Luminescent lanthanide complexes with 4-acetamidobenzoate: Synthesis, supramolecular assembly via hydrogen bonds, crystal structures and photoluminescence

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Abstract

Four new luminescent complexes, namely, [Eu(aba)2(NO3)(C2H5OH)2] (1), [Eu(aba)3(H2O)2]·0.5 (4, 4′-bpy)·2H2O (2), [Eu2(aba)4(2, 2′-bpy)2(NO3)2]·4H2O (3) and [Tb2(aba)4(phen)2(NO3)2]·2C2H5OH (4) were obtained by treating Ln(NO3)3·6H2O and 4-acetamidobenzoic acid (Haba) with different coligands (4, 4′-bpy=4, 4′–bipyridine, 2, 2′-bpy=2, 2′-bipyridine, and phen=1, 10-phenanthroline). They exhibit 1D chains (12) and dimeric structures (34), respectively. This structural variation is mainly attributed to the change of coligands and various coordination modes of aba molecules. Moreover, the coordination units are further connected via hydrogen bonds to form 2D even 3D supramolecular networks. These complexes show characteristic emissions in the visible region at room temperature. In addition, thermal behaviors of four complexes have been investigated under air atmosphere. The relationship between the structures and physical properties has been discussed.

Graphical abstract

Structure variation of four complexes is attributed to the change of coligands and various coordination modes of aba molecules. Moreover, they show characteristic emissions in the visible region.

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Highlights

► Auxiliary ligands have played the crucial roles on the structures of the resulting complexes. ► Isolated structure units are further assembled via H-bonds to form supramolecular networks. ► These solid-state complexes exhibit strong, characteristic emissions in the visible region.

Introduction

The development of luminescent probes and sensors based on lanthanide complexes [1], [2], [3] have gained great recognition in materials, biological and medical science over the past years, due to many advantages of f–f transitions in fluorescence spectra, such as extremely narrow emission profiles, large Stokes shifts and long fluorescence lifetimes [4], [5]. These make luminescent materials including Ln(III) ions very attractive for a variety of applications, such as effective light conversion devices, fluorescent labels and probes for high sensitive time-resolved fluorimetric immunoassays [6], [7], [8], [9], [10], [11]. Since the f–f transitions are parity-forbidden [4], [5], free Ln(III) ions have low extinction coefficients leading to low luminescence intensity. It is well documented that sensitizers (chromophoric ligands), energy transfer from the sensitizers to the central ions, the types of solvent, coordination environment and spatial structures of the complexes have a significant influence on the emission characteristics of Ln(III) ions [12], [13], [14], [15], [16]. Especially, the design of multifunctional chromophoric ligands [9], [10], [11] has played a crucial role in the construction of lanthanide complexes with desirable luminescent properties.

Aromatic carboxylic acids and their derivates have multifunctional coordination sites with chelating and bridging ability and have been widely adopted to construct the fascinating network topologies in the field of crystal engineering [17], [18], [19], [20], [21]. In addition, chromophoric groups of these ligands can be able to absorb the photons provided by the light source and transfer it efficiently to the emitting levels of the Ln(III) ions. That is to say, these ligands can enhance the f–f electronic transitions through an intersystem energy transfer process. On the other hand, some aromatic diimines such as phen, 2, 2′-bpy and 4, 4′-bpy molecules may remarkably regulate structural topologies and fluorescent properties of resulting complexes [22], [23], [24], [25], [26]. 4-Acetamidobenzoic (Haba) (Scheme 1) attracted our attention due to two typical characteristics: (1) it contains only one carboxyl group, which is much easier to form complexes with lower dimension than multi-carboxylate ligands and may help us elucidate the relationship between the complexes' structures and physical properties; and (2) the acetamido group acts as a hydrogen bonding donor and an acceptor, which may facilitate the crystallization and result in diverse supramolecular networks. Herein, we introduce this ligand to synthesize four new luminescent complexes 1–4. Synthesis, crystal structures and physical properties of these complexes will be presented in this work.

Section snippets

Materials and physical measurements

4-Acetamidobenzoic acid (Haba) was prepared according to the literature [27]. Lanthanide nitrate hydrates were prepared by dissolving the respective oxides (99.5%) in 1:1 HNO3 (v/v) followed by drying. All the other reagents were of analytical grade and used without any further purification. Elemental (C, H, N) analyses were preformed on a Thermo FlashEA112 elemental analyzer. IR spectra were recorded using Perkin–Elmer Spectrum One spectrometer with KBr pellets in the range 4000–400 cm−1. X-ray

Syntheses and IR spectra

In our previous studies [27], hydrothermal reactions of Ln(III) ions with Haba ligands and phen resulted in lanthanide complexes with mononuclear or dimeric structure features, which may be mainly ascribed to the lanthanide contraction effect. In this work, four new complexes 14 were obtained by treating the corresponding nitrates with Haba molecules and different coligands (4, 4′-bpy, 2, 2′-bpy and phen). These crystalline solids are stable in air, soluble in DMF and DMSO and sparingly

Conclusion

In summary, the reactions of Ln(III) ions with 4-acetamidobenzoate (aba) and different coligands resulted in four new luminescent complexes 14. They exhibit two kinds of structural characteristics, such as 1D chains (12) and dimeric structures (34), attributed to the change of the coligands and various coordination modes of aba anions. In addition, they show strong, characteristic emission of Ln(III) ions in the visible region at room temperature and the results reveal that the types of

Acknowledgment

This work was granted financial support by the National Natural Science Foundation of China (No. 20771040), the Ministry of Science and Technology of China (No. 10C26214412704) and Guangdong Science and Technology Department (No. 2010B090300031).

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