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A quantum gate: A system of two coupled electron spins that is useful for simple quantum computing operations has been prepared by synthesis of a biradical 1 and co-crystallization with an isomorphous host molecule. The two weakly exchange-coupled quantum bits (target qubit blue and control qubit red) span four electron spin states. The electron spin transition is denoted by two black arrows.

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Stable gable radical: 1,3‐Diazaphenalenyl radical, a typical example of an isoelectronic mode of modification for phenalenyl, has been isolated for the first time as a crystalline solid by introducing bulky substituents (see picture, tert‐butyl groups are omitted from the crystal structure). The gable syn dimer with a column motif shows an extremely strong antiferromagnetic exchange coupling of 2J/kB=−4.19(2)×103 K.

The full configuration interaction (full-CI) method is capable of providing the numerically best wave functions and energies of atoms and molecules within basis sets being used, although it is intractable for classical computers. Quantum computers can perform full-CI calculations in polynomial time against the system size by adopting a quantum phase estimation algorithm (QPEA). In the QPEA, the preparation of initial guess wave functions having sufficiently large overlap with the exact wave function is recommended. The Hartree–Fock (HF) wave function is a good initial guess only for closed shell singlet molecules and high-spin molecules carrying no spin-β unpaired electrons, around their equilibrium geometry, and thus, the construction of multiconfigurational wave functions without performing post-HF calculations on classical computers is highly desired for applying the method to a wide variety of chemistries and physics. In this work, we propose a method to construct multiconfigurational initial guess wave functions suitable for QPEA-based full-CI calculations on quantum computers, by utilizing diradical characters computed from spin-projected UHF wave functions. The proposed approach drastically improves the wave function overlap, particularly in molecules with intermediate diradical characters.

A carboxylate group of D1-Glu-189 in photosystem II has been proposed to serve as a direct ligand for the manganese cluster. Here we constructed a mutant that eliminates the carboxylate by replacing D1-Glu-189 with Gln in the cyanobacterium Synechocystis sp. PCC 6803, and we examined the resulting effects on the structural and functional properties of the oxygen-evolving complex (OEC) in photosystem II. The E189Q mutant grew photoautotrophically, and isolated photosystem II core particles evolved oxygen at ϳ70% of the rate of control wild-type particles. The E189Q OEC showed typical S 2 state electron spin resonance signals, and the spin center distance between the S 2 state manganese cluster and the Y D (D2-Tyr-160), detected by electron-electron double resonance spectroscopy, was not affected by this mutation. However, the redox potential of the E189Q OEC was considerably lower than that of the control OEC, as revealed by the elevated peak temperature of the S 2 state thermoluminescence bands. The mutation resulted in specific changes to bands ascribed to the putative carboxylate ligands for the manganese cluster and to a few carbonyl bands in mid-frequency (1800 to 1100 cm ؊1 ) S 2 /S 1 Fourier transform infrared difference spectrum. Notably, the low frequency (650 to 350 cm ؊1 ) S 2 /S 1 Fourier transform infrared difference spectrum was also uniquely changed by this mutation in the frequencies for the manganese cluster core vibrations. These results suggested that the carboxylate group of D1-Glu-189 ligates the manganese ion, which is influenced by the redox change of the oxidizable manganese ion upon the S 1 to S 2 transition.Photosynthetic water oxidation occurs in an oxygen-evolving complex (OEC), 4 which contains a catalytic center composed of four manganese ions and an associated Ca 2ϩ ion located on the luminal side of the D1/D2 heterodimer in photosystem (PS) II (1). The OEC converts two water molecules to one oxygen molecule through a light-driven reaction cycle with five intermediate states denoted S n (n ϭ 0 -4), where n represents the number of stored oxidizing equivalents. In the dark, the OEC is predominantly in a thermally stable S 1 state and advances to the next oxidation state, S 2 , by absorbing a photon. The OEC reaches the highest oxidation state, S 4 , by accumulating four oxidizing equivalents after three flashes and then relaxes to the lowest oxidation state, S 0 , concurrent with the release of an oxygen molecule (2, 3).Site-directed mutagenesis studies using the cyanobacterium Synechocystis sp. PCC 6803 (reviewed in Refs. 4 and 5) have revealed that replacement of amino acid residues by another residue affects the properties of the OEC. These amino acids include Asp-170, Glu-189, His-190, His-332, Glu-333, His-337, Asp-342, and Ala-344 of the D1 protein (6 -11). Some residues are arranged in close proximity to the manganese cluster, as shown by x-ray crystallographic models (12-15), suggesting that the manganese cluster is coordinated mainly with carboxylate and imidazole groups fro...

We have first achieved the synthesis of triple-stranded metallo-helicates composed of 4,4':2',2'':4'',4'''-quaterimidazole (Qim) and Mn(II) or Zn(II) ions, which serve as synthetic electron spin qubits (quantum bits). In the crystal structure, a hydrogen-bonding network through counteranions and/or crystal solvents was constructed by the outward N-H hydrogen-bonding functional groups intrinsic to the imidazole skeleton. Importantly, these helicates showed high stability even in a solution state at room temperature. These salient features of triple helicates of Qim are different from those of reported metallo-helicates. These chemical properties of the Qim-based triple helicates allow us to synthesize magnetically diluted single crystals composed of Mn(II) (S = 5/2) and diamagnetic Zn(II) complexes of Qim in an appropriate Mn(II)/Zn(II) ratio. The magnetically diluted crystals can afford to build up the prototype of electron-spin qubits of Lloyd's one-dimensional periodic system, which gives a practical approach to scalable quantum computers/quantum information processing systems (QCs/QIPSs). The experiments have proven the practical capability of oligo(imidazole)s as a component of Lloyd's system which has nonequivalent g-tensors within the helicate (g-engineering). The helical symmetry plays an important role in giving a prototype of the synthetic spin qubits of the formidable Lloyd model. This result links supramolecular chemistry to the field of QCs/QIPSs.