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Photoselective DNA Hairpin Spin Switches
Abstract
DNA hairpins having both a tethered anthraquinone (Aq) end-capping group and a perylenediimide (PDI) base surrogate were synthesized, wherein Aq and PDI are each separated from a G-C base pair hole trap by A-T and I-C base pairs (G = guanine, A = adenine, T= thymine, C = cytosine, I = inosine). Selective photoexcitation of PDI at 532 nm generates a singlet radical ion pair (RP), (1)(G(+•)-PDI(-•)), while selective photoexcitation of Aq at 355 nm generates the corresponding triplet RP, (3)(G(+•)-Aq(-•)). Subsequent radical pair intersystem crossing within these spin-correlated RPs leads to mixed spin states that exhibit spin-polarized, time-resolved EPR spectra in which the singlet- and triplet-initiated RPs have opposite phases. These results demonstrate that a carefully designed DNA hairpin can serve as a photodriven molecular spin switch based on wavelength-selective formation of the singlet or triplet RP without significant competition from undesired energy transfer processes.
... In particular, recent experiments have reported the spin effects of DNA hairpins. Wasielewski et al. have prepared spin-entangled radical pairs within synthetic DNA hairpins possessing donor and acceptor chromophores, and probed their spin dynamics using electron paramagnetic resonance spectroscopy [46][47][48][49]. These DNA hairpins can serve as molecular spin switches and are regarded as a promising platform for applications in quantum information science. ...
... Although a variety of ssDNA molecules and organic groups have been used as the hairpin loops [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56], they can be divided into two categories, i.e., the conducting loops and the insulating ones. When conductive organics are covalently linked to the hairpin stem as the loop, such as the acceptor chromophores, the electrons can propagate through the loop. ...
The chirality-induced spin selectivity (CISS), demonstrated in diverse chiral molecules by numerous experimental and theoretical groups, has been attracting extensive and ongoing interest in recent years. As the secondary structure of DNA, the charge transfer along DNA hairpins has been widely studied for more than two decades, finding that DNA hairpins exhibit spin-related effects as reported in recent experiments. Here, we propose a setup to demonstrate directly the CISS effect in DNA hairpins contacted by two nonmagnetic leads at both ends of the stem. Our results indicate that DNA hairpins present a pronounced CISS effect and the spin polarization could be enhanced by using conducting molecules as the loop. In particular, DNA hairpins exhibit several intriguing features. First, the local spin currents can flow circularly and assemble into a number of vortex clusters when the electron energy locates in the left/right electronic band of the stem. The chirality of vortex clusters in each band is the same and will be reversed by switching the electron energy from the left band to the right one, inducing the sign reversal of the spin polarization. Interestingly, the local spin currents can be greater than the corresponding spin component of the source-drain current. Second, both the conductance and the spin polarization can increase with molecular length as well as dephasing strength, contrary to the physical intuition that the transmission ability of molecular wires should be poorer when suffering from stronger scattering. Third, we unveil the optimal contact configuration of efficient electron transport and that of the CISS effect, which are distinct from each other and can be controlled by dephasing strength. The experimental realization of these results is discussed and the underlying physical mechanism is illustrated.
... Subnanosecond photoinduced electron transfer in a structurally well-defined molecular donor-acceptor system can be used to produce an entangled spin qubit (radical) pair (RP) in a pure initial singlet state fulfilling this criterion. Photogenerated spin-entangled RPs occur in diverse molecular systems, including photosynthetic reaction centre proteins 14 , electron donors and acceptors in micelles 15,16 , covalent fixed-distance donor-acceptor molecules 17-19 , cryptochromes implicated in the mechanism of the avian compass 20,21 and donor-acceptor systems that are part of DNA hairpin structures 22,23 . In many of these systems, multi-step electron transfer results in electron spin coherence being maintained over several nanometres for tens of microseconds, making it possible to synthesize and use complex molecular assemblies for quantum information applications. ...
... Subnanosecond photoinduced electron transfer in a structurally well-defined molecular donor-acceptor system can be used to produce an entangled spin qubit (radical) pair (RP) in a pure initial singlet state fulfilling this criterion. Photogenerated spin-entangled RPs occur in diverse molecular systems, including photosynthetic reaction centre proteins 14 , electron donors and acceptors in micelles 15,16 , covalent fixed-distance donor-acceptor molecules [17][18][19] , cryptochromes implicated in the mechanism of the avian compass 20,21 and donor-acceptor systems that are part of DNA hairpin structures 22,23 . In many of these systems, multi-step electron transfer results in electron spin coherence being maintained over several nanometres for tens of microseconds, making it possible to synthesize and use complex molecular assemblies for quantum information applications. ...
Quantum teleportation transfers the quantum state of a system over an arbitrary distance from one location to another through the agency of quantum entanglement. Because quantum teleportation is essential to many aspects of quantum information science, it is important to establish this phenomenon in molecular systems whose structures and properties can be tailored by synthesis. Here, we demonstrate electron spin state teleportation in an ensemble of covalent organic donor–acceptor–stable radical (D–A–R•) molecules. Following preparation of a specific electron spin state on R• in a magnetic field using a microwave pulse, photoexcitation of A results in the formation of an entangled electron spin pair D•+–A•−. The spontaneous ultrafast chemical reaction D•+–A•−–R• → D•+–A–R⁻ constitutes the Bell state measurement step necessary to carry out spin state teleportation. Quantum state tomography of the R• and D•+ spin states using pulse electron paramagnetic resonance spectroscopy shows that the spin state of R• is teleported to D•+ with high fidelity.
... Moreover, photoactive compounds can be easily arranged on DNA. [17][18][19][20] 1,4-Disubstituted 9,10-anthraquinone is well known to form a supramolecular complex intercalating into DNA 21 and for this reason display interesting biological activity. 22,23 By coupling an aromatic 9,10-anthraquinone to a single nitroxide stable radical, via the amide linkages in position 1 or, indistin-guishably, 4 (molecule 1 in Scheme 1), we obtained a system that can interact with DNA, has a paramagnetic spin state in the ground state, and absorbs light in the visible range where DNA does not absorb. ...
An anthraquinone modified with a nitroxide radical and able to intercalate into DNA has been synthesized to obtain a molecule the spin state of which can be manipulated by visible light and DNA binding. The doublet ground state of the molecule can be photo-switched to either a strongly coupled spin state (quartet+doublet), when isolated, or to an uncoupled spin state (triplet and doublet), when bound to DNA. The different spin state that is obtained upon photoexcitation depends on the intercalation of the quinonic core into double-stranded DNA which changes the conformation of the molecule, thereby altering the exchange interaction between the excited state localized on the quinonic core and the nitroxide radical. The spin state of the system has been investigated using both continuous-wave and time-resolved EPR spectroscopy.
The inherent spin polarization present in photogenerated spin-correlated radical pairs makes them promising candidates for quantum computing and quantum sensing applications. The spin states of these systems can be probed and manipulated with microwave pulses using electron paramagnetic resonance spectrometers. However, to date, there are no reports on magnetic resonance-based spin measurements of photogenerated spin-correlated radical pairs hosted on quantum dots. In the current work, we prepare dye molecule-inorganic quantum dot conjugates and show that they can produce photogenerated spin-polarized states. The dye molecule, D131, is chosen for its ability to undergo efficient charge separation, and the nanoparticle materials, ZnO quantum dots, are chosen for their promising spin properties. Transient and steady state optical spectroscopy performed on ZnO quantum dot-D131 conjugates shows that reversible photogenerated charge separation is occurring. Transient and pulsed electron paramagnetic resonance experiments are then performed on the photogenerated radical pair, which demonstrate that (1) the radical pair is polarized at moderate temperatures and well modeled by existing theories and (2) the spin states can be accessed and manipulated with microwave pulses. This work opens the door to a new class of promising qubit materials that can be photogenerated in polarized states and hosted by highly tailorable inorganic nanoparticles.
Photon-to-spin quantum transduction, i.e., the exchange of coherence between photons and spins, is important for establishing links between localized molecular qubit systems performing logic and/or sensing operations and similar systems at different physical locations. We are developing molecular systems to explore how transduction can be implemented by using entangled photons to produce entangled spin pairs by ultrafast electron transfer leading to spin-correlated radical pairs (SCRPs). We have prepared and systematically evaluated electron transfer dyads and triads based on a 9-(N-piperidinyl)perylene-3,4-dicarboximide (6PMI) donor, a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor, and a tetrathiofulvalene (TTF) secondary donor, which are prepared with and without a phenyl spacer between the 6PMI and NDI to give 6PMI-NDI, 6PMI-Ph-NDI, TTF-6PMI-NDI, and TTF-6PMI-Ph-NDI. We used transient absorption, fluorescence-detected classical two-photon-absorption, and entangled two-photon-absorption (ETPA) spectroscopies to characterize the optical properties of these systems, while time-resolved EPR spectroscopy was used to characterize their photogenerated SCRPs. The results show that the ETPA cross section depends entirely on the presence of 6PMI and NDI in the 6PMI-NDI and 6PMI-Ph-NDI dyads, but in the triads TTF-6PMI-NDI and TTF-6PMI-Ph-NDI, the presence of the Ph spacer significantly affects the ETPA cross section. These results provide important information for designing molecular systems to implement photon-to-spin quantum transduction using entangled photons.
The photo-reactive acylhydrazone derivatives (4-FBAc, 4-ClBAc, 4-BrBAc and 4-IBAc) had been prepared, and they could undergo reversible E-Z isomerization reactions in the crystals under UV (365 nm) irradiation and heating condition. Meanwhile, the significant photomechanical behaviors were observed for the crystals with different shapes. The crystals of 4-FBAc (block-like), 4-ClBAc (block-like) and 4-BrBAc (rod-like) showed light-induced cracking, salient and cracking behaviors, respectively. Interestingly, the crystals of 4-FBAc (needle-like) and 4-IBAc (needle-like) bent away from the UV light (E→Z isomerization) firstly, then bent toward the source with the increasing of the irradiation or heating time (structure relaxation and Z→E isomerization). The theoretical study also showed that the molecules could undergo reversible E-Z isomerization reactions under UV irradiation and heating condition. Interestingly, the passive optical (635 nm) output direction of the crystal of 4-FBAc could be controlled by UV light. When mechanical force was applied or removed, the needle-like crystals of 4-FBAc and 4-IBAc exhibited reversible elastic bending properties. Analysis of the single crystal data suggested that C-H…π and H-bonds were responsible for the flexibility. So, the elastic organic molecular crystals based on acylhydrazone derivatives could be used as the candidate for crystal actuators and position-controllable optical transducer.
The designing of tunable molecular systems that can host spin qubits is a promising strategy for advancing the development of quantum information science (QIS) applications. Photogenerated radical pairs are good spin qubit pair (SQP) candidates because they can be initialized in a pure quantum state that exhibits relatively long coherence times. DNA is a well-studied molecular system that allows for control of energetics and spatial specificity through careful design and thus serves as a tunable scaffold on which to control multispin interactions. Here, we examine a series of DNA hairpins that use naphthalenediimide (NDI) as the hairpin linker. Photoexcitation of the NDI leads to subnanosecond oxidation of guanine (G) within the duplex or a stilbenediether (Sd) end-cap to give NDI•--G•+ or NDI•--Sd•+ SQPs, respectively. A 2,2,6,6-tetramethylpiperdinyl-1-oxyl (TEMPO) stable radical is covalently attached to the hairpin at varying distances from the SQP spins. While TEMPO has a minimal effect on the SQP formation and decay dynamics, EPR spectroscopy indicates that there are significant spin-spin dipolar interactions between the SQP and TEMPO. We also demonstrate the ability to implement more complex spin manipulations of the NDI•--Sd•+-TEMPO system using pulse-EPR techniques, which is important for developing DNA hairpins for QIS applications.
The synthetic tunability, flexibility, and rich spin physics of semiconductor quantum dots make them promising candidates for quantum information science applications. However, the rapid spin relaxation observed in colloidal quantum dots limits their functionality. In the current work, we demonstrate a method to harness photoexcited spin states in QDs to produce long-lived spin polarization on an appended organic ligand molecule. We present a system composed of CdSe/CdS core/shell QDs, covalently linked to naphthalenediimide (NDI) electron accepting molecules. The electron transfer dynamics from photoexcited QDs to the appended NDI ligands is explored as a function of both shell thickness and number of NDIs per quantum dot. Transient EPR spectroscopy shows that the photoexcited QDs strongly spin polarize the NDI radical anion, which is interpreted in the context of both the radical pair and triplet mechanisms of spin polarization. This work serves an initial step towards using photoexcited QDs to strongly spin polarize organic radicals having long spin relaxation times to serve as spin qubits in quantum information science applications.
Photoinduced electron transfer can produce radical pairs having two quantum entangled electron spins that can act as spin qubits in quantum information applications. Manipulation of these spin qubits requires selective addressing of each spin using microwave pulses. In this work, photogenerated spin qubit pairs are prepared within chromophore-modified DNA hairpins with varying spin qubit distances, and are probed using transient EPR spectroscopy. By performing pulse-EPR measurements on the shortest hairpin, selective addressing of each spin qubit comprising the pair is demonstrated. Furthermore, these spin qubit pairs have coherence times of more than 4 µs, which provides a comfortable time window for performing complex spin manipulations for quantum information applications. The applicability of these DNA-based photogenerated two-qubit systems is discussed in the context of quantum gate operations, specifically the controlled-NOT gate.
Implementation of the two-qubit controlled-NOT (CNOT) gate is necessary to develop a complete set of universal gates for quantum computing. Here, we demonstrate that a photogenerated radical (spin qubit) pair within a covalent donor-chromophore-acceptor molecule can be used to successfully execute a CNOT gate with high fidelity. The donor is tetrathiafulvalene (TTF), the chromophore is 8-aminonaphthalene-1,8-dicarboximide (ANI), and the acceptor is pyromellitimide (PI). Selective photoexcitation of ANI with a 416 nm laser pulse results in subnanosecond formation of the TTF•+-ANI-PI•− radical (spin qubit) pair at 85 K having a 1.8 µs phase memory time. This is sufficiently long to execute a CNOT gate using a sequence of five microwave pulses followed by a sequence of two pulses that read out all the elements of the density matrix. Comparing these data to a simulation of the data that assumes ideal conditions results in a fidelity of 0.97 for the execution of the CNOT gate. These results show that photogenerated molecular spin qubit pairs can be used to execute this essential quantum gate at modest temperatures, which affords the possibility that chemical synthesis can be used to develop structures to execute more complex quantum logic operations using electron spins.
The photo-reactive 5-chloro-2-(naphthalenylvinyl)benzo[d]oxazols (BOV1N, BOV1NF, BOV1NM, BOV2N and BOV2NM) have been prepared. Interestingly, their molecular crystals exhibit significant photomechanic effects, and ¹H NMR spectral changes for the microcrystals before and after UV irradiation suggest the photodimerization is the driving force for the light-induced macroscopic mechanic movements. The needle-like crystals of BOV1N bend away from UV light upon irradiation for several seconds. Such rapid elastic deformation could be repeated for several times. It should be noted that β-type and α-type cyclobutane derivatives D-BOV1N (the dimer of BOV1N) are afforded as main product and byproduct, respectively, after the microcrystals were exposed to 365 nm light for 5 min. Since the photo-induced [2+2] cycloaddition shortens the distance of olefin pairs and makes the terminal units to stretch outside the molecular long axle, the strain in the phototropic surface of the crystal can be yielded and accumulated. The release of the accumulated strain leads to the bending of the needle-like crystals backwards UV light. Thus, it was reasonable that the thinner fibers exhibited more rapid and significant movements on account of the high topo-photochemical reaction efficiency. The molecular crystals of BOV1NF, BOV2N and BOV2NM exhibited similar photomechanic effects. We also found that the parallel UV irradiation to the long axis of crystals would not drive the crystals to move. Differently, the light-induced slipping and swinging of the needle-like crystal of BOV1NM are observed due to the low dimerization efficiency. Thus, the molecular crystals based on naphthylvinylbenzoxazole derivatives become new platform for the effcient light-mechanical energy conversion.
The ability to prepare physical qubits in specific initial quantum states is a critical requirement for their use in quantum information science (QIS). Sub-nanosecond photoinduced electron transfer in a structurally well-defined donor-acceptor system can be used to produce an entangled spin qubit (radical) pair in a pure initial singlet state fulfilling this criterion. Synthetic DNA is a promising platform on which to build spin qubit arrays with fixed spatial relationships; therefore, we have prepared a series of DNA hairpins in which naphthalenediimide (NDI) is the chromophore/acceptor hairpin linker, variable length diblock A- and G-tracts are intermediate donors, and a stilbenediether (Sd) is the terminal donor. Photoexcitation of NDI in these DNA hairpins generates high yield, long-lived, entangled spin qubit pairs at 85 K, and time-resolved and pulse electron paramagnetic resonance (EPR) spectroscopies are used to probe their spin dynamics. Specifically, measurements of the distance-dependent dipolar coupling between the two spins is used to obtain the average spin qubit pair distance and reveals that one of the spins is fully delocalized across up to five adjacent guanines in a G-tract on the EPR timescale. We have recently shown that extensive spin hopping between degenerate sites accessible to one spin of the pair may result in spin decoherence. However, we observe a strong out-of-phase electron spin echo envelope modulation (OOP-ESEEM) signal from the NDI•- - Sd•+ spin qubit pair in DNA hairpins showing that spin coherence is maintained across a two adenine A-tract followed by a 2-4 guanine G-tract as a result of rapid spin transport to Sd. These results demonstrate that pulse-EPR can manipulate coherent spin states in DNA hairpins, which is essential for quantum gate operations relevant to QIS applications.
A Hall device based on a GaN/AlGaN high electron mobility structure was used for measuring spin polarization on electrons that are either reorganized within the molecules or transmitted through the self-assembled monolayers of DNA adsorbed on its surface. Two types of experiments were conducted, charge and spin polarization and spin-dependent electrochemical studies. Employing this setup, we were able to observe spin-dependent charge polarization and charge transport through double-stranded DNA of various lengths, in one case, and through double-stranded DNA containing oxidative damage. We found enhancement in the spin-dependent transport through oxidatively damaged DNA. This phenomenon can be rationalized either by assuming that the damaged DNA is characterized by a higher barrier for conduction or by charge transfer through the DNA being conducted through at least two channels, one involves the bases and is highly conductive but less spin selective, while the other pathway is mainly through the ribophosphate backbone and it is the minor one in terms of charge transmission efficiency, but it is highly spin selective.
Ultrafast photo-driven electron transfer reactions starting from an excited singlet state in an organic donor-acceptor molecule can generate a spin-correlated radical pair (RP) with an initially entangled spin state that may prove useful as a two-qubit pair in quantum information protocols. Here we investigate the effects of modulating electron-nuclear hyperfine coupling by rapidly transferring an electron between two equivalent sites comprising the reduced acceptor of the RP. A covalent electron donor-acceptor molecule including a tetrathiafulvalene (TTF) donor, a 4-aminonaphthalene-1,8-imide (ANI) chromophoric primary acceptor, and a m-xylene bridged cyclophane having two equivalent pyromellitimides (PI2), TTF-ANI-PI2, as a secondary acceptor was synthesized along with the analogous molecule having one pyromellitimide (PI) acceptor, TTF-ANI-PI. Photoexcitation of ANI within each molecule results in sub-nanosecond formation of TTF+•-ANI-PI-• and TTF+•-ANI-PI2-•. The effect of reducing electron-nuclear hyperfine interactions in TTF+•-ANI-PI2-• relative to TTF+•-ANI-PI-• on decoherence of multiple-quantum coherences has been measured in tandem by pulse-EPR spectroscopy. The theoretical prediction of the contribution from an ensemble of hyperfine interactions to decoherence of the quantum coherences in these RPs is shown to be less than the full width at half maximum of the quantum beat frequencies measured experimentally. Pulse bandwidth and off-resonant excitation by square microwave pulses are proposed as larger contributors to decoherence in these molecules than the hyperfine interactions.
A novel Hall circuit design that can be incorporated into a working electrode, which is used to probe spin‐selective charge transfer and charge displacement processes, is reviewed herein. The general design of a Hall circuit based on a semiconductor heterostructure, which forms a shallow 2D electron gas and is used as an electrode, is described. Three different types of spin‐selective processes have been studied with this device in the past: i) photoinduced charge exchange between quantum dots and the working electrode through chiral molecules is associated with spin polarization that creates a local magnetization and generates a Hall voltage; ii) charge polarization of chiral molecules by an applied voltage is accompanied by a spin polarization that generates a Hall voltage; and iii) cyclic voltammetry (current–voltage) measurements of electrochemical redox reactions that can be spin‐analyzed by the Hall circuit to provide a third dimension (spin) in addition to the well‐known current and voltage dimensions. The three studies reviewed open new doors into understanding both the spin current and the charge current in electronic materials and electrochemical processes. A novel Hall circuit design is used as a working electrode. Three different types of spin‐selective processes have been studied with this device in the past: photoinduced charge exchange between quantum dots and the working electrode through chiral molecules, charge polarization, and cyclic voltammetry measurements of electrochemical redox reactions that can be spin‐analyzed.
Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems.
Folding-unfolding imparts fluorescence dual color switching, thus a novel concept to switch fluorescence between two distinct colors while avoiding traditional bond rupturing and bond forming in photoswitching. Because folding and unfolding minimize the wear and tear on molecular structures, the new systems have excellent reversibility and fatigue resistance.
This chapter reviews the literature published in 2013 relating to modified oligonucleotides. The literature continues the trend of previous years in that modified oligonucleotides continue to play a key role in many biochemical and structural studies. A broad range of modifications to the internucleotide linkage, the sugar and the nucleobase are included in this review for many different biochemical applications, as are nucleic acid nanostructures and devices. Nucleic acid structures, including single-molecule studies, are one of the most studied areas, and the systems described becoming more complex as novel techniques emerge.
Shortly after their formation, transient free radicals trapped in micelles exhibit electron spin resonance (ESR) spectra with “antiphase” lineshapes, i.e. lines with their low-field halves in emission and high-field halves in absorption. It is demonstrated that this hitherto unexplained effect is consistent with the detection of ESR transitions in geminate radical pairs before the radicals have had time to diffuse apart.
The distance dependence of photoinduced electron transfer in duplex DNA was determined for a family of synthetic DNA hairpins
in which a stilbene dicarboxamide forms a bridge connecting two oligonucleotide arms. Investigation of the fluorescence and
transient absorption spectra of these hairpins established that no photoinduced electron transfer occurs for a hairpin that
has six deoxyadenosine-deoxythymidine base pairs. However, the introduction of a single deoxyguanosine-deoxycytidine base
pair resulted in distance-dependent fluorescence quenching and the formation of the stilbene anion radical. Kinetic analysis
suggests that duplex DNA is somewhat more effective than proteins as a medium for electron transfer but that it does not function
as a molecular wire.
Regardless of its position within the DNA film, cross-linked daunomycin (DM) is efficiently reduced electrochemically, indicating that the electron transfer exhibits a shallow distance dependence. Upon the introduction of an intervening cytosine–adenine (CA) mismatch, the electrochemical response is dramatically attenuated (shown schematically). Therefore, the DNA double helix can facilitate long-range electron transfer, but only in the presence of a well-stacked pathway.
Spin polarization conservation during intramolecular triplet-triplet energy transfer was studied for the phthalimide derivatives. It is shown that spin polarization transfer is useful for determining the conformation and the nature of the lowest triple (T-1) states of the donor and acceptor moieties. The polarization pattern of the acceptor triplet state was well reproduced taking into account the polarization of the donor and the mutual orientation of the donor and acceptor moieties. This technique clarified the order of the triplet sublevels in energy of the phthalimide chromophore. The T-1 states of stilbene derivatives, which have a low quantum yield of intersystem crossing under direct excitation, were also detected.
Exact analytical expressions for the evolution of a non-diffusing spin-correlated radical pair in a strong magnetic field are presented. The recombination of singlet and triplet pairs at different rates, and the coherent evolution caused by magnetic interactions, lead to radical pair concentrations which are the sum of four time-dependent terms : two decaying exponentials and two exponentially damped oscillations. The predicted time dependence is used to illuminate the recombination dynamics of the primary radical pair in photosynthetic bacterial reaction centres. Although mono-exponential decay is expected 20 ns after the formation of each individual radical pair, the inhomogeneous distribution of Larmor frequencies of the two electron spins leads to non-exponential kinetics and different time constants for the disappearance of singlet and triplet pairs, and the formation of singlet and triplet recombination products.
A series of donor-chromophore-acceptor-stable radical (D-C-A-R•) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D+•-C-A-• spin-correlated radical pair (RP) to the stable radical R•. Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2JDA) of D+•-C-A-•. Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R• molecules described here, D = 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C = 4-aminonaphthalene-1,8-dicarboximide, and A = naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R•) (1B-6B). Photoexcitation of C with 416 nm laser pulses results in two-step charge separation to yield D+•-C-A-•-(R•). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin echo detection) are used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R• radical. The TREPR spectra show that |2JDA| for D+•-C-A-• decreases in the order MeOAn+• > DioxAn+• > BDXAn+• as a result of their spin density distributions, while the spin-spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin echo detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R• depends on |2JDA|.
Determining the electronic coupling matrix element, V, for an electron transfer reaction is challenging both experimentally and theoretically. The magnitude of the singlet−triplet splitting (spin−spin exchange interaction), 2J, within a radical ion pair (RP) is directly related to the sum of the squares of the matrix elements that couple the RP state to the ground state and to other energetically proximate excited and ionic states. Each term in this sum is weighted by the reciprocal of the energy gap between the RP state and the particular state to which it is coupled. We present here a series of intramolecular triads with linear, rodlike structures that undergo very efficient two-step electron transfer following direct excitation of a 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophore. Attachment of a p-methoxyaniline (MeOAn) donor by means of the piperazine bridge and naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI) acceptors, either directly or through a 2,5-dimethylphenyl (Me2Ph) spacer to 6ANI results in the triads MeOAn-6ANI-NI, MeOAn-6ANI-PI, MeOAn-6ANI-Me2Ph-NI, and MeOAn-6ANI-Me2Ph-PI. The two-step charge separation from the lowest excited singlet state of 6ANI yields singlet radical ion pairs in which the charges are separated by 14 to 19 Å and whose lifetimes range from about 15 to 200 ns. These lifetimes are long enough such that radical pair intersystem crossing occurs to form the triplet radical ion pair, which then recombines to form the ground state and a neutral excited triplet state, which is localized either on 6ANI or NI. The yield of this locally excited triplet state, monitored by nanosecond transient absorption as a function of applied magnetic field strength, exhibits distinct resonances that directly yield 2J. The value of 2J is used to estimate VCR for charge recombination of the radical ion pair. These measurements provide a highly sensitive method of determining the dependence of the electronic coupling on the structure of the radical ion pair.
Supramolecular systems synthesized to model the photosynthetic reaction center (RC) are designed to mimic several key properties of the RC protein. Thus far, most RC models fulfill only a subset of these criteria, with very few reports employing time-resolved electron paramagnetic resonance spectroscopy (TREPR). We now report TREPR results on a photosynthetic model system (1) in a nematic liquid crystal (LC) that does not contain the natural pigments, yet closely mimics the spin dynamics of triplet state formation found only in photosynthetic RCs. The design of supermolecule 1 follows criteria established for promoting high quantum yield charge separation in glassy media. The observation of this triplet state in 1 by TREPR demonstrates that most of the electronic states found in the primary photochemistry of photosynthetic RCs can be mimicked successfully in synthetic models interacting with LCs.
A model for polarized EPR spectra generated in radical pair reactions in micelles is being proposed. The model is based on electron spin-spin interactions which remain observable because of limited diffusion in micelles. The experimental observable is a doubling of hyperfine transitions, split by the magnitude of the interaction. The polarization is generated by the nonadiabatic generation of the radical pair with a triplet or singlet precursor. The time evolution of the wave function leads to a non-Boltzmann distribution of the populations among the four energy levels. The theory is tested by comparison with experiments, previously reported and repeated in this laboratory, obtained by laser flash photolysis of benzophenone in sodium dodecyl sulfate (SDS) micelles. Simulations of the shape of the spectra and their time dependence give excellent agreement with experiment. The model is further supported by experiments in micelles modified by different salt concentrations as well as different chain lengths of the micelle-forming molecules.
Regardless of its position within the DNA film, cross-linked daunomycin (DM) is efficiently reduced electrochemically, indicating that the electron transfer exhibits a shallow distance dependence. Upon the introduction of an intervening cytosine–adenine (CA) mismatch, the electrochemical response is dramatically attenuated (shown schematically). Therefore, the DNA double helix can facilitate long-range electron transfer, but only in the presence of a well-stacked pathway.
A series of DNA hairpins (AqGn) possessing a tethered anthraquinone (Aq) end-capping group were synthesized in which the distance between the Aq and a guanine-cytosine (G-C) base pair was systematically varied by changing the number (n - 1) of adenine-thymine (A-T) base pairs between them. The photophysics and photochemistry of these hairpins were investigated using nanosecond transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Upon photoexcitation, (1*)Aq undergoes rapid intersystem crossing to yield (3*)Aq, which is capable of oxidizing purine nucleobases resulting in the formation of (3)(Aq(-•)Gn(+•)). All (3)(Aq(-•)Gn(+•)) radical ion pairs exhibit asymmetric TREPR spectra with an electron spin polarization phase pattern of absorption and enhanced emission (A/E) due to their different triplet spin sublevel populations, which are derived from the corresponding non-Boltzmann spin sublevel populations of the (3*)Aq precursor. The TREPR spectra of the (3)(Aq(-•)Gn(+•)) radical ion pairs depend strongly on their spin-spin dipolar interaction and weakly on their spin-spin exchange coupling. The anisotropy of (3)(Aq(-•)Gn(+•)) makes it possible to determine that the π systems of Aq(-•) and G(+•) within the radical ion pair are parallel to one another. Charge recombination of the long-lived (3)(Aq(-•)Gn(+•)) radical ion pair displays an unusual bimodal distance dependence that results from a change in the rate-determining step for charge recombination from radical pair intersystem crossing for n < 4 to coherent superexchange for n > 4.
Although the interest in experimental evidence of magnetic field effects (MFEs) on the kinetics of chemical reactions, which might be characterized by the term magnetokinetics, has a long tradition, an impressive evolution of the field took place only after the discovery and understanding of nuclear and electronic spin polarization phenomena during chemical reactions (CIDNP, CIDEP) in the late 1960s. The so-called radical pair mechanism lying at the heart of these phenomena turned out to be a most valuable key for systematically tracing out MFEs on chemical yields and kinetics. Nevertheless one should be aware that other mechanisms, too, with pairs of triplets, triplet-doublet pairs, or individual triplets, which originated at about the same time and were initially developed for explaining magnetic phenomena on luminescence in organic solids, also have their implication on chemical, particularly on photochemical, kinetics. Phenomenologically, the basic mechanisms of magnetic-field-dependent reaction mechanisms may become apparent in fields and systems as different as the gas phase, the solid and liquid states, interfaces, and microheterogeneous systems such as micelles and in billogical systems. In all of these applications they have specific experimental and theoretical characteristics. Also, the techniques applied to study magnetokinetic phenomena span a large variety, ranging from magnetic resonance detection of spin polarization (CIDNP, CIDEP, ODMR) through simple detection of magneticfield-dependent reaction yields and magnetic isotope effects (MIE) to reaction-yield-detected magnetic resonance (RYDMR). Thus the field of magnetokinetic chemical and related physical phenomena appears as a tree with several roots and many branches. Although each of these branches has been reviewed from time to time (cf. Table l), most of the treatments have been rather specialized, and it is not easy to provide oneself with a broad and general view of the scope, objectives, and achievements of the field. Thus we have found it worthwhile to write this survey, developing the different aspects from a fairly general point of view (cf. section 11), and to review, as comprehensively as possible, the original experimental (section IV) and theoretical (section V) work published since the early 1970s, providing whenever possible a systematic compilation in the form of tables. Furthermore, in section I11 an outline of the various experimental techniques applied in the field is given. Of course, the goals of completeness and compactness were not attainable without compromise. Thus the large field of chemically induced spin polarization phenomena would have been beyond the scope of this review. We have, however, attempted to include those theoretical papers in the field that have a general bearing on the understanding of magnetokinetic effects in general. We felt that, especially where photochemistry is concerned, the borderline between truly chemical and purely physical phenomena should not be defined too formally, since from the mechanistic and theoretical point of view they may be closely related. In order to account for this we included what has been termed related phenomena in the title of this review. Of course. the problem of delimitation cannot be solved without arbitrariness. The more photophysical aspects are mainly to be found in the sections on gas-phase and solid-state phenomena. In the solid state our attention has been mainly directed on work with organic molecular crystals. Only some representative references on inorganic solids and semiconductors are given. We hope that this review may provide a welcome guide to the present body of literature on magnetokinetics, that it may help those working in the field to assess the achievements of current original work, and that it may be a useful framework of orientation for those who want to get into it or get an impression of the present scope of magnetokinetics.
For development of the molecular solar-energy conversion systems, it is crucial to investigate how both the molecular geometry and electronic structure of electron donor-bridge-acceptor (D-B-A) molecules contribute to the electronic coupling for the charge-separation (CS) and charge-recombination (CR) processes. In a D-B-A system of a porphyrin-fullerene dyad (ZnP-C(60)) bridged by a diphenyldisilane spacer, we have characterized one specific folded molecular conformation in the CS state among several existing conformations using the time-resolved electron paramagnetic resonance (TREPR) method at low temperature. To determine the molecular conformation and spin-spin exchange coupling of the CS state, we have considered (1) the electron spin polarization transfer from the excited triplet state of the C(60) moiety to the CS state and (2) the sublevel-selective spin relaxations and CR in the CS state. In the CS state of this conformation, although the ZnP cation and C(60) anion radicals are in close proximity, direct overlap between their singly occupied molecular orbitals is small, resulting in detection of the long-lived CS state which has a totally different conformation from the optically detected, charge-transfer (CT) complex. It has been demonstrated that, among several folded and extended molecular conformations created by the flexibility of the -Si-Si- bridge, the EPR conformation can play a role on the prevention of the energy-wasting CR.
A perylenediimide chromophore (P) was incorporated into DNA hairpins as a base-pair surrogate to prevent the self-aggregation of P that is typical when it is used as the hairpin linker. The photoinduced charge-transfer and spin dynamics of these hairpins were studied using femtosecond transient absorption spectroscopy and time-resolved EPR spectroscopy (TREPR). P is a photooxidant that is sufficiently powerful to quantitatively inject holes into adjacent adenine (A) and guanine (G) nucleobases. The charge-transfer dynamics observed following hole injection from P into the A-tract of the DNA hairpins is consistent with formation of a polaron involving an estimated 3-4 A bases. Trapping of the (A 3-4) (+*) polaron by a G base at the opposite end of the A-tract from P is competitive with charge recombination of the polaron and P (-*) only at short P-G distances. In a hairpin having 3 A-T base pairs between P and G ( 4G), the radical ion pair that results from trapping of the hole by G is spin-correlated and displays TREPR spectra at 295 and 85 K that are consistent with its formation from (1*)P by the radical-pair intersystem crossing mechanism. Charge recombination is spin-selective and produces (3*)P, which at 85 K exhibits a spin-polarized TREPR spectrum that is diagnostic of its origin from the spin-correlated radical ion pair. Interestingly, in a hairpin having no G bases ( 0G), TREPR spectra at 85 K revealed a spin-correlated radical pair with a dipolar interaction identical to that of 4G, implying that the A-base in the fourth A-T base pair away from the P chromophore serves as a hole trap. Our data suggest that hole injection and transport in these hairpins is completely dominated by polaron generation and movement to a trap site rather than by superexchange. On the other hand, the barrier for charge injection from G (+*) back onto the A-T base pairs is strongly activated, so charge recombination from G (or even A trap sites at 85 K) most likely proceeds by a superexchange mechanism.
Our understanding of oxidative damage to double helical DNA and the design of DNA-based devices for molecular electronics is crucially dependent upon elucidation of the mechanism and dynamics of electron and hole transport in DNA. Electrons and holes can migrate from the locus of formation to trap sites, and such migration can occur through either a single-step "superexchange" mechanism or a multistep charge transport "hopping" mechanism. The rates of single-step charge separation and charge recombination processes are found to decrease rapidly with increasing transfer distances, whereas multistep hole transport processes are only weakly distance dependent. However, the dynamics of hole transport has not yet been directly determined. Here we report spectroscopic measurements of photoinduced electron transfer in synthetic DNA that yield rate constants of approximately 5 x 10(7) s(-1) and 5 x 10(6) s(-1), respectively, for the forward and return hole transport from a single guanine base to a double guanine base step across a single adenine. These rates are faster than processes leading to strand cleavage, such as the reaction of guanine cation radical with water, thus permitting holes to migrate over long distances in DNA. However, they are too slow to compete with charge recombination in contact ion pairs, a process which protects DNA from photochemical damage.
We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet.
Functional molecular wires are essential for the development of molecular electronics. Charge transport through molecules occurs primarily by means of two mechanisms, coherent superexchange and incoherent charge hopping. Rates of charge transport through molecules in which superexchange dominates decrease approximately exponentially with distance, which precludes using these molecules as effective molecular wires. In contrast, charge transport rates through molecules in which incoherent charge hopping prevails should display nearly distance independent, wirelike behavior. We are now able to determine how each mechanism contributes to the overall charge transport characteristics of a donor-bridge-acceptor (D-B-A) system, where D = phenothiazine (PTZ), B = p-oligophenylene, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), by measuring the interaction between two unpaired spins within the system's charge separated state via magnetic field effects on the yield of radical pair and triplet recombination product.
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