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.

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

Triplet excited state chemistry has enabled a range of important organic transformations by accessing reaction pathways inaccessible to photoredox chemistry. Such photoreactions are triggered by triplet photosensitizers, which absorb visible-light photons and transfer the energy to the substrate or to a co-catalyst through triplet-triplet energy transfer (TT EnT). The most popular triplet photosensitizers, metal complexes and organic chromophores, have proven useful in a range of pericyclic reactions, bond dissociations, and isomerizations, but they have several characteristics related to their chemical and electronic structure that limit their selectivity, energy efficiency, and sustainability. This Perspective describes some ways that colloidal quantum dots (QDs) address the limitations of molecular photocatalysts for TT EnT-driven organic transformations. These sub-5-nm particles have the large catalytic surface area and electronic/optical tunability of homogeneous catalysts, and the easy separation and surface templating effects of heterogeneous catalysts. Their optical and electronic properties, small singlet-triplet energy splitting, narrow emission line widths, and high photostability enhance their performance as triplet photosensitizers. This Perspective describes these advantages in the context of published and ongoing investigations of TT EnT-driven reactions, and then highlights the advantages and challenges associated with using related emerging materials, specifically lead halide perovskite QDs and quasi-2D nanoplatelets, as photocatalysts for triplet excited state chemistry.

Thermodynamics and Mechanism of a Photocatalyzed Stereoselective [2 + 2] Cycloaddition on a CdSe Quantum Dot.

Colloidal quantum dots (QDs) have shown promise over the last few decades for a range of applications including single photon emission, in vivo imaging, and photocatalysis. Recent experiments demonstrated that QDs impart stereoselectivity to triplet excited-state [2 + 2] cycloaddition reactions of alkenes photocatalyzed by the QD through self-assembly of the reagent molecules on the QD surface, but these experiments did not reveal the precise geometries of surface-bound molecules or their interactions with surface atoms. Here, a theoretical mechanistic approach is used to study such interactions for [2 + 2] cycloadditions of 4-vinylbenzoic acid derivatives on CdSe QDs. Spin-polarized periodic density functional theory (DFT) and nonperiodic DFT calculations are deployed to determine the origin of the selectivity for the syn diastereomer of the resultant tetrasubstituted cyclobutane product via atomistic modeling of the CdSe surface and substrates, determination of the thermodynamic energies of reactions for each step, the intermolecular interactions between the substrates, and the triplet state reaction paths. The calculations indicate that reaction selectivity arises from preferred binding of pairs through intermolecular interactions of substrate molecules on the QD surface in a syn-precursor structure followed by dimerization after triplet excitation. These mechanisms are generalizable to other metal-enriched QD surfaces that have a similar surface structure as that of CdSe, such as InSe or CdTe. Design principles for anti diastereomer derivatives are also discussed.

Viewpoint: Challenges in Colloidal Photocatalysis and Some Strategies for Addressing Them.

Colloidal semiconductor nanocrystals, or "quantum dots" (QDs), have several optical and chemical properties that give them the potential to enable nonincremental increases in the efficiencies of many types of photocatalytic reactions relevant for energy conversion and organic synthesis. Colloidal photocatalysts have many desirable characteristics of both heterogeneous and homogeneous catalysts but come with their own particular set of challenges. This viewpoint outlines some of the obstacles one first encounters when driving reactions with these colloids and offers some strategies for overcoming these obstacles, including ways to extend their excited state lifetimes, prevent corrosion by photogenerated holes, and choose a surface chemistry and buffering system for maximum colloidal stability over a range of environmental conditions.

Tunable Photothermal Actuators Based on a Pre-programmed Aligned Nanostructure.

For various applications, it is challenging but essential to obtain complex tunable mechanical actuations in response to environmental stimuli. Here, a general and effective strategy is developed to produce multiple types of photomechanical actuation (from phototropic/apheliotropic bending to three-dimensional helical buckling) by manipulating the orientation of one-dimensional nanomaterials. These materials are manipulated to mimic plants that generate diverse mechanical motions through the orientation of cellulose fibrils. The photomechanical actuations can be completed in milliseconds and can be performed reversibly without detectable fatigue after 100 000 cycles. This capacity to produce multiple types of photomechanical actuation is further developed to produce complex integrated movements, as demonstrated by a light-manipulated robotic arm and a solar energy harvesting system.

Realizing both high energy and high power densities by twisting three carbon-nanotube-based hybrid fibers.

Energy storage devices, such as lithium-ion batteries and supercapacitors, are required for the modern electronics. However, the intrinsic characteristics of low power densities in batteries and low energy densities in supercapacitors have limited their applications. How to simultaneously realize high energy and power densities in one device remains a challenge. Herein a fiber-shaped hybrid energy-storage device (FESD) formed by twisting three carbon nanotube hybrid fibers demonstrates both high energy and power densities. For the FESD, the energy density (50 mWh cm(-3) or 90 Wh kg(-1) ) many times higher than for other forms of supercapacitors and approximately 3 times that of thin-film batteries; the power density (1 W cm(-3) or 5970 W kg(-1) ) is approximately 140 times of thin-film lithium-ion battery. The FESD is flexible, weaveable and wearable, which offers promising advantages in the modern electronics.

Self-healable electrically conducting wires for wearable microelectronics.

Electrically conducting wires play a critical role in the advancement of modern electronics and in particular are an important key to the development of next-generation wearable microelectronics. However, the thin conducting wires can easily break during use, and the whole device fails to function as a result. Herein, a new family of high-performance conducting wires that can self-heal after breaking has been developed by wrapping sheets of aligned carbon nanotubes around polymer fibers. The aligned carbon nanotubes offer an effective strategy for the self-healing of the electric conductivity, whereas the polymer fiber recovers its mechanical strength. A self-healable wire-shaped supercapacitor fabricated from a wire electrode of this type maintained a high capacitance after breaking and self-healing.

Chemo- and Stereoselective Intermolecular [2+2] Photocycloaddition of Conjugated Dienes using Colloidal Nanocrystal Photocatalysts.

The use of visible-light photosensitizers to power [2+2] photocycloadditions that produce complex tetrasubstituted cyclobutanes is a true success of photochemistry, but the scope of this reaction has been limited to activated α, ß-unsaturated carbonyls. This paper describes selective intermolecular homo- and hetero-[2+2] photocycloadditions of terminal and internal aryl conjugated dienes - substrates historically unsuited for this reaction because of their multiple possible reaction pathways and product configurations - through triplet-triplet energy transfer from CdSe nanocrystal photocatalysts, to generate valuable and elusive syn-trans aryl vinylcyclobutanes. The negligible singlet-triplet splitting of nanocrystals' excited states allows them to drive the [2+2] pathway over the competing [4+2] photoredox pathway, a chemoselectivity not achievable with any known molecular photosensitizer. Reversible tethering of the cyclobutane product to the nanocrystal surface results in near quantitative yield of the syn-trans product. Flat colloidal CdSe nanoplatelets produce cyclobutanes coupled at the terminal alkenes of component dienes with up to 89% regioselectivity.

Regio- and diastereoselective intermolecular [2+2] cycloadditions photocatalysed by quantum dots.

Light-driven [2+2] cycloaddition is the most direct strategy to build tetrasubstituted cyclobutanes, core components of many lead compounds for drug development. Significant advances in the chemoselectivity and enantioselectivity of [2+2] photocycloadditions have been made, but exceptional and tunable diastereoselectivity and regioselectivity (head-to-head versus head-to-tail adducts) is required for the synthesis of bioactive molecules. Here we show that colloidal quantum dots serve as visible-light chromophores, photocatalysts and reusable scaffolds for homo- and hetero-intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, including aryl-conjugated alkenes, with up to 98% switchable regioselectivity and 98% diastereoselectivity for the previously minor syn-cyclobutane products. Transient absorption spectroscopy confirms that our system demonstrates catalysis triggered by triplet-triplet energy transfer from the quantum dot. The precisely controlled triplet energy levels of the quantum dot photocatalysts facilitate efficient and selective heterocoupling, a major challenge in direct cyclobutane synthesis.

Outstanding catalytic performance in the semi-hydrogenation of acetylene in a front-end process by establishing a "hydrogen deficient" phase.

Pd-[Bmim][Cl] phase was immobilized onto Al2O3 to neutralize the excessive hydrogen in the gas phase to prevent the over-hydrogenation of acetylene, thereby achieving a high selectivity for ethylene.

Insight into the dispersion behavior of 1-butyl-3-methylimiazolium chloride confined in nanoscale pores of carbon materials.

It is important to understand the behaviour of ionic liquids (ILs) in nanoscale pores, as application of supported ionic liquid phase (SILP) materials has attracted much attention. The main intention of this investigation is to study the dispersion performance of ionic liquid on activated carbons with different surface oxygen groups. Several active carbons with diverse oxygenated surface groups were obtained by oxidation and reduction methods. All samples were impregnated with [Bmim]Cl at different loadings. The blocked porosity of ionic liquid on SILP materials mainly depends on the amount of ionic liquid and surface oxygen content. In addition, stability of the supported IL in water was tested by analyzing the leached amount of IL. We found that the supported IL is not easily leached from the interface of carbon with low amount of surface oxygen. This may be due to the high free π-electron density of oxygen-free supports, forming more CH-π bonds with H atoms at the C2 position of the acidic cation. In addition, we propose that dispersion of ionic liquid in the pores depends on the density of surface free π-electrons of carbon materials.

Reversible Modulation of the Electrostatic Potential of a Colloidal Quantum Dot through the Protonation Equilibrium of Its Ligands.

This Letter describes the reversible modulation of the electrostatic potential at the interface between a colloidal PbS quantum dot (QD) and solvent, through the protonation equilibrium of the QD's histamine-derivatized dihydrolipoic acid (DHLA) ligand shell. The electrostatic potential is sensitively monitored by the yield of photoinduced electron transfer from the QD to a charged electron acceptor, 9,10-anthraquinone-2-sulfonate (AQ). The permeability of the DHLA coating to the AQ progressively increases as the average degree of protonation of the ligand shell increases from 0 to 92%, as quantified by 1H NMR, upon successive additions of p-toluenesulfonic acid; this increase results in a decrease in the photoluminescence (PL) intensity of the QDs by a factor of 6.7. The increase in permeability is attributable to favorable electrostatic interactions between the ligands and AQ. This work suggests the potential of the combination of near-IR-emitting QDs and molecular quenchers as robust local H+ sensors.

Electrochemical Capacitors with High Output Voltages that Mimic Electric Eels.

A new family of energy-storage devices is created by mimicking the electric eel to obtain a high output voltage. These novel energy-storage devices are flexible, stretchable, and weavable fibers, which satisfies the needs of next-generation portable and wearable electronics. The devices are fabricated via a continuous fabrication technology to effectively power electronic watches and light-emitting diodes as two examples.

A Novel Slicing Method for Thin Supercapacitors.

Thin and flexible supercapacitors with low cost and individual variation are fabricated by a new and efficient slicing method. Tunable output voltage and energy can be realized with a high specific capacitance of 248.8 F g(-1) or 150.8 F cm(-3) , which is well maintained before and after bending.

Large-Area Supercapacitor Textiles with Novel Hierarchical Conducting Structures.

Supercapacitor textiles with large sizes are developed to satisfy practical applications by designing a novel hierarchical conducting structure. They display remarkable capacitance, power, and energy of 69.3 F, 80.7 mW, and 5.4 mW h, respectively, representing an important step toward real-world applications.

Cross-stacking aligned carbon-nanotube films to tune microwave absorption frequencies and increase absorption intensities.

Aligned carbon-nanotube (CNT) sheets are used as building blocks to prepare light-weight, frequency-tunable and high-performance microwave absorbers, and the absorption frequency can be accurately controlled by stacking them with different intersectional angles. A remarkable reflection loss of -47.66 dB is achieved by stacking four aligned CNT sheets with an intersectional angle of 90° between two neighboring ones. The incorporation of a second phase such as a metal and a conducting polymer greatly enhances the microwave-absorption capability.