From Mononuclear to Dinuclear Iridium(III) Complex: Effective Tuning of the Optoelectronic Characteristics for Organic Light-Emitting Diodes

Sky-blue emitting bridged diiridium complexes: beneficial effects of intramolecular π-π stacking

The potential of intramolecular π-π interactions to influence the photophysical properties of diiridium complexes is an unexplored topic, and provides the motivation for the present study. A series of diarylhydrazide-bridged diiridium complexes functionalised with phenylpyridine (ppy)-based cyclometalating ligands is reported. It is shown by NMR studies in solution and single crystal X-ray analysis that intramolecular π-π interactions between the bridging and cyclometalating ligands rigidify the complexes leading to high luminescence quantum efficiencies in solution and in doped films. Fluorine substituents on the phenyl rings of the bridge promote the intramolecular π-π interactions. Notably, these non-covalent interactions are harnessed in the rational design and synthesis of the first examples of highly emissive sky-blue diiridium complexes featuring conjugated bridging ligands, for which they play a vital role in the structural and photophysical properties. Experimental results are supported by computational studies.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S₁ and T₁ excited state energetics and S₁ ⇔ T₁ intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Highly luminescent 2-phenylpyridine-free diiridium complexes with bulky 1,2-diarylimidazole cyclometalating ligands

While a number of highly emissive dinuclear Ir(iii) complexes have been reported, they have generally been restricted to structures based on 2-phenylpyridine (Hppy) cyclometalates.

A hybrid inorganic–organic light-emitting diode using Ti-doped ZrO2 as an electron-injection layer

We have fabricated stable efficient iridium(iii)-bis-5-(1-(naphthalene-1-yl)-1H-phenanthro[9,10-d]imidazole-2-yl) benzene-1,2,3-triol (acetylacetonate) [Ir(NPIBT)₂ (acac)] doped inverted bottom-emissive green organic light-emitting diodes using Ti-doped ZrO₂ nanomaterials as the electron injection layer. The current density (J) and luminance (L) of the fabricated devices with Ti-doped ZrO₂ deposited between an indium tin oxide cathode and an Ir(NPIBT)₂ (acac) emissive layer increased significantly at a low driving voltage (V) compared with control devices without Ti-doped ZrO₂. The Ti-doped ZrO₂ layer can facilitate the electron injection effectively and enhances the current efficiency (η_c) of 2.84 cd A⁻¹ and power efficiency (η_p) of 1.32 lm W⁻¹

Ti-doped ZrO₂ facilitates electron injection effectively, leading to enhanced current efficiency of 2.84 cd A⁻¹ and power efficiency of 1.32 lm W⁻¹

Multimetallic and Mixed Environment Iridium(III) Complexes: A Modular Approach to Luminescence Tuning Using a Host Platform

Mononuclear and trinuclear bis‐cyclometallated Ir^III complexes of the host ligands tris(4‐[4′‐methyl‐2,2′‐bipyridyl]methyl)cyclotriguaiacylene (L1) and tris(4‐(4′‐methyl‐2,2′‐ bipyridyl)carboxy)cyclotriguaiacylene (L2) have been prepared. Complexes [{Ir(ppy)₂}₃(L1)](PF₆)₃ (1.1), [{Ir(ppy)₂}(L1)](PF₆)₃ (1.2), [{Ir(ppy)₂}₃(L2)](PF₆)₃ (2.1) and [{Ir(ppy)₂}(L2)](PF₆)₃ (2.2) (where ppy=phenylpyridinato) showed distinct photophysical properties depending on the L ligand. Complexes featuring the L1 ligand were comparatively blue‐shifted in solution, with longer lifetimes and higher quantum yields. The mixed bis‐cyclometallated Ir^III complexes [{Ir(ppy)₂}{Ir(dFppy)₂}₂(L1)](PF₆)₃ (1.3), [{Ir(ppy)₂}{Ir(dFppy)₂}₂(L2)](PF₆)₃ (2.3), [{Ir(ppy)₂}₂{Ir(dFppy)₂}(L1)](PF₆)₃ (1.4) and [{Ir(ppy)₂}₂{Ir(dFppy)₂}(L2)](PF₆)₃ (2.4) (where dFppy=2,4‐difluorophenylpyrinato) were also synthesised. Steady‐state and time‐resolved spectroscopy, along with electrochemical investigations, show that the Ir(III) chromophores within these mixed Ir‐environment species behave as isolated centres, with no energy transfer or electronic communication between them.

Luminescent ion pairs with tunable emission colors for light-emitting devices and electrochromic switches

Most recently, stimuli-responsive luminescent materials have attracted increasing interest because they can exhibit tunable emissive properties which are sensitive to external physical stimuli, such as light, temperature, force, and electric field. Among these stimuli, electric field is an important external stimulus. However, examples of electrochromic luminescent materials that exhibit emission color change induced by an electric field are limited. Herein, we have proposed a new strategy to develop electrochromic luminescent materials based on luminescent ion pairs. Six tunable emissive ion pairs (IP1-IP6) based on iridium(iii) complexes have been designed and synthesized. The emission spectra of ion pairs (IPs) show concentration dependence and the energy transfer process is very efficient between positive and negative ions. Interestingly, IP6 displayed white emission at a certain concentration in solution or solid state. Thus, in this contribution, UV-chip (365 nm) excited light-emitting diodes showing orange, light yellow and white emission colors were successfully fabricated. Furthermore, IPs displayed tunable and reversible electrochromic luminescence. For example, upon applying a voltage of 3 V onto the electrodes, the emission color of the solution of IP1 near the anode or cathode changed from yellow to red or green, respectively. Color tunable electrochromic luminescence has also been realized by using other IPs. Finally, a solid-film electrochromic switch device with a sandwiched structure using IP1 has been fabricated successfully, which exhibited fast and reversible emission color change.

Photoluminescence and electroluminescence of an iridium(iii) complex with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol ligands

Efficient greenish yellow OLEDs based on an iridium(iii) complex containing electron transport ligands of 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-phenyl-1,3,4-thiadiazol-2-yl)phenol show a maximum current efficiency of 54.5 cd A⁻¹and a maximum external quantum efficiency of 16.1%.

Photoluminescence and electroluminescence of iridium(iii) complexes with 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 1,3,4-oxadiazole/1,3,4-thiadiazole derivative ligands

Highly efficient OLEDs based on green and orange iridium(iii) complexes based on 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine and 2-(5-(4-(trifluoromethyl)phenyl)-1,3,4-oxadiazol-2-yl)phenol show peak current efficiencies of 74.8 and 41.0 cd A⁻¹, respectively, with low efficiency roll-off.

Highly efficient electroluminescent PtII ppy-type complexes with monodentate ligands

Functional Pt^II ppy-type complexes (ppy = 2-phenylpyridine anion) with pyridine and chloride monodentate ligands are prepared, which show high electroluminescence efficiencies.

Efficiency of photoinduced electron transfer in mono- and di-nuclear iridium complexes: a comparative study

Photoinduced electron transfer in mono- and dinuclear Ir(iii) complexes is dominated by the mononuclear as compared to the dinuclear complex.

Photoluminescence and electroluminescence of deep red iridium(iii) complexes with 2,3-diphenylquinoxaline derivatives and 1,3,4-oxadiazole derivatives ligands

OLEDs using efficient deep red iridium(iii) complexes display good electroluminescence performances with maximum current efficiency and external quantum efficiency of up to 14.0 cd A⁻¹and 17.8%, respectively, and the efficiency roll-off is mild.

Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes for High Performance Organic Light-Emitting Diodes

Containing two nitrogen atoms, the electron-deficient pyrimidine ring has excellent coordinating capability with transition metal ions. However, compared with the widely used pyridine ring, applications of the pyrimidine ring in phosphorescent Ir(III) complexes are rare. In this research, two highly emissive pyrimidine-based mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes were prepared with a simple one-pot reaction. The incorporation of the second Ir(III) center can lead to dramatic differences of both photophysical and electrochemical properties between the mono- and dinuclear complexes. Besides, these properties can also be fine-tuned with different substituents. Theoretical calculations have also been performed to understand their photophysical behaviors. The electroluminescent investigations demonstrate that the pyrimidine-based mono- and dinuclear Ir(III) complexes could show impressive device performance. The vacuum-deposited organic light-emitting diode (OLED) based on the mononuclear Ir(III) complex exhibited an external quantum efficiency (EQE) of 16.1% with almost no efficiency roll-off even at 10 000 cd m^-2. More encouragingly, the solution-processed OLED based on the dinuclear Ir(III) complex achieved the outstanding EQE, current efficiency (CE), and power efficiency (PE) of 17.9%, 52.5 cd A^-1, and 51.2 lm W^-1, respectively, representing the highest efficiencies ever achieved by OLEDs based on dinuclear Ir(III) complexes.

Novel Design of Iridium Phosphors with Pyridinylphosphinate Ligands for High-Efficiency Blue Organic Light-emitting Diodes

Due to the high quantum efficiency and wide scope of emission colors, iridium (Ir) (III) complexes have been widely applied as guest materials for OLEDs (organic light-emitting diodes). Contrary to well-developed Ir(III)-based red and green phosphorescent complexes, the efficient blue emitters are rare reported. Like the development of the LED, the absence of efficient and stable blue materials hinders the widely practical application of the OLEDs. Inspired by this, we designed two novel ancillary ligands of phenyl(pyridin-2-yl)phosphinate (ppp) and dipyridinylphosphinate (dpp) for efficient blue phosphorescent iridium complexes (dfppy)₂Ir(ppp) and (dfppy)₂Ir(dpp) (dfppy = 2-(2,4-difluorophenyl)pyridine) with good electron transport property. The devices using the new iridium phosphors display excellent electroluminescence (EL) performances with a peak current efficiency of 58.78 cd/A, a maximum external quantum efficiency of 28.3%, a peak power efficiency of 52.74 lm/W and negligible efficiency roll-off ratios. The results demonstrated that iridium complexes with pyridinylphosphinate ligands are potential blue phosphorescent materials for OLEDs.

A Method for Reducing the Singlet-Triplet Energy Gaps of TADF Materials for Improving the Blue OLED Efficiency

We have successfully synthesized a series of blue thermally activated delayed fluorescence emitters, BPy-pC, BPy-pTC, BPy-p2C, and BPy-p3C, bearing a 4-benzoylpyridine core as the electron-accepting unit and carbazolyl, tert-butylcarbazolyl, dicarbazolyl, and tercarbazolyl groups as the electron-donating units, respectively. The density functional theory calculation shows that all of the compounds have their lowest unoccupied molecular orbitals on the benzoylpyridine moiety. However, the highest occupied molecular orbital (HOMO) of BPy-p3C is widely dispersed to the whole tercarbazolyl group, while the HOMOs of BPy-pC and BPy-pTC are mainly on the carbazolyl and extended to the phenyl ring. As a result, ΔE_ST is reduced from 0.29 eV for BPy-pC to 0.05 eV for BPy-p3C, and the organic light-emitting diodes using these materials as dopants emit blue light and their maximum external quantum efficiencies (EQEs) increase from 4.2% to 23.9% for BPy-pC and BPy-p3C, respectively. The EQE of the BPy-p3C-based device increases 2 times more than that of the BPy-pTC-based device without a significant change in the color coordinates.