Synthesis, structural characterization and luminescence properties of 1-carboxymethyl-3-ethylimidazolium chloride

Elucidating structure–property relationships in imidazolium-based halide ionic liquids: crystal structures and thermal behavior

A set of imidazolium-based ionic liquids (ILs), 1-(2-hydroxyethyl)-3-methylimidazolium chloride (1), 1,3-bis-(2-hydroxyethyl)-imidazolium chloride (2), and 1-butyl-2,3,4,5-tetramethylimidazolium bromide (3), has been synthesized and their structural and thermal behavior studied. Organic halides are well-known IL formers with imidazolium halides being the most prominent ones. Functionalization of the imidazolium cation by enhancing its hydrogen bonding capacity, i.e. through introduction of –OH groups or by diminishing it, i.e. through substitution of the ring hydrogen atoms by methyl groups is expected to change the inter-ionic interactions. Consequently, the solid-state structures of 1–3 have been characterized with means of single X-ray diffraction to shed light on preferential inter-ionic interactions for obtaining valuable information on anti-crystal engineering, i.e. designing ion combinations that favor a low melting point and exhibit a low tendency for crystallization. The study reveals that endowing IL forming ions with an enhanced hydrogen bonding capacity leads to a depression in melting points and kinetically hinders crystallization. This study provides hints towards new design concepts for IL design, similar to the common strategy of employing conformationally flexible ions.

Solvatochromism and mechanochromism observed in a triphenylamine derivative

A donor-π-acceptor fluorescent dye, 4-[N,N-di(4-phenyl)amino]benzaldehyde 4-chlorobenzoyl hydrazone (TPA-CBH), based on a triphenylamine derivative (TPA) and 4-chlorobenzoyl hydrazine (CBH) was designed and synthesized. The optical properties of this luminogen were investigated in solutions as well as in the solid states. In the intramolecular charge-transfer (CT) mechanism, TPA-CBH exhibits solvatochromism when dissolved in various polar solvents. Aggregation-induced emission was observed with the changes of tetrahydrofuran/water ratios. In the solid state, mechanochromic fluorescence is observed when the samples are stressed under different conditions. Based on structural analyses and theoretical calculations, it is found that intra- and intermolecular CT processes play a key role in the diversity of fluorescent properties.

Highly Luminescent Ionic Liquids Based on Complex Lanthanide Saccharinates

Four strongly luminescent ionic liquids with complex lanthanide saccharinate anions, [C₄mim]₃[Eu(Sac)₆(H₂O)₂] (1), {C₄mpy}₅{[Ln(Sac)₆(H₂O)₂][Ln(Sac)₅(H₂O)₃]}{(H₂O)₂(CH₃CN)₂} (Ln = Sm for 2a; Ln = Eu for 2b), and [C₄mpy]₃[Eu(Sac)₆][CH₃CN] (3) (C₄mim = 1-butyl-3-methylimidazolium; C₄mpy = N-butyl-4-methylpyridinium; Sac = saccharinate), have been obtained by reacting the ionic liquids 1-butyl-3-methylimidazolium saccharinate, [C₄mim][Sac], and N-butyl-4-methylpyridinium saccharinate, [C₄mpy][Sac], with the respective lanthanide saccharinates. Single-crystal X-ray diffraction analyses reveal the respective lanthanide center to be six- or eight-coordinated by five or six saccharinate anions and two or three aqua ligands in the cases of 1 and 2 when lanthanide saccharinated hydrate was employed as the starting material. Coordination of water to the lanthanide can be avoided when using the anhydrous lanthanide saccharinate as shown by the structure of 3. Using a co-solvent, acetonitrile, to facilitate the reaction led to incorporation of solvent molecules into the crystal structure of the final materials (2 and 3). Differential scanning calorimetry analyses reveal that 1 is an ionic liquid whereas 2 and 3 are low-temperature molten salts. The three europium(III)-containing compounds (1, 2b, and 3) all show characteristic intense red emissions of Eu(III) upon excitation into levels of the saccharinate ligands (321 nm) or Eu(III) (393 nm). At room temperature, the decay times of 1 and 2b are all ∼0.5 ms, whereas the decay time of 3 amounts to 3.85 ms due to removal of aqua ligands in the first coordination sphere; accordingly, the quantum efficiencies of 1, 2b, and 3 were determined to be 15.9%, 22.95%, and 57.75%, respectively. The CIE chromaticity coordinates for all Eu compounds are in the red region and approach the NTSC standard CIE values when the temperature is increased. Sm(III)-containing compound 2a shows characteristic Sm(III) emission peaks. As expected, the CIE coordinates of samarium compound 2a fall in the orange-red region.