Rotation Restricted Emission and Antenna Effect in Single Metal–Organic Frameworks

Abstract

Aggregation induced-emission (AIE) and antenna effects are important luminescence behaviors. Thus, investigating their emission mechanisms and revealing their behaviors have become critical but challenging. Here we design and prepare metal–organic frameworks (MOFs) with an AIE ligand (i.e., tetrakis­(4-carboxyphenyl)­pyrazine (L1)) and Ln3+ ions (including Eu3+, Tb3+, and Gd3+). The emission from L1 is gradually enhanced during the formation of the MOFs because coordination restricts the intramolecular rotation. Thus, the emission is called as coordination-induced emission (CIE) with the same restriction of intramolecular rotation mechanism as AIE. Meanwhile, benzene rings twist to adapt to the MOFs’ rigid structure, so the emission blueshifts gradually, as an additional evidence of CIE. Both AIE and CIE are “rotation-restricted emission (RRE)”. Eu3+ ions exhibit the strongest emission with gradually enhanced intensity during the formation of L1-Eu MOF. Combined with emission properties from Tb3+ and Gd3+ ions, the antenna effect is verified. We also validate the conditions for the efficient sensitization of Ln3+ ions experimentally and refresh the threshold value of the energy gap between triplet state of a ligand and excited state of Ln3+ ions to 3000 cm–1. Thus, RRE and antenna effects are revealed and validated simultaneously. Because CIE of L1 and antenna effect emission from Eu3+ ions are enhanced simultaneously as strong dual emissions, ratiometric fluorescence detection is realized with the detection of arginine as a model. Our results incorporate AIE and CIE into RRE, which provides explicit information for the construction and application of emission systems with AIE ligands as building blocks. MOFs are also extended to explore the emission mechanism and the energy transfer between ligands and metal ions.

A new family of lanthanide coordination polymers based on 3,3′-[(5-carboxylato-1,3-phenylene)bis(oxy)]dibenzoate: synthesis, crystal structures and magnetic and luminescence properties

“…Among all types of ligands, aromatic polycarboxylic acids are the most frequently used due to their variety of coordination modes, as well as their structuring effects through -stacking (Shang et al, 2018;Wang et al, 2015). Moreover, they can link to metal ions and form interesting structures while simultaneously offering an-tenna effects, which transfer the absorbed energy to the Ln ions through intersystem crossing for circularly polarized (CP) luminescence (Yin et al, 2019). In recent years, our group has synthesized many novel Ln-CPs, such as [Eu(C 20 H 17 O 8 ) 3 (-CH 3 OH) 3 ] n [C 20 H 17 O 8 is (+)-di-p-toluoyl-d-tartaric acid; Han et al, 2019], [Ln(HPIDC)(C 2 O 4 )(H 2 O) 0.5 ] n [HPIDC is 2-(pyridin-2-yl)-1H-imidazole-4,5-dicarboxylic acid; Ln = La and Ce] and {[Ln 2 (HPIDC)(C 2 O 4 ) 2 (H 2 O) 3 ]Á2H 2 O} n (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er) (Zhang et al, 2017), and so on.…”

Recent Advances in Developing Lanthanide Metal–Organic Frameworks for Ratiometric Fluorescent Sensing

“…In the case of partial energy transfer from the ligand to lanthanide ions, both characteristic emissions from lanthanide ion and ligand can be recorded. Such dual emissive Ln-Ligand LnMOF system can be helpful in constructing ratiometric sensors once exposed to analytes (Yang et al, 2017;Yue et al, 2018;Li et al, 2019b;Yin et al, 2019). For example, Qian et al prepared ZJU-136-Ce 1-x Eu x with 1,1′; 4′,1″-terphenyl-2′,4,4″,5′-tetracarboxylic acid (TPTC) and Ce 4+ and Eu 3+ ions (Yue et al, 2018).…”

Energy Transfer in Metal–Organic Frameworks and Its Applications

“…[ 70 ] More importantly, precisely tailoring the Eu 3+ /Tb 3+ ratio leads to the dual characteristic emission bands derived from Eu 3+ and Tb 3+ and the luminescent color transitions of the Ln‐MOFs (i.e., red, orange, yellow, and green) caused by mixed emissions from both Tb 3+ and Eu 3+ ions (Figure 8c). [13d,71] …”