Decoupling oxygen sensitivity and aggregation effects in a dual-emissive platinum(II) chlorin lifetime probe

Here, we systematically investigate the interplay between aggregation and oxygen quenching on the photophysical properties of a platinum(II) 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorin (Pt(II)-chlorin). In air-saturated dimethyl sulfoxide (DMSO), Pt(II)-chlorin displays a low photoluminescence quantum yield (ϕ_PL ∼ 0.006), which increases nearly tenfold (ϕ_PL ∼ 0.05) upon deaeration. The fluorescence lifetimes (τ_F,1 ≈ 0.2 ns, ∼60%; τ_F,2 ≈ 1.8 ns, ∼40%) remain largely unaffected by the presence of molecular oxygen, whereas the phosphorescence lifetime (τ_Ph) increases from 3.1 μs to 28.6 μs under oxygen-free conditions. In DMSO : water mixtures, increasing the water fraction up to 1 : 9 (v/v) induces pronounced photoluminescence quenching (Φ_PL ≈ 0.0016) and a decrease in both average τ_F and τ_Ph values, attributed to solvent-induced aggregation, as confirmed by dynamic light scattering. This differentiation enables clear distinction between effects arising from solvent composition, probe concentration, and surrounding oxygen fluctuations. Incorporation of Pt(II)-chlorin into Zeonex® and PMMA polymer matrices (used as model systems to mimic cellular compartments) also results in a concentration-dependent decreases in the Φ_PL, τ_F, and τ_Ph values. In Zeonex®, a matrix with higher oxygen permeability, the probe exhibits a faster and more pronounced luminescence response to oxygen concentration changes. Notably, in vitro cell studies demonstrate that Pt(II)-chlorin retains its non-aggregated photophysical characteristics intracellularly, indicating that aggregation induced in the cell culture medium is reversed upon membrane permeation. Overall, these findings establish Pt(II)-chlorin as an effective dual-lifetime probe for intracellular oxygen sensing and a promising candidate for bioimaging and photodynamic therapy applications.