Dissipative Formation of Covalent Basket Cages

Triply Adaptive Libraries of Dynamic Covalent Macrocycles: Switching between Sorted and Unsorted States

Dynamic noncovalent and covalent chemistries have enabled the constitutional modulation of chemical entities within chemical dynamic systems. The switching between order and disorder, i.e., self-sorted and unsorted states of constitutional dynamic libraries, remains challenging. Herein, we study the adaptive behaviors of a dynamic library of imine macrocycles generated from dialdehydes and diamines, seeking ways to exert control over sorting and unsorting processes. The distribution of constituents in the present library of dynamic macrocycles is modulated in response to internal and external effectors (e.g., time, metal cations, and chemical fuels), resulting in the transient amplification of self-sorted constituents in out-of-equilibrium states. The present study showcases higher complexity in systems subject to multiple adaptation through the dynamic interconversion between singularity/order and diversity/disorder.

Dynamic covalent synthesis

Dynamic covalent synthesis aims to precisely control the assembly of simple building blocks linked by reversible covalent bonds to generate a single, structurally complex, product. In recent years, considerable progress in the programmability of dynamic covalent systems has enabled easy access to a broad range of assemblies, including macrocycles, shape-persistent cages, unconventional foldamers and mechanically-interlocked species (catenanes, knots, etc.). The reversibility of the covalent linkages can be either switched off to yield stable, isolable products or activated by specific physico-chemical stimuli, allowing the assemblies to adapt and respond to environmental changes in a controlled manner. This activatable dynamic property makes dynamic covalent assemblies particularly attractive for the design of complex matter, smart chemical systems, out-of-equilibrium systems, and molecular devices.

Switched "On" Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site, and then dethreading), with the onset of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes.

Conformational control enables boroxine-to-boronate cage metamorphosis

The discovery of molecular organic cages (MOCs) is inhibited by the limited organic-chemical space of the building blocks designed to fulfill strict geometric requirements for efficient assembly. Using intramolecular attractive or repulsive non-covalent interactions to control the conformation of flexible systems can effectively augment the variety of building blocks, ultimately facilitating the exploration of new MOCs. In this study, we introduce a set of boronic acid tripods that were designed using rational design principles. Conformational control was induced by extending the tripod's arms by a 2,3-dimethylbenzene unit, leading to the efficient formation of a tetrapodal nanometer-sized boroxine cage. The new building block's versatility was demonstrated by performing cage metamorphosis upon adding an aromatic tetraol. This led to a quantitative boroxine-to-boronate transformation and a topological shift from tetrahedral to trigonal bipyramidal.

Crystalline Nanoparticles of Water-Soluble Covalent Basket Cages (CBCs) for Encapsulation of Anticancer Drugs

We herein describe the preparation, assembly, recognition characteristics, and biocompatibility of novel covalent basket cage CBC-11, composed of four molecular baskets linked to four trivalent aromatic amines through amide groups. The cage is tetrahedral in shape and similar in size to small proteins (Mw =8637 g/mol) with a spacious nonpolar interior for accommodating multiple guests. While 24 carboxylates at the outer surface of CBC-11 render it soluble in aqueous phosphate buffer (PBS) at pH=7.0, the amphiphilic nature prompts its assembly into nanoparticles (d=250 nm, DLS). Cryo-TEM examination of nanoparticles revealed their crystalline nature with wafer-like shapes and hexagonally arranged cages. Nanoparticulate CBC-11 traps anticancer drugs irinotecan and doxorubicin, with each cage binding up to four drug molecules in a non-cooperative manner. The inclusion complexation resulted in nanoparticles growing in size and precipitating. In media containing mammalian cells (HCT 116, human colon carcinoma), the IC50 value of CBC-11 was above 100 μM. While this work presents the first example of a large covalent organic cage operating in water at the physiological pH and forming crystalline nanoparticles, it also demonstrates its biocompatibility and potential to act as a polyvalent binder of drugs for their sequestration or delivery.

Fast and slow walking driven by chemical fuel

We demonstrate the fast forward and slow backward motion of a biped on a tetrahedral track using chemical fuel, cooperative binding and kinetic selectivity. Walking of the biped is based on its dibenzyl amine feet that bind to zinc porphyrin units and, upon protonation, to dibenzo 24-crown-8 sites affording pseudorotaxane linkages.

Dissipative Systems Driven by the Decarboxylation of Activated Carboxylic Acids

ConspectusThe achievement of artificial systems capable of being maintained in out-of-equilibrium states featuring functional properties is a main goal of current chemical research. Absorption of electromagnetic radiation or consumption of a chemical species (a "chemical fuel") are the two strategies typically employed to reach such out-of-equilibrium states, which have to persist as long as one of the above stimuli is present. For this reason such systems are often referred to as "dissipative systems". In the simplest scheme, the dissipative system is initially found in a resting, equilibrium state. The addition of a chemical fuel causes the system to shift to an out-of-equilibrium state. When the fuel is exhausted, the system reverts to the initial, equilibrium state. Thus, from a mechanistic standpoint, the dissipative system turns out to be a catalyst for the fuel consumption. It has to be noted that, although very simple, this scheme implies the chance to temporally control the dissipative system. In principle, modulating the nature and/or the amount of the chemical fuel added, one can have full control of the time spent by the system in the out-of-equilibrium state.In 2016, we found that 2-cyano-2-phenylpropanoic acid (1a), whose decarboxylation proceeds smoothly under mild basic conditions, could be used as a chemical fuel to drive the back and forth motion of a catenane-based molecular switch. The acid donates a proton to the catenane that passes from the neutral state A to the transient protonated state B. Decarboxylation of the resulting carboxylate (1acb), generates a carbanion, which, being a strong base, retakes the proton from the protonated catenane that, consequently, returns to the initial state A. The larger the amount of the added fuel, the longer the time spent by the catenane in the transient, out-of-equilibrium state. Since then, acid 1a and other activated carboxylic acids (ACAs) have been used to drive the operation of a large number of dissipative systems based on the acid-base reaction, from molecular machines to host-guest systems, from catalysts to smart materials, and so on. This Account illustrates such systems with the purpose to show the wide applicability of ACAs as chemical fuels. This generality is due to the simplicity of the idea underlying the operation principle of ACAs, which always translates into simple experimental requirements.

Rapid Access to Chiral and Tripodal Cavitands from β-Pinene

We report Pd-catalyzed cyclotrimerization of (+)-α-bromoenone, obtained from monoterpene β-pinene, into an enantiopure cyclotrimer. This C₃ symmetric compound has three bicyclo[3.1.1]heptane rings fused to its central benzene with each ring carrying a carbonyl group. The cyclotrimer undergoes diastereoselective threefold alkynylation with the lithium salts of five terminal alkynes (41-63 %, de=4-83 %). The addition enabled a rapid synthesis of a small library of novel chiral cavitands that, in shape, resemble a tripod stand. These molecular tripods include a tris-bicycloannelated benzene head attached to three alkyne legs twisted in one direction to form a nonpolar cavity with polar groups as feet. Tripods with methylpyridinium and methylisoquinolinium legs, respectively, form inclusion complexes with anti-inflammatory and chiral drugs (R)/(S)-ibuprofen and (R)/(S)-naproxen. The mode of binding shows drug molecules docked in the cavity of the host through ion-ion, cation-π, and C-H-π contacts that, in addition of desolvation, give rise to complexes having millimolar to micromolar stability in water. Our findings open the door to creating a myriad of enantiopure tripods with tunable functions that, in the future, might give novel chemosensors, catalysts or sequestering agents.

Construction of transient supramolecular polymers controlled by mass transfer in biphasic systems

Inspired by life assembly systems, the construction of transient assembly systems with spatiotemporal control is crucial for developing intelligent materials. A widely adopted strategy is to couple the self-assembly with chemical reaction networks. However, orchestrating the kinetics of multiple reactions and assembly/disassembly processes without crosstalk in homogeneous solutions is not an easy task. To address this challenge, we propose a generic strategy by separating components into different phases, therefore, the evolution process of the system could be easily regulated by controlling the transport of components through different phases. Interference of multiple components that are troublesome in homogeneous systems could be diminished. Meanwhile, limited experimental parameters are involved in tuning the mass transfer instead of the complex kinetic matching and harsh reaction selectivity requirements. As a proof of concept, a transient metallo-supramolecular polymer (MSP) with dynamic luminescent color was constructed in an oil-water biphasic system by controlling the diffusion of the deactivator (water molecules) from the water phase into the oil phase. The lifetime of transient MSP could be precisely regulated not only by the content of chemical fuel, but also factors that affect the efficiency of mass transfer in between phases, such as the volume of the water phase, the stirring rate, and the temperature. We believe this strategy can be further extended to multi-compartment systems with passive diffusion or active transport of components, towards life-like complex assembly systems.

Photoresponsive Dissipative Macrocycles Using Visible-Light-Switchable Azobenzenes

Visible light can be used to shift dynamic covalent imine assemblies out of equilibrium. We studied a fluorinated azobenzene building block that reliably undergoes geometric isomerism upon irradiation. The building block was used in combination with two different amines, ethylenediamine and R,R-1,2-diaminocyclohexane, to create a library of imine macrocycles. Whereas the simple amine can be used to access a polymeric state and a defined bowl-shaped macrocycle, the chiral amine gives access to a rich network of macrocycles that undergo both isomerisation as well as interconversion between different macrocyclic species, thereby allowing for control over the number of monomers involved in the cyclo-oligomerization; ¹ H- and ¹⁹ F-DOSY NMR, MALDI-MS measurements, and UV/Vis spectroscopy were used to study the processes.