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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. [ABSTRACT FROM AUTHOR]

Up‐conversion of triplet into singlet exciton in the emitting layer is believed to be one of the ways that thermally activated delayed fluorescent (TADF) materials may employ to reduce triplet exciton density hence preventing device quenching by triplet excitons. Yet, two donor‐acceptor type molecules; 5‐(4‐(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)phenyl)‐12‐(3‐(triphenylsilyl) phenyl)‐5,12‐dihydroindolo[3,2‐a]carbazole (SiPhCzTrz) and 5‐phenyl‐12‐(4‐(4‐phenyl‐6‐(3‐(triphenylsilyl) phenyl)‐1,3,5‐triazin‐2‐yl)pheny‐l)‐5,12‐dihydroindolo [3,2‐a]carbazole (SiTrzPhCz), which exhibited different TADF properties depending on the relative positions of their electron donor unit (PhCz) and electron acceptor unit (Trz), show opposite behaviors. These materials are used as sensitizer in phosphorescent solution‐processed organic light emitting diodes (s‐OLEDs) showing moderately high current efficiencies of 19.3 and 20 cd/A, respectively. SiTrzPhCz exhibits stronger TADF properties compared to SiPhCzTrz; however, these stronger TADF characteristics lead to a more pronounced efficiency roll‐off, mainly due to the longer residence time of excitons in SiTrzPhCz, leading to exciton quenching. In contrast, due to their twisted structures, the efficiency roll‐off is efficiently suppressed, particularly for SiTrzPhCz, when both materials are used as hosts. Their twisted structures promote aggregate‐induced emission and prevent aggregation‐caused quenching. Further analysis of exciton dynamics reveals faster decay rate for both singlet and triplet densities in SiPhCzTrz compared to SiTrzPhCz, indicated by its higher fast prompt emission, kFRET and knrT. [ABSTRACT FROM AUTHOR]

The lack of viable solid‐state annihilators is one of the greatest hurdles in perovskite‐sensitized triplet–triplet annihilation upconversion (UC). Unfavorable singlet and triplet energy surfaces in the solid state have limited the successful implementation of many conventional solution‐based annihilators. To date, rubrene is still the best‐performing annihilator; however, this comes at the cost of a limited apparent anti‐Stokes shift. To this point, anthracene derivatives are promising candidates to increase the apparent anti‐Stokes shift. The well‐known green glowstick dye 9,10‐(bisphenylethynyl)anthracene (BPEA) and its chlorinated derivatives have already shown promise in solution‐based UC applications. Due to favorable band alignment of the perovskite and triplet energy levels of BPEA, it is conceivable that a wide variety of BPEA derivatives can be compatible with the perovskite‐based UC system. Here, the properties of the parent molecule BPEA and its derivatives 1‐chloro‐9,10‐(bisphenylethynyl)anthracene and 2‐chloro‐9,10‐(bisphenylethynyl)anthracene are investigated. Despite similar optical properties in solution, the different molecules exhibit vastly different properties in thin films. UC studies in lead halide perovskite/BPEA bilayer devices demonstrate the importance of intermolecular coupling on the resulting properties of the upconverted emission. [ABSTRACT FROM AUTHOR]

The kinetics of singlet fission (SF) (i.e., spontaneous splitting of the excited singlet state into a pair of triplet (T) excitons (TT-pair)) is known to be significantly affected by TT-annihilation (TTA), which manifests itself in magnetic field effects on the TTA and, in particular, in the magnetic-field dependence of the SF-kinetics. In this study, in the two-state model (TSM), a method is proposed for treating the magnetic field effects on the SF-kinetics, which allows for the correct description of the manifestation of the stochastic migration of T-excitons assuming that the manifestation results from the transition from two states of coupled and freely diffusing T-excitons. In the TSM, the analytical expression for the magnetic-field-dependent part of the SF-kinetics is derived. This expression is applied to the analysis of the SF-kinetics measured in amorphous rubrene films in the absence of a magnetic field and in the field B = 8 kGs. [ABSTRACT FROM AUTHOR]

Triplet–triplet annihilation (TTA), or triplet fusion, is a biexcitonic process in which two triplet‐excited molecules can combine their energy to promote one into an excited singlet state. To alleviate the dependence of the TTA rate and yield on triplet diffusion in both solid and solution environments, intramolecular TTA (intra‐TTA) has been recently proposed in conjugated molecular systems able to hold multiple triplet excitons simultaneously. Developing from the previous demonstration of TTA performance enhancement in sensitized upconversion solutions, here similar improvements in triplet harvesting in solid‐state films are reported under electrical excitation in organic light emitting diodes (OLEDs). At low dye concentration and low current densities, the intra‐TTA active OLED shows a +40% improved external quantum efficiency with respect to the reference device, and a TTA spin‐statistical factor f 4DPA of 0.4, close to that determined in fluid solution for the individual chromophore (0.45). These results therefore indicate the utility of this molecular design strategy across a wider range of TTA applications, and with particular utility in the further development of low‐power TTA‐enhanced OLEDs. [ABSTRACT FROM AUTHOR]

Crystalline host matrix (CHM)‐nanoaggregates (NA) structure combines the advantages of a crystalline host matrix and high‐performance luminescent materials. This structure emerges as a highly effective strategy for achieving efficient blue organic light‐emitting diodes (OLEDs). This approach can tackle the current challenge of achieving low voltage and high brightness in amorphous OLEDs. In this work, triplet‐triplet annihilation (TTA) material NA are embedded into CHM and sensitize the fluorescent dopant (D) BD1. As a result, a CHM‐TTANA‐D structure device is created, demonstrating an outstanding external quantum efficiency (EQE) of 8.46%. Notably, the efficiency roll‐off is mitigated. The device maintains an EQE of ≈7.50% at 1000 cd m−2, which is the highest values among blue crystalline OLEDs. The device leverages the benefits of CHM and exhibits swift turn‐on and a rapid increase in brightness and current density. This leads to a significant improvement in blue photon output and a reduced series‐resistance Joule heat loss ratio. The findings underscore the promising nature of the CHM‐TTANA‐D structure as a favorable design paradigm for high‐performance crystalline thin‐film OLEDs (C‐OLEDs). [ABSTRACT FROM AUTHOR]

Triplet‐triplet annihilation upconversion (TTA‐UC) has made significant progress in recent years in several key applications, including solar energy harvesting, photocatalysis, stereoscopic 3D printing, and disease therapeutics. In TTA‐UC research, photosensitizers serve the vital function of harvesting low‐energy photons. The photophysical characteristics of photosensitizers, including absorbance, triplet state quantum yield, triplet state energy level, triplet state lifetime, etc. determine the performance of TTA‐UC. Thus, the study of photosensitizers has been a key aspect of TTA‐UC. In recent years, multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have received extensive attention due to their excellent photophysical properties and electroluminescent device performance. MR‐TADF molecules not only present a narrow energy gap between the singlet and triplet excited states, but also have stronger absorption and better wavelength regulation than conventional TADF molecules. Nowadays, the preliminary attempts in TTA‐UC using MR‐TADF molecules as photosensitizers have resulted in the development of green to ultraviolet, blue to ultraviolet, and even near‐infrared to blue emission. This concept will summarize the research progress of MR‐TADF molecules as photosensitizers in TTA‐UC, analyzing the challenges and giving possible solutions. Finally, we prospect the future development of MR‐TADF molecules as photosensitizers, including the molecular design as well as the possible application areas. [ABSTRACT FROM AUTHOR]

Nonlinear photochromic reactions that work with weak incoherent light are important for molecular operations with high spatial resolution and multiple photofunctions based on single molecules. However, nonlinear photochromic compounds generally require complex molecular design, restricting accessibility in various fields. Herein, we report nonlinear photochromic properties in a perylene‐substituted rhodamine spirolactam derivative (Rh−Pe), which is synthesized from rhodamine B in facile procedures. Direct excitation of Rh−Pe produces the triplet excited state via the charge‐transfer (CT) state. The triplet excited state causes triplet–triplet annihilation to bring the generation of the intensely colored ring‐open form with nonlinear behavior. Furthermore, green‐ and red‐light‐induced photochromism was achieved in Rh−Pe using triplet sensitizers, although Rh−Pe can be directly excited only by ultraviolet and blue light. Our findings are expected to contribute to the development of photofunctional materials showing nonlinear behavior and low‐energy light responsivity. [ABSTRACT FROM AUTHOR]

The triplet annihilator is a critical component for triplet–triplet annihilation upconversion (TTA-UC); both the photophysical properties of the annihilator and the intermolecular orientation have pivotal effects on the overall efficiency of TTA-UC. Herein, we synthesized two supramolecular annihilators A-1 and A-2 by grafting 9,10-diphenylanthracene (DPA) fragments, which have been widely used as triplet annihilators for TTA-UC, on a macrocyclic host—pillar[5]arenes. In A-1, the orientation of the two DPA units was random, while, in A-2, the two DPA units were pushed to a parallel arrangement by intramolecular hydrogen-bonding interactions. The two compounds showed very similar photophysical properties and host–guest binding affinities toward electron-deficient guests, but showed totally different TTA-UC emissions. The UC quantum yield of A-2 could be optimized to 13.7% when an alkyl ammonia chain-attaching sensitizer S-2 was used, while, for A-1, only 5.1% was achieved. Destroying the hydrogen-bonding interactions by adding MeOH to A-2 significantly decreased the UC emissions, demonstrating that the parallel orientations of the two DPA units contributed greatly to the TTA-UC emissions. These results should be beneficial for annihilator designs and provide a new promising strategy for enhancing TTA-UC emissions. [ABSTRACT FROM AUTHOR]

The enhancement in the efficiency of triplet-triplet annihilation upconversion (TTA-UC) is mainly determined by the triplet energy transfer (TET) and triplet-triplet annihilation (TTA) between the sensitizers and annihilators. The TET process works efficiently by adjusting the concentration ratio of the sensitizers and annihilators. The efficiency of TTA is determined by the properties of the annihilator. Because TTA is a Dexter-type energy transfer and is affected by the diffusion rate, the energy levels of the excited states and the molecular size are both crucial in TTA. In this study, four isomerized dimers of 9,10-diphenlanthracene (DPA) and anthracene (An) were designed and prepared as annihilators for TTA-UC. The singlet and triplet energy levels could be adjusted by altering the connection position while maintaining the molecular weight and size. When PtOEP was used as the sensitizer, the maximum upconversion efficiency of 9-[4-(9-anthracenyl)phenyl]-10-phenylanthracene (9DPA-9An) was ∼11.18%. This is four times higher than that of 9,10-diphenyl-2,9 ' -bianthracene (2DPA-9An, 2.63%). The calculation of the energies of T1 and the higher triplet state (T3, because E (T2) is similar to the E (T1) of these dimers) for these dimers has provided insights into the underlying reasons. These indicated that the energy gap value of 2 × E (T1) - E (T3) is the determining factor for TTA efficiency. This work may provide a better understanding of the excited-state energy levels, which is crucial for designing novel annihilators to enhance the TTA-UC efficiency. [ABSTRACT FROM AUTHOR]

Photon upconversion is a process that combines low-energy photons to form useful high-energy photons. There are potential applications in photovoltaics, photocatalysis, biological imaging, etc. Semiconductor quantum dots (QDs) are promising for the absorption of these low-energy photons due to the high extinction coefficient of QDs, especially in the near infrared (NIR). This allows the intriguing use of diffuse light sources such as solar irradiation. In this review, we describe the development of this organic-QD upconversion platform based on triplet-triplet annihilation, focusing on the dark exciton in QDs with triplet character. Then we introduce the underlying energy transfer steps, starting from QD triplet photosensitization, triplet exciton transport, triplet-triplet annihilation, and ending with the upconverted emission. Design principles to improve the total upconversion efficiency are presented. We end with limitations in current reports and proposed future directions. This review provides a guide for designing efficient organic-QD upconversion platforms for future applications, including overcoming the Shockley-Queisser limit for more efficient solar energy conversion, NIR-based phototherapy, and diagnostics in vivo. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The kinetics of the decay (splitting) of the excited singlet -state of rubrene molecules into a pair of triplet-excitons (T-excitons) in rubrene films, usually represented in terms of the kinetics ps(t) of the decay of fluorescence (KDF) from the -state, is analyzed in detail. The KDF is known to be significantly controlled by the process of diffusive migration and annihilation of the generated T-excitons. In the analysis, two migration models are considered: the two-state model (TSM), treating the migration effect as a result of transitions between the [TT] state of coupled T-excitons (at small TT-distances r) and the [T+T]-state of freely migrating T-excitons (at large distances r), as well as the free migration model (FMM), neglecting the effect of the [TT] state. Within the TSM and FMM, the expressions for ps(t) are derived, which are applied to describe the KDF , measured in amorphous rubrene films. Within the experimentally investigated range of times, 0.4–200 ns, the TSM is shown to reproduce the behavior of the experimental KDF much more accurately than the FMM. At longer times t 103 ns, a substantial difference () between and the FMM-predicted KDF ps(t) is found, which is far beyond the experimental error ( 3%). [ABSTRACT FROM AUTHOR]

Image contrast is often limited by background autofluorescence in steady‐state bioimaging microscopy. Upconversion bioimaging can overcome this by shifting the emission lifetime and wavelength beyond the autofluorescence window. Here we demonstrate the first example of triplet‐triplet annihilation upconversion (TTA‐UC) based lifetime imaging microscopy. A new class of ultra‐small nanoparticle (NP) probes based on TTA‐UC chromophores encapsulated in an organic–inorganic host has been synthesised. The NPs exhibit bright UC emission (400–500 nm) in aerated aqueous media with a UC lifetime of ≈1 μs, excellent colloidal stability and little cytotoxicity. Proof‐of‐concept demonstration of TTA‐UC lifetime imaging using these NPs shows that the long‐lived anti‐Stokes emission is easily discriminable from typical autofluorescence. Moreover, fluctuations in the UC lifetime can be used to map local oxygen diffusion across the subcellular structure. Our TTA‐UC NPs are highly promising stains for lifetime imaging microscopy, affording excellent image contrast and potential for oxygen mapping that is ripe for further exploitation. [ABSTRACT FROM AUTHOR]

"Hot exciton" molecules that allow the transition of excitons from a high‐lying triplet state (Tn, n≥ 2) to singlet states (Sm, m≥1) by a reverse intersystem crossing (hRISC) process have become an effective strategy to achieve high efficiency fluorescence organic light emitting diodes (OLEDs). However, the understanding of the dynamic behavior of the "hot exciton" process is still very lacking. Herein, the exciton dynamics of an aggregation‐induced emission (AIE) molecule TPA‐An‐mPhCz with the "hot exciton" property are deeply investigated, and the proportion between hRISC and internal inversion (IC) of excitons on Tn is successfully quantified by in situ transient electroluminescent measurements and theoretical calculation. It is found that the IC process increases severe energy loss of Tn excitons. By introducing a triplet–triplet annihilation up‐conversion layer in the emissive layer to efficiently recapture the IC excitons and further doping a blue fluorescent emitter in the TPA‐An‐mPhCz layer to achieve high photoluminescence quantum yield (PLQY), the resulting OLED achieves a maximum external quantum efficiency of 12.5% with negligible efficiency roll‐off. More impressively, the operational lifetime LT75 (lifetime to 75% of the initial luminance) of the device with efficient triplets utilization performs a remarkable 116 h under 1000 cd m−2. This work provides the fundamentals for the design of hot exciton materials with efficient exciton utilization to develop efficient blue fluorescence OLEDs. [ABSTRACT FROM AUTHOR]

Non‐fullerene acceptors (NFAs) have enabled power conversion efficiencies exceeding 19% in organic solar cells (OSCs). However, the open‐circuit voltage of OSCs remains low relative to their optical gap due to excessive non‐radiative recombination, and this now limits performance. Here, an important aspect of OSC design is considered, namely management of the triplet exciton population formed after non‐geminate charge recombination. By comparing the blends PM6:Y11 and PM6:Y6, it is shown that the greater crystallinity of the NFA domains in PM6:Y11 leads to a higher rate of triplet‐triplet annihilation (TTA). This is attributed to the four times larger ground state dipole moment of Y11 versus Y6, which improves the long range NFA out‐of‐plane ordering. Since TTA converts a fraction of the non‐emissive triplet states into bright singlet states, it has the potential to reduce non‐radiative voltage losses. Through a kinetic analysis of the recombination processes under 1‐Sun illumination, a framework is provided for determining the conditions under which TTA may improve OSC performance. If these could be satisfied, TTA has the potential to reduce non‐radiative voltage losses by up to several tens of millivolts. [ABSTRACT FROM AUTHOR]

The utilization of triplet excitons is of great importance for organic light‐emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one of the effective tactics to achieve high efficiency deep‐blue organic electroluminescence emitters by converting two triplet excitons into one singlet exciton. Whereas, in addition to the 25% electrogenerated singlet excitons, the proportion of radiative singlet excitons (RSE) produced by the TTA process is usually only 15%; thus the total radiative excitons are 40%. In this study, ≈35% of RSE is achieved by the TTA process (total 60%) with two deep‐blue emitters based on the isomeric naphthoimidazole (NI) unit and anthracene bridge. As a result, non‐doped OLEDs based on the two NI derivatives as emitting layers achieve maximum external quantum efficiencies of 10.9% and 11.2% with an identical deep‐blue emission peak of 452 nm, which are the best TTA OLEDs with a Commission Internationale de l'Eclairage chromaticity Y coordinate below 0.15. Theoretical and experimental results demonstrate that the TTA process can be improved owing to the efficient spin–orbit interactions, even though the energy levels of the triplet pairs are higher than the calculated second triplet excited states. [ABSTRACT FROM AUTHOR]

Aggregation‐induced emission (AIE) materials are attractive for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs) owing to the "hot exciton" process by reverse intersystem crossing (hRISC) and high photoluminescence quantum yields (PLQY). However, the internal conversion (IC) from the high‐lying triplet excitation states (Tn, n≥2) to the lower excited triplet state (Tn‐1) is inevitable, resulting in severe exciton losses. Herein, an effective device structure is designed that reuses the lost triplet excitons caused by IC and realizes the breakthrough in the efficiency of blue fluorescence OLEDs based on AIE molecule as an emitter. The maximum external quantum efficiency reached as high as 14.8% and is kept at 14.4% at the luminance of 1000 cd m−2. In the designed device, a triplet–triplet annihilation (TTA) up‐conversion material 1‐[2,5‐dimethyl‐4‐(1‐pyrenyl)phenyl]pyrene (DMPPP) is introduced into the AIE emitter as a triplet sensitizer to receive the lost triplet excitons, and a thin TTA up‐conversion layer 9‐[4‐(10‐phenyl‐9‐anthryl)phenyl]‐9H‐carbazole (CzPA) is introduced in the middle of the doped layer to form the emissive layer (EML). It is found that the hRISC process of iTPB‐2AC greatly enhances the utilization efficiency of TTA intermediate state ([TT]*) excitons on CzPA to iTPB‐2AC so that the utilization of the lost excitons is maximized. This work establishes physical insights into the AIE emission materials and device fabrication of high‐efficiency blue fluorescence OLEDs. [ABSTRACT FROM AUTHOR]

The molecular processes taking place during triplet‐triplet annihilation (TTA) in solvents at room temperature are examined in detail. Special attention is paid to modelling of the nonlinear kinetic reactions. Using time‐ and spectrally resolved spectroscopy of DPA and Pt/Pd‐OEP based sensitizer and annihilators, it is shown how the kinetic of parameters, such as the triplet energy transfer (TET) and TTA of the rate‐reactions, can be simulated, measured and fitted, without approximations, using a numerical scheme. Studies of DPA in the solvents DMSO, THF and toluene at room temperature revealed that viscosity/diffusion together with the excited triplet lifetime of the annihilator are the most crucial parameters for high TTA induced delayed fluorescence. From an analysis of the experimentally determined rates an efficiency of 16–40 % was determined for the combined TET and TTA processes using THF, DMSO and toluene as solvents. (DPA=9,10‐diphenylanthracene, OEP=2,3,7,8,12,13,17,18‐octaethyl‐21H,23H‐porphyrin) [ABSTRACT FROM AUTHOR]

In this study, the specific features of the kinetics of singlet fission (SF)—i.e., spontaneous splitting of the excited singlet state into a pair of triplet (T) excitons (TT-pair)—in anisotropic molecular crystals are analyzed in detail. These features are known to be primarily determined by the TT-annihilation of the created TT-pairs (migrating in the crystals). In our analysis, the kinetics of annihilation-affected SF processes is described in the two-state model (TSM), in which the interaction of migrating T-excitons is associated with transitions between two kinetic states of TT-pairs: [TT]-state of coupled TT-pairs and [T+T]-state of freely migrating T-excitons. The TSM makes it possible to represent the effects of migration and interaction on SF-kinetics in terms of the lattice Green's functions, for which the analytical formulas are obtained in this study. The TSM is applied to the analysis of SF-kinetics in the rubrene single crystals recently measured in a wide time range. The analysis provides detailed information on some characteristic kinetic properties of SF processes in anisotropic crystals. It is shown, for example, that the formation of the [TT]-state in the SF process results in some distortion of the shape of the SF kinetic dependence at short times (of the order of the primary-stage time of SF kinetics). Is also demonstrated that the anisotropy of T-exciton migration manifests itself in some characteristic features of SF kinetics at long times. [ABSTRACT FROM AUTHOR]

The true molecular conformation and the crystal structure of benzo[e]dinaphtho[2,3-a;1′,2′,3′,4′-ghi]fluoranthene, 7,14-diphenylnaphtho[1,2,3,4-cde]bisanthene and 7,16-diphenylnaphtho[1,2,3,4-cde]helianthrene were determined ab initio by 3D electron diffraction. All three molecules are remarkable polycyclic aromatic hydrocarbons. The molecular conformation of two of these compounds could not be determined via classical spectroscopic methods due to the large size of the molecule and the occurrence of multiple and reciprocally connected aromatic rings. The molecular structure of the third molecule was previously considered provisional. These compounds were isolated as by-products in the synthesis of similar products and were at the same time nanocrystalline and available only in very limited amounts. 3D electron diffraction data, taken from submicrometric single crystals, allowed for direct ab initio structure solution and the unbiased determination of the internal molecular conformation. Detailed synthetic routes and spectroscopic analyses are also discussed. Based on many-body perturbation theory simulations, benzo[e]dinaphtho[2,3-a;1′,2′,3′,4′-ghi]fluoranthene may be a promising candidate for triplet–triplet annihilation and 7,14-diphenylnaphtho[1,2,3,4-cde]bisanthene may be a promising candidate for intermolecular singlet fission in the solid state.

A new class of cis‐stilbene/fluorene (STIF) and 10,11‐benzo‐ and imidazole‐fused dibenzosuberane/fluorene (Bz‐STIF and Imd‐STIF) spiro hybrid systems with paired cyanophenyl, pyridyl, and/or pyrimidyl units was synthesized as electron transporting materials (ETMs) for blue fluorescent OLEDs (FOLEDs) and phosphorescent organic light emitting diodes (PhOLEDs). Their photophysical (UV‐Vis absorption and emission), electrochemical (Eox/Ered), morphological (glassy transition temperature, Tg), and thermal stability (decomposition temperature, Td) properties are systematical compared. Their correlations regarding the effects of the fused unit/substituent(s) in the STIF core on the morphological/thermal stability, HOMO/LUMO energy level, π‐electron distribution profiles by DFT calculations, and electron mobility are established. Blue FOLEDs are fabricated by using them and TmPyPB (a control) as ETMs. The effects of their LUMO energy levels and electron mobilities on the device turn‐on voltage, performance efficiencies including external quantum efficiency/current efficiency (EQE/CE), power efficiency (PE), and device lifetime at 5% luminescence decay (T95) are correlated. Among them, three best ETMs and TmPyPB (a control) are selected for further green and blue PhOLED fabrications. The effects of their LUMO energy levels and electron mobilities on the device turn‐on voltage and performance efficiencies are confirmed, allowing for potential commercial applications. [ABSTRACT FROM AUTHOR]

We prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×104 M−1 cm−1 at 570 nm), and the singlet oxygen quantum yield (ΦΔ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S1 and T1 states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet‐triplet energy transfer (TTET) and triplet‐triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions. [ABSTRACT FROM AUTHOR]

Triplet‐triplet annihilation upconversion (TTA‐UC) with near‐infrared (NIR) photosensitizers is highly desirable for a variety of emerging applications. However, the development of NIR‐to‐blue TTA‐UC with a large anti‐Stokes shift is extremely challenging because of the energy loss during the intersystem crossing (ISC). Here, we develop the first NIR‐absorbing B,N‐heteroarene‐based sensitizer (BNS) with multi‐resonance thermally activated delayed fluorescence (MR‐TADF) characters to achieve efficient NIR‐to‐blue TTA‐UC. The small energy gap between the singlet and triplet excited states (0.14 eV) of BNS suppresses the ISC energy loss, and its long‐delayed fluorescence lifetime (115 μs) contributes to efficient triplet energy transfer. As a result, the largest anti‐Stokes shift (1.03 eV) among all heavy‐atom‐free NIR‐activatable TTA‐UC systems is obtained with a high TTA‐UC quantum yield of 2.9 % (upper limit 50 %). [ABSTRACT FROM AUTHOR]

Ultraviolet (UV, λ<400 nm) light is essential for various photochemical reactions, but its intensity in the solar spectrum is very low, and light sources that artificially generate high‐energy UV light are inefficient and environmentally unfriendly. A solution to this problem is photon upconversion (UC) from visible (vis, λ>400 nm) light to UV light. Among several mechanisms, UC based on triplet‐triplet annihilation (TTA‐UC) in particular has made remarkable progress in recent years. The development of new chromophores has enabled highly efficient conversion of low‐intensity visible light into UV light. In this review, we summarize the recent development of visible‐to‐UV TTA‐UC, from the development of chromophores and their production into films to their application in various photochemical processes such as catalysis, bond activation and polymerization. Finally, challenges and opportunities in future material development and applications will be discussed. [ABSTRACT FROM AUTHOR]

Triplet–triplet annihilation (TTA) can double the energy of excited states. We utilized a charge transfer state at an organic semiconductor interface as a precursor for efficient TTA. As a result, a solid‐state photon upconversion from near‐infrared to visible light is achieved with an efficiency that is two orders of magnitude higher than those demonstrated by conventional systems. Furthermore, it is demonstrated that an efficient organic light‐emitting diode (OLED) can be operated by only a 1.5 V battery. [ABSTRACT FROM AUTHOR]

Designing optimized systems for triplet‐triplet annihilation upconversion (TTA‐UC) with high UC emission efficiency at low power densities remains challenging. To improve the lifetime of triplet photosensitizers for TTA‐UC, donor‐acceptor‐heavy atoms were used to synthesize rotationally restricted PY‐MBD‐BR and unrestricted PY‐BD‐BR photosensitizers. The singlet oxygen quantum yields of both photosensitizers was ≈0.60 due to their structural similarity, except for the degree of rotation between the donor and acceptor. The triplet lifetime of PY‐BD‐BR and PY‐MBD‐BR was 352 μs and 1,503 μs, respectively. The latter may be among the highest reported for BODIPY photosensitizers. The prolonged triplet lifetime enabled excellent green‐to‐blue upconversion emission of PY‐MBD‐BR, with a high upconversion emission efficiency (ηUC) of 8.8 % and low threshold intensity (Ith) of 90 mW/cm2 compared to PY‐BD‐Br (low ηUC of 4.0 % and high Ith of 381 mW/cm2). The rigid molecular design of triplet photosensitizers based on donor‐acceptor‐heavy atom systems has potential for efficient TTA‐UC. [ABSTRACT FROM AUTHOR]

[Display omitted] The characteristic properties of the effects of migration-controlled triplet–triplet annihilation on the kinetics of singlet fission (the splitting of an excited singlet state S 1 * into a pair of triplet excitons) in molecular semiconductors are thoroughly analyzed. The analysis assumes that the short-time and long-time parts of the kinetics, i.e. , the kinetics of fluorescence I(t) from the S 1 * state, are governed by the corresponding short-time (geminate) and long-time (bulk) annihilation stages. Within the analysis, a simple formula is proposed that accurately describes the experimental singlet-fission kinetics over a wide range of times. [ABSTRACT FROM AUTHOR]

Photon upconversion (UC) is a technology that converts lower-energy photons (longer wavelength light) into higher-energy photons (shorter wavelength light), the opposite of fluorescence. Thus, UC is expected to open a vast domain of photonic applications that are not otherwise possible. Recently, UC by triplet−triplet annihilation (TTA) between organic molecules has been studied because of its applicability to low-intensity light, although the majority of such studies have focused on liquid samples in the form of organic solvent solutions. To broaden the range of applications, solid-state UC materials have been an active area of research. We recently developed air-stable, high-performance molecular UC crystals that utilize a stable solid-solution phase of bicomponent organic crystals. This article begins with a brief overview of previous challenges in developing and improving solid-state TTA–UC materials. Then, we briefly review and explain the concept as well as advantages of our molecular solid-solution UC crystals. We applied these organic crystals for the first time to a water environment. We observed blue UC emission upon photoexcitation at 542 nm (green–yellow light) and then measured the excitation intensity dependence as well as the temporal stability of the UC emission in air-saturated water. In nondegassed water, these organic crystals were stable, functioned with a low excitation threshold intensity of a few milliwatts per square centimeter, and exhibited high photo-irradiation durability at least over 40 h; indicating that the developed organic crystals are also viable for aqueous conditions. Therefore, the organic crystals presented in this report are expected to extend the domain of UC-based photonic applications in practical water systems including in vivo diagnostic, clinical, and therapeutic applications.

In this work, we report an energy upconversion system based on triplet-triplet annihilation comprised of rose bengal dye as the sensitizer and a highly fluorescent 9-(triphenyl)ethynyl-10-phenylanthracene (TPE-PAn) as the triplet annihilator. The energy upconversion can be observed with a low-power laser excitation at 532 nm or noncoherent Xe arc lamp excitaion at 540 nm. A delayed fluorescence from (TPE-PAn) at 418 nm was observed with an energy upconversion up to 0.64 eV and quantum yield of 0.8 %. A white-light emission was achieved as a result of combination of delayed fluorescence from TPE-PAn and residual fluorescence from rose bengal dye. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The "hot exciton" mechanism based on high-lying reverse intersystem crossing process has great advantages in achieving high-performance deep-blue organic light-emitting diodes (OLEDs). Nevertheless, how to solve the loss of high-lying excitons to improve device performance further is a pressing and challenging issue. To break through this shackle, a novel deep-blue emitter based on "exciton recovery" strategy is successfully design, namely CAT. By combining the transient absorption spectrum, theoretical calculation, magneto-electroluminescence, and transient-electroluminescence measurements, the multi-channeled pathways of excitons utilization via hot exciton and triplet-triplet annihilation processes is comprehensively demonstrated, and the proportion of singlet excitons by each channel. Finally, a high exciton utilization efficiency is successfully achieved, and the non-doped OLED based on CAT exhibit an excellent external quantum efficiency of 10.39% with the CIE coordinates of (0.15, 0.087). Furthermore, the sensitized blue fluorescent OLED by CAT as host also achieves excellent performance. More importantly, the operational lifetime of the "multi-channel" sensitized device is evaluated for the first time, performing the remarkable LT50 (lifetime to 50% of the initial luminance) of 320 h at 540 cd m-2. These results fully reveal the significance of the "exciton recovery" strategy and give new inspiration for exploring highperformance blue OLEDs. [ABSTRACT FROM AUTHOR]

Green‐to‐blue triplet–triplet annihilation photon upconversion with the well‐studied upconversion pair 9,10‐diphenylanthracene (DPA)/platinum octaethylporphyrin (PtOEP) was used to reversibly drive the photoisomerization of diarylethene (DAE) photoswitches by using visible light. By carefully selecting the kinetic and spectral properties of the molecular system as well as the experimental geometry, a single green light source can be used to selectively trigger both the ring‐opening and the ring‐closing reactions, whilst also inducing fluorescence from the colored closed isomer that can be used as a readout to monitor the isomerization process in situ. The upconversion solution and the DAE solution are kept physically separated, allowing them to be characterized both concomitantly and individually without further separation processes. The ring‐closing reaction using upconverted photons was quantified and compared to the efficiency of direct isomerization with ultraviolet light. [ABSTRACT FROM AUTHOR]

[Display omitted] • A ratiometric luminescent nanosensor is developed based on down/up-conversion luminescence. • The TTA-UC nanoprobes offer the benefits of no background fluorescence interference and self-calibration. • A smartphone-assisted colorimetric detection strategy based on the ratiometric nanoprobes is achieved. • Trace detection of Methyl Orange has been achieved in real water samples. A ratiometric nanosensor was developed for detecting methyl orange (MO) based on down/up-conversion luminescence achieved by a triplet–triplet annihilation upconversion luminescence (TTA-UCL) system. The probe, utilizing sensitizer and annihilator fluorophores encapsulated in nanomicelles, demonstrated high sensitivity and selectivity for MO detection. The energy transfer from UCL to MO endowed the sensor with responsive capabilities. The unaffected triplet–triplet energy transfer process maintained the phosphorescence signal constant, serving as a reference to construct the ratiometric sensor along with the UCL signal. Additionally, a smartphone-assisted colorimetric detection method was also developed based on the ratiometric sensor, enabling rapid and convenient detection of MO without the need for a spectrometer. The performance of the nanosensor in real water samples confirmed its potential for practical environmental applications. [ABSTRACT FROM AUTHOR]

The low-voltage operation of blue organic light-emitting diodes (OLEDs) is critical for reducing power consumption and improving device lifetime. This review briefly summarizes the history and origin of low-voltage electroluminescence and its realization in recent reports. In particular, upconversion OLEDs, which use the charge transfer state as an intermediate state to facilitate blue emission via triplet–triplet annihilation, present a promising solution. Thus, continued efforts aimed at reducing the driving voltage, including the exploration of new materials and device structures, are expected to significantly contribute to expanding the applications of blue OLEDs. • Turn-on voltages of blue OLEDs are reviewed. • Minimizing charge injection barriers is vital to reduce turn-on voltage. • Extremely low turn-on is realized in upconversion OLED via charge transfer state. [ABSTRACT FROM AUTHOR]

Two electron-transport host materials are synthesized by linking anthracene with benzimidazole derivatives through a unit of methyl substituted phenyl. The methyl substitution generates a large dihedral angle between the benzimidazole plane and bridging phenyl, keeping the excited state distribution and energy levels of T 1 and S 1 unchanged. The electron-only devices reveal that the two hosts have obviously enhanced electron transport ability than that of Ph-AN-MBP without benzimidazole. As a result, the devices with Ph-AN-BIZ and N1-AN-BIZ as hosts and a blue multiple resonance material of t BuCz-DABNA as emitter exhibit high external quantum efficiency of 10.1 % and 9.1 %, respectively, and low turn-on voltage of 2.7 V, which is better than those of Ph-AN-MBP based device (7.9 %, 3.4 V). And their electroluminescence spectra have a very small full-width at half-maximum of 16.2 nm and pure blue color with CIE coordinates of (0.12, 0.11). [Display omitted] • Benzimidazole derivative enhances electron-transport ability of the TTA hosts. • Twisted molecular structure makes characteristics of excited states unchanged. • Energy level degeneracy between [TT]3 and T 4 facilitates exciton upconversion. • Ph-AN-BIZ based device achieves high EQE of 10.1 % and low turn-on voltage of 2.7 V. [ABSTRACT FROM AUTHOR]

Abstract 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.

Triplet–triplet annihilation‐based photon upconversion (TTA‐UC) can efficiently generate higher energy photons at low relative fluences. Bulk metal halide perovskites have offered promise in efficiently sensitizing molecular triplet states in the solid state, necessary for the integration of TTA‐UC into device‐based applications. Recent work focused on TTA‐UC from a rubrene triplet annihilator sensitized by perovskite thin films has established relatively efficient charge extraction from the perovskite, forming the triplet exciton in rubrene. Yet, the specifics underpinning charge transfer at the perovskite/rubrene interface are not fully elucidated. To improve device performance and study the properties governing charge transfer at the interface, various organic solvents are explored to treat the perovskite surface. Scanning tunneling microscopy and spectroscopy show a difference in the electronic band structure, where both n‐ and p‐type terminated perovskite surfaces are observed depending on the solvent used. Supported by optical spectroscopy, the impact of the perovskite electronic structure is monitored, indicating that n‐type perovskite sensitizers feature higher TTA‐UC efficiencies due to favorable band bending resulting in efficient hole‐mediated triplet formation. Overall, the tuning of the electronic structure of the perovskite sensitizer through solvent treatment is shown to be a key force in tuning the mechanism of efficient triplet generation. [ABSTRACT FROM AUTHOR]

Light-driven phenomena in organic molecular aggregates underpin several mechanisms relevant to optoelectronic applications. Modeling these processes is essential for aiding the design of new materials and optimizing optoelectronic devices. In this review, we cover the use of different atomistic models, excited-state dynamics, and transport approaches for understanding light-activated phenomena in molecular aggregates, including radiative and nonradiative decay pathways. We consider both intra- and intermolecular mechanisms and focus on the role of conical intersections as facilitators of internal conversion. We explore the use of the exciton models for Frenkel and charge transfer states and the electronic structure methods and algorithms commonly applied for excited-state dynamics. Throughout the review, we analyze the approximations employed for the simulation of internal conversion, intersystem crossing, and reverse intersystem crossing rates and analyze the molecular processes behind single fission, triplet-triplet annihilation, Dexter energy transfer, and Förster energy transfer. [ABSTRACT FROM AUTHOR]

Photochemical upconversion is a process whereby two lower-energy photons are converted into a higher-energy photon by sensitized triplet–triplet annihilation. While recent interest in this process has been motivated by improving the efficiencies of solar cells, many applications are being explored. In this review, we address the underlying physicochemical phenomena that are responsible for photochemical upconversion. We review their kinetics, and the requirements for annihilators and sensitizers to design efficient upconversion systems. We discuss the spin physics of the bi-excitonic interactions and how the spin character of the triplet pairs can fundamentally limit the upconversion efficiency and give rise to the magnetic field effect on delayed photoluminescence. Finally, we address light-matter coupling phenomena that could be employed to enhance photochemical upconversion. [ABSTRACT FROM AUTHOR]

Herein, new deep-blue triplet-triplet annihilation (TTA) molecules, namely 4-(10-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)anthracen-9-yl)benzonitrile (TPIAnCN) and 4-(12-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)chrysen-6-yl)benzonitrile (TPIChCN), are designed, synthesized, and investigated as emitters for organic light-emitting diodes (OLED). TPIAnCN and TPIChCN are composed of polyaromatic hydrocarbons of anthracene (An) and chrysene (Ch) as the cores functionalized with tetraphenylimidazole (TPI) and benzonitrile (CN) moieties, respectively. The experimental and theoretical results verify their excellent thermal properties, photophysical properties, as well as electrochemical properties. Particularly, their emissions are in the deep blue region, with TTA emissions being observed in their thin films. By utilization of these molecules as emitters, deep blue TTA OLEDs with CIE coordinates of (0.15, 0.05), high external quantum efficiency of 6.84%, and high exciton utilization efficiency (ηs) of 48% were fabricated. This result manifests the potential use of chrysene as an alternate building block to formulate new TTA molecules for accomplishing high-performance TTA OLEDs. [ABSTRACT FROM AUTHOR]

Photophysical processes occurring within organic semiconductors is important for designing and fabricating organic solar cells. Copper phthalocyanine (CuPc) is a typical electron acceptor. In this work, the triplet exciton lifetime is prolonged by altering the molecular stacking pattern of the CuPc film. For CuPc thin films, the excited state decays are mainly determined by the triplet-triplet annihilation process. The ultrafast transient absorption measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics show a clearly time-dependent annihilation rate constant with γ∝t−1/2. Annihilation rate constants are determined to be γ0 = (2.87±0.02)×10−20 cm3·s−1/2 and (1.42±0.02)×10−20 cm3·s−1/2 for upright and lying-down configurations, respectively. Compared to the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows longer exciton lifetime and higher absorbance, which are beneficial to organic solar cells. The results in this work have important implications on the design and mechanistic understanding of organic optoelectronic devices. [ABSTRACT FROM AUTHOR]

3D direct-writing via photopolymerization based on two-photon absorption (TPA) can achieve excellent out-of-plane resolution. The key to this technology is a quadratic intensity dependence in photoexciting the TPA photosensitizers. Triplet-triplet annihilation (TTA) also has similar nonlinear light-intensity dependence. As a result, TTA can also generate spatially confined excitation near the beam focus. Combining the photopolymerization reaction with the TTA system composed of palladium porphyrin and diphenylanthracene, 3D direct-writing micro-fabrication based on photopolymerization and the TTA was realized. The out-of-plane resolution can reach 10 µm under continuous-wave laser excitation. TTA-based 3D direct-writing technology does not need an expensive femtosecond pulsed laser, showing the potential of a next-generation 3D printing technology. [ABSTRACT FROM AUTHOR]

Highly spatiotemporal-resolved photomodulation demonstrates promise for investigating key biological events in vivo and in vitro, such as cell signaling pathways, neuromodulation, and tumor treatment without side effects. However, enhancing the performance of photomodulation tools remains challenging due to the limitations of the physicochemical properties of the photoactive molecules. Here, a compact, stable intramolecular π-π stacking conformation forming between the target molecule (naproxen) and the perylene-based photoremovable protecting group is discovered to confine the motion of the photolabile bond and then enhance the photocleavage quantum yield. In conjunction with a red-absorbing photosensitizer, the photocleavage wavelength is extended to the red region via triplet-triplet annihilation. In particular, the triplet lifetime of the prodrug can be extended via the linked steric hindrance to improve the conversion yield via TTA. Using the new photomodulation tool, it is precisely photoreleased cyclooxygenase-2 inhibitors for tumor vascular growth suppression in vivo. In combination with cisplatin, over 90% efficient inhibition of malignant breast tumors is observed via the synergistic tumor treatment strategy. These findings provide a new concept for the rational design of efficient photocleavage and have implications for photomodulating cell signaling pathways in tumor therapy, as well as laying a solid foundation for the development of phototherapeutic approaches., (© 2024 Wiley‐VCH GmbH.)