Cooperativity in Photofoldamer Chloride Double Helices Turned On with Sequences and Solvents, Around with Guests, and Off with Light

Photofoldamers are sequence-defined receptors capable of switching guest binding on and off. When two foldamer strands wrap around the guest into 2:1 double helical complexes, cooperativity emerges, and with it comes the possibility to switch cooperativity with light and other stimuli. We use lessons from nonswitchable sequence isomers of aryl-triazole foldamers to guide how to vary the sequence location of azobenzenes from the end (F_END) to the interior (F_IN) and report their impact on the cooperative formation of 2:1 complexes with Cl^-. This sequence change produces a 125-fold increase from anti-cooperative (α = 0.008) for F_END to non-cooperative with F_IN (α = 1.0). Density functional theory (DFT) studies show greater H-bonding and a more relaxed double helix for F_IN. The solvent and guest complement the synthetic designs. Use of acetonitrile to enhance solvophobicity further enhances cooperativity in F_IN (α = 126) but lowers the difference in cooperativity between sequences. Surprisingly, the impact of the sequence on cooperativity is inverted when the guest size is increased from Cl^- (3.4 Å) to BF₄^- (4.1 Å). While photoconversion of interior azobenzenes was poor, the cis-cis isomer forms 1:1 complexes around chloride consistent with switching cooperativity. The effect of the guest, solvent, and light on the double-helix cooperativity depends on the sequence.

Multiple Azoarenes Based Systems - Photoswitching, Supramolecular Chemistry and Application Prospects

In the recent decades, the investigations on photoresponsive molecular systems with multiple azoarenes are quite popular in diverse perspectives ranging from fundamental understanding of multiple photoswitches, supramolecular chemistry, and various application prospects. In fact, several insightful and conceptual designs of such systems were investigated with architectural distinctions. In particular, the demonstration of applications such as data storage with the help of multistate or orthogonal photoswitches, light modulation of catalysis via cooperative switching, sensors using supramolecular host-guest interactions, and materials such as liquid crystals, grating, actuators, etc. are some of the milestones in this area. Herein, we cover the recent advancements in the research areas of multiazoarenes containing systems that have been classified into Type-1 {linear, non-linear, and core-based (A)}, Type-2 {tripodal C₃ -symmetric (C3)} and Type-3 {macrocyclic (M)} structural motifs.

Non-uniform Photoinduced Unfolding of Supramolecular Polymers Leading to Topological Block Nanofibers

Synthesis of one-dimensional nanofibers with distinct topological (higher-order structural) domains in the same main chain is one of the challenging topics in modern supramolecular polymer chemistry. Non-uniform structural transformation of supramolecular polymer chains by external stimuli may enable preparation of such nanofibers. To demonstrate feasibility of this post-polymerization strategy, we prepared a photoresponsive helically folded supramolecular polymers from a barbiturate monomer containing an azobenzene-embedded rigid π-conjugated scaffold. In contrast to previous helically folded supramolecular polymers composed of a more flexible azobenzene monomer, UV-light induced unfolding of the newly prepared helically folded supramolecular polymers occurred nonuniformly, affording topological block copolymers consisting of folded and unfolded domains. The formation of such blocky copolymers indicates that the photoinduced unfolding of the helically folded structures initiates from relatively flexible parts such as termini or defects. Spontaneous refolding of the unfolded domains was observed after visible-light irradiation followed by aging to restore fully folded structures.

Polarity-Tolerant Chloride Binding in Foldamer Capsules by Programmed Solvent-Exclusion

Persistent anion binding in a wide range of solution environments is a key challenge that continues to motivate and demand new strategies in synthetic receptor design. Though strong binding in low-polarity solvents has become routine, our ability to maintain high affinities in high-polarity solvents has not yet reached the standard set by nature. Anions are bound and transported regularly in aqueous environments by proteins that use secondary and tertiary structure to isolate anion binding sites from water. Inspired by this principle of solvent exclusion, we created a sequence-defined foldameric capsule whose global minimum conformation displays a helical folded state and is preorganized for 1:1 anion complexation. The high stability of the folded geometry and its ability to exclude solvent were supported by solid-state and solution phase studies. This capsule then withstood a 4-fold increase in solvent dielectric constant (ε_r) from dichloromethane (9) to acetonitrile (36) while maintaining a high and solvent-independent affinity of 10⁵ M^-1; ΔG ∼ 28 kJ mol^-1. This behavior is unusual. More typical of solvent-dependent behavior, Cl^- affinities were seen to plummet in control compounds, such as aryl-triazole macrocycles and pentads, with their solvent-exposed binding cavities susceptible to dielectric screening. Finally, dimethyl sulfoxide denatures the foldamer by putative solvent binding, which then lowers the foldamer's Cl^- affinity to normal levels. The design of this capsule demonstrates a new prototype for the development of potent receptors that can operate in polar solvents and has the potential to help manage hydrophilic anions present in the hydrosphere and biosphere.

Extended enantiopure ortho-phenylene ethylene (o-OPE)-based helical systems as scaffolds for supramolecular architectures: a study of chiroptical response and its connection to the CISS effect

Versatile enantiopure helical systems are described and are of interest owing to their intense chiroptical responses, their attractive architecture for metallosupramolecular chemistry and CISS effect.

The Many Ways to Assemble Nanoparticles Using Light

The ability to reversibly assemble nanoparticles using light is both fundamentally interesting and important for applications ranging from reversible data storage to controlled drug delivery. Here, the diverse approaches that have so far been developed to control the self-assembly of nanoparticles using light are reviewed and compared. These approaches include functionalizing nanoparticles with monolayers of photoresponsive molecules, placing them in photoresponsive media capable of reversibly protonating the particles under light, and decorating plasmonic nanoparticles with thermoresponsive polymers, to name just a few. The applicability of these methods to larger, micrometer-sized particles is also discussed. Finally, several perspectives on further developments in the field are offered.

An MS-CASPT2 study of the photodecomposition of 4-methoxyphenyl azide: role of internal conversion and intersystem crossing

Photoexcitation of 4-methoxyphenyl azide at 266 nm yields triplet nitrene after decaying through an intersystem crossing (ISC1, 2¹A′/2³A′′) in a first step followed by internal conversion (CI2, 2³A′′/1³A′′).

Synergistic Assembly of Covalent and Supramolecular Polymers

Integrating irreplaceable features of both covalent chemistry and noncovalent interactions into a single entity to maximize the applicability is highly desired. Here, a discovery of this type of hybrid, developed by Stupp and co-workers, is developed, where a synergistic combination of covalent and noncovalent compartments enables them to assemble by each other perfectively. The covalent compartments can grow into polymer chains assisted by a supramolecular compartment. The supramolecular compartments can be reversibly removed and re-formed to reconstitute the hybrid structure. The obtained soft materials can serve as functional platforms for molecular delivery or self-repairing materials.

Remote control over folding by light

Integrating photoswitchable unit into helical macromolecules allows their conformations to be externally controlled by light and therefore provides a versatile strategy to design photoresponsive materials.

Attraction or Repulsion? London Dispersion Forces Control Azobenzene Switches

Large substituents are commonly seen as entirely repulsive through steric hindrance. Such groups have additional attractive effects arising from weak London dispersion forces between the neutral atoms. Steric interactions are recognized to have a strong influence on isomerization processes, such as in azobenzene-based molecular switches. Textbooks indicate that steric hindrance destabilizes the Z isomers. Herein, we demonstrate that increasing the bulkiness of electronically equal substituents in the meta-position decreases the thermal reaction rates from the Z to the E isomers. DFT computations revealed that attractive dispersion forces essentially lower the energy of the Z isomers.

A photoswitchable bis-azo derivative with a high temporal resolution

The novel photoswitchable bis-azo derivative reported herein shows a high temporal resolution of 2 × 10(8) times between the thermal relaxation rates of its two constituting photochromes. Moreover, the slow and fast azo building blocks of this molecular construct can be triggered by using UV and visible light, respectively.

Imidazole-peptide foldamers: parabolic dependence of the folding process on the water content of the solvent

Oligomers of azole peptides have been isolated from a multitude of marine organisms. Up to now, these azole-containing dipeptide-analogue oligomers have only been found as cyclic n-mers (mostly tri- and tetramers) in nature. Herein, we show that imidazole-containing pseudopeptides form helixlike secondary structures in different solvents. The screw sense of the helix can be determined by attaching a single chiral imidazole unit to the N terminus of the oligomer. Investigation by means of circular dichroism (CD) spectroscopy showed that the folding process of the helix depends on the water content of the solvent in a parabolic way. In a pure organic medium, the helix is stabilized by hydrogen bonds between the hydrogen atoms of the amide groups and the nitrogen atoms of the azole ring. In aqueous solution, the formation of the helix is driven by dispersion interactions. The formation of the helix is more pronounced in aqueous solution than in organic solvents.

Donor–acceptor interaction-driven folding of linear naphthalene–glycol oligomers templated by a rigid bipyridinium rod

The construction of folded and helical supramolecular structures through the self-assembly of a series of flexible linear oligomers induced by a rigid rod-like template has been demonstrated.

Reversible photoswitching of RNA hybridization at room temperature with an azobenzene C-nucleoside

Photoregulation of RNA remains a challenging task as the introduction of a photoswitch entails changes in the shape and the stability of the duplex that strongly depend on the chosen linker strategy. Herein, the influence of a novel nucleosidic linker moiety on the photoregulation efficiency of azobenzene is investigated. To this purpose, two azobenzene C-nucleosides were stereoselectively synthesized, characterized, and incorporated into RNA oligonucleotides. Spectroscopic characterization revealed a reversible and fast switching process, even at 20 °C, and a high thermal stability of the respective cis isomers. The photoregulation efficiency of RNA duplexes upon trans-to-cis isomerization was investigated by using melting point studies and compared with the known D-threoninol-based azobenzene system, revealing a photoswitching amplitude of the new residues exceeding 90 % even at room temperature. Structural changes in the duplexes upon photoisomerization were investigated by using MM/MD calculations. The excellent photoswitching performance at room temperature and the high thermal stability make these new azobenzene residues promising candidates for in-vivo and nanoarchitecture photoregulation applications of RNA.

Selective uncaging of DNA through reaction rate selectivity

The synthesis and use of the new nucleobase-caged nucleotides dT(pHP) and dT(NDEACM) is reported. Through a combination of time and wavelength selectivity four levels of selective uncaging with only two cages, and only two wavelengths, were obtained. The new residue dT(pHP) can be uncaged at 313 nm without the formation of unwanted cyclic pyridine dimers.