Tuning the luminescence lifetimes of ruthenium(ii) polypyridine complexes and its application in luminescent oxygen sensing

Abstract

Ru(Phen)(bpy)₂ (1) and its new derivatives (2–5) with pyrenyl or ethynylated pyrene and phenyl units appended to the 3-position of the phenanthroline (Phen) ligand were prepared and these complexes generate long-lived room temperature phosphorescence in the red and near IR range (600–800 nm). The photophysical properties of these complexes were investigated by UV-Vis absorption, luminescence emission, transient absorption spectra and DFT/TDDFT calculations. We found the luminescence lifetime (τ)can be drastically extended by ligand modification (increased up to 140-fold), e.g. τ = 58.4 μs for complex 3 (with pyrenyl ethynylene appendents) was found, compared to τ = 0.4 μs for the reference complex 1. Ethynylated phenyl appendents alter the τ also (complex 2, τ = 2.4 μs). With pyrenyl appendents (4 and 5), lifetimes of 2.5 μs and 9.2 μs were observed. We proposed three different mechanisms for the lifetime extension of 2, 3, 4 and 5. For 2, the stabilization of the ³MLCT state by π-conjugation is responsible for the extension of the lifetime. For 3, the emissive state was assigned as an intra-ligand (IL) long-lived ³π–π* state (³IL/³LLCT, intraligand or ligand-to-ligand charge transfer), whereas a C–C single bond linker results in a triplet state equilibrium between ³MLCT state and the pyrene localized ³π–π* triplet state (³IL, e.g.4 and 5). DFT/TDDFT calculations support the assignment of the emissive states. The effects of the lifetime extension on the oxygen sensing properties of these complexes were studied in both solution and polymer films. With tuning the emissive states, and thus extension of the luminescence lifetimes, the luminescent O₂ sensing sensitivity of the complexes can be improved by ca. 77-fold in solution (I₀/I₁₀₀ = 1438 for complex 3, vs. I₀/I₁₀₀ = 18.5 for complex 1). In IMPES-C polymer films, the apparent quenching constant K_SV^app is improved by 150-fold from 0.0023 Torr⁻¹ (complex 1) to 0.35 Torr⁻¹ (complex 3). The K_SV^app value of complex 3 is even higher than that of PtOEP under similar conditions (0.15 Torr⁻¹).