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ESR and 1H-,19F-ENDOR/TRIPLE Study of Fluorinated Diphenylnitroxides as Synthetic Bus Spin-Qubit Radicals with Client Qubits in Solution Tomohiro Yoshino,†,|| Shinsuke Nishida,†,|| Kazunobu Sato,*,†,|| Shigeaki Nakazawa,†,|| Robabeh D. Rahimi,† Kazuo Toyota,†,|| Daisuke Shiomi,†,|| Yasushi Morita,*,‡,|| Masahiro Kitagawa,§,|| and Takeji Takui*,†,|| †
Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan § Department of System Innovation, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan Japan Science and Technology Agency (JST), Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan
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bS Supporting Information ABSTRACT: Electron and nuclear spins as quantum bits (qubits) have been the focus of current issues in quantum information science/technology and related fields. From the viewpoint of chemistry, synthetic spin qubits are emerging. Diphenylnitroxide (DPNO) and its novel fluorine-substituted radicals are characterized as synthetic electron bus spin-qubits by continuous-wave ESR and 1H-,19F-ENDOR/TRIPLE spectroscopy in solution and by DFT calculations. The partially fluorinated DPNOs have been synthesized to illustrate that they are candidates for the synthetic bus spin-qubits with well-defined client qubits. The fluorinated DPNOs undergo spin delocalization, dominating the robust spin polarization in the π-conjugation of phenyl rings, serving to increase the number of distinguishable client qubits from three to six. SECTION: Molecular Structure, Quantum Chemistry, General Theory synthetic work have appeared.6-8,14,15 Well-defined open-shell molecular systems in ensemble, such as stable neutral radicals, can afford resources of matter spin-qubits, in which an electron spin and nuclear spins play their own role in QC/QIP as a bus qubit26 and client qubits, respectively. In view of pulse-based current electron magnetic resonance technology, client nuclei with large nuclear gyromagnetic ratios, such as protons, take great advantage of spin manipulation in implementing any quantum gates.14 Fluorine nuclei embedded in the open-shell systems can be excellent candidates for client qubits with NMR frequencies distinguishable from protons and in terms of natural abundance. Among possible open-shell molecular entities with such client nuclei, termed synthetic/molecular bus spin-qubits, we have revisited diphenylaminoxyl (DPNO: diphenylnitroxide)27 as a neutral radical of π-conjugation. The π-conjugation in DPNO plays a role of bus network, and the nuclei with nonzero nuclear spin quantum numbers couple with the bus electron spin via hyperfine interactions.14 In the bus spin-qubits, hyperfine couplings are used to implement quantum gate operations. In solid-state QC/ QIP, all of the protons in DPNO are available as 10 client qubits distinguishable from each other, in addition to the nitrogen nucleus at the spin-bearing NO site.28 This availability arises from the
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ntil recently, stable organic open-shell entities have attracted continuous attention from the viewpoint of moleculebased exotic magnetic properties since organic ferromagnetism underlain by high-spin hydrocarbons in the ground state was proposed as early as the 1960s.1-4 Among the diverse topics in the related interdisciplinary fields, recent trends in open-shell chemistry are relevant to exotic molecular functionalities such as molecular spin batteries,5-12 organic spintronics,13 and molecular spin-qubits (spin quantum bits).14,15 Molecular spins as matter spin-qubits have been the latest arrival in the implementation of quantum computers or quantum information processing systems.13-16 Quantum entanglements between an electron and nuclear spin-qubits17,18 and superdense coding19 have been achieved in organic open-shell frames.14 Interestingly, the spinor as an intrinsic nature of an electron has experimentally been demonstrated by invoking the entanglement between an electron and proton nucleus in malonyl radicals in the crystal,17 for the first time.20 It is noteworthy that the quest for new molecular functionalities as materials has been deeply underlain by well-defined electronic and molecular structures of relevant open-shell molecular systems.6-8 In the emerging field of quantum computing/quantum information processing (QC/ QIP),21-25 physically realizable scalable qubits have been the focus of current issues.14,15 In this context, synthetic chemistry for open-shell molecules takes part in this field, and tailor-made molecular design and r 2011 American Chemical Society
Received: December 7, 2010 Accepted: January 24, 2011 Published: February 11, 2011 449
dx.doi.org/10.1021/jz101650z | J. Phys. Chem. Lett. 2011, 2, 449–453
The Journal of Physical Chemistry Letters
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anisotropic hyperfine tensors of the protons and the noninversion symmetry of DPNO in the solid state. On the other hand, in molecular spin-bus-based QC/QIP in solution, the number of available proton client qubits in DPNO is only two because all the ortho- and para-protons become equivalent.29,30 Liquid-phase electron spin-qubit-based QC/QIP technology, in its own right, provides the execution of fast gate operations compared with the NMR counterpart,21 although both liquid-phase versions of QC/ QIP are subject to the difficulty of initialization for QC/QIP.14 To increase the number of distinguishable client qubits even in solution, we have attempted to introduce fluorines in the π-electron network of DPNO31,32 and synthesized novel partially fluorinated DPNOs, 2a, 2b, and 3, as given below, checking the potential of DPNOs as candidates for molecular bus spin qubits in the liquid phase.
Scheme 1. Synthesis of 2 and 3
Figure 1. 1H-,19F-ENDOR and TRIPLE spectra of fluorinated DPNO 2a (a) and 2b (b) at 190 K, and 3 (c) at 200 K in oxygen-free ethylbenzene solutions (
