<i>3</i>‐Ethoxysalicylaldimine‐based symmetrical azine‐linked luminogen exhibiting ESIPT and bright orange colour AIE behaviour with live cell bioimaging application

Herein, we describe the design and development of a new cell-permeable aggregation-induced emission (AIE) active 3-ethoxysalicylaldimine-based symmetrical azine molecule HDBE. The synthesized compound underwent comprehensive investigation of different spectroscopic methods, like NMR, mass and single crystal X-ray diffraction analysis. The fluorophore HDBE exhibited the bright orange colour AIE behaviour in THF-H₂O mixture. The drastic enhancement of emission was achieved upon adding the water to the THF solution of HDBE, with a concentration of 90%. Along with the dynamic light scattering (DLS) and quantum yield measurements, the formation of aggregates was also verified by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Further, HDBE demonstrated excited state intramolecular proton transfer (ESIPT) characteristics in different polarity of solvents, which was corroborated by absorption, emission and lifetime spectroscopical investigations. The detailed scrutiny of X-ray structure of HDBE displayed the two strong intramolecular hydrogen bonding interactions, while solid-state fluorescent spectra showed dual emission that corresponds to enol and keto form confirming the ESIPT feature. Further, the synthesized AIE molecule was non-toxic and cell-permeable, making it easy to label as a biomarker in live HeLa cells via fluorescent bioimaging. These studies offer a quick and easy way to develop both AIE and ESIPT-coupled molecules for live cell bioimaging applications.