[7]Helicene-backboned quaternary ammonium salts: synthesis, photophysical investigation, and lysosomal tracking

Polyaromatic quaternary ammonium salts (PQASs) are of significant interest due to their promising applications in biological and materials sciences. The incorporation of a heteroatom significantly modifies the electronic and chemical properties of these molecules, influencing their absorption and emission characteristics, as well as the HOMO–LUMO gap. Herein, we report the synthesis and photophysical investigation of [7]helicene-backboned quaternary ammonium salts. These compounds exhibit excellent stability, absorb light in the visible region with λ_abs ranging from 385 to 395 nm, and show emission in the green wavelength spectrum at λ_em between 541 and 552 nm. Also, they are redox-active and feature a structurally defined double helical axis. Single-crystal X-ray diffractometry has demonstrated the presence of a double helical structure within the crystal packing, which is characterized by the linkage of M–P and P–M heterodimers. Additionally, they show photoluminescence capability (ϕ_f) of up to 0.57, with fluorescence lifetimes in the range of 1.81–3.17 ns. Notably, these fluorophores turned out to be potential cell imaging agents. Colocalization studies that utilized LysoTracker Red probes as standard lysosomal trackers demonstrated that the [7]helicene QAS probe is efficacious in specifically labeling lysosomes in the neuroblastoma (N2a) cell line and RAW 264.7 macrophage cells. Additionally, to elucidate their electronic profiles, we employed time-dependent density functional theory calculations.