Delayed Fluorescence Molecules Research Articles

Aggregation-induced delayed fluorescence molecules with mechanochromic behaviors for efficient blue organic light-emitting diodes

Aggregation-induced delayed fluorescence (AIDF) materials combining the advantages of aggregation-induced emission and thermally activated delayed fluorescence have attracted considerable attention. However, blue AIDF materials with satisfactory electroluminescence efficiencies are still few. In this work, two blue luminescent molecules (XT-DPDBA and XT-BDPDBA) with evident AIDF characteristics and high photoluminescence quantum yields are developed by using rigid electron acceptor xanthenone and weak electron donor 10-dihydrodibenzo[b,e][1,4]azasiline. Both molecules exhibit prominent mechanochromic luminescence (MCL) behaviors originating from conformational interconversion between quasi-axial and quasi-equatorial conformations. The quasi-equatorial conformation has small singlet–triplet energy splitting and thus delayed fluorescence characteristic. Efficient blue organic light-emitting diodes are fabricated based on both molecules, providing high external quantum efficiencies of up to 27% and small efficiency roll-offs. These blue AIDF molecules with MCL property have good application potentials in various optical and electronic devices.

Theoretical insights into molecular design of hot-exciton based thermally activated delayed fluorescence molecules

We examine and analyse the core concepts in molecular design for hot-exciton-based TADF molecules using DFT methods. We demonstrated the structure-property relationships and suggested a design strategy for producing multiple hot-exciton channels to improve RISC efficiency.

High-Efficiency Emissive Dendritic Phosphorescent Iridium (Iii) Complex with Thermally Activated Delayed Fluorescence Molecules as Functional Light-Harvesting Moieties

High-Efficiency Emissive Dendritic Phosphorescent Iridium (Iii) Complex with Thermally Activated Delayed Fluorescence Molecules as Functional Light-Harvesting Moieties

A QM/MM study on through space charge transfer-based thermally activated delayed fluorescence molecules in the solid state

A QM/MM study on through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules with sky-blue emission.

Trifluoromethylated thermally activated delayed fluorescence molecule as a versatile photocatalyst for electron-transfer- and energy-transfer-driven reactions.

In this study, we propose that the trifluoromethylated thermally activated delayed fluorescent molecule 4[Cz(CF3)2]IPN is a versatile organic photocatalyst that can be used for electron-transfer-driven reactions requiring a photocatalyst with high oxidizing power and energy-transfer-driven reactions that require an Ir photocatalyst. 4[Cz(CF3)2]IPN was used in radical reactions via electron transfer and dearomative cycloaddition reactions via energy transfer.

A theoretical perspective of the relationship between the structures and luminescence properties of red thermally activated delayed fluorescence molecules.

Orange and red thermally activated delayed fluorescence (TADF) emitters have shown promising applications in organic light emitting diodes (OLEDs) and the bio-medical field. However, both the species and amounts of orange and red molecules are far from meeting the requirement for practical applications; this is due to the lack of systematic studies on the relationship between molecular structures and luminescence properties. Herein, the excited state dynamic processes and photophysical properties of six donor-acceptor (D-A) type orange-red TADF molecules, which possess the same acceptor, are theoretically studied in toluene by using the polarizable continuum model (PCM). Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method, the adiabatic singlet-triplet energy gaps, natural transition orbital properties, reorganization energies, hole and electron distributions, and the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are theoretically analyzed. The results indicate that remarkable geometric changes between the lowest singlet excited state (S1) and the ground state (S0) are mainly caused by the rotation of the donor unit for NAI-R2, NAI-R3 and NAI-DPAC, and the reorganization energy is mainly contributed by the dihedral angle. However, for NAI-DMAC, BTDMAc-NAI and BFDMAc-NAI, remarkable geometric changes are found in the acceptor unit with large contribution of reorganization energy by bond length. These variations bring different non-radiative energy consumption processes. Moreover, small energy gaps between S1 and the lowest triplet excited state (T1) are determined for all studied molecules and an efficient RISC process is detected. Furthermore, enhanced conjugacy in the donor unit and remarkable intramolecular interactions are determined for BTDMAc-NAI and BFDMAc-NAI, which is helpful to promote the up-conversion process. Our investigations give reasonable explanations for previous experimental measurements and the relationship between basic structures and luminescence properties is revealed, which could facilitate the development of new efficient TADF emitters.

Structure–property relationship study of blue thermally activated delayed fluorescence molecules with different donor and position substitutions: theoretical perspective and molecular design

Blue-efficient thermally-activated delayed fluorescence emitters are widely desired in organic light-emitting diodes due to their advantages in both improving display resolution and providing better pixels.

Theoretical studies on the excited-state properties of thermally activated delayed fluorescence molecules with aggregation induced emission

Recently, thermally activated delayed fluorescence (TADF) molecules with aggregation induced emission (AIE) features have attracted much attention.

High-Performance Orange-Red Organic Light-Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl-Containing Delayed Fluorescence Molecules.

Developing orange to red purely organic luminescent materials having external quantum efficiencies (η exts) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (Φ PL), and large optical outcoupling efficiency (Φ out) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin‐11‐yl(phenyl)methanone, is created and employed for constructing orange–red delayed fluorescence molecules with various acridine‐based electron donors. The incorporation of benzoyl leads to red‐shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased Φ PLs, and the adoption of spiro‐structured acridine donors promotes horizontal dipole orientation and thus renders high Φ outs. Consequently, the state‐of‐the‐art orange–red organic light‐emitting diodes are achieved, providing record‐high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A−1, and 93.5 lm W‒1. By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm−1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.

Thermally activated delayed fluorescence (TADF) organic molecules for efficient X-ray scintillation and imaging.

X-ray detection, which plays an important role in medical and industrial fields, usually relies on inorganic scintillators to convert X-rays to visible photons; although several high-quantum-yield fluorescent molecules have been tested as scintillators, they are generally less efficient. High-energy radiation can ionize molecules and create secondary electrons and ions. As a result, a high fraction of triplet states is generated, which act as scintillation loss channels. Here we found that X-ray-induced triplet excitons can be exploited for emission through very rapid, thermally activated up-conversion. We report scintillators based on three thermally activated delayed fluorescence molecules with different emission bands, which showed significantly higher efficiency than conventional anthracene-based scintillators. X-ray imaging with 16.6 line pairs mm-1 resolution was also demonstrated. These results highlight the importance of efficient and prompt harvesting of triplet excitons for efficient X-ray scintillation and radiation detection.

Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation.

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (ΦPL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (ηout), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase ηout and external quantum efficiency (ηext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent ΦPL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high ηext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

Realizing Record‐High Electroluminescence Efficiency of 31.5 % for Red Thermally Activated Delayed Fluorescence Molecules

AbstractTailor‐made red thermally activated delayed fluorescence (TADF) molecules comprised of an electron‐withdrawing pyrazino[2,3‐f][1,10]phenanthroline‐2,3‐dicarbonitrile core and various electron‐donating triarylamines are developed. They can form intramolecular hydrogen‐bonding, which is conducive to improving emission efficiency and promoting horizontal orientation and show near infrared (NIR) emissions (692–710 nm) in neat films and red delayed fluorescence (606–630 nm) with high photoluminescence quantum yields (73–90%) in doped films. They prefer horizontal orientation with large horizontal dipole ratios in films, rendering high optical out‐coupling factors (0.39–0.41). Their non‐doped OLEDs exhibit NIR lights (716–748 nm) with maximum external quantum efficiencies (ηext,max) of 1.0–1.9%. And their doped OLEDs radiate red lights (606–648 nm) and achieve record‐beating ηext,max of up to 31.5%. These new red TADF materials should have great potentials in display and lighting devices.

Realizing Record‐High Electroluminescence Efficiency of 31.5 % for Red Thermally Activated Delayed Fluorescence Molecules

Tailor-made red thermally activated delayed fluorescence (TADF) molecules comprised of an electron-withdrawing pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile core and various electron-donating triarylamines are developed. They can form intramolecular hydrogen-bonding, which is conducive to improving emission efficiency and promoting horizontal orientation and show near infrared (NIR) emissions (692-710 nm) in neat films and red delayed fluorescence (606-630 nm) with high photoluminescence quantum yields (73-90%) in doped films. They prefer horizontal orientation with large horizontal dipole ratios in films, rendering high optical out-coupling factors (0.39-0.41). Their non-doped OLEDs exhibit NIR lights (716-748 nm) with maximum external quantum efficiencies (ηext,max ) of 1.0-1.9%. And their doped OLEDs radiate red lights (606-648 nm) and achieve record-beating ηext,max of up to 31.5%. These new red TADF materials should have great potentials in display and lighting devices.

Highly Efficient Triplet‐Triplet‐Annihilation Upconversion Sensitized by a Thermally Activated Delayed Fluorescence Molecule in Optical Microcavities

AbstractTriplet‐triplet‐annihilation (TTA) upconversion sensitized by a new thermally activated delayed fluorescence (TADF) molecule, BTZ‐DMAC‐4Br, is investigated in whispering gallery mode (WGM) microcavities made of toluene solutions filled into quartz capillaries. Since the utilization of the excitation light is improved in WGM microcavities through total internal reflection, the upconversion efficiency is enhanced greatly and an upconversion quantum yield of 24.6% is observed, which is a dozen times higher than that measured in conventional cuvettes. To the best of knowledge, this is the highest upconversion efficiency achieved in TADF sensitized TTA systems. Not only is a new material designed, but also the results provide a novel way to improve the TTA upconversion performance by utilizing the optical cavity effect, and the TTA microcavities may be used to construct highly efficient and low‐threshold photonic devices in many applications.

Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

Dihedral Angle Distribution of Thermally Activated Delayed Fluorescence Molecules in Solids Induces Dual Phosphorescence from Charge-Transfer and Local Triplet States

Creating Side Transport Pathways in Organic Solar Cells by Introducing Delayed Fluorescence Molecules

Creating Side Transport Pathways in Organic Solar Cells by Introducing Delayed Fluorescence Molecules

Structure-property relationship for triarylboron-based thermally activated delayed fluorescence molecules: A theoretical perspective

In this work, a total of sixteen TAB-based molecules are studied theoretically based on first-principles calculations. Our results show that the two theoretical calculation methods are system dependence for different TADF molecules especially for these with large geometry changes. Moreover, we find that OTR-SDFT is more advantageous when calculating the emission wavelength, while the system calculated by OHF can have stronger charge transfer, smaller orbital overlap and S1-T1 energy gap. As the electron-donating ability of the group increases, the degree of the orbital overlaps progressively decreases, this brings the gradual reduction of the S1-T1 energy gap and the redshift of the absorption and emission wavelengths. Furthermore, our investigation shows that the Cz substitution can be adopted for designing efficient TADF molecules. Our word could promote the understanding of the similarities and differences between the two theoretical methods and help to design efficient TAB-based TADF emitters in organic light emitting diodes.

A TADF Emitter Featuring Linearly Arranged Spiro-Donor and Spiro-Acceptor Groups: Efficient Nondoped and Doped Deep-Blue OLEDs with CIEy

Reported herein is a molecular design strategy of deep-blue emitters for resolving the lack of highly efficient deep-blue organic light-emitting diodes (OLEDs) featuring CIEy (Commission Internationale de l'Eclairage) color coordinates matching the display requirements (<0.1). The strategy is to combine weak spiro-donor and spiro-acceptor groups into a linear donor-π-acceptor type of thermally-activated delayed fluorescence molecule through a sterically bulky π-spacer. The strategy endows an emitter with deep-blue emission, a narrower emission bandwidth (51 nm in toluene), a high photoluminescence quantum yield (0.95 in toluene), weak concentration quenching, and efficient triplet-exciton utilization, which are all attractive characteristics for emitters of deep-blue OLEDs with lower CIEy coordinates. Owing to the rational design, the emitter has realized not only highly efficient doped deep-blue OLEDs with external quantum efficiencies (EQEs) up to 25.4 % and CIEy less than 0.1 but also so far the most efficient nondoped deep-blue OLED (EQE up to 22.5 %) with CIEy less than 0.1.

A TADF Emitter Featuring Linearly Arranged Spiro‐Donor and Spiro‐Acceptor Groups: Efficient Nondoped and Doped Deep‐Blue OLEDs with CIEy&lt;0.1

AbstractReported herein is a molecular design strategy of deep‐blue emitters for resolving the lack of highly efficient deep‐blue organic light‐emitting diodes (OLEDs) featuring CIEy(Commission Internationale de l'Eclairage) color coordinates matching the display requirements (&lt;0.1). The strategy is to combine weak spiro‐donor and spiro‐acceptor groups into a linear donor‐π‐acceptor type of thermally‐activated delayed fluorescence molecule through a sterically bulky π‐spacer. The strategy endows an emitter with deep‐blue emission, a narrower emission bandwidth (51 nm in toluene), a high photoluminescence quantum yield (0.95 in toluene), weak concentration quenching, and efficient triplet‐exciton utilization, which are all attractive characteristics for emitters of deep‐blue OLEDs with lower CIEycoordinates. Owing to the rational design, the emitter has realized not only highly efficient doped deep‐blue OLEDs with external quantum efficiencies (EQEs) up to 25.4 % and CIEyless than 0.1 but also so far the most efficient nondoped deep‐blue OLED (EQE up to 22.5 %) with CIEyless than 0.1.