Synthesis of Chiral Pyrene-Based 1,4-Dithiins.

The 1,4-dithiin motif is known for its reversible redox properties to generate radical cations and diradical dications and thus is interesting for organic electronic applications. However, examples where this motif is embedded into chiral larger fused aromatic compounds are very rare. Here we describe the syntheses of several structurally related pyrene fused dithiins and their spectroscopic investigations with a focus on tuning circular dichroism, with respect to the g values, depending on their connectivity.

Single Crystals of Insoluble Porous Salicylimine Cages.

Porous organic cages (POCs) are meanwhile an established class of porous materials. Most of them are soluble to a certain extend and thus processable in or from solution. However, a few of larger salicylimine cages were reported to be insoluble in any organic solvents and thus characterized as amorphous materials. These cages were now synthesized as single-crystalline materials to get insight into packing motifs and preferred intermolecular interactions. Furthermore, the pairs of crystalline and amorphous materials for each cage allowed to compare their gas-sorption properties in both morphological states.

Synthesis of a Benzotrisazulene via Trioxobenzotrisazulene.

Polycyclic aromatic hydrocarbons (PAHs) containing odd-membered rings (such as pentagons or heptagons) are fascinating synthetic targets. A special case is the introduction of five- and seven-membered rings in form of an azulene unit. Azulene itself is an aromatic compounds known for its deep blue color, which is a result of its internal dipole moment. When azulene is embedded into PAHs it can change the optoelectronic properties of the PAH significantly. Herein, the synthesis and characterization of a PAH containing three azulene units is reported via reduction and elimination of its trioxo derivative.

Palladium-Catalyzed Cyclization of a Pyryne Precursor to Higher Pyrenylenes.

The palladium catalyzed cyclotrimerization of ortho-silylaryl triflates as aryne precursors is meanwhile an established method to synthesize polycyclic aromatic hydrocarbons (PAHs) with triphenylene cores. During the palladium-catalyzed reaction of a pyrene with an o-silylaryl triflate moiety in the K-region higher homologues with central eight- and ten-membered rings (the pyrenylenes) were found, besides the expected trimer and a protocol was developed to isolate all members of this series. This unprecedented new class of PAHs was fully investigated by all means, including X-ray diffraction of single-crystals, UV/Vis and fluorescence spectroscopy and theoretical calculations. Supported by density-functional theory (DFT) calculations, a mechanism of all higher cyclooligomers is proposed.

Solvent-Controlled Quadruple Catenation of Giant Chiral [8+12] Salicylimine Cubes Driven by Weak Hydrogen Bonding.

Mechanically interlocked structures are fascinating synthetic targets and the topological complexity achieved through catenation offers numerous possibilities for the construction of new molecules with exciting properties. In the structural space of catenated organic cage molecules, only few examples have been realized so far, and control over the catenation process in solution is still barely achieved. Herein, we describe the formation of a quadruply interlocked catenane of giant chiral [8+12] salicylimine cubes. The formation could be controlled by the choice of solvent used in the reaction. The interlocked structure was unambiguously characterized by single crystal X-ray diffraction and weak hydrogen bonding was identified as a central driving force for the catenation. Furthermore, scrambling experiments using partially deuterated cages were performed, revealing that the catenane formation occurs through mechanical interlocking of preformed single cages.

Dimeric and trimeric catenation of giant chiral [8 + 12] imine cubes driven by weak supramolecular interactions.

Mechanically interlocked structures, such as catenanes and rotaxanes, are fascinating synthetic targets and some are used for molecular switches and machines. Today, the vast majority of catenated structures are built upon macrocycles and only a very few examples of three-dimensional shape-persistent organic cages forming such structures have been reported. However, the catenation in all these cases was based on a thermodynamically favoured π-π-stacking under certain reaction conditions. Here, we show that catenane formation can be induced by adding methoxy or thiomethyl groups to one of the precursors during the synthesis of chiral [8 + 12] imine cubes, giving dimeric and trimeric catenated organic cages. To elucidate the underlying driving forces, we reacted 11 differently 1,4-disubstituted terephthaldehydes with a chiral triamino tribenzotriquinacene under various conditions to study whether monomeric cages or catenated cage dimers are the preferred products. We find that catenation is mainly directed by weak interactions derived from the substituents rather than by π-stacking.

Highly Selective Adsorption of Perfluorinated Greenhouse Gases by Porous Organic Cages.

Anthropogenic greenhouse gases contribute to global warming. Among those gases, perfluorocarbons (PFCs) are thousands to tens of thousands of times more harmful to the environment than comparable amounts of carbon dioxide. To date, materials that selectively adsorb perfluorocarbons in favor of other less harmful gases have not been reported. Here, a series of porous organic cage compounds with alkyl-, fluoroalkyl-, and partially fluorinated alkyl groups is presented. Their isomorphic crystalline states allow the study of the structure-property relationship between the degree of fluorination of the alkyl chains and the gas sorption properties for PFCs and their selective uptakes in comparison to other, nonfluorinated gases. By this approach, one compound having superior selectivities of PFCs versus N2 or CO2 under ambient conditions is identified.

[2+3] Amide Cages by Oxidation of [2+3] Imine Cages - Revisiting Molecular Hosts for Highly Efficient Nitrate Binding.

The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD2 Cl2 /CD3 CN (1 : 3). NMR experiments as well as single-crystal structure comparison of host-guest complexes give insight into structure-property-relationships.

A Series of Soluble Thieno-Fused Coronene Nanoribbons of Precise Lengths.

Among graphene nanoribbons (GNRs), reports on coronene-based GNRs were very rare, despite the unique optoelectronic properties of coronene. Herein, the synthesis of a series of structurally precise and soluble thieno-fused coronene nanoribbons (CR-1 to CR-4) with up to four coronene units connected through benzene rings along their K-regions is described by two different synthetic approaches. Due to the triptycene end-caps, all CRs are soluble, allowing the characterization of such structures in solution and the study of the length-dependent photophysical properties that are supported by theoretical calculations.

Cucurbitimines - imine cages with concave walls.

The variety of shape-persistent organic cages by imine bond formation has tremendously enlarged in recent years by using different building blocks (aldehydes and amines) in the condensation reactions. Here, we describe the use of a kinked tetraldehyde to generate pumpkin-shaped cages with concave walls, similar to cucurbiturils.

Contorted Heteroannulated Tetraareno[a,d,j,m]coronenes.

Fused polycyclic aromatic compounds are interesting materials for organic electronics applications. To fine-tune photophysical or electrochemical properties, either various substituents can be attached or heteroatoms (such as N or S) can be incorporated into the fused aromatic backbone. Coronenes and heterocoronenes are promising compounds in this respect. Up until now, the possibilities for varying the attached fused heteroaromatics at the coronene core were quite limited, and realizing both electron-withdrawing and -donating rings at the same time was very difficult. Here, a series of pyridine, anisole and thiophene annulated tetraareno[a,d,j,m]coronenes has been synthesized by a facile two-step route that is a combination of Suzuki-Miyaura cross-coupling and a following cyclization step, starting from three different diarenoperylene dibromides. The contorted molecular π-planes of the obtained cata-condensed tetraarenocoronenes were analyzed by single-crystal X-ray crystallography, and the photophysical and electrochemical properties were systematically investigated by UV/Vis spectroscopy and cyclovoltammetry.

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages.

One of the most applied reaction types to synthesize shape-persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n-hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.

Chiral Self-sorting of Giant Cubic [8+12] Salicylimine Cage Compounds.

Chiral self-sorting is intricately connected to the complicated chiral processes observed in nature and no artificial systems of comparably complexity have been generated by chemists. However, only a few examples of purely organic molecules have been reported so far, where the self-sorting process could be controlled. Herein, we describe the chiral self-sorting of large cubic [8+12] salicylimine cage compounds based on a chiral TBTQ precursor. Out of 23 possible cage isomers only the enantiopure and a meso cage were observed to be formed, which have been unambiguously characterized by single crystal X-ray diffraction. Furthermore, by careful choice of solvent the formation of meso cage could be controlled. With internal diameters of din =3.3-3.5 nm these cages are among the largest organic cage compounds characterized and show very high specific surface areas up to approx. 1500 m2 g-1 after desolvation.

Benzo-Fused Perylene Oligomers with up to 13 Linearly Annulated Rings.

The longer acenes with more than six linearly fused six-membered rings are still fascinating chemists and physicists because of their unique photophysical properties and their high potential for organic electronics applications. Unfortunately, with increasing size (seven and more rings) these compounds rapidly lose chemical stability. Besides kinetic and chemical stabilization approaches introducing either bulky or electron-withdrawing groups or both, such systems also have been stabilized by peri-annulation. Although strictly spoken, these peri-annulated compounds are no longer real acenes, they have fascinating properties as well. Herein, we describe the first synthesis of a new series of peri-annulated acenes with up to 13 linearly fused rings, which is unprecedented till date. Furthermore, this new series contains perylene units connected through benzene rings along their [b,k]edges, responsible for unique absorption and emission properties.

Triptycene End-Capping as Strategy in Materials Chemistry to Control Crystal Packing and Increase Solubility.

In materials chemistry of polycyclic aromatic compounds (PACs) the kind of aggregation and the spatial arrangement of the π-planes are of utmost importance, e. g. for charge transport properties. Unfortunately, controlling these during crystallization is not trivial. In the past decade, we have introduced one-fold triptycene end-capping of quinoxalinophenanthrophenazines (QPPs) and other related structures to overcome this problem. When two instead of one triptycene end-caps are introduced, packing is largely suppressed, making typical PACs or pigments soluble in common organic solvents - which is another important property for such compounds to be processable from solution. In this account an overview of our research on using triptycene end-capping as dual strategy is given.

Triptycene End-Capped Indigo Derivatives - Turning Insoluble Pigments to Soluble Dyes.

The synthesis of a highly soluble triptycene end-capped indigo and its bay annulated derivative is reported. Both compounds have been studied by absorption and emission spectroscopy cyclic voltammetry, as well as theoretical calculations and compared to the parent indigo and bay annulated indigo. Besides a large improvement of solubility in organic solvents by the factor of approx. 70(!) the compounds also show a pronounced tendency to form crystals. Both properties, making these compounds promising electron acceptors for organic electronics.

Synthesis and Optoelectronic Properties of a Quinoxalino-Phenanthrophenazine (QPP) Extended Tribenzotriquinacene (TBTQ).

A six-step synthesis towards a tribenzotriquinacene (TBTQ) bearing three quinoxalinophenanthrophenazine (QPP) units is presented. The optoelectronic properties are investigated and the effect of the three-dimensional arrangement of the individual QPP planes is examined using optical spectroscopy, electrochemical analysis and quantum-chemical calculations.

A Giant [8+12] Boronic Ester Cage with 48 Terminal Alkene Units in the Periphery for Postsynthetic Alkene Metathesis.

Dynamic covalent chemistry (DCC) is a powerful synthetic tool to construct large defined molecules in one step from rather simple precursors. The advantage of the intrinsic dynamics of the applied reversible reaction steps is a self-correction under the chosen conditions, to achieve high yields of the target compound. To date, only a few examples are known, in which DCC was used to build up a molecular defined but larger product that was chemically transferred to a more stable congener in a second (irreversible) step. Here, we present a nanometer-sized [8+12] boronic ester cage containing 48 peripheral terminal alkene units which allows to put a hydrocarbon exoskeleton around the cage via alkene metathesis.

Homoconjugation and Intramolecular Charge Transfer in Extended Aromatic Triptycenes with Different π-Planes.

Homoconjugation and intramolecular "through-space" charge transfers are molecular phenomena that have been studied since the 1960s. A detailed understanding and control of these effects would provide a tool to tune the optoelectronic properties of organic molecules in respect of the necessities for applications such as for organic electronics. Triptycene is a perfect candidate to investigate homoconjugation effects due to its three-dimensional alignment of three aromatic phenylene units, separated by two methine bridges. Here, a series of 16 π-extended triptycenes with up to three different permuted electron-accepting units and an electron-rich veratrole unit are studied in detail by UV/vis spectroscopy and cyclovoltammetry in combination with DFT calculations to get a deeper understanding of homoconjugation and charge-transfer processes of triptycenes. Furthermore, the gained knowledge can be exploited to construct triptycene-based electron acceptors with fine-tuned adjustment of electronic properties, such as electron affinities, by thorough choice of the aromatic blades that interact through homoconjugation.