Secondary Electron Transfer Research Articles

Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF)

1,3-Dipolar cycloadditions of TTF-azomethine ylides (TTF = tetrathiafulvalene) to C60 have been used to synthesize a series of novel donor−bridge−acceptor dyads. In these dyads the pyrrolidine[3‘,4‘:1,2][60]fullerene is covalently attached to the electron donor TTF either directly (5) or alternatively through one (2a) or two (7) vinyl groups. In the ground state, dyads 2a, 5, and 7 undergo four quasireversible one-electron reductions and two reversible oxidation steps. The former are associated with the reduction of the C60 core, whereas the latter correspond to the formation of the radical cation and dication of the TTF moiety, respectively. Semiempirical PM3 calculations reveal donor−acceptor distances of 4.8 Å (5), 7.6 Å (2a), and 10.5 Å (7), and a deviation from planarity between the TTF fragment and the vinylogous spacer. In relation to an N-methylfulleropyrrolidine, the emission of the fullerene singlet excited state in dyads 2a, 5, and 7 is substantially reduced. Furthermore, the fluorescence quantum yield correlates well with the solvent dielectric constant and also with the spatial separation of the donor and acceptor moieties in the dyads. These correlation suggest that intramolecular electron-transfer processes evolving from the fullerene singlet excited state generate the (C60•-)−(TTF•+) pair. Pico- and nanosecond-resolved transient spectroscopy further substantiate a rapid transformation of the initially formed singlet excited state into the charge-separated radical pair with intramolecular rates ranging between 1.17 × 1010 s-1 and 1.47 × 109 s-1. In all cases, the product of back electron transfer is the triplet excited state, which is generated with markedly high quantum yields (0.61−0.97). The latter is, in addition to the rapid primary intramolecular electron transfer, subject to a slower, secondary intermolecular electron transfer with rate constants of 7 × 108 M-1s-1 (5) in benzonitrile and 1.6 × 109 M-1 s-1 (5) in toluene.

Investigation of the primary electron transfer processes in some rigidly bridged dyads and triads by (sub)picosecond pump–probe experiments

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

Low-Temperature Optical and Resonance Raman Spectra of a Carotenoid Cation Radical in Photosystem II

Low-temperature (77 K) illumination of manganese-depleted Synechocystis PCC 6803 photosystem II core complexes caused the reversible photooxidation of a carotenoid, forming a carotenoid cation radical with an absorbance maximum at 984 nm. Resonance FT-Raman spectra obtained with 1064 nm excitation gave a spectrum characteristic of carotenoid cation radicals in solution. This is the first example of a resonance Raman spectrum of a carotenoid cation radical in a protein. The carotenoid photooxidation requires prior chemical oxidation of cytochrome b559 which implicates the carotenoid in the secondary electron-transfer pathway of photosystem II that may play a role in photoprotection. The possible nature of the pathway and the structure of the carotenoid cation radical are discussed.

Revealing the structure of the oxygen-evolving core dimer of photosystem II by cryoelectron crystallography.

Here we present cryoelectron crystallographic analysis of an isolated dimeric oxygen-evolving complex of photosystem II (at a resolution of approximately 0.9 nm), revealing that the D1-D2 reaction center (RC) proteins are centrally located between the chlorophyll-binding proteins, CP43 and CP47. This conclusion supports the hypothesis that photosystems I and II have similar structural features and share a common evolutionary origin. Additional density connecting the two halves of the dimer, which was not observed in a recently described CP47-RC complex that did not include CP43, may be attributed to the small subunits that are involved in regulating secondary electron transfer, such as PsbH. These subunits are possibly also required for stabilization of the dimeric photosystem II complex. This complex, containing at least 29 transmembrane helices in its asymmetric unit, represents one of the largest membrane protein complexes studied at this resolution.

Electron spin echo envelope modulation by electronic spin–spin interactions in radical pairs undergoing electron transfer

Electron spin echo envelope modulation caused by the electronic spin–spin interaction is investigated for the case of a light-induced radical pair undergoing secondary electron transfer. Analytical expressions reveal contributions of both the primary and secondary radical pair to the out-of-phase component of the signal, while only the secondary radical pair contributes to the in-phase component. Spectra exhibit lifetime broadening for the primary pair and dispersive contributions for the secondary pair, but no frequency shifts. Numerical calculations illustrate the dependence of the spectra on the electron transfer rate and on the delay between laser and microwave pulses.

Generation of Polyphenylene Radical Cations and Their Cosensitization Ability in the 9,10-Dicyanoanthracene-Sensitized Photochemical Chain Reactions of 1,2-Bis(4-methoxyphenyl)cyclopropane

Cosensitization effects of polyphenylene compounds (PP) such as biphenyl (BP), terphenyls (o-, m-, p-TP), and phenanthrene (Phen) in photoinduced electron-transfer reactions were examined. The 9,10-dicyanoanthracene (DCA)-sensitized cis−trans photoisomerization of 1,2-bis(4-methoxyphenyl)cyclopropane (CP), which proceeds in a chain reaction via free radical cation of CP (CP•+) as a chain carrier, was accelerated by adding PP, particularly by TP. A similar accelerating effect was observed in the DCA-sensitized photooxygenation of CP as another example. BP and TP were more stable under the oxygenation condition than phenanthrene and naphthalene, which also accelerate the photooxygenation CP. CP•+ is generated by the direct electron transfer from CP to the excited singlet state of DCA (1DCA*) and also by the secondary electron transfer from CP to PP•+, which is generated by the primary electron transfer from PP to 1DCA*. The laser flash photolysis study revealed that the quantum yield for the formation of fr...

Excitations and excitons in photosystem I

The elementary requirements for energy transfer in a photosynthetic antenna are discussed. Energy migration between chlorophyll molecules in the PSI antenna reaction centre complex was modelled using a geometry based on the recent X–ray structure. The migration is trap–limited and an intrinsic electron transfer rate constant of 1.3 ps−1 was obtained by modifying the model to account for experimental results obtained from antennae of different sizes. Exciton coupling between pairs of Chlorophyll molecules was calculated and it is found that to prevent self–quenching their interplanar separation must be greater than or equal to 5 A. In the PSI reaction centre, exciton coupling between adjacent chlorophylls was found to be around 300 cm−1 with no particularly large value between the special pair being necessary to describe the experimentally observed transient spectra of P +700− P 700 and A −− A . Electrochromic interaction is particularly important in reproducing the A −− A spectrum.

Type B Semiconductor Photocatalysis: The Synthesis of Homoallyl Amines by Cadmium Sulfide‐Catalyzed Linear Photoaddition of Olefins and Enol/allyl Ethers to N‐Phenylbenzophenone Imine

AbstractHomoallyl amines were synthesized by visible‐light irradiation of CdS powder in the presence of N‐phenylbenzophenone imine and cyclohexene, 2,3‐dihydrofuran, 2,5‐dihydrofuran, 3,4‐dihydropyran, 2‐pentene, cyclopentene, 1‐methylcyclohexene, or α‐pinene. The structures of the products from the last three olefins were determined by single‐crystal X‐ray analysis to prove that C‐alkylation of the imine had occurred. Thus, the reaction is formally an insertion of the imine into an allylic C‐H bond of the olefin. It is proposed that a photogenerated electron–hole pair reduces the imine to an α‐aminodiphenylmethyl radical and oxidizes the olefin with concomitant deprotonation to the corresponding allyl radical. Heterocoupling of these intermediates affords the final addition product. The overall reaction is therefore classified as type B semiconductor photocatalysis. The presence of acetic acid accelerates the reaction by rendering the reduction potential of the imine more positive. In the series 2,5‐dihydrofuran/cyclopentene/3,4‐dihydropyran, the decrease in apparent quantum yield with increasing driving force of olefin oxidation points to a significant contribution of secondary back electron transfer.

Visible Light Initiated Photosensitized Electron Transfer Cyclizations of Aldehydes and Ketones to Tethered α,β-Unsaturated Esters: Stereoselective Synthesis of Optically Pure C-Furanosides

Photosensitized one-electron reductive activation of aldehydes/ketones tethered with activated olefins leads to efficient cyclization to give diastereoselective cycloalkanols in high yield. The activation is promoted by secondary and dark electron transfer from visible light (405 nm) initiated photosensitized electron transfer generated 9,10-dicyanoanthracene radical anion (DCA•-). The DCA•- is produced by electron transfer using either triphenylphosphine (Ph3P) as sacrificial electron donor (PS-A) or 1,5-dimethoxynaphthalene (DMN) as primary electron donor and ascorbic acid as sacrificial electron donor (PS-B), to light-absorbing DCA. The cyclization is suggested to involve ketyl radical intermediate. High trans diastereoselectivity is observed during the formation of cycloalkanols. This cyclization strategy is further extended for the stereoselective synthesis of optically pure C-furanoside (41), starting from naturally occuring l-tartaric acid. The stereochemistry of 41 is suggested based on the single-crystal X-ray diffraction data.

Electron transfer photochemistry of 7-(spirocyclopropane)quadricyclane: capture of a radical cation and sequential cyclopropylcarbinyl rearrangements

The electron transfer photo-sensitized reaction of 7-(spirocyclopropane)quadricyclane (1) with methanol produces two rearranged mono-methanol adducts, 2 and 3, and a bis-methanol adduct, 4. The products reveal that 1·+ reacts by stereo- and regiospecific attack of methanol on one trisubstituted cyclopropane ring. The resulting free radical rapidly undergoes one or two (consecutive) cyclopropylcarbinyl to butenyl rearrangements. The mono-adducts are formed by net hydrogen abstraction, the di-adduct via a (secondary) electron transfer reaction of 3. © 1997 John Wiley & Sons, Ltd.

Electron transfer photochemistry of 7-(spirocyclopropane)quadricyclane: capture of a radical cation and sequential cyclopropylcarbinyl rearrangements

The electron transfer photo-sensitized reaction of 7-(spirocyclopropane)quadricyclane (1) with methanol produces two rearranged mono-methanol adducts, 2 and 3, and a bis-methanol adduct, 4. The products reveal that 1·+ reacts by stereo- and regiospecific attack of methanol on one trisubstituted cyclopropane ring. The resulting free radical rapidly undergoes one or two (consecutive) cyclopropylcarbinyl to butenyl rearrangements. The mono-adducts are formed by net hydrogen abstraction, the di-adduct via a (secondary) electron transfer reaction of 3. © 1997 John Wiley & Sons, Ltd.

Spectroscopic characterization of quinone-site mutants of the bacterial photosynthetic reaction center

Site-specific mutations in the quinone binding sites of the photosynthetic reaction center (RC) protein complexes of Rhodobacter (R.) capsulatus caused pronounced effects on sequential electron transfer. Conserved residues that break the twofold symmetry in this region of the RC – M246Ala and M247Ala in the QA binding pocket, and L212Glu and L213Asp in the QB binding pocket – were targeted. We constructed a QB-site mutant, L212Glu-L213Asp → Ala-Ala, and a QA-site mutant, M246Ala–M247Ala → Glu-Asp, to partially balance the differences in charge distribution normally found between the two quinone binding sites. In addition, two photocompetent revertants were isolated from the photosynthetically-incompetent M246Glu-M247Asp mutant: M246Ala–M247Asp and M246Gly–M247Asp. Sequential electron transfer was investigated by continuous light excitation and time-resolved electron paramagnetic resonance (EPR), and time-resolved optical techniques. Several lines of EPR evidence suggested that the forward electron transfer rate to QA, kQ, was slowed in those strains containing altered QA sites. The slower rates of secondary electron transfer were confirmed by time-resolved optical results with the M246Glu-M247Asp mutations in the QA site resulting in a dramatically lowered secondary electron transfer efficiency [kQ < (2 ns)-1] in comparison with either the native R. capsulatus RC or the QB site mutant [kQ ≈ (200 ps)-1]. Secondary electron transfer in the two revertants was intermediate between that of the native RC and the QA mutant. The P+ QA- → PQA charge recombination rates were also changed in the strains that carried altered QA sites. We show that local mutations in the QA site, presumably through local electrostatic changes, significantly alter binding and electron transfer properties of QA.

The deactivation of singlet excited all-trans-1,6-diphenylhexa-1,3,5-triene by intermolecular charge transfer processes. 1. Mechanisms of fluorescence quenching and of triplet and cation formation

Abstract The photophysical and fluorescence quenching properties of all-trans-1,6-diphenulhexa-1,3,5-triene (DPH) were investigated in toluene and acetonitrile solution and the influences of the external heavy-atom effect (HAE) and of charge transfer (CT) processes on the fluorescence quenching mechanism were characterized. Particular emphasis was placed upon the elucidation of the electron transfer reactions in the quenching of DPH fluorescence by p -dicyanobenzene (p-DCB) in acetonitrile. It was shown that the combination of stationary and time-resolved fluorescence measurements, laser flash photolysis and CIDNP experiments allows a comprehensive description of the primary and secondary electron transfer reactions in the fluorescence quenching process. In particular, the characterization of the kinetics of genuine charge recombination (CR) reactions of the DPH p-DCB singlet and triplet ion pairs, of the spin evolution process and of the degenerate electron transfer between neutral and cationic DPH was achieved.

Design and characteristics of a magnetic collimating lens spectrometer for electron beam probing

A new objective lens design for an electron beam probe is reported. The object of the design was a sharp(50 meV) secondary electron transfer function with low aberration coefficients for the primary beam. The design involved extensive numerical simulation utilizing a simulation system developed specifically for this design effort.

Photoinduced Electron Transfer in Eosin-Modified Co(II)-Protoporphyrin IX Reconstituted Myoglobin and α- or β-Hemoglobin Subunits: Photocatalytic Transformations by the Reconstituted Photoenzymes1

A series of hemo-protein-derived photocatalysts, prepared by reconstitution of the respective apo-proteins with Co(II)-protoporphyrin IX and chemical modification of the protein with the eosin chromophore, is presented. Apo-myoglobin, Apo-Mb, was reconstituted with Co(II)-protoporphyrin IX and further modified with eosin-isothiocyanate (3) to yield the photocatalyst Eo2--Mb-Co(II). The protein is loaded by two eosin chromophore units. Photoexcitation of Eo2--Mb-Co(II) yields the electron transfer species Eo•--Mb-Co(I) formed by direct oxidative quenching of TEo2--Mb-Co(II), kq = 5.2 × 104 s-1, and via an indirect path where self-quenching of the eosin-chromophore units yields the intermediate redox products (Eo•3- + Eo•-)-Mb-Co(II) that, in the presence of Na2EDTA, generate the Eo•--Mb-Co(I) in a secondary dark electron transfer, kr = 330 s-1. The reconstituted protein Eo2--Mb-Co(II) reveals photocatalytic features and its steady-state illumination in the presence of Na2EDTA yields hydrogen evolution, φ =...

Secondary electron transfer processes in membranes of Heliobacillus mobilis.

Picosecond transient absorption difference spectroscopic experiments were performed on membranes of the antenna/reaction center complex of Heliobacillus mobilis to study the electron transfer processes. Particular emphasis was placed on the blue spectral region, where the difference spectra of iron-sulfur centers and quinones are significantly different. Spectra were measured at room temperature in the wavelength region from 400 to 470 nm and from 630 to 730 nm. Laser excitation was into the 788 nm Q gamma band of the bacteriochlorophyll g of the reaction center complex. Global analysis in both wavelength regions reveals three kinetic components. A 25 ps phase originates from the decay of the excited state of antenna to form the primary charge-separated state P798+A0-; a 600 ps component is assigned to the electron transfer from the primary electron acceptor A0 to a secondary electron acceptor; a nondecaying component on the time scale measured represents the formation of the secondary charge-separated state. When the secondary electron acceptors were reduced by adding dithionite at pH 11, the 600 ps component disappeared. Only a 25 ps component and a constant were observed in the 630-730 nm region. The 25 ps component is assigned to the excitation decay in the antenna and the formation of P798+A0-, just as in the nonreduced sample. In the reduced sample, the P798+A0- state does not decay on the time scale measured. In the 400-470 nm region, the same kinetic behavior was observed. The absorption difference spectra of the primary and the secondary electron acceptor were constructed from different charge-separated states.(ABSTRACT TRUNCATED AT 250 WORDS)

Multistep Electron Transfer between Porphyrin Modules Assembled around a Ruthenium Center

A new strategy has been devised for the construction of photoactive multicomponent arrays based on metal ion chelation whereby bisporphyrins have been assembled around a central ruthenium(II) bis(terpyridyl) complex. One of the terminal subunits is a gold (III) porphyrin while the second terminus is selected from a gold (III), zinc(II), or free-base porphyrin. Photophysical properties have been measured for each of the tripartitic compounds using ultrafast transient absorption and emission spectroscopy. Excitation into the central ruthenium(II) bis(terpyridyl) complex is followed by rapid intramolecular triplet energy transfer to one of the appended porphyrins. Direct excitation into the gold(III) porphyrin subunit generates the corresponding triplet excited state which is unreactive toward energy- or electron-transfer processes. In contrast, excitation into the zinc(II) or free-base porphyrin produces the corresponding excited singlet state which transfers an electron to the adjacent ruthenium(II) bis(terpyridyl) complex. Secondary electron transfer to the appended gold(III) porphyrin competes with reverse electron transfer such that the redox equivalents become separated by about 30 A. The original ground-state system is restored by relatively slow interporphyrin electron transfer. The energetics for each of these electron-transfer steps have been evaluated. 51 refs., 10 figs., 1 tab.

Photophysical and fluorescence quenching properties of 1,7-diazaperylene in solution

The photophysical and fluorescence quenching properties of 1,7-diazaperylene (DAP) were determined in acetonitrile and benzene solutions. The absorption spectra of the triplet molecules ( 3DAP ∗) and of the radical anions and cations were recorded. The quantum yield of intersystem crossing from the S 1 state was determined to be Φ ISC=0.015 (in methylcyclohexane at room temperature). In acetonitrile, radical anions 2DAP − were detected as primary products of the reductive fluorescence quenching of singlet excited DAP ( 1DAP ∗) with anthracene as quencher molecule. The DAP radical cation 2DAP + was generated by the method of secondary electron transfer with photoexcited 9,10-dicyanoanthracene ( 1DCA ∗) as energy donor.

A comparison of the photochemical activity of two forms of Photosystem II reaction centre isolated from sugar beet

Both time-resolved fluorescence and absorption measurements have been conducted on two different forms of Photosystem II reaction centre isolated from sugar beet. One form, called RC IIa, contained 6 chlorophylls and 2 β-carotenes per 2 pheophytins, while the other, called RC Iib, contained 4 chlorophylls and 1 β-carotene per 2 pheophytins. Single photon-counting fluorescence decay obtained from the two preparations showed similar charge recombination fluorescence lifetimes which could be resolved into two components of 46.1 and 14.2 ns. Analysis of the amplitude of the fluorescence of the fast component of 5.6 ns, which largely originates from non-functional chlorophyll, gave an estimate of the relative activity for RC IIb which was only a 5.5% lower compared to that of RC IIa. This small relative difference in photochemical activity was also confirmed by measuring the extent of primary charge separation activity using flash induced absorption spectroscopy. In this case the amplitude of the long-lived component, attributed to primary radical-pair formation and recombination, was 16% lower in RC IIb as compared with RC IIa. When the secondary electron transfer activity of the two forms of reaction centre were measured using MnCl 2 and silicomolibdate as electron donor and acceptor respectively, RC IIb was 16% less active than RC IIa. From the data we conclude that the removal of 2 chlorophylls and 1 β-carotene molecules from the isolated Photosystem II reaction centre only slightly impair its functional activity with respect to primary charge separation. This conclusion seems to suggest that photochemically active isolated reaction centres of Photosystem II and purple bacteria can have the same minimum pigment stoichiometry of 4 chlorophylls and 1 carotenoid per 2 pheophytins.

Primary processes in isolated bacterial reaction centers from Rhodobacter sphaeroides studied by picosecond fluorescence kinetics

Picosecond to nanosecond fluorescence kinetics as a function of emission wavelength is reported for isolated reaction centers of the purple bacterium Rb. sphaeroides. New kinetic components of 12 ps and about 100 ps lifetime have been found in the fluorescence decay kinetics of the primary donor P*. Various kinetic models of the primary and secondary electron transfer processes are discussed which could possibly explain these new components. The data indicate either the necessity for more complex electron transfer schemes than assumed so far or a substantial heterogeneity in the rates of both the primary and secondary electron transfer steps.