Time-resolved ESR Spectroscopy Research Articles

Termination Kinetics of Surface-Initiated Radical Polymerization Measured by Time-Resolved ESR Spectroscopy after Laser-Pulse Initiation

The single-pulse–pulsed-laser polymerization–electron spin resonance (SP-PLP-ESR) method was used to study the termination reaction in polymerization of n-butyl methacrylate (n-BMA) from silica nanoparticles. The polymerization from the surface was initiated by the surface-attached photoinitiator 2,2-dimethoxy-2-phenyl-1-(2-(2-(trimethoxysilyl)ethyl)phenyl)ethane-1-one (DMPTS). The obtained termination rate coefficient at 335 K of kt = (5.4 ± 0.7) × 106 L mol–1 s–1, which could unambiguously be assigned to the termination reaction between two surface-attached macroradicals, is about 2 orders of magnitude smaller than the value found in homogeneous n-BMA polymerization. Surprisingly, no chain-length dependence of the termination rate coefficient could be observed. This fact and temperature-dependent studies of kt, which yield an activation energy that is in perfect agreement with the activation energy of propagation of n-BMA, indicate that the termination between surface-attached macroradicals proceeds under reaction diffusion control, as their translational diffusion is suppressed.

Unusually Large Dynamic Electron Polarization in an O2(1Δg)−2,2,6,6-Tetramethylpiperidine-1-oxyl Radical System

Net absorptive (Abs) chemically induced dynamic electron polarization (CIDEP) is observed in the system of the lowest excited singlet molecular oxygen O2(1Δg) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in benzene by means of the time-resolved ESR (TR-ESR) spectroscopy. As a result of (1) the analysis of CIDEP time profiles by the modified Bloch and kinetic equations and (2) the quenching of CIDEP by triethylamine, it is confirmed that the Abs CIDEP is created through the interaction of O2(1Δg) with TEMPO. The creation of Abs CIDEP is reasonably interpreted in terms of the radical-triplet pair mechanism (RTPM) of the O2(3Σg-)−TEMPO pair with doublet precursor. The absolute magnitude of net Abs CIDEP created on TEMPO is experimentally estimated to be 340 ± 40 in the unit of Boltzmann polarization. To our knowledge, this is one of the largest magnitudes of CIDEP measured so far by the TR-ESR method. Based upon the RTPM theory, the origin of this unusually large CIDEP is attributed to the large ...

Time resolved ESR study on energy difference of quartet and doublet states in radical-triplet encounter pairs

The energy difference J of quartet and doublet states in a radical-triplet pair (RTP) has been studied in pairs of triplet naphthalene (Np) derivative/galvinoxyl(Galv) and triplet biphenyl (Biph) derivative/Galv systems by time resolved ESR spectroscopy. Most of the systems studied show an unusual positive sign for J that corresponds to the relative location of the quartet state being below the doublet state. In order to explain this unusual sign of J in RTPs, the effects of intermolecular charge transfer (CT) states were examined. The observation was explained by introducing configuration interaction between zero-order RTP (RTP0) and CT (CT0) states in the encounter pair, where the RTP0 state is located above the CT0 state. Among the RTP systems examined, triplet 1-chloro-Np/Galv, triplet 1-bromo-Np/Galv and triplet 4-cyano-Biph/Galv systems show a normal negative value of J. On the basis of the estimated energy gap between the RTP0 and CT0 states, it was concluded that configuration interaction is not significant in these three RTP systems. The value of J in these RTP systems is dominated by exchange interaction, giving a negative sign. The solvent effect on the sign of J in a triplet Np/Galv system is also reported, as evidence that the sign of J is controlled by the CT0 state.

Spectroscopy, Electrochemistry, and Molecular Orbital Calculations of Metal-Free Tetraazaporphyrin, -chlorin, -bacteriochlorin, and -isobacteriochlorin

Metal-free tetraazachlorin (TAC), -bacteriochlorin (TAB), and -isobacteriochlorin (TAiB) were characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence, and time-resolved ESR (TR-ESR) spectroscopy, and by cyclic voltammetry. The results are compared with those of metal-free tetraazaporphyrin (TAP). The potential difference DeltaE between the first oxidation and reduction couples decreases in the order TAP>TAiB>TAC>TAB. The splitting of both the Q and Soret bands decreases in the order TAB>TAC>TAP>TAiB. Corresponding to the split absorption bands, MCD spectra show a minus-to-plus pattern with increasing energy in both the Q and Soret regions, which suggests that the energy difference between the HOMO and second HOMO is larger than that between the LUMO and second LUMO. These spectroscopic properties and redox potentials were reproduced by molecular orbital calculations using the ZINDO/S Hamiltonian. The fluorescence quantum yields of the reduced species are much smaller than that of TAP. The zero-field splitting (ZFS) parameters D and E of the excited triplet states (T1) of these species decrease and increase, respectively, on going from TAP to TAC and further to TAB. The D and E values of TAiB are larger than those of the other species. The results are supported by the absence of interaction between the spin over reduced pyrrole moieties of the HOMO and over the LUMO, and by calculations of ZFS under a half-point-charge approximation.

Computational characterization of sulfur-cxygen-bonded sulfuranyl radicals derived from alkyl- and (carboxyalkyl)thiopropionic acids: evidence for sigma-type radicals.

Sulfide radical cations are known to stabilize via sulfur-oxygen bond formation with carboxylic acid functions. However, structural information on the resulting sulfuranyl radicals has so far only been provided by ESR spectroscopy for more stable, aromatic-substituted species, while the rather short-lived aliphatic sulfuranyl radicals have largely been characterized by time-resolved UV spectroscopy only. Therefore, we have obtained theoretical structural information on S-O-bonded sulfuranyl radicals from three model compounds, tert-butyl 2-(methylthio)peroxybenzoate, 3-methylthiopropionic acid, and 3,3'-thiodipropionic acid, using density functional theory, semiempirical, and molecular mechanics methods. All S-O-bonded species exist predominantly as three-electron-bonded sigma-radicals with an estimated heterolytic bond dissociation energy of the S therefore O bond on the order of 23-27 kcal mol(-1). Characteristic optical absorption bands, vibrational frequencies, and hyperfine coupling tensors are evaluated to facilitate the identification of such radicals by time-resolved UV, IR, and ESR spectroscopy.

Identification of Radicals and Determination of Their Yields in the Radiolytic Oxidation of Glycine. Time-resolved ESR Methodology

Both aminomethyl (•CH2NH2) and H2N−•CH−CO2- radicals were identified by time-resolved ESR spectroscopy in the oxidation of glycine anions by •OH. Previous ESR measurements had detected only the H2N−•CH−CO2- radical in these systems. The rise times of the ESR lines for these two different radicals are consistent with both of the radicals being formed in the initial reaction of •OH with the glycine anions. The yields of these two C-centered radicals relative to the total yield of •OH are approximately 29% for aminomethyl radicals and 53% for H2N−•CH−CO2- radicals. The yield of aminomethyl radicals from methlyamine was 53%. No ESR lines attributable to aminyl radicals could be directly detected in solutions of glycine (HN•−CH2−CO2-) at high pH or low pH, or in solutions of methylamine (HN•−CH3). However, clear evidence for the presence of the aminyl radicals was obtained in spin-trapping experiments using the aci-anion form of nitromethane with both methylamine and glycine. The rapid appearance of aminomethyl radicals in a complementary experiment with hydrated electrons reacting with N-chloroglycine could be simply explained on the basis of the formation of N-centered radicals in this situation. The ESR method for determining the yields was tested in the •OH oxidation of methylamine and succinic acid. The implications of the findings of this work are discussed in the context of the recently proposed scheme for the oxidation of glycine anions by •OH.

Propagating Radicals in Photo-Initiated Polymerization as Detected by Time-Resolved CW- and FT-ESR Spectroscopy

Abstract The primary propagating radicals in photo-initiated polymerization of isoprene and MMA were directly detected and assigned for the first time by use of time-resolved ESR spectroscopy. Based on the kinetic analysis including the spin-lattice relaxation, the rate constant of the formation and the relaxation time were determined for the primary propagating radicals by simulating time-profile of the FT-ESR intensity.

Exchange Interaction in Radical−Triplet Pairs: Evidences for CIDEP Generation by Level Crossings in Triplet−Doublet Interactions

Chemically induced dynamic electron polarization (CIDEP) generated through interaction of the excited triplet state of 1-chloronaphthalene, benzophenone, benzil, and Buckminsterfullerene (C60) with 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by using time-resolved ESR spectroscopy. We carefully examined what factors affect the CIDEP intensities. By comparing CIDEP intensities of TEMPO in the 1-chloronaphthalene, benzophenone, and benzil systems with that obtained in the C60−TEMPO system, the absolute magnitude of net emissive polarization was determined to be −2.2, −6.9, and −8.0, respectively, in the units of Boltzmann polarization. In the 1-chloronaphthalene−TEMPO system, the viscosity effect on the magnitude of net polarization was studied by changing the temperature (226−275 K) in 2-propanol. The emissive polarization was concluded to result from the state mixing between quartet and doublet manifolds in a radical−triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. The magnitude of net polarization is much larger than the polarization calculated with the reported theory that the CIDEP is predominantly generated in the region where the exchange interaction is smaller than the Zeeman energy. Our experimental results are quantitatively explained by the theory that the CIDEP is generated predominantly in the regions where the quartet and doublet levels cross. We propose a theoretical treatment to calculate the magnitude of net polarization generated by the level crossings in the radical−triplet pair mechanism under highly viscous conditions and perform a numerical analysis of the net RTPM polarization with the stochastic-Liouville equation. The viscosity dependence of the net polarization indicates that the back transition from the doublet to quartet states sufficiently occurs in the level-crossing region under highly viscous conditions. The estimated large exchange interaction suggests that the quenching of the excited triplet molecules by TEMPO proceeds via the electron exchange interaction.

CIDEP measurement of the electron spin-lattice relaxation time of triplet benzophenone in liquid solution

The relaxation of chemically induced electron polarization (CIDEP) in photoexcited triplet benzophenone is measured in competition with triplet quenching by 2,2′-azobis[isobutyronitrile] (AIBN). The spin polarization, transferred to the resulting 2-cyano-2-propyl radicals, is determined by time-resolved ESR spectroscopy for different AIBN concentrations. The rate constant for quenching is found to be k Q = 4.4 × 10 9 M −1 s −1, and 3 T 1 = 0.8 ns is obtained for the electron spin-lattice relaxation time of triplet benzophenone in 1,2-epoxypropane at room temperature.

Absolute magnitude of spin polarization in the radical-triplet pair mechanism: CIDEP generation by level crossings in a triplet-doublet interaction

Chemically induced dynamic electron polarization (CIDEP) generated through the interaction of the excited triplet state of benzophenone with the 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by time-resolved ESR spectroscopy in benzene solution. We carefully examined what factors control the CIDEP intensities. By comparing the CIDEP intensity of TEMPO obtained in the triplet benzophenone-TEMPO system with the intensity in the C 60-TEMPO system, the absolute magnitude of net emissive polarization was determined to be −6.9 in the unit of Boltzmann polarization. The emissive polarization is attributed to state mixing between a quartet and a doublet in the radical-triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. Our result is quantitatively explained by the theory that the net CIDEP is generated predominantly in regions where the quartet and doublet levels cross. This indicates that the quenching of the excited triplet benzophenone by TEMPO in benzene can proceed via an electron-exchange interaction.

Time-resolved Esr spectroscopy (Cidep) of ketones

Time-resolved Esr spectroscopy (Cidep) of ketones

A small protein in model membranes: a time-resolved fluorescence and ESR study on the interaction of M13 coat protein with lipid bilayers.

Model membranes with unsaturated lipid chains containing various amounts of M13 coat protein in the alpha-helical form were studied using time-resolved fluorescence and ESR spectroscopy. The lipid-to-protein (L/P) ratios used were > 12 to avoid protein-protein contacts and irreversible aggregation leading to beta-polymeric coat protein. In the ESR spectra of the 12-SASL probe in dioleoyl phosphatidylcholine (DOPC) bilayers no second protein induced component is observed upon incorporation of M13 coat protein. However, strong effects are detected on the ESR lineshapes upon changing the protein concentration. The ESR lineshapes are simulated by assuming a fixed ratio between the parallel (D parallel) and perpendicular (D perpendicular) diffusion coefficients of 4, and an order parameter equal to zero. It is found that increasing the protein concentration from L/P infinity to L/P 15 results in a decrease of the rotational diffusion coefficient D perpendicular from 3.4 x 10(7) to 1.9 x 10(7) s-1. In the time-resolved fluorescence experiments with DPH-propionic acid as a probe, it is observed that increasing the M13 coat protein concentration causes an increase of the two fluorescent lifetimes, indicating an increase in bilayer order. Analysis of the time-resolved fluorescence anisotropy decay allows one to quantitatively determine the order parameters <P2> and <P4>, and the rotational diffusion coefficient D perpendicular of the fluorescent probe. The order parameters <P2> and <P4> increase from 0.34 to 0.55 and from 0.59 to 0.77, respectively, upon adding M13 coat protein to DOPC bilayers with an L/P ratio of 35.(ABSTRACT TRUNCATED AT 250 WORDS)

CIDEP studies of the photolysis of the lignin model compound α-guaiacoxylacetoveratrone: the role of triplet reactions in aqueous and hydroxylic solvents

Free radical reactions induced by the photolysis of the lignin model compound α-guaiacoxylacetoveratrone have been studied by conventional and time-resolved ESR spectroscopy. In the presence of efficient hydrogen donors such as aqueous and hydroxylic solvents the primary reaction involves photoreduction of the triplet phenacyl ether to form the ketyl radical followed by rapid cleavage to the phenacyl radical and guaiacol. Subsequent formation of polarized guaiacoxyl radicals is due to secondary photo-oxidation. The cleavage of the ketyl radical is retarded in basic media and accelerated in acetic acid. Minor reaction pathways involving excited singlets cannot be ruled out by the current CIDEP observations.

Introduction to pulsed ESR spectroscopy

Time resolved ESR spectroscopy makes possible direct studies of paramagnetic systems as organic radicals and metallic complexes. Spin dynamics may be directly studied and short lifetime radicals may be observed. Spin echoes are the basis of pulsed ESR spectroscopy. They are characteristic of the interactions of the electronic spins with its environment. These interactions may be studied by either one or multidimensional spectroscopies.

Cidep after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism

Benzil ketyl radicals are generated by laser flash irradiation of benzil in 2-propanol at T = -50 °C and are observed by time-resolved ESR spectroscopy. Their electron spin polarization is found to consist of a fast and slowly rising emissive component. The fast component is due to polarized ketyl radicals formed by a two-photon process from an excited triplet state. The slow one is attributed to ketyl radicals which are generated by a slow photoreduction of benzil in its lowest triplet state. Their emissive polarization stems predominantly from the radical-triplet pair mechanism (RTPM). Rate constants of the relevant processes are determined.

ChemInform Abstract: Spin Polarization Conservation During Triplet‐Triplet Energy Transfer in Fluid Solution as Studied by Time‐Resolved ESR Spectroscopy

ChemInformVolume 21, Issue 32 Physical Organic Chemistry ChemInform Abstract: Spin Polarization Conservation During Triplet-Triplet Energy Transfer in Fluid Solution as Studied by Time-Resolved ESR Spectroscopy K. AKIYAMA, K. AKIYAMA Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorA. KANEKO, A. KANEKO Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorS. TERO-KUBOTA, S. TERO-KUBOTA Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorY. IKEGAMI, Y. IKEGAMI Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this author K. AKIYAMA, K. AKIYAMA Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorA. KANEKO, A. KANEKO Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorS. TERO-KUBOTA, S. TERO-KUBOTA Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this authorY. IKEGAMI, Y. IKEGAMI Chem. Res. Inst. Non-Aqueous Solutions, Tohoku Univ., Sendai 980, Jap.Search for more papers by this author First published: August 7, 1990 https://doi.org/10.1002/chin.199032039Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue32August 7, 1990 RelatedInformation

Spin polarization conservation during triplet-triplet energy transfer in fluid solution as studied by time-resolved ESR spectroscopy

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSpin polarization conservation during triplet-triplet energy transfer in fluid solution as studied by time-resolved ESR spectroscopyKimio Akiyama, Atsuko Kaneko, Shozo Tero-Kubota, and Yusaku IkegamiCite this: J. Am. Chem. Soc. 1990, 112, 9, 3297–3301Publication Date (Print):April 1, 1990Publication History Published online1 May 2002Published inissue 1 April 1990https://doi.org/10.1021/ja00165a006RIGHTS & PERMISSIONSArticle Views134Altmetric-Citations10LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (575 KB) Get e-Alerts Get e-Alerts

A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution

Transient alkyl radicals and triplet-state molecules are generated simultaneously in solution by laser flash irradiation, and chemically induced electron polarization (CIDEP) of the radicals is studied by time-resolved ESR spectroscopy. The presence of triplet-state molecules is found to induce emissive ESR polarizations. The phenomenon is attributed to triplet-state quenching by radicals as well as to quartet-doublet mixing and splitting in radical-triplet encounters.

Electron spin polarization of free radicals induced by electron-nuclear cross relaxation

Transient radicals •C (CH 3) 2X are generated in solution by laser flash photolysis, and chemically induced electron polarization (CIDEP) is investigated by time-resolved ESR spectroscopy. The sign reversal of multiplet polarization at longer times, observed for t-butyl and 2-hydroxy-2-propyl radicals is studied as a function of temperature and solvent and is explainable by efficient electron-nuclear cross relaxation due to modulation of the nonplanarity at the radical centre.

Two-photon processes in ketone photochemistry observed by time-resolved ESR spectroscopy

Laser flash excitation of the photopolymerization initiator ω,ω-dimethoxy-ω-phenylacetophenone at 308 nm in toluene yields benzoyl and α,α-dimethoxybenzyl radicals. The latter fragment onabsorption of a second photon to methylbenzoate and methyl radicals. This explains previous contradictory results on the photochemistry of the system. Further, excitation of triplet acetone yields α-cleavage even in 2-propanol solution, where photoreduction of normal triplets is fast. The advantages of directly detected ESR to study such two-photon processes are discussed.