Strategic Synchronization of 7,7-Dimethyl-5,7-dihydroindeno[2,1-b]carbazole for Narrow-Band, Pure Violet Organic Light-Emitting Diodes with an Efficiency of > 5% and a CIE y Coordinate of < 0.03

Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

Exploring Efficient Dual-Phase Emissive Fluorophores with High Mobility by Integrating a Rigid Donor and Flexible Acceptor

Developing luminogens with a high emission efficiency in both single-molecule and aggregate states, as well as high mobility, shows promise for advancing the iteration and update of organic optoelectronic materials. However, achieving a delicate balance between the plane configuration of luminophores and the strong exciton interactions of aggregates is a formidable task from the molecular design perspective. This dilemma was overcome by integrating a rigid donor and flexible acceptor to establish donor-acceptor (D-A) type emitters. The π-conjugate-extended donor ensures the substantial planarity of these molecules, allowing strong emission in solution with photoluminescence quantum yield values of 86% and 75%. Furthermore, the restricted molecular motion of the aggregation-induced emission moiety and the formation of J-aggregates reduce the quenching effect, leading to a high emissive efficiency of 85% and 91% in the aggregate state. The mildly distorted D-A geometry builds moderate electrostatic interaction, resulting in high mobility with μM,h of 7.12 × 10-5 and 3.27 × 10-4 cm2/V s. Additionally, an improved synthesized procedure for terminal E-configured acrylonitrile with metal-free and concise reaction conditions is presented. The successful application of the synthesized compounds in organic light-emitting diode devices demonstrates the practicability of the molecular design strategy with connecting a rigid donor and flexible acceptor.

Novel π-Extended Indolocarbazole-Based Deep-Blue Fluorescent Emitter with Remarkably Narrow Bandwidth for Solution-Processed Organic Light-Emitting Diodes

In solution-processed organic light-emitting diodes (OLEDs), achieving high color purity and efficiency is as important as that in vacuum processes. Emitters suitable for solution processing must have excellent solubility in organic solvents, high molecular weight, and compatibility with the host materials. In this study, we synthesized a deep-blue emitter that satisfies the above conditions by introducing a 1,4-bis(indolo[3,2,1-jk]carbazol-2-yl)benzene-based planar emitting core (DICz) structure and four 3,6-di-tert-butyl-9-phenyl-9H-carbazole (tCz) peripheral units, namely, 4tCz-DICz. A comparative compound, 4Hex-DICz, incorporating hexyl phenyl groups was synthesized. In contrast to 4Hex-DICz, 4tCz-DICz exhibited exceptional solubility in organic solvents and superior film-forming properties attributed to the presence of tCz units. Additionally, in the film state, the effective encapsulation of the emitting core (DICz) by the tCz units in 4tCz-DICz helps prevent undesirable molecular aggregation. The solution-processed OLEDs employing the CH-2D1 film, doped with 5 wt % 4tCz-DICz as the emitting layer, exhibited a deep-blue emission at 424 nm, characterized by a narrow bandwidth of 22 nm, and achieved a maximum external quantum efficiency (EQE) of approximately 4.0%. In contrast, the 4Hex-DICz-based device demonstrated an EQE of 2.91%. Consequently, we have successfully demonstrated that the introduction of four bulky tCz units into the DICz core is a promising molecular design strategy for the development of soluble indolocarbazole-based emitters, especially those used in high-performance deep-blue fluorescent OLEDs.

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

Organic light-emitting diodes (OLEDs) using conventional fluorescent emitters are currently attracting considerable interests due to outstanding stability and abundant raw materials. To construct high-performance narrowband fluorophores to satisfy requirements of ultra-high-definition displays, a strategy fusing multi-resonance BN-doped moieties to naphthalene is proposed to construct two novel narrowband fluorophores. Green Na-sBN and red Na-dBN, manifest narrow full-width at half-maxima of 31 nm, near-unity photoluminescence quantum yields and molecular horizontal dipole ratios above 90 %. Their OLEDs exhibit the state-of-the-art performances including high external quantum efficiencies (EQE), ultra-low efficiency roll-off and long operational lifetimes. The Na-sBN-based device achieves EQE as high as 28.8 % and remains 19.8 % even at luminance of 100,000 cd m-2 , and Na-dBN-based device acquires a record-high EQE of 25.2 % among all red OLEDs using pure fluorescent emitters.

Resolving the Photophysics of Nitrogen-Embedded Multiple Resonance Emitters: Origin of Color Purity and Emitting Efficiency

The emerging nitrogen-embedded multiple resonance (MR) emitters with an indolo[3,2,1-jk] carbazole (ICz) unit have exhibited promising performance for high-resolution organic light-emitting diode (OLED) devices, while the underlying photophysics has been rarely reported. In this work, the optical spectra, color purity, and emitting efficiency of ICz-based MR emitters were investigated by using electronic structure and thermal vibration correlation function (TVCF) calculations. Unlike B-N MR emitters, the high color purity of investigated ICz-based MR emitters was mainly contributed by considerable structural rigidity, which also greatly affects the radiative decay rate and fluorescence quantum yield of the S1 state. For the majority of investigated emitters, potential reverse intersystem crossing (RISC) channels (T1 → S1 and T2 → S1) are limited by thermally inaccessible ΔEST* or insufficient spin-orbital coupling (SOC), which can be distinguished by the calculated temperature-dependent RISC rate pattern. We provided a systematic photophysical picture for ICz-based MR emitters that might be interesting for the OLED design and application community.

The Novel Organic Emitters for High-Performance Narrow-Band Deep Blue OLEDs

Narrow-band deep-blue organic light-emitting diodes (OLEDs) have played a key role in the field of high-quality full-color displays. However, because of the considerable challenges of inherent band gaps, unbalanced carrier injection and the lack of molecular structures, narrow-band deep-blue emitters develop slowly compared with red- and green-emitting materials. Encouragingly, with the continuous efforts of scientists in recent years, great progress has been made in the molecule design and material synthesis of highly efficient narrow-band deep-blue emitters. The typical deep-blue emitters which exhibit narrow emission with a full width at half maximum of < 50 nm are summarized in this article. They are divided into the three categories: fluorescence, phosphorescence and thermally activated delayed fluorescence. The methods of molecular design for realizing narrow-band deep-blue emission are described in detail and future research directions are also discussed in this article.

Facile Functionalization of Ambipolar, Nitrogen-Doped PAHs toward Highly Efficient TADF OLED Emitters

Despite promising optoelectronic features of N-doped polycyclic aromatic hydrocarbons (PAHs), their use as functional materials remains underdeveloped due to their limited post-functionalization. Facing this challenge, a novel design of N-doped PAHs with D-A-D electronic structure for thermally activated delayed fluorescence (TADF) emitters was performed. Implementing a set of auxiliary donors at the meta position of the protruding phenyl ring of quinoxaline triggers an increase in the charge-transfer property simultaneously decreasing the delayed fluorescence lifetime. This, in turn, contributes to a narrow (0.04-0.28 eV) singlet-triplet exchange energy split (ΔEST) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m2 which is the highest reported to date for N-doped PAHs investigated in the OLED domain.

Multi-Resonant Indolo[3,2,1-jk]carbazole-Based Host for Blue Phosphorescent Organic Light-Emitting Diodes

In organic light-emitting diodes, positive and negative charge carriers mostly migrate at different rates. This could result in excitons formed in the EML often migrating to the vicinity of the hole transport layer and the electron transport layer. To address this, it is important to design high-quality multi-resonance hosts that can balance the migration rate of carriers. Here, we report two newly developed multi-resonance hosts, m-ICzPBI and o-ICzPBI. The hosts contain an indolo[3,2,1-jk]carbazole (ICz) motif, which functionalized as either a donor or an acceptor unit. The hosts exhibit extremely high molecular rigidity and thermal stability. Devices A and B were constructed using FIrpic as a phosphorescent emitter with m-ICzPBI or o-ICzPBI as a host. Device A achieved high maximum values of EQE, PE and CE of 13.4%, 24.8 lm W-1 and 31.6 cd A-1, respectively, and low efficiency roll-off at 5000 cd m-2 of 8.6%, 10.6 lm W-1 and 20.3 cd A-1, respectively.

Toward Narrowband and Efficient Blue Fluophosphors by Locking the Stretching Vibration of Indolocarbazole Skeletons

Developing efficient and color-saturated deep-blue emitting molecules with small Commission Internationale de L'Eclairage (CIE) y values is challenging and has great potential for wide-color gamut displays. Herein, we introduce an intramolecular locking strategy to restrain molecular stretching vibrations of the emission spectral broadening. By cyclizing rigid fluorenes and connecting electron-donating groups to the indolo[3,2,1-jk]-indolo[1',2',3':1,7]indolo[2,3-b]carbazole (DIDCz) framework, the in-plane swing of peripheral bonds and stretching vibrations of the indolocarbazole skeleton are restricted due to an increased steric hindrance from cyclized groups and diphenylamine auxochromophores. As a result, reorganization energies at the high-frequency region (1300-1800 cm-1) are reduced, realizing pure blue emission with a small full-width-at-half-maximum (FWHM) of 30 nm by suppressing shoulder peaks of polycyclic aromatic hydrocarbon (PAH) frameworks. The fabricated bottom-emitting organic light-emitting diode (OLED) exhibits an efficient external quantum efficiency (EQE) of 7.34% and deep-blue coordinates of (0.140, 0.105) at a high brightness of 1000 cd/m2. The FWHM of the electroluminescent spectrum is only 32 nm, which is one of the narrowest electroluminescent emissions among the reported intramolecular charge transfer fluophosphors. Our current findings provide a new molecular design strategy to conceive efficient and narrowband emitters with small reorganization energies.

A Deep-Blue-Emitting Heteroatom-Doped MR-TADF Nonacene for High-Performance Organic Light-Emitting Diodes

We present a p- and n-doped nonacene compound, NOBNacene, that represents a rare example of a linearly extended ladder-type multiresonant thermally activated delayed fluorescence (MR-TADF) emitter. This compound shows efficient narrow deep blue emission, with a λ_PL of 410 nm, full width at half maximum, FWHM, of 38 nm, photoluminescence quantum yield, Φ_PL of 71 %, and a delayed lifetime, τ_d of 1.18 ms in 1.5 wt % TSPO1 thin film. The organic light-emitting diode (OLED) using this compound as the emitter shows a comparable electroluminescence spectrum peaked at 409 nm (FWHM=37 nm) and a maximum external quantum efficiency (EQE_max ) of 8.5 % at Commission Internationale de l'Éclairage (CIE) coordinates of (0.173, 0.055). The EQE_max values were increased to 11.2 % at 3 wt % doping of the emitter within the emissive layer of the device. At this concentration, the electroluminescence spectrum broadened slightly, leading to CIE coordinates of (0.176, 0.068).

Indolo[3,2,1-jk]carbazole-Derived Narrowband Violet-Blue Fluorophores: Tuning the Optical and Electroluminescence Properties by Chromophore Juggling

The development of rigid polyaromatic building blocks for narrowband violet fluorophores has received tremendous attention. Herein, we designed and synthesized two new triangle-shaped rigid building blocks, namely, 2,5-di-tert-butylindolo[3,2,1-jk]carbazole (tBuICz) and 2,11-di-tert-butylindolo[3,2,1-jk]carbazole-4-carbonitrile (tBuICzCN), and tethered them with different chromophores to yield a series of violet-blue fluorophores, viz., ICzTPA-ICzPICN, and studied their structure-function relationship. The appended chromophores and cyano unit played a vital role in controlling the optical and electrical properties of the compounds. Except triphenylamine-substituted derivatives, the compounds showed pure violet emission (λ_em ≤ 403 nm). Intriguingly, the compounds exhibited narrow-band emission with a full-width at half-maximum ≤ 40 nm, attributed to the rigidity of the ICz core. The emission of the compounds displayed positive solvatochromism, which is ascribed to the photoinduced intramolecular charge transfer in the excited state. The compounds revealed excellent thermal robustness with T_5d ≥ 363 °C. The triphenylamine-featuring derivatives displayed a high-lying HOMO compared to their congeners due to their electron-rich nature. When we applied these materials in organic light-emitting diodes, ICzPI outperformed in the series with an EQE_max of 3.07% and a current efficiency of 1.04 cd/A. Notably, its CIEy ∼ 0.046 precisely matched with the Rec.2020 standard of deep-blue color (CIEy ∼ 0.046).

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.

Diindolocarbazole - achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units

In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes4 possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔE_ST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔE_ST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes4 as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered.

Robust Luminescent Molecules with High-Level Reverse Intersystem Crossing for Efficient Near Ultraviolet Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) radiating near ultraviolet (NUV) light are of high importance but rarely reported due to the lack of robust organic short-wavelength emitters. Here, we report a short π-conjugated molecule (POPCN-2CP) with high thermal and morphological stabilities and strong NUV photoluminescence. Its neat film exhibits an electroluminescence (EL) peak at 404 nm with a maximum external quantum efficiency (η_ext,max ) of 7.5 % and small efficiency roll-off. The doped films of POPCN-2CP in both non-polar and polar hosts at a wide doping concentration range (10-80 wt%) achieve high-purity NUV light (388-404 nm) and excellent η_ext,max s of up to 8.2 %. The high-level reverse intersystem crossing improves exciton utilization and accounts for the superb η_ext,max s. POPCN-2CP can also serve as an efficient host for blue fluorescence, thermally activated delayed fluorescence and phosphorescence emitters, providing excellent EL performance via Förster energy transfer.