Controlling the Energy-Transfer Processes in a Nanosized Molecular Upconverter to Tap into Luminescence Thermometry Application.

Photon upconversion (UC) in molecular species remains a highly sought‐after property with vast potential applications in many fields. Until now, a few reports on molecular upconverters are limited to demonstrating upconversion. The low UC quantum yields (QY) and nuclearities hindered the application capabilities for molecular upconverters. To overcome these limitations, we report the use of a molecular cluster‐aggregate (MCA) containing 20 lanthanide ions to target YbIII‐TbIII‐based cooperative UC. Upconversion quantum yield value of 1.04×10−4 %, among the highest value observed for a molecular cooperative UC, was attained for the{Gd11Tb2Yb7}composition. Substitution of GdIIIions for EuIIIcenters opens a YbIII→TbIII→EuIIIenergy‐transfer pathway, allowing the first proof‐of‐concept of potential application for molecular UC. This report on upconversion‐based luminescence thermometry in a molecular species endorses further development of upconversion properties of nanoscale MCAs.

Photon upconversion (UC) is one kind of anti-Stokes shift process, which generally requires a combination of two or more low-energy photons to produce one high-energy photon. However, another inter...

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

Photon upconversion (UC) via triplet–triplet annihilation enables the conversion of near‐infrared (NIR) photons into visible light, offering opportunities for solar energy harvesting, photocatalysis, and biophotonics. However, progress has been limited by the lack of triplet sensitizers capable of fully exploiting rubrene, the representative annihilator/emitter for NIR‐to‐visible UC. Here, we report Au 42 (PET) 32 ( Au 42 ; PET = 2‐phenylethanethiolate), a highly anisotropic, needle‐shaped gold nanocluster that unlocks the annihilator potential of rubrene, enabling high‐performance NIR‐to‐visible UC. The Au 42 /rubrene pair achieves record‐setting UC quantum yields ( Φ UC , 50% maximum) of 16.5% (reabsorption‐corrected quantum yield Φ UCg of 21.4%) with a low threshold intensity ( I th ) of 0.14 W cm −2 under 808 nm excitation and 12.3% ( Φ UCg = 15.0%) under 936 nm excitation—over two orders of magnitude higher than previously reported values above 850 nm. Quantitative analysis revealed a high spin‐statistical factor ( f = 0.58) for rubrene, suggesting an attainable Φ UC maximum of ∼30% in rubrene‐based systems. The remarkable performances arise from the unique electronic structure of Au 42 , which combines strong NIR absorption with high visible transparency, minimizing losses of the S 1 annihilator and the UC photons. These findings establish Au 42 as a benchmark sensitizer and exemplify a design principle for realizing highly efficient, low‐threshold NIR‐to‐visible UC.

Upconversion (UC) is a promising option to enhance the efficiency of solar cells by conversion of sub-bandgap infrared photons to higher energy photons that can be utilized by the solar cell. The UC quantum yield is a key parameter for a successful application. Here the UC luminescence properties of Er3+-doped Gd2O2S are investigated by means of luminescence spectroscopy, quantum yield measurements, and excited state dynamics experiments. Excitation into the maximum of the 4I15/2 → 4I13/2 Er3+ absorption band around 1500 nm induces very efficient UC emission from different Er3+ excited states with energies above the silicon bandgap, in particular, the emission originating from the 4I11/2 state around 1000 nm. Concentration dependent studies reveal that the highest UC quantum yield is realized for a 10% Er3+-doping concentration. The UC luminescence is compared to the well-known Er3+-doped β-NaYF4 UC material for which the highest UC quantum yield has been reported for 25% Er3+. The UC internal quantum yields were measured in this work for Gd2O2S: 10%Er3+ and β-NaYF4: 25%Er3+ to be 12 ± 1% and 8.9 ± 0.7%, respectively, under monochromatic excitation around 1500 nm at a power of 700 W/m2. The UC quantum yield reported here for Gd2O2S: 10%Er3+ is the highest value achieved so far under monochromatic excitation into the 4I13/2 Er3+ level. Power dependence and lifetime measurements were performed to understand the mechanisms responsible for the efficient UC luminescence. We show that the main process yielding 4I11/2 UC emission is energy transfer UC.

Dual Triplet Sensitization Strategy for Efficient and Stable Triplet–Triplet Annihilation Upconversion Perovskite Solar Cells

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has the potential to enhance significantly photovoltaic and photocatalytic efficiencies by harnessing sub-bandgap photons, but the progress of this field is held back by the chemistry problem of how to preorganize multiple chromophores for efficient UC under weak solar irradiance. Recently, the first maximization of UC quantum yield at solar irradiance was achieved using fast triplet energy migration (TEM) in metal-organic frameworks (MOFs) with ordered acceptor arrays, but at the same time, a trade-off between fast TEM and high fluorescence efficiency was also found. Here, we provide a solution for this trade-off issue by developing a new strategy, triplet energy migration, annihilation and upconverted singlet energy collection (TEM-UPCON). The porous structure of acceptor-based MOF crystals allows triplet donor molecules to be accommodated without aggregation. The surface of donor-doped MOF nanocrystals is modified with highly fluorescent energy collectors through coordination bond formation. Thanks to the higher fluorescence quantum yield of surface-bound collectors than parent MOFs, the implementation of the energy collector greatly improves the total UC quantum yield. The UC quantum yield maximization behavior at ultralow excitation intensity was retained because the TTA events take place only in the MOF acceptors. The TEM-UPCON concept may be generalized to collectors with various functions and would lead to quantitative harvesting of upconverted energy, which is difficult to achieve in common molecular diffusion-based systems.

Efficient photon upconversion (UC) is a promising way to utilize the infrared photons, where upconversion quantum yield (QY) is a key parameter to find the potential application of these UC materials for many applications. Here we are reporting the absolute upconversion QYs for different concentrations of M2O2S:Yb3+/Er3+ (M = Gd, Y, La) in comparison with β-NaYF4:20%Yb3+/2%Er3+ for the UC emissions centered at 410, 550, 667 and 822 nm at varying excitation power densities under 980 nm excitation. Concentration dependent upconversion QY analysis has revealed that La2O2S:1%Yb3+/1%Er3+ and Y2O2S:1%Yb3+/1%Er3+ possess almost similar upconversion QY compared to that of 9%Yb3+/1%Er3+:La2O2S. Measurements under identical conditions with respect to the reported most efficient upconverting phosphor NaYF4:20%Yb3+/2%Er3+ show that M2O2S:Yb3+/Er3+ yields a higher absolute upconversion QY at lower excitation power densities compared to that of β-NaYF4:20%Yb3+/2%Er3+. Furthermore, UC spectral studies and lifetime measurements were performed for a series of M2O2S:Yb3+/Er3+ phosphors to analyze the UC mechanisms in these phosphors. We show that upconversion emission from 4F9/2 is mainly influenced by the longer lifetime of metastable state 4I13/2.

Photon upconversion from the near-IR (NIR) to the visible (VIS) is potentially interesting for many applications. The wellknown process of second-harmonic generation in nonlinear optical materials is widely used for the conversion of NIR laser radiation into VIS or UV radiation. It requires coherent radiation. In this paper we are talking about a different nonlinear upconversion (UC) process, which does not require coherent radiation. Instead, it requires the existence of a metastable intermediate state in the NIR, which is reached with the first photon. The second photon then promotes the system to a higher excited state, followed by emission from a second metastable state in the VIS. The first UC step can be followed by a second and third UC step, if there exist metastable states at higher energies. Since the early reviews by Auzel 1 and Wright 2 about UC processes, there has been a lot of work on UC processes and UC materials. Both fundamental studies of new materials and the mechanisms of upconversion and investigations of the application potential of the UC process have been reported. These potential applications are in the areas of lighting, lasers, displays, and imaging systems as well as NIR photon counters. 3-5 Most of this work is based on lanthanide-doped crystals and glasses. f-f excited states of lanthanides are often metastable because of the shielding of the f-electrons and the resulting small electron-phonon coupling. Er 3+ ,T m 3+ ,N d 3+ , and Ho 3+ are

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

The demand for more efficient and complete sorting techniques for plastic waste is growing, and one possible solution isbased on fluorescent labeling. Novel fluorescent labels based on trivalent lanthanide (Ln3+) activated inorganic up‐conversion (UC) materials offer a promising technological solution for plastic recycling. UC is a nonlinear, anti‐Stokes process of combining two or more low energy near‐infrared (NIR) photons to obtain the emission of a single higher energy photon. While Ln3+ based UC materials possess one key disadvantage –low quantum yield, they also exhibit many unique features, such as high signal/noise ratio, tailored emission color, long photo‐luminescent lifetime, and low toxicity. These unique features endear them for a diverse range of applications and offer many new opportunities. Herein, we review the recent advances in the Ln3+ activated inorganic micro‐sized UC materials from the perspective of tailoring UC emission color and intensity for plastic recycling applications.

Metal nanoclusters have emerged as new triplet sensitizers of photo‐upconversion, while the strategy of engineering their electronic properties for efficient utilization of triplet energy remains elusive. Au16Cu6(tBuPhC≡C)18 (Au16Cu6) metal nanocluster shows near‐unity near‐infrared (NIR) phosphorescent quantum yield at room temperature and near‐unity intersystem crossing quantum yield, which is an ideal platform for understanding the mechanism of triplet energy transfer (TET). Here, a model system is designed to probe the TET from Au16Cu6 to different organic acceptors and obtained kTET from Stern–Volmer plots. Transient absorption spectroscopy clearly shows the formation of T1 signals in organic molecules accompanied by the decay of T1 signals in Au16Cu6, which indicates a direct TET mechanism. Among four organic molecules, the TET between Au16Cu6 and perylene possesses the highest TET quantum yield. Moreover, a relatively small threshold intensity of 0.4 W cm−2 is obtained with 532 nm excitation and a large anti‐Stokes shift of 0.68 eV with 635 nm excitation in triplet‐triplet annihilation (TTA) upconversion (UC). The efficient TET and UC in Au16Cu6 nanoclusters and the underlying mechanism will pave the way for optimizing their performance and expanding their utilization in optoelectronic devices.

A new methodology of single‐component photon upconversion (UC) based on S 0 ‐to‐T 1 (S−T) absorption is demonstrated. Although the S−T absorption is spin‐forbidden, modification of anthracene chromophores with heavy bromine atoms allows the direct population of the excited triplets in crystals upon near‐infrared (NIR) excitation with incident light intensities of ∼W cm −2 . The consequent triplet energy migration (TEM) and triplet‐triplet annihilation (TTA) events in crystals lead to blue UC emission with a large anti‐Stokes shift over 0.9 eV. The generality of the S−T absorption‐based UC is further confirmed with crystals of brominated perylene‐derivatives that show a green UC emission under a longer‐wavelength NIR excitation ( λ ex =856 nm). The single‐component UC with large anti‐Stokes shifts observed for brominated aromatic crystals compensates for the shortcomings of conventional two‐component solid UC system, i. e., energy losses intrinsic to the intersystem crossing of the sensitizers and the triplet energy transfer from sensitizer to emitter. It offers a simple and complementary UC methodology to the conventional two‐photon absorption (TPA) and TTA‐UC.

Application of singlet sink approach for matrix-free amorphous photon upconversion films

Realization of efficient photon upconversion (UC) mediated by triplet–triplet annihilation (TTA) in a solid state is a great challenge, even though order of magnitude higher efficiencies are routinely reported in a solution/liquid state. To address this issue in a typical emitter/sensitizer UC system, a combined approach is introduced based on 1) thorough emitter purification for reduced exciton quenching and 2) UC film fabrication via melt‐processing for attaining large emitter concentrations with suppressed aggregation. Emitter purification via vacuum sublimation is shown to reduce the number of both singlet and triplet quenchers as confirmed by fluorescence and UC quantum yield measurements along with the respective transient measurements performed on nanosecond and millisecond time domains. Application of this approach to the benchmark TTA‐UC system DPA/PtOEP is demonstrated to yield a record‐high UC efficiency (=8 ± 1%, out of maximum 50%) achieved at 40 wt% of DPA homogeneously dispersed in the PMMA host. Importantly, such high efficiency is accomplished in large‐area amorphous films, the most preferred for practical applications, and featuring low UC threshold (=5 mW cm−2) that is close to the solar irradiance. The presented approach describes the guidelines for boosting UC efficiency in the solid state, and generally, is applicable to any conventional TTA‐UC system.