Long-Lived Room-Temperature Deep-Red-Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its Application in Triplet-Triplet Annihilation-Based Upconversion

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Abstract

Cyclometalated IrIII complexes with acetylide ppy and bpy ligands were prepared (ppy=2-phenylpyridine, bpy=2,2′-bipyridine) in which naphthal (Ir-2) and naphthalimide (NI) were attached onto the ppy (Ir-3) and bpy ligands (Ir-4) through acetylide bonds. [Ir(ppy)3] (Ir-1) was also prepared as a model complex. Room-temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir-3 and Ir-4 showed strong absorption in the visible range (ε=39600 M−1 cm−1 at 402 nm and ε=25100 M−1 cm−1 at 404 nm, respectively), long-lived triplet excited states (τT=9.30 μs and 16.45 μs) and room-temperature red emission (λem=640 nm, Φp=1.4 % and λem=627 nm, Φp=0.3 %; cf. Ir-1: ε=16600 M−1 cm−1 at 382 nm, τem=1.16 μs, Φp=72.6 %). Ir-3 was strongly phosphorescent in non-polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir-4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non-polar solvents. Emission of Ir-1 and Ir-2 was not solvent-polarity-dependent. The T1 excited states of Ir-2, Ir-3, and Ir-4 were identified as mainly intraligand triplet excited states (3IL) by their small thermally induced Stokes shifts (ΔEs), nanosecond time-resolved transient difference absorption spectroscopy, and spin-density analysis. The complexes were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir-2 and Ir-3, respectively, whereas the upconversion was negligible for Ir-1 and Ir-4. These results will be useful for designing visible-light-harvesting transition-metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.

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