Excitation wavelength-dependent multi-coloured and white-light emissive pyrene-based hydrazones: suppression of Kasha's rule.

Multi-coloured and white-light emissions from pyrene-based hydrazones are described. They exhibit excitation wavelength-dependent emissions in solution due to the suppression of Kasha's rule. Interestingly, in dimethylformamide, 1-3 emit light that covers all the regions of primary colours as a function of excitation wavelength, and 1 and 2 emit white light (λex = 420 nm) in isopropanol.

Metal and Proton Relay-Controlled Hierarchical Multistep Switching Cascade.

Transition metals play an important role in many biological processes including cellular regulation and signal transduction. Emulating such processes on the molecular level, while challenging, can help us learn how to manipulate intermolecular communication, an important requirement for the development of solution-based molecular machines. In this work, we demonstrate a transition metal-based artificial multistep switching cascade that exhibits intrinsic hierarchical level control. The process starts with Zn(II), which initiates a transition metal relay by displacing a macrocycle-encapsulated Pd(II). The latter then binds to a hydrazone switch leading to coordination-coupled deprotonation (CCD). Finally, the proton generated through CCD activates the E/Z isomerization of a second noncoordinating pH-sensitive hydrazone switch. This whole multistep process can be reset to the original state by removing the Pd(II) from the system.

Carbohydrate recognition using metal-ligand assemblies.

Carbohydrate-binding proteins, known as lectins, play a wide range of vital roles in cellular and pathological processes. Mimicking lectins to achieve specific molecular recognition of carbohydrates in organic and aqueous media using artificial receptors is challenging due to the synthetic hurdles of receptors and structural similarities between sugars. Carbohydrate recognition using non-covalent interactions remains a vast topic. This review summarises the recognition of carbohydrates using metal-ligand assemblies, including metallosupramolecules, macrocycles, and cages. It also highlights the challenges and future directions in the field.

Hydrazone Photoswitches for Structural Modulation of Short Peptides.

Molecules that undergo light-driven structural transformations constitute the core components in photoswitchable molecular systems and materials. Among various families of photoswitches, photochromic hydrazones have recently emerged as a novel class of photoswitches with superb properties, such as high photochemical conversion, spectral tunability, thermal stability, and fatigue resistance. Hydrazone photoswitches have been adopted in various adaptive materials at different length scales, however, their utilization for modulating biomolecules still has not been explored. Herein, we present new hydrazone switches that can photomodulate the structures of short peptides. Systematic investigation on a set of hydrazone derivatives revealed that installation of the amide group does not significantly alter the photoswitching behaviors. Importantly, a hydrazone switch comprising an upper phenyl ring and a lower quinolinyl ring was effective for structural control of peptides. We anticipate that this work, as a new milestone in the research of hydrazone switches, will open a new avenue for structural and functional control of biomolecules.

Light-mediated chiroptical switching of an achiral foldamer host in presence of a carbohydrate guest.

A photoresponsive diarylethene was incorporated in an achiral helical foldamer container. A carbohydrate guest was found to induce opposite handedness upon binding to the open and closed forms of the diarylethene-containing foldamer, thus enabling chiroptical switching of an achiral host mediated by a chiral guest.

Three-state switching in a double-pole change-over nanoswitch controlled by redox-dependent self-sorting.

The four-arm nanomechanical switch 1 with four different terminals exhibits two switching arms (contacts A and D) and two distinct stations for binding (contacts B and C). In switching State I, the azaterpyridine arm is intramolecularly coordinated to a zinc(ii) porphyrin station (connection A ↔ B) while contact D (a ferrocenylbipyridine unit) and contact C (phenanthroline) remain disconnected. After addition of copper(i) ions (State II) both connections A ↔ B and C ↔ D are established. Upon one-electron oxidation, double-pole change-over switching cleaves both connections A ↔ B & C ↔ D and establishes the new connection A ↔ C (State III). Fully reversible three-state switching (State I → State II → State III → State II → State I) was achieved by adding appropriate chemical and redox stimuli.

Catalytically active nanorotor reversibly self-assembled by chemical signaling within an eight-component network.

A catalytically active three-component nanorotor is reversibly self-assembled and disassembled by remote control. When zinc(ii) ions (2 equiv.) are added as an external chemical trigger to the mixture of transmitter [Cu(1)]+ and pre-rotor assembly [(S)·(R)], two equiv. of copper(i) ions translocate from [Cu(1)]+ to the two phenanthroline sites of [(S)·(R)]. As a result, [Zn(1)]2+ forms along with the three-component assembly [Cu2(S)(R)]2+, which is both a nanorotor (k298 = 46 kHz, ΔH‡ = 49.1 ± 0.4 kJ mol-1, ΔS‡ = 9.5 ± 1.7 J mol-1 K-1) and a catalyst for click reactions (catalysis ON: A + B→AB). Removal of zinc from the mixture reverts the translocation sequence and thus commands disassembly of the catalytically active rotor (catalysis OFF). The ON/OFF catalytic cycle was run twice in situ in the full network.

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

NetState I of the communication-catalysis protocol is defined by a 1 : 1 mixture of the nanoswitches [Cu(1)]+ and 2. Upon one-electron oxidation at the ferrocenyl unit of the switch [Cu(1)]+, copper(i) ions are released that after translocation toggle nanoswitch 2 → [Cu(2)]+ (NetState II) within 4 min. NetState I was fully reset within 1 min by reduction of 1+ → 1. Running this redox-triggered switching protocol in the presence of 4-nitrobenzaldehyde, diethyl malonate and piperidine (catalyst) allows toggling of a catalyzed Knoevenagel addition from ON to OFF and back to ON.

Photochromic Hydrazone Switches with Extremely Long Thermal Half-Lives.

A family of easily accessible light-activated hydrazone switches has been developed having thermal half-lives of up to 2700 years! Structure-property analysis shows that replacing the rotor pyridyl group of our typical hydrazone switch with a phenyl one leads to the long-lived negative photochromic compounds. The switching properties of the hydrazones in both toluene and DMSO were assessed offering insights into the kinetics and thermodynamics of the switching process.

Networking Nanoswitches for ON/OFF Control of Catalysis.

The nanoswitches 1 and 2 are interdependently linked in so-called network states (NetStates). In NetState I, defined by presence of [Cu(1)]+ and 2, the organocatalyst N-methylpyrrolidine catalyzes a conjugate addition. Addition of iron(II) ions as an external chemical trigger to NetState I discharges Cu+ from [Cu(1)]+. The liberated copper(I) ion acts as a second messenger and changes the toggling state at nanoswitch 2. The resulting nanoswitch [Cu(2)]+ captures the catalytically active species from solution and the conjugate addition is turned OFF. Removal of the original trigger reverses the sequence and turns catalysis ON. The ON/OFF catalytic cycle was run three times in situ.

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.

Redox-dependent self-sorting toggles a rotary nanoswitch.

The pyridine-pyrimidine (py-pym) arm as the moving part of the two-state nanomechanical rotary switch [Cu(1)](+) is toggled reversibly between two stations using one-electron oxidation/reduction. In state I, the arm is attached via Cu(+) complexation to a sterically encumbered phenanthroline and in state II to a zinc porphyrin station. Toggling is realised by charging and discharging an external input signal, the ferrocene-appended diimine ligand 2. Addition of 2 leads to formation of the intermolecular complex [Cu(1)(2)](+) paralleled by a move of the py-pym arm to the zinc porphyrin station. Upon oxidation at the ferrocenyl unit, 2(+) detaches from [Cu(1)(2)](2+) so that [Cu(1)](+) is formed in state I. Switching was ascertained by NMR, UV-Vis spectroscopy and cyclic voltammetry.

A toggle nanoswitch alternately controlling two catalytic reactions.

Reversible switching between two states of the triangular nanoswitch [Cu(1)](+) was accomplished by alternate addition of 2-ferrocenyl-1,10-phenanthroline (2) and copper(I) ions. The two switching states regulate the binding and release of two distinct catalysts, piperidine and [Cu(2)](+), in a fully interference-free manner and allow alternating on/off switching of two orthogonal catalytic processes. In switching state I, piperidine is released from the nanoswitch and catalyzes a Knoevenagel addition between 4-nitrobenzaldehyde and diethyl malonate (ON-1 and OFF-2), while in state II the released [Cu(2)](+) catalyzes a click reaction between 4-nitrophenylacetylene and benzylazide (OFF-1 and ON-2). Upon addition of one equivalent of 2 to the (OFF-1 and ON-2)-state, both catalytically active processes are shut down (OFF-1 and OFF-2).

A trio of nanoswitches in redox-potential controlled communication.

A potential-controlled two-step bidirectional communication protocol between the nanoswitches [Cu(1)](+), 2 and 3 is set up, in which ligand followed by metal-ion oxidation drives two subsequent metal ion translocations (self-sorting) changing the switching state at each switch. The communication is reset to its starting point by a two-electron reduction.

A monomer-dimer nanoswitch that mimics the working principle of the SARS-CoV 3CLpro enzyme controls copper-catalysed cyclopropanation.

A triangular framework with a terpyridine and shielded phenanthroline at its termini constitutes an open/close nanoswitch that is toggled by chemical inputs. In the presence of copper(I) ions, the open triangular framework (OPEN-I) firmly closes to a catalytically inactive heteroleptic [Cu(phen)(terpy)](+) complex (CLOSE). Reversible switching between CLOSE and OPEN-I states was demonstrated by successive addition and removal of Cu(+). In contrast, after addition of iron(II) ions to the CLOSE state a bishomoleptic dimeric [Fe(terpy)2](2+) complex is formed with the copper(I) ions placed in the phenanthroline cavities (OPEN-II). Due to its coordinatively unsaturated [Cu(Phen)](+) sites the dimeric iron complex is able to serve as a catalyst in the cyclopropanation of Z-cyclooctene using ethyl diazoacetate.

Bidirectional chemical communication between nanomechanical switches.

The interplay of biological machines depends critically on the bidirectionality of chemical information exchange. The implementation of such a communication procedure for abiological systems is achieved using two nanoswitches that both operate as transmitters and receivers by transfering copper ions in oxidation states +I and +II. Even at micromolar concentrations, communication in both directions is remarkably fast, occurring at t1/2 =2-3 min. Metal ion translocation triggers a 20 Šrelocation of the toggle at both nanoswitches, entailing major geometric and electronic changes.

Dual coordination in ditopic azabipyridines and azaterpyridines as a key for reversible switching.

Dual coordination at two distinct nitrogen binding sites in various ditopic azabipyridines and azaterpyridines was evaluated by preparing 3- and 4-component complexes. The 4-aza-2,2'-bipyridine unit was implemented into a nanoswitch and used for reversible switching between open and closed forms.

Orthogonality in discrete self-assembly--survey of current concepts.

Over years, mathematicians, biologists and chemists have capitalised on the highly useful concept of orthogonality for developing sophisticated complex systems. The use of orthogonal pairs ensures that any modification made on one pair does not propagate any effect onto the other. While the concept equally pertains to dynamic supramolecular interactions, interference-free self-assembly built on multiple orthogonal interactions is still limited and the underlying notions are not yet firmly established. Herein, we identify, classify and evaluate dynamic interactions in various orthogonal settings in order to distill out general recommendations for reliable dynamic orthogonality. Our classification has to exclude templating, allosteric and/or cooperative effects as the latter are specific for individual cases only.

A reversible nanoswitch as an ON-OFF photocatalyst.

The two states of a new nanomechanical switch were quantitatively and reversibly populated in several subsequent switching cycles using either Cu(+) or cyclam as chemical inputs. State II was demonstrated to cis-trans isomerise diazastilbene upon irradiation selectively in the presence of stilbene, and even to operate as a photocatalyst.

Spontaneous and catalytic fusion of supramolecules.

Using principles of completive and integrative self-sorting, a clean supramolecule-to-supramolecule transformation is realised that involves fusion of a 3-component rectangle and a 2-component equilateral triangle into a 5-component scalene triangle. While the spontaneous process takes 15 h at 25 °C, the catalytic transformation is completed within 1 h.