Boron-nitrogen dative bonds provide a suitable motif for reversible, yet strong and directed interactions, leading to the highly efficient self-assembly of small organic building blocks into supramolecular cage structures. A bipyramidal [2+3] assembly, as the first example of a supramolecular cage mediated by BN dative bonds that exists as a discrete species in solution, is quantitatively obtained from a tribenzotriquinacene-based trisboronate ester and 1,4-diazabicyclo[2.2.2]octane. Thermodynamic equilibria of cage formation are investigated by isothermal titration calorimetry and fully reversible cage opening can be observed at elevated temperatures.

Dynamic covalent self-assembly of small and rigid precursors into cage-type architectures can serve as a powerful strategy for the formation of molecular porous units. In order to enhance the functionality of suchlike nanostructures, formation of complex multicomponent assemblies with high spatial precision, and on-demand control of both reversible assembly and disassembly is highly desirable. Here we highlight some of our most recent achievements on the size-specific synthesis and self-sorting properties within a series of covalent organic cage compounds and the stimuli-responsive assembly of supramolecular cages assembled by boron-nitrogen dative bonds. 1 Introduction 2 Self-Sorting of Covalent Organic Cages 3 Stimuli-Responsive Cages 4 Conclusion and Outlook Key words cage compounds, dynamic covalent chemistry, self-sorting, boronate esters, boron-nitrogen dative bonds, self-assembly, supramolecular chemistry Florian Beuerle (center) studied chemistry at Friedrich-Alexander University in Erlangen (Germany) and graduated with a diploma in 2005. After working with Prof. Andreas Hirsch on the regioselective functionalization and antioxidant properties of [60]fullerene derivatives, he obtained his PhD in 2008. Afterwards, he moved as a Feodor Lynen postdoctoral fellow of the Humboldt Foundation to the group of Sir Fraser Stoddart at Northwestern University in Evanston, IL, USA. Since 2010, he has been a junior research group leader at Julius-Maximilian University in Würzburg (Germany) supported by a Liebig fellowship of the Fonds der Chemischen Industrie. His current research interests include covalent organic cage compounds, porous materials, and nanosystems chemistry. Stefanie Klotzbach (right) studied chemistry at the Julius-Maximilian University in Würzburg (Germany) and obtained her diploma in 2011. Since then, she is working towards her PhD in the group of Florian Beuerle, where she is currently studying the shape-selective synthesis and self-sorting properties of covalent organic cage compounds. Ayan Dhara (left) graduated from the Indian Institute of Technology Kanpur (India) with a Master's degree in 2012. He is currently pursuing his doctoral research on supramolecular and covalent organic cage compounds in the Beuerle group.

Fluorophores are powerful tools for the study of chemistry, biology, and physics. However, fluorescence is severely impaired when concentrations climb above 5 μM as a result of effects like self-absorption and chromatic shifts in the emitted light. Herein, we report the creation of a charge-transfer (CT) fluorophore and the discovery that its emission color seen at low concentrations is unchanged even at 5 mM, some 3 orders of magnitude beyond typical limits. The fluorophore is composed of a triphenylamine-substituted cyanostar macrocycle, and it exhibits a remarkable Stokes shift of 15 000 cm–1 to generate emission at 633 nm. Crucial to the performance of this fluorophore is the observation that its emission spectrum shows near-zero overlap with the absorption band at 325 nm. We propose that reducing the spectral overlap to zero is a key to achieving full fluorescence across all concentrations. The triphenylamine donor and five cyanostilbene acceptor units of the macrocycle generate an emissive CT state. Unlike closely related donor–acceptor control compounds showing dual emission, the cyanostar framework inhibited emission from the second state to create a zero-overlap fluorophore. We demonstrated the use of emission spectroscopy for characterization of host–guest complexation at millimolar concentrations, which are typically the exclusive domain of NMR spectroscopy. The binding of the PF6 – anion generates a 2:1 sandwich complex with blue-shifted emission. Distinct from twisted intramolecular charge-transfer (TICT) states, experiment-supported density functional theory shows a 67° twist inside an acceptor unit in the CT state instead of displaying a twist between the donor and acceptor; it is TICT-like. Inspired by the findings, we uncovered similar concentration-independent behavior from a control compound, strongly suggesting this behavior may be latent to other large Stokes-shift fluorophores. We discuss strategies capable of generating zero-overlap fluorophores to enable accurate fluorescence characterization of processes across all practical concentrations.

The introduction of one alkyne moiety at the central carbon atom of the tripodal tribenzotriquinacene scaffold allows easy access to a great variety of apically functionalized derivatives. The spatially well-separated arrangement of different functional units on the convex face and outer rim was further proven by single-crystal X-ray studies. Subsequent modifications that feature a general protecting group-free strategy for the demethylation of protected catechols in the presence of a terminal alkyne group, an azide-alkyne Huisgen cycloaddition, and Sonogashira cross-coupling reactions showcase the high synthetic potential of this modular approach for tribenzotriquinacene derivatization.

A highly crystalline material comprised of [2]rotaxanes shows large amplitude motion of the interlocked macrocycle as evidenced by variable-temperature (VT) 2H solid-state nuclear magnetic resonance (SSNMR).

Subcomponent self-assembly relies on cation coordination whereas the roles of anions, often only emerging during self-assembly, are growing. When sites for anions are instead pre-programmed, they have the potential to...

No abstract

The rigid molecular scaffold of the tribenzotriquinacenes (TBTQs) has emerged as a versatile structural platform that possesses unique geometrical features and allows for an orthogonal arrangement of organic functional substituents or convex-concave interactions. In this review, we summarize and discuss important synthetic strategies for a regioselective functionalization at the four distinct positions of the TBTQ basic framework, namely, apical, bridgehead, ortho, and outer rim.1 Introduction2 Structure and Synthesis of TBTQs3 Bridgehead Functionalization4 Outer Rim Functionalization5 ortho-Functionalization6 Apical Functionalization7 Conclusion and Outlook