Jiao Wang

Unusual Aggregation Arrangement of Eu-Containing Polyoxometalate Hybrid in a Protic Ionic Liquid with Improved Luminescence Property

Hybridization of polyoxometalates (POMs) with cationic surfactants offers the opportunity to greatly improve their functionalities as well as processabilities. Here, a surfactant-encapsulated Eu-containing POM complex (SEP) was formed via electrostatic interaction between 1-octadecyl-3-methylimidazolium bromide (OB) and Na9(EuW10O36)·32H2O (EuW10). SEP was first self-assembled in a protic ionic liquid to prepare the soft aggregates to fundamentally avoid the fluorescence quenching by water molecules. The structures and photophysical properties of SEP or aggregates were investigated thoroughly by NMR and FTIR spectroscopy, optical and electron microscopy, small-angle X-ray scattering, and fluorescence measurements. The formed gel-like aggregates were found to compose of three-dimensional networks of microribbons with an interdigitated layered molecular packing of SEP, which was different from the usual inverse bilayer model of POM hybrids in common organic solvents. Compared to EuW10 solid or its aqueous solution, both SEP and its aggregates exhibited intense red luminescence with much improved lifetime and quantum efficiency. In addition, the soft aggregates exhibited an efficient energy transfer and an obviously enhanced monochromaticity, owning to the organized arrangement of EuW10 units and a confined microenvironment to isolate them from each other between adjacent layers. The obtained results will not only present a useful reference to the aggregation behavior of POM hybrids in ionic liquids, but also provide an easy way to design EuW10 luminescent soft materials based on the nonaqueous media.

Highly luminescent and stable lyotropic liquid crystals based on a europium β-diketonate complex bridged by an ethylammonium cation

Soft lanthanide luminescent materials are impressive because of their tunable and self-assembling characteristics, which make them an attractive emerging materials field of research. In this report, novel luminescent lyotropic liquid crystals (LLCs) with four different mesophases have been fabricated by a protic ionic liquid (IL) based europium β-diketonate complex EA[Eu(TTA)4] (EA = ethylammonium, TTA = 2-thenoyltrifluoro-acetone) and an amphiphilic block copolymer (Pluronic P123). The protic IL, ethylammonium nitrate (EAN), was used as both the solvent and linkage to stabilize the doped complexes. Analyses by single-crystal X-ray diffraction for EA[Eu(TTA)4] and Fourier transform infrared spectroscopy for the LLC materials reveal convincingly that the ethylammonium cations establish an effective connection with both the carbonyl group of the β-diketonate ligand and the EO blocks of the amphiphilic block copolymer P123 via strong hydrogen bonding interactions. Due to this, an extremely long decay time of the excited state is obtained in EA[Eu(TTA)4] and excellent photostability of the luminescent LLCs could be achieved. The long-period ordered structures of the luminescent LLCs have been investigated by small-angle X-ray scattering measurements and the best luminescence performance was found in the most organized mesophase. Noteworthy, the LLCs could yield an effective confining effect on the europium complex accompanied by a sizeable elongation of the excited-state lifetime and an enhancement of the energy transfer efficiency, which reaches a remarkably high value of 52.6%. More importantly, the modulated luminescence properties observed in the four mesophase structures offer the potential and powerful possibility for these unique composite LLCs to be used in the fabrication of soft luminescent materials with tunable functions.

Enhanced full color tunable luminescent lyotropic liquid crystals from P123 and ionic liquid by doping lanthanide complexes and AIEgen

HYPOTHESIS The soft materials from ionic liquid mediated lyotropic liquid crystals (LLCs) containing europium complexes have exhibited enhanced luminescence efficiencies and photo-stabilities. The combination with aggregation-induced emissive compounds (AIEgens), however, may produce multicolor and even white emitting LLCs. EXPERIMENTS Here, we have fabricated highly luminescent hexagonal (H1) LLC containing a red-emitting trisdipicolinate lanthanide complex [choline]3[Eu(DPA)3] (Eu-DPA) from Pluronic 123 and 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6). Then, a typical AIEgen, tetrakis(4-hydroxyphenyl)ethane (TPE-OH) showing blue light was doped into H1 matrix, accompanying with a green-emitting Tb-DPA. FINDINGS The emission color of such LLCs could be finely-tuned through changing the molar ratio of Eu-DPA to Tb-DPA. Remarkably, the white emitting H1 LLC with CIE coordinate of (0.328, 0.315) has been prepared by accurately adjusting the relative contents of TPE-OH, Eu-DPA and Tb-DPA. Further, the emission color could be switched between bright white and blue upon tuning UV light from 254 to 365 nm repeatedly. To our best knowledge, such full color tunable luminescent LLCs with improved luminescent performances for both AIEgen and lanthanide complexes have not been reported. This facile approach is universal and various kinds of luminophores can be thus encapsulated into LLC matrices to fabricate soft materials with rich luminescent properties.

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.

Multicompartment-like aggregates formed by a redox-responsive surfactant encapsulated polyoxometalate in DMF/butanol mixed solvent

The unique and interesting aggregation behaviors of a novel surfactant-encapsulated complex (SEC), composed of a Keggin-type polyoxometalate (POM), phosphotungstic acid and a redox-responsive ferrocene-containing cationic surfactant, dimethyldioctadecylammonium bromide (BFDMA), have been investigated in N,N-dimethylformamide (DMF)/butanol mixed solvent. By carefully tuning the composition of the solvent, hollow spherical aggregates with multilayered shells were initially observed with a diameter of 55 ± 9 nm using transmission or scanning electron microscopes. Owing to the solvophobic effect and π–π stacking interaction between alkyl chains or ferrocene groups, these spherical aggregates were found to gradually fuse together into aggregates with flower-like and finally compartment-like morphologies with an average size of 120–150 nm. Based on X-ray diffraction measurements and the molecular configuration data of SEC, the multilayers in aggregates were composed of bilayers of SEC with the POM inside and three rearranged BFDMA molecules outside. The morphology of such aggregates was partially reversible under electrochemical modulation. Applying an anodic oxidation potential would induce the disassembling of compartment-like aggregates into irregular spheres with a size of about 200 nm, which could be turned back after applying a cathodic reduction potential. The obtained results here should shed light on the design of novel nanostructures from such inorganic/organic hybrid clusters.

Micelle Formation of Morpholinium Bromide Surfactants in Aqueous Solution

Abstract The micelle formation of N-alkyl-N-methylmorpholinium bromide (CnMMB, n = 12, 14, 16) surfactants in aqueous solution has been investigated. Their critical micelle concentration (CMC), effectiveness and efficiency of the surface tension reduction, and the maximum surface excess concentration were derived from the surface tension curves. Thermodynamic parameters were then evaluated in the temperature range of 25 °C ∼ 45 °C. The results showed that with increasing the alkyl chain length, the CMC values were decreased gradually, which is consistent with the enhancement of hydrophobic effect. Due to the existence of an oxygen atom in the headgroup, an unusual surface activity of CnMMB surfactants was obtained. The obtained results should help to better understand the effect of the hydrophilic headgroup on the micelle behavior of cationic surfactants containing saturated nitrogen-containing heterocycles.