Visible Light-Induced Cytotoxicity of a Thiophene-Benzothiadiazole-Based Conjugated Oligoelectrolyte in Cancer Cells

We report the in vitro visible-light-induced cytotoxic activity of a donor-acceptor-donor π-conjugated oligoelectrolyte M1, in which a 4,7-dithiophene benzothiadiazole core is flanked by phenyl rings with ionic side chains. The π-electron delocalized framework with electron-rich and electron-deficient moieties provides M1 with strong light absorption in the midvisible region (∼500 nm). The presence of a hydrophobic core with cationic quaternary ammonium side chains at its six terminals introduces amphiphilicity to M1, making it water-soluble and hence biologically compatible. M1 displays efficient penetration into breast and ovarian cancer cells (HCC1937, OVCAR-8, and SK-OV-3) with localization determined by confocal laser scanning microscopy, indicative of endosomal trafficking. Light activation of cancer cells preloaded with 10 μM M1 using a 525 nm source (4.5 mW/cm²) for 5 min promoted a significant reduction in cell viability that was not observed under dark conditions, when cells were subject to light in the absence of M1, or when M1 was activated prior to cellular uptake. Spectroscopic investigations point toward the ability of M1 to sensitize singlet oxygen (¹O₂) as the in situ generated reactive oxygen species (Φ ∼ 0.15) that effectively leads to cancer cell death, specifically in cells containing intracellular M1 at the time of light activation, providing a platform that can be targeted to cancer cells to provide selective phototoxicity at wavelengths compatible with biological systems.