Absorbance-detected Magnetic Resonance Research Articles

Dynamics of flavin containing radical pairs in SDS micellar media probed by static and pulse magnetic field effect and pulse ADMR

ABSTRACTThe dynamics of the radical pairs generated from the electron transfer reaction from indole derivatives to flavin derivatives are studied by three techniques, time resolved magnetic field effect (TR-MARY) and pulse absorption detected magnetic resonance (ADMR), and absorption detected switching of external magnetic field techniques (AD-SEMF). The three techniques complementarily work for the precise analysis of the radical pair kinetics. The results by all three techniques reflect the difference of the hydrophobic nature of the radicals in the lifetime of RP in a micelle. Overcoming the difficulty of the determination of the short RP lifetime under the nearly quasi steady state due to slow and inhomogeneous decay kinetics of the precursor triplet excited state, AD-SEMF analysis enabled us to determine the kinetic parameters, which is consistent with the qualitatively observed by the other techniques.

Magnetometry with nitrogen-vacancy ensembles in diamond based on infrared absorption in a doubly resonant optical cavity

We propose to use an optical cavity to enhance the sensitivity of magnetometers relying on the detection of the spin state of high-density nitrogen-vacancy ensembles in diamond using infrared optical absorption. The role of the cavity is to obtain a contrast in the absorption-detected magnetic resonance approaching unity at room temperature. We project an increase in the photon shot-noise limited sensitivity of two orders of magnitude in comparison with a single-pass approach. Optical losses can limit the enhancement to one order of magnitude which could still enable room temperature operation. Finally, the optical cavity also allows to use smaller pumping power when it is designed to be resonant at both the pump and the signal wavelength.

Anomalous photoinduced absorption of conjugated polymer/fullerene mixtures at low temperatures and high frequencies

Light induced charge transfer from conjugated polymers and their oligomers to different electron acceptors, e.g. fullerenes or tetracyano-p-quinodimethane (TCNQ) derivatives has been studied extensively [1–5]. From time-resolved photoinduced absorption (PIA) studies it is well known that the charge-transfer process from a conjugated polymer to C60 or C60 derivatives takes place on an ultrafast time scale (<100 fs) [2]. On the other hand, the back electron transfer is remarkably slow; lifetimes on the order of milliseconds can be observed for the charge separated states in such systems. This very efficient charge separation has been utilized in polymeric photovoltaic devices [6–9]. Photoexcitation of conjugated polymers in liquid solutions or solid state films across the – ∗ energy gap produces a metastable triplet state by intersystem crossing from the excited singlet manifold [10,11]. These triplet states were identified by their dipole allowed T1–T2 transition, which exhibit a red shift with increasing chain length [10]. The lifetimes of these triplet states are on the order of 10–100 ms. In composite films of these polymers mixed with electron accepting molecules the triplet–triplet absorption is quenched due to a prior charge transfer. Absorption detected magnetic resonance (ADMR) experiments on films of pristine poly[2-methoxy-(5-2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have shown that the origin of the photoinduced absorption in the range between 1.1 and 1.6 eV is due to excited triplet states (spin 1), whereas mixed films of MEH-PPV and C60 show in the same energetic region PIA

Multichannel Flash Spectroscopy of the Reaction Centers of Wild-type and Mutant Rhodobacter sphaeroides: BacteriochlorophyllB-mediated Interaction Between the Carotenoid Triplet and the Special Pair†¶

Multichannel flash spectroscopy (with microsecond time resolution) has been applied to carotenoid (Car)-containing and Car-less reaction centers (RC) of Rhodobacter sphaeroides with a view to investigate the interaction between the Car and its neighboring pigments at room temperature. Under neutral redox potential conditions, where the primary quinone acceptor (QA) is oxidized, the light-induced spectral changes in the 350-1000 nm region are attributed to the photochemical oxidation of the special pair (denoted here as P870), the generation of P870(+)QA(-), and the attendant electrochromism of adjacent chromophores. A bathochromic shift of <1 nm in the visible absorption region of Car reveals the sensitivity of Car to the P870 photooxidation. Under low redox potential conditions, where QA is reduced, P870 triplets (P870(+)) are formed. The time-resolved triplet-minus-singlet (TmS) spectrum of Car-less RC shows a deep bleaching at 870 nm, which belongs to P870(+), and additional (but smaller) bleaching at 800 nm; the entire spectrum decays at the same rate (with a lifetime of about 50 micros). The bleaching at 800 nm arises from the pigment interaction between P870(+) and the accessory bacteriochlorophylls on A and B branches (BA,B). In Car-containing RC, the TmS spectra of Car are accompanied by two smaller, negative signals--a sharp peak at 809 +/- 2 nm and a broad band at 870 nm--which decay at the same rate as the TmS spectrum of Car (ca 10 micros). The former is ascribed to the perturbation, by Car(+), of the absorption spectrum of BB; the latter, to the TmS spectrum of P870(+), a species that appears to be in approximate thermal equilibrium with Car(+). These assignments are consistent with the absorption-detected magnetic resonance spectra obtained by other workers at low temperatures.

Observation and reaction control of a singlet born radical pair formed photochemically in an SDS micelle by pulsed microwave irradiation

The photochemical reaction of tetraphenylhydrazine in an SDS micellar solution is studied using a transient absorption detected magnetic resonance (ADMR) method. This system is photo-dissociated via the singlet excited state and forms a transient radical pair. Strong microwave irradiation of this system under an × band EPR magnetic field provides an ADMR spectrum of the singlet born radical pair as the optical absorbance change of the diphenylaminyl radical. The employment of a short duration microwave pulse that efficiently flips the electron spin quantum (Δm s = 1) controls the reaction of the radical pair, and the reaction rate constant of this system is determined. Furthermore, changing the microwave duration shows the quantum beat of this system induced by the electromagnetic field. This beat signal carries both a single beat that corresponds to the Rabi frequency and a double-frequency beat that is due to simultaneous two-spin controlling.

Absorbance Detected Magnetic Resonance Spectra of the FMO Complex ofProsthecochlorisaestuariiReconsidered: Exciton Simulations

The isotropic and linear dichroic triplet−singlet difference spectra of the Fenna−Matthews−Olson (FMO) complex of the green sulfur bacterium Prosthecochloris aestuarii measured with absorbance detected magnetic resonance (ADMR) are presented. The absorption, linear dichroic (LD), circular dichroic (CD), triplet-minus-singlet (T−S), and LD(T−S) spectra are simultaneously analyzed by means of an exciton model. In this analysis the dependence of the T−S(α) spectrum on the angle α between the optical and microwave transition moments was taken into account. The present simulation procedure yields fits of the absorption and LD spectra of FMO that are about equally good as those in the literature, while the fits of the CD, T−S, and LD(T−S) spectra show significant improvements. We find that the interaction energies are roughly 25% higher than in previous analyses. The simultaneous fit of all available optical (difference) spectra with correct procedures offers a precise and detailed description of pigment−pigment interactions in the FMO complex.

Polarons in Ladder-Type Polymer Films; Recombination Channels and Electron−Phonon Coupling

We applied the photomodulation spectroscopy for probe-photon energies from 0.05 to 2.5 eV in films of ladder-type poly(p-phenylene) and identified a photoinduced absorption (PA) band related to triplet excitons and two correlated PA bands due to charged polarons. The low energy polaron band peaks at a relatively low photon energy of ∼0.1 eV, showing small polaronic relaxation energy. We employed photoinduced absorption-detected magnetic resonance spectroscopy to study the polaron recombination channels and found that spin 1/2 magnetic resonance enhances the bimolecular recombination of polarons to singlet byproducts (singlet excitons and bipolarons). However, it reduces polaron recombination to triplet byproducts, such as triplet excitons. The low-energy polaron PA shows antiresonances due to its overlap with Raman- and IR-active vibrations. The antiresonances spectrum is compared to the IR and Raman spectra and found to also contain modes that are silent in the latter spectra.

Orientation Effects in Triplet−Singlet Difference Spectroscopy Measured with Absorbance-Detected Magnetic Resonance

Absorbance-detected magnetic resonance (ADMR) triplet−singlet difference (T−S) spectra are often measured with unpolarized light. We show that under nonsaturating conditions ADMR spectra are a func...

Linear dichroism in triplet–singlet difference spectra of photosynthetic pigment–protein complexes, monitored with absorbance-detected magnetic resonance: correction for non-ideal optical effects

The non-sinusoidal modulation of the photo-elastic modulator used for measurements of the linear dichroism in absorption-detected magnetic resonance (LD-AMDR) causes an experimental error in determining the angles of the triplet spin-sublevel axes relative to the optical dipole transition moment that can be be as large as 20°. We have obtained an analytical correction for this error. The error analysis was applied to LD-ADMR experiments on the carotenoidless mutant R26 of Rhodobacter (Rb.) sphaeroides. The corrected angles of the triplet x- and y-axis with respect to the optical transition dipole moment at 896 nm of R26 are 86°±4° and 22°±3°, respectively.

Absorption detected magnetic resonance of bacteriochlorophyll a and related molecules in low external magnetic fields

The | D|±| E| signals of the bacteriochlorophyll a (BChl a) triplet state were measured with absorption detected magnetic resonance (ADMR) in low magnetic fields up to 100 G in MTHF glass at 1.2 K. The lineshapes were modeled theoretically, taking into account magneto- and photo-selection as well as saturation effects. An increase of up to 200% in integrated ADMR intensity upon the application of low external magnetic fields was found. Other pigments like deuterated BChl a, Zn-bacteriopheophytin a (Zn-BPhe a), pyro-bacteriochlorophyll a (Pyro-BChl a), and anthracene show similar effects.

Toward an Integral Interpretation of the Optical Steady-State Spectra of the FMO-Complex of Prosthecochloris aestuarii. 1. An Investigation with Linear-Dichroic Absorbance-Detected Magnetic Resonance

In this paper we present results of (linear-dichroic) absorbance-detected magnetic resonance experiments on the FMO-complex of Prosthecochloris aestuarii at 1.2 K. We have used these results as add...

Toward an Integral Interpretation of the Optical Steady-State Spectra of the FMO-Complex of Prosthecochloris aestuarii. 2. Exciton Simulations

We present a simultaneous simulation of various experimental steady-state spectra of the FMO-complex of green sulfur bacteria. The simulations are based on exciton calculations using a significantly lower dipolar interaction energy as compared to exciton calculations on the FMO-complex in the literature. Decrease of the interaction energy was suggested by comparing our results obtained with linear-dichroic absorbance-detected magnetic resonance spectroscopy with exciton simulations using parameters taken from literature. By considering a single subunit only, we arrive at a minimal set of parameters, consisting of reduced interaction energies between the bacteriochlorophyll molecules, seven different site energies, and a common line width for all transitions of 80 cm-1. With such a minimal set of parameters, we have achieved an unequalled match between the simulations and the experimental spectra, including the absorption, the linear dichroic, the circular dichroic, the triplet-minus-singlet, as well as the linear-dichroic triplet-minus-singlet spectra. We conclude that the structure in the various steady-state spectra is mainly determined by the variation in site energy and nearly all interaction energies are substantially less than the inhomogeneous width of the individual transitions within the QY band of the FMO-complex.

Excitonic Interactions between the Reaction Center and Antennae in Purple Photosynthetic Bacteria

The interaction between the light-harvesting antenna (LH1) and the reaction center (RC) in the photosynthetic purple bacteria Rhodobacter sphaeroides R26 and strain 2.4.1 was studied with low-temperature (1.5 K) absorbance-detected magnetic resonance (ADMR) experiments combined with computer simulations. The triplet-minus-singlet (T−S) spectra of chromatophores of Rb. sphaeroides R26 and strain 2.4.1 show an extra positive component at approximately 884 and 887 nm, respectively, and an extra negative component at 897 and 901 nm, respectively, when compared to RCs of Rb. sphaeroides R26 and strain RCO1. Computer simulations using a simple model of the RC encircled by the antenna of LH1 show that the dipole strength and energies of the second-lowest exciton component of the antenna and the RC change when the coupling term between the RC and antenna is included in the Hamiltonian of the system. Thus the difference in the shape of the ADMR-detected T−S spectra of isolated RCs (or strain RCO1 chromatophores) and chromatophores is plausibly caused by a change in the excitonic interaction between the RC and antenna.

The time-resolved ADMR in the photolysis of the polymethylene-linked system of xanthone and xanthene: Power dependence

The time-resolved ADMR (absorption-detected magnetic resonance) spectrum of the polymethylene linked biradical generated in the photolysis of α-(xanthene-2-carbonyloxy)-ω-(xanthene-2-carbonyloxy) dodecane is observed at various microwave power. When the lower microwave power is low, the ADMR spectrum is totally negative (the absorbance of xanthyl radical decreases by the microwave irradiation) and is interpreted in terms of the hyperfine structure of the terminal radicals. In contrast with this, the ADMR spectrum obtained at high microwave field consists of two components: central component and wing component. The central component is positive and is interpreted in terms of the deceleration of the intersystem crossing by microwave, which is known as the spin locking. The wing component is negative and the linewidth is much larger than that of the central one. It is ascribed to the transitions from |T+1> and |T−1> states to singlet state having a slight |T0> character. It is proposed that the width of the time resolved spectrum is due to the fluctuation of the exchange integral (modulated exchange integral) by the motion of the polymethylene chain. The time dependence of the central and wing components and the conditions for observation of them are discussed.

The time-resolved absorption-detected magnetic resonance spectrum of the polymethylene linked biradical: effect of the exchange integral

The time-resolved absorption-detected magnetic resonance spectrum of the polymethylene linked biradical generated in the photolysis of α-(xanthene-2-carbonyloxy)- ω-(xanthene-2-carbonyloxy)dodecane was observed. The central part of the spectrum shows an increase in the absorbance by applying high microwave power. This is rationalized by the deceleration of the intersystem crossing due to the state locking that serves to slow down the intramolecular recombination reaction. Both wing portions show an increase in the recombination rate by microwave radiation, i.e. acceleration of the intersystem crossing. This is ascribed to transitions from the |T +1> and |T −1> states to a singlet state having a slight |T 0> character. We propose that the broadness of the time resolved spectrum is due to the wide distribution of the exchange integral and its fluctuation (modulated exchange integral) by the motion of the polymethylene chain.

The triplet state of the primary donor in reaction centers of the HL(L173) and HL(M202) heterodimer mutants of Rhodobacter sphaeroides

The triplet state of reaction centers of the HL(L173) and HL(M202) mutants of Rhodobacter sphaeroides, whose primary donor is a bacteriochlorophyll-bacteriopheophytin (BChl-BPh) heterodimer instead of a BChl-BChl homodimer, has been studied with absorbance-detected magnetic resonance in zero-magnetic field. The zero-field splitting parameters of the triplet state of the two heterodimers are close to those of monomeric 3BChl a, indicating that the triplet state is largely localized on the BChl-half of the heterodimers, but not identical, demonstrating that the two BChls of the native primary donor are inequivalent. The microwave-induced triplet-minus-singlet absorbance-difference spectra of the two mutant reaction centers differ in the Q Y-absorption region of the BChls that are located close to the dimer. The latter difference is attributed to differences in interaction between the dimer-BChls and the two adjacent BChls for the two mutants.

Interactions between chromophores in reaction centers of purple bacteria. A reinterpretation of the triplet-minus-singlet spectra of Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis

The orientations of the optical transition moments contributing to the Q Y-region of the triplet-minus-singlet absorbance-difference spectrum of the reaction center of Rhodobacter sphaeroides R26 have been determined with respect to the triplet axes of the primary donor with linear-dichroic absorbance-detected magnetic resonance. The orientation of the S 1 ← S 0 transition moment of the reaction center relative to the triplet x- and y-axis was found to be 85 ± 5° and 18 ± 3°, respectively. A comparison of the (linear-dichroic) triplet-minus-singlet absorbance-difference spectra is made with those obtained previously for the reaction center of Rhodopseudomonas viridis (E.J. Lous and A.J. Hoff (1987) Proc. Natl. Acad. Sci. USA 84, 6147–6151). For both types of reaction centers the transitions in the absorption region of the accessory bacteriochlorophylls appear to shift to lower energies upon triplet formation on the primary donor. The orientations of the transition moments, however, do not change upon excitation of the primary donor to its triplet state, from which we conclude that the band shifts in the triplet-minus-singlet spectrum cannot be explained by a change of dipolar interactions between the transitions of the dimer and the adjacent bacteriochlorophylls.

An LD-ADMR study on reaction centers of the LH(L131) and LH(M160) hydrogen-bonding mutants of Rhodobacter sphaeroides

Reaction centers of the LH(L131) and LH(M160) mutants of Rhodobacter sphaeroides, which have a leucine near the primary donor, a bacteriochlorophyll dimer, changed into a histidine, have been investigated with linear-dichroic absorbance-detected magnetic resonance, and the results are compared with those obtained for native reaction centers. The microwave-induced triplet-minus-singlet absorbance-difference spectra of the three reaction centers upon triplet formation of the primary donor show small differences in band shifts in the Q Y-region of the accessory bacteriochlorophylls. The orientations of the Q Y-transition moments with respect to the dipolar axes of the triplet state of the primary donor were found to be equal for all reaction centers, indicating that the orientation of the triplet axes with respect to the reaction center coordinate frame is the same for all three reaction centers. From this we conclude that the electronic composition of the triplet state and the Q Y transitions are basically the same for the mutant and native reaction centers.

Characterization of Carotenoid Triplet States in the Light-Harvesting Complex B800–850from the Purple Bacterium Rubrivivax gelationsus

The carotenoid triplet states in the light-harvesting complex B800–850from purple bacterium Rubrivivax gelatinosus were characterized by absorption-detected magnetic resonance in zero magnetic field (ADMR) spectroscopy. Detailed HPLC analysis of carotenoids from B800–850demonstrated the presence of several carotenoids bound to the complex: the major ones are hydroxyspheroidene and spheroidene (together 80%), followed by neurosporene and hydroxyneurosporene (7%), spheroidenone and hydroxyspheroidenone (7.5%) and two other minor carotenoids that could be 3,4-dihydrospheroidenone and 3,4-dihydrohydroxyspheroidenone (5.5%). Three triplet states originating from carotenoids present in the B800–850were observed. The identical T-S spectra recorded at selectively chosen 2|E| transitions of carotenoids indicated that all these triplet states can be attributed to three different populations of one carotenoid family, probably to spheroidene and to hydroxyspheroidene, with different out-of-plane distortions of their polyene chain due to a different protein environment. Triplet states of the neurosporene and the spheroidenone families are probably not observed because of the low signal amplitude.

Structure of the Reaction Center of Photosystem I of Plants. An Investigation with Linear-Dichroic Absorbance-Detected Magnetic Resonance

Photosystem I particles from spinach were studied with linear-dichroic absorbance-detected magnetic resonance in zero-magnetic field. The microwave-induced triplet-minus-singlet (T−S) spectra and the linear-dichroic (LD) T−S spectra were recorded for the |D| + |E| and the |D| − |E| microwave transitions of the triplet state of the primary donor, 3P700. From these data the directions of the optical transition moments contributing to the T−S spectra in the 600−750 nm region, with respect to the triplet x- and y-axes of 3P700 were obtained. The orientation of the optical QY-transition moment of P700 relative to the triplet x- and y-axis is found to be 35 ± 2° and 56 ± 1°, respectively. A comparison is made with data obtained for monomeric chlorophyll (Chl) a in two glasses. The orientation of the QY-transition moment with respect to the in-plane triplet x- and y-axes of 3P700 differs from monomeric Chl a in the two glasses. This difference is ascribed to the different environments of P700 and Chl a, rather t...