Rong-Huei Yi

Publication date: 15 June 2023

Source: Chemical Engineering Journal, Volume 466

Author(s): Qinqin Peng, Wei Yang, Nengquan Li, Shaolong Gong, Xiang Gao, Changqing Ye, Yang Zou, Chuluo Yang

The triimidazole-carbazole derivative TT-Ph-Cz here synthesized possesses high conformational freedom resulting in rigidochromic and multi-stimuli responsive emissive behavior. The origin of the compound stimuli-responsiveness is interpreted through X-ray diffraction and DFT-TDDFT studies. Interconversion between TT-Ph-Cz solvated and unsolvated phases occurs through solvent fuming and/or thermal treatment. Phosphorescent aqueous nanoaggregates are prepared from solvent/non-solvent solutions.

Abstract

Stimuli responsive luminescent materials possessing room temperature phosphorescence (RTP) are extremely desirable for various applications. The here investigated derivative of cyclic triimidazole (TT) functionalized with carbazole (Cz), namely TT-Ph-Cz, belongs to this class. TT-Ph-Cz possesses high conformational freedom resulting in rigidochromic and multi-stimuli responsive emissive behavior. It has been isolated as MeOH-solvated and de-solvated forms characterized by distinctive emissive features. In particular, the solvated form, in which hydrogen bonds with MeOH inhibit competitive non-radiative deactivation channels, possesses a higher quantum yield associated with a strong phosphorescence contribution which is preserved in DMSO/water solutions.

Three multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters containing sulfide (S), sulfoxide (SO), and sulfone (SO₂) in para boron position are designed and synthesized. The influence of electron structure transformation from para-D-π-B to para-A-π-B on photophysical properties and device performance of the MR emitters are systematically investigated. Consequently, BN(p)SCH₃-based organic light-emitting diodes (OLEDs) show highly efficient blue emission with external quantum efficiency (EQE) exceeding 26% and excellent color purity.

Abstract

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been studied to address the issue of the broadband emission in organic light-emitting diodes (OLEDs). Herein, the authors have systematically investigated the effect of electron-donating or -withdrawing units in the para position of B atom on the optoelectronic emission modulation BCz-BN MR-TADF emitters. Due to the enhanced spin-orbit coupling (SOC) effect, BN(p)SCH₃ with electron structure of para-D-π-B is synthesized by introducing a heavy S atom into the para position of B atom of BCz-BN. By oxidizing BN(p)SCH₃, BN(p)SOCH₃ and BN(p)SO₂CH₃ with electron structure of para-A-π-B have been synthesized. The quantum simulations and photophysical studies have illustrated BN(p)SCH₃ can exhibit large reverse intersystem crossing rate constant (k _RISC) of 6.4 × 10⁴ s⁻¹ due to the large SOC constants and small singlet-triplet energy splitting (ΔE _ST) of 0.12 eV. BN(p)SOCH₃ and BN(p)SO₂CH₃ with electron structure of para-A-π-B displayed red-shift emissions with smaller full-width at half-maximum (FWHM) values of ≈21 nm and k _RISC values owing to enhanced ΔE _ST and the low emission contribution of the triplet excitons in contrast to those of BN(p)SCH₃ with electron structure of para-D-π-B. Consequently, BN(p)SCH₃-based OLEDs show highly efficient blue emission with an external quantum efficiency (EQE) of 26.2% and excellent color purity.

A pure-red multi-resonance emitter fused with indolocarbazole segments is developed, simultaneously redshifting and narrowing the emission color. The corresponding organic light-emitting diode realizes BT.2020 red coordinates of (0.708, 0.292), also achieving an impressive maximum external quantum efficiency of 34.4% and an ultralong LT95 of over 10 000 h at an initial luminance of 1000 cd m⁻².

Abstract

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.

The unusual photophysics of a class of curved chiral nanographenes (CNG) consisting of a corannulene and a tert-butylhexa-peri-hexabenzocoronene is investigated by time-resolved and temperature-dependent spectroscopy. Dual fluorescence and dual phosphorescence are found, as well as non-Kasha emission and thermally activated delayed fluorescence (TADF) in a narrow temperature range. Quantum-chemical calculations provide detailed understanding of the experimental findings.

Abstract

The intriguing and rich photophysical properties of three curved nanographenes (CNG 6, 7, and 8) are investigated by time-resolved and temperature-dependent photoluminescence (PL) spectroscopy. CNG 7 and 8 exhibit dual fluorescence, as well as dual phosphorescence at low temperature in the main PL bands. In addition, hot bands are detected in fluorescence as well as phosphorescence, and, in the narrow temperature range of 100–140 K, thermally activated delayed fluorescence (TADF) with lifetimes on the millisecond time-scale is observed. These findings are rationalized by quantum-chemical simulations, which predict a single minimum of the S₁ potential of CNG 6, but two S₁ minima for CNG 7 and CNG 8, with considerable geometric reorganization between them, in agreement with the experimental findings. Additionally, a higher-lying S₂ minimum close to S₁ is optimized for the three CNG, from where emission is also possible due to thermal activation and, hence, non-Kasha behavior. The presence of higher-lying dark triplet states close to the S₁ minima provides mechanistic evidence for the TADF phenomena observed. Non-radiative decay of the T₁ state appears to be thermally activated with activation energies of roughly 100 meV and leads to disappearance of phosphorescence and TADF at T > 140 K.

Transformation of a deep-blue MR-TADF emitter, DIDOBNA-N, to a narrowband near-UV emitter, MesB-DIDOBNA-N is demonstrated. Efficient deep-blue OLEDs with EQE_max and CIE _y coordinate of 15.3% and 0.073 with DIDOBNA-N and 16.2% and 0.049 with MesB-DIDOBNA-N illustrate the promise of the molecular design of these emitters.

Abstract

Two multiresonant thermally activated delayed fluorescence (MR-TADF) emitters are presented and it is shown how further borylation of a deep-blue MR-TADF emitter, DIDOBNA-N, both blueshifts and narrows the emission producing a new near-UV MR-TADF emitter, MesB-DIDOBNA-N, are shown. DIDOBNA-N emits bright blue light (Φ_PL = 444 nm, FWHM = 64 nm, Φ _PL = 81%, τ _d = 23 ms, 1.5 wt% in TSPO1). The deep-blue organic light-emitting diode (OLED) based on this twisted MR-TADF compound shows a very high maximum external quantum efficiency (EQE_max) of 15.3% for a device with CIE _y of 0.073. The fused planar MR-TADF emitter, MesB-DIDOBNA-N shows efficient and narrowband near-UV emission (λ _PL = 402 nm, FWHM = 19 nm, Φ _PL = 74.7%, τ _d = 133 ms, 1.5 wt% in TSPO1). The best OLED with MesB-DIDOBNA-N, doped in a co-host, shows the highest efficiency reported for a near-UV OLED at 16.2%. With a CIE _y coordinate of 0.049, this device also shows the bluest EL reported for a MR-TADF OLED to date.

The indolocarbazole-based emitters have recently emerged as a class of acceptor-free multiple resonance emitters, exhibiting extremely small FWHMs, good color-tunability from violet to deep red and excellent stability in OLED devices, which are ideal as end emitters in sensitizer-type OLEDs.

Abstract

Narrowband emitters based on the multiple resonance (MR) effect have shown great promises for applications in high-definition displays. Though most reported MR emitters adopt the boron- and nitrogen-doped triangulene-like skeleton following the design of the DABNA series, MR materials with the nitrogen-only indolocarbazole structures have recently emerged and demonstrated good color tunability from violet to deep red and impressive electroluminescence performances in terms of both efficiency and stability. Herein, we will summarize indolocarbazole-based MR materials in recent reports, with emphasis on their molecular design, synthesis, photophysical and electroluminescence properties as well as some future research directions to unlock the full potential of this fascinating class of materials.

Y-shaped D-A type molecules possessed two independent ICT states inside, resulted from the two independent D−A conjugation from their small donor arms to the acceptor. As their two D−A torsion angles are very sensitive to aggregated states, they showed interesting mechano-induced TADF and high-contrast MRL properties.

Abstract

High-contrast mechano-responsive luminescence (MRL) materials with mechano-induced emission enhancement properties are fascinating candidates but few, for applications in rewritable media and recording devices. Here, an interesting design strategy of “Y-shape” donor-acceptor (D−A) type molecules for high-contrast MRL materials was presented, based on substituted diphenylamine donor and planar acceptor. Interestingly, their D−A torsion angles are small in crystals but increased after ground, resulted in planar and twist intramolecular charge transfer (PICT and TICT) states, respectively. Therefore, high-contrast MRL switching between weak blue (450 nm) fluorescence and bright yellow (552 nm) thermally activated delayed fluorescence (TADF) can be achieved for compound TXDO (4,4′-dimethoxydiphenylamine donor), which photoluminescence quantum yield increased from 2.8 % to 54.7 % after ground. Most importantly, the two independent D−A conjugation dihedral angles are actually independent in the “Y-shape” molecules. Especially for compound TXDT (4,4′-di-tert-butyldiphenylamine donor), its crystal exhibited both PICT and TICT processes inside, resulted from the different dihedral angles of 11.8° and 35.5°, respectively. The TXDT crystal thus showed dual-peak emission, including both TICT fluorescence and PICT room-temperature phosphorescence. Therefore, this strategy of “Y-shape” D−A type molecules provide a new approach to design advanced luminescent materials with mechano-induced TADF feature, for high-contrast MRL and single-component white luminescence.

Two novel NIR TADF organic emitters, namely OPDC-DTPA and OPDC-DBBPA, were first designed and compared in parallel. Neat films of OPDC-DTPA and OPDC-DBBPA present real NIR emission with peaks at 962 and 1003 nm, respectively. Encouragingly, the solution processable doped NIR OLEDs based on OPDC-DBBPA exhibits a maximum of 0.103 % with an emission at a peak wavelength of 906 nm.

Abstract

Near-infrared (NIR) organic light-emitting diodes (OLEDs) suffer from the low external electroluminescence (EL) quantum efficiency (EQE), which is a critical obstacle for potential applications. Herein, 1-oxo-1-phenalene-2,3-dicarbonitrile (OPDC) is employed as an electron-withdrawing aromatic ring, and by incorporating with triphenylamine (TPA) and biphenylphenylamine (BBPA) donors, two novel NIR emitters with thermally activated delayed fluorescence (TADF) characteristics, namely OPDC-DTPA and OPDC-DBBPA, are first developed and compared in parallel. Intense NIR emission peaks at 962 and 1003 nm are observed in their pure films, respectively. Contributed by the local excited (LE) characteristics in the triplet (T₁) state in synergy with the charge transfer (CT) characteristics for the singlet (S₁) state to activate TADF emission, the solution processable doped NIR OLEDs based on OPDC-DTPA and OPDC-DBBPA yield EL peaks at 834 and 906 nm, accompanied with maximum EQEs of 0.457 and 0.103 %, respectively, representing the state-of-the-art EL performances in the TADF emitter-based NIR-OLEDs in the similar EL emission regions so far. This work manifests a simple and effective strategy for the development of NIR TADF emitters with long wavelength and efficiency synchronously.

BASHY dyes, such as the one drawn in the foreground, are prepared in a multicomponent reaction. This is symbolized by the background, which shows part of a beach that is made of sea-washed stones and which is located in the North of Portugal (Praia de Belinho). The BASHY dyes discussed in the Research Article by P. M. P. Gois, U. Pischel and co-workers (DOI: 10.1002/chem.202300579) show several photophysical features such as fluorescence, intramolecular charge transfer (ICT), and photoinduced electron transfer (PeT), which jointly orchestrate the function of the dye.

A red-emitting aggregation-induced emission luminogen (AIEgen) is realized by introducing a bridging structure into push-pull distyrylbenzene.

Abstract

Multiple emission colors in solid-state organic fluorophores with the same main skeleton are essential for improving the performance of light-emitting devices/materials. Specifically, emission in the red/near-infrared region is of great importance in the biological field. Previously, we developed di-bridged-distyrylbenzene DBDB[7] with high-brightness solid-state blue and aggregation-induced emissions (AIE) by introducing the bridging structures of a seven-membered ring into the vinylene groups of distyrylbenzene (DSB). Herein, we synthesize MNDBDMeODB[7] (1), which features substituted methoxy and malononitrile groups as donor and acceptor groups, respectively, in DBDB[7]. In solvents more polar than THF, MNDSDMeOB (3), which has the same main skeleton as 1 but without bridges, shows no emission in the solid state, whereas 1 exhibits highly bright red-orange emission in the solid state owing to the suppression of intermolecular electronic interactions by the bridges and the AIE property. We also synthesize MNDSD(EHO)B (2) in which the methoxy groups of 3 are replaced by ethylhexyloxy groups, thus disrupting the crystallinity of the molecule. 2 exhibits positive fluorescence solvatochromism and has a high fluorescence quantum yield in the solid state as a red-emitting DSB derivative. The solid-state emission properties of 1 and 2 will improve the applicability of DSBs and functionalities of light-emitting devices/materials.

Prompt and delayed emission: A series of 3-ligated or bridged phenothiazinyl-terephthalonitrile dyads and triads are accessed in a one-pot fashion and photophysical studies in solution, solid-state and in polymer matrix reveal TADF (thermally activated delayed fluorescence) as estimated experimentally for four dyes and confirmed by DFT/MRCI calculations for one dyad.

Abstract

The bromine-lithium exchange-borylation-Suzuki sequence efficiently furnishes phenothiazine-terephthalonitrile donor-acceptor dyads and triads in high yields. In contrast to most phenothiazine-acceptor conjugates the title compounds are ligated in p-position to the phenothiazine nitrogen atom. Moreover, the acceptors are either directly linked or ligated by an arylene bridge and p-anisyl N-substituents on the phenothiazine are chosen to lock the tricycle into an intra-configuration. Cyclic voltammetry reveals effects of bridging and ligation of the N-substituent. Optical spectroscopy likewise displays similar band gaps, large Stokes shifts and substantial to high quantum yields in solution, in the solid state and in PMMA matrix. Time-resolved fluorescence spectroscopy indicates quite long fluorescence decay times in solution and emission components in the microsecond time range. TADF properties are further assessed by fluorescence increase in deoxygenated solution, gated emission spectroscopy and temperature-dependent determination of phosphorescence. The nature of the electronically excited states is investigated by DFT/MRCI. While for the directly ligated dyad a singlet-triplet energy gap Δ of 0.24 eV can be estimated and is consistently confirmed by quantum chemical calculations on the lowest energy conformer, even lower of 0.029 and 0.008 eV are estimated for the investigated dyads and the triad in the solid state and in PMMA matrix.

The architecture of the ring fuses and extends π-skeleton based on high-order B/N-embedded steric rooted and structurally rigid hypsochromic-shifted narrow-band pure blue multi-resonant thermally activated delayed fluorescent emitter for efficient and ultralow-efficiency roll-off hyperfluorescent-organic light-emitting diodes.

Abstract

Multi-resonant thermally activated delayed fluorescent (MR-TADF) materials are blooming for high-resolution organic light-emitting diodes (OLEDs). However, boron/nitrogen (B/N)-integrated MR-TADF emitters suffer severe efficiency roll-off from their strong inter-molecular π–π interactions. Herein, versatile narrowband pure blue emitters (mono-mx-CzDABNA and tri-mx-CzDABNA) are demonstrated featuring a ring-fused extended π-skeleton: a classic steric hindrance and rigidity accessed by integrating with meta-xylene (mx) rotors. tri-mx-CzDABNA shows a narrowband (FWHM, 26 nm) pure blue emission (λ _max, 462 nm) with substantial hypsochromic shift (12 nm) while maintaining MR-TADF characteristics. The key solid-state analyses conclude that they conceivably suppress the non-radiative energy loss, thus improving the photoluminescence quantum yield (PLQY > 90%) and rate of reverse intersystem crossing (RISC) (k _RISC ≈2.85 × 10⁵ s⁻¹). The integration of tri meta-xylene significantly leads to an enhanced horizontal dipole ratio (HDR) from 65% to 85%. Hyperfluorescent-OLEDs are fabricated using designed MR-TADF as terminal emitter, achieving a narrowband (FWHM, 34 nm) pure blue electroluminescence (λ _max, 472 nm) and maximum external quantum efficiency (EQE_max) of 26.97% with magnificently suppressed efficiency roll-off (7.8%) at 1000 cd m⁻². So, it is believed that regulation of internal efficiencies and high color purity can amplify the route to achieving a narrowband pure blue emission through new synthetic MR-TADF approaches.

The integration of exciton donor and acceptor in a single emitting molecule with zero radius of intramolecular energy transfer (ZRIET) is proposed for efficient and stable organic light-emitting diodes. The blue-emitting nondoped device with ZRIET mechanism exhibits comparable efficiency and color purity as a conventional host–dopant emitting system and 220% enhancement in device lifetime.

Abstract

Multimaterial emission layer (EML) with host and dopant is known for its advantages in efficiency and lifetime for organic light-emitting diodes (OLEDs). However, as the compositions and the mixing ratio in the EML diversify, complex device physics and photophysics and complicated fabrication processes become challenging issues. Herein, a new concept to integrate exciton donor (host) and acceptor (dopant) into a single emitting molecule is suggested. 9-(8,9,10,11-Tetradeuterospiro[benzo[de]anthracene-7,9′-fluoren]-2'-yl)-9H-carbazole (SBAF-Cz) and 8,9,10,11-tetradeutero-N-(9,9-dimethyl-9H-fluoren-2-yl)-N-phenylspiro[benzo[de]anthracene-7,9′-fluoren]-3-amine (FPA-SBAF) are used as host and dopant, respectively. By integrating two units, 8,9,10,11-tetradeutero-2′-(9H-carbazol-9-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-N-phenylspiro[benzo[de]anthracene-7,9′-fluoren]-3-amine (FPA-SBAF-Cz) is synthesized, and photophysical and electrical properties are characterized. Density functional theory and experimental results consistently reveal that SBAF-Cz and FPA-SBAF are photophysically and electrically independent. The electrical properties of FPA-SBAF-Cz behave independently, while the photophysical properties are the sum of those of SBAF-Cz and FPA-SBAF, which is attributed to the zero radius of intramolecular energy transfer (ZRIET) between donor and acceptor in FPA-SBAF-Cz. The single-component blue OLED with FPA-SBAF-Cz exhibits a maximum external quantum efficiency of 4.18%, which is higher than those of OLEDs with SBAF-CZ:FPA-SBAF multicomponent EMLs. Surprisingly, the device lifetime to 50% decrease from initial luminance is 114 h in FPA-SBAF-Cz OLED, which is 2.2 times longer than that of SBAF-Cz:FPA-SBAF OLEDs.

Two novel near-ultraviolet (NUV) organic emitters are reported by incorporating extra charge-transfer (CT) channel into multi-resonance skeleton to activate hybridized local and CT emission. Both emitters exhibit high-efficiency narrow-band NUV emissions at 414 nm with a FWHM about 32 nm in their doped, solution-processed organic light-emitting diodes. The highest external quantum efficiency of 9.15% and a low CIE _y of 0.034 are obtained.

Abstract

Hybridized local and charge-transfer (HLCT) excited-state emitters can effectively utilize non-radiative triplet excitons through high-lying reverse intersystem crossing (hRISC), but they are mostly limited to the donor-π-acceptor type molecules. It is a great challenge to develop high-performance near-ultraviolet (NUV) emitters with narrow-band emission by HLCT due to the large carrier injection barriers and high triplet energy. In this work, by incorporating planar multi-resonance (MR) skeleton of oxygen-bridged triphenylborane (BO) and weak electron-donating unit of tetraphenyl-silane (TPS), two NUV emitters of tBOSi and tBOSiCz are developed, where an extra weak charge transfer (CT) channel between BO skeleton and peripheral phenyl in TPS is observed to increase CT component and activate hot exciton channel of hRISC. As a result, tBOSi and tBOSiCz show an outstanding narrow-band NUV emission at 414 nm with a FWHM of about 32 nm in solution-processed organic light-emitting diodes (OLEDs), even at a heavy doping ratio. The best NUV electroluminescent properties are further achieved in the optimal tBOSi-doped OLEDs with a record efficiency of 9.15% and a low CIE _y of 0.034. This work provides a profound guidance for developing high-performance narrow-band NUV emitters by an extra weak CT channel into MR skeleton to activate HLCT emission.

Space-confined through-space charge transfer emitters are developed with different bridges, which show strong through-space donor/acceptor (D/A) interactions, fast radiative decays, and high luminescent efficiencies, as an optimal D–A distance is achieved. Organic light-emitting diodes using the emitters show high external quantum efficiencies up to 23.1% and low efficiency roll-offs.

Abstract

Through-space charge transfer (TSCT) emitters featuring thermally-activated delayed fluorescence (TADF) are extensively researched but suffer from low radiative decay rates (k _r,s) due to insufficient through-space donor/acceptor interactions. Here, space-confined TSCT TADF emitters 1–3 with a chemically fixed benzophenone acceptor and a triphenylamine donor on different bridges, that is, 1-methyl-9,10-dihydroanthracene for 1, 4-methyl-9H-xanthene for 2 and 4-methyl-9H-thioxanthene for 3, which exhibit reinforced donor/acceptor interactions with shortened donor–acceptor distances, are reported. It is unveiled that there exists an optimal distance between the fixing sites of donor and acceptor. The emitter 2 with such an optimal distance shows both strong donor/acceptor interactions and high molecular rigidity, whereas the emitter 3 with a too short distance exhibits a twisted molecular structure and increased non-radiative deactivation. In solution, 1–3 shows high k _r,s up to 3.0 × 10⁷ s⁻¹. In doped films, 1–3 exhibits green emission with high k _r,s up to 8.3 × 10⁶ s⁻¹ and photoluminescent efficiency up to 0.96. Organic light-emitting diodes based on 1–3 afford high external quantum efficiencies up to 23.1% and largely alleviated efficiency roll-offs. The work demonstrates that using rigid bridges that render an optimal donor–acceptor distance is crucial to the development of highly efficient TSCT emitters with fast radiative decays for electroluminescence applications.

Using the strategy of chiral donor-acceptor copolymerization, two novel chiral thermally activated delayed fluorescent polymers are synthesized. The circularly polarized polymer light-emiiting diode devices are then prepared through a solution process where intense deep-red-emission circularly polarized electroluminescence signals from the corresponding devices fabricated with R-P and S-P are clearly detected at the wavelength of 662 nm.

Abstract

Circularly polarized electroluminescence (CPEL) from thermally activated delayed fluorescence (TADF) emitters have been widely reported in recent years. However, a universal strategy for designing deep-red chiral TADF emitters is still unexplored. Herein, to address the problem, a pair of chiral donor intermediates are designed and synthesized for preparing deep-red chiral TADF emitters, and two novel chiral TADF polymers R-P and S-P are developed through this strategy. The polymers exhibit intense mirror-image circularly polarized luminescence (CPL) signals in toluene dilute solution. Besides, a series of solution-processed circularly polarized polymer light-emitting diode (CP-PLED) devices are fabricated with R-P and S-P. The corresponding devices achieve persuasive performance with external quantum efficiencies (EQEs) of 6.2% and 5.8% at the wavelength of 662 nm, respectively. Besides, mirror-image CPEL signals are detected with electroluminescence dissymmetry factors (gEL) of +1.6 × 10⁻³ and −1.7 × 10⁻³ from the corresponding CP-PLED devices, respectively.

Benefiting from the rational intramolecular charge transfer introduced into the multi-resonance skeleton, fine modulation of emission color is achieved from 395 to 421 nm while retaining small full-width at half-maximums of 28 and 33 nm (214 and 237 meV), enabling the bright violet organic light-emitting diode (OLED) with the peak at 412 nm and the deep-blue OLED with the peak at 427 nm and an ultralow CIE_y of 0.027, both of which show alleviated efficiency roll-off.

Abstract

For purely nitrogen-based multi-resonance emitters (N-MR), strategies to integrate emission modulation with high color purity remain exclusive, especially in near-ultraviolet (NUV) regions. Herein, it is demonstrated for the first time that rationally introducing weak intramolecular charge transfer (ICT) to the MR skeleton to modulate the emission from violet to deep-blue while retaining high color purity is feasible. By replacing the middle phenyl moiety with the pyridine or benzonitrile unit in the solely nitrogen-based violet MR skeleton of tDIDCz, two proof-of-concept emitters, Nm-ICz and CNm-ICz, emphasizing mixed excited states of localized excited (LE) transition from the original MR skeleton and emergent transition channel with charge-transfer (CT) character, successfully realize bathochromic-shift and polarity-insensitive fluorescence from 395 nm to 404–407 and 419–421 nm, respectively, while retaining small full-width at half-maximums (FWHMs) of 28–37 and 33–43 nm (214-281 and 237–310 meV). Furthermore, Nm-ICz shows bright and violet electroluminescence (EL) spectrum with the peak at 412 nm, while CNm-ICz shows high color purity deep-blue EL spectrum with the peak at 427 nm, a small FWHM of 42 nm (286 meV), an ultralow y color coordinate of 0.027, and a maximum external quantum efficiency (EQE_max) of 3.7%.

Two donor–accepter molecules, Mes*BA-Cz and ^FXylBA-Cz, exhibit fluorescence-phosphorescence dual-emission in solid state and mechanochromism phenomenon. Dual-emission results in white light photoluminescence in ^FXylBA-Cz (5%) doped poly (methyl methacrylate) film. A white organic light-emitting diode based on ^FXylBA-Cz is manufactured with CIE coordinates of (0.31, 0.33) and an external quantum efficiency (EQE) of 0.49%.

Abstract

Organic molecules with fluorescence-phosphorescence dual-emission have attracted significant attention for their potential applications in white organic light-emitting diodes (WOLEDs). Herein, donor–accepter molecules, Mes*BA-Cz and ^FXylBA-Cz, are designed and synthesized, and the photoluminescence of these two compounds in solid state exhibits fluorescence-phosphorescence dual-emission. Through the single crystal study, the phosphorescent emission is mainly caused by the molecular stacking in the crystalline state, which is beneficial for aggregation-induced intersystem crossing. Dual emission behavior further caused the mechanochromism of the two compounds, with emission color changed between amorphous and crystalline state. White light photoluminescence with CIE coordinates of (0.33, 0.34) is achieved by doping ^FXylBA-Cz in the poly (methyl methacrylate) film. Furthermore, a WOLED based on ^FXylBA-Cz is manufactured with CIE coordinates of (0.31, 0.33), and the external quantum efficiency (EQE) reached 0.49%.

A series of room temperature red afterglow materials at 650 nm with delayed lifetimes of 11–83 ms and quantum yields of 16.2–22.1% are constructed using polyvinylpyrrolidone as the host, isoquinolines as the guests, and triphenylamine-based dicyanomethylene-4H-pyran derivative as the third component and energy acceptor. The delayed fluorescence is caused by triplet-to-singlet Förster-resonance energy transfer from isoquinolines to dicyanomethylene-4H-pyran derivative.

Abstract

A newly emerged and attractive strategy to obtain afterglow is the use of the Förster-resonance energy transfer (FRET) from an energy donor with room-temperature phosphorescence (RTP) to an energy acceptor with fluorescence. Due to the transfer of energy between molecules with different emissions, it is possible to develop the ultralong and long-wavelength afterglow materials. However, there are few reports on red afterglow materials with emission wavelengths up to 650 nm because of the difficulty of accurate design of chemical structures. Herein, a series of red afterglow materials with emission wavelengths of 650 nm are constructed using polyvinylpyrrolidone as the host, multisubstituted isoquinolines as the guests, and triphenylamine-based dicyanomethylene-4H-pyran derivative as the energy acceptor. Two-component host-guest materials exhibit yellow-green, yellow, and orange RTP with delayed lifetimes of 205-301 ms and phosphorescence quantum yields of 5.3-13.2%, which originate from the guests in a rigid microenvironment provided by the host polymer. Three-component doped materials exhibit red afterglow with a delayed lifetime of 11-83 ms and an emission quantum yield of 16.2-22.1%, which is determined to be delayed fluorescence caused by triplet-to-singlet FRET from isoquinolines to dicyanomethylene-4H-pyran derivative. This work provides inspiration for the development of doped materials with long-wavelength room-temperature afterglow.

A class of delayed fluorescent macrocycles is successfully synthesized via a modular strategy. Among them, MC-XT and MC-X show perfect photoluminescence quantum yields (≈100%) due to more ideal macrocyclic structure and larger oscillator strength. Therefore, organic light-emitting diodes (OLEDs) based on MC-XT and MC-X achieve record-high maximum external quantum efficiencies of 26.9% and 31.6%, respectively.

Abstract

Several thermally activated delayed fluorescence (TADF) materials have been studied and developed to realize high-performance organic light-emitting diodes (OLEDs). However, TADF macrocycles have not been sufficiently investigated owing to the synthetic challenges, resulting in limited exploration of their luminescent properties and the corresponding highly efficient OLEDs. In this study, a series of TADF macrocycles is synthesized using a modularly tunable strategy by introducing xanthones as acceptors and phenylamine derivatives as donors. A detailed analysis of their photophysical properties combined with fragment molecules reveals characteristics of high-performance macrocycles. The results indicate that: a) the ideal structure decreases the energy loss, which in turn reduces the non-radiative transitions; b) reasonable building blocks increase the oscillator strength providing a higher radiation transition rate; c) the horizontal dipole orientation (Θ) of the extended macrocyclic emitters is increased. Owing to the high photoluminescence quantum yields of ≈100% and 92% and excellent Θ of 80 and 79% for macrocycles MC-X and MC-XT in 5 wt% doped films, the corresponding devices exhibit record-high external quantum efficiencies of 31.6% and 26.9%, respectively, in the field of TADF macrocycles.