A lithium superionic conductor for millimeter-thick battery electrode.

No design rules have yet been established for producing solid electrolytes with a lithium-ion conductivity high enough to replace liquid electrolytes and expand the performance and battery configuration limits of current lithium ion batteries. Taking advantage of the properties of high-entropy materials, we have designed a highly ion-conductive solid electrolyte by increasing the compositional complexity of a known lithium superionic conductor to eliminate ion migration barriers while maintaining the structural framework for superionic conduction. The synthesized phase with a compositional complexity showed an improved ion conductivity. We showed that the highly conductive solid electrolyte enables charge and discharge of a thick lithium-ion battery cathode at room temperature and thus has potential to change conventional battery configurations.

Structural modification on copper(I)-pyridylpyrimidine complexes for modulation of rotational dynamics, redox properties, and phototriggered isomerization.

The redox properties of copper pyridylpyrimidine complexes, which undergo linkage isomerism based on pyrimidine ring rotation, were compared under different coordination environments. A newly synthesized compound, [Cu(Mepypm)(L(Mes))]BF4 (1·BF4, Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine, L(Mes) = 2,9-dimesityl-1,10-phenanthroline) was compared with previously reported complexes of [Cu(MepmMepy)(L(Mes))]BF4 (2·BF4, MepmMepy = 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine), Cu(Mepypm)(DPEphos)]BF4 (3·BF4, DPEphos = bis[2-(diphenylphosphino)phenyl]ether), [Cu(Mepypm)(L(Anth))]BF4 (4·BF4, L(Anth) = 2,9-bis(9-anthryl)-1,10-phenanthroline), and [Cu(Mepypm)(L(Macro))]BF4 (5·BF4). Isomer ratios, isomerization dynamics, redox properties, and photoelectron conversion functions varied with the coordination structure. Methyl substituents on the 6-position of the pyridine moiety increased steric repulsion and contributed to quicker rotation, enhanced photoluminescence, and increased photodriven rotational isomerization.

Solvated-ion-pairing-sensitive molecular bistability based on copper(I)-coordinated pyrimidine ring rotation.

We describe herein the effect of solvated ion pairing on the molecular motion of a pyrimidine ring coordinated on a copper center. We synthesized a series of heteroleptic copper(I) complex salts bearing an unsymmetrically substituted pyridylpyrimidine and a bulky diphosphine. Two rotational isomers of the complexes were found to coexist and interconvert in solution via intramolecular ligating atom exchange of the pyrimidine ring, where the notation of the inner (i-) and outer (o-) isomers describes the orientation of the pyrimidine ring relative to the copper center. The stability of the pyrimidine orientation was solvent- and counterion-sensitive in both 2·BF(4) {2(+) = [Cu(Mepypm)(dppp)](+), where Mepypm = 4-methyl-2-(2'-pyridyl)pyrimidine and dppp = 1,3-bis(diphenylphosphino)propane} and previously reported 1·BF(4), which possesses a bulky diphosphine ligand (1(+) = [Cu(Mepypm)(DPEphos)](+), where DPEphos = bis[2-(diphenylphosphino)phenyl] ether). Two rotational isomers of 2(+) were separately obtained as single crystals, and the structure of each isomer was examined in detail. Both the enthalpy and entropy values for the rotation of 2·BF(4) in CDCl(3) (ΔH = 6 kJ mol(-1); ΔS = 25 J K(-1) mol(-1)) were more positive than that tested under other conditions, such as in more polar solvents CD(2)Cl(2), acetone-d(6), and CD(3)CN. The reduced contact of the anion to the cation in a polar solvent seems to contribute to the enthalpy, entropy, and Gibbs free energy for rotational isomerization. This speculation based on solvated ion pairing was further confirmed by considering the rotational behavior of 2(+) with a bulky counterion, such as B(C(6)F(5))(4)(-). The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Reversible copper(II)/(I) electrochemical potential switching driven by visible light-induced coordinated ring rotation.

We here describe the first metal complex system in which electronic signals can be repeatedly extracted by converting bistable states related to an intramolecular ligand rotational motion, which is fueled by visible light. The molecular structure for relating an electron transfer and a motion consists of a copper center and a coordinated unsymmetrically substituted pyrimidine derivative, whose rotational isomerization causes an electrochemical potential shift. To harness light energy effectively through metal-to-ligand charge transfer (MLCT) excitation, we prepared a simple copper(I) complex coordinated by a 4-methyl-2-(6'-methyl-2'-pyridyl)pyrimidine and a bulky diimine. The thermodynamic and kinetic parameters of redox and rotational reactions were analyzed by cyclic voltammograms at variable temperatures, by considering four stable isomers related to copper(II)/(I) states and rotational isomeric states. The key feature of this compound is that the rotation is frozen in the copper(I) state (rate constant for the rotation, k(Ii→o) = 10(-4) s(-1)) but is active in the copper(II) state (k(IIi→o) = 10(-1) s(-1)) at 203 K. The compound makes a bypass route to the isomeric metastable copper(I) state, via a tentative copper(II) state formed by photoelectron transfer (PET) in the presence of a redox mediator, decamethylferrocenium ion (DMFc(+)), or upon a partial oxidation of the complex. Light- and heat-driven rotation in the copper(I) state with a potential shift (ΔE°' = 0.14 V) was analyzed by electrochemical measurements of the complex in the solution state. The rotor could be reset to the initial state by heating, thereby completing the cycle and enabling repeated operation fueled by light energy. A significant redox potential shift associated with the copper(II)/(I) transition accompanied the rotation, thereby providing a new type of molecular signaling system.

Dual emission caused by ring inversion isomerization of a 4-methyl-2-pyridyl-pyrimidine copper(I) complex.

We developed a new convertible copper(I) complex using 2-pyridyl-4-methylpyrimidine and diphosphine as ligands. This complex exhibited mechanical bistability based on the inversion motion of the pyrimidine ring, leading to dual luminescence behavior. The inversion dynamics was strongly dependent on temperature and solvent. Variable-temperature (1)H NMR spectra revealed that the two isomers interconverted in solution via ring inversion, and the motion was frozen below 200 K. The complex exhibited characteristic CT absorption and emission bands in solution. Emission lifetime measurements demonstrated that the emission could be deconvoluted into two components. The fast and slow components were assigned to the two isomers, the excited states of which were characterized by different structural relaxation process and/or additional solvent coordination properties. The emission properties of the two isomers differed not only in lifetime and wavelength but also in heat sensitivity. The molar ratio of the two isomers varied with the polarity of the solvent via electrostatic interactions with the counteranion. The rate of inversion was affected by solvent, suggesting that inversion was promoted by solvent coordination.

Intramolecular electron arrangement with a rotative trigger.

We have constructed a single molecule system, consisting of a ferrocene-tethered copper complex, in which electron transfer between redox centers is triggered by molecular rotational motion. In the compound, an asymmetric methyl-substituted 2,2'-pyridylpyrimidine ligand, tethered to the ferrocene moiety, has two isomeric ring-inversion coordination conformations around the copper center. Both isomeric structures were characterized by X-ray crystallography. (1)H NMR and electrochemical measurements revealed that these isomers interconvert through rotation of the pyrimidine at room temperature, but the process is frozen below 233 K in the solution state. The two isomers undergo different redox processes, and the identity of the first oxidation center alternates between the copper center and ferrocene, as confirmed by chemical oxidation monitored by EPR and UV-vis absorption spectroscopy. Oxidation of the compound causes spontaneous isomerization of the pyrimidine due to the different relative stabilities of the isomers in the monovalent and divalent states. Oxidation in the motionless state at low temperatures extracts the first electron from the ferrocene center. When molecular motion is released by warming, the electron moves from the copper center to the ferrocene, leading to an enhancement of the copper(II) signal in the EPR spectrum. The synchronized motion/electron migration process was observed as a one-step UV-vis absorption spectral conversion.

A single molecular system gating electron transfer by ring inversion of a methylpyridylpyrimidine ligand on copper.

We have succeeded in constructing an electron-transfer gating system involving a copper complex, 1.BF(4), that is regulated by the rotational motion of a pyridylpyrimidine ligand. 4-Methyl-2-(2'-pyridyl)pyrimidine confined between two bulky groups underwent a dynamic process derived from pyrimidine-ring coordination inversion between inner and outer isomers, and these isomers interconverted with each other in solution with a barrier of 73 kJ mol(-1) at 293 K. As the ring-inversion process induces a change in redox potential on the copper center, electron transfer between 1(+) and the electrode can be gated through on/off control of the inversion by changing the temperature, resulting in a -0.14 V shift of the electrode potential.