Axially Chiral TADF‐Active Enantiomers Designed for Efficient Blue Circularly Polarized Electroluminescence

The use of a chiral, emitting skeleton for axially chiral enantiomers showing activity in thermally activated delayed fluorescence (TADF) with circularly polarized electroluminescence (CPEL) is proposed. A pair of chiral stable enantiomers, (?)‐(S)‐Cz‐Ax‐CN and (+)‐(R)‐Cz‐Ax‐CN, was designed and synthesized. The enantiomers, both exhibiting intramolecular π‐conjugated charge transfer (CT) and spatial CT, show TADF activities with a small singlet–triplet energy difference (ΔE_ST) of 0.029 eV and mirror‐image circularly polarized luminescence (CPL) activities with large g_lum values. Notably, CP‐OLEDs based on the enantiomers feature blue electroluminescence centered at 468 nm with external quantum efficiencies (EQEs) of 12.5 and 12.7 %, and also show intense CPEL with g_EL values of ?1.2×10^?2 and +1.4×10^?2, respectively. These are the first CP‐OLEDs based on TADF‐active enantiomers with efficient blue CPEL.     

Axially Chiral TADF-Active Enantiomers Designed for Efficient Blue Circularly Polarized Electroluminescence

The use of a chiral, emitting skeleton for axially chiral enantiomers showing activity in thermally activated delayed fluorescence (TADF) with circularly polarized electroluminescence (CPEL) is proposed. A pair of chiral stable enantiomers, (−)-(S)-Cz-Ax-CN and (+)-(R)-Cz-Ax-CN, was designed and synthesized. The enantiomers, both exhibiting intramolecular π-conjugated charge transfer (CT) and spatial CT, show TADF activities with a small singlet–triplet energy difference (ΔE_ST) of 0.029 eV and mirror-image circularly polarized luminescence (CPL) activities with large g_lum values. Notably, CP-OLEDs based on the enantiomers feature blue electroluminescence centered at 468 nm with external quantum efficiencies (EQEs) of 12.5 and 12.7 %, and also show intense CPEL with g_EL values of −1.2×10⁻² and +1.4×10⁻², respectively. These are the first CP-OLEDs based on TADF-active enantiomers with efficient blue CPEL.     

TADF-Sensitized Fluorescent Enantiomers: A New Strategy for High-Efficiency Circularly Polarized Electroluminescence**

A promising strategy of thermally activated delayed fluorescence (TADF) sensitized circularly polarized luminescence (CPL) has been proposed for improving the electroluminescence efficiencies of circularly polarized fluorescent emitters. Compared with chiral TADF emitters which suffer from the dilemma of small ΔE_ST accompanied by small k_r, the TADF-sensitized CPL (TSCP) strategy using TADF molecules as sensitizers and CP-FL molecules as emitters might be the most promising method to construct high-performance circularly polarized organic light-emitting diodes (CP-OLEDs). Consequently, by taking advantage of the theoretically 100 % exciton utilization of TADF sensitizers, especially, by designing CP-FL emitters with high PLQY, narrow FWHM and large g_lum values, TSCP-type CP-OLEDs with excellent overall performances can be realized.     

Remarkable mechanochromism and force-induced thermally activated delayed fluorescence enhancement from white-light-emitting organic luminogens with aggregation-induced emission

The development of organic materials with white-light emission and thermally activated delayed fluorescence (TADF) properties in the solid state remain a challenge. Herein, a series of white-light-emitting organic luminogens have been developed and are found to show aggregation-induced delayed fluorescence (AIDF) characteristics. The AIDF emitters present dual-emission consisted of prompt fluorescence and TADF in the crystalline state. Their white-light emissions can be easily tuned by altering the chemical structure and connecting position of the heterocyclic aromatic substituent. Under the stimuli of mechanical force and solvent vapor, the compounds exhibit remarkable and reversible mechanochromism, in which their emission colors are switchable between white and yellow. Upon grinding, they also display linearly tunable luminescence colors, as well as force-induced TADF enhancement, which may be associated with the more compact molecular packing and the restriction of intramolecular motions. The results from time-resolved emission scanning and theoretical calculation suggest that the dual-emission of the AIDF luminogens likely results from the twisted intramolecular charge transfer transitions of the molecules, and the reversible mechanochromism properties probably stem from the interconversion of the quasi-axial and the quasi-equatorial conformations.     

基于卤素取代邻苯二甲酰亚胺新型有机发光材料的合成及其性能研究(英文)

方便地合成了三个含有卤素取代邻苯二甲酰亚胺与咔唑基团的新型有机发光材料Br-Al-Cz,Cl-Al-Cz和F-AI-Cz,发现它们不仅具有强的聚集诱导发光效应,而且显示膜态下热激活延迟荧光以及晶态诱导的室温磷光性质.尤其是化合物Br-Al-Cz表现出肉眼可见的长余辉室温磷光现象,因此在数据加密等中具有潜在用途.     

Versatile electrochemiluminescent organic emitters

Organic luminophores for electrochemiluminescence (ECL), namely polycyclic aromatic hydrocarbons, have been the first molecules investigated since the beginning of ECL studies. Moving from organic solvents to water-based solutions in view of analytical applications, the attention on ECL emitters shifted to soluble inorganic complexes, which prevailed in both fundamental and applied research. However, the investigation of organic molecules has recently revived owing to new synthetic procedures and concepts. Polymeric nanoparticles, surface functionalisation, aggregation-induced emission (AIE), and thermally activated delayed fluorescence (TADF) sparked the research with renovated interest for organic molecules. Here, we introduce and summarise these new concepts behind organic emitters for ECL.     

Aggregation-induced emission and thermally activated delayed fluorescence of 2,6-diaminobenzophenones

Exploration of novel organic luminophores that exhibit thermally activated delayed fluorescence (TADF) in the aggregated state is very crucial for advance of delayed luminescence-based applications such as time-gated bio-sensing and temperature sensing. We report herein that synthesis, photophysical properties, molecular and crystal structures, and theoretical calculations of 2,6-bis (diarylamino)benzophenones. Absorption spectra in solution and calculations using density functional theory (DFT) method revealed that the optical excitation took place through intramolecular charge-transfer from one diarylamino moiety to an aroyl group. While the benzophenones did not luminesce in solution, the solids of the benzophenones emitted green light with moderate-to-good quantum yields. Thus, the benzophenones exhibit aggregation-induced emission. Based on the lifetime measurement, the green emission of the solids was found to include TADF. The emergence of the TADF is supported by the small energy gap between the excited singlet and triplet states, which was estimated by time-dependent DFT calculations. Thin films of poly(methyl methacrylate) doped by the benzophenones also showed green prompt and delayed fluorescence whose lifetimes were in the order of microseconds. Linear correlation between logarithm value of TADF lifetime and temperature was observed with the benzophenone in powder, suggesting that the benzophenones can serve as molecular thermometers workable under aqueous conditions.     

Stable Enantiomers Displaying Thermally Activated Delayed Fluorescence: Efficient OLEDs with Circularly Polarized Electroluminescence

Aromatic‐imide‐based thermally activated delayed fluorescent (TADF) enantiomers, (+)‐(S,S)‐ CAI‐Cz and (?)‐(R,R)‐ CAI‐Cz , were efficiently synthesized by introducing a chiral 1,2‐diaminocyclohexane to the achiral TADF unit. The TADF enantiomers exhibited high PLQYs of up to 98 %, small ΔE_ST values of 0.06 eV, as well as obvious temperature‐dependent transient PL spectra, thus demonstrating their excellent TADF properties. Moreover, the TADF enantiomers showed mirror‐image CD and CPL activities. Notably, the CP‐OLEDs with CPEL properties based on the TADF enantiomers not only achieved high EQE values of up to 19.7 and 19.8 %, but also displayed opposite CPEL signals with g_EL values of ?1.7×10^?3 and 2.3×10^?3, which represents the first CP‐OLEDs, based on the enantiomerically pure TADF materials, having both high efficiencies and intense CPEL.     

Asymmetric Thermally Activated Delayed Fluorescence Materials Rendering High-performance OLEDs Through both Thermal Evaporation and Solution-processing

Exploring high-efficiency thermally activated delayed fluorescence(TADF) materials is of great importance regarding to organic light-emitting diode(OLED). Herein, we present a design strategy for developing asymmetric TADF materials based on a diphenyl sulfone-phenoxazine structure, resulting in efficient TADF emitters(CzPXZ and t-CzPXZ) with aggregation-induced emission properties, while t-CzPXZ is modified with tert-butyl groups. The two compounds exhibit high solid-state luminescence, efficient TADF, and significantly impressive device performances by both thermal evaporation and solution processing. For an instance, CzPXZ and t-CzPXZ enable the thermally-evaporated OLEDs with high external quantum efficiencies(EQEs) of over 20%. Meanwhile, t-CzPXZ allows the solution-processed device with a high EQE of 16.3% with low-efficiency roll-off, attributing to the enhanced molecular solubility and suppressed excitons quenching through tert-butyl modification on t-CzPXZ. The results reveal that the proposed asymmetric structure is a promising approach for developing high-efficiency TADF materials and OLEDs.     

Luminescent Chiral Exciplexes with Sky-Blue and Green Circularly Polarized-Thermally Activated Delayed Fluorescence

Luminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with g_lum up to 7×10⁻³, one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone (g_lum ∼7×10⁻⁴). This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge-transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor-acceptor model. Interestingly, blue, green-yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi-color CP-TADF emitters.     

Polymerized Thermally Activated Delayed-Fluorescence Small Molecules: Long-Axis Polymerization Leads to a Nearly Concentration-Independent Luminescence

Nowadays numerous thermally activated delayed fluorescence (TADF) polymers have been developed for PLEDs to realize high device performance and tunable emission colors. However, they often possess a strong concentration dependence on their luminescence including aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE). Herein, we first report a nearly concentration-independent TADF polymer based on the strategy of polymerized TADF small molecules. It is found that when a donor-acceptor-donor (D-A-D) type TADF small molecule is polymerized through its long-axis direction, the triplet state is distributed along the polymeric backbone to effectively suppress the unwanted concentration quenching. Unlike the short-axis one with an ACQ effect, the photoluminescent quantum yield (PLQY) of the resultant long-axis polymer remains almost unchanged with the increasing doping concentration. Accordingly, a promising external quantum efficiency (EQE) up to 20 % is successfully achieved in a whole doping control window of 5–100 wt. %.     

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

Organic emitting compounds that are based on π‐conjugated skeletons have emerged as promising next‐generation materials for application in optoelectronic devices. In this Minireview, recent advances in the development of organic emitters that irradiate room‐temperature phosphorescence and/or thermally activated delayed fluorescence with extraordinary luminescence properties, such as aggregation‐induced emission, mechanochromic luminescence, and circularly polarized luminescence, are discussed.     

Small-molecule based thermally activated delayed fluorescence materials with dual-emission characteristics

Organic thermally activated delayed fluorescence(TADF)emitters have attracted increasing concerns,owing to their atypical photophysical features that can pave the way to the innovative engineering applications.As cutting-edge type of luminescent molecules,however,most of them only exert a single-wavelength emission from the lowest excited state,according to Kasha’s rule.To develop their potential applications in multicolor luminescence and multi-functional luminescent probes for biological imaging,researchers have begun to turn their attention to design organic TADF molecules with dual-emission characteristics,by employing an additional fluorescence,phosphorescence,or TADF signal within a single-component system.We herein summarized the design principles as well as the luminescence mechanism of organic donor-acceptor TADF compounds with dual-emission characteristics,the superiority of which can cover unique material applications in modern luminescencerelated fields.     

Reversible switching between normal and thermally activated delayed fluorescence towards “smart” and single compound whitelight luminescence via controllable conformational distribution

An organic compound exhibiting simultaneously reversible switch between its emission colors and luminescence mechanisms, possessing high contrast from deep blue normal fluorescence (NF) to yellow thermally activated delayed fluorescence (TADF), is reported. Based on these two complementary colors, white-light emission combining NF and TADF from a single compound can be achieved in various states. Experimental results and density functional theory calculations indicate that the controllable conformational distribution under thermal and mechanical activation is the mechanism responsible for the reversible switching behavior.     

Controlling Singlet–Triplet Energy Splitting for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters

The development of efficient metal‐free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep‐blue emission is still challenging. A new family of deep‐blue TADF emitters based on a donor–acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep‐blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).     

Mechanochromic luminescence of soft crystals: Recent systematic studies in controlling the molecular packing and mechanoresponsive properties

Soft crystals are a class of smart materials that can switch their photophysical or mechanical properties in response to gentle external stimuli. A representative stimuli-responsive behavior of soft crystals is mechanochromic luminescence (MCL), i.e., a reversible color change of solid-state photoluminescence induced by external mechanical stimuli. Together with the rapid growth in the area of solid-state photoluminescence including fluorescence, room-temperature phosphorescence (RTP), thermally activated delayed fluorescence (TADF), white-light emission (WLE), and circularly polarized luminescence (CPL), a number of soft crystals that exhibit MCL behaviors have been reported during the past decade. In the typical MCL of soft crystals, the emission color switches in the bathochromic direction upon amorphization by grinding and recovers to the original color upon recrystallization by heating or exposure to organic solvents. Relatively few are known to exhibit hypsochromically shifted MCL, two-step MCL, self-recovering MCL, or mechanical-stimuli-induced single-crystal-to-single-crystal (SCSC) transitions. Rational design guidelines to control the mechanoresponsive properties of soft crystals have not yet been established. This review summarizes the systematic studies on the substituent effect to control the MCL properties of soft crystals. Recent studies provide useful insights into the effects of electronic and steric differences of substituents on crystal structure, luminescence properties, and mechanoresponsive behaviors.     

甲基修饰的咔唑/二苯砜基AIE-TADF蓝光材料及其OLED器件

以四甲基咔唑为电子给体(D)、 二苯砜为电子受体(A)构建了具有D-A-D结构的纯有机咔唑/二苯砜衍生物——9,9'-[磺酰基双(3,1-亚苯基)]双(1,3,6,8-四甲基-9H-咔唑)(TMe-mSOCz). 对所合成材料的光物理性能研究表明, TMe-mSOCz表现出明显的聚集诱导发射(AIE)和热激活延迟荧光(TADF), 延迟寿命和延迟荧光占比分别为2.26 μs和47.7%, 并具有良好的电化学稳定性和热稳定性. 基于TMe-mSOCz作为非掺杂发光层制备了有机发光二极管(OLED)器件, 其启亮电压(V_on)为3.5 V, 最大外量子效率为5.63%, 国际照明委员会(CIE)坐标为(0.18, 0.26). 在1000 cd/m²亮度下, 非掺杂器件的效率滚降非常小(7.1%), 色彩稳定性较好, 其具有窄的半峰宽(FWHM=72 nm). 研究结果表明, 在传统TADF分子给受体间引入甲基修饰有利于开发具有AIE特性与更高效的D-A-D型TADF分子, 这为基于AIE-TADF分子开发新型OLED器件提供了新途径.     

Simultaneous enhancement of efficiency and stability of OLEDs with thermally activated delayed fluorescence materials by modifying carbazoles with peripheral groups

Science China Chemistry - Albeit their high efficiencies, the operational stability of the organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters is...     

1,1'-Binaphthol annulated perylene diimides: Aggregation-induced emission enhancement and chirality inversion

Aggregation-induced emission enhancement and aggregation-induced chirality inversion are two individual phenomena for the enantiomerically pure organic dyes in the aggregates. Herein we reported for the first time that these two interesting phenomena could be observed simultaneously in the aggregated states of enantiomerically pure S/R-1,1’-binaphthol annulated perylene diimides, in which two perylene diimides moieties were bridged by S/R-1,1’-binaphthol(BINOL) at the bay positions. Owing to the...     

Manipulating exciton dynamics of thermally activated delayed fluorescence materials for tuning two-photon nanotheranostics

Rational manipulation of energy utilization from excited-state radiation of theranostic agents with a donor–acceptor structure is relatively unexplored. Herein, we present an effective strategy to tune the exciton dynamics of radiative excited state decay for augmenting two-photon nanotheranostics. As a proof of concept, two thermally activated delayed fluorescence (TADF) molecules with different electron-donating segments are engineered, which possess donor–acceptor structures and strong emissions in the deep-red region with aggregation-induced emission characteristics. Molecular simulations demonstrate that change of the electron-donating sections could effectively regulate the singlet–triplet energy gap and oscillator strength, which promises efficient energy flow. A two-photon laser with great permeability is used to excite TADF NPs to perform as theranostic agents with singlet oxygen generation and fluorescence imaging. These unique performances enable the proposed TADF emitters to exhibit tailored balances between two-photon singlet oxygen generation and fluorescence emission. This result demonstrates that TADF emitters can be rationally designed as superior candidates for nanotheranostic agents by the custom controlling exciton dynamics.

Exciton dynamics can be manipulated rationally in the design of TADF materials for nanotheranostics. Regulating the ΔE_ST and f promises efficient energy flow for tailoring balances between singlet oxygen generation and fluorescence emission.