Dual-emissive 2-(2'-hydroxyphenyl)oxazoles for high performance organic electroluminescent devices: discovery of a new equilibrium of excited state intramolecular proton transfer with a reverse intersystem crossing process.
ABSTRACT: The photoluminescence (PL) and electroluminescence (EL) properties of two highly efficient excited state intramolecular proton transfer (ESIPT) molecules, 2-(2'-hydroxyphenyl)oxazoles containing one triphenylamine (TPA) (1) and two TPAs (2) respectively, are studied systematically. The enol-forms of both 1 and 2 possess highly hybridized local and charge transfer (HLCT) excited state character, while their excited-state keto-forms are not of obvious HLCT character. A 1-based device exhibits green-white electroluminescence with Commission Internationale d'Eclairage (CIE) coordinates of (0.25, 0.41) and a high external quantum efficiency (EQE) up to 5.3%, which is the highest EQE value recorded for single molecular white light-emitting materials. A 2-based device shows sky-blue emission with CIE coordinates of (0.18, 0.16) and an EQE of 8.0%, which is the highest EQE in the reported HLCT materials. The fluorescence intensities of the enol-forms of 1 and 2 in EL spectra are increased remarkably relative to their PL spectra. Experimental data and theoretical calculations reveal a new ESIPT equilibrium with a reverse intersystem crossing (RISC) process arising from the HLCT character. In EL, the RISC of the enol-form excitons from the triplet state to the singlet state triggers an increase in the number of enol-form singlet excitons, which further leads to a shift of the ESIPT equilibrium towards an enhanced enol-form emission. Thus, the difference between the ESIPT equilibria in PL and EL may be ascribed to the HLCT character of the enol-form excited state.
Project description:Throwing light upon the structure-property relationship of the excited state properties for next-generation fluorescent materials is crucial for the organic light emitting diode (OLED) field. Herein, we designed and synthesized three donor-acceptor (D-A) structure compounds based on a strong spin orbit coupling (SOC) acceptor benzo[a, c]phenazine (DPPZ) to research on the three typical types of excited states, namely, the locally-excited (LE) dominated excited state (CZP-DPPZ), the hybridized local and charge-transfer (HLCT) state (TPA-DPPZ), and the charge-transfer (CT) dominated state with TADF characteristics (PXZ-DPPZ). A theoretical combined experimental research was adopted for the excited state properties and their regulation methods of the three compounds. Benefiting from the HLCT character, TPA-DPPZ achieves the best non-doped device performance with maximum brightness of 61,951 cd m-2 and maximum external quantum efficiency of 3.42%, with both high photoluminescence quantum efficiency of 40.2% and high exciton utilization of 42.8%. Additionally, for the doped OLED, PXZ-DPPZ can achieve a max EQE of 9.35%, due to a suppressed triplet quenching and an enhanced SOC.
Project description:In this work, we revealed a new approach for the development of efficient violet-blue emitting materials featuring a hybrid local and charge transfer (HLCT) excited state through the incorporation of naphthyl group(s) as a weak n-type ? spacer in a donor-?-acceptor (D-?-A) system. The resulting materials (TPINCz and TPIBNCz) show improved intramolecular charge transfer properties and highly efficient violet-blue fluorescence. It is demonstrated that the pattern of the ? spacers has significant influence on the photophysical properties. The incorporation of a naphthyl/binaphthyl spacer between the donor and acceptor moieties can alleviate the common dilemma that enhancing device performance by increasing the charge transfer excited properties often leads to red-shifted emissions. A device using TPINCz as an emissive dopant shows a violet-blue emission with CIE coordinates of (0.153, 0.059) and a record high EQE of 6.56 ± 0.11% at a brightness of 1000 cd m-2. To the best of our knowledge, this performance is the highest among the reported devices with CIE y ?0.08. Our study provides a new pathway for the design of high-performance violet-blue emitters with a D-?-A architecture in organic electroluminescence applications.
Project description:The intensive measures of luminescence in a GaN/InGaN multiple quantum well system are used to examine the thermodynamics and phenomenological structure. The radiative /nonradiative transitions along with absorbed or emitted phonons that occur between the different quantum states of the electrons and holes associated with these processes make the quantum efficiency of a semiconductor nanosystem in an equilibrium state an extensive property. It has long been recognized that tuning of the indium (In) composition in InGaN interlayers gives the potential to obtain a spectrum in the near-infrared to near-ultraviolet spectral range. The thermodynamic intensive properties, including the Debye temperature, carrier temperature, and junction temperature, are the most appropriate metrics to describe the optical-related interactions inherent in a given heterostructure and so can be used as the state variables for understanding the quantum exchange behaviors. The energetic features of the quantum processes are characterized based on analysis of the intensive parameters as determined by means of electroluminescence (EL) and photoluminescence (PL) spectroscopy and current-voltage measurement and then correlated with the designed InGaN/GaN microstructures. According to the McCumber-Sturge theory, the EL and PL Debye temperatures obtained experimentally signal the strength of the electron-phonon and photon-phonon interaction, respectively, while the EL and PL carrier/junction temperatures correspond to the carrier localization. Higher EL Debye temperatures and lower EL carrier/junction temperatures reflect significantly higher luminescence quantum yields, indicative of electron-phonon coupling in the transfer of thermal energy between the confined electrons and the enhancement by excited phonons of heat-assisted emissions. On the other hand, the observation of low luminescence efficiency, corresponding to the lower PL Debye temperatures and higher PL carrier/junction temperatures, is attributed to photon-phonon coupling. These findings are in good accordance to the dependence of the EL and PL quantum efficiency on the In-content of the InGaN/GaN barriers, suggesting that the characteristic Debye and carrier/junction temperatures are intensive parameters useful for assessing the optical properties of a nano-engineered semiconductor heterostructure.
Project description:In this paper, high quality green-emitting CsPbBr3 quantum dots (QDs) are successfully synthesized by hot-injection method. Different injection temperatures are tested to optimize the synthesis conditions. High brightness with the photoluminescence (PL) quantum yields (QYs) up to 90% and narrow size-distribution with the full width at half-maximum (FWHM) of 18.5 nm are obtained under the optimized conditions. Green light emitting diodes (LEDs) based on the CsPbBr3 QDs are successfully demonstrated by combining solution method with vapor deposition method. Composite films of poly[9,9-dioctylfluorene-co- N-[4-(3-methylpropyl)]-diphenylamine] (TFB) and bathocuproine (BCP) layers are chosen as the hole-transporting and the electron-transporting layers, respectively. The highly bright green QD-based light-emitting devices (QLEDs) showing maximum luminance up to 46,000 cd/m2 with a low turn on voltage of 2.3 V, and peak external quantum efficiency (EQE) of 5.7%, corresponding to 19.9 cd/A in luminance efficiency. These devices also show high color purity for electroluminescence (EL) with FWHM <20 nm, and no redshift and broadening with increasing voltage as well as a spectral match between PL and EL.
Project description:A novel near ultraviolet (NUV) emitter with a meta-linked donor-acceptor (D-A) structure between triphenylamine (TPA) and phenanthroimidazole (PPI), mTPA-PPI, was designed and synthesized. This molecular design is expected to resolve the conflict between the non-red-shifted emission and the introduction of a charge-transfer (CT) state in the D-A system, aiming at NUV organic light-emitting diodes (OLEDs) with high-efficiency and colour-purity. Theoretical calculations and photophysical experiments were implemented to verify the unique excited state properties of mTPA-PPI. The mTPA-PPI device exhibited excellent NUV electroluminescence (EL) performance with an emission peak at 404 nm, a full width at half maximum (FWHM) of only 47 nm corresponding to a CIE coordinate of (0.161, 0.049), and a maximum external quantum efficiency (EQE) of 3.33%, which is among the best results for NUV OLEDs. This work not only demonstrates the promising potential of mTPA-PPI in NUV OLEDs, but also provides a valuable strategy for the rational design of NUV materials by using the meta-linked D-A architecture.
Project description:Theoretical design of conjugated proton cranes, based on 7-hydroxyquinoline as a tautomeric sub-unit, has been attempted by using ground and excited state density functional theory (DFT) calculations in various environments. The proton crane action request existence of a single enol tautomer in ground state, which under excitation goes to the excited keto tautomer through a series of consecutive excited-state intramolecular proton transfer (ESIPT) steps with the participation of the crane sub-unit. A series of substituted pyridines was used as crane sub-units and the corresponding donor-acceptor interactions were evaluated. The results suggest that the introduction of strong electron donor substituents in the pyridine ring creates optimal conditions for 8-(pyridin-2-yl)quinolin-7-ols to act as proton cranes.
Project description:Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor-acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.
Project description:Intermolecular electron-hole coupling in organic semiconductor excited states plays important roles in organic light-emitting diodes and organic photovoltaics, and the distance of the coupling is typically only on the order of a few nanometers. Here, we report exceptionally long-distance coupled exciplex emissions between electron-donor and electron-acceptor molecules even with a 70 nm-thick spacer layer. Donor/spacer (?70 nm)/acceptor-type stacked films showed a low-energy band emission, which is not ascribed to the emission of the donor, spacer, and acceptor themselves, but well corresponds to the energy difference between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Delayed transient photoluminescence (PL) and electroluminescence (EL) decays and PL quenching by oxygen at the low-energy band were observed and are consistent with the characteristics of the exciplex species.
Project description:Cesium lead halide perovskite nanocrystals (NCs) have unique optical properties such as high color purity and high photoluminescence (PL) efficiency. However, the external quantum efficiency (EQE) of the corresponding light-emitting diodes (LEDs) is low, primarily as a result of the NC surface defects. Here, we report a method to reduce the surface defects by capping CsPbI3 NCs with PbS. This passivation significantly enhanced the PL efficiency, reduced the Stokes shift, narrowed the PL bandwidth, and increased the stability of CsPbI3 NCs. At the same time, CsPbI3 NC films switched from n-type behavior to nearly ambipolar by PbS capping, which allowed us to fabricate electroluminescence LEDs using p-i-n structures. The thus-fabricated LEDs exhibited dramatically improved storage and operation stability, and an EQE of 11.8%. These results suggest that, with a suitable surface passivation strategy, the perovskite NCs are promising for next-generation LED and display applications.
Project description:Time-dependent density functional theory (TDDFT) and atoms in molecules (AIM) theory are combined to study the photoinduced excited state intramolecular proton transfer (ESIPT) dynamics for eight anthraquinones (AQs) derivatives in solution. The calculated absorption and emission spectra are consistent with the available experimental data, verifying the suitability of the theory selected. The systems with the excited-state exothermic proton transfer, such as 1-HAQ, 1,5-DHAQ and TFAQ, emit completely from transfer structure (T), while the reactions for those without ESIPT including 1,4-DHAQ and AAAQ appear to be endothermic. Three reaction properties of three systems (1,8-DHAQ, DCAQ and CAAQ) are between the exothermic and endothermic, sensitive to the solvent. Energy scanning shows that 1,4-DHAQ and AAAQ exhibit the higher ESIPT energy barriers compared to 1-HAQ, 1,5-DHAQ and TFAQ with the "barrierless" ESIPT process. The ESIPT process is facilitated by the strengthening of hydrogen bonds in excited state. With AIM theory, it is observed that the change in electrons density ?(r) and potential energy density V(r) at BCP position between ground state and excited state are crucial factors to quantitatively elucidate the ESIPT.