Novel Carbazole/Fluorene-Based Host Material for Stable and Efficient Phosphorescent OLEDs

Theoretical studies of a novel carbazole-fluorene macrocycle as a host material for phosphorescent organic light-emitting diodes and the effects of substituents

Phosphorescent organic light-emitting diodes (PhOLEDs) have great potential in display applications due to their high luminous efficiency. Using density functional theory, a series of host molecules labelled as carbazole-fluorene macrocycles (CzFMCs) were designed and characterized to possess high triplet energy levels for application in PhOLEDs, and their accuracy was verified by comparing with experimental values. Our systematic investigation focuses on how the number and position of cyano, ethynyl, and fluoro substitutions affect various electronic properties of these hosts, including triplet energy (ET), frontier orbital energy levels, absorption spectra, charge-injection barriers, and reorganization energies. These properties were compared with those of reference hosts, emissive layers, and hole and electron transport materials. Our findings highlight that cyano-substituted host molecules exhibit higher ET, balanced charge transport and low charge injection barriers, making them promising candidates for blue PhOLEDs. Among the host molecules examined in the study, four were identified as the most effective for blue PhOLEDs, showcasing superior electronic properties compared to traditional reference hosts. This research offers theoretical insights into a novel macrocycle design for host materials, warranting further experimental exploration.

Exploring the sensing properties of pH-sensitive carbazole-based AIE emitters and their applications in paper strip sensing

A novel carbazole-coupled phenanthridine molecule with an intense blue emissive fluorescence was produced through a two-step synthesis of 5-(4-(9H-carbazol-9-yl)phenyl)-7,8,13,14-tetrahydrodibenzo[a,i]phenanthridine (DSPH). This blue probe shows a good thermal and electrochemical stability. It has been used for sensing trifluoracetic acid (TFA) with a limit of detection (LoD) value of 198 pM. The probe tends to show aggregation induced emission (AIE) characteristics. Additionally, the experimental data were supported by DFT analysis.

Carbazolylpyridine (cp)-based tetradentate platinum(II) complexes containing fused 6/5/6 metallocycles

A series of carbazolylpyridine (cp)-based 6/5/6 Pt(II) complexes featuring tetradentate ligands with nitrogen or oxygen atoms as bridging groups was designed and synthesized, and the bridging nitrogen atoms were derived from acridinyl (Ac), azaaceridine (AAc) and carbazole (Cz). Systematic experimental and theoretical studies reveal that the ligand structures have a significant effect on the electrochemical, photophysical and excited state properties of these complexes. Their oxidation processes mainly occur on the carbazole-Pt moieties, whereas the reduction processes typically occur on the electron-deficient pyridine (Py) moieties. Time-dependent density functional theory (TD-DFT) and natural transition orbital (NTO) calculations reveal that the cp-based Pt(II) complexes have a metal-to-ligand charge transfer (3MLCT) state mixed with ligand-centered (3LC) and intra-ligand charge-transfer (3ILCT) characteristics. Pt(cp-1) shows strong red luminescence with a dominant peak at 611 nm and an excited-state lifetime of 10.7 μs in dichloromethane at room temperature, 602 nm and 10.9 μs in toluene, and 602 nm and 8.2 μs in PMMA films. It also exhibits high photoluminescence quantum efficiencies of 85%, 84% and 60% in dichloromethane, toluene and PMMA, respectively. These studies indicate the potential application of the cp-based Pt(II) complexes as phosphorescent emitters in the field of organic light-emitting diodes (OLEDs).

Effect of the Number of Phenylcarbazole Units Adorned to the Silicon Atom for High Triplet Energy with High Charge Mobility

Increasing the number of phenylcarbazole (PC) units attached to the silicon atom in organic solid-state thin films led to a remarkable enhancement in charge mobility. Specifically, the charge mobility values exhibited an increase from 1.32 × 10-4 cm2/Vs for 3PCBP to 4.39 × 10-4 cm2/Vs for 2MCBP, ultimately reaching 1.16 × 10-3 cm2/Vs for MCBP. Notably, these enhancements were achieved while maintaining a high triplet energy of 3.01 eV. DFT calculations on the spin density distribution provided insights into the nature of the improved mobility while preserving the triplet energy. The accuracy of the DFT calculations was validated by comparing the results with experimental data from photoemission spectroscopy (PES). Mobility measurements, as contemplated by DFT, allowed for a comprehensive understanding of the factors influencing enhanced mobility while keeping the triplet energy constant. This study suggested that intramolecular charge transfers played a crucial role in reducing reorganization energy, showing an inverse dependence on the number of PCs. Consequently, it was inferred that the manipulation of PC units could effectively optimize charge transfer mechanisms, offering a promising avenue for tailoring organic thin films with improved electronic properties.

Push-Pull Carbazole-Based Dyes: Synthesis, Strong Ultrafast Nonlinear Optical Response, and Effective Photoinitiation for Multiphoton Lithography

The present work reports on the ultrafast nonlinear optical (NLO) properties of a series of D-π-Α and D-A push-pull carbazole-based dyes and establishes a correlation between these properties and their efficiency for potential photonic and optoelectronic applications such as multiphoton lithography (MPL). The ultrafast NLO properties of the studied dyes are determined by two distinct experimental techniques, Z-scan and pump-probe optical Kerr effect (OKE), employing 246 fs laser pulses at 515 nm. The results indicate that chemical functionalization of the carbazole moiety with various strong electron-donating and/or electron-withdrawing groups, such as benzene, styrene, 4-bromostyrene, nitrobenzene, trimethyl isocyanurate, methyl, and indane-1,3-dione, can result in a controlled and significant enhancement of the NLO absorptive and refractive responses. In the context of potential applications, the efficiency of carbazole-based organic materials as photoinitiators (PIs) for MPL applications is demonstrated. The fabricated woodpile microstructure using chemically functionalized carbazole as a PI demonstrates improvements in both feature size and MPL efficiency compared to that using unfunctionalized carbazole as a PI. This is attributed to the efficient charge transfer resulting from chemical functionalization, which leads to a substantial increase (approximately 1 order of magnitude) in the values of the imaginary part of the second-order hyperpolarizability (Imγ) and the two-photon absorption cross section (σ). The achieved feature size of 280 nm is comparable to that obtained with other widely used PIs in MPL applications. Additionally, owing to the strong NLO properties of the studied functionalized carbazole, they could also be promising candidates for further applications in photonics and optoelectronics.

A novel central seven-membered BOPYOs: Synthesis, optical properities and optimization of BF2 removal

Many efforts have been made to enrich the variety of BF2 complexes because of their excellent optical properties. However, the investigation on seven-membered ring N, O-chelated BF2 complexes is rare due to their instability with the removal of BF2 unit. Herein, a novel seven-membered ring N, O-chelated BF2 complexes (BOPYOs) with dual-state emission has been synthesized via a facile method. The results of optical properties showed that the fluorescence quantum yield of BOPYO-2 with donor group on 1 and 2-position of 1-indanone unit is much higher than that of BOPYO-1, 3-5 in toluene. The emission spectra of BOPYO-6 or 7 have redshift phenomenon compared with BOPYO-1-5 with weak fluorescence intensity due to their highly distorted structure or intramolecular charge transfer (ICT) effect. BOPYOs show relatively moderate solid emission from orange to deep red color with 596 nm to 686 nm. On the contrary, fluorescence quantum yield of BOPYO-2 in solid is the lowest. The optical properties in solution and solid states are further supported by the single-crystal structure and DFT calculation. Furthermore, the investigation on optimization of BF2 removal shows that the corresponding precursors of BOPYOs could be obtained in protic solvents without adding other catalysts.

Double boron-oxygen-fused polycyclic aromatic hydrocarbons: skeletal editing and applications as organic optoelectronic materials

An efficient one-pot strategy for the facile synthesis of double boron-oxygen-fused polycyclic aromatic hydrocarbons (dBO-PAHs) with high regioselectivity and efficient skeletal editing is developed. The boron-oxygen-fused rings exhibit low aromaticity, endowing the polycyclic aromatic hydrocarbons with high chemical and thermal stabilities. The incorporation of the boron-oxygen units enables the polycyclic aromatic hydrocarbons to show single-component, low-temperature ultralong afterglow of up to 20 s. Moreover, the boron-oxygen-fused polycyclic aromatic hydrocarbons can also serve as ideal n-type host materials for high-brightness and high-efficiency deep-blue OLEDs; compared to single host, devices using boron-oxygen-fused polycyclic aromatic hydrocarbons-based co-hosts exhibit dramatically brightness and efficiency enhancements with significantly reduced efficiency roll-offs; device 9 demonstrates a high color-purity (Commission International de l'Eclairage CIEy = 0.104), and also achieves a record-high external quantum efficiency (28.0%) among Pt(II)-based deep-blue OLEDs with Commission International de l'Eclairage CIEy < 0.20; device 10 achieves a maximum brightnessof 27219 cd/m2 with a peak external quantum efficiency of 27.8%, which representes the record-high maximum brightness among Pt(II)-based deep-blue OLEDs. This work demonstrates the great potential of the double boron-oxygen-fused polycyclic aromatic hydrocarbons as ultralong afterglow and n-type host materials in optoelectronic applications.

Recent synthetic strategies for the construction of functionalized carbazoles and their heterocyclic motifs enabled by Lewis acids

This article demonstrates recent innovative cascade annulation methods for preparing functionalized carbazoles and their related polyaromatic heterocyclic compounds enabled by Lewis acid catalysts. Highly substituted carbazole scaffolds were synthesized via Lewis acid mediated Friedel-Crafts arylation, electrocyclization, intramolecular cyclization, cycloaddition, C-N bond-formations, aromatization and cascade domino reactions, metal-catalyzed, iodine catalyzed reactions and multi-component reactions. This review article mainly focuses on Lewis acid-mediated recent synthetic methods to access a variety of electron-rich and electron-poor functional groups substituted carbazole frameworks in one-pot reactions. Polyaromatic carbazole and their related nitrogen-based heterocyclic compounds were found in several synthetic applications in pharma industries, energy devices, and materials sciences. Moreover, the review paper briefly summarised new synthetic strategies of carbazole preparation approaches will assist academic and pharma industries in identifying innovative protocols for producing poly-functionalized carbazoles and related highly complex heterocyclic compounds and discovering active pharmaceutical drugs or carbazole-based alkaloids and natural products.

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed.

An aggregation induced emission active bis-heteroleptic ruthenium(II) complex for luminescence light-up detection of pyrophosphate ions

A red emissive ruthenium(II) complex 1[PF6]2 of an amino ethanol substituted 1,10-phenanthroline-based ligand (L1) has been developed and characterized by spectroscopic analysis and single-crystal X-ray diffraction. Complex 1 shows an aggregation-induced emission (AIE) enhancement and forms nano-aggregates in the poor solvent water and highly dense polyethylene glycol (PEG) media. The possible reason behind the AIE properties may be the rigidity gained through weak supramolecular interactions between neighbouring phenanthroline ligands and PF₆^- counterions. The AIE properties were supported by UV-vis and photoluminescence (PL) spectroscopy and dynamic light scattering (DLS) studies to substantiate the formation of nano-aggregates and to understand the morphology of the aggregated particles, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were performed. Compound 1[PF6]2 was highly selective towards pyrophosphate ions (PPi) over other phosphates such as ATP, ADP, AMP and H₂PO₄^- ions and other competitive anions in the PL spectroscopic channel in acetonitrile. The PL titrations of 1[PF6]2 with PPi in CH₃CN furnished the association constant K_a = 1.08 × 10⁴ M^-1 and the detection limit was calculated as low as 1.54 μM. The PPi detection has been established through the unique H-bonding interaction, supported by ¹H NMR titration. Finally, the cytotoxicity study and bioimaging were carried out for biological application. The complex shows very low cytotoxicity and good biocompatibility and is suitable for intracellular PPi imaging.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

Fused 6/5/6 Metallocycle-Based Tetradentate Pt(II) Emitters for Efficient Green Phosphorescent OLEDs

Pt(II) complexes are promising phosphorescent materials for organic light-emitting diode (OLED) applications in the fields of display, lighting, healthcare, aerospace, and so on. A series of novel biphenyl (bp)-based tetradentate 6/5/6 Pt(II) emitters using oxygen or carbon as a linking atom was designed and developed. The intermolecular interactions in crystal packing, electrochemical, and photophysical properties of the bp-based Pt(II) emitters and also their excited-state properties were systematically studied, which could be effectively regulated by ligand modification through linking group control; however, their emission spectra nearly showed no change. All the bp-based Pt(II) emitters exhibited vibronically featured emission spectra with dominant peaks at 502-505 nm and photoluminescent quantum yields of 24-34% in dichloromethane solution. Green OLED using Pt(bp-12) as an emitter achieved a maximum brightness (L_max) of 16,644 cd/m².

Indolo[3,2,1-jk]carbazole-Derived Narrowband Violet-Blue Fluorophores: Tuning the Optical and Electroluminescence Properties by Chromophore Juggling

The development of rigid polyaromatic building blocks for narrowband violet fluorophores has received tremendous attention. Herein, we designed and synthesized two new triangle-shaped rigid building blocks, namely, 2,5-di-tert-butylindolo[3,2,1-jk]carbazole (tBuICz) and 2,11-di-tert-butylindolo[3,2,1-jk]carbazole-4-carbonitrile (tBuICzCN), and tethered them with different chromophores to yield a series of violet-blue fluorophores, viz., ICzTPA-ICzPICN, and studied their structure-function relationship. The appended chromophores and cyano unit played a vital role in controlling the optical and electrical properties of the compounds. Except triphenylamine-substituted derivatives, the compounds showed pure violet emission (λ_em ≤ 403 nm). Intriguingly, the compounds exhibited narrow-band emission with a full-width at half-maximum ≤ 40 nm, attributed to the rigidity of the ICz core. The emission of the compounds displayed positive solvatochromism, which is ascribed to the photoinduced intramolecular charge transfer in the excited state. The compounds revealed excellent thermal robustness with T_5d ≥ 363 °C. The triphenylamine-featuring derivatives displayed a high-lying HOMO compared to their congeners due to their electron-rich nature. When we applied these materials in organic light-emitting diodes, ICzPI outperformed in the series with an EQE_max of 3.07% and a current efficiency of 1.04 cd/A. Notably, its CIEy ∼ 0.046 precisely matched with the Rec.2020 standard of deep-blue color (CIEy ∼ 0.046).

Design and synthesis of yellow- to red-emitting gold(III) complexes containing isomeric thienopyridine and thienoquinoline moieties and their applications in operationally stable organic light-emitting devices

A new class of yellow- to red-emitting carbazolylgold(III) complexes containing isomeric thienopyridine or thienoquinoline moieties in the cyclometalating ligand has been designed and synthesized, which showed high photoluminescence quantum yields of over 80% in solid-state thin films. The isomeric effect and extended π-conjugation of the N-heterocycles have been found to remarkably perturb the photophysical, electrochemical and electroluminescence properties of the gold(III) complexes. In particular, the operational lifetimes of organic light-emitting devices based on that incorporated with thieno[2,3-c]pyridine are almost three orders of magnitude longer than that incorporated with thieno[3,2-c]pyridine. This has led to long device operational stability with a LT₇₀ value of up to 63 200 h at a luminance of 100 cd m^-2 and a long half-lifetime of 206 800 h, as well as maximum external quantum efficiencies of up to 8.6% and 14.5% in the solution-processed and vacuum-deposited devices, respectively. This work provides insights into the development of robust and highly luminescent gold(III) complexes and the identification of stable molecular motifs for designing efficient emitters.

Realization of Efficient Phosphorescent Organic Light-Emitting Devices Using Exciplex-Type Co-Host

High-performance phosphorescent organic light-emitting devices with an exciplex-type co-host were fabricated. The co-host is constituted by 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene, and 4,4,4-tris (N-carbazolyl) triphenylamine, and has obvious virtues in constructing efficient devices because of the thermally activated delayed fluorescence (TADF) resulting from a reverse intersystem crossing (RISC) process. The highest external quantum efficiency and luminance are 14.60% and 100,900 cd/m2 for the optimal co-host device. For comparison, 9.22% and 25,450 cd/m2 are obtained for a device employing 4,4,4-tris (N-carbazolyl) triphenylamine as a single-host. Moreover, the efficiency roll-off is notably alleviated for the co-host device, indicated by much higher critical current density of 327.8 mA/cm2, compared to 120.8 mA/cm2 for the single-host device. The alleviation of excitons quenching resulting from the captured holes and electrons, together with highly sufficient energy transfer between the co-host and phosphorescent dopant account for the obvious boost in device performances.

Enthalpy formation of fluorene: a challenging problem for theory or experiment?

The large discrepancy between the experimental enthalpy of formation of fluorene and theoretical value calculated by the G3(MP2) method was revealed more than ten years ago. Three years later, a new experimental study of this compound was undertaken to ascertain whether there is any significant error in the thermochemical data. However, after this research, the agreement between theory and experiment was improved only slightly. In this work we decided to calculate the enthalpy of formation of fluorene using the high-level DLPNO-CCSD(T₁)/CBS method which shows better results compared to Gn theories. To examine the accuracy of the available experimental data, the calculations were performed not only for fluorene but also for eleven fluorene derivatives. The discrepancy of about 9 kJ mol^-1 between the experimental and theoretical enthalpies of formation of fluorene was confirmed by the present calculations, whereas good agreement was observed for the fluorene derivatives. It is highly unlikely that this discrepancy may disappear when using a higher-level theory. The possible reason for such inconsistency might be the experimental difficulty associated with the glass transition discovered in the stable crystalline state of fluorene. In this case, new experiments using the latest methods, such as differential scanning calorimetry combined with X-ray powder diffraction, are needed to gain deeper insight into the solid phase transformations of fluorene.

Tetradentate Platinum(II) and Palladium(II) Complexes Containing Fused 6/6/6 or 6/6/5 Metallocycles with Azacarbazolylcarbazole-Based Ligands

A series of novel tetradentate Pt(II) and Pd(II) complexes containing fused 6/6/6 or 6/6/5 metallocycles employing azacarbazolylcarbazole (ACzCz)-based ligands was developed. Systematic experimental and theoretical studies suggest that both the ligand structures and the central metal ions have great influences on the electrochemical and photophysical properties of the complexes. The time-dependent density functional theory (TD-DFT) calculations and natural transition orbital (NTO) analyses reveal that the Pt(II) complexes possess 10.8-15.2% metal-to-ligand charge transfer (³MLCT) mixed with ligand-centered (³LC) characters, by contrast, the Pd(II) complexes exhibit significantly decreased 4.2-7.1% ³MLCT characters and enhanced ³LC compositions. All of the Pt(II) and Pd(II) complexes possess various channels for the intersystem crossing (ISC) on the basis of small energy gaps ΔE_S1-Tn and matching transition orbital compositions; moreover, Pd(ACzCz-1) and Pd(ACzCz-2) also possess efficient reverse intersystem crossing (RISC) to show both delayed fluorescence (DF) and phosphorescence in PMMA films at room temperature (RT). Pt(ACzCz-3) has Φ_PL values of 57% with a τ of 5.1 μs in dichloromethane at RT and 50% with 3.9 μs in PMMA at RT. Notably, Pd(ACzCz-1) exhibits ultralong low-temperature phosphorescence with a τ of 1307 μs. Pt(ACzCz-2)-based green OLED employing 26mCPy as the host demonstrated a peak EQE of 8.2% and a L_max of 24065 cd/m².

Highly Efficient Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/6 Metallocycles

A series of neutral tetradentate Pt(II) complexes with fused 6/5/6 metallocycles and biphenyl (bp)-containing ligands have been designed and synthesized. All bridging atoms adopt nitrogens designed as an acridinyl group (Ac), an aza acridinyl group (AAc), and an aza carbazolyl group (ACz), which can effectively tune their LUMO energy levels. Their HOMO energy levels can be well-controlled through molecular modifications on the bp moieties with electron-donating and electron-withdrawing groups. These molecular modifications also have profound effects on the electrochemical and photophysical properties and photostabilities of the Pt(II) complexes. The ground-states and excited states are systematically studied by density functional theory (DFT), time-dependent density functional theory (TD-DFT), and natural transition orbital (NTO) calculations. All the Pt(II) complexes exhibit admixed ³(LC/MLCT) characters in T₁ states with various proportions, which are strongly structure-dependent. These 6/5/6 Pt(II) complexes demonstrate high quantum efficiencies in dichloromethane solutions (Φ_PL = 27-51%) and in doped PMMA films (Φ_PL = 36-52%) at room temperature with short luminescence lifetimes of 1.6-9.5 μs and 7.6-9.0 μs, respectively. They emit green light with dominant peaks of 512-529 nm in solutions and 512-524 nm in doped PMMA films, respectively. Importantly, Pt(bp-2) exhibits highly stable emission colors with the same dominant peaks at 512 nm in various matrixes and also demonstrates a long photostability lifetime, LT₈₀, at 80% of initial luminance, of 190 min, which is doped in polystyrene films (5 wt %) excited by UV light of 375 nm at 500 W/m². These studies indicate that these 6/5/6 Pt(II) complexes can act as good phosphorescent emitters for OLED applications and should provide a viable route for the development of efficient and stable Pt(II)-based phosphorescent emitters.

A theoretical study on the electronic structure and phosphorescence properties of two series of iridium(iii) complexes with a four-membered Ir–S–C–S chelating ring

The electronic structure and photophysical properties of two series of iridium(iii) complexes with a four-membered Ir–S–C–S chelating ring have been theoretically studied using the DFT and TDDFT method.