a molecule used to mimic the behavior of the sensor.     

From molecular chemistry to supramolecular chemistry to superdupermolecular chemistry. Controlling covalent bond formation through non-covalent and magnetic interactions

The reactions of carbon centered radical pairs often involve diffusion controlled combination and/or disproportionation reactions which are non-selective. A triplet geminate pair of radicals is produced by the photolysis of suitable ketones. The reactions of such geminate pairs can be controlled though the application of supramolecular concepts which emphasize non-covalent interaction to "steer" the geminate pair toward a selected pathway. In addition, "superdupermolecular" concepts, which emphasize the control of radical pair reactions through the orientation of electron spins, can be employed to further control the course of geminate pair reactions. Examples of control of a range of the selectivity of geminate radical combinations, which form strong covalent bonds, through supramolecular and superdupermolecular effects will be presented for the photolysis of ketones adsorbed in the supercages of zeolites.     

Supramolecular effects on the dynamics of radicals in MFI zeolites: a direct EPR investigation

Photolysis of the supramolecular complexes (dibenzyl ketones@ZSM-5) produced supramolecular complexes of benzyl radicals@ZSM-5, which were directly detected by CW-EPR spectroscopy, and provided information on the dynamics of the radicals. The lifetimes of the radicals increased as the group X attached to the carbon atom at the radical center increases from X = H (t(1/2) ca. 2 min) to X = (CH(2))(4)CH(3) (t(1/2) > 200 min). In addition, line broadening of the EPR signal was observed as the group X increases. Experiments involving cation-exchanged zeolites (MZSM-5; M = Li, Na, K, Rb, Cs) showed a strong dependence of the radical lifetime on the size of the cation (t(1/2) ca. 10 min for Li and t(1/2) > 200 min for Cs). The results are discussed in terms of supramolecular steric effects on the radical-radical reactions in the zeolite supercages.     

Aminoxyl radicals as crosslinks for macromolecules of polyvinylpyrrolidone

Using polyvinylpyrrolidone as an example, it has been shown that photolysis of ceric ammonium nitrate at room temperature can result in crosslinking of macromolecules. This process correlates with the formation of stable aminoxyl radicals, which are registered by EPR. The mechanism involves photodissociation of nitrate radicals produced in the primary reaction into nitric oxide or nitrogen dioxide depending on the wavelength of the light, and simultaneous formation of macroradicals. The accumulation of aminoxyl radicals occurs owing to the acceptance of macroradicals by nitroso or nitro groups according to which mechanism of the nitrate radical photodissociation prevails. Similar radical reactions are observed in N-methyl-2-pyrrolidone.     

Dramatic Effect of Magnetite Particles on the Dynamics of Photogenerated Free Radicals

Abstract— Polymeric particles of about 1 u.m in diameter and containing around 40% magnetite have a dramatic influence on the dynamics of triplet radical pair reactions occurring in micelles in their vicinity. The effect has been monitored with laser flash photolysis techniques and results in a decrease of the number of radicals that separate from their geminate partner and an acceleration of radical pair geminate reactions. The effect involves remote interactions because the solution contains ca 10¹¹micelles for each polymeric particle. Magnetite also perturbs the way in which the radicals interact with an external magnetic field.     

Photooxidative damage of guanine in DG and DNA by the radicals derived from the alpha cleavage of the electronically excited carbonyl products generated in the thermolysis of alkoxymethyl-substituted dioxetanes and the photolysis of alkoxyacetones.

On thermolysis of the methoxy (MeO-TMD), tert-butoxy (tBuO-TMD), and hydroxy (HO-TMD) derivatives of 3,3,4,4-tetramethyl-1,2-dioxetane (TMD) in the presence of dG and calf-thymus DNA, the guanine is oxidized considerably more efficiently than the parent TMD. The same trend in the oxidative reactivity is observed for the photolysis of the corresponding oxy-substituted ketones versus acetone. The oxidative reactivity order in the dioxetane thermolysis, as well as in the ketone photolysis, parallels the ability of the excited ketones to release radicals (determined by spin trapping with DMPO and EPR spectroscopy) upon alpha cleavage (Norrish-type-I reaction). In the presence of molecular oxygen, the carbon-centered radicals are scavenged to produce peroxyl radicals, which are proposed as the reactive species in the oxidation of the guanine in dG and calf-thymus DNA.     

A comparative photomechanistic study (spin trapping, EPR spectroscopy, transient kinetics, photoproducts) of nucleoside oxidation (dG and 8-oxodG) by triplet-excited acetophenones and by the radicals generated from alpha-oxy-substituted derivatives through Norrish-type I cleavage

The photooxidation of 2'-deoxyguanosine (dG) and its derivative 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by a series of acetophenones (AP-X) and benzophenone (BP) has been studied.The favorable absorption characteristics of the benzoyl chromophore enables time-resolved spectroscopy of the triplet ketones to assess their quenching kinetics by dG and 8-oxodG. Whereas the photolysis of acetophenone (AP), 2-acetoxyacetophenone (AP-OAc), and benzophenone (BP) does not produce radicals (group A ketones), the oxymethyl-substituted derivatives 2-hydroxyacetophenone (AP-OH) and 2-tert-butoxyacetophenone (AP-O(t)Bu) lead to carbon-centered radicals by alpha cleavage (group B ketones). For the latter ketones, this was confirmed by EPR studies with the spin trap 5,5-dimethylpyrroline N-oxide (DMPO) and by their triplet lifetimes that were shorter than those for the unsubstituted acetophenone. Both groups of ketones photooxidize dG and 8-oxodG; the oxidation products are spiroiminodihydantoin and guanidine-releasing products (GRP) in the case of dG and AP-OH also 8-oxodG. In the presence of O(2), the photooxidation by the group A ketones is efficient at high dG or 8-oxodG concentrations, whereas the group B ketones photooxidize dG and 8-oxodG also at low substrate concentrations. These results imply that peroxyl radicals are responsible for the photooxidation by the group B ketones, which are formed by alpha cleavage of the triplet ketone and subsequent O(2) trapping of the carbon-centered radicals. At higher dG concentrations, direct electron transfer from dG to the triplet ketone, as observed for the group A ketones, competes with the radical activity.     

Comment on “Persistent Room-Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry” and on “Persistent Radical Pairs between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV/Vis Absorption”

EPR spectroscopic evidence for intramolecular electron transfer in anionic N-substituted naphthalimides to yield persistent diradical anions and intermolecular electron transfer from a variety of carbanions to 6-bromo-N-phenyl-naphthalimide to yield persistent radical–radical anion pairs was recently claimed in two papers by Zhang et al. In this comment, it is shown that the EPR spectra published in both papers do not agree with the proposed triplet-state species. Rather, the spectra are due to various doublet-state radicals, deriving from minor side reactions. The misinterpretations invalidate the general conclusions of the papers.     

Spectroscopic properties of tyrosyl radicals in dipeptides

Redox-active tyrosine residues play important roles in long-distance electron reactions in enzymes, including prostaglandin H synthase, galactose oxidase, ribonucleotide reductase, and photosystem II. Magnetic resonance and vibrational spectroscopy provide methods with which to study the structures of redox-active amino acids in proteins. In this report, ultraviolet photolysis was used to generate tyrosyl radicals from polycrystalline tyrosinate or dipeptides, and the structure of the radical was investigated with EPR and reaction-induced FT-IR spectroscopy at 77 K. Photolysis at 77 K is expected to generate a neutral tyrosyl radical through oxidation of the aromatic ring. EPR and FT-IR results obtained from (13)C-labeled tyrosine were consistent with that expectation. Surprisingly, labeling of the tyrosyl amino group with (15)N also resulted in isotope-shifted bands in the photolysis spectrum. The force constant of a NH deformation mode increased when the tyrosyl radical was generated. These data suggest an interaction between the pi system of the tyrosyl radical and the amino group. In spectra acquired from the dipeptides, evidence for a sequence-dependent interaction between the tyrosyl radical and the amide bond of the dipeptide was also obtained. We postulate that perturbation of the amino or the amide/imide groups may occur through a spin polarization mechanism, which is indirectly detected as a change in NH force constant. This conclusion is supported by density functional calculations, which suggest a conformationally sensitive delocalization of spin density onto the amino and carboxylate groups of the tyrosyl radical. These experiments provide a step toward a detailed spectral interpretation for protein-based tyrosyl radicals.     

Kinetics of the reaction C2H5 + HO2 by time-resolved mass spectrometry

The overall rate constant for the radical-radical reaction C2H5 + HO2 --> products has been determined at room temperature by means of time-resolved mass spectrometry using a laser photolysis/flow reactor combination. Excimer laser photolysis of gas mixtures containing ethane, hydrogen peroxide, and oxalyl chloride was employed to generate controlled concentrations of C2H5 and HO2 radicals by the fast H abstraction reactions of the primary radicals Cl and OH with C2H6 and H2O2, respectively. By careful adjustments of the radical precursor concentrations, the title reaction could be measured under almost pseudo-first-order conditions with the concentration of HO2 in large excess over that of C2H5. From detailed numerical simulations of the measured concentration-time profiles of C2H5 and HO2, the overall rate constant for the reaction was found to be k1(293 K) = (3.1 +/- 1.0) x 10(13) cm3 mol(-1) s(-1). C2H5O could be confirmed as a direct reaction product.     

Structure of the Paramagnetic Products of Thiocyanate Photolysis at 77 K

Solid paramagnetic products of thiocyanate photolysis were studied at 77 K by EPR and diffuse reflectance spectroscopy. Ab initio calculations of the electronic structure of these products have been carried out. Direct UV irradiation can generate the SCN⁰ radical due to ionization of the thiocyanate ion. Interaction of the SCN⁰ radical with the neighboring thiocyanate ion forms another radical product of photolysis — (SCN)₂ ^- radical ion.     

Probing the reactivity of photoinitiators for free radical polymerization: time-resolved infrared spectroscopic study of benzoyl radicals

A series of substituted benzoyl radicals has been generated by laser flash photolysis of alpha-hydroxy ketones, alpha-amino ketones, and acyl and bis(acyl)phosphine oxides, all of which are used commercially as photoinitiators in free radical polymerizations. The benzoyl radicals have been studied by fast time-resolved infrared spectroscopy. The absolute rate constants for their reaction with n-butylacrylate, thiophenol, bromotrichloromethane and oxygen were measured in acetonitrile solution. The rate constants of benzoyl radical addition to n-butylacrylate range from 1.3 x 10(5) to 5.5 x 10(5) M(-1) s(-1) and are about 2 orders of magnitude lower than for the n-butylacrylate addition to the counterradicals that are produced by alpha-cleavage of the investigated ketones. Density functional theoretical calculations have been performed in order to rationalize the observed reactivities of the initiating radicals. Calculations of the phosphorus-centered radicals generated by photolysis of an acyl and bis(acyl)phosphine oxide suggest that P atom Mulliken spin populations are an indicator of the relative reactivities of the phosphorus-centered radicals. The alpha-cleavage of (2,4,6-trimethylbenzoyl)phosphine oxide was studied by picosecond pump-probe and nanosecond step-scan time-resolved infrared spectroscopy. The results support a mechanism in which the alpha-cleavage occurs from the triplet excited state that has a lifetime less than or equal to the singlet excited state.     

Photoinduced Electron Transfer from Xanthates to Acyl Azoliums: Divergent Ketone Synthesis via N-Heterocyclic Carbene Catalysis

Considering the prevalence of alcohols and carboxylic acids, their fragment cross-coupling reactions could hold significant implications in organic synthesis. Herein, we report a versatile method for synthesizing a diverse range of ketones from alcohols and carboxylic acid derivatives via N-heterocyclic carbene (NHC) catalysis. Mechanistic investigations revealed that photoexcited xanthates and acyl azoliums undergo single electron transfer (SET) under photocatalyst-free conditions, generating NHC-derived ketyl radicals and alkyl radicals. These open-shell intermediates subsequently undergo the radical-radical cross-coupling reaction, yielding valuable ketones. Furthermore, this approach can be employed in three-component reactions involving alkenes and enynes, resulting in structurally diverse cross-coupled ketones. The unified strategy offers a unique opportunity for the fragment coupling of a diverse range of alcohols and carboxylic acid derivatives, accommodating diverse functional groups even in complex settings.     

Oxygen uptake and involvement of superoxide radicals upon photolysis of ketones in air-saturated aqueous alcohol, formate, amine or ascorbic acid solutions

The photolysis of acetophenone, benzophenone, 4-carboxybenzophenone and benzil was studied in air-saturated aqueous solution in the presence of alcohols. The overall reaction is an oxidation of 2-propanol to acetone. The quantum yield of oxygen uptake (Phi(-O(2))) increases with increasing 2-propanol concentration up to 0.9. The photoreaction can also be initiated by quenching of the ketone triplet state by ascorbic acid, formate or an amine e.g. triethylamine. Subsequent reactions of the involved radicals with oxygen yield the superoxide radical and eventually hydrogen peroxide. For the ketones in the presence of 3-30 mM ascorbic acid or triethylamine Phi(-O(2)) = 0.3-0.9. The specific properties of ketones, including 4-methoxyacetophenone and 2-acetonaphthone, the radicals involved and the pH and concentration dependences of Phi(-O(2)) are discussed.     

Hyperfine structure observed in EPR spectra of polyphenylacetylene solutions

The high-temperature (>120C) electron paramagnetic resonance (EPR) spectrum of solutions of polyphenylacetylene have been deconvoluted into the spectra of two separate radicals, a delocalized π radical, whose EPR spectrum consists of a single 15-G wide Gaussian line comprising about 90% of the total signal and a second, more localized π radical exhibiting complex hyperfine structure in its EPR spectrum. Some possible structures for the minor component radical are suggested and their hyperfine splitting constants calculated using molecular orbital theory.     

Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.     

Magnetic Field Effects on the Dynamics of Radical Pairs in Micelles: A New Approach to Understanding the "Cage Effect"

The effect of temperature on the photolysis of dibenzyl ketone and 4-methyldibenzyl ketone in sodium dodecyl sulfate micelles was studied by laser flash photolysis and product distributions derived from steady-state photolysis. At high temperatures, the product distribution and radical decay kinetics are primarily due to random encounters of radicals, and the "cage effect" cannot be rationalized by geminate recombination reactions that occur before the radicals escape from the micelles. A mechanism is proposed in which the enhancement of the crosstermination product derived from random encounters is due to the different partitioning of each radical species between the micelles and the aqueous phase, thereby leading to different rates for the self-termination reactions.