Optical Initialization of Molecular Qubit Spin States Using Weak Exchange Coupling to Photogenerated Fullerene Triplet States

The ability to initialize an electron spin qubit into a well-defined state is an important criterion for quantum information applications. To achieve this goal, a chromophore photoexcited to its triplet state is used to strongly spin polarize a nearby stable radical in a series of C60 fullerene derivatives containing a covalently linked α,γ-bisdiphenylene-β-phenylallyl (BDPA) radical. Selective photoexcitation of C60 results in up to 20-fold enhancement of the BDPA spin polarization observed by pulse electron paramagnetic resonance spectroscopy at room temperature. The sign of the spin polarization depends on the nature of the molecular spacer between C60 and BDPA. In addition, transient absorption spectroscopy and pulse-EPR measurements reveal that the BDPA spin polarization is derived from spin polarization transfer from the C60 triplet state by weak exchange coupling over a 1 nm distance

Spin Frustration in the Triradical Trianion of a Naphthalenediimide Molecular Triangle

Crystalline supramolecular frameworks consisting of charged molecules, held together by hydrogen bonds and Coulomb interactions, have attracted great interest because of their unusual structural, chemical, electronic, and magnetic properties. Herein, we report the preparation, structure, and magnetic properties of the triradical trianion of a shape-persistent chiral equilateral molecular triangle having three naphthalene-1,4:5,8-bis­(dicarboximide)­s ((+)-NDI-Δ^3(−•)). Single-crystal X-ray diffraction of its tris­(cobalto­cenium) salt ([(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃]) reveals accessible one-dimensional tubular cavities, and variable-temperature electron paramagnetic resonance spectroscopy shows that a dilute solution of [(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃] in an organic glass has a spin-frustrated doublet ground state and a thermally accessible quartet state. Furthermore, SQUID magnetometry from 5 to 300 K of solid [(+)-NDI-Δ^3(−•)­(CoCp₂⁺)₃] shows ferromagnetic ordering with a Curie temperature <i>T</i>_C = 20 K. The successful preparation of hybrid ionic materials comprising macrocyclic triradical trianions with spin-frustrated ground states and accessible 1D pores offers routes to new organic spintronic materials

Probing Nuclear Spin Effects on Electronic Spin Coherence via EPR Measurements of Vanadium(IV) Complexes

Quantum information processing (QIP) has the potential to transform numerous fields from cryptography, to finance, to the simulation of quantum systems. A promising implementation of QIP employs unpaired electronic spins as qubits, the fundamental units of information. Though molecular electronic spins offer many advantages, including chemical tunability and facile addressability, the development of design principles for the synthesis of complexes that exhibit long qubit superposition lifetimes (also known as coherence times, or <i>T</i>₂) remains a challenge. As nuclear spins in the local qubit environment are a primary cause of shortened superposition lifetimes, we recently conducted a study which employed a modular spin-free ligand scaffold to place a spin-laden propyl moiety at a series of fixed distances from an <i>S</i> = ¹/₂ vanadium­(IV) ion in a series of vanadyl complexes. We found that, within a radius of 4.0(4)–6.6(6) Å from the metal center, nuclei did not contribute to decoherence. To assess the generality of this important design principle and test its efficacy in a different coordination geometry, we synthesized and investigated three vanadium tris­(dithiolene) complexes with the same ligand set employed in our previous study: K₂[V­(C₅H₆­S₄)₃] (<b>1</b>), K₂[V­(C₇H₆­S₆)₃] (<b>2</b>), and K₂[V­(C₉H₆­S₈)₃] (<b>3</b>). We specifically interrogated solutions of these complexes in DMF-<i>d</i>₇/toluene-<i>d</i>₈ with pulsed electron paramagnetic resonance spectroscopy and electron nuclear double resonance spectroscopy and found that the distance dependence present in the previously synthesized vanadyl complexes holds true in this series. We further examined the coherence properties of the series in a different solvent, MeCN-<i>d</i>₃/toluene-<i>d</i>₈, and found that an additional property, the charge density of the complex, also affects decoherence across the series. These results highlight a previously unknown design principle for augmenting <i>T</i>₂ and open new pathways for the rational synthesis of complexes with long coherence times

DFT and ENDOR Study of Bixin Radical Cations and Neutral Radicals on Silica–Alumina

Bixin, a carotenoid found in annatto (<i>Bixa orellana</i>), is unique among natural carotenoids by being water-soluble. We stabilized free radicals from bixin on the surface of silica–alumina (Si–Al) and characterized them by pulsed electron–nuclear double resonance (ENDOR). DFT calculations of unpaired electron spin distribution for various bixin radicals predict the EPR hyperfine couplings. Least-square fitting of experimental ENDOR spectra by spectra calculated from DFT hyperfine couplings characterized the radicals trapped on Si–Al. DFT predicts that the <i>trans</i> bixin radical cation is more stable than the <i>cis</i> bixin radical cation by 1.26 kcal/mol. This small energy difference is consistent with the 26% <i>trans</i> and 23% <i>cis</i> radical cations in the ENDOR spectrum. The remainder of the ENDOR spectrum is due to several neutral radicals formed by loss of a H⁺ ion from the 9, 9′, 13, or 13′ methyl group, a common occurrence in all water-insoluble carotenoids previously studied. Although carboxyl groups of bixin strongly affect its solubility relative to other natural carotenoids, they do not alter properties of its free radicals based on DFT calculations and EPR measurements which remain similar to typical water-insoluble carotenoids

Synthetic Approach To Determine the Effect of Nuclear Spin Distance on Electronic Spin Decoherence

Nuclear–electronic interactions are a fundamental phenomenon which impacts fields from magnetic resonance imaging to quantum information processing (QIP). The realization of QIP would transform diverse areas of research including accurate simulation of quantum dynamics and cryptography. One promising candidate for the smallest unit of QIP, a qubit, is electronic spin. Electronic spins in molecules offer significant advantages with regard to QIP, and for the emerging field of quantum sensing. Yet relative to other qubit candidates, they possess shorter superposition lifetimes, known as coherence times or <i>T</i>₂, due to interactions with nuclear spins in the local environment. Designing complexes with sufficiently long values of <i>T</i>₂ requires an understanding of precisely how the position of nuclear spins relative to the electronic spin center affects decoherence. Herein, we report the first synthetic study of the relationship between nuclear spin–electron spin distance and decoherence. Through the synthesis of four vanadyl complexes, (Ph₄P)₂[VO­(C₃H₆S₂)₂] (<b>1</b>), (Ph₄P)₂[VO­(C₅H₆S₄)₂] (<b>2</b>), (Ph₄P)₂[VO­(C₇H₆S₆)₂] (<b>3</b>), and (Ph₄P)₂[VO­(C₉H₆S₈)₂] (<b>4</b>), we are able to synthetically place a spin-laden propyl moiety at well-defined distances from an electronic spin center by employing a spin-free carbon–sulfur scaffold. We interrogate this series of molecules with pulsed electron paramagnetic resonance (EPR) spectroscopy to determine their coherence times. Our studies demonstrate a sharp jump in <i>T</i>₂ when the average V–H distance is decreased from 6.6(6) to 4.0(4) Å, indicating that spin-active nuclei sufficiently close to the electronic spin center do not contribute to decoherence. These results illustrate the power of synthetic chemistry in elucidating the fundamental mechanisms underlying electronic polarization transfer and provide vital principles for the rational design of long-coherence electronic qubits

Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical

Photoinduced electron transfer reactions in organic donor–acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor − electron acceptor − stable radical system consisting of a <i>meta</i>-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA–mPD^+•–ANI^–• triradical in which the mPD^+•–ANI^–• RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA–¹(mPD^+•–ANI^–•) to BDPA–³(mPD^+•–ANI^–•), resulting in accelerated RP recombination via the triplet channel to produce BDPA–mPD–^3*ANI as compared to a reference molecule lacking the BDPA radical. The RP recombination rates observed are much faster than those previously reported for weakly coupled triradical systems. Time-resolved EPR spectroscopy shows that this process is also associated with strong spin polarization of the stable radical. Overall, these results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale

Large Dipolar Spin–Spin Interaction in a Photogenerated U‑Shaped Triradical

Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin–spin interactions in a novel U-shaped electron donor–chromophore–acceptor–radical (D–C–A–R^•) system in which a xanthene bridge holds a <i>tert</i>-butylphenyl nitroxide (BPNO^•) radical in close proximity to a naphthalene-1,8:4,5-bis­(dicarboximide) (NDI) acceptor. Photoexcitation of the 4-aminonaphthalene-1,8-dicarboximide (ANI) chromophore results in rapid, two-step electron transfer to generate the triradical (D^+•–C–A^–•–R^•). The large through-bond distance between A^–• and R^• makes their spin–spin exchange interaction (2<i>J</i>_AR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (<i>D</i>_AR), which is observed as a set of broad lines in the TREPR spectra of D^+•–C–A^–•–R^• in solid toluene solution at 85 K. Transient nutation experiments show that these transitions belong to a species with spin <i>S</i> = 1, whereas experiments on D^+•–C–A^–•–R^• in the oriented nematic liquid crystal 4-cyano-4′-<i>n</i>-pentylbiphenyl at 85 K demonstrate the anisotropy of <i>D</i>_AR

Photogenerated Quartet State Formation in a Compact Ring-Fused Perylene-Nitroxide

We report on a novel small organic molecule comprising a perylene chromophore fused to a six-membered ring containing a persistent nitroxide radical to give a perylene-nitroxide, or <b>PerNO</b>^•. This molecule is a robust, compact molecule in which the radical is closely bound to the chromophore but separated by saturated carbon atoms, thus limiting the electronic coupling between the chromophore and radical. We present both ultrafast transient absorption experiments and time-resolved EPR (TREPR) studies to probe the spin dynamics of photoexcited <b>PerNO</b>^<b>•</b> and utilize X-ray crystallography to probe the molecular structure and stacking motifs of <b>PerNO</b>^<b>•</b> in the solid state. The ability to control both the structure and electronic properties of molecules having multiple spins as well as the possibility of assembling ordered solid state materials from them is important for implementing effective molecule-based spintronics

1,2,3-Triazole–Heme Interactions in Cytochrome P450: Functionally Competent Triazole–Water–Heme Complexes

In comparison to imidazole (IMZ) and 1,2,4-triazole (1,2,4-TRZ), the isosteric 1,2,3-triazole (1,2,3-TRZ) is unrepresented among cytochrome P450 (CYP) inhibitors. This is surprising because 1,2,3-TRZs are easily obtained via “click” chemistry. To understand this underrepresentation of 1,2,3-TRZs among CYP inhibitors, thermodynamic and density functional theory computational studies were performed with unsubstituted IMZ, 1,2,4-TRZ, and 1,2,3-TRZ. The results indicate that the lower affinity of 1,2,3-TRZ for the heme iron includes a large unfavorable entropy term likely originating in solvent–1,2,3-TRZ interactions; the difference is not solely due to differences in the enthalpy of heme–ligand interactions. In addition, the 1,2,3-TRZ fragment was incorporated into a well-established CYP3A4 substrate and mechanism-based inactivator, 17-α-ethynylestradiol (17EE), via click chemistry. This derivative, 17-click, yielded optical spectra consistent with low-spin ferric heme iron (type II) in contrast to 17EE, which yields a high-spin complex (type I). Furthermore, the rate of CYP3A4-mediated metabolism of 17-click was comparable to that of 17EE, with a different regioselectivity. Surprisingly, continuous-wave electron paramagnetic resonance (EPR) and HYSCORE EPR spectroscopy indicate that 17-click does not displace water from the sixth axial ligand position of CYP3A4 as expected for a type II ligand. We propose a binding model in which 17-click pendant 1,2,3-TRZ hydrogen bonds with the sixth axial water ligand. The results demonstrate the potential for 1,2,3-TRZ to form metabolically labile water-bridged low-spin heme complexes, consistent with recent evidence that nitrogenous type II ligands of CYPs can be efficiently metabolized. The specific case of [CYP3A4·17-click] highlights the risk of interpreting CYP–ligand complex structure on the basis of optical spectra

Effect of Magnetic Coupling on Water Proton Relaxivity in a Series of Transition Metal Gd^III Complexes

A fundamental challenge in the design of bioresponsive (or bioactivated) Gd^III-based magnetic resonance (MR) imaging probes is the considerable background signal present in the “preactivated” state that arises from outer-sphere relaxation processes. When sufficient concentrations of a bioresponsive agent are present (i.e., a detectable signal in the image), the inner- and outer-sphere contributions to <i>r</i>₁ may be misinterpreted to conclude that the agent has been activated, when it has not. Of the several parameters that determine the observed MR signal of an agent, only the electron relaxation time (<i>T</i>_1e) impacts both the inner- and outer-sphere relaxation. Therefore, strategies to minimize this background signal must be developed to create a near zero-background (or truly “off” state) of the agent. Here, we demonstrate that intramolecular magnetic exchange coupling when Gd^III is coupled to a paramagnetic transition metal provides a means to overcome the contribution of second- and outer-sphere contributions to the observed relaxivity. We have prepared a series of complexes with the general formula LMLn­(μ-O₂CCH₃)­(O₂CCH₃)₂ (M = Co, Cu, Zn). Solid-state magnetic susceptibility measurements reveal significant magnetic coupling between Gd^III and the transition metal ion. Nuclear magnetic relaxation dispersion (NMRD) analysis confirms that the observed differences in relaxivity are associated with the modulation of <i>T</i>_1e at Gd^III. These results clearly demonstrate that magnetic exchange coupling between Gd^III and a transition metal ion can provide a significant decrease in <i>T</i>_1e (and therefore the relaxivity of Gd^III). This design strategy is being exploited to prepare new generations of <i>preclinical</i> bioresponsive MR imaging probes with near zero-background