Observation of the room temperature phosphorescence of Bodipy in visible light-harvesting Ru(ii) polyimine complexes and application as triplet photosensitizers for triplet–triplet-annihilation upconversion and photocatalytic oxidation

Abstract

Two Ru(II) polyimine complexes containing a boron-dipyrromethene (Bodipy) chromophore were prepared. The two complexes are different in the linker which connects the Bodipy part and the Ru(II) coordination centre. The Bodipy core and the Ru(II) centre are in π-conjugation in Ru-1, whereas in Ru-2 the Bodipy part is linked in a non-conjugated way to the Ru(II) centre. Ru(bpy)₃[PF₆]₂ (Ru-3) was used as a reference complex. Both Ru-1 and Ru-2 show strong absorption in the visible region (ε = 65 200 M⁻¹ cm⁻¹ at 528 nm for Ru-1 and ε = 76 700 M⁻¹ cm⁻¹ at 499 nm for Ru-2). The fluorescence of the Bodipy ligands was almost completely quenched in Ru-1 and Ru-2. Ru-1 shows room temperature phosphorescence of the Bodipy chromophore, as well as the residual fluorescence of the Bodipy ligand. Ru-2 shows only the residual fluorescence of the Bodipy ligand. A long-lived Bodipy-localized triplet excited state was observed for both Ru-1 and Ru-2 upon visible light excitation (τ_T is up to 279.7 μs, the longest T₁ state lifetime observed for the Bodipy moiety in the transition metal complex). Application of the complexes in triplet–triplet-annihilation upconversion and singlet oxygen (¹O₂)-mediated photo-oxidation proved that Ru-1 is more efficient (e.g. singlet oxygen quantum yield Φ_Δ = 0.93) as a triplet photosensitizer than Ru-2 (Φ_Δ = 0.64). Therefore, direct connection of the π-core of the Bodipy chromophore to the coordination centre, i.e. by establishing π-conjugation between the visible light-harvesting chromophore and the metal coordination centre is essential to enhance the effective visible light-harvesting of the Ru(II) complexes.