Hybrid Molecule-Nanocrystal Photon Upconversion Across the Visible and Near-Infrared

Multifold Enhanced Photon Upconversion in a Composite Annihilator System Sensitized by Perovskite Nanocrystals

Photon upconversion via triplet-triplet annihilation (TTA-UC) provides a pathway to overcoming the thermodynamic efficiency limits in single-junction solar cells by allowing the harvesting of sub-bandgap photons. Here, we use mixed halide perovskite nanocrystals (CsPbX3, X = Br/I) as triplet sensitizers, with excitation transfer to 9,10-diphenylanthracene (DPA) and/or 9,10-bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-An) which act as the triplet annihilators. We observe that the upconversion efficiency is five times higher with the combination of both annihilators in a composite system compared to the sum of the individual single-acceptor systems. Our work illustrates the importance of using a composite system of annihilators to enhance TTA upconversion, demonstrated in a perovskite-sensitized system, with promise for a range of potential applications in light-harvesting, biomedical imaging, biosensing, therapeutics, and photocatalysis.

Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 μs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

Axial Coordinated Manganese(III) Porphyrin/Tetraazaporphyrin - 4-(10-phenylanthracen-9-yl)Pyridine Dyads: Self-Assembly, Structure and Spectral Properties in Ground and Excited States

Self-assembly of new donor-acceptor systems based on (5,10,15,20-tetraphenylporphinato)manganese(III)/(5,10,15,20-tetra-4-tert-butylphenylporphinato)manganese(III)/(octakis(4-tert-butylphenyl)tetraazaporphinato)manganese(III) acetate ((AcO)MnTPP/(AcO)MnTBPP/(AcO)MnTAP) and 4-(10-phenylanthracen-9-yl)pyridine (PyAn) was studied using fluorescence spectroscopy and mass spectrometry. It was found that the coordination complexes of 1 : 1 composition (dyads) are formed in toluene. The spectral properties, the chemical structures and redox behavior of the dyads were described using 1H NMR, IR, ESR spectroscopy and cyclic voltammetry, respectively. The dynamic processes and the characteristics in the excited state of the dyads were obtained using the femtosecond transient absorption spectroscopy method. Density functional theory (DFT), time-dependent DFT methods were used to elucidate the dyad electronic structures and to establish the differences in their frontier molecular orbitals. The analysis of the lambda parameter and the distance of hole-pair interaction was indicated more favorable charge transfer between the macrocycle and the axial PyAn fragment in (AcO)(PyAn)MnTAP. The calculated values of the zero-field splitting parameters D and E/D, together with the g tensors of the lowest spin-orbit state for (AcO)MnTPP and (AcO)(PyAn)MnTPP were obtained using the combination of DFT and Multireference Perturbation Theory (CASSCF/NEVPT2) simulations. The data obtained develop the fundamental basis in the field of photovoltaics and show the prospects for the study of molecular systems of this class.

Near Infrared-II Excited Triplet Fusion Upconversion with Anti-Stokes Shift Approaching the Theoretical Limit

The anti-Stokes shift represents the capacity of photon upconversion to convert low-energy photons to high-energy photons. Although triplet exciton-mediated photon upconversion presents outstanding performance in solar energy harvesting, photoredox catalysis, stereoscopic 3D printing, and disease therapeutics, the interfacial multistep triplet exciton transfer leads to exciton energy loss to suppress the anti-Stokes shift. Here, we report near infrared-II (NIR-II) excitable triplet exciton-mediated photon upconversion using a hybrid photosensitizer consisting of lead sulfide quantum dots (PbS QDs) and new surface ligands of thiophene-substituted diketopyrrolopyrrole (Th-DPP). Under 1064 nm excitation, this photon upconversion revealed a record-corrected upconversion efficiency of 0.37% (normalized to 100%), with the anti-Stokes shift (1.07 eV) approaching the theoretical limit (1.17 eV). The observation of this unexpected result is due to our discovery of the presence of a weak interaction between the sulfur atom on Th-DPP and Pb2+ on the PbS QDs surface, facilitating electronic coupling between PbS QDs and Th-DPP, such that the realization of triplet exciton transfer efficiency is close to 100% even when the energy gap is as small as 0.04 eV. With this premise, this photon upconversion as a photocatalyst enables the production of standing organic gel via photopolymerization under 1064 nm illumination, displaying NIR-II photon-driven photoredox catalysis. This research not only establishes the foundation for enhancing the performance of NIR-II excitable photonic upconversion but also promotes its development in photonics and photoredox catalysis.

Enhancing NIR-to-visible upconversion in a rigidly coupled tetracene dimer: approaching statistical limits for triplet-triplet annihilation using intramolecular multiexciton states

Important applications of photon upconversion through triplet-triplet annihilation require conversion of near-IR photons to visible light. Generally, however, efficiencies in this spectral region lag behind bluer analogues. Herein we consider potential benefits from a conformationally well-defined covalent dimer annihilator TIPS-BTX in studies that systematically compare function to a related monomer model TIPS-tetracene (TIPS-Tc). TIPS-BTX exhibits weak electronic coupling between chromophores juxtaposed about a polycyclic bridge. We report an upconversion yield ϕUC for TIPS-BTX that is more than 20× larger than TIPS-Tc under comparable conditions (0.16%). While the dimer ϕUC is low compared to bluer champion systems, this yield is amongst the largest so-far reported for a tetracenic dimer system and is achieved under unoptimized conditions suggesting a significantly higher ceiling. Further investigation shows the ϕUC enhancement for the dimer is due exclusively to the TTA process with an effective yield more that 30× larger for TIPS-BTX compared to TIPS-Tc. The ϕTTA enhancement for TIPS-BTX relative to TIPS-Tc is indicative of participation by intramolecular multiexciton states with evidence presented in spin statistical arguments that the 5TT is involved in productive channels. For TIPS-BTX we report a spin-statistical factor f = 0.42 that matches or exceeds values found in champion annihilator systems such as DPA. At the same time, the poor relative efficiency of TIPS-Tc suggests involvement of non-productive bimolecular channels and excimeric states are suspected. Broadly these studies indicate that funneling of photogenerated electronic states into productive pathways, and avoiding parasitic ones, remains central to the development of champion upconversion systems.

Ultraviolet Circularly Polarized Luminescence in Chiral Perovskite Nanoplatelet-Molecular Hybrids: Direct Binding Versus Efficient Triplet Energy Transfer

The development of ultraviolet circularly polarized light (UVCPL) sources has the potential to benefit plenty of practical applications but remains a challenge due to limitations in available material systems and a limited understanding of the excited state chirality transfer. Herein, by constructing hybrid structures of the chiral perovskite CsPbBr3 nanoplatelets and organic molecules, excited state chirality transfer is achieved, either via direct binding or triplet energy transfer, leading to efficient UVCPL emission. The underlying photophysical mechanisms of these two scenarios are clarified by comprehensive optical studies. Intriguingly, UVCPL realized via the triple energy transfer, followed by the triplet-triplet annihilation upconversion processes, demonstrates a 50-fold enhanced dissymmetry factor glum . Furthermore, stereoselective photopolymerization of diacetylene monomer is demonstrated by using such efficient UVCPL. This study provides both novel insights and a practical approach for realizing UVCPL, which can also be extended to other material systems and spectral regions, such as visible and near-infrared.

Electronic State Engineering in Perovskite-Cerium-Composite Nanocrystals toward Enhanced Triplet Annihilation Upconversion

Wavelength conversion based on hybrid inorganic-organic sensitized triplet-triplet annihilation upconversion (TTA-UC) is promising for applications such as photovoltaics, light-emitting-diodes, photocatalysis, additive manufacturing, and bioimaging. The efficiency of TTA-UC depends on the population of triplet excitons involved in triplet energy transfer (TET), the driving force in TET, and the coupling strength between the donor and acceptor. Consequently, achieving highly efficient TTA-UC necessitates the precise control of the electronic states of inorganic donors. However, conventional covalently bonded nanocrystals (NCs) face significant challenges in this regard. Herein, a novel strategy to exert control over electronic states is proposed, thereby enhancing TET and TTA-UC by incorporating ionic-bonded CsPbBr3 and lanthanide Ce3+ ions into composite NCs. These composite-NCs exhibit high photoluminescence quantum yield, extended single-exciton lifetime, quantum confinement, and uplifted energy levels. This engineering strategy of electronic states engendered a comprehensive impact, augmenting the population of triplet excitons participating in the TET process, enhancing coupling strength and the driving force, ultimately leading to an unconventional, dopant concentration-dependent nonlinear enhancement of UC efficiency. This work not only advances fundamental understanding of hybrid TTA-UC but also opens a door for the creation of other ionic-bonded composite NCs with tunable functionalities, promising innovations for next-generation optoelectronic applications.

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.

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

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

Fluorescence Intermittency of Quantum Dot-Organic Dye Conjugates: Implications for Alternative Energy and Biological Imaging

Quantum dot (QD)-organic dye couple chromophores are topical due to their applications in biology, catalysis, and energy. The maximization of energy transfer efficiency can be guided by the underlying Förster or Dexter mechanisms; however, the impact of fluorescence intermittency must also be considered. Here we demonstrate that the average ⟨t_on⟩ and ⟨t_off⟩ times of dye acceptors in coupled QD-dye chromophores are substantially affected by the donors' blinking behavior. With regard to biological imaging, this effect beneficially minimizes the photobleaching of the acceptor dye. The implications for alternative energy are less encouraging as the acceptors' capacity to store energy, using ⟨t_on⟩/⟨t_off⟩ as a metric, was reduced by as much as ∼95%. These detrimental effects can be mitigated by suppressing QD blinking via surface treatment. This study also demonstrates several instances of the nonconformity of QD blinking dynamics to a power law distribution, as a robust examination of the off times reveals log-normal behavior that is consistent with the Albery model.

Colloidal-ALD-Grown Metal Oxide Shells Enable the Synthesis of Photoactive Ligand/Nanocrystal Composite Materials

Colloidal nanocrystals (NCs) are ideal materials for a variety of applications and devices, which span from catalysis and optoelectronics to biological imaging. Organic chromophores are often combined with NCs as photoactive ligands to expand the functionality of NCs or to achieve optimal device performance. The most common methodology to introduce these chromophores involves ligand exchange procedures. Despite their ubiquitous nature, ligand exchanges suffer from a few limitations, which include reversible binding, restricted access to binding sites, and the need for purification of the samples, which can result in loss of colloidal stability. Herein, we propose a methodology to bypass these inherent issues of ligand exchange through the growth of an amorphous alumina shell by colloidal atomic layer deposition (c-ALD). We demonstrate that c-ALD creates colloidally stable composite materials, which comprise NCs and organic chromophores as photoactive ligands, by trapping the chromophores around the NC core. As representative examples, we functionalize semiconductor NCs, which include PbS, CsPbBr₃, CuInS₂, Cu_2-xX, and lanthanide-based upconverting NCs, with polyaromatic hydrocarbons (PAH) ligands. Finally, we prove that triplet energy transfer occurs through the shell and we realize the assembly of a triplet exciton funnel structure, which cannot be obtained via conventional ligand exchange procedures. The formation of these organic/inorganic hybrid shells promises to synergistically boost catalytic and multiexcitonic processes while endowing enhanced stability to the NC core.

How Pendant Groups Dictate Energy and Electron Transfer in Perovskite-Rhodamine Light Harvesting Assemblies

Energy and electron transfer processes allow for efficient manipulation of excited states within light harvesting assemblies for photocatalytic and optoelectronic applications. We have now successfully probed the influence of acceptor pendant group functionalization on the energy and electron transfer between CsPbBr₃ perovskite nanocrystals and three rhodamine-based acceptor molecules. The three acceptors─rhodamine B (RhB), rhodamine isothiocyanate (RhB-NCS), and rose Bengal (RoseB)─contain an increasing degree of pendant group functionalization that affects their native excited state properties. When interacting with CsPbBr₃ as an energy donor, photoluminescence excitation spectroscopy reveals that singlet energy transfer occurs with all three acceptors. However, the acceptor functionalization directly influences several key parameters that dictate the excited state interactions. For example, RoseB binds to the nanocrystal surface with an apparent association constant (K_app = 9.4 × 10⁶ M^-1) 200 times greater than RhB (K_app = 0.05 × 10⁶ M^-1), thus influencing the rate of energy transfer. Femtosecond transient absorption reveals the observed rate constant of singlet energy transfer (k_EnT) is an order-of-magnitude greater for RoseB (k_EnT = 1 × 10¹¹ s^-1) than for RhB and RhB-NCS. In addition to energy transfer, each acceptor had a subpopulation of molecules (∼30%) that underwent electron transfer as a competing pathway. Thus, the structural influence of acceptor moieties must be considered for both excited state energy and electron transfer in nanocrystal-molecular hybrids. The competition between electron and energy transfer further highlights the complexity of excited state interactions in nanocrystal-molecular complexes and the need for careful spectroscopic analysis to elucidate competitive pathways.

Nanoengineering Triplet-Triplet Annihilation Upconversion: From Materials to Real-World Applications

Using light to control matter has captured the imagination of scientists for generations, as there is an abundance of photons at our disposal. Yet delivering photons beyond the surface to many photoresponsive systems has proven challenging, particularly at scale, due to light attenuation via absorption and scattering losses. Triplet-triplet annihilation upconversion (TTA-UC), a process which allows for low energy photons to be converted to high energy photons, is poised to overcome these challenges by allowing for precise spatial generation of high energy photons due to its nonlinear nature. With a wide range of sensitizer and annihilator motifs available for TTA-UC, many researchers seek to integrate these materials in solution or solid-state applications. In this Review, we discuss nanoengineering deployment strategies and highlight their uses in recent state-of-the-art examples of TTA-UC integrated in both solution and solid-state applications. Considering both implementation tactics and application-specific requirements, we identify critical needs to push TTA-UC-based applications from an academic curiosity to a scalable technology.

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals

Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface-anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from k_quench = 0.0096 ns^-1 ligand^-1 to 0.064 ns^-1 ligand^-1) when the barrier to hole-first sequential transfer is lowered from 100 ± 25 meV to 50 ± 25 meV. This acceleration is 5.1× the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to ∼98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.

Thermally Activated Delayed Near-Infrared Photoluminescence from Functionalized Lead-Free Nanocrystals

As an analogue to thermally activated delayed fluorescence (TADF) of organic molecules, thermally activated delayed photoluminescence (TADPL) observed in molecule-functionalized semiconductor nanocrystals represents an exotic mechanism to harvest energy from dark molecular triplets and to obtain controllable, long-lived PL from nanocrystals. The reported TADPL systems have successfully covered the visible spectrum. However, TADF molecules already emit very efficiently in the visible, diminishing the technological impact of the less-efficient nanocrystal-molecule TADPL. Here we report bright, near-infrared TADPL in lead-free CuInSe₂ nanocrystals functionalized with carboxylated tetracene ligands, which results from efficient triplet energy transfer from photoexcited nanocrystals to ligands, followed with thermally activated reverse energy transfer from ligand triplets back to nanocrystals. This strategy prolonged the nanocrystal exciton lifetime from 100 ns to 60 μs at room temperature.

Mechanistic insight into CdSe nanoplatelet-sensitized upconversion: size and stacking induced effects

CdSe nanoplatelets (NPLs) have been reported as triplet sensitizers for photon upconversion (UC). However, their UC quantum yields lag behind more conventional systems. Here, we take advantage of their one-dimensional quantum confinement to decouple effects caused by the energetic driving force and lateral size. A surprising anti-correlation between the power threshold I_th and the UC quantum yield based on the NPL size is found. We attribute this result to two distinct triplet-triplet annihilation mechanisms based on the NPL lateral dimension and degree of NPL stacking-mediated either by molecular diffusion or triplet energy diffusion.

Aggregation of Charge Acceptors on Nanocrystal Surfaces Alters Rates of Photoinduced Electron Transfer

Semiconductor nanocrystals (NCs) interfaced with molecular ligands that function as charge and energy acceptors are an emerging platform for the design of light-harvesting, photon-upconverting, and photocatalytic materials. However, NC systems explored for these applications often feature high concentrations of bound acceptor ligands, which can lead to ligand-ligand interactions that may alter each system's ability to undergo charge and energy transfer. Here, we demonstrate that aggregation of acceptor ligands impacts the rate of photoinduced NC-to-ligand charge transfer between lead(II) sulfide (PbS) NCs and perylenediimide (PDI) electron acceptors. As the concentration of PDI acceptors is increased, we find the average electron transfer rate from PbS to PDI ligands decreases by nearly an order of magnitude. The electron transfer rate slowdown with increasing PDI concentration correlates strongly with the appearance of PDI aggregates in steady-state absorption spectra. Electronic structure calculations and molecular dynamics (MD) simulations suggest PDI aggregation slows the rate of electron transfer by reducing orbital overlap between PbS charge donors and PDI charge acceptors. While we find aggregation slows electron transfer in this system, the computational models we employ predict ligand aggregation could also be used to speed electron transfer by producing delocalized states that exhibit improved NC-molecule electronic coupling and energy alignment with NC conduction band states. Our results demonstrate that ligand aggregation can alter rates of photoinduced electron transfer between NCs and organic acceptor ligands and should be considered when designing hybrid NC:molecule systems for charge separation.

Gas-phase grafting for the multifunctional surface modification of silicon quantum dots

Photon upconversion in systems incorporating inorganic quantum dots (QDs) is of great interest for applications in solar energy conversion, bioimaging, and photodynamic therapy. Achieving high up-conversion efficiency requires not only high-quality inorganic nanoparticles, but also precise control of their surface functional groups. Gas-phase surface functionalization provides a new pathway towards controlling the surface of small inorganic nanoparticles. In this contribution, we utilize a one-step low-temperature plasma technique for the synthesis and in-flight partial functionalization of silicon QDs with alkyl chains. The partially functionalized surface is then modified further with 9-vinylanthracene via thermal hydrosilylation resulting in the grafting of 9-ethylanthracene (9EA) groups. We have found that the minimum alkyl ligand density necessary for quantum dot solubility also gives the maximum upconversion quantum yield, reaching 17% for silicon QDs with Si-dodecyl chains and an average of 3 9EA molecules per particle.

Triplet transfer from PbS quantum dots to tetracene ligands: is faster always better?

Quantum dot-organic semiconductor hybrid materials are gaining increasing attention as spin mixers for applications ranging from solar harvesting to spin memories. Triplet energy transfer between the inorganic quantum dot (QD) and organic semiconductor is a key step to understand in order to develop these applications. Here we report on the triplet energy transfer from PbS QDs to four energetically and structurally similar tetracene ligands. Even with similar ligands we find that the triplet energy transfer dynamics can vary significantly. For TIPS-tetracene derivatives with carboxylic acid, acetic acid and methanethiol anchoring groups on the short pro-cata side we find that triplet transfer occurs through a stepwise process, mediated via a surface state, whereas for monosubstituted TIPS-tetracene derivative 5-(4-benzoic acid)-12-triisopropylsilylethynyl tetracene (BAT) triplet transfer occurs directly, albeit slower, via a Dexter exchange mechanism. Even though triplet transfer is slower with BAT the overall yield is greater, as determined from upconverted emission using rubrene emitters. This work highlights that the surface-mediated transfer mechanism is plagued with parasitic loss pathways and that materials with direct Dexter-like triplet transfer are preferred for high-efficiency applications.

Efficient solid-state infrared-to-visible photon upconversion on atomically thin monolayer semiconductors

Upconverting infrared light into visible light via the triplet-triplet annihilation process in solid state is important for various applications including photovoltaics, photodetection, and bioimaging. Although inorganic semiconductors with broad absorption and negligible exchange energy loss have emerged as promising alternative to molecular sensitizers, currently, they have exclusively suffered from low efficiency and contained toxic elements in near-infrared (NIR)-to-visible upconversion. Here, we report an ultrathin bilayer film for NIR-to-visible upconversion based on atomically thin two-dimensional (2D) monolayer semiconductors. The atomic flatness and strong light absorption of 2D monolayer semiconductors enable ultrafast energy transfer and robust NIR-to-visible emission with a high upconversion quantum yield (1.1 ± 0.2%) at modest incident power (260 mW cm^-2). Increasing layer thickness adversely quenches the upconversion emission, highlighting the 2D advantage. Considering the whole library of 2D semiconductors, the facile large-scale production and the ultrathin solid-state architecture open a new research field for solid-state upconversion applications.

Upconversion Nanostructures Applied in Theranostic Systems

Upconversion (UC) nanostructures, which can upconvert near-infrared (NIR) light with low energy to visible or UV light with higher energy, are investigated for theranostic applications. The surface of lanthanide (Ln)-doped UC nanostructures can be modified with different functional groups and bioconjugated with biomolecules for therapeutic systems. On the other hand, organic molecular-based UC nanostructures, by using the triplet-triplet annihilation (TTA) UC mechanism, have high UC quantum yields and do not require high excitation power. In this review, the major UC mechanisms in different nanostructures have been introduced, including the Ln-doped UC mechanism and the TTA UC mechanism. The design and fabrication of Ln-doped UC nanostructures and TTA UC-based UC nanostructures for theranostic applications have been reviewed and discussed. In addition, the current progress in the application of UC nanostructures for diagnosis and therapy has been summarized, including tumor-targeted bioimaging and chemotherapy, image-guided diagnosis and phototherapy, NIR-triggered controlled drug releasing and bioimaging. We also provide insight into the development of emerging UC nanostructures in the field of theranostics.

A Sensitizer of Purpose: Generating Triplet Excitons with Semiconductor Nanocrystals

The process of photon upconversion promises importance for many optoelectronic applications, as it can result in higher efficiencies and more effective photon management. Upconversion via triplet-triplet annihilation (TTA) occurs at low incident powers and at high efficiencies, requirements for integration into existing optoelectronic devices. Semiconductor nanocrystals are a diverse class of triplet sensitizers with advantages over traditional molecular sensitizers such as energetic tunability and minimal energy loss during the triplet sensitization process. In this Perspective, we review current progress in semiconductor nanocrystal triplet sensitization, specifically focusing on the nanocrystal, the ligand shell which surrounds the nanocrystal, and progress in solid-state sensitization. Finally, we discuss potential areas of improvement which could result in more efficient upconversion systems sensitized by semiconductor nanocrystals. Specifically, we focus on the development of solid-state TTA upconversion systems, elucidation of the energy transfer mechanisms from nanocrystal to transmitter ligand which underpin the upconversion process and propose novel configurations of nanocrystal-sensitized systems.

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr₃ with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.

The Rise and Future of Discrete Organic-Inorganic Hybrid Nanomaterials

Hybrid nanomaterials (HNs), the combination of organic semiconductor ligands attached to nanocrystal semiconductor quantum dots, have applications that span a range of practical fields, including biology, chemistry, medical imaging, and optoelectronics. Specifically, HNs operate as discrete, tunable systems that can perform prompt fluorescence, energy transfer, singlet fission, upconversion, and/or thermally activated delayed fluorescence. Interest in HNs has naturally grown over the years due to their tunability and broad spectrum of applications. This Review presents a brief introduction to the components of HNs, before expanding on the characterization and applications of HNs. Finally, the future of HN applications is discussed.

Quantifying the Ligand-Induced Triplet Energy Transfer Barrier in a Quantum Dot-Based Upconversion System

During photon upconversion, quantum dots (QDs) transfer energy to molecules in solution through a long ligand shell. This insulating ligand shell imparts colloidal stability at the expense of efficient photosensitization. For the first time, we quantify the barrier these aliphatic ligands pose for triplet energy transfer in solution. Using transient absorption spectroscopy, we experimentally measure a small damping coefficient of 0.027 Å^-1 for a ligand exceeding 10 carbons in length. The dynamic nature of ligands in solution lowers the barrier to charge or energy transfer compared to organic thin films. In addition, we show that surface ligands shorter than 8 carbons in length allow direct energy transfer from the QD, bypassing the need for a transmitter ligand to mediate energy transfer, leading to a 6.9% upconversion quantum yield compared with 0.01% for ligands with 18 carbons. This experimentally derived insight will enable the design of efficient QD-based photosensitizers for catalysis and energy conversion.

Entropy-Powered Endothermic Energy Transfer from CsPbBr3 Nanocrystals for Photon Upconversion

Colloidal semiconductor nanocrystals as triplet photosensitizers are characterized by a negligible intersystem crossing energy loss as compared to that of traditional molecular sensitizers. This property in principle allows for a large apparent anti-Stokes shift in sensitized triplet-triplet annihilation photon upconversion (TTA-UC) for a variety of applications. In previous systems, however, this advantage is largely erased by the energy loss associated with energy transfer from nanocrystals to surface-anchored triplet transmitter molecules. Here we report visible-to-ultraviolet TTA-UC from 473 to 355 nm, corresponding to an apparent anti-Stokes shift of 0.87 eV, with a quantum efficiency that reaches 4.5% (normalized at 100%). The system consists of CsPbBr₃ nanocrystal sensitizers, phenanthrene transmitters, and diphenyloxazole annihilators. Time-resolved spectroscopy reveals that triplet energy transfer from CsPbBr₃ nanocrystals to phenanthrene can be endothermic yet efficient thanks to a sizable entropic gain. This study exemplifies how entropic effects can be harnessed to enhance or control a plethora of applications with nanocrystals as photosensitizers.

Green-to-UV photon upconversion enabled by new perovskite nanocrystal-transmitter-emitter combination

The first example of triplet-triplet annihilation-based photon upconversion (TTA-UC) from green light to ultraviolet (UV) light sensitized by lead halide perovskite nanocrystals is demonstrated. The combination of a new transmitter that extracts triplet energy from perovskite and a UV emitter with a low triplet energy level lengthens the excitation wavelength of perovskite-sensitized upconverted UV emission.

Directing Energy Transfer in Halide Perovskite-Chromophore Hybrid Assemblies

Directing the flow of energy and the nature of the excited states that are produced in nanocrystal-chromophore hybrid assemblies is crucial for realizing their photocatalytic and optoelectronic applications. Using a combination of steady-state and time-resolved absorption and photoluminescence (PL) experiments, we have probed the excited-state interactions in the CsPbBr₃-Rhodamine B (RhB) hybrid assembly. PL studies reveal quenching of the CsPbBr₃ emission with a concomitant enhancement of the fluorescence of RhB, indicating a singlet-energy-transfer mechanism. Transient absorption spectroscopy shows that this energy transfer occurs on the ∼200 ps time scale. To understand whether the energy transfer occurs through a Förster or Dexter mechanism, we leveraged facile halide-exchange reactions to tune the optical properties of the donor CsPbBr₃ by alloying with chloride. This allowed us to tune the spectral overlap between the donor CsPb(Br_1-xCl_x)₃ emission and acceptor RhB absorption. For CsPbBr₃-RhB, the rate constant for energy transfer (k_ET) agrees well with Förster theory, whereas alloying with chloride to produce chloride-rich CsPb(Br_1-xCl_x)₃ favors a Dexter mechanism. These results highlight the importance of optimizing both the donor and acceptor properties to design light-harvesting assemblies that employ energy transfer. The ease of tuning optical properties through halide exchange of the nanocrystal donor provides a unique platform for studying and tailoring excited-state interactions in perovskite-chromophore assemblies.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

Photon upconversion is a strategy to generate high-energy excitations from low-energy photon input, enabling advanced architectures for imaging and photochemistry. Here, we show that ultra-small PbS nanocrystals can sensitize red-to-blue triplet-fusion upconversion with a large anti-Stokes shift (ΔE = 1.04 eV), and achieve max-efficiency upconversion at near-solar fluences (I th = 220 mW cm-2) despite endothermic triplet sensitization. This system facilitates the photo-initiated polymerization of methyl methacrylate using only long-wavelength light (λ exc: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Time-resolved spectroscopy and kinetic modelling clarify key loss channels, highlighting the benefit of long-lifetime nanocrystal sensitizers, but revealing that many (48%) excitons that reach triplet-extracting carboxyphenylanthracene ligands decay before they can transfer to free-floating acceptors-emphasizing the need to address the reduced lifetimes that we determine for molecular triplets near the nanocrystal surface. Finally, we find that the inferred thermodynamics of triplet sensitization from these ultra-small PbS quantum dots are surprisingly favourable-completing an advantageous suite of properties for upconversion photochemistry-and do not vary significantly across the ensemble, which indicates minimal effects from nanocrystal heterogeneity. Together, our demonstration and study of red-to-blue upconversion using ultra-small PbS nanocrystals in a quasi-equilibrium, mildly endothermic sensitization scheme offer design rules to advance implementations of triplet fusion, especially where large anti-Stokes wavelength shifts are sought.

CdSe nanocrystal sensitized photon upconverting film

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters were assembled to address several challenges to practical applications for solution-based photon upconversion. By using poly(9-vinylcarbazole) as a phosphorescent host in this film, volatile organic solvents are eliminated, the spontaneous crystallization of the emitter is significantly retarded, and ∼1.5% photon upconversion quantum yield (out of a maximum of 50%) is obtained. Transient absorption spectroscopy on nanosecond-to-microsecond time scales reveals this efficiency is enabled by an exceptionally long triplet lifetime of 3.4 ± 0.3 ms. Ultimately, we find the upconversion efficiency is limited by incomplete triplet–triplet annihilation, which occurs with a rate 3–4 orders of magnitude slower than in solution-phase upconversion systems.

Here, films using CdSe nanocrystal (NC) triplet photosensitizers in conjunction with diphenylanthracene (DPA) emitters doe for the conversion of green to blue light.

Kinetics of photon upconversion by triplet-triplet annihilation: a comprehensive tutorial

This perspective article provides a comprehensive but organized tutorial introduction of the kinetics related to photon upconversion (UC) by triplet-triplet annihilation (TTA) (TTA-UC). The field of TTA-UC is multi-disciplinary and rapidly growing with the involvement of researchers from diverse backgrounds. TTA-UC consists of a series of tangled photophysical processes, so a solid understanding of the kinetic features and consequences is important to develop and evaluate materials for TTA-UC. This tutorial starts with an introduction of the standard model of TTA-UC along with the assumptions used in the model. The essential concept of the spin statistics for TTA and how this concept is related to the singlet branching ratio, which directly affects the efficiency of UC, are then explained through step-by-step analyses. Using these foundations, solutions for the steady-state behaviors are derived, featuring the universal curve that describes the excitation intensity dependence of the UC quantum yield for any sample type. Various useful functions for analyzing experimental data are also introduced and summarized. The transient behaviors of TTA-UC are then discussed along with their equations, and the usefulness for analyzing transient experimental data is explained using examples. In this article, self-consistent derivations and relevant references are provided for an easy understanding of the advanced discussion and analyses.

Entropy-Gated Thermally Activated Delayed Emission Lifetime in Phenanthrene-Functionalized CsPbBr3 Perovskite Nanocrystals

Charge and electronic energy transfer form the basis of many natural and artificial energy transduction systems. The energy landscapes that drive these transfer processes are often constructed from enthalpy changes. In contrast, the entropic effect, although occasionally invoked to explain some excited-state dynamics, has rarely been used to actively control charge/energy flow. Here we derive a generic formula describing how entropy can quantitatively gate the thermally activated delayed emission lifetime in semiconductor nanocrystal-molecular triplet acceptor complexes and experimentally verify the model using highly emissive, quantum-confined CsPbBr₃ nanocrystals surface-functionalized with multiple phenanthrene triplet acceptors. Triplet energy transfer from photoexcited CsPbBr₃ nanocrystals to phenanthrene is followed by thermally activated repopulation of nanocrystal excitons, leading to delayed nanocrystal emission. The lifetime of delayed emission increases with the phenanthrene/nanocrystal ratio, due to lowering of the free energy of the acceptor state by entropic gain. This study points toward a direction of using entropy to artificially design donor-acceptor light-emitting materials with predetermined excited-state lifetimes.

Multi-function PtCo nanozymes/CdS nanocrystals@graphene oxide luminophores and K2S2O8/H2O2 coreactants-based dual amplified electrochemiluminescence immunosensor for ultrasensitive detection of anti-myeloperoxidase antibody

Background

Anti-myeloperoxidase antibody (anti-MPO) is an important biomarker for anti-neutrophil cytoplasm antibody (ANCA)-associated vasculitides (AAVs). However, the complicated operation procedures and insufficient sensitivity of conventional anti-MPO detection methods limit their application in monitoring efficacy of AAVs in clinical diagnosis. Herein, a dual amplified electrochemiluminescence (ECL) immunosensor based on multi-function PtCo nanozymes/CdS nanocrystals@graphene oxide (PtCo/CdS@GO) luminophores and K₂S₂O₈/H₂O₂ coreactants has been fabricated for ultrasensitive detection of anti-MPO.

Results

PtCo/CdS@GO luminophores as novel signal amplification labels and nanocarriers to load rabbit anti-mouse IgG were synthesized by co-doping with Pt and Co nanozymes simultaneously with several considerable advantages, including astonishing peroxidase-like catalytic activity, high-efficiency luminescence performance and superior stability in aqueous solutions. Meanwhile, upon the K₂S₂O₈/H₂O₂ coreactants system, benefiting from the efficient peroxidase-like activity of the PtCo/CdS@GO toward H₂O₂, massive of transient reactive intermediates could react with K₂S₂O₈, thus obtaining higher ECL emission. Therefore, the developed ECL immunosensor for anti-MPO detection displayed good analytical performance with good concentration linearity in the range of 0.02 to 1000 pg/mL and low detection limit down to 7.39 fg/mL.

Conclusions

The introduction of multi-function PtCo/CdS@GO luminophores into the established ECL immunoassay not only was successfully applied for specific detection of anti-MPO in clinical serum samples, but also provided a completely new concept to design other high-performance luminophores. Meaningfully, the ECL immunoassay strategy held wide potential for biomarkers detection in clinical diagnosis.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00968-4.

Down and up conversion luminescence of the lead-free organic metal halide material: (C9H8NO)2SnCl6·2H2O

The present work deals with the optical properties of hybrid organic metal halide material namely (C9H8NO)2SnCl6·2H2O. Its structure is built up from isolated [SnCl6]2- octahedral dianions surrounded by Hydroxyl quinolinium organic cations (C9H8NO)+, abbreviated as [HQ]+. Unlike the usual hybrid materials, where metal halide ions are luminescent semiconductors while the organic ones are optically inactive, [HQ]2SnCl6·2H2O contains two optically active entities: [HQ]+ organic cations and [SnCl6]2- dianions. The optical properties of the synthesized crystals were studied by optical absorption spectroscopy, photoluminescence measurements and DFT calculations of electronic density of states. These studies have shown that both organic and inorganic entities have very close HOMO-LUMO gaps and very similar band alignments favoring the resonant energy transfer process. In addition, measurements of luminescence under variable excitations reveal an intense green luminescence around 497 nm under UV excitation (down conversion) and infrared excitation (up conversion luminescence). The down conversion luminescence is assigned to the π-π* transition within the [HQ] + organic cations involving charge transfer between the organic and inorganic entities, whereas the up-conversion luminescence is based on the triplet-triplet annihilation mechanism (TTA).

Generating Triplets in Organic Semiconductor Tetracene upon Photoexcitation of Transition Metal Dichalcogenide ReS2

We studied the dynamics of transfer of photoexcited electronic states in a bilayer of the two-dimensional transition metal dichalcogenide ReS₂ and tetracene, with the aim to produce triplets in the latter. This material combination was used as the band gap of ReS₂ (1.5 eV) is slightly larger than the triplet energy of tetracene (1.25 eV). Using time-resolved optical absorption spectroscopy, transfer of photoexcited states from ReS₂ to triplet states in tetracene was found to occur within 5 ps with an efficiency near 38%. This result opens up new possibilities for heterostructure design of two-dimensional materials with suitable organics to produce long-lived triplets. Triplets are of interest as sensitizers in a wide variety of applications including optoelectronics, photovoltaics, photocatalysis, and photon upconversion.

Controlling Thermally Activated Delayed Photoluminescence in CdSe Quantum Dots through Triplet Acceptor Surface Coverage

Quantum-dot/molecule composites (QD/mol) have demonstrated useful photochemical properties for many photonic and optoelectronic applications; however, a comprehensive understanding of these materials remains elusive. This work introduces a series of cadmium(II) selenide/1-pyrenecarboxylic acid (CdSe/PCA) nanomaterials featuring bespoke PCA surface coverage on CdSe585 (coded by the peak of the first exciton absorption band) to glean insight into the QD/mol photophysical behavior. Tailoring the energy gap between the CdSe585 first exciton band (2.1 eV) and the lowest PCA triplet level (T₁ = 2.0 eV) to be nearly isoenergetic, strong thermally activated delayed photoluminescence (TADPL) is observed resulting from reverse triplet-triplet energy transfer. The resultant average decay time constant (τ_obs) of the photoluminescence emanating from CdSe585 is deterministically controlled with surface-bound PCA_n chromophores (n = average number of adsorbed PCA molecules) by shifting the triplet excited state equilibrium from the CdSe585 to the PCA molecular triplet reservoir as a function of n.

Bidirectional triplet exciton transfer between silicon nanocrystals and perylene

Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light's energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon's biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system's utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion.

We demonstrate that silicon quantum dots can exchange spin triplet excitons with molecules covalently attached to their surface. Such hybrid materials can enable systems that upconvert incoherent far-red light into the visible spectral range.