Comparative photodynamic and sonodynamic properties of indolenine, benzothiazole, and benzoselenazole heptamethine cyanine dyes

Cyanine dyes are widely used near-infrared (NIR) chromophores, yet the impacts of structural rigidity and heteroatom substitution on their photo- and sonochemical behavior remain underexplored. Here, linear (HITC, Cy7S, Cy7Se) and cyclohexene (IR786, IR2S, IR2Se, IR3S, IR3Se) cyanines were systematically compared for their photophysical characteristics, ROS generation, photo- and sonostability, and antimicrobial photodynamic (APDT) and sonodynamic (ASDT) efficacies. Cy dyes absorb and emit at slightly shorter wavelengths with larger Stokes shifts and higher brightness than IR analogs. Heavy-atom substitution induces moderate red-shifts but quenches fluorescence. IR dyes are generally more photo- and sonostable than Cy dyes. S and Se atoms reduce photostability in methanol but enhance it in saline while lowering sonostability in both media. Light-induced singlet-oxygen generation is stronger for Cy dyes, whereas ultrasound-induced ROS production shows no consistent trend across the series in either methanol or saline. S and Se substitution boosts ROS generation in methanol under both activation modes. Biologically, IR dyes exhibit mostly similar sonodynamic but stronger photodynamic activity against S. aureus and E. coli. Cy7S, Cy7Se, and IR2S act as dual-mode (APDT/ASDT) antibacterial agents, while HITC, IR786, IR2Se, and IR3Se perform well only in APDT and IR3S solely in ASDT. Aggregation and cell uptake complicate relationships between cytotoxicity and structure, especially upon heavy-atom introduction. Notably, light-irradiated HITC completely eradicates S. aureus at nanomolar levels, cautioning its use as a “benign” probe. Thus, molecular rigidity and heteroatom content mutually define cyanine efficacy under photo- and ultrasound activation, guiding rational sensitizer design.