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Article Cite This: ACS Omega 2019, 4, 9843−9849
http://pubs.acs.org/journal/acsodf
Luminescent Vesicles and Lyotropic Liquid Crystals in Ethylammonium Nitrate from a Partially Amphiphilic Eu Complex Qingrun Li,† Sijing Yi,‡ and Xiao Chen*,† †
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, China College of Arts and Sciences, Shanxi Agricultural University, Taigu 030801, China
‡
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ABSTRACT: 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 βdiketonate 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 lanthanidecontaining luminescent soft materials in ionic liquids but also provide reference to better understand the effect of organized aggregates on luminescence properties. matrices.5,6 In addition, the organized soft aggregates like vesicles, lyotropic liquid crystals (LLCs), and gels have also become more and more recognized because of their rich phase structures.7−9 As examples, an amphiphilic Tb3+ complex has been prepared by Liu et al. which could spontaneously selfassemble into vesicles in water. The addition of adenosine triphosphate could significantly amplify the luminescence intensity mainly because of the replacement of water molecules to reduce fluorescence quenching.7 By introducing the europium complexes into an amphiphilic block polymerbased LLC system, we observed their improved luminescent properties because of the specially organized LLC microenvironment and thus induced confinement effect.8 The novel luminescent gels reported by Li et al., however, could be fabricated via reaction of Eu3+-functionalized ionic liquids (ILs) with imidazolium salts.9 Thus, the organized soft aggregates exhibit good potential as matrices to fabricate lanthanide hybrid luminescent materials. To further improve the luminescence performance and stabilities of lanthanide complexes, the ILs have been chosen
1. INTRODUCTION Because of its unique 4f electronic layer structure, the lanthanides exhibit abundant properties like luminescence, electricity, magnetism, and catalysis, which make them to be praised as “treasure house of new materials”.1 Therefore, the lanthanide-based luminescent materials have been paid much attention and widely used in many fields, such as biological probes and information display.2,3 To solve the problem of poor luminescence efficiency and easy annihilation of lanthanide ions, the organic ligands are often introduced to form complexes. Then, they could present excellent luminescent properties because of the good antenna effect of ligands, which could improve ligand-to-metal energy transfer or sensitization of lanthanide ions luminescence.4 This is also why the lanthanide β-diketonate complexes are much attractive as high-performance luminescent materials because they display good monochromaticity, long lifetimes, and high luminescence quantum yields. However, their intrinsic defects like poor thermal stability or easily degradable under UV irradiation may block their practical applications. To overcome these difficulties, the luminescent hybrid materials by immobilizing or dispersing the lanthanide complexes in certain matrices through covalent or noncovalent interactions have realized some achievements, especially for those in solid © 2019 American Chemical Society
Received: April 28, 2019 Accepted: May 24, 2019 Published: June 5, 2019 9843
DOI: 10.1021/acsomega.9b01215 ACS Omega 2019, 4, 9843−9849
ACS Omega
Article
complexes in different soft aggregates and expand their applications.
more and more as ideal dispersion media because of their excellent properties such as better thermal stability, nonvolatile, noninterference in the visible and near-IR spectral regions.10−12 Meanwhile, ILs are also ideal self-assembling media for organized aggregate fabrication. Therefore, such dual benefits of ILs have promoted the construction of IL-based lanthanide luminescent materials.13−15 By this motivation, we have also prepared IL-based LLCs with doped lanthanide complexes, which exhibited remarkably improved luminescence properties.16,17 However, in most studies for IL-based luminescent materials, the lanthanide salts or complexes were just simply dispersed or doped, which might limit their uses because of poor dispersibility or solubility of dopants in matrices.18−24 Then, whether the lanthanide complexes could directly participate the aggregate formation to increase their dispersion becomes a challenge. For this aim, we herein report the synthesis and selfassembly of a partially amphiphilic europium complex, 1dodecyl-3-methylimidazolium tetrakis(thenoyltrifluoroacetonato)europate ([C12mim][Eu(TTA)4], denoted as Eu(III)), which contains a hydrophobic long alkyl chain and a hydrophilic alkylimidazolium cation, with an europium β-diketonate complex as the counterion (see Scheme 1). Eu(III) was then co-assembled with the corresponding
2. RESULTS AND DISCUSSION 2.1. Phase Behavior of Self-Assembled Eu[III] Aggregates in EAN. We characterized first on the phase structure of aggregates formed only by [C12mim]Br itself. As we know, the formation of micelle of [C12mim]Br in aqueous solution has been observed previously.26 Therefore, is it possible to form the micelle also by [C12mim]Br in EAN? At lower [C12mim]Br concentrations, C12 (
