Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet–Triplet Annihilation Upconversion

Abstract

We report molecular designing strategies to enhance the effective visible-light absorption of cyclometalated Ir­(III) complexes. Cationic cyclometalated Ir­(III) complexes were prepared in which boron–dipyrromethene (Bodipy) units were attached to the 2,2′-bipyridine (bpy) ligand via −CC– bonds at either the <i>meso</i>-phenyl (<b>Ir-2</b>) or 2 position of the π core of Bodipy (<b>Ir-3</b>). For the first time the effect of π conjugating (<b>Ir-3</b>) or tethering (<b>Ir-2</b>) of a light-harvesting chromophore to the coordination center on the photophysical properties was compared in detail. Ir­(ppy)₂(bpy) (<b>Ir-1</b>; ppy = 2-phenylpyridine) was used as model complex, which gives the typical weak absorption in visible range (ε < 4790 M^–1 cm^–1 in region > 400 nm). <b>Ir-2</b> and <b>Ir-3</b> showed much stronger absorption in the visible range (ε = 71 400 M^–1 cm^–1 at 499 nm and 83 000 M^–1 cm^–1 at 527 nm, respectively). Room-temperature phosphorescence was only observed for <b>Ir-1</b> (λ_em = 590 nm) and <b>Ir-3</b> (λ_em = 742 nm). <b>Ir-3</b> gives RT phosphorescence of the Bodipy unit. On the basis of the 77 K emission spectra, nanosecond transient absorption spectra, and spin density analysis, we proposed that Bodipy-localized long-lived triplet excited states were populated for <b>Ir-2</b> (τ_T = 23.7 μs) and <b>Ir-3</b> (87.2 μs). <b>Ir-1</b> gives a much shorter triplet-state lifetime (0.35 μs). Complexes were used as singlet oxygen (¹O₂) photosensitizers in photooxidation. The ¹O₂ quantum yield of <b>Ir-3</b> (Φ_Δ = 0.97) is ca. 2-fold of <b>Ir-2</b> (Φ_Δ = 0.52). Complexes were also used as triplet photosensitizer for TTA upconversion; upconversion quantum yields of 1.2% and 2.8% were observed for <b>Ir-2</b> and <b>Ir-3</b>, respectively. Our results proved that the strong absorption of visible light of <b>Ir-2</b> failed to enhance production of a triplet excited state. These results are useful for designing transition metal complexes that show <i>effective</i> strong visible-light absorption and long-lived triplet excited states, which can be used as ideal triplet photosensitizers in photocatalysis and TTA upconversion