The impact of metal ions on the photo-antibacterial efficiency of metalloporphyrins with triphenylphosphonium units

The influence of metal ions on the photochemical, photophysical, and antibacterial properties of three cationic porphyrin-triphenylphosphonium conjugates was investigated for the first time. Coordination with Zn(II), Pd(II) and Co(II) enabled fine-tuning of these conjugates properties, with Zn(II) complexes demonstrating particular promise. While Pd(II) and Co(II) complexes failed to generate singlet oxygen (¹O₂), thereby reducing their efficacy in bacteria photoinactivation, Zn(II) complexes exhibited efficient ¹O₂ generation and strong bacterial adhesion. Among these, the Zn(II) metalloporphyrin 2-Zn, featuring a well-optimized structure with three triphenylphosphonium units and six positive charges, showed exceptional effectiveness against Gram-negative Escherichia coli at lower concentrations. It significantly outperformed its free-base counterpart, reducing the required irradiation time by more than 65 %. Furthermore, 2-Zn demonstrates to be safe, exhibiting no cytotoxicity towards Vero cells. These findings highlight the potential of Zn(II) porphyrin-triphenylphosphonium complexes as efficient photosensitizers for antimicrobial photodynamic therapy (aPDT), offering a promising approach to address the growing challenge of antibiotic bacterial resistance in both clinical and environmental contexts.