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1.
J Am Chem Soc ; 2020 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-32650641

RESUMO

The synthetic tunability, flexibility, and rich spin physics of semiconductor quantum dots (QDs) 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 QD. 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 the triplet mechanisms of spin polarization. This work serves as an initial step toward using photoexcited QDs to strongly spin polarize organic radicals having long spin relaxation times to serve as spin qubits in quantum information science applications.

2.
J Phys Chem A ; 2020 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-32551620

RESUMO

Switchable coupling between two qubits is important for quantum information science (QIS). As a proof of concept, a series of mesosubstituted porphyrins have been synthesized with a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl stable free radical (SFR) appended and metalated with Cu(II), Ni(II), and Zn(II) in order to explore the interaction between the SFR doublet state and metalloporphyrin. The spin state of the porphyrin varies upon metal insertion, where Zn(II) is a diamagnetic metal, Cu(II) is paramagnetic, and Ni(II) can be switched from a diamagnetic square-planar structure to a paramagnetic octahedral state by complexation with a solvent (i.e., pyridine or tetrahydrofuran). Time-resolved electron paramagnetic resonance (EPR) measurements reveal that upon photoexcitation, the Zn(II) and free-base porphyrin species demonstrate different magnetic exchange regimes between the porphyrin triplet excited states and the SFR doublet state, with the Zn derivative populating a quartet state (i.e., moderate magnetic exchange), whereas the free-base derivative remains a triplet (i.e., weak magnetic exchange). Transient absorption measurements corroborate the TREPR results, demonstrating a 66% increase in the singlet excited-state decay rate due to enhanced intersystem crossing for the Zn(II) derivative in comparison to a modest 14% enhancement for the free-base porphyrin. These results enable the realization of a switchable qubit coupler, depending upon Zn metal insertion to the free-base porphyrin, which has potential QIS applications.

3.
J Am Chem Soc ; 142(7): 3346-3350, 2020 Feb 19.
Artigo em Inglês | MEDLINE | ID: mdl-32009396

RESUMO

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.

4.
J Chem Phys ; 152(1): 014503, 2020 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-31914753

RESUMO

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.

5.
J Am Chem Soc ; 141(47): 18727-18739, 2019 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-31580664

RESUMO

A series of donor-acceptor (D-A) naphthalene-viologen-based cyclophanes of different shapes, sizes, and symmetries have been synthesized and characterized. Solution optical studies on these cyclophanes reveal the existence of photoinduced intramolecular charge transfer (CT) at 465 nm from naphthalene (D) to viologen (A) units, resulting in a conformational change in the viologen units and the emergence of an emission at 540 nm. The D-A cyclophanes with box-like and hexagon-like shapes offer an opportunity to control the arrangement within 2D layers where D-A interactions direct the superstructures. While a box-like 2,6-disubstituted naphthalene-based tetracationic cyclophane does not form square tiling patterns, a truncated hexagon-like congener self-assembles to form a hexagonal superstructure which, in turn, adopts a hexagonal tiling pattern. Tessellation of the more rigid and highly symmetrical 2,7-disubstituted naphthalene-based cyclophanes leads to the formation of 2D square and honeycomb tiling patterns with the box-like and hexagon-like cyclophanes, respectively. Co-crystallization of the box-like cyclophanes with tetrathiafulvalene (TTF) results in the formation of D-A CT interactions between TTF and viologen units, leading to tubular superstructures. Co-crystallization of the hexagon-like cyclophane with TTF generates well-ordered and uniform tubular superstructures in which the TTF-viologen CT interactions and naphthalene-naphthalene [π···π] interactions propagate with 2D topology. In the solid state, the TTF-cyclophane co-crystals are paramagnetic and display dual intra- and intermolecular CT behavior at ∼470 and ∼1000 nm, respectively, offering multi-responsive materials with potential pathways for electron transport.

6.
J Am Chem Soc ; 141(44): 17783-17795, 2019 Nov 06.
Artigo em Inglês | MEDLINE | ID: mdl-31526001

RESUMO

Tessellation of organic polygons though [π···π] and charge-transfer (CT) interactions offers a unique opportunity to construct supramolecular organic electronic materials with 2D topologies. Our approach to exploring the 3D topology of 2D tessellations of a naphthalene diimide-based molecular triangle (NDI-Δ) reveals that the 2D molecular arrangement is sensitive to the identity of the solvent and solute concentrations. Utilization of nonhalogenated solvents, combined with careful tailoring of the concentrations, results in NDI-Δ self-assembling though [π···π] interactions into 2D honeycomb triangular and hexagonal tiling patterns. Cocrystallization of NDI-Δ with tetrathiafulvalene (TTF) leads systematically to the formation of 2D tessellations as a result of superstructure-directing CT interactions. Different solvents lead to different packing arrangements. Using MeCN, CHCl3, and CH2Cl2, we identified three sets of cocrystals, namely CT-A, CT-B, and CT-C, respectively. Solvent modulation plays a critical role in controlling not only the NDI-Δ:TTF stoichiometric ratios and the molecular arrangements in the crystal superstructures, but also prevents the inclusion of TTF guests inside the cavities of NDI-Δ. Confinement of TTF inside the NDI-Δ cavities in the CT-A superstructure enhances the CT character with the observation of a broad absorption band in the NIR region. In the CT-B superstructure, the CHCl3 lattice molecules establish a set of [Cl···Cl] and [Cl···S] intermolecular interactions, leading to the formation of a hexagonal grid of solvent in which NDI-Δ forms a triangular grid. In the CT-C superstructure, three TTF molecules self-assemble, forming a supramolecular isosceles triangle TTF-Δ, which tiles in a plane alongside the NDI-Δ, producing a 3 + 3 honeycomb tiling pattern of the two different polygons. Solid-state spectroscopic investigations on CT-C revealed the existence of an absorption band at 2500 nm, which on the basis of TDDFT calculations, was attributed to the mixed-valence character between two TTF•+ radical cations and one neutral TTF molecule.

7.
Nat Chem ; 11(11): 981-986, 2019 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-31548665

RESUMO

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.

8.
J Phys Chem B ; 123(41): 8823-8828, 2019 10 17.
Artigo em Inglês | MEDLINE | ID: mdl-31549504

RESUMO

Nitrogenase is activated for N2 reduction through the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co: Fe7S9MoC; homocitrate) to form the key Janus intermediate, denoted E4(4H), whose lowest-energy structure contains two [Fe-H-Fe] bridging hydrides and two protons bound to the sulfurs that also bridge the Fe pairs. In the critical step of catalysis, a H2 complex transiently produced by reductive elimination (re) of the hydrides of E4(4H), denoted E4(H2;2H), undergoes H2 displacement by N2, which then undergoes the otherwise energetically unfavorable cleavage of the N≡N triple bond. In pursuing the study of the re activation process, we have employed a photochemical approach to obtaining its atomic-level details. Continuous 450 nm irradiation of the ground state of the dihydride Janus intermediate, denoted E4(4H)a, in an EPR cavity at cryogenic temperatures causes photoinduced re of H2 to generate E4(H2;2H). We here extend this photochemical approach with time-resolved EPR studies of the photolysis process on the ns time scale. These studies reveal an additional intermediate in the catalytic reductive elimination process, an isomer of the E4(4H) FeMo-co metal-ion core that is formed prior to E4(H2;2H) and is thought to be created by breaking an Fe-SH bond, thus further integrating the calculational and structural studies into the experimentally determined mechanism by which nitrogenase is activated to cleave the N≡N triple bond.

9.
J Phys Chem B ; 123(36): 7731-7739, 2019 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-31418270

RESUMO

There has been increasing interest in the excited states of stable diradicals as means of manipulating their spin states for potential applications in quantum information science (QIS). In this work, we examine a set of diradicals composed of two stable naphthalene-1,8:4,5-bis(dicarboximide) radical anions (NDI•-) bound either directly at their imide nitrogen atoms or through a series of benzene spacers resulting in diradicals with either singlet or triplet ground states. We use time-resolved near-UV, visible, near-IR, and mid-IR spectroscopy to show that the population in the singlet ground state can undergo photoinduced electron transfer upon excitation of one of the NDI•- radicals to produce the NDI0-NDI2- moiety, while the corresponding triplet population cannot. In particular, spectroscopy in the wavelength region 330-450 nm and in the energy range 1450-1750 cm-1 is critical to distinguishing the two populations. By varying the connectivity between the two radical anions, we vary both the sign and magnitude of the singlet-triplet energy splitting (2J) of the diradicals, thereby varying the proportion of singlet and triplet ground state populations that are detected optically. EPR spectroscopy provides corroborating evidence for the ground spin state of the diradicals. This result has implications for using photoexcitation to manipulate the spin states of diradicals for QIS applications.

10.
Chem Sci ; 10(27): 6707-6714, 2019 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-31367325

RESUMO

Harnessing synthetic chemistry to design electronic spin-based qubits, the smallest unit of a quantum information system, enables us to probe fundamental questions regarding spin relaxation dynamics. We sought to probe the influence of metal-ligand covalency on spin-lattice relaxation, which comprises the upper limit of coherence time. Specifically, we studied the impact of the first coordination sphere on spin-lattice relaxation through a series of four molecules featuring V-S, V-Se, Cu-S, and Cu-Se bonds, the Ph4P+ salts of the complexes [V(C6H4S2)3]2- (1), [Cu(C6H4S2)2]2- (2), [V(C6H4Se2)3]2- (3), and [Cu(C6H4Se2)2]2- (4). The combined results of pulse electron paramagnetic resonance spectroscopy and ac magnetic susceptibility studies demonstrate the influence of greater M-L covalency, and consequently spin-delocalization onto the ligand, on elongating spin-lattice relaxation times. Notably, we observe the longest spin-lattice relaxation times in 2, and spin echos that survive until room temperature in both copper complexes (2 and 4).

11.
J Am Chem Soc ; 141(20): 8306-8314, 2019 May 22.
Artigo em Inglês | MEDLINE | ID: mdl-31083934

RESUMO

The understanding of the catalyst-support interactions has been an important challenge in heterogeneous catalysis since the supports can play a vital role in controlling the properties of the active species and hence their catalytic performance. Herein, a series of isostructural mesoporous metal-organic frameworks (MOFs) based on transition metals, lanthanides, and actinides (Zr, Hf, Ce, Th) were investigated as supports for a vanadium catalyst. The vanadium species was coordinated to the oxo groups of the MOF node in a single-ion fashion, as determined by single-crystal X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and diffuse reflectance UV-vis spectroscopy. The support effects of these isostructural MOFs were then probed using the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The turnover frequency was found to be correlated with the electronegativity and oxidation state of the metal cations on the supporting MOF nodes, highlighting an important consideration when designing catalyst supports.

12.
Proc Natl Acad Sci U S A ; 116(17): 8178-8183, 2019 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-30948629

RESUMO

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T1T1) state whose spin dynamics influence the successful generation of uncorrelated triplets (T1). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T1T1) state. The spin dynamics of the (T1T1) state and the processes leading to uncoupled triplets (T1) were studied at room temperature for TDI2 aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T1T1) state has mixed 5(T1T1) and 3(T1T1) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the 3(T1T1) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T1 state. The presence of the 5(T1T1) state at room temperature and its relationship with the 1(T1T1) and 3(T1T1) states emphasize that understanding the relationship among different (T1T1) spin states is critical for ensuring high-yield T1 formation from singlet fission.

13.
Chem Sci ; 10(6): 1702-1708, 2019 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-30842834

RESUMO

Synthetic chemistry offers a pathway to realize atomically precise arrays of qubits, the smallest unit of a quantum information science system. We harnessed framework chemistry to create an array of qubit candidates, featuring one qubit every 13.6 Å, by synthesizing the new copper(ii) variant of the porphyrinic metal-organic framework PCN-224. We subjected the framework to pulse-electron paramagnetic resonance (EPR) measurements, establishing spin coherence at temperatures up to 80 K within a fully spin concentrated framework. Observation of Rabi oscillations further support the viability of the qubits within these arrays. To interrogate the spin dynamics of qubit arrays, we investigated spin-lattice relaxation, T 1, through a combination of pulse-EPR and alternating current (ac) magnetic susceptibility measurements. These data revealed distinct vibrational environments within the frameworks that contribute to spin dynamics. The aggregate results establish a pathway for a synthetic approach to create spatially precise networks of qubits.

14.
J Am Chem Soc ; 141(15): 6191-6203, 2019 04 17.
Artigo em Inglês | MEDLINE | ID: mdl-30854854

RESUMO

We have designed and used four different spacers, denoted A-D, to connect two pentacenes and to probe the impact of intramolecular forces on the modulation of pentacene-pentacene interactions and, in turn, on the key steps in singlet fission (SF), that is, the 1(S1S0)-to-1(T1T1) as well as 1(T1T1)-to-5(T1T1) transitions by means of transient absorption and electron paramagnetic resonance measurements. In terms of the 1(S1S0)-to-1(T1T1) transition, a superexchange mechanism, that is, coupling to a higher-lying CT state to generate a virtual intermediate, enables rapid SF in A-D. Sizeable electronic coupling in A and B opens, on one hand, an additional pathway, that is, the population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping. In turn, A and B feature much higher 1(T1T1) quantum yields than C and D, with a maximum value of 162% for A. In terms of the 1(T1T1)-to-5(T1T1) transition, the sizable electronic coupling in A and B is counterproductive, and C and D give rise to higher 5(T1T1)-to-(T1 + T1) quantum yields than A and B, with a maximum value of 85% for D.

15.
J Am Chem Soc ; 141(11): 4678-4686, 2019 03 20.
Artigo em Inglês | MEDLINE | ID: mdl-30807125

RESUMO

PmoD, a recently discovered protein from methane-oxidizing bacteria, forms a homodimer with a dicopper CuA center at the dimer interface. Although the optical and electron paramagnetic resonance (EPR) spectroscopic signatures of the PmoD CuA bear similarities to those of canonical CuA sites, there are also some puzzling differences. Here we have characterized the rapid formation (seconds) and slow decay (hours) of this homodimeric CuA site to two mononuclear Cu2+ sites, as well as its electronic and geometric structure, using stopped-flow optical and advanced paramagnetic resonance spectroscopies. PmoD CuA formation occurs rapidly and involves a short-lived intermediate with a λmax of 360 nm. Unlike other CuA sites, the PmoD CuA is unstable, decaying to two type 2 Cu2+ centers. Surprisingly, NMR data indicate that the PmoD CuA has a pure σu* ground state rather than the typical equilibrium between σu* and πu of all other CuA proteins. EPR, ENDOR, ESEEM, and HYSCORE data indicate the presence of two histidine and two cysteine ligands coordinating the CuA core in a highly symmetrical fashion. This report significantly expands the diversity and understanding of known CuA sites.

16.
J Am Chem Soc ; 141(5): 2152-2160, 2019 02 06.
Artigo em Inglês | MEDLINE | ID: mdl-30636401

RESUMO

The ability to prepare physical qubits in specific initial quantum states is a critical requirement for their use in quantum information science (QIS). Subnanosecond 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 are used to obtain the average spin qubit pair distance in the absence of the terminal Sd donor and reveal that one of the spins is fully delocalized across up to five adjacent guanines in a G-tract on the EPR time scale. 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 2 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.

17.
J Am Chem Soc ; 141(3): 1290-1303, 2019 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-30537816

RESUMO

The development of rigid covalent chiroptical organic materials, with multiple, readily available redox states, which exhibit high photoluminescence, is of particular importance in relation to both organic electronics and photonics. The chemically stable, thermally robust, and redox-active perylene diimide (PDI) fluorophores have received ever-increasing attention owing to their excellent fluorescence quantum yields in solution. Planar PDI derivatives, however, generally suffer from aggregation-caused emission quenching in the solid state. Herein, we report on the design and synthesis of two chiral isosceles triangles, wherein one PDI fluorophore and two pyromellitic diimide (PMDI) or naphthalene diimide (NDI) units are arranged in a rigid cyclic triangular geometry. The optical, electronic, and magnetic properties of the rigid isosceles triangles are fully characterized by a combination of optical spectroscopies, X-ray diffraction (XRD), cyclic voltammetry, and computational modeling techniques. Single-crystal XRD analysis shows that both isosceles triangles form discrete, nearly cofacial PDI-PDI π-dimers in the solid state. While the triangles exhibit fluorescence quantum yields of almost unity in solution, the dimers in the solid state exhibit very weak-yet at least an order of magnitude higher-excimer fluorescence yield in comparison with the almost completely quenched fluorescence of a reference PDI. The triangle containing both NDI and PDI subunits shows superior intramolecular energy transfer from the lowest excited singlet state of the NDI to that of the PDI subunit. Cyclic voltammetry suggests that both isosceles triangles exhibit multiple, easily accessible, and reversible redox states. Applications beckon in arenas related to molecular optoelectronic devices.

18.
Chempluschem ; 84(9): 1432-1438, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31944060

RESUMO

Singlet and triplet excited-state dynamics of anthanthrene and anthanthrone derivatives in solution are studied. Triisopropylsilyl- (TIPS) or H-terminated ethynyl groups are used to tune the singlet and triplet energies to enable their potential applications in singlet fission and triplet fusion processes. Time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies are used to obtain a mechanistic understanding of triplet formation. The anthanthrene derivatives form triplet states efficiently at a rate (ca. 107  s-1 ) comparable to radiative singlet fluorescence processes with approximately 30 % triplet yields, despite their large S1 -T1 energy gap (>1 eV) and the lack of carbonyl groups. In contrast, anthanthrone has a higher triplet yield (50±10 %) with a faster intersystem crossing rate (2.7 × 108  s-1 ) because of the n-π* character of the S1 ←S0 transition. Analysis of time-resolved spin-polarized EPR spectra of these compounds reveals that the triplet states are primarily generated by the spin-orbit-induced intersystem crossing mechanism. However, at high concentrations, the EPR spectrum of the 4,6,10,14-tetrakis(TIPS-ethynyl)anthanthrene triplet state shows a significant contribution from a non-Boltzmann population of the ms =0 spin sublevel, which is characteristic of triplet formation by singlet fission.

19.
J Phys Chem A ; 122(49): 9392-9402, 2018 Dec 13.
Artigo em Inglês | MEDLINE | ID: mdl-30428667

RESUMO

Ultrafast photodriven 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 an m-xylene bridged cyclophane having two equivalent pyromellitimides (PI2), TTF-ANI-PI2, as a secondary acceptor was synthesized along with an 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 by pulse-EPR spectroscopy. This contribution is especially relevant in the absence of modulation of exchange or dipolar interactions, as with the RP at a fixed distance in the molecules in this work. The theoretical prediction of the contribution from an ensemble of hyperfine interactions to decoherence 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, and specific pulse shapes relevant to arbitrary waveform generation are introduced.

20.
J Am Chem Soc ; 140(40): 13011-13021, 2018 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-30211549

RESUMO

Ultrafast photodriven electron transfer reactions starting from an excited singlet state in an organic donor-acceptor molecule generate a radical pair (RP) in which the two spins are initially entangled and, in principle, can serve as coupled spin qubits in quantum information science (QIS) applications, provided that spin coherence lifetimes in these RPs are long. Here we investigate the effects of electron transfer between two equivalent sites comprising the reduced acceptor of the RP. A covalent electron donor-acceptor molecule (D-C-A24+) including a p-methoxyaniline donor (D), a 4-aminonaphthalene-1,8-imide chromophoric primary acceptor (C), and a m-xylene bridged cyclophane having two equivalent phenyl-extended viologens (A24+) as a secondary acceptor was synthesized along with the analogous molecule having one phenyl-extended viologen acceptor and a second, more difficult to reduce 2,5-dimethoxyphenyl-extended viologen in a very similar cyclophane structure (D-C-A4+). Photoexcitation of C within each molecule results in subnanosecond formation of D+•-C-A23+• and D+•-C-A3+•. The spin dynamics of these RPs were characterized by time-resolved EPR spectroscopy and magnetic field effects on the RP yield in both CH3CN and CD3CN. The data show that rapid electron hopping within A23+• promotes spin decoherence in D+•-C-A23+• relative to D+•-C-A3+• having a monomeric acceptor, while the interaction of the RP electron spins with the nuclear spins of the solvent have little or no effect on the spin dynamics. These observations provide important information for designing and understanding novel molecular assemblies of spin qubits with long coherence times for QIS applications.

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