Efficient Triplet-triplet Annihilation Upconversion Research Articles

Electronic State Engineering in Perovskite-Cerium-Composite Nanocrystals toward Enhanced Triplet Annihilation Upconversion.

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.

Origin of the Delayed Fluorescence by Triplet–Triplet Annihilation in Solids with a Power Law Exponent of between −1/2 and −1

Triplet–triplet annihilation (TTA) merges low electronic energies in two molecules to high electronic energy in one molecule, which, following triplet sensitization, allows us to achieve high-efficiency conversion using low-intensity light. Although efficient TTA up-conversion in solutions has been reported, the TTA up-conversion in solid matrixes is preferred for practical applications. However, the emission decay kinetics after pulsed-light excitation is more complex in solids than in liquids, and the process underlying the different TTA kinetics has not yet been elucidated. Herein, we report that the complexity in the TTA kinetics in solid matrixes can originate from processes such as slow mixing of triplet excitons by migration, strong dependence on the initial distribution of triplet excitons, and an anisotropic random walk of excitons. We show theoretically that power-law fluorescence with an exponent of between −1/2 and −1 can originate from the slow diffusional mixing of excitons in one and three dimensions, where the initial exciton generation is uniformly distributed. By analyzing experimental data, we show that rich fluorescence kinetics observed experimentally by varying the temperature in molecular solids can be deeply understood in terms of bulk and nonbulk TTA under dispersive as well as normal diffusion.

Optically Coupled PtOEP and DPA Molecules Encapsulated into PLGA-Nanoparticles for Cancer Bioimaging

Triplet-triplet annihilation upconversion (TTA-UC) nanoparticles (NPs) have emerged as imaging probes and therapeutic probes in recent years due to their excellent optical properties. In contrast to lanthanide ion-doped inorganic materials, highly efficient TTA-UC can be generated by low excitation power density, which makes it suitable for clinical applications. In the present study, we used biodegradable poly(lactic-co-glycolic acid) (PLGA)-NPs as a delivery vehicle for TTA-UC based on the heavy metal porphyrin Platinum(II) octaethylporphyrin (PtOEP) and the polycyclic aromatic hydrocarbon 9,10-diphenylanthracene (DPA) as a photosensitizer/emitter pair. TTA-UC-PLGA-NPs were successfully synthesized according to an oil-in-water emulsion and solvent evaporation method. After physicochemical characterization, UC-efficacy of TTA-UC-PLGA-NPs was assessed in vitro and ex vivo. TTA-UC could be detected in the tumour area 96 h after in vivo administration of TTA-UC-PLGA-NPs, confirming the integrity and suitability of PLGA-NPs as a TTA-UC in vivo delivery system. Thus, this study provides proof-of-concept that the advantageous properties of PLGA can be combined with the unique optical properties of TTA-UC for the development of advanced nanocarriers for simultaneous in vivo molecular imaging and drug delivery.

Efficient Triplet-Triplet Annihilation Upconversion in Solution and Hydrogel Enabled by an S-T Absorption Os(II) Complex Dyad with an Elongated Triplet Lifetime.

A new Os(II) complex dyad featuring direct singlet-to-triplet (S-T) absorption and intramolecular triplet energy transfer (ITET) with lifetime up to 7.0 μs was designed to enhance triplet energy transfer efficiency during triplet-triplet annihilation upconversion (TTA-UC). By pairing with 9,10-bis(phenylethynyl)anthracene (BPEA) as a triplet acceptor, intense upconverted green emission in deaerated solution was observed with unprecedented TTA-UC emission efficiency up to 26.3% (with a theoretical maximum efficiency of 100%) under photoexcitation in the first biological transparency window (650-900 nm). Meanwhile, a 7.1% TTA-UC emission efficiency was acquired in an air-saturated hydrogel containing the photosensitizer and a newly designed hydrophilic BPEA derivative. This ITET mechanism would inspire further development of a highly efficient TTA-UC system for biological fields and renewable energy production.

Application of Triplet-Triplet Annihilation Upconversion in Organic Optoelectronic Devices: Advances and Perspectives.

Organic semiconductor materials have been widely used in various optoelectronic devices due to their rich optical and/or electrical properties, which are highly related to their excited states. Therefore, how to manage and utilize the excited states in organic semiconductors is essential for the realization of high-performance optoelectronic devices. Triplet-triplet annihilation (TTA) upconversion is a unique process of converting two non-emissive triplet excitons to one singlet exciton with higher energy. Efficient optical-to-electrical devices can be realized by harvesting sub-bandgap photons through TTA-based upconversion. In electrical-to-optical devices, triplets generated after the combination of electrons and holes also can be efficiently utilized via TTA, which resulted in a high internal conversion efficiency of 62.5%. Currently, many interesting explorations and significant advances have been demonstrated in these fields. In this review, a comprehensive summary of these intriguing advances on developing efficient TTA upconversion materials and their application in optoelectronic devices is systematically given along with some discussions. Finally, the key challenges and perspectives of TTA upconversion systems for further improvement for optoelectronic devices and other related research directions are provided. This review hopes to provide valuable guidelines for future related research and advancement in organic optoelectronics.

Highly Ordered 2D-Assemblies of Phase-Segregated Block Molecules for Upconverted Linearly Polarized Emission.

Materials based on the laminar ordering of self-assembled molecules have a unique potential for applications requiring efficient energy migration through densely packed chromophores. Here, employing molecular assemblies of coil-rod-coil block molecules for triplet-triplet annihilation upconversion (TTA-UC) based on triplet energy migration with linearly polarized emission is reported. By covalently attaching discrete-length oligodimethylsiloxane (oDMS) to 9,10-diphenylanthracene (DPA), highly ordered 2D crystalline DPA sheets separated by oDMS layers are obtained. Transparent films of this material doped with small amounts of triplet sensitizer PtII octaethylporphyrin show air-stable TTA-UC under non-coherent excitation. Upon annealing, an increase in TTA-UC up to two orders of magnitude is observed originating from both an improved molecular ordering of DPA and an increased dispersion of the sensitizer. The molecular alignment in millimeter-sized domains leads to upconverted linearly polarized emission without alignment layers. By using a novel technique, upconversion imaging microscopy, the TTA-UC intensity is spatially resolved on a micrometer scale to visually demonstrate the importance of molecular dispersion of sensitizer molecules for efficient TTA-UC. The reported results are promising for anti-counterfeiting and 3D night-vision applications, but also exemplify the potential of discrete oligodimethylsiloxane functionalized chromophores for highly aligned and densely packed molecular materials.

Triplet-triplet annihilation photon upconversion materials coated with TiO<sub>2</sub> nanocapsules

Triplet-triplet annihilation (TTA) upconversion is a technology that can convert low-energy light waves to high-energy light waves. The system is mainly composed of triplet sensitizers and energy receptors. The conversion mainly takes place in solution and is difficult to use as a whole molecule, which will greatly limit its application field. In this paper, by using the micelle soft template method to encapsulate the sensitizer and annihilant into the titanium dioxide shell, a highly efficient upconversion nanocapsule (TTA-UCNP) based on triplet-triplet annihilation upconversion was successfully prepared. In this study, BDP-I2 was used as the triplet sensitizer, perylene as the energy receptor, and soybean oil as the solvent. These newly synthesized nanocapsules can not only keep the up-conversion system in a solution environment, but also effectively block the quenching of triplet states by oxygen, and successfully achieve efficient upconversion luminescence in the aqueous phase. In the preparation process of the upconversion nanocapsules, we studied the effects of the type of surfactant, the amount of acid and the amount of butyl titanate on the upconversion luminescence intensity of the nanocapsules. The structure of nanocapsules was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), and a highly efficient TTA upconversion luminescent material coated with titanium dioxide nanocapsules was successfully prepared.

Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion.

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5'-position of the complexes. For the first time, a long lived triplet excited state (τT =296 μs) along with efficient ISC ability (ΦΔ =71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT. The results revealed that molecular geometry and energy gap between the charge transfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes. An efficient triplet-triplet annihilation upconversion system was devised for the first time using a SBCT architecture as triplet photosensitizer, reaching a high upconversion quantum yield of 6.2 %. Our findings provide a blueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionary photochemical applications.

Highly efficient triplet-triplet annihilation upconversion in high viscosity phthalate ester media

Movement of molecules are always affected by the surrounding solvent molecules, so solvents have an essential effect in triplet-triplet annihilation (TTA) upconversion. In this article, the TTA upconversion system with Diiodo-BODIPY (MBDP-2I) as sensitizer and perylene as acceptor in six solvents of differing viscosity were investigated (toluene, 1,4-dioxane and four high viscosity phthalate ester solvents DMP, DBP, DNOP and DOP). The upconversion efficiency was positively correlated with the solvent viscosity. Among the six solvents, the highest upconversion quantum efficiency of 9.59% was observed in high viscosity DOP (η = 72.4 mPa s), almost 1.5 times higher than that in 1,4-dioxane (η = 3.2 mPa s, ΦUC = 6.36%). The efficient upconversion was attributed to the long-lived triplet excited states of MBDP-2I, efficient fluorescence quantum yield of perylene and efficient TTA in high viscosity phthalate esters solvents. The mechanism underlying the highly efficient TTA upconversion in the high viscosity phthalate esters solvents was investigated by solvent cage effect. In addition, significant room temperature phosphorescence of MBDP-2I at 765 nm was observed in the viscous phthalate esters solvents.

Precharging Photon Upconversion: Interfacial Interactions in Solution-Processed Perovskite Upconversion Devices.

Recent advances in perovskite-sensitized photon upconversion via triplet-triplet annihilation (TTA) in rubrene have yielded several unanswered questions about the underlying mechanism and processes occurring at the interface. In particular, the near-infrared perovskite emission is not significantly quenched and a rapid reversible "photobleach" of the upconverted emission can be observed under fairly low excitation densities of 3.2 mW/cm2. In this contribution, we investigate the perovskite/rubrene interface in more detail and conclude that noncovalent interactions between the organic layer and the perovskite result in surface trap passivation. In addition, band bending results in a space charge region at the perovskite/rubrene interface, which "precharges" the rubrene with holes. Upon initial illumination, electrons can rapidly transfer to the excited triplet state of rubrene, followed by efficient TTA upconversion. As the device is continuously illuminated, the existing holes are consumed and a new equilibrium is reached, resulting in the previously investigated steady-state upconversion efficiency.

Efficient Triplet-Triplet Annihilation Upconversion with an Anti-Stokes Shift of 1.08 eV Achieved by Chemically Tuning Sensitizers.

A series of Pt(II)-Schiff base complexes were synthesized as triplet sensitizers for the purpose of tuning the singlet and triplet energy levels so as to minimize energy loss during triplet-triplet annihilation (TTA) upconversion (UC). A deep-red to blue TTA-UC was achieved with an unprecedentedly large anti-Stokes shift of 1.08 eV. UC quantum yields of up to 21% (with a theoretical maximum efficiency of 50%) were observed in solution. The complexes also showed efficient UC emission in air-saturated hydrogels with a UC quantum yield up to 14.8%, which is much higher than the highest previously reported value. The low threshold excitation intensity provided by the present system offers promising potential for application in terrestrial solar energy conversion.

Flexible and Micropatternable Triplet-Triplet Annihilation Upconversion Thin Films for Photonic Device Integration and Anticounterfeiting Applications.

Triplet-triplet annihilation upconversion (TTA-UC) has recently drawn widespread interest for its capacity to harvest low-energy photons and to broaden the absorption spectra of photonic devices, such as solar cells. Although conceptually promising, effective integration of TTA-UC materials into practical devices has been difficult due to the diffusive and anoxic conditions required in TTA-UC host media. Of the solid-state host materials investigated, rubbery polymers facilitate the highest TTA-UC efficiency. To date, however, their need for long-term oxygen protection has limited rubbery polymers to rigid film architectures that forfeit their intrinsic flexibility. This study introduces a new multilayer thin-film architecture, in which scalable solution processing techniques are employed to fabricate flexible, photostable, and efficient TTA-UC thin films containing layers of oxygen barrier and host polymers. This breakthrough material design marks a crucial advance toward TTA-UC integration within rigid and flexible devices alike. Moreover, it introduces new opportunities in unexplored applications such as anticounterfeiting. Soft lithography is incorporated into the film fabrication process to pattern TTA-UC host layers with a broad range of high-resolution microscale designs, and superimposing host layers with customized absorption, emission, and patterning ultimately produces proof-of-concept anticounterfeiting labels with advanced excitation-dependent photoluminescent security features.

Room-Temperature High-Efficiency Solid-State Triplet-Triplet Annihilation Up-Conversion in Amorphous Poly(olefin sulfone)s.

Triplet-triplet annihilation up-conversion (TTA-UC) is a developing technology that can enable spectral conversion under sunlight. Previously, it was found that efficient TTA-UC can be realized in polymer hosts for temperatures above the polymer's glass transition (T > Tg). In contrast, TTA-UC with high quantum yield for temperatures below Tg is rarely reported. In this article, we report new polymer hosts in which efficient TTA-UC is observed well below Tg, when the polymer is in a fully solid state. The four poly(olefin sulfone) hosts were loaded with upconversion dyes, and absolute quantum yields of TTA-UC (ηTTA-UC) were measured. The highest value of ηTTA-UC = 2.1% was measured for poly(1-dodecene sulfone). Importantly, this value was the same in vacuum and at ambient conditions, indicating that the host material acts as a good oxygen barrier. We performed time-resolved luminescence experiments in order to elucidate the impact of elementary steps of TTA-UC. In addition to optical characterization, we used magic angle spinning solid-state NMR experiments to estimate the T2 transverse relaxation time. Relatively long T2 times measured for poly(olefin sulfone)s indicate an enhanced nanoscale fluidity in the studied (co)polymers, which unexpectedly coexists with a rigidity on the macroscale. This would explain the exceptional triplet energy transfer between the guest molecules, despite the macroscopic rigidity.

Highly Efficient Triplet Photosensitizers: A Systematic Approach to the Application of Ir(III) Complexes containing Extended Phenanthrolines.

A series of Ir(III) complexes, based on 1,10-phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross-coupling reactions using a "chemistry-on-the-complex" method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light-harvesting group. Intense UV/Vis absorption was observed for the Ir(III) complexes with two light-harvesting groups at the 3- and 8-positions of the phenanthroline. The asymmetric Ir(III) complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time-resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet-oxygen ((1) O2 ) sensitization and triplet-triplet annihilation (TTA)-based upconversion were explored. Highly efficient TTA upconversion (ΦUC =28.1 %) and (1) O2 sensitization (ΦΔ =97.0 %) were achieved for the asymmetric Ir(III) complex, which showed intense absorption in the visible region (λabs =482 nm, ϵ=50900 m(-1) cm(-1) ) and had a long-lived triplet excited state (53.3 μs at RT).

Enhanced Precision of Nanoparticle Phototargeting in Vivo at a Safe Irradiance.

A large proportion of the payload delivered by nanoparticulate therapies is deposited not in the desired target destination but in off-target locations such as the liver and spleen. Here, we demonstrate that phototargeting can improve the specific targeting of nanoparticles to tumors. The combination of efficient triplet-triplet annihilation upconversion (TTA-UC) and Förster resonance energy transfer (FRET) processes allowed in vivo phototargeting at a safe irradiance (200 mW/cm(2)) over a short period (5 min) using green light.

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

A series of directly meso-meso-linked Pd-porphyrin oligomers (PdDTP-M, PdDTP-D, and PdDTP-T) have been prepared. The absorption region and the light-harvesting ability of the Pd-porphyrin oligomers are broadened and enhanced by increasing the number of Pd-porphyrin units. Triplet-triplet annihilation upconversion (TTA-UC) systems were constructed by utilizing the Pd-porphyrin oligomers as the sensitizer and 9,10-diphenylanthracene (DPA) as the acceptor in deaerated toluene and green-to-blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet-triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP-M]/DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respectively, under an excitation power density of 500 mW cm(-2) . The photophysical processes of the TTA-UC systems have been investigated in detail. The higher triplet-triplet annihilation upconversion quantum efficiency observed in the [PdDTP-D]/DPA system can be rationalized by the enhanced light-harvesting ability of PdDTP-D at 532 nm. Under the same experimental conditions, the [PdDTP-D]/DPA system produces more (3) DPA* than the other two TTA-UC systems, benefiting the triplet-triplet annihilation process. This work provides a useful way to develop efficient TTA-UC systems with broad spectral response by using Pd-porphyrin oligomers as sensitizers.

C70] Fullerene-sensitized triplet–triplet annihilation upconversion

We herein report the first instance of using pristine C70 as a heavy-atom free organic sensitizer for efficient triplet-triplet annihilation upconversion (UC) for both green-to-blue and red-to-green UC using 9,10-bis(phenylethynyl)anthracene and perylene as acceptors, respectively. C70 achieved quantum yields of 8% and 0.8% for green-to-blue and red-to-green UC, 25 to 35 times higher than C60, and showed improved stability under continuous laser irradiation compared to the benchmark platinum(II)-octaethylporphyrin.

Thienyl-substituted BODIPYs with strong visible light-absorption and long-lived triplet excited states as organic triplet sensitizers for triplet–triplet annihilation upconversion

Thienyl-substituted BODIPY derivatives were prepared as organic triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion. The photophysical properties of the sensitizers were fully studied with steady state and time-resolved spectroscopy, as well as density functional theory (DFT) calculations. Sensitizers with both 2-monothienyl substituted (BI-1) and 2,6-dithienyl substituted BODIPY cores (BI-2) were prepared. These sensitizers show strong absorption in the visible range. Interestingly, the sensitizers show large Stokes shifts (up to 86 nm) vs. small Stokes shifts (ca. 15 nm) for the normal BODIPY derivatives. DFT/TDDFT calculations show that the large Stokes shifts are due to the remarkable geometry relaxation of the sensitizers upon photoexcitation. The sensitizers show long-lived triplet excited states (up to 95.2 μs at room temperature), which is the longest T1 state lifetime of organic sensitizers used for TTA upconversion. DFT calculations indicate that the energy level T1 state does not decrease, although the absorption–emission wavelengths of the thiophene-substituted sensitizers are red-shifted compared to the unsubstituted BODIPY, which is beneficial for TTA upconversion. The organic sensitizers were used for TTA upconversion and upconversion quantum yields up to 16.5% were observed, compared to the quantum yields of 0.6% and 6.1% observed previously with organic triplet sensitizers. The efficient TTA upconversion is attributed to the enhanced triplet–triplet-energy-transfer (TTET) process, confirmed by the lifetime quenching experiments of the photosensitizers. Our results are useful for the design of new efficient organic triplet photosensitizers to replace the currently used phosphorescent transition metal complex sensitizers for TTA upconversion and other appropriate photophysical processes, such as photocatalysis, photodynamic therapy, etc.