Green light-excitable naphthalenediimide acetylide-containing cyclometalated Ir(iii) complex with long-lived triplet excited states as triplet photosensitizers for triplet–triplet annihilation upconversion

Abstract

Naphthalenediimide (NDI) was connected to the ligand of a cyclometalated Ir(III) complex (Ir-1) via a CC triple bond to enhance the absorption in the visible region and to access long-lived triplet excited states. Ir(ppy)₂(bpy)[PF₆] (Ir-2, ppy = 2-phenylpyridine and bpy = 2,2′-bipyridine) was used as a model complex. The photophysical properties of the complexes were studied with steady state and time-resolved spectroscopy. Ir-1 shows strong absorption in the visible region (ε = 11 000 M⁻¹ cm⁻¹ at 542 nm) and in comparison Ir-2 shows typically weak absorption in the visible region (ε < 3000 M⁻¹ cm⁻¹ above 400 nm). Room temperature near IR emission at 732 nm (Φ_P = 0.1%) was observed for Ir-1, which is attributed to the NDI localized emissive triplet excited state, by transient absorption spectra and DFT calculations on the spin density surface. The lifetime of the NDI-localized triplet excited state is up to 130.0 μs, which is rarely reported for Ir(III) complexes. In comparison, Ir-2 shows phosphorescence at 578 nm and the triplet state lifetime is a typical value of 0.3 μs. The complexes were used as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion and an upconversion quantum yield of 6.7% was observed with Ir-1. No upconversion was observed with Ir-2 as the triplet photosensitizer at the same experimental conditions.