Long-lived emissive intra-ligand triplet excited states (3IL): next generation luminescent oxygen sensing scheme and a case study with red phosphorescent diiminePt(ii) bis(acetylide) complexes containing ethynylated naphthalimide or pyrene subunits

Abstract

The long-lived room temperature (RT) intra-ligand phosphorescence (³IL) of dbbpy Pt(II) bis(acetylide) (dbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) complexes Pt-1 (λ_em = 629 nm, τ = 118 μs, quantum yield ϕ = 17.5%) and Pt-3 (λ_em = 658 nm, τ = 73.6 μs, ϕ = 2.1%) (dbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), containing naphthalimide (NI) and pyrene subunits, respectively, were used for enhanced luminescent oxygen sensing, compared to the model complex dbbpyPt (bisphenylacetylide) (Pt-2, λ_em = 559 nm, τ = 0.7 μs, ϕ = 49.6%) with the normal ³MLCT excited state (metal-to-ligand-charge-transfer). The luminescent lifetimes of Pt-1 and Pt-3 are greatly extended by 168-fold and 105-fold, respectively, when compared to that of Pt-2. The ³IL features of the photoluminescence of Pt-1 and Pt-3 are supported by DFT/TDDFT calculations, which indicated a NI localized triplet excited state but a normal ³MLCT/³LLCT excited state for Pt-2. The luminescent oxygen sensing properties of the complexes in solution as well as in polymer films were studied. In polymer films, the O₂ sensitivity of Pt-1 (quenching constant K_SV = 0.085 Torr⁻¹) and Pt-3 (K_SV = 0.062 Torr⁻¹) is 70-fold and 50-fold of Pt-2 (K_SV = 0.0012 Torr⁻¹), respectively.