Inducing AIE in planar styrylpyridinium dyes through counterion exchange and liposomal encapsulation for live-cell imaging

Aggregation-caused quenching (ACQ) significantly limits the performance of planar fluorescent probes in aqueous environments, thereby restricting their applicability in live-cell imaging. Although styrylpyridinium (SP) dyes possess favorable donor–π–acceptor architectures and attractive photophysical properties, their planar structure makes them prone to aggregation-induced fluorescence loss. Therefore, this study aimed to enhance the emission efficiency of planar SP derivatives without major structural modifications by employing counterion exchange and lipid-based encapsulation strategies. A series of 17 novel mono-SP and bis-SP derivatives was synthesized and systematically evaluated to assess how molecular architecture, counterion identity (BF₄⁻, PF₆⁻, ClO₄⁻, Br⁻), and liposomal encapsulation into DOPC:CHEM liposomes influence photophysical behavior. Counterion variation affected fluorescence intensity; however, no single anion proved universally optimal for all derivatives. In contrast, liposomal encapsulation significantly enhanced fluorescence in aqueous environments for all compounds by suppressing ACQ, with all synthesized compounds exhibiting large Stokes shifts and emission in the green spectral region. Notably, bis-SP derivatives demonstrated more pronounced aggregation-induced emission (AIE) behavior due to increased restriction of intramolecular motions compared to their mono-SP analogues, with fluorescence enhancement of up to 17.6-fold and Stokes shifts exceeding 200 nm upon liposomal encapsulation. Biological evaluation revealed low cytotoxicity and rapid cellular uptake in live cells. Furthermore, all SP bromide derivatives demonstrated superior photostability compared to the commercially available nuclear stain DAPI under identical illumination conditions. Overall, the combination of counterion engineering and liposomal encapsulation provides an effective strategy to induce AIE behavior in planar styrylpyridinium dyes, enhancing fluorescence performance without extensive structural modification and offering promising candidates for live-cell imaging applications.