A Yellow-Emitting Homoleptic Iridium(III) Complex Constructed from a Multifunctional Spiro Ligand for Highly Efficient Phosphorescent Organic Light-Emitting Diodes

J-Aggregate Promoting NIR-II Emission for Fluorescence/Photoacoustic Imaging-Guided Phototherapy

J-aggregate is a promising strategy to enhance second near-infrared window (NIR-II) emission, while the controlled synthesis of J-aggregated NIR-II dyes is a huge challenge because of the lack of molecular design principle. Herein, bulk spiro[fluorene-9,9'-xanthene] functionalized benzobisthiadiazole-based NIR-II dyes (named BSFX-BBT and OSFX-BBT) are synthesized with different alkyl chains. The weak repulsion interaction between the donor and acceptor units and the S…N secondary interactions make the dyes to adopt a co-planar molecular conformation and display a peak absorption >880 nm in solution. Importantly, BSFX-BBT can form a desiring J-aggregate in the condensed state, and femtosecond transient absorption spectra reveal that the excited states of J-aggregate are the radiative states, and J-aggregate can facilitate stimulated emission. Consequently, the J-aggregated nanoparticles (NPs) display a peak emission at 1124 nm with a high relative quantum yield of 0.81%. The efficient NIR-II emission, good photothermal effect, and biocompatibility make the J-aggregated NPs demonstrate efficient antitumor efficacy via fluorescence/photoacoustic imaging-guided phototherapy. The paradigm illustrates that tuning the aggregate states of NIR-II dye via spiro-functionalized strategy is an effective approach to enhance photo-theranostic performance.

Phosphorescent Platinum(II) Complexes with a Spiro-fused Xanthene Unit: Synthesis and Photophysical Properties

Novel platinum(II) complexes, derived from the spiro[fluorene-9,9'-xanthene] (SFX) motif, were synthesized and combined with different auxiliary ligands such as acetylacetonate (acac), bis(2,4,6-trimethylphenyl)propane-1,3-dionate (mesacac) and dihydrobis(3,5-dimethylpyrazole-1-yl) borate. The final products were obtained in yields of up to 36 % and characterized by NMR, X-ray and combustion analysis. These complexes have structured green-blue emission spectra with Commission Internationale de l'Éclairage (CIExy) coordinates of (0.21;0.46). Excellent photoluminescence quantum yields (PLQYs) ranging from 87 %-91 % were found. The emission lifetimes vary from 33 μs to 43 μs. Calculations on the B3LYP/6-311++G** level of theory reveal, that the nature of the emissive state is dependent on the positional regioisomerism of the SFX motif. The 2-SFX complexes demonstrate ligand-centered (3LC) emission, while the 2'-SFX regioisomer with the mesacac ligand shows a strong 3MLCT character.

Two Synthetic Tools to Deepen the Understanding of the Influence of Stereochemistry on the Properties of Iridium(III) Heteroleptic Emitters

Two complementary procedures are presented to prepare cis-pyridyl-iridium(III) emitters of the class [3b+3b+3b'] with two orthometalated ligands of the 2-phenylpyridine type (3b) and a third ligand (3b'). They allowed to obtain four emitters of this class and to compare their properties with those of the trans-pyridyl isomers. The finding starts from IrH5(PiPr3)2, which reacts with 2-(p-tolyl)pyridine to give fac-[Ir{κ2-C,N-[C6MeH3-py]}3] with an almost quantitative yield. Stirring the latter in the appropriate amount of a saturated solution of HCl in toluene results in the cis-pyridyl adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} stabilized with p-tolylpyridinium chloride, which can also be transformed into dimer cis-[Ir(μ-OH){κ2-C,N-[C6MeH3-py]}2]2. Adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} directly generates cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-Isoqui]}] and cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-py]}] by transmetalation from Li[2-(isoquinolin-1-yl)-C6H4] and Li[py-2-C6H4]. Dimer cis-[Ir(μ-OH){κ2-C,N-[C6MeH3-py]}2]2 is also a useful starting complex when the precursor molecule of 3b' has a fairly acidic hydrogen atom, suitable for removal by hydroxide groups. Thus, its reactions with 2-picolinic acid and acetylacetone (Hacac) lead to cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,N-[OC(O)-py]} and cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,O-[acac]}. The stereochemistry of the emitter does not significantly influence the emission wavelengths. On the contrary, its efficiency is highly dependent on and associated with the stability of the isomer. The more stable isomer shows a higher quantum yield and color purity.

Unraveling the Position Effect of Spiroxanthene-Based n-Type Hosts for High-Performance TADF-OLEDs

For developing high-performance organic light-emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) emitters, the diphenyltriazine (TRZ) unit was introduced onto the 2'- and 3'-positions of xanthene moiety of spiro[fluorene-9,9'-xanthene] (SFX) to construct n-type host molecules, namely 2'-TRZSFX and 3'-TRZSFX. The outward extension of the TRZ unit, induced by the meta-linkage, resulted in a higher planarity between the TRZ unit and xanthene moiety in the corresponding 3'-TRZSFX. Additionally, this extension led to a perched T1 level, as well as a lower unoccupied molecular orbital (LUMO) level when compared with 2'-TRZSFX. Meanwhile, the 3'-TRZSFX molecules in the crystalline state presented coherent packing along with the interaction between TRZ units; the similar packing motif was spaced apart from xanthene moieties in the 2'-TRZSFX crystal. These endowed 3'-TRZSFX superior electron transport capacity in single-carrier devices relative to the 2'-TRZSFX-based device. Hence, the 3'-TRZSFX-based TADF-OLED showed remarkable electroluminescent (EL) performance under the operating luminance from turn-on to ca. 1000 cd·m-2 with a maximum external quantum efficiency (EQEmax) of 23.0%, thanks to its matched LUMO level with 4CzIPN emitter and better electron transport capacity. Interestingly, the 2'-TRZSFX-based device, with an EQEmax of 18.8%, possessed relatively low roll-off and higher efficiency when the operating luminance exceeded 1000 cd·m-2, which was attributed to the more balanced carrier transport under high operating voltage. These results were elucidated by the analysis of single-crystal structures and the measurements of single-carrier devices, combined with EL performance. The revealed position effect of the TRZ unit on xanthene moiety provides a more informed strategy to develop SFX-based hosts for highly efficient TADF-OLEDs.

Incorporation of Fluorene and Its Heterocyclic Spiro Derivatives To Realize High-Performance and Stable Sky-Blue-Emitting Arylgold(III) Complexes

A series of arylgold(III) complexes of tridentate diphenylpyridine ligand incorporated with fluorene and its heterocyclic spiro derivatives, spiro[fluorene-9,9'-xanthene] and spiro[acridine-9,9'-fluorene], as auxiliary ligands has been prepared. This class of complexes exhibits high decomposition temperatures of up to 387 °C, excellent film morphologies in solid-state thin films with a root-mean-square roughness smaller than 0.20 nm, as well as high photoluminescence quantum yields of up to 0.72 in solid-state thin films. Solution-processed organic light-emitting devices (OLEDs) fabricated from this series of complexes as dopants show intense electroluminescence in the sky-blue region with maximum external quantum efficiencies of 10.0%. Taking advantage of their high thermal stability, vacuum-deposited OLEDs have also been fabricated and satisfactory operational lifetimes of ∼300 h have been recorded.

Phospho-rescent mono- and diiridium(III) complexes cyclo-metalated by fluorenyl- or phenyl-pyridino ligands with bulky substituents, as prospective OLED dopants

The crystal structures of tris-[9,9-dihexyl-2-(5-meth-oxy-pyridin-2-yl-κN)-9H-fluoren-3-yl-κC 3]iridium pentane monosolvate, [Ir(C31H38NO)3]·C5H12, (I), di-μ2-chlorido-bis-{bis-[2-(5-fluoro-pyridin-2-yl)-9,9-dihexyl-9H-fluoren-3-yl]iridium} pentane 0.3-solvate, [Ir2(C30H35FN)4Cl2]·0.3C5H12, (II), di-μ2-cyanato-bis-{bis-[9,9-dihexyl-2-(5-meth-oxy-pyridin-2-yl)-9H-fluoren-1-yl]iridium} pentane monosolvate, [Ir2(C31H38NO)4(NCO)2(NCO)2]·C5H12, (III), and {μ-N,N'-bis-[3,5-bis-(tri-fluoro-meth-yl)phen-yl]oxamidato}bis(bis{2-[4-(2,4,6-trimethylphenyl)pyridin-2-yl]phenyl-κ2 C 1,N'}iridium)-chloro-benzene-pentane (1/2.3/0.4), [Ir2(C20H19N)4(C18H6F12N2O2)]·2.3C6H5Cl·0.4C5H12, (IV), synthesized in the quest for organic light-emitting devices, were determined. The bis-μ2-chloro and bis-μ2-cyanato complexes have ΔΔ and ΛΛ configurations of the distorted octa-hedral Ir centres in racemic crystals, whereas the oxamido complex has a centrosymmetric (meso) structure with the ΔΛ configuration. The bridging oxamido moiety has a nearly planar anti geometry. All structures show substantial disorder of both host mol-ecules and solvents of crystallization.