Small Molecule Host Research Articles

Polymer Hosts Containing Carbazole-Dibenzothiophene-Based Pendants for Application in High-Performance Solution-Processed TADF-OLEDs.

The film-forming capability of the host plays a crucial role in effectively forming a light-emitting layer through a solution process in organic light-emitting diodes (OLEDs). In this study, we synthesized two side-chain polymer hosts, PCz-DBT and P2Cz-DBT, consisting of carbazole and dibenzothiophene. The synthesis was carried out through radical polymerization using styrene-based host monomers. Their photophysical characteristics and molecular energy levels are similar to those of the reference small molecule hosts, namely, Cz-DBT and 2Cz-DBT. However, compared to the small-molecule hosts Cz-DBT and 2Cz-DBT, the two polymer hosts showed high thermal stability and good film-forming properties in the neat and host-emitter blend films. Specifically, bluish-green multiple-resonance (MR) thermally activated delayed fluorescence (TADF) OLEDs, fabricated via solution processing with an emissive layer based on P2Cz-DBT, exhibited remarkable performance. These devices achieved a maximum external quantum efficiency of 17.4% without utilizing a hole transport layer. This polymer host design strategy is considered to significantly contribute to enhancing the performance of TADF-OLEDs fabricated through solution processing.

Understanding Spin-Triplet Excited States in Carbene-Metal-Amides.

Carbene-metal-amides (CMAs) are emerging delayed fluorescence materials for organic light-emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time-resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge-transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host : guest films of CMAs and small-molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited-state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor-moiety 3LE states to spectral features, with no strong evidence for a low-lying acceptor-centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Brilacidin, a host defense peptide mimetic, potentiates ibrexafungerp antifungal activity against the human pathogenic fungus Aspergillus fumigatus.

Invasive fungal infections have a high mortality rate causing more deaths annually than tuberculosis or malaria. Aspergillus fumigatus causes a series of distinct invasive fungal infections have a high mortality rate causing more deaths annually than tuberculosis or malaria. A. fumigatus causes a spectrum of distinct clinical entities named aspergillosis, which the most severe form is the invasive pulmonary aspergillosis. There are few therapeutic options for treating aspergillosis and searching for new antifungal agents against this disease is very important. Here, we present brilacidin (BRI) as a synergizer o fibrexafungerp (IBX) against A. fumigatus. BRI is a small molecule host defense peptide mimetic that has previously exhibited broad-spectrum immunomodulatory/anti-inflammatory activity against bacteria and viruses. We propose the combination of BRI and IBX as a new antifungal combinatorial treatment against aspergillosis.

Brilacidin, a novel antifungal agent against Cryptococcus neoformans.

Invasive fungal infections have a high mortality rate causing more deaths annually than tuberculosis or malaria. Cryptococcosis, one of the most prevalent fungal diseases, is generally characterized by meningitis and is mainly caused by two closely related species of basidiomycetous yeasts, Cryptococcus neoformans and Cryptococcus gattii. There are few therapeutic options for treating cryptococcosis, and searching for new antifungal agents against this disease is very important. Here, we present brilacidin (BRI) as a potential antifungal agent against C. neoformans. BRI is a small molecule host defense peptide mimetic that has previously exhibited broad-spectrum immunomodulatory/anti-inflammatory activity against bacteria and viruses. BRI alone was shown to inhibit the growth of C. neoformans, acting as a fungicidal drug, but surprisingly also potentiated the activity of caspofungin (CAS) against this species. We investigated the mechanism of action of BRI and BRI + CAS against C. neoformans. We propose BRI as a new antifungal agent against cryptococcosis.

Tetrakis(benzoxazine) calix[4]resorcinarenes as hosts for small molecules

Tetrakis(benzoxazine) calix[4]resorcinarenes as hosts for small molecules

Solution-processable red phosphorescent OLEDs based on Ir(dmpq)2(acac) doped in small molecules as emitting layer

Stable red phosphorescent materials are of high importance in OLED applications and a promising scenario relies on the organometallic emitters, such as phosphorescent dopants, constituting an effective emissive layer of OLEDs, which is usually embedded in an appropriate host matrix. Iridium (III) complexes are the most widely used dopants in Phosphorescent OLEDs because of their high internal quantum efficiency. In this work, Bis(2-(3,5-dimethylphenyl)quinoline-C,N)(acetylacetonato)iridium(III), (Ir(dmpq)2(acac)), was doped in four different small molecule host materials, such as 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-Bis(N-carbazolyl)benzene (mCP), 1,1-Bis[(di-4-tolylamino) phenyl]cyclohexane (TΑPC) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), with doping concentrations of 4%, 6%, and 8%. Τhe net host materials and the Ir(dmpq)2(acac) were first characterized in respect of their optical and photophysical properties. Following, the doped thin films were implemented as the active layers in OLED devices, formed via the solution deposition method. It was found that OLED devices fabricated for all studied cases emit red light, a characteristic of Ir(dmpq)2(acac), with a maximum wavelength of approximately 620 nm. By exploring the different combinations between the host and the dopant, as well as the different doping concentrations, we aim to provide insights into the selection of the best performing host materials for PhOLEDs.

Structure-guided design of direct-acting antivirals that exploit the gem-dimethyl effect and potently inhibit 3CL proteases of severe acute respiratory syndrome Coronavirus-2 (SARS-CoV-2) and middle east respiratory syndrome coronavirus (MERS-CoV)

The high morbidity and mortality associated with SARS-CoV-2 infection, the etiological agent of COVID-19, has had a major impact on global public health. Significant progress has been made in the development of an array of vaccines and biologics, however, the emergence of SARS-CoV-2 variants and breakthrough infections are an ongoing major concern. Furthermore, there is an existing paucity of small-molecule host and virus-directed therapeutics and prophylactics that can be used to counter the spread of SARS-CoV-2, and any emerging and re-emerging coronaviruses. We describe herein our efforts to address this urgent need by focusing on the structure-guided design of potent broad-spectrum inhibitors of SARS-CoV-2 3C-like protease (3CLpro or Main protease), an enzyme essential for viral replication. The inhibitors exploit the directional effects associated with the presence of a gem-dimethyl group that allow the inhibitors to optimally interact with the S4 subsite of the enzyme. Several compounds were found to potently inhibit SARS-CoV-2 and MERS-CoV 3CL proteases in biochemical and cell-based assays. Specifically, the EC50 values of aldehyde 1c and its corresponding bisulfite adduct 1d against SARS-CoV-2 were found to be 12 and 10 nM, respectively, and their CC50 values were >50 μM. Furthermore, deuteration of these compounds yielded compounds 2c/2d with EC50 values 11 and 12 nM, respectively. Replacement of the aldehyde warhead with a nitrile (CN) or an α-ketoamide warhead or its corresponding bisulfite adduct yielded compounds 1g, 1eand1f with EC50 values 60, 50 and 70 nM, respectively. High-resolution cocrystal structures have identified the structural determinants associated with the binding of the inhibitors to the active site of the enzyme and, furthermore, have illuminated the mechanism of action of the inhibitors. Overall, the high Safety Index (SI) (SI=CC50/EC50) displayed by these compounds suggests that they are well-suited to conducting further preclinical studies.

Untargeted Multimodal Metabolomics Investigation of the Haemonchus contortus Exsheathment Secretome.

In nematodes that invade the gastro-intestinal tract of the ruminant, the process of larval exsheathment marks the transition from the free-living to the parasitic stages of these parasites. To investigate the secretome associated with larval exsheathment, a closed in vitro system that effectively reproduces the two basic components of an anaerobic rumen environment (CO2 and 39 °C) was developed to trigger exsheathment in one of the most pathogenic and model gastrointestinal parasitic nematodes, Haemonchus contortus (barber‘s pole worm). This study reports the use of multimodal untargeted metabolomics and lipidomics methodologies to identify the metabolic signatures and compounds secreted during in vitro larval exsheathment in the H. contortus infective third-stage larva (iL3). A combination of statistical and chemoinformatic analyses using three analytical platforms revealed a panel of metabolites detected post exsheathment and associated with amino acids, purines, as well as select organic compounds. The major lipid classes identified by the non-targeted lipidomics method applied were lysophosphatidylglycerols, diglycerides, fatty acyls, glycerophospholipids, and a triglyceride. The identified metabolites may serve as metabolic signatures to improve tractability of parasitic nematodes for characterizing small molecule host–parasite interactions related to pathogenesis, vaccine and drug design, as well as the discovery of metabolic biomarkers.

Production of the Extremolyte Cyclic 2,3-Diphosphoglycerate Using Thermus thermophilus as a Whole-Cell Factory

Osmolytes protect microbial cells against temperature, osmolarity and other stresses. The osmolyte cyclic 2,3-diphosphoglycerate, originally isolated from the thermophilic archaeon Methanothermus fervidus, naturally protects cellular proteins under extreme conditions. The biosynthetic pathway for cyclic 2,3-diphosphoglycerate has been introduced into the thermophilic bacterium Thermus thermophilus. The two enzymes in this synthetic pathway, 2-phosphoglycerate kinase and cyclic diphosphoglycerate synthetase, were incorporated into a newly designed modular BioBricks vector. The expression of this two-enzyme cascade resulted in the whole cell production of cyclic 2,3 diphosphoglycerate. In vivo production of cyclic 2,3-diphosphoglycerate was confirmed by mass spectrometry to a concentration up to 650 µM. This study demonstrates the feasibility of using this well studied thermophilic bacterium as a host in a whole-cell factory approach to produce cyclic 2,3 diphosphoglycerate. This raises the potential for commercialisation of cDPG for cosmetic and healthcare applications. Our work demonstrates the application of Thermus thermophilus as an alternative host for other high value small organic molecules of industrial interest.

Dimers or Solid‐State Solvation? Intermolecular Effects of Multiple Donor–Acceptor Thermally Activated Delayed Fluorescence Emitter Determining Organic Light‐Emitting Diode Performance

AbstractOrganic light‐emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid‐state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6‐penta(carbazol‐9‐yl)benzoate (5CzCO2Me), is provided. Steady‐state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid‐state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light‐emitting diode (OLED) in terms of current–voltage–luminance and external quantum efficiency (EQE) parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5%. It is shown that the emitter transports holes and that charge‐carrier recombination does not take place on the bandgap of the host, but rather, a mixed host–guest concentration‐dependent recombination is seen. The hole‐transporting properties of 5CzCO2Me allow for a new dimension in tuning the device performance by controlling the emitter concentration.

Palladium-Catalyzed Generation of ortho-Quinone Methides. A Three-Component Synthesis of L-Shaped Dimeric Steroidal Scaffolds.

A series of hybrid dimers having orthogonal steroidal cores bridged by a chroman ketal moiety were obtained by Pd-catalyzed three-component reactions of steroid alkynols, 2-formylestradiol 17-monoacetate, and methyl orthoformate, via ortho-quinone methide intermediates. One of the obtained L-shaped scaffolds showed an inefficient crystal packing featuring large channels within the crystal array. Monte Carlo simulations indicate that these voids preferentially allocate n-hexane, opening the way to explore further applications of similar organic crystalline materials as selective hosts for small molecules.

Rational Design of Carbazole- and Carboline-Based Polymeric Host Materials for Realizing High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diode.

Recently, various host materials have been developed for solution-processed thermally activated delayed fluorescent organic light-emitting diodes (TADF-OLEDs). Compared with small-molecule hosts, polymeric hosts are advantageous for inducing a uniform distribution and segregation of dopant molecules in the emissive layer without undesired large-scale phase separation. In this study, new polymer hosts were demonstrated, in which a bipolar conjugative moiety consisting of a carbazole (Cz) donor and an α-carboline (α-Cb) acceptor was bound to the polystyrene backbone through a non-conjugated linker. They exhibited high triplet energies of >2.8 eV, and their emission spectra overlapped with the absorption spectrum of a green TADF emitter, which allowed facile energy transfer from the polymeric host to the small-molecule dopants. High device performance was observed, with external quantum efficiencies (EQEs) of 13.65, 17.09, and 17.48% for solution-processed green TADF-OLEDs using PSCzCz, PSCzCb, and PSCbCz, respectively, as hosts for the EML. The EQEs of bipolar host (PSCzCb and PSCbCz)-based devices were higher than those of unipolar host (poly(N-vinylcarbazole) and PSCzCz)-based devices owing to the well-balanced charge-carrier transport. According to these results, the polymeric host bearing a bipolar Cz and α-Cb coupled moiety is a promising material for solution-processable TADF-OLEDs.

Inkjet printing a small-molecule binary emitting layer for organic light-emitting diodes

Inkjet printing a synthesized small-molecule host material and its blend for OLEDs with a single solvent.

Cyclohexane-cored dendritic host materials with high triplet energy for efficient solution-processed blue thermally activated delayed fluorescence OLEDs

Two dendritic host materials (CH-2D1 and CH-2D2) are developed by using non-conjugated cyclohexane core and two first/second generation carbazole dendrons, which show excellent solubility in organic solvents, high thermal stability and high triplet energy (ET) of 2.78–2.84 eV, making them suitable for fabrication of solution-processed blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Compared to CH-2D1 with the first-generation carbazole dendron which shows low-lying highest occupied molecular orbital (HOMO) energy level of −5.48 eV, CH-2D2 with the second-generation carbazole dendrons exhibits higher HOMO level of −5.37 eV, which is more favourable for hole injection from the anode to the emissive layers. Consequently, solution-processed blue TADF OLEDs utilizing CH-2D2 as host show promising device performance with a low turn-on voltage of 3.4 V, maximum external quantum efficiency of 17.8%, maximum luminous efficiency of 41.3 cd A−1 and power efficiency of 33.9 lm W−1, which can compare with the most efficient solution-processed blue TADF OLEDs based on small-molecule and polymer hosts. These results indicate that dendritic host materials with well-defined chemical structure and good solubility are an attractive approach―beyond the soluble small-molecule hosts and polymer hosts―for the development of efficient solution-processed TADF OLEDs.

31.2: Invited Paper: Advanced Materials Development for Ink‐Jet Printed OLEDs

High‐performance, ink‐jet printable OLED materials have been successfully developed. Our wide libraries of platforms and small molecule hosts were effective for improving efficiency and lifetime of OLEDs. For wide color gamut, new emitters with deep in color and narrow full‐width at half maximum were developed. Further, solvent systems having physical properties suitable for improving film uniformity in pixels were found.

Exploring the structural landscape of 2-(thiophen-2-yl)-1,3-benzothiazole: high-Z' packing polymorphism and cocrystallization with calix[4]tube.

We report here for the first time a cocrystal of the so-called neutral calix[4]tube, which is two tail-to-tail-arranged and partially deprotonated tetrakis(carboxymethoxy)calix[4]arenes, including three sodium ions, with 2-(thiophen-2-yl)-1,3-benzothiazole, namely trisodium bis(carboxymethoxy)bis(carboxylatomethoxy)calix[4]arene tris(carboxymethoxy)(carboxylatomethoxy)calix[4]arene-2-(thiophen-2-yl)-1,3-benzothiazole-dimethyl sulfoxide-water (1/1/2/2), 3Na+·C36H30O122-·C36H31O12-·C11H7NS2·2C2H6OS·2H2O, which provides a new approach into the host-guest chemistry of inclusion complexes. Three packing polymorphs of the same benzothiazole with high Z' (one with Z' = 8 and two with Z' = 4) were also discovered in the course of our desired cocrystallization. The inspection of these polymorphs and a previously known polymorph with Z'= 2 revealed that Z' increases as the strength of intermolecular contacts decreases. Also, these results expand the frontier of invoking calixarenes as a host for nonsolvent small molecules, besides providing knowledge on the rare formation of high-Z' packing polymorphs of simple molecules, such as the target benzothiazole.

Bulkyl diphenylamine functionalized spiro[fluorene-9,9′- xanthene] based solution-processible small molecule host in red PhOLEDs for investigating structure-property relationship

A novel diphenylamine/spiro[fluorene-9,9′-xanthene] (SFX) hybrid (SFX27DPA) was constructed successfully by incorporating diphenylamine into SFX via Pd-catalyzed CN cross-coupling to ingeniously develop state-of-the-art models of solution-processible small molecule host for uncovering the structure-property relationship of molecular structure change in the application of PhOLEDs, which is vital to rational cumulative enhancement in device performance by purposive and feasible structure modifications. SFX27DPA shows outstanding thermal stability (Td, 425 °C) compared to those of SFXDPA ((Spiro[fluorene-9,9′-xanthen]-2-yl)-diphenylamine) (Td, 338 °C) and SFX-Cz ((2-carbazolyl-spiro[fluorene-9,9′-xanthene]) (Td, 345 °C) by increasing the molecular weight but owns weak morphological stability due to its low melting point (Tc, 60.5 °C). The HOMO (−5.28 eV), LUMO (−2.08 eV), and energy gap (Eg, 3.20 eV) were obtained by cyclic voltammetry measurements (CVs). The high T1 (T1, 2.98 eV) and narrow energy gap (Eg, 3.20 eV) are beneficial to achieving high efficient device. The compound not only possesses a high triplet energy level but also exhibits good solubility in common solvents. Two SFX-based compounds (SFX27DPA, SFX-Cz) for red PhOLEDs have been fabricated successfully by solution process for deep researching the relationship between molecular structure and device performance. An enhanced performance of device based on SFX27DPA with maximum external quantum efficiencies (EQEs) of 10.9% under lower driving voltage was obtained compared to that of SFX-Cz-based red PhOLEDs (1.5%). This investigation offers good reference for future molecular design to achieve high performance through accumulative structure modification.

Analysis of organic multilayer structures using a combined grazing incidence X-ray fluorescence/X-ray reflectometry approach

Organic light-emitting diodes (OLEDs) continue to attract research interest due to their application in bright displays and solid-state lighting. The highest efficiency values are reached with multilayer devices. The morphology of these layers is important for device performance. A current field of development is the application of solution-based deposition techniques. In general, solution processed devices do not yet reach the performance of OLEDs with vapor deposited materials. In this study, we used typical OLED materials with a sulfur based small molecule host, which can be vapor deposited as well as solution processed, to prepare single and multilayer samples. Samples consisted of the host layer deposited on a hole transport and a buffer layer and were investigated with a combination of grazing incidence X-ray fluorescence (GIXRF) and X-ray reflectometry (XRR). Measurements were performed using a diffractometer with an added fluorescence detector. The angle curves obtained have been evaluated with the software package JGIXA. Significant differences between differently prepared samples have been observed for single layer and multilayer samples. Sulfur could be detected in the layers beneath the solution processed host layer, but not in the layers below the vapor deposited host. The obtained results demonstrate the combination of GIXRF and XRR as a suitable nondestructive analysis method for the characterization of organic thin films with little to no sample preparation required.

Ionic Organic Small Molecules as Hosts for Light-Emitting Electrochemical Cells.

Light-emitting electrochemical cells (LEECs) from ionic transition-metal complexes (iTMCs) offer the potential for high-efficiency electroluminescence in a simple, single-layer device. However, LEECs typically rely on the use of rare metal complexes. This has limited their cost effectiveness and put constraints on their applicability. With a view to leveraging the efficient emission of these complexes while mitigating costs, we describe here a host/guest LEEC strategy that relies on the use of carbazole (Cz)-based organic small-molecule hosts and iTMC guests. Three cationic host molecules were prepared via the coupling of 1-(4-bromophenyl)-2-phenylbenzimidazole (PBI-Br) with Cz. This has allowed a comparison between the hosts bearing methoxy (PBI-CzOMe) and tert-butyl (PBI-Cz tBu) substituents, as well as an unsubstituted analogue (PBI-CzH). Cyclic voltammetry and UV-visible absorption revealed that allthree host materials have wide band gaps characterized by reversible oxidation and irreversible reduction events. On the basis of electronic structure calculations, the host highest occupied molecular orbital (HOMO) resides primarily on the Cz moiety, whereas the lowest unoccupied molecular orbital (LUMO) is located primarily on the phenyl-benzimidazolium unit. Photoluminescence analysis of thin-film blends of PBI-CzH with iTMC guests confirmed that the emission was blue-shifted relative to pristine iTMC films, which is consistent with what was seen in dilute dichloromethane solution.LEEC devices were prepared based on thin films of the pristine hosts, pristine guests, and 90%/10% (w/w) host/guest blends. Among these host/guest blends, LEECs based on PBI-CzH displayed the best performance, particularly when an iridium complex was used as the guest. The system in question yielded a luminance maximum of 624 cd/m2 at an external quantum efficiency of 3.80%. This result stands in contrast to what is seen with typical organic light-emitting diode host studies, where tert-butyl substitution of the host generally leads to a better performance. To rationalize the present observations, the host materials were subject to single-crystal X-ray diffraction analysis. The resulting structures revealed clear head-to-tail interactions in the case of both PBI-CzH and PBI-CzOMe. No such interactions were evident in the case of PBI-Cz tBu. Furthermore, PBI-CzH showed a relatively smaller spacing between the successive HOMO and successive LUMO levels relative to PBI-CzOMe and PBI-Cz tBu, a finding consistent with more favorable charge transport and energy transfer. The results presented here can help inform the design and preparation of host materials suitable for use in single-layer iTMC LEECs.

Electroluminescence Properties of IrQ(ppy)2 Dual-Emitter Organometallic Compound in Organic Light-Emitting Devices

This paper reports the influence of the charge carrier mobility on the electroluminescent properties of a dual-emitter organometallic compound dispersed in two conjugated organic small-molecule host materials and embedded in organic light-emitting devices (OLEDs). The electroluminescent processes in OLEDs are strongly influenced by the host–guest interaction. The charge carrier mobility in the host material plays an important role in the electroluminescent processes but also depends on the triplet–triplet interaction with the organometallic compound. The low charge carrier mobility in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host material reduces the electroluminescent processes, but they are slightly enhanced by the triplet–triplet exothermic charge transfer. The higher charge carrier mobility in the case of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) host material influences the electroluminescent processes by the endothermic energy transfer at room temperature, which facilitates the triplet–triplet harvesting in the host–guest system. The excitation is transferred to the guest molecules by triplet–triplet interaction as a Dexter transfer, which occurs by endothermic transfer from the triplet exciton in the host to the triplet exciton in the guest.