Phenylene Bridge Research Articles

Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host.

Up to now, the most efficient blue phosphorescent organic light-emitting diode (PhOLED) was achieved with a maximum external quantum efficiency (ηext) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9'-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a ηext of 29.4% at a practical luminance of 100 cd m-2 and a maximum ηext of 34.6% (at 23 cd m-2). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A-1) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9'-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

Backbone‐Acceptor/Pendant‐Donor Strategy for Efficient Thermally Activated Delayed Fluorescence Conjugated Polymers with External Quantum Efficiency Close to 25% and Emission Peak at 608 nm

AbstractA feasible strategy, backbone‐acceptor and pendant‐donor, for constructing thermally activated delayed fluorescence (TADF) conjugated polymer is presented. In the obtained polymers PSAQFx, the backbone consists of dibenzothiophene‐S,S‐dioxide (S) and 6,7‐difluoroquinoxaline (QF), while two 9,10‐dihydroacridine (A) groups as pendants are grafted on QF acceptor through phenylene bridges. Compared to the model compound AQF, the polymers exhibit redshifted emission owing to extended conjugation along the acceptor backbone. Photoluminescence quantum yields can be improved by managing the molar content (x) of the AQF unit and reach the maximum of 91% for PSAQF5 in neat film. Solution‐processed organic light‐emitting diodes (OLEDs) using these polymers as the emitters achieve a maximum external quantum efficiency (EQE) of 24.8% and a maximum power efficiency of 68.5 lm W−1 with the emission peak at 608 nm and luminance at 58 cd m−2. This is the first example of achieving EQE above 20% with emission peak over 600 nm in the polymer TADF OLEDs.

Reversibly Switchable Phase-Dependent Emission of Quinoline and Phenothiazine Derivatives towards Applications in Optical Sensing and Information Multicoding.

Three new quinoline and di-tert-butyl phenothiazine isomeric derivatives were synthesized and characterized towards applications for oxygen sensing and optical information multicoding. The compounds with phenylene linker showed outstanding phase-dependent reversibility between ON/OFF states (low and high emission intensity, drastic shifting of emission colors, short- and long-lived fluorescence) in systematic grinding/fuming cycles, as required for multichannel memory devices based on optical information multicoding. The conformational diversity of the phenothiazine unit resulted in dual emission of the doped films implemented by the different luminescence mechanisms with peaks located at 414/530, 416/540, and 440/582 nm. The presence of a phenylene linker and thus two rotational degrees of freedom resulted in quenching of the delayed fluorescence of quasi-equatorial conformers in the solid state. The compound containing no phenylene bridge was characterized by two different driving photoluminescence mechanisms of the doped films: short fluorescence of the quasi-axial conformer and thermally activated delayed fluorescence of the quasi-equatorial form. This compound showed oxygen sensitivity with a Stern-Volmer constant of 7.5×10-4 ppm-1 .

Homobivalent Lamellarin-Like Schiff Bases: In Vitro Evaluation of Their Cancer Cell Cytotoxicity and Multitargeting Anti-Alzheimer's Disease Potential.

Marine alkaloids belonging to the lamellarins family, which incorporate a 5,6-dihydro-1-phenylpyrrolo[2,1-a]isoquinoline (DHPPIQ) moiety, possess various biological activities, spanning from antiviral and antibiotic activities to cytotoxicity against tumor cells and the reversal of multidrug resistance. Expanding a series of previously reported imino adducts of DHPPIQ 2-carbaldehyde, novel aliphatic and aromatic Schiff bases were synthesized and evaluated herein for their cytotoxicity in five diverse tumor cell lines. Most of the newly synthesized compounds were found noncytotoxic in the low micromolar range (<30 μM). Based on a Multi-fingerprint Similarity Search aLgorithm (MuSSeL), mainly conceived for making protein drug target prediction, some DHPPIQ derivatives, especially bis-DHPPIQ Schiff bases linked by a phenylene bridge, were prioritized as potential hits addressing Alzheimer’s disease-related target proteins, such as cholinesterases (ChEs) and monoamine oxidases (MAOs). In agreement with MuSSeL predictions, homobivalent para-phenylene DHPPIQ Schiff base 14 exhibited a noncompetitive/mixed inhibition of human acetylcholinesterase (AChE) with Ki in the low micromolar range (4.69 μM). Interestingly, besides a certain inhibition of MAO A (50% inhibition of the cell population growth (IC50) = 12 μM), the bis-DHPPIQ 14 showed a good inhibitory activity on self-induced β-amyloid (Aβ)1–40 aggregation (IC50 = 13 μM), which resulted 3.5-fold stronger than the respective mono-DHPPIQ Schiff base 9.

Fast Delayed Emission in New Pyridazine-Based Compounds.

Three novel donor-acceptor molecules comprising the underexplored pyridazine (Pydz) acceptor moiety have been synthesized and their structural, electrochemical and photophysical properties thoroughly characterized. Combining Pydz with two phenoxazine donor units linked via a phenyl bridge in a meta configuration (dPXZMePydz) leads to high reverse intersystem crossing rate kRISC = 3.9 · 106 s−1 and fast thermally activated delayed fluorescence (TADF) with <500 ns delayed emission lifetime. Efficient triplet harvesting via the TADF mechanism is demonstrated in OLEDs using dPXZMePydz as the emitter but does not occur for compounds bearing weaker donor units.

Design, Synthesis and Study of Non-Linear Optical Properties of Phenyl Bridged Diphenylamine-s-triazine Based Donor-Acceptor Triads Containing Different π-Acceptor Groups

Novel donor-acceptor triads of starburst D-A-A type incorporating electron deficient triazine moiety as a non-conjugating π-spacer/acceptor with two acceptor/anchoring arms comprising of cyanoacetic acid (DTP-CYA), rhodanine-3-acetic acid (DTP-RHA), barbituric acid (DTP-BA) or thiobarbituric acid (DTP-TBA) linked to triazine core via a phenyl bridge have been synthesized. Diphenylamine is used as the donor moiety and the role of the π-spacer on the absorption spectra and other electronic properties were studied. All the compounds were tested for their non-linear optical properties and determined the non-linear absorption coefficient (β) and non-linear refractive index (n2) in DMF solutions. Improved non-linear optical properties were obtained when compared to the compounds with triazine moiety as a non-conjugating π-spacer/acceptor with rhodanine-3-acetic acid (DTOP-RHA), barbituric acid (DTOP-BA) or thiobarbituric acid (DTOP-TBA) as anchoring/acceptor groups. DTOP series compounds are in the geometry of the molecule where the phenyl π-bridge and the triazine unit are coplanar with the arylidne acceptor unit. The enhanced performance may be due to the structural variants, which expected to improve through bond coupling between the donor and the terminal acceptor parts of the molecules and all compounds show asymmetry in the electron density due to the basic donor-acceptor nature of the structure.

Triphenylethylene-based emitters exhibiting aggregation induced emission enhancement and balanced bipolar charge transport for blue non-doped organic light-emitting diodes

New triphenylethylene-based derivatives, containing 2,6-di-tert-butylcarbazole or 9,9-dimethyl-9,10-dihydroacridine as donor moieties and phenylene or tetrafluorophenylene linking bridges were synthesised by Heck coupling and characterized. The synthesized compounds emitted blue light in the solid state with emission maxima in the range of 478–487 nm and photoluminescence quantum yields of 16–29%. In thermogravimetric analysis they demonstrated sublimation with 5% weight loss temperatures in the range of 307–355 °C. They exhibited glass-forming properties with glass-transition temperatures of 89–123 °C. The solid-state ionization potentials and electrochemical band gaps ranged from 5.88 to 6.37 eV and from 3.30 to 3.57 eV, respectively. Three compounds demonstrated bipolar semiconducting properties. Derivative of triphenylethylene and 2,6-di-tert-butylcarbazole with phenylene bridge showed highest hole and electron drift mobilities reaching 3.07 × 10−3 cm2/Vs and 4.2 × 10−3 cm2/Vs respectively at high electric field. Utilizing compounds in organic light‐emitting diodes as emitters external quantum efficiency of 2% was observed.

(Keynote) Radical Ion Excited States As Super-Redox Agents for Artificial Photosynthesis

We will discuss several examples in which photoexcitation of a stable radical anion or cation with visible or NIR light results in production of a high-potential radical ion excited state that can carry out difficult redox reactions relevant to artificial photosynthesis.First, we will show that selective excitation of a naphthalenediimide radical anion (NDI•−) covalently linked to the 4-, 5, or 6-positions of the bipyridine (bpy) in the Re(bpy)(CO)3X (X = Cl or pyridine) carbon dioxide reduction catalyst results in electron transfer from 2*NDI•− to Re(bpy)(CO)3X to form Re(bpy•−)(CO)3X, the first intermediate in the photocatalytic reduction of CO2. Femtosecond UV/Vis, near-IR and mid-IR spectroscopy on these constitutional isomers show that systematically varying the electronic coupling as well as the reaction free energy increases the lifetime of Re(bpy•−)(CO)3X by an order of magnitude when the NDI chromophore is attached to the 6-position of bpy.Electrochemical reduction of the corresponding Mn(bpy)(CO)3X CO2 reduction catalyst is thought to proceed by the initial reduction of MnI to Mn0. We have covalently attached a naphthalenediimide radical anion (NDI•-) chromophore to the 4-, 5-, or 6-positions of the bpy via a phenyl bridge to produce Mn(NDI•--bpy)(CO)3X, where X = Br, CH3CN, or DMF, and have used femtosecond and nanosecond transient IR spectroscopy to directly observe the intermediates produced by two electron transfer reactions following selective photo-excitation of NDI•−. In complexes where NDI•− is attached at the 4- or 5-positions of bipyridine, only the reaction Mn(2*NDI•−-bpy)(CO)3X → Mn(NDI-bpy•−)(CO)3X is observed, while in the complex where NDI•− is attached to the 6-position of bipyridine, the reaction sequence: Mn(2*NDI•−-bpy)(CO)3X → Mn(NDI-bpy•−)(CO)3X → Mn0(NDI-bpy)(CO)3 is observed. Moreover, in the complexes with an NDI•- bound to the 6-position of bipyridine, Mn0(NDI-bpy)(CO)3 exhibits a lifetime that is ~105 times longer than those in complexes with an NDI•- bound at the 4- or 5- positions of the bipyridine.On the oxidative side, we will discuss the 10-phenyl-10H-phenothiazine radical cation (PTZ+•), which has a manifold of excited doublet states accessible using visible and NIR light that can serve as super-photo-oxidants with excited state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photo-excitation of PTZ+• in CH3CN with a 517 nm laser pulse populates a Dn electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D1' (τ < 0.3 ps), followed by relaxation to D1 (τ = 10.9 ± 0.4 ps), which finally decays to D0 (τ = 32.3 ± 0.8 ps). D1' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D1' → D1 relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ+• photoexcited doublet states, PTZ+• was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenyl-anthracene (DPA), and 10-phenyl-9-anthracene-carbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs. SCE, respectively. In all cases, photoexcitation of PTZ+• result in ultrafast oxidation of Per, DPA, and ACN.The photoexcited peri-xanthenoxanthene radical cation (PXX+•) is another super-oxidant that has a 124 ps electronic excited state (D1) lifetime and can deliver +2.1 V vs. SCE of oxidizing potential. Photoexcitation of PXX+• covalently attached to electron deficient 9,10-bis(trifluoromethyl)anthracene (TMFA) using an 885 nm laser pulse drives oxidation of TFMA with unity quantum yield. Extending the PXX+•-TFMA dyad to a molecular triad having a 9,10-diphenylanthracene terminal hole acceptor, PXX+•-TFMA-DPA, and selectively exciting PXX+• results in formation of PXX-TFMA-DPA+• with a 46% quantum yield and a τ = 11.5 ± 0.6 ns lifetime. This work demonstrates that the PXX+• D1 electronic excited state can serve as a promising super-oxidant for challenging oxidation reactions relevant to solar-energy applications.

Theoretical investigation of non-Förster exciton transfer mechanisms in perylene diimide donor, phenylene bridge, and terrylene diimide acceptor systems.

The rates of exciton transfer within dyads of perylene diimide and terrylene diimide connected by oligophenylene bridge units have been shown to deviate significantly from those of Förster's resonance energy transfer theory, according to single molecule spectroscopy experiments. The present work provides a detailed computational and theoretical study investigating the source of such a discrepancy. Electronic spectroscopy data are calculated by time-dependent density functional theory and then compared with experimental results. Electronic couplings between the exciton donor and the acceptor are estimated based on both the transition density cube method and transition dipole approximation. These results confirm that the delocalization of the exciton to the bridge parts contributes to significant enhancement of donor-acceptor electronic coupling. Mechanistic details of exciton transfer are examined by estimating the contributions of the bridge electronic states, vibrational modes of the dyads commonly coupled to both donor and acceptor, inelastic resonance energy transfer mechanism, and dark exciton states. These analyses suggest that the contribution of common vibrational modes serves as the main source of deviation from Förster's spectral overlap expression.

Evolution from Tunneling to Hopping Mediated Triplet Energy Transfer from Quantum Dots to Molecules.

Efficient energy transfer is particularly important for multiexcitonic processes like singlet fission and photon upconversion. Observation of the transition from short-range tunneling to long-range hopping during triplet exciton transfer from CdSe nanocrystals to anthracene is reported here. This is firmly supported by steady-state photon upconversion measurements, a direct proxy for the efficiency of triplet energy transfer (TET), as well as transient absorption measurements. When phenylene bridges are initially inserted between a CdSe nanocrystal donor and anthracene acceptor, the rate of TET decreases exponentially, commensurate with a decrease in the photon upconversion quantum efficiency from 11.6% to 4.51% to 0.284%, as expected from a tunneling mechanism. However, as the rigid bridge is increased in length to 4 and 5 phenylene units, photon upconversion quantum efficiencies increase again to 0.468% and 0.413%, 1.5-1.6 fold higher than that with 3 phenylene units (using the convention where the maximum upconversion quantum efficiency is 100%). This suggests a transition from exciton tunneling to hopping, resulting in relatively efficient and distance-independent TET beyond the traditional 1 nm Dexter distance. Transient absorption spectroscopy is used to confirm triplet energy transfer from CdSe to transmitter, and the formation of a bridge triplet state as an intermediate for the hopping mechanism. This first observation of the tunneling-to-hopping transition for long-range triplet energy transfer between nanocrystal light absorbers and molecular acceptors suggests that these hybrid materials should further be explored in the context of artificial photosynthesis.

Aggregation-induced emission enhancement -active triarylamine-based polyamides containing fused ring groups towards electrochromic smart window and sensor for HCl and TNP

A series of new polyamides (PAs) were designed and synthesized based on two novel triarylamines (TAA) with fused ring groups via phenyl bridge through the Suzuki coupling reaction. These novel PAs have amorphous structure and excellent thermal stability, with a carbon residue rate of up to 70% at 800 °C in nitrogen atmosphere. In the process of electrochemical oxidation, there is no obvious recessionary in current and transmittance over 1000s with a cycle time of 10s, which showing high contrast and excellent cycle stability. And the electrofluorochromic (EFC) performance can also be adjusted by being applied voltage. 1NAT-CA and 9AAT-CA have solvatochromic effect in absorption, fluorescence (PL) in solvents, 1NAT-6F and 1NAT-BA have aggregation-induced emission enhancement (AIEE) properties in solid state. The kind of multifunctional integrated PAs show characteristics of responding to voltage, explosive and hazardous gas stimuli and the stimuli-responsive mechanisms are investigated, which have the prospect of application in photoelectric sensors, explosive detectors and high-contrast displays.

High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions...

Simultaneous Quantification of Lamivudine, Tenofovir Disoproxil Fumarate and Doravirine in Pharmaceutical Dosage Form by Liquid Chromatography with Diode Array Detection

A new simple, precise and robust isocratic reverse-phase high performance liquid chromatography (RP-HPLC) method was developed and validated for simultaneous determination of lamivudine, tenofovir disoproxil fumarate (TDF), and doravirine in bulk and pharmaceutical dosage form. The validation included specificity, linearity, system suitability, precision, robustness, LOD and LOQ characteristics. The chromatographic separation was achieved on C18X bridge phenyl column (150 × 4.6 mm, 3 μm particle size) eluted with acetonitrile and hexane-1-sulfonic acid (pH 2.5; 50:50, v/v) at a flow rate of 0.8 mL/min and monitored at 243 nm over a run time of 12 min. The retention times of lamivudine, TDF, and doravirine were found to be 2.45, 7.3, and 8.79 min. respectively. The method was linear in the range of 5 – 100 μg/mL (r2 = 0.999) for lamivudine and TDF and in the range of 1.75 – 35 μg/mL (r2 = 0.999) for doravirine. The percentage recoveries of three drugs were within the acceptable limits (98 – 102%). The method was found to be precise as confirmed by % RSD < 0.6. Forced degradation study was conducted as per ICH guidelines, and the three drugs showed degradation within 21.4 – 33.8% under acidic, basic, oxidative, photolysis, and hydrolysis conditions. The proposed RP-HPLC method can be used for the quantification of lamivudine, TDF, and doravirine in API and tablets without any interference from excipients.

High Triplet Energy Host Materials for Blue TADF OLEDs-A Tool Box Approach.

The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.

Imidazo[1,2-a]pyridine as an Electron Acceptor to Construct High-Performance Deep-Blue Organic Light-Emitting Diodes with Negligible Efficiency Roll-Off.

Two novel bipolar deep-blue fluorescent emitters, IP-PPI and IP-DPPI, featuring different lengths of the phenyl bridge, were designed and synthesized, in which imidazo[1,2-a]pyridine (IP) and phenanthroimidazole (PI) were proposed as an electron acceptor and an electron donor, respectively. Both of them exhibit outstanding thermal stability and high emission quantum yields. All the devices based on these two materials showed negligible efficiency roll-off with increasing current density. Impressively, non-doped organic light-emitting diodes (OLEDs) based on IP-PPI and IP-DPPI exhibited external quantum efficiencies (EQEs) of 4.85 % and 4.74 % with CIE coordinates of (0.153, 0.097) and (0.154, 0.114) at 10000 cd m-2 , respectively. In addition, the 40 wt % IP-PPI doped device maintained a high EQE of 5.23 % with CIE coordinates of (0.154, 0.077) at 10000 cd m-2 . The doped device based on 20 wt % IP-DPPI exhibited a higher deep-blue electroluminescence (EL) performance with a maximum EQE of up to 6.13 % at CIE of (0.153, 0.078) and maintained an EQE of 5.07 % at 10000 cd m-2 . To the best of our knowledge, these performances are among the state-of-the art devices with CIEy ≤0.08 at a high brightness of 10000 cd m-2 . Furthermore, by doping a red phosphorescent dye Ir(MDQ)2 (MDQ=2-methyldibenzo[f,h]quinoxaline) into the IP-PPI and IP-DPPI hosts, high-performance red phosphorescent OLEDs with EQEs of 20.8 % and 19.1 % were achieved, respectively. This work may provide a new approach for designing highly efficient deep-blue emitters with negligible roll-off for OLED applications.

Emission shift of an imidazole bridged diethylaminocoumarin and diphenyl

Diethylaminocoumarin with imidazole as the bridge unit and phenyl rings appended was synthesized by conventional condensing method and routinely characterized by nuclear magnetic resonance and high resolution mass spectra technology. The absorption and emission behavior were studied in various polar environment. It exhibit highly emissive character in methanol with 89% emission quantum yield. In solid state, the fluorescence color shifts to yellow part with emission peak around 563 nm. Emission decay in solid state demonstrates the single index character of the excited dye molecules. Together with the conformation derived from single crystal structure, balanced driving force between regular and irregular packing trend leads to the highly emissive character.

Triarylboron/Triarylamine-Functionalized 2,2'-Bipyridine Ligands and Their Copper(I) Complexes.

We report herein three new either -BMes2 (Mes = mesityl) or -NPh2 group-functionalized 2,2'-bipyridine ligands-4,4'-(p-BMes2-phenyl-C≡C)2-2,2'-bpy (BB-bpy), 4-(p-BMes2-phenyl-C≡C)-4'-(p-NPh2-phenyl-C≡C)-2,2'-bpy (BN-bpy), and 4,4'-(p- NPh2-phenyl-C≡C)2-2,2'-bpy (NN-bpy)-along with their heteroleptic copper(I) complexes Cu(L)(PPh3)2(BF4) (L = BB-bpy, BN-bpy, and NN-bpy, respectively). The electron-donor and -acceptor units are connected to the bipyridine core via acetylene linkers. The incorporation of acetylene linkers decreases the dihedral angle between the bridging phenyl and pyridine rings, resulting in a nearly coplanar geometry of the ligands. All free ligands display temperature-dependent luminescence changes, which may be explained by the twisted intramolecular charge-transfer mechanism. Binding with Cu(I) results in significantly red-shifted emission maxima for the heteroleptic complexes Cu(NNbpy)(PPh3)2(BF4) [Cu(NNbpy)] and Cu(BNbpy)(PPh3)2(BF4) [Cu(BNbpy)] relative to those of the free ligands. The electronic and photophysical properties of all compounds were investigated by electrochemical, absorption, and emission spectroscopic analyses as well as density functional theory calculations.

Effects of Electron Affinity and Steric Hindrance of the Trifluoromethyl Group on the π-Bridge in Designing Blue Thermally Activated Delayed Fluorescence Emitters.

To explore the correlation of the acceptor electron affinity and the molecular conformation to the thermally activated delayed fluorescence (TADF) feature, a series of d-π-A molecules were designed and synthesized with triazine (Trz) as the acceptor (A) and carbazole (Cz) or tert-butylcarbazole (BuCz) as the donor (D). On the phenylene bridge between D and A, methyl or trifluoromethyl was incorporated close either to D or to A to tune the molecular conformation and the electron-withdrawing ability of acceptor. Both the twist angles and the singlet and triplet energy difference (ΔEST ) were observed strongly dependent on the type and position of the substituent on the π-bridge. Only those molecules with trifluoromethyl locating close to the D side, namely TrzCz-CF3 and TrzBuCz-CF3 , exhibit TADF feature, verifying that both sufficient electron affinity of the A unit and large dihedral angle between D and the π-bridge are necessary to ensure the occurrence of TADF. The blue organic light-emitting diodes fabricated with TrzCz-CF3 and TrzBuCz-CF3 achieved external quantum efficiencies of 9.40 % and 14.22 % with CIE coordinates of (0.19, 0.23) and (0.18, 0.29) respectively. This study provides practical design strategy for blue TADF materials particularly when planar and less crowded group is used as donor.

Mono- and di-substituted pyrene-based donor-π-acceptor systems with phenyl and thienyl π-conjugating bridges

The synthesis of a series of four donor-π-acceptor (D-π-A) compounds involving pyrene derivatized with hexylcyanoacetate acceptor groups connected by phenyl or thienyl π-conjugating spacers is described. The electronic properties are characterized by UV–Vis absorption spectroscopy, cyclic voltammetry theoretical calculations and solvatochromism experiments. The results indicate that the thienyl connectors leads to D-π-A systems with smaller optical gaps and stronger internal charge transfers attributed to the combined effects of the lower resonance energy of thiophene, the smaller twist angles between the pyrene block and the π-connector and the deformability of the thiophene ring.

Elevating the triplet level of carbazolyl benzonitrile-based dendritic hosts by suppressing intramolecular charge transfer for solution-processed blue thermally activated delayed fluorescence OLEDs

Realizing appropriate and excellent solution-processed thermally activated delayed fluorescence host materials is of great consequence to the development of low-cost, large-area OLED displays, as the doped devices mostly yield better efficiency than the undoped devices. Herein, two carbazolyl benzonitrile-based dendritic host materials were designed and synthesized by inserting phenyl bridges to 2,4,6-tri(9H-carbazol-9-yl) benzonitrile (3CzBN) and encapsulating flexible alkyl chains. The phenyl bridge extended the backbone of acceptor unit and donor unit respectively, allowed for a better separation of the HOMO and LUMO orbital and weakened intramolecular charge transfer, which endowed high triplet level of 3.01 eV, 2.85 eV for 3CzPhCN-O-Cz and 3PhCzCN-O-Cz. The introduction of the peripheral alkyl chains resulted in better solubility and film-forming ability. These features made the host materials suitable for fabricating blue solution-processed devices. The experimental results proved that the device performance was better when the phenyl bridge was inserted between the accepter unit and the center benzene ring than when it was introduced between the donor unit and the center benzene ring, which paved the way toward achieving high triplet level host materials for solution-processed OLEDs.