Organic Light-emitting Materials Research Articles

Recent Advances in Thermally Activated Delayed Fluorescent Polymer-Molecular Designing Strategies.

Thermally activated delayed fluorescent (TADF) materials, as the third generation of organic electroluminescent materials, have many advantages over other organic light-emitting diodes (OLEDs) materials, such as 100% internal quantum efficiency, no doping of heavy metals, and avoiding the shortages of ordinary fluorescent materials and phosphorescent materials. So it is considered to be the most competitive organic light-emitting materials, and has great application prospects in the field of OLEDs. So far, small-molecule TADF materials have achieved high quantum yield and full-color range of red, green, and blue. However, TADF polymers suitable for low-cost and easily scalable solution processing are less developed, which are confined by the preparation methods and polymers designing, and there are still challenges of increasing quantum efficiency and strengthening device performance. This review mainly summarizes different synthesis strategies of TADF polymers and the latest development in the field. Special attention is focused on illustrating the designing and structure-property relationship of TADF polymers, and finally, an outlook is given for the design and application prospect of TADF polymers in the future.

Rational construction of AIEgens with wide color tunability and their specific lipid droplet imaging applications

Organic light-emitting materials with aggregation-induced emission (AIE) character have experienced a rapid development in imaging, visualization and sensing. In this paper, by installing rotors to an aggregation-caused quenching (ACQ) compound, we constructed an AIE luminogen (AIEgens) system (A2-A4, B1-B3) with wide emission tunability, which covers almost all regions of visible light (400-780 nm). The calculated energy gap of the compounds is consistent with the value of the absorption transition. In the biological experiments, B3 showed excellent lipid droplet (LD) targeting ability in PC12 cells, and higher photostability compared with the commercial LDs bioprobe. Moreover, as a hydrophobic compound, B3 can distinguish blood from a hyperlipidemia patient and normal people. Thus, this study provides a strategy to construct red/NIR fluorescent materials with AIE character, and further apply it to identify the blood of people with hyperlipidemia.

Formation mechanism of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials

Formation mechanism of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials

Formation mechanism of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials

Formation mechanism of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials

Synthesis of naphthoxindoles, anthraoxindoles and phenanthrofuranones from arynes

abstract An efficient process for the synthesis of naphthoxindoles, anthraoxindoles and phenanthrofuranones by Diels−Alder reactions and dehydrogenation aromatization reactions of benzyne or naphthyne with methyleneindolinones or methylenebenzofuranones in good yields. The photophysical, redox, and thermal properties of oxadisilole fused naphthoxindoles and anthraoxindoles have been determined. This compounds show potential applications for organic light-emitting materials due to their high fluorescence quantum yields and thermal stabilities.

Theoretical Study of the Mechanism of Aggregation-Caused Quenching in Near-Infrared Thermally Activated Delayed Fluorescence Molecules: Hydrogen-Bond Effect

Aggregation-caused quenching (ACQ) has long been a problem that inhibits the application of organic light-emitting materials in organic light-emitting diodes, especially near-infrared (NIR) materia...

Aptamer-Anchored Rubrene-Loaded Organic Nanoprobes for Cancer Cell Targeting and Imaging

Aptamer-Anchored Rubrene-Loaded Organic Nanoprobes for Cancer Cell Targeting and Imaging

Construction of Efficient Deep-Red/Near-Infrared Emitter Based on a Large π-Conjugated Acceptor and Delayed Fluorescence OLEDs with External Quantum Efficiency of over 20%

Organic light-emitting materials with thermally activated delayed fluorescence (TADF) are promising for promoting the efficiency of organic light-emitting diodes (OLEDs) without any precious metals...

Theoretical study and experimental validation on the optical emission processes in "free" and "locked" pyrazine derivatives.

The excited-state properties of the "free" and "locked" pyrazine derivatives are investigated in solution. DCFP with "free" phenyls is theoretically calculated to be non-emissive due to the non-radiative energy dissipation through strong Duschinsky rotation effect, in agreement with the available experimental result. Surprisingly, DCBP with "bi-locked" phenyls is also calculated to be nonluminous. The emission of DCAP with "conjoined" architecture is predicted to be weaker than DCPP with "single-locked" phenyls, quite contrary to our intuition but further validated by the experimental measurement. The construction of four-, five- and six-membered ring respectively in DCBP, DCAP and DCPP is found to be the major structural origin for the descending relaxation energy in these "locked" systems, thus giving rise to the ascending luminescence order. Our work not only provides strategy for the molecular design of efficient organic light-emitting materials, but also offers valuable insight into the aggregation-induced emission phenomena.

Realization of high-efficiency fluorescent organic light-emitting diodes with low driving voltage

It is commonly accepted that a full bandgap voltage is required to achieving efficient electroluminescence (EL) in organic light-emitting diodes. In this work, we demonstrated organic molecules with a large singlet-triplet splitting can achieve efficient EL at voltages below the bandgap voltage. The EL originates from delayed fluorescence due to triplet fusion. Finally, in spite of a lower quantum efficiency, a blue fluorescent organic light-emitting diode having a power efficiency higher than some of the best thermally activated delayed fluorescent and phosphorescent blue organic light-emitting diodes is demonstrated. The current findings suggest that leveraging triplet fusion from purely organic molecules in organic light-emitting diode materials offers an alternative route to achieve stable and high efficiency blue organic light-emitting diodes.

Theoretical studies on charge transport and optical properties of diarylmaleic anhydride derivatives as organic light-emitting materials

Abstract The charge transport and optical properties of aromatic ring substituents symmetrically-substituted diarylmaleic anhydrides is studied based on the Marcus–Hush theory and quantum chemistry calculations. The molecule with benzene substituents (1) exhibits the balanced hole and electron charge transport properties. Among three molecules, molecule with thiophene substituents (2) shows the maximum carrier mobility. The frontier molecular orbitals analysis turned out that molecule with indole substituents (3) is characterized as intramolecular charge transfer. The absorption and emission spectra indicate that the aromatic substituents can affect optical properties of molecules. 2 might be promising luminescent material for organic light-emitting diodes.

Cocrystals with tunable luminescence colour self-assembled by a predictable method

Organic light-emitting materials play an essential role in the field of luminescent materials because they are easily prepared and they have controllable luminescent properties. Here it is shown that intermolecular interactions and luminescent properties of cocrystals can be predicted using the molecular surface electrostatic potential of selected π-hole/σ-hole...π bonding donor haloperfluorobenzenes and acceptor acenaphthene (AC). Single-crystal X-ray diffraction data reveal that actual bonding patterns in five cocrystals assembled from haloperfluorobenzenes and AC are in accordance with predictable patterns of intermolecular interactions; that is π-hole...π bond is the major interaction in AC–octafluoronaphthalene, AC–1,4-dibromotetrafluorobenzene and AC–1,3,5-tribromo-2,4,6-trifluorobenzene cocrystals, whereas σ-hole...π bond is the major interaction in AC–1,4-diiodotetrafluorobenzene and AC–1,3,5-trifluoro-2,4,6-triiodobenzene cocrystals. The luminescent properties of the cocrystals are affected by the bonding patterns between AC and haloperfluorobenzenes; the π-hole...π bond leads to weak phosphorescence, whereas the σ-hole...π bond results in weak delayed fluorescence and relatively strong phosphorescence.

High-Performance Organic Electroluminescence: Design from Organic Light-Emitting Materials to Devices.

Organic electroluminescence is considered as the most competitive alternative for the future solid-state displays and lighting techniques owing to many advantages such as self-luminescence, high efficiency, high contrast, high color rendering index, ultra-thin thickness, transparency, flat and flexibility, etc. The development of high-performance organic electroluminescence has become the continuing focus of research. In this personal account, a brief overview of representative achievements in our study on the design of highly efficient novel organic light-emitting materials (including fluorescent materials, phosphorescent iridium(III) complexes and conjugated polymers bearing phosphorescent iridium(III) complex) and high-performance device structures together with working principles are given. At last, we will give some perspectives on this fascinating field, and also try to provide some potential directions of research on the basis of the current stage of organic electroluminescence.

Enhancing the Electroluminescent Efficiency of Acridine-Based Donor–Acceptor Materials: Quasi-Equivalent Hybridized Local and Charge-Transfer State

Excited-state properties (characteristics and composition) play a crucial role in luminescence properties of the new-generation organic light-emitting materials. As a special excited species, the hybridized local and charge-transfer (HLCT) state is composed of locally excited (LE) state and charge-transfer (CT) state, which can harvest simultaneous high photoluminescence (PL) efficiency and high exciton utilization in organic light-emitting diodes. In this work, we designed and synthesized three donor (D)–acceptor (A) compounds with different donor units and a typical electron-deficient unit acridine as the acceptor unit to investigate the different hybridization statuses among them. As is revealed by the molecular design, the three compounds precisely show the different hybridization statuses: LE-dominated hybridization (CZP-1AC), quasi-equivalent HLCT (qHLCT, TPA-1AC), and CT-dominated hybridization (PTZ-1AC). As a result, TPA-1AC exhibits the highest PL efficiency and multifold device performance, meriting from the most effective suppression of nonradiative processes that originated from a qHLCT state due to the strong interstate coupling and the small energy gap between LE and CT states. This work not only provides a comprehensive insight into the hybridization formation and a fine modulation in HLCT state composition but also provides a valuable strategy to design new-generation, low-cost, high-performance organic electroluminescence materials based on lowly emissive chromophores of D or A.

Observation of spontaneous orientation polarization in evaporated films of organic light-emitting diode materials

Abstract The molecular orientation in organic semiconductor films determines device performances. In particular, the spontaneous orientation of a permanent dipole moment (PDM) along the surface normal direction induces a polarization charge at the hetero-interfaces of stacked multilayer devices, and the interface charge dominates the charge accumulation and injection properties. Spontaneous orientation polarization (SOP) has been observed in the “randomly oriented” films of several organic semiconductor materials, and is potentially inherent in many common materials. Herein, we report that 11 additional molecules of organic light-emitting diode materials, including thermally activated delayed fluorescence emitters, and horizontally oriented emitters and electron transporters, exhibit SOP in their evaporated films. The experimental results clearly indicate that SOP frequently occurs in “horizontally oriented” films as well as “randomly oriented” films. The factors contributing to SOP formation are discussed in terms of the figure of merit per PDM. We found that strong intermolecular interactions tend to reduce the figure of merit. Moreover, we suggest the impact of SOP on device performances.

Molecular-Barrier-Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency.

Singlet-triplet conversion in organic light-emitting materials introduces non-emissive (dark) and long-lived triplet states, which represents a significant challenge in constraining the optical properties. There have been considerable attempts at separating singlets and triplets in long-chain polymers, scavenging triplets, and quenching triplets with heavy metals; nonetheless, such triplet-induced loss cannot be fully eliminated. Herein, a new strategy of crafting a periodic molecular barrier into the π-conjugated matrices of organic aromatic fluorophores is reported. The molecular barriers effectively block the singlet-to-triplet pathway, resulting in near-unity photoluminescence quantum efficiency (PLQE) of the organic fluorophores. The transient optical spectroscopy measurements confirm the absence of the triplet absorption. These studies provide a general approach to preventing the formation of dark triplet states in organic semiconductors and bring new opportunities for the development of advanced organic optics and photonics.

Theoretical investigations on the electron-vibration couplings and description of opto-electronic properties of materials

One of the challenging topics for physical chemistry is the theoretical modeling of atomistic dynamics at quantum mechanical level, including the processes for chemical bond breaking and formation. If the atomic motions are simply slight deviations away from the equilibrium, their couplings with electrons will determine the physical properties related to mechanical, including opto-, electric/magnetic, and thermal functions. Taking organic light-emitting materials as examples, we developed a vibration correlation function formalism to elucidate the essential role of electron-vibration couplings in evaluating the optical spectrum and light-emitting quantum efficiency starting from electronic structures. Such formalism has been successfully applied to explain the exotic aggregation induced emission phenomena and help molecular design of highly efficient organo-metallic ligand compounds. The predicted aggregation causing isotope effect has been validated by experiment.

Organic planar pn heterojunction light-emitting diodes: Materials and devices

Organic light-emitting diodes (OLEDs) have been widely applicable in the flat-panel display products, such as mobile phones and television. They also possess more outstanding application advantages in the area of wearable display products in the future and have a great potential to serve as a key competitor in the next-generation solid-state lighting technology. For the commercial OLED products, phosphorescent materials that contain the rare metals are essential. This is because that phosphorescent OLEDs can give more efficient radiation, as the result of harvesting both singlet and triplet excitons for radiative utilization, in contrast to traditional fluorescent emitters. However, because of the limited resources of the rare metals, e.g., indium, on the earth, the phosphorescent materials are rather expensive, and their sustainable supply are also questionable. Besides, the long-term stability of blue phosphorescent emitters are also challenging. To further lower the fabrication cost of materials and devices, numerous research efforts have been devoted to developing rare-metal-free, purely organic light-emitting materials, which could be more low-cost and still exhibit excellent electroluminescence performances. In this review, we firstly introduce the evolution of OLED materials and devices. The OLED materials could be roughly classified into three generations. For the 1st generation traditional fluorescent emitters, three quarters of electro-generated excitons are wasted as heat release, due to the spin forbidden rule of triplets. This suggests that the OLEDs employing traditional fluorescent emitters could only achieve a maximum internal quantum efficiency of 25%. For the 2nd generation phosphorescent emitters, the spin orbital coupling due to heavy atom effect results in the mixing of singlet and triplet energy levels, and make triplet radiative transition feasible. Thereby, phosphorescent OLEDs can obtain a maximum internal quantum efficiency close to 100%. Recently, the 3rd generation hyperfluorescence emitters with the thermally activated delayed fluorescence (TADF) character could also make the devices thereof give an internal quantum efficiency close to 100%. This paves a road to realize low-cost and high-performance OLEDs based on purely organic semiconductors. We also outline the evolution of the OLED device configuration. For single-, double- and multi-layer device configuration of conventional OLEDs, their performances are mainly dependent on the emission layer (EML). The current methodology to develop high-performance OLEDs is to maximize the light emission ability of EML. Based on the understanding of the excitonic property of organic semiconductors, we show that efficient EML-free planar pn heterojunction light-emitting diodes (pn-OLEDs) should be realizable, given that the planar heterojunction property is well modulated. This indicates a way to directly construct efficient fluorescent OLEDs without an EML from the perspective of planar heterojunction composed of p type and n type charge transport materials. As following, the progress of OLEDs based on the layer-to-layer charge transfer mechanism using purely organic semiconductors is introduced. Main discussion focuses on pn-OLEDs, including that how this concept was proposed and the progress of relevant materials and devices. We also discuss the challenges of developing high performance pn-OLEDs and their potential applications in other optoelectronic areas, such as organic light-emitting transistors and electrically-pumped organic laser.

Synthesis and characterization of 9,10-[di-p-(7-diethylamino- coumarin-3-yl) thiopheneyl]anthracene as fluorescent material

ABSTRACTA novel N-coumarin derivative, namely 9,10-[di-p-(7-diethylamino-coumarin-3-yl) thiopheneyl]anthracene ((CTh)2A), containing anthracene as the core and 3-thiophene N-coumarin as the substituent was synthesized, and its structure was confirmed by 1H NMR and IR spectroscopies. The optical, electrochemical and thermal properties were investigated. Thermogravimetric analysis and PL spectra reveal the high thermal and good photoluminescence characteristics. The coumarin derivative exhibits blue photoluminescence with high fluorescence quantum yield in solution (up to 40%). The results show that the derivative would serve as promising organic light-emitting diode luminescent material.

Light-Emission Enhancement in a Flexible and Size-Controllable ZnO Nanowire/Organic Light-Emitting Diode Array by the Piezotronic Effect

Enhancing the light emission of LED arrays by the piezotronic effect of ZnO NWs has recently attracted wide attention for the visual mapping of pressure. Here, to overcome deficiencies of ZnO LEDs, such as uncontrollable emission and poor emission efficiency, we designed a flexible, patterned ZnO/organic light-emitting device by combining the advantages of organic light-emitting materials and the piezoelectric effect of ZnO nanowires (NWs). The spatial resolution of the device can be tuned by the pattern size of the ZnO NW array. In addition, the light-emission character is solely determined by the organic light-emitting layer. The light-emission performance of the organic device is enhanced by the piezotronic transistors of the ZnO NW array through the modification of the energy band and reducing the Schottky barrier at the interface of the electrode and the semiconductor. By combining the advantages of the organic material and the piezotronic effect of ZnO NWs, this device shows potential for many appli...