Synthesis, Crystal Structure, and Visible Light Sensitized Red Luminescence of a Novel Salen-Bridged Pentanuclear Eu-Na
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Synthesis, Crystal Structure, and Visible Light Sensitized Red Luminescence of a Novel Salen-Bridged Pentanuclear Eu-Na Complex Ming Zhaoa, * and Xiaoli Zhoua aSchool
of Chemical Engineering and Food Science, Zhengzhou Institute of Technology, Zhengzhou, 450044 China *e-mail: [email protected] Received August 12, 2019; revised February 27, 2020; accepted March 2, 2020
Abstract—The europium complex 1 ([Eu2Na3(bsalen)2(L)2(CH3O)(NO3)2] ⋅ 3H2O) with unusual trigonal bipyramid-shaped core [Na3Eu2] has been formed via the reaction of Eu(NO3)3 with N,N'-bis(3-methoxysalicylidene)-ethylenediamine (bsalen2– ligand), 2-hydroxy-4-methoxybenzophenone (HL) and NaOH in the ratio of 1 : 1 : 1 : 3.5 in methanol. Crystal structure reveals that each Eu(III) ion is encapsulated in the inner cavity of bsalen2– ligand. Photoluminescence (PL) measurement results show that the europium complex exhibits intense red emission (611 nm) under blue light excitation (417 nm) with quantum efficiency of 16.9% and lifetime of 1.02 ms. Keywords: Shiff-base ligand, benzophenone derivative, blue-light-excitable europium(III) complex, heteronuclear complex, X-ray crystal structure DOI: 10.1134/S0036023620080203
INTRODUCTION The design and synthesis of the luminescent europium(III)-based complexes have attracted considerable attention in recent decades due to their unique optical properties comprising large Stokes shifts and red emission with high color purity [1–4]. However, due to the parity-forbidden intra-4f transitions of Eu3+ ion, the molar absorption coefficient (ε) of f–f transitions is very low (ε < 10 mol L–1 cm–1)[5–7]. In order to overcome this shortcoming, organic ligands with high light-harvesting abilities (ε > 103 mol L–1 cm–1) are employed as antenna chromophores to sensitize Eu3+ ion through the ligand-to-metal energy-transfer process, which is also called the “antenna effect” [8, 9]. At the present stage, various types of antenna ligands such as β-diketones, aromatic carboxylic acids, heterocyclic compounds, Schiff-bases, etc., have been reported [10–14]. Notably, based on these organic chromophores, the optical excitation windows appear to be limited to less than 380 nm, that usually results in photodecomposition of the Eu(III) complexes [15, 16]. Meanwhile, the ultraviolet (UV) excitation may cause living tissue damage in the field of bio-analysis, and unwanted high-voltage-driven emitter in the optoelectronic technology [17–20]. Therefore, it is of fundamental importance to design suitable ligands to sensitize the Eu(III) ion under excitation of visible light. However, Eu(III) complexes excitable in the visible light are still limited [21–23]. Moreover,
several examples could not be used in the solid state and the photoluminescence quantum efficiency (PLQY) usually decreases drastically with increasing wavelength of the excitation light [24–27]. It is worth noting that 2-hydroxybenzophenone derivatives have absorptions in the near UV or visible spectrum, [28] which presents the advantage to realize t
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