Triplet Emitters Research Articles

Investigating the role of heavy metal complexes as efficient triplet emitters for OLED applications

We present here two iridium metal complexes, namely complex A and B, based on two different cyclometalated ligands 2-(4-fluorophenyl) H-imidazo [1,2-a] pyridine (pip), and 2-phenylpyrdine (ppy), and an ancillary ligand acetylacetone (acac) ligands for their use in organic light-emitting diodes. The structural and purity analysis of the complexes have been estimated via NMR spectroscopy and elemental analysis. UV–visible and photoluminescence spectroscopy were used to investigate the optical characteristics of the molecules. Both complexes exhibit nearly identical absorption spectra comprising of sharp bands in the ultraviolet region i.e., 245 nm–300 nm for the complex A and 232 nm and 304 nm for the complex B which are assigned to spin allowed π – π* transitions of cyclometalated (C^N) ligand and relatively weaker bands in the lower energy range i.e., 300 nm to 450 nm for the complex A and 403 nm and 457 nm for the complex B, which are ascribed to metal-ligand charge transfer (MLCT) transitions. Upon excitation, both of the complexes reveal green photoluminescence with their emission peaks at 535 nm and 520 nm for the complexes A and B, respectively. The thermal analysis of both complexes performed under a nitrogen atmosphere exhibit excellent thermal stability up to 300 °C. The electrochemical behavior of the complexes has been analyzed through cyclic voltammetry measurements and the corresponding values of HOMO/LUMO levels were found to be 5.09/2.52 eV for the complex A and 5.21/2.92 eV for the complex B. Furthermore, the diffusion of ions at the surface of electrodes and the corresponding values of diffusion coefficients have been estimated using the Randles-Sevcik equation.

Deep Blue Phosphorescence from Platinum Complexes Featuring Cyclometalated N-Pyridyl Carbazole Ligands with Monocarborane Clusters (CB11H12-).

The utilization of deep blue phosphorescent materials in high-performance displays and solid-state lighting requires high quantum efficiencies and color purities. Here, we describe the preparation and luminescent properties of novel platinum triplet emitters featuring cyclometalated N-pyridyl-carbazole ligands functionalized with closo-monocarborane clusters [CB11H12]-. All reported complexes were fully characterized by using standard small molecule techniques (UV-vis, cyclic voltammetry, nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS)), and their solid-state structures were elucidated by X-ray diffraction. These platinum phosphors emit in the blue region of the visible wavelength spectrum in both the solid and solution states. Complex 4a exhibits the highest luminous efficiency at λem = 439 nm with a photoluminescent quantum yield (PLQY) of 60% by dispersing in a PMMA matrix. Electrochemical and computational studies of complexes 4a and 4b revealed that the blue phosphorescence originates mainly from intraligand 3π → π* (3ILCT) transitions with relatively small 3MLCT mixing. A deep-blue OLED containing 4a as the light-emitting dopant was successfully fabricated using a solution-processed method, and the device exhibited blue photoluminescence with CIE coordinates of (0.17, 0.15) and a maximum external quantum efficiency (EQEmax) value of 6.2%. This article represents the pioneering study of a deep blue PhOLED using a Pt complex bearing a closo-monocarborane anion substituent, providing a new avenue into the preparation of novel triplet emitters based on boron-rich cluster anions.

Property study of triplet emitters based on selenophenol-substituted phenothiazine derivatives

<p indent="0mm">In this article, we designed and synthesized ACN-PTZ and ACN-Se-PTZ by introducing phenylacetonitrile and selenophenol groups to the phenothiazine (PTZ) structure aiming to enhance spin-orbit coupling (SOC) for room-temperature phosphorescence. Because of the introduction of the donor-acceptor (D-A) structure, the luminous efficiency of ACN-PTZ increased by two times compared with that of PTZ. ACN-Se-PTZ showed obvious fluorescence quenching after introducing selenophenol; its photoluminescence quantum yield was significantly lower than those of PTZ and ACN-PTZ. In addition, the long lifetimes of ACN-Se-PTZ in solution at low temperature (77 K) and room temperature are 269.93 ms and 12.17 μs, respectively, which were higher than those of ACN-PTZ (138.19 ms and 11.73 μs). Selenophene with a heavy atom effect substituted to the benzene ring of phenothiazine acted as an electron donor and quenched the fluorescence due to the promoted intersystem crossing (ISC) process. Various photophysical test results show that the introduced selenophene group destroyed the intramolecular charge transfer of the phenothiazine precursor, resulting in only localized excited state transitions in ACN-Se-PTZ.

Very sensitive probes for quantitative and organoleptic detection of oxygen based on conformer-induced room-temperature phosphorescence enhancement of the derivative of triazatruxene and phenothiazine

Organic materials which show room-temperature phosphorescence (RTP) are of interest for many applications including chemical qubits which are the core units of a quantum information systems, organic light emitting diodes, chemical sensors. In contrast to triplet emitters based on noble metals (e.g. Ir, Rh), quantum yields of RTP of noble-atom-free organic emitters are much lower than unity. In this work, we report on a new approach to conformer-induced RTP enhancement of organic triplet emitters. The synthesis, thermal, photophysical, charge-transporting and oxygen sensing properties of the derivative of phenothiazine and triazatriazatruxene in which phenothiazine moieties are linked to nitrogen atoms of triazatriazatruxene via C-2 positions are presented. This compound exhibit stable RTP with a quantum yield of 54% which is more than six times higher than that of a reference material. For 1% solid solution of the compound in Zeonex, the ratio of intensity of RTP observed under vacuum and fluorescence intensity recorded in air reached the value of 19 which allows quantitative detection of oxygen with the high Stern-Volmer quenching constant of 7.83 × 10−3 ppm−1. The observed change of emission colour depending on the oxygen concentration can be used for the simple detection of oxygen by human eye.

Realization of room temperature electro-phosphorescence from an iridium metal based efficient novel triplet emitter

Realization of room temperature electro-phosphorescence from an iridium metal based efficient novel triplet emitter

Synthesis, Mesomorphism, Photophysics, and Device Properties of Liquid-Crystalline Pincer Complexes of Gold(III) Containing Semiperfluorinated Chains.

Gold(III) complexes of C∧N∧C-coordinating 2,6-diphenylpyridine pincer ligands with arylacetylide co-ligands are known triplet emitters at room temperature. We have reported previously that by functionalizing both the pincer ligand and the phenylacetylene with alkoxy chains, liquid crystallinity may be induced, with the complexes showing columnar mesophases. We now report new derivatives in which the phenylacetylene incorporates one, two, or three 1H,1H,2H,2H-perfluoroalkyl chains. In terms of intermolecular interactions, solution 1H NMR experiments suggest that the semiperfluoroalkyl chains promote a parallel, head-to-head arrangement of neighboring molecules relative to one another, rather than the anti-parallel, head-to-tail orientation found for the all-hydrocarbon materials. In terms of the liquid crystal properties, the complexes show columnar phases, with the addition of the more rigid fluorocarbon chains leading to a stabilization of both the crystal and liquid crystal mesophases. Mesophase temperature ranges were also wider. Interestingly, the amphiphilic nature of these complexes is evident through the observation of a frustrated columnar nematic phase between a Colr and a Colh phase, an observation recently reported in detail for one compound (Liq. Cryst., 2022, doi: 10.1080/02678292.2021.1991017). While calculation shows that, despite the “electronic insulation” provided by the dimethylene spacer group in the semiperfluoroalkyl chains, a small hypsochromic shift in one component of the absorption band is anticipated, experimentally this effect is not observed in the overall absorption envelope. Complexes with substituents in the 3,3′,4,4′-positions of the phenyl rings of the pincer ligand once more show higher-luminescence quantum yields than the analogues with substituents in the 4,4′-positions only, associated with the lower-energy-emissive state in the former. However, in contrast to the observations with all-hydrocarbon analogues, the luminescence quantum yield of the complexes with 3,3′,4,4′-substitution on the pincer increases as the number of semiperfluoroalkyl chains on the phenylacetylide increases, from 20% (one chain) to 34% (three chains). External quantum efficiencies in fabricated OLED devices are, however, low, attributed to the poor dispersion in the host materials on account of the fluorinated chains.

Strategy to improve the performance of solution-processed phosphorescent organic light-emitting diodes using heteroleptic green Ir(III) complexes bearing multi-functional units

For solution-processed phosphorescence organic light-emitting diodes (PHOLEDs), two multi-functional units functionalized green triplet emitters Ir(ET)-Pic and Ir(HT)-Pic were designed and synthesized. They were synthesized by employing a 2-ethylhexyl substituted amide-bridged phenylpyridine moiety applied as main ligand and picolinate (Pic) as an ancillary ligand. In addition, the 1,3-bis(N-carbazolyl)benzene (mCP) and 1,3,4-oxadiazole (OXD) groups were introduced into 3-position of main ligand via non-conjugated benzyl ether linkage. Ir(ET)-Pic and Ir(HT)-Pic exhibited an intensive green photoluminescence (PL) emission at 502 nm, narrowed full-width at half maximum (FWHM), high PL quantum yields (PLQYs) and good solubility in organic solvents. Hence, the hole injection layer optimized device using Ir(ET)-Pic and Ir(HT)-Pic as dopants showed excellent performances with a maximum external quantum efficiency (EQEmax) of 21.5 and 19.6%, and the maximum current efficiency (CEmax) of 79.4 and 69.2 cd A−1 with Commission International L'Eclairage (CIE x,y) color coordinates of (0.36, 0.60) and (0.31,0.62), respectively. In addition, we found that the presence of bulky mCP unit in the Ir(III) complex affect the hole transport characteristics in the emissive layer. The result findings, the construction of multi-functional Ir(III) complexes are an attractive design strategy for fabrication of highly efficient green solution-processed PHOLEDs.

Enhancing quantum efficiency in Pt-based emitters using a pendant closo-monocarborane cluster to enforce charge neutrality

The use of organometallic triplet emitters in OLED applications may be complicated when the luminescent complex bears a formal charge. By designing ligands containing pendant closo-monocarborane clusters [CB11H11]−, it is possible to neutralize the otherwise cationic cores of cyclometalated Pt(II) complexes bearing bis-carbenes, resulting in significantly enhanced photophysical properties. Herein, we report four Pt(II) complexes decorated with a monoanionic closo-monocarborane by coupling the cluster to the classic cyclometallating moiety, phenylpyridine (ppy). All four complexes were fully characterized by NMR, HR-MS, UV–Vis, and cyclic voltammetry. Complexes 1b, 1c and 2a were characterized by single-crystal X-ray diffraction. Moreover, all four complexes show interesting photophysical behaviors, with complex 1a exhibiting a photoluminescent quantum yield of 97% in a 2 wt% PMMA film. This high efficiency prompted the incorporation of 1a and 1c into an OLED device using solution processing, resulting in green emission with CIE coordinates of (0.28, 0.55) for 1a and yellow emission with CIE coordinates of (0.44, 0.50) for 1c. The measured maximum external quantum efficiency (EQEmax) was 18.51% for 1a, an EQEmax that is among the highest for solution-processed OLED devices containing Pt(II) complexes with simple cyclometalated ppy ligands.

Metal‐Free Organic Triplet Emitters with On–Off Switchable Excited State Intramolecular Proton Transfer

AbstractMetal‐free organic triplet emitters are an emerging class of organic semiconducting material. Among them, molecules with tunable emission responsive to environmental stimuli have shown great potential in solid‐state lighting, sensors, and anti‐counterfeiting systems. Here, a novel excited‐state intramolecular proton transfer (ESIPT) system is proposed showing the activation of thermally activated delayed fluorescence (TADF) or room‐temperature phosphorescence (RTP) simultaneously from both keto and enol tautomers. The prototype ESIPT triplet emitters exhibit up to 50% delayed emission quantum yield. Their enol–keto tautomerization can be switched by controlling the matrix acidity in doped polymer films. Taking advantage of these unique properties, “on‐off” switchable triplet emission systems controlled by acid vapor annealing, as well as photopatterning systems capable of generating facile and high‐contrast emissive patterns, are devised.

Isoelectronic Pt(II) complexes of cyclometalating C^N^N ligands with phenyl/(benzo)thiophenyl and pyridyl/(benzo)thiazolyl moieties.

A series of cyclometalated Pt(II) complexes [Pt(C^N^N)X] (X = Cl, CCPh, CCC6F5) was synthesised from the protoligands HC^N^N containing either phenyl (ph), naphthyl (na) or (benzo)thiophenyl (b(th)) C aryl functions and either pyridyl (py) or (benzo)thiazolyl ((b)tz) peripheral N units, alongside the central 4-phenyl-pyridyl (ppy) or tBu2-phenyl-pyridyl (tbppy) N group. Depending on the combination of the peripheral N or C aryl building blocks, these square planar complexes reveal very different electrochemical, UV-vis absorption and emission behaviour. The reversible reductions shift anodically along the series th/py < ph/tz ≈ th/tz < ph/btz while the irreversible oxidations shift cathodically along the series Cl ≈ CCC6F5 < CCPh. Similar trends were observed for the long-wavelength UV-vis absorption and photoluminescence properties. The emission maxima range from 605 to 675 nm at 298 K in CH2Cl2 solution and from 555 to 655 nm at 77 K in glassy frozen CH2Cl2/MeOH matrices. Large differences in amplitude-weighted average lifetimes τav (up to 0.9 μs at 298 K, up to 12 μs at 77 K) and photoluminescence quantum yields ΦL (up to 0.15 at 298 K and up to 0.82 at 77 K) were found. TD-DFT calculations showed that the decomposition of the triplet excited states into LC (π-π*, centred in the individual parts of the C^N^N ligand) and LLCT (π-π*, between the individual parts of the C^N^N ligand + X-π* from coligand to C^N^N) contributions for the ligand-centred states as well as MLCT (dPt-to-π*C^N^N) and LMCT (pCl or πCCR-to-dPt) character for the charge-transfer states involving the metal is beneficial to assess the participation of the individual heteroaryl groups of the C^N^N ligands. In view of the modular synthesis of these ligands, this will allow the realisation of tailor-made Pt(II) triplet emitters in future work.

Low-power supralinear photocurrent generation via excited state fusion in single-component nanostructured organic photodetectors

Columnar arrangement of triplet emitters in the photoactive layer of vertically-configured photodetectors enables photocurrent generation via triplet–triplet annihilation of dimer species.

Control of photoluminescence quantum yield and long-lived triplet emission lifetime in organic alloys.

Two-component crystalline organic alloys with a wide range of compositional ratios (from 30% to 90% of one component) are employed to tune excited-state lifetimes and photoluminescence quantum yields (PLQYs). Alloy crystals exhibit homogeneous distribution of parent compounds by X-ray crystallography and differential scanning calorimetry. The alloys display a 1.5- to 5-fold enhancement in thermally activated delayed fluorescence (TADF) lifetime, compared to the parent compounds. PLQYs can also be tuned by changing alloy composition. The reverse intersystem crossing and long-lived lifetime of the parent compounds give rise to long-lived TADF in the alloys. Organic alloys enable tunability of both lifetime and efficiency, providing a new perspective on the development of organic long-lived emissive materials beyond the rules established for host–guest doped systems.

Purely Organic Triplet Emitters: From Fundamental Molecular Design to Performance Amplification in Modern Applications

Purely Organic Triplet Emitters: From Fundamental Molecular Design to Performance Amplification in Modern Applications

2,7(3,6)-Diaryl(arylamino)-substituted Carbazoles as Components of OLEDs: A Review of the Last Decade.

Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.

High Performance Solution‐Processed Deep‐Blue Phosphorescence Organic Light‐Emitting Diodes with EQE Over 24% by Employing New Carbenic Ir(III) Complexes

AbstractSolution‐processed phosphorescence organic light‐emitting diodes (PHOLEDs) are an increasingly attractive option as compared with vacuum‐processed PHOLEDs due to their lower costs, large areas, and flexible production. Currently, the majority of reported solution‐processed PHOLEDs are produced using red and green triplet emitters. The earlier reports on blue solution‐processed PHOLEDs describe poor Commission International de l'Éclairage color coordinates (CIE(x, y)) and low external quantum efficiencies (EQEs). It is difficult to produce efficient solution‐processed deep‐blue PHOLEDs that meet high EQE and CIE y color coordinate ≤ 0.15 requirements. The authors design and synthesize three new carbenic homoleptic deep‐blue emitting Ir(III) complexes for solution‐processed PHOLEDs. The introduction of bulky tert‐butyl and trifluoromethyl (CF3) substituents at a suitable position on the benzylated pyridoimidazole moiety effectively improves solution‐processed device performance without compromising CIE(x, y) color purity. The deep‐blue PHOLEDs exhibit remarkable maximum EQE (EQEmax) of 25.3% and excellent CIE(x, y) color of (0.16, 0.10) at a mer‐Ir(ptbbp)3 doping ratio of 50 wt%. In particular, deep‐blue PHOLEDs containing mer‐Ir(CF3pbp)3 as dopant exhibit excellent CIE(x, y) color coordinates of (0.16, 0.05) and an EQEmax of 21.2%. This is the first report of deep‐blue solution‐processed PHOLEDs containing carbene‐based Ir(III) complexes with extremely high EQE values and CIE y coordinate requirement.

Organic Light-Emitting Diodes Based on Luminescent Self-Assembled Materials of Copper(I)

Over the past decade, we have seen the rapid advancement in the technology of organic light-emitting diodes (OLEDs) toward practical applications. The high color contrast, wide viewing angles, good flexibility, and low power consumption of OLEDs have rendered them promising candidates for next-generation displays and solid-state lighting applications. Parallel to the optimization in device structures, of equal importance is the search for new classes of emitters. While conventional OLEDs mainly rely on triplet emitters based on precious metal complexes, such as those of platinum(II) and iridium(III), copper(I) self-assembled materials represent attractive alternatives with relatively low cost but comparable emission tunability. In this mini review, the recent progress in the development of copper(I) self-assembled materials, including copper(I) clusters and coordination polymers, as electroluminescent materials in OLEDs will be discussed.

Ambient Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence from a Core-Substituted Pyromellitic Diimide Derivative.

Triplet harvesting under ambient conditions plays a crucial role in improving the luminescence efficiency of purely organic molecular systems. This requires elegant molecular designs that can harvest triplets either via room temperature phosphorescence (RTP) or by thermally activated delayed fluorescence (TADF). In this context, here we report a donor core-substituted pyromellitic diimide (acceptor) derivative as an efficient charge-transfer molecular design from the arylene diimide family as a triplet emitter. Solution-processed thin films of carbazole-substituted CzPhPmDI display both RTP- and TADF-mediated twin emission with a long lifetime and high efficiency under ambient conditions. The present study not only sheds light on the fundamental photophysical process involved in the triplet harvesting of donor-acceptor organic systems, but also opens new avenues in exploring an arylene diimide class of molecules as potential organic light-emitting materials.

Configuring device architecture with new solution-processable host for high performance low color-temperature OLEDs with ultra-low driving voltage

After the revelation of nonvisual lighting impact on human health, the lighting engineers are more concerned about the human benign light with low cost and high efficiency. The solution-processed fabrication technique with smart device engineering for high efficacy OLED devices is being anticipated to drastically reduce the fabrication cost leading to affordable desired end product. The co-host matrix could be a potential solution to improve device performance multi-folds with suitable band-gap engineering and most effective energy transfer from mixed host to guest. Here, rationally configured device architecture with two novel host materials possessing wide energy gap, high triplet energy, and excellent thermal and morphological stability resulted in highly-efficient solution-processed green, red and low color temperature (CT) OLEDs with sub-bandgap level driving voltage of 2.1 V i.e. record lowest within its own category. Perfect triplet energy match of our newly developed host materials with commercial p-type host m-MTDATA and the common phosphors enabled efficient energy transfer with very low energy loss led to high efficiency of resulting OLED. The resultant solution-processed triplet emitter based green and red OLED devices displayed a maximum efficacy of 75.0 and 30.2 lm W−1 without using light extraction out-coupling techniques, respectively. The designed blue-hazard free 2048 K low CT OLED exhibited a ηPE of 44.5 lmW−1, a ηCE of 47.6 cdA-1 and an EQE of 20.7% as the maximum value, the highest known so far within its own category. The melatonin suppression sensitivity (MSS) of the engineered low CT OLED is only 1.0% to that of the 480 nm blue light, which is much safer compared to other light sources. The impact of our design engineering was established by fabricating 1 cm × 1 cm device area prototype.

Solution-Processed Efficient Blue Phosphorescent Organic Light-Emitting Diodes (PHOLEDs) Enabled by Hole-Transport Material Incorporated Single Emission Layer.

Solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a single emission layer with small-molecule hole-transport materials (HTMs) are demonstrated. Various HTMs have been readily incorporated by solution-processing to enhance hole-transport properties of the polymer-based emission layer. Poly(N-vinylcarbazole) (PVK)-based blue emission layer with iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) triplet emitter blended with solution-processed 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) gave luminous efficiency of 21.1 cd/A at a brightness of 6220 cd/m2 with an external quantum efficiency (EQE) of 10.6%. Blue PHOLEDs with solution-incorporated HTMs turned out to be 50% more efficient compared to the reference device without HTMs. The high hole mobility, high triplet energy of HTM, and favorable energy transfer between HTM blended PVK host and FIrpic blue dopant were found to be important factors for achieving high device performance. The results are instructive to design and/or select proper hole-transport materials in solution-processed single emission layer.

Extended ligand conjugation and dinuclearity as a route to efficient platinum-based near-infrared (NIR) triplet emitters and solution-processed NIR-OLEDs

A complex in which two Pt(ii) ions are cyclometallated to bis-dithienylpyrimidine displays intense luminescence in the NIR region, both in solution and in an OLED.