A processable ionogel with thermo-switchable conductivity

We report an ionogel with thermo-switchable conductivity and high processability based on physical self-assembly of poly(styrene-b-ethylene oxide-b-styrene) (PS-PEO-PS) in mixed ionic liquids composed of thermo-responsive 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide and polymerizable 1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide, and subsequent chemical crosslinking of the polymerizable component.

Controlling Ionic Conductivity in Organometallic Ionic Liquids through Light-Induced Coordination Polymer Formation and Thermal Reversion

Owing to their high ionic conductivity and negligible vapor pressure, ionic liquids (ILs) find applications in various electronic devices. However, fabricating IL-based photocontrollable devices remains a challenge. In this study, we developed organometallic ILs with reversible light- and heat-controlled ionic conductivities for potential use in tunable devices. The physical properties and stimulus responses of ILs containing a cationic sandwich Ru complex with two coordinating substituents were investigated. UV photoirradiation of these ILs triggered cation photodissociation, resulting in their transformation into viscoelastic coordination polymers wherein the coordinating substituents bridged the Ru centers. Owing to the anion coordination, salts with coordinating anions such as CF3SO2NCN-, B(CN)4-, and BF2(CN)2- exhibited faster response and higher conversion than those with noncoordinating anions including (FSO2)2N- and (CF3SO2)2N-. All photoproducts reverted to their original ILs upon heating, except for the photoproduct of the BF2(CN)2 salt, which decomposed upon heating. Light- and heat-induced reversible changes occur in most cases between the high-ionic-conductive IL state and low-ionic-conductive coordination polymer state. Unlike previously reported ILs with three or one cation substituent, the current ILs exhibited both high reactivity and large ionic conductivity changes.

Organometallic Ionic Liquids Containing Sandwich Complexes: Molecular Design, Physical Properties, and Chemical Reactivities

Ionic liquids (ILs) are salts with low melting points and are useful as electrolytes and solvents. We have developed ILs containing cationic metal complexes, which form a family of functional liquids that exhibit unique physical properties and chemical reactivities originating from metal complexes. Our study explores the liquid chemistry in the field of coordination chemistry, where solid-state chemistry is currently the main focus. This review describes the molecular design, physical properties, and reactivities of organometallic ILs containing sandwich or half-sandwich complexes. This paper mainly covers stimuli-responsive ILs, whose magnetic properties, solvent polarities, colors, or structures change by the application of external fields, such as light, heat, and magnetic fields, or by reaction with coordinating molecules.

Thermal Properties and Solvent Polarities of Mixed-Valence Ionic Liquids Containing Cationic Biferrocenylene Derivatives

Ionic liquids (ILs) containing cationic mixed-valence biferrocenylene derivatives were synthesized with an octanoyl or octyl substituent in each cation. Their melting points ranged between 25 and 39 °C, and the octanoyl derivatives exhibited higher melting points than the octyl derivatives. In addition, each IL exhibited a glass transition in the temperature ranging from -66 to -45 °C after melting. Their melting points were ∼10 °C higher than those of mononuclear octamethylferrocenium salts bearing the same substituents. The solvent polarity (E_T^N) and Kamlet-Taft parameters (π*, α, and β) of these dinuclear and mononuclear ILs were then examined. The dinuclear ILs bearing octanoyl substituents exhibited significant increases in E_T^N and π* and a decrease in α with the decreasing temperature, whereas the other ILs exhibited a significantly less pronounced temperature dependence. Finally, the intervalence charge-transfer (or charge-resonance) bands of the octanoyl dinuclear ILs exhibited red shifts with the decreasing temperature, which can be regarded as self-thermosolvatochromism.

On-demand gelation of ionic liquids using photoresponsive organometallic gelators

The reversible formation of ionic liquid gels, or ionogels, upon external stimuli could improve their versatility and expand their application scope in electronic, biomedical, and micro-engineering systems. Herein, we developed organometallic compounds that release low-molecular-weight gelators upon photoirradiation, which facilitate the on-demand photogelation of ionic liquids (ILs). The chemical formulae of the gelator-coordinated complexes are [Ru(C₅H₅)L]X (L = C₆H₅NHCONHC₁₂H₂₅; X = PF₆, B(CN)₄). Each of the complexes were ILs that are easy to synthesize and miscible in ILs. By adding a small amount of the complex, various ILs were transformed to gels upon UV photoirradiation. The PF₆ salt allowed the photogelation of ILs with coordinating substituents, whereas the B(CN)₄ salt allowed the photogelation of non-coordinating ILs, albeit the reaction was slower. These gels underwent the reverse reaction and liquefied back when heated, and the photogelation was repeatable for ILs with coordinating cations.

Photoinduced and Thermal Linkage Isomerizations of an Organometallic Ionic Liquid Containing a Half-Sandwich Ruthenium Thiocyanate Complex

Metal complexes with thiocyanate (SCN^-) ligands typically exhibit S- or N-coordinated linkage isomers. In this study, to explore ionic liquids that exhibit stimuli-responsiveness based on linkage isomerization, we synthesized an ionic liquid containing a cationic half-sandwich thiocyanate complex, [Ru(C₆H₆)(NCS)L]Tf₂N (L = N-hexyl-2-pyridinemethanimine, Tf₂N = bis(trifluoromethanesulfonyl)amide anion). The as-synthesized ionic liquid was a 0.7:0.3 mixture of N- and S-coordinated isomers, presenting as an extremely viscous liquid exhibiting a glass transition at 0 °C. Isomerization from the N- to the S-coordinated isomer occurred upon UV photoirradiation or heating, although thermal isomerization was accompanied by significant decomposition. The N- and S-coordinated isomers were separated into brown and orange liquids, respectively, using gel permeation chromatography. Each isomer exhibited a small solvatochromic absorption shift in organic solvents, with different solvent dependences observed for the two isomers.

Reversible formation of soft coordination polymers from liquid mixtures of photoreactive organometallic ionic liquid and bridging molecules

The reversible switching of bonding modes in coordination polymers through the application of external stimuli leads to versatile mechanical and electronic functions. However, the exploration of such a system remains a great challenge. In this study, we designed liquid mixtures comprising a photoreactive organometallic ionic liquid and a bridging ligand, which form intermolecular coordination bonds upon photoirradiation. The liquid mixture of an ionic liquid [Ru(C5H5){Ph(CH2)3CN}][(SO2F)2N] (1) and a tridentate ligand N(C2H4CN)3 was transformed into an elastomer of an amorphous coordination polymer upon ultraviolet photoirradiation. By contrast, the photoirradiation of the mixture of 1 and a bidentate ligand NC(CH2)4CN produced a highly viscous liquid comprising coordination-bonded oligomers. In these reactions, photoirradiation causes dissociation of the organometallic cation, followed by the formation of intermolecular coordination bonds via the bridging ligands. The photoproducts underwent reverse reactions thermally. Based on coordination transformation, the ionic conductivity and viscoelasticity of these materials were reversibly controlled by the application of light and heat.

Thermal properties, crystal structures, and phase diagrams of ionic plastic crystals and ionic liquids containing a chiral cationic sandwich complex

Salts of a chiral ruthenium sandwich complex with various anions were synthesized and their phase diagrams were investigated.