Dynamic Signaling Cascades: Reversible Covalent Reaction-Coupled Molecular Switches

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Single Dynamic Covalent Bond Tailored Responsive Molecular Junctions

Responsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break-junction technique (STM-BJ) and the eutectic Ga-In technique (EGaIn). We found that the single-DCB-tailored molecular devices exhibited acid-base and/or photo-thermal response with three well-defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single-molecule junctions and ≈3 for the SAMs-based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single-molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Toward Molecular Cybernetics - the Art of Communicating Chemical Systems

The emerging field of molecular cybernetics has the potential to widely broaden our perception of chemistry. Chemistry will develop beyond its current focus that is mainly concerned with single transformations, pure compounds, and/or defined mixtures. On this way, chemistry will become autonomous, networked and smart through communicating molecules each of which serves a control engineering purpose, like the set of wheels in the machinery of life. The present personal account describes our latest developments in this field.

Fluorodynamers Displaying Tunable Fluorescence on Constitutional Exchanges in Solution and at Solid Film-Solution Interface

Dynamic covalent polymers-dynamers-are adaptive materials that offer timely variant adaptive macroscopic organization across extended scales. In the current study, imine exchange reactions and fluorescence transfer can occur at the interfaces between various solutions and solid state dynameric films. The fluorescence quenching upon imine formations for designed fluorogen was successfully demonstrated, and this tunable fluorescence was further used to study the re-composition of a solid film. Moreover, the dynamic covalent films also exhibited responsiveness to competing amines and acid/base conditions, both in solutions and solid film-solution interface. This work can provide more insights into interface dynamic chemistry and holds great potential for further applications in optical and biomedical materials.

New Acylhydrazone Photoswitches with Quantitative Conversion and High Quantum Yield but without Hydrogen Bond Stabilizing ( Z)-Isomer

Hydrazones are recently attracting increasing interest because of their facile synthesis and high addressability, fatigue resistance, and modifiability as molecular switches. However, this new class of switches generally suffers from low conversion from E- to Z-configuration. Here, novel benzoylhydrazones were synthesized by condensation of 2-methoxynaphthaldhyde and benzoylhydrazine. In this hydrazone system, both sides of the imine double bond had large steric hindrance, so that the ( E)-isomer of the benzoylhydrazones was less stable and easily converted into the ( Z)-isomer even without an intramolecular hydrogen bond. Up to 99% conversion efficiency and 89% quantum yield were obtained, in addition to excellent addressability and high fatigue resistance. Outstandingly, the crystal structure of one ( Z)-isomer disclosed no intermolecular hydrogen bonds between the molecules of the ( Z)-isomer but strong and sequential hydrogen bonds between those of the ( E)-isomer. Therefore, the ( E)-isomer was less soluble in solvents than the ( Z)-isomer. This molecular switch system could be easily modified by both hydrophilic pentaethylene glycol chains and hydrophobic octyl chains. Under light irradiation, the resultant amphiphilic acylhydrazone could be transferred from ( E)-isomer to ( Z)-isomer in more than 90% yield even in water after light irradiation. Meanwhile, the self-assembled big nanospheres could rearrange into much smaller vesicles because of the solubility difference of ( Z)- and ( E)-isomers. After the anticancer drug procarbazine was loaded by this kind of acylhydrazone in water, it could be released by light irradiation, showing potential application in photocontrollable drug release.

A high-speed network of nanoswitches for on/off control of catalysis

Copper(i) ion translocation is the key for fast and reliable communication between networked devices in the catalytic machinery.

A Pyridoindole-Based Multifunctional Bioprobe: pH-Induced Fluorescence Switching and Specific Targeting of Lipid Droplets

A versatile fluorescent probe, PITE, based on alkyl-substituted pyridoindole (PI) and tetraphenylethylene (TE), which exhibits facile pH-induced fluorescence switching in solution, as nanoparticles, and in the solid state, is presented. Strong fluorescence in the solid state, as well as in solution and the aggregated state, allow sensing of toxic acid vapors. Fluorescence "off-on" switching of PITE through exposure to trifluoroacetic acid and triethylamine vapor is visualized by the naked eye. A unified picture of the switchable fluorescence of PITE is obtained by comprehensive spectroscopic investigations coupled with quantum mechanical calculations. Strong fluorescence, a large Stokes shift, high photostability, and biocompatibility of PITE make it a viable probe for subcellular imaging. Extensive fluorescence microscopic studies by employing organisms including lower and higher eukaryotes reveal specific localization of PITE to lipid droplets (LDs). LDs are dynamic subcellular organelles linked to various physiological processes and human diseases. Hence, the specific detection of LDs in diverse organisms is important to biomedical research and healthcare. Isolation of LDs and subsequent colocalization studies ascertain selective targeting of LDs by the easily affordable, lipophilic bioprobe, PITE. Thus, PITE is a promising multifunctional probe for chemosensing and the selective tracking of LDs.

Antiparallel Dynamic Covalent Chemistries

The ability to design reaction networks with high, but addressable complexity is a necessary prerequisite to make advanced functional chemical systems. Dynamic combinatorial chemistry has proven to be a useful tool in achieving complexity, however with some limitations in controlling it. Herein we introduce the concept of antiparallel chemistries, in which the same functional group can be channeled into one of two reversible chemistries depending on a controllable parameter. Such systems allow both for achieving complexity, by combinatorial chemistry, and addressing it, by switching from one chemistry to another by controlling an external parameter. In our design the two antiparallel chemistries are thiol-disulfide exchange and thio-Michael addition, sharing the thiol as the common building block. By means of oxidation and reduction the system can be reversibly switched from predominantly thio-Michael chemistry to predominantly disulfide chemistry, as well as to any intermediate state. Both chemistries operate in water, at room temperature, and at mildly basic pH, which makes them a suitable platform for further development of systems chemistry.

Real-time dark-field light scattering imaging to monitor the coupling reaction with gold nanorods as an optical probe

Gold nanorods (GNRs) have opened up promising applications based on their reshaping, due to the fact that a tiny change in shape or size could directly lead to optical changes. Herein, we report chemical reshaping of GNRs induced by the coupling reaction between Au, ferric chloride and thiourea. In the coupling reaction, Fe³⁺ oxidizes the GNRs to yield Au(i), which complexes with the thiourea ligand, lowering the Gibbs free energy of the gold species and promoting the reaction equilibrium to enable the chemical reshaping of the GNRs. This coupling reaction process was monitored using a light-scattering dark-field microscopy (DFM) imaging technique and scanning electron microscopy (SEM). The light scattering underwent a colour change from bright red to yellow and finally to green, and the GNRs underwent a morphological change from rod-shaped to fusiform and finally to spherical, which is somewhat different from the results of other chemical etching processes of GNRs. It is believed that the coupling reaction induced chemical reshaping of GNRs not only provides an alternative way to monitor the coupling reaction, but also offers a facile way to obtain a desirable GNR morphology, which is important for the preparation of fusiform nanostructures.

Hydrazone Switch-Based Negative Feedback Loop

A negative feedback loop that relies on the coordination-coupled deprotonation (CCD) of a hydrazone switch has been developed. Above a particular threshold of zinc(II), CCD releases enough protons to the environment to trigger a cascade of reactions that yield an imine. This imine sequesters the excess of zinc(II) from the hydrazone switch, hence lowering the effective amount of protons, and switching the cascade reactions "OFF", thus establishing the negative feedback loop.

Ion-mediated single-molecular optical switching and sensing based on the fluorophore-tethered calix[4]pyrrole

The first asymmetrically meso-aryl substituted “two-walled” calix[4]pyrrole is explored as a single-site sequential ‘on–off–on–off’ fluorescent switch with sub-nanomolar sensitivity.