TTA-UC System Research Articles

Near-Infrared-to-Visible Photon Upconversion with Efficiency Exceeding 21% Sensitized by InAs Quantum Dots.

Upconversion (UC) of incoherent near-infrared (NIR) photons to visible photons through sensitized triplet-triplet annihilation (TTA) shows great potential in solar energy harvesting, photocatalysis, and bioimaging. However, the efficiencies of NIR-to-visible TTA-UC systems lag considerably behind those of their visible-to-visible counterparts. Here, we report a novel NIR-to-yellow TTA-UC system with a record quantum yield (QY) of 21.1% (out of a 100% maximum) and a threshold intensity of 20.2 W/cm2 by using InAs-based colloidal quantum dots (QDs) as triplet photosensitizers. The key to success is the epitaxial growth of an ultrathin ZnSe shell on InAs QDs that passivates the surface defects without impeding triplet energy transfer (TET) from QDs to surface-bound tetracene. Transient absorption spectroscopy verifies efficient TET efficiency of more than 80%, along with sufficiently long triplet lifetime of tetracene molecules, leading to high-performance UC. Moreover, high UC QYs (>18%) remain when larger InAs-based QDs─of which the absorption peak is red-shifted by more than 50 nm─are used as sensitizers, indicating the great potential of InAs QDs to utilize NIR photons with lower energy.

Ambient solid-state triplet-triplet annihilation upconversion in ureasil organic-inorganic hybrid hosts.

Triplet-triplet-annihilation upconversion (TTA-UC) has attracted significant attention as an approach to harvest low energy solar photons that cannot be captured by conventional photovoltaic devices. However, device integration requires the design of solid-state TTA-UC materials that combine high upconversion efficiency with long term stability. Herein, we report an efficient solid-state TTA-UC system based on organic-inorganic hybrid polymers known as ureasils as hosts for the archetypal sensitiser/emitter pair of palladium(ii) octaethylporphyrin and diphenylanthracene. The role of the ureasil structure on the TTA-UC performance was probed by varying the branching and molecular weight of the organic precursor to tune the structural, mechanical, and thermal properties. Solid-state green-to-blue UC quantum yields of up to 1.86% were observed under ambient conditions. Notably, depending on the ureasil structure, UC emission could be retained for >70 days without any special treatment, including deoxygenation. Detailed analysis of the structure-function trends revealed that while a low glass transition temperature is required to promote TTA-UC molecular collisions, a higher inorganic content is the primary factor that determines the UC efficiency and stability, due to the inherent oxygen barrier provided by the silica nanodomains.

A new sensitization strategy for achieving organic RTP in aqueous solution: tunable RTP and UC emission in supramolecular TTA-UC systems

A new sensitization strategy for achieving organic RTP in aqueous solution: tunable RTP and UC emission in supramolecular TTA-UC systems

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends.

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Generating spin-triplet states at the bulk perovskite/organic interface for photon upconversion.

Perovskite-sensitized triplet-triplet annihilation (TTA) upconversion (UC) holds potential for practical applications of solid-state UC ranging from photovoltaics to sensing and imaging technologies. As the triplet sensitizer, the underlying perovskite properties heavily influence the generation of spin-triplet states once interfaced with the organic annihilator molecule, typically polyacene derivatives. Presently, most reported perovskite TTA-UC systems have utilized rubrene doped with ∼1% dibenzotetraphenylperiflanthene (RubDBP) as the annihilator/emitter species. However, practical applications require a larger apparent anti-Stokes than is currently achievable with this system due to the inherent 0.4 eV energy loss during triplet generation. In this minireview, we present the current understanding of the triplet sensitization process at the perovskite/organic semiconductor interface and introduce additional promising annihilators based on anthracene derivatives into the discussion of future directions in perovskite-sensitized TTA-UC.

Photon Upconversion Systems Based on Triplet-Triplet Annihilation as Photosensitizers for Chemical Transformations.

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) is considered one of the most attractive methodologies for switching wavelengths from lower to higher energy. This two-photon process, which requires the involvement of a bimolecular system, has been widely used in numerous fields such as bioimaging, solar cells, displays, drug delivery, and so on. In the last years, we have witnessed the harnessing of this concept by the organic community who have developed new strategies for synthetic purposes. Interestingly, the generation of high-energetic species by this phenomenon has provided the opportunity not only to photoredox activate compounds with high-energy demanding bonds, expanding the reactivity window that lies outside the energy window of the initial irradiation wavelength, but also to sensitized conventional photocatalysts through energy transfer processes even employing infrared irradiation. Herein, an overview of the principal examples found in literature is described where TTA-UC systems are found to be suitable photosensitizers for several chemical transformations.

Stimuli-Responsive Molecular Photon Upconversion.

The addition of stimuli-responsiveness to anti-Stokes emission provides a unique platform for biosensing and chemosensing. Particularly, stimuli-responsive photon upconversion based on triplet-triplet annihilation (TTA-UC) is promising due to its occurrence at low excitation intensity with high efficiency. This Minireview summarizes the recent developments of TTA-UC switching by external stimuli such as temperature, oxygen, chemicals, light, electric field, and mechanical force. For the systematic understanding of the underlying general mechanisms, the switching mechanisms are categorized into four types: 1) aggregation-induced UC; 2) assembly-induced air-stable UC; 3) diffusion-controlled UC; and 4) energy-transfer-controlled UC. The development of stimuli-responsive smart TTA-UC systems would enable sensing with unprecedented sensitivity and selectivity, and expand the scope of TTA-UC photochemistry by combination with supramolecular chemistry, materials chemistry, mechanochemistry, and biochemistry.

Triplet-Triplet Annihilation Upconversion in a MOF with Acceptor-Filled Channels.

Photon upconversion has enjoyed increased interest in the last years due to its high potential for solar‐energy harvesting and bioimaging. A challenge for triplet–triplet annihilation upconversion (TTA‐UC) processes is to realize these features in solid materials without undesired phase segregation and detrimental dye aggregation. To achieve this, we combine a palladium porphyrin sensitizer and a 9,10‐diphenylanthracene annihilator within a crystalline mesoporous metal–organic framework using an inverted design. In this modular TTA system, the framework walls constitute the fixed sensitizer, while caprylic acid coats the channels providing a solventlike environment for the mobile annihilator in the channels. The resulting solid material shows green‐to‐blue delayed upconverted emission with a luminescence lifetime of 373±5 μs, a threshold value of 329 mW cm−2 and a triplet–triplet energy transfer efficiency of 82 %. The versatile design allows straightforward changing of the acceptor amount and type.

Steric hindrance boosted upconversion for low-power imaging in vivo

Upconversion based on triplet-triplet annihilation (TTA-UC) has received much attention due to its unique ability to effectively harvest low-energy photons in the solar spectrum for widespread investigations involving energy, environment, and bioapplication. However, the utilization of conventional TTA-UC systems for low-power bioimaging in vivo is still a challenge due to their incompatibility and moderate efficiency in aqueous media. Herein, we pioneer the molecular modification strategy by introducing steric hindrance to the BODIPY annihilators to realize robust TTA-UC in biocompatible nanocapsule. The improved molecules could maintain excellent luminescence even at a very high concentration wherein the separation distance between neighbouring chromophores was reduced to facilitate the pivotal process of Dexter energy transfer. In this way, the encapsulated TTA-UC system became highly efficient in biological circumstance with quantum efficiency ФUC of 6.3%. Consequently, the constructed nanocapsule was capable of operating at low-power with high contrast for in vivo imaging.

(Invited) Enhancement of Triplet-Triplet Annihilation-Based Upconversion Emission By Localized Surface Plasmon Resonance

The upconversion emission systems using triplet-triplet annihilation (TTA-UC emission) has attracted much attention because these systems can be driven by a noncoherent light with low intensity (e.g. sunlight). Namely, the achievement of efficient TTA-UC emission systems directly leads to the development of high-performance solar energy devices. On the other hand, since the Dexter energy transfer between the sensitizers and the emitters is contained in the process of TTA-UC emission, the emission efficiency is quite low in solid state. We have tried to enhance the emission in solid state TTA-UC systems by interacting with the localized surface plasmon (LSP) resonance of metal nanoparticles. The anisotropic triangular silver nanoplates (silver nanoprisms: AgPRs) were employed as a plasmonic metal nanoparticle because of the generating wavelength tunability of their LSP resonance and the generation of strong local electromagnetic fields. The hybrids of AgPRs and TTA-UC systems were fabricated by spin-coating the polymer solution containing the sensitizer (palladium(II) tetraphenyltetrabenzoporphyrin: Pd-TPTBP) and the emitter (9,10-bis(phenylethynyl)anthracene: BPEA) onto the AgPRs-immobilized glass plates. When the LSP resonance band of AgPRs spectrally overlapped with both the photoexcitation wavelength of Pd-TPTBP and the fluorescence band of BPEA, the TTA-UC emission was efficiently enhanced. It is reasonable to consider that the former spectral overlap provides the enhancement of TTA-UC emission through the photoexcitation enhancement of Pd-TPTBP (Figure 1). In addition, the threshold excitation intensity (the figure-of-merit in TTA-UC systems) significantly decreased by overlapping the LSP resonance band with the photoexcitation wavelength. Also, it was suggested by the fluorescence lifetime measurements that the enhancement of TTA-UC emission caused by overlapping with the fluorescence band was induced by the efficient nonradiative energy transfer from the singlet excited BPEA to the LSP of AgPRs (Figure 1). On the other hand, when the LSP resonance band overlapped well with the phosphorescence band of Pd-TPTBP, the TTA-UC emission was rather quenched. These results suggest that the nonradiative energy transfer by the LSP resonance from the triplet excited Pd-TPTBP to the LSP became dominant, as compared with the triplet-triplet energy transfer from the Pd-TPTBP to the BPEA. Namely, it was demonstrated that the LSP resonance of AgPRs has both of the positive and negative effects on the solid state TTA-UC emission systems.[1] Furthermore, we investigated the optical interaction between the singlet excited palladium porphyrin and the LSP resonance of AgPRs. Consequently, we will discuss the detailed effects of LSP resonance of the AgPRs on the complicated TTA-UC emission processes. [1] S. Jin, K. Sugawa et al., ACS Photon., 2018, 5, 5025-5037. Figure 1

Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals.

Photon upconversion (UC) based on sensitized triplet-triplet annihilation (TTA), TTA-UC, can potentially alleviate the transmission loss of below-band-gap photons in solar energy conversion. TTA-UC across various spectral windows has been demonstrated, but efficient visible-to-ultraviolet (UV) UC remains a big challenge primarily due to the lack of suitable triplet sensitizers. Here we report a TTA-UC system sensitized by quantum-confined CsPbBr3 perovskite nanocrystals (NCs) that simultaneously achieves a high photon energy gain of up to 0.7 eV (443-355 nm) and a high UC efficiency up to 10.2%. Time-resolved spectroscopy studies reveal that the performance is mainly enabled by ultrafast and efficient triplet energy transfer from the strongly confined NC sensitizers to triplet acceptors.

Hetero-bichromophore Dyad as a Highly Efficient Triplet Acceptor for Polarity Tuned Triplet–Triplet Annihilation Upconversion

Triplet-triplet annihilation upconversion (TTA UC) was intensively investigated for developing efficient photosensitizers and emitters. But an emission wavelength tunable TTA UC system with only one emitter was rarely reported. A novel hetero-bichromophore dyad, HB-An, showing solvatochromic emission and high fluorescence quantum yields in weakly polar solvents (such as n-hexane, dichloromethane (DCM), and so on) was used as triplet energy acceptor/emitter for polarity tuned TTA UC. A high TTA UC quantum yield up to 25% was achieved and the UC emission wavelengths can be fine-tuned from cyan to yellow by changing the media polarity. This information will be useful for constructing efficient emitting-light-tunable TTA UC system.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Chiral self-assembly regulated photon upconversion based on triplet-triplet annihilation

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) in quasi-solid or solid state has been attracting much research interest due to its great potential applications. To get effective UC, precisely controlled donor-acceptor interaction is vitally important. Chiral self-assembly provides a powerful approach for sophisticated regulation of molecular interaction. Here we report a chiral self-assembly controlled TTA-UC system composed of chiral acceptor and achiral donor. It is found that racemic mixture of acceptors could form straight fibrous nanostructures, which show strong UC emission, while chiral assemblies for homochiral acceptors emit weak upconverted light. The racemic assemblies allow efficient triplet-triplet energy transfer (TTET) and further realize efficient UC emission, while the homochiral assemblies from chiral acceptor produce twisted nanostructures, suppressing efficient triplet energy transfer and annihilation. The establishment of such chiral self-assembly controlled UC system highlights the potential applications of triplet fusion in optoelectronic materials and provides a new perspective for designing highly effective UC systems.

Harnessing Molecular Photon Upconversion in a Solar Cell at Sub-solar Irradiance: Role of the Redox Mediator.

Self-assembled bilayers offer a promising strategy to directly harness photon upconversion via triplet-triplet annihilation (TTA-UC) and increase maximum theoretical solar cell efficiencies from 33% to >43%. Here we demonstrate that the choice of redox mediator in these solar cells has a profound influence on both the light harvesting and TTA-UC efficiency. Devices with CoII/III(phen)3 as the redox mediator produced the highest photocurrent yet generated from TTA-UC (0.158 mA cm-2) under 1 sun. Despite generating less photocurrent, CoII/III(pz-py-pz)2 devices achieved maximum TTA-UC efficiency at excitation intensities well below solar irradiance (0.8 mW cm-2), which is on par with the lowest value yet reported for any TTA-UC system. The large variation in performance with respect to mediator is attributed to triplet excited-state quenching via (1) energy transfer or paramagnetic quenching by the CoII species and (2) excited-state electron transfer to CoIII species.

Triplet-triplet annihilation based upconversion in silica matrices

Triplet-triplet annihilation based upconversion emission (TTA-UC), through energy transfer processes among organic dyes, has been achieving great attentions for the potential applications in different fields; an important step forward the application of TTA-UC systems in real devices is the incorporation of the dye couple into solid supports.In this work mesoporous silica (SBA) with regular pores and silica nanoparticles (SNs) with core-shell structure were prepared and loaded with 2,3,7,8,12,13,17,18–octaethyl–21H,23H–porphine platinum(II) (PtOEP) and 1,3,6,8–Tetraphenylpyrene (TPPy) which act as antenna/sensitizer of the TTA-UC process and as light emitting species, respectively. The samples were fully characterized by TEM imaging, XRDP, steady-state and time resolved fluorescence and phosphorescence measurements. No upconverted emission could be detected for the SBA samples because the mesoporous matrix offered a rigid location to the dyes resulting in aggregate and excimer-like species with modified electronic properties. On the other hand, TTA-UC was recorded on SNs samples; in the latter case, steady-state and time resolved phosphorescence and fluorescence measurements indicated that PtOEP was entrapped in monomeric form and TPPy was mainly present as monomer.The careful and detailed photophysical characterization of the obtained nanostructured materials enables the optimization of the conditions to achieve light upconversion in solid matrices.

Enhanced Triplet–Triplet Annihilation Upconversion in Dual-Sensitizer Systems: Translating Broadband Light Absorption to Practical Solid-State Materials

Photochemical upconversion (UC) of low-energy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet–triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue ...

Molecularly Dispersed Donors in Acceptor Molecular Crystals for Photon Upconversion under Low Excitation Intensity

For real-world applications of photon upconversion based on the triplet-triplet annihilation (TTA-UC), it is imperative to develop solid-state TTA-UC systems that work effectively under low excitation power comparable to solar irradiance. As an approach in this direction, aromatic crystals showing high triplet diffusivity are expected to serve as a useful platform. However, donor molecules inevitably tend to segregate from the host acceptor crystals, and this inhomogeneity results in the disappointing performance of crystalline state TTA-UC. In this work, a series of cast-film-forming acceptors was developed, which provide both regular acceptor alignment and soft domains of alkyl chains that accommodate donor molecules without segregation. A typical triplet sensitizer, PtII octaethylporphyrin (PtOEP), was dispersed in these acceptor crystals without aggregation. As a result, efficient triplet energy transfer from the donor to the acceptor and diffusion of triplet excitons among regularly aligned anthracene chromophores occurred. It resulted in TTA-UC emission at low excitation intensities, comparable to solar irradiance.

Photon-Upconverting Ionic Liquids: Effective Triplet Energy Migration in Contiguous Ionic Chromophore Arrays.

Inspired by the bicontinuous ionic-network structure of ionic liquids (ILs), we developed a new family of photofunctional ILs which show efficient triplet energy migration among contiguously arrayed ionic chromophores. A novel fluorescent IL, comprising an aromatic 9,10-diphenylanthracene 2-sulfonate anion and an alkylated phosphonium cation, showed pronounced interactions between chromophores, as revealed by its spectral properties. Upon dissolving a triplet sensitizer, the IL demonstrated photon upconversion based on triplet-triplet annihilation (TTA-UC). Interestingly, the TTA-UC process in the chromophoric IL was optimized at a much lower excitation intensity compared to the previous nonionic liquid TTA-UC system. The superior TTA-UC in this IL system is characterized by a relatively high triplet diffusion constant (1.63×10(-6) cm(2) s(-1)) which is ascribed to the presence of ionic chromophore networks in the IL.

Triple-Emulsion Microcapsules for Highly Efficient Multispectral Upconversion in the Aqueous Phase

This study reports a new triplet-emulsion-based architecture for highly efficient triplet–triplet annihilation (TTA) upconversion (UC) in the aqueous phase. This encapsulation system separates the upconversion-emitting solvent core from the polymeric shell phase by utilizing an inner water phase. As a result, undesirable phosphorescence interference and chromophore loss were eliminated to achieve the aqueous upconversion efficiency close to those attainable in typical deoxygenated organic solvent systems. This system further allows various unconventional spectral shifting schemes, i.e., multicolor emission from monochromatic excitation (e.g., red to green and blue) or, conversely, frequency amplification of broad-band visible spectra to near-monochromatic shorter wavelength emission (e.g., green and red to blue), with minimal spectral impurities and interferences, representing a significant advance in encapsulated TTA-UC systems suitable for various aqueous-based applications.