Colloidal Quantum Dots as Photocatalysts for Triplet Excited State Reactions of Organic Molecules

Deciphering the Energy Transfer Mechanism Across Metal Halide Perovskite-Phthalocyanine Interfaces

Energy transfer processes in nanohybrids are at the focal point of conceptualizing, designing, and realizing novel energy-harvesting systems featuring nanocrystals that absorb photons and transfer their energy unidirectionally to surface-immobilized functional dyes. Importantly, the functionality of these dyes defines the ultimate application. Herein, CsPbBr3 perovskite nanocrystals (NCs) are interfaced with zinc phthalocyanine (ZnPc) dyes featuring carboxylic acid. The functionality is the photosensitization of singlet oxygen. The CsPbBr3@ZnPc nanohybrid is to the best of our knowledge the first example, in which an unusual Dexter-type singlet energy transfer between metal halide perovskite nanocrystals and phthalocyanine dyes enables singlet oxygen generation as a proof-of-concept application. A detailed temporal picture of the singlet energy transfer mechanism is made possible by combining key time-resolved spectroscopic techniques, that are, femtosecond, nanosecond, and microsecond transient absorption spectroscopy as well as time-correlated single photon counting, and target analyses. In fact, three excitonic components in the NCs govern a concerted Dexter-type energy transfer. The work illustrates the potential of CsPbBr3@ZnPc as a singlet photosensitizer of ZnPc to produce singlet oxygen (1O2) almost quantitatively while photoexciting CsPbBr3.

Carrier dynamics competition in the nanocrystal-molecule complex for triplet generation studied by transient-absorption spectroscopy

Owing to the long-lived decay of triplet excited state, extensive efforts have been devoted to efficient triplet generation for applications covering triplet-triplet annihilation for photon upconversion, photocycloaddition and photoredox catalysis. Among the candidates, nanocrystal-molecule complexes have received tremendous attention for triplet generation because of easier spin flip and negligible energy loss during intersystem crossing. However, the triplet energy transfer (TET) from nanocrystals (NCs) to molecules can be very complicated in actual situation due to intricate energy level alignment and inevitable defect states, which often involves various decay pathes of the excited state competing with TET. Understanding the detailed carrier dynamics in such complexes is strongly necessary for related applications. Here, a CdSe-TCA (5-tetracene carboxylic acid) complex with a Type-II like energy level alignment is synthesized through precisely adjusting the dimension of CdSe NC. Based on series of spectral measurements, especially the transient absorption (TA) spectroscopy, the results show various carrier dynamics including hole-transfer-mediated TET, Förster resonance energy transfer (FRET) and carrier trapping. Although the carrier trapping by defect states in CdSe NC is revealed not associated with the TET from CdSe to TCA, the FRET is proved to competing with the TET process. Both the FRET and defect states should be refrained for efficient TET in such complexes. This study could provide further insight for understanding the carrier dynamics competition in NC-molecule complexes for triplet generation and benefit related optoelectronics applications.

Pyridinethiolate-Capped CdSe Quantum Dots for Red-Light-Driven H2 Production in Water

The utilization of low-energy sunlight to produce renewable fuels is a subject of great interest. Here we report the first example of metal chalcogenide quantum dots (QDs) capped with a pyridinethiolate carboxylic acid (pyS-COOH) for red-light-driven H2 production in water. The precious-metal-free system is robust over 240 h, and achieves a turnover number (TON) of 43910±305 (vs Ni) with a rate of 31570±1690 μmol g-1 h-1 for hydrogen production. In contrast to the inactive QDs capped with other thiolate ligands, the CdSe-pyS-COOH QDs give a significantly higher singlet oxygen quantum yield [ΦΔ (1O2)] in solution.

Molecular Triplet Generation Enabled by Adjacent Metal Nanoparticles

High-efficiency generation of spin-triplet states in organic molecules is of great interest in diverse areas such as photocatalysis, photodynamic therapy, and upconversion photonics. Recent studies have introduced colloidal semiconductor nanocrystals as a new class of photosensitizers that can efficiently transfer their photoexcitation energy to molecular triplets. Here, we demonstrate that metallic Ag nanoparticles can also assist in the generation of molecular triplets in polycyclic aromatic hydrocarbons (PAHs), but not through a conventional sensitization mechanism. Instead, the triplet formation is mediated by charge-separated states resulting from hole transfer from photoexcited PAHs (anthracene and pyrene) to Ag nanoparticles, which is established through the rapid formation and subsequent decay of molecular anions revealed in our transient absorption measurements. The dominance of hole transfer over electron transfer, while both are energetically allowed, could be attributed to a Marcus inverted region of charge transfer. Owing to the rapid charge separation and the rapid spin-flip in metals, the triplet formation yields are remarkably high, as confirmed by their engagement in production of singlet oxygen with a quantum efficiency reaching 58.5%. This study not only uncovers the fundamental interaction mechanisms between metallic nanoparticles and organic molecules in both charge and spin degrees of freedom but also greatly expands the scope of triplet "sensitization" using inorganic nanomaterials for a variety of emerging applications.

Demystifying Triplet-Triplet Annihilation Mechanism in the CsPbI3-Rubrene-DBP Upconversion System

A triplet-triplet annihilation-based upconversion (TTA-UC) system, employing a multichromophore assembly, is convenient to harvest low-energy photons for light energy conversion and optoelectronic applications. The primary donor in the TTA-UC system, typically a low-bandgap semiconductor, captures the low-energy photons and transfers triplet energy to an annihilator dye molecule, which in turn generates a high-energy singlet excited state via T-T annihilation. We have now succeeded in revealing kinetic and mechanistic details of multistep energy transfer processes in the CsPbI3-rubrene-perylene derivative (DBP) films by analyzing time-resolved emission and absorption measurements. The initial triplet energy transfer between CsPbI3 and rubrene occurs with an efficiency of 70% and a rate constant of 9 × 108 s-1. The rubrene triplets undergo T-T annihilation via simple second-order kinetics to form an excited singlet state exhibiting a delayed emission up to 10 μs, which is significantly greater than the intrinsic lifetime of 15 ns. The emitter DBP (a perylene derivative) captures the singlet energy quite effectively and delivers the upconverted emission in sync with the delayed emission of rubrene. The quadratic dependence of DBP emission on the excitation light intensity shows the importance of the T-T annihilation process in dictating the overall upconversion process. The kinetic parameters evaluated in this study, which divulge the critical steps dictating energy transfer in a TTA-UC system, should aid in the design of new light harvesting assemblies.

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.

Ternary heterostructure-driven photoinduced electron-hole separation enhanced oxidative stress for triple-negative breast cancer therapy

Zinc oxide nanoparticles (ZnO NPs) stand as among the most significant metal oxide nanoparticles in trigger the formation of reactive oxygen species (ROS) and induce apoptosis. Nevertheless, the utilization of ZnO NPs has been limited by the shallowness of short-wavelength light and the constrained production of ROS. To overcome these limitations, a strategy involves achieving a red shift towards the near-infrared (NIR) light spectrum, promoting the separation and restraining the recombination of electron-hole (e--h+) pairs. Herein, the hybrid plasmonic system Au@ZnO (AZ) with graphene quantum dots (GQDs) doping (AZG) nano heterostructures is rationally designed for optimal NIR-driven cancer treatment. Significantly, a multifold increase in ROS generation can be achieved through the following creative initiatives: (i) plasmonic Au nanorods expands the photocatalytic capabilities of AZG into the NIR domain, offering a foundation for NIR-induced ROS generation for clinical utilization; (ii) elaborate design of mesoporous core-shell AZ structures facilitates the redistribution of electron-hole pairs; (iii) the incorporation GQDs in mesoporous structure could efficiently restrain the recombination of the e--h+ pairs; (iv) Modification of hyaluronic acid (HA) can enhance CD44 receptor mediated targeted triple-negative breast cancer (TNBC). In addition, the introduced Au NRs present as catalysts for enhancing photothermal therapy (PTT), effectively inducing apoptosis in tumor cells. The resulting HA-modified AZG (AZGH) exhibits efficient hot electron injection and e--h+ separation, affording unparalleled convenience for ROS production and enabling NIR-induced PDT for the cancer treanment. As a result, our well-designed mesoporous core-shell AZGH hybrid as photosensitizers can exhibit excellent PDT efficacy.

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

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.

CsPbBr3 Quantum Dots Promoted Depolymerization of Oxidized Lignin via Photocatalytic Semi-Hydrogenation/Reduction Strategy

Due to the demanding depolymerization conditions and limited catalytic efficiency, enhancing lignin valorization remains challenging. Therefore, lowering the bond dissociation energy (BDE) has emerged as a viable strategy for achieving mild yet highly effective cleavage of bonds. In this study, a photocatalytic semi-hydrogenation/reduction strategy utilizing CsPbBr3 quantum dots (CPB-QDs) and Hantzsch ester (HEH2 ) as a synergistic catalytic system was introduced to reduce the BDE of Cβ -O-Ar, achieving effective cleavage of the Cβ -O-Ar bond. This strategy offers a wide substrate scope encompassing various β-O-4 model lignin dimers, preoxidized β-O-4 polymers, and native oxidized lignin, resulting in the production of corresponding ketones and phenols. Notably, this approach attained a turnover frequency (TOF) that is 17 times higher than that of the reported Ir-catalytic system in the photocatalytic depolymerization of the lignin model dimers. It has been observed via meticulous experimentation that HEH2 can be activated by CPB-QDs via single electron transfer (SET), generating HEH2 ⋅+ as a hydrogen donor while also serving as a hole quencher. Moreover, HEH2 ⋅+ readily forms an active transition state with the substrates via hydrogen bonding. Subsequently, the proton-coupled electron transfer (PCET) from HEH2 ⋅+ to the carbonyl group of the substrate generates a Cα ⋅ intermediate.

Extension of molecules with an inverted singlet-triplet gap with conjugated branches to alter the oscillator strength

Molecules that violate Hund's rule and possess negative singlet-triplet gaps (ΔEST) have been actively studied for their potential usage in organic light emitting diodes without the need for thermal activation. However, the weak oscillator strength from the symmetry of such molecules has been recognized as their shortcoming for their application in optoelectronic devices. A group of molecules with a common structural motif involving the original molecule with an inverted gap having branches consisting of conjugated molecules of varied structures and extent of conjugation have been predicted to have desirable oscillator strength, but only few detailed and comprehensive studies regarding the form of excited states and the reason behind the improved oscillator strength have been carried out. We show in this work a series of analyses that suggest that the increase of oscillator strength is correlated with the nature of the excited state changing from a localized excitation to a delocalized excitation involving the central molecule and the branches. The resulting oscillator strength thus depends on the energetic matching of the branching molecule and the central molecule, rather than solely the oscillator strength of the central molecule. From the ΔEST inversion point of view, the static correlation with low-lying doubly excited configurations, the key mechanism behind the inversion in the localized excited state, weakens as the excited states delocalize. As a consequence, the dynamic correlation has a more decisive effect in determining the singlet-triplet gap.

Tuning Energy Transfer Pathways in Halide Perovskite-Dye Hybrids through Bandgap Engineering

Lead halide perovskite nanocrystals, which offer rich photochemistry, have the potential to capture photons over a wide range of the visible and infrared spectrum for photocatalytic, optoelectronic, and photon conversion applications. Energy transfer from the perovskite nanocrystal to an acceptor dye in the form of a triplet or singlet state offers additional opportunities to tune the properties of the semiconductor-dye hybrid and extend excited-state lifetimes. We have now successfully established the key factors that dictate triplet energy transfer between excited CsPbI3 and surface-bound rhodamine dyes using absorption and emission spectroscopies. The pendant groups on the acceptor dyes influence surface binding to the nanocrystals, which in turn dictate the energy transfer kinetics, as well as the efficiency of energy transfer. Of the three rhodamine dyes investigated (rhodamine B, rhodamine B isothiocyanate, and rose Bengal), the CsPbI3-rose Bengal hybrid with the strongest binding showed the highest triplet energy transfer efficiency (96%) with a rate constant of 1 × 109 s-1. This triplet energy transfer rate constant is nearly 2 orders of magnitude slower than the singlet energy transfer observed for the pure-bromide CsPbBr3-rose Bengal hybrid (1.1 × 1011 s-1). Intriguingly, although the single-halide CsPbBr3 and CsPbI3 nanocrystals selectively populate singlet and triplet excited states of rose Bengal, respectively, the mixed halide perovskites were able to generate a mixture of both singlet and triplet excited states. By tuning the bromide/iodide ratio and thus bandgap energy in CsPb(Br1-xIx)3 compositions, the percentage of singlets vs triplets delivered to the acceptor dye was systematically tuned from 0 to 100%. The excited-state properties of halide perovskite-molecular hybrids discussed here provide new ways to modulate singlet and triplet energy transfer in semiconductor-molecular dye hybrids through acceptor functionalization and donor bandgap engineering.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Size-Regulated Hole and Triplet Energy Transfer from CdSe Quantum Dots to Organic Acceptors for Enhancing Singlet Oxygen Generation

Triplet energy transfer (TET) from semiconductor quantum dots (QDs) is an emerging strategy for sensitizing molecular triplets that have great potential in many applications. Here, CdSe QDs with varying sizes and 1-pyrenecarboxylic acid (PCA) are selected as the triplet donor and acceptor, respectively, to study the TET and charge transfer dynamics as well as enhanced singlet oxygen (1O2) generation properties. The results from static and transient spectroscopy measurements demonstrate that both the TET and hole transfer occur at the QDs-PCA interface. The observed significant drop in TET efficiency from 52 to 8% with increasing QD size results from the reduced TET driving force between the QDs and PCA, which is further confirmed by the more efficient sensitization of the anthracene derivative with a large TET driving force. In contrast, the hole transfer efficiency displays a small decrease with an increasing QD size due to a slight change in the hole driving force. The sensitized PCA triplets show a good ability of 1O2 generation, and the 1O2 formation rate increases 10-fold as the QD size decreases from 3.3 to 2.4 nm. These findings provide a profound understanding of the TET and hole transfer mechanism from QDs to molecules and are significant in designing efficient 1O2 generation systems based on semiconductor QDs and triplet molecules.

Variations on the Bergman Cyclization Theme: Electrocyclizations of Ionic Penta-, Hepta-, and Octadiynes

The Bergman cyclization of (Z)-hexa-3-ene-1,5-diyne to form the aromatic diradical p-benzyne has garnered attention as a potential antitumor agent due to its relatively low cyclization barrier and the stability of the resulting diradical. Here, we present a theoretical investigation of several ionic extensions of the fundamental Bergman cyclization: electrocyclizations of the penta-1,4-diyne anion, hepta-1,6-diyne cation, and octa-1,7-diyne dication, leveraging the spin-flip formulation of the equation-of-motion coupled cluster theory with single and double substitutions (EOM-SF-CCSD). Though the penta-1,4-diyne anion exhibits a large cyclization barrier of +66 kcal mol-1, cyclization of both the hepta-1,6-diyne cation and octa-1,7-diyne dication along a previously unreported triplet pathway requires relatively low energy. We also identified the presence of significant aromaticity in the triplet diradical products of these two cationic cyclizations.

Colloidal Quantum Dots as an Emerging Vast Platform and Versatile Sensitizer for Singlet Molecular Oxygen Generation

Singlet molecular oxygen (1O2) has been reported in wide arrays of applications ranging from optoelectronic to photooxygenation reactions and therapy in biomedical proposals. It is also considered a major determinant of photodynamic therapy (PDT) efficacy. Since the direct excitation from the triplet ground state (3O2) of oxygen to the singlet excited state 1O2 is spin forbidden; therefore, a rational design and development of heterogeneous sensitizers is remarkably important for the efficient production of 1O2. For this purpose, quantum dots (QDs) have emerged as versatile candidates either by acting individually as sensitizers for 1O2 generation or by working in conjunction with other inorganic materials or organic sensitizers by providing them a vast platform. Thus, conjoining the photophysical properties of QDs with other materials, e.g., coupling/combining with other inorganic materials, doping with the transition metal ions or lanthanide ions, and conjugation with a molecular sensitizer provide the opportunity to achieve high-efficiency quantum yields of 1O2 which is not possible with either component separately. Hence, the current review has been focused on the recent advances made in the semiconductor QDs, perovskite QDs, and transition metal dichalcogenide QD-sensitized 1O2 generation in the context of ongoing and previously published research work (over the past eight years, from 2015 to 2023).

Gram-scale synthesis of MIL-125 nanoparticles and their solution processability

Although metal-organic framework (MOF) photocatalysts have become ubiquitous, basic aspects of their photoredox mechanisms remain elusive. Nanosizing MOFs enables solution-state techniques to probe size-dependent properties and molecular reactivity, but few MOFs have been prepared as nanoparticles (nanoMOFs) with sufficiently small sizes. Here, we report a rapid reflux-based synthesis of the photoredox-active MOF Ti8O8(OH)4(terephthalate)6 (MIL-125) to achieve diameters below 30 nm in less than 2 hours. Whereas MOFs generally require ex situ analysis by solid-state techniques, sub-30 nm diameters ensure colloidal stability for weeks and minimal light scattering, permitting in situ analysis by solution-state methods. Optical absorption and photoluminescence spectra of free-standing colloids provide direct evidence that the photoredox chemistry of MIL-125 involves Ti3+ trapping and charge accumulation onto the Ti-oxo clusters. Solution-state potentiometry collected during the photochemical process also allows simultaneous measurement of MOF Fermi-level energies in situ. Finally, by leveraging the solution-processability of these nanoparticles, we demonstrate facile preparation of mixed-matrix membranes with high MOF loadings that retain the reversible photochromism. Taken together, these results demonstrate the feasibility of a rapid nanoMOF synthesis and fabrication of a photoactive membrane, and the fundamental insights they offer into heterogeneous photoredox chemistry.

Chiral Perovskite Nanocrystals for Asymmetric Reactions: A Highly Enantioselective Strategy for Photocatalytic Synthesis of N-C Axially Chiral Heterocycles

Catalytic approaches to generate enantiospecific chiral centers are the major premise of modern organic chemistry. Heterogeneous catalysis is responsible for the vast majority of chemical transformations, yet the direct employment of chiral solid catalysts for asymmetric synthesis is mostly overlooked. Here, we demonstrated that a heterogeneous metal-halide perovskite nanocrystal (NC) catalyst is active for asymmetric organic synthesis under visible-light activation. Chiral 1-phenylethylamine (PEA)-hybridized perovskite PEA/CsPbBr3 NC photocatalysts exhibit an enantioselective (up to 99% enantiomer excess, ee) avenue to produce N-C axially chiral N-heterocycles, i.e., N-arylindoles from N-arylamine photo-oxidation. Mechanistic investigation indicated a discriminated prochiral binding of the N-arylamine substrates onto the chiral-NC surface with ca. -2.4 kcal/mol enantiodifferentiation. Our perovskite NC heterogeneous catalytic system not only demonstrates a promising strategy to address the long-term challenges in atroposelective pharmaceutical scaffold synthesis but also paves the road to directly employ chiral solids for asymmetric synthesis.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

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.

Quantum Dot Metal Salt Interactions Unraveled by the Sphere of Action Model

Postsynthetic metal salt treatments are frequently employed in the luminescence enhancement of quantum dots (QDs); however, its microscopic picture remains unclear. CsPbBr₃-QDs, featuring strong excitonic absorption and high photoluminescence (PL) quantum yield, are ideal QDs to unravel the intricate interaction between QDs and such surface-bound metal salts. Herein, we study this interaction based on the controlled PL quenching of CsPbBr₃-QDs with BiBr₃. Upon the addition of BiBr₃, an instant and complete PL quenching is observed, which can be fully recovered after the addition of an excess of PbBr₂. This, together with the complete preservation of the excitonic absorption suggests a surface-driven adsorption equilibrium. Additionally, time-resolved studies reveal a non-homogeneous surface trap formation. Based on the so-called sphere of action model for the adsorption process, we show that already a single BiBr₃ adsorption suffices to completely quench a QD's luminescence. This approach is expanded to analyze size-, ligand-, and metal-dependent quenching dynamics. Facet junctions are identified as regions of enhanced surface reactivity. A Langmuir-type ligand coverage is exposed with a strong impact on adsorption. Our results provide a detailed mechanistic insight into postsynthetic interaction of QDs with metal salts, opening pathways for future surface manipulations.

Colloidal CsPbX3 Nanocrystals with Thin Metal Oxide Gel Coatings

Lead halide perovskite (LHP) nanocrystals (NCs) have gathered much attention as light-emitting materials, particularly owing to their excellent color purity, band gap tunability, high photoluminescence quantum yield (PLQY), low cost, and scalable synthesis. To enhance the stability of LHP NCs, bulky strongly bound organic ligands are commonly employed, which counteract the extraction of charge carriers from the NCs and hinder their use as photoconductive materials and photocatalysts. Replacing these ligands with a thin coating is a complex challenge due to the highly dynamic ionic lattice, which is vulnerable to the commonly employed coating precursors and solvents. In this work, we demonstrate thin (<1 nm) metal oxide gel coatings through non-hydrolytic sol-gel reactions. The coated NCs are readily dispersible and highly stable in short-chain alcohols while remaining monodisperse and exhibiting high PLQY (70-90%). We show the successful coating of NCs in a wide range of sizes (5-14 nm) and halide compositions. Alumina-gel-coated NCs were chosen for an in-depth analysis, and the versatility of the approach is demonstrated by employing zirconia- and titania-based coatings. Compact films of the alumina-gel-coated NCs exhibit electronic and excitonic coupling between the NCs, leading to two orders of magnitude longer photoluminescence lifetimes (400-700 ns) compared to NCs in solution or their organically capped counterparts. This makes these NCs highly suited for applications where charge carrier delocalization or extraction is essential for performance.

Multifunctional behavior of a carbazole derivative: Red phosphorescent emission, aggregation-induced long-life exciton and light-emitting diode application

A D-π-A typed cyanyl-carboxylic derivative (named as CECZA) merely produced prompt fluorescence with lifetime at nanosceond scale in dilute solutions, whose solid-state luminescence exhibited 3.36 μs lifetime with 13.80 % quantum yield (QY, captured at 522 nm for powder at nanometer scale) at 298 K and 43.36 ms lifetime with 30.46 % QY (650 nm, 80 K, tiny crystals). Femtosecond transient absorption, Raman spectroscopy and quantum chemical calculation provided valid clues to reveal its excitonic transition mechanism. The results indicated that the restricted vibration of benzene ring on carbazole group and alkyl chain weakened the vibrational modes of CECZA molecule and strengthened inter-molecular interactions between adjacent molecules at low temperatures, which promoted the persistent phosphorescent emission. Due to strong UV-vis absorption, high quantum efficiency and excellent thermal stability, CECZA can be used as a potential candidate in light-emitting diode (LED) application. Combined with a commercial InGaN blue-emitting chip, CECZA-InGaN emitted daylight white light.

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.

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.

Unambiguous spectral characterization on triplet energy transfer from quantum dots mediated by hole transfer competing with other carrier dynamics

Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called "back transfer charge recombination" (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.

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.

Confined Excitons in Spherical-Like Halide Perovskite Quantum Dots

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies. While strongly confined III-V and II-VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr₃ QDs with diameters >6 nm, excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV). The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr₃ QDs and gives a detailed understanding of their linear and nonlinear optical spectra.

Comparative Study of Covalent and van der Waals CdS Quantum Dot Assemblies from Many-Body Perturbation Theory

Quantum dot (QD) assemblies are nanostructured networks made from aggregates of QDs and feature improved charge and energy transfer efficiencies compared to discrete QDs. Using first-principles many-body perturbation theory, we systematically compare the electronic and optical properties of two types of CdS QD assemblies that have been experimentally investigated: (i) QD gels, where individual QDs are covalently connected via di- or polysulfide bonds, and (ii) QD nanocrystals, where individual QDs are bound via van der Waals interactions. Our work illustrates how the electronic and optical properties evolve when discrete QDs are assembled into 1D, 2D, and 3D gels and nanocrystals, as well as how the one-body and many-body interactions in these systems impact the trends as the dimensionality of the assembly increases. Furthermore, our work reveals the crucial role of the di- or polysulfide covalent bonds in the localization of the excitons, which highlights the difference between QD gels and QD nanocrystals.

Molecular Wires for Efficient Long-Distance Triplet Energy Transfer

We propose design rules for building organic molecular bridges that enable coherent long-distance triplet-exciton transfer. Using these rules, we describe example polychromophoric structures with low inner-sphere exciton reorganization energies, low static and dynamic disorder, and enhanced π-stacking interactions between nearest-neighbor chromophores. These features lead to triplet-exciton eigenstates that are delocalized over several units at room temperature. The use of such bridges in donor-bridge-acceptor assemblies enables fast triplet-exciton transport over very long distances that is rate-limited by the donor-bridge injection and bridge-acceptor trapping rates.

Zinc Carboxylate Surface Passivation for Enhanced Optical Properties of In(Zn)P Colloidal Quantum Dots

Indium phosphide (InP) colloidal quantum dots (CQDs) have generated great interest as next-generation light-emitting materials owing to their narrow emission spectra and environment-friendly components. The minimized surface defects is essential to achieve narrow full-width at half-maximum (FWHM) and high photoluminescence quantum yield (PLQY). However, InP CQDs are readily oxidized in ambient condition, which results in formation of oxidation defect states on the surface of InP CQDs. Herein, we introduce a strategy to successfully passivate the surface defects of InP core by zinc complexes. The zinc carboxylates passivation reduces FWHM of InP CQDs from 130 nm to 70 nm and increases PLQY from 1% to 14% without shelling. Furthermore, the photoluminescence (PL) peak has shifted from 670 nm to 510 nm with an increase of zinc carboxylates passivation, which suggests that excessive zinc carboxylates functions as a size-regulating reagent in the synthesis.

Thiolumazines as Heavy-Atom-Free Photosensitizers for Applications in Daylight Photodynamic Therapy: Insights from Ultrafast Excited-State Dynamics

Finding appropriate photosensitizers (PSs) for daylight photodynamic therapy (dPDT) applications is extremely challenging, even though heavy-atom-free photosensitizers (HAFPSs) such as thiocarbonyl-modified nucleobases have shown a ray of hope. Few attempts have been made to find alternative natural products for dPDT applications. Pteridine heterocycles consisting of a pyrazine ring and a pyrimidine ring, such as lumazine, which exhibit many structural similarities to the alloxazine ring of the flavin molecule, could be an option for HAFPSs. The photophysical and quantum mechanical studies of the thio-modified lumazines revealed that sequential thiomodifications in lumazine result in a bathochromic shift. Additionally, higher tissue penetration depths were observed for thiolumazines. The fluorescence quenching in the case of thiomodified lumazines was explained using triplet state formation, whereas the contribution from the photoinduced electron transfer process cannot be ignored. It was also noticed that a strong one-photon absorption influenced the two-photon absorption (TPA) process, leading to a self-focusing effect in the visible spectral region. The higher tissue penetration and larger TPA cross section are the hallmark characteristics of the thiolumazines to be considered as potential HAFPSs for dPDT applications.

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.

Coherent InP/ZnS core@shell quantum dots with narrow-band green emissions

We report, for the first time, that the coherent growth of zinc sulfide (ZnS) on a colloidal indium phosphide (InP) quantum dot (QD) yields a InP/ZnS core/shell structure with a single lattice constant of 0.563 nm. Compared to the bulk crystal of zinc-blend (cubic) InP, the lattice of the core QD is compressed by 4.1%. In contrast, the lattice of the shell expands by 4.1% relative to the bulky ZnS crystal throughout the core/shell QD if the shell is thinner than or equal to 0.81 nm and the diameter of the core QD is smaller than 2.64 nm. Under these conditions, the bandgap of the core QD increases, resulting in a blueshift of absorption and photoluminescence (PL) spectra. The PL peak is centered at 523 nm. Furthermore, the PL quantum yield is enhanced up to 70% and the PL bandwidth narrows to 36 nm based on the strengthened quantum confinement effect. The temperature dependence of the PL properties is investigated to discuss the effect of the core/shell lattice coherency on the improved PL performances.

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.

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes

Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI₂ are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li⁰ and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.

Triplet Energy Transfer from Lead Halide Perovskite for Highly Selective Photocatalytic 2 + 2 Cycloaddition

Triplet excitons are generally confined within a semiconductor. Hence, solar energy utilization via direct triplet energy transfer (TET) from semiconductors is challenging. TET from lead halide perovskite semiconductors to nearby organic molecules has been illustrated with ultrafast spectroscopy. Direct utilization of solar energy, i.e., visible light, via TET for photocatalysis is an important route but has not yet been demonstrated with lead halide perovskite semiconductors. Here, we show that a photocatalytic reaction, focusing on a 2 + 2 cycloaddition reaction, can been successfully demonstrated via TET from lead halide perovskite nanocrystals (PNCs). The triplet excitons are shown to induce a highly diastereomeric syn-selective 2 + 2 cycloaddition starting from olefins. Such photocatalytic reactions probe the TET process previously only observed spectroscopically. Moreover, our observation demonstrates that bulk-like PNCs (size, >10 nm; PL = 530 nm), in addition to quantum-confined smaller PNCs, are also effective for TET. Our findings may render a new energy conversion pathway to employ PNCs via direct TET for photocatalytic organic synthesis.

Two-dimensional transition metal dichalcogenides as an emerging platform for singlet fission solar cells

Singlet fission, a rapid exciton doubling process via inverse Auger recombination, is recognized as one of the most practical and feasible means for overcoming the Shockley-Queisser limit. Singlet fission solar cells are generally developed by integrating photon downconversion organic semiconductors into conventional photovoltaic devices to break the maximum photovoltaic response of the host semiconductors by virtue of extra triplet excitons. In this regard, proper matching of two different semiconductors and heterointerface engineering are both crucial for highly efficient singlet fission solar cells. Therefore, the aim of this study is to review the prerequisite conditions for efficient triplet transfer at the heterointerfaces and thus highlight the robust spin and valley degrees of freedom of transition metal dichalcogenides with the ultimate goal of stimulating research into next-generation singlet fission solar cells.

Bright State Sensitized Triplet Energy Transfer from Quantum Dot to Molecular Acceptor Revealed by Temperature Dependent Energy Transfer Dynamics

Quantum dot (QD) sensitized molecular triplet excited state generation has been a promising alternative for traditional triplet state harvesting schemes. However, the correlation between QD bright/dark states and QD sensitized triplet energy transfer (TET) has been unclear. Herein, we studied the bright/dark states contribution to TET with CdSe/CdS core/shell QD-oligothiophene as the model system. Equilibrium between QD bright and dark states was tuned by changing temperature, and TET dynamics were monitored with transient absorption spectroscopy. Analysis of acceptor triplet excited state growth kinetics yields rates of TET from bright and dark states as 0.492 ± 0.011 ns^-1 and 0.0271 ± 0.0014 ns^-1 at 5 K, suggesting significant contribution of bright states to TET. The result was rationalized by bright state wave function components with the same electron/hole spin projections leading to nonzero TET probability. The study provides new insights into QD sensitized TET mechanisms and inspiration for future TET efficiency optimization through QD exciton engineering.

Structural changes in selected human proteins induced by exposure to quantum dots, their biological relevance and possible biomedical applications

Quantum dots (QDs) are semi-conductor luminescent nanocrystals usually of 2-10 nm diameter, attracting the significant attention in biomedical studies since emerged. Due to their unique optical and electronic properties, i.e. wide absorption spectra, narrow tunable emission bands or stable, bright photoluminescence, QDs seem to be ideally suited for multi-colour, simultaneous bioimaging and cellular labeling at the molecular level as new-generation probes. A highly reactive surface of QDs allows for conjugating them to biomolecules, what enables their direct binding to areas of interest inside or outside the cell for biosensing or targeted delivery. Particularly protein-QDs conjugates are current subjects of research, as features of QDs can be combined with protein specific functionalities and therefore used as a complex in variety of biomedical applications. It is known that QDs are able to interact with cells, organelles and macromolecules of the human body after administration. QDs are reported to cause changes at proteins level, including unfolding and three-dimensional structure alterations which might hamper proteins from performing their physiological functions and thereby limit the use of QD-protein conjugates in vivo. Moreover, these changes may trigger unwanted cellular outcomes as the effect of different signaling pathways activation. In this review, characteristics of QDs interactions with certain human proteins are presented and discussed. Besides that, the following manuscript provides an overview on structural changes of specific proteins exposed to QDs and their biological and biomedical relevance.

Quantum Dots Photocatalyze Intermolecular [2 + 2] Cycloadditions of Aromatic Alkenes Adsorbed to their Surfaces via van der Waals Interactions

Triplet excited state-initiated photochemistry is a mild and selective route to cycloadditions, radical rearrangements, couplings, fragmentations, and isomerizations. Colloidal quantum dots are proven visible-light photosensitizers and structural scaffolds for triplet-initiated reactions of molecules that are functionalized (with carboxylates) to anchor on the QD surface. Here, with the aid of polyaromatic energy shuttles that act as noncovalent adsorption sites for substrates on the QD surface, the scope of QD-photocatalyzed intermolecular [2 + 2] cycloadditions is extended to freely diffusing substrates (no anchoring groups). QD-shuttle complexes photocatalyze homo- and heterointermolecular [2 + 2] photocycloadditions of benzalacetone, chalcone and its derivatives with up to 94% yield; the yields for all reactions are comparable to those achieved by Ir(ppy)₃ but with the advantages of a factor of 2.5 lower catalyst loading, superior stability, and the ability to recover the catalyst by simple centrifugation and reuse it for multiple reaction cycles. Experiments imply a two-step triplet-triplet energy transfer mechanism, one energy transfer from the QD to the energy shuttle followed by a second energy transfer from the shuttle to the transiently adsorbed substrate.

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.

The structural and luminescence properties of plexcitonic structures based on Ag2S/l-Cys quantum dots and Au nanorods

A technique of obtaining plexitonic structures based on Ag2S quantum dots passivated with l-cysteine (Ag2S/l-Cys QDs) in the presence of Au nanorods passivated with cetyltrimethylammonium bromide molecules (Au/CTAB NRs) with controlled luminescence properties has been developed. The structural and luminescence properties of Ag2S/l-Cys QDs with Au/CTAB NRs are studied. The effect of plasmonic Au/CTAB NRs on IR trap state luminescence (750 nm) is considered. It has been found that the direct interaction between the components of the plexcitonic nanostructure leads to a significant luminescence quenching of Ag2S/l-Cys QDs, with the luminescence lifetime being constant. This is the evidence for photoinduced charge transfer. The spatial separation of the components of plexcitonic nanostructures due to the introduction of a polymer - poly(diallyldimethylammonium chloride) (polyDADMAC) provides a means to change their mutual arrangement and achieve an increase in the IR trap state luminescence intensity and a decrease in the luminescence lifetime from 7.2 ns to 4.5 ns. With weak plexcitonic coupling in the nanostructures [Ag2S QD/l-Cys]/[polyDADMAC]/[Au/CTAB NRs], the possibility of increasing the quantum yield of trap state luminescence for Ag2S QDs due to the Purcell effect has been demonstrated. In the case of formation [Ag2S QD/l-Cys]/[polyDADMAC]/[Au/CTAB NRs] a transformation of shallow trap state structure was established using the thermostimulated luminescence method.

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.