Multiple Resonance Effect Research Articles

Visualization of Multiple-Resonance-Induced Frontier Molecular Orbitals in a Single Multiple-Resonance Thermally Activated Delayed Fluorescence Molecule.

The spatial distribution and electronic properties of the frontier molecular orbitals (FMOs) in a thermally activated delayed fluorescence (TADF) molecule contribute significantly to the TADF properties, and thus, a detailed understanding and sophisticated control of the FMOs are fundamental to the design of TADF molecules. However, for multiple-resonance (MR)-TADF molecules that achieve spatial separation of FMOs by the MR effect, the distinctive distribution of these molecular orbitals poses significant challenges for conventional computational analysis and ensemble averaging methods to elucidate the FMOs' separation and the precise mechanism of luminescence. Therefore, the visualization and analysis of electronic states with the specific energy level of a single MR-TADF molecule will provide a deeper understanding of the TADF mechanism. Here, scanning tunneling microscopy/spectroscopy (STM/STS) was used to investigate the electronic states of the DABNA-1 molecule at the atomic scale. FMOs' visualization and local density of states analysis of the DABNA-1 molecule clearly show that MR-TADF molecules also have well-separated FMOs according to the internal heteroatom arrangement, providing insights that complement existing theoretical prediction methods.

Narrowband room temperature phosphorescence of closed-loop molecules through the multiple resonance effect

Luminescent materials with narrowband emission show great potential for diverse applications in optoelectronics. Purely organic phosphors with room-temperature phosphorescence (RTP) have made significant success in rationally manipulating quantum efficiency, lifetimes, and colour gamut in the past years, but there is limited attention on the purity of the RTP colours. Herein we report a series of closed-loop molecules with narrowband phosphorescence by multiple resonance effect, which significantly improves the colour purity of RTP. Phosphors show narrowband phosphorescence with full width at half maxima (FWHM) of 30 nm after doping into a rigid benzophenone matrix under ambient conditions, of which the RTP efficiency reaches 51.8%. At 77 K, the FWHM of phosphorescence is only 11 nm. Meanwhile, the colour of narrowband RTP can be tuned from sky blue to green with the modification of methyl groups. Additionally, the potential applications in X-ray imaging and display are demonstrated. This work not only outlines a design principle for developing narrowband RTP materials but also makes a major step forward extending the potential applications of narrowband luminescent materials in optoelectronics.

Metastructure based broadband structural stealth with material-structure-function integration

Broadband microwave absorption is difficult to be realized in traditional coating form as multiple electromagnetic resonances are difficult to be generated even with dielectric-magnetic loss composites. Narrow absorption band confines the practical usage of the novel electromagnetic nano composites. Herein, the structural stealth concept is proposed to overcome the narrow-band absorption problem with the effect carrier of metastructure. The gradient honeycomb metastructure (GHM) was designed and optimized with module stack large mutation genetic algorithms. The structural design, fabrication, experimental verification and parameter adjustment were included in the closed loop with material-structure-function integration. Multiple resonance effects were introduced in the gradient designs. GHM achieved −10 dB absorption bandwidth in 1.92–17.6 GHz and three deep absorption peaks were introduced by three layers of electromagnetic resonant honeycomb. Broadband absorption in oblique incidence from 30° to grazing angle 85° was achieved to overcome the oblique absorption degeneration problems of traditional nano lossy composites. The structural mechanical performance was high with the maximum equivalent tensile strength of 108.6 MPa and the maximum flexural load of 0.873 kN. The results showed the importance of structural stealth with material-structure-function integration to design metastructure for broadband microwave absorption, which provided a promising approach to achieve broadband microwave absorption.

Broadband, thin thickness and low filling ratio microwave absorption by constructing Sm2Fe17/Fe(Sm) hard/soft magnetic nanoparticles in Sm2O3-doped porous carbon matrix

Exchange-coupled hard/soft magnetic composites are supposed to be promising microwave absorber, due to the multiple-resonance peaks in permeability. However, due to insufficient magnetization and dielectric loss, soft/hard ferrite magnets are difficult to achieve further breakthroughs in microwave absorption performance. In this work, the special hard/soft magnetic composites are successfully fabricated, in which Sm2Fe17/Fe(Sm) hard/soft hybrid nanoparticles are embedded in ∼5 nm Sm2O3-doped porous carbon matrix. The hard/soft magnetic exchange-coupling result in the multiple-resonance peaks beyond Snoek’s limitation at broad wave band and the Sm2O3-doped carbon supplements dielectric loss capacity. Multiple-resonance effect is modified by adjusting the ratio of hard and soft magnetic components and the dielectric loss capacity is supplemented by Sm2O3-doped carbon. Herein, the hard/soft magnetic exchange-coupling and dielectric matrix are dexterously integrated to achieve broad absorption band with thin matching thickness and low filling ratio. By varying the ratio of Fe3+/Sm3+ in raw material, 20 wt% Sm2Fe17/Fe(Sm)-paraffin composites can reach the maximum effective absorption bandwidth of 6.8 GHz with matching thickness of 1.6 mm, the minimal reflection loss of −47.15 dB with 1.5 mm and the optimal microwave absorption efficiency of 71.1 dB·GHz/mm with 1.4 mm. Overall, this work provides a simple approach to introduce multi-resonance effect, which broadens the path for advanced absorbers with broad band and thin thickness and low filling ratio.

Photophysics and photochemistry of thermally activated delayed fluorescence emitters based on the multiple resonance effect: transient optical and electron paramagnetic resonance studies.

The photochemistry of two representative thermally activated delayed fluorescence (TADF) emitters based on the multiple resonance effect (MRE) (DABNA-1 and DtBuCzB) was studied. No significant TADF was observed in fluid solution, although the compounds have a long-lived triplet state (ca. 30 μs). We found that these planar boron molecules bind with Lewis bases, e.g., 4-dimethylaminopyridine (DMAP) or an N-heterocyclic carbene (NHC). A new blue-shifted absorption band centered at 368 nm was observed for DtBuCzB upon formation of the adduct; however, the fluorescence of the adduct is the same as that of the free DtBuCzB. We propose that photo-dissociation occurs for the DtBuCzB-DMAP adduct, which is confirmed by femtosecond transient absorption spectra, implying that fluorescence originates from DtBuCzB produced by photo-dissociation; the subsequent in situ re-binding was observed with nanosecdon transient absorption spectroscopy. No photo-dissociation was observed for the NHC adduct. Time-resolved electron paramagnetic resonance (TREPR) spectra show that the triplet states of DABNA-1 and DtBuCzB have similar zero field splitting (ZFS) parameters (D = 1450 MHz). Theoretical studies show that the slow ISC is due to small SOC and weak Herzberg-Teller coupling, although the S1/T1 energy gap is small (0.14 eV), which rationalizes the lack of TADF.

Design Strategy for Thermally Activated Delayed Fluorescence Materials with Multiple Resonance Effect

Design Strategy for Thermally Activated Delayed Fluorescence Materials with Multiple Resonance Effect

Tunable multimode emission induced by charge transfer and multiple resonance effect

Single-component luminescent materials with dual emission properties show great promise in material and life science. Here, we report a novel thermally activated delayed fluorescence (TADF) emitter BO-PST based on a rigid B-embedded dioxygen-bridged acceptor (A) and flexible phenothiazine donor (D). The flexibility of D unit contributes to different charge transfer characteristic and interesting multimode emission in various environment, including D-A-type TADF emission, multi-resonant (MR) fluorescence emission and MR induced afterglow with long lifetime of 1.55 s. Single-crystal structures and theoretical calculations prove that dual emission originates from the quasi-equatorial and quasi-axial conformers of the phenothiazine unit, respectively. The dynamic multicolor properties demonstrate the potential of the novel BO-PST molecule in the volatile organic compounds (VOCs) detection and the stimulus-responsive application.

One-Shot Construction of BN-Embedded Heptadecacene Framework Exhibiting Ultra-narrowband Green Thermally Activated Delayed Fluorescence

BN-embedded nonacene, tridecacene, and heptadecacene frameworks were constructed using one-shot quadruple, sextuple, and octuple borylation reactions, respectively. The key to success is the judicious choice of borylating reagents and long-chain alkyl-substituted carbazolyl groups as boron-trapping groups, which suppressed the decrease in HOMO energy and insolubilization associated with borylation. Based on the product yields, each electrophilic C-H borylation proceeded in >99% yield, which is the best efficiency reported so far for C-H borylation reactions. Owing to the multiple resonance effects of boron and nitrogen, the prepared acenes exhibited ultra-narrowband green thermally activated delayed fluorescence with full-width at half-maximum of 12-16 nm; moreover, their kRISC values were in the order of 105 s-1. We fabricated an organic light-emitting diode by employing the nonacene as an emitter, which exhibited high external quantum efficiency (EQE) of 28.7%. The device also showed a minimum efficiency roll-off with an EQE of 25.8% at 1000 cd m-2.

A Series of Singlet‐Triplet InVerted TADF Fluorescent Probes with High Stability, Low Molecular Weight, and Synthesis Accessibility

AbstractThermally activated delayed fluorescence (TADF)‐based fluorescent biological probes embedded with near infrared (NIR) emission have some unique properties, such as minimal photodamage, low light scattering, and deep tissue penetration. Typical TADF materials are based on electron donors and acceptors, all of them have broad emission and high molecular weight, which is not ideal for biological applications. The cycl[3,3,3]azine is a multiple resonance effect (MRE)‐like TADF fluorescent compound, with narrow NIR emission, inverted singlet‐triplet gaps, and low molecular weight, which is an ideal platform for biological applications. However, the low stability of cycl[3,3,3]azine and its anti‐Kasha's rule nature limit its biological applications. In this work, the authors have rationally investigated the decomposition process of cycl[3,3,3]azine, and a series of NIR and MRE‐like molecules are designed. These molecules would be stable in the biological environment and NIR fluorescence can also be emitted from the first excited state, the synthesis route has also been proposed. In a word, this work provides a series of biological probes with inverted singlet‐triplet gaps, good stability, and low molecular weight for further study.

Highly Efficient and Stable Blue OLEDs Based on B—N Bonds Embedded 6,12‐Diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole with Narrowband Emission and Extended Lifetime†

Comprehensive SummaryEmitters with narrowband spectra are of great importance nowadays because of the growing demand for ultra‐high‐definition displays in the fields of panel displays and solid‐state lighting. Though the reported multiple resonance (MR) emitters have been widely studied with extremely sharp spectra and color‐tunable emissions, the electrophilic borylation synthetic strategies and stable narrowband blue devices still face great challenges. Herein, by virtue of the advantages of both the MR effect of conventional indolocarbazole skeleton and the easy‐to‐access of B—N covalent bonds, the linear tert‐butyl benzene and carbazole groups modified 6,12‐diphenyl‐5,11‐dihydroindolo[3,2‐b]carbazole derivatives (tPh[BN] and Cz[BN]) containing two B—N covalent bonds are easily constructed through amine‐directed double borylation strategy. Both emitters behaved bright blue emission with full‐width‐at‐half‐maximum (FWHM) of 20—27 nm in solution, high fluorescence quantum efficiency over 90%, and excellent stability toward moisture. By employing triplet‐triplet annihilation electroluminescent mechanism, blue OLED devices based on them showed a small FWHM of 48 nm and maximum external quantum efficiency of ~7.1% with negligible efficiency roll‐off, and long operational lifetime (LT80) of 87 h at an initial brightness of 1000 cd·m−2, demonstrating the enormous application potential of B—N bonds embedded emitters in achieving ideal narrowband blue emitters.

Multiple Fusion Strategy for High-Performance Yellow OLEDs with Full Width at Half Maximums Down to 23nm and External Quantum Efficiencies up to 37.4.

The pursuit of ideal narrowband yellow multiple resonance (MR) emitters is hampered by the mutual constraints of effective spectral redshift and maintaining a small full width at half maximum (FWHM) value. Here, a novel multiple fusion molecular design strategy is reported to break this trade-off. Compared with the selected narrowband parent core, the specific multiple MR effects in target molecules can simultaneously extend the π-conjugation length, increase the rigidity of the structure, and reduce the vibrational frequency. Proof-of-the-concept emitters BN-DICz and DBN-ICz show bright yellowish green to yellow emissions in dilute toluene solutions with peaks at 533-542nm and extremely small FWHMs of ≤20nm. Notably, BN-DICz-based electroluminescent device exhibits excellent efficiencies of 37.4%, 136.6cd A-1 , and 119.2lm W-1 with an FWHM of merely 23nm, representing the best performance for yellow MR organic light-emitting diodes.

Boron‐, Sulfur‐ andNitrogen‐DopedTridecacyclic Aromatic Emitters with Multiple Resonance Effect for Narrowband Red Emission†

Comprehensive SummaryTwo novel boron (B), sulfur (S), nitrogen (N)‐doped polycyclic aromatic hydrocarbon multiple resonance emitters (DBNS and DBNS‐tBu) are designed and synthesized for narrowband red emission by embedding two pairs of S and N atoms and two B atoms inpara‐positions of central benzene rings within a tridecacyclic aromatic skeleton to form donor‐π‐donor (D‐π‐D) and acceptor‐π‐acceptor (A‐π‐A) structures, which not only exhibit emission maximum at 641 nm with small full width at half maximum of 39 nm, but also combine high photoluminescent quantum efficiency (85%) and rapid reverse intersystem crossing (kRISC = 2.2 × 105s−1), giving rise to red electroluminescence with color coordinates of (0.65, 0.34) and maximum external quantum efficiency of 7.8% via solution process.

Toward a BT.2020 green emitter through a combined multiple resonance effect and multi-lock strategy

Color-saturated green-emitting molecules with high Commission Internationale de L’Eclairage (CIE) y values have great potential applications for displays and imaging. Here, we linked the outer phenyl groups in multiple-resonance (MR)-type blue-emitting B (boron)-N (nitrogen) molecules through bonding and spiro-carbon bridges, resulting in rigid green emitters with thermally activated delayed fluorescence. The MR effect and multiple interlocking strategy greatly suppressed the high-frequency vibrations in the molecules, which emit green light with a full-width at half-maximum of 14 nm and a CIE y value of 0.77 in cyclohexane. These were the purest green molecules with quantum efficiency and color purity that were comparable with current best quantum dots. Doping these emitters into a traditional green-emitting phosphorescence organic light-emitting diode (OLED) endowed the device with a Broadcast Service Television 2020 color-gamut, 50% improved external quantum efficiency, and an extremely high luminescence of 5.1 × 105 cd/m2, making it the greenest and brightest OLED ever reported.

Amine-Directed Formation of B-N Bonds for BN-Fused Polycyclic Aromatic Multiple Resonance Emitters with Narrowband Emission.

Despite the remarkable multiple resonance (MR) optoelectronic properties of organic nanographenes with boron and nitrogen atoms disposed para to each other, the synthetic procedures are sophisticated with low yields and the molecular skeletons are limited. Here, a new paradigm of easy-to-access MR emitter is constructed by simplifying the multiborylation through amine-directed formation of B-N bonds while introducing an additional para-positioned nitrogen atom to trigger the MR effect. The proof-of-concept molecules exhibit narrowband emissions at 480 and 490 nm, with full width at half maxima (FWHMs) of only 29 and 34 nm. The devices based on them showed external quantum efficiencies (EQE) of >33.0 %, which remained above 24.0 % even at a high brightness of 5000 cd m-2 . This is the first example of MR emitters with a B-N covalent bond, not only decreasing the synthesis difficulty but also increasing the diversity of MR skeletons for emerging new optoelectronic properties.

Amine‐Directed Formation of B−N Bonds for BN‐Fused Polycyclic Aromatic Multiple Resonance Emitters with Narrowband Emission

AbstractDespite the remarkable multiple resonance (MR) optoelectronic properties of organic nanographenes with boron and nitrogen atoms disposedparato each other, the synthetic procedures are sophisticated with low yields and the molecular skeletons are limited. Here, a new paradigm of easy‐to‐access MR emitter is constructed by simplifying the multiborylation through amine‐directed formation of B−N bonds while introducing an additionalpara‐positioned nitrogen atom to trigger the MR effect. The proof‐of‐concept molecules exhibit narrowband emissions at 480 and 490 nm, with full width at half maxima (FWHMs) of only 29 and 34 nm. The devices based on them showed external quantum efficiencies (EQE) of >33.0 %, which remained above 24.0 % even at a high brightness of 5000 cd m−2. This is the first example of MR emitters with a B−N covalent bond, not only decreasing the synthesis difficulty but also increasing the diversity of MR skeletons for emerging new optoelectronic properties.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Narrowband Deep-Blue Multi-Resonance Induced Thermally Activated Delayed Fluorescence: Insights from the Theoretical Molecular Design.

Multi-resonance thermal activated delayed fluorescence (MR-TADF) has been promising with large oscillator strength and narrow full width at half maxima of luminescence, overcoming the compromise of emission intensity and energy criteria of traditional charge transfer TADF frameworks. However, there are still limited theoretical investigations on the excitation mechanism and systematic molecular manipulation of MR-TADF structures. We systematically study the highly localized excitation (LE) characteristics based on typical blue boron-nitrogen (BN) MR-TADF emitters and prove the potential triangular core with theoretical approaches. A design strategy by extending the planar π-conjugate core structure is proposed to enhance the multiple resonance effects. Moreover, several substituted groups are introduced to the designed core, achieving color-tunable functions with relatively small energy split and strong oscillator strength simultaneously. This work provides a theoretical direction for molecular design strategy and a series of potential candidates for highly efficient BN MR-TADF emitters.

An electron acceptor featuring a B-N covalent bond and small singlet-triplet gap for organic solar cells.

BNTT2F, an electron acceptor featuring a B-N covalent bond and singlet-triplet gap as low as 0.20 eV via the multiple resonance effect, is developed for organic solar cells. The optimized device based on BNTT2F offered an efficiency of 8.3%, suggesting the great prospect of B-N covalent bond-containing π-conjugated molecules for photovoltaics.

One-Shot Synthesis of Expanded Heterohelicene Exhibiting Narrowband Thermally Activated Delayed Fluorescence.

An expanded heterohelicene consisting of three BN2-embedded [4]helicene subunits (V-DABNA-Mes) has been synthesized by one-shot triple borylation. The key to success is the excessive use of boron tribromide in an autoclave. Based on the multiple resonance effect of three boron and six nitrogen atoms, V-DABNA-Mes exhibited a narrowband sky-blue thermally activated delayed fluorescence with a full width at half-maximum of 16 nm. The resonating π-extension minimized the singlet-triplet energy gap and enabled rapid reverse intersystem crossing with a rate constant of 4.4 × 105 s-1. The solution-processed organic light-emitting diode device, employed as an emitter, exhibited a narrowband emission at 480 nm with a high external quantum efficiency of 22.9%.

Simple Molecular Design Strategy for Multiresonance Induced TADF Emitter: Highly Efficient Deep Blue to Blue Electroluminescence with High Color Purity

AbstractThermally activated delayed fluorescence (TADF) materials based on multiple resonance (MR) effect exhibit enormous potentials in organic light‐emitting diodes (OLEDs) with high color purity due to their intrinsically narrow emission. However, most of MR‐TADF emitters are limited to the boron‐nitrogen‐based rigid skeleton. In this work, three novel MR‐TADF emitters, namely CzBNO, DMAcBNO and DPAcBNO, are elaborately constructed, the TADF properties of which are realized by virtue of opposite MR effect of boron and nitrogen/oxygen atom. CzBNO‐based deep blue‐emitting OLEDs achieve a maximum external quantum efficiency (EQE) of 13.6% with a small FWHM of 36 nm, as well as a Commission Internationale de l'Eclairage (CIE) coordinate of (0.14,0.08). While the other two emitter‐based devices exhibit blue emission with a maximum EQE of up to 23.0%. To further improve the OLEDs performances, DMAcBNO and DPAcBNO‐based devices assisted by a sensitizer exhibit an excellent EQE of up to 29.6% with a relatively small efficiency roll‐off.