High-Performance Circularly Polarized Electroluminescence with Simultaneous Narrowband Emission, High Efficiency, and Large Dissymmetry Factor

Recent advances in circularly polarized luminescence of planar chiral organic compounds

Circularly polarized luminescence (CPL), as an important chiroptical phenomenon, can not only directly characterize excited-state structural information about chiroptical materials but also has great application prospects in 3D optical displays, information storage, biological probes, CPL lasers and so forth. Recently, chiral organic small molecules with CPL have attracted a lot of research interest because of their excellent luminescence efficiency, clear molecular structures, unique flexibility and easy functionalization. Planar chiral organic compounds make up an important class of chiral organic small molecular materials and often have rigid macrocyclic skeletons, which have important research value in the field of chiral supramolecular chemistry (e.g., chiral self-assembly and chiral host-guest chemistry). Therefore, research into planar chiral organic compounds has become a hotspot for CPL. It is time to summarize the recent developments in CPL-active compounds based on planar chirality. In this feature article, we summarize various types of CPL-active compounds based on planar chirality. Meanwhile, we overview recent research in the field of planar chiral CPL-active compounds in terms of optoelectronic devices, asymmetric catalysis, and chiroptical sensing. Finally, we discuss their future research prospects in the field of CPL-active materials. We hope that this review will be helpful to research work related to planar chiral luminescent materials and promote the development of chiral macrocyclic chemistry.

Switchable Dual Circularly Polarized Luminescence in Through-Space Conjugated Chiral Foldamers

Organic materials with switchable dual circularly polarized luminescence (CPL) are highly desired because they can not only directly radiate tunable circularly polarized light themselves but also induce CPL for guests by providing a chiral environment in self-assembled structures or serving as the hosts for energy transfer systems. However, most organic molecules only exhibit single CPL and it remains challenging to develop organic molecules with dual CPL. Herein, novel through-space conjugated chiral foldamers are constructed by attaching two biphenyl arms to the 9,10-positions of phenanthrene, and switchable dual CPL with opposite signs at different emission wavelengths are successfully realized in the foldamers containing high-polarizability substitutes (cyano, methylthiol and methylsulfonyl). The combined experimental and computational results demonstrate that the intramolecular through-space conjugation has significant contributions to stabilizing the folded conformations. Upon photoexcitation in high-polar solvents, strong interactions between the biphenyl arms substituted with cyano, methylthio or methylsulfonyl and the polar environment induce conformation transformation for the foldamers, resulting in two transformable secondary structures of opposite chirality, accounting for the dual CPL with opposite signs. These findings highlight the important influence of the secondary structures on the chiroptical property of the foldamers and pave a new avenue towards efficient and tunable dual CPL materials.

Circularly Polarized Organic Light-Emitting Diodes Based on Chiral Hole Transport Enantiomers

The circularly polarized organic light-emitting diodes (CP-OLEDs) demonstrate promising application in 3D display due to the direct generation of circularly polarized electroluminescence (CPEL). But the chiral luminescence materials face challenges as intricated synthetic route, enantiomeric separation, etc. Herein, fresh CP-OLEDs are designed based on chiral hole transport material instead of chiral emitters. A pair of hole transport enantiomers (R/S-NPACZ) exhibit intense dissymmetry factors (|gPL|) about 5.0 × 10-3. With R/S-NPACZ as hole transport layers, CP-OLEDs are fabricated employing six achiral phosphorescence and thermally activated delayed fluorescence (TADF) materials with different wavelengths, in consistence with the generated CPEL spectra. The CP-OLEDs based on achiral red, green, and blue iridium(III) complexes exhibit external quantum efficiencies (EQEs) of 14.9%, 30.7%, and 14.1% with |gEL| factors of 8.8 × 10-4, 2.3 × 10-3, and 2.0 × 10-3, respectively. Moreover, the devices using achiral blue, blueish-green, and green TADF materials display EQEs of 24.1%, 17.9%, and 25.4% with |gEL| factors of 1.0 × 10-3, 3.6 × 10-3, and 2.2 × 10-3, respectively. As far as known, it is the first example of CP-OLEDs based on chiral hole transport materials, which act as the organic circularly polarizers and have potential to generate CPEL from achiral luminescence materials.

Integration of fine-tuned chiral donor with hybrid long/short-range charge-transfer for high-performance circularly polarized electroluminescence

The synergistic integration of a fine-tuned chiral donor with a hybrid long/short-range charge-transfer mechanism offers an accessible pathway to construct highly efficient circularly polarized emitters. Consequently, a notable dissymmetry factor of 1.6 × 10-3, concomitantly with a record-setting maximum external quantum efficiency of 37.4%, is synchronously realized within a single embodiment.

A chiral bipolar host for efficient solution-processed circularly polarized OLEDs via a chirality energy transfer process

Chiral bipolar hosts of (R/S)-L-2mCPCN are synthesized, which show high singlet/triplet energy levels and clear circularly polarized luminescence. Employing racemic phosphorescent and TADF materials as emitting guest molecules, solution-processable CP-OLEDs based on such chiral hosts are obtained with an EQEmax of 10.7% and |gEL| values of 5.0 × 10-3.

Liquid-crystalline circularly polarised fluorescent emitters with a high luminescence dissymmetry factor

Chiral liquid-crystalline emitters based on 9,9-dimethyl-10-(4-(phenylsulfonyl)phenyl)-9,10-dihydroacridine and a functionalised binaphthol show smectic liquid crystal phases and circularly polarised blue fluorescence with a high luminescence dissymmetry factor |glum| of 0.13. Solution-processable organic light-emitting diodes (OLEDs) based on the enantiomers were explored.

A donor-acceptor cage for circularly polarized TADF emission

Herein, we report the first example of chiral donor-acceptor cage DA-2 displaying efficient circularly polarized thermally activated delayed fluorescence (CP-TADF) with |glum| values up to 2.1 × 10-3 and PLQY of 32%. A small ΔEST of 0.051 eV and quasi-parallel (θ = 6°) transition electric and magnetic dipole moments were realized from the through-space charge transfer interaction between the parallelly aligned donor and acceptor in DA-2. This D-A cage configuration has provided a novel design strategy for discovering potential efficient CP-TADF emitters.

High-Performance Ultraviolet Organic Light-Emitting Diodes Enabled by Double Boron-Oxygen-Embedded Benzo[m]tetraphene Emitters

Ultraviolet organic light-emitting diodes (UV OLEDs) have attracted increasing attention because of their promising applications in healthcare, industry, and agriculture; however, their development has been hindered by the shortage of robust UV emitters. Herein, we embedded double boron-oxygen units into nonlinear polycyclic aromatic hydrocarbons (BO-PAHs) to regulate their molecular configurations and excited-state properties, enabling novel bent BO-biphenyl (BO-bPh) and helical BO-naphthyl (BO-Nap) emitters with hybridized local and charge-transfer (HLCT) characteristics. They could be facilely synthesized in gram-scale amounts via a highly efficient two-step route. BO-bPh and BO-Nap showed strong UV and violet-blue photoluminescence in toluene with full width at half-maximum values of 25 and 37 nm, along with quantum efficiencies of 98 and 99%, respectively. A BO-bPh-based OLED showed high color purity UV electroluminescence peaking at 394 nm with Commission Internationale de l'Eclairage (CIE) coordinates of (0.166, 0.021). Moreover, the device demonstrated a record-high maximum external quantum efficiency (EQE) of 11.3%, achieved by successful hot exciton utilization. This work demonstrates the promising potential of double BO-PAHs as robust emitters for future UV OLEDs.

Helical-caging enables single-emitted large asymmetric full-color circularly polarized luminescence

Colorful circularly polarized luminescence materials are desired for 3D displays, information security and asymmetric synthesis, in which single-emitted materials are ideal owing to self-absorption avoidance, evenly entire-visible-spectrum-covered photon emission and facile device fabrication. However, restricted by the synthesis of chiral broad-luminescent emitters, the realization and application of high-performing single-emitted full-color circularly polarized luminescence is in its infancy. Here, we disclose a single-emitted full-color circularly polarized luminescence system (spiral full-color emission generator), composed of whole-vis-spectrum emissive quantum dots and chiral liquid crystals. The system achieves a maximum luminescence dissymmetry factor of 0.8 and remains an order of 10-1 in visible region by tuning its photonic bandgap. We then expand it to a series of desired customized-color circularly polarized luminescence, build chiral devices and further demonstrate the working scenario in the photoinduced enantioselective polymerization. This work contributes to the design and synthesis of efficient chiroptical materials, device fabrication and photoinduced asymmetric synthesis.

Time-Dependent Colorful Circularly Polarized Organic Ultralong Room Temperature Phosphorescence from a Single-Component Chiral Molecule

Colorful circularly polarized organic ultralong room temperature phosphorescence (CP-OURTP) materials have attracted much attention due to their superior optoelectronic properties for various applications. However, the development of colorful CP-OURTP materials in a single-component molecular system is currently facing great challenges. Herein, a feasible strategy is proposed to develop colorful CP-OURTP material from a single-component chiral molecule by introducing a chiral unit into the phosphorescence chromophore. A dual CP-OURTP band originated from inherent triplet excitons emission showing a lifetime of 946.44 ms and triplet-triplet annihilation induced delayed emission with a short lifetime of 209.91 ms as well as maximum asymmetry factors of ≈10-3 are realized. Owing to the changed OURTP intensity ratios between inherent CP-OURTP and delayed emission at different delayed times, time-dependent colorful CP-OURTP turned from yellow to green is obtained. This study provides a potential platform to prepare circularly polarized material systems showing colorful luminescent properties.

Constructing Highly Efficient Circularly Polarized Multiple-Resonance Thermally Activated Delayed Fluorescence Materials with Intrinsically Helical Chirality

Advanced circularly polarized multiple-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials synergize the advantages of circularly polarized luminescence (CPL), narrowband emission, and the TADF characteristic, which can be fabricated into highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with high color purity, directly facing the urgent market strategic demand of ultrahigh-definition and 3D displays. In this work, based on an edge-topology molecular-engineering (ETME) strategy, a pair of high-performance CP-MR-TADF enantiomers, (P and M)-BN-Py, is developed, which merges the intrinsically helical chirality into the MR framework. The optimized CP-OLEDs with (P and M)-BN-Py emitters and the newly developed ambipolar transport host PhCbBCz exhibit pure green emission with sharp peaks of 532 nm, full-width at half-maximum (FWHM) of 37 nm, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.29, 0.68). Importantly, they achieve remarkable maximum external quantum efficiencies (EQEs) of 30.6% and 29.2%, and clear circularly polarized electroluminescence (CPEL) signals with electroluminescence dissymmetry factors (gEL s) of -4.37 × 10-4 and +4.35 × 10-4 for (P)-BN-Py and (M)-BN-Py, respectively.

Pivotal role of transition density in circularly polarized luminescence

Realizing high luminescence dissymmetry factor (g) in circularly polarized luminescence (CPL) materials remains a big challenge, which necessitates understanding systematically how their molecular structure controls the CPL. Here we investigate representative organic chiral emitters with different transition density distributions and reveal the pivotal role of transition density in CPL. We rationalize that to obtain large g-factors, two conditions should be simultaneously satisfied: (i) the transition density for the S₁ (or T₁)-to-S₀ emission must be delocalized over the entire chromophore; and (ii) the chromophore inter-segment twisting must be restricted and tuned to an optimal value (∼50°). Our findings offer molecular-level insights into the CPL of organic emitters, with potential applications in the design of chiroptical materials and systems with strong CPL effects.

Energy-Efficient Stable Hyperfluorescence Organic Light-Emitting Diodes with Improved Color Purities and Ultrahigh Power Efficiencies Based on Low-Polar Sensitizing Systems

Multi-resonance (MR) molecules with thermally activated delayed fluorescence (TADF) are emerging as promising candidates for high-definition displays because of their narrow emission spectra. However, the electroluminescence (EL) efficiencies and spectra of MR-TADF molecules are highly sensitive to hosts and sensitizers when applied to organic light-emitting diodes (OLEDs), and the highly polar environments in devices often lead to significantly broadened EL spectra. In this study, a proof-of-concept TADF sensitizer (BTDMAC-XT) with low polarity, high steric hindrance, and concentration-quenching free feature is constructed, which acts as a good emitter in doped and non-doped OLEDs with high external quantum efficiencies (η_ext s) of 26.7% and 29.3%, respectively. By combining BTDMAC-XT with conventional low-polarity hosts, low-polarity sensitizing systems with a small carrier injection barrier and full exciton utilization are constructed for the MR-TADF molecule BN2. Hyperfluorescence (HF) OLEDs employing the low-polar sensitizing systems successfully improve the color quality of BN2 and afford an excellent η_ext of 34.4%, a record-high power efficiency of 166.3 lm W^-1 and a long operational lifetime (LT₅₀  = 40309 h) at an initial luminance of 100 cd m^-2 . These results provide instructive guidance for the sensitizer design and device optimization for energy-efficient and stable HF-OLEDs with high-quality light.

Binaphthol-based chiral host molecules for efficient solution-processed circularly polarized OLEDs

Two kinds of chiral hosts, named (R/S)-BN-mCP and (R/S)-BN-2mCP, are prepared. Solution processable circularly polarized organic light-emitting diodes (CP-OLEDs) based on the chiral hosts and achiral emitter Ir(mypp)₃ present the maximum external quantum efficiency (EQE_max) and dissymmetry factor values (g_EL) of 12.7%/-1.7 × 10^-3 and 17.1%/-1.3 × 10^-3, respectively. Using (R)-BN-2mCP as the chiral host and Ir(mypp)₃ and Ir(piq)₂(acac) as the achiral emitters, the solution-processed OLED exhibits a broad emission spectrum with the EQE_max of 12.1% and g_EL of -1.1 × 10^-3.

Multicolor hyperafterglow from isolated fluorescence chromophores

High-efficiency narrowband emission is always in the central role of organic optoelectronic display applications. However, the development of organic afterglow materials with sufficient color purity and high quantum efficiency for hyperafterglow is still great challenging due to the large structural relaxation and severe non-radiative decay of triplet excitons. Here we demonstrate a simple yet efficient strategy to achieve hyperafterglow emission through sensitizing and stabilizing isolated fluorescence chromophores by integrating multi-resonance fluorescence chromophores into afterglow host in a single-component copolymer. Bright multicolor hyperafterglow with maximum photoluminescent efficiencies of 88.9%, minimum full-width at half-maximums (FWHMs) of 38 nm and ultralong lifetimes of 1.64 s under ambient conditions are achieved. With this facilely designed polymer, a large-area hyperafterglow display panel was fabricated. By virtue of narrow emission band and high luminescent efficiency, the hyperafterglow presents a significant technological advance in developing highly efficient organic afterglow materials and extends the domain to new applications.

Induced CPL-Active Materials Based on Chiral Supramolecular Co-Assemblies

Circularly polarized luminescence (CPL) has attracted much interest due to its potential applications on chiral photonic techniques and optoelectronic materials science. As known, dissymmetry factor (g_em ) of CPL is one essential factor for evaluating the features of CPL-active materials. Much attention has focused on how to increase the g_em value, which is one of the most important issues for CPL practical applications. Recently, more and more works have demonstrated that chiral supramolecular could provide the significant strategy to improve the g_em value through the orderly helical superstructure of chiral building blocks. Normally, this kind of chiral supramolecular assembly process can be accompanied by chirality transfer and induction mechanism, which can promote the amplification effect on the induced CPL of achiral dyes. In this review, we fully summarized recent advances on the induced CPL-active materials of chiral supramolecular co-assemblies, their applications in circularly polarized organic light-emitting diodes (CP-OLEDs) and current challenges.

One-Pot Synthesis of Acetylacetonate-Based Isomeric Phosphorescent Cyclometalated Iridium(III) Complexes via Random Cyclometalation: A Strategy for Excited-State Manipulation

The excited-state manipulation of the phosphorescent iridium(III) complexes plays a vital role in their photofunctional applications. The development of the molecular design strategy promotes the creative findings of novel iridium(III) complexes. The current molecular design strategies for iridium(III) complexes mainly depend on the selective cyclometalation of the ligands with the iridium(III) ion, which is governed by the steric hindrance of the ligand during the cyclometalation. Herein, a new molecular design strategy (i.e., random cyclometalation strategy) is proposed for the effective excited-state manipulation of phosphorescent cyclometalated iridium(III) complexes. Two series of new and separable methoxyl-functionalized isomeric iridium(III) complexes are accessed by a one-pot synthesis via random cyclometalation, resulting in a dramatic tuning of the phosphorescence peak wavelength (∼57 nm) and electrochemical properties attributed to the high sensitivity of their excited states to the position of the methoxyl group. These iridium(III) complexes show intense phosphorescence ranging from the yellow (567 nm) to the deep-red (634 nm) color with high photoluminescence quantum yields of up to 0.99. Two deep-red emissive iridium(III) complexes with short decay lifetimes are further utilized as triplet emitters to afford efficient solution-processed electroluminescence with reduced efficiency roll-offs.

Remarkable White Circularly Polarized Electroluminescence Based on Chiral Co-assembled Helix Nanofiber Emitters

White circularly polarized organic light-emitting diodes (CP-WOLEDs) are of great significance in potential lighting sources and full-color 3D displays. However, High-performance white CP-EL sources are almost unexplored. We have constructed full-color CP-EL devices based on chiral co-assemblies by using three achiral conjugated pyrene-based dyes (BP, w-WP and c-WP) doped with chiral binaphthyl-based enantiomers (S-/R-M) as the EMLs through an intermolecular chirality induction mechanism. (S-/R-M)_0.2 -(c-WP)_0.8 films exhibit regular helix nanofibers under annealing treatment and emit strong white CPL. Significantly, remarkable CP-WOLEDs based on (S-/R-M)_0.2 -(c-WP)_0.8 were achieved with |g_EL | values as high as 6.2×10^-2 and an excellent CRI of 98 at the CIE coordinates of (0.33, 0.33). These are the highest g_EL and CRI values of reported CP-WOLEDs to date. This is the first achievement of CP-WOLEDs based on chiral co-assembled helix nanofiber emitters, and provides a valuable strategy with which to develop white CP-EL for future practical applications.