Triplet Annihilation Upconversion Research Articles

Low blue-hazard white-light emission based on color-tunable triplet-triplet annihilation upconversion

The commonly used artificial light sources, such as fluorescent lamps and white light-emitting diodes, often have a high ratio of blue light emission, which poses potential blue light hazards, especially one of the main culprits leading to eye diseases. Therefore, developing novel white lighting sources with low blue-hazard is highly appreciated. In this work, an air-stable and color-tunable triplet–triplet annihilation upconversion (TTA-UC) mechanism was proposed to realize the low blue-hazard white-light emission. The proposed design was composed of three primary RGB colors from the annihilator (9,10-diphenylanthracene, DPA), the laser excitation source, and the photosensitizer (palladium (II) octaetylporphyrin, PdOEP), respectively. The introduction of oil-in-water (o/w) microemulsion can effectively block the potential oxygen-induced triplet-quenching and benefit high UC efficiency. Moreover, either raising ambient temperatures or adding isobutanol can activate the UC process to yield white-light emission. Notably, the white-light emission with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.33, 0.33) as well as a low ratio of blue emission (14.2 %) was achieved at an ambient temperature of 42 °C. Therefore, the proposed air-stable TTA-UC mechanism can significantly lower the blue-hazard and provide a novel solution for applications in lighting and display.

Advances in functionalized annihilators to extend triplet–triplet annihilation upconversion performances

Triplet–triplet annihilation upconversion (TTA-UC) has made major advances in many emerging fields in recent years, such as solar light harvesting, photocatalysis, biological imaging, and sensing. TTA-UC consists of photosensitizers and annihilators. In addition to acting as emitters, chemical modification of annihilators has expanded their roles to include the formation of organic gel to avoid oxygen-mediated triplet quenching, amplifying the asymmetry factor of circularly polarized luminescence, constructing an upconversion sensor as recognition units, serving as photoremovable protecting groups, and photocatalysts to realize long-wavelength light-driven organic transformations. Here, we will focus on the significant applications of functionalized annihilators other than photoluminescence, which are manifested via chemical modification with other functional units. Finally, we will elaborate on the existent issues with TTA-UC, including challenges in molecular design, material development, and emerging field applications. In accordance with our research experience, we will propose potential solutions.

Photophysical properties and excited-state dynamics of donor–acceptor–heavy-atom molecules and their application in triplet–triplet annihilation upconversion

A novel series of donor–acceptor–heavy-atom (D–A–H) molecules that simultaneously implement both photoinduced electron transfer and heavy atom effect strategies.

Indication of Intramolecular Triplet–Triplet Annihilation Upconversion in Organic Light‐Emitting Diodes

AbstractIt is well known that triplet–triplet annihilation upconversion (TTU) can enhance electroluminescence (EL) efficiency by converting two low‐energy triplet excitons into one high‐energy singlet exciton. However, conventional intermolecular TTU requires high doping concentration to activate, limiting device architecture. Here, six dimer molecules comprised of two anthracene units connected by a spacer unit are investigated, which have a possibility of the intramolecular TTU process. As a result, two dimer molecules show high TTU efficiencies close to the theoretical maximum even in dispersed, diluted conditions. By comparing these molecular conformations, a correlation is found between the dimer structures of two anthracene units and the TTU activity in the diluted condition, i.e., the 3D molecular structure with the exciton utilizing efficiency.

Π‐Radical Photosensitizer for Highly Efficient and Stable Near‐Infrared Photon Upconversion

AbstractNear‐infrared (NIR)‐to‐blue triplet–triplet annihilation upconversion (TTA‐UC) exhibits substantial potential for diverse applications, encompassing photovoltaics, photocatalysis, bioimaging, and photodynamic therapy. A major challenge in this field, however, is attaining high upconversion quantum yields (ΦUC) without using transition metals as NIR photosensitizers. This research presents an innovative organic π‐radical photosensitizer (TTM‐TPA) featuring extended absorption in the NIR region (≈760 nm) that is capable of generating pronounced anti‐Stokes emissions when coupled with suitable triplet acceptors. By eliminating energy loss associated with intersystem crossing and promoting rapid doublet–triplet energy transfer processes, the binary TTM‐TPA/perylene system achieves ΦUC values of up to 6.8% and an anti‐Stokes shift of 0.93 eV. Notably, the TTA‐UC system demonstrates exceptional stability when subjected to intense 733 nm laser irradiation (4 W cm−2), maintaining nearly constant upconversion intensity after 4 h. These findings underscore the considerable potential of doublet‐sensitized TTA‐UC for a broad array of practical applications.

Bodipy−corrole tetrads as heavy−atom−free triplet photosensitizers: Study of the energy transfer and application in triplet−triplet annihilation upconversion

Free base corrole can generate triplet state without invoking of heavy−atom effect, however, its weak and narrow absorption band in the visible spectral range make corrole not an ideal triplet photosensitizer. Herein, novel Bodipy−corrole tetrads were prepared by attaching visible light-harvesting antennae of Bodipy or electron spin converter with fullerene units (2B−Tur−Cor and B−C60−Tur−Cor) to solve the above problems, which show a stronger and broader absorption band in the visible region, and the maximum molar absorption coefficient is up to 2.58 × 105 M−1 cm−1 at 504 nm for 2B−Tur−Cor. By using of fluorescence spectroscopy, steady-state and time-resolved transient absorption spectroscopic methods, the singlet energy transfer progress from the Bodipy moiety to the corrole moiety was confirmed. Through fluorescence lifetimes and fluorescence excitation spectrum study, the rate constant of the singlet energy transfer was calculated as 9.58 × 107 s−1 for 2B−Tur−Cor and 2.45 × 108 s−1 for B−C60−Tur−Cor, respectively. The TD−DFT computation suggests that the first and second triplet state is localized in the corrole moiety and the Bodipy moiety for 2B−Tur−Cor, respectively, and the triplet state energy was calculated to be 1.37 eV and 1.61 eV, respectively. Nanosecond time−resolved transient difference absorption spectra show that the triplet excited states of these tetrads are localized on corrole unit, the lifetime of 2B−T−Cor is up to 143 μs, however, the lifetime of B−C60−Tur−Cor is only 3.3 μs. Long-lived triplet excited states play important role in applications of the triplet photosensitizers in photocatalysis or other photophysical processes concerning triplet–triplet−energy−transfer. As a proof of concept, these tetrads were used as heavy−atom−free organic triplet photosensitizers for triplet−triplet annihilation based upconversion. The upconversion quantum yield was up to 5.86% for 2B−Tur−Cor, however, it is only 0.52% for B−C60−Tur−Cor.

Temperature dependent triplet-exciton relay from thermally activated delayed fluorescence (TADF) to self-sensitized triplet–triplet annihilation (TTA) of asymmetrical diphenylsulfone-based blue emitter

The utilization of triplet excitons is greatly concerned in organic luminophores. For thermally activated delayed fluorescence (TADF) emitters, the reverse intersystem crossing (RISC) which activates the triplet excitons at low temperature remains challenging without thermal stimulation. Herein, a temperature-dependent promotion relay of the triplet excitons from TADF to self-sensitized triplet − triplet annihilation up-conversion (TTA-UC) was demonstrated in an asymmetrical TADF luminophore based on 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor and diphenylfonyl (DPS) acceptor. Single-crystal structure analysis combined with theoretical calculations revealed that the alternate strong-and-weak contact of the molecules reduced the singlet–triplet splitting energy (ΔEST), amplified the singlet–triplet coupling (SOC) strength and exciton coupling (EC), leading to highly efficient RISC process over a wide range of temperatures. Moreover, benefiting from the extreme small ΔEST value and large SOC strength, highly efficient blue OLEDs were achieved with the highest CEmax, PEmax and EQEmax of 32.0 cd A-1, 33.5 lm W−1 and 14.7%, and low efficiency roll-off. This work provides a design guidance for high utilization of the triplet excitons through RISC to improve the luminescence efficiency and reduce the device efficiency roll-off.

Exploring the Key Factors of TADF Materials as Sensitizers: Toward High‐performance Triplet Fusion Upconversion

AbstractEmploying metal‐free thermally activated delayed fluorescence (TADF) materials as novel photosensitizers (PSs) for triplet–triplet annihilation upconversion (TTA‐UC) systems has received great attention. Nevertheless, how to choose suitable TADF materials is still a gap that deserves further in‐depth research. Herein, by systematically investigating the three TADF‐based TTA‐UC systems, it is found that TADF materials with multiple resonance (MR) effect are more suitable PSs for high‐performance TTA‐UC systems than those with intramolecular charge transfer (ICT) properties. Therefore, unlike a negligible maximum UC quantum efficiency (ΦUC’, with a theoretical maximum of 100%) of 0.05% for ICT‐1 with a highly twisted molecular structure, MR‐TADF materials MR‐1 and MR‐2 offer superior UC performance with ΦUC’ values above 12.7%. All these results suggest that employing MR‐TADF materials as the PS is the key to achieving efficient TTA‐UC performance.

Nanoengineering Triplet–Triplet Annihilation Upconversion: From Materials to Real-World Applications

Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet-triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

Truxene-linked di-coumarin-mono-corrole tetrad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

Triplet−triplet annihilation upconversion (TTA-UC) has drawn widespread interest because of its great potential applications. However, it is still a challenge to design and synthesize efficient triplet photosensitizers without heavy atoms for TTA-UC. In this work, a triplet photosensitizer tetrad 2C-T-Cor based on two coumarin moieties and one corrole moiety with a truxene as the bridge was synthesized and used in TTA-UC as a novel heavy atom-free organic photosensitizer (perylene bisimide (PBI) was used as triplet acceptor). The photophysical properties were investigated by means of steady-state UV–Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, three-dimensional excitation emission matrix fluorescence spectra, and cyclic voltammetry. With photo-excitation of the coumarin unit at 420 nm, the intramolecular singlet–singlet energy transfer takes place from the coumarin moiety to the corrole unit with a transfer efficiency of 63.9%. Electrochemical study indicated that the intramolecular electron transfer was thermodynamically prohibited in toluene owing to the positive ΔGCS for charge-separation. A triplet excited state was investigated for 2C-T-Co via nanosecond time-resolved transient absorption and the triplet lifetime is 118 μs. 2C-T-Co can act as the triplet photosensitizer for the application of TTA-UC, and the upconversion quantum yield of 0.98% was observed.

Boosting photocatalytic hydrogen evolution via triplet–triplet annihilation upconversion

To enhance the light-harvesting ability of photocatalytic splitting water system, triplet–triplet annihilation upconversion (TTA-UC) is integrated with photocatalysis directly for the first time via a detergent-free ternary toluene/isopropanol/water microemulsion system. TTA-UC pairs containing PtOEP/PdTPTBP/PdTPNEt2P and perylene are combined with Cd0.5Zn0.5S for photocatalytic hydrogen production. These TTA-UC pairs convert visible light with the wavelength >510 nm that cannot be absorbed directly by Cd0.5Zn0.5S into blue light with smaller wavelength. Cd0.5Zn0.5S is then sensitized by this upconverted blue light to produce hydrogen. More importantly, the hydrogen production rate (8.44 mmol g-1h−1) of this integrated photocatalytic system is twice as high as that of the pure Cd0.5Zn0.5S microemulsion (4.04 mmol g-1h−1) under visible light (λ > 420 nm) irradiation, which is the best performance for the TTA-UC integrated photocatalytic system for hydrogen evolution ever reported so far. The high upconversion efficiency and high utilization efficiency of upconversion emission in microemulsion are responsible for the significantly promoted catalytic performance of this integrated photocatalytic system. The result demonstrates a simple strategy to integrate TTA-UC with photocatalyst for enhancing the hydrogen production efficiency. Considering the easily-tunable excitation/emission wavelength of TTA-UC pairs, this method could be used to integrate TTA-UC with a variety of photocatalysts.

Recent Advances in the Photoreactions Triggered by Porphyrin-Based Triplet-Triplet Annihilation Upconversion Systems: Molecular Innovations and Nanoarchitectonics.

Triplet–triplet annihilation upconversion (TTA-UC) is a very promising technology that could be used to convert low-energy photons to high-energy ones and has been proven to be of great value in various areas. Porphyrins have the characteristics of high molar absorbance, can form a complex with different metal ions and a high proportion of triplet states as well as tunable structures, and thus they are important sensitizers for TTA-UC. Porphyrin-based TTA-UC plays a pivotal role in the TTA-UC systems and has been widely used in many fields such as solar cells, sensing and circularly polarized luminescence. In recent years, applications of porphyrin-based TTA-UC systems for photoinduced reactions have emerged, but have been paid little attention. As a consequence, this review paid close attention to the recent advances in the photoreactions triggered by porphyrin-based TTA-UC systems. First of all, the photochemistry of porphyrin-based TTA-UC for chemical transformations, such as photoisomerization, photocatalytic synthesis, photopolymerization, photodegradation and photochemical/photoelectrochemical water splitting, was discussed in detail, which revealed the different mechanisms of TTA-UC and methods with which to carry out reasonable molecular innovations and nanoarchitectonics to solve the existing problems in practical application. Subsequently, photoreactions driven by porphyrin-based TTA-UC for biomedical applications were demonstrated. Finally, the future developments of porphyrin-based TTA-UC systems for photoreactions were briefly discussed.

Tetrathienothiophene Porphyrin as a Metal-Free Sensitizer for Room-Temperature Triplet-Triplet Annihilation Upconversion.

Optically excited triplet states of organic molecules serve as an energy pool for the subsequent processes, either photon energy downhill, such as room temperature phosphorescence, or photon energy uphill process—the triplet–triplet annihilation upconversion (TTA-UC). Manifestation of a high intersystem crossing coefficient is an unavoidable requirement for triplet state formation, following the absorption of a single photon. This requirement is even more inevitable if the excitation light is non-coherent, with moderate intensity and extremely low spectral power density, when compared with the light parameters of 1 Sun (1.5 AM). Coordination of a heavy atom increases substantially the probability of intersystem crossing. Nevertheless, having in mind the global shortage in precious and rare-earth metals, identification of metal-free organic moieties able to form triplet states becomes a prerequisite for environmental friendly optoelectronic technologies. This motivates us to synthesize a metal-free thienothiophene containing porphyrin, based on a condensation reaction between thienothiophene-2-carbaldehyde and pyrrole in an acidic medium by modified synthetic protocol. The upconversion couple tetrathienothiophene porphyrin/rubrene when excited at λ = 658 nm demonstrates bright, delayed fluorescence with a maximum emission at λ = 555 nm. This verifies our hypothesis that the ISC coefficient in thienothiophene porphyrin is efficient in order to create even at room temperature and low-intensity optical excitation densely populated organic triplet ensemble and is suitable for photon energy uphill processes, which makes this type of metal-free sensitizers even more important for optoelectronic applications.

Applying triplet-triplet annihilation upconversion in degradation of oxidized lignin model with good selectivity

TTA-UC photocatalyst achieved selective depolymerization of oxidized lignin model with high efficiency under excitation light of wavelength > 510 nm. • TTA-UC system is used as catalyst for the first time in lignin model degradation. • Achieved a high degradation efficiency, better selectivity when TTA-UC as catalyst. • TTA-UC should be a group of very promising catalysts for the decomposition of lignin. Extraction of higher value products from lignin via photoredox catalysis is crucial to the economic viability of integrated biorefineries. Triplet–triplet annihilation upconversion (TTA-UC) as a photon modulation technique, including a sensitizer and an emitter, could convert of long-wavelength light into higher-energy photons. In this work, photocatalysts based on TTA-UC are used in oxidized lignin model degradation for the first time. Under the irradiation of light with wavelength > 510 nm, TTA-UC catalyst comprised of 5,10,15,20-tetra(N,N-diethylaniline)porphyrin palladium (PdTPNEt2P) and perylene achieves a comparable high degradation efficiency with that perylene as catalyst used directly (irradiated at λ > 420 nm), but a higher product selectivity is obtained. Compared with conventional tetraphenyl porphyrin palladium (PdTPP), PdTPNEt2P is a better sensitizer of TTA-UC catalyst with a 13 times improvement on the lignin degradation efficiency (from 7% to 99%). That is because the introduction of four electron-donating groups (N,N-diethyl amino) onto PdTPNEt2P could remarkably reduce its oxidation potential and thus the electron transfer from reductive sacrificial reagents to PdTPNEt2P is efficiently hindered and ensured the occurrence of TTA-UC. It suggests that the redox potential of TTA-UC sensitizer should be considered when applying it to redox reactions. This work also indicates that TTA-UC catalyst could enhance the product selectivity with a relatively lower energy photon excitation via avoiding side reactions induced by high energy excitation. Considering the flexibility of the molecular structure of the TTA-UC sensitizer and emitter, the application of TTA-UC in lignin photocatalytic decomposition should be very promising.

Photon-upconverters for blue organic light-emitting diodes: a low-cost, sky-blue example.

In the research ecosystem's quest towards having deployable organic light-emitting diodes with higher-energy emission (e.g., blue light), we advocate focusing on fluorescent emitters, due to their relative stability and colour purity, and developing design strategies to significantly improve their efficiencies. We propose that all triplet–triplet annihilation upconversion (TTA-UC) emitters would make good candidates for triplet fusion-enhanced OLEDs (“FuLEDs”), due to the energetically uphill nature of the photophysical process, and their common requirements. We demonstrate this with the low-cost sky-blue 1,3-diphenylisobenzofuran (DPBF). Having satisfied the criteria for TTA-UC, we show DPBF as a photon upconverter (Ith 92 mW cm−2), and henceforth demonstrate it as a bright emitter for FuLEDs. Notably, the devices achieved 6.5% external quantum efficiency (above the ∼5% threshold without triplet contribution), and triplet-exciton-fusion-generated fluorescence contributes up to 44% of the electroluminescence, as shown by transient measurements. Here, triplet fusion translates to a quantum yield (ΦTTA-UC) of 19%, at an electrical excitation of ∼0.01 mW cm−2. The enhancement is meaningful for commercial blue OLED displays. We also found DPBF to have decent hole mobilities of ∼0.08 cm2 V−1 s−1. This additional finding can lead to DPBF being used in other capacities in various printable electronics.

Highly efficient triplet–triplet annihilation upconversion in polycaprolactone: application to 3D printable architectures and microneedles

This research reports the next generation of solid-state triplet–triplet annihilation upconversion (TTA-UC) host material (polycaprolactone, PCL) for highly efficient, processable, and biocompatible solid-state TTA-UC.

Luminescence Ratiometric Nanothermometry Regulated by Tailoring Annihilators of Triplet–Triplet Annihilation Upconversion Nanomicelles

AbstractTriplet–triplet annihilation (TTA) upconversion is a special non‐linear photophysical process that converts low‐energy photons into high‐energy photons based on sensitizer/annihilator pairs. Here, we constructed a novel luminescence ratiometric nanothermometer based on TTA upconversion nanomicelles by encapsulating sensitizer/annihilator molecules into a temperature‐sensitive amphiphilic triblock polymer and obtained good linear relationships between the luminescence ratio (integrated intensity ratio of upconverted luminescence peak to the downshifted phosphorescence peak) and the temperature. We also found chemical modification of annihilators would rule out the interference of the polymer concentration and stereochemical engineering of annihilators would readily regulate the thermal sensitivity.

Luminescence Ratiometric Nanothermometry Regulated by Tailoring Annihilators of Triplet–Triplet Annihilation Upconversion Nanomicelles

AbstractTriplet–triplet annihilation (TTA) upconversion is a special non‐linear photophysical process that converts low‐energy photons into high‐energy photons based on sensitizer/annihilator pairs. Here, we constructed a novel luminescence ratiometric nanothermometer based on TTA upconversion nanomicelles by encapsulating sensitizer/annihilator molecules into a temperature‐sensitive amphiphilic triblock polymer and obtained good linear relationships between the luminescence ratio (integrated intensity ratio of upconverted luminescence peak to the downshifted phosphorescence peak) and the temperature. We also found chemical modification of annihilators would rule out the interference of the polymer concentration and stereochemical engineering of annihilators would readily regulate the thermal sensitivity.

Spin Statistics for Triplet-Triplet Annihilation Upconversion: Exchange Coupling, Intermolecular Orientation, and Reverse Intersystem Crossing.

Triplet–triplet annihilation upconversion (TTA-UC) has great potential to significantly improve the light harvesting capabilities of photovoltaic cells and is also sought after for biomedical applications. Many factors combine to influence the overall efficiency of TTA-UC, the most fundamental of which is the spin statistical factor, η, that gives the probability that a bright singlet state is formed from a pair of annihilating triplet states. The value of η is also critical in determining the contribution of TTA to the overall efficiency of organic light-emitting diodes. Using solid rubrene as a model system, we reiterate why experimentally measured magnetic field effects prove that annihilating triplets first form weakly exchange-coupled triplet-pair states. This is contrary to conventional discussions of TTA-UC that implicitly assume strong exchange coupling, and we show that it has profound implications for the spin statistical factor η. For example, variations in intermolecular orientation tune η from to through spin mixing of the triplet-pair wave functions. Because the fate of spin-1 triplet-pair states is particularly crucial in determining η, we investigate it in rubrene using pump–push–probe spectroscopy and find additional evidence for the recently reported high-level reverse intersystem crossing channel. We incorporate all of these factors into an updated model framework with which to understand the spin statistics of TTA-UC and use it to rationalize the differences in reported values of η among different common annihilator systems. We suggest that harnessing high-level reverse intersystem crossing channels in new annihilator molecules may be a highly promising strategy to exceed any spin statistical limit.

Photoinduced energy transfer in truxene-linked zinc porphyrin–fullerene-corrole tetrad and its application in triplet−triplet annihilation upconversion

Triplet photosensitizers (PSs) have attracted widespread interest due to their application in photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water splitting, and triplet–triplet annihilation (TTA) upconversion. Herein, the triad T-P-Co and tetrad T-C60-P-Co were synthesized, characterized, and applied in triplet–triplet annihilation (TTA) upconversion. The photophysical processes were investigated with steady-state UV–visible absorption and fluorescence spectroscopies, nanosecond time-resolved transient absorption spectroscopy and electrochemical characterization. T-P-Co and T-C60-P-Co show efficient intramolecular singlet–singlet energy transfer from the porphyrin moiety to corrole/fullerene moiety. The energy transfer efficiency is nearly 100% for T-P-Co and 69.4% for T-C60-P-Co, respectively. Electrochemical investigation shows that the photo-induced intramolecular electron transfer of the excited triad T-P-Co and tetrad T-C60-P-Co is thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of T-P-Co and T-C60-P-Co are both distributed on the corrole moiety. The triplet lifetimes of T-P-Co and T-C60-P-Co are 86.9 μs and 73.3 μs, respectively. The triad T-P-Co and tetrad T-C60-P-Co were used as triplet photosensitizers for TTA upconversion, and upconversion quantum yields of 0.76% and 0.84% were observed, respectively.