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A blend of two organic molecules excited by a simple LED light source can release the stored excitation energy slowly as ‘long persistent luminescence’ over periods of up to an hour. Long life light The ability of 'glow-in-the-dark' materials to slowly release energy as light over extended periods of time is used in several contexts, from emergency signage to optical imaging. Most of these materials make use of rare inorganic elements and require extreme processing conditions during fabrication. Ryota Kabe and Chihaya Adachi offer a different solution. They use the power of chemistry to control the excitation properties of organic molecules—which are already harnessed to great effect in the field of organic light-emitting diodes—and show how a blend of two simple molecules can be engineered to slowly release their stored energy as light over times spanning seconds to hours. Stability issues remain to be resolved (perhaps by encapsulation), but the transparent, soluble, flexible and colour-tunable nature of these new materials is attractive for a range of applications. Long persistent luminescence (LPL) materials—widely commercialized as ‘glow-in-the-dark’ paints—store excitation energy in excited states that slowly release this energy as light 1 . At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl 2 O 4 ) doped with europium and dysprosium, and exhibit emission for more than ten hours 2 . However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl 2 O 4 powders limits the transparency of LPL paints 1 . Here we show that an organic LPL (OLPL) system of two simple organic molecules that is free from rare elements and easy to fabricate can generate emission that lasts for more than one hour at room temperature. Previous organic systems, which were based on two-photon ionization, required high excitation intensities and low temperatures 3 . By contrast, our OLPL system—which is based on emission from excited complexes (exciplexes) upon the recombination of long-lived charge-separated states—can be excited by a standard white LED light source and generate long emission even at temperatures above 100 degrees Celsius. This OLPL system is transparent, soluble, and potentially flexible and colour-tunable, opening new applications for LPL in large-area and flexible paints, biomarkers, fabrics, and windows. Moreover, the study of long-lived charge separation in this system should advance understanding of a wide variety of organic semiconductor devices 4 .

Organic long-persistent luminescence (LPL) is an organic luminescence system that slowly releases stored exciton energy as light. Organic LPL materials have several advantages over inorganic LPL materials in terms of functionality, flexibility, transparency, and solution-processability. However, the molecular selection strategies for the organic LPL system still remain unclear. Here we report that the energy gap between the lowest localized triplet excited state and the lowest singlet charge-transfer excited state in the exciplex system significantly controls the LPL performance. Changes in the LPL duration and spectra properties are systematically investigated for three donor materials having a different energy gap. When the energy level of the lowest localized triplet excited state is much lower than that of the charge-transfer excited state, the system exhibits a short LPL duration and clear two distinct emission features originating from exciplex fluorescence and donor phosphorescence. Long-persistent luminescence can be realized in all-organic systems (OLPL) but still with unsatisfactory performance. Here the authors investigate the relationships between energy levels and emission properties in a series of OLPL materials and propose a design strategy for efficient OLPL performance.

Organic long-persistent-luminescent (OLPL) materials demonstrating hour-long photoluminescence have practical advantages in applications owing to their flexible design and easy processability. However, the energy absorbed in these materials is typically stored in an intermediate charge-separated state that is unstable when exposed to oxygen, thus preventing persistent luminescence in air unless oxygen penetration is suppressed through crystallization. Moreover, OLPL materials usually require ultraviolet excitation. Here we overcome such limitations and demonstrate amorphous OLPL systems that can be excited by radiation up to 600 nm and exhibit persistent luminescence in air. By adding cationic photoredox catalysts as electron-accepting dopants in a neutral electron-donor host, stable charge-separated states are generated by hole diffusion in these blends. Furthermore, the addition of hole-trapping molecules extends the photoluminescence lifetime. By using a p-type host less reactive to oxygen and tuning the donor–acceptor energy gap, our amorphous blends exhibit persistent luminescence stimulated by visible light even in air, expanding the applicability of OLPL materials. Organic blends based on cationic photoredox catalyst dopants in neutral donor hosts show p-type charge transport behaviour. This favours reduced reactivity to oxygen in organic long-persistent luminescence materials responsive to visible light.

Triplet–triplet upconversion, in which two triplet excitons are converted to one singlet exciton, is a well-known approach to exceed the limit of electroluminescence quantum efficiency in conventional fluorescence-based organic light-emitting diodes. Considering the spin multiplicity of triplet pairs, upconversion efficiency is usually limited to 20%. Although this limit can be exceeded when the energy of a triplet pair is lower than that of a second triplet excited state, such as for rubrene, it is generally difficult to engineer the energy levels of higher triplet excited states. Here, we investigate the upconversion efficiency of a series of new anthracene derivatives with different substituents. Some of these derivatives show upconversion efficiencies close to 50% even though the calculated energy levels of the second triplet excited states are lower than twice the lowest triplet energy. A possible upconversion mechanism is proposed based on the molecular structures and quantum chemical calculations. Though triplet-triplet upconversion is a promising strategy for designing new deep blue-emitting organic materials, maximizing the efficiency of this process remains difficult. Here, the authors report the upconversion efficiency in anthracene derivatives based on a spin-orbit coupling mechanism.

Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which an activation energy as well as spin–orbit coupling between excited states with different spin multiplicities, were simultaneously controlled. An extremely fast exciton lifetime of 750 ns was realized in a donor–acceptor-type molecular structure without heavy metal elements. An OLED utilizing this TADF emitter displayed deep-blue electroluminescence (EL) with CIE chromaticity coordinates of (0.14, 0.18) and a high maximum EL quantum efficiency of 20.7%. Further, the high maximum efficiency were retained to be 20.2% and 17.4% even at high luminance. Deep-blue emitting organic materials with low exciton lifetime are required to realize efficient organic light-emitting diodes (OLEDs) at high brightness. Here, the authors report deep-blue OLEDs featuring thermally activated delayed fluorescence molecules with subnano-second exciton lifetime.

Light emission from organic light-emitting diodes that make use of fluorescent materials have an internal quantum efficiency that is typically limited to no more than 25% due to the creation of non-radiative triplet excited states. Here, we report the use of electron-donating and electron-accepting molecules that allow a very high reverse intersystem crossing of 86.5% between non-radiative triplet and radiative singlet excited states and thus a means of achieving enhanced electroluminescence. Organic light-emitting diodes made using m -MTDATA as the donor material and 3TPYMB as the acceptor material demonstrate that external quantum efficiencies as high as 5.4% can be achieved, and we believe that the approach will offer even higher values in the future as a result of careful material selection. High-efficiency fluorescent organic light-emitting diodes have been realized by employing custom-designed molecules that make it possible to convert non-radiative triplet states into radiative singlet states.

Organic light-emitting diodes (OLEDs) are a promising light-source technology for future generations of display1,2. Despite great progress3–12, it is still challenging to produce blue OLEDs with sufficient colour purity, lifetime and efficiency for applications. Here, we report pure-blue (Commission Internationale de l’ Eclairage (CIE) coordinates of 0.13, 0.16) OLEDs with high efficiency (external quantum efficiency of 32 per cent at 1,000 cd m−2), narrow emission (full-width at half-maximum of 19 nm) and good stability (95% of the initial luminacnce (LT95) of 18 hours at an initial luminance of 1,000 cd m−2). The design is based on a two-unit stacked tandem hyperfluorescence OLED with improved singlet-excited-state energy transfer from a sky-blue assistant dopant exhibiting thermally activated delayed fluorescence (TADF) called hetero-donor-type TADF(HDT-1) to a pure-blue emitter. With stricter control of device fabrication and procedures it is expected that device lifetimes will further improve to rival commercial fluorescent blue OLEDs.Pure-blue organic LEDs with narrow emission and improved stability show promise for display applications.

Blue emission is crucial for quality white lighting sources. Delving into precise exciton management in the subtle chemical design using organic materials and understanding key insights into organic light-emitting diode efficiency is of prime importance. Herein, we envision a simple design strategy combining short-range charge transfer with main long-range charge transfer and synchronizing their complementary advantages to afford a new paradigm of multi-resonance charge transfer-type thermally activated delayed fluorescence emitters. Modifying the auxiliary push-pull connection between a carbazole donor and a boron-based acceptor core, we construct C-C bond extended DBACzPh and DBADCzPh emitters with strong thermally activated delayed fluorescence emission in solution and thin films. Preferential 100% horizontal molecular orientation results in balanced bipolar carrier transport and effective host-to-dopant energy transfer in a high-polarity host matrix. Thereby, DBADCzPh significantly boosts outcoupling efficiency. This design achieves sky-blue electroluminescence with a very high external quantum efficiency of over 40%, paving the way for future advancements in emitter designs for high-efficiency OLEDs. Delving into precise exciton management through subtle chemical design of materials is of prime importance for organic light-emitting devices. Here, the authors modify the push-pull connection between the donor and acceptor core, achieving sky-blue electroluminescence with efficiency of over 40%.

Organic light-emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) have emerged as cheaper alternatives to high-performance phosphorescent OLEDs with noble-metal-based dopants. However, the efficiencies of blue TADF OLEDs are still low at high luminance, limiting full-colour display. Here, we report a blue OLED containing a 9,10-dihydroacridine/diphenylsulphone derivative that has a comparable performance to today's best phosphorescent OLEDs. The device offers an external quantum efficiency of 19.5% and reduced efficiency roll-off characteristics at high luminance. Through computational simulation, we identified six pretwisted intramolecular charge-transfer (CT) molecules with small singlet–triplet CT state splitting but different energy relationships between 3 CT and locally excited triplet ( 3 LE) states. Systematic comparison of their excited-state dynamics revealed that CT molecules with a large twist angle can emit efficient and short-lifetime (a few microseconds) TADF when the emission peak energy is high enough and the 3 LE state is higher than the 3 CT state. Blue organic light-emitting diodes that harness thermally activated delayed fluorescence are realized with an external quantum efficiency of 19.5% and reduced roll-off at high luminance.

A class of metal-free organic electroluminescent molecules is designed in which both singlet and triplet excitons contribute to light emission, leading to an intrinsic fluorescence efficiency greater than 90 per cent and an external electroluminescence efficiency comparable to that achieved in high-efficiency phosphorescence-based organic light-emitting diodes. Efficient fluorescence-based OLEDs One successful way of enhancing the efficiency of organic light-emitting diodes (OLEDs) is to incorporate additional phosphorescent metal-organic molecules that are powered by the normally non-emitting 'triplet' excitons (triplet excitons typically account for 75% of the injected charge carriers). Now Hiroki Uoyama and colleagues describe an alternative strategy in which the electronic properties of the organic host material are tuned by molecular design to achieve the same net result without the need for adding phosphorescent entities. The new method makes use of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, so that triplet excitons are efficiently converted into states that can contribute effectively to the overall emissions. Their devices reach levels of efficiency in excess of 19%, comparable to those of phosphorescence-based OLEDs. The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules 1 to those using phosphorescent molecules 2 , 3 . In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio 1 ; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency 2 , 3 . Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design 4 , thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10 6 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs 3 .