High-concentration annihilators for efficient green-to-blue photon upconversion and photocatalytic production of hydroxyl radicals

Photon upconversion in nanocrystals shows significant promise in versatile frontier applications; however, it has remained a huge challenge to overcome the long-standing concentration quenching effect. Here we report a conceptual model to boost upconversion through selective control of interfacial energy transfer (IET) in core-shell-based nanocrystals. Such a design enables both activator and sensitizer to achieve 100% doping in a single nanocrystal in contrast to conventional nanoparticles. Importantly, it is able to greatly minimize the nonradiative energy loss by the atomic-level spatial manipulation of interfacial interactions between Er and Yb sublattices, resulting in 2 orders of magnitude enhancement in luminescence intensity. We further demonstrate that the IET channels can also mediate up-transition dynamics, which contributes to temporal tuning of real-time red-to-green switchable emission colors. Our findings provide new insights into the upconversion physics involved in the heavy doping nanomaterials and offer new opportunities for infrared photodetection and potentially other photonic applications.

In this work, triplet-triplet annihilation upconversion (TTA-UC) was employed as a sensitive tool to study the influence of media organization on the efficiency of bimolecular processes. For this purpose, an archetypal sensitizer/emitter pair of platinum octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA) was probed in oleic (OA) and elaidic acid (EA), chosen as isomeric phase change materials (PCMs). TTA-UC was monitored below and above the melting points of the PCMs. The results point out that the arrangement of solid- and liquid-phase media imposes constraints able to assist TTA-UC, resulting in higher efficiency in the solid phase of OA. A green-to-blue UC quantum yield of up to 3.6% was observed under ambient conditions with incoherent and low power excitation. The role and the effects of media constraints were monitored through steady-state and time-resolved luminescence, small-angle X-ray scattering (SAXS), Raman and UV-vis spectroscopy and were rationalized in terms of PtOEP aggregation.

Triplet sensitization of semiconductor nanocrystals is important in photocatalysis and photodynamic therapy. However, current photosensitizers often contain toxic or precious metals, making the development of environmentally benign nanocrystal-based systems challenging due to scarce suitable materials and insufficient understanding of excited-state transfer. Herein, we investigate triplet energy transfer from CuInS2/ZnS nanorods to surface-anchored 9-anthracene carboxylic acid acceptors, with a rate constant of 1.02 × 1010 s-1 and 90% efficiency. Besides, the transferred spin-triplet states undergo triplet-triplet annihilation in the interfaced annihilator molecules of diphenyl anthracene, enabling photon upconversion with a threshold of 23.26 W/cm² and an efficiency of 6.8%. These findings highlight CuInS2/ZnS nanorods as nontoxic triplet sensitizers and provide a viable strategy for sustainable upconversion optoelectronic devices.

Detection of trace gases with high sensitivity and weak excitation power is highly desired for long-range remote sensing. Here, we report the detection of the greenhouse gas nitrous oxide (N2O) with the power of excitation light down to picowatts, by converting the mid-infrared laser to near-infrared photons through an intra-cavity-enhanced sum-frequency upconversion system. The intra-cavity-enhanced pumping power of 1064.0 nm reaches about 200.0 W, resulting in the conversion of the 4514.6 nm mid-infrared laser to 861.1 nm with an efficiency up to 73.4% under optimal conditions. The upconverted light is then detected by a single-photon avalanche detector, followed by a time-correlated single-photon counting module, which can measure the arrival time of each upconverted photon. By performing discrete Fourier transformations of the arrival time of the detected photons, the frequency spectrum can be determined. By using frequency modulation, this method can suppress background noise significantly. Consequently, the excitation power can be brought down to about 100 pW with the concentration of N2O being 10 ppm. As a demonstration of application, the presented system is also used for N2O sensing in an open-path geometry, highlighting the potential for stand-off leak detection. Our proposal offers promising applications to monitor trace gases over long distances with weak excitation powers.

The triplet-triplet annihilation (TTA)-assisted photon upconversion (PUC) properties of a solution system containing the newly developed acceptor di(p-anisyl)tetrahydropentalene TP-An, containing a rigidly constrained planar diene subunit, were elucidated. Due to its symmetric and rigid structure, TP-An undergoes rapid fluorescence emission and slow nonradiative processes, respectively, resulting in its unity fluorescence quantum yield and its short singlet excited state lifetime of 1.13 ns in degassed toluene. Its rigidity also affords a long triplet excited state lifetime of 654 μs, essential for efficient TTA-PUC. Moreover, the experimentally determined singlet (ES, 3.22 eV) and triplet (ET, 1.62-1.79 eV) excited state energy levels of TP-An meet the requirements for exergonic TTA (ES ≲ 2ET). Analysis of the green-to-purple TTA-PUC behavior (anti-Stokes shift: 0.89 eV) reveals that a toluene solution of TP-An and platinum octaethylporphyrin displays a quite low threshold excitation intensity (10 mW cm-2) and high PUC quantum yield (ca 23%).

Triplet energy transfer (TET) underlies key applications in energy storage, conversion, and utilization such as photovoltaics, photon upconversion, singlet fission, and photocatalysis. Fast and long-distance TET is generally desirable in these applications to enhance performance and limit back transfer. However, conventional TET in the weak coupling regime only occurs over short distances between donor and acceptor as Dexter-type electronic coupling for TET decreases exponentially with increasing separation. One way to achieve long-distance TET is to enhance the electronic coupling between donor and acceptor by designing conjugated linking bridges. Here, we reveal three new silicon quantum dot (Si QD):Anthracene hybrid systems with variable-length -[SiMe2] n - (n = 2-4) linkers as bridges to promote long-distance TET. Transient absorption experiments and density functional theory calculations show that electronic coupling in each of these four systems is intermediate between non-conjugated ethyl and π-conjugated vinyl bridges. In addition, the TET rates between Si QDs and anthracene facilitated by -[SiMe2] n - (n = 1-4) linkers do not show the expected exponential decay trend with increasing separation. Rather we observe an increase in the rate of TET when n is increased from 2 to 3, which we propose arises from greater bridge chain flexibility that opens access to geometries where the anthracene can directly engage the Si QD surface via through-space van der Waals interactions. By controlling the average number of tethered anthracene transmitters, we are able to optimize the performance of Si QD:Anthracene hybrids as photosensitizers for triplet-triplet annihilation photon upconversion, obtaining efficiencies of 6.2 ± 0.4%, 3.4 ± 0.1%, 4.1 ± 0.2% and 3.9 ± 0.1% (out of 100%), respectively for n = 1-4. This work provides insight into the role that electronic coupling plays in hybrid materials to move triplet excitons across semiconductor junctions, which sheds light on material design principles for applications in optoelectronics and photocatalysis.

Strategies for aqueous triplet-triplet annihilation upconversion using nanostructured materials

Charge Transfer States in Donor–Acceptor Bulk‐Heterojunctions as Triplet–Triplet Annihilation Sensitizer for Solid‐State Photon Upconversion (Adv. Mater. Interfaces 8/2026)

Solid-state near-infrared (NIR)-to-visible triplet-triplet annihilation upconversion (TTA-UC) at the 1 μm edge is attractive for deep-tissue photonics and NIR energy harvesting but remains limited by sensitizer losses and restricted triplet transport in condensed media. Here we demonstrate porous poly(vinyl alcohol) (PVA)/rubrene films sensitized by spin-forbidden Ru complexes (DX1m-DX3m) with appreciable NIR absorption. Photon-flux-normalized action spectra show sensitizer-dependent red-edge response across the series, and DX3m affords quantifiable upconversion under 1000 nm femtosecond and 980 nm continuous-wave excitation, with detectable spectra to 1030 nm. Because spin-forbidden Ru sensitizers offer molecular tunability yet face threshold limitations from red-edge absorption and short triplet lifetimes, we examined what governs the operating thresholds in porous films. Transient kinetics indicate that excitation range is sensitizer-controlled, whereas thresholds are governed by triplet survival and encounter kinetics in porous domains rather than sensitizer-to-annihilator triplet-triplet energy transfer alone. These results establish a Ru-based route to 1-μm-class solid-state TTA-UC in polymer films.

Photon upconversion, a nonlinear process converting near-infrared light into visible emission, has attracted growing interest for photovoltaics, bioimaging, sensing, and photonic devices. Conventional lanthanide-doped nanomaterials have enabled this field, yet their low-quantum yields and high excitation thresholds limit their applicability. Recent strategies such as core–shell designs, plasmonic coupling, and hybrid composites have significantly enhanced efficiency by suppressing surface quenching and amplifying local electromagnetic fields. Emerging materials, including perovskite quantum dots, carbon quantum dots, and metal–organic frameworks, offer tunable bandgaps, high photostability, and versatile energy transfer pathways, particularly for triplet–triplet annihilation. Complementary theoretical models using rate equations, density functional theory, and Monte Carlo simulations provide insights into dopant optimization and nonradiative losses, accelerating material design. Despite these advances, scalable fabrication, reproducibility, and mitigation of photothermal losses remain key challenges. This review outlines progress in next-generation PUC nanomaterials, bridging experimental and computational approaches to realize efficient, stable, and multifunctional systems for next-generation energy and biomedical technologies.

Threshold excitation intensity is a key parameter for characterizing triplet-triplet annihilation photon upconversion (TTA-UC) systems, expected for diverse applications, including management of the sunlight spectrum. However, its measurement is a laborious and time-consuming process. In this article, we propose a simple and quick method for determining the threshold intensity from a snapshot of the emission image. The proposed method significantly shortens the measurement time compared to that for the conventional, widely used intensity-by-intensity method; it can be within a second. Its accuracy was confirmed to be comparable to that of the conventional method.

Activating inorganic hosts with lanthanide ions enables nonlinear phenomena, such as photon upconversion, photon avalanche, and quantum cutting, thereby expanding the potential of nanomaterial-based technologies. To maximize the optical performance of lanthanide dopants, it is essential to develop optimal host materials, especially those with low cutoff phonon energies. Heavy halide compositions, with their low phonon energies, have tantalized researchers with the potential to boost lanthanide brightness but are hampered by inherent stability issues. However, thanks to advancements in colloid and surface chemistry, these issues can now be addressed. In this Perspective, we outline research on heavy halide hosts favorable for lanthanide doping and photon upconversion, highlight the outstanding challenges when introducing lanthanides into these hosts, and identify as-yet unexplored directions that may lead to significant performance gains in brightness, nonlinear process efficiency, and color-tuning. Inspired by early research on lanthanide upconversion in bulk heavy halides and ongoing studies on lead halide perovskites, we provide insight into the development of lanthanide-activated heavy halides - from nanocrystal synthesis and surface and composition engineering to leveraging emerging approaches in tuning optical properties, including automated and computational materials discovery. Ongoing research on doped heavy halides will expand the variety of lanthanide-based nonlinear materials, unlock unique optical properties, and deliver greater value to applications in color and energy conversion, imaging, security, and photonics.

ConspectusOne of the most central questions in chemistry is how a starting material can be converted as simply and efficiently as possible into a product. The answer may include photocatalysis, and if the reaction proceeds well, one might argue that understanding the underlying mechanism is not essential. Even if the reaction does not perform as anticipated, condition screening may still provide the operationally simplest and most effective path to the desired outcome, while mechanistic aspects can remain largely unexamined. Given the large parameter space typically associated with modern photocatalytic reactions, this approach is both plausible and justified, particularly when product synthesis is the primary goal.A complementary perspective on modern photocatalysis focuses on the conceptual advancement of photochemistry and a deeper understanding of its elementary steps and their interplay. This type of research begins with classical mechanistic elucidation to break down complex processes into individual elementary events. Once sufficient understanding has been achieved, it can lead to the mechanistic design of photoreactions. At that stage, the sequence of photophysical and chemical events triggered by light, and consequently the overall outcome of the reaction, can become rationally predictable, at least in principle.In this Account, we examine how the cross-fertilization between synthetically oriented photoredox catalysis, which is primarily concerned with the activation and functionalization of organic molecules, and mechanistically driven research from the physical-inorganic domain has advanced the field of photochemistry. This interaction has often been catalyzed by controversial discussions surrounding the mechanistic details of reactions that have attracted significant synthetic interest. As a result, this interplay has propelled significant advances across several critical areas of modern molecular photocatalysis, including the reactivity of excited-state organic radicals and solvated electrons, the mechanisms underlying multiphoton excitation processes such as photon upconversion, the puzzling light-independent energy-loss phenomenon known as "cage escape", and even the possibility of challenging Kasha's rule, a foundational principle in photophysics with profound implications for photochemistry.The knowledge accumulated through this work has brought the field closer to achieving mechanistically guided design in photocatalysis, extending far beyond the initial light-induced step. Central to this advancement are modern time-resolved spectroscopic methods, which have provided crucial insights into transient species and reaction dynamics. This conceptual strategy opens new opportunities and highlights challenges in redefining thermodynamic and kinetic limits. Ultimately, combining mechanistic insight with the practical expertise of synthetic chemists offers great potential for continued innovation in photoredox catalysis at the intersection of organic and physical-inorganic chemistry. With this Account, we aim to bridge the gap between those who prioritize the synthetic perspective and those who emphasize mechanistic and conceptual approaches, fostering greater integration between organic chemists and physical-inorganic chemists.

Photon upconversion based on triplet–triplet annihilation using thermally activated delayed fluorescence sensitizers

ConspectusPhoton upconversion, which converts low-energy near-infrared light into higher-energy emission, has emerged as a powerful tool at the intersection of photophysics, materials science, and biosensing. The nonlinear excitation, large anti-Stokes shifts, minimal background autofluorescence, high photostability, and effective tissue penetration of photon upconversion make it particularly attractive for probing biological systems under physiologically relevant conditions.Lanthanide-doped nanoparticles constitute a dominant class of upconversion systems. Encapsulation of lanthanide ions within crystalline hosts shields their 4f electronic states from environmental perturbations, enabling spectrally stable and temporally persistent emission. Nevertheless, their relatively low quantum yields under biologically safe irradiation often necessitate higher excitation power densities, which limit their in vivo applications. To address this challenge, advances in materials design, most notably core-shell architectures that regulate energy migration and suppress surface quenching, have substantially boosted upconversion efficiency and spectral tunability. Complementary surface engineering via chemical modifications has further enhanced colloid stability, biocompatibility, and targeting specificity. In parallel, optical field engineering strategies, including superlensing effects and plasmonic coupling, have expanded the functional scope of upconversion platforms beyond conventional luminescence. Together, these developments have established upconversion nanoparticles as a robust physical interface between optical excitation and biological response.In this Account, we focus on recent progress in integrating upconversion nanoparticles with diverse physical modalities for biosensing and biointerfacing. We first outline the photophysical principles underlying photon upconversion and summarize key strategies for enhancing efficiency and signal fidelity. We then survey upconversion nanoparticle-based platforms that couple optical emission with electrical, mechanical, and thermal readouts. In optical microscopy, upconversion nanoparticles enable long-term single-particle tracking of neuronal transport and support super-resolution imaging through nonlinear emission processes and surface-migration depletion. When interfaced with electrophysiological measurements, these nanoparticles allow real-time monitoring of transmembrane water transport including flux through ion channels. Upconversion-assisted optogenetics further enables noninvasive neuromodulation without implanting optical fibers. Besides optical and electrical modalities, upconversion nanoparticles have been applied to force sensing over a wide dynamic range and to subcellular thermometry with high spatial precision. Incorporation of upconversion nanoparticles into device architectures extends these capabilities to stochastic photoluminescence encoding, infrared vision through retinal nanoantennas, and biocompatible contact lenses for near-infrared color perception.Looking ahead, the main challenges include further improving quantum yield under biologically permissible excitation, reducing excitation power requirements while maintaining high brightness, establishing long-term biosafety, and advancing toward clinical translation. Emerging directions such as data-driven materials design, artificial intelligence-guided optimization, and integration with regenerative medicine and microrobotics may help overcome these hurdles. By uniting advances in photophysical control with biological sensing and actuation, upconversion nanoplatforms hold strong potential to transform imaging, diagnostics, and therapeutic intervention.

ABSTRACT Triplet–triplet annihilation upconversion (TTA‐UC) is a promising strategy for converting low‐energy photons into high‐energy photons. Among emitter molecules, 9,10‐diphenylanthracene ( DPA ) is widely used owing to its rigid aromatic structure, high fluorescence quantum yield (Φ FL ), and favorable triplet energy levels for efficient TTA‐UC. However, only a few systematic studies have examined how chemical substitutions at the 4‐positions of DPA affect their UC efficiency. Here, we investigate the effect of electron‐donating and electron‐accepting substituents at the 4‐positions of DPA on its fluorescence and TTA‐UC properties. DPA derivatives with substituents at the two 4‐positions of the phenyl rings were synthesized. The fluorescence properties of solutions of these DPA derivatives and the TTA‐UC properties of mixed solutions containing the DPA s and a sensitizer molecule were evaluated under a nitrogen atmosphere. The Φ FL of DCl‐DPA was the highest, followed by DCN‐DPA . The UC quantum efficiency ( η UC ) was estimated, with the highest value obtained for unsubstituted DPA , followed by DCl‐DPA and DCN‐DPA . Moreover, the saturated UC quantum efficiency ( η UC ∞ ), the quantum yield of triplet–triplet energy transfer, and the quantum yield of TTA (Φ TTA ) were also estimated. The results indicate that the Φ TTA value primarily governs the η UC ∞ values. These findings provide insight into the rational design of emitter molecules for optimizing TTA‐UC efficiency.

Aromatic ketones have been long adopted as photocatalysts for photochemical synthesis owing to their efficient intersystem crossing and high triplet energy. Most of these ketones, however, absorb at ultraviolet wavelengths with low extinction coefficients, limiting the efficiency and scope of accessible photochemical reactions. Herein, we report the use of low-toxicity (Cd, Pb-free) blue ZnSe-based quantum dots (QDs) to sensitize the triplets of their surface-anchored thioxanthone molecules with high triplet energy of 2.75eV. Ultrafast spectroscopy unambiguously reveals a stepwise, electron-transfer-mediated triplet energy transfer mechanism. This unusual photosensitization, in conjunction with the unique chemical reactivity of the thioxanthone triplets, allows us to use blue light to drive multiple challenging photochemical and photocatalytic applications: i) visible-to-ultraviolet B photon upconversion with large anti-Stokes shift of 0.8eV through triplet-triplet annihilation; ii) energy transfer photocatalysis such as the disulfide-ene reaction; iii) reductive aryl dechlorination and C-N coupling enabled by hydrogen abstraction by thioxanthone triplets. This environmentally benign QD-ketone hybrid system will inspire the design of advanced photocatalysts for a variety of energy-demanding photochemical applications.

ABSTRACT 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.

ABSTRACT Photon Upconversion in molecular hetero‐metallic lanthanide systems is challenged by the lack of chemical diversity displayed by the lanthanide ions. Here, we report the multi‐photon photophysical properties of a series of molecular hetero‐trimetallic lanthanide complexes Yb 2 Ln (Ln = Eu 3+ , Gd 3+ , Tb 3+ ) assembled from kinetically inert building blocks providing site‐specific chemical control regarding introduction of differing lanthanide ions. The hetero‐trimetallic complex Yb 2 Tb shows efficient Yb 2 → Tb photon upconversion via cooperative sensitization in both D 2 O and H 2 O. By contrast, Yb 2 Eu does not show Yb 2 → Eu upconversion, while Yb 2 Gd has been used as a spectroscopic blank. We find that the Yb 2 → Tb energy transfer appears to be independent of OH quenching from the solvent. Additionally, we report the intermetallic distances in the complex using density functional theory and molecular dynamics simulations. We find that the Yb 2 → Tb cooperative sensitization upconversion energy transfer remains effective despite relatively long intermetallic distances between donor pairs (13.5–25 Å) and between the Yb donors and the Tb acceptor (11.5–13.5 Å).