Hybridized Local And Charge-transfer Research Articles

Novel electro-fluorescent materials with hybridized local and charge-transfer (HLCT) excited state for highly efficient non-doped pure blue OLEDs

Two molecules with HLCT characteristics were synthesized through fine-tuning the substituent on PPI based on the PPI–TAZ skeleton, and the non-doped device based on PPITZCN achieved higher efficiency with an EQEmax of 7.5% and CIE of (0.16, 0.10).

Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices

AbstractThe front cover artwork is provided by Phattananawee Nalaoh, VISTEC (Thailand). The image shows the solution‐processable hybridized local and charge‐transfer (HLCT) fluorophore as an efficient non‐doped emitter for solution‐processed double‐layered organic light‐emitting diode. Read the full text of the Research Article at 10.1002/cptc.202200154.

Front Cover: Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices (ChemPhotoChem 12/2022)

Front Cover: Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices (ChemPhotoChem 12/2022)

Charge transfer of cocrystal complex featuring efficient aggregation-induced emission and self-powered photo-electrochemical photodetector

A novel cocrystal complex CZIPA-AD assembled by twisting D-A molecule 5-(9H-carbazol-9-yl)isophthalic (CZIPA) and acridine (AD) through a facial solution volatilization process is reported. The combination of photophysical investigation and DFT analysis verify a reasonable cooperation of hybridized local and charge transfer (HLCT) and aggregation-induced emission (AIE) character. The orbital overlap on CZIPA (for local excited (LE) state) and orbital spatial separation between CZIPA and AD (for charge transfer (CT) state) satisfy the formation of HLCT character. The presence of AD molecules preclude the formation of π-stacking between the carbazole halves, preventing the quenching of PL emission in the aggregate state. It shows intense orange emission in aggregated solid state with high PLQY of 80% and prolonged PL lifetime of 38.57 ns. A self-powered photo-electrochemical photodetector fabricated from the cocrystal thin film exhibits a remarkable detectivity (4.78 × 1010 Jones) in neutral aqueous solution, which is at lest one order of magnitude higher than other reported inorganic materials measured in alkaline solution with additional bias potential.

Achieving high-performance non-doped sky-blue OLEDs with negligible efficiency roll-off by combining AIE, HLCT and mechanoluminescence features

The hot exciton materials with high-lying reverse intersystem crossing provide great potential for the realization of efficient, low-cost and stable organic light-emitting diodes (OLEDs). However, it is highly desired but still challenging to achieve high-performance non-doped blue OLEDs with hot exciton luminogens. In this work, a new hybridized local and charge transfer (HLCT) luminogen 2-(4-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-phenanthro[9,10-d]imidazole-1-yl)phenyl)phenanthro[9,10-d]oxazole (abbreviated as TPE-PI-PO) with the incorporation of tetraphenylethylene is obtained for constructing the high-performance non-doped blue OLEDs. TPE-PI-PO possesses the multifunctional features of aggregation-induced emission (AIE), HLCT and mechanoluminescence with high photoluminescence quantum yield (58%) in the neat film. The non-doped OLEDs based on TPE-PI-PO exhibits sky-blue emission, high luminance of 13,020 cd m−2 and the maximum efficiencies of 11.62 cd A−1, 5.08% with the negligible efficiency roll-off as low as 1.6% at 1000 cd m−2. The electroluminescent performance is highly competitive with the best reported non-doped blue fluorescent OLEDs based on AIE luminogens. These results can shed light on a feasible design principle to develop high-quality HLCT-type luminogens for high-performance non-doped blue OLEDs.

D–π–A–π–D-type Fluorophores based on Pyridal[2,1,3]thiadiazole acceptor with hybridized local and charge-transfer excited-state for high-efficiency OLEDs

Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which are characterized by high photoluminescence quantum yield (PLQY) and reverse intersystem crossing (RISC) from high-lying triplets to singlets, has emerged advantages in organic light-emitting diodes (OLEDs) application. Although blue and green HLCT fluorophores are well developed, the long wavelength ones (orange to red) are still behind. Here, based on more electron-deficient Pyridal[2,1,3]thiadiazole (PT) acceptor, a D–π–A–π–D-type HLCT emitter, namely 2CZP-PT, with orangered emission is designed and synthesized. Due to the less steric hindrance of pyridine nitrogen in PT, a small twist angle between the π bridge and acceptor on the nitrogen side is obtained, which endows the whole molecule with relative planar conformation. Thus 2CZP-PT exhibits HLCT characteristics with high PLQY (77% in THF solution and 44% in neat film), as well as energy levels arrangement favoring the RISC process from high-lying triplet levels to singlet levels. The doped devices based on 2CZP-PT reached a maximum external quantum efficiency (EQEs) of 9.57% with an electroluminescence (EL) peak at 568 nm. While the non-doped OLEDs based on 2CZP-PT emitter achieved a maximum EQEs of 1.34% with red EL peaked around 600 nm, whose efficiency roll-off is only 1.5% at 1000 cd m−2. These device performances demonstrate PT based D–π–A–π–D-type fluorophores are promising for their long-wavelength OLEDs application.

Dual Hybridized Local and Charge Transfer Characteristic Fluorophore as an Efficient Nondoped Emitter for Electroluminescent Device

Hybridized local and charge transfer (HLCT) is an attractive strategy for achieving efficient electroluminescence (EL) in electrofluorescent devices by the conversion of triplet‐state (T) to singlet‐state (S) excitons via a reverse intersystem crossing, giving rise to a high exciton utilization efficiency exceeding the spin statistical limit. Excellent device performance is reached using thermally evaporated emissive layers. However, for potential large‐scale commercialization, it is crucial to attain comparable device performances using low‐cost solution‐processing techniques. Herein, a new concept of dual HLCT characteristic fluorophore (BCBF) with a high solid‐state fluorescence as a nondoped emitter for a simple structured solution‐processed EL device is presented. BCBF is effectively formulated by π‐conjugation linking two HLCT fragments with a highly soluble hole‐transporting aromatic moiety. Its dual HLCT and photoluminescence (PL) properties are experimentally and theoretically probed by the solvatochromic effect and density functional theory (DFT) calculations. The molecule exhibits an intense yellow–green emission with a good solution‐processed film‐forming quality and a high solid‐state fluorescence quantum yield of 80%. BCBF is successfully utilized as a nondoped emissive layer in a solution‐processed double‐layered organic light‐emitting diode (OLED), which shows excellent EL performance (brightness of 47 580 cd m−2, current efficiency of 15.78 cd A−1, and external quantum efficiency of 7.20%).

Benzothiadiazole‐Based Green Hybridized Local and Charge‐Transfer (HLCT) Fluorophores as Solution‐Processed Non‐Doped Emitters for Simple Structured Electroluminescent Devices

AbstractHerein, two new solution‐processable HLCT fluorophores (CBzF and CBzFC) based on benzothiadiazole (Bz) derivatives were designed and synthesized (HLCT=hybridized local and charge transfer). Their optical and photophysical properties were experimentally and theoretically studied and verified by the solvatochromic effect and density functional theory calculations. The two molecules demonstrate high solubility, HLCT features, and an intense green fluorescence with solid‐state photoluminescence quantum yields of 63–72 %. Both fluorophores are effectively applied as non‐doped emitters for solution‐processed double‐layered OLEDs, which produce intense green emission colors with low turn‐on voltages (3.0–3.2 V). Specifically, the CBzFC‐based solution‐processed device achieves a high maximum luminescence (24400 cd m−2), high maximum efficiencies (EQEmax=5.59 %, CEmax=12.24 cd A−1, and PEmax=10.54 lm W−1), and decent efficiency low‐off.

A new amine‒carbonyl fused emitter with hybridized excited state for green OLED with high luminance and low efficiency roll-off

In this work, a novel green emitter, namely 16,16-dimethylbenzo[a]quinolino[3,2,1-de]acridin-10(16H)-one (GACQ), with amine‒carbonyl fused structure has been designed and synthesized. The single-crystal structure analysis confirmed that the fused structure shows non-planar conformation with dihedral angle of 48.97° from the front-view direction. The photophysical and theoretical results confirmed that GACQ exhibited hybridized local and charge-transfer (HLCT) feature with strong green emissions of 513 and 535 nm in solution and film, respectively. The non-doped device based on GACQ achieved high luminance (15810 cd m−2) and good efficiency stability with negligible external quantum efficiency (EQE) roll-off. Furthermore, the doped device exhibited high maximum EQE of 4.8% with CIE coordinate of (0.38, 0.58).

Controllable synthesis of conjugated microporous polymer films for ultrasensitive detection of chemical warfare agents

Nerve agents, one of the most toxic chemical warfare agents, seriously threaten human life and public security. The high toxicity of nerve agents makes the development of fluorescence sensors with suitable limit of detection challenging. Here, we propose a sensor design based on a conjugated microporous polymer film for the detection of diethyl chlorophosphate, a substitute of Sarin, with low detection limit of 2.5 ppt. This is due to the synergy of the susceptible on-off effect of hybridization and de-hybridization of hybrid local and charge transfer (HLCT) materials and the microporous structure of CMP films facilitating the inward diffusion of DCP vapors, and the extended π-conjugated structure. This strategy provides a new idea for the future development of gas sensors. In addition, a portable sensor is successfully integrated based on TCzP-CMP films that enables wireless, remote, ultrasensitive, and real-time detection of DCP vapors.

Tuning Hybridized Local and Charge-Transfer Mixing for Efficient Hot-Exciton Emission with Improved Color Purity

Delayed fluorescence (DF) emitters with high color purity are of high interest for applications in high-resolution displays. However, the charge transfer required by high emitting efficiency usually conflicts with the expected color purity. In this work, we investigated the S1/S0 conformational relaxation, spin-orbital coupling (SOC), and vibronic coupling of hot-exciton emitters while hybrid local and charge transfer (HLCT) state tuning was achieved by a structural meta-effect. The meta-linkage leads to suppressed S1/S0 conformational relaxation and weakened vibronic coupling, while the unsacrificed emitting efficiency is largely ensured by multiple rISC channels (Tn → Sm) with thermally accessible triplet-singlet energy gap (ΔEST) and effective SOC. We demonstrated that the unique excited-state mechanism provides opportunities to improve the emitting color purity of hot-exciton emitters without sacrificing emitting efficiency by HLCT state tuning with simple chemical structural modification, for which hot-exciton emitters might play a more important role for high-resolution organic light-emitting diode displays.

Recent Progress of Novel Organic Near‐Infrared‐Emitting Materials

Organic near‐infrared (NIR) emissive materials integrating the intrinsic NIR light advantages and organic semiconductor characters have generated immense scientific interest in fundamental science and practical application, including optical communications, night vision, surveillance, and biomedical applications. Nevertheless, the intrinsic energy gap law engenders a huge challenge to simultaneously meet longer emission wavelength and higher luminescence efficiency for organic NIR emitters. Herein, it is suggested that the purposefully molecular design and crystal engineering for novel organic NIR materials are urgently studied and explored. Herein, recent advances in novel organic NIR materials, mainly focusing on the hybridized local and charge transfer (HLCT) compounds, thermally activated delayed fluorescence (TADF) emitters, CT cocrystals, neutral π‐radicals, and excited‐state intramolecular proton transfer (ESIPT)‐active materials, are summarized. It is hoped that this perspective can afford a new insight into the fine design and synthesis of the desired organic NIR emitters.

Efficient red hybridized local and charge-transfer OLEDs by rational isomer engineering

Developing highly efficient red emitters with emission beyond 600 nm is full of challenges for organic light-emitting diodes (OLEDs). Herein, two novel red isomers of 11-DTPA-BBI and 3-DTPA-BBI manifesting hybridized local and charge-transfer (HLCT) state were designed and synthesized, based on triphenylamine (TPA) as donor and 7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (BBI) as acceptor. The charge-transfer (CT) component in HLCT state ensures their red emission and high excitons utilization, while the locally excited (LE) component guarantees the high fluorescence radiative rate. Further, the non-radiative transition is suppressed through regulating molecular vibration modes by isomer engineering, resulting in the high fluorescence quantum efficiency of 53% for 3-DTPA-BBI. Consequently, a maximum external quantum efficiency of 3.0% is achieved with emission peak at 604 nm in 3-DTPA-BBI based OLEDs. This contribution demonstrates a comprehensive structure-property relationship which may provide an effective guidance to design high performance red electroluminescent emitters.

Excited-state regulated electroluminescence performance from thermally-activated delayed fluorescence (TADF) to hybridized local and charge-transfer (HLCT) emission

In this work, two novel D-π-A compounds (PXZ-FR-DRZ and CZ-FR-DRZ) are designed and synthesized, aiming at the excited-state regulation from thermally-activated delayed fluorescence (TADF) to hybridized local and charge-transfer (HLCT) emission. This change of excited-state nature can be ascribed to the weakened electron-donating ability of carbazole relative to phenoxazine, together with the decreased twisting angle between carbazole and fluorine units. As a result of twisted charge-transfer (CT) excited state, the doped organic light-emitting diode (OLED) of PXZ-FR-DRZ demonstrates a blue-green TADF electroluminescence (EL) with a maximum external quantum efficiency (EQEmax) of 11.5%, while its non-doped device shows a much lower EQEmax. Due to HLCT state of CZ-FR-DRZ, both its doped and non-doped OLEDs achieve the deep-blue non-delayed EL with the satisfied external quantum efficiency (EQE), especially for the excellent efficiency roll-off. Overall, this excited-state transformation between TADF and HLCT allows us to harvest the better comprehensive performance of OLED emitters for the practical applications, which provides a feasible solution for the development of OLED industry.

Near-infrared piezochromism of AIE-active luminophore in hybridized local and charge-transfer excited state—the effect of shortened donor-acceptor distance

A close-bonded D-A-D′ luminophore BOTPA was designed via direct coupling of secondary arylamine derivatives (D, D′) with benzothiadiazole cyanostilbene group (A). Both near-infrared (NIR) aggregation-induced emission (AIE) and hybridized local and charge-transfer (HLCT) effects, which endow it with highly efficient NIR piezochromic performance, are observed owing to its especially short D-A distance and twisted conformation. The luminophore shows bright fluorescence at 740 nm with an absolute photoluminescence quantum yield (PLQY) as large as 13.2%. Remarkable piezochromism with high sensitivity (11.0 nm•Gpa−1) completely in the NIR region (700–840 nm) could be observed from BOTPA crystals, which is rarely found in piezochromic materials (PCMs). The effects of the shortened D-A distance towards the highly efficient NIR piezochromism are confirmed via single-crystal analysis, in-situ FTIR and theoretical calculations. This work provides a new strategy for the development of high-efficiency NIR PCMs which possess strong penetrating capacity.

Enhanced blue-emissive electroluminescence performance with rational donor substitution

High performance blue emitters with nice color purity are highly desired in the commercialization of organic light-emitting diodes (OLEDs). Herein, two blue-emissive hybridized local and charge-transfer (HLCT) phenanthroimidazole (PPI) derivatives were designed and synthesized. Comparing to the 9,9-dimethyl-9,10-dihydroacridine (DMAC) substituted DPCN, the 10H-spiro[acridine-9,9′-fluorene] (SAF) substituted SPCN demonstrates higher photoluminescence quantum yield (PLQY) and improved radiative transition rate (kr). With the same device structure, despite that SPCN and DPCN show similar electroluminescence (EL) spectra, the EL efficiency of SPCN is about 1.5 times that of DPCN. Especially the non-doped OLED based on SPCN, the maximum external quantum efficiency (EQE) is 7.1%, which is a competitive result among the conventional blue-emissive fluorescent emitters. More importantly, this work provides a general strategy to achieve high performance blue/deep-blue OLED by rational donor optimization.

Hydroxy-Tetraphenylimidazole Derivatives as Efficient Blue Emissive Materials for Electroluminescent Devices.

Herein, three hydroxy-tetraphenylimidazole (HPI)-based fluorophores (HPI-TPA, HPI-PCz, and HPI-CzP) are designed and synthesized by disubstituted HPI core with arylamine units of triphenylamine (TPA), phenyl carbazole (PCz), and carbazole phenyl (CzP) at 3,5-positions of the N-phenyl ring of HPI, respectively. Their photophysical properties are theoretically and experimentally examined. HPI-TPA shows a hybridized local and charge transfer (HLCT) excited state characteristic and emits deep blue color via an HLCT mechanism, while both HPI-PCz and HPI-CzP exhibit excited-state intramolecular proton transfer (ESIPT) property and display pure keto form emissions. They possess high thermal stability and are successfully fabricated as emitters in organic light-emitting diodes (OLEDs). All devices exhibit intense blue color emissions with low turn-on voltages (3.5-3.7 V). Particularly, HPI-TPA-based OLED emits light in the deep-blue region with a high maximum external quantum efficiency (EQEmax ) of 3.77% and a decent efficiency roll-off.

Anthracene-based blue fluorescence materials utilized in non-doped OLEDs with high luminance and a low efficiency roll-off

In this work, we designed and synthesized two novel blue-emitting compounds (m-TAC and p-TAC) with a twisted D-π-A configuration by incorporation of 2,4,6-triphenyl-1,3,5-triazine as an acceptor and carbazole as a donor through an anthracene π-spacer. Therein, p-TAC exhibits a hybridized local and charge-transfer (HLCT) characteristic with a sky-blue emission at 462 nm in toluene while m-TAC shows emission maximum at 431 nm. The two compounds exhibit superb thermal stability in the company of a thermal decomposition temperature (Td) as high as 507 and 516 °C. It is noticed that the non-doped devices with m-TAC and p-TAC as blue emitters display a turn-on voltage (Von) of 3.2 V and achieve sky-blue emission at 460 and 476 nm. In particular, the optimized device based on p-TAC realizes a maximum luminance above 10 000 cd m−2 and the maximum external quantum efficiency (EQEmax) reaching 5.03%. As a sky-blue HLCT emitter, p-TAC reveals high-efficiency electroluminescence (EL) performance, providing a new insight for further industrial applications.

Mild donor-π-mild acceptor (mD-π-mA) benzimidazole-based deep blue fluorophores with hybridized local and charge transfer (HLCT) excited states for OLEDs

The design of an efficient pure blue emitter to achieve stable, long operating organic light-emitting diode (OLED) devices still poses a significant challenge. Although through phosphorescence and thermally activated delayed fluorescence concepts, efficient and stable monochrome green and red OLEDs can be realized, the design of stable and efficient blue emitter has been a significant challenge. All efforts so far have resulted in severe efficiency roll-off and limited device lifetime. Therefore, developing efficient blue-emitting fluorescence materials with little or no efficiency roll-off is of great importance for commercial display OLEDs. In this context, we have designed and synthesized two pure blue fluorescence light-emitting materials that are thermally stable and have improved photophysical properties: 2-(4″-(1-(4-(tert-butyl)phenyl)-4,5-diphenyl-1H-imidazol-2-yl)-[1,1′:4′,1′′-terphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (PTBIBI) and 2-(4′′-(4,5-diphenyl-1-(3-(trifluoromethyl)phenyl)-1H-imidazol-2-yl)-[1,1′:4′,1′′-terphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (MCFBIBI). These emitters possess a hybrid local and charge-transfer (HLCT) state and have high photoluminescence quantum yields (>90%). The doped devices based on PTBIBI display a reasonably good device performance with the Commission International de l’Eclairage (CIE) coordinates of (0.15, 0.06) in the deep blue region and g maximum luminance of 6559 cd m−2 at a very low turn-on voltage (3.2 V) corresponding to the bandgap value of the blue emitter.

Highly Efficient Sensitized Chiral Hybridized Local and Charge‐Transfer Emitter Circularly Polarized Electroluminescence

AbstractThis work describes the first hot exciton fluorescent material based on benzo[c][1,2,5]thiadiazole and chiral binaphthol enabling circularly polarized luminescence (CPL) through a chiral perturbation strategy. The new molecular architecture displays CPL, hybridized local and charge transfer (HLCT) properties concurrently. Utilizing it as the emitter, circularly polarized organic light‐emitting diodes (CP‐OLEDs) achieve an external quantum efficiency (EQE) of 7.2% with a good exciton utilization (36%) and a moderate circularly polarized electroluminescence (CPEL) dissymmetry factor (gEL, 2.1 × 10−3). In addition, the CP‐HLCT molecule is sensitized by a thermally activated delayed fluorescence material, significantly ameliorating the efficiency of HLCT fluorescent CP‐OLEDs. Excellent performances of twofold maximum EQE (EQEmax) of 15.3% and 82% exciton utilization are obtained in the sensitized device, regarding an extremely low‐efficiency roll‐off of 2.6% at 1000 cd m−2 as well as CPEL with a gEL value of 2.0 × 10−3.