Showing papers in "Angewandte Chemie in 2012"

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

Abstract: Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Challenges Facing Lithium Batteries and Electrical Double‐Layer Capacitors

Abstract: Energy-storage technologies, including electrical double-layer capacitors and rechargeable batteries, have attracted significant attention for applications in portable electronic devices, electric vehicles, bulk electricity storage at power stations, and “load leveling” of renewable sources, such as solar energy and wind power. Transforming lithium batteries and electric double-layer capacitors requires a step change in the science underpinning these devices, including the discovery of new materials, new electrochemistry, and an increased understanding of the processes on which the devices depend. The Review will consider some of the current scientific issues underpinning lithium batteries and electric double-layer capacitors.

C-H bond functionalization: emerging synthetic tools for natural products and pharmaceuticals

Abstract: The direct functionalization of C-H bonds in organic compounds has recently emerged as a powerful and ideal method for the formation of carbon-carbon and carbon-heteroatom bonds. This Review provides an overview of C-H bond functionalization strategies for the rapid synthesis of biologically active compounds such as natural products and pharmaceutical targets.

Palladium-Catalyzed Cross-Coupling: A Historical Contextual Perspective to the 2010 Nobel Prize

Abstract: In 2010, Richard Heck, Ei-ichi Negishi, and Akira Suzuki joined the prestigious circle of Nobel Laureate chemists for their roles in discovering and developing highly practical methodologies for C-C bond construction. From their original contributions in the early 1970s the landscape of the strategies and methods of organic synthesis irreversibly changed for the modern chemist, both in academia and in industry. In this Review, we attempt to trace the historical origin of these powerful reactions, and outline the developments from the seminal discoveries leading to their eminent position as appreciated and applied today.

Sulfur and Nitrogen Dual-Doped Mesoporous Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance†

Abstract: Doping duo: Mesoporous graphene doped with both N and S atoms (N-S-G) was prepared in one step and studied as an electrochemical catalyst for the oxygen reduction reaction (ORR). The catalyst shows excellent ORR performance comparable to that of commercial Pt/C. The outstanding activity of N-S-G results from both the large number and the synergistic effect of the dopant heteroatoms.

Visible-light photoredox catalysis.

Abstract: In the last few years, visible-light initiated organic transformations have attracted increasing attention. The development of visible-light-promoted photocatalytic reactions, which enable rapid and efficient synthesis of fine chemicals, is highly desirable from the viewpoint of cost, safety, availability, and environmental friendliness. In this Minireview, recent advances made in this fast developing area of research are discussed.

Small molecule organic semiconductors on the move: promises for future solar energy technology.

Abstract: This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.

Zirconium-Metalloporphyrin PCN-222: Mesoporous Metal–Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts†

Abstract: In nature, metalloporphyrins are well known for performing many biological functions in aqueous media, such as light harvesting, oxygen transportation, and catalysis. Heme, the iron–porphyrin derivative, is the cofactor for many enzyme/ protein families, including peroxidases, cytochromes, hemoglobins, and myoglobins. Using synthetic systems to mimic natural enzymes with high catalytic activity and substrate selectivity has been a sought-after goal in the last decade. Direct application of a heme as an oxidation catalyst in aqueous solution is usually challenging due to the formation of catalytically inactive dimers and catalyst self-destruction in the oxidizing reaction media. One promising approach is to load heme on supports, such as zeolites, clays, nanoparticles, hydrogels, or carbon materials, a practice which inevitably dilutes the density of active sites. An alternative approach is to protect the heme center by modifying the porphyrin to produce dendrimers or molecular crystals, which is a synthetically demanding method. Herein, we propose a unique strategy employing heme-like active centers as structural motifs for the assembly of highly stable porous materials, which should possess well-defined mesochannels and ultrahigh stability in aqueous solution. Metal-organic frameworks (MOFs) are a new class of crystalline porous materials with fascinating structures and intriguing properties, such as permanent porosity, high surface area, and uniform open cavities. The availability of various building blocks consisting of metals and organic linkers makes it possible to construct MOFs with unique properties for diverse applications. However, these desirable features of MOFs have rarely been applied to an enzymatic mimic, especially for catalysis in an aqueous medium, despite the fact that the assembly of ligands bearing high-density active sites into 3D frameworks may provide an ideal system to both enhance the catalytic activity and protect the cofactors. One of the main reasons is the lack of water-stable MOFs containing redox-active metal centers. Furthermore, most MOFs are microporous (pore size< 2 nm). Although they are suitable for gas storage, the small pore size slows down diffusion and limits the access of large substrate molecules to the active sites inside a MOF. Therefore, MOFs with mesopores, accessible redox sites, and ultrahigh stability, especially in aqueous media, are indispensible for any successful biomimetic attempt. Herein we have employed Fe-TCPP (TCPP= tetrakis(4carboxyphenyl)porphyrin) as a heme-like ligand and chosen highly stable Zr6 clusters as nodes for the assembly of stable Zr-MOFs. With carefully selected starting materials, we have successfully constructed a 3D heme-like MOF, designated as PCN-222(Fe) (Figure 1; PCN= porous coordination net-

Beyond directing groups: transition-metal-catalyzed C-H activation of simple arenes.

Abstract: The use of coordinating moieties as directing groups for the functionalization of aromatic CH bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of CH activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non-chelate-assisted CH activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.

Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

Abstract: Liquid hydrocarbon fuels play an essential part in the global energy chain, owing to their high energy density and easy transportability. Olefins play a similar role in the production of consumer goods. In a post-oil society, fuel and olefin production will rely on alternative carbon sources, such as biomass, coal, natural gas, and CO(2). The methanol-to-hydrocarbons (MTH) process is a key step in such routes, and can be tuned into production of gasoline-rich (methanol to gasoline; MTG) or olefin-rich (methanol to olefins; MTO) product mixtures by proper choice of catalyst and reaction conditions. This Review presents several commercial MTH projects that have recently been realized, and also fundamental research into the synthesis of microporous materials for the targeted variation of selectivity and lifetime of the catalysts.

An amine-functionalized titanium metal-organic framework photocatalyst with visible-light-induced activity for CO2 reduction.

Abstract: Let your light shine: the photocatalytic reduction of carbon dioxide to the formate anion under visible light irradiation is for the first time realized over a photoactive Ti-containing metal-organic framework, NH(2)-MIL-125(Ti), which is fabricated by a facile substitution of ligands in the UV-responsive MIL-125(Ti) material.

Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions

Abstract: Molybdenum boride (MoB) and carbide (Mo2C) are excellent catalysts for electrochemical hydrogen evolution at both pH 0 and pH 14.

Hydrogen-evolution catalysts based on non-noble metal nickel-molybdenum nitride nanosheets.

Abstract: Hydrogen production through splitting of water has attracted great scientific interest because of its relevance to renewable energy storage and its potential for providing energy without the emission of carbon dioxide. Electrocatalytic systems for H2 generation typically incorporate noble metals such as Pt in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high costs and limited world-wide supply of these noble metals make their application in viable commercial processes unattractive. Several non-noble metal materials, such as transition-metal chalcogenides, carbides, and complexes as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in the evolution of hydrogen. Nitrides of early transition-metals have been shown to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with low-cost alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition-metal nitrides are governed by their bulk and surface structure and stoichiometry. While there is some information concerning the effect of the bulk composition on the catalytic properties of this material, there is currently little known about the effects of the surface nanostructure. Nickel and nickel–molybdenum are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form they exhibited exchange current densities between 10 6 and 10 4 Acm , compared to 10 3 Acm 2 for Pt. Jaksic et al. postulated a hypo-hyper-d-electronic interactive effect between Ni and Mo that yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.With the objective of exploiting the decrease in the overpotential by carrying out the HER in acidic media, we have developed a low-cost, stable, and active molybdenum-nitride-based electrocatalyst for the HER. Guided by the “volcano plot” in which the activity for the evolution of hydrogen as a function of the M H bond strength exhibits an ascending branch followed by a descending branch, peaking at Pt, we designed a material on the molecular scale combining nickel, which binds H weakly, with molybdenum, which binds H strongly. Here we report the first synthesis of NiMo nitride nanosheets on a carbon support (NiMoNx/C), and demonstrate the high HER electrocatalytic activity of the resulting NiMoNx/C catalyst with low overpotential and small Tafel slope. The NiMoNx/C catalyst was synthesized by reduction of a carbon-supported ammonium molybdate [(NH4)6Mo7O24·4H2O] and nickel nitrate (Ni(NO3)2·4H2O) mixture in a tubular oven in H2 at 400 8C, and subsequent reaction with NH3 at 700 8C. During this process, the (NH4)6Mo7O24 and Ni(NO3)2 precursors were reduced to NiMo metal particles by H2, and then they were mildly transformed to NiMoNx nanosheets by reaction with ammonia. The atomic ratio of Ni/Mo was 1/4.7 determined by energy dispersive X-ray spectroscopy (EDX) on the NiMoNx/ C sample. The transmission electron microscopy (TEM) images, as shown in Figure 1a, display NiMo particles that are mainly spherical. The high-resolution TEM image, as shown in the inset of Figure 1a, corroborated the presence of an amorphous 3 to 5 nm Ni/Mo oxide layer (see Figure S4 in the Supporting Information for resolved image), whereas NiMoNx is characterized by thin, flat, and flaky stacks composed of nanosheets with high radial-axial ratios (Figure 1b and Figure S5 in the Supporting Information for a magnified image). Figure 1c shows that some of the nanosheets lay flat on the graphite carbon (as indicated by the black arrows), and some have folded edges that show different layers of NiMoNx sheets (white arrows). The thickness of the sheets ranged from 4 to 15 nm. The average stacking number of sheets measured from Figure 1b is about [*] Dr. W.-F. Chen, Dr. K. Sasaki, Dr. J. T. Muckerman, Dr. R. R. Adzic Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: ksasaki@bnl.gov

BCN Graphene as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction

Abstract: : The cathodic oxygen reduction reaction (ORR) is an important process in fuel cells and metal air batteries.[1 3] Although Pt-based electrocatalysts have been commonly used in commercial fuel cells owing to their relatively low overpotential and high current density, they still suffer from serious intermediate tolerance, anode crossover, sluggish kinetics, and poor stability in an electrochemical environment. This, together with the high cost of Pt and its limited nature reserves, has prompted the extensive search for alternative low-cost and high-performance ORR electrocatalysts. In this context, carbon-based metal-free ORR electrocatalysts have generated a great deal of interest owing to their low-cost, high electrocatalytic activity and selectivity, and excellent durability.[4 9] Of particular interest, we have previously prepared vertically aligned nitrogendoped carbon nanotubes (VA-NCNTs) as ORR electrocatalysts, which are free from anode crossover and CO poisoning and show a threefold higher catalytic activity and better durability than the commercial Pt/C catalyst.[4]

A Biocompatible Fluorescent Ink Based on Water-Soluble Luminescent Carbon Nanodots

Abstract: Carbon nanodots (C-dots) are fascinating carbon materialsthat are attracting increasing interest because they possessdistinct benefits, such as chemical inertness, a lack of opticalblinking, low photobleaching, low cytotoxicity, and excellentbiocompatibility, compared with organic dyes and othersemiconductor nanodots with heavy metal cores.

Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries

Abstract: Rechargeable lithium–sulfur (Li–S) batteries are attracting increasing attention due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li-ion batteries based on intercalation chemistry. Since the electronic conductivity of sulfur is extremely low, conductive carbon materials with high accessible porosity to “wire” and contain the sulfur are an essential component of the positive electrode. During the past decades, attempts have been made to fabricate C/S composites using carbon black, activated carbons (ACs), and carbon nanotubes (CNTs). Although improvements resulted, the cathodes suffered from inhomogeneous contact between the active material and the electronic conductors. A major step forward in fabricating a uniform C/S composite was reported in 2009. Some of us employed CMK-3, an ordered mesoporous carbon (OMC) featuring high specific surface area and large pore volume as a scaffold. As much as 70 wt% sulfur was incorporated into the uniform 3–4 nm mesopores, and the cells exhibited reversible capacities up to 1350 mAhg , albeit at moderate rates. Inspired by this, another OMC, a bulk bimodal mesoporous carbon (BMC-1) was investigated as a Li-S cathode. The favorable pore dimensions and large pore volume greatly improved the rate performance. An electrode with 40 wt% S showed a high initial discharge capacity of 1135 mAhg 1 at a current rate of 1 C (defined as discharge/ charge in one hour). However, similar to other reports, the capacity is sensitive to the sulfur ratio, dropping to 718 mAhg 1 at a sulfur content of 60 wt%. These results suggest that the texture of the mesoporous carbon could be further enhanced. Recently, Archer et al. reported nanoscale hollow porous C/S spheres prepared through vapor infusion. These materials displayed good cyclability and capacity at a C/5 rate, illustrating the advantages of nanosized porous carbon in the sulfur cathodes. Here we report the synthesis of unique nanoscale spherical OMCs with extremely high bimodal porosities. The particles were investigated as a cathode material and sulfur host in Li–S batteries where they showed high initial discharge capacity and good cyclability without sacrificing rate capability. Unlike bulk porous carbons, these carbon– sulfur sphere electrodes did not display significant capacity fading with the increase of sulfur content in the cathodes. We show that the nanoscale morphology of these materials is of key importance for ensuring very efficient use of the sulfur content even at high cycling rates. Morphology control is a central issue in OMC synthesis. There are numerous examples of mesoporous bulk materials obtained either by hard-templating or soft-templating, including thin films, membranes or free fibers. Most syntheses use evaporation-induced self-assembly (EISA) followed by thermal treatment for template-removal and carbonization. It is a challenge to either create solution-based OMC nanoparticle syntheses or to adapt the established EISA methods to nanoparticles. Only few examples of OMC nanoparticles have been reported so far which are mostly unsuitable for applications in Li–S cells due to low pore volume and/or surface area. Approaches include templating with PMMA colloidal crystals or mesoporous silica nanoparticles, aerosol-assisted synthesis, ultrasonic emulsification or hydrothermal synthesis. Ordered arrays of fused mesoporous carbon spheres were reported by Liu et al. using a macroporous silica as template. Recently Lei et al. reported the synthesis of 65 nm mesoporous carbon nanospheres, with both 2.7 nm mesopores and high textural porosity (surface area of 2400 mg ). These showed promising supercapacitor properties. Our spherical OMC nanoparticles of 300 nm in diameter, prepared by a novel method, can be dispersed in water by sonification to form stable colloidal suspensions. The spherical mesoporous carbon nanoparticles were obtained in a twostep casting process. An opal structure of PMMA spheres was cast with a silica precursor solution to form a silica inverse opal. The inverse opal was then used as template for a triconstituent precursor solution containing resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor and the block copolymer Pluronic F127 as a structure-directing agent. Carbonization was followed by etching of the silica template and the silica in the carbon/silica nanocomposite, resulting in the formation of OMC with hierarchical porosity. Through the presence of silica in the [*] J. Schuster, B. Mandlmeier, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5–13 (Gerhard Ertl Building), 81377 Munich (Germany) E-mail: tbein@cup.uni-muenchen.de Homepage: http://bein.cup.uni-muenchen.de G. He, T. Yim, K. T. Lee, Prof. Dr. L. F. Nazar Department of Chemistry, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1 (Canada) E-mail: lfnazar@uwaterloo.ca [] These authors contributed equally to this work.

Nanotechnology-Enabled Energy Harvesting for Self-Powered Micro-/Nanosystems

Abstract: Health, infrastructure, and environmental monitoring as well as networking and defense technologies are only some of the potential areas of application of micro-/nanosystems (MNSs). It is highly desirable that these MNSs operate without an external electricity source and instead draw the energy they require from the environment in which they are used. This Review covers various approaches for energy harvesting to meet the future demand for self-powered MNSs.

Catalytic Asymmetric Dearomatization Reactions

Abstract: This Review summarizes the development of catalytic asymmetric dearomatization (CADA) reactions. The CADA reactions discussed herein include oxidative dearomatization reactions, dearomatization by DielsAlder and related reactions, the alkylative dearomatization of electron-rich arenes, transition-metal-catalyzed dearomatization reactions, cascade sequences involving asymmetric dearomatization as the key step, and nucleophilic dearomatization reactions of pyridinium derivatives. Asymmetric dearomatization reactions with chiral auxiliaries and catalytic asymmetric reactions of dearomatized substrates are also briefly introduced. This Review intends to provide a concept for catalytic asymmetric dearomatization.

A "Renaissance" in radical trifluoromethylation

Abstract: This Minireview highlights recent developments in radical trifluoromethylation reactions. The trifluoromethyl group belongs to the privileged moieties in medicinal chemistry. Many drugs and drug candidates contain a trifluoromethyl substituent. Also in agrochemicals, the CF3 moiety often appears. The present article addresses the radical trifluoromethylation of alkenes and arenes mainly focussing on recent achievements. However, important earlier work in this field is also covered.

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells.

Abstract: Higher efficiency in the end-use of energy requires substantial progress in lighting concepts. All the technologies under development are based on solid-state electroluminescent materials and belong to the general area of solid-state lighting (SSL). The two main technologies being developed in SSL are light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), but in recent years, light-emitting electrochemical cells (LECs) have emerged as an alternative option. The luminescent materials in LECs are either luminescent polymers together with ionic salts or ionic species, such as ionic transition-metal complexes (iTMCs). Cyclometalated complexes of IrIII are by far the most utilized class of iTMCs in LECs. Herein, we show how these complexes can be prepared and discuss their unique electronic, photophysical, and photochemical properties. Finally, the progress in the performance of iTMCs based LECs, in terms of turn-on time, stability, efficiency, and color is presented.

N-heterocyclic carbene catalyzed domino reactions.

Abstract: While organocatalyzed domino reactions or "organocascade catalysis" developed into an important tool in synthetic chemistry during the past decade, the utility of N-heterocyclic carbenes (NHCs) as catalysts in domino reactions has only received growing attention in the past three years. Taking into account the unique activation modes of the substrates by NHC catalysts, it is often difficult to distinguish between a single chemical transformation and a sequential one-pot transformation. Therefore, herein we present a critical consideration of domino, cascade, and tandem catalysis in the case of NHC catalysts and highlight recent publications in this area.

Multivalency as a chemical organization and action principle.

Abstract: Multivalent interactions can be applied universally for a targeted strengthening of an interaction between different interfaces or molecules. The binding partners form cooperative, multiple receptor-ligand interactions that are based on individually weak, noncovalent bonds and are thus generally reversible. Hence, multi- and polyvalent interactions play a decisive role in biological systems for recognition, adhesion, and signal processes. The scientific and practical realization of this principle will be demonstrated by the development of simple artificial and theoretical models, from natural systems to functional, application-oriented systems. In a systematic review of scaffold architectures, the underlying effects and control options will be demonstrated, and suggestions will be given for designing effective multivalent binding systems, as well as for polyvalent therapeutics.

Maximizing the Potency of siRNA Lipid Nanoparticles for Hepatic Gene Silencing In Vivo

Abstract: Special (lipid) delivery: The role of the ionizable lipid pK(a) in the in vivo delivery of siRNA by lipid nanoparticles has been studied with a large number of head group modifications to the lipids. A tight correlation between the lipid pK(a) value and silencing of the mouse FVII gene (FVII ED(50) ) was found, with an optimal pK(a) range of 6.2-6.5. The most potent cationic lipid from this study has ED(50) levels around 0.005 mg kg(-1) in mice and less than 0.03 mg kg(-1) in non-human primates.

A Versatile, Ultralight, Nitrogen‐Doped Graphene Framework

Abstract: Graphene lite: a density of (2.1 ± 0.3) mg cm(-3), the lowest to date for a graphene framework architecture, is achieved by preparing an ultralight, N-doped, 3D graphene framework (see photo of a block of the material balancing on a dandelion). Its adsorption capacity for oils and organic solvents is much higher than that of the best carbonaceous sorbents, and it is a promising electrode material for supercapacitors (484 F g(-1)) and as a metal-free catalyst for the oxygen reduction reaction.

Confining Sulfur in Double-Shelled Hollow Carbon Spheres for Lithium-Sulfur Batteries

Abstract: New sulfur—carbon nanocomposites are prepared by confining sulfur in double-shelled hollow carbon spheres, which are obtained from SnO2 hollow spheres dispersed in aqueous glucose solution (autoclave, 180 °C, 4 h).

The birth of the lithium-ion battery.

Abstract: Throughout the 1980s, the development of portable electronic products such as video cameras, notebook computers, and cell phones led to a growing need for rechargeable batteries with greater capacity, or reduced size and weight for a given capacity. However, conventional rechargeable batteries such as lead–acid batteries and nickel–cadmium batteries, as well as nickel–metalhydride batteries, which were under development at the time, posed limitations to reduction in size and weight. Thus a need remained for a new, small, and lightweight rechargeable battery to be put into practical use. The two main classifications for batteries are disposable (primary) and rechargeable (secondary), and batteries may also be classified by the type of electrolyte employed, either aqueous or nonaqueous. Some common battery types are shown in Figure 1 in accordance with these classifications. Aqueous-electrolyte batteries have a disadvantage in that the

Piezochromic Luminescence Based on the Molecular Aggregation of 9,10-Bis((E)-2-(pyrid-2-yl)vinyl)anthracene†

Abstract: Luminescent materials sensitive to environmental stimuli are of great interest from a scientific viewpoint owing to their potential applications in fluorescent switches and optical devices. Pressure is one of the most common natural external stimuli, and thus piezochromic materials, which show color changes resulting from external pressure or mechanical grinding, can be used as pressure-sensing and optical-recording systems. However, whereas pH-, light-, and temperature-sensitive materials are relatively well understood, studies of piezochromic materials remain inadequate owing to the absence of an effective mechanism to explain the relationship between changes in molecular assembly or packing and the corresponding luminescence properties of the material. Until now, successful systems have generally made use of transformations between monomeric and dimeric/excimeric states through hydrogen-bonding interactions as the mechanism to cause changes in luminescence. It is well-known that in the condensed phase, the luminescence properties of a given molecular system usually undergo significant variation according to the molecular aggregation state or stacking mode, since intermolecular interactions invariably alter photophysical processes. Therefore, an understanding of and the ability to control the molecular aggregation state and the consequent intermolecular interactions are still very important for the development of piezochromic materials. Herein we report an effective mechanism of piezochromic luminescence on the basis of the molecular aggregation state of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (BP2VA). BP2VA exhibited spectacular luminescence characteristics: grinding and the exertion of external pressure on the powder led to a change in its photoluminescence color from green to red. Three crystal polymorphs of BP2VA with different stacking modes involving gradually enhanced p–p interactions in the three crystalline states provided further insight into the origin of luminescence changes under the external stimulae. BP2VA was synthesized in a straightforward manner by a one-step Witting–Horner reaction according to a previously reported method, and the purified material was characterized by spectroscopic methods (see the Supporting Information). BP2VA powder exhibited a strong green emission at lmax= 528 nm, in contrast to its weak orange emission at lmax= 583 nm as a solution in THF. The weak orange emission was ascribed to a conformational relaxation in solution, which was reflected by the corresponding photoluminescence (PL) spectrum of BP2VA at low temperature (77 K; see Figure S1 in the Supporting Information). Furthermore, the emission of BP2VA aggregation as a solution in THF/water was blueshifted to lmax= 570 nm (from the value lmax= 583 nm in THF; see Figure S2). Interestingly, after being ground, BP2VA powder showed a strong red shift with a yellow emission (lmax= 561 nm) under UV light with a wavelength of 365 nm, and after being heated above 160 8C, the ground powder recovered its initial green emission (lmax= 528 nm; Figure 1b,c). The interconversion of the two states with their distinct emission colors is completely reversible through grinding and heating. The red shift of 33 nm in fluorescence emission upon grinding and the recovery of the initial state upon heating is a significant piezochromic effect. To gain further understanding of the piezochromic effect, we investigated the influence of applied pressure on the luminescence of BP2VA. The powder was placed in the holes (diameter: 200 mm) of a T301 steel gasket, which was preindented to a thickness of 50 mm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration according to the R1 ruby fluorescence method. A 4:1 mixture of methanol and ethanol was used as a pressuretransmitting medium (PTM). The hydrostatic pressure on the powder was determined by monitoring the widths and separation of the R1 and R2 lines. The photoluminescence measurements under high pressure were performed on a QuantaMaster 40 spectrometer in the reflection mode. The 405 nm line of a violet diode laser with a spot size of 20 mm and a power of 100 mW was used as the excitation source. The diamond anvil cell (DAC) containing the sample was put on a Nikon fluorescence microscope to focus the laser on the sample. The emission spectra were recorded with a monochromator equipped with a photomultiplier. All experiments were conducted at room temperature. [*] Y. J. Dong, Dr. B. Xu, J. B. Zhang, L. J. Wang, J. L. Chen, Dr. H. G. Lv, Dr. S. P. Wen, Dr. B. Li, Dr. L. Ye, Prof. Dr. W. J. Tian State Key Laboratory of Supramolecular Structure and Materials Jilin Unversity Qianjin Street No. 2699, Changchun 130012 (China) E-mail: wjtian@jlu.edu.cn X. Tan, Prof. Dr. B. Zou State Key Laboratory of Superhard Materials Jilin University (China) E-mail: zoubo@jlu.edu.cn [] These authors contributed equally. [**] This research was supported by the 973 program (2009CB623605), the NSFC (Grant No. 20874035, 21074045, 21073071), and the Project of Jilin Province (Grant No.20100704). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201204660. . Angewandte Communications