[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.