Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore

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.

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.

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.

Effective Long Afterglow Amplification Induced by Surface Coordination Interaction

Long-persistent luminescent (LPL) materials have attracted considerable research interest due to their extensive applications and outstanding afterglow performance. However, the performance of red LPL materials lags behind that of green and blue materials. Therefore, it is crucial to explore novel red LPL materials. This study introduces a straightforward and viable strategy for organic-inorganic hybrids, wherein the organic ligand 1,3,6,8-Tetrakis(4-carboxyphenyl)pyrene (TCPP) is coordinated to the surface of a red persistent phosphor Sr0.75 Ca0.25 S:Eu2+ (R) through a one-step method. TCPP serves as an antenna, facilitating the transfer of absorbed light energy to R via triplet energy transfer (TET). Notably, the initial afterglow intensity and luminance of R increase by twofold and onefold, respectively, and the afterglow duration extends from 9 to 17 min. Furthermore, this study involves the preparation of a highly flexible film by mixing R@TCPP with high-density polyethylene (HDPE) to create a sound-controlled afterglow lamp. This innovative approach holds promising application prospects in flexible large-area luminescence, flexible wearables, and low-vision lighting.

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.

Rethinking Electronic Effects in Photochemical Hydrogen Evolution Using CuInS2@ZnS Quantum Dots Sensitizers

Molecular catalysts based on coordination complexes for the generation of hydrogen via photochemical water splitting exhibit a large versatility and tunability of the catalytic properties through chemical functionalization. In the present work, we report on light-driven hydrogen production in an aqueous solution using a series of cobalt polypyridine complexes as hydrogen evolving catalysts (HECs) in combination with CuInS₂@ZnS quantum dots (QDs) as sensitizers, and ascorbate as the electron donor. A peculiar trend in activity has been observed depending on the substituents present on the polypyridine ligand. This trend markedly differs from that previously recorded using [Ru(bpy)₃]²⁺ (where bpy = 2,2'-bipyridine) as the sensitizer and can be ascribed to different kinetically limiting pathways in the photochemical reaction (viz. protonation kinetics with the ruthenium chromophore, catalyst activation via electron transfer from the QDs in the present system). Hence, this work shows how the electronic effects on light-triggered molecular catalysis are not exclusive features of the catalyst unit but depend on the whole photochemical system.

Thermally Activated Bright-State Delayed Blue Photoluminescence from InP Quantum Dots

Thermally activated delayed photoluminescence (TADPL) generated from organic chromophore-functionalized quantum dots (QDs) is potentially beneficial for persistent light generation, QD-based PL sensors, and photochemical synthesis. While previous research demonstrated that naphthoic acid-functionalized InP QDs can be employed as low-toxicity, blue-emissive TADPL materials, the electron trap states inherent in these nanocrystals inhibited the observation of TADPL emerging from the higher-lying bright states. Here, we address this challenge by employing the heterocyclic aromatic compound 8-quinolinecarboxylic acid (QCA), whose triplet energy is strategically positioned to bypass the electron trap states in InP QDs. Transient absorption and photoluminescence spectroscopies revealed the generation of bright-state TADPL from QCA-functionalized InP QDs resulting from a nearly quantitative Dexter-like triplet-triplet energy transfer (TTET) from photoexcited InP QDs to surface-anchored QCA chromophores followed by reverse TTET from these bound molecules to the InP QDs. This modification resulted in a 119-fold increase in the average PL intensity decay time with respect to the as-synthesized InP QDs.

Construction of Hybrid Fluorescent Sensor for Cu2+ Detection Using Fluorescein-functionalized CdS Quantum Dots Via FRET

A new hybrid fluorescent nanosensor (Flu@Mea-CdS) for the Cu²⁺ detection in aqueous solution was constructed through fluorescence resonance energy transfer (FRET). The Flu@Mea-CdS was fabricated by amide linkage between CdS quantum dots capped with cysteamine (Mea-CdS) and fluorescein. With the formation of FRET process from Mea-CdS quantum dots to fluorescein, the fluorescence intensity of fluorescein at 520 nm was significantly enhanced. In addition, the sensor based on FRET has high selectivity for Cu²⁺ ions detection. With the presence of Cu²⁺ ions, Cu²⁺ ions were transferred to Cu₂S by the reaction with Flu@Mea-CdS, which caused the inhibition of FRET process and quenched the fluorescence signal of 520 nm. Compared with Mea-CdS quantum dots, the Flu@Mea-CdS sensor has a lower detection limit for Cu²⁺. The linear range is 4-14 μM, and the detection limit is 0.17 μM. The sensor has been successfully applied to the detection of Cu²⁺ ions in practical samples, which shows its potential application value in environmental monitoring.

Engineering Long-Lived Blue Photoluminescence from InP Quantum Dots Using Isomers of Naphthoic Acid

Leveraging triplet excitons in semiconductor quantum dots (QDs) in concert with surface-anchored molecules to produce long-lifetime thermally activated delayed photoluminescence (TADPL) continues to emerge as a promising technology in diverse areas including photochemical catalysis and light generation. All QDs presently used to generate TADPL in QD/molecule constructs contain toxic metals including Cd(II) and Pb(II), ultimately limiting potential real-world applications. Here, we report newly conceived blue-emitting TADPL-producing nanomaterials featuring InP QDs interfaced with 1- and 2-naphthoic acid (1-NA and 2-NA) ligands. These constitutional isomers feature similar triplet energies but disparate triplet lifetimes, translating into InP-based TADPL processes displaying two distinct average lifetime ranges upon cooling from 293 to 193 K. The time constants fall between 4.4 and 59.2 μs in the 2-NA-decorated InP QDs while further expanding between 84.2 and 733.2 μs in the corresponding 1-NA-ligated InP materials, representing a 167-fold time window. The resulting long-lived excited states enabled facile bimolecular triplet sensitization of ¹O₂ phosphorescence in the near-IR and promoted sensitized triplet-triplet annihilation photochemistry in 2,5-diphenyloxazole. We speculate that the discovery of new nanomaterials exhibiting TADPL lies on the horizon as myriad QDs can be readily derivatized using isomers of numerous classes of surface-anchoring chromophores yielding precisely regulated photophysical properties.

Hybrid EuIII Coordination Luminophore Standing on Two Legs on Silica Nanoparticles for Enhanced Luminescence

In this study, we have demonstrated a two-legged, upright molecular design method for monochromatic and bright red luminescent Ln^III -silica nanomaterials. A novel Eu^III -silica hybrid nanoparticle was developed by using a doubly binding TPPO-Si(OEt)₃ (TPPO: triphenyl phosphine oxide) linker. The TPPO-Si(OEt)₃ was confirmed by ¹ H, ³¹ P, ²⁹ Si NMR spectroscopy and single-crystal X-ray analysis. Luminescent Eu(hfa)₃ and Eu(tfc)₃ moieties (hfa: hexafluoroacetylacetonate, tfc: 3-(trifluoromethylhydroxymethylene)camphorate) were fixed onto TPPO-Si(OEt)₃ -modified silica nanoparticles, producing Eu(hfa)₃ (TPPO-Si)₂ -SiO₂ and Eu(tfc)₃ (TPPO-Si)₂ -SiO₂ , respectively. Eu(hfa)₃ (TPPO-Si)₂ -SiO₂ exhibited the higher intrinsic luminescence quantum yield (93 %) and longer emission lifetime (0.98 ms), which is much larger than those of previously reported Eu^III -based hybrid materials. Eu(tfc)₃ (TPPO-Si)₂ -SiO₂ showed an extra-large intrinsic emission quantum yield (54 %), although the emission quantum yield for the precursor Eu(tfc)₃ (TPPO-Si(OEt)₃ )₂ was found to be 39 %. These results confirmed that the TPPO-Si(OEt)₃ linker is a promising candidate for development of Eu^III -based luminescent materials.

Thermally Activated Delayed Photoluminescence: Deterministic Control of Excited-State Decay

Thermally activated photophysical processes are ubiquitous in numerous organic and metal-organic molecules, leading to chromophores with excited-state properties that can be considered an equilibrium mixture of the available low-lying states. Relative populations of the equilibrated states are governed by temperature. Such molecules have been devised as high quantum yield emitters in modern organic light-emitting diode technology and for deterministic excited-state lifetime control to enhance chemical reactivity in solar energy conversion and photocatalytic schemes. The recent discovery of thermally activated photophysics at CdSe nanocrystal-molecule interfaces enables a new paradigm wherein molecule-quantum dot constructs are used to systematically generate material with predetermined photophysical response and excited-state properties. Semiconductor nanomaterials feature size-tunable energy level engineering, which considerably expands the purview of thermally activated photophysics beyond what is possible using only molecules. This Perspective is intended to provide a nonexhaustive overview of the advances that led to the integration of semiconductor quantum dots in thermally activated delayed photoluminescence (TADPL) schemes and to identify important challenges moving into the future. The initial establishment of excited-state lifetime extension utilizing triplet-triplet excited-state equilibria is detailed. Next, advances involving the rational design of molecules composed of both metal-containing and organic-based chromophores that produce the desired TADPL are described. Finally, the recent introduction of semiconductor nanomaterials into hybrid TADPL constructs is discussed, paving the way toward the realization of fine-tuned deterministic control of excited-state decay. It is envisioned that libraries of synthetically facile composites will be broadly deployed as photosensitizers and light emitters for numerous synthetic and optoelectronic applications in the near future.

Self-Assembly of Semiconductor Quantum Dots using Organic Templates

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.

TIPS-pentacene triplet exciton generation on PbS quantum dots results from indirect sensitization

Many fundamental questions remain in the elucidation of energy migration mechanisms across the interface between semiconductor nanomaterials and molecular chromophores.

Many fundamental questions remain in the elucidation of energy migration mechanisms across the interface between semiconductor nanomaterials and molecular chromophores. The present transient absorption study focuses on PbS quantum dots (QDs) of variable size and band-edge exciton energy (ranging from 1.15 to 1.54 eV) post-synthetically modified with a carboxylic acid-functionalized TIPS-pentacene derivative (TPn) serving as the molecular triplet acceptor. In all instances, selective excitation of the PbS NCs at 743 nm leads to QD size-dependent formation of an intermediate with time constants ranging from 2–13 ps, uncorrelated to the PbS QD valence band potential. However, the rate constant for the delayed formation of the TPn triplet excited state markedly increases with increasing PbS conduction band energy, featuring a parabolic Marcus free energy dependence in the normal region. These observations provide evidence of an indirect triplet sensitization process being inconsistent with a concerted Dexter-like energy transfer process. The collective data are consistent with the generation of an intermediate resulting from hole trapping of the initial PbS excited state by midgap states, followed by formation of the TPn triplet excited state whose rate constant and yield increases with decreasing quantum dot size.

Synthesis of folic acid conjugated photoluminescent carbon quantum dots with ultrahigh quantum yield for targeted cancer cell fluorescence imaging

Folic acid functionalized carbon quantum dots (FA-CQDs) with ultrahigh quantum yield (50 %) were synthesized by one-pot hydrothermal route using citric acid. The synthesized CQDs were characterized by fluorescence spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS) and X-ray diffraction. The cell viability of about 95 % and 97 % were obtained for MTT assay of the CQDs and FA-CQDs toward MCF-7 cells after 24 h of incubation respectively. The FA-CQDs were successfully applied for targeted imaging of ovarian cancer (type HeLa) and human breast adenocarcinoma (type MCF7) cells using fluorescence microscope.

Semiconductor Quantum Dots as Components of Photoactive Supramolecular Architectures

Luminescent quantum dots (QDs) are colloidal semiconductor nanocrystals consisting of an inorganic core covered by a molecular layer of organic surfactants. Although QDs have been known for more than thirty years, they are still attracting the interest of researchers because of their unique size-tunable optical and electrical properties arising from quantum confinement. Moreover, the controlled decoration of the QD surface with suitable molecular species enables the rational design of inorganic-organic multicomponent architectures that can show a vast array of functionalities. This minireview highlights the recent progress in the use of surface-modified QDs - in particular, those based on cadmium chalcogenides - as supramolecular platforms for light-related applications such as optical sensing, triplet photosensitization, photocatalysis and phototherapy.

Mechanisms of triplet energy transfer across the inorganic nanocrystal/organic molecule interface

The mechanisms of triplet energy transfer across the inorganic nanocrystal/organic molecule interface remain poorly understood. Many seemingly contradictory results have been reported, mainly because of the complicated trap states characteristic of inorganic semiconductors and the ill-defined relative energetics between semiconductors and molecules used in these studies. Here we clarify the transfer mechanisms by performing combined transient absorption and photoluminescence measurements, both with sub-picosecond time resolution, on model systems comprising lead halide perovskite nanocrystals with very low surface trap densities as the triplet donor and polyacenes which either favour or prohibit charge transfer as the triplet acceptors. Hole transfer from nanocrystals to tetracene is energetically favoured, and hence triplet transfer proceeds via a charge separated state. In contrast, charge transfer to naphthalene is energetically unfavourable and spectroscopy shows direct triplet transfer from nanocrystals to naphthalene; nonetheless, this "direct" process could also be mediated by a high-energy, virtual charge-transfer state.

Photon upconversion utilizing energy beyond the band gap of crystalline silicon with a hybrid TES-ADT/PbS quantum dots system

The recent introduction of inorganic semiconductor quantum dots (QDs) as triplet sensitizers for molecular semiconductors has led to significant interest in harvesting low energy photons, which can then be used for photon upconversion (PUC), via triplet-triplet annihilation (TTA). A key goal is the harvesting of photons from below the bandgap of crystalline silicon 1.12 eV (≈1100 nm) and their upconversion into the visible region. In practice, the systems demonstrated so far have been limited to harvesting photons with energies above 1.2 eV (≈1 μm), due to two reasons: firstly the need to use transmitter ligands which allow efficient energy harvesting from the QD but introduce an energy loss of larger than 200 meV in transmission from the QD to the annihilator, and secondly due to the use of molecules such as tetracene which cannot accept smaller energy than 1.2 eV. Here, we introduce a new strategy to overcome these difficulties by using a low energy triplet annihilator that also harvests excitations efficiently from QDs. Specifically, we show that 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT, triplet energy of 1.08 eV: ca. 1150 nm) functions as a triplet annihilator (20% TTA efficiency) while also rapidly extracting triplet excitons from lead sulfide (PbS) QDs with a rate constant of k = ca. 2 × 10^-8 s^-1 with an excitation at 1064 nm. This rate is consistent with an orbital overlap between TES-ADT and PbS QDs, which we propose is due to the thiophene group of TES-ADT, which enables a close association with the PbS surface, allowing this system to function both as annihilator and transmitter. Our results pave the way for the design of triplet annihilators that can closely associate with the QD surface and harvest low energy excitons with minute losses in energy during the TET process, with the ultimate goal of efficiently utilizing photon energy beyond the bandgap of crystalline silicon.

Photoactive Molecular-Based Devices, Machines and Materials: Recent Advances

Molecular and supramolecular-based systems and materials that can perform predetermined functions in response to light stimulation have been extensively studied in the past three decades. Their investigation continues to be a highly stimulating topic of chemical research, not only because of the inherent scientific value related to a bottom-up approach to functional nanostructures, but also for the prospective applications in diverse fields of technology and medicine. Light is an important tool in this context, as it can be conveniently used both for supplying energy to the system and for probing its states and transformations. In this microreview we recall some basic aspects of light-induced processes in (supra)molecular assemblies, and discuss their exploitation to implement novel functionalities with nanostructured devices, machines and materials. To this aim we illustrate a few examples from our own recent work, which are meant to illustrate the trends of current research in the field.

Semiconductor Nanocrystal Light Absorbers for Photon Upconversion

Semiconductor nanocrystals (NCs) can initiate energy and charge transfer in multiple applications with their unique optical and electronic properties. In particular, NCs are excellent light absorbers for initiating triplet energy transfer (TET) to organic molecules, a key step in triplet-fusion-based photon upconversion. Triplet energy transfer across this inorganic-organic interface is one of the bottlenecks that currently limits the overall photon upconversion quantum yield. In this Perspective, we summarize the progress made in the past three years on this hybrid photon upconversion platform. We discuss the effects of NC size, composition, and surface states on TET. Nanocrystal surface engineering may address the loss mechanisms arising from defect states and exciton-phonon coupling. Alternative materials for NC triplet photosensitizers that do not contain toxic heavy metals will be especially useful for various biological applications.

Multi-wavelength tailoring of a ZnGa2O4 nanosheet phosphor via defect engineering

The multi-wavelength luminescence tailoring of an individual phosphor free of external dopants is of great interest and technologically important for practical applications. Using ZnGa2O4 nanosheets as a target phosphor, we demonstrate how to artificially control the luminescence wavelength centers and their emission intensities to simultaneously emit ultraviolet/blue, green and red light via a feasible defect engineering strategy. Simple high-temperature annealing of hydrothermally synthesized ZnGa2O4 nanosheets leads to the effective tunability of their emission process to present multi-wavelength luminescence due to the structural distortion and the formation of oxygen vacancies. Controlling the annealing temperature and time can further precisely modulate the wavelengths and their corresponding intensities. It is speculated that the migration of Ga into the [GaO4] tetrahedron and the O vacancy are responsible for the multi-wavelength luminescence of the ZnGa2O4 nanosheet phosphor. Finally, the tentative multi-wavelength luminescence behavior of the ZnGa2O4 nanosheet phosphor via defect engineering is discussed based on a series of evidenced experimental observations of XRD, XPS, HRTEM and CL.

Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self-assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 μs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.