Versatile side chain turns property and functions of α-cyanostilbene derivatives with near-infrared emission and two-photon property for mitochondrial imaging

Developing red materials with bright near-infrared emission and excellent two-photon performance by a simple preparation process is a meaningful and challenging work. Herein, a series of nitro functionalized D-pi-A type alpha-cyanostilbene derivatives with different side chains (ZH, ZOM, ZOE, and ZOB) have been designed and synthesized by a one-pot method. The compounds show weak emissions and almost the same photophysical properties in solutions. In contrast, in the aggregation state, ZOM, ZOE, and ZOB with side chains of methoxy, ethoxy and benzyloxy emit strong fluorescence, while ZH with no side chain shows weak emission. The aggregate emission mechanism is ascribed to the side chain induced the molecular planarization, and formation of unique bricks H-aggregates and staircase J-aggregates, investigated by single-crystal analysis and DFT calculations. Considering the excellent photophysical properties of the side chain modified compounds, the triphenylphos-phonium is employed to the side chain to obtain a mitochondrial targeting probe Z-TPP, which performs high fluorescence and selectively targeted the mitochondria in live cell and zebrafish under one/two-photon excita-tion, and further successfully applied to observe the complex mitochondrial network with high signal-to-noise ratio under stimulated emission depletion (STED) nanoscopy.

Automated summary

Developing red materials with bright near-infrared emission and excellent two-photon performance through a simple preparation process is a meaningful and challenging work. In this study, a series of nitro functionalized D-pi-A type alpha-cyanostilbene derivatives with different side chains were designed and synthesized. The fluorescent properties of the compounds were found to be enhanced in the aggregation state due to side chain induced molecular planarization and formation of unique aggregate structures. Furthermore, side chain modified compounds were employed for the preparation of a mitochondrial targeting probe, which exhibited high fluorescence and selective targeting of mitochondria in live cells and zebrafish, and enabled the observation of complex mitochondrial networks under STED nanoscopy.