Copper(I) photosensitisers for antimicrobial photodynamic therapy: from solution studies to silica-immobilised...

Antimicrobial photodynamic therapy (aPDT) is an emerging approach to treat localised drug-resistant infections by generating reactive oxygen species with spatial and temporal control. Herein, we report the systematic assessment of six heteroleptic Cu(I) diimine-diphosphine photosensitisers of the type [Cu(N^N)(P^P)]⁺ and their transition from solution studies to silica-immobilised composite materials (Cu(I)@SiO₂). Targeted ligand variations (steric bulk, polarity and π-extension) within a comparable coordination framework result in markedly different absorption and emission characteristics across the series. Antibacterial screening against representative Gram-positive (methicillin-resistant Staphylococcus aureus, MRSA) and Gram-negative (Escherichia coli) strains revealed no measurable activity against E. coli up to 100 μM, neither in the dark nor under irradiation. However, several complexes showed pronounced activity against MRSA with enhanced potency upon 405 nm irradiation. Cubiipo displayed the strongest light-dependent response (minimum inhibitory concentration, MIC_light = 6.25 μM, phototoxicity index >16), while Cuphen and Cuneo exhibited measurable dark activity in the low-micromolar range. Immobilisation on silica using an established adsorption protocol afforded Cu(I)@SiO₂ composite materials that retained light-enhanced antibacterial activity against MRSA (e.g.Cubiipo@SiO2 MIC_light = 0.62-1.23 mg mL^-1), albeit with reduced dark/light differences compared to the molecular complexes. These results demonstrate that small ligand modifications can cause strongly differentiated aPDT-relevant behaviour and that silica-immobilised Cu(I) photosensitisers offer a promising option for light-mediated surface disinfection/sanitation applications.