Long-lived room-temperature deep-red-emissive intraligand triplet excited state of naphthalimide in cyclometalated Ir ~(III) complexes and its application in triplet-triplet annihilation-based upconversion

摘要

Cyclometalated Ir ~(III) 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 (τ _T=9.30 μs and 16.45 μs) and room-temperature red emission (? _(em)=640 nm, ∥ _p=1.4 and ? _(em)=627 nm, ∥ _p=0.3 ; cf. Ir-1: ε=16600 M ~(-1) cm ~(-1) at 382 nm, τ _(em)=1.16 μs, ∥ _p=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 T _1 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 (?E _s), 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.