Stimuli-Responsive Chiroptical Switching

Switchable supramolecular helices for asymmetric stereodivergent catalysis

Despite recent developments on the design of dynamic catalysts, none of them have been exploited for the in-situ control of multiple stereogenic centers in a single molecular scaffold. We report herein that it is possible to obtain in majority any amongst the four possible stereoisomers of an amino alcohol by means of a switchable asymmetric catalyst built on supramolecular helices. Hydrogen-bonded assemblies between a benzene-1,3,5-tricarboxamide (BTA) achiral phosphine ligand coordinated to copper and a chiral BTA comonomer are engaged in a copper-hydride catalyzed hydrosilylation and hydroamination cascade process. The nature of the product stereoisomer is related to the handedness of the helices and can thus be directed in a predictable way by changing the nature of the major enantiomer of the BTA comonomer present in the assemblies. The strategy allows all stereoisomers to be obtained one-pot with similar selectivities by conducting the cascade reaction in a concomitant manner, i.e. without inverting the handedness of the helices, or sequentially, i.e. by switching the handedness of the supramolecular helices between the hydrosilylation and hydroamination steps. Supramolecular helical catalysts appear as a unique and versatile platform to control the configuration of molecules or polymers embedding several stereogenic centers.

Photocontrolled chiroptical switch based on the self-assembly of azobenzene-bridged bis-tryptophan enantiomers

Chirality dynamic tuning plays fundamental roles in chemistry, material science and biological system. Herein, a pair of azobenzene-bridged bis-tryptophan enantiomers (Azo-di-d/l-Trp) were designed and synthesized via simple reactions. With the fuel of glucono-δ-lactone (GdL), releasing protons during its hydrolysis, the alkaline solution of Azo-di-d/l-Trp gradually self-assembled into contrast chiral helical structures and displayed magnitude and mirror image of circular dichroism (CD) signals. While the chiral helices converted to CD silent nanoparticles when the azobenzene moiety isomerized from trans- to cis-form under UV irradiation. More importantly, this chiroptical switch, displaying reversible interconversion between chiral amplification and silent, can be smartly controlled via photoirradiation at various wavelengths.

A Cyclotriveratrylene Solvent-Dependent Chiral Switch

Chiral molecular switches are attracting attention as they could pave the way to chiral molecular machines. Herein, we report on the design and synthesis of a single molecule chiral switch based on a cyclotriveratrylene scaffold, in which the chirality inversion is controlled by the solvent. Hemicryptophanes are built around a C3 cyclotriveratrylene chiral unit, with either M or P handedness, connected to another tripod and usually displaying an "out" configuration. Here, we demonstrate that solvents are able to control the "in" and "out" configurations of the CTV unit, creating a chiral molecular switch from (M/P)"in" to (P/M)"out" handedness. The full characterization of the "in" and "out" configurations and of the chirality switch were made possible by combining NMR, HPLC, ECD, DFT and molecular dynamics. Interestingly, bulky aromatic solvents such as 2-t-butylphenol favor the "in" configuration while polar aprotic solvents such as acetone favor the "out" configuration. This chiral switch was found to be fully reversible allowing the system to oscillate between two different M and P configurations several times upon the action of solvents stimuli.

Stimuli-Responsive Properties on a Bisbenzofuropyrazine Core: Mechanochromism and Concentration-Controlled Vapochromism

Stimulus-responsive organic materials with luminescence switching properties have attracted considerable attention for their practical applications in sensing, security, and display devices. In this paper, bent-type bisbenzofuropyrazine derivatives, Bent-H and Bent-sBu, with good solubilities were synthesized, and their physical and optical properties were investigated in detail. Bent-H gave three crystalline polymorphs, and they showed different luminescence properties depending on their crystal packing structures. In addition, Bent-H exhibited mechanochromic luminescence in spite of its rigid skeleton. Bent-sBu exhibited unique concentration-dependent vapochromic luminescence. Ground Bent-sBu was converted to blue-emissive, green-emissive, and green-emissive high-viscosity solution states at low, moderate, and high concentrations of CHCl3 vapor, respectively. This finding represents a concentration-dependent multi-phase transition with an organic solvent, which is of potent interest for application in sensing systems.

Enabling Stereochemical Communication and Stimuli-Responsive Chiroptical Properties in Biphenyl-Capped Cyclodextrins

Chiroptical materials are gaining increasing interest due to their innovative character and their applications in optoelectronics and data encryption technologies. Fully harnessing the potential of building blocks from the "chiral pool", such as native cyclodextrins (CDs), as they often lack chromophores suitable for the construction of materials with significant chiroptical properties. Here, we present the synthesis and characterization of a two-level molecular stack consisting of a point-chiral element (CD) and an axially chiral element (biphenyl), capable of effectively translating the overall stereochemical information contained in CDs into stimuli-responsive chiroptical properties. α- and β-permethylated CDs were efficiently capped with two different 2,2'-difunctionalized 1,1'-biphenyl units. In CD derivatives containing the rigid 2,2'-dihydroxy-1,1'-biphenyl cap, two intramolecular hydrogen bonds act synergistically as stereoselective actuators, enabling effective communication between the two levels and the transfer of nonchromophoric stereochemical information from the cyclic-oligosaccharide to the atropoisomeric cap. The chiroptical properties can be finely tuned by external stimuli such as temperature and solvent. The way chirality is transferred from the CD platform to the biphenyl cap was revealed thanks to crystallographic and computational analyses, together with electronic circular dichroism (ECD) studies.

Alignment and Dynamic Inversion of Planar Chirality in Pillar[n]arenes

Pillar[n]arenes are symmetrical macrocyclic compounds composed of benzene panels with para-methylene linkages. Each panel usually exhibits planar chirality and prefers chirality-aligned states. Because of this feature, pillar[n]arenes are attractive scaffolds for chiroptical materials that are easy to prepare and optically resolve and show intense circular dichroism (CD) signals. In addition, rotation of the panels endows the chirality of pillar[n]arenes with a dynamic nature. The chirality in tubular oligomers and supramolecular assemblies sometimes show time- and procedure-dependent alignment phenomena. Furthermore, the CD signals of some pillar[n]arenes respond to the addition of chiral guests when their dynamic chirality is coupled with host-guest properties. By using diastereomeric pillar[n]arenes with additional chiral structures, the response can also be caused by achiral guests and changes of the environment, providing molecular sensors.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies g_abs of up to 2.6 × 10^-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Thermal Annealing Triggered Chirality Inversion through Solvent Migration

Solvent strategy is a powerful tool to manipulate chirality and self-assembly over hierarchical levels, yet the solvent dynamics during thermal annealing in controlling chirality and chiroptical features remain a mystery. Here, we show how solvent migration affects molecular folding and chirality through thermal annealing. Pyrene segments were conjugated to a 2,6-diamide pyridine skeleton, where intramolecular hydrogen bonds anchor the chiral geometry. The orientation of pyrene blades adopted π···π and CH···π stacking, respectively, in organic solvents (dimethyl sulfoxide, DMSO) and aqueous media, leading to the chiroptical inversion. Thermal annealing treatment of the DMSO/H₂O mixture homogenized distribution of solvents that further altered the molecular folding from CH···π to π···π modality. Solvent migration from aggregates to bulky phases was evidenced by the nuclear magnetic resonance and molecular dynamic simulations, leading to the rearrangement of molecular packing with luminescent changes. It realized a consecutive chiroptical inversion using solvent strategy and thermal annealing.

Hydrogen Bonded Foldamers with Axial Chirality: Chiroptical Properties and Applications

Folding phenomenon refers to the formation of a specific conformation widely featured by the intramolecular interactions, which broadly exist in biomacromolecules, and are closely related to their structures and functions. A variety of oligomeric folded molecules have been designed and synthesized, namely "foldamer", exhibiting potentials in pharmaceutical and catalysis. Molecular folding is a promising strategy to transfer chirality from substituents to the whole skeleton, when chirality transfer, amplification, evolution, and other behaviors could be achieved. Investigating chirality using foldamer model deepens the understanding of the structure-function correlation in biomacromolecules and expands the molecular toolbox towards chiroptical and asymmetrical chemistry. Substitutes with abundant hydrogen bonding sites conjugated to a rotatable aryl group afford a parallel β-sheet-like conformation, which enables the emergence and manipulation of axial chirality. This concept aims to give a brief introduction and summary of the hydrogen bonded foldamers with anchored axial chirality, by taking some recent cases as examples. Design principles, control over axial chirality and applications are also reviewed.

Indane Based Molecular Motors: UV-Switching Increases Number of Isomers

We describe azophenylindane based molecular motors (aphin-switches) which have two different rotamers of trans-configuration and four different rotamers of cis-configuration. The behaviors of these motors were investigated both experimentally and computationally. The conversion of aphin-switch does not yield single isomer but a mixture of these. Although the trans to cis conversion leads to the increase of the system entropy some of the cis-rotamers can directly convert to each other while others should convert via trans-configuration. The motion of aphin-switches resembles the work of a mixing machine with indane group serving as a base and phenol group serving as a beater. The aphin-switches presented herein may provide a basis for promising applications in advanced biological systems or particularly in cases where on demand disordering of molecular packing has value, such as lipid bilayers.

Recent Advances in Fuel-Driven Molecular Switches and Machines

The molecular switches and machines arena has entered a new phase in which molecular machines operate under out-of-equilibrium conditions using appropriate fuel. Unlike the equilibrium version, the dissipative off-equilibrium machines necessitate only one stimulus input to complete each cycle and decrease chemical waste. Such a modus operandi would set significant steps towards mimicking the natural machines and may offer a platform for advancing new applications by providing temporal control. This review summarises the recent progress and blueprint of autonomous fuel-driven off-equilibrium molecular switches and machines.