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

Abstract

The long-lived room temperature (RT) intra-ligand phosphorescence ((3)IL) 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 (3)MLCT 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 (3)IL 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 (3)MLCT/(3)LLCT 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 O(2) 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.