Photon Upconversion Research Articles

High-Level Reverse Intersystem Crossing and Molecular Rigidity Improve Spin Statistics for Triplet-Triplet Annihilation Upconversion.

The structural factors affecting triplet-triplet annihilation (TTA) at the molecular level are not well-understood. Here, our steady-state photoluminescence and transient absorption results demonstrate that the spin statistical factor, η, decreases from 0.60 to 0.46 and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and 2,6-DPA isomers, respectively, during photon upconversion with a platinum octaethylporphyrin sensitizer. Density functional theory (DFT) shows that η depends on the energetics of hot triplet states and molecular rigidity. The significantly high conical intersection energy between the S0 and T1 states for 9,10-DPA gives its longer triplet lifetime. Time-dependent DFT calculations show that 9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossing from their T2 and T3 states, respectively, to the bright S1 state, increasing the limit of the spin statistical factor. Both factors ultimately serve to enhance the TTA efficiency. This work provides insight into designing molecules for efficient light-emitting and photon upconversion applications.

Efficient photon upconversion enabled by strong coupling between silicon quantum dots and anthracene.

Hybrid structures formed between organic molecules and inorganic quantum dots can accomplish unique photophysical transformations by taking advantage of their disparate properties. The electronic coupling between these materials is typically weak, leading photoexcited charge carriers to spatially localize to the dot or to a molecule at its surface. However, we show that by converting a chemical linker that covalently binds anthracene molecules to silicon quantum dots from a carbon-carbon single bond to a double bond, we access a strong coupling regime where excited carriers spatially delocalize across both anthracene and silicon. By pushing the system to delocalize, we design a photon upconversion system with a higher efficiency (17.2%) and lower threshold intensity (0.5 W cm-2) than that of a corresponding weakly coupled system. Our results show that strong coupling between molecules and nanostructures achieved through targeted linking chemistry provides a complementary route for tailoring properties in materials for light-driven applications.

Bright photon upconversion in LiYbF4:Tm3+@LiYF4 nanoparticles and their application for singlet oxygen generation and in immunoassay for SARS-CoV-2 nucleoprotein

Photon upconversion is an intensively investigated phenomenon in the materials sciences due to its unique applications, mainly in biomedicine for disease prevention and treatment. This study reports the synthesis and properties of tetragonal LiYbF4:Tm3+@LiYF4 core@shell nanoparticles (NPs) and their applications. The NPs had sizes ranging from 18.5 to 23.7 nm. As a result of the energy transfer between Yb3+ and Tm3+ ions, the synthesized NPs show intense emission in the ultraviolet (UV) range up to 347 nm under 975 nm excitation. The bright emission in the UV range allows for singlet oxygen generation in the presence of hematoporphyrin on the surface of NPs. Our studies show that irradiation with a 975 nm laser of the functionalized NPs allows for the production of amounts of singlet oxygen easily detectable by Singlet Oxygen Sensor Green. The high emission intensity of NPs at 800 nm allowed the application of the synthesized NPs in an upconversion-linked immunosorbent assay (ULISA) for highly sensitive detection of the nucleoprotein from SARS-CoV-2, the causative agent of Covid-19. This article proves that LiYbF4:Tm3+@LiYF4 core@shell nanoparticles can be perfect alternatives for the most commonly studied upconverting NPs based on the NaYF4 host compound and are good candidates for biomedical applications.

Searching for low-mass axions using resonant upconversion

We present new results of a room temperature resonant AC haloscope, which searches for axions via photon upconversion. Traditional haloscopes require a strong applied DC magnetic background field surrounding the haloscope cavity resonator, the resonant frequency of which is limited by available bore dimensions. UPLOAD, the UPconversion Low-Noise Oscillator Axion Detection experiment, replaces this DC magnet with a second microwave background resonance within the detector cavity, which upconverts energy from the axion field into the readout mode, accessing axions around the beat frequency of the modes. Furthermore, unlike the DC case, the experiment is sensitive to a newly proposed quantum electromagnetodynamical axion coupling term $g_{aBB}$. Two experimental approaches are outlined - one using frequency metrology, and the other using power detection of a thermal readout mode. The results of the power detection experiment are presented, which allows exclusion of axions of masses between 1.12 $-$ 1.20 $\mu eV$ above a coupling strength of both $g_{a\gamma\gamma}$ and $g_{aBB}$ at $3 \times 10^{-6}$ 1/GeV, after a measurement period of 30 days, which is a three order of magnitude improvement over our previous result.

9‐1: Invited Paper: Triplet‐Triplet Annihilation at Organic Semiconductor Interface for Efficient Solid‐State Photon Upconversion and Organic Light‐Emitting Diodes with Low Driving Voltage

Triplet–triplet annihilation (TTA) can double the energy of excited states. We utilized a charge transfer state at an organic semiconductor interface as a precursor for efficient TTA. As a result, a solid‐state photon upconversion from near‐infrared to visible light is achieved with an efficiency that is two orders of magnitude higher than those demonstrated by conventional systems. Furthermore, it is demonstrated that an efficient organic light‐emitting diode (OLED) can be operated by only a 1.5 V battery.

Luminescent Dynamics of Perovskite Quantum Dots Encapsulated in Metal–Organic Frameworks

The encapsulation of perovskite quantum dots (PQDs) in metal–organic frameworks (MOF) has emerged as a promising strategy to improve their environmental stability and tune their optical property. Although energy or charge transfer processes between PQDs and the MOF matrix have already been reported, the triplet-energy transfer (TET) process from MOF to PQD, which is useful for applications such as photon upconversion and photoredox catalysis, still remains unexplored. Herein, we encapsulated CH3NH3Pb(Br/I)3 PQDs (MAPb(Br/I)3) into a zinc isophthalate MOF (denoted as Y346) to construct a solid hybrid composite (PQDs@Y346). Compared with traditional colloid PQDs, the photostability of PQDs in PQDs@Y346 is enhanced by about hundreds of times due to the protection of Y346 skeleton, accompanied with a prominent photoinduced self-healing behavior. Furthermore, a slow but efficient (∼57.8%) Y346-to-PQDs TET process is observed in PQDs@Y346 composites, leading to a long-lived (∼3 ms) delayed emission from PQDs. Our findings suggest PQDs@Y346 composites hold great promise for future long-term optoelectronic and photochemical applications.

Bandgap Engineering for Ultralow‐Voltage Operation of Organic Light‐Emitting Diodes

AbstractThe minimum voltage required for a light‐emitting diode (LED) to emit light has been suggested to depend on the bandgap of the emitter. However, the correlation between the bandgap of the emitter and the operating voltage in organic LEDs (OLEDs) has remained unclear because OLEDs require a complex multilayer configuration that includes many materials other than the emitter. Here, a simple equation describing the minimum voltage required for OLED operation, expressed by using the bandgap of the emitter and other energies relevant to OLEDs, is reported. The equation is derived from the characteristics of simple‐structure OLEDs consisting of only electron‐donating (D) and electron‐accepting (A) molecules showing exciplex emission from the green to near‐infrared regions. The minimum voltage required for OLED operation is demonstrated to be more than 0.7 V lower than the emitter's bandgap divided by the elementary charge. The relatively high exciton binding energy, which is unique to organic materials, contributes to lowering the operating voltage. On the basis of these findings, D/A molecules have been newly designed for the ultralow‐voltage operation of OLEDs, and green and blue OLEDs that emit light at ≈1.5 V are realized via triplet–triplet annihilation photon upconversion.

Photon Upconversion in a Vapor Deposited 2D Inorganic-Organic Semiconductor Heterostructure.

Energy transfer processes may be engineered in van der Waals heterostructures by taking advantage of the atomically abrupt, Å-scale, and topologically tailorable interfaces within them. Here, we prepare heterostructures comprised of 2D WSe2 monolayers interfaced with dibenzotetraphenylperiflanthene (DBP)-doped rubrene, an organic semiconductor capable of triplet fusion. We fabricate these heterostructures entirely through vapor deposition methods. Time-resolved and steady-state photoluminescence measurements reveal rapid subnanosecond quenching of WSe2 emission by rubrene and fluorescence from guest DBP molecules at 612 nm (λexc = 730 nm), thus providing clear evidence of photon upconversion. The dependence of the upconversion emission on excitation intensity is consistent with a triplet fusion mechanism, and maximum efficiency (linear regime) of this process occurs at threshold intensities as low as 110 mW/cm2, which is comparable to the integrated solar irradiance. This study highlights the potential for advanced optoelectronic applications employing vdWHs which leverage strongly bound excitons in monolayer TMDs and organic semiconductors.

Power Dependence of the Magnetic Field Effect on Triplet Fusion: A Quantitative Model

Two strategies for improving solar energy efficiencies, triplet fusion and singlet fission, rely on the details of triplet-triplet interactions. In triplet fusion, there are several steps, each of which is a possible loss mechanism. In solution, the parameters describing triplet fusion collisions are difficult to inspect. Here we show that these parameters can be determined by examining the magnetic field dependence of triplet fusion upconversion. We show that there is a reduction of the magnetic field effect for perylene triplet fusion as the system moves from the quadratic to linear annihilation regimes with an increase in laser power. Our data are modeled with a small set of parameters that characterize the triplet fusion dynamics. These parameters are cross-validated with molecular dynamics simulations. This approach can be applied to both solution and solid state materials, providing a tool for screening potential annihilators for photon upconversion.

Quasi-Reversible Photoinduced Displacement of Aromatic Ligands from Semiconductor Nanocrystals.

Organic-inorganic nanohybrids using semiconductor nanocrystals (NCs) coordinated with aromatic organic molecules have been widely studied in the fields of optoelectronic materials, such as solar cells, photocatalysis, and photon upconversion. In these materials, coordination bonds of ligand molecules are usually assumed to be stable during optical processes. However, this assumption is not always valid. In this study, we demonstrate that the coordination bonds between ligand molecules and NCs by carboxyl groups are displaced quasi-reversibly by light irradiation using zinc sulfide (ZnS) NCs coordinated with perylenebisimide (PBI) as a model system. Time-resolved spectroscopy over a wide range of time from tens-of femtosecond to second timescales and density functional theory calculations show that the photoinduced ligand displacement is driven by ultrafast hole transfer from PBI to ZnS NCs and that the dissociated radical anion of PBI survives over the second timescale. Photoinduced ligand displacements are important to be considered in various organic-inorganic nanohybrids and offer a possibility of NCs covered by nonphotoresponsive organic ligands for advanced photofunctional materials.

Broadband-sensitive up-conversion in Ni2+/Er3+ co-doped Gd2MgTiO6

Ni2+ doped and Ni2+/Er3+ co-doped Gd2MgTiO6 (GMT) phosphors have been synthesized by conventional high-temperature solid-state reaction method. First-principles calculation and X-ray diffraction analysis proves that Ni2+ ions occupy the Mg2+ sites in the Gd2MgTiO6 matrix at low Ni2+ doping concentration and begin to occupy Ti4+ sites at high Ni2+ doping concentration. The near infrared emissions of Ni2+ ions occupying Mg2+ and Ti4+ sites peak at 1344 nm and 1412 nm, respectively, forming a broad near infrared emission band from 1100 nm to 1700 nm. It is found that Ni2+/Er3+ co-doped Gd2MgTiO6 phosphor is capable to up-convert near infrared photons in the wavelength range of 870–1700 nm to visible 532 nm, 555 nm and 666 nm emission, caused by the energy transfer sensitization process from Ni2+ to Er3+. The Ni2+ and Er3+ concentration dependent up-conversion properties have been investigated and it is found that the Gd2MgTiO6: 2%Ni2+/5%Er3+ phosphor shows the strongest up-conversion emission, in which the energy transfer efficiencies from the Ni2+ ions occupying Mg2+ and Ti4+ sites to the Er3+ ions are 81.5% and 79.3%, respectively. The Gd2MgTiO6: 2%Ni2+/5%Er3+ phosphor shows yellow emission under the irradiation of 1342 nm and 1550 nm lasers, and green and orange-red emissions under the irradiation of 980 nm and 1064 nm laser, respectively. The broad band excitability and multi-color up-conversion performance of the Ni2+/Er3+ co-doped Gd2MgTiO6 phosphor suggest that it has wide application prospect in anti-counterfeiting, solar cell efficiency enhancement, infrared detection and so on.

Engineering Orthogonal Upconversion through Selective Excitation in a Single Nanoparticle (Adv. Funct. Mater. 18/2023)

Photon Upconversion In article number 2212037, Bo Zhou and co-workers describe a conceptual design to realize the orthogonal upconversion through the selective excitation strategy. The switchable red/green, red/blue, and green/blue together with full-color output are easily achieved at the single nanoparticle level, showing great potential in multi-level anti-counterfeiting, information security and biological applications.

Photonic Bandgap and Excitation Wavelength Modulated Multicolor Upconverted Circularly Polarized Luminescence in Liquid Crystal Microcapsule

AbstractUpconverted circularly polarized luminescence (UC‐CPL) which is coined by integrating two concepts of photon upconversion and circularly polarized luminescence, has aroused enormous attention from scientists owing to the great performance and potential applications in various fields. To date, although various types of UC‐CPL systems are reported, constructing a UC‐CPL system with multicolor emission remains a big challenge. In this work, by incorporating lanthanide‐doped upconversion nanoparticles (UCNPs) into chiral nematic liquid crystal (N*LC), multicolor UC‐CPL emission can be realized upon two kinds of modulations: the regulation of photonic bandgap (PBG) and adjustment of excitation wavelength of the doped UCNPs. More importantly, this kind of multicolor UC‐CPL system can be used as internal materials for constructing polymer‐shell microcapsules. The PBG effect can still be well preserved in the microcapsules, providing a kind of easy processable luminescent material with multicolor UC‐CPL properties, which can be used as an advanced anti‐counterfeiting material.

Photon upconversion from one-photon absorption to triplet-triplet annihilation based on halofluoresceins and dual-channel upconversion ion detection

Halofluorescein derivatives were found to exhibit one-photon absorption upconversion (oPUC) at ∼585 nm under the excitation of 655 nm diode laser (∼1.0 W⋅cm−2); on the other hand, they can induce emitter 9,10-diphenylanthracene (DPA) to emit triplet-triplet annihilation upconversion (TTA-UC) at ∼430 nm under the excitation of 532 nm diode laser (∼0.5 W⋅cm−2). Inspired by such unique feature, a turn-on upconversion (UC) chemsensor (i.e, halofluorescein thiospirolactone) was synthesized to develop dual-channel ion detection. The results have shown that the limit of detection (LOD) to Hg2+ can be as low as nanomolar concentration (1.4 nM) based on the TTA-UC probing, while the upper detection limit to Hg2+ can reach as high as millimolar concentration range (0.5 mM) based on the oPUC probing. The importance in this work may provide a new strategy that the high concentration probing with the low detection limit can be realized by one probe, based on simultaneously using the oPUC and TTA-UC detection methods.

Triplet–Triplet Annihilation Upconversion in a Porphyrinic Molecular Container

Triplet-triplet annihilation-based molecular photon upconversion (TTA-UC) is a photophysical phenomenon that can yield high-energy emitting photons from low-energy incident light. TTA-UC is believed to fuse two triplet excitons into a singlet exciton through several consecutive energy-conversion processes. When organic aromatic dyes─i.e., sensitizers and annihilators─are used in TTA-UC, intermolecular distances, as well as relative orientations between the two chromophores, are important in an attempt to attain high upconversion efficiencies. Herein, we demonstrate a host-guest strategy─e.g., a cage-like molecular container incorporating two porphyrinic sensitizers and encapsulating two perylene emitters inside its cavity─to harness photon upconversion. Central to this design is tailoring the cavity size (9.6-10.4 Å) of the molecular container so that it can host two annihilators with a suitable [π···π] distance (3.2-3.5 Å). The formation of a complex with a host:guest ratio of 1:2 between a porphyrinic molecular container and perylene was confirmed by NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry (ITC) as well as by DFT calculations. We have obtained TTA-UC yielding blue emission at 470 nm when the complex is excited with low-energy photons. This proof-of-concept demonstrates that TTA-UC can take place in one supermolecule by bringing together the sensitizers and annihilators. Our investigations open up some new opportunities for addressing several issues associated with supramolecular photon upconversion, such as sample concentrations, molecular aggregation, and penetration depths, which have relevance to biological imaging applications.

Role of Semiconductor Nanostructures in Photon Upconversion Applications

Role of Semiconductor Nanostructures in Photon Upconversion Applications

Photon up-conversion in Er3+ ion-doped ZnO-Al2O3-BaO-B2O3 glass for enhancing the performance of dye-sensitized solar cells

One of the main limitations in the development of dye-sensitized solar cell (DSSC) is contributed by the optical losses that occur within the absorber layer due to spectral mismatch. DSSC typically only absorbs solar light within the visible region (i.e. up to 800 nm), while the photon energy in the infrared region remain unutilized. Here we report the integration of Er3+ ion-doped glass as up-conversion (UC) element in DSSC and study their effect on the photovoltaic performances. Several characterizations were conducted including physical, structural, optical, luminescence, and radiative properties on the glass sample, while current-voltage and incident photon-to-current conversion efficiency characterization were conducted to analyze the electrical properties of DSSC. The UC luminescence spectra of the glass sample under 800 nm excitation showed green light emissions that were located between 531 and 553 nm. The UC luminescence band of glass sample was confirmed suitable with the absorption band of Z907 dye. After placing the UC glass below the DSSC’s counter electrode side, the power conversion efficiency of DSSC were mostly positively affected by the presence of UC glass, where the highest improvement of around 7.21 % was obtained for DSSC coupled with 2.0 mol% of Er3+ ion-doped glass. The enhancement was primarily attributed to the improved light harvesting due to the UC mechanism within the device as indicated by the increase in external quantum efficiency and fill factor. Our contribution provides a simple yet versatile approach in improving DSSC performance via the application of a free standing Er3+ ion-doped up-conversion glass.

Visible‐to‐UV Photon Upconversion: Recent Progress in New Materials and Applications

AbstractUltraviolet (UV, λ<400 nm) light is essential for various photochemical reactions, but its intensity in the solar spectrum is very low, and light sources that artificially generate high‐energy UV light are inefficient and environmentally unfriendly. A solution to this problem is photon upconversion (UC) from visible (vis, λ>400 nm) light to UV light. Among several mechanisms, UC based on triplet‐triplet annihilation (TTA‐UC) in particular has made remarkable progress in recent years. The development of new chromophores has enabled highly efficient conversion of low‐intensity visible light into UV light. In this review, we summarize the recent development of visible‐to‐UV TTA‐UC, from the development of chromophores and their production into films to their application in various photochemical processes such as catalysis, bond activation and polymerization. Finally, challenges and opportunities in future material development and applications will be discussed.

Visible-to-UV Photon Upconversion: Recent Progress in New Materials and Applications.

Ultraviolet (UV, λ<400 nm) light is essential for various photochemical reactions, but its intensity in the solar spectrum is very low, and light sources that artificially generate high-energy UV light are inefficient and environmentally unfriendly. A solution to this problem is photon upconversion (UC) from visible (vis, λ>400 nm) light to UV light. Among several mechanisms, UC based on triplet-triplet annihilation (TTA-UC) in particular has made remarkable progress in recent years. The development of new chromophores has enabled highly efficient conversion of low-intensity visible light into UV light. In this review, we summarize the recent development of visible-to-UV TTA-UC, from the development of chromophores and their production into films to their application in various photochemical processes such as catalysis, bond activation and polymerization. Finally, challenges and opportunities in future material development and applications will be discussed.

High-Efficiency Near-Infrared-to-Visible Photon Upconversion in Poly(vinyl alcohol) Porous Film.

Triplet-triplet annihilation photon upconversion (TTA-UC) has received significant attention in energy harvesting applications such as solar cells. The realization of high upconversion (UC) performance in the form of films is a crucial factor for the incorporation of this technology into large-area devices. Herein, we propose a porous UC film prepared by an emulsification method with a poly(vinyl alcohol) (PVA) aqueous solution and a toluene solution of chromophores (rubrene/Pd-tetraphenyltetraanthraporphyrin pair) that achieved considerable UC performance in the near-infrared (NIR) (810 nm) to visible (560 nm) range with a maximum quantum yield of 3.7% (out of 50%). Notably, the films displayed a UC emission when using an NIR light-emitting diode as a low-power-density noncoherent light source, which was confirmed by the naked eye. Excitation-power-dependent time-resolved photoluminescence measurements showed almost identical triplet lifetimes of emitter species for the films and toluene solutions; however, lower threshold intensities (Ith = 1-2 W/cm2) were observed for the films than those of the solutions (Ith = ∼10 W/cm2). An evaluation of the remaining toluene in the film and UC emission behavior in liquid nitrogen suggested that the chromophores exist as an amorphous solid in the pores, thus enabling hybrid triplet energy transfer (chromophore mobility based and exciton migration) in this unique film. The presented methodology can be generalized to other wavelengths and can enable diverse applications of the TTA-UC technology.