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.

Multifunctionality of the [C2mim][Ln(fod)4] series (Ln = Nd–Tm except Pm): magnetic, luminescence and thermochemical studies

The [C2mim][Ln(fod)4] series presents a rare and reversible polymorphism, NIR and visible emission as well as SMM behavior. The varieties of physical behaviors make these compounds suitable for potential technological and/or biomedical applications.

Luminescent Ln-Ionic Liquids beyond Europium

Searching in the Web of Knowledge for “ionic liquids” AND “luminescence” AND “lanthanide”, around 260 entries can be found, of which a considerable number refer solely or primarily to europium (90%, ~234). Europium has been deemed the best lanthanide for luminescent applications, mainly due to its efficiency in sensitization, longest decay times, and the ability to use its luminescence spectra to probe the coordination geometry around the metal. The remaining lanthanides can also be of crucial importance due to their different colors, sensitivity, and capability as probes. In this manuscript, we intend to shed some light on the existing published work on the remaining lanthanides. In some cases, they appear in papers with europium, but frequently in a subordinate position, and in fewer cases then the main protagonist of the study. All of them will be assessed and presented in a concise manner; they will be divided into two main categories: lanthanide compounds dissolved in ionic liquids, and lanthanide-based ionic liquids. Finally, some analysis of future trends is carried out highlighting some future promising fields, such as ionogels.

Reverse Hofmann-Type Spin-Crossover Compound Showing a Multichannel Controllable Color Change in an Ambient Environment

Materials that demonstrate a multichannel controllable color change in response to external stimuli are fascinating for their potential applications in sensoring and displaying devices. Herein we report a Fe^II spin-crossover (SCO) compound that exhibits both solvatochromism and thermochromism under an ambient environment. This Hofmann-type compound possesses two different pores where the solvent guests can be removed in a two-step process. Because the loss of solvent guests modifies the spin state of magnetic centers, an unusual yellow-red-yellow two-step color change of crystals was detected. Moreover, because of the strong cooperativity of the spin centers, a dramatic red-to-yellow color change of crystals in response to a minute thermal perturbation around 303 K is triggered by an abrupt spin transition of the metal centers. The multichannel controllable dramatic color change demonstrated in the present compound highlights the sensoring and displaying roles of SCO materials.

Lipophilic Fluorescent Dye Liquids: Förster Resonance Energy Transfer-Based Fluorescence Amplification for Ion Selective Optical Sensors Based on a Solvent Polymeric Membrane

Optical sensors based on solvent polymeric membranes have the potential to measure analytes present in an aqueous solution through the development of a tailored method for a specific target. However, limits in the concentrations of the component dyes have prevented improvements in sensitivity. We propose a Förster resonance energy transfer (FRET)-based fluorescence amplification system for ion-selective optical sensors using a highly fluorescent liquid material composed of a lipophilic phosphonium cation and a pyrene modifying sulfonate anion ([P₆₆₆₁₄][HP-SO₃]), as both the plasticizer and donor, in addition to a combination of the lipophilic phosphonium cation and the fluorescein dodecyl ester anion ([P₆₆₆₁₄][12-FL]) as the fluorescent sensing dye acceptor. For ion extraction-based sensing, the donor and acceptor were retained in the plasticized PVC membrane with negligible leaching upon exposure to acidic and basic aqueous solutions. Systematic investigation of the donor and acceptor ratios clarified the effect of the amplification factor and the sensitivity of the sensor. At an acceptor doping level of 0.5 mol % (vs donor), an approximately 22-fold higher sensitivity was obtained compared to that of a conventional PVC membrane optical sensor. During ion measurement based on the coextraction of protons and anions, selectivity following the Hofmeister order was observed, which was controlled by the addition of ionophores. The proposed FRET system based on a lipophilic fluorescent liquid material has the potential to significantly improve the sensitivities of optical sensors using solvent polymeric membranes with high selectivities for various target analytes.

A Europium(III) Complex Embedded in a Polysulfone Host Matrix: A Flexible Film with Temperature-Responsive Ratiometric Behaviour

An emissive europium(III) complex [C₂ mim][Eu(fod)₄ ] (1; C₂ mim=1-ethyl-3-methyl-imidazolium; fod=1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionate) was prepared. The complex shows ratiometric thermal behaviour up to 155 °C. These unusual temperature-dependent properties arise from a solid-solid phase transition that promotes increased contact between the anion and the cation, affecting the emission profile of the emissive anion in two different ratiometric relations. A ultrabright and flexible emissive photopolymer film was obtained using polysulfone (PSU) as the host matrix of 10 % (w/w) of 1, that also induced changes on the lanthanide emissive profile with temperature. A temperature-responsive luminescent film 1/PSU is sensitivr to heating between 100 and 155 °C. Also, the emission lifetime of 1 was not affected by confinement in PSU, while its emission quantum yield was reduced from 82 to 59 %.

Thermochromism of Highly Luminescent Photopolymer Flexible Films Based On Eu (III) Salts Confined in Polysulfone

Here we discuss the influence of two different cations on the emissive properties of the highly emissive [Eu(fod)₄]^- anion. The studied Eu(III) salts were [C₁₆Pyr][Eu(fod)₄] (1), and the previously reported [Chol][Eu(fod)₄]. C₁₆Pyr stands for N-cetylpyridinium, Chol for cholinium and fod for 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionate. 1 is classified as ionic liquid, with melting point close to 60 °C, and presented a luminescence quantum yield of (ϕ) 100%. Ultrabright emissive photopolymers were obtained for the first time using polysulfone as the host matrix. The films were prepared with incorporation of 10% (w/w) of 1 and [Chol][Eu(fod)₄] in the polymeric matrix, which improved its thermal stability. Additionally, the luminescence of CholEu(fod)₄/PSU presented a strong temperature dependence with a ratiometric thermal behavior.

A lipophilic ionic liquid-based dye for anion optodes: importance of dye lipophilicity and application to heparin measurement

Herein, a fully lipophilic ionic liquid (IL) comprising a lipophilic fluorescein anion and a trihexyltetradecylphosphonium cation was synthesized and used as the plasticizer for a plasticized poly(vinyl chloride) (PVC) membrane optode. Systematic investigation of the alkyl chain length of the fluorescein anion proved the significance of lipophilicity for obtaining the reversible absorbance measurements. A PVC membrane fabricated with the synthesized lipophilic IL was observed to comprise an unusually high dye concentration (915 mmol kg^-1) and exhibited good sensitivity as well as response time in its sensor performance. The sensitivity of the presented PVC membrane was 26-fold higher than that of a conventional optode membrane with the same membrane thickness and the same lipophilic dye of typical dye content (1 wt%). The response time was observed to be >120-fold faster by using a significantly thinner PVC membrane (approx. 140 nm). Heparin is known to be a polyanionic anticoagulant, and the presented PVC membrane exhibited an extremely fast response (20-150 seconds) to the heparin in diluted serum within the required concentration region. Thus, the lipophilic IL-based dye could significantly improve the sensor performance in conventional optodes, especially for an analyte showing slow diffusion, such as macromolecular heparin.

A Europium(III) Complex with an Unusual Anion-Cation Interaction: A Luminescent Molecular Thermometer for Ratiometric Temperature Sensing

An unusual thermally sensitive anion-cation interaction, which is characteristic of the anion [Eu(FOD)₄ ]^- , occurs in the complex [CHOL][Eu(FOD)₄ ] (1; CHOL=choline; FOD=1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate) and affects both quantum yield and thermochromic behavior. This prompted the design of an Eu³⁺ -based ratiometric thermometer that functions at temperatures up to 95 °C through a thermally excited state absorption of the Eu³⁺ ion. The reusable temperature-sensitive luminescent complex showed a range of relative sensitivity between 0.45 % C^-1 at 25 °C, with an increase to 7.0 % C^-1 at 95 °C. Confinement of compound 1 in a transparent film of polysulfone resulted in a higher thermal stability of 1 while its luminescence showed a strong temperature dependence.

Enzyme-responsive Fluorescent Ionic Liquid

Herein, we describe the development of a novel material, "enzyme-responsive fluorescent ionic liquid", which enabled a highly sensitive detection of alkaline phosphatase (ALP). We prepared a plasticized poly(vinyl chloride) (PVC) membrane using this new material as a plasticizer and quantified ALP in aqueous solutions. Preliminary results suggested that the PVC membrane responded to ALP at an interface between the membrane and the sample solution with anion extraction to maintain electroneutrality in the membrane phase. The developed PVC membrane showed an approximately six-times higher sensitivity than the conventional membrane, thereby demonstrating highly sensitive ALP detection. These results suggested the potential applicability of the proposed membrane for highly sensitive protein detection by using ALP-labeled antibodies.

An ionic liquid composed of purely functional sensing molecules: a colorimetrically calcium responsive ionic liquid

A highly lipophilic ionic liquid (IL) consisting of stoichiometrically equal amounts of two purely functional chemical sensing molecules, an anionic calcium ionophore and a cationic dye, was synthesized for the first time, and used as a component for a poly(vinyl chloride)-based or neat liquid membrane optical sensor. This is the first report of an IL consisting of purely functional chemical sensing molecules, which is completely different from the previously reported ionic liquids typically consisting of imidazolium or other lipophilic cations. Since the present IL contained an extremely high concentration of dyes, the IL-based sensor showed dramatically enhanced sensitivity (13 times higher compared to that of a conventional sensor), and fully reversible and selective response to Ca2+. Preliminary investigation on the unique response characteristics of the present liquid membrane IL-based sensor was performed with the responses to Ca2+ and Na+.

Dinuclear lanthanide–lithium complexes based on fluorinated β-diketonate with acetal group: magnetism and effect of crystal packing on mechanoluminescence

A convenient and straightforward synthesis of a novel type of Ln–Li β-diketonates with a discrete molecular structure is presented.

Lanthanide doped coordination polymers with tunable afterglow based on phosphorescence energy transfer

Lanthanide ion doped coordination polymers (CPs) exhibit an unusual red/green afterglow with long photoemission lifetimes (10.54 ms for Eu³⁺ and 57.66 ms for Tb³⁺) due to the phosphorescence energy transfer at room temperature.