Energy‐Transfer Photocatalysis Using Lead Halide Perovskite Nanocrystals: Sensitizing Molecular Isomerization and Cycloaddition

Facilitating Energy and Charge Transfer from CsPbBr3 Perovskite Nanocrystals via Ligand Shell Reconstruction

Efficiently extracting photon energy from colloidal lead halide perovskite nanocrystals (PNCs) as excitons and charge carriers is a crucial step in many applications of these materials. We herein report a functionalization strategy based on reconstructing the surface chemical environment of CsPbBr3 PNCs to strengthen the binding of acceptor motifs and, thereby, enhance energy and charge carrier transfer efficiency. A zwitterion ligand, 2-ammonium benzenesulfonate, was employed to protect the integrity of the PNC surface during a purification step for removing excess original synthetic ligands. Heterocyclic-carboxylate structures with strong chelating binding effects were utilized as the anchoring motifs to couple the acceptors to the PNC surface. Compared to directly applying the acceptors to as-synthesized PNCs, the new method achieved at least a 6-fold increase in transportation efficiency for both an oligothiophene triplet energy acceptor and a quinoline-derivative electron acceptor. NMR spectroscopy systematically analyzed the binding conditions of different surface ligands in each step of functionalization. The improved functionalization was attributed to the diminishment of competitive adsorption after the purification step. We identified the N-heterocyclic-carboxylate structure as the most effective anchoring group. Transient absorption spectroscopy was employed to monitor the triplet energy transfer and charge carrier migration processes in the PNC-acceptor complexes and evaluate their rate constants. Spectral and dynamic features for distinguishing the electron transfer process from triplet energy transfer were summarized. Our surface reconstruction strategy will benefit the development of PNC-based optoelectronics and promote the application of perovskite materials as photosensitizers in different photophysical and photochemical processes.

Combining Quantum Dots and Photochromic Molecular Switches: Next-Generation Light-Responsive Materials

Quantum dots (QDs), with the unique merits of narrow and tunable photoluminescence (PL) wavelength, high PL quantum yield, have gained significant interest in fields such as display, solar energy conversion, bioimaging, and encrypted quantum communication. On the other hand, photochromic molecular switches (PMS) can undergo reversible interconversion between (at least) two distinct states at the molecular scale upon light irradiation. When combining QDs and PMS, the resulting hybrid systems exhibit synergistic functionalities and light responsiveness, enabling precise and reversible modulation over PL intensity/color, energy/electron transfer, and motion with high temporal and spatial resolution in a non-invasive manner. This perspective explores the recent advancements in the combination method, light-responsive mechanism, and functions of QD-PMS hybrids. The applications of QD-PMS hybrids are also highlighted as light-responsive materials in bioimaging, information processing, sensing, optoelectrical devices, and discuss future challenges, opportunities, and directions for enhancing performance and exploring applications in next-generation light-responsive materials and smart optoelectronic devices.

Halide Perovskite Photocatalysts for Clean Fuel Production and Organic Synthesis: Opportunities and Challenges

The need to constrain the use of fossil fuels causing global warming is motivating the development of a variety of photocatalysts for solar-to-fuel generation and chemical synthesis. In particular, semiconductor-based photocatalysts have been extensively exploited in solar-driven organic synthesis, carbon dioxide (CO2) conversion into value-added products, and hydrogen (H2) generation from water (H2O) splitting. Recently, metal halide perovskites (MHPs) have emerged as an important class of semiconductors for heterogeneous photocatalysis owing to their interesting properties. Despite key issues with long-term stability and degradation in polar solvents due to their ionic character, there has been significant progress in halide perovskite-based photocatalysts with improving their stability and performance in the gas and liquid phases. This review discusses the state-of-the-art for using halide perovskite-based photocatalysts and photoelectrocatalysis in hydrogen production from water and halogen acid solutions, CO2 reduction into value-added chemicals, and various organic chemical transformations. The different types of halide perovskites used, design strategies to overcome the instability issues in polar solvents, and the efficiencies achieved are discussed. Furthermore, the outstanding challenges associated with the use of polar electrolytes and how the stability and performance can be improved are discussed.

An excitation-wavelength-dependent organic photoluminescent molecule with high quantum yield integrating both ESIPT and PCET mechanisms

Excitation wavelength-dependent (Ex-De) chromophores, which exhibit changes in spectral composition with varying excitation wavelengths, have garnered significant interest. However, the pursuit of novel photoluminescence (PL) mechanisms and high luminescence quantum yields is facing huge challenges. Here, we discover that the introduction of a spinacine moiety to 2-(2-hydroxy-5-methylphenyl)benzothiazole, a traditional excited-state intramolecular proton transfer (ESIPT) fluorophore, results in a novel Ex-De PL molecule. The luminescent color of this compound can be effectively modulated from greenish-blue to yellow-green by adjusting either the excitation wavelength or temperature. Transient absorption and spectroelectrochemistry spectra elucidate the underlying mechanism, demonstrating the roles of ESIPT and proton-coupled electron transfer (PCET). When embedded in a poly(vinyl alcohol) film, the composite exhibits remarkable Ex-De PL behavior, achieving absolute fluorescence quantum yields of 55.6% (λ ex: 396 nm) and 69.6% (λ ex: 363 nm), as well as phosphorescence at room temperature. These properties highlight its potential for multiple encryption features, enhancing its application in anti-counterfeiting technologies.

Ligand Influence on the Performance of Cesium Lead Bromide Perovskite Quantum Dots in Photocatalytic C(sp3)-H Bromination Reactions

Lead halide perovskite quantum dots (LHP QDs) CsPbX3 generate immense interest as narrow-band emitters for displays, lasers, and quantum light sources. All QD applications rely on suited engineering of surface capping ligands. The first generation of LHP QDs employed oleic acid/oleyl amine capping and have found only a limited use in photoredox catalysis. These catalysts have been reported to be unstable and decompose over the course of the reaction, thus reducing turnover numbers (TONs) and limiting their synthetic ability. Herein, the impact of eight distinct surface ligands on monodisperse CsPbBr3 QDs is reported, affording a thorough comprehension of their performance in photocatalytic C-H brominations. These efforts yielded QDs operating at extremely low catalyst loadings (<100 ppb) with TONs over 9,000,000 per LHP QD. We emphasize that the optimal catalytic performance requires increased QD surface accessibility without compromising the QD structural and colloidal integrity. Control experiments indicated that well-known photoredox catalysts such as Ir(ppy)3, Ru(bpy)3Cl2, or 4CzlPN are ineffective in the same reaction. Mechanistic studies reveal that the C-Br bond reduction in CH2Br2 is the rate-limiting step and is likely facilitated through interaction with the CsPbBr3 QD surface. This work outlines a holistic approach toward the design of practically useful photocatalysts out of QDs comprising structurally soft QD cores and dynamically bound capping ligands.

Functionalized Violet-Emitting Cd, Pb-Free Quantum Dots with Thermally Activated Delayed Photoluminescence for Efficient Photochemical Reactions

Colloidal quantum dots (QDs) are excellent luminescent materials, but their short exciton lifetime, typically in the nanosecond range, restricts their applications in photochemical reactions. By designing QD-molecule conjugates, the exciton energy can be cycled between the QD exciton state and the molecular triplet state through triplet energy transfer, thereby prolonging the exciton lifetime to hundreds of microseconds, which is similar to the mechanism of thermally activated delayed fluorescence in molecules. Currently, QD-molecule-based thermally activated delayed photoluminescence (TADPL) systems have covered the blue to near-infrared spectral range. Here, we extend the reach of TADPL to the violet band, including slight penetration into the ultraviolet, by using Cd, Pb-free ZnSe/ZnS core/shell QDs functionalized with biphenyl ligands, which exhibit the highest TADPL energy (up to 3.0 eV) reported to date for QD-molecule conjugates. The high exciton energy, long lifetime (80 μs), and high TADPL quantum yield (23.7%) of the ZnSe/ZnS-biphenyl system enable very high efficiency in a variety of QD-sensitized photochemical reactions.

Metal Halide Perovskites for Photocatalysis: Performance and Mechanistic Studies

Metal halide perovskites, both lead-based and lead-free variants, have emerged as highly versatile materials with widespread applications across various fields, including photovoltaics, optoelectronics, and photocatalysis. This review provides a succinct overview of the recent advancements in the utilization of lead and lead-free halide perovskites specifically in photocatalysis. We explore the diverse range of photocatalytic reactions enabled by metal halide perovskites, including organic transformations, carbon dioxide reduction, pollutant degradation, and hydrogen production. We highlight key developments, mechanistic insights, and challenges in the field, offering our perspectives on the future research directions and potential applications. By summarizing recent findings from the literature, this review aims to provide a timely resource for researchers interested in harnessing the full potential of metal halide perovskites for sustainable and efficient photocatalytic processes.

An efficient all-visible light-activated photoswitch based on diarylethenes and CdS quantum dots

All-visible light-activated diarylethene (DAE) photoswitches are highly attractive for applications in smart photoresponsive materials. The photocyclization of DAE via the low-lying excited triplet state through triplet energy transfer (TET) from a sensitizer has been proven to be an effective approach for the realization of this scheme. However, the TET process is sensitive to oxygen and typically requires more than one sensitizer per photoswitch to facilitate sensitized photocyclization. Herein, we present a bi-component system comprising carboxylic acid-functionalized DAEs and CdS quantum dots (QDs) to achieve all-visible light-activated photoswitching. Due to the large surface area-to-volume ratio of CdS QDs and surface anchored DAEs, one CdS QD can activate at least 18 DAE molecules in the solution without oxygen exclusion. The efficiency of photocyclization of DAEs under visible light irradiation through energy transfer from CdS QDs is nearly comparable to that of direct UV light irradiation. Moreover, our strategy is adaptable for solid-state applications in the presence of air, enabling reversible writing and erasing of color and patterns by adjusting irradiation wavelengths in the visible region.

All-Visible-Light-Activated Diarylethene Photoswitches

Photochromic compounds have attracted much attention for their potential applications in photo-actuators, optoelectronic devices and optical recording techniques. This interest is driven by their key photochemical and photophysical properties, which can be reversibly modulated by light irradiation. Among them, diarylethene compounds have garnered extensive investigation due to their excellent thermal stability of both open- and closed-form isomers, robust fatigue resistance, high photocyclization quantum yield and good photochromic performance in both solution and solid phases. However, a notable limitation in expanding the utility of diarylethene compounds is the necessity for ultraviolet light to induce their photochromism. This requirement poses challenges, as ultraviolet light can be detrimental to biological tissues, and its penetration is often restricted in various media. This review provides an overview of design strategies employed in the development of visible-light-responsive diarylethene compounds. These design strategies serve as a guideline for molecular design, with the potential to significantly broaden the applications of all-visible-light-activated diarylethene compounds in the realms of materials science and biomedical science.

Double Layer SiO2-Coated Water-Stable Halide Perovskite as a Promising Antimicrobial Photocatalyst under Visible Light

Halide perovskite nanocrystals (HPNCs) have emerged as promising materials for various light harvesting applications due to their exceptional optical and electronic properties. However, their inherent instability in water and biological fluids has limited their use as photocatalysts in the aqueous phase. In this study, we present highly water-stable SiO2-coated HPNCs as efficient photocatalysts for antimicrobial applications. The double SiO2 layer coating method confers long-term structural and optical stability to HPNCs in water, while the in situ synthesis of lead- and bismuth-based perovskite NCs into the SiO2 shell enhances their versatility and tunability. We demonstrate that the substantial generation of singlet oxygen via energy transfer from HPNCs enables efficient photoinduced antibacterial efficacy under aqueous conditions. More than 90% of Escherichia coli was inactivated under mild visible light irradiation for 6 h. The excellent photocatalytic antibacterial performance suggests that SiO2-coated HPNCs hold great potential for various aqueous phase photocatalytic applications.

Emerging Opportunities of Colloidal Quantum Dots for Photocatalytic Organic Transformations

Colloidal quantum dots (QDs) have emerged as a versatile photocatalyst for a wide range of photocatalytic transformations owing to its high absorption coefficient, large surface-to-volume ratio, high stability, and efficient charge and energy transfer dynamics. The past decades have witnessed a rapid development of QDs for artificial photocatalysis. In this review, the unique characteristics of QDs are focused on, including quantum size effect, compositional and structural diversity, tunable surface chemistry, and photophysics, that can be utilized for photocatalytic transformations. The recent advancements in photocatalytic organic transformations enabled by QDs photocatalysts are summarized. The unique opportunities of QDs are highlighted to tackle organic reactions that are previously unattainable with small molecule photocatalysts. Lastly, an outlook is provided for future directions in this field.

Recent progress of metal halide perovskite materials in heterogeneous photocatalytic organic reactions

Photocatalytic technology is widely regarded as an important way to utilize solar energy and achieve carbon neutrality, which has attracted considerable attentions in various fields over the past decades. Metal halide perovskites (MHPs) are recognized as "superstar" materials due to their exceptional photoelectric properties, readily accessible and tunable structure, which made them intensively studied in solar cells, light-emitting diodes, and solar energy conversion fields. Since 2018, increased attention has been focused on applying the MHPs as a heterogeneous visible light photocatalyst in catalyzing organic synthesis reactions. In this review, we present an overview of photocatalytic technology and principles of heterogeneous photocatalysis before delving into the structural characteristics, stability, and classifications of MHPs. We then focus on recent developments of MHPs in photocatalyzing various organic synthesis reactions, such as oxidation, cyclization, C-C coupling etc., based on their classifications and reported reaction types. Finally, we discuss the main limitations and prospects regarding the application of metal halide perovskites in organic synthesis.

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.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Antimony-Based Halide Perovskite Nanoparticles as Lead-Free Photocatalysts for Controlled Radical Polymerization

Metal halide perovskites have emerged as versatile photocatalysts to convert solar energy for chemical processes. Perovskite photocatalyzed polymerization draws special attention due to its straightforward synthesis process and the ability to create advanced perovskite-polymer nanocomposites. Herein, this work employs Cs3Sb2Br9 perovskite nanoparticles (NPs) as a lead-free photocatalyst for light-controlled atom transfer radical polymerization (ATRP). Cs3Sb2Br9 NPs exhibit high reduction potential and interact with electronegative bromide initiator with Lewis acid Sb sites, enabling efficient photoinduced reduction of initiators and controlled polymerization under blue light irradiation. Methacrylate monomers with various functional groups are successfully polymerized, and the resulting polymer showcased a dispersity (Đ) as small as 1.27. The living nature of polymerization is confirmed by high chain end fidelity and kinetic studies. Moreover, Cs3Sb2Br9 NPs serve as heterogeneous photocatalysts, demonstrating recyclability and reusability for up to four cycles. This work presents a promising approach to overcome the limitations of lead-based perovskites in photoinduced controlled radical polymerization, offering a sustainable and efficient alternative for the synthesis of well-defined polymeric materials.

Surface-Engineered Cesium Lead Bromide Perovskite Nanocrystals for Enabling Photoreduction Activity

In recent years, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have exhibited such intriguing light absorption properties to be contemplated as promising candidates for photocatalytic conversions. However, for effective photocatalysis, the light harvesting system needs to be stable under the reaction conditions propaedeutic to a specific transformation. Unlike photoinduced oxidative reaction pathways, photoreductions with LHP NCs are challenging due to their scarce compatibility with common hole scavengers like amines and alcohols. In this contribution, it is investigated the potential of CsPbBr3 NCs protected by a suitably engineered bidentate ligand for the photoreduction of quinone species. Using an in situ approach for the construction of the passivating agent and a halide excess environment, quantum-confined nanocubes (average edge length = 6.0 ± 0.8 nm) are obtained with a low ligand density (1.73 ligand/nm2) at the NC surface. The bifunctional adhesion of the engineered ligand boosts the colloidal stability of the corresponding NCs, preserving their optical properties also in the presence of an amine excess. Despite their relatively short exciton lifetime (τAV = 3.7 ± 0.2 ns), these NCs show an efficient fluorescence quenching in the presence of the selected electron accepting quinones (1,4-naphthoquinone, 9,10-phenanthrenequinone, and 9,10-anthraquinone). All of these aspects demonstrate the suitability of the NCs for an efficient photoreduction of 1,4-naphthoquinone to 1,4-dihydroxynaphthalene in the presence of triethylamine as a hole scavenger. This chemical transformation is impracticable with conventionally passivated LHP NCs, thereby highlighting the potential of the surface functionalization in this class of nanomaterials for exploring new photoinduced reactivities.

Amplifying dual-visible-light photoswitching in aqueous media via confinement promoted triplet-triplet energy transfer

Achieving visible-light photochromism is a long-term goal of chemists keen to exploit the opportunities of molecular photoswitches in multi-disciplinary research studies. Triplet-sensitization offers a flexible approach to building diverse visible-light photoswitches using existing photochromic scaffolds, circumventing the need for sophisticated molecular design and synthesis. Unfortunately, distance-dependence and environment-sensitivity of triplet-excited species remain as key challenges that severely impair sensitization efficiency and limit their practical availability. We present herein a nature-inspired nanoconfinement strategy in which a triplet-sensitized visible-light photoswitch/sensitizer system is assembled into nanoconfined micelles (d ∼ 40 nm). A ca. 10-fold efficiency increase of triplet-triplet energy transfer for photochromism as well as an amplified fluorescence on/off contrast upon bi-directional visible-light excitation (470/560 nm) was achieved in full aqueous media. By virtue of this, the hybrid photoswitchable system is successfully applied for both flash information encryption and multiple dynamic cell imaging assays, further proving its versatility in materials and life science.

Quantifying Förster Resonance Energy Transfer from Single Perovskite Quantum Dots to Organic Dyes

Colloidal quantum dots (QDs) are promising regenerable photoredox catalysts offering broadly tunable redox potentials along with high absorption coefficients. QDs have thus far been examined for various organic transformations, water splitting, and CO2 reduction. Vast opportunities emerge from coupling QDs with other homogeneous catalysts, such as transition metal complexes or organic dyes, into hybrid nanoassemblies exploiting energy transfer (ET), leveraging a large absorption cross-section of QDs and long-lived triplet states of cocatalysts. However, a thorough understanding and further engineering of the complex operational mechanisms of hybrid nanoassemblies require simultaneously controlling the surface chemistry of the QDs and probing dynamics at sufficient spatiotemporal resolution. Here, we probe the ET from single lead halide perovskite QDs, capped by alkylphospholipid ligands, to organic dye molecules employing single-particle photoluminescence spectroscopy with single-photon resolution. We identify a Förster-type ET by spatial, temporal, and photon-photon correlations in the QD and dye emission. Discrete quenching steps in the acceptor emission reveal stochastic photobleaching events of individual organic dyes, allowing a precise quantification of the transfer efficiency, which is >70% for QD-dye complexes with strong donor-acceptor spectral overlap. Our work explores the processes occurring at the QD/molecule interface and demonstrates the feasibility of sensitizing organic photocatalysts with QDs.

Structural Modification Strategies, Interfacial Charge-Carrier Dynamics, and Solar Energy Conversion Applications of Organic-Inorganic Halide Perovskite Photocatalysts

Over the past few decades, organic-inorganic halide perovskites (OIHPs) as novel photocatalyst materials have attracted intensive attention for an impressive variety of photocatalytic applications due to their excellent photophysical (chemical) properties. Regarding practical application and future commercialization, the air-water stability and photocatalytic performance of OIHPs need to be further improved. Accordingly, studying modification strategies and interfacial interaction mechanisms is crucial. In this review, the current progress in the development and photocatalytic fundamentals of OIHPs is summarized. Furthermore, the structural modification strategies of OIHPs, including dimensionality control, heterojunction design, encapsulation techniques, and so on for the enhancement of charge-carrier transfer and the enlargement of long-term stability, are elucidated. Subsequently, the interfacial mechanisms and charge-carrier dynamics of OIHPs during the photocatalytic process are systematically specified and classified via diverse photophysical and electrochemical characterization methods, such as time-resolved photoluminescence measurements, ultrafast transient absorption spectroscopy, electrochemical impedance spectroscopy measurements, transient photocurrent densities, and so forth. Eventually, various photocatalytic applications of OIHPs, including hydrogen evolution, CO2 reduction, pollutant degradation, and photocatalytic conversion of organic matter.

Extending Infrared Emission via Energy Transfer in a CsPbI3-Cyanine Dye Hybrid

Directing energy flow in light harvesting assemblies of nanocrystal-chromophore hybrid systems requires a better understanding of factors that dictate excited-state processes. In this study, we explore excited-state interactions within the CsPbI3-cyanine dye (IR125) hybrid assembly through a comprehensive set of steady-state and time-resolved absorption and photoluminescence (PL) experiments. Our photoluminescence investigations reveal the quenching of CsPbI3 emission alongside the simultaneous enhancement of IR125 fluorescence, providing evidence for a singlet energy transfer. The evaluation of both photoluminescence (PL) quenching and PL decay measurements yield ∼94% energy transfer efficiency for the CsPbI3-IR125 hybrid assembly. Transient absorption spectroscopy further unveils that this singlet energy transfer process operates on an ultrafast time scale, occurring within 400 ps with a rate constant of energy transfer of 1.4 × 1010 s-1. Our findings highlight the potential of the CsPbI3-IR125 hybrid assembly to extend the emission of halide perovskites into the infrared region, paving the way for light energy harvesting and display applications.

Photocatalysis Based on Metal Halide Perovskites for Organic Chemical Transformations

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C-X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations.

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.

Exploring Solar Cells Based on Lead- and Iodide-Deficient Halide Perovskite (d-HP) Thin Films

Perovskite solar cells have become more and more attractive and competitive. However, their toxicity induced by the presence of lead and their rather low stability hinders their potential and future commercialization. Reducing lead content while improving stability then appears as a major axis of development. In the last years, we have reported a new family of perovskite presenting PbI⁺ unit vacancies inside the lattice caused by the insertion of big organic cations that do not respect the Goldschmidt tolerance factor: hydroxyethylammonium HO-(CH₂)₂-NH₃⁺ (HEA⁺) and thioethylammonium HS-(CH₂)₂-NH₃⁺ (TEA⁺). These perovskites, named d-HPs for lead and halide-deficient perovskites, present a 3D perovskite corner-shared Pb_1-xI_3-x network that can be assimilated to a lead-iodide-deficient MAPbI₃ or FAPbI₃ network. Here, we propose the chemical engineering of both systems for solar cell optimization. For d-MAPbI₃-HEA, the power conversion efficiency (PCE) reached 11.47% while displaying enhanced stability and reduced lead content of 13% compared to MAPbI₃. On the other hand, d-FAPbI₃-TEA delivered a PCE of 8.33% with astounding perovskite film stability compared to classic α-FAPI. The presence of TEA⁺ within the lattice impedes α-FAPI degradation into yellow δ-FAPbI₃ by direct degradation into inactive Pb(OH)I, thus dramatically slowing the aging of d-FAPbI₃-TEA perovskite.

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.

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.