Rong-Huei Yi

This study proposes a discrete supramolecular dimerization strategy for achieving efficient chiral multi-resonance thermally activated delayed fluorescence emitters featuring aggregation-induced circularly polarized luminescence enhancement. The resulting electroluminescence devices achieve narrowband emission with full width at half maximum down to 26 nm and a record-breaking figure of merit (|g _EL| × EQE) of 4.12 × 10⁻³.

Abstract

Achieving narrowband emission, high efficiency, and circularly polarized luminescence (CPL) in organic light-emitting diodes (OLEDs) remains a significant challenge. In this study, a discrete supramolecular dimerization strategy is presented to overcome this limitation. By incorporating a helical arylamine with a sterically demanding configuration into a multi-resonance narrowband emitter, the formation of a unique dimeric structure in the solid state is enabled. Unlike conventional multi-resonance emitters prone to aggregation-caused quenching and continuous stacking, the CPL emitters form discrete, well-separated dimers. This distinct supramolecular arrangement not only preserves high photoluminescence quantum yield and narrowband emission but also amplifies CPL signals by optimizing intermolecular electronic coupling. OLEDs incorporating these enantiomers at a 10 wt.% doping level exhibit outstanding performances, including a narrow full-width at half-maximum of 30 nm, maximum external quantum efficiencies (EQE) of 33.5% and 32.4%, and impressive electroluminescence dissymmetry factors (g _EL) of +8.7 × 10⁻³ and −9.1 × 10⁻³, respectively. Remarkably, increasing the doping concentration to 20 wt.% further boosts the g _EL values to +1.6 × 10⁻² and −1.8 × 10⁻². This enhancement leads to Figures of Merit (EQE × |g _EL|) of 3.71 × 10⁻³ and 4.12 × 10⁻³, among the highest values for CPL devices.

A new class of luminescent organic quantum materials is introduced, formed by attaching a stable radical to TADF chromophores. Due to ferromagnetic exchange coupling, a high-spin quartet state forms upon excitation, which can thermally access a bright state. ≈200 nm tunability of the emission color is reported, and coherent spin manipulations with microwaves are achieved.

Abstract

High-spin states in organic molecules offer promising tuneability for quantum technologies. Photogenerated quartet excitons are an extensively studied platform, but their applications are limited by the absence of optical read-out via luminescence. Here, a new class of synthetically accessible molecules with quartet-derived luminescence is demonstrated, formed by appending a non-luminescent TEMPO radical to thermally activated delayed fluorescence (TADF) chromophores previously used in OLEDs. The low singlet-triplet energy gap of the chromophore opens a luminescence channel from radical-triplet coupled states. A set of design rules is established by tuning the energetics in a series of compounds based on a naphthalimide (NAI) core. Generation of quartet states is observed and the strength of radical-triplet exchange is measured. In DMAC-TEMPO, up to 72% of detected photons emerge after reverse intersystem crossing from the quartet state repopulates the state with singlet character. This design strategy does not rely on a luminescent radical to provide an emission pathway from the high-spin state. The large library of TADF chromophores promises a greater pallet of achievable emission colours.

Dibenzooxasiline-derived mOSiTrz and mOSiCzTrz are developed as high-triplet-energy n-type hosts for blue phosphorescent organic light-emitting diodes. They are used as n-type hosts in a mixed host to assist positive-polaron dispersion in the p–n mixed host to achieve high efficiency and long device lifetime.

Abstract

Dibenzooxasiline-derived compounds, 2,4-bis(10,10-diphenyl-10H-dibenzo[b,e][1,4]oxasilin-2-yl)-6-phenyl-1,3,5-triazine (mOSiTrz) and 9-(4,6-bis(10,10-diphenyl-10H-dibenzo[b,e][1,4]oxasilin-2-yl)-1,3,5-triazin-2-yl)-9H-carbazole (mOSiCzTrz), are developed as high-triplet-energy n-type hosts for blue phosphorescent organic light-emitting diodes (PhOLEDs). They are used as n-type hosts in a mixed host to assist positive-polaron dispersion in the p–n mixed host to achieve high efficiency and long device lifetime. The dibenzooxasiline-based n-type hosts are mixed with a p-type 9-(3-(triphenylsilyl)phenyl)-9H-3,9′-bicarbazole host and performed better than n-type hosts built on a conventional tetraphenylsilane moiety. The external quantum efficiency of the blue PhOLEDs with the mOSiTrz and mOSiCzTrz hosts is over 20%, and their device lifetimes are extended 1.5- and 4-fold, respectively, relative to the well-known 2-phenyl-4,6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazine device due to the enhanced positive-polaron transport and dispersion abilities of the mOSiTrz and mOSiCzTrz hosts assisted by the dibenzooxasiline group.

A pair of novel CPL-active Shibasaki-type Yb(III) enantiomers supported by 3,3′-fluorinated binaphthol (F₂BINOL) was designed and optically investigated, showing enhanced photophysical properties and CPL metrics compared to the nonfluorinated counterparts.

Abstract

Chiral lanthanide complexes hold great potential in developing advanced circularly polarized luminescence (CPL) materials in the near-infrared (NIR) wavelength range. While various ligands, such as β-diketonates and biphenols, have been successfully used to sensitize NIR emission of lanthanide ions, little attention has been paid to regulating CPL behaviors via ligand modification. In this study, we report the synthesis, structure, luminescence, and chiroptical properties of a pair of novel air-stable Shibasaki-type Yb(III) enantiomers supported by 3,3′-fluorinated binaphthol (F₂BINOL). Compared to their nonfluorinated counterparts, these fluorinated complexes exhibit larger dihedral angles between the two naphthyl moieties and more distorted octahedral coordination environments around the Yb(III) ion, leading to increased overall crystal field splitting. These structural modifications result in enhanced photophysical properties: the luminescence lifetime (τ _obs), sensitization efficiency (η _sens), and quantum yield (QY) improve from 2.1 µs, 38%, and 0.7% to 2.7 µs, 63%, and 1.8%, respectively. Additionally, the dissymmetry factor (g _lum) and CPL brightness (B _CPL) at 963 nm increase by 32% and one order of magnitude, respectively. The scope of such chemical modification is broad, potentially encompassing a variety of BINOL derivatives, offering a unique platform to further elucidate the relationship between structural and electronic properties and CPL activity in lanthanide systems.

The TOC showcases the chemical structure of the emitter tCzBT2B as an “MVP athlete” in a stadium surrounded by. spotlights and cheering fans celebrating its impressive performance in organic light-emitting diodes (OLEDs). The stats for this emitter and its device are detailed below.

Abstract

This study explores the impact of the regioisomerism of a heavy chalcogen atom on the photophysical properties of multi-resonant thermally activated delayed fluorescence (MR-TADF) materials. Two pairs of isomeric MR-TADF emitters containing different benzothienocarbazole moieties, tDPABT1B/tDPABT2B and tCzBT1B/tCzBT2B, are synthesized. Theoretical calculations indicate that tDPABT2B and tCzBT2B possess higher spin–orbital coupling values (0.27 and 0.60 cm⁻¹) compared to their respective isomers. The photophysical study reveals that tDPABT2B and tCzBT2B have twofold faster reverse intersystem crossing rate constants of 0.5 × 10⁵ and 2.7 × 10⁵ s⁻¹, respectively, than their isomeric counterparts. The sensitizer-free organic light-emitting diodes (OLEDs) with tCzBT1B and tCzBT2B exhibit green emissions [CIE coordinates of (0.12, 0.54)] and show high maximum external quantum efficiencies (EQE_max) of 34.9 and 34.3%, respectively. Notably, the device with tCzBT2B demonstrates a reduced efficiency roll-off (34% decrease at 1000 cd cm⁻²) compared to that with tCzBT1B (48% decrease at 1000 cd cm⁻²), highlighting the distinct benefits and importance of the regiochemistry of the heavy atom in contributing to an enhancing device performance.

An exceptional EQE_max of 19.8% for MePAB is achieved, with high yield of 87%. The bottom-emitting device achieves ultra-low CIEy values of 0.044 and 0.046 for the emitters 2FPAB and MePAB, respectively. The potential application of MePAB as a deep-blue sensitizer is investigated in a TADF-sensitized-TADF (TST) device.

Abstract

Due to higher exciton energy, the deep-blue organic light-emitting diodes (OLEDs) are the most challenging among trichromatic emitters. In this work, three novel blue emitters with multi-resonance (MR) effects are reported based on N,N,5,9-tetraphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (PAB) skeleton, incorporating fluorine and methyl groups. Compound 2FPAB and MePAB exhibit ultra-pure deep-blue emission peaks at 436 and 448 nm respectively. The device based on MePAB achieves a maximum external quantum efficiency of 19.8% with an ultralow CIEy value of 0.046 and a full-width-at-half-maximum (FWHM) of 29 nm in the bottom emitting device. Additionally, the peripheral cladding of fluorine atoms extends the conjugation framework and slightly enhances the electron acceptance character of the boron atom at the ortho-position. This leads to a redshift in the emission spectrum and enhanced photoluminescence quantum yield (PLQY) of up to 93% for MePABF. Furthermore, the potential application of MePAB as a deep-blue sensitizer for MePABF is examined by fabricating a TADF-sensitized-TADF (TST) device. The results show a significantly reduced efficiency roll-off and improved device performance, with a maximum external quantum efficiency (EQE_max) of 25.1% and a FWHM of 29 nm.

Phosphorescent OLEDs suffer from efficiency roll-off due to triplet-polaron quenching (TPQ). This study demonstrates for a large set of host-guest combinations a spectroelectrochemical method to measure the absorption of charged molecules, enabling determining TPQ Förster radii (2.5–4 nm) from the spectral overlap. The methodology helps optimize material combinations and validate quantum-chemical models for quantifying TPQ.

Abstract

The quantum efficiency of phosphorescent organic light-emitting diodes (OLEDs) shows a decrease with increasing current density and luminance (“roll-off”). A major contribution to the roll-off is triplet-polaron quenching (TPQ), upon which a triplet exciton on an emitter molecule (“donor”) is lost after exciting an electron or hole polaron on another molecule (“acceptor”), followed by non-radiative decay. The microscopic mechanism is not well understood. Within a Förster-type dipole-dipole interaction model, the TPQ rate is determined by the overlap of the donor photoluminescence spectrum and the absorption spectrum of the positively or negatively charged acceptor molecules. In this work, a spectroelectrochemical method is demonstrated for measuring the absorption spectra of charged molecules, so that the interaction rate, expressed in terms of the TPQ Förster radius, can be determined. Typical values of these radii are found to be in the range of 2.5−4 nm. For often used OLED designs, in which emitter molecules (“guests”) are embedded at a small concentration in a matrix material (“host”), this study enables obtaining in a rational manner optimal host-guest combinations. The methodology is also expected to provide an experimental benchmark for emerging open-shell quantum-chemical methods for quantifying TPQ.

This work reports the design of four novel fluorophores by incorporating an aggregation-induced emission (AIE) active moiety tetraphenylethylene (TPE) and a phenyl spacer into a hybridized local and charge-transfer (HLCT)-type phenanthroimidazole (PI) core. Through precise tuning of electron-donating and withdrawing groups, these fluorophores demonstrate improved non-doped organic light-emitting diode (OLED) performance.

Abstract

Organic light-emitting diodes (OLEDs) employing hybridized local and charge-transfer (HLCT) state emitters have exhibited commendable external quantum efficiencies (EQE). However, these emitters typically suffer from reduced photoluminescence quantum yields (PLQYs) in their aggregated states, which diminishes device efficiency. In this work, by introducing an aggregation-induced emission (AIE) active moiety tetraphenylethene (TPE) and a phenyl spacer to the HLCT-typed core, phenanthroimidazole (PI) unit, four fluorophores with different substituents at the N1-position (PI-Ph-p-CH₃-TPE, PI-Ph-TB-TPE, PI-Ph-m-CF₃-TPE, and PI-Ph-m-CN-TPE) are obtained. The high PLQYs of these HLCT-AIEgens in aggregates are ensured by their AIE properties. The effects of a phenyl spacer between PI and TPE and different substituents at the N1-position of PI on the regulation of the locally excited and charge transfer components are also revealed. Theoretical calculations indicate that their exciton conversion channels can be promoted by tuning the excited state, validated by enhancement of their EQE. The non-doped OLED using PI-Ph-m-CF₃-TPE, as an emitter, exhibit the best electroluminescence performance with an excellent EQE approaching 15%, maximum luminance of 4584 cd m⁻², current efficiency of 15.75 cd A⁻¹, and power efficiency of 13.02 lm W⁻¹. Strategically designing HLCT-AIE materials and tuning excited states, high-efficiency OLED emitters can be developed.

The newly developed 1,3,4,6,9b-pentaazaphenalene (5AP) based emitters with donor and acceptor units have succeeded in converting nonemissive 5AP to highly luminescent emitters by breaking the forbidden transition. These emitters exhibited photoluminescence quantum yields exceeding 80% and 90%; both are the highest among the 5AP derivatives reported to date.

Abstract

1,3,4,6,9b-pentaazaphenalene (5AP) derivatives are of growing interest because of their potential for exhibiting thermally activated delayed fluorescence and inverted singlet–triplet excited state properties. However, a major challenge has been the nonemissive nature of 5AP. This study reports a donor-5AP-acceptor-type molecular design for converting nonemissive 5AP into highly emissive molecules. The newly designed molecules, 2,5-di(1-pyrrolidino)-7,9-bis(4-(trifluoromethyl)phenyl)-1,3,4,6,9b-pentaazaphenalene (Pyr-5AP-CF₃ ) and 2,5-di(1-pyrrolidino)-7,9-bis(4-benzonitrile)-1,3,4,6,9b-pentaazaphenalene (Pyr-5AP-CN), exhibited delayed fluorescence and achieved high photoluminescence quantum yields of 83.5% and 90.6%, respectively, in solid films. These values dramatically exceed those of previously reported 5AP derivatives with only 8% or less. Furthermore, Pyr-5AP-CF₃ and Pyr-5AP-CN exhibited the fastest radiative decays and the narrowest emission spectra among all the 5AP based materials reported to date. This study provides a promising solution to the nonemissive nature of 5AP, leading to the development of a class of highly luminescent materials for future organic light-emitting diodes.

Herein, through regiospecific bromination on a helical 7H-dibenzo[c,g]carbazole-based SAM (CbzNaph) featuring a stronger dipole, we study the properties related to intrinsic stability, electrostatic potential (ESP) distribution, and changes in the molecular dipole of the derived SAM molecules. Bromination at the chemically inert sites of 7H-dibenzo[c,g]carbazole (JJ26) helps maximize molecular dipole while maintaining superior intrinsic stability. Together with the dense assembly promoted by enhanced intermolecular interactions and synergistic effects of stronger crystallinity, JJ26 efficiently modulates the work function (WF) of indium tin oxide (ITO) and enhances the stability of SAM under external pressure. The OSC device adopting JJ26 demonstrates significantly improved performance, achieving an efficiency of 19.35% along with notably enhanced stability.

Abstract

Halogenated carbazole-derived self-assembled monolayers (SAMs) are promising hole-extraction materials in conventional organic solar cells (OSCs). While halogenation helps optimize the molecular dipole, intermolecular interactions, and energetics of SAM, the highly polarizable carbon-halogen bonds can be reactive and prone to photocleavage depending on their regiochemistry. Herein, we study the regiospecific properties, including the intrinsic stability, electrostatic potential (ESP) distribution, and changes in molecular dipole of the brominated SAM molecules by brominating a helical 7H-dibenzo[c,g]carbazole-based SAM (CbzNaph) featuring a stronger dipole. Additionally, a correlation between the intrinsic molecular stability and the derived SAM surface stability is established to determine the performance and stability of the OSCs. Notably, the bromination at the chemically inert sites of 7H-dibenzo[c,g]carbazole (JJ26) helps maximize molecular dipole while maintaining superior intrinsic stability. Together with dense assembly promoted by the synergistically enhanced intermolecular interactions and crystallinity, JJ26 can efficiently modulate the work function (WF) of indium tin oxide (ITO) and enhance the stability of SAM under external stress. Consequently, the JJ26 derived OSC shows significantly improved performance, achieving an efficiency of 19.35% along with notably enhanced stability. This work shows that the precise modulation of the regiochemistry of SAM molecules is critical for improving their quality and derived device performance.

A general donor fusion strategy is proposed to extend frontier molecular orbital delocalization to construct long-wavelength narrowband emitters. Two proof-of-concept molecules, BN-PhAzCz and BN-tCzAzPh, are constructed. BN-PhAzCz achieves the best device performances, exhibiting green emission at 528 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.69), maximum external quantum efficiency (EQE) of 38.2%, and efficiency roll-off of 28.0% at 1000 cd m⁻².

Abstract

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising candidates for ultra-high-definition organic light-emitting diodes (OLEDs) displays. Here, we propose a general strategy for post-functionalizing MR core, introducing donors on the lowest unoccupied molecular orbital (LUMO) position via Suzuki coupling and then fusing the donors to the highest occupied molecular orbital (HOMO) position via Scholl cyclization, which can extend frontier molecular orbitals delocalization to construct long-wavelength narrowband emitters. Two proof-of-concept molecules, BN-PhAzCz and BN-tCzAzPh, are constructed, achieving large spectral red-shifts while maintaining narrowband emission. The distorted configuration resulting from the hetero[6]helicene structure introduces twisted π–π* transition, which effectively enhances spinorbit coupling, ultimately enabling molecule BN-PhAzCz to achieve a fast reverse intersystem crossing rate (kRISC = 6.4 × 104 s⁻¹). BN-PhAzCz-based single-host OLED exhibits green emission at 528 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.69) and maximum external quantum efficiency (EQE) of 38.2% with efficiency roll-off of 28.0% at 1000 cd m⁻² under 5 wt% doping concentration. Even at 20 wt% doping concentration, the maximum EQE remains at 29.6% with little change in the spectral profile.

A dibenzoselenophene-based multiple resonance thermally activated delayed fluorescence emitter, DBSeBN, combines narrow-band emission and rapid reverse intersystem crossing. In addition to functioning as an emitter, it can serve as a sensitizer in sensitized organic light-emitting diodes or as a blue emitter in white organic light-emitting diodes, consistently delivering high efficiency and suppressed efficiency roll-off.

Abstract

Selenium-containing multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with ultra-fast reverse intersystem crossing (RISC) have emerged as a promising solution for mitigating efficiency roll-off in organic light-emitting diodes (OLEDs). In this work, we introduce DBSeBN, the first MR-TADF emitter incorporating a rigid five-membered dibenzoselenophene unit. This design simultaneously achieves a narrow full width at half maximum of 23 nm across a wide range of doping concentrations in films, along with an ultra-fast RISC rate of 1.1 × 10⁷ s⁻¹, which is two orders higher than that of its sulfur-containing counterpart, DBTBN, due to the enhanced spin-orbit coupling via the heavy atom effect of selenium. As an OLED emitter, DBSeBN demonstrates exceptional performance, achieving a maximum external quantum efficiency of 31.6% and retaining 23.3% at 1000 cd m⁻², surpassing DBTBN in suppressing efficiency roll-off. Its remarkably fast RISC and insensitivity to doping concentration enable unprecedented versatility in advanced OLED architectures. As a sensitizer in sensitized green-fluorescent OLEDs, it surpasses Ir-based complex sensitizers in reducing efficiency roll-off. As a blue emitter in bi-color white OLEDs, it effectively harnesses high-energy triplet excitons to minimize efficiency roll-off. DBSeBN thus expands the scope of MR-TADF materials across various kinds of OLED applications while suppressing efficiency roll-off.

Spatially folded A–D|D–A type TSCT-TADF emitters (DCT-1 and DCT-2) with dual-donor fusion design have been developed, which achieve the accelerated RISC rates (up to 1.05 × 10⁶ s⁻¹) through dense excited-state engineering, enabling solution-processed organic light-emitting diodes with high EQEs (23.9%) and low efficiency roll-off.

Abstract

The development of through-space charge transfer (TSCT)-thermally activated delayed fluorescence (TADF) material is defective in relatively low reverse intersystem crossing (RISC) rates (commonly <5 × 10⁵ s⁻¹). Herein, we fuse two 3,6-dimethyl-8H-indolo[3,2,1-de]acridine (IAc) donor units to obtain large planar donors (m-bIAc and p-bIAc) for forming spatially folded A–D|D–A configured TSCT emitters (DCT-1 and DCT-2). The configuration of highly parallel and large-plane intramolecular multiple π-stacking has been achieved. The symmetrical multi-channel charge transfer networks of emitters induce multiple energetically proximal excited states within a small energy range (<0.12 eV) at the lowest excited state, creating additional configuration interaction and spin-orbit coupling channels to accelerate the RISC process. This molecular configuration yields enhanced RISC rates of 6.19 × 10⁵ s⁻¹ for DCT-1 and 1.05 × 10⁶ s⁻¹ for DCT-2. Solution-processed organic light-emitting diodes employing these emitters achieve maximum external quantum efficiencies of 18.9% (DCT-1, 474 nm sky-blue emission) and 23.9% (DCT-2, 498 nm green emission), with attenuated efficiency roll-offs of DCT-2 (12% at 1000 cd m⁻²). This work provides a critical pathway for manipulating dense excited states to address the bottleneck of the RISC rates while maintaining structural rigidity, promoting further advancement of TSCT-TADF materials.

An electron-transporting material with sub-second triplet excited state lifetime but low energy level, can be utilized as assistant host to recycle excitons in the emitting layer by doping at extremely low concentrations. This strategy facilitates the recovery and management of high-energy triplet excitons and plays a role in achieving carrier balance, thereby enhancing the overall performance of the OLEDs.

Abstract

In organic light-emitting diodes (OLEDs), the confinement of triplet excitons is essential for achieving efficient and stable devices. Recently, an electron-transporting material (ETM) with sub-second triplet lifetime is reported that can effectively achieve triplet exciton confinement, even with a lower triplet energy (E_T) of 0.32 eV than that of the phosphorescent emitter, which is named the long lifetime triplet exciton reservoir (LTER) effect. Due to the challenge that confining triplet excitons in the emitting layer (EML) typically requires host materials with higher energy level, which leads to accelerated degradation, the possibility of LTER effect in the EML is further explored. The results show that directly using LTER molecule as the host only leads to severe quenching. However, when doped at low-concentration (e.g., 1 wt.%) as assistant host in the carrier recombination zone (RZ), device performance is improved unexpectedly by the LTER effect. Besides, the RZ of carriers is shifted and expanded within the EML, contributing to improved carrier balance due to its intrinsic electron transport properties. As a result, an increase in device external quantum efficiency (EQE) to 24.5% is achieved, along with a 1.5-fold increase in device lifetime.

We report the first indolocarbazole-embedded dual-boron-containing multiple resonance thermally activated delayed fluorescence (MR-TDAF) emitters with emission peaks below 500 nm which employ meta-N–π–N-type indolecarbazole as the core framework. These emitters exhibit remarkably ultra-narrow full-widths at half-maximum (FWHM) of 18 nm in toluene solution. Organic light-emitting diodes (OLEDS) demonstrate excellent electroluminescent performance, with maximum external quantum efficiency (EQE) of 40.0% and FWHM of 21 nm.

Abstract

The demand for ultra-high-definition display technology has spurred the prosperity of multiple resonance induced thermally activated delayed fluorescence (MR-TADF) materials with narrow full-width at half-maximum (FWHM) and high efficiency, making them highly promising candidates for high-color-purity organic light-emitting diodes (OLEDs) displays. Indolocarbazole, a highly rigid aza-polycyclic aromatic hydrocarbon framework, has shown significant potential as a building block for constructing MR-TADF emitters with ultra-narrowband emission (<20 nm). However, it remains a great challenge to construct ultra-narrowband indolocarbazole-embedded MR-TADF emitters with emission maxima less than 500 nm. Here, two MR-TADF emitters, DBN-amICz and DBN-bmICz, are constructed by adopting meta-N–π–N-type indolocarbazole as core framework and achieve ultra-narrowband blue–green emission in toluene solution with peaks of both 490 nm and FWHMs of 18 and 19 nm, respectively. OLEDs incorporating emitters DBN-amICz and DBN-bmICz demonstrate excellent electroluminescence (EL) performances, with maximum external quantum efficiencies (EQEs) of 40.1% and 35.5%, and FWHMs of 21 and 24 nm, respectively. This study represents the first report of dual-boron-containing MR emitters derived from indolocarbazole with emission below 500 nm, filling a gap in the development of indolocarbazole-embedded dual-boron-containing blue–green MR-TADF emitters.

The soliton-like electronic structure features of compound 2AcPh-PhF are enhanced by introducing peripheral substituted groups on the parent 2AcPh, which is considered to be the intrinsic reason for its narrowed emission spectra. A sky-blue organic light-emitting diode based on 2AcPh-PhF has achieved an electroluminescence spectrum as narrow as 25 nm and an external quantum efficiency of up to 29.17%.

Abstract

The effect of substituents on the spectral behavior of aromatic heterocyclic systems is a significant yet highly complex issue. Inspired by cyanine dyes, it was found that emission spectrum narrowing can be realized by regulating the terminal substituents to enhance the soliton-like characteristics of emitters. Herein, compared to the parent compound 2AcPh (full-width at half-maximum, FWHM = 0.124 eV), the introduction of fluorobenzene-group substituents in 2AcPh-PhF narrows the emission spectral FWHM to 0.108 eV. Single crystal structures reveal more homogeneous carbon–carbon bond lengths in 2AcPh-PhF compared to 2AcPh. Theoretical calculations show that the charge magnitudes on carbon atoms of 2AcPh-PhF also become homogenized, but the charges on adjacent carbon atoms are opposite. These results suggest that the soliton-like electronic structural characteristics in 2AcPh-PhF are enhanced after introducing peripheral substituents, which is considered to be the reason for the narrower emission spectrum of 2AcPh-PhF. The sky-blue organic light-emitting diode devices based on 2AcPh-PhF demonstrated a maximum external quantum efficiency of 29.17% and a small electroluminescent spectral FWHM of 25 nm.

A new class of high-performance electron acceptors for short-wavelength infrared organic photodetectors (SWIR OPDs) has been demonstrated, following a strategy that is distinctly different from the general quinoidal and donor–acceptor strategies. The acceptor utilizes four thiophene-fused BODIPY units to achieve SWIR absorption, while pentafluorophenyl end groups enable efficient exciton dissociation.

Abstract

Device performance of photodiode-type short-wavelength infrared (SWIR) organic photodetectors (OPDs) is largely limited by poor exciton dissociation. In this work, we reported that downshifted highest occupied molecular orbital energy level (E _HOMO) and increased electrostatic potential (ESP) of electron acceptor lead to improved exciton dissociation and consequently enhanced SWIR OPD device performance. Tetramers of thiophene-fused 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (TF-BODIPY) unit represent a new kind of electron acceptors with SWIR photoresponse. By endcapping the TF-BODIPY tetramer with electron-deficient pentafluorophenyl groups, we downshift the E _HOMO of the electron acceptor by 0.06 eV and increase the ESP of the electron acceptor by 89 meV. As a result, the OPD devices of the electron acceptor exhibit SWIR photoresponse in the wavelength range of 0.3–1.3 µm with a maximum specific detectivity (D*) of 1.04 × 10¹² Jones and a responsivity (R) of 0.16 A W⁻¹ at 1.12 µm. This performance is among the highest reported for SWIR OPDs.

By strategically integrating spiro polycyclic aromatic hydrocarbons (spiro-PAHs) either in their entirety or partially into the boron/nitrogen- multiple resonances (B/N-MR) cores, it is feasible to develop highly efficient and narrow-band MR-thermally activated delayed fluorescence (MR-TADF), while simultaneously tuning their emission wavelengths.

Abstract

Departing from conventional molecular design strategies that rely on spiro units merely as peripheral components (side chains, terminal groups, or linkage units), we fully or partially incorporate the rigid 9,9′-spirobi[fluorene] (SF) unit into the boron/nitrogen multiple resonances (B/N-MR) emitting core, thereby successfully developing a series of proof-of-concept isomerized multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, namely SF-BN1, SF-BN2, SF-BN3, and SF-BN4. Remarkably, these novel emitters exhibit exceptionally narrow full-width at half-maximum (FWHM) values of 15–21 nm in dilute toluene solutions and high photoluminescence quantum yields (PLQYs) of up to 90% in doped films. The corresponding organic light-emitting diode (OLED) based on SF-BN1 achieved high external quantum efficiency (EQE) of up to 29.0%, with CIE coordinates of (0.13, 0.08), closely aligning with the BT.2020 blue emission standard. Sky-blue OLEDs based on SF-BN3 can achieve a high EQE of 29.8%, with a narrow FWHM value of 18 nm; the hyperfluorescent (HF) OLEDs based on SF-BN3 improved the EQE of 35.5%. Moreover, we elucidated subtle variations in the connectivity of chemical functional groups within emitters and the polar environment and doping concentrations of OLEDs, which can significantly impact these isomers' optical and electroluminescent (EL) properties.

Ir(III) complexes with contrasting MLCT or TSCT transition characters were synthesized, among which the TSCT emitter f-ct13b gave PhOLED device with an EQE_max of 22.2% and CIE _xy of (0.155, 0.120). Additionally, hyper-OLED bearing another TSCT sensitizer, f-ct13c and v-DABNA terminal emitter, achieved a narrowband blue hyperphosphorescence with EQE_max of 28.2% and CIE _xy of (0.123, 0.129), highlighting the effectiveness of TSCT phosphor in realizing blue electroluminescence.

Abstract

Through-space charge transfer (TSCT), rather than the commonly postulated metal-to-ligand charge transfer (MLCT) process, was proposed in getting the lowest lying excited state of newly designed Ir(III) blue phosphors. Accordingly, two benzo[d]imidazolylidene pro-chelates, L12H₂ ⁺ and L13H₂ ⁺ , one with two cyano groups at the peri-benzo and N-aryl pendent and the other with its peri-cyano group being replaced with methyl substituent, were employed in syntheses of Ir(III) complexes f-ct12b,c and f-ct13b,c. Notably, complexes f-ct12b,c exhibited the traditional MLCT process, while f-ct13b,c were dominated by the TSCT transition, resulting in a smaller S₁–T₁ energy gap ΔE _ST. Next, it prompted us to explore whether their long-lived emission originated from phosphorescence or thermally activated delayed fluorescence (TADF). Although temperature-dependent emission studies favor TADF, the unresolved concerns are still discussed in depth. For application, OLED with the TSCT-based dopant f-ct13b delivered a maximum external quantum efficiency (EQE) of 22.2% and a max. luminance of 10 000 cd m^‒2, together with CIE _xy of (0.155, 0.120). Moreover, the hyper-OLED with f-ct13c sensitizer and v-DABNA terminal emitter exhibited a max. EQE of 28.2% and CIE _xy of (0.123, 0.129), demonstrating a new approach in developing efficient Ir(III) blue phosphors.

An effective approach, i.e., blocking orbital π-conjugation via either molecular-structure distortion or meta-arrangement of chemical substituents is proposed, to boost spin-orbit couplings (SOC) in carbonyl-embedded polycyclic heteroatomic emitters. This work not only provides a deep insight into the structure-property relationship on SOC but also opens the door for designing more efficient OLED emitters with large SOCs.

Abstract

Both reducing singlet-triplet energy gaps (ΔE _S1T1) and enhancing spin-orbit couplings (SOCs) are key to improving reverse intersystem crossing rates (k _RISC) in thermally activated delayed fluorescence (TADF) materials. While considerable efforts have focused on reducing ΔE _S1T1, investigations on SOCs remain limited. Here, blocking π-conjugation in carbonyl-embedded polycyclic heteroaromatic (PHA) molecules as potential approach to elevate ππ* excitation energy, allowing its hybridization with nπ* excitation, thereby increasing SOCs is proposed. Two proof-of-concept isomers, DNDK-1 and DNDK-2 are synthesized, with different orientations of carbonyl units. DNDK-1 adopts a heavily twisted structure that hinders π-conjugation, while DNDK-2 remains quasi-planar, maintaining stronger π-conjugation. Experimental measurements reveals stark differences in their photophysical properties, with DNDK-1 exhibiting faster k _RISC and much higher electroluminescence efficiency. The ab-initio calculations elucidate that hindered conjugation in DNDK-1 elevates ππ* excitation energy, enabling nπ*-ππ* mixing, thus significantly boosting SOCs. In contrast, smooth π-conjugation in DNDK-2 leads to marginal nπ*-ππ* mixing. In addition, utilizing groups composed of meta-arranged carbonyl-Ar-carbonyl and meta-arranged N-Ar-N units emerges as another approach to block π-conjugation and enhance SOCs. This joint experimental and theoretical work provides promising pathways to enhance SOCs by blocking π-conjugation, offering crucial insights for designing carbonyl-embedded PHA emitters with larger SOCs.

A set of calamitic axially chiral luminescent liquid crystals with high-performance circularly polarized luminescence (CPL) were synthesized. A near-unity photoluminescence quantum yield (98.4%) and high g _lum value (up to 2.1 × 10⁻²) were achieved with an exceptional figure of merit of 0.02. Crucially, in situ CPL switching is realized through pathway-dependent phase control, which driven by slight excited-state molecular packing differences, enables reversible CPL tuning without altering molecular chirality.

Abstract

Chiral luminescent materials have garnered increasing attention for their exceptional ability to emit circularly polarized luminescence (CPL) along with their excellent applications. Here, a cyclohexylidene scaffold was conceptualized as a chiral source for developing higher-performance CPL materials in terms of simultaneously enhanced quantum yields (PLQYs) and dissymmetry factor. It was found that the axially chiral scaffold attached with a cyanostilbene showed a pathway-dependent assembly route to form chiral luminescent liquid crystals and crystals upon fast and slow cooling, respectively. A significant enhancement of PLQYs (98.4%) and a dissymmetry factor (g _lum) value (2.1 × 10⁻²), and consequently, a high figure of merit (FM) of up to 0.02 was achieved in the chiral liquid crystal phase. Moreover, the liquid crystal and crystal phases showed the opposite CPL signals while maintaining the same circular dichroism signs. Through a thorough evaluation of UV absorption, CPL emission, wide-angle X-ray diffraction, and theoretical calculations, it was revealed that the reversal of the CPL sign was linked to distinct phases of excited state molecular packing. This research utilized a novel intrinsically axially chiral source to develop a pathway-dependent and higher-performance CPL materials.

Contrasting mechanochromic luminescence (MCL) has been observed for enantiopure and racemic crystals of pyrenylprolinamides with different substituents. Enantiopure crystals exhibited circularly polarized luminescence (CPL), and mechanochromic CPL was achieved due to the formation of hydrogen-bonded excimers in the amorphous state. For the first time, the amorphous-state CPL was elucidated based on the excimer chirality rule.

Abstract

Circularly polarized luminescence (CPL) and mechanochromic luminescence (MCL) have independently made substantial progress in recent years. However, the exploration of MCL in solid-state CPL materials, which holds practical significance, is still in its infancy. Herein, we report the MCL properties of readily accessible chiral pyrenylprolinamides bearing tert-butoxycarbonyl (Boc) or 2,2,2-trichloroethoxycarbonyl (Troc) groups. Enantiopure crystals of the Boc derivative display a greater MCL wavelength shift than racemic crystals, while the Troc derivative exhibit the opposite trend. Most notably, the enantiopure crystals show mechanochromic CPL. Unlike in previous examples, where CPL is quenched upon amorphization, robust CPL spectra were observed even in the amorphous states. By applying the excimer chirality rule, we have, for the first time, acquired insights into the excited-state structures within mechanically generated amorphous states. These findings offer a novel design strategy for developing mechanochromic CPL materials, paving the way for the future advancements in this emerging field.

Molecules with large gaps between their first singlet and triplet excited states (ΔEST) are key components of various modern technologies, most prominently singlet fission photovoltaics and triplet-triplet annihilation upconversion. The design of these molecules is hampered by the fact that only limited rules for maximizing ΔEST exist, other than increasing the overlap between the frontier molecular orbitals (FMO). Here we suggest a new strategy for tuning and maximizing ΔEST based on a detailed analysis of the underlying quantum mechanical energy terms. We present a model based on the transition density and derive three straightforward design rules: ΔEST values can be maximized by (i) minimizing the overall number of π-electrons, (ii) reducing delocalization, and (iii) optimizing specific geometric interactions. The validity of these rules is first exemplified for a set of 18 hydrocarbon backbones before proceeding to a varied set of dye molecules highlighting their transferability to realistic settings. We believe that the developed rules will provide an enormous boost to the field enabling rational design instead of trial-and-error screening. More generally, this work demonstrates the power of going beyond the FMO approximation in designing advanced molecular materials.