Enhanced fluorescence quenching for p-nitrophenol in imidazolium ionic liquids using a europium-based fluorescent probe

p-Nitrophenol (PNP) is a toxic contaminant in water, the detection of which has attracted considerable attention. Since ionic liquids (ILs) have been widely used as popular solvents in both extraction and catalysts for PNP, the remediation of PNP is not limited to water and traditional organic solvents. Thus, it is significant to develop approaches for the detection of PNP in ILs. Accordingly, the present work is focused on the detection of PNP in a series of imidazolium-based ILs, 1-hexyl-3-methyl-imidazolium bromide ([Hmim]Br), 1-butyl-3-methyl-imidazolium tetrafluoroborate ([Bmim]BF4), 1-butyl-3-methyl-imidazolium trifluoromesulfonate ([Bmim]TfO), 1-butyl-3-methyl-imidazolium trifluoroacetate ([Bmim]TA), and 1-butyl-3-methyl-imidazolium nitrate ([Bmim]NO3), using a europium-based fluorescent probe, Na3[Eu(DPA)3] (DPA = 2, 6-pyridinedicarboxylic acid). This fluorescent probe showed excellent selectivity and sensitivity toward [Bmim]NO3 in aqueous solution. Further studies showed that not only the fluorescence performance of the europium complex was enhanced in the other four ILs compared with that in water, but also the detecting capability for PNP was improved. The order of the quenching efficiency in different solvents was: [Bmim]BF4 > [Bmim]TfO > [Hmim]Br > [Bmim]TA > water. The higher sensitivity for PNP in ILs was proven to be related to the efficient energy transfer of the europium complex and lower solvent polarity of the ILs. The quenching mechanism for the detection of PNP was established as being due to the ground state electrostatic interactions between the fluorescent probe and analyte, photoinduced electron transfer (PET) and inner filter effect (IFE).

NMR and luminescence experiments reveal the structure and symmetry adaptation of a europium ionic liquid to solvent polarity

By combining NMR data (nuclear Overhauser effect and pseudocontact shifts) with luminescence measurements, we uncover how the structure of an anionic europium complex adapts to different solvent polarities as a result of the different relative proximities of the ion pairs. In nonpolar solvents, the detected contact ion pairs, CIPs, indicate that the ions remain in proximity, with the molecular cation, and then perturbing and distorting the coordination polyhedron of the anion complex to a low symmetry configuration, which promotes luminescence. Differently, solvent separated ion pairs occur in polar solvents, indicating that the molecular ions have been disconnected. Thus, in polar solvents, the europium complex anion becomes free from the close influence of the molecular cation, allowing the coordination polyhedron to increase its symmetry, which in turn reduces the luminescence of the anionic europium complex. This reduction of coordination polyhedron symmetry by the close proximity of the molecular cation in nonpolar solvents was confirmed by additional photophysical measurements combined with quantum chemical RM1 calculations, suggesting that, in nonpolar solvents, the symmetry point group of the coordination polyhedron is C1, whereas in polar solvents it is either D2d or S4. The nonpolar solvents used were benzene, chloroform and dichloromethane; and the polar ones were acetone and acetonitrile. The synthesized ionic liquids were tetrakis [C5mim][La(BTFA)4] and [C5mim][Eu(BTFA)4], where BTFA stands for 4,4,4-trifluoro-1-phenyl-1,3-butanedione, lanthanoids (La3+ and Eu3+) and C5mim stands for 1-methyl-3-isopentylimidazolium. They were synthesized by a microwave methodology that is both fast and green (the synthetic reaction takes about 15 min) and also leads to more easily purifiable crystals.

Luminescent Vesicles Self-assembled Directly from an Amphiphilic Europium Complex in an Ionic Liquid

Novel luminescent vesicles with enhanced emission were successfully achieved for the first time by an amphiphilic europium complex through its spontaneously self-assembly in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆). The complex was prepared by europium ions coordinated with terpyridine ligands, which were modified with the hydrophilic ethoxy chains. The enhanced absolute quantum yield and prolonged fluorescence lifetime of complex in vesicles were observed because of the effective shielding of the quench effects caused by both solvent and complex concentration. Compared to the aggregates formed in other solvents, the vesicles obtained in [Bmim]PF₆ showed the best luminescence intensity with the quantum efficiency (37.74%) and luminescent emission lifetime (1.915 ms) both increased about 10 times more. Furthermore, this europium complex was designed to show unsaturated coordination, which made the vesicle luminescence easily quenched when contacting with water. The fluorescence sensing of water with this vesicle as probe was therefore possible, where several unique properties like high sensitivity, low detection limit (0.05 vol %), visible color change, and fast response had been observed. Such designed systems are expected to provide strategies to develop novel supramolecular aggregates in ionic liquids and offer guidance for luminescence detection with facile and wide applications.

A luminescent lyotropic liquid crystal with UV irradiation induced photochromism

Multicolored photoluminescence induced by UV irradiation in soft materials has invaluable potential in display and detection applications. Here, an ionic liquid (ethylammonium nitrate, EAN)-mediated luminescent lyotropic liquid crystal (LLC) has been fabricated, where an europium complex with two ligands (Eu(DBM)₃BQ, DBM = dibenzoylmethane and BQ = 2,2'-biquinoline) was doped. The obtained LLC exhibited enhanced fluorescence intensity and lifetime compared to those of Eu(DBM)₃BQ in EAN solution. Interestingly, the luminescent LLC color from the initial red emission of the complex was changed gradually to green when the UV exposure was extended. Though the DBM ligand displayed the typical photo-degradation, the emission intensity of BQ increased drastically. A mechanism based on an UV-induced trans-to-cis isomerization of BQ was proposed to explain such an unusual luminescence phenomenon. Both the experimental and computational results indicated an intra ligand charge transfer (ILCT) accompanied with the BQ isomerization under continuous UV irradiation, which resulted in the enhanced green light emission. The results obtained here are referable for better understanding the interplay between weak interactions and modulation of novel lanthanide-based photochromism systems under UV exposure.

Luminescent Vesicles and Lyotropic Liquid Crystals in Ethylammonium Nitrate from a Partially Amphiphilic Eu Complex

Soft luminescent materials have attracted much attention because of their self-assembled and controllable properties. To explore their facile and effective fabrication ways, we report here the self-assembling of luminescent vesicles and lyotropic liquid crystals (LLCs) in a protic ionic liquid, ethylammonium nitrate, by a partially amphiphilic europium β-diketonate complex (Eu(III)) with a 1-dodecyl-3-methylimidazolium cation as the counter ion. An interesting result came from the complex-induced vesicle formation of corresponding amphiphile, 1-dodecyl-3-methylimidazolium bromide ([C₁₂mim]Br), which has been rarely reported in the past. It was the interaction between the Eu(III) and imidazolium group that changed the critical packing parameter of [C₁₂mim]Br, which finally resulted in the occurrence of vesicles. The obtained vesicle aggregates exhibited enhanced fluorescence intensity and lifetime compared to those of Eu(III) solution. Meanwhile, a hexagonal LLC phase with better fluorescence properties was found at higher [C₁₂mim]Br concentration. The obtained photophysical data confirmed that the order degree of Eu(III)-containing aggregates could effectively increase the energy transition efficiency of ligands. The better luminescent properties of LLC resulted from the stronger stabilizing and binding effects on Eu(III) in LLC than that in vesicles, which might be caused by closer molecular packing in LLC. The results presented here will not only expand the strategy of constructing lanthanide-containing luminescent soft materials in ionic liquids but also provide reference to better understand the effect of organized aggregates on luminescence properties.

Luminescent Sodium Deoxycholate Ionogel Induced by Eu3+ in Ethylammonium Nitrate

Hydrogels based on bile salts and lanthanide ions have been reported for their easy gelation. However, the weak mechanical properties and water quenching to luminescence of lanthanide ions limit their applications in practice. Hence, a supramolecular ionogel has been prepared here through simply mixing of sodium deoxycholate and europium nitrate in a protic ionic liquid, ethylammonium nitrate (EAN). The prepared ionogel was characterized by scanning electron microscopy, X-ray energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction, fluorescence spectroscopy, and rheological measurements. Such an ionogel resulted synergistically from metal coordination and hydrogen bonding. The effect of the solvent structure on gel properties was also explored by comparison with those formed in alkylammonium nitrates with longer chains. EAN was found to behave more effectively both as a solvent and a bridge to enhance the ionogel mechanical strength. The ionogels also exhibited better fluorescent properties than those of the corresponding hydrogels. The obtained results should expand the applications of lanthanide-containing luminescent soft materials in nonaqueous media. It is expected to apply in the fields of solid electrolytes, biosensors, and optics response.

Flexible and enhanced multicolor-emitting films co-assembled by lanthanide complexes and a polymerizable surfactant in aqueous solution

Lanthanide complex doped lyotropic liquid crystals (LLCs) are soft materials which are impressive due to their excellent luminescence efficiencies and stabilities. The introduction of lanthanide complexes into polymerizable LLCs, however, may produce organized films with better optical and mechanical properties through in situ photopolymerization. An environmentally friendly strategy to fabricate flexible multicolor-emitting films has been developed through co-assembling red-/green-emitting trisdipicolinate lanthanide complexes [choline]3[Ln(DPA)3] (Ln-DPA, Ln = Eu, Tb) into LLC matrices mainly via electrostatic interaction and further photopolymerization. The LLCs were constructed from a polymerizable surfactant, 3-dodecyl-1-vinylimidazolium bromide (C12VIMBr), in aqueous solution. The maintenance of the well-defined LLC nanostructures in the luminescent films was validated by small-angle X-ray scattering (SAXS) as well as scanning electron microscopy (SEM) measurements. Remarkably, the lifetime and absolute luminescence quantum efficiency of such films have been improved significantly compared with those of the corresponding solid Eu-DPA complex or in aqueous solution and LLC matrices. Through tuning the molar ratio of Eu-DPA to Tb-DPA complexes, the emission color of the films could be finely-tailored between red and green in the CIE chromaticity diagram. Furthermore, the films also possessed certain mechanical strength and stability against pH, metal ions, and temperature, indicating their potential application as robust luminescent materials.

"Rigid" Luminescent Soft Materials: Europium-Containing Lyotropic Liquid Crystals Based on Polyoxyethylene Phytosterols and Ionic Liquids

Soft materials of europium β-diketonate complexes constructed in lyotropic liquid crystals (LLCs) mediated by ionic liquids (ILs) are impressive for their excellent luminescence performance and stability. For the aim to further improve their mechanical processability and luminescent tunablility, the polyoxyethylene phytosterols (BPS-n) were introduced here as structure directing agents to prepare relatively "rigid" lamellar luminescent LLCs in 1-butyl-3-methyl-imidazolium hexafluorophosphate by doping europium β-diketonate complexes with different imidazolium counterions. As a result of the solvophobic sterol ring structure of BPS-n, the more effective isolation and confinement effects of europium complexes could be achieved. The longest fluorescence lifetime and the highest quantum efficiency reported so far for europium containing lyotropic organized soft materials were thus obtained. Changing the molecular structures of BPS-n with different oxyethylene chains or doped complexes with imidazolium counterions of different alkyl chain lengths, the spacings of lamellar LLC matrixes and position of dispersed complexes became tunable. The measured luminescent and rheological properties for such composite LLCs showed a dependence on the rigidity and isolation capability afforded by sterol molecules. It was also found that the increase of counterion alkyl chain length would weaken the LLC matrix's confinement and isolation effects and therefore exhibit the deteriorated luminescence performance. The enhanced luminescence efficiency and stability of doped BPS-n LLCs reflected the excellent segregation of europium complexes from each other and therefore the reduced self-quenching process. The obtained results here present the designability of LLC matrixes and their great potential to promote achieving the luminescence tunability of soft materials.