Nucleophilic Substitution of Fluorine Atoms in 2,6-Difluoro-3-(pyridin-2-yl)benzonitrile Leading to Soluble Blue-Emitting Cyclometalated Ir(III) Complexes

Positional Isomeric Cyano-Substituted Bis(2-phenylpyridine)(acetylacetonate)iridium Complexes for Efficient Organic Light-Emitting Diodes with Extended Color Range

Bis(2-phenylpyridine)(acetylacetonate)iridium, Ir(ppy)2(acac), is a benchmark green emitter for phosphorescent organic light-emitting diodes (PhOLEDs). In this work, we reported three positional isomeric cyano-substituted Ir(ppy)2(acac) complexes, i.e., Ir(3-CN), Ir(4-CN), and Ir(10-CN), with the emission in the yellow to red region (544-625 nm). Through theoretical investigation and single-crystal analysis, it was found that the introduction of cyano substitution at various positions of the ppy ligand allows for tuning the electron distribution and coordination bond length of Ir complexes. Therefore, the charge transfer property of Ir complexes is enhanced such that the energy gap of the cyano-substituted Ir(ppy)2(acac) complexes was reduced. In addition, Ir(3-CN), Ir(4-CN), and Ir(10-CN) exhibited high PLQYs of 83, 54, and 75%, respectively, with the phosphorescence lifetime in the range of 0.79-2.08 μs. Notably, the device utilizing Ir(3-CN) as the emitter exhibited a maximum external quantum efficiency (EQE) of 25.4%, current efficiency of 56.9 cd A-1, power efficiency of 68.7 lm W-1, and brightness of 61,340 cd m-2 at 8 V. The EQE of this device remained 24.3 and 19.9% at luminances of 1,000 and 10,000 cd m-2, corresponding to the efficiency roll-off of 4.3 and 21.7%, respectively. Comparing to the Ir complexes using the ligand with an extended conjugated structure, our results demonstrated a simple molecular design strategy for phosphorescence emitters with reduced molecular weight for efficient PhOLEDs in the yellow to red color region.

Versatile Electroluminescence Color-Tuning Strategy of an Efficient Light-Emitting Electrochemical Cell (LEC) by an Ionic Additive

The color-tuning strategies of solid-state light-emitting devices (ss-LEDs) are mainly focused on engineering molecular structures. In this paper, for the first time, we developed a facile strategy for tuning the electroluminescence (EL) color from orange to green through the addition of the ionic additive TBAP (tetrabutylammonium perchlorate). To achieve the active ionic emissive compound for use in a light-emitting electrochemical cell (LEC), the neutral biscyclometalated bromo tetrazole iridium(III) [Ir(ppy)₂(BrTz)] was exchanged to its cationic complex, [Ir(ppy)₂(BrTz-Me)]ClO₄ (ppy = 2-phenyl pyridine, BrTz = 4-bromo-2-pyridine tetrazole, BrTz-Me = 4-bromo-2-pyridine methyl tetrazole) with a new synthetic strategy. This method allows employing neutral Ir-cyclometalated complexes, which are ruled out for use in LECs because of their non-ionic behaviors. In the following, an LEC based on the new cationic [Ir(ppy)₂(BrTz-Me)]ClO₄ as the emissive layer was fabricated between the FTO (fluorine-doped tin oxide) anode and Ga:In alloy cathode without using any additive or polymers, which makes this configuration the simplest ss-LED so far. By adding the ionic additives, the electroluminescence characteristics of [Ir(ppy)₂(BrTz-Me)]ClO₄ were dramatically increased, including luminance (L) from 162.8 cd/m² for the device with an additive to 212.9 and 355.9 cd/m² for devices containing LiTFSI (bis(trifluoromethane)sulfonamide lithium salt) and TBAP, respectively. In particular, when TBAP was added to the [Ir(ppy)₂(BrTz-Me)]ClO₄ complex, the irradiance was significantly increased from 166.4 to 220.8 μW/cm² with an efficacy of 1.78 cd/A and external quantum efficiency (EQE) value of 2.14%. The obtained EL results clearly showed that adding TBAP and LiTFSI significantly improved the electroluminescence characteristics and tuned the electroluminescence color.

On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy)₂(bpy)]⁺, where Hppy is 2-phenylpyridine and bpy is 2,2′-bipyridine (complex 1), and [Ir(diFppy)₂(dtb-bpy)]⁺, where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine (complex 2), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex 2 being derived from complex 1 through the addition of fluoro and tert-butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.196 for complex 1 in dichloromethane and 0.71 for complex 2 in acetonitrile) has been evaluated by characterizing both radiative and nonradiative decay paths. It has emerged that the probability of decaying through the nonradiative triplet metal-centered state, normally associated with the loss of the emission quantum yield, does not appear to be the reason behind the reported substantially different emission efficiency. A more critical factor appears to be the ability of complex 2 to emit from both the usual metal-to-ligand charge-transfer state and from two additional ligand-centered states, as supported by the fact that the respective minima belong to the potential energy surface of the lowest triplet T₁ state and that their phosphorescence lifetimes are in the same order of magnitude. In contrast, the emission of complex 1 can be originated only from the metal-to-ligand charge-transfer state, being the only emissive T₁ minimum. The results constitute a significant case in which the emission from ligand-centered states is the key for determining the high emission quantum yield of a complex.

The reasons behind the significant increase in the emission quantum yield of the [Ir(diFppy)₂(dtb-bpy)]⁺ complex with respect to [Ir(ppy)₂(bpy)]⁺ are rationalized on the basis of DFT, TDDFT, and TDDFT-SOC calculations, revealing the key role that low-lying LC states can play in the emission properties of the complex.

Dual emission from an iridium(III) complex/counter anion ion pair

[Ir(tpy)2](PF6)3 (tpy = 2,2':6',2''-terpyridine) dissolved in CH3CN was found to exhibit dual color luminescent emission depending on the excitation wavelength. Specifically, blue and green emissions were obtained with excitation at 350 and 410 nm, respectively. Because the associated emission spectra were consistent with those of [Ir(tpy)2]Cl3 in water and [Ir(tpy)2](PF6)3 in the crystalline state, respectively, this dual emission is attributed to emissions from the [Ir(tpy)2]3+ cation and its ion pair [Ir(tpy)2]3+·PF6-. The emission is assigned to the 3π-π* transition of the ligands based on time-dependent density functional theory (TD-DFT) calculations. Conversely, [Ir(tpy)2]I3 in CH3CN shows emission due to [Ir(tpy)2]3+ but not [Ir(tpy)2]3+·I-, while crystalline [Ir(tpy)2]I3 emits red luminescence at 77 K that is inconsistent with that from [Ir(tpy)2]3+. Since the emission energies of crystalline [Ir(tpy)2]X3 (X- = Cl-, Br- or I-) show a good correlation with the electron affinity of X, the emissions are assigned to a counter anion to complex ion charge-transfer transition. This hypothesis is supported by TD-DFT calculations regarding [Ir(tpy)2]3+·X-.

Roles of Ancillary Chelates and Overall Charges of Bis-tridentate Ir(III) Phosphors for OLED Applications

A series of charge-neutral bis-tridentate Ir(III) complexes (1, 3, and 4) were prepared via employing three distinctive tridentate prochelates, that is, (pzp^tBph^FO)H₂, [(phpyim)H₂·(PF₆)], and [(pimb)H₃·(PF₆)₂], which possess one dianionic pzp^tBph^FO, together with a second monoanionic tridentate chelate, namely, (pzp^tBph^FO)H, phpyim, and pimb, respectively. Moreover, a homoleptic, charge-neutral complex 2 was obtained by methylation of chelating (pzp^tBph^FO)H of 1 in basic media, while closely related cationic complexes 5-7 were obtained by further methylation of the remaining pyrazolate unit of previously mentioned neutral complexes 2-4, followed by anion metatheses. All of these Ir(III) metal complexes showed a broadened emission profile with an onset at ∼450 nm, a result of an enlarged ligand-centered ππ* transition gap, but with distinct efficiencies ranging from 0.8% to nearly unity. Comprehensive spectroscopic and computational approaches were executed, providing a correlation for the emission efficiencies versus energy gaps and between the metal-to-ligand charge transfer/ππ* emitting excited state and upper-lying metal-centered dd quenching state. Furthermore, Ir(III) complexes 3 and 4 were selected as dopant emitters in the fabrication of sky-blue phosphorescent organic light-emitting diodes, affording maximum external quantum efficiencies of 16.7 and 14.6% with CIE_x,y coordinates of (0.214, 0.454) and (0.191, 0.404) at a current density of 10² cd/m², respectively. Hence, this research highlights an inherent character of bis-tridentate Ir(III) complexes in achieving high phosphorescence quantum yield at the molecular level.

Azole-containing cationic bis-cyclometallated iridium(iii) isocyanide complexes: a theoretical insight into the emission energy and emission efficiency

Using a density functional theory approach, we explore the emission properties of a family of bis-cyclometallated cationic iridium(iii) complexes of general formula [Ir(C^N)2(CN-tert-Bu)2]+ that have tert-butyl isocyanides as neutral auxiliary ligands. Taking the [Ir(ppy)2(CN-tert-Bu)2]+ complex (Hppy = 2-phenylpyridine) as a reference, the effect of replacing the pyridine ring in the cyclometallating ppy ligand by a five-membered azole ring has been examined. To this end, two series of complexes differing by the nature of the atom (either nitrogen or carbon) linking the azole to the phenyl ring of the cyclometallating ligand have been designed. Each series is composed of three molecules having an increasing number of nitrogen atoms (2 to 4) in the azole ring. The emission energies computed for the azole-containing [Ir(C^N)2(CN-tert-Bu)2]+ complexes show a generalized blue-shift compared to [Ir(ppy)2(CN-tert-Bu)2]+, in agreement with the experimental data available for two of the six complexes designed here. The electronic nature of the lowest-lying triplet (T1) is clearly established as a ligand-centred (3LC) state associated with the cyclometallating ligands, and cannot be described as a simple HOMO → LUMO promotion. Therefore, no clear trend based on the sole use of molecular orbital energies can be inferred to predict the emission properties. The significant oscillation in the emission quantum yield (ranging from 0.1% to 52%) experimentally reported is rationalized by the energy gap between the emitting T1 state and a non-radiative triplet state having metal-centred (3MC) d-d* nature. On the basis of such a model, two of the here proposed systems are expected to display significant emission quantum yields in the blue region of the visible spectra, which make them good candidates for electroluminescent applications.

Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(iii) complexes and solution-processable PhOLEDs

A blue phosphorescent emitter for PhOLEDs with brightness of 5400 cd m⁻² at 10 V and λELmax 460 nm, CIE_x,y (0.15, 0.21).

Emission energy of azole-based ionic iridium(iii) complexes: a theoretical study

Tuning the emission color of azole-based iridium(iii) complexes by changing the number and position of nitrogen atoms in the azole ring.

Synthesis of most polyene natural product motifs using just 12 building blocks and one coupling reaction

The inherent modularity of polypeptides, oligonucleotides and oligosaccharides has been harnessed to achieve generalized synthesis platforms. Importantly, like these other targets, most small-molecule natural products are biosynthesized via iterative coupling of bifunctional building blocks. This suggests that many small molecules also possess inherent modularity commensurate with systematic building block-based construction. Supporting this hypothesis, here we report that the polyene motifs found in >75% of all known polyene natural products can be synthesized using just 12 building blocks and one coupling reaction. Using the same general retrosynthetic algorithm and reaction conditions, this platform enabled both the synthesis of a wide range of polyene frameworks that covered all of this natural-product chemical space and the first total syntheses of the polyene natural products asnipyrone B, physarigin A and neurosporaxanthin b-D-glucopyranoside. Collectively, these results suggest the potential for a more generalized approach to making small molecules in the laboratory.