DinuclearIr(III)/Ru(II)/Os(II) Bis-terpyridine ComplexesTethered by Tetrathienyl-Substituted Diketopyrrolopyrrole: Synthesis,Photophysics, and Antimicrobial Photodynamic Therapy

Three dinuclear bis-terpyridine Ir(III)/Ru(II)/Os(II) complexes DPP-M (M = Ir, Ru, Os) tethered by tetrathienyl-substituted diketopyrrolopyrrole were synthesized and characterized as near-IR photosensitizers. Photophysical studies, including UV–vis absorption, emission, and transient absorption, were conducted to investigate the singlet and triplet excited states of the complexes and to understand how different metals influence their photophysical properties. All complexes demonstrated panchromatic absorption at 280–750 nm and near-IR emission in various organic solvents and PBS solutions at room temperature. The nature of the emission varied among the complexes, with DPP-Ir and DPP-Ru exhibiting fluorescence, while DPP-Os displayed phosphorescence. Their triplet excited-state lifetimes ranged from 1.74 μs for DPP-Os to 5.10 μs for DPP-Ru. DPP-M can generate singlet oxygen and superoxide anion radical upon 525 nm light activation, with singlet oxygen generation efficiencies between 0.08 and 0.53 in CH3CN, increasing in the order of DPP-Ir < DPP-Ru < DPP-Os. The antimicrobial photodynamic therapy (aPDT) activity of DPP-M toward Staphylococcus aureus was assessed using 500 nM DPP-M and a 20 min drug-light interval under 525 nm light irradiation. DPP-Ir displayed the strongest aPDT effect, achieving a remarkable 3.9-log reduction at 15 J/cm2 irradiation, which may be attributed to its higher positive charge density, leading to enhanced bacterial uptake and increased aPDT efficacy.