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

Abstract

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=39,600 M(-1) cm(-1) at 402 nm and ε=25,100 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 (λ(em)=640 nm, Φ(p)=1.4 % and λ(em)=627 nm, Φ(p)=0.3 %; cf. Ir-1: ε=16,600 M(-1) cm(-1) at 382 nm, τ(em)=1.16 μs, Φ(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 ((3)IL) by their small thermally induced Stokes shifts (ΔE(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.

Syntheses, Spectroscopic, Electrochemical, and Third‐Order Nonlinear Optical Studies of a Hybrid Tris{ruthenium(alkynyl)/(2‐phenylpyridine)}iridium Complex

“…In cyclometalated iridium complexes, the excitation processes can be both ligand‐centred and MLCT in nature 2. 14, 18, 20, 21b, 22 The presence of vibrational structure in the emission spectra of 6 – 10 (Figure S31–S36) indicates that the relevant excited state in these complexes may possess significant ligand‐centered character, and likely resulting from mixing the 3 MLCT states with π→π* states as previously demonstrated for 13 14. 20…”

Green to Blue Annihilated Upconversion from a Simple Iridium(III) Sensitizer with Carboxylic Group

“…[14, 16] The upconversion efficiency (Φ uc ) of Ir–COOH/DPA in deaerated DMF reached its maximum value of 9.5 % wherein the ratio of [Ir–COOH]/[DPA] is at 8 μM/2.8 mM. Moreover, the single DPA in deaerated DMF is also excited by the same laser and no blue emission is obtained as well (shown in Figure S3 in supporting information) under the identical experimental condition, which indicate the observed blue emission of Ir–COOH/DPA system in this work was all from the triplet‐triplet annihilation upconversion step, rather than from the direct excitation of DPA . The inset picture of Figure is the photograph of Ir–COOH/DPA (8 μM/2.8 mM) in deaerated DMF under 532 nm laser irradiation (60 mW/cm 2 ), we could find green to blue upconversion from Ir–COOH/DPA is obviously visible under the naked eyes, which directively proves that Ir–COOH is an efficient triplet sensitizer for TTA upconversion.…”

Transition-Metal Complexes for Triplet–Triplet Annihilation-Based Energy Upconversion

“…Two neutral (Ir-6) and cationic (Ir-7) complexes have been synthesized [24]. By contrast, Ir(III) complex with a naphthal ligand (Ir-8) and the model complex (Ir-1) were also prepared.…”