Fluorescence-detected Magnetic Resonance Research Articles

Electron Paramagnetic Resonance: Elementary Theory and Practical Applications

Basic Principles of Electron Paramagnetic Resonance Magnetic Interactions Between Particles Isotropic Hyperfine Effects in EPR Spectra g-Anisotropy in Solids Hyperfine Anisotropy in Solids Systems with More than One Unpaired Electron Paramagnetic Species in Gas Phase Transition-Group Ions The Interpretation of EPR Parameters Relaxation Times, Linewidths and Kinetic Phenomena Time-Dependent Excitation of Spins Double-Resonance Techniques Other Topics.

FDMR spectroscopy of peridinin-chlorophyll-a protein from Amphidinium carterae

Fluorescence detected magnetic resonance (FDMR) is used to investigate peridinin-chlorophyll-a protein (PCP), the peripheral antenna of the photosystem of Amphidinium carterae. Peridinin triplet resonances are obtained monitoring chlorophyll-a fluorescence in agreement with efficient energy transfer between the two pigments. Two sites are revealed for peridinin having slightly different triplet energy (about 190 cm −1), and D values averaging 0.045 cm −1. Spin exchange occurs between the two sites at a rate of about 5 × 10 7 exp (−67/ T) s −1.

FDMR in Low-Temperature Solids. 2. Observation of Weakly Bound Electrons

Time-resolved fluorescence-detected magnetic resonance (FDMR) was used to study radiolysis of organic solids at low temperatures (4-30 K). The mobility of charges generated in the primary events of solid-state radiolysis may be accounted for by two mechanisms: single-step tunneling and relatively slow (10[sup 6]-10[sup 7] s[sup [minus]1]) resonant charge transfer involving the solvent hole. Acting together, these mechanisms engender a variety of unusual and previously unknown spectral features. Although the tunneling satisfactorily explains FDMR behavior in general, quantitative agreement is poor, particularly for donor-acceptor systems. It appears that the origin of this discrepancy lies in an underestimation of the tunneling radii in reactions of negative charges. The latter is rationalized in terms of a selectivity of FDMR toward mobile weakly bound electrons and excited radical anions formed after electron scavenging. The possible nature of these weakly bound electronic states is discussed. 50 refs., 13 figs., 2 tabs.

FDMR in Low-Temperature Solids. 1. Theory

Various mechanisms of geminate recombination in spin-correlated radical ion pairs produced by radiolysis of organic solids under cryogenic conditions are considered and compared with experimental data. Single-step electron tunneling and slow resonant hole transfer are examined from the viewpoint of their manifestation in pulsed time-resolved fluorescence-detected magnetic resonance (FDMR). Two theoretical models treating the phenomenon are developed, and their predictions are compared with the experimental results. These models are found to be consistent with many observed features: t[sup [minus]1] decay kinetics, unusual evolution of the FDMR spectra with time, narrow radial distribution of the FDMR-active pairs, and efficient spin memory transfer from the primary pairs. However, with very few exceptions the tunneling theories fail to reproduce the exact scale of the effect and its dependence on solute concentration and thermalization distance of electrons. This discrepancy suggests that the negative charge is more mobile than predicted by the single-step tunneling model. 31 refs., 8 figs.

FDMR of chlorophyll triplets in integrated particles and isolated reaction centres of Photosystem II. Identification of P680 triplet

Fluorescence detected magnetic resonance (FDMR) of various chlorophyll a triplets in PS II large particles was studied with the aim of identifying the nature of the different components underlying the complex line shape of the signals. The latter are deconvoluted into sums of Gaussians in different experimental conditions, and the charge recombination triplet is unequivocally identified as the source of two lines with negative signs at 722.5 and 992 MHz |D| = 0.0286 cm −1, |E| = 0.0043 cm −1). The origin of the other triplet signals from the integrated particles is discussed in terms of functional antenna components and products from alteration of the native environment of pigments. FDMR signals also arise from isolated reaction centre complexes and are all to be attributed to different triplet populations of the primary electron donor P680. The results are compared with those obtained by other authors on ADMR. The heterogeneity of P680 pigments in D 1D 2-cyt- b 559 complexes is shown to arise during preparation procedures.

Fluorescence detected magnetic resonance (FDMR) of green sulfur photosynthetic bacteria Chlorobium sp.

Fluorescence Detected Magnetic Resonance (FDMR) spectra have been measured for whole cells and isolated chlorosomal fractions for the green photosyntheic bacteria Chlorobium phaeobacteroides (containing bacteriochlorophyll e, and isorenieratene as major carotenoid) and Chlorobium limicola (containing bacteriochlorophyll c, and chlorobactene as major carotenoid). The observed transition at 237 MHz (identical in both bacteria) and > 1100 MHz can be assigned, by analogy with published data on other carotenoids, to the 2E and D + E transitions, respectively, of Chlorobium carotenoids. Their zero field splitting (ZFS) parameters are estimated to be: |D|=0.0332 cm(-1) and |E|=0.0039 cm(-1) (chlorobactene), and |D|=0.0355 cm(-1) and |E|=0.0039 cm(-1) (isorenieratene). In the intermediate frequency range 300-1000 MHz the observed transitions can be assigned to chlorosomal bacteriochlorophylls c and e, and to bacteriochlorophyll a located in the chlorosome envelope and water-soluble protein. The bacteriochlorophyll e triplet state measured in 750 nm fluorescence (aggregated chlorosomal BChl e) is characterised by the ZFS parameters: |D|=0.0251 cm(-1) and |E|=0.0050 cm(-1).

Solid-state FDMR from sundry poly(oxyethylene) alkylphenyl ethers: a verification of the single-step electron tunneling as a cause of the charge mobility in frozen solids at 4-30 K

Observations of the time-resolved fluorescence-detected magnetic resonance (FDMR) from bridged radical ion pairs generated by pulse radiolysis of micellar aqueous and homogeneous alcohol solutions of poly(oxyethylene) (chain length 5-40) alkylphenyl ethers at 430 K are reported. Theoretical analysis of the spectra favors single-step through-bond electron tunneling as a mechanism of the geminate recombination in the pairs at these low temperatures

Optically Detected ESR (OD ESR) of ion-radical Pairs in coloured solutions. Observation of transienttrans-azobenzene radical cation

Trans-azobenzene dissolved in different liquid hydrocarbons absorbs fluorescence arising from all acceptors previously used in Fluorescence Detected Magnetic Resonance (FDMR) and Optically Detected ESR (OD ESR) spectroscopy making optical detection impossible. In this report a new acceptor,rubrene, having sufficient quantum yield of fluorescence in the red band 550–620 nm, has been proven successful. OD ESR spectra of the radical-ion pairtrans-azobenzene+/rubrene− were detected in liquid squalane (2,6,10,15,19,23-hexamethyl-tetracosane) solution in the temperature range 294–243 K. The experimental isotropic hyperfine splittings of the radical cation oftrans-azobenzene (a N=1.4 mT) have been compared with those from MNDO/INDO calculations and with those of earlier work using freon matrix studies.

Solvent effects in nonpolar solvents: Radical anion reactions

Dimer radical anions, formed by anion—molecule reactions between octafluoronaphthalene and 1,2,4,5-tetrafluorobenzene and their respective radical anions, are observed in n-hexane solvent. The radical anions are characterized by their EPR spectra obtained by the time-resolved fluorescence-detected magnetic resonance technique. In toluene solvent the dimer anions are not formed. Rapid electron-exchange reactions occur instead in at least one case. Possible reasons why the solvent affects the anion reaction mechanism are discussed.

FDMR of Carotenoid and Chlorophyll triplets in light-harvesting complex LHCII of spinach

Fluorescence detected magnetic resonance (FDMR) of the light-harvesting complex LHCII of the spinach photosynthetic machinery revealed triplet contributions both from Carotenoids and Chlorophylls. All three carotenoids present in the complex (lutein, neoxanthin and violaxanthin) are evidenced as triplet states in the FDMR signals obtained as variation of the emission intensity of the Chlorophylls in the 680 nm region. The triplets show ⋎D⋎ values of 0.0401, 0.0388 and 0.0382 cm−1. A comparison with the results obtained by ADMR (Absorption Detected Magnetic Resonance) is made and discussed. An interesting concentration effect is discovered and discussed in terms of specific interactions between carotenoids and chlorophyll molecules. Signals are also obtained by microwave sweeping in the Chlorophyll regions and one triplet is detected (⋎D⋎=0.028−0.029 cm−1). The polarization of the carotenoid signals is discussed in terms of singlet-singlet and triplet-triplet energy transfer between carotenoids and chlorophylls, also with the help of double resonance experiments. Double resonance experiments involving both carotenoids and chlorophylls signals gave negative results. It is not possible as a consequence to assess that the chlorophyll whose triplets levels are scanned in the FDMR spectra are functionally connected to the carotenoids.

Fluorescence detected magnetic resonance of the primary donor and inner core antenna chlorophyll in Photosystem I reaction centre protein: Sign inversion and energy transfer

The Photosystem I reaction centre protein CP1, isolated from barley using polyacrylamide gel electrophoresis showed an EPR (Electron Paramgnetic Resonance) spectrum with the polarisation pattern AEEAAE, typical of the primary donor triplet state (3)P700, created via radical pair formation and recombination. (3)P700 could also be detected by Fluorescence Detected Magnetic Resonance (FDMR) at λf > 700 nm even in the presence of a large number of chlorophyll antennae. Its zero field splitting parameters, D=282.5×10(-4) cm(-1) and E=38.5×10(-4) cm(-1), were independent of the detection wavelength, and agreed with ADMR (Absorption Detected Magnetic Resonance) and EPR values. The signs of the (3)P700 D+E and D-E transitions were positive (increase in fluorescence intensity on applying a resonance microwave field). In contrast, in the emission band 685 < λf < 700 nm FDMR spectra with negative D+E and D-E transitions were detected, and the D value was wavelength-dependent. These FDMR results support an excitation energy transfer model for CP1, derived from time-resolved fluorescence studies, in which two chlorophyll antenna forms are distinguished, with fluorescence at 685 < λf < 700 nm (inner core antennae, F690), and λf > 700 nm (low energy antenna sites, F720), in addition to the P700. The FDMR spectrum in F690 emission can be interpreted as that of (3)P700, observed via reverse singlet excitation energy transfer and added to the FDMR spectrum of the antenna triplet states generated via intramolecular intersystem crossing. This would indicate that reversible energy transfer between F690 and P700 occurs even at 4.2 K.

Radical-cation complexes formed by .pi.-lone pair interactions

Time-resolved fluorescence-detected magnetic resonance (FDMR) is used to observe radical cations of thioethers in alkane and aromatic hydrocarbon solvents. Monomeric sulfide radical cations are stabilized at temperatures less than {approximately} 250 K in aromatic solvents such as toluene by electron donation from the solvent. The resulting arene-thioether complex radical cations are examples of mixed complex radical cations involving the interaction between lone-pair and {pi}-orbitals. The solvent interaction inhibits reactions of thioether radical cations (dimer formation and probably proton loss) and retards other radical cation reactions, such as dimer exchange. 39 refs., 7 figs., 4 tabs.

Medium effect on the Jahn-Teller distortions of the tetramethylallene radical cation

The Jahn-Teller distortion of the delocalized {pi}-radical cation tetramethylallene{sm bullet}{sup +} (TMA{sm bullet}{sup +}) was found to be medium dependent. TMA{sup {sm bullet}} was studied in liquid hydrocarbons (195-295 K) by time-resolved fluorescence-detected magnetic resonance (FDMR) spectroscopy and in freon matrices (80-160 K) and zeolite Na-Y (80-298 K) by EPR spectroscopy. The observed coupling constants for the twelve protons of the four methyl groups of the radical cation are 11.3, 8.1, and 14.4 G, respectively. AM1 and INDO calculations suggest that different coupling constants can be explained by different twist angles in TMA{sup {sm bullet}+}. It is suggested that interaction between TMA{sup {sm bullet}+} and the chlorine or fluorine atoms of freon matrices or between TMA{sup {sm bullet}+} and the sodium atoms of the zeolite can influence the charge and spin distribution in the radical cation so that the structures that differ from that found in hydrocarbon solution can be stabilized. This is the first example of a Jahn-Teller-active radical cation that has been studied in several media.

Study of radical ions in the condensed phase by fluorescence-detected magnetic resonance

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStudy of radical ions in the condensed phase by fluorescence-detected magnetic resonanceD. W. Werst and A. D. TrifunacCite this: J. Phys. Chem. 1991, 95, 9, 3466–3477Publication Date (Print):May 1, 1991Publication History Published online1 May 2002Published inissue 1 May 1991https://doi.org/10.1021/j100162a007RIGHTS & PERMISSIONSArticle Views127Altmetric-Citations36LEARN 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 (2 MB) Get e-Alerts

Transient alkylaminium radicals in n-hexane: condensed-phase ion-molecule reactions

Time-resolved fluorescence detected magnetic resonance (FDMR) is used to observe alkylaminium radicals formed in n-hexane solutions by electron pulse radiolysis. The ease of observation of aminium radical FDMR signals increases with increasing alkyl substitution of the amine solutes. The results are discussed in terms of the ion-molecule reactions, such as proton transfer, which compete with the electron-transfer processes, i.e, the electron transfer from solute molecules to n-hexane radical cations and geminate recombination.

Primary events in hydrocarbon radiolysis: Real-time studies of radical-cation chemistry

Recent studies of the radiolysis of liquid alkanes by picosecond emission and absorption spectroscopy and by Fluorescence Detected Magnetic Resonance (FDMR) are discussed. These studies establish that the parent radical cations of smaller alkanes have short lifetimes (< 10 ns). Ion-molecule and fragmentation reactions of alkane radical cations exhibit surprising diversity. Cyclohexane and trans-decalin radical cations are the shortest-lived. Cyclohexane +· has a lifetime of ≈ 0.3 ns and also has a great propensity to transfer a proton, rather than a charge, to other additives. These observations are discussed in the context of past work on hydrocarbon radiolysis and recent measurements of G-values of solute excited states. An important conclusion is that such radical-cation reactions are quite important in any analysis of experiments designed to probe early events in hydrocarbon radiolysis where the method depends on the integrity of the solvent radical cation. Because geminate radical cation-electron recombination is typically faster than ion-molecule reactions of the radical cation, the effect of such radical cation reactions of final products is expected to be minor.

ChemInform Abstract: The Fate of Alkane Radical Cations in Liquid and Solid Hydrocarbons. Time‐Resolved Fluorescence Detected Magnetic Resonance.

Time-resolved fluorescence detected magnetic resonance (FDMR) is used to observe alkane radical cations generated by electron radiolysis in liquid and solid alkane solutions. The ease of observation of the alkane radical cations (on the time scale of tens to hundreds of nanoseconds) depends strongly on the alkane under study as well as the conditions of temperature and concentration. Ion-molecule reactions such as proton transfer or H-atom transfer are responsible for the very transient nature of the alkane radical cations and possibly account for much of the diversity of hydrocarbon radiation chemistry.

Radical ion recombination in photoionization: observations by fluorescence detected magnetic resonance

Observations of time-resolved fluorescence detected magnetic resonance (FDMR) from the fluorescent recombination of geminate radical ion pairs produced by laser photoionization of scintillators in alcohols and ethers are described. The FDMR spectrum of the geminate radical ion pair produced from N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine (TMPD) in 2-propanol was resolved and assigned to the TMPD radical cation and the solvated electron. An FDMR signal was observed in hydrocarbons with small amounts of added alcohol, but not in neat hydrocarbons. In all cases, the FDMR signal observed was a resonant decrease in fluorescence intensity, indicating that photoionization occurs predominantly from singlet excited states. It is concluded that stabilization of he geminate ion pair by electron solvation is a key requirement for the observation of FDMR from the recombination of geminate ion pairs produced by photoionization.

Proton transfer in alkane radical cations. FDMR pulse radiolysis study

Time-resolved fluorescence detected magnetic resonance (FDMR) is used to study EPR spectra of cis- and trans-decalin radical cations in neat and mixed alkane solutions in solid state. The conditions which determine the lifetime of radical cations in alkanes are explained by proton transfer reactions between radical cations and the surrounding solvent molecules. The wide difference in observed proton transfer of cis- and trans-decalin may be accounted for in terms of orientation and proximity of proton acceptor sites around the alkane radical cation. Proton transfer may be the key to the diversity of hydrocarbon radiation chemistry.

Solid state FDMR studies of ion-molecule reactions in radiolysis of saturated hydrocarbons

Studies of radical cations produced in pulse radiolysis of frozen hydrocarbon solutions have been carried out using time-resolved fluorescence-detected magnetic resonance (FDMR). These first time-resolved solid-state FDMR experiments were performed with a spectrometer modified to allow sample translation and cooling to near liquid-helium temperature. The stability of solvent radical cations in decalins increases at lower temperatures in solid phase. The contrasting behavior of the cis- and transdecalin radical cations is indicative of the importance of ion-molecule reactions of alkane radical cations. Experiments of this type provide information about the dynamics of charge pair recombination in solids.