Supramolecular Antibiotic Switches: A Potential Strategy for Combating Drug Resistance

Bacterial infectious disease is a serious public health concern throughout the world. Pathogen drug resistance, arising from both rational use and abuse/misuse of germicides, complicates the situation. Aside from developing novel antibiotics and antimicrobial agents, molecular approaches have become another significant method to overcome the problem of pathogen drug resistance. Established supramolecular systems, the antibiotic properties of which can be switched “on” and “off” through host–guest interactions, show great potential in combating issues regarding antibiotic resistance in the long term, as indicated by several recent studies. In this Concept, recently developed strategies for antibacterial regulation are summarized and further directions for research into antibiotic switches are proposed.     

Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2‐substituted dibenzofulvene molecular actuators based on (2,2,2‐triphenylethylidene)fluorene (TEF) are reported. The 2‐substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X‐ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2‐substituents on steric gearing. The addition–elimination reaction of Z‐ 9 with trityl anion to form Z‐ 10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266–355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near‐UV, making it a particularly promising drive rotor moiety for light‐powered molecular devices.     

Dynamic Responsive Systems for Catalytic Function

Responsive systems have recently gained much interest in the scientific community in attempts to mimic dynamic functions in biological systems. One of the fascinating potential applications of responsive systems lies in catalysis. Inspired by nature, novel responsive catalytic systems have been built that show analogy with allosteric regulation of enzymes. The design of responsive catalytic systems allows control of catalytic activity and selectivity. In this Review, advances in the field over the last four decades are discussed and a comparison is made amongst the dynamic responsive systems based on the principles underlying their catalytic mechanisms. The catalyst systems are sorted according to the triggers used to achieve control of the catalytic activity and the distinct catalytic reactions illustrated.     

A Photoswitchable Fluorophore for the Real‐Time Monitoring of Dynamic Events in Living Organisms

This study reports the synthesis of a photoactivatable fluorophore with optimal photochemical and photophysical properties for the real‐time tracking of motion in vivo. The photoactivation mechanism designed into this particular compound permits the conversion of an emissive reactant into an emissive product with resolved fluorescence, under mild illumination conditions that are impossible to replicate with conventional switching schemes based on bleaching. Indeed, the supramolecular delivery of these photoswitchable probes into the cellular blastoderm of Drosophila melanogaster embryos allows the real‐time visualization of translocating molecules with no detrimental effects on the developing organisms. Thus, this innovative mechanism for fluorescence photoactivation can evolve into a general chemical tool to monitor dynamic processes in living biological specimens.     

A pH‐Dependent,Mechanically Interlocked Switch: Organometallic [2]Rotaxane vs. Organic [3]Rotaxane

We present the first [2]rotaxane featuring a functional organometallic host. In contrast to the known organic scaffolds, this assembly shows a high post‐synthetic modifiability. The reactivity of the Ag₈ pillarplex host is fully retained, as is exemplified by the first transmetalation in a rotaxane framework to provide the respective Au₈ analogue. Additionally, a transformation under acidic conditions to give a purely organic [3]rotaxane is demonstrated which is reversible upon addition of a suitable base, rendering the assembly a pH‐dependent switch. Hereby, it is shown that the mechanically interlocked nature of the system enhances the kinetic stability of the NHC host complex by a factor of >1000 and corresponds to the first observation of a stabilizing “rotaxand effect”.     

Tautomerization‐Mediated Molecular Switching Between Six‐ and Seven‐Membered Rings Stabilized by Hydrogen Bonding

1,3,4,6‐Tetraketones typically undergo keto–enol tautomerism forming bis‐enols stabilized by intramolecular hydrogen bonding in two six‐membered rings. However, 1,3,4,6‐tetraketones derived from the terpene ketone camphor and norcamphor exist as isomers with two distinguishable modes of intramolecular hydrogen bonding, namely, the formation of six‐ or seven‐membered rings. The structural requirements for this so far unknown behavior were investigated in detail by synthesis and comparison of structural analogues. Both isomers of such 1,3,4,6‐tetraketones were fully characterized in solution and in the solid state. Intriguingly, they slowly interconvert in solution by means of tautomerism–rotation cascades, as was corroborated by DFT calculations. The influence of temperature and complexation with the transition metals Pd, Rh, and Ir on the interconversion process was investigated.     

Spatially Close Azo Dyes with Sub‐Nanosecond Switching Speeds and Exceptional Temporal Resolution

Photoswitchable bis‐azo dyes with an outstanding temporal resolution of 10¹⁵ times between the thermal relaxation rates of their two constituting photochromes are reported. Remarkably, the close spatial proximity of both azo photochromes in these molecular assemblies translates in an unprecedented 10³‐fold acceleration of the thermal isomerization rate of their faster azo unit compared to the one displayed by the isolated counterpart. Indeed, the relaxation time of the fast‐isomerizing platform of the herein reported bis‐azobenzenes is as low as 200 ps under ambient conditions. In the wake of these results, the bis‐azo dyes described herein are invaluable chromophores for the design of multifunctional light‐addressable materials in which simultaneous switching in two very different timescales might be essential.     

Electro-optical investigations and effect of asymmetry in bent-core liquid crystals

We report the synthesis and mesomorphic properties of a homologous series (10a–10g) of bent-core molecules constructed through covalent linkage of structurally non-symmetrical rod-like mesogens connected with a 1, 3-phenylene unit. The study of homologous series underlines the importance of length and nature of terminal chains. The homologues of shorter chains show a typical non-switchable rectangular columnar B₁ phase, while the switchable lamellar (B₂) phase is induced on moving to higher homologues. X-ray diffraction patterns indicate the presence of B₁ and B₂ mesophases. Polarised optical microscopy investigations under electric field in the B₂ phases revealed the existence of anticlinic antiferroelectric texture. The measured spontaneous polarisation value in one of the compounds is 936 nC cm^?2, a high polarisation value in bent-core liquid crystals.     

Control of the Helical Chirality of Enantiopure Sulfinyl (Z)‐Azobenzene‐Based Photoswitches

A new class of enantiopure ortho,ortho‐disubstituted azobenzene photoswitches has been synthesized from (S)‐2‐(p‐tolylsulfinyl)benzoquinone and arylhydrazines. The sulfoxide acts as a unidirectional controller of the helical chirality that arises in the Z isomer after photoisomerization. Highly congested E‐azobenzenes 5 c showed two atropisomeric diastereoconformers in the solid state that converged upon irradiation into a unique Z isomer with defined helicity (M), as evident in the X‐ray structure. The chiroptical properties of this three‐state enantiopure switch can be externally tuned both photochemically and/or thermally. Theoretical CD spectra calculated by using time‐dependent DFT methods support the existence of two atropoisomeric E isomers and only one Z isomer with (M) helicity. Complementary to the classical azobenzene‐based switches, the photoswiching event is promoted under green/blue light and do not occur under UV irradiation.