Toward Efficient Halogen- and Heavy Metal-Free Photosensitizers: Alkoxy/Aryloxy-BODIPYs for Effective Singlet Oxygen...

This study investigates the singlet oxygen O₂(¹Δ_g) generation capabilities and photophysical properties of eight synthesized alkoxy- and aryloxy-substituted BODIPY derivatives across eight different solvents. The production of O₂(¹Δ_g) was confirmed by monitoring its characteristic near-infrared (NIR) phosphorescence and through chemical trapping experiments using 1,3-diphenylisobenzofuran. Among the series, phenanthrenyloxy-BODIPY demonstrated outstanding performance, achieving a singlet oxygen quantum yield (Φ_Δ) of up to 0.85. Photophysical analysis indicated that this high efficiency originates from a photoinduced electron transfer (PET) process within the molecule. All synthesized compounds exhibit significantly enhanced O₂(¹Δ_g) generation over the unsubstituted BODIPY reference (Φ_Δ ≈ 0.10). The Φ_Δ was found to be modulated by solvent polarity, while the carbon chain length of the alkoxy substituents has only a marginal influence. Density functional theory calculations, which reveals the spatial distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO), provides further evidence for the proposed PET mechanism in phenanthrenyloxy-BODIPY. The established structure-property relationships offer valuable insights for designing efficient, heavy-atom-free BODIPY photosensitizers, with promising applications in photodynamic therapy for cancer.