Design,Synthesis And Photoelectric Properties Of AIE-active Iridium(?) Complexes

Rapid and highly selective detection of explosives has become an important and urgent issue in modern society because of its broad applications in both homeland security and environmental safety. So far various detection techniques have been developed for the purpose of detecting the explosives, including gas chromatography, Raman spectroscopy, cyclic voltammetry, fluorescence sensing and so on. Especially, fluorescence sensing of explosives offers several advantages such as simplicity, sensitivity, cost-effectiveness and short response time. In this regard, numerous explosive sensors based on polymers, metal-organic frameworks, nanomaterials and organic dyes have been designed. Despite these advances, the selective detection of nitro explosives still remains a challenge as they usually have similar electron affinity. It is in great demand to design and synthesize luminophores that are sensitive and selective for nitro-explosives.Recently, the use of phosphorescent iridium(?) complexes as chemosensors has gained increasing attention due to their rich photophysical properties. However, the traditional aggregation caused quenching (ACQ) effect in solid state and/or aquatic systems results in drastically negative effects on the efficiency and sensitivity of the sensors. The aggregation-induced emission (AIE) phenomenon firstly reported by Tang in 2001 is precisely opposite to the ACQ effect, which provides a new strategy to construct efficient luminescent materials. Since then, great efforts have been devoted to the design and synthesis of AIE-active fluorophores and the exploration of their potential applications. Very recently, iridium(?) complexes exhibiting the phosphorescent AIE phenomenon have been investigated and used as chemosensors and in cell imaging. The recent studies reported by Reddy and our group have demonstrated the potential of AIE-active iridium(?)complexes in explosive detection. Although their high sensitivity towards nitro-explosives has been demonstrated, selective detection has not been achieved so far. Among the nitro-explosives, the power of 2,4,6-trinitrophenol (TNP) is somewhat superior to that of 2,4,6-trinitroluene (TNT), but far less attention has been paid to detection of TNP. Moreover, TNP is widely used in fireworks, dyes, matches and so on, whose release into the environment leads to the contamination of soil and aquatic systems. Consequently, the development of efficient and selective sensors for TNP is highly desirable.Herein, we report the design and synthesis of a AIE-active iridium(?) complex for detection of a nitro explosive. For the first time, the selective detection of TNP using an AIE-active iridium(?) complex has been realized.We preliminarily speculate that the emission quenching towards TNP is probably attributed to the combination photoinduced electron transfer (PET) and resonance energy transfer (RET). However, the sensor exhibits relatively low detection limit and sensitivity, which may be ascribed to the incomplete energy transfer. It is conjectured that adjusting the emission wavelength of the materials to much higher energy gap, the efficient RET will be occurred and thus a more highly sensitive sensor for TNP enables to be constructed. In addition, the mechanism of AIE-active iridium(III) complexes sensing TNP should be clearly depicted in order to provide the theoretical basis for the design of novel molecules.