Near-IR Emitting Iridium(III) Complexes with Heteroaromatic β-Diketonate Ancillary Ligands for Efficient Solution-Processed OLEDs: Structure-Property Correlations

Azaacene containing iridium(III) phosphors: elaboration of the π-conjugation effect and application in highly efficient solution-processed near-infrared OLEDs

Azaacenes have attracted wide research interest due to their tremendous potential in organic electronics. However, near-infrared (NIR) light-emitting iridium(III) phosphors bearing azaacene derivatives are rarely investigated. In this contribution, two solution-processable heteroleptic iridium(III) complexes, namely DBPzIr and PPzIr, are rationally designed and synthesized, and they contain a rigid phenanthrene- or pyrene-fused diazaacene core and two peripheral groups of 4-tert-butyl-phenyl attached at the 12,13-positions in the core, respectively. The effects of the diazaacene core and appending groups on the optoelectronic properties of both complexes are systematically investigated. A dramatically red-shifted NIR emission peak at 789 nm with a photoluminescence quantum yield (PLQY) of 14% is observed in PPzIr compared with the 746 nm emission with a PLQY of 40% in DBPzIr. Taking advantage of their photophysical properties, the solution-processed device doped with DBPzIr achieves a maximum external quantum efficiency (EQEmax) of 8.00% with a radiance of 54 866 mW Sr-1 m-2 at 716 nm and the device doped with PPzIr exhibits a significantly red-shifted emission at 772 nm with an EQEmax of 3.53%. The achieved device performance is among the best values in the reported NIR-OLEDs based on iridium(III) complexes via a solution process at the same color gamut. Our study indicates that the reasonable collocation of the rigid diazaacene chelating core and flexible peripheral groups in the iridium(III) complex is of great significance in designing highly efficient NIR emitters.

Unprecedented hetero-coordinated Ir(C^N)2tmd complexes containing both five- and six-membered Ir-(C^N) rings based on phenanthrylpyridine ligands: syntheses, crystal structures and photophysical properties

By using 2-(9-phenanthryl)pyridine (phpyr) and its derivatives as cyclometalated ligands, we synthesized a set of isomeric red-emitting complexes Ir(phpyr-R)2tmd (R = -H, -CF3, -F and -CH3, tmd = 2,2,6,6-tetramethylheptane-3,5-dione) with different coordinated modes, including bis-five-membered and five- + six-membered Ir-(C^N) ring chelating modes. The latter are the first examples of hetero-coordinated Ir(C^N)2(L^X)-type complexes containing both five- and six-membered Ir-(C^N) metallocycles. Their coordination geometries were distinctly determined using X-ray crystallographic analysis. Compared to typical bis-five-membered ring-chelated complexes, these novel hetero-coordinated isomers show bathochromic emission and lower quantum yields. On careful analysis of their electrochemical behavior and DFT calculations, it has been found that the regulatory effects of the solitary six-membered metallocycles in Ir(phpyr-R)2tmd could not only stabilize the LUMO but also destabilize the HOMO, leading to a narrower energy gap. More importantly, DFT calculations of the relative energies of these isomeric complexes demonstrated that bis-five-membered and five- + six-membered chelating modes are more stable compared to bis-six-membered rings, consistent with experiments. This work provides guidance for the structural design of Ir(C^N)2(L^X)-type complexes.

One-Pot Synthesis of Acetylacetonate-Based Isomeric Phosphorescent Cyclometalated Iridium(III) Complexes via Random Cyclometalation: A Strategy for Excited-State Manipulation

The excited-state manipulation of the phosphorescent iridium(III) complexes plays a vital role in their photofunctional applications. The development of the molecular design strategy promotes the creative findings of novel iridium(III) complexes. The current molecular design strategies for iridium(III) complexes mainly depend on the selective cyclometalation of the ligands with the iridium(III) ion, which is governed by the steric hindrance of the ligand during the cyclometalation. Herein, a new molecular design strategy (i.e., random cyclometalation strategy) is proposed for the effective excited-state manipulation of phosphorescent cyclometalated iridium(III) complexes. Two series of new and separable methoxyl-functionalized isomeric iridium(III) complexes are accessed by a one-pot synthesis via random cyclometalation, resulting in a dramatic tuning of the phosphorescence peak wavelength (∼57 nm) and electrochemical properties attributed to the high sensitivity of their excited states to the position of the methoxyl group. These iridium(III) complexes show intense phosphorescence ranging from the yellow (567 nm) to the deep-red (634 nm) color with high photoluminescence quantum yields of up to 0.99. Two deep-red emissive iridium(III) complexes with short decay lifetimes are further utilized as triplet emitters to afford efficient solution-processed electroluminescence with reduced efficiency roll-offs.

Cell proliferation effect of deep-penetrating microcavity tandem NIR OLEDs with therapeutic trend analysis

Long wavelengths that can deeply penetrate into human skin are required to maximize therapeutic effects. Hence, various studies on near-infrared organic light-emitting diodes (NIR OLEDs) have been conducted, and they have been applied in numerous fields. This paper presents a microcavity tandem NIR OLED with narrow full-width half-maximum (FWHM) (34 nm), high radiant emittance (> 5 mW/cm²) and external quantum efficiency (EQE) (19.17%). Only a few papers have reported on biomedical applications using the entire wavelength range of the visible and NIR regions. In particular, no biomedical application studies have been reported in the full wavelength region using OLEDs. Therefore, it is worth researching the therapeutic effects of using OLED, a next-generation light source, and analyzing trends for cell proliferation effects. Cell proliferation effects were observed in certain wavelength regions when B, G, R, and NIR OLEDs were used to irradiate human fibroblasts. The results of an in-vitro experiment indicated that the overall tendency of wavelengths is similar to that of the cytochrome c oxidase absorption spectrum of human fibroblasts. This is the first paper to report trends in the cell proliferation effects in all wavelength regions using OLEDs.

Effects of Ancillary Ligands on Deep Red to Near-Infrared Cyclometalated Iridium Complexes

The design of organometallic compounds with efficient phosphorescence in the deep red to near-infrared portions of the spectrum is a long-standing fundamental challenge. Here we describe a series of heteroleptic bis-cyclometalated iridium complexes with phosphorescence in these low-energy regions of the spectrum. The cyclometalating ligands in this study feature a metalated benzothiophene aryl group substituted with a quinoline, isoquinoline, or phenanthridine heterocycle. Increasing the conjugation on the heterocycle stabilizes the ligand-centered LUMO, decreases the HOMO-LUMO gap, and enables phosphorescence to occur at long wavelengths. These cyclometalating ligands are paired with a variety of electron-rich ancillary ligands, such as dithiocarbamate (dipdtc), β-ketoiminate (acNac), β-diketiminate (NacNac), amidinate (dipba), and hexahydropyrimidopyrimidine (hpp), some of which have significant influences on the phosphorescence wavelength and excited-state dynamics. The syntheses of seven compounds in this series are described, three of which are structurally validated by single-crystal X-ray diffraction. Cyclic voltammetry reveals the effects of ligand modification on the frontier orbital energies. The photophysical properties of all compounds are thoroughly characterized by UV-vis absorption spectroscopy and steady-state photoluminescence at room-temperature and 77 K. Photoluminescence quantum yields and lifetimes of all compounds are reported.

Recent Progress in Near-Infrared Organic Electroluminescent Materials

Near-infrared (NIR) refers to the section of the spectrum from 650 to 2500 nm. NIR luminescent materials are widely employed in organic light-emitting diodes (OLEDs), fiber optic communication, sensing, biological detection, and medical imaging. This paper reviews organic NIR electroluminescent materials, including organic NIR electrofluorescent materials and organic NIR electrophosphorescent materials that have been investigated in the past 6 years. Small-molecule, polymer NIR fluorescent materials and platinum(II) and iridium(III) complex NIR phosphorescent materials are described, and the limitations of the development of NIR luminescent materials and future prospects are discussed.

Iridium(III) Complexes with [-2, -1, 0] Charged Ligand Realized Deep-Red/Near-Infrared Phosphorescent Emission

A series of [-2, -1, 0] charged-ligand based iridium(III) complexes of [Ir(bph)(bpy)(acac)] (1), [Ir(bph)(2MeO-bpy)(acac)] (2), [Ir(bph)(2CF₃ -bpy)(acac)] (3), [Ir(bph)(bpy)(2^t Bu-acac)] (4) and [Ir(bph)(bpy)(CF₃ -acac)] (5), which using biphenyl as dianionic ligand [-2], acetylacetone (or its derivatives) as monoanionic ligand [-1], and 2,2'-bipyridine (or its derivatives) as neutral ligand [0] were designed and synthesized. The chemical structures were well characterized. All of the ligands have simple chemical structures, thus further making the complexes have excellent thermal stability and are easy to sublimate and purify. Phosphorescent characteristics with short emission lifetime were demonstrated for these emitters. Notably, all of the complexes exhibit remarkable deep red/near infrared emission, which is quite different from the reported [-1, -1, -1] charged-ligand based iridium(III) complexes. The photophysical properties of these complexes are regularly improved by introducing electron-donating or -withdrawing groups into [-1] or [0] charged-ligand. The related organic light-emitting diodes exhibited deep red/near infrared emission with acceptable external quantum efficiency and low turn-on voltage (<2.6 V). This work provides a new idea for the construction of new type phosphorescent iridium(III) emitters with different valence states of [-2, -1, 0] charged ligands, thus offering new opportunities and challenges for their optoelectronic applications.

Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence

A molecular design to high-performance red and near-infrared (NIR) organic light-emitting diodes (OLEDs) emitters remains demanding. Herein a series of dinuclear platinum(II) complexes featuring strong intramolecular Pt⋅⋅⋅Pt and π-π interactions has been developed by using N-deprotonated α-carboline as a bridging ligand. The complexes in doped thin films exhibit efficient red to NIR emission from short-lived (τ=0.9-2.1 μs) triplet metal-metal-to-ligand charge transfer (³ MMLCT) excited states. Red OLEDs demonstrate high maximum external quantum efficiencies (EQEs) of up to 23.3 % among the best Pt^II -complex-doped devices. The maximum EQE of 15.0 % and radiance of 285 W sr^-1  m^-2 for NIR OLEDs (λ_EL =725 nm) are unprecedented for devices based on discrete molecular emitters. Both red and NIR devices show very small efficiency roll-off at high brightness. Appealing operational lifetimes have also been revealed for the devices. This work sheds light on the potential of intramolecular metallophilicity for long-wavelength molecular emitters and electroluminescence.

Near-infrared emitting iridium complexes: Molecular design, photophysical properties, and related applications

Organic light-emitting diodes (OLEDs) have become popular displays from small screens of wearables to large screens of televisions. In those active-matrix OLED displays, phosphorescent iridium(III) complexes serve as the indispensable green and red emitters because of their high luminous efficiency, excellent color tunability, and high durability. However, in contrast to their brilliant success in the visible region, iridium complexes are still underperforming in the near-infrared (NIR) region, particular in poor luminous efficiency according to the energy gap law. In this review, we first recall the basic theory of phosphorescent iridium complexes and explore their full potential for NIR emission. Next, the recent advances in NIR-emitting iridium complexes are summarized by highlighting design strategies and the structure-properties relationship. Some important implications for controlling photophysical properties are revealed. Moreover, as promising applications, NIR-OLEDs and bio-imaging based on NIR Ir(III) complexes are also presented. Finally, challenges and opportunities for NIR-emitting iridium complexes are envisioned.

Iridium(III) complexes with 1-phenylisoquinoline-4-carbonitrile units for efficient NIR organic light-emitting diodes

Achieving the high external quantum efficiency (EQE) of near-infrared (NIR) emission in iridium(III) complexes still remains a challenge owing to their unsteady excited states which easily decay to the ground states through the nonradiative pathways. Herein, three Ir(III) phosphors in which the cyclometalated ligand 1-phenylisoquinoline-4-carbonitrile (piq-CN) is functionalized with the cyano, tert-butyl, and dimethyl groups are developed (CN-CNIr, Bu-CNIr, and DM-CNIr, respectively). Three simple synthetic steps can afford this class of deep red to NIR Ir(III) emitters. The organic light-emitting diodes (OLEDs) based on Bu-CNIr and DM-CNIr attain the maximum EQEs of 7.1% and 7.2% with the emission peaks at 695 and 714 nm, respectively. This strategy using substituted piq-CN derivatives as the cyclometalated ligands can offer an effective approach to promote the radiative rate of NIR-emitting Ir(III) materials. An insight into how the electron-withdrawing and electron-donating substituents on ligands influence the optoelectronic properties of their Ir(III) complexes is also provided.

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Efficient NIR-emitting Ir(III) phosphors are attained via three synthetic steps

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Cyano and alkyl groups exert different effects on the properties of complexes

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NIR OLEDs exhibit a maximum EQE of 7.2% with an emission peak at 714 nm

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High current densities are recorded as peak EQEs are dropped to their half values

Non-toxic near-infrared light-emitting diodes

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.

Sterically hindered phenanthroimidazole ligands drive the structural flexibility and facile ligand exchange in cyclometalated iridium(III) complexes

A series of bis-cyclometalated iridium(iii) complexes with 2-arylphenanthroimidazole "antenna" ligands containing electron-donor or withdrawing substituents and a more flexible ancillary aromatic β-diketone bearing the "anchoring" carboxymethyl function has been prepared. Thorough X-ray study of the complexes revealed significant structural strains caused by bulky cyclometalated 2-arylphenanthroimidazoles resulting in dramatic distortions of the iridium octahedron and even in twist of the phenanthrene fragment. The crystal data were corroborated by gas-phase DFT calculations whereby the geometry of the complexes was distorted in the same way. While redox potentials, absorption and emission maxima of the complexes displayed expected change upon the variation of the electron-donating ability of the cyclometalated ligands, the complexes readily exchanged the bidentate ancillary ligand in the presence of a negligible amount of protons that was inspected in solution by UV-Vis spectroscopy. Moreover, after hydrolysis of the carboxymethyl group the resulting complexes readily react with the surface of titanium dioxide giving unique binuclear structures in which the deprotonated carboxy group of the coordinated β-diketonate binds the second bis-cyclometalated unit by forming a four-membered metallacycle. Though the enhanced reactivity of the complexes is contrary to the common idea of the high inertness of iridium(iii) compounds it can be seen as a consequence of the interplay between the steric hindrance induced by the ligands and the strong preference of the iridium(iii) ion for octahedral geometry. This study demonstrates that the use of bulky ligands provides access to light-harvesting iridium(iii) complexes with required extent of lability which may be promising as photocatalysts and biologically active molecules.

An Effective Approach to Obtain Near-Infrared Emission from Binuclear Platinum(II) Complexes Involving Thiophenpyridine-Isoquinoline Bridging Ligand in Solution-Processed OLEDs

Bimetallic complexes have become an emerging hot topic in field of luminous applications in recent years. Unlike the traditional modification on a cyclometalated ligand, grafting an additional metal ion provides a novel approach to tune molecular conjugation as well as the spin orbital coupling (SOC). Herein, we demonstrate a new kind of binuclear platinum(II) complex Pt-3 that possesses an asymmetric thiophenpyridine-isoquinoline bridging ligand. Compared to its mononuclear analogues of Pt-1 and Pt-2, an extremely large redshift emission from 576 and 618 nm to 721 nm was observed in solution. Binding of two metal ions helps to enhance molecular planarity, extend conjugation and suppress excited state distortion. However, their quantum yields tend to remarkably decrease with increasing red-shift emission as following the "energy gap law". The relatively larger HOMO/LUMO separation that induced by the second platinum ion also decreases the oscillator strength at the lowest singlet state, and goes against the fast radiative decay process. Solution-processed organic light-emitting diodes (OLEDs) based on Pt-1, Pt-2 and Pt-3 achieved external quantum efficiencies (EQEs) and luminance/radiant emittance of 13.6% and 13640 cd/m² , 3.5% and 3754 cd/m² , 0.9% and 7981 mW/Sr/m² with the corresponding electroluminescent (EL) emission peaked at 580 nm, 625 nm and 708 nm, respectively. This work emphasizes the complement argument of the commonly largely reported symmetric binuclear configurations, and provides a new view to photophysical mechanism and design strategies for bimetallic species.

Near-infrared cyclometalated iridium(iii) complexes with bipolar features for efficient OLEDs via solution-processing

A novel bipolar NIR iridium(iii) complex (CH3OTPA-BTz-Iq)2Ir(pic-OXD) with both a hole transporting (HT) triphenylamine (TPA) group and an electron transporting (ET) oxadiazole (OXD) group was designed and synthesized. It was observed that the incorporation of OXD and TPA into the ligand (CH3OTPA-BTz-Iq)2Ir(pic-OXD) improved the optophysical and electroluminescence performance in comparison with the parent iridium(iii) complex (CH3OTPA-BTz-Iq)2Irpic. In (CH3OTPA-BTz-Iq)2Ir(pic-OXD)-based OLEDs, a maximum external quantum efficiency (EQEmax) of 1.15% at 716 nm was obtained, which is much superior than that of the (CH3OTPA-BTz-Iq)2Irpic-based OLEDs (0.41% at 723 nm).

More efficient spin-orbit coupling: adjusting the ligand field strength to the second metal ion in asymmetric binuclear platinum(ii) configurations

Two types of asymmetric binuclear platinum(ii) complexes (Pt-1 and Pt-3) bearing bridging ligands of 2-(2,4-difluorophenyl)-5-(pyridin-2-yl)pyridine and 2-(2,4-difluorophenyl)-4-(pyridin-2-yl)pyridine as well as their corresponding mononuclear counterparts (Pt-2, Pt-4, and Pt-5) were synthesized and characterized. Different chelating constructions of the second platinum(ii) ions and the bridging ligands in Pt-1 and Pt-3 gave rise to two kinds of electron-transition pathway during their photophysical processes. The meta-/para-carbon of nitrogen on the center pyridyl segments set different levels of ligand field strength to the second platinum(ii) ions, lowering their occupied d orbital to varying degrees. Pt-1 showed an enhanced spin-orbit coupling (SOC), caused by the additional metal component through direct orbital hybridization at higher states, where the fixed molecular skeleton induced by the additional metal-ligand bonding also helped to suppress molecular distortion in the excited state, ensuring a high quantum yield (Φ, 0.89 in toluene), which is among the best results in bimetallic complexes. While the second platinum(ii) ion in Pt-3 seemed to make no contribution to the radiative transition, and only contributed to the HOMO, it provided a benefit by enlarging the conjugate system. Solution-processed organic lighting emitting devices (OLEDs) fabricated with the bimetallic Pt-1 emitter achieved superior efficiencies and up to 21% external quantum efficiency (EQE) in the Kelly-green region.

The Quantum Efficiency Roll-Off Effect in Near-Infrared Organic Electroluminescent Devices with Iridium Complexes Emitters

The electroluminescence quantum efficiency roll-off in iridium(III)-based complexes, namely Ir(iqbt)₂(dpm) and Ir(iqbt)₃ (iqbt = 1 (benzo[b]thiophen-2-yl)-isoquinolinate, dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate) utilized as near-infrared emitters in organic light emitting diodes with remarkable external quantum efficiencies, up to circa 3%, 1.5% and 1%, are measured and analyzed. With a 5–6 weight% of emitters embedded in a host matrix, the double-layer solution-processed structure as well as analogous three-layer one extended by a hole-conducting film are investigated. The triplet-polaron, the Onsager electron-hole pair dissociation and the triplet-triplet annihilation approaches were used to reproduce the experimental data. The mutual annihilation of triplets in iridium emitters was identified as prevailingly controlling the moderate roll-off, with the interaction between those of iridium emitters and host matrixes found as being less probable. Following the fitting procedure, the relevant rate constant was estimated to be (0.5−12)×10−12 cm³/s, values considered to be rather too high for disordered organic systems, which was assigned to the simplicity of the applied model. A coexistence of some other mechanisms is therefore inferred, ones, however, with a less significant contribution to the overall emission quenching.

Ab Initio Many-Body Perturbation Theory Calculations of the Electronic and Optical Properties of Cyclometalated Ir(III) Complexes

Cyclometalated Ir(III) compounds are the preferred choice as organic emitters in organic light-emitting diodes. In practice, the presence of the transition metal surrounded by carefully designed ligands allows fine-tuning of the emission frequency as well as good efficiency of the device. To support the development of new compounds, experimental measurements are generally compared with absorption and emission spectra obtained from ab initio calculations. The standard approach for these calculations is time-dependent density functional theory (TDDFT) with a hybrid exchange-correlation functional like B3LYP. Because of the size of these compounds, the application of more complex quantum chemistry approaches can be challenging. In this work, we used many-body perturbation theory approaches, in particular the GW approximation with the Bethe-Salpeter equation (BSE) implemented in Gaussian basis sets, to calculate the quasiparticle properties and the absorption spectra of six cyclometalated Ir(III) complexes, going beyond TDDFT. In the presented results, we compared standard TDDFT simulations with BSE calculations performed on top of perturbative G₀W₀ and accounting for eigenvalue self-consistency. Moreover, in order to investigate in detail the effect of the DFT starting point, we concentrated on Ir(ppy)₃ and performed GW-BSE simulations starting from different DFT exchange-correlation potentials.

A simple and efficient approach toward deep-red to near-infrared-emitting iridium(iii) complexes for organic light-emitting diodes with external quantum efficiencies of over 10

While the external quantum efficiency (EQE) of iridium(iii) (Ir(iii)) phosphor based near-infrared organic light-emitting diodes (NIR OLEDs) has been limited to 5.7% to date, there is no significant EQE improvement for these types of OLEDs due to the lack of efficient Ir(iii) emitters. Here, a convenient approach within three synthetic steps is developed to afford two novel and efficient deep-red to near-infrared (DR-NIR) emitting phosphors (CNIr and TCNIr), in which a cyano group is added into a commercial red emitter named Ir(piq)2(acac) to significantly stabilize the lowest unoccupied molecular orbitals of the newly designed Ir(iii) complexes. They emit strong DR-NIR phosphorescence emissions at a wavelength of around 700 nm, with relatively high absolute quantum efficiencies of around 45% for their doped films. DR-NIR OLEDs made using CNIr and TCNIr exhibit high-efficiencies, affording peak EQEs of 10.62% and 9.59% with emission peak wavelengths of 690 and 706 nm, respectively. All these devices represent the most efficient Ir(iii)-based DR-NIR OLEDs with a similar color gamut. The simplified synthesis procedure of the DR-NIR-emitting phosphors in conjunction with their excellent performance in OLEDs confirms our efficient strategy to achieve the DR-NIR-emitting Ir(iii) phosphors.

Deep Red Iridium(III) Complexes Based on Pyrene-Substituted Quinoxaline Ligands for Solution-Processed Phosphorescent Organic Light-Emitting Diodes

In this paper, we systemically investigated the photoelectric properties of three new deep-red quinoxaline-based iridium(III) complexes: Ir-0, Ir-1, and Ir-2. (MPQ)₂Ir(dpm) (Ir-0) bore a 2-methyl-3-phenylquinoxaline cyclometalated ligand, while (c-PyMPQ)₂Ir(dpm) (Ir-1) and (t-PyMPQ)₂Ir(dpm) (Ir-2) possessed a 1-pyrene substituent that connected at the 6/7 position of the corresponding ligands. The configurations of the latter two complexes were well-confirmed by single-crystal X-ray diffraction, and both of them had large dihedral angles between the quinoxaline and pyrene units, preventing the emission peaks of the three complexes from being altered too much. Based on the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, we concluded that the emission of all complexes originated predominantly from the triplet metal-to-ligand/intraligand charge transfer (³MLCT/³ILCT) state of the non-pyrene-substituted counterpart Ir-0 core. Interestingly, we also obtained another type of pyrene-stacking characteristic crystal of Ir-1, which had an emission resembled the phosphorescence observed in thin film. The easily formed pyrene-stacking configuration would most probably limit their device performance at a higher concentration. Moreover, the fabricated organic light-emitting diodes (OLEDs) using these materials achieved considerable device performance at a low doping concentration of 0.5 wt %. This work provides an approach for reasonably designing large fused-ring-substituted quinoxaline ligands of iridium complexes.

Enhancing singlet oxygen generation in semiconducting polymer nanoparticles through fluorescence resonance energy transfer for tumor treatment

Photosensitizers (PSs) are of particular importance for efficient photodynamic therapy (PDT). Challenges for PSs simultaneously possessing strong light-absorbing ability, high 1O2 generation by effective intersystem crossing from the singlet to the triplet state, good water-solubility and excellent photostability still exist. Reported here are a new kind of dual-emissive semiconducting polymer nanoparticles (SPNs) containing fluorescent BODIPY derivatives and near-infrared (NIR) phosphorescent iridium(iii) complexes. In the SPNs, the BODIPY units serve as the energy donors in the fluorescence resonance energy transfer (FRET) process for enhancing the light absorption of the SPNs. The NIR emissive iridium(iii) complexes are chosen as the energy acceptors and efficient photosensitizers. The ionized semiconducting polymers can easily self-assemble to form hydrophilic nanoparticles and homogeneously disperse in aqueous solution. Meanwhile, the conjugated backbone of SPNs provides effective shielding for the two luminophores from photobleaching. Thus, an excellent overall performance of the SPN-based PSs has been realized and the high 1O2 yield (0.97) resulting from the synergistic effect of BODIPY units and iridium(iii) complexes through the FRET process is among the best reported for PSs. In addition, owing to the phosphorescence quenching of iridium(iii) complexes caused by 3O2, the SPNs can also be utilized for O2 mapping in vitro and in vivo, which assists in the evaluation of the PDT process and provides important instructions in early-stage cancer diagnosis.

Recent Advances on Metal-Based Near-Infrared and Infrared Emitting OLEDs

During the past decades, the development of emissive materials for organic light-emitting diodes (OLEDs) in infrared region has focused the interest of numerous research groups as these devices can find interest in applications ranging from optical communication to defense. To date, metal complexes have been most widely studied to elaborate near-infrared (NIR) emitters due to their low energy emissive triplet states and their facile access. In this review, an overview of the different metal complexes used in OLEDs and enabling to get an infrared emission is provided.

Enhancing Molecular Aggregations by Intermolecular Hydrogen Bonds to Develop Phosphorescent Emitters for High-Performance Near-Infrared OLEDs

Phosphorescent near-infrared (NIR) organic light-emitting devices (OLEDs) have drawn increasing attention for their promising applications in the fields such as photodynamic therapy and night-vision readable displays. Here, three simple phosphorescent Pt(II) complexes are synthesized, and their intermolecular interactions are investigated in crystals and neat films by X-ray single crystal diffraction and grazing-incidence wide-angle X-ray scattering, respectively. The photophysical properties, molecular aggregation (including Pt-Pt interaction), molecular packing orientation, and electron transport ability are all influenced by the strong intermolecular hydrogen bonds. Consequently, the nondoped OLEDs based on tBu-Pt and F-Pt show electroluminescent emissions in NIR region with the highest external quantum efficiencies of 13.9% and 16.7%, respectively.

B- and N-embedded color-tunable phosphorescent iridium complexes and B-N Lewis adducts with intriguing structural and optical changes

A novel family of B- and N-embedded phosphorescent iridium complexes has been prepared. Single crystal structures indicate that the B-embedded polycyclic unit exhibits better planarity than the N-embedded polycyclic unit, which leads to different π-π-stacking and electrical characteristics. More importantly, by controlling the number of boron or nitrogen atoms embedded, solution-processed OLED devices incorporating these emitters as emitting layers can achieve a phosphorescence color variation from green to deep red (638 nm) and show low-efficiency roll-off and turn-on voltage. In particular, the B-embedded complex Ir-BB shows good color purity with a narrow full width at half maximum (1211 cm-1) and CIE coordinates (0.67, 0.31) in the deep red light region. Notably, B-embedded iridium complexes can also react with two different Lewis bases (pyridine and DMAP) to form intriguing B-N Lewis adducts through different coordination modes. During this process, significantly different structural and optical changes are triggered by the structure and electronic properties of Lewis bases, as confirmed by X-ray crystallographic, 1H NMR and spectral analysis.

Metal-complex chromophores for solar hydrogen generation

Solar H₂ generation from water has been intensively investigated as a clean method to convert solar energy into hydrogen fuel. During the past few decades, many studies have demonstrated that metal complexes can act as efficient photoactive materials for photocatalytic H₂ production. Here, we review the recent progress in the application of metal-complex chromophores to solar-to-H₂ conversion, including metal-complex photosensitizers and supramolecular photocatalysts. A brief overview of the fundamental principles of photocatalytic H₂ production is given. Then, different metal-complex photosensitizers and supramolecular photocatalysts are introduced in detail, and the most important factors that strictly determine their photocatalytic performance are also discussed. Finally, we illustrate some challenges and opportunities for future research in this promising area.

Dual phosphorescence emission of dinuclear platinum(ii) complex incorporating cyclometallating pyrenyl-dipyridine-based ligand and its application in near-infrared solution-processed polymer light-emitting diodes

Two novel mono- and binuclear cyclometalated platinum(ii) complexes of (BuPyrDPy)Pt(dpm) and (BuPyrDPy)[Pt(dpm)]₂ incorporating a pyrenyl-dipyridine-based cyclometalated ligand were synthesized and characterized, respectively. Single-crystal X-ray diffraction of the two materials revealed each complex's coordination mode; their photophysical, electrochemical as well as electroluminescent properties were also investigated. Both complexes exhibited good solubility and excellent thermal stability. (BuPyrDPy)[Pt(dpm)]₂ presented dual phosphorescence emissive character at room-temperature and showed an increased quantum efficiency compared to that of (BuPyrDPy)Pt(dpm). Density functional theory (DFT) calculations were carried out to model their photophysical process, and found a significant contribution of the second Pt center to the LUMO plot, giving the T₁ and T₂ states considerable LMCT nature, which is quite rare in metallic complexes. A device with the structure of ITO/PEDOT (40 nm)/PVK : 30 wt% OXD-7 : 16 wt% (BuPyrDPy)[Pt(dpm)]₂ (60 nm)/TPBI (30 nm)/Ba (4 nm)/Al (100 nm) showed a stable NIR emission peak at 695 nm accompanied by two shoulders at 599 nm and 762 nm, with a maximum external quantum efficiency (EQE) of 0.31% and a radiance of 26.9 mW cm^-2, which are about 2 and 1.4 times higher than those of (BuPyrDPy)Pt(dpm)-doped devices. This study provides an efficient strategy to simultaneously design novel biluminescent materials and achieve NIR emission through adjusting the emissive triplet states by introducing a second metal into an asymmetric bimetallic system.

β-Diketonate ancillary ligands in heteroleptic iridium complexes: a balance between synthetic advantages and photophysical troubles

How the triplet energy of β-diketonate ancillary ligands in Ir(iii) complexes affects the phosphorescence emission: photochemical and electrochemical investigations and DFT calculations shed light on the dark triplet excited states.

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.

A near-infrared organic light-emitting diode based on an Yb(iii) complex synthesized by vacuum co-deposition

Yb(DBM)₃(DPEPO), an emitter, was directly synthesized on a substrate by the vacuum co-deposition of the precursor Yb(DBM)₃(H₂O)₂ and the ligand DPEPO.

Near-infrared roll-off-free electroluminescence from highly stable diketopyrrolopyrrole light emitting diodes

Organic light emitting diodes (OLEDs) operating in the near-infrared spectral region are gaining growing relevance for emerging photonic technologies, such as lab-on-chip platforms for medical diagnostics, flexible self-medicated pads for photodynamic therapy, night vision and plastic-based telecommunications. The achievement of efficient near-infrared electroluminescence from solution-processed OLEDs is, however, an open challenge due to the low photoluminescence efficiency of most narrow-energy-gap organic emitters. Diketopyrrolopyrrole-boron complexes are promising candidates to overcome this limitation as they feature extremely high photoluminescence quantum yield in the near-infrared region and high chemical stability. Here, by incorporating suitably functionalized diketopyrrolopyrrole derivatives emitting at ~760 nm in an active matrix of poly(9,9-dioctylfluorene-alt-benzothiadiazole) and without using complex light out-coupling or encapsulation strategies, we obtain all-solution-processed NIR-OLEDs with external quantum efficiency as high as 0.5%. Importantly, our test-bed devices show no efficiency roll-off even for high current densities and high operational stability, retaining over 50% of the initial radiant emittance for over 50 hours of continuous operation at 10 mA/cm², which emphasizes the great applicative potential of the proposed strategy.