Delayed Fluorescence Properties Research Articles

Theoretical exploration on the combined effect of isoelectron B-N bonds and B-N resonance skeleton on the thermally activated delayed fluorescence property

The multiple resonance thermally activated delayed fluorescence (MR-TADF) materials has become a hot spot in recent years depending on their potential in achieving an internal quantum efficiency of 100%. The combined effect of para B-N resonance skeleton and isoelectron B-N bonds on the TADF property was investigated in details by employing density functional theory/time-dependent density functional theory (DFT/TDDFT) in this work based on four polycyclic aromatic molecules. The results show that isoelectron B-N bonds are favourable to the enhancement of spin orbital coupling (SOC) constants between the first singlet state (S1) and the first triplet state (T1), while para B-N resonance skeleton is more conducive to reducing the energy gap between S1 and T1 (ΔEST) through realizing short-range charge transfer character and keeping electron and hole localized at different atoms. Accordingly the combination of isoelectron B-N bonds and para B-N resonance skeleton in m[B-N]N1 and m[B-N]N2 could realize faster intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes through larger SOC and lower ΔEST compared with BCz-BN and NBN-2 which only contain para B-N resonance skeleton and isoelectron B-N bonds, respectively. At the same time, the para B-N resonance skeleton help to stabilize the structure of polycyclic aromatic hydrocarbons and localize the vibration in low frequency region during emission from S1 to ground state. Thus larger nonradiative rates induced by the rotation of carbazole and tert-butyl carbazole in low frequency region in m[B-N]N1 and m[B-N]N2 could be easily reduced by replacing them by small functional group such as electron-donating (-CH3) and electron-withdrawing (-CN) groups. Therefore, we can obtain high TADF efficiency through combining para B-N resonance skeleton and isoelectron B-N bonds and suppressing low-frequency vibrations as in our designed molecules m[B-N]CH3 and m[B-N]CN.

Influence of Donor–Acceptor Conformation on Thermally Activated Delayed Fluorescence Properties of Two-Coordinate Carbene–Au–Amide Complexes

Influence of Donor–Acceptor Conformation on Thermally Activated Delayed Fluorescence Properties of Two-Coordinate Carbene–Au–Amide Complexes

Peripheral Selenium Modification of Multi‐Resonance Thermally Activated Delayed Fluorescence Molecules for High‐Performance Blue Organic Light‐Emitting Diodes

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have attracted much attention in the academia owing to their unique photoelectrical properties. However, MR‐TADF emitters usually show slow reverse intersystem crossing (RISC) rate, resulting in high efficiency roll‐off of organic light‐emitting diodes (OLEDs) and seriously limiting their further development. Here, a peripheral selenium (Se) modification is presented for MR‐TADF molecules to promote the RISC process while keeping the narrowband emission for high‐performance blue OLEDs. Compared to the parent molecules (NBN and tBuNBN), SeNBN and SetBuNBN exhibited narrower full‐width at half maximum (FWHM) value of 23 nm and more obvious delayed fluorescence properties with a high efficiency of delayed fluorescence up to 86%, shorter delayed lifetime of 2.4 µs as well as a faster RISC rate of 3.34×105 s−1. Therefore, high‐performance OLEDs based on these two Se modified MR‐TADF emitters are achieved with a high maximum external quantum efficiency (EQE) up to 25.5% and extremely suppressed efficiency roll‐offs of 3.9% at 100 cd m−2 and 24.4% at 1000 cd m−2. This work demonstrated that the introduction of peripheral Se atom can achieve high‐performance organic semiconductors with both narrowband emission and fast RISC rate constant for high‐performance organic optoelectronic devices.

Sulphur-induced structural rearrangement in the self-sensitization photo-oxidation behaviour of phenothiazine-imidazole molecules.

A green photo-oxidation reaction was discovered in which the imidazole ring is oxidized and rearranged to generate imidazolinone derivatives with thermal activation delayed fluorescence properties. The excellent photo-oxidation properties of p-PTZ-PIM can be utilized for detecting trace oxygen in the water phase and the sealing of food packaging bags.

Color-Tunable Dual-Mode Organic Afterglow for White-Light Emission and Information Encryption Based on Carbazole Doping.

Dual-mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white-light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white-light emission. In this study, a carbazole-based host-guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long-lived white-light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which further boosted the delayed fluorescence properties of the doping system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white-light afterglow.

Color‐Tunable Dual‐Mode Organic Afterglow for White‐Light Emission and Information Encryption Based on Carbazole Doping

AbstractDual‐mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white‐light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white‐light emission. In this study, a carbazole‐based host–guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long‐lived white‐light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which facilitating the delayed fluorescence properties of host–guest system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white‐light afterglow.

Solvation‐Mediated Self‐Assembly from Crystals to Helices of Protic Acyclic Carbene AuI‐Enantiomers with Chirality Amplification

AbstractConstructing chiral supramolecular assembly and exploring the underlying mechanism are of great significance in promoting the development of circularly polarized luminescence (CPL)‐active materials. Herein, we report a solvation‐mediated self‐assembly from single‐crystals to helical nanofibers based on the first protic acyclic (methoxy)(amino)carbenes (pAMACs) AuI‐enantiomers driven by a synergetic aurophilic interactions and H‐bonds. Their aggregation‐dependent thermally activated delayed fluorescence properties with high quantum yields (ΦFL) up to 95 % were proved to be attributed to packing modes of Au⋅⋅⋅Au dimers with π‐stacking or one‐dimensional extended Au⋅⋅⋅Au chains. Via drop‐casting method, supramolecular P‐ or M‐helices were prepared. Detailed studies on the helices demonstrate that formations of extended helical Au⋅⋅⋅Au molecular chains amplify supramolecular chirality, leading to strong CPL with high dissymmetry factor (|glum|=0.030, ΦFL=67 %) and high CPL brightness (BCPL) of 4.87×10−3. Our findings bring new insights into the fabrication of helical structures to improve CPL performance by modifying aurophilic interactions.

Solvation-Mediated Self-Assembly from Crystals to Helices of Protic Acyclic Carbene AuI -Enantiomers with Chirality Amplification.

Constructing chiral supramolecular assembly and exploring the underlying mechanism are of great significance in promoting the development of circularly polarized luminescence (CPL)-active materials. Herein, we report a solvation-mediated self-assembly from single-crystals to helical nanofibers based on the first protic acyclic (methoxy)(amino)carbenes (pAMACs) AuI -enantiomers driven by a synergetic aurophilic interactions and H-bonds. Their aggregation-dependent thermally activated delayed fluorescence properties with high quantum yields (ΦFL ) up to 95 % were proved to be attributed to packing modes of Au⋅⋅⋅Au dimers with π-stacking or one-dimensional extended Au⋅⋅⋅Au chains. Via drop-casting method, supramolecular P- or M-helices were prepared. Detailed studies on the helices demonstrate that formations of extended helical Au⋅⋅⋅Au molecular chains amplify supramolecular chirality, leading to strong CPL with high dissymmetry factor (|glum |=0.030, ΦFL =67 %) and high CPL brightness (BCPL ) of 4.87×10-3 . Our findings bring new insights into the fabrication of helical structures to improve CPL performance by modifying aurophilic interactions.

Acceptor engineering for modulating circularly polarized luminescence and thermally activated delayed fluorescence properties

As the new type of organic photoelectric device, circularly polarized thermally activated delayed fluorescence (CP-TADF) organic light-emitting diodes (OLEDs) with the advantages of high exciton utilization and full color saturation show wide applications in 3D display, encrypted information storage and quantum computing. However, due to the conflict between high luminescence efficiency and large luminescence asymmetry factor (glum), the species of efficient CP-TADF molecules are limited. Herein, theoretical studies are performed by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods coupled with thermal vibration correlation function, the effects of different acceptors on circularly polarized luminescence (CPL) and TADF properties are investigated, inner mechanisms between basic molecular structures and luminescence properties are revealed. By calculating intersystem crossing rate, reverse intersystem crossing rate, radiative decay rate and non-radiative decay rate, we determine that the SPHCN molecule exhibited dominate TADF properties. Additionally, the electronic circular dichroism (ECD) spectrum and glum values are quantitatively calculated to measure the CPL properties. Based on our findings, we conclude that lengthening the acceptor unit is a feasible strategy for improving the luminescence properties of CP-TADF molecules. Our study provides a novel approach for understanding the molecular luminescence mechanism of CP-TADF, which could promote the development of CP-OLEDs.

Conformation- and Coordination Mode-Dependent Stimuli-Responsive Salicylaldehyde Hydrazone Zn(II) Complexes.

Luminescent Zn(II) complexes that respond to external stimuli are of wide interest due to their potential applications. Schiff base with O,N,O-hydrazone shows excellent luminescence properties with multi-coordination sites for different coordination modes. In this work, three salicylaldehyde hydrazone Zn(II) complexes (1, 2a, 2b) were synthesized and their stimuli-responsive behaviors in different states were explored. Only complex 1 exhibits reversible and self-recoverable photochromic and photoluminescence properties in solution. This may be due to the configuration eversion and the excited-state intramolecular proton transfer (ESIPT) process. In the solid state, 2a has obvious mechanochromic luminescence property, which is caused by the destruction of intermolecular interactions and the transformation from crystalline state to amorphous state. 2a and 2b have delayed fluorescence properties due to effective halogen bond interactions in structures. 2a could undergo crystal-phase transformation into its polymorphous 2b by force/vapor stimulation. Interestingly, 2b shows photochromic property, which can be attributed to the electron transfer and generation of radicals induced by UV irradiation. Due to different conformations and coordination modes, the three Zn(II) complexes show different stimuli-responsive properties. This work presents the multi-stimuli-responsive behaviors of salicylaldehyde hydrazone Zn(II) complexes in different states and discusses the response mechanism in detail, which may provide new insights into the design of multi-stimuli-responsive materials.

Polymer‐Based Circularly Polarized Luminescent Materials

AbstractChiral materials capable of generating circularly polarized luminescence (CPL) have gained rapidly growing research interest by virtue of their peculiar photophysical characteristics and far‐reaching applications. Amongst various CPL materials, polymer‐based CPL materials have constituted an important class owing to their abundant types, ease of manufacture, high thermal stability, as well as, tunable properties. The present work reviews the latest advances in constructing polymer‐based CPL materials, which are classified based on the interaction modes between chiral and luminescent components, that is, covalent interaction, non‐covalent interaction, as well as, without covalent and non‐covalent interactions. The newly emerging categories of polymer‐based CPL materials, that is, nonconventional fluorescence‐based CPL materials, circularly polarized room temperature phosphorescence, and polymer‐based CPL materials with thermally activated delayed fluorescence properties are also presented. Typical applications of the polymer‐based CPL emitters in organic light‐emitting diodes, information security encryption, and optical sensors are further discussed. Additionally, the current challenges and future perspectives in this field are summarized. This review article is expected to stimulate more unprecedented achievements derived from polymer‐based CPL materials, thus further promoting their future practical applications.

A red thermally activated delayed fluorescence emitter based on benzo[c][1,2,5]thiadiazole

Herein, a series of materials (BTD-PXZ, BTD-PTZ, and BTD-DMAC) were designed and synthesized with the merit of the facile synthesis process, high synthesis yield, and small molecular weight through directly connecting electron donor to position 5 of benzo[c][1,2,5]thiadiazole (BTD) rather than adopting extensive conjugated structure strategy. The thermally activated delayed fluorescence property of BTD-PXZ was proven and the photophysical characteristics were discussed through the single crystal, computational simulation, and photophysical experiments. The doped organic light-emitting diode with the BTD-PXZ as the emissive layer shows an emitting peak of 620 nm, Commission Internationale de L'Eclairage coordinate of (0.65, 0.25), and external quantum efficiency of 2.4%.

Molecular design of phenazine-5,10-diyl-dibenzonitriles and the impact on their thermally activated delayed fluorescence properties

The impact of the molecular design on observable thermally activated delayed fluorescence is discussed for three compounds consisting of a dihydrophenazine donor core and differently substituted benzonitrile acceptor units.

Efficient thermally activated delayed fluorescence organic light-emitting device based on an exciplex.

An exciplex with significant thermally activated delayed fluorescence properties was realized, comprising diphenyl-[3'-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-biphenyl-4-yl]-amine as a donor and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine as an acceptor. A very small energy difference between the singlet and triplet levels and a large rate constant of the reverse intersystem crossing were attained simultaneously, contributing to the efficient upconversion of triplet excitons from the triplet state to the singlet state and thermally activated delayed fluorescence emission. A high-efficiency organic light-emitting device based on the exciplex was fabricated, which exhibited a maximum current efficiency, power efficiency, external quantum efficiency, and exciton utilization efficiency of 23.1 cd/A, 24.2 lm/W, 7.32%, and 54%, respectively. The efficiency roll-off of the exciplex-based device was slight, as illustrated by a large critical current density of 34.1 mA/cm2. This efficiency roll-off was ascribed to triplet-triplet annihilation, as confirmed by the triplet-triplet annihilation model. We proved the high binding energy of the excitons and excellent charge confinement within the exciplex by performing transient electroluminescence measurements.

Aggregation-Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene-CuI -Amide Enantiomers.

The field of luminescent carbene-metal-amide (CMA) complexes and chiroptical-active materials has been blossoming in recent years, although chiroptical-active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI -based CMA enantiomers, (R,R)-PSIPr*-Cu-DMAC and (S,S)-PSIPr*-Cu-DMAC, have been developed by using chiral phenyl-substituted N-heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI -based CMA enantiomers exhibited aggregation-induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand-ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation-dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Aggregation‐Dependent Circularly Polarized Luminescence and Thermally Activated Delayed Fluorescence from Chiral Carbene‐CuI‐Amide Enantiomers

AbstractThe field of luminescent carbene‐metal‐amide (CMA) complexes and chiroptical‐active materials has been blossoming in recent years, although chiroptical‐active CMA complexes have not been reported so far. For the first time, a pair of chiral CuI‐based CMA enantiomers, (R,R)‐PSIPr*‐Cu‐DMAC and (S,S)‐PSIPr*‐Cu‐DMAC, have been developed by using chiral phenyl‐substituted N‐heterocyclic carbenes as acceptor ligands in the CMA motif. The CuI‐based CMA enantiomers exhibited aggregation‐induced circularly polarized luminescence with a large luminescence dissymmetry factor of up to +0.027, the first reported for CMA complexes. This success originates from the limited ligand‐ligand rotation freedom and asymmetrical packing pattern (helical structure) of the CMA enantiomers in the crystals. Moreover, these CuI enantiomers displayed inspiring aggregation‐dependent thermally activated delayed fluorescence properties. These findings bring new insights into the optical properties of chiral CMA complexes from the perspective of aggregation states.

Theoretical perspective for the relationship between molecular structures and circularly polarised thermally activated delayed fluorescence properties

Circularly polarised thermally activated delayed fluorescence (CP-TADF) molecules producing circularly polarised light which is beneficial to human eyes are widely used in circularly polarised organic light-emitting diodes. However, the relationship between the molecular structures and their luminescence properties of these CP-TADF molecules has not been well explained. Herein, the chiral molecule (R)-SFST with properties of circularly polarised luminescence (CPL) and thermally activated delayed fluorescence (TADF) is studied in detail. Firstly, we found two conformations by using molecular dynamic conformation search for (R)-SFST, namely (R)-SFST-a and (R)-SFST-b. The molecular geometric structures of singlet and triplet states are optimised by utilising density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Combining the thermal vibration correlation function methods, their photophysical properties are studied simultaneously. The processes of excited-state energy decay are revealed. The results show that the chiral units are more important to the molecular orbitals when the molecule has a more distorted configuration, which is beneficial to obtain CPL properties. However, when the molecular has rigid configuration, the excitons are easier to convert between S1 and T1, and the molecular has better TADF properties. Our results reveal the influence of different molecular conformations on CPL and TADF properties.

Multifunctional luminophores with dual emitting cores: TADF emitters with AIE properties for efficient solution- and evaporation-processed doped and non-doped OLEDs

• Three dual emitting core emitters are developed by coupling single core emitters. • The emitters possess high absorption coefficients and PLQY values in neat films. • Photophysical investigations demonstrate both AIE and TADF properties. • Highly efficient OLEDs with ultra-low efficiency roll-offs are achieved. Three novel emitters with dual emitting cores, namely 3,3′,5,5′-tetra(triphenylamine-4-yl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DDTPAIPN), 3,3′-di(triphenylamine-4-yl)-5,5′-di(3,5-(9,9′-dicarbazolyl)phenyl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DTPAmCPIPN), and 3,3′,5,5′-tetra(3,5-(9,9′-dicarbazolyl)phenyl)-[1,1′-biphenyl]-2,2′,6,6′-tetracarbonitrile (DDmCPIPN) are designed and synthesized by coupling their respective single core emitters. Compared with their single core counterparts, the three novel compounds exhibit significantly improved thermal stability, absorption coefficients, and photoluminescence efficiencies in neat films. Photophysical measurements show that these three emitters exhibit both aggregation-induced emission and thermally activated delayed fluorescence properties, which is beneficial for non-doped organic light-emitting diodes (OLEDs). The new compounds show relatively balanced charge carrier transport abilities as revealed by unipolar devices. The solution- and evaporation-processed doped and non-doped OLEDs made from these three luminophores achieve excellent electroluminescence (EL) performances. The peak current, power, and external quantum efficiencies of a DTPAmCPIPN-based solution-processed doped device reach 59.2 cd A −1 , 61.3 lm W −1 , and 17.2%, respectively, with an emission peak at 548 nm. The evaporation-processed doped device based on DTPAmCPIPN exhibits maximum EL efficiencies of up to 63.7 cd A −1 , 61.1 lm W −1 , and 19.2%. Moreover, the non-doped OLEDs also possess superior EL efficiencies with extremely small efficiency roll-offs. The molecular design strategy in this work is demonstrated to be effective in developing new emitters for efficient and stable OLEDs.

A novel thermally activated delayed fluorescence macrocycle.

A novel luminescent macrocycle has been conveniently synthesized, which displays flexible conformations and interesting solid-state host-guest properties. Besides, the macrocycle exhibits excellent thermally activated delayed fluorescence (TADF) emission with a photoluminescence quantum yield of 80%, which represents the highest value among reported TADF macrocycles.

Enhancement of thermally activated delayed fluorescence properties by substitution of ancillary halogen in a multiple resonance-like diplatinum(ii) complex

Metathesis of the halide co-ligands X from chloro to iodo improves the TADF properties in a diplatinum(ii) complex of the form (N^C^N–N^C^N)Pt2X2, leading to a ∼3-fold increase of radiative decay rate constant.