Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Recent Advances in Bulk Heterojunction Polymer Solar Cells

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.

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

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Recent advances in organic thermally activated delayed fluorescence materials.

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Reversible, Metal-Free Hydrogen Activation

Polymer-based bulk-heterojunction solar cells have shown some of the highest photoconversion efficiencies in organic photovoltaics, but polymer polydispersity impacts their performance. A small-molecule donor is now reported that enables the fabrication of bulk-heterojunction devices with low acceptor content and photoconversion efficiencies of up to 6.7%.

Solution-processed small-molecule solar cells with 6.7% efficiency

The facile heterolytic cleavage of H2 is readily achieved at room temperature by the cooperative action of the Lewis acidic borane and Lewis basic phosphine, where the steric congestion precludes quenching of the acid and base via adduct formation.

Facile heterolytic cleavage of dihydrogen by phosphines and boranes.

Hydrogenation is the addition of hydrogen to unsaturated organic compounds. Such reactions are used for the production of a myriad of chemical products worldwide, from largescale operations including the upgrading of crude oil and the production of bulk commodity materials to the synthesis of a variety of fine chemicals used in the food, agricultural, and pharmaceutical industries. The process of hydrogen addition to unsaturated precursors is mediated by either homogeneous or heterogeneous transition-metal-based catalysts. Blaser et al. place the importance of this chemistry in context, stating that “hydrogen is the cleanest reducing agent and hydrogenation is arguably the most important catalytic method in synthetic organic chemistry both on the laboratory and the production scale”. Historically, hydrogenation began with Sabatier%s 1897 discovery that traces of nickel could mediate the catalytic hydrogenation of olefins and culminated in a share of the 1912 Nobel Prize with Grignard. In the 1960s, the advent of organometallic chemistry gave rise to homogeneous transition-metal-based hydrogenation catalysts for a variety of substrates. The operation of these catalysts hinges on the key step of oxidative addition of hydrogen. More recently, transition-metal systems that effect heterolytic cleavage of hydrogen at a metal center have been uncovered. In these cases, a metal hydride is formed with concurrent protonation of an amido ligand. Non-transition-metal catalysts for hydrogenation reactions are all but unknown. KOtBu has been shown to act as a catalyst effecting the addition of H2 to benzophenone under forcing conditions of 200 8C and greater than 100 bar H2. [6] Organocatalysts have been developed for hydrogenations of enones and imines; however, such systems do not employ H2 directly but rather a surrogate such as a Hantzsch ester as the stoichiometric source of hydrogen. The development of nonmetal hydrogenation catalysts hinges on the discovery of systems that react cleanly with H2, but few are known. Power and co-workers reported the hydrogenation of Ge2–alkyne analogues to give a mixture of Ge2 and primary germane products. Recently we have introduced the concept of “frustrated Lewis pairs”, bulky Lewis acids and bases which are sterically precluded from forming simple Lewis adducts. Mixtures of “frustrated” phosphines and boranes can heterolytically cleave H2, forming phosphonium borates of the form [R3PH][BHR’3]. [14] In very recent work, Bertrand and co-workers have demonstrated that selected carbenes exhibit transition-metal-like reactivity and cleave hydrogen or ammonia to effect a formal oxidative addition of the carbene C atom. Last year, we reported the only nonmetal system known to reversibly activate and liberate H2. The phosphonium borate (2,4,6-Me3C6H2)2PH(C6F4)BH(C6F5)2 (1) is formed by reaction of the phosphine–borane species (2,4,6-Me3C6H2)2P(C6F4)B(C6F5)2 (3) with H2 while heating of the zwitterion 1 above 100 8C liberates hydrogen and regenerates the phosphine–borane 3. Herein, we demonstrate that this system and a related system provide the first metalfree hydrogenation catalysts that effect the addition of molecular H2 to imines, nitriles, and aziridines to produce primary and secondary amines in high yields under relatively mild reaction conditions. The reduction of imines and nitriles is one of the best synthetic methods to generate secondary and primary amines, and has found tremendous importance in the pharmaceutical and fine chemicals industry. The airand moisture-stable phosphonium borates (R2PH)(C6F4)BH(C6F5)2 (R= 2,4,6Me3C6H2 (1) [16] and tBu (2)) are active catalysts for the hydrogenation of C N multiple bonds with H2. For example, imines are reduced in toluene to the corresponding amines cleanly and in high yield at slightly elevated temperatures (80–140 8C) and H2 pressures (1–5 atm) in sealed glass bombs (Table 1, entries 1–6). The amine products are readily separated from residual catalyst by filtration through a plug of silica gel; no other side products are observed in the NMR spectra of the crude reaction mixtures. In the case of a sterically less demanding imine (Table 1, entry 6) no catalytic turnover was noted. Similarly, nitriles are not catalytically reduced, as these donors intervene in the catalytic cycle by binding strongly to the B center of the catalyst. Sequestering the N lone pair by coordination of Ph(H)C=NCH2Ph to B(C6F5)3 (Table 1, entry 7) allowed catalytic imine reduction to proceed. In a similar fashion, by employing the more active 2 as the catalyst, alkyl and aryl B(C6F5)3-bound nitriles are also successfully reduced and isolated as the corresponding primary amine–borane adducts (Table 1, entries 8–10). In these cases, partial reduction of nitriles to the corresponding imines can not be intercepted or observed. It is noteworthy that the bis-borane adduct of adiponitrile is also fully reduced under similar conditions to give the bis-borane adduct of 1,6diaminohexane (Table 1, entry 10). Attempts to reduce similar B(C6F5)3–isonitrile adducts were unsuccessful. However, catalytic reductive ring opening of an unactivatedN-aryl aziridine functionality is achieved under similar conditions (Table 1, entry 11). [*] P. A. Chase, G. C. Welch, T. Jurca, Prof. Dr. D. W. Stephan Department of Chemistry and Biochemistry University of Windsor Windsor, Ontario, N9B 3P4 (Canada) Fax: (+1)519-973-7098 E-mail: stephan@uwindsor.ca

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.200790249

Metal‐Free Catalytic Hydrogenation

Organic semiconductors incorporated into solar cells using a bulk heterojunction (BHJ) construction show promise as a cleaner answer to increasing energy needs throughout the world. Organic solar c...

Gregory C. Welch

Papers

Reversible, Metal-Free Hydrogen Activation

Solution-processed small-molecule solar cells with 6.7% efficiency

Facile heterolytic cleavage of dihydrogen by phosphines and boranes.

Metal‐Free Catalytic Hydrogenation

Design and synthesis of molecular donors for solution-processed high-efficiency organic solar cells.