Project description:Compounds PTZ-MBZ (methyl 3-(10H-phenothiazin-10-yl)benzoate) and DMAC-MBZ (methyl 3-(9,9-dimethylacridin-10(9H)-yl)benzoate) were conveniently synthesized, and they exhibited TADF properties with lifetimes of 0.80 and 2.17 μs, respectively. The spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet-triplet energy gap of 0.0152 eV and 0.0640 eV, respectively. Thermally activated delayed fluorescence materials with short lifetime could be used as promising luminescent materials for organic light-emitting diodes.
Project description:We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.
Project description:Organic scintillators, materials with the ability to exhibit luminescence when exposed to X-rays, have aroused increasing interest in recent years. However, the enhancement of radioluminescence and improving X-ray absorption of organic scintillators lie in the inherent dilemma, due to the waste of triplet excitons and weak X-ray absorption during scintillation. Here, we employ halogenated thermally activated delayed fluorescence materials to improve the triplet exciton utilization and X-ray absorption simultaneously, generating efficient scintillation with a low detection limit, which is one order of magnitude lower than the dosage for X-ray medical diagnostics. Through experimental study and theoretical calculation, we reveal the positive role of X-ray absorption, quantum yields of prompt fluorescence, and intersystem crossing in promoting the radioluminescence intensity. This finding offers an opportunity to design diverse types of organic scintillators and expands the applications of thermally activated delayed fluorescence.
Project description:Ultra-narrowband blue multi-resonance-induced thermally activated delayed fluorescence (MR-TADF) materials (V-DABNA and V-DABNA-F), consisting of three DABNA subunits possessing phenyl or 2,6-difluorophenyl substituents on the peripheral nitrogen atoms are synthesized by one-shot triple borylation. Benefiting from the inductive effect of fluorine atoms, the emission maximum of V-DABNA-F (464 nm) is blueshifted from that of the parent V-DABNA (481 nm), while maintaining a small full width at half maximum (FWHM, 16 nm) and a high rate constant for reverse intersystem crossing (6.5 × 105 s-1 ). The organic light-emitting diodes (OLEDs) using V-DABNA and V-DABNA-F as emitters are fabricated by vapor deposition and exhibit blue emission at 483 and 468 nm with small FWHMs of 17 and 15 nm, corresponding to Commission Internationale d'Éclairage coordinates of (0.09, 0.27) and (0.12, 0.10), respectively. Both devices achieve high external quantum efficiencies of 26.2% and 26.6% at the maximum with minimum efficiency roll-offs of 0.9% and 3.2%, respectively, even at 1000 cd m-2 , which are record-setting values for blue MR-TADF OLEDs.
Project description:We investigate a series of D-A molecules consisting of spiro[acridan-9,9'-fluorene] as the donor and 2-phenylenepyrimidine as the acceptor. In two of the materials, a spiro center effectively electronically isolates the D unit from (consequently) optically innocent yet structurally influential adamantyl side groups. In a third material, adamantyl groups attached directly to the acceptor strongly influence the electronic properties. Steady-state and time-resolved photophysical studies in solution, Zeonex polymer matrix, and neat films reveal that the substituents impact the efficiency of vibronic coupling between singlet and triplet states relevant to reverse intersystem crossing (rISC) and thermally activated delayed fluorescence (TADF), without significantly changing the singlet-triplet gap in the materials. The adamantyl groups serve to raise the segmental mass and inertia, thereby damping intramolecular motions (both vibrational and rotational). This substitution pattern reveals the role of large-amplitude (primarily D-A dihedral angle rocking) motions on reverse intersystem crossing (rISC), as well as smaller contributions from low-amplitude or dampened vibrations in solid state. We demonstrate that rISC still occurs when the high-amplitude motions are suppressed in Zeonex and discuss various vibronic coupling scenarios that point to an underappreciated role of intersegmental motions that persist in rigid solids. Our results underline the complexity of vibronic couplings in the mediation of rISC and provide a synthetic tool to enable future investigations of vibronic coupling through selective mechanical dampening with no impact on electronic systems.
Project description:Compounds MBZ-mPXZ, MBZ-2PXZ, MBZ-oPXZ, EBZ-PXZ, and TBZ-PXZ were conveniently synthesized, and they were found to exhibit TADF properties with lifetimes of 857, 575, 561, 768, and 600 ns, respectively. These short lifetimes of the compounds might be due to the combination of small singlet-triplet splitting energy (ΔEST) and benzoate group, which could be an efficient strategy for the further design of short-lifetime TADF materials.
Project description:Organic light-emitting devices (OLEDs) containing organic molecules that exhibit thermally activated delayed fluorescence (TADF) produce high efficiencies. One challenge to the commercialization of the TADF OLEDs that remains to be addressed is their operational stability. Here we investigate the molecular factors that govern the stability of various archetypal TADF molecules based on a cycloamino donor–acceptor platform. Our results reveal that the intrinsic stability depends sensitively on the identity of the cycloamino donors in the TADF compounds. The rates and photochemical quantum yields of the degradation are positively correlated with the operation lifetimes of the devices. Our research shows that the stability is governed by the conformeric heterogeneity between the pseudo-axial and pseudo-equatorial forms of the cycloamino donor. Spontaneous bond dissociation occurs in the former (i.e., the pseudo-axial form), but the cleavage is disfavored in the pseudo-equatorial form. These findings provide valuable insights into the design of stable TADF molecules. Organic molecules that exhibit thermally activated delayed fluorescence allow for high efficiencies in OLEDs, but their operational stability remains a challenge to their commercialization. Here the molecular factors that govern the stability of various dyads based on a cycloamino donor–acceptor platform are studied.
Project description:The construction of lanthanide multicolor luminescent materials with tunable photoluminescence properties has been developed as one of the increasingly significant topics and shown inventive applications in miscellaneous fields. However, fabricating such materials based on synergistically assembly-induced emission rather than simple blending of different fluorescent dyes together still remains a challenge. Herein, we report a europium-based noncovalent polymer with tunable full-color emission, which is constructed from the 2,6-pyridinedicarboxylic acid-bearing bromophenylpyridinium salt. This rationally designed bifunctional component can concurrently serve as a guest molecule and a chelating ligand to associate with cucurbit[8]uril and europium ions, thus leading to the formation of a trichromatic (red-green-blue, RGB) photoluminescent polypseudorotaxane-type noncovalent polymer in aqueous solution. Meanwhile, the full-color emission enclosed within the RGB color triangle could be readily produced by simply tuning the molar ratio of cucurbit[8]uril and europium ions. The lanthanide supramolecular polymer featuring tricolor emission, long lifetime, high photoluminescence efficiency and low cytotoxicity could be further applied in multicolor imaging in a cellular environment. These results provide a new and feasible strategy for the construction of full-color single lanthanide self-assembled nanoconstructs.
Project description:A new method of supramolecular polymerization at the water-oil interface is developed. As a demonstration, an oil-soluble supramonomer containing two thiol end groups linked by two ureidopyrimidinone units and a water-soluble monomer bearing two maleimide end groups are employed. Supramolecular interfacial polymerization can be implemented by a thiol-maleimide click reaction at the water-chloroform interface to obtain supramolecular polymeric films. The glass transition temperature of such supramolecular polymers can be well-tuned by simply changing the polymerization time and temperature. It is highly anticipated that this work will provide a facile and general approach to realize control over supramolecular polymerization by transferring the preparation of supramolecular polymers from solutions to water-oil interfaces and construct supramolecular materials with well-defined properties.
Project description:The 3rd-Gen OLED materials employing thermally-activated delayed fluorescence (TADF) combine advantages of first two for high-efficiency and low-cost devices. Though urgently needed, blue TADF emitters have not met stability requirement for applications. It is essential to elucidate the degradation mechanism and identify the tailored descriptor for material stability and device lifetime. Here, via in-material chemistry, we demonstrate chemical degradation of TADF materials involves critical role of bond cleavage at triplet state rather than singlet, and disclose the difference between bond dissociation energy of fragile bonds and first triplet state energy (BDE-ET1) is linearly correlated with logarithm of reported device lifetime for various blue TADF emitters. This significant quantitative correlation strongly reveals the degradation mechanism of TADF materials have general characteristic in essence and BDE-ET1 could be the shared "longevity gene". Our findings provide a critical molecular descriptor for high-throughput-virtual-screening and rational design to unlock the full potential of TADF materials and devices.