Knotting Two Donor−π-Acceptor AIEgens Using a Nonconjugated Linker: Tunable and Switchable Fluorescence and Fingerprinting and Live Cell Imaging Applications

Abstract

Aggregation-induced emissive fluorophores (AIEgens) have drawn widespread attention over the years because of their structure-controlled tunable and external-stimuli-responsive solid-state fluorescence, which have stimulated rapid development in the optoelectronics and biomedical fields. However, precise control of organic π-conjugated molecular fluorescence remains a great challenge and depends on the molecular structure and organization. Herein, we have interlinked two triphenylamine-based donor−π acceptor (D−π-A) AIEgens using a nonconjugate spacer (xylene) and explored the effect of interlinking, positional isomerism, and acceptor structure on the tunable and switchable solid-state fluorescence. A change of the acceptor group resulted in tuning of fluorescence between 534 and 614 nm. Interestingly, interspacing modulation from o-xylene (1) to the meta (2) and para isomers (3) showed a decrease of fluorescence efficiency in the solid state. A closer spacing between the AIEgens and tighter molecular packing could restrict the intramolecular rotation in ortho isomers that resulted in enhanced fluorescence in comparison to para isomers. In contrast, more spatial freedom in para isomers allowed the molecules to respond to external pressure and produced mechanofluorochromism (MFC) in comparison to ortho isomers. Strong tunable solid-state fluorescence of ortho isomers was used to obtain clear latent fingerprinting images on three different surfaces with tunable fluorescence color. 1a,b,e (orange, green, and red, respectively) were further utilized for live cell imaging applications using E. coli cells that demonstrated clear labeling of the cell membrane and cytoplasm with strong green/red fluorescence in the cell structures. In vitro and in vivo toxicity studies of 1a,b,e confirmed the nontoxic nature of these fluorophores and indicated their potential for live cell imaging applications.