Tuning the emission properties of cyclometalated platinum(II) complexes by intramolecular electron-sink/arylethynylated ligands and its application for enhanced luminescent oxygen sensing

We have synthesized five novel cyclometalated Pt(II) complexes (aryl-ppy)Pt(acac) (ppy = 2-phenyl pyridine, aryl = N-butyl naphthalimide (NI) ethynylene for Pt-1, N-butyl naphthalimide (NI)–CH2 –CO– for Pt-2, 4-cyanophenyl – CH2 – CO– for Pt-3, naphthal ethynylene for Pt-4 and naphthal-diketo for Pt-5). For the first time, π-conjugation of the ppy ligands was extended via the CC bond. Deep red/near IR emission (638 nm–700 nm) was observed for the complex containing naphthalimide ethynylene subunit (Pt-1), whereas the close analogue Pt-2 (in which the linker between the NI and the ppy subunit is a –CH2CO– group) shows a relatively blue-shifted emission (540 nm–570 nm) but much longer luminescent lifetime (τ = 25.5 μs) than Pt-1 (τ = 6.6 μs). Simultaneous fluorescence/phosphorescence emissions were observed for Pt-1 and Pt-2, but other complexes show sole phosphorescent emission. The red-shifted phosphorescence of the complexes compared to the model complex ppyPt(acac) (486 nm) was attributed to either the significant electron-sink effect of the NI fragment (Pt-1) (for which the electron withdrawing effect is stronger than the previously reported fluoren-9-one), or the extended π-conjugation of the ppy ligand (via CC bond) (e.g.Pt-4). The substantial tuning of the emission color and the luminescent lifetimes (0.86 μs–25.5 μs) of the complexes were rationalized by theoretical calculations (DFT/TDDFT), i.e. the emissive triplet excited states were assigned as the normal 3MLCT state (give smaller τ values) or the novel ligand-localized 3IL emissive state (give larger τ values). With tuning the luminescent lifetimes, the luminescent O2 sensitivity of the complexes was improved by 117-fold (Stern–Volmer quenching constants KSV = 0.234 Torr−1 for Pt-2vs. KSV = 0.002 Torr−1 for Pt-5).