Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes

Charge transporting and thermally activated delayed fluorescence materials for OLED applications

The design and synthesis of effective charge transporting (CT) and thermally activated delayed fluorescence (TADF) materials are in high demand to obtain high-performing OLED devices. Recently, the significant development in the field of OLEDs has led to the creation of numerous charge transporting and TADF materials with diverse structures. To further improve the device performance, a better understanding of the structural characteristics and structure-property relationships of these materials is essential. Moreover, to enhance the efficiency of OLEDs, all the electrogenerated excitons should be constrained in EMLs. The TADF mechanism can theoretically register 100% IQE through a potent up-conversion method from non-radiative triplet excitons to radiative singlet excitons. In this review, the structural importance, classification, physical properties, and electroluminescence data of some recent charge transporting and TADF materials are summarized and discussed. Moreover, their molecular structural dependence on functional groups and linkers is classified, which can enhance their charge transporting or emitting ability. To offer a potential roadmap for the further development of charge transporting and TADF materials, it is hoped that this study will encourage researchers to acknowledge their important role in OLEDs.

Molecularly induced order promotes charge separation through delocalized charge-transfer states at donor-acceptor heterojunctions

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer (CT) states into free charge carriers (FCC) in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by molecular arrangement. We experimentally determine the CT dissociation properties of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors above 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher structural order of B4PYMPM molecules leads to more delocalized CT states. Furthermore, we find no correlation between the quantum efficiency of FCC radiative recombination and the bound or unbound nature of the CT states. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient FCC generation and shows that less bound CT states do not preclude efficient radiative recombination.

Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design

The complex photokinetics of donor-acceptor-donor triads with varying flexible spacer lengths (n = 4-10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady-state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in a liquid solution, dynamic charge-transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli-responsive high-contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in-depth analysis of complex photophysics can be put to practical use in materials science.

Highly Efficient OLEDs by Using a Brominated Thermally Activated Delayed Fluorescent Host due to Balanced Carrier Transport and Enhanced Exciton Upconversion

Thermally activated delayed fluorescent (TADF) materials are naturally bipolar and can potentially serve as hosts. However, triplet excitons in TADF materials are long-lived and prone to unfavorable bimolecular processes. Implementing an efficient reverse system intersection (RISC) process is an effective solution. Moreover, although the general TADF host is bipolar, polarity differences still cause a mobility imbalance. In this work, we designed and synthesized a novel TADF host material, 11-(3-(4-(3-bromophenyl)-6-phenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (Br-DMIC-TRZ). The upconversion of the TADF host and its doped films is facilitated due to enhanced spin-orbit coupling (SOC) induced by bromine, which exhibits a higher rate of RISC. This progress facilitates the involvement of more triplet excitons in luminescence. Meanwhile, the attachment of bromine to the acceptor fragment of TADF enhances the electron mobility, where hole mobility and electron mobility are more comparable. Enhanced exciton upconversion and balanced carrier transport allow devices formed based on brominated TADF hosts to outperform other hosts. The Br-TADF-based devices with three dopants sensitized achieved improvements of 29.8, 21.4, and 24.4% compared to the DMIC-TRZ-based device. This work provides a feasible molecular design strategy for further developing efficient hosts.

Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Recombination of X-ray-Generated Radical Ion Pairs in Alkane Solution Assembles Optically Inaccessible Exciplexes from a Series of Perfluorinated para-Oligophenylenes with N,N-Dimethylaniline

We demonstrate that a series of perfluorinated para-oligophenylenes C₆F₅-(C₆F₄)_n-C₆F₅ (n = 1-3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7-4.6 × 10⁴ M^-1cm^-1, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters.

Dibenzodipyridophenazines with Dendritic Electron Donors Exhibiting Deep-Red Emission and Thermally Activated Delayed Fluorescence

The development of deep-red thermally activated delayed fluorescence (TADF) emitters is important for applications such as organic light-emitting diodes (OLEDs) and biological imaging. Design strategies for red-shifting emission include synthesizing rigid acceptor cores to limit nonradiative decay and employing strong electron-donating groups. In this work, three novel luminescent donor-acceptor compounds based on the dibenzo[a,c]dipyrido[3,2-h:20-30-j]-phenazine-12-yl (BPPZ) acceptor were prepared using dendritic carbazole-based donors 3,3″,6,6″-tetramethoxy-9'H-9,3':6',9″-tercarbazole (TMTC), N³,N³,N⁶,N⁶-tetra-p-tolyl-9H-carbazole-3,6-diamine (TTAC), and N³,N³,N⁶,N⁶-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (TMAC). Here, dimethoxycarbazole, ditolylamine, and bis(4-methoxyphenyl)amine were introduced at the 3,6-positions of carbazole to increase the strength of these donors and induce long-wavelength emission. Substituent effects were investigated with experiments and theoretical calculations. The emission maxima of these materials in toluene were found to be 562, 658, and 680 nm for BPPZ-2TMTC, BPPZ-2TTAC, and BPPZ-2TMAC, respectively, highlighting the exceptional strength of the TMAC donor, which pushes the emission into the deep-red region of the visible spectrum as well as into the biological transparency window (650-1350 nm). Long-lived emission lifetimes were observed in each emitter due to TADF in BPPZ-2TMC and BPPZ-2TTAC, as well as room-temperature phosphorescence in BPPZ-2TMAC. Overall, this work showcases deep-red emissive dendritic donor-acceptor materials which have potential as bioimaging agents with emission in the biological transparency window.

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative

The exciplex-thermally activated delayed fluorescence (exciplex-TADF) system is an excellent candidate for the fabrication of high-efficiency organic light-emitting diodes (OLEDs) because of its more easily achieved small singlet-triplet energy splitting (ΔE_ST) and doping control. However, exciplex-TADF is still faced with the problems of low external quantum efficiency (η_ext) and unclear effect of structure modification in electron acceptors. Herein, we provide a steric hindrance increase strategy to obtain high-efficiency exciplex emissions. Through introducing a 9-phenylfluorene group into N-ethylcarbazole of the dicyano-substituted 9-phenylfluorene, an electron acceptor material with increased steric hindrance is obtained, which helps the exciplex harvest a larger driving force and higher emission efficiencies. Encouragingly, the obtained OLED displays a maximum η_ext of 25.8%, which is one of the best efficiency values among reported exciplex-OLEDs, simultaneously possessing excellent current efficiency of 83.6 cd A^-1 and power efficiency of 93.7 lm W^-1. It is expected that this work will offer a new avenue for designing electron acceptors for highly efficient exciplex emissions.

Cationic Zinc(II) Complexes with Carbazole-Type Counter-Anions: Intracomplex Donor/Acceptor Pairs Affording Exciplexes with Thermally Activated Delayed Fluorescence

Two cationic zinc(II) complexes with carbazole-type counter-anions, namely, [Zn(tpy)₂]²⁺[CAZ-p-BF₃^-]₂ (Zn-p) and [Zn(tpy)₂]²⁺[CAZ-o-BF₃^-]₂ (Zn-o), have been designed and synthesized, where tpy is 2,2':6',2″-terpyridine, CAZ-p-BF₃^- is 4-((9H-carbazol-9-yl)phenyl)trifluoroborate, and CAZ-o-BF₃^- is (2-(9H-carbazol-9-yl)phenyl)trifluoroborate. The complex cation [Zn(tpy)₂]²⁺ (as the acceptor) and the carbazole-type counter-anion CAZ-p-BF₃^- or CAZ-o-BF₃^- (as the donor) form an intracomplex donor/acceptor pair. Single-crystal structures reveal that compared to Zn-p, Zn-o exhibits a stronger π-π stacking interaction between the carbazole group (as the donor unit) of the counter-anion and the tpy ligand (as the acceptor unit) of [Zn(tpy)₂]²⁺ because of the different anchoring position of the BF₃^- anion in the counter-anion. In a doped film, Zn-p and Zn-o afford an isolated exciplex formed between the carbazole group and the tpy ligand within the single complex, which gives green-yellow emission with a thermally activated delayed fluorescence (TADF) feature. In crystalline states, Zn-p and Zn-o afford exciplexes with blue emission centered at 468 nm and green-blue emission centered at 508 nm, respectively. The Zn-p crystalline sample shows a relatively large singlet-triplet energy gap (ΔE_ST) (0.33 eV) and no TADF, whereas the Zn-o crystalline sample exhibits a small ΔE_ST (0.06 eV) and distinct TADF, with a reverse intersystem crossing rate at 3.3 × 10⁵ s^-1. Zn-p and Zn-o both exhibit intriguing mechanochromic luminescence, with largely red-shifted (by over 70 nm) emission and modulated TADF properties upon mechanically grinding the crystalline samples. The work demonstrates that donor/acceptor pairs affording exciplexes can be formed within cationic metal complexes using counter-anions with donor nature, which opens a new avenue toward photo-active metal complexes with rich photophysical properties.

Delayed Luminescence in 2-Methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole Derivatives

2,5-Diphenyl-1,3,4-oxadiazole has been widely used as an acceptor portion of donor-acceptor fluorophores that exhibit thermally activated delayed fluorescence (TADF), but analogous 2-alkyl-5-phenyl-1,3,4-oxadiazoles have been much less widely investigated. Here the properties of carbazole-substituted 2-methyl-5-phenyl-1,3,4-oxadiazoles are compared to those of their 2,5-diphenyl analogues. The fluorescence of each of the former compounds is blue-shifted by ca. 50-100 meV relative to that in the latter, while similar estimated values of the singlet-triplet energy separation (ΔE_ST) are maintained. In particular, 2-methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole and 2-methyl-5-(penta(3,6-di-tert-butyl-9-carbazolyl)phenyl)-1,3,4-oxadiazole exhibit solution fluorescence maxima of 466 and 485 nm and estimated ΔE_ST values of 0.12 and 0.03 eV, respectively. In both cases the reverse intersystem crossing (RISC) rates inferred from their solution fluorescence behavior are over twice those of the corresponding 2-phenyl derivatives. Organic light-emitting diodes (OLEDs) in which the 2-methyl derivatives are used as emitters yield external quantum efficiency (EQE) values of up to 23%. OLEDs with 2-methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole and 2-methyl-5-(penta(3,6-di-tert-butyl-9-carbazolyl)phenyl)-1,3,4-oxadiazole emitters show reduced efficiency rolloff at high current densities relative to their 2-phenyl counterparts, the latter exhibiting an EQE of 16% at 1000 cd m^-2.

Mode-tunable, micro/nanoscale electrohydrodynamic deposition techniques for optoelectronic device fabrication

The rapid development of fascinating new optoelectronic materials and devices calls for the innovative production of micro/nanostructures in a high-resolution, large-scale, low-cost fashion, preferably compatible with flexible/wearable applications. Powerful electrohydrodynamic (EHD) deposition techniques, which generate micro/nanostructures using high electrical forces, exhibit unique advantages in high printing resolution (<1 μm), tunable printing modes (electrospray for films, electrospinning for fibers and EHD jet printing for dots), and wide material applicability (viscosity 1-10 000 cps), making them attractive in the fabrication of high-density and high-tech optoelectronic devices. This review highlights recent advances related to EHD-deposited optoelectronics, ranging from solar cells, photodetectors, and light-emitting diodes, to transparent electrodes, with detailed descriptions of the EHD-based jetting mechanism, ink formulation requirements and corresponding jetting modes to obtain functional micro/nanostructures. Finally, a brief summary and an outlook on the future perspectives are proposed.

Advances in Blue Exciplex–Based Organic Light-Emitting Materials and Devices

Exciplexes possessing thermally activated delayed fluorescence (TADF) characteristics have received much attention in the fields of organic light-emitting materials and devices over the past decade. In general, an exciplex is a physical mixture between a donor (D) with hole transport properties and an acceptor (A) with electron transport characteristics, and the energy difference between the lowest excited singlet state and the lowest excited triplet state is usually fairly small in terms of the long-range charge-transfer process from D to A. In the processes of photoluminescence and electroluminescence, triplet excitons can be converted to singlet excitons through reverse intersystem crossing and then radiate photons to achieve TADF. As a consequence, triplet excitons can be effectively harvested, and the exciton utilization can be significantly enhanced. Up to now, a large number of exciplexes have been developed and applied to organic light-emitting devices. Notably most of them showed green or red emission, while blue exciplexes are relatively few owing to the spectrum characteristics of the large red-shift and broadened emission. In this study, the latest progress of blue exciplex–based organic light-emitting materials and devices is briefly reviewed, and future research is prospected.

Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers

In recent years, phosphorescent dyes forming ultrathin light-emitting layers (<1 nm, UEMLs) have been widely applied to fabricate monochromatic and white organic light-emitting diodes (OLEDs) owing to its merits of simplified device structure and preparation process, more flexible design, lower material consumption, and complete exciton utilization. In addition, it was demonstrated that the OLEDs with UEMLs achieved high electroluminescence performance comparable to the conventional doping-based devices. Structurally, OLEDs were structured with phosphorescent UEMLs inserted into nonluminous materials, heterojunction interface as well as into luminescent materials including phosphorescent, conventional fluorescent, thermally activated delayed fluorescence, and exciplex emitters. We carefully reviewed the successful applications of UEMLs in OLEDs and underlying working mechanism of corresponding devices, and also emphasized the representative achievements about OLEDs with UEMLs, aimed at forming a comprehensive summary of the present research for UEMLs-based OLEDs. In the end, we also gave an outlook for the future development of UEMLs-based OLEDs

Modulating the optical properties and functions of organic molecules through polymerization

Organic functional materials with advanced optical properties have attracted much attention due to their broad applications, such as in light-emitting diodes, solar cells, anti-counterfeiting, photocatalysis, and even disease diagnosis and treatment. Recent research has revealed that many optical properties of organic molecules can be improved through simple polymerization. In this review, we discuss the phenomenon, mechanism, and impact of polymerization on the properties of materials, including the polymerization-induced spectral shift, polymerization-enhanced photosensitization, polymerization-enhanced two-photon absorption, polymerization-enhanced photocatalytic efficiency, polymerization-induced room temperature phosphorescence, polymerization-induced thermally activated delayed fluorescence, and polymerization-induced emission using specific examples with different applications. The new opportunities arising from polymerization in designing high performance optical materials are summarized in the future perspective.

Lower limits for non-radiative recombination loss in organic donor/acceptor complexes

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = ∼22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, k_nr, is on the order of 10⁵ s^-1, approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing k_nr. Furthermore, despite the high radiative efficiency and already comparatively slow k_nr, we find that k_nr is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

The mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for high-performance nondoped OLEDs are summarized. Their molecular structures depending on the functional A groups are further classified.

Theoretical studies on boron dimesityl-based thermally activated delayed fluorescence organic emitters: excited-state properties and mechanisms

At 300 K, S1 excitons could emit fluorescence or undergo ISC to T1, where rISC exceeds the phosphorescence emission enabling TADF.

Through-Space Charge-Transfer Emitters Developed by Fixing the Acceptor for High-Efficiency Thermally Activated Delayed Fluorescence

Through-space charge-transfer (TSCT) emitters have been extensively explored for thermally activated delayed fluorescence (TADF), but arranging various donors and acceptors into rigid cofacial conformations for various efficient TSCT TADF emitters has remained challenging. Here, we report a "fixing acceptor" design to reach various efficient TSCT TADF emitters. By chemically fixing the acceptor (benzophenone) with a rigid spiro-structure and cofacially aligning various donors with the fixed acceptor, a series of efficient TSCT TADF emitters have been developed. Single-crystal structures and theoretical calculations have verified closely packed cofacial donor/acceptor conformations and favorable TSCT in the emitters. In doped films, the emitters afford sky blue to yellow TADF emission, with high photoluminescence efficiencies up to 0.92 and reverse intersystem crossing rates up to 1.0 × 10⁶ s^-1. Organic light-emitting diodes using the emitters afford sky blue to yellow electroluminescence with high external quantum efficiencies up to 20.9%. The work opens a new avenue toward a wide variety of efficient TSCT TADF emitters.

Increasing donor-acceptor spacing for reduced voltage loss in organic solar cells

The high voltage losses ([Formula: see text]), originating from inevitable electron-phonon coupling in organic materials, limit the power conversion efficiency of organic solar cells to lower values than that of inorganic or perovskite solar cells. In this work, we demonstrate that this [Formula: see text] can in fact be suppressed by controlling the spacing between the donor (D) and the acceptor (A) materials (DA spacing). We show that in typical organic solar cells, the DA spacing is generally too small, being the origin of the too-fast non-radiative decay of charge carriers ([Formula: see text]), and it can be increased by engineering the non-conjugated groups, i.e., alkyl chain spacers in single component DA systems and side chains in high-efficiency bulk-heterojunction systems. Increasing DA spacing allows us to realize significantly reduced [Formula: see text] and improved device voltage. This points out a new research direction for breaking the performance bottleneck of organic solar cells.

Identifying lifetime as one of the key parameters responsible for the low brightness of lanthanide-based OLEDs

OLEDs based on lanthanide complexes have decisive optical advantages but are hampered by low brightness. Despite the efforts to optimize several parameters such as quantum yield and charge carrier mobility, there seems to be another key parameter that hinders their performances. Experimental data are therefore collected for mixed-ligand europium complexes with bathophenanthroline and different classes of anionic ligands and screened to identify the key parameter responsible for this situation, which turns out to be the long lifetime of their excited states. A broad literature search supports this conclusion, showing that lanthanide complexes are inferior to other classes of OLED emitters often because of their long lifetimes; furthermore, among a series of lanthanide complexes, the best results are achieved for those with the shortest lifetimes, even though they suffer from low quantum yields.

Efficient Exciplex-Based Deep-Blue Organic Light-Emitting Diodes Employing a Bis(4-fluorophenyl)amine-Substituted Heptazine Acceptor

The realization of a deep-blue-emitting exciplex system is a herculean task in the field of organic light-emitting diodes (OLEDs) on account of a large red-shifted and broadened exciplex emission spectrum in comparison to those of the corresponding single compounds. Herein, 2,5,8-tris(di(4-fluorophenyl)amine)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3FDPA) was designed as an electron acceptor by integrating three bis(4-fluorophenyl)amine groups into a heptazine core, while 1,3-di(9H-carbazol-9-yl)benzene (mCP) possessing two electron-donating carbazole moieties was chosen as the electron donor. Excitingly, the exciplex system of 8 wt% HAP-3FDPA:mCP exhibited deep-blue emission and a high photoluminescence quantum yield of 53.2%. More importantly, an OLED containing this exciplex system as an emitting layer showed deep-blue emission with Commission Internationale de l'Eclairage coordinates of (0.16, 0.12), a peak luminance of 15,148 cd m-2, and a rather high maximum external quantum efficiency of 10.2% along with a low roll-off. This study not only reports an efficient exciplex-based deep-blue emitter but also presents a feasible pathway to construct highly efficient deep-blue OLEDs based on exciplex systems.

Heptazine-Based π-Conjugated Materials for Light-Emitting

On the basis of planar and relatively rigid nitrogen-rich heterocyclic system of the heptazine core, heptazine-based π-conjugated materials have aroused widespread attention over the past decade by virtue of the fascinating electronic, optical, thermal, and mechanical properties in the fields of light-emitting, photocatalysis, sensors, environmental remediation, and so forth. However, there are still several obstacles to be solved before practical applications, such as low photoluminescence quantum efficiencies for light-emitting and weak visible absorption for photocatalysis. To further enhance various properties of heptazine-based π-conjugated materials, a series of strategies have been developed, including ingenious molecular design and modification, novel synthetic, and preparation methods. In this review, the significant progress of monomeric and polymeric heptazine-based π-conjugated materials and their applications typically in light-emitting are reviewed, which is beneficial for the acceleration of practical applications of heptazine-based materials and devices.

Positive impact of chromophore flexibility on the efficiency of red thermally activated delayed fluorescence materials

Rigid electron donors (D) and acceptors (A) have been widely used in recent years for the construction of D-A type thermally activated delayed fluorescence (TADF) materials. However, the chromophore robustness does not always make a positive contribution to the high efficiency of TADF materials. Here, the comparison study of two D-A type red TADF compounds (PT-TPA and PT-Az) demonstrated, for the first time, the positive impact of chromophore flexibility on the efficiency of TADF materials. In PT-Az, the rotation of terminal phenyl groups is restrained by an ethylene linker, leading to its inferior photoluminescence quantum yield (PLQY). In contrast, PT-TPA with free rotation of the phenyl groups showed a low reorganization energy and a large transition dipole moment for the S₁→ S₀ transition, which resulted in a high fluorescence radiative decay rate. As a result, the optimized devices based on PT-TPA gave a maximum external quantum efficiency (EQE) of 29.7% (632 nm) when doped in a single host and an EQE of 28.8% (648 nm) in an exciplex host. This study provided an insight into the impact of chromophore flexibility on the photophysical properties and device efficiency of TADF materials, and these results may provide valuable guidance for the molecular design of efficient emitters.