Multifunctional AIEgen-based luminescent metal–organic frameworks with coordination-induced emission for chemical sensing

Abstract

Aggregation-induced emission luminogens (AIEgens) offer a timely remedy to tackle aggregation-caused quenching of traditional organic fluorescence probes. Metal–organic frameworks (MOFs) receive tremendous attention due to their easy-to-functionalize surface and tunable porosity. If AIEgens are used to construct MOFs, they are stitched together through coordinate bonds and placed in specific geometric with spatial arrangements. Thus, MOFs are a powerful platform to investigate the underlying AIE mechanism and for luminescent sensing applications. Herein, a symmetrical tetra-carboxylic AIE-active ligand, 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine (TCPP), was prepared for luminescent MOFs with Fe, Eu, and Cu as metal nodes, separately. During the formation of MOFs, the emission of TCPP was enhanced as coordination-induced emission (CIE) to share the same restriction of intramolecular rotation mechanism to AIE. However, the emission was blue-shifted in the presence of Cu, had a constant wavelength with Fe, and was red-shifted in the Eu–TCPP system. The different emission behaviors of TCPP in different systems revealed the different electronic states and coordination modes of different metal ions. The results also illustrated that the electronic property of the ions affected the luminescence of the MOFs differently. Interestingly, antenna effect emission was observed from Eu–TCPP MOF as dual-emission to study the energy and charge transfer. Considering the stronger emission and high stability of Fe–MOFs than those of Cu–MOFs, Cu–MOFs were used as a probe for the “turn-on” sensing of Fe³⁺. The combination of AIEgen and MOFs provides a chance to reveal the emission mechanism and develop the sensing application, while CIE is used as the additional evidence for the restriction of intramolecular rotation emission mechanism, the same as AIE.