Accessing the long-lived emissive 3IL triplet excited states of coumarin fluorophores by direct cyclometallation and its application for oxygen sensing and upconversion

Abstract

We studied four cyclometallated Pt(II) complexes, in which the thiazo-coumarin ligands (Pt-2, Pt-3 and Pt-4) or the phenylthiazo ligand (Pt-1) were directly cycloplatinated. Pt-2 shows intense absorption in visible region but other complexes show blue-shifted absorption. Room temperature phosphorescence was observed for all the complexes, and the emission wavelength is dependent on the size of the π-conjugation, not the intramolecular charge transfer (ICT) feature of the C⁁N ligands. Pt-2 shows longer phosphorescence lifetime (τ = 20.3 μs) than other complexes (below 2.0 μs). Furthermore, Pt-2 shows phosphorescence quantum yield Φ = 0.37, whereas Pt-3 and Pt-4 show much smaller Φ values (0.03 and 0.01, respectively). DFT/TDDFT calculations indicate ³IL triplet excited states for the complexes. The complexes were used as for luminescence O₂ sensing and triplet–triplet-annihilation (TTA) based upconversion. Stern–Volmer quenching constant K_SV = 0.026 Torr⁻¹ was observed for Pt-2, ca. 89-fold of that of Pt-3. TTA upconversion is achieved with Pt-2 (λ_em = 400 nm with λ_ex = 473 nm, anti-Stokes shift is 0.47 eV, excitation power density is at 70 mW cm⁻²). The upconversion quantum yield with Pt-2 as triplet sensitizer is up to 15.4%. The TTET efficiency (K_SV = 1.33 × 10⁵ M⁻¹, k_q = 6.57 × 10⁹ M⁻¹s⁻¹. DPA as quencher) of Pt-2 is 34-fold of the model complex [Ru(dmb)₃][PF₆]₂. Our results show that the ³IL state can be readily accessed by direct cyclometallation of organic fluorophores and this approach will be useful for preparation and applications of transition metal complexes that show intense absorption in visible region and the long-lived emissive ³IL excited states.