Over the LEC rainbow: Colour and stability tuning of cyclometallated iridium(III) complexes in light-emitting electrochemical cells

Breaking the deep-red light absorption barrier of iridium(III)-based photosensitizers

Activating photosensitizers with long-wavelength excitation is an important parameter for effective photodynamic therapy due to the minimal toxicity of this light, its superior tissue penetration, and excellent spatial resolution. Unfortunately, most Ir(III) complexes suffer from limited absorption within the phototherapeutic window, rendering them ineffective against deep-seated and/or large tumors, which poses a significant barrier to their clinical application. To address this issue, several efforts have been recently made to shift the absorption of Ir(III) photosensitizers to the deep-red/near-infrared region by using different strategies: functionalization with organic fluorophores, including porphyrinoid compounds, and ligand design via π-extension and donor-acceptor interactions. In this Frontier, we highlight such new developments and the ongoing challenges in this field.

Phosphorescent Chiral Cationic Binuclear Iridium(III) Complexes: Boosting the Circularly Polarized Luminescence Brightness

We report the synthesis and characterization of two chiral binuclear iridium(III) complexes (ΛΛ and ΔΔ) prepared from enantiopure building blocks [μ-Cl2(Δ-Ir(C^N)2)2] and [μ-Cl2(Λ-Ir(C^N)2)2]. These building blocks have been obtained by chiral preparative high-performance liquid chromatography of the neutral iridium(III) complex Irpiv (piv = 2,2,6,6-tetramethylheptane-3,5-dionate) followed by selective degradation of the ancillary ligand. For comparison purposes, we also synthesized a monomer (IrL1) and a dimer (Ir2L2, mixture). All the complexes exhibit similar emission properties, emitting in the orange-red region of the spectra with a good photoluminescence quantum yield (λmax = 610-625 nm, Φ ∼ 25%, τ ∼ 800-900 ns). However, the ΛΛ and ΔΔ complexes are optically active, indicating that no isomerization occurred during the different synthetic steps, as evidenced by both the circular dichroism spectra and their circularly polarized luminescence (CPL). The capital gain of the dimers (Ir2L2, ΛΛ, and ΔΔ) is a 4-fold brightness (B380 = ε380 nm × Φ) compared to the monomer (IrL1) and the CPL brightness (BCPL = B380 × glum/2) of the binuclear complexes being among the highest reported to date for chiral iridium(III) complexes.

Trigonal NHC bis-pyridyl silver(I) complexes: a beacon of light in the darkness of light-emitting electrochemical cells?

Complex [Ag(IPr)(3-Medpa)][PF6] (1), possessing the same combination of ligands as previously reported [Cu(IPr)(3-Medpa)][PF6] (2) applied in a blue-emitting light-emitting electrochemical cell (LEC), has been synthesized and fully structurally and photophysically characterized both in powder and thin-film form. In detail, temperature-dependent steady-state, time-resolved emission experiments, and computational calculations have been performed to understand the impact of the nature of the metal ion center on the photophysical and electroluminescent properties. Here, a direct comparison between the complexes reveals (i) a distinct emission behavior, such as fluorescence (1) vs. thermally activated delay fluorescence (2), caused by the changes of the nature of the emitting excited states from ligand centered (1) to metal-to-ligand charge transfer (2), (ii) an unforeseen crystallinity-dependent emission in 1 that leads to either smooth and non-emissive thin-films or phase aggregated and emissive thin-films, and (iii) 1-based LECs with a stable electrical behavior over 100 h, which contrasts with the prior start-of-the-art value of a few minutes (2).

Binuclear cyclometalated Ir(III) complexes with bis-bidentate butterfly-shaped ligands: synthesis, characterization, and application in efficient yellow-orange light-emitting electrochemical cells

Two yellow-to-orange-emitting binuclear iridium complexes of the form [Ir2(ppy)4(N^N)](ClO4)2 (namely IrL1 and IrL2) were designed and synthesized, using phenylpyridine (ppy) as a cyclometalating ligand (C^N) and phenanthroimidazole-based bridging compounds as ancillary ligands (N^N), to explore the effect of sterically hindered bulky bridging ligands and their substituents on the electronic and emission properties of the related ionic transition metal complexes (iTMCs) and the performance of iTMC-based light-emitting electrochemical cells (iTMC-LECs). In CH2Cl2 solution, complexes IrL1 and IrL2 afford yellow-to-orange emission centered at 570 and 582 nm with PLQYs of 27% and 36%, respectively. The reversible and quasi-reversible redox behaviors of both complexes are shown to reveal the excellent stability of the emitters in solution. The electrochemical and photophysical data indicate that the substitution at the C2 position of the imidazole moiety does not significantly affect the electronic properties of the complexes. IrL1 and IrL2-based LECs show yellow-to-orange electroluminescence centered at 586 and 598 nm, with a maximum luminance (Lmax) of 430 and 570 cd m-2 and current efficiencies of 3.45 and 4.50 cd A-1, respectively. However, compared with their corresponding archetypecomplex, IrL1, and IrL2-based LECs exhibit several fold higher Lmax, which highlights the role of these ligands in the design of emitters.

Unveiling the key role of metal coordination mode and ligand's side groups on the performance of deep-red light-emitting electrochemical cell

Three novel deep-red to near-infrared (DR to NIR) emitters based on mononuclear and dinuclear ruthenium(II) complexes with bulky structures were presented herein. For the first time, the unusual effects of metal coordination mode on the electroluminescence properties of a binuclear emitter were investigated. Unexpectedly, the mononuclear complexes showed superior performance in deep-red light-emitting electrochemical cells (DR-LEC) compared to the dinuclear complex. Likewise, substituting various ancillary ligands improved the radiance and lifetime of devices by 2.5 and 1.5 times, respectively. To the best of our knowledge, the obtained efficiency is among the best reported to date for DR-LECs based on ruthenium polypyridyl complexes.

Tuning Excited State Character in Iridium(III) Photosensitizers and Its Influence on TTA-UC

A series of mixed ligand, photoluminescent organometallic Ir(III) complexes have been synthesized to incorporate substituted 2-phenyl-1H-naphtho[2,3-d]imidazole cyclometalating ligands. The structures of three example complexes were categorically confirmed using X-ray crystallography each sharing very similar structural traits including evidence of interligand hydrogen bond contacts that account for the shielding effects observed in the 1H NMR spectra. The structural iterations of the cyclometalated ligand provide tuning of the principal electronic transitions that determine the visible absorption and emission properties of the complexes: emission can be tuned in the visible region between 550 and 610 nm and with triplet lifetimes up to 10 μs. The nature of the emitting state varies across the series of complexes, with different admixtures of ligand-centered and metal-to-ligand charge transfer triplet levels evident. Finally, the use of the complexes as photosensitizers in triplet-triplet annihilation energy upconversion (TTA-UC) was investigated in the solution state. The study showed that the complexes possessing the longest triplet lifetimes showed good viability as photosensitizers in TTA-UC. Therefore, the use of an electron-withdrawing group on the 2-phenyl-1H-naphtho[2,3-d]imidazole ligand framework can be used to rationally promote TTA-UC using this class of complex.

A Semi-Automated, High-Throughput Approach for the Synthesis and Identification of Highly Photo-Cytotoxic Iridium Complexes

The discovery of new compounds with pharmacological properties is usually a lengthy, laborious and expensive process. Thus, there is increasing interest in developing workflows that allow for the rapid synthesis and evaluation of libraries of compounds with the aim of identifying leads for further drug development. Herein, we apply combinatorial synthesis to build a library of 90 iridium(III) complexes (81 of which are new) over two synthesise-and-test cycles, with the aim of identifying potential agents for photodynamic therapy. We demonstrate the power of this approach by identifying highly active complexes that are well-tolerated in the dark but display very low nM phototoxicity against cancer cells. To build a detailed structure-activity relationship for this class of compounds we have used density functional theory (DFT) calculations to determine some key electronic parameters and study correlations with the experimental data. Finally, we present an optimised semi-automated synthesise-and-test protocol to obtain multiplex data within 72 hours.

Cationic Ir(III) Complexes with 4-Fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile as the Cyclometalating Ligand: Synthesis, Characterizations, and Application to Ultrahigh-Efficiency Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) promise low-cost, large-area luminescence applications with air-stabilized electrodes and a versatile fabrication that enables the use of solution processes. Nevertheless, the commercialization of LECs is still encountering many obstacles, such as low electroluminescence (EL) efficiencies of the ionic materials. In this paper, we propose five blue to yellow ionic Ir complexes possessing 4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile (ppfn) as a novel cyclometalating ligand and use them in LECs. In particular, the device within di[4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile]-4,4'-di-tert-butyl-2,2'-bipyridyl iridium(III) hexafluorophosphate (DTBP) shows a remarkable photoluminescence quantum yield (PLQY) of 70%, and by adjusting the emissive-layer thickness, the maximal external quantum efficiency (EQE) reaches 22.15% at 532 nm under the thickness of 0.51 μm, showing the state-of-the-art value for the reported blue-green LECs.

Neutral 2-phenylbenzimidazole-based iridium(III) complexes with picolinate ancillary ligand: tuning the emission properties by manipulating the substituent on the benzimidazole ring

We report the synthesis and characterization of ten neutral bisheteroleptic iridium(III) complexes with 2-phenylbenzimidazole cyclometallating ligand and picolinate as ancillary ligand. The 2-phenylbenzimidazole has been modified by selected substituents introduced on the cyclometallating ring and/or on the benzimidazole moiety. The integrity of the complexes has been assessed by NMR spectroscopy, by high-resolution mass spectrometry and by elemental analysis. The complexes are demonstrated to be highly phosphorescent at room temperature and a luminescence study with comprehensive ab initio calculations allow us to determine the lowest emitting excited state which depends on the substituent nature and its position on the cyclometallating ligand.

Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh3 and SbPh3

In the frame of our research aiming to develop efficient triplet-emitting materials, we are exploring the concept of introducing additional heavy atoms into cyclometalated transition metal complexes to enhance intersystem-crossing (ISC) and thus triplet emission through increased spin-orbit coupling (SOC). In an in-depth proof-of-principle study we investigated the double cyclometalated Pt(II) complexes [Pt(C^N^C)(PnPh3)] (HC^N^CH = 2,6-diphenyl-pyridine (H2dpp) or dibenzoacridine (H2dba); Pn = pnictogen atoms P, As, Sb, or Bi) through a combined experimental and theoretical approach. The derivatives containing Pn = P, As, and Sb were synthesised and characterised comprehensively using single crystal X-ray diffraction (scXRD), UV-vis absorption and emission spectroscopy, transient absorption (TA) spectroscopy and cyclic voltammetry (CV). Across the series P < As < Sb, a red-shift is observed concerning absorption and emission maxima as well as optical and electrochemical HOMO-LUMO gaps. Increased photoluminescence quantum yields ΦL and radiative rates kr from mixed metal-to-ligand charge transfer (MLCT)/ligand centred (LC) triplet states are observed for the heavier homologues. Transient absorption spectroscopy showed processes in the ps range that were assigned to the population of the T1 state by ISC. The heavy PnPh3 ancillary ligands are found to enhance the emission efficiency due to both higher Pt-Pn bond strength and stronger SOC related to increased MLCT character of the excited states. The experimental findings are mirrored in hybrid (TD-)DFT calculations. This allowed for extrapolation to the rather elusive Bi derivatives, which were synthetically not accessible. This shortcoming is attributed to the transmetalation of phenyl groups from BiPh3 to Pt, as supported by experimental NMR/MS as well as DFT studies.

2-(Thienyl)quinoxaline derivatives and their application in Ir(III) complexes yielding tuneable deep red emitters

The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are described. These species display variable fluorescence properties in the visible region (λem = 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(III) to yield [Ir(C^N)2(bipy)]PF6 (where C^N = the cyclometalated ligand; bipy = 2,2'-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(III) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates via a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved via both the thienyl and quinoxaline moieties.

Mononuclear copper(I) complexes bearing a 3-phenyl-5-(pyridin-4-yl)-1,2,4-triazole ligand: synthesis, crystal structure, TADF-luminescence, and mechanochromic effects

Three new mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh3)2X] (X = Cl, Br, I), based on 3-phenyl-5-(pyridin-4-yl)-1,2,4-triazole (L) and triphenylphosphine (PPh3) ligands, have been prepared by reaction of CuX (X = Cl, Br, I), L and PPh3 in a molar ratio of 1 : 1 : 2 in MeCN solutions. The synthesized complexes exhibit blue light emission in solutions and bright green emission in the crystal state with quantum yields of up to 100%. The luminescence decay analysis and density functional theory calculations revealed that the emission of solid samples at room temperature corresponds to the thermally activated delayed fluorescence, while that at 77 K is assigned to phosphorescence. Utilizing the studied complexes in OLED heterostructures resulted in high-performing green-emitting devices with an external quantum efficiency of up to 13.4%.

Bis(2-phenylpyridinato,-C2′,N)[4,4′-bis(4-Fluorophenyl)-6,6′-dimethyl-2,2′-bipyridine] Iridium(III) Hexafluorophosphate

A new bis cyclometallated Ir(III) phosphor, [Ir(ppy)2L]PF6 (ppy = 2-phenylpyridine, L = 4,4′-bis(4-fluorophenyl)-6,6′-dimethyl-2,2′-bipyridine was prepared and structurally characterized in the solid state (X-ray diffraction) and solution (1 and 2D NMR spectroscopy). The compound exhibited yellow photoluminescence (λem = 562 nm). The quantum yield Φ was solvent-dependent (5% in acetonitrile and 19% in dichloromethane solutions, respectively).

Phenanthroimidazole as molecularly engineered switch for efficient and highly long-lived light-emitting electrochemical cell

Light-emitting electrochemical cells (LECs) based on Ir(III) complexes owing to the superior advantages exhibit high potential for display and lighting applications. Herein, a series of Ir(III) complexes based on phenanthroimidazole (PI) as an ancillary ligand were synthesized to achieve efficient and highly stable yellow-to-orange LEC devices with fast response. These complexes exhibit appropriate electrochemical stability and significant suppression of concentration quenching in the thin films compared to the archetype complex. The fabricated LECs showed remarkably long device lifetimes over 1400 and 2100 h and external quantum efficiency of 2 and 3% for yellow and orange-LECs, respectively. The obtained t1/2 for yellow LEC is much higher than archetype [Ir(ppy)2(phen)]+ and their phenanthroline-based analogues reported so far. The incorporation of an ionic tethered functional group on PI, improved the mobility of the emissive layer and reduced the device turn-on time by 75-88%. This study shows a facile functionalization and characterization of the PI ligand as well as its potential application in optoelectronic devices (OLED).

Cationic Iridium(III) Complexes with Benzothiophene-Quinoline Ligands for Deep-Red Light-Emitting Electrochemical Cells

Three new cationic cyclometalated iridium(III) complexes equipped with differently substituted benzo[b]thiophen-2-ylquinoline cyclometalating ligands and with a sterically demanding tert-butyl-substituted 2,2'-bipyridine ancillary ligand were synthesized and structurally characterized by NMR and X-ray diffraction techniques. To tune the electronic properties of such complexes, the quinoline moiety of the cyclometalating ligands was kept pristine or equipped with electron-withdrawing phenyl and -CF₃ substituents, leading to complexes 1, 2, and 3, respectively. A complete electrochemical and photophysical investigation, supported by density functional theory calculations, permits a deep understanding of their electronic properties. The emission of all complexes arises from ligand-centered triplet states in the spectral range between 625 and 950 nm, with excited-state lifetimes between 2.10 and 6.32 μs at 298 K. The unsubstituted complex (1) exhibits the most blue-shifted emission in polymeric matrix at 298 K (λ_max = 667 nm, photoluminescence quantum yield (PLQY) = 0.25 and τ = 5.32 μs). The phenyl-substituted complex (2) displays the highest photoluminescent quantum yields (up to 0.30 in polymeric matrix), while the CF₃-substituted counterpart (3) shows the most red-shifted emission, peaking at approx. 720 nm, but with lower quantum yields (e.g., 0.10 in polymeric matrix at 298 K). Complexes 1 and 2 were tested in single-layer nondoped light-emitting electrochemical cells (LEECs), using a nozzle-printing technique; both devices display deep-red electroluminescence with an external quantum efficiency close to 20%.

Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C2− heterocyclic ligand dba2− (H2dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh3, CNtBu and Me2Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh3, CNtBu and Me2Imd derivatives and characterized them by elemental analysis, 1H (and 31P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between −1.89 and −2.10 V and increasing along the series Me2Imd < dmso ≈ PPh3 < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me2Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me2Imd < CNtBu < PPh3 < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields ΦL ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high ΦL and long τ, the dmso and PPh3 complexes outperform the CNtBu and Me2Imd derivatives. This is remarkable in view of the higher ligand strength of Me2Imd, compared with all other coligands, as concluded from the electrochemical data.

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h’]diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around −1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around −1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around −2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed ³LC/³MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields Φ_L massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed ³LC/³MLCT with an emission lifetime of around 3 µs.

Rational Design of Mono- and Bi-Nuclear Cyclometalated Ir(III) Complexes Containing Di-Pyridylamine Motifs: Synthesis, Structure, and Luminescent Properties

Heteroleptic cyclometalated iridium (III) complexes (1–3) containing di-pyridylamine motifs were prepared in a stepwise fashion. The presence of the di-pyridylamine ligands tunes their electronic and optical properties, generating blue phosphorescent emitters at room temperature. Herein we describe the synthesis of the mononuclear iridium complexes [Ir(ppy)₂(DPA)][OTf] (1), (ppy = phenylpyridine; DPA = Dipyridylamine) and [Ir(ppy)₂(DPA-PhI)][OTf] (2), (DPA-PhI = Dipyridylamino-phenyliodide). Moreover, the dinuclear iridium complex [Ir(ppy)₂(L)Ir(ppy)₂][OTf]₂ (3) containing a rigid angular ligand “L = 3,5-bis[4-(2,2′-dipyridylamino)phenylacetylenyl]toluene” and displaying two di-pyridylamino groups was also prepared. For comparison purposes, the related dinuclear rhodium complex [Rh (ppy)₂(L)Rh(ppy)₂][OTf]₂ (4) was also synthesized. The x-ray molecular structure of complex 2 was reported and confirmed the formation of the target molecule. The rhodium complex 4 was found to be emissive only at low temperature; in contrast, all iridium complexes 1–3 were found to be phosphorescent in solution at 77 K and room temperature, displaying blue emissions in the range of 478–481 nm.

Sulfur-bridged chromophores for photofunctional materials: using sulfur oxidation state to tune electronic and structural properties

The use of a heteroatom, such as sulfur, as a linker or bridge, in π-conjugated materials has advantages over purely carbon-based ones due to the accessibility of higher oxidation states as a result of hypervalence. Materials containing a sulfide bridge (S) can be systemically oxidized into sulfoxides (SO) and sulfones (SO2), each of which can then influence how a material interacts with light, playing a large role in dictating the photophysical and sometimes photochemical properties. In this perspective, we summarize the progress that our group and others have made, showing how oxidation of a sulfur bridge in symmetric bichromophoric dimers and in diimine ligands can influence the excited state behavior in organic π-conjugated materials and metal complexes.

Theoretical Approach for the Luminescent Properties of Ir(III) Complexes to Produce Red-Green-Blue LEC Devices

With an appropriate mixture of cyclometalating and ancillary ligands, based on simple structures (commercial or easily synthesized), it has been possible to design a family of eight new Ir(III) complexes (1A, 1B, 2B, 2C, 3B, 3C, 3D and 3E) useful as luminescent materials in LEC devices. These complexes involved the use of phenylpyridines or fluorophenylpyridines as cyclometalating ligands and bipyridine or phenanthroline-type structures as ancillary ligands. The emitting properties have been evaluated from a theoretical approach through Density Functional Theory and Time-Dependent Density Functional Theory calculations, determining geometric parameters, frontier orbital energies, absorption and emission energies, injection and transport parameters of holes and electrons, and parameters associated with the radiative and non-radiative decays. With these complexes it was possible to obtain a wide range of emission colours, from deep red to blue (701-440 nm). Considering all the calculated parameters between all the complexes, it was identified that 1B was the best red, 2B was the best green, and 3D was the best blue emitter. Thus, with the mixture of these complexes, a dual host-guest system with 3D-1B and an RGB (red-green-blue) system with 3D-2B-1B are proposed, to produce white LECs.

Phosphorescent Ir(III) complexes derived from purine nucleobases

We report the preparation and the study of new types of neutral and cationic phosphorescent heteroleptic Ir(III) complexes derived from 6-phenylpurine nucleosides and nucleotides. Neutral complexes of general formula Ir(C^N)₂(acac) 7, and 8a-c (HC^N = 9-substituted-6-phenyl purine) are orange-red emissive upon photoexcitation, with short lifetimes and good quantum yields (0.42-0.65) in both PMMA films and 2-MeTHF at room temperature. In turn, cationic complexes [Ir(C^N)₂(dtb-bpy)][PF₆] 9, 12a and 12c (dtb-bpy = 4,4'-di-tert-butyl-2,2'-dipyridine) are yellow-green emitters with moderate quantum yields (0.24-0.32).

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S1) and excited triplet (T1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]+ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)2(N^N)]+ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N3)]+, [Cu(P)(N^N)(N)]+ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]+ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]+ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]+ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]+ and two-coordinate [Cu(N)(NHC)]+ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

The overlooked NIR luminescence of Cr(ppy)3

Cr(ppy)₃, a structural analog of the green phosphorescent Ir(ppy)₃, emits even in solution at room temperature from a weakly distorted spin-flip state at 910 nm (Hppy = 2-phenylpyridine). The low energy arises from an enhanced covalence of the Cr-C bonds as compared to Cr-N bonds. Lower temperature reduces thermally activated decay increasing the emission intensity.

Chemiluminescent 1,2-Dioxetane Iridium Complexes for Near-Infrared Oxygen Sensing

Chemiluminescent iridium-based sensors which demonstrate oxygen dependent responses have been developed. The molecular probes, named IrCL-1, IrCL-2 and IrCL-3 consist of oxygen-sensitive iridium complexes attached to a spiroadamantane 1,2 dioxetane and operate via energy transfer from the chemiexcited benzoate to the corresponding iridium(III) complex. Complexing the iridium(III) center with π-extended ligands results in emission in the biologically relevant, near-infrared (NIR) region. All probes demonstrate varying oxygen tolerance, with IrCL-1 being the most oxygen sensitive. These probes have been further utilized for in vitro ratiometric imaging of oxygen, as well as for intraperitoneal, intramuscular and intratumoral imaging in live mice. To our knowledge, these are the first iridium-based chemiluminescent probes that have been employed for in vitro ratiometric oxygen sensing, and for in vivo tumor imaging.

Bis-Heteroleptic Cationic Iridium(III) Complexes Featuring Cyclometalating 2-Phenylbenzimidazole Ligands: A Combined Experimental and Theoretical Study

In this report, we investigate a new family of cationic iridium(III) complexes featuring the cyclometalating ligand 2-phenylbenzimidazole and ancillary ligand 4,4'-dimethyl-2,2'-bipyridine. Our benchmark complex IrL¹₂ (L¹ = 2-phenylbenzimidazole) displays emission properties similar to those of the archetypical complex 2,2'-dipyridylbis(2',4'-phenylpyridine)iridium(III) in deaerated CH₃CN (Φ = 0.20, λ_em = 584 nm and Φ = 0.14, λ_em = 585 nm, respectively) but exhibits a higher photoluminescence quantum yield in deaerated CH₂Cl₂ (Φ = 0.32, λ_em = 566 nm and Φ = 0.20, λ_em = 595 nm, respectively) and especially a lower nonradiative constant (k_nr = 6.6 × 10⁵ s^-1 vs k_nr = 1.4 × 10⁶ s^-1, respectively). As a primary investigation, we explored the influence of the introduction of electron-donating and electron-withdrawing groups on the benzimidazole moiety and the synergetic effect of the substitution of the cyclometalating phenyl moiety at the para position with the same substituents. The emission energy displays very good correlation with the Hammett constants of the introduced substituents as well as with ΔE_redox values, which allow us to ascribe the phosphorescence of these series to emanate mainly from a mixed metal/ligand to ligand charge transfer triplet excited state (³M/LLCT*). Two complexes (IrL⁵₂ and IrL⁸₂) display a switch of the lowest triplet excited state from ³M/LLCT* to ligand centered (³LC*), from the less polar CH₂Cl₂ to the more polar CH₃CN. The observed results are supported by (TD)-DFT computations considering the vibrational contributions to the electronic transitions. Chromaticity diagrams based on the maximum emission wavelength of the recorded and simulated phosphorescence spectra demonstrate the strong promise of our complexes as emitting materials, together with the very good agreement between experimental and theoretical results.

Tuning the luminescence of transition metal complexes with acyclic diaminocarbene ligands

Organometallics featuring acyclic diaminocarbene ligands have recently emerged as powerful emitters for use in electroluminescent technologies.

A counterion study of a series of [Cu(P^P)(N^N)][A] compounds with bis(phosphane) and 6-methyl and 6,6'-dimethyl-substituted 2,2'-bipyridine ligands for light-emitting electrochemical cells

The syntheses and characterisations of a series of heteroleptic copper(I) compounds [Cu(POP)(Mebpy)][A], [Cu(POP)(Me2bpy)][A], [Cu(xantphos)(Mebpy)][A] and [Cu(xantphos)(Me2bpy)][A] in which [A]- is [BF4]-, [PF6]-, [BPh4]- and [BArF4]- (Mebpy = 6-methyl-2,2'-bipyridine, Me2bpy = 6,6'-dimethyl-2,2'-bipyridine, POP = oxydi(2,1-phenylene)bis(diphenylphosphane), xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane), [BArF4]- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are reported. Nine of the compounds have been characterised by single crystal X-ray crystallography, and the consequences of the different anions on the packing interactions in the solid state are discussed. The effects of the counterion on the photophysical properties of [Cu(POP)(N^N)][A] and [Cu(xantphos)(N^N)][A] (N^N = Mebpy and Me2bpy) have been investigated. In the solid-state emission spectra, the highest energy emission maxima are for [Cu(xantphos)(Mebpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BPh4] (λemmax = 520 nm) whereas the lowest energy λemmax values occur for [Cu(POP)(Mebpy)][PF6] and [Cu(POP)(Mebpy)][BPh4] (565 nm and 563 nm, respectively). Photoluminescence quantum yields (PLQYs) are noticeably affected by the counterion; in the [Cu(xantphos)(Me2bpy)][A] series, solid-state PLQY values decrease from 62% for [PF6]-, to 44%, 35% and 27% for [BF4]-, [BPh4]- and [BArF4]-, respectively. This latter series of compounds was used as active electroluminescent materials on light-emitting electrochemical cells (LECs). The luminophores were mixed with ionic liquids (ILs) [EMIM][A] ([EMIM]+ = [1-ethyl-3-methylimidazolium]+) containing the same or different counterions than the copper(I) complex. LECs containing [Cu(xantphos)(Me2bpy)][BPh4] and [Cu(xantphos)(Me2bpy)][BArF4] failed to turn on under the LEC operating conditions, whereas those with the smaller [PF6]- or [BF4]- counterions had rapid turn-on times and exhibited maximum luminances of 173 and 137 cd m-2 and current efficiencies of 3.5 and 2.6 cd A-1, respectively, when the IL contained the same counterion as the luminophore. Mixing the counterions ([PF6]- and [BF4]-) of the active complex and the IL led to a reduction in all the figures of merit of the LECs.

Study of a phosphorescent cationic iridium(III) complex displaying a blue-shift in crystals

We report the synthesis and the characterization of a new cationic iridium(III) complex featuring two 1-(p-methoxyphenyl)-5-methoxybenzimidazole cyclometallating ligands and a dimethylbipyridine ancillary ligand. The complex has been fully characterized by 1D and 2D NMR (¹H, ¹³C, ¹⁹F and ³¹P), elemental analysis and high-resolution mass spectrometry (HRMS). The photoluminescence studies performed in a solution, on amorphous powder and on crystals revealed an unexpected behavior. Indeed, the emission spectra observed in both solution (CH₂Cl₂) and amorphous powder samples are centered at around 580 nm, whereas in crystals the emission displays a large hypsochromic shift of ∼800 cm^-1 (λ_em = 558 nm). X-ray diffraction experiments, photophysical studies and DFT calculations allow for rationalizing the hypsochromic shift.

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH)

The three complexes [M(Me₂dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me₂dpb^-) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal d_yz, phenyl C4, C2, C1, and C6, and Cl p_z orbitals were found. As expected, the d_z² (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding d_x²_-y² orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced d_z²-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the d_z² orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(d_yz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me₂dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called "dark", i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the "dark" states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

Towards rainbow photo/electro-luminescence in copper(i) complexes with the versatile bridged bis-pyridyl ancillary ligand

The synthesis and characterization of a family of copper(i) complexes bearing a bridged bis-pyridyl ancillary ligand is reported, highlighting how the bridge nature impacts the photo- and electro-luminescent behaviours within the family. In particular, the phosphonium bridge led to copper(i) complexes featuring good electrochemical stability and high ionic conductivity, as well as a stark blue-to-orange luminescence shift compared to the others. This resulted in high performance light-emitting electrochemical cells reaching stabilities of 10 mJ at ca. 40 cd m^-2 that are one order of magnitude higher than those of the other complexes. Overall, this work sheds light onto the crucial role of the bridge nature of the bis-pyridyl ancillary ligand on the photophysical features, film forming and, in turn, on the final device performances.

Photophysics and Excited State Dynamics of Cyclometalated [M(Phbpy)(CN)] (M = Ni, Pd, Pt) Complexes: A Theoretical and Experimental Study

Cyclometalated complexes [M(Phbpy)(CN)] (HPhbpy = 6-phenyl-2,2'-bipyridine) of the group 10 metals (Ni, Pd, and Pt) bearing a carbanionic ^-C^∧N^∧N pincer ligand were synthesized and studied in a combined experimental and computational DFT approach. All three complexes were crystallographically characterized showing closely packed dimers with head-to-tail stacking and short metal-metal contacts in the solid state. The computational models for geometries, excited states, and electronic transitions addressed both monomeric (Ni-mono, Pd-mono, and Pt-mono) and dimeric (Ni-dim, Pd-dim, and Pt-dim) entities. Photophysical properties and excited state dynamics of all title complexes were investigated in solution and in the solid at 298 and 77 K. [Ni(Phbpy)(CN)] and [Pd(Phbpy)(CN)] are virtually nonemissive in solution at 298 K, whereas [Pt(Phbpy)(CN)] shows phosphorescence in CH₂Cl₂ (DCM) solution (λ_em = 562 nm) stemming from a mixed ³MLCT/ILCT (metal-to-ligand charge transfer/intraligand charge transfer) state. At 77 K in a glassy frozen DCM:MeOH matrix, [Pd(Phbpy)(CN)] shows a remarkable emission (λ_em = 571 nm) with a photoluminescence quantum yield reaching almost unity, whereas [Ni(Phbpy)(CN)] is again nonemissive. Calculations on the monomeric models M-mono show that low-lying metal-centered states (MC, i.e., d-d* configuration) with dissociative character quench the photoluminescence. In the solid state, the complexes [M(Phbpy)(CN)] show defined photoluminescence bands (λ_em = 561 nm for Pd and 701 nm for Pt). Calculations on the dimeric models M-dim shows that the axial M···M interactions alter the photophysical properties of Pd-dim and Pt-dim toward MMLCT (metal-metal-to-ligand charge transfer) excited states with Pd-dim showing temperature-dependent emission lifetimes, suggesting thermally activated delayed fluorescence, whereas Pt-dim displayed phosphorescence with excimeric character. The metal-metal interactions were analyzed in detail with the quantum theory of atoms in molecules approach.

All-Solution-Processed Multilayered White Polymer Light-Emitting Diodes (WPLEDs) Based on Cross-Linked [Ir(4-vb-PBI)2(acac)]

All-solution-processed multilayered white polymer light-emitting diodes (WPLEDs) are promising candidates for low-cost and large-area flexible full-color flat-panel displays and solid-state lighting. However, it is still challenging to improve their performance. In this work, based on an elegant strategy of orthogonal materials, the utilization of the cross-linked Ir³⁺ polymer film poly(NVK-co-[Ir(4-vb-PBI)₂(acac)]-co-NVK) (NVK = N-vinyl-carbazole; 4-vb-HPBI = 1-(4-vinylbenzyl)-2-phenyl-1H-benzo[d]imidazole; and Hacac = acetylacetone) as the emitting layer (EML) between a hydrophilic polymer film poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole injection layer (HIL) and a hydrophobic polymer film poly(vinyl-PBD) (vinyl-PBD = 2-(4-(tert-butyl)phenyl)-5-(4'-vinyl-[1,1'-biphenyl]-4-yl)-2,5-dihydro-1,3,4-oxadiazole) as the electron transport layer (ETL) led to the successful fabrication of reliable all-solution-processed multilayered WPLEDs. The device exhibits a η_CE^Max of 18.19 cd/A, a η_PE^Max of 8.16 lm/W, and a η_EQE^Max of 9.32% with stable white light (Commission International De L'Eclairage (CIE) coordinates x = 0.269-0.283, y = 0.317-0.330; corrected color temperatures (CCTs) of 7237-8199 K, and CRIs (color rendering indices) of 63-72) under a wide applied-voltage range. Its high performance, especially with record efficiencies among those of reported all-solution-processed WPLEDs, renders cross-linked Ir³⁺ polymers a new platform to all-solution-processed multilayered WPLEDs.

Synthesis, Study, and Application of Pd(II) Hydrazone Complexes as the Emissive Components of Single-Layer Light-Emitting Electrochemical Cells

For the first time, square planar Pd(II) complexes of hydrazone ligands have been investigated as the emissive components of light-emitting electrochemical cells (LECs). The neutral transition metal complex, [Pd(L¹)₂]·2CH₃OH (1), (HL¹ = (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinhydrazide), was prepared and structurally characterized. Complex 1 displays quasireversible redox properties and is emissive at room temperature in solution with a λ_max of 590 nm. As a result, it was subsequently employed as the emissive material of a single-layer LEC with configuration FTO/1/Ga/In, where studies reveal that it has a yellow color with CIE(x, y) = (0.33, 0.55), a luminance of 134 cd cm^-2, and a turn-on voltage of 3.5 V. Protonation of the pendant pyridine nitrogen atoms of L¹ afforded a second ionic complex [Pd(L¹H)₂](ClO₄)₂ (2) which is also emissive at room temperature with a λ_max of 611 nm, resulting in an orange LEC with CIE(x, y) = (0.43, 0.53). The presence of mobile anions and cations in the second inorganic transition metal complex resulted in more efficient charge injection and transport which significantly improved the luminance and turn-on voltage of the device to 188.6 cd cm^-2 and 3 V, respectively. This study establishes Pd(II) hydrazone complexes as a new class of materials whose emissive properties can be chemically tuned and provides proof-of-concept for their use in LECs, opening up exciting new avenues for potential applications in the field of solid state lighting.

A new application of terahertz time-domain absorption spectra in luminescent complexes: characterization of the C-Hπ weak interactions in Cu(I) complexes

Six Cu(i) complexes, [Cu(2,3-f)(bdppmapy)]BF4 (1), [Cu(2,3-f)(bdppmapy)]ClO4 (2), [Cu(2,3-f)(bdppmapy)]CF3SO3 (3), [Cu(imidazo[4,5-f])(bdppmapy)]BF4 (4), [Cu(imidazo[4,5-f])(bdppmapy)]ClO4 (5), and [Cu(imidazo[4,5-f])(bdppmapy)]CF3SO3·MeOH (6·MeOH) (bdppmapy = N,N-bis[(diphenylphosphino)methyl]-2-pyridinamine, 2,3-f = pyrazine[2,3-f][1,10]-phenanthroline, and imidazo[4,5-f] = 1H-imidazo[4,5-f][1,10]-phenanthroline), have been synthesized to explore the effects of counteranions on their crystal structures, photophysical properties, and terahertz (THz) spectra. Time-dependent density functional theory (TD-DFT) shows that the luminescence performance of these complexes is attributed to the metal-to-ligand charge transfer (MLCT) in combination with ligand-to-ligand charge transfer (LLCT). In complexes 1-3, the characteristic peak at 1.4 THz is mainly related to the C-Hπ interaction formed by the H atom on the 4#/5# position of 2,3-f and the benzene ring from the bdppmapy on the adjacent asymmetric unit. The common C-Hπ interaction enhances the rigidity of the structure and has non-negligible influence on the photoluminescence quantum yields (PLQYs): the stronger the C-Hπ interaction is, the higher the quantum yield (QY) is. In complexes 4-6, similar absorption peaks (1.10-1.30 THz) are mainly related to the C-Hπ interactions, and strong absorption peaks (1.50-1.90 THz) are affected by the typical hydrogen bonds N-HF/O and O-HO. These results show that some weak interactions can be characterized by THz time-domain spectroscopy (THz-TDS). So, the THz spectroscopy method would make it possible to tune some of the weak interactions in complex structures to regulate the luminescence of materials.

Bis-cyclometalated iridium(iii) complexes with terpyridine analogues: syntheses, structures, spectroscopy and computational studies

Two ligands based upon a 2,6-disubstituted pyridine bridge introduce bis-quinoxalinyl units in a fashion that yields analogues to the archetypal terdentate ligand, 2,2′:6′,2′′-terpyridine. The ligands were synthesised from the key intermediate 2,6-bis(bromoacetyl)pyridine: a new, high-yielding route is described for this reagent. Two ligand variants (differentiated by H/Me substituents on the quinoxaline ring) were explored as coordinating moieties for iridium(iii) in the development of luminescent complexes. Computational studies (DFT approaches employing B3LYP, B3LYP/LANL2DZ, and M062X/LANL2DZ levels) were used to investigate the geometric and coordination mode preferences of the new ligands and two possibilities arose from theoretical investigations: [Ir(N^N^N)₂]³⁺ and [Ir(N^N^C)₂]⁺, with the former predicted to be more energetically favourable. Upon synthesis and isolation of the Ir(iii) complexes, X-ray crystallographic studies revealed coordination spheres that were cyclometalated, the structures both showing a [Ir(N^N^C)₂]PF₆ arrangement. Further spectroscopic characterization via NMR confirmed the ligand arrangements in the complexes, and photophysical studies, supported by DFT, showed that a mixture of metal-to-ligand charge transfer (MLCT) and intra-ligand charge transfer (ILCT) character is likely to contribute to the emission features of the complexes, which phosphoresce orange-red (λ_em = 580–618 nm). The emission wavelength was influenced by the substituents on the quinoxaline ring (H vs. Me), thereby implying further tuneability is possible with future ligand iterations.

Bis-terdentate Ir(iii) complexes incorporate two cyclometalated N^N^C bis-quinoxalinyl type ligands derived from the condensation of 2,6-bis(bromoacetyl)pyridine and different o-phenylenediamines.

Cationic iridium complexes with an alkyl-linked bulky group at the cyclometalating ligand: synthesis, characterization, and suppression of phosphorescence concentration-quenching

Cationic iridium complexes with a bulky 2,2-diphenylethyl group at the cyclometalating ligand show remarkably suppressed phosphorescence concentration-quenching.

Green to blue-green-emitting cationic iridium complexes with a CF3-substituted phenyl-triazole type cyclometalating ligand: synthesis, characterization and their use for efficient light-emitting electrochemical cells

Green to blue-green-emitting cationic iridium complexes free of sp2 C-F bonds, namely [Ir(CF3-dPhTAZ)2(bpy)]PF6 (1), [Ir(CF3-dPhTAZ)2(dmebpy)]PF6 (2) and [Ir(CF3-dPhTAZ)2(phpyim)]PF6 (3), have been designed and synthesized with 3,4-diphenyl-5-(trifluoromethyl)-4H-1,2,4-triazole (CF3-dPhTAZ) as the cyclometalating ligand (C^N) and 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmebpy) or 2-(1-phenyl-1H-imidazol-2-yl)pyridine (phpyim) as the ancillary ligand (N^N). In CH3CN solution, complexes 1-3 afford green to blue-green emission centered at 521, 508 and 498 nm, respectively. The electron-withdrawing CF3 group attached at the triazole ring in CF3-dPhTAZ largely blue-shifts (by over 20 nm) the emission of the complex through stabilizing the highest occupied molecular orbital. In doped films, the complexes afford sky-blue emission with near-unity phosphorescent efficiencies. In neat films, the complexes show largely suppressed phosphorescence concentration-quenching, with phosphorescent efficiencies of up to 0.66. Theoretical calculations reveal that the emission of the complexes can arise from either charge-transfer (Ir → C^N/C^N → N^N) or C^N/N^N-centered 3π-π* states, depending on the local environment of the complexes. Solid-state light-emitting electrochemical cells (LECs) based on the complexes afford green to blue-green electroluminescence centered at 525, 517 and 509 nm, respectively, with high current efficiencies of up to 35.1 cd A-1. The work reveals that CF3-dPhTAZ is a promising C^N ligand free of sp2 C-F bonds for constructing efficient cationic iridium complexes with blue-shifted emission.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based ¹π,π* transitions and broad structureless charge-transfer (¹CT) absorption bands at 400-700 nm. The energies or intensities of these ¹CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)₂⁺ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) ³MLCT (metal-to-ligand charge-transfer)/³LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR ³ILCT (intraligand charge-transfer)/³LMCT (ligand-to-metal charge-transfer) emission in CH₂Cl₂. In CH₃CN, 1-3 displayed an additional emission band at ca. 590 nm (³ILCT/³LMCT/³MLCT/³π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)₂⁺ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH₂Cl₂/ethyl acetate or CH₂Cl₂/hexane solutions of these ligands.

Electrochemiluminescence and Photoluminescence of Carbon Quantum Dots Controlled by Aggregation-Induced Emission, Aggregation-Caused Quenching, and Interfacial Reactions

Carbon quantum dots (CQDs) show promise in optoelectronics as a light emitter due to simple synthesis, biocompatibility and strong tunable light emissions. However, CQDs commonly suffer from aggregation caused quenching (ACQ), inhibiting the full potential of these light emitters. Studies into different ideal light emitters have shown enhancements when converting common ACQ effects to aggregation induced emission (AIE) effects. We report CQD synthesis using citric acid and high/low thiourea concentrations, or sample 2/1. These two CQDs exhibited AIE and ACQ PL effects, respectively. CQD characterizations and photoluminescence interrogations of CQD films and solutions revealed that these unique emission mechanisms likely arose from different S incorporations into the CQDs. Furthermore, it was discovered that sample 2 emitted electrochemiluminescence (ECL) more intensely than sample 1 in a homogenous solution with S₂O₈^2- as a coreactant, due to aggregation and interactions of CQD species in solution. Very interestingly, sample 1's CQD film|S₂O₈^2- system achieved an ECL efficiency of 26% and emitted roughly 26 times more efficiently than sample 2 in the same conditions. Predominant interfacial reactions and surface state emission produced intense white light with a correlated color temperature of 2000 K. Spooling ECL spectroscopy was utilized to investigate emission mechanisms. Sample 2's CQD film|TPrA system had four times higher ECL intensity than that of sample 1, most likely due to π-cation interactions leading to a strong CQD^•+ stability, thereby, enhancing ECL. It is anticipated that ECL enhancement of CQD films or solutions by means of AIE will lead to wide CQD optoelectronic applications.

Cationic Iridium Complexes with 3,4,5-Triphenyl-4H-1,2,4-Triazole Type Cyclometalating Ligands: Synthesis, Characterizations, and Their Use in Light-Emitting Electrochemical Cells

Cationic iridium complexes that show blue-shifted emission and high phosphorescent efficiency have been pursued for their optoelectronic applications. Five cationic iridium complexes with 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ) type cyclometalating ligands (C^N) and 2,2'-bipyridine or 2-(pyridin-2-yl)-1H-benzo[d]imidazole type ancillary ligands (N^N) have been designed and synthesized. Their structures have been confirmed by X-ray crystallography, and their photophysical and electrochemical properties have been comprehensively characterized. In solution and thin films, the complexes afford efficient yellow to blue-green emission. The highest occupied molecular orbitals (HOMOs) of these complexes are delocalized over the C^N ligand and the iridium ion, and compared with the conventional 2-phenylpyridine (Hppy) ligand, the tPhTAZ ligand largely shifts the emission of the complex toward blue by over 40 nm through stabilizing the HOMO. Moreover, the peripheral phenyl rings in tPhTAZ provide steric hindrance to the complexes, which suppresses phosphorescence concentration-quenching of the complexes, leading to high luminescent efficiencies in neat films. Theoretical calculations have shown that the emission of the complexes originates from either the charge-transfer state (Ir/C^N → N^N) or the C^N/N^N-centered ³π-π* state, depending on the local surrounding of the complex. The complexes exhibit good electrochemical stability with reversible oxidation and reduction processes in solution. Solid-state light emitting electrochemical cells (LECs) using the complexes afford yellow to blue-green emission, with peak current efficiencies of up to 34.7 cd A^-1 and maximum brightness of up to 256 cd m^-2 at 3.0 V, which are among the highest for LECs based on cationic iridium complexes reported so far, indicating the great potential for the use of tPhTAZ-type C^N ligands in construction of cationic iridium complexes for LEC applications.

Sky-blue-emitting cationic iridium complexes with carbazole-type counter-anions and their use for efficient solution-processed organic light-emitting diodes

The control of counter-anion is a facile approach to tune the overall properties of cationic iridium complexes for optoelectronic applications. Here, we report for the first time the use of electron-rich carbazole-type counter-anions in cationic iridium complexes and the use of such complexes as dopants in solution-processed organic light-emitting diodes (OLEDs). PF6-, 4-(9H-carbazol-9-yl)benzenesulfonate (CAZ-SO3-), and 4-(9'H-[9,3':6',9''-tercarbazol]-9'-yl)benzenesulfonate (TCAZ-SO3-) have been employed as the counter-anions of sky-blue-emitting complexes R, 1 and 2, respectively. The carbazole-type counter-anions do not largely disturb the phosphorescence of the cations and the complexes show similar emission properties in solution and films. The close proximity of the carbazole-type anions to the phosphorescent cations allows efficient energy-transfer from the former to the latter in films. When used as dopants in solution-processed OLEDs, complexes 1 and 2 show higher performances than complex R because the hole-trapping effects of the carbazole-type counter-anions largely improve the carrier-recombination balance in the emissive layers. In particular, the double-layer sky-blue device based on complex 2 with strong hole-trapping by TCAZ-SO3- affords a high peak current efficiency of 27.1 cd A-1. The work reveals that electron-rich carbazole-type anions are promising counter-anions for cationic iridium complexes toward optoelectronic applications.

Bright, stable, and efficient red light-emitting electrochemical cells using contorted nanographenes

This work rationalizes, for the first time, the electroluminescent behavior of a representative red-emitting contorted nanographene -i.e., hexabenzoovalene derivative - in small molecule light-emitting electrochemical cells (SM-LECs). This new emitter provides devices with irradiances of ca. 220 μW cm^-2 (242 cd m^-2), external quantum efficiencies (EQE) of 0.78% (<25% loss of the maximum theoretical EQE), and stabilities over 200 h. Upon optimizing the device architecture, the stability increased up to 3600 h (measured) and 13 000 h (extrapolated) at a high brightness of ca. 30 μW cm^-2 (34 cd m^-2). This represents a record stability at a high brightness level compared to the state-of-the-art SM-LECs (1000 h at 0.3 μW cm^-2). In addition, we rationalized one of the very rare LEC examples in which the changes of the electroluminescence band shape relates to the dependence of the relative intensity of the vibrational peaks with electric field, as corroborated by dynamic electrochemical impedance spectroscopy assays. Nevertheless, this exclusive electroluminescence behavior does not affect the device color, realizing one of the most stable, bright, and efficient red-emitting SM-LECs up to date.