Functionalized Salen ligands linking with non-conjugated bridges: unique and colorful aggregation-induced emission, mechanism, and applications

A series of novel, simple, and colorful Salen ligands (56 samples), salicylaldehyde-based bis-Schiff bases, linking with different non-conjugated alkyl bridges ((CH2)(n), n = 2-9, 12; cyclohexyl) and containing different electron-accepting (-NO2, -F, and -Cl), electron-donating (-OMe, -OH, and -NEt2), or sterically hindering (-t-butyl) substituents or a pi-extended system (naphthalene ring) have been designed and synthesized. The photophysical properties of these Salen ligands can be well-tuned by the introduction of side functional substituents, p-extended systems, and central N-alkyl chain bridges. It is unusual that they contain a small pi-conjugated system but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 162 nm) and high fluorescence quantum yields (up to 0.44 and 0.75 in water and in solid, respectively). Combining with their advantages of AIE and good stability and biocompatibility, the Salen ligands can be potentially used in mechanofluorochromism (crystal-defect-induced emission) and living cell imaging. Moreover, the inherent relationships between their chemical structures and AIE properties are studied, which provide unequivocal insights for the design of AIE-active dyes.