Carbon nitride frameworks and dense crystalline polymorphs

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3 bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold-coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2

Carbon nitride frameworks and dense crystalline polymorphs

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3-bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2.Engineering and Physical Sciences Research Council (EPSRC) (Grant IDs: EP/J017639/1, EP/G007489/2, EP/K013688/1, EP/K014560/1, EP/L01709/1), Royal Society (Wolfson Research Merit Award), Czech Science Foundation (Junior Grant (CAMs – 16-21151Y)), European Research Council (Starting Grant scheme (BEGMAT – 678462))This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.94.09410

Trapping virtual pores by crystal retro-engineering

Stable guest-free porous molecular crystals are uncommon. By contrast, organic molecular crystals with guest-occupied cavities are frequently observed, but these cavities tend to be unstable and collapse on removal of the guests—this feature has been referred to as ‘virtual porosity’. Here, we show how we have trapped the virtual porosity in an unstable low-density organic molecular crystal by introducing a second molecule that matches the size and shape of the unstable voids. We call this strategy ‘retro-engineering’ because it parallels organic retrosynthetic analysis, and it allows the metastable two-dimensional hexagonal pore structure in an organic solvate to be trapped in a binary cocrystal. Unlike the crystal with virtual porosity, the cocrystal material remains single crystalline and porous after removal of guests by heating

Dicyano- and tetracyanopentacene: foundation of an intriguing new class of easy-to-synthesize organic semiconductors

Cyanated pentacenes are very promising candidate materials for ambipolar and n-type transistors. However, only a few examples have been obtained to date – all requiring lengthy, multi-step processes. Herein, we present the first preparation of 5,7,12,14-tetracyanopentacene (TCP) and a facile, scaled-up preparation of 6,13-dicyanopentacene (DCP). Both compounds are prepared by a one-pot synthesis using cheap quinones as starting materials. Detailed crystallographic investigations evince that the bulk assemblies of both cyanated pentacenes are dominated by non-covalent interactions, resulting in a dense, stable, face-to-face packing and in an intriguing packing motif for TCP. Very low frontier molecular orbital energy levels and a reversible bleaching of TCP are revealed by cyclic voltammetry. Finally, both cyanated pentacenes are used in proof-of-concept organic thin-film transistors (OTFTs) operating under ambient conditions. This work highlights the potential of cyanation for larger acenes and presents a straightforward route to the rational design of this promising class of materials

Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds

Covalent organic frameworks are a class of crystalline organic porous materials that can utilize p–p-stacking interactions as a driving force for the crystallization of polygonal sheets to form layered frameworks and ordered pores. However, typical examples are chemically unstable and lack intrasheet p-conjugation, thereby significantly limiting their applications. Here we report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, in which the p conjugation-spans the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of properties: chemical stability, extended p-delocalization, ability to host guest molecules and hole mobility. We show that the p-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy may constitute a step towards realizing ordered semiconducting porous materials for innovations based on two-dimensionally extended p systems.11521611sciescopu