Narrowband Fluorescent Emitters Based on BN-Doped Polycyclic Aromatic Hydrocarbons for Efficient and Stable Organic Light-Emitting Diodes

Cyano-Modified Multi-Resonance Thermally Activated Delayed Fluorescent Emitters Towards Pure-Green OLEDs with a CIEy Value of 0.74

The development of pure-green organic emitters with ideal emission peaks and ultra-narrow full-widths at half-maximum (FWHMs) remains a formidable challenge. Herein, we report two new green emitters, CNBN and MCNBN, which achieve extremely narrow FWHMs by synergistic rigid π-extension and cyano-substitution of a sky-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) core. The introduction of cyano groups induces red-shifts in the emission to the green region and dramatically minimizes the FWHMs. In toluene solution, CNBN and MCNBN exhibit narrowband emission with a maximum at 501 nm and 510 nm with ultra-narrow FWHMs of 14 nm/0.066 eV and 15 nm/0.071 eV, respectively. Given the near-unity photoluminescence quantum yields and almost 100 % horizontal dipole orientation, the electroluminescent (EL) devices based on CNBN and MCNBN deliver external quantum efficiencies (EQEs) exceeding 30 % with FWHMs of 16 nm/0.072 eV and 17 nm/0.080 eV, respectively. Notably, the MCNBN-based device achieves pure-green emission with a maximum at 517 nm with Commission Internationale de l'Éclairage coordinates of (0.17, 0.74), closely aligning with the BT.2020 green standard.

Clar's Aromatic π-Sextet Rule for the Construction of Red Multiple Resonance Emitter

Despite the proliferation of multiple resonance (MR) materials in the blue to green spectral ranges, red MR emitters remain scarce in the literature, an area that certainly warrants attention for future applications. Here, through a clever application of classic Clar's aromatic π-sextet rule, we triumphantly constructed the first red MR emitter by substituting the conventional benzene ring core with anthracene (fewer π-sextets). Theoretical studies indicate that the quantity of π-sextets ultimately determines the optical band gap of a molecule, rather than the number of fused benzene rings. Benefiting from the high photoluminescence quantum yield of ~94 % and horizontal dipole ratio of ~90 %, the corresponding narrowband red (luminescence wavelength: 608 nm) organic light-emitting diode shows a high external quantum efficiency of 27.3 %, with only a slight decrease of 3.7 % at an elevated luminance level of 100,000 cd/m2.

Multiple Resonance Quasi-fluorescence from BN-Doped Aromatic Compounds Modified with "Naphthalene" Units Approaches the BT.2020 Green Light Standard

Developing fluorophores that conform to the Broadcast Service Television 2020 (BT.2020) standard presents a formidable challenge. Here, we propose an innovative approach that integrates two and three-boron/nitrogen (BN2)-embedded [4]helicene subunits with naphthalene, resulting in the synthesis of two novel narrowband bright green quasi-fluorescent emitters, NT-2B and NT-3B for ultra-high-definition displays. These emitters exhibit minimal reorganization energy and Huang-Rhys factor, emitting at 510 and 511 nm in dilute toluene solution with exceptionally narrow full width at half maximum values of 15 and 14 nm, respectively. Notably, NT-2B demonstrates an impressive photoluminescence quantum yield of 92.5 %, rapid radiative decay rate, and slow non-radiative decay rate. Owing to their narrowband emission characteristics and outstanding optoelectronic properties, corresponding OLEDs based on NT-2B demonstrate a high external quantum efficiency of 30.6 %, with an FWHM value of 21.5 nm and a CIEy of 0.74, positioning it as one of the leading narrow-band green emitters.

Sulfur-locked multiple resonance emitters for high performance orange-red/deep-red OLEDs

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are preferred for their high efficiency and high colour purity in organic light-emitting diodes (OLEDs). However, the design strategies of MR-TADF emitters in the red region are very limited. Herein, we propose a concept for a paradigm shift in orange-red/deep-red MR emitters by linking the outer phenyl groups in a classical MR framework through intramolecular sulfur (S) locks. Endowed with the planar architectural feature of the MR mother core, the proof-of-concept S-embedded emitters S-BN and 2S-BN also exhibit considerable flatness, which proves critical in avoiding the direct establishment of potent charge transfer states and inhibiting the non-radiative decay process. The emission maxima of S-BN and 2S-BN are 594 nm and 671 nm, respectively, and both have a high photoluminescence quantum yield of ~100%, a rapid radiative decay rate of around 107 s-1, and a remarkably high reverse intersystem crossing rates of about 105 s-1. Notably, maximum external quantum efficiencies of 39.9% (S-BN, orange-red) and 29.3% (2S-BN, deep-red) were also achieved in typical planar OLED structures with ameliorated efficiency roll-offs.

High-Performance Narrowband Pure-Green OLEDs with Gamut Approaching BT.2020 Standard: Deuteration Promotes Device Efficiency and Lifetime Simultaneously

In order to fulfill the demand for ultrahigh definition organic light-emitting diodes (OLEDs), pure-green emitters with Commission Internationale de l'Éclairage (CIE) y-coordinate over 0.71 are in urgent demand. Meanwhile, the high device efficiency, small efficiency roll-off, and operational lifetime also remain challenging issues. In this work, a series of narrowband pure-green fluorescent emitters based on a double-boron (B) doped polycyclic aromatic hydrocarbons (PAHs) framework fused with naphthalene units is reported. The newly designed emitters realize green emission with peaks of 512-521 nm and extremely narrow full-width at half-maxima (FWHMs) of 16-17 nm in toluene solution. Utilizing these emitters in a phosphor-sensitized fluorescence (PSF) system, the resulting OLEDs exhibit pure green emission with peaks in the range of 514-526 nm and very narrow FWHMs of 19-20 nm. Notably, the devices based on the partially deuterated emitter, DBN-NaPh-d, not only achieve a maximum external quantum efficiency (EQEmax) as high as 35.2%, small efficiency roll-off, along with a CIEy of 0.74 but also showcase impressive operational stability with a long lifetime (LT50) of over 3000 h at an initial luminance of 1000 cd m-2. This work represents one of the highest device efficiencies and superior device stability among fluorescence emitter-based OLEDs.

BN-Acene Ladder with Enhanced Charge Transport for Organic Field-Effect Transistors

The in-depth research on the charge transport properties of BN-embedded polycyclic aromatic hydrocarbons (BN-PAHs) still lags far behind studies of their emitting properties. Herein, we report the successfully synthesis of novel ladder-type BN-PAHs (BCNL1 and BCNL2) featuring a highly ordered BC3N2 acene unit, achieved via a nitrogen-directed tandem C-H borylation. Single-crystal X-ray diffraction analysis unambiguously revealed their unique and compact herringbone packing structures. Micro-sized single-crystalline organic field-effect transistors (OFETs) demonstrated that an enhanced charge transport capability, with BCNL2 achieving a hole mobility of up to 0.62 cm2 V-1 s-1-three orders of magnitude higher than that of BCNL1 (μh max=6 × 10-4 cm2 V-1 s-1), ranking among the highest values for BN-PAHs-based OFETs. Detailed calculations attribute this significant enhancement in the hole mobility to the marked reduction in reorganization energy (λ) of BCNL2, resulting from the five-membered pyrrole ring annulation and molecular skeleton elongation. This work provides insight into molecular design principles for potential BN-PAHs in optoelectronic applications.

Syntheses of multi-resonance frameworks towards narrowband organic light-emitting diodes

Multi-resonance (MR)-type organic emitters are highly attractive in the field of organic light-emitting diodes because of their narrowband emission and thermally activated delayed fluorescence (TADF) properties. Compared with conventional TADF emitters, MR-featured emitters have more complex chemical structures and building logics. The core structures of MR emitters are MR frameworks, i.e., polyaromatic frameworks, precisely embedded with electron-donating and electron-withdrawing atoms/groups. Generally, electron-donating units can be easily introduced through the dedicated design of the precursors/intermediates, while integrating electron-withdrawing units is the key point and bottleneck for the synthesis of the MR framework. In this review, we briefly summarize the synthetic strategies of MR frameworks, focusing on the means of introducing various electron-withdrawing atoms/groups, which will aid the further exploration of MR-TADF emitters and their applications in OLEDs.

Multiple Enol-Keto Isomerization and Excited-State Unidirectional Intramolecular Proton Transfer Generate Intense, Narrowband Red OLEDs

A novel series of excited-state intramolecular proton transfer (ESIPT) emitters, namely, DPNA, DPNA-F, and DPNA-tBu, endowed with dual intramolecular hydrogen bonds, were designed and synthesized. In the condensed phase, DPNAs exhibit unmatched absorption and emission spectral features, where the minor 0-0 absorption peak becomes a major one in the emission. Detailed spectroscopic and dynamic approaches conclude fast ground-state equilibrium among enol-enol (EE), enol-keto (EK), and keto-keto (KK) isomers. The equilibrium ratio can be fine-tuned by varying the substitutions in DPNAs. Independent of isomers and excitation wavelength, ultrafast ESIPT takes place for all DPNAs, giving solely KK tautomer emission maximized at >650 nm. The spectral temporal evolution of ESIPT was resolved by a state-of-the-art technique, namely, the transient grating photoluminescence (TGPL), where the rate of EK* → KK* is measured to be (157 fs)-1 for DPNA-tBu, while a stepwise process is resolved for EE* → EK* → KK*, with a rate of EE* → EK* of (72 fs)-1. For all DPNAs, the KK tautomer emission shows a narrowband emission with high photoluminescence quantum yields (PLQY, ∼62% for DPNA in toluene) in the red, offering advantages to fabricate deep-red organic light-emitting diodes (OLED). The resulting OLEDs give high external quantum efficiency with a spectral full width at half-maximum (FWHM) as narrow as ∼40 nm centered at 666-670 nm for DPNAs, fully satisfying the BT. 2020 standard. The unique ESIPT properties and highly intense tautomer emission with a small fwhm thus establish a benchmark for reaching red narrowband organic electroluminescence.

Nonplanar structure accelerates reverse intersystem crossing of TADF emitters: nearly 40% EQE and relieved efficiency roll off

Exploring strategies to enhance reverse intersystem crossing (RISC) is of great significance to develop efficient thermally activated delayed fluorescent (TADF) molecules. In this study, we investigate the substantial impact of nonplanar structure on improving the rate of RISC (k RISC). Three emitters based on spiroacridine donors are developed to evaluate this hypothesis. All molecules exhibit high photoluminescent quantum yields (PLQYs) of 96-98% due to their rigid donor and acceptor. Leveraging the synergistic effects of heavy element effect and nonplanar geometry, S2-TRZ exhibits an accelerated k RISC of 24.2 × 105 s-1 compared to the 11.1 × 105 s-1 of S1-TRZ, which solely incorporates heavy atoms. Additionally, O1-TRZ possesses a further lower k RISC of 9.42 × 105 s-1 because of the absence of these effects. Remarkably, owing to the high PLQYs and suitable TADF behaviors, devices based on these emitters exhibit state-of-the-art performance, including a maximum external quantum efficiency of up to 40.1% and maximum current efficiency of 124.7 cd A-1. More importantly, devices utilizing S2-TRZ as an emitter achieve a relieved efficiency roll-off of only 7% under 1000 cd m-2, in contrast to the 12% for O1-TRZ and 11% for S1-TRZ, respectively. These findings advance our fundamental understanding of TADF processes for high-performance electroluminescent devices.

Expanding Multiple-Resonance Structure of a Double-Borylated Skeleton by Fusing with Indolocarbazole a Multiple-Resonance Donor for Narrowband Green Emission

Double-borylated multiple-resonance (MR) skeletons are promising templates for high performance, while the chemical design space is relatively limited. Peripheral segments are often used to decorate/fuse MR skeletons and modulate the photophysics but they can also cause unwanted spectral broadening. Herein, a narrowband MR emitter ICzDBA by fusing an MR-featured donor segment indolocarbazole into a double-borylated MR skeleton is developed. In ICzDBA, the nitrogen atom located away from the core benzene ring can also contribute to the generation of the overall MR-featured distribution through the long-range conjugation effect, along with the other boron/nitrogen atoms on the phenyl center. Thus, ICzDBA in toluene displays a narrowband emission peaking at 507 nm with a full width at half maximum of merely 20 nm (0.09 eV). Moreover, organic light-emitting diode devices using ICzDBA emitter exhibit ultrapure green emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.27, 0.70) and a high external quantum efficiency of 32.5%. These results manifest the importance of MR characters of peripheral decorations/fusions in preserving the narrowband features of MR skeletons, which provides a solution for further expanding MR structures with well-maintained narrowband characters.

Carbonyl- and Nitrogen-Embedded Multi-Resonance Emitter with Ultra-Pure Green Emission and High Electroluminescence Efficiencies

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrow emission spectra have garnered significant attention in future organic light-emitting diode (OLED) displays. However, current C=O/N-embedded MR-TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N-embedded green MR-TADF emitter, featuring two acridone units incorporated in a sterically protected 11-ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation-induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N-based MR-TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l'Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N-based MR-TADF emitters and holds profound implications for the design and synthesis of other MR-TADF systems.