Fluorescence-detected Magnetic Resonance Research Articles

An unusual triplet population pathway in the Reaction Centre of the Chlorophyll-d binding Photosystem I of A. marina, as revealed by a combination of TR-EPR and ODMR spectroscopies

Photo-induced Chlorophyll (Chl) triplet states in the isolated Photosystem I (PSI) of Acaryochloris marina, that harbours Chl d as its main pigment, were investigated by Optically Detected Magnetic Resonance (ODMR) and Time-Resolved Electron Paramagnetic Resonance (TR-EPR), and as a function of pre-illumination of the sample under reducing redox poising. Fluorescence Detected Magnetic Resonance (FDMR) allowed resolving four Chl d triplet (3Chl d) populations (T1-T4) both in untreated and illuminated samples in the presence of ascorbate and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). The FDMR signals increased following the pre-illumination treatment, particularly for the T3 and T4 populations, which are therefore sensitive to the redox state of PSI cofactors. Microwave-induced Triplet minus Singlet (TmS) spectra were detected in the |D|-|E| resonance window of the T3 and T4 triplets. These showed a broad singlet bleaching centred at 740 nm and also displayed complex spectral structure with several derivative-like features, indicating that both the T3 and T43Chl d populations are associated with the PSI reaction centre (RC) triplet, P3740. Parallel measurements by TR-EPR demonstrated that triplet signals observed under all conditions investigated are dominated by an electron spin polarisation (esp), which is typical of intersystem crossing, differently from what expected for recombination triplet states formed from a radical pair precursor. Moreover, stronger reductant conditions obtained by pre-illumination of the samples in the presence of dithionite and 5-methylphenazinium methyl sulfate (PMS) did not lead to a recombination triplet state esp, but rather to a decrease of the whole signal intensity. The energetics of A. marina PSI and the possible occurrence of distributions of cofactors redox properties are discussed in order to address the unexpected P3740 esp.

Luminescent Organic Triplet Diradicals as Optically Addressable Molecular Qubits.

Optical-spin interfaces that enable the photoinitialization, coherent microwave manipulation, and optical read-out of ground state spins have been studied extensively in solid-state defects such as diamond nitrogen vacancy (NV) centers and are promising for quantum information science applications. Molecular quantum bits (qubits) offer many advantages over solid-state spin centers through synthetic control of their optical and spin properties and their scalability into well-defined multiqubit arrays. In this work, we report an optical-spin interface in an organic molecular qubit consisting of two luminescent tris(2,4,6-trichlorophenyl)methyl (TTM) radicals connected via the meta-positions of a phenyl linker. The triplet ground state of this system can be photoinitialized in its |T0⟩ state by shelving triplet populations as singlets through spin-selective excited-state intersystem crossing with 80% selectivity from |T+⟩ and |T-⟩. The fluorescence intensity in the triplet manifold is determined by the ground-state polarization, and we show successful optical read-out of the ground-state spin following microwave manipulations by fluorescence-detected magnetic resonance spectroscopy. At 85 K, the lifetime of the polarized ground state is 45 ± 3 μs, and the ground state phase memory time is Tm = 5.9 ± 0.1 μs, which increases to 26.8 ± 1.6 μs at 5 K. These results show that luminescent diradicals with triplet ground states can serve as optically addressable molecular qubits with long spin coherence times, which marks an important step toward the rational design of spin-optical interfaces in organic materials.

Chlorophyll triplet states in thylakoid membranes of Acaryochloris marina. Evidence for a triplet state sitting on the photosystem I primary donor populated by intersystem crossing.

Photo-induced triplet states in the thylakoid membranes isolated from the cyanobacterium Acaryocholoris marina, that harbours Chlorophyll (Chl) d as its main chromophore, have been investigated by Optically Detected Magnetic Resonance (ODMR) and time-resolved Electron Paramagnetic Resonance (TR-EPR). Thylakoids were subjected to treatments aimed at poising the redox state of the terminal electron transfer acceptors and donors of Photosystem II (PSII) and Photosystem I (PSI), respectively. Under ambient redox conditions, four Chl d triplet populations were detectable, identifiable by their characteristic zero field splitting parameters, after deconvolution of the Fluorescence Detected Magnetic Resonance (FDMR) spectra. Illumination in the presence of the redox mediator N,N,N',N'-Tetramethyl-p-phenylenediamine (TMPD) and sodium ascorbate at room temperature led to a redistribution of the triplet populations, with T3 (|D|= 0.0245cm-1, |E|= 0.0042cm-1) becoming dominant and increasing in intensity with respect to untreated samples. A second triplet population (T4, |D|= 0.0248cm-1, |E|= 0.0040cm-1) having an intensity ratio of about 1:4 with respect to T3 was also detectable after illumination in the presence of TMPD and ascorbate. The microwave-induced Triplet-minus-Singlet spectrum acquired at the maximum of the |D|-|E| transition (610MHz) displays a broad minimum at 740nm, accompanied by a set of complex spectral features that overall resemble, despite showing further fine spectral structure, the previously reported Triplet-minus-Singlet spectrum attributed to the recombination triplet of PSI reaction centre, [Schenderlein M, Çetin M, Barber J, et al. Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium Acaryochloris marina. Biochim Biophys Acta 1777:1400-1408]. However, TR-EPR experiments indicate that this triplet displays an eaeaea electron spin polarisation pattern which is characteristic of triplet sublevels populated by intersystem crossing rather than recombination, for which an aeeaae polarisation pattern is expected instead. It is proposed that the observed triplet, which leads to the bleaching of the P740 singlet state, sits on the PSI reaction centre.

Differential Roles of Carotenes and Xanthophylls in Photosystem I Photoprotection.

Carotenes and their oxygenated derivatives, xanthophylls, are structural elements of the photosynthetic apparatus and contribute to increasing both the light-harvesting and photoprotective capacity of the photosystems. β-Carotene is present in both the core complexes and light-harvesting system (LHCI) of Photosystem (PS) I, while xanthophylls lutein and violaxanthin bind exclusively to its antenna moiety; another xanthophyll, zeaxanthin, which protects chloroplasts against photooxidative damage, binds to the LHCI complexes under conditions of excess light. We functionally dissected various components of the xanthophyll- and carotene-dependent photoprotection mechanism of PSI by analyzing two Arabidopsis mutants: szl1 plants, with a carotene content lower than that of the wild type, and npq1, with suppressed zeaxanthin formation. When exposed to excess light, the szl1 genotype displayed PSI photoinhibition stronger than that of wild-type plants, while removing zeaxanthin had no such effect. The PSI-LHCI complex purified from szl1 was more photosensitive than the corresponding wild-type and npq1 complexes, as is evident from its faster photobleaching and increased rate of singlet oxygen release, suggesting that β-carotene is crucial in controlling chlorophyll triplet formation. Accordingly, fluorescence-detected magnetic resonance analysis showed an increase in the amplitude of signals assigned to chlorophyll triplets in β-carotene-depleted complexes. When PSI was fractioned into its functional moieties, it was revealed that the boost in the rate of singlet oxygen release caused by β-carotene depletion was greater in LHCI than in the core complex. We conclude that PSI-LHCI complex-bound β-carotene elicits a protective response, consisting of a reduction in the yield of harmful triplet excited states, while accumulation of zeaxanthin plays a minor role in restoring phototolerance.

Zeaxanthin Protects Plant Photosynthesis by Modulating Chlorophyll Triplet Yield in Specific Light-harvesting Antenna Subunits

Plants are particularly prone to photo-oxidative damage caused by excess light. Photoprotection is essential for photosynthesis to proceed in oxygenic environments either by scavenging harmful reactive intermediates or preventing their accumulation to avoid photoinhibition. Carotenoids play a key role in protecting photosynthesis from the toxic effect of over-excitation; under excess light conditions, plants accumulate a specific carotenoid, zeaxanthin, that was shown to increase photoprotection. In this work we genetically dissected different components of zeaxanthin-dependent photoprotection. By using time-resolved differential spectroscopy in vivo, we identified a zeaxanthin-dependent optical signal characterized by a red shift in the carotenoid peak of the triplet-minus-singlet spectrum of leaves and pigment-binding proteins. By fractionating thylakoids into their component pigment binding complexes, the signal was found to originate from the monomeric Lhcb4-6 antenna components of Photosystem II and the Lhca1-4 subunits of Photosystem I. By analyzing mutants based on their sensitivity to excess light, the red-shifted triplet-minus-singlet signal was tightly correlated with photoprotection in the chloroplasts, suggesting the signal implies an increased efficiency of zeaxanthin in controlling chlorophyll triplet formation. Fluorescence-detected magnetic resonance analysis showed a decrease in the amplitude of signals assigned to chlorophyll triplets belonging to the monomeric antenna complexes of Photosystem II upon zeaxanthin binding; however, the amplitude of carotenoid triplet signal does not increase correspondingly. Results show that the high light-induced binding of zeaxanthin to specific proteins plays a major role in enhancing photoprotection by modulating the yield of potentially dangerous chlorophyll-excited states in vivo and preventing the production of singlet oxygen.

NPQ activation reduces chlorophyll triplet state formation in the moss Physcomitrella patens

Plants live in variable environments in which light intensity can rapidly change, from limiting to excess conditions. Non-photochemical quenching (NPQ) is a regulatory mechanism which protects plants from oxidative stress by dissipating excess Chl singlet excitation. In this work, the physiological role of NPQ was assessed by monitoring its influence on the population of the direct source of light excess damage, i.e., Chl triplets (3Chl*). 3Chl* formation was evaluated in vivo, with the moss Physcomitrella patens, by exploiting the high sensitivity of fluorescence-detected magnetic resonance (FDMR). A dark adapted sample was compared with a pre-illuminated sample in which NPQ was activated, the latter showing a strong reduction in 3Chl* yield. In line with this result, mutants unable to activate NPQ showed only a minor effect in 3Chl* yield upon pre-illumination.The decrease in 3Chl* yield is equally experienced by all the Chl pools associated with PSII, suggesting that NPQ is effective in protecting both the core and the peripheral antenna complexes. Moreover, the FDMR results show that the structural reorganization in the photosynthetic apparatus, required by NPQ, does not lead to the formation of new 3Chl* traps in the LHCs. This work demonstrates that NPQ activation leads to effective photoprotection, promoting a photosystem II state characterized by a reduced probability of 3Chl* formation, due to a decreased singlet excited state population, while maintaining an efficient quenching of the 3Chl* eventually formed by carotenoids.

Fluorescence-detected magnetic resonance in organic systems: A pair-density matrix formalism approach

A theoretical approach, based on a pair-density matrix formalism, is developed for the study of the dynamic triplet-pair annihilation in organic materials. Stochastic Liouville equation, taking into account effects of static and dynamic magnetic fields, with superoperators representing coherent evolution, spin-independent annihilation rate, spin-dependent recombination, and diffusion of triplet exciton pairs, is used. This approach is applied to analyze fluorescence detected magnetic resonance (FDMR) spectra of one-dimensional and two-dimensional exciton motion systems. The nearest-neighbor and the long-range mutual annihilations of triplets are taken in account, and the ${S}_{0}{Q}_{0}$ mixing, in triplet-pair states, is pointed out. The long-range annihilation rate $\ensuremath{\lambda}$ and the triplet effective decay rate $\ensuremath{\beta}$ are determined from the best fit with experimental FDMR spectra.

Spectroscopy and Photophysics of Self-Organized Zinc Porphyrin Nanolayers. 3. Fluorescence Detected Magnetic Resonance of Triplet States

Fluorescence detected magnetic resonance (FDMR) has been applied to approximately 25-nm-thick porphyrin films, containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides. Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar(+) laser produces at least two different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene solution, the zero-field splitting (ZFS) parameters were determined as |D| = (316.9 +/- 0.1) x 10(-4) cm(-1) and |E| = (32.0 +/- 0.5) x 10(-4) cm(-1). By exposure of the spin-coated films to chloroform vapor at room temperature, triplet i is converted into species ii, with |D| = (295 +/- 3) x 10(-4) cm(-1) and |E| = (121 +/- 3) x 10(-4) cm(-1). For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| = (295 +/- 1) x 10(-4) cm(-1) and |E| = (91 +/- 1) x 10(-4) cm(-1) are found. From a combined study of the FDMR- and microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species, respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species with ZFS parameters very similar to those of ligated ZnTOPP. At approximately 5 K, the lifetime of triplet i is considerably shortened compared to that of ZnTOPP in a glass at the same temperature.

Carotenoid Triplet States Associated with the Long-Wavelength-Emitting Chlorophyll Forms of Photosystem I in Isolated Thylakoid Membranes

The carotenoid triplet populations associated with the long-wavelength-emitting chlorophyll forms of photosystem I (PS I)(dagger) have been investigated in isolated spinach thylakoids by means of fluorescence-detected magnetic resonance in zero field. The spectra collected in the 730-800 nm emission range can be globally fitted assuming the presence of four different carotenoid triplet states coupled to long-wavelength-emitting forms of PS I, having zero-field-splitting parameters /D/ = 0.0359 cm(-1) and /E/ = 0.00371 cm(-1), /D/ = 0.0382 cm(-1) and /E/ = 0.00388 cm(-1), /D/ = 0.0395 cm(-1) and /E/ = 0.00397 cm(-1), and /D/ = 0.0405 cm(-1) and /E/ = 0.00411 cm(-1). On the basis of the triplet-associated fluorescence emission profile, it is suggested that those triplets are associated with light-harvesting complex I, the peripheral antenna complex of PS I.

Analysis of photosystem II triplet states in thylakoids by fluorescence detected magnetic resonance in relation to the redox state of the primary quinone acceptor QA

The dependence of PSII triplet populations as a function of the primary quinone acceptor Q A redox state has been investigated by means of the fluorescence detected magnetic resonance (FDMR) technique in isolated thylakoid membranes. The results show that the population level of a microsecond decaying component having zero field splitting parameters | D |=0.0292 cm −1 and | E |=0.0037 cm −1 is inversely correlated to that of the recombination triplet (| D |=0.0285 cm −1 | E |=0.045 cm −1 ). The results could be interpreted in terms of the fast (50–150 μs) decaying population being either the recombination triplet when the primary quinone Q A is singly reduced or a triplet located on a chlorophyll which is in tight singlet transfer coupling with the primary donor.

Magnetic Resonance Studies and Molecular Orbital Calculations on the Doublet and Triplet States of Bacteriopurpurin: a Potential Second-Generation Photosensitizer for Photodynamic Therapy

Bacteriopurpurin is a macrocyclic compound at the saturation level of bacteriochlorins that was specifically synthesized as a potential photosensitizer in photodynamic therapy. It exhibits a long-wavelength Qy absorption band at 843 nm, which is a necessary prerequisite for a deep tissue penetration. Magnetic resonance spectroscopic properties of bacteriopurpurin have been examined by fluorescence-detected magnetic resonance (FDMR) and electron paramagnetic resonance (EPR) in conjunction with molecular orbital calculations. The electronic structure of the radical cation state obtained by oxidation of bacteriopurpurin with iodine is mapped by the determination of isotropic proton hyperfine coupling constants from electron−nuclear double-resonance spectroscopy at 298 K. A tentative assignment of the couplings to the various proton positions in the molecule is achieved by comparison of the experimental values with simulated ones based on semiempirical intermediate neglect of differential overlap calculations...

Validity and possibility of photoconductivity-detected magnetic resonance (PCDMR) method as one of reaction-yield-detected magnetic resonance (RYDMR) methods

Fluorescence-detected magnetic resonance and photoconductivity-detected magnetic resonance are simultaneously applied to the system of a radical-ion pair formed in the photoinduced electron transfer reaction between pyrene and 1,3-dicyanobenzene. The former monitors the exciplex fluorescence via the singlet radical-ion pair, while the latter monitors free ionic radicals escaping from the radical-ion pair. Both spectra show good coincidence, which shows that the population of the singlet state is proportional to the total radical-ion pair one. The data obtained from the photoconductivity-detected magnetic resonance method suggest the faster quenching reaction of the triplet state of the radical-ion pair than that of the singlet state.

Theory of non-Markovian relaxation of single triplet electron spins using time- and frequency-domain magnetic resonance spectroscopy measured via optical fluorescence: Application to single pentacene molecules in crystallinep-terphenyl

Fluorescence-detected magnetic resonance (FDMR) allows one to monitor magnetic resonance phenomena via fluorescence. Experimental FDMR data obtained using single triplet-state chromophore guest molecules in a low-temperature organic host matrix are analyzed using a stochastic approach to describe triplet electron spin dephasing resulting from frequency fluctuations ${U}^{t}$ induced by host-matrix proton spin dynamics. Modeling the fluctuations ${U}^{t}$ by a sum of N independent random telegraph processes with the same jump rate $\ensuremath{\nu}$ but different variances ${\ensuremath{\sigma}}_{k}$ we construct an exact set of equations for the density matrix of a five-level molecule averaged over fluctuation histories ${U}^{t}.$ These equations provide a basis to study non-Markovian effects of microwave- (MW-) field-dependent dephasing in the FDMR response of a molecule undergoing slow fluctuations ${U}^{t} ({\ensuremath{\sigma}}^{2}/{\ensuremath{\nu}}^{2}>~1, {\ensuremath{\sigma}}^{2}=\ensuremath{\sum}{\ensuremath{\sigma}}_{k}^{2})$ to a MW field that is resonant with a transition between triplet spin substates. Both frequency- and time-domain FDMR phenomena such as (i) power-broadened FDMR line shapes, (ii) FDMR Hahn echo signals, and (iii) FDMR free induction decay are studied. Analytical expressions for the FDMR response are obtained in the case $\ensuremath{\nu}\ensuremath{\gg}{k}_{j}^{i}$ where ${k}_{j}^{i}$ is an intersystem crossing rate. Experimental data on power-broadened line shapes for a pentacene+p-terphenyl pair which demonstrate a pronounced effect of MW-field-suppressed dephasing are explained in the context of the theory.

The P700 triplet state in an intact environment detected by ODMR: A well resolved triplet minus singlet spectrum

Optically detected magnetic resonance (ODMR) of P700 triplet state has been performed in large photosystem I (PSI) particles. The `intactness' of the P700 environment in the PSI particles has been proved by the comparison of the spectra with those obtained in thylakoids and leaves. Effect of degradation of the samples on the spectra has also been tested. When detected in `native' PSI centers the microwaves induced triplet-minus-singlet absorption spectra (T−S) show very sharp features which allow to estimate the minimum number of bands involved in the change of the electronic state of the primary donor (singlet to triplet), and can be interpreted in terms of interactions among pigments in the reaction center. The analogy with the reaction centers of purple bacteria is discussed. The previously published spectra (H.J. den Blanken, A.J. Hoff, Biochim. Biophys. Acta 724 (1983) 52–61; J. Vrieze, P. Gast, A.J. Hoff, J. Phys. Chem. 100 (1996) 9960–9967) obtained in particles having smaller antenna size, resemble the ones obtained in our degraded samples: it is suggested that the isolation procedure to small particles may produce an increase in the heterogeneity of the environment of the primary donor and/or a change of the mutual orientations and interactions among pigments. The fluorescence detected magnetic resonance (FDMR) in large PSI particles supports an excitation energy transfer model derived from time resolved optical studies (R. Van Grondelle, J.P. Dekker, T. Gillbro, V. Sundstrom, Biochim. Biophys. Acta 1187 (1994) 1–65) and adapted for helium temperature, characterized by the presence of antenna pools emitting at 690, 720 and 735 nm, respectively. The FDMR signal of P700 changes sign (from positive to negative) when detected at emission wavelength shorter than 700 nm. Comparison of FDMR of P700 triplet state in large PSI-200 particles, in thylakoids and, for the first time, in leaves has been done.

Fluorescence detected magnetic resonance of monomers and aggregates of bacteriochlorophylls of green sulfur bacteria Chlorobium sp.

Fluorescence detected magnetic resonance (FDMR) was used to study the lowest triplet state of bacteriochlorophylls (BChls) c and d in Chlorobium (Chl.) tepidum and Chl. vibrioforme, respectively. These pigments were studied both in the oligomeric form (in whole cells) and in the monomeric form (after conversion using a 1% 1-hexanol treatment). Fluorescence spectra show the presence of lower-state aggregates, apart from monomers, in samples treated with 1-hexanol. Values of the zero field splitting (ZFS) parameter D, obtained from FDMR spectra, were found to decrease with an increasing aggregate size. The observed ZFS trends are explained by a delocalization of the triplet spins, including a charge resonance (CR) contribution, over the aggregate. A simple model is presented relating the changes of D and E as a result of monomer aggregation to the aggregate geometry. Application of this model to BChls c and d indicates approximately diagonal stacking of the monomers in the dimer. Results for oligomeric BChl c and d were compared with those previously obtained for oligomeric BChl e. FDMR transitions of BChls c, d and e differ both in frequencies and in signs. The D and E values of Car's and BChl a (in whole cells) agree well with those reported for Chl. phaeobacteroides and Chl. limicola.

Triplet state magnetic resonance and fluorescence spectroscopy of metal-substituted hemoglobins

Fluorescence detected magnetic resonance (FDMR) spectra detected at 596 nm of zinc-substituted hemoglobins at 4.2 K show a split D-E transition, which is not observed for zinc protoporphyrins ligated by methylimidazole in glasses. Incorporation of the zinc heme into the globin pocket is also accompanied by a blue shift of the fluorescence of 20 nm at 4.2 K. FDMR spectra recorded at 576 nm do not show the D-E splitting. The D-E splitting and the huge blue shift are not observed for the magnesium-substituted hemoglobins. Fluorescence measurements at 4.2 K and 77 K, and EPR measurements at 110 K, were carried out to obtain information about the ligation states of the zinc and magnesium protoporphyrins in glasses and in hemoglobin. The results are explained by considering ligation effects and distortion of the porphyrin plane.

Self-exchange reactions of radical anions in n-hexane

The formation and reactions of radical anions in n-hexane at 190 K were investigated by pulse radiolysis and time-resolved fluorescence-detected magnetic resonance (FDMR). Electron attachment was found to occur for compounds with gas-phase electron affinities (EA) more positive than −1.1 ± 0.1 eV. The FDMR concentration and time dependence are interpreted as evidence for self-exchange electron-transfer reactions, indicating that formation of dimer radical anions is not prevalent for the range of molecules studied. FDMR detection of radical anions is mainly restricted to electron acceptors with EA less than approximately 0.5 eV.

Electron Transfer Reactions and Mobile Holes in Radiolysis of Squalane. Time-Resolved FDMR Study

Fluorescence detected magnetic resonance (FDMR) is applied to study ion−molecule reactions in squalane (2,6,10,15,19,23-hexamethyltetracosane). Several time-resolved pulsed FDMR experiments are suggested to measure rate constants of scavenging by aromatic solutes. These constants vary from 5 × 108 to 1.6 × 109 mol-1 dm3 s-1. The ion−molecule reactions of radical cations are somewhat slower than reactions of radical anions. The scavenging of the alkane hole is significantly faster than ion−molecule reactions of molecular cations. The mobile hole decays on a time scale of several tens of nanoseconds. These FDMR results are in accord with our recent work on transient absorption spectroscopy in radiolysis of squalane.

(Iso)Pagodane Radical Cations in Liquid Hydrocarbons: “Time-Resolved Fluorescence-Detected Magnetic Resonance” Study of Valence Isomeric Radical Cations

The `tight` (T), cyclobutanoid, and the `extended` (E), {pi}-complex-like, four-centred-three-electron radical cations (4c/3e) derived from (iso)pagodanes and from the valence isomeric (iso)dienes can be viewed as well-separated stationary points on the reaction coordinate for the ethylene/ethylene radical cation [2+1]cycloaddition. So far, experimentally verified examples (ESR, CV, PE) are the extended [1.1.1.1](2{sup .+}) and iso-[2.2.1.1](4{sup .+})ions, persistent in fluid solution at room temperature, and the tight iso-[1.1.1.1]{sup .+} and [2.2.1.1]{sup .+} ions, observable only in Freon matrix (CFCl{sub 3}, {gamma}-rays, 77K). According to calculations, these are the thermodynamically more stable ones of the respective pairs of valence isomers. In this communication the direct observation of tight 1{sup .+} and the activation barrier for its isomerization into extended 2{sup .+} are reported. 16 refs., 3 figs., 1 tab.

Magnetic resonance and spin dynamics in radical ion pairs: Pulsed time-resolved fluorescence detected magnetic resonance

Pulsed time-resolved fluorescence detected magnetic resonance (FDMR) was applied to examine spin dynamics in radical ion pairs. Two time-domain FDMR phenomena were examined; microwave-induced quantum beats and the formation of the ‘‘echo’’ signal. These experiments demonstrate the fast decay of the first-order coherence generated by a microwave field on the time scale of 0.05–1 μs. This decay originates through the phase relaxation in the triplet manifold of the pairs and is probably caused by electron dipole–dipole interaction of radical ions in spurs.