Ultrabright organic fluorescent probe for quantifying the dynamics of cytosolic/nuclear lipid droplets.

Lipid droplets (LDs) are extremely active organelles that play a crucial role in energy metabolism, membrane formation, and the production of lipid-derived signaling molecules by regulating lipid storage and release. Nevertheless, directly limited by the lack of superior fluorescent probes, studies of LDs dynamic motion velocity have been rarely reported, especially for nuclear LDs. Herein, a novel organic fluorescent probe Lipi-Bright has been rationally developed based on bridged cyclization of distyrylbenzene. The fully ring-fused molecule structure endows the probe with high photostability. Moreover, this new fluorescent probe displays the features of excellent LDs staining specificity as well as ultrahigh fluorescence brightness. Lipi-Bright labeled LDs was dozens of times brighter than representative probes BODIPY 493/503 or Nile Red. Consequently, by in-situ time-lapse fluorescence imaging, the dynamics of LDs have been quantitatively studied. For instance, the velocities of cytosolic LDs (37 ± 15 nm/s) are found to be obviously faster than those of nuclear LDs (24 ± 4 nm/s), and both the cytosolic LDs and the nuclear LDs would be moved faster or slower depend on the various stimulations. Overall, this work providing plentiful information on LDs dynamics will greatly facilitate the in-depth investigation of lipid metabolism.

Full-Color Sterically Shielded Boron Difluoride Emitters with Efficient and Ultrapure Electroluminescence via Sensitized Fluorescence.

Emitters with narrowband emissions are essential to improve the color purity of organic light-emitting diodes (OLEDs). Boron difluoride (BF) derivatives have preliminarily exhibited small full width at half-maximum (FWHM) values in electroluminescent devices, which, however, still face formidable challenges in recycling triplet excitons and realizing full-color emissions covering the whole visible spectra. Here, a systematic molecular engineering on the aza-fused aromatic emitting core and peripheral substitutions is made, affording a family of full-color BF emitters spanning from blue (461 nm) to red (635 nm), with high photoluminescence quantum yields of >90% and a small FWHM of 0.12 eV. The device architectures are delicately manipulated to form effective thermally activated sensitizing emissions, first affording the highest maximum external quantum efficiency of >20% for BF-based OLEDs with negligible efficiency roll-off.

High-Efficiency Narrowband Multi-Resonance Emitter Fusing Indolocarbazole Donors for BT. 2020 Red Electroluminescence and Ultralong Operation Lifetime.

Polycyclic heteroaromatics with multi-resonance (MR) characteristics are attractive materials for narrowband emitters in wide-color-gamut organic light-emitting diodes. However, MR emitters with pure-red colors are still rare and usually exhibit problematic spectral broadening when redshifting emission. Here, a narrowband pure-red MR emitter is reported by fusing indolocarbazole segments into a boron/oxygen-embedded skeleton, realizing BT.2020 red electroluminescence for the first time together with a high efficiency and an ultralong lifetime. The rigid indolocarbazole segment possesses a strong electron-donating ability due to its para-positioned nitrogen-π-nitrogen backbone and also enlarges the π-extension of the MR skeleton to suppress structural displacement during radiation, achieving concurrently redshifted and narrowed emission spectrum. An emission maximum at 637 nm with a full width at half-maxima of merely 32 nm (0.097 eV) is recorded in toluene. The corresponding device simultaneously exhibits CIE coordinates of (0.708, 0.292) precisely matching the BT.2020 red point, a high external quantum efficiency of 34.4% with low roll-off and an ultralong LT95 (time to 95% of the initial luminance) of >10 000 h at 1000 cd m-2 . These performance characteristics are superior even to those of state-of-the-art perovskite and quantum-dot-based devices for this specific color, paving the way toward practical applications.

Record-High Electron Mobility Exceeding 16 cm2 V- 1 s- 1 in Bisisoindigo-Based Polymer Semiconductor with a Fully Locked Conjugated Backbone.

Polymer semiconductors with mobilities exceeding 10 cm2 V- 1 s- 1 , especially ambipolar and n-type polymer semiconductors, are still rare, although they are of great importance for fabricating polymer field-effect transistors (PFETs) toward commercial high-grade electronics. Herein, two novel donor-acceptor copolymers, PNFFN-DTE and PNFFN-FDTE, are designed and synthesized based on the electron-deficient bisisoindigo (NFFN) and electron-rich dithienylethylenes (DTE or FDTE). The copolymer PNFFN-DTE, containing NFFN and DTE, possesses a partially locked polymeric conjugated backbone, whereas PNFFN-FDTE, containing NFFN and FDTE, has a fully locked one. Fluorine atoms in FDTE not only induce the formation of additional CH∙∙∙F hydrogen bonds, but also lower frontier molecular orbitals for PNFFN-FDTE. Both PNFFN-DTE and PNFFN-FDTE form more ordered molecular packing in thin films prepared from a polymer solution in bicomponent solvent containing 1,2-dichlorobenzene (DCB) and 1-chloronaphthalene (with volume ratio of 99.2/0.8) than pure DCB. The two copolymers-based flexible PFETs exhibit ambipolar charge-transport properties. Notably, the bicomponent solvent-processed PNFFN-FDTE-based PFETs afford a high electron mobility of 16.67 cm2 V-1 s-1 , which is the highest electron-transport mobility for PFETs reported so far. The high electron mobility of PNFFN-FDTE is attributed to its fully locked conjugated backbone, dense molecular packing, and much matched LUMO energy level.

Multiple Fusion Strategy for High-Performance Yellow OLEDs with Full Width at Half Maximums Down to 23 nm 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 533-542 nm and extremely small FWHMs of ≤20 nm. Notably, BN-DICz-based electroluminescent device exhibits excellent efficiencies of 37.4%, 136.6 cd A-1 , and 119.2 lm W-1 with an FWHM of merely 23 nm, representing the best performance for yellow MR organic light-emitting diodes.

Highly efficient and stable deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure.

High-efficiency and stable deep-blue bottom-emitting organic light-emitting diodes with Commission Internationale de l'Eclairage y coordinates (CIE y s) < 0.08 remain exclusive in the literature owing to the high excited-state energy of the emitters. Here, we propose the utilization of narrowband emitters to lower the excited-state energy for stable deep-blue devices by taking advantage of their high color purity. Two proof-of-concept deep-blue emitters with nitrogen-containing spiro-configured polycyclic frameworks are thereafter developed to introduce a multi-resonance effect for narrow emissions and sterically orthogonal configurations for alleviated molecular interactions. Both emitters show bright ultrapure deep-blue emissions with an extremely small full-width-at-half-maxima of only 18-19 nm, which can be maintained even in heavily doped films. Small CIE y s of 0.054 and 0.066 are therefore measured from the corresponding electroluminescence devices with peak energies of only 2.77 eV (448 nm) and 2.74 eV (453 nm), accounting for the remarkably long LT80s (lifetime to 80% of the initial luminance) of 18 900 and 43 470 hours at 100 cd m-2, respectively. Furthermore, by adopting a thermally activated delayed fluorescence sensitizer, impressive maximum external quantum efficiencies of 25% and 31% are recorded respectively, representing state-of-the-art performances for deep-blue devices.

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.

Constructing Organic Electroluminescent Material with Very High Color Purity and Efficiency Based on Polycyclization of the Multiple Resonance Parent Core.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) compounds have set off an upsurge of research because of their tremendous application prospects in the field of wide color gamut display. Herein, we propose a novel MR-TADF molecular construction paradigm based on polycyclization of the multiple resonance parent core, and construct a representative multiple resonance polycyclic aromatic hydrocarbon (MR-PAH) based on the para-alignment of boron and nitrogen atoms into a six-membered ring (p-BNR). Through the retrosynthesis analysis, a concise synthesis strategy with wide applicability has been proposed, encompassing programmed sequential boron esterification, Suzuki coupling and Scholl oxidative coupling. The target model molecule BN-TP shows green fluorescence with an emission peak at 523 nm and a narrow full-width at half-maximum (FWHM) of 34 nm. The organic light-emitting diode (OLED) employing BN-TP as an emitter exhibits ultrapure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.70), and achieves a maximum external quantum efficiency (EQE) of 35.1 %.

Fusion of Multi-Resonance Fragment with Conventional Polycyclic Aromatic Hydrocarbon for Nearly BT.2020 Green Emission.

Herein, we report a general strategy for achieving ultra-pure green emissions by suppressing the shoulder peaks in the emission spectra of conventional polycyclic aromatic hydrocarbons (PAHs). Through precise synthetic fusion of multi-resonance (MR) fragments with conventional PAH, extended π-conjugation lengths, increased molecular rigidity, and reduced vibrational frequency could be simultaneously realized. The proof-of-concept emitters exhibited ultra-pure green emissions with dominant peaks at ca. 521 nm, photoluminescence quantum yields that are greater than 99 %, a small full-width-at-half-maximum of 23 nm, and CIE coordinates of (0.16, 0.77). The bottom-emitting organic light-emitting diode (OLED) exhibited a record-high CIEy value of 0.74 and a high maximum external quantum efficiency of 30.5 %. The top-emitting OLED not only achieved a BT.2020 green color (CIE: 0.17, 0.78) for the first time but also showed superior performance among all green OLED devices, with a current efficiency of 220 cd A- .

Highly Efficient and Stable Blue Organic Light-Emitting Diodes based on Thermally Activated Delayed Fluorophor with Donor-Void-Acceptor Motif.

Thermally activated delayed fluorophores (TADF) with donor-acceptor (D-A) structures always face strong conjugation between donor and acceptor segments, rendering delocalized new molecular orbitals that go against blue emission. Developing TADF emitters with blue colors, high efficiencies, and long lifetimes simultaneously is therefore challenging. Here, a D-void-A structure with D and A moieties connected at the void-position where the frontier orbital from donor and acceptor cannot be distributed, resulting in nonoverlap of the orbitals is proposed. A proof-of-the-concept TADF emitter with 3,6-diphenyl-9H-carbazole (D) connected at the 3'3-positions of 9H-xanthen-9-one (A), the void carbon-atom with no distribution of the highest occupied molecular orbital (HOMO) of A-segment, realizes more efficient and blue-shifted emission compared with the contrast D-A isomers. The deeper HOMO-2 of A is found to participate into conjugation rather than HOMO, providing a wider-energy-gap. The corresponding blue device exhibits a y color coordinate (CIEy ) of 0.252 and a maximum external quantum efficiency of 27.5%. The stability of this compound is further evaluated as a sensitizer for a multiple resonance fluorophore, realizing a long lifetime of ≈650 h at an initial luminance of 100 cd m-2 with a CIEy of 0.195 and a narrowband emission with a full-width-at-half-maxima of 21 nm.

Nitrogen-Embedded Multi-Resonance Heteroaromatics with Prolonged Homogeneous Hexatomic Rings.

Nitrogen-containing polycyclic heteroaromatics have exhibited fascinating multi-resonance (MR) characteristics for efficient narrowband emission, but strategies to bathochromic shift their emissions while maintaining the narrow bandwidths remain exclusive. Here, homogeneous hexatomic rings are introduced into nitrogen-embedded MR skeletons to prolong the π-conjugation length for low-energy electronic transitions while retaining the non-bonding character of the remaining parts. The proof-of-the-concept emitters exhibit near unity photoluminescence quantum yields with peaks at 598 nm and 620 nm and small full-width-at-half-maximums of 28 nm and 31 nm, respectively. Optimal organic light-emitting diodes exhibit a high external quantum efficiency of 18.2 %, negligible efficiency roll-off, and ultra-long lifetime with negligible degradation at an initial luminance of 10 000 cd m-2 after 94 hours.

Sterically Wrapped Multiple Resonance Fluorophors for Suppression of Concentration Quenching and Spectrum Broadening.

Multiple resonance (MR) emitters are promising for highly efficient organic light-emitting diodes (OLEDs) with narrowband emission; however, they still face intractable challenges with concentration-caused emission quenching, exciton annihilation, and spectral broadening. In this study, sterically wrapped MR dopants with a fluorescent MR core sandwiched by bulk substituents were developed to address the intractable challenges by reducing intermolecular interactions. Consequently, high photo-luminance quantum yields of ≥90 % and small full width at half maximums (FWHMs) of ≤25 nm over a wide range of dopant concentrations (1-20 wt %) were recorded. In addition, we demonstrated that the sandwiched MR emitter can effectively suppress Dexter interaction when doped in a thermally activated delayed fluorescence sensitizer, eliminating exciton loss through dopant triplet. Within the above dopant concentration range, the optimal emitter realizes remarkably high maximum external quantum efficiencies of 36.3-37.2 %, identical small FWHMs of 24 nm, and alleviated efficiency roll-offs in OLEDs.

Multi-Resonance Deep-Red Emitters with Shallow Potential-Energy Surfaces to Surpass Energy-Gap Law*.

Efficient organic emitters in the deep-red region are rare due to the "energy gap law". Herein, multiple boron (B)- and nitrogen (N)-atoms embedded polycyclic heteroaromatics featuring hybridized π-bonding/ non-bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B-phenyl-B and N-phenyl-N structures enhances the electronic coupling of those para-positioned atoms, forming restricted π-bonds on the phenyl-core for delocalized excited states and thus a narrow energy gap. The mutually ortho-positioned B- and N-atoms also induce a multi-resonance effect on the peripheral skeleton for the non-bonding orbitals, creating shallow potential energy surfaces to eliminate the high-frequency vibrational quenching. The corresponding deep-red emitters with peaks at 662 and 692 nm exhibit narrow full-width at half-maximums of 38 nm, high radiative decay rates of ca. 108  s-1 , ≈100 % photo-luminescence quantum yields and record-high maximum external quantum efficiencies of ca. 28 % in a normal planar organic light-emitting diode structure, simultaneously.

Highly Efficient Electrofluorescence Material Based on Pure Organic Phosphor Sensitization*.

Pure organic room-temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal complexes to fabricate highly efficient organic light-emitting devices (OLEDs). However, applications of the reported RTP materials in OLEDs are seriously impeded by their low photoluminescence quantum yields (PLQYs) in a thin film state. To overcome these obstacles, we established a new strategy to construct highly efficient OLEDs based on a pure organic RTP material sensitized fluorescence emitter by selecting benzimidazole-triazine molecules (PIM-TRZ), 2,6-di(phenothiazinyl)naphthalene (ß-DPTZN), and 5,6,11,12-tetraphenylnaphthacene (rubrene) as host, phosphor sensitizer, and fluorescent emitter, respectively. The perfect combination of host, phosphorescent sensitizer, and fluorescent emitter in the emitting layer ensure the outstanding performance of the devices with an external quantum efficiency (EQE) of 15.7 %.

Indolo[3,2,1-jk]carbazole Embedded Multiple-Resonance Fluorophors for Narrowband Deep-blue Electroluminescence with EQE≈34.7 % and CIEy ≈0.085.

Multiple-resonance (MR) organic emitters bearing small full-width at half-maximum (FWHMs) are of general interest in organic light-emitting diodes. Indolo[3,2,1-jk]carbazole (ICz) embedded MR-fluorophors have demonstrated extremely small FWHMs, yet in the violet region with low electroluminescence efficiency. Herein, a strategic implementation of ICz subunits into MR fluorophors is proposed by taking advantage of the synergetic effect of para-positioned nitrogen atoms to enhance electronic coupling to decrease emitting energy gap. Deep blue emitters peaking at 441 and 447 nm with FWHMs of only 18 and 21 nm are thereof obtained, respectively, accompanied by ≈90 % photo-luminance quantum yields. With the assistance of a thermally activated delayed fluorescence sensitizer to recycle excitons, the corresponding narrowband electroluminescent devices show unprecedent high maximum external quantum efficiencies of 32.0 % and 34.7 % with CIEy of 0.10 and 0.085, respectively.

Achieving Pure Green Electroluminescence with CIEy of 0.69 and EQE of 28.2% from an Aza-Fused Multi-Resonance Emitter.

Pure green emitters are essential for realizing an ultrawide color gamut in next-generation displays. Herein, by fusing the difficult-to-access aza-aromatics onto B (boron)-N (nitrogen) skeleton, a hybridized multi-resonance and charge transfer (HMCT) molecule AZA-BN was successfully synthesized through an effective one-shot multiple cyclization method. AZA-BN shows pure green fluorescence with photoluminance quantum yield of 99.7 %. The corresponding green device exhibits a maximum external quantum efficiency and power efficiency of 28.2 % and 121.7 lm W-1 , respectively, with a full width half maximum (FWHM) of merely 30 nm and Commission Internationale de l'Eclairage (CIE) coordinate y of 0.69, representing the purest green bottom-emitting organic light-emitting diode.

Constructing Charge-Transfer Excited States Based on Frontier Molecular Orbital Engineering: Narrowband Green Electroluminescence with High Color Purity and Efficiency.

The design and synthesis of organic materials with a narrow emission band in the longer wavelength region beyond 510 nm remain a great challenge. For constructing narrowband green emitters, we propose a unique molecular design strategy based on frontier molecular orbital engineering (FMOE), which can integrate the advantages of a twisted donor-acceptor (D-A) structure and a multiple resonance (MR) delayed fluorescence skeleton. Attaching an auxiliary donor to a MR skeleton leads to a novel molecule with twisted D-A and MR structure characteristics. Importantly, a remarkable red-shift of the emission maximum and a narrowband spectrum are achieved simultaneously. The target molecule has been employed as an emitter to fabricate green organic light-emitting diodes (OLEDs) with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.69) and a maximum external quantum efficiency (EQE) of 27.0 %.

Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands.

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3-3.6 V for practical luminance of 1000 cd m-2 , as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.

Multi-Resonance Induced Thermally Activated Delayed Fluorophores for Narrowband Green OLEDs.

High-color-purity emissions with small a full-width at half-maximum (FWHM) are an ongoing pursuit for high-resolution displays. Though the flourishment of narrow-band emissive materials with multi-resonance induced thermally activated delayed fluorescence (MR-TADF) in the blue region, such materials have not validated their potential in other color regions. By amplifying the influence of skeleton and peripheral units, a series of highly efficient green-emitting MR-TADF materials are firstly reported. Peripheral units with electron-deficit properties can significantly narrow the energy gap for bathochromic emission without compromising the color fidelity. MR-TADF emitters with photo-luminance quantum yields of above 90 % with FWHMs of ≤25 nm are developed. The corresponding organic light-emitting diodes show maximum external quantum efficiency/ power efficiency of 22.02 %/ 69.82 lm W-1 with excellent long-term stability.