Photon Upconversion Systems Research Articles

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

Solid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (&lt; 500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s‐1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

(Invited) Reshaping Light with Hybrid Quantum Dot: Molecule Systems

Photon upconversion is an energy conversion process wherein a material absorbs two or more low-energy photons and uses their energy to generate high-energy photons. Upconversion systems that convert near-infrared light into the visible range can address current challenges in solar energy capture and near-infrared sensor design while materials that operate at higher energy, producing UV photons from visible light, can enable applications in photocatalysis and light-based 3D printing. Due to their high extinction coefficients and size-tunable optical properties, quantum dots have emerged as ideal photosensitizers for photon upconversion systems. In these systems, light absorbed by a quantum dot is passed to a molecule at its surface, placing the molecule into a spin-triplet state. Upconversion is achieved when two molecules in their triplet state encounter one another and undergo triplet fusion, a process that deexcites one molecule and promotes the other to a high-energy, emissive spin-singlet state. In this presentation, I will present spectroscopic measurements and electronic structure calculations that identify energy transfer rates and key intermediates involved in two quantum dot systems that respectively demonstrate red-to-blue and blue-to-UV photon upconversion. In the first system, which consists of silicon quantum dots functionalized with anthracene ligands, we find that by controlling the chemical structure of molecular tethers that covalently link anthracene to silicon, we can produce strongly coupled states wherein excited charge carriers are shared between silicon and anthracene. By controlling the energy of these states, we can optimize the system’s performance, achieving an upconversion quantum yield of 17.2%. In the second system, we use CsPbBr3 perovskite quantum dots to drive triplet energy transfer to naphthalene ligands. This energy transfer process is found to be highly sensitive to the structure of the chemical linker that binds naphthalene to CsPbBr3, which we attribute to modulation of the degree of wavefunction overlap between the states of the quantum dot energy donor and naphthalene energy acceptor.

Tunable Intracavity Coherent Up-Conversion with Giant Nonlinearity in a Polar Fluidic Medium.

The study has demonstrated a novel microcavity-based flexible photon up-conversion system using second harmonic generation (SHG) from a polar nematic fluidic medium doped with a laser dye. The idea is based on coherent light generation via stimulated emission (lasing) and simultaneous frequency doubling inside a microcavity. The polar nematic fluid equips very high even-order optical nonlinearity due to its polar symmetry and large dipole moment along the molecular long axis. At the same time, its inherent fluidic nature allows to easily functionalize the media just by doping, in the present case, with an emissive laser dye. The demonstrated system exhibits a giant nonlinear optical response to input light, while enabling spectral narrowing and multiple-signal output of up-converted light, which is not attainable through the simple SH-conversion of input light. Furthermore, the susceptibility of the liquid crystal offers dynamic modulation capabilities by an external stimulus, such as signal switching by the application of electric field or wavelength tuning through temperature variation. Such a brand-new type of simple coherent flexible up-conversion system must be promising as a new principle for easily accessible and down-scalable wavelength conversion devices.

(Invited) Rational Design of Quantum Dot/Mediator Interfaces for Triplet Energy Transfer and Photon Upconversion

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems. Ongoing detailed mechanistic studies of the TET1 process shows that both QD bright and dark exciton states contribute to the TET process and higher lying bridge states play significant role in enhancing TET from core/shell QDs.

(Keynote) Rational Design of Quantum Dot/Mediator Interfaces for Triplet Energy Transfer and Photon Upconversion

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems. Ongoing detailed mechanistic studies of the TET1 process shows that both QD bright and dark exciton states contribute to the TET process and higher lying bridge states play significant role in enhancing TET from core/shell QDs.

Excited-State Transient Chemistry of Rubrene: A Whole Story.

The ability to manipulate low-energy triplet excited states into higher-energy emissive singlet states, a process known as photon upconversion (UC), has potential applications in bioimaging, photocatalysis, and in increasing the efficiency of solar cells. However, the overall UC mechanism is complex and can involve many intermediate states, especially when semiconductors such as lead halide perovskites are used to sensitize the required triplet states. Using a combination of pulse radiolytic and electrochemical techniques, we have now explored the transient features of rubrene─a commonly employed triplet annihilator in UC systems. The rubrene triplet, radical anion, and radical cation species yield unique spectra that can serve as spectral fingerprints to distinguish between transient species formed during UC processes. Using detailed kinetic studies, we have succeeded in establishing that the rubrene triplets are susceptible to self-quenching (kquench = 3.6 × 108 M-1 s-1), and as the triplets decay, an additional transient feature is observed in the transient absorption spectra. This new feature indicates a net electron transfer process occurs to form the radical cation and anion as the triplets recombine. Taken together, this work provides a comprehensive picture of the excited state and transient features of rubrene and will be crucial for understanding the mechanism(s) of photon upconversion systems.

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.

Efficiency of Thermally Activated Delayed Fluorescence Sensitized Triplet Upconversion Doubled in Three-Component System.

As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti-Stokes shift) while diminishing the cost of production. In this vein, studies employing metal-free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion is utilized with the TADF sensitizer 2,4,5,6-tetrakis(carbazol-9-yl)isophthalonitrile (4CzIPN), resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p-terphenyl as the emitter. In this ternary blend, the sensitizer 4CzIPN is paired with an intermediate acceptor, 1-methylnaphthalene, in addition to the emitter molecule, p-terphenyl, yielding a normalized upconversion quantum yield of 7.6% while maintaining the 0.83eV anti-Stokes shift. These results illustrate the potential benefits of utilizing this strategy of energy-funneling, previously used only with heavy-metal based sensitizers, to increase the performance of these photon upconversion systems.

Efficient Visible‐to‐UV Photon Upconversion Systems Based on CdS Nanocrystals Modified with Triplet Energy Mediators

AbstractDeveloping high‐performance visible‐to‐UV photon upconversion systems based on triplet–triplet annihilation photon upconversion (TTA‐UC) is highly desired, as it provides a potential approach for UV light‐induced photosynthesis and photocatalysis. However, the quantum yield and spectral range of visible‐to‐UV TTA‐UC based on nanocrystals (NCs) are still far from satisfactory. Here, three different sized CdS NCs are systematically investigated with triplet energy transfer to four mediators and four annihilators, thus substantially expanding the available materials for visible‐to‐UV TTA‐UC. By improving the quality of CdS NCs, introducing the mediator via a direct mixing fashion, and matching the energy levels, a high TTA‐UC quantum yield of 10.4% (out of a 50% maximum) is achieved in one case, which represents a record performance in TTA‐UC based on NCs without doping. In another case, TTA‐UC photons approaching 4 eV are observed, which is on par with the highest energies observed in optimized organic systems. Importantly, the in‐depth investigation reveals that the direct mixing approach to introduce the mediator is a key factor that leads to close to unity efficiencies of triplet energy transfer, which ultimately governs the performance of NC‐based TTA‐UC systems. These findings provide guidelines for the design of high‐performance TTA‐UC systems toward solar energy harvesting.

(Invited) Rational Design of Quantum Dot/Mediator Interfaces for Efficient Triplet- Annihilation-Based Photon Upconversion Systems

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems.

Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation.

Strong light–matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in photochemistry and photophysics. In this article, we use strong light–matter coupling to transform an endothermic triplet–triplet annihilation process into an exothermic one. The resulting gradual on–off photon upconversion experiment demonstrates a direct conversion between molecular states and hybrid light–matter states. Our study provides a direct evidence that energy can relax from nonresonant low energy molecular states directly into hybrid light–matter states and lays the groundwork for tunable photon upconversion systems that modify molecular properties in situ by optical cavities rather than with chemical modifications.

Steering Nanohelix and Upconverted Circularly Polarized Luminescence by Using Completely Achiral Components.

Enormous attention has been paid to upconverted circularly polarized luminescence (UC-CPL). However, so far, chiral species are still needed in UC-CPL materials, either through the covalent or noncovalent bond. Here, we report a general supramolecular coassembly approach for the fabrication of UC-CPL systems from completely achiral components. We have found that an achiral C3-symmetric molecule could form a chiral nanohelix through symmetry breaking, which could serve as a general helical platform to endow achiral guests with induced chirality and CPL activity. Two different photon upconversion systems, namely, triplet-triplet annihilation photon upconversion (TTA-UC) donor/acceptor pairs and inorganic lanthanide upconversion nanoparticles (UCNPs), are selected. When these two systems coassembled with the chiral nanohelix made from an achiral C3-symmetric molecule, hybrid nanohelix structures formed and UC-CPL activity was induced. Through such an approach, we demonstrated that the fabrication of the UC-CPL materials does not require any chiral molecules. Moreover, we have shown that the polarization of UC-CPL can be tuned by the helicity of the nanohelix, which could be controlled through the seeded vortex. Our work provides a general approach for designing tunable UC-CPL materials from completely achiral motifs, which largely expands the research scope of the CPL materials.

Recent Advances of Circularly Polarized Luminescence in Photon Upconversion Systems

Abstract Upconverted circularly polarized luminescence (UC-CPL) is coined by combining two individual concepts of photon upconversion (UC) and circularly polarized luminescence (CPL). It has exhibited great performance and potential applications in various fields, arousing enormous attention from scientists. The emergence, amplification and modulation of UC-CPL can be adjusted through regulation of the UC process and assembled state of emitters. Benefitting from the abundant upconverted photoluminescence mechanisms, we can flexibly build UC-CPL systems with distinct properties. In addition, based on different photophysical processes, the circular polarization of some UC-CPL can be promoted. Furthermore, the practical application of UC-CPL has been confirmed in enantioselective photopolymerization. Thus, in this review, we will briefly highlight the recent development and advances of UC-CPL. We envisage providing a guide for the development of those remarkable chiroptical materials.

Upconversion Model for Directly Determining the Microscopic Energy-Transfer Parameters for β-NaYF4:Er3+

Lanthanide-doped photon upconversion systems are widely used to convert two or more low-energy photons into one high-energy photon. Theoretical upconversion models are needed to further understand ...

Tuning the Quantum Dot (QD)/Mediator Interface for Optimal Efficiency of QD-Sensitized Near-Infrared-to-Visible Photon Upconversion Systems.

Lead sulfide (PbS) quantum dots (QDs) have shown promising performance as a sensitizer in infrared-to-visible photon upconversion systems. To investigate the key design rules, we compare three PbS-sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Using transient absorption spectroscopy, we directly measure the triplet energy-transfer rates and efficiencies from the QD to the mediator and from the mediator to the emitter. With increasing distance between the mediator and PbS QD, the efficiency of the first triplet energy transfer from the QD to the mediator decreases because of a decrease in the rate of this triplet energy-transfer step, while the efficiency of the second triplet energy transfer from the mediator to the emitter increases because of a reduction in the QD-induced mediator triplet state decay. The latter effect is a result of the slow rate constant of the second triplet energy-transfer process, which is 3 orders of magnitude slower than the diffusion-limited value. The combined results lead to a net decrease of the steady-state upconversion quantum yield with distance, which could be predicted by our kinetic model. Our result shows that the QD/mediator interface affects both the first and second triplet energy transfer processes in the photon upconversion system, and the QD/mediator distance has an opposite effect on the efficiencies of the first and second triplet energy transfer. These findings provide important insight for the further rational improvement of the overall efficiency of QD-based upconversion systems.

Number of Surface-Attached Acceptors on a Quantum Dot Impacts Energy Transfer and Photon Upconversion Efficiencies

Composites of organic molecules and inorganic quantum dots (QDs) have emerged as attractive photon upconversion systems that use triplet–triplet annihilation upconversion (TTA-UC). However, the upc...

Low temperature radical initiated hydrosilylation of silicon quantum dots.

The photophysics of silicon quantum dots (QDs) is not well understood despite their potential for many optoelectronic applications. One of the barriers to the study and widespread adoption of Si QDs is the difficulty in functionalizing their surface, to make, for example, a solution-processable electronically-active colloid. While thermal hydrosilylation of Si QDs is widely used, the high temperature typically needed may trigger undesirable side-effects, like uncontrolled polymerization of the terminal alkene. In this contribution, we show that this high-temperature method for installing aromatic and aliphatic ligands on non-thermal plasma-synthesized Si QDs can be replaced with a low-temperature, radical-initiated hydrosilylation method. Materials prepared via this low-temperature route perform similarly to those created via high-temperature thermal hydrosilylation when used in triplet fusion photon upconversion systems, suggesting the utility of low-temperature, radical-initiated methods for creating Si QDs with a range of functional behavior.

Heavy-Atom-Free Red-to-Yellow Photon Upconversion in a Thiosquaraine Composite

We demonstrate a completely heavy-atom-free red-to-yellow triplet–triplet annihilation (TTA) photon upconversion system using a thionated squaraine sensitizer, both in fluid solution and in a solid...

Future and challenges for hybrid upconversion nanosystems

To improve the efficiency of photon upconversion, a hybrid approach of combining organic dyes and inorganic nanoparticles is proving successful, especially in the form of dye-sensitized lanthanide-doped upconversion nanoparticles, nanoparticle-sensitized molecular triplet–triplet annihilation systems and metal–organic-framework nanoparticles. In this Review, we survey the latest advances and examine the key factors affecting upconversion performance, such as spectral overlap, core–shell design and the management of triplet excitons and quenchers at the interface between materials. Although issues such as stability, triplet-state quenching, concentration quenching and reabsorption must still be overcome, smart designs of hybrid nanosystems offer exciting opportunities for applications such as solar photovoltaic devices, deep-tissue biomedical imaging, optogenetics and nanomedicine among others. The prospects for improving photon upconversion systems by combining organic dyes and inorganic nanoparticles are reviewed.