Singlet-triplet Energy Splitting Research Articles

Modulating Tri-Mode Emission for Single-Component White Organic Afterglow.

Achieving single-component white organic afterglow remains a great challenge owing to the difficulties in simultaneously supporting long-lived emissions from varied excited states of a molecule for complementary afterglow. Here, an extraordinary tri-mode emission from the radiative decays of singlet (S1 ), triplet (T1 ), and stabilized triplet (T1 * ) excited states was proposed to afford white afterglow through modulating the singlet-triplet splitting energy (ΔEST ) and exciton trapping depth (ETD ). Low-lying T1 * for yellow afterglow was constructed by H-aggregation engineering with large ETD and trace isomer doping, while high-lying T1 and S1 for blue afterglow with thermally activated emission feature were realized by reducing ΔEST through donor-acceptor molecular design. Therefore, the single-component white afterglow with high efficiency of 14.1 % and a lifetime of 0.61 s was achieved by rationally regulating the afterglow intensity ratios of complementary emissions from S1 , T1 , and T1 *.

Thermally Activated Delayed Fluorescence Amorphous Molecular Materials for High-Performance Organic Light-Emitting Diodes.

Small-molecule thermally activated delayed fluorescence (TADF) materials have been extensively developed to actualize efficient organic LEDs (OLEDs). However, organic small molecules generally compromise thin film quality and stability due to the tendency of crystallization, aggregation, and phase separation, which hence degrade the efficiency and long-term stability of the OLEDs. Here, for the first time, we exploit the unique molecular configuration of the bimesitylene scaffold to design two highly efficient TADF amorphous molecular materials with excellent thermal and morphological stabilities. The twisted and rigid bimesitylene scaffold thwarts regular molecular packing and crystallization, thereby guaranteeing homogeneous and stable amorphous thin films. Meanwhile, the highly twisted geometry of the bimesitylene scaffold efficiently breaks the molecular conjugation and thus conserves the high energies of the lowest locally excited triplet states (3LE) above the lowest charge transfer states (1CT and 3CT), leading to small singlet-triplet energy splitting and fast reverse intersystem crossing. These TADF emitters exhibit high photoluminescence quantum yields of 0.90 and 0.69 and short TADF lifetimes of 4.94 and 1.44 μs in doped films, based on which the greenish-blue and greenish-yellow OLEDs achieve external quantum efficiencies of 23.2 and 16.2%, respectively, with small efficiency roll-off rates and perfect color stability.

Blue TADF Emitters Based on B-Heterotriangulene Acceptors for Highly Efficient OLEDs with Reduced Efficiency Roll-Off.

The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)-acceptor (A)-type blue TADF compounds (1-3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D-A connectivity and a completely planar geometry for the B-heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet-triplet energy splitting (<0.1 eV) in their doped host films. The compounds exhibit a fast reverse intersystem crossing rate (kRISC ≈ 106 s-1) with short-lived delayed fluorescence (τd ≈ 2 μs), which is found to be promoted by the strong spin-orbit coupling between the local triplet excited state (3LE, T2) and singlet (S1) states. Using compounds 1-3 as the emitters, highly efficient blue TADF-OLEDs are realized. The devices based on the emitters with B-heterotriangulenes exhibit better performances than the device incorporating a singly bridged reference emitter over the whole luminance range. Notably, the device based on the fully dimethylmethylene-bridged emitter (2) achieves the highest maximum external quantum efficiency (EQE) of 28.2% and the lowest efficiency roll-off, maintaining a high EQE value of 21.2% at 1000 cd/m2.

Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes.

Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising, and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (ΦPL), short delayed luminescence lifetime, high external quantum efficiency (EQE), and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX (spiro-(fluorene-9,9'-xanthene))-based emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg-Teller mechanism in the excited state. They possess short excited-state delayed lifetimes (<10 μs), high photoluminescence quantum yields (ΦPL ∼ 70%), and small singlet-triplet splitting energies (ΔEST < 0.10 eV) in the doped films in an mCP host matrix. The OLEDs showed some of the highest EQEs using spiro-containing emitters where maximum external quantum efficiencies (EQEmax) of 11 and 16% were obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a spiro-based emitter coupled with a low efficiency roll-off (19% at 100 cd m-2) was attained with SFX-PO-DPA-Me.

Promoting Room Temperature Phosphorescence through Electron Transfer from Carbon Dots to Promethazine.

Room temperature phosphorescence (RTP) as a fascinating phenomenon shows great potential toward multiple applications. Howbeit, it is challengeable to improve the phosphorescence efficiency of carbon dots (CDs) owing to their short lifetime. Herein, we proposed a facile, rapid, and gram-scale strategy to synthesize the cross-linked carbon dots (named N-CDs) with both bright blue fluorescence and green RTP emissions. To be specific, the polymer of polyethylenimine (PEI) served as the cross-linking agent and carbon source, during which process phosphoric acid accelerated the formation of the compact carbon core within 30 s. Subsequently, the cross-linked carbon dots with the rigid network formed a small singlet-triplet energy splitting (ΔEST) of 0.490 eV, thus exhibiting a long RTP lifetime of 429.880 ms while coated on the filter paper through the hydrogen bonds. Taking advantage of the double luminescence, we successfully achieved the dual-channel detection of promethazine by N-CDs. The fluorescence of N-CDs was obviously quenched by promethazine through the electron-transfer process, displaying the linear range from 0.4 to 8 mM. Significantly, the electron transfer (ET) from carbon dots to promethazine boosted their phosphorescence efficiency and prolonged the lifetime to 565.190 ms, and the enhanced phosphorescence facilitated the sensitive recognition of promethazine with the concentration range of 1-3000 μM. Meanwhile, the possible autofluorescence interference from biological samples could be avoided through this RTP assaying mode, providing the more accurate results. Also, their RTP and fluorescence endowed the current N-CDs with the ability of dual-signal painting and imaging. This strategy may broaden the new approaches to produce the long-lifetime and high-efficiency RTP material toward the sensing purpose.

Machine-Learned Energy Functionals for Multiconfigurational Wave Functions.

We introduce multiconfiguration data-driven functional methods (MC-DDFMs), a group of methods which aim to correct the total or classical energy of a qualitatively accurate multiconfigurational wave function using a machine-learned functional of some featurization of the wave function such as its density, on-top density, or both. On a data set of carbene singlet-triplet energy splittings, we show that MC-DDFMs are able to achieve near-benchmark performance on systems not used for training with a robust degree of active-space independence. Beyond demonstrating that the density and on-top density hold the information necessary to correct the singlet-triplet energy splittings of multiconfigurational wave functions, this approach shows great promise for the development of functionals for MC-PDFT because corrections to the classical energy appear to be more transferable to types of molecules not included in the training data than corrections to total energies such as those yielded by CASSCF or NEVPT2.

High-efficiency orange thermally activated delayed fluorescence by secondary acceptor modification

The secondary donor/acceptor has significant effects on the excited-state properties and efficiencies of thermally activated delayed fluorescent (TADF) emitters. Here, we use two electron-withdrawing units (cyanobenzene and trifluoromethylbenzene) as a secondary acceptor to control the electron-withdrawing ability of the dibenzo[a,c]phenazine (BP) acceptor core based on the BP-triphenylamine (TPA) backbone. The synthesized orange-red TADF emitters (CN-BP-TPA and CF3-BP-TPA) exhibit the singlet-triplet energy splitting (ΔEST) values of 0.19 and 0.23 eV, respectively. Both emitters show a high photoluminescence quantum yield up to 92%–98% in the doped films. The organic light-emitting diodes (OLEDs) based on CN-BP-TPA achieve the external quantum efficiency of 26.0%, power efficiency of 76.8 lm/W, and current efficiency of 61.1 cd/A as compared with 16.6%, 59.2 lm/W, and 50.9 cd/A of the CF3-BP-TPA counterpart. In addition, the non-doped OLED involving the non-doped CN-BP-TPA emitter shows an ultralow turn-on voltage of 2.2 V and an external quantum efficiency of 5.0%.

Heavy-atom effect promotes multi-resonance thermally activated delayed fluorescence

As one type of latest emitters with simultaneous high efficiency and color-purity, the development of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials represents an important advancement for organic light-emitting diodes (OLEDs). We herein present a new strategy to improve the performance of MR-TADF emitters by fusing sulfur element into the B-N based framework, aiming to utilize the non-metal heavy-atom effect in accelerating the reverse intersystem crossing (RISC) process of the emitter. Two compounds, namely 2PTZBN and 2PXZBN, were developed in this work through rigidifying the DABNA-1 skeleton by sulfur or oxygen atoms. The theoretical calculations and photoluminescence studies reveal that the sulfur-incorporated 2PTZBN enable considerable rate constant of RISC (kRISC) up to 2.8 × 105 s−1 in toluene due to larger spin-orbital coupling (SOC) values and smaller singlet–triplet energy splitting (ΔEST) compared with 2PXZBN. Consequently, organic light-emitting diodes based on 2PTZBN exhibited highly efficient green emission with maximum external quantum efficiency (EQE) of 25.5%.

Single-Shot Readout of a Driven Hybrid Qubit in a GaAs Double Quantum Dot.

We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs tunneling time (≈50) and singlet-triplet splitting vs thermal energy (≈20) allows energy-selective tunneling-based spin-to-charge conversion with a readout visibility of ≈92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs. Further, we discuss the generality of the method for use in other materials, including silicon.

Deep-red thermally activated delayed fluorescence emitters based on a phenanthroline-containing planar acceptor

Organic luminescent materials possessing thermally activated delayed fluorescence (TADF) feature have aroused the tremendous attention. In this work, two novel TADF emitters, 10-(acenaphtho [1′,2':5,6]pyrazino [2,3-f] [1,10]phenanthrolin-12-yl)-10H-phenoxazine (APPT-PXZ) and 7-(acenaphtho [1′,2':5,6]pyrazino [2,3-f] [1,10]phenanthrolin-12-yl)-7H-benzo [c]phenoxazine (APPT-BPXZ), are developed via combining the strong electron-donating and electron-withdrawing units. Both target compounds exhibit orthogonal configurations to decrease singlet-triplet splitting energy (ΔEST) for the remarkable TADF property. Furthermore, APPT-PXZ and APPT-BPXZ-based devices realize the maximum external quantum efficiencies of 7.2% and 2.0% with the electroluminescence peaks at 640 nm and 665 nm, respectively. This work facilitates the development of the deep-red organic light-emitting diodes.

Photochemical Carbene Transfer Reactions of Aryl/Aryl Diazoalkanes-Experiment and Theory*.

Controlling the reactivity of carbene intermediates is a key parameter in the development of selective carbene transfer reactions and is usually achieved by metal complexes via singlet metal‐carbene intermediates. In this combined experimental and computational studies, we show that the reactivity of free diaryl carbenes can be controlled by the electronic properties of the substituents without the need of external additives. The introduction of electron‐donating and ‐withdrawing groups results in a significant perturbation of singlet triplet energy splitting of the diaryl carbene intermediate and of activation energies of consecutive carbene transfer reactions. This strategy now overcomes a long‐standing paradigm in the reactivity of diaryl carbenes and allows the realization of highly chemoselective carbene transfer reactions with alkynes. We could show that free diaryl carbenes can be readily accessed via photolysis of the corresponding diazo compounds and that these carbenes can undergo highly chemoselective cyclopropenation, cascade, or C−H functionalization reactions. Experimental and theoretical mechanistic analyses confirm the participation of different carbene spin states and rationalize for the observed reactivity.

Exploring the x-ray photoabsorption spectrum of sodium in the energy range 1070–1090 eV by elucidating the inner shell excitation transition probabilities

The researchers report on a new study of the x-ray photoabsorption spectrum of sodium near the K-edge, involving the calculations of the excitation energies and the excitation transition probabilities for K-shell and M-shell electron excitations transitions in atomic sodium. The calculations were carried out by employing the ‘multi configuration Dirac–Fock method’, Breit interactions and ‘finite nuclear size corrections’ are included in the calculations, the transition probabilities were convoluted into ‘Breit–Wigner’ line shapes, and single 1s photoionization was also taken into account. In addition, the [1s]np and the [1s3s]ns n′p singlet–triplet energy splitting was calculated. Results of the calculations were compared with previous measurements of the photoabsorption spectrum of sodium near the K-edge, and a very good agreement was observed.

Sterically-Locked Donor-Acceptor Conjugated Polymers Showing Efficient Thermally Activated Delayed Fluorescence.

Donor-acceptor (D-A) conjugated polymers often possess a significant frontier molecular orbital overlap because of the conjugation elongation, leading to no thermally activated delayed fluorescence (TADF) caused by a large singlet-triplet energy splitting (▵EST ). Herein a novel steric locking strategy is proposed by incorporating methyl groups into D-A conjugated polymers. Benefitting from the methyl hindrance, the torsion between the donor and acceptor can be well tuned to form a sterically-locked conformation, so that the unwanted relaxation toward planarity and thus conjugation elongation is prevented to boost hole-electron separation. The resultant D-A conjugated polymer achieves an extremely low ΔEST of 0.09 eV to enable efficient TADF. The corresponding doped and non-doped devices are fabricated via a solution process, revealing a record-high external quantum efficiency (EQE) of 24.0 % (79.4 cd A-1 , 75.0 lm W-1 ) and 15.3 % (50.9 cd A-1 , 47.3 lm W-1 ).

Design Approach of Lifetime Extending Thermally Activated Delayed Fluorescence Sensitizers for Highly Efficient Fluorescence Devices.

In this work, a design approach of three thermally activated delayed fluorescence (TADF) emitters to extend the device lifetime of the TADF sensitized fluorescent devices was studied. Three TADF materials, 5-{4,6-bis[4-(tert-butyl)phenyl]-1,3,5-triazin-2-yl}-2-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)benzonitrile (tTCNTruX), 4-[3-cyano-4-(10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl)phenyl]-2,6-diphenylpyrimidine-5-carbonitrile (PCNTruX) and 4-(4-{10,15-bis[4-(tert-butyl)phenyl]-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazol-5-yl}-3-cyanophenyl)-2,6-diphenylpyrimidine-5-carbonitrile (PCNtTruX), were synthesized as sensitizers for TADF-sensitized fluorescent organic light-emitting diodes. The two tTCNTruX and PCNtTruX TADF emitters were designed to have Dexter energy transfer with blocking groups either in the donor or acceptor unit of the donor-acceptor-type TADF sensitizer. The TADF materials showed small singlet-triplet energy splitting and a high reverse intersystem crossing (RISC) rate for effective sensitization of the fluorescent emission of the fluorescent emitter. tTCNTruX- and PCNtTruX-sensitized fluorescent devices showed maximum external quantum efficiencies (EQEs) of 17.7 % and 11.5 % in the yellow and red devices, respectively, which were higher than those of TADF-sensitized devices with the corresponding TADF sensitizer without a blocking group. Moreover, the device lifetime was also extended by employing the tTCNTruX and PCNtTruX sensitizers. This work demonstrated that the tTCNTruX and PCNtTruX sensitizers are effective to improve the maximum EQE and device lifetime of TADF-sensitized fluorescent devices.

Multi‐Layer π‐Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters

AbstractMulti‐layer π‐stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high‐efficiency thermally activated delayed fluorescence (TADF). In this case, dual donor moieties and a single acceptor moiety are introduced to form two three‐dimensional (3D) emitters, DM‐BD1 and DM‐BD2, which rely on spatial charge transfer (CT). Owing to the enforced face‐to‐face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet–triplet energy splitting (ΔEST) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM‐BD1 and DM‐BD2, respectively, which are higher than those of their D/A‐type analogues.

Mechanistic insights into the insertion and addition reactions of group 13 analogues of the six-membered N-heterocyclic carbenes: interplay of electrophilicity, basicity, and aromaticity governing the reactivity.

Three fundamental concepts (aromaticity/basicity/electrophilicity), being heavily used in modern chemistry, have been applied in this work to study the chemical reactivity of six-membered-ring group 13 N-heterocyclic carbenes (G13-6-Rea; G13 = group 13 elements) using density functional theory (BP86-D3(BJ)/def2-TZVP). G13-6-Rea is isolobal to benzene. Two model reactions have been used in the present study: the insertion reaction of G13-6-Rea with methane and the [1 + 2] cycloaddition reaction of G13-6-Rea with ethene. Our theoretical analysis reveals that the chemical reactivity of B-6-Rea, Al-6-Rea, and Ga-6-Rea is governed by their HOMO (the sp2-σ lone pair orbital on the G13 element), and thus they can be considered nucleophiles. Conversely, the chemical behavior of In-6-Rea and Tl-6-Rea is determined by their LUMO (the vacant p-π orbital on the G13 element), and thus they can be considered electrophiles. On the basis of the VBSCD (valence bond state correlation diagram) model and ASM (activation strain model), this theoretical evidence demonstrates that the origin of activation barriers for the above model reactions is due to the atomic radius of the pivotal group 13 element in the six-membered-ring of G13-6-Rea. Accordingly, our theoretical conclusions suggest that the lower the atomic number and the smaller the atomic radius of the G13 atom, the higher the aromaticity of the six-membered-ring of G13-6-Rea and the smaller the singlet–triplet energy splitting ΔEst of this N-heterocyclic carbene analogue, which will facilitate its chemical reactions. The theoretical findings originating from this study allow many predictions in experiments to be made.

Excitation energies through Becke’s exciton model within a Cartesian-grid KS DFT

Photon-induced electronic excitations are ubiquitously observed in organic chromophore. In this context, we present a simple, alternative time-independent DFT procedure, for the computation of single-particle excitation energies, in particular, the lower bound excited singlet states, which are of primary interest in photochemistry. This takes inspiration from recently developed Becke’s exciton model, where a key step constitutes the accurate evaluation of correlated singlet–triplet splitting energy. It introduces a non-empirical model, from both “adiabatic connection theorem” and “virial theorem” to analyze the role of 2e\(^-\) integral in such calculations. The latter quantity is efficiently mapped onto a real grid and computed accurately using a purely numerical strategy. Illustrative calculations are performed on 10 \(\pi \)-electron organic chromophores within a Cartesian grid implementation of pseudopotential Kohn–Sham (KS) DFT, developed in our laboratory, taking SBKJC-type basis functions within B3LYP approximation. The triplet and singlet excitation energies corresponding to first singly excited configuration are found to be in excellent agreement with TD-B3LYP calculations. Further, we perform the same for a set of larger molecular systems using the asymptotically corrected LC-BLYP, in addition to B3LYP. A systematic comparison with theoretical best estimates demonstrates the viability and suitability of current approach in determining optical gaps, combining predictive accuracy with moderate computational cost.

Engineering silicon-carbide quantum dots for third generation photovoltaic cells.

Interested in the recent development of the building up of photovoltaic devices using graphene-like quantum dots as a novel electron acceptor; we study in this work the optoelectronic properties of edge-functionalized SiC quantum dots using the first principles density functional. For an accurate quantitative estimation of key parameters, a many-body perturbation theory within GW approximation is also performmed. We examine the ability to tailor the electronic gap and optical absorption of the new class of QDs through hydroxylation and carboxylation of seam atoms, in order to improve their photovoltaic efficiency. The HOMO-LUMO energy gap was significantly altered in terms of the type, the concentration and the position of functional groups. The spatial charge separation and charge transfer characterizing our systems seem very prominent to use as dye-sensitized solar cells. Furthermore, the optical band gap of all our compounds is in the NIR-visible energy window, and exhibits a magnitude smaller than that calculated in the pristine case, which enhances the photovoltaic efficiency. Likewise, absorption curves, exciton binding energy and singlet-triplet energy splitting have been broadly modified by functionalization confirming the great luminescent yield of SiCQDs. Depending on the size, SiC quantum dots absorb light from the visible to the near-infrared region of the solar spectrum, making them suitable for third generation solar cells.

Solution-processed multi-resonance organic light-emitting diodes with high efficiency and narrowband emission

With excellent color purity (full-width half maximum (FWHM) < 40 nm) and high quantum yield, multi-resonance (MR) molecules can harvest both singlet and triplet excitons for highly efficient narrowband organic light-emitting diodes (OLEDs) owing to their thermally activated delayed fluorescence (TADF) nature. However, the highly rigid molecular skeleton with the oppositely positioned boron and nitrogen in generating MR effects results in the intrinsic difficulties in the solution-processing of MR-OLEDs. Here, we demonstrate a facile strategy to increase the solubility, enhance the efficiencies and modulate emission color of MR-TADF molecules by extending aromatic rings and introducing tert-butyls into the MR backbone. Two MR-TADF emitters with smaller singlet-triplet splitting energies (ΔEST) and larger oscillator strengths were prepared conveniently, and the solution-processed MR-OLEDs were fabricated for the first time, exhibiting efficient bluish-green electroluminescence with narrow FWHM of 32 nm and external quantum efficiency of 16.3%, which are even comparable to the state-of-the-art performances of the vacuum-evaporated devices. These results prove the feasibility of designing efficient solution-processible MR molecules, offering important clues in developing high-performance solution-processed MR-OLEDs with high efficiency and color purity.

Spatial donor/acceptor architecture for intramolecular charge-transfer emitter

Charge transfer via electron hopping from an electron donor (D) to an acceptor (A) in nanoscale, plays a crucial role in optoelectronic materials, such as organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). Here, we propose a strategy for binding D/A units in space, where intramolecular charge-transfer can take place. The resulted material DM-Me-B is able to give bright emission in this molecular architecture because of the good control of D/A interaction and conformational rigidity. Moreover, DM-Me-B presents small singlet-triplet splitting energy, enabling thermally activated delayed fluorescence. Therefore, the DM-Me-B exhibits ∼20% maximum external quantum efficiency and low efficiency roll-off at 1000 cd/m2, certifying an effective strategy in controlling D/A blocks through space.