Rational Design of Efficient Organometallic Ir(III) Complexes for High-Performance, Flexible, Monochromatic, and White Light-Emitting Electrochemical Cells

White light-emitting electrochemical cells based on metal-free TADF emitters

The attainment of white emission from a light-emitting electrochemical cell (LEC) is important, since it enables illumination and facile color conversion from devices that can be cost-efficient and sustainable. However, a drawback with current white LECs is that they either employ non-sustainable metals as an emitter constituent or are intrinsically efficiency limited by that the emitter only converts singlet excitons to photons. Organic compounds that emit by thermally activated delayed fluorescence (TADF) can address these issues since they can harvest all excitons for light emission while being metal free. Here, we report on the first white LEC based on solely metal-free TADF emitters, as accomplished through careful tuning of the energy-transfer processes and the electrochemically formed doping structure in the single-layer active material. The designed TADF-LEC emits angle-invariant white light (color rendering index = 88) with an external quantum efficiency of 2.1 % at a luminance of 350 cd/m2.

Long-Wavelength Light-Emitting Electrochemical Cells: Materials and Device Engineering

Long-wavelength light-emitting electrochemical cells (LECs) are potential deep-red and near infrared light sources with solution-processable simple device architecture, low-voltage operation, and compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long-wavelength LECs over the past two decades. The materials designed the for long-wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down-conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long-wavelength LECs. In this review, recent advances in the long-wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long-wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long-wavelength LECs in the applications of displays, bio-imaging, telecommunication, and night-vision displays.

White Light-Emitting Electrochemical Cells Employing Phosphor-Sensitized Thermally Activated Delayed Fluorescence to Approach All-Phosphorescent Device Efficiencies

Solid-state light-emitting electrochemical cells (LECs) show promising advantages of simple device architecture, low operation voltage, and insensitivity to the electrode work functions such that they have high potential in low-cost display and lighting applications. In this work, novel white LECs based on phosphor-sensitized thermally activated delayed fluorescence (TADF) are proposed. The emissive layer of these white LECs is composed of a blue-green phosphorescent host doped with a deep-red TADF guest. Efficient singlet-to-triplet intersystem crossing (ISC) on the phosphorescent host and the subsequent Förster energy transfer from the host triplet excitons to guest singlet excitons can make use of both singlet and triplet excitons on the host. With the good spectral overlap between the host emission and the guest absorption, 0.075 wt.% guest doping is sufficient to cause substantial energy transfer efficiency (ca. 40 %). In addition, such a low guest concentration also reduces the self-quenching effect and a high photoluminescence quantum yield of up to 84 % ensures high device efficiency. The phosphor-sensitized TADF white LECs indeed show a high external quantum efficiency of 9.6 %, which is comparable with all-phosphorescent white LECs. By employing diffusive substrates to extract the light trapped in the substrate, the device efficiency can be further improved by ca. 50 %. In the meantime, the intrinsic EL spectrum and device lifetime of the white LECs recover since the microcavity effect is destroyed. This work successfully demonstrates that the phosphor-sensitized TADF white LECs are potential candidates for efficient white light-emitting devices.

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%.

Aptamer recognition-promoted specific intercalation of iridium complexes in G-quadruplex DNA for label-free and enzyme-free phosphorescence analysis of kanamycin

In consideration of relevance of antibiotic with food security, it is extremely desirable to propose sensitive and credible methods for antibiotic screening. Nevertheless, most of known approaches are developed based on fluorescence technique, which suffered from the interferences of background fluorescence and autoluminescence, and tedious labeling procedures, ascribing to the deficiency of high-performance and multifunctional dyes. Herein, we developed a novel iridium (III) complex (Ir-QAU)-based aptamer-promoted phosphorescence sensor for label-free, enzyme-free and highly sensitive detection of target antibiotic (kanamycin, Kan) based on target-switched hybridizing chain reaction (HCR). Ir-QAU was elaborately devised to present a signal-on response to G-quadruplex (G4) DNA against other DNAs due to its specific intercalation in G4 DNA and subsequent restriction of intra-molecular rotation. The recognition of H1 by Kan promoted the formation of Kan@H1 complexes, which hybridized with H2 and H3 via toehold-mediated hybridization reaction, subsequently switching HCR to produce large numbers of G4 DNA. Compared to Kan absence, abundant Ir-QAU was locked in G4 DNA to yield a significantly increased luminescence, which switches the luminescence analysis process of Kan with a limit of detection down to 0.38 pM. Furthermore, the Ir-QAU-based sensor was triumphantly applied to detect Kan in milk sample. We anticipate this work will disclose a new way to development of high-efficiency and practical luminescence sensor, and show a great potential for antibiotic-related food security.

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.

Near-Infrared Emitting Ir(III) Complexes Bearing a Dipyrromethene Ligand for Oxygen Imaging of Deeper Tissues In Vivo

Phosphorescence lifetime imaging microscopy (PLIM) using a phosphorescent oxygen probe is an innovative technique for elucidating the behavior of oxygen in living tissues. In this study, we designed and synthesized an Ir(III) complex, PPYDM-BBMD, that exhibits long-lived phosphorescence in the near-infrared region and enables in vivo oxygen imaging in deeper tissues. PPYDM-BBMD has a π-extended ligand based on a meso-mesityl dipyrromethene structure and phenylpyridine ligands with cationic dimethylamino groups to promote intracellular uptake. This complex gave a phosphorescence spectrum with a maximum at 773 nm in the wavelength range of the so-called biological window and exhibited an exceptionally long lifetime (18.5 μs in degassed acetonitrile), allowing for excellent oxygen sensitivity even in the near-infrared window. PPYDM-BBMD showed a high intracellular uptake in cultured cells and mainly accumulated in the endoplasmic reticulum. We evaluated the oxygen sensitivity of PPYDM-BBMD phosphorescence in alpha mouse liver 12 (AML12) cells based on the Stern-Volmer analysis, which gave an O₂-induced quenching rate constant of 1.42 × 10³ mmHg^-1 s^-1. PPYDM-BBMD was administered in the tail veins of anesthetized mice, and confocal one-photon PLIM images of hepatic tissues were measured at different depths from the liver surfaces. The PLIM images visualized the oxygen gradients in hepatic lobules up to a depth of about 100 μm from the liver surfaces with a cellular-level resolution, allowing for the quantification of oxygen partial pressure based on calibration results using AML12 cells.

New Molecularly Engineered Binuclear Ruthenium (II) Complexes for Highly Efficient Near-Infrared Light-Emitting Electrochemical Cell (NIR-LEC)

Abstract: From practical point of view, the stability, response time and efficiency of near-infrared light-emitting electrochemical cell (NIR-LEC) are key factors. By using the high potential of chemical modification potential...

High-Efficiency Deep-Red Light-Emitting Electrochemical Cell Based on a Trinuclear Ruthenium(II)-Silver(I) Complex

Turn-on time is a key factor for lighting devices to be of practical application. To decrease the turn-on time value of a deep-red light-emitting electrochemical cells (DR-LECs), two novel approaches based on molecularly engineered ruthenium phenanthroimidazole complexes were introduced. First, we found that with the incorporation of ionic methylpyridinium group to phenanthroimidazole ligand, the turn-on time of the DR-LECs device was dramatically reduced, from 79 to 27 s. By complexation of ruthenium emitter with Ag⁺, the turn-on time was improved by 85%, and the EQE of DR-device was increased from 0.62 to 0.71%. These results open a new avenue in decreasing the turn-on time without adding ionic electrolytes, leading to an efficient LEC.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

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.

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.

Synthesis and photophysics of gold(i) alkynyls bearing a benzothiazole-2,7-fluorenyl moiety: a comparative study analyzing influence of ancillary ligand, bridging moiety, and number of metal centers on photophysical properties

Three new gold(i) alkynyl complexes (Au-ABTF(0-2)) containing a benzothiazole fluorenyl moiety, with either an organic phosphine or N-heterocyclic carbene as ancillary ligand, have been synthesized and photophysically characterized. All three complexes display highly structured ground-state absorption and luminescence spectra. Dual-luminescence is observed in all three complexes at room temperature in toluene after three freeze-pump-thaw cycles. The phosphine complexes (Au-ABTF(0-1)) exhibit similar photophysics with fluorescent quantum yields ∼0.40, triplet-state quantum yields ∼0.50, and fluorescent lifetimes ∼300 ps. The carbene complex Au-ABTF2 displays different behavior; having a fluorescent quantum yield of 0.23, a triplet-state quantum yield of 0.61, and a fluorescent lifetime near 200 ps, demonstrating that the ancillary ligand alters excited-state dynamics. The compounds exhibit strong (on the order of 105 M-1 cm-1) and positive excited-state absorption in both their singlet and triplet excited states spanning the visible region. Delayed fluorescence resulting from triplet-triplet annihilation is also observed in freeze-pump-thaw deaerated samples of all the complexes in toluene. DFT calculations (both static and time-resolved) agree with the photophysical data where phosphine complexes have slightly larger S1-T2 energy gaps (0.28 eV and 0.26 eV) relative to the carbene complex (0.21 eV). Comparison of the photophysical properties of Au-ABTF(0-2) to previously published dinuclear gold(i) complexes and mononuclear gold(i) aryl complexes bearing the same benzothiazole-2,7-fluorenyl moiety are made. Structure-property relationships regarding ancillary ligand, bridging moiety, and number of metal centers are drawn.

Mono and dinuclear iridium(iii) complexes featuring bis-tridentate coordination and Schiff-base bridging ligands: the beneficial effect of a second metal ion on luminescence

Ditopic bis-N^N^O-coordinating ligands, prepared by Schiff base chemistry, lead to dinuclear iridium complexes that emit much more brightly than their mononuclear counterparts.