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Singlet fission is a photophysical process that provides a pathway for more efficient harvesting of solar energy in photovoltaic devices. The design of singlet fission candidates is non-trivial and requires careful optimization of two key criteria: (1) correct energetic alignment and (2) appropriate intermolecular coupling. Meanwhile, this optimization must not come at the cost of molecular stability or feasibility for device applications. Cibalackrot is a historic and stable organic dye which, although it has been suggested to have ideal energetics, does not undergo singlet fission due to large interchromophore distances, as suggested by single crystal analysis. Thus, while the energetic alignment is satisfactory, the molecule does not have the desired intermolecular coupling. Herein, we improve this characteristic through molecular engineering with the first synthesis of an aza-cibalackrot and show, using ultrafast transient spectroscopy, that singlet fission is successfully "turned on."

Crystal engineering has, so far, exclusively focused on the development of advanced materials based on small organic molecules. We demonstrate how the cocrystallization of a polymer significantly enhances its thermal stability without compromising its mechanical flexibility, while isomorphous replacement of one of the cocrystal components enables the formation of solid solutions with melting points that can be readily fine-tuned over a practically wide temperature range. The results of this study credibly extend the scope of crystal engineering and cocrystallization from small molecules to polymers.

Co‐crystallisation is widely explored as a route to improve the physical properties of pharmaceutical active ingredients, but little is known about the fundamental mechanisms of the process. Herein, we apply a hyphenated differential scanning calorimetry—X‐ray diffraction technique to mimic the commercial hot melt extrusion process, and explore the heat‐induced synthesis of a series of new co‐crystals containing isonicotinamide. These comprise a 1:1 co‐crystal with 4‐hydroxybenzoic acid, 2:1 and 1:2 systems with 4‐hydroxyphenylacetic acid and a 1:1 crystal with 3,4‐dihydroxyphenylactic acid. The formation of co‐crystals during heating is complex mechanistically. In addition to co‐crystallisation, conversions between polymorphs of the co‐former starting materials and co‐crystal products are also observed. A subsequent study exploring the use of inkjet printing and milling to generate co‐crystals revealed that the synthetic approach has a major effect on the co‐crystal species and polymorphs produced., Co‐crystallisation is widely explored as a route to improve the physical properties of pharmaceutical active ingredients, but little is known about the fundamental mechanisms of the process. In this study, X‐ray diffraction was used to provide an unprecedented insight into pharmaceutical co‐crystallisation processes (see figure).

A correlation between Young's modulus, as determined by using nanoindentation atomic force microscopy (AFM), and atomic polarizability is observed for members of a series of cocrystals based on systematic changes to one cocrystal component. Time domain spectroscopy over terahertz frequencies (THz-TDS) is used for the first time to directly measure the polarizability of macro- and nanosized organic solids. Cocrystals of both macro- and nanodimensions with highly polarizable atoms result in softer solids and correspondingly higher polarizabilities.

L-Arabinose is an abundant resource available as a waste product of the sugar beet industry. Through use of a hydrazone-based strategy, L-arabinose was selectively dehydrated to form a chiral tetrahydrofuran on a multi-gram scale without the need for protecting groups. This approach was extended to other biomass-derived reducing sugars and the mechanism of the key cyclization investigated. This methodology was applied to the synthesis of a range of functionalized chiral tetrahydrofurans, as well as a formal synthesis of 3R-3-hydroxymuscarine.

We present the synthesis and characterization of a series of encapsulated diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials display extremely high fluorescence quantum yields in both solution (>70%) and thin film (>20%). Both the absorption and emission spectra show clearer, more defined features compared to their naked counterparts demonstrating the suppression of inter and intramolecular aggregation. We find that the encapsulation results in decreased energetic disorder and a dramatic increase in backbone colinearity as evidenced by scanning tunnelling microscopy. This study paves the way for diketopyrrolopyrrole to be used in emissive solid state applications and demonstrates a novel method to reduce structural disorder in conjugated polymers.

The [4+2] cycloaddition of 3-alkoxyfurans with N-substituted maleimides provides the first general route for preparing endo-cantharimides. Unlike the corresponding reaction with 3H furans, the reaction can tolerate a broad range of 2-substitued furans including alkyl, aromatic, and heteroaromatic groups. The cycloaddition products were converted into a range of cantharimide products with promising lead-like properties for medicinal chemistry programs. Furthermore, the electron-rich furans are shown to react with a variety of alternative dienophiles to generate 7-oxabicyclo[2.2.1]heptane derivatives under mild conditions. DFT calculations have been performed to rationalize the activation effect of the 3-alkoxy group on a furan Diels–Alder reaction.

We describe a conceptually novel “accelerated aging” approach for the synthesis of metal–organic materials. This approach, inspired by natural mineral weathering processes, enables the synthesis of metal–organic structures from simple and inexpensive solid reactants upon exposure to catalytic amounts of an ammonium salt under conditions of high humidity and mild temperatures (up to 45 °C). Accelerated aging exploits the inherent mobility of molecules and is entirely different from solution-based (precipitation, solvothermal synthesis) or other solvent-free (mechanochemical synthesis) approaches to metal–organic materials that require either bulk solvent and/or thermo- or mechanochemical intervention. The present proof-of-principle study of accelerated aging demonstrates the catalysed and topologically specific transformation of ZnO into unusual close-packed varieties of zeolitic imidazolate frameworks (ZIFs) in a static, non-agitated reaction mixture. The reactivity is readily scaled up, as demonstrated by performing selected syntheses of quartz- and diamondoid-topology close-packed ZIFs in ten gram amounts. The latter framework, previously obtained only by using a large excess of reagents under hydrothermal conditions, is transformed into the well-known open framework ZIF-8 by exposure to methanol vapours at room temperature, suggesting an alternative to both solvothermal and mechanochemical approaches to these materials. A tentative proton-transfer mechanism underpinning the catalytic effect in accelerated aging is proposed, involving protonated imidazole as an intermediate.

A supramolecular protecting group strategy has been applied to achieve solid-state photodimerizations of olefins lined with a combination of hydrogen-bond-donor and -acceptor groups. Esters were used as protecting groups to generate head-to-head photodimers that were readily converted into diacids. A protected olefin equipped with a stilbene unit exhibits enhanced reactivity that is ascribed to pedal motions in the solid state (orange hexagons: template: blur circles: recognition sites). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

[Figure Presented] Chemical mechanics: Hydrogen-bonddriven self-assembly and mechanochemistry are used to facilitate supramolecular catalysis in the solid state. Mortar-andpestle grinding proves to be an efficient means to achieve co-crystal formation and turnover using a physical mixture composed of an olefin and catalytic amounts of a ditopic template (see scheme). © 2010 Wiley-VCH Verlag GmbH &. Co. KGaA.

We show that a molecule constructed from an organic solid-state synthesis, namely, rctt-1,3-bis(2-pyridyl)-2,4-bis(4-pyridyl)cyclobutane (ht-2,4′-tpcb) upon reaction with Cu(II) and Zn(II) ions gives one- (1D) and two-dimensional (2D) metal–organic frameworks (MOFs) of composition [Cu(hfac)2(2,4′-tpcb-ht)]∞ (where: hfac=hexafluoroacetylacetonate) (1) and [ Zn ( 2 , 4 ′ ‐ tpcb ‐ ht ) 2 ( H 2 O ) 2 ] ∞ 2 + (2). Each MOF possesses uncoordinated, or free, 2-pyridyl groups that decorate the walls of each extended structure.

A bicyclobutyl that bears six carboxylic acid groups results from a trimerisation of a diene diacid in the solid state. Powder X-ray diffraction and a co-crystallisation are used to solve the structure of the diene and elucidate the stereochemistry of the bicyclobutyl, respectively.

Nearly twenty years ago, Dunitz and Bernstein described a selection of intriguing cases of polymorphs that disappear. The inability to obtain a crystal form that has previously been prepared is indeed a frustrating and potentially serious problem for solid-state scientists. This Review discusses recent occurrences and examples of disappearing polymorphs (as well as the emergence of elusive crystal forms) to demonstrate the enduring relevance of this troublesome, but always captivating, phenomenon in solid-state research. A number of these instances have been central issues in patent litigations. This Review, therefore, also highlights the complex relationship between crystal chemistry and the law.

Ultrasound-assisted crystallization (sonocrystalliza- tion) was used to prepare a mixture of nano- and micrometer- sized crystals of the monoclinic form of paracetamol—a widely used analgesic known for its particularly problematic mechan- ical behavior under compression (i.e. poor tabletability). The nano- and micrometer-sized crystals yielded a powder which exhibits elastic moduli and bulk cohesions that are significantly higher than those observed in samples consisting of macro- meter-sized crystals, thus leading to enhanced tabletability without the use of excipients, particle coating, salt, or cocrystal formation. Experimental compaction and finite element anal- ysis were utilized to rationalize the significantly improved compaction behavior of the monoclinic form of paracetamol. The current demand for organic nanomaterials (1, 2) has triggered the development of fabrication methods for organic nanocrystals. (3-10) Recent pharmaceutical and biomedical research has shown that molecular nanocrystals are highly valuable in the context of drug delivery, biolabeling, and biosensing. (11, 12) Nanosizing of molecular crystals has also become a particularly popular approach in drug development with poorly soluble pharmaceutical agents (13) and is mainly used to enhance the solubility and bioavailability of the drug candidate. (14, 15) Surprisingly, other applications of molecular crystals, despite their unique physicochemical properties, have been relatively unexplored in biomedical (and other) sciences. We show herein through the use of sonocrystalliza- tion, (16, 17) that is, the use of ultrasound to facilitate crystal

A survey of the Cambridge Structural Database (CSD) revealed the existence of a new supramolecular homosynthon in oxovanadium coordination compounds (OCCs) that is based on non-covalent V=O...C interactions. V=O...C close contacts were found in 38.7% of structures containing vanadyl or pervanadyl groups. The V=O...C interaction is clearly distinguishable from a putative V=O...H–C hydrogen bond upon analysis of OCC crystal structures deposited in the CSD. In the majority of structures, the OCCs assemble to form dimers or polymers. The V=O...C interactions were also found to lead to self-assembly of two new oxovanadium β-diketonate complexes into dimers in the solid state.

A discrete dinuclear Ag(I) complex undergoes a single-crystal-to-single-crystal transformation to generate a 1D coordination polymer with a polycyclobutane backbone.

Multi-component organic nanocrystals that are comprised of two or more supramolecular building blocks can be used to extend the design and assembly scope of solid molecular materials. Herein, we report the use of ultrasonication to prepare halogen-bonded stilbene-based nano-cocrystals that exhibit different photoemission properties, including one- and two-phonon emission and fluorescence lifetimes, relative to those of macrodimensional crystals. The structural transformation from nano-cocrystals into nanocrystals upon heating results in a luminescence red-shift from greenish blue to yellow. The temperature-dependent ratiometric luminescence may allow such nano-cocrystals to be used as fluorescent sensors and thermosensitive materials.

Here, we highlight recent research involving atomic force microscopy investigations of molecular crystals, and focus particularly on the latest relevant advances in our knowledge of crystal-growth mechanisms and structure-property relationships in organic crystals. This brief survey features the importance of incorporating AFM into solid-state research as an essential tool for the informed design and construction of crystalline materials.

Nano- and micrometer-scale crystals of a self-assembled hexamer have been synthesized via sonochemistry. The application of ultrasonic irradiation afforded hollow rhombic-dodecahedral crystals of the C-methylcalix[4]resorcinarene hexamer. The formation of the hollow crystals is attributed to a reversed crystal growth mechanism heretofore described only in the synthesis of inorganic-based materials.

A ditopic hydrogen-bond-donor template in the form of resorcinol facilitates a [2+2] cross-photodimerisation of 4-Cl-stilbazole and 4-Me-stilbazole in a rare cocrystal solid solution. A photoreaction does not proceed with the olefins individually or as a solid solution composed solely of the two olefins.

Facile reactivity of hydrazides and aldehydes was explored as potential coupling partners for incorporation into M(CO)(3) (M = Re, (99m)Tc) based radiopharmaceuticals. Both 'click, then chelate' and 'prelabel, then click' synthetic routes produced identical products in high yields and lacked metal-hydrazide/-hydrazone interactions, highlighting the potential of this click strategy.

Metallogels form from Cu(II) ions and tetratopic ligand rctt-1,2-bis(3-pyridyl)-3,4-bis(4-pyridyl)cyclobutane. The tetrapyridyl cyclobutane has been synthesized in the organic solid state. The gel forms with a variety of counteranions and gels water. The hydrogel is thixotropic and is composed of nanoscale metal-organic particles (NMOPs), a high surface area of which likely accounts for the gelation of the polar aqueous medium. A shear stress profile of the thixotropic hydrogel gave a yield value of 8.33 Pa. A novel combination of atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) is used to assess the densities of individual NMOPs. A density of 1.37 g/cm(3) has been determined. A single-crystal X-ray diffraction study demonstrates the ability of the unsymmetrical cyclobutane 3,4'-tpcb to self-assemble with Cu(II) ions in [Cu(2)(hfac)(4)(3,4'-tpcb)](∞) (where hfac is hexafluoroacetylacetonate) to form a solvated 1D coordination polymer.

Reaction of 4-vinylpyridine with AgClO(3) yields a crystalline solid that supports a stereospecific and quantitative [2+2] photodimerisation of two terminal olefins. Structural dynamics of the -CH[double bond, length as m-dash]CH(2) group, nature of the counterion, and extended packing influence the photoreactivity.

Supermolecules with olefins organized by hydrogen-bond donor and acceptor templates and that react in the solid state rapidly form co-crystals via solvent-free and liquid-assisted grinding.

An achiral ligand, synthesized in the solid state via a coded hydrogen bond-directed organic synthesis, self-assembles with Cu(II) ions to form a chiral tetrahedral capsule that hosts an anion as a guest.

Co-crystal formation between caffeine and adipic acid has been explored over the years without success; utilizing the newly developed co-crystal screening method, we have finally discovered this “hidden” caffeine and adipic acid co-crystal.

Metal-organic polyhedra with surface-exposed organic groups have been designed. The polyhedra are based on concentric shells of alternating negative-positive-negative charges and have been used to design homochiral hosts.

The unsymmetrical tetratopic ligand rctt-1,2-bis(3-pyridyl)-3,4-bis(4-pyridyl)cyclobutane self-assembles with zinc(II) nitrate to form a 2D metal–organic framework with three distinct cavities.

Tetratopic rctt-tetrakis(4-pyridyl)cyclobutane acts as an angular bifunctional ligand that yields a single-stranded helicate coordination polymer based on a Ln(III) ion.