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Studies of delayed electroluminescence in highly efficient fluorescent organic light-emitting diodes (OLEDs) of many dissimilar architectures indicate that the triplet-triplet annihilation (TTA) significantly increases yield of excited singlet states-emitting molecules in this type of device thereby contributes substantially to their efficiency. Towards the end of the 2000s, the essential role of TTA in realizing highly efficient fluorescent devices was widely recognized. Analysis of a diverse set of fluorescent OLEDs shows that high efficiencies are often cor-related to TTA extents. It is therefore likely that it is the long-term empirical optimization of OLED efficiencies that has resulted in fortuitous emergence of TTA as a large and ubiquitous contributor to efficiency. TTA contributions as high as 20-30% are common in the state-of-the-art OLEDs, and even become dominant in special cases, where TTA is shown to substantially exceed the spin-statistical limit. The fundamental features of OLED efficiency enhancement via TTA-molecular structure-dependent contributions, current density-dependent intensities in practical devices and frequently observed antagonistic relationships between TTA extent and OLED lifetime-came to be understood over the course of the next few years. More recently, however, there was much less reported progress with respect to all-important quantitative details of the TTA mechanism. It should be emphasized that, to this day and despite the decades of work on improving blue phosphorescent OLEDs as well as the recent advent of thermally activated delayed fluorescence OLEDs, the majority of practical blue OLEDs still rely on TTA. Considering such practical importance of fluorescent blue OLEDs, the design of blue OLED-compatible materials capable of substantially exceeding the spin-statistical limit in TTA, elimination of the antagonistic relationship between TTA-related efficiency gains and lifetime losses, and designing devices with an extended range of current densities producing near-maximum TTA electroluminescence are the areas where future improvements would be most beneficial.

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet–triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.

Background The worldwide occurrence of triplet pregnancy is estimated to be 0.093%, with a natural incidence of approximately 1 in 8000. This study aims to analyze the neonatal health status and birth weight discordance (BWD) of triplets based on chorionicity from birth until discharge. Methods This was a retrospective study. We reviewed a total of 136 triplet pregnancies at our tertiary hospital between January 1, 2001, and December 31, 2021. Maternal and neonatal outcomes, inter-triplet BWD, neonatal morbidity, and mortality were analyzed. Results Among all cases, the rates of intrauterine death, neonatal death, and perinatal death were 10.29, 13.07, and 24.26%, respectively. Thirty-seven of the cases resulted in fetal loss, including 13 with fetal anomalies. The maternal complications and neonatal outcomes of the 99 triplet pregnancies without fetal loss were compared across different chorionicities, including a dichorionic (DC) group (41 cases), trichorionic (TC) group (37 cases), and monochorionic (MC) group (21 cases). Neonatal hypoproteinemia ( P  < 0.001), hyperbilirubinemia ( P  < 0.019), and anemia ( P  < 0.003) exhibited significant differences according to chorionicity, as did the distribution of BWD ( P  < 0.001). More than half of the cases in the DC and TC groups had a BWD < 15%, while those in the MC group had a BWD < 50% (47.6%). TC pregnancy decreased the risk of neonatal anemia (adjusted odds ratio [AOR] = 0.084) and need for blood transfusion therapy after birth (AOR = 0.119). In contrast, a BWD > 25% increased the risk of neonatal anemia (AOR = 10.135) and need for blood transfusion after birth (AOR = 7.127). TC pregnancy, MCDA or MCTA, and BWD > 25% increased neonatal hypoproteinemia, with AORs of 4.629, 5.123, and 5.343, respectively. Conclusions The BWD differed significantly according to chorionicity. Additionally, TC pregnancies reduced the risk of neonatal anemia and need for blood transfusion, but increased the risk of neonatal hypoproteinemia. In contrast, the BWD between the largest and smallest triplets increased the risk of neonatal anemia and the need for blood transfusion. TC pregnancy, MCDA or MCTA, and BWD > 25% increased the risks of neonatal hypoproteinemia. However, due to the limited number of triplet pregnancies, further exploration of the underlying mechanism is warranted.

Wavelength conversion based on hybrid inorganic–organic sensitized triplet–triplet annihilation upconversion (TTA‐UC) is promising for applications such as photovoltaics, light‐emitting‐diodes, photocatalysis, additive manufacturing, and bioimaging. The efficiency of TTA‐UC depends on the population of triplet excitons involved in triplet energy transfer (TET), the driving force in TET, and the coupling strength between the donor and acceptor. Consequently, achieving highly efficient TTA‐UC necessitates the precise control of the electronic states of inorganic donors. However, conventional covalently bonded nanocrystals (NCs) face significant challenges in this regard. Herein, a novel strategy to exert control over electronic states is proposed, thereby enhancing TET and TTA‐UC by incorporating ionic‐bonded CsPbBr3 and lanthanide Ce3+ ions into composite NCs. These composite‐NCs exhibit high photoluminescence quantum yield, extended single‐exciton lifetime, quantum confinement, and uplifted energy levels. This engineering strategy of electronic states engendered a comprehensive impact, augmenting the population of triplet excitons participating in the TET process, enhancing coupling strength and the driving force, ultimately leading to an unconventional, dopant concentration‐dependent nonlinear enhancement of UC efficiency. This work not only advances fundamental understanding of hybrid TTA‐UC but also opens a door for the creation of other ionic‐bonded composite NCs with tunable functionalities, promising innovations for next‐generation optoelectronic applications. It is discovered that electronic state engineering of perovskite nanocrystals leads to size focusing with greatly increased fluorescent quantum yield. This strategy can control triplet population, energy‐level alignment, and exciton lifetime, which significantly enhanced triplet–triplet annihilation upconversion (TTA‐UC) efficiency (≈284%). This work clarifies the functions of electronic state engineering for inorganic sensitizers and TTA‐UC. It will promote the fundamental research of electronic state engineering in energy transfer systems and facilitate the practical applications of TTA‐UC systems.

Triplet‐triplet annihilation photon upconversion (TTA‐UC) has emerged as a promising strategy for enhancing solar energy harvesting efficiency by converting two low‐energy, long‐wavelength photons into a high‐energy, short‐wavelength photon. In recent years, semiconductor nanocrystals have gained significant attention as efficient photosensitizers for TTA‐UC due to their excellent triplet energy transfer efficiency and the ability to tune their bandgap across the solar spectrum. This review focuses on the mechanism of NC‐based TTA‐UC, emphasizing key parameters to evaluate the performance of TTA‐UC systems. The influence of various material‐related factors on the overall NC‐based TTA‐UC performance is thoroughly discussed. Moreover, recent advances in solid‐state approaches for NC‐based TTA‐UC are highlighted, along with an overview of the current status of applications in this field. Lastly, this review identifies the challenges and opportunities that lie ahead in the future development of NC‐based TTA‐UC, providing insights into the potential advancements and directions for further research. “Semiconductor nanocrystals‐based triplet‐triplet annihilation photon‐upconversion”

The electrochemiluminescence (ECL) behavior of a multiresonance thermally activated delayed fluorescence molecule has been investigated for the first time by means of ECL‐voltage curves, newly designed ECL‐time observatory, and ECL spectroscopy. The compound, Mes3DiKTa, shows complex ECL behavior, including a delayed onset time of 5 ms for ECL generation in both the annihilation pathway and the coreactant route, which we attribute to organic long‐persistent ECL (OLECL). Triplet‐triplet annihilation, thermally activated delayed fluorescence and uncompensated solution resistance cannot be ruled out as contributing mechanisms to the ECL. A very long ECL emission decay was attributed to OLECL as well. The absolute ECL efficiencies of Mes3DiKTa were enhanced and reached 0.0013% in annihilation route and 1.1% for the coreactant system, which are superior to those of most other organic ECL materials. It is plausible that ECL materials with comparable behavior as Mes3DiKTa are desirable in applications such as ECL sensing, imaging, and light‐emitting devices. Newly designed time‐resolved electrochemiluminescence (ECL) experiments were employed to demonstrate unusual delayed ECL onset and long ECL decay of an multiresonance thermally activated delayed fluorescence (TADF) molecule, which arise from the contribution of organic long‐persistent ECL (OLECL). ECL spectroscopy provides evidence of a combination of monomer and aggregates emissions. The TADF and OLECL lead to an emission enhancement with superior ECL efficiency.

The realization of highly operationally stable blue organic light‐emitting diodes (OLEDs) is a challenge in both academia and industry. This paper describes the development of anthracene–dibenzofuran host materials, 2‐(10‐(naphthalen‐1‐yl)anthracen‐9‐yl)naphtho[2,3‐b]benzofuran (Host 1) and 2‐(10‐([1,1′‐biphenyl]‐2‐yl)anthracen‐9‐yl)naphtho[2,3‐b]benzofuran (Host 2), namely for use in the emissive layer of an OLED stack. A multiple‐resonance thermally activated delayed serves as the blue fluorescence emitter and exhibits an initial luminance of 1000 cd m−2 and long operational stability (i.e., time to decay to 90% of initial luminance) of 249 h. Furthermore, a deep‐blue OLED with an optimized top‐emitting architecture with a high current efficiency of 154.3 cd A−1, is fabricated and calibrated to a Commission International de l’Éclairage y chromaticity coordinate of 0.048. Moreover, the emission spectrum of this OLED has a narrowband peak at 476 nm with a full width at half maximum (FWHM) of 16 nm. This work provides valuable insights into the design of anthracene‐based host materials and highlights the importance of host optimization in improving the operational stability of OLEDs. This paper explores the demonstration of highly stable blue organic light‐emitting diodes (OLEDs) using novel anthracene–based host materials. Integrating a multiple‐resonance emitter, the OLEDs exhibit exceptional long‐operational stability (LT90 = 249 h). Additionally, a top‐emitting deep‐blue OLED with superior current efficiency and precise emission characteristics underscores the significance of host material optimization for enhancing OLED stability.

Significance We use transient spectroscopy to investigate the mechanism of singlet exciton fission, a quantum mechanical phenomenon in some organic molecules in which a spin-singlet excited state can split into two spin-triplet states. This process may be harnessed to boost solar cell efficiencies, but the underlying mechanism remains poorly understood. Central to most models is a triplet pair state, consisting of two triplets entangled into an overall spin-singlet configuration, but it has never before been optically detected. In a solution-based system, we detect a state with simultaneous singlet and triplet exciton character that dissociates to form triplet excitons in 120% yield. We consider that this intermediate constitutes a triplet pair state, and its observation allows important insight into the nature of triplet exciton coupling. Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]—tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.

Energy transfer with an associated spin change of the donor and acceptor, Dexter energy transfer, is critically important in solar energy harvesting assemblies, damage protection schemes of photobiology, and organometallic opto-electronic materials. Dexter transfer between chemically linked donors and acceptors is bridge mediated, presenting an enticing analogy with bridge-mediated electron and hole transfer. However, Dexter coupling pathways must convey both an electron and a hole from donor to acceptor, and this adds considerable richness to the mediation process. We dissect the bridge-mediated Dexter coupling mechanisms and formulate a theory for triplet energy transfer coupling pathways. Virtual donor–acceptor charge-transfer exciton intermediates dominate at shorter distances or higher tunneling energy gaps, whereas virtual intermediates with an electron and a hole both on the bridge (virtual bridge excitons) dominate for longer distances or lower energy gaps. The effects of virtual bridge excitons were neglected in earlier treatments. The two-particle pathway framework developed here shows how Dexter energy-transfer rates depend on donor, bridge, and acceptor energetics, as well as on orbital symmetry and quantum interference among pathways.

Magnetic field effects (MFEs) in thermally activated delayed fluorescence (TADF) materials have been shown to influence the reverse intersystem crossing (RISC) and to impact on electroluminescence (EL) and conductivity. Here, we present a novel model combining Cole–Cole and Lorentzian functions to describe low and high magnetic field effects originating from hyperfine coupling, the g mechanism, and triplet processes. We applied this approach to organic light-emitting devices of third generation based on tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), exhibiting blue emission, to unravel their loss mechanisms. The quality of the regression function was evaluated using k-fold cross-validation. The scoring was compared to various alternative fitting functions, which were previously proposed in literature. Density functional theory calculations, photoluminescence, and electroluminescence studies validated the formation of a TADF exciplex system. Furthermore, we propose successful encapsulation using a semi-permeable polymer, showing promising results for magnetic field sensing applications on arbitrary geometry. This study provides insights into the origin of magnetic field effects in exciplex-TADF materials, with potential applications in optoelectronic devices and sensing technologies.