Visible light-harvesting cyclometalated Ir(iii) complexes as triplet photosensitizers for triplet–triplet annihilation based upconversion

Abstract

Visible light-harvesting cyclometalated Ir(III) complexes with 3-(2-benzothiazoly)-7-diethylaminocoumarin as the C^N cyclometalation ligands were prepared. The ancillary N^N ligand is either 6-piperidine naphthalimide-phenanthroline (Ir-1) or 9-aminophenanthroline (Ir-3). Ir(ppy)₂(Phen) was prepared as model complex (Ir-2). Ir-1 and Ir-3 show strong absorption of visible light (ε = 109 000 M⁻¹ cm⁻¹ or 112 000 M⁻¹ cm⁻¹ at 486 or 484 nm, respectively). All the complexes show room temperature phosphorescence with drastically different phosphorescence quantum yields (Φ_P = 4.3%, 44.3% and 46.0% for Ir-1, Ir-2 and Ir-3, respectively). With steady state and time-resolved spectra, as well as DFT calculations, the T₁ excited states of Ir-1 and Ir-3 were proposed to be the ³IL state, whereas the ³MLCT state was proposed for Ir-2. Long-lived emissive triplet excited states (7.6 μs and 54.5 μs) were observed for Ir-1 and Ir-3, compared to the short T₁ excited state lifetime of Ir-2 (1.2 μs). The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion and upconversion quantum yields (Φ_UC) of 19.3% and 12.7% were observed for Ir-1 and Ir-3, respectively. No upconversion was observed for Ir-2 under the same experimental conditions.