Exploring London Dispersion and Solvent Interactions at Alkyl-Alkyl Interfaces Using Azobenzene Switches

Advancing oil-water separation: Design and efficiency of amphiphilic hyperbranched demulsifiers

HYPOTHESIS

An innovative strategy for designing high-performance demulsifiers is proposed. It hypothesizes that integrating mesoscopic molecular simulations with macroscopic physicochemical experiments can enhance the understanding and effectiveness of demulsifiers. Specifically, it is suggested that amphiphilic hyperbranched polyethyleneimine (CHPEI) could act as an efficient demulsifier in oil-water systems, with its performance influenced by its adsorption behaviors at the oil-water interface and its ability to disrupt asphaltene-resin aggregates.

EXPERIMENTS

Several coarse-grained models of oil-water systems, with CHPEI, are constructed using dissipative particle dynamics (DPD) simulation. Following the insights gained from the simulations, a series of CHPEI-based demulsifiers are designed and synthesized. Demulsification experiments are conducted on both simulated and crude oil emulsions, with the process monitored using laser scanning confocal microscopy. Additionally, adsorption kinetics and small angle X-ray scattering are employed to reveal the inherent structural characteristics of CHPEI demulsifiers.

FINDINGS

CHPEI demonstrates over 96.7 % demulsification efficiency in high acid-alkali-salt systems and maintains its performance even after multiple reuse cycles. The simulations and macroscopic experiments collectively elucidate that the effectiveness of a demulsifier is largely dependent on its molecular weight and the balance of hydrophilic and hydrophobic groups. These factors are crucial in providing sufficient interfacial active functional groups while avoiding adsorption sites for other surfactants. Collaborative efforts between DPD simulation and macroscopic measurements deepen the understanding of how demulsifiers can improve oil-water separation efficiency in emulsion treatment.

Solvent Attenuation of London Dispersion in Polycyclic Aromatic Stacking

Solvent competition for London dispersion attenuates its energetic significance in molecular recognition processes. By varying both the stacked contact area and the solvent, here we experimentally deconvolute solvent attenuation using molecular balances. Experimental stacking energies (phenyl to pyrene) correlated strongly with calculations only when dispersion was considered. Such calculations favoured stacking by up to -27 kJ mol-1 in the gas phase, but it was weakly disfavoured in our solution-phase experiments (+0.5 to +4.6 kJ mol-1). Nonetheless, the propensity for stacking increased with contact area and in solvents with lower bulk polarisabilities that compete less for dispersion. Experimental stacking energies per unit change in solvent accessible area ranged from -0.02 kJ mol-1 Å-2 in CS2, to -0.05 kJ mol-1 Å-2 in CD2Cl2, but were dwarfed by the calculated gas-phase energy of -0.6 kJ mol-1 Å-2. The results underscore the challenge facing the exploitation of dispersion in solution. Solvent competition strongly but imperfectly cancels dispersion at molecular recognition interfaces, making the energetic benefits difficult to realise.

In-situ Oxidation and Coupling of Anilines towards Unsymmetric Azobenzenes Using Flow Chemistry

Molecular switches, especially azobenzenes, are used in numerous applications, such as molecular solar thermal storage (MOST) systems and photopharmacology. The Baeyer-Mills reaction of anilines and nitrosobenzenes has been established as an efficient synthetic method for non-symmetric azobenzenes. However, nitrosobenzenes are not stable, depending on their substitution pattern and pose a health risk. An in-situ oxidation of anilines with Oxone® was optimized under continuous flow conditions avoiding isolation and contact. The in-situ generated nitrosobenzene derivatives were subjected to a telescoped Baeyer-Mills reaction in flow. That way azobenzenes with a broad substituent spectrum were made accessible.

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Intramolecular London Dispersion Interactions in Single-Molecule Junctions

This work shows the first example of using intramolecular London dispersion interactions to control molecular geometry and quantum transport in single-molecule junctions. Flexible σ-bonded molecular junctions typically occupy straight-chain geometries due to steric effects. Here, we synthesize a series of thiomethyl-terminated oligo(dimethylsilmethylene)s that bear [CH2-Si(CH3)2]n repeat units, where all backbone dihedral states are sterically equivalent. Scanning tunneling microscopy break-junction (STM-BJ) measurements and theoretical calculations indicate that in the absence of a strong steric bias concerted intramolecular London dispersion interactions staple the carbosilane backbone into coiled conformations that remain intact even as the junction is stretched to its breakpoint. As these kinked conformations are highly resistive to electronic transport, we observe record-high conductance decay values on an experimental junction length basis (β = 1.86 ± 0.12 Å-1). These studies reveal the potential of using intramolecular London dispersion interactions to design single-molecule electronics.

Molecular Wind-Up Meter for the Quantification of London Dispersion Interactions

ConspectusThe experimental quantification of interactions on the molecular level provides the necessary basis for the design of functional materials and chemical processes. The interplay of multiple parameters and the small quantity of individual interactions pose a special challenge for such endeavors. The common method is the use of molecular balances, which can exist in two different states. Thereby, a stabilizing interaction can occur in one of the states, favoring its formation and thus affecting the thermodynamic equilibrium of the system. One challenge is determining the change in this equilibrium since various analytical methods could not be applied to fast-changing equilibria. A new and promising method for quantifying molecular interactions is the use of Molecular Wind-up Meters (MWM) in which the change in kinetics, rather than the effect on thermodynamics, is investigated. An MWM is transformed with an energy input (e.g. irradiation) into a metastable state. Then, the rate of thermal transformation back to the ground state is measured. The strength of interactions present in the metastable state controls the kinetics of the back reactions, allowing direct correlation. The advantage of this approach lies in the high sensitivity (energy differences can be larger by 1 order of magnitude) and, in general, allows the use of a broader range of solvents and analytical methods. An Azobenzene-based MWM has been established as a powerful tool to quantify London dispersion interactions. London dispersion (LD) represents the attractive part of the van der Waals potential. Although neglected in the past due to its weak character, it has been shown that the influence of LD on the structure, stability, and reactivity of matter can be decisive. Especially in larger molecules, its energy contribution increases overproportionately with the number of atoms, which has sparked increasing interest in the use of so-called dispersion energy donors (DED) as a new structural element. Application of the azobenzene-based MWM not only allowed the differentiation of bulkiness, but also systematically addressed the influence of the length of n-alkyl chains. Additionally, the solvent influence on LD was studied. Based on the azobenzene MWM, an increment system has been proposed, allowing a rough estimate of the effect of a specific DED.

Empirical Model of Solvophobic Interactions in Organic Solvents

An empirical model was developed to predict organic solvophobic effects using N-phenylimide molecular balances functionalized with non-polar alkyl groups. Solution studies and X-ray crystallography confirmed intramolecular alkyl-alkyl interactions in their folded conformers. The structural modularity of the balances allowed systematic variation of alkyl group lengths. Control balances were instrumental in isolating weak organic solvophobic effects by eliminating framework solvent-solute effects. A 19 F NMR label enabled analysis across 46 deuterated and non-deuterated solvent systems. Linear correlations were observed between organic solvophobic effects and solvent cohesive energy density (ced) as well as changes in solvent-accessible surface areas (SASA). Using these empirical relationships, a model was constructed to predict organic solvophobic interaction energy per unit area for any organic solvent with known ced values. The predicted interaction energies aligned with recent organic solvophobic measurements and literature values for the hydrophobic effect on non-polar surfaces confirmed the model's accuracy and utility.

Contributions of London Dispersion Forces to Solution-Phase Association Processes

ConspectusDespite their ubiquity and early discovery, London dispersion forces are often overlooked. This is due, in part, to the difficulty in assessing their contributions to molecular and polymeric structure, stability, properties, and reactivities. However, recent advances in modeling have revealed that dispersion interactions play an important role in many important chemical and biological processes. Experimental confirmation of their impact in solution has been challenging, leading to controversies about their relative importance.In the course of studying noncovalent interactions using molecular devices, our understanding and appreciation for the importance of dispersion interactions have evolved. This Account follows this intellectual journey by using examples from the literature. The goals are twofold: to describe recent advances in understanding the interaction and to provide guidance to researchers studying weak noncovalent interactions. However, first, the experimental methods for measuring the effects of dispersion interactions and the strategies for isolating their influence are described. These include the design of molecular devices to measure these weak noncovalent interactions and the strategies to disentangle the solvation, solvophobic, and dispersion components of the resulting equilibria.The literature examples are organized around five fundamental questions. (1) Do dispersion interactions have a measurable effect on solution equilibria? (2) To what extent do solvents attenuate or compensate for dispersion interactions? (3) To what extent do the solvation and solvophobic terms influence the dispersion equilibria? (4) Can we predict whether a system will form attractive dispersion or repulsive steric interactions? (5) Can the dispersion term be isolated and interrogated? We were often surprised by the answers to these questions. In each case, we describe how the systems were designed to address these questions and discuss possible interpretations of the results.While dispersion interactions in solution were weak (usually <1 kcal/mol), their influence on complexation and conformational equilibria can be observed and measured. This underscores the significance of these interactions in molecular recognition, coordination chemistry, reaction design, and catalysis. The solvent components of the dispersion equilibria can also be significant. Therefore, the isolation of the dispersion contributions from the solvation and solvophobic effects represents an ongoing challenge. The experimental studies also provide important benchmarks and offer valuable insights to help refine the next generation of computational solvent models.

The Decay of Dispersion Interaction and Its Remarkable Effects on the Kinetics of Activation Reactions Involving Alkyl Chains

The importance of dispersion interactions in many chemical processes is well recognized. It is known that the dispersion strength would decay with the increasing separation between the interacting groups; however, its effects on chemical reactivity have not been well understood. Here we reveal the decay law of dispersion interactions along the n-alkyl chain, its effective interaction ranges for common functional groups, and their remarkable effects on the kinetics of activation reactions involving alkyl chains. This is achieved by DLPNO-CCSD(T) calculations and the local energy decomposition analysis and is supported by experimental findings. In particular, our calculations indicate that the lifetime of alkyl-substituted cis-azobenzenes increases with the alkyl chain length but reaches a steady value when alkyl chains are longer than butyl groups, which is in satisfactory accordance with experimental measurements. We also propose a concise expression to describe the dispersion decay, which shows excellent agreement with our computed results.

An NMR Spectroscopy View on London Dispersion in Catalysis: Detection, Quantification, and Application in Ion Pair and Transition Metal Catalysis

ConspectusThe energetic contribution of London dispersion (LD) can cover a broad range from very few to hundreds of kJ mol-1 for extended interaction interfaces due to its pairwise additivity. However, for a designed and successful application of LD in chemical catalysis, there are still many obstacles and questions that remain. In principle, LD can be regarded as the attractive part of the van der Waals potential. Thus, considering the whole van der Waals potential, including the repulsive part (steric repulsion), the ideal solution to the problem in catalysis would be to design compatible interaction interfaces at exactly the correct distance. In the case of a self-assembled, flexible structure arrangement, entropic contributions and solvent interactions might be detrimental. In the case of a rigid catalyst pocket, steric hindrance might not allow for large substituents that are usually applied as dispersion energy donors (DEDs). For a working catalytic system, the following question arises: how is it possible to dissect the complex interaction interfaces in terms of energetic contributions? Usually, the energetic contribution of LD to catalysis is addressed by using calculations. However, adequately computing the correct energetic contributions can be extremely challenging for a vast conformational space with all kinds of intermolecular interactions. Thus, experimental data are essential for comparison or benchmarking.Therefore, in this Account, we describe our quest for detailed experimental data obtained via NMR spectroscopy to experimentally dissect and quantify LD in catalytic systems. In addition, we address the question of whether bulky substituents used as DEDs can be used in confined catalytic pockets. With the example of Pd phosphoramidite complexes, we show how it is possible to experimentally dissect and quantify the contribution of individual interaction areas in complicated transition metal complexes. Furthermore, a correlation between conformational rigidity and heterodimer preference clearly reveals that LD can only unfold its full potential in cases where entropic contributions are minimized. This finding can also explain the small contribution of LD in flexible and solvent-exposed molecular balances. In the field of Brønsted acid catalysis, we demonstrated that LD has a strong influence on the structures, stability, and populations of confined catalytic intermediates. LD is key for populating higher aggregates such as dimers. In addition, offsets between the experimental and computational results were observed and attributed to solvent-solute dispersion interactions. We studied the delicate interplay of attractive and repulsive interactions by adding bulky DED substituents onto a substrate, which can function as a molecular balance system. Intriguingly, the effect of LD on the free substrate was straightforwardly transferred onto the highly confined intermediates. Furthermore, this effect could even be read out in the enantioselectivities of the underlying reaction. This conceptualized a general approach regarding how LD can be used beneficially in catalysis to convert from moderate/good to excellent stereoselectivities. It showcased that bulky groups such as tert-butyl must not only be regarded as occupied volumes.

A Photopatternable Conjugated Polymer with Thermal-Annealing-Promoted Interchain Stacking for Highly Stable Anti-Counterfeiting Materials

Photoresponsive polymers can be conveniently used to fabricate anti-counterfeiting materials through photopatterning. However, an unsolved problem is that ambient light and heat can damage anti-counterfeiting patterns on photoresponsive polymers. Herein, photo- and thermostable anti-counterfeiting materials are developed by photopatterning and thermal annealing of a photoresponsive conjugated polymer (MC-Azo). MC-Azo contains alternating azobenzene and fluorene units in the polymer backbone. To prepare an anti-counterfeiting material, an MC-Azo film is irradiated with polarized blue light through a photomask, and then thermally annealed under the pressure of a photonic stamp. This strategy generates a highly secure anti-counterfeiting material with dual patterns, which is stable to sunlight and heat over 200 °C. A key for the stability is that thermal annealing promotes interchain stacking, which converts photoresponsive MC-Azo to a photostable material. Another key for the stability is that the conjugated structure endows MC-Azo with desirable thermal properties. This study shows that the design of photopatternable conjugated polymers with thermal-annealing-promoted interchain stacking provides a new strategy for the development of highly stable and secure anti-counterfeiting materials.

Menthol and Fatty Acid-Based Hydrophobic Deep Eutectic Solvents as Media for Enzyme Activation

This research aims to provide insights into the biological efficacy of a newly formed hydrophobic deep eutectic solvent (DES). A DES based on menthol was successfully synthesized with fatty acids. The DESs’ properties as enzyme activators were examined against a neat counterpart. The menthol:decanoic acid (1:1) combination showed improved thermal stability, strong catalytic activity, and reusability for up to four subsequent cycles under ideal conditions (pH 7.0, 40 °C for 2 h). The hydrophobic DES replaced hexane in ester synthesis, where RNL@DES5 showed better fatty acid conversion compared to neat RNL. This study demonstrated promising applications of hydrophobic DESs in non-aqueous organic reactions.

Probing the Size Limit of Dispersion Energy Donors with a Bifluorenylidene Balance: Magic Cyclohexyl

We report the synthesis of 14 2,2'-disubstituted 9,9'-bifluorenylidenes as molecular balances for the quantification of London dispersion interactions between various dispersion energy donors. For all balances, we measured ΔG_Z/E at 333 K using ¹H NMR in seven organic solvents. For various alkyl and aryl substituents, we generally observe a preference for the "folded" Z-isomer due to attractive London dispersion interactions. The cyclohexyl-substituted system shows the largest Z-preference in this study with ΔG_Z/E = -0.6 ± 0.05 kcal mol^-1 in all solvents, owing to the rotational freedom of cyclohexyl groups paired with their large polarizability that maximizes London dispersion interactions. On the other hand, rigid and sterically more demanding substituents like tert-butyl unexpectedly favor the unfolded E-isomer. This is a result of the close relative position in which the functional groups are positioned in this molecular balance. This close proximity is the reason for the increase of Pauli repulsion in the Z-isomers with large rigid substituents (tert-butyl, adamantyl, and diamantyl) which leads to an equilibrium shift toward the unfolded E-form. While we were able to reproduce most of our experimental trends qualitatively using contemporary computational chemistry methods, quantitative accuracy of the employed methods still needs further improvement.

Tilting the Balance: London Dispersion Systematically Enhances Enantioselectivities in Brønsted Acid Catalyzed Transfer Hydrogenation of Imines

London dispersion (LD) is attracting more and more attention in catalysis since LD is ubiquitously present and cumulative. Since dispersion is hard to grasp, recent research has concentrated mainly on the effect of LD in individual catalytic complexes or on the impact of dispersion energy donors (DEDs) on balance systems. The systematic transfer of LD effects onto confined and more complex systems in catalysis is still in its infancy, and no general approach for using DED residues in catalysis has emerged so far. Thus, on the example of asymmetric Brønsted acid catalyzed transfer hydrogenation of imines, we translated the findings of previously isolated balance systems onto confined catalytic intermediates, resulting in a systematic enhancement of stereoselectivity when employing DED-substituted substrates. As the imine substrate is present as Z- and E-isomers, which can, respectively, be converted to R- and S-product enantiomers, implementing tert-butyl groups as DED residues led to an additional stabilization of the Z-imine by up to 4.5 kJ/mol. NMR studies revealed that this effect is transferred onto catalyst/imine and catalyst/imine/nucleophile intermediates and that the underlying reaction mechanism is not affected. A clear correlation between ee and LD stabilization was demonstrated for 3 substrates and 10 catalysts, allowing to convert moderate-good to good-excellent enantioselectivities. Our findings conceptualize a general approach on how to beneficially employ DED residues in catalysis: they clearly showcase that bulky alkyl residues such as tert-butyl groups must be considered regarding not only their repulsive steric bulk but also their attractive properties even in catalytic complexes.

Silyl Groups Are Strong Dispersion Energy Donors

We present an experimental and computational study to investigate noncovalent interactions between silyl groups that are often employed as "innocent" protecting groups. We chose an extended cyclooctatetraene (COT)-based molecular balance comprising unfolded (1,4-disubstituted) and folded (1,6-disubstituted) valance bond isomers that typically display remote and close silyl group contacts, respectively. The thermodynamic equilibria were determined using nuclear magnetic resonance measurements. Additionally, we utilized Boltzmann weighted symmetry-adapted perturbation theory (SAPT) at the sSAPT0/aug-cc-pVDZ level of theory to dissect and quantify noncovalent interactions. Apart from the extremely bulky tris(trimethylsilyl)silyl "supersilyl" group, there is a preference for the folded 1,6-COT valence isomer, with London dispersion interactions being the main stabilizing factor. This makes silyl groups excellent dispersion energy donors, a finding that needs to be taken into account in synthesis planning.

London Dispersion Favors Sterically Hindered Diarylthiourea Conformers in Solution

We present an experimental and computational study on the conformers of N,N'-diphenylthiourea substituted with different dispersion energy donor (DED) groups. While the unfolded anti-anti conformer is the most relevant for thiourea catalysis, intramolecular noncovalent interactions counterintuitively favor the folded syn-syn conformer, as evident from a combination of low-temperature nuclear magnetic resonance measurements and computations. In order to quantify the noncovalent interactions, we utilized local energy decomposition analysis and symmetry-adapted perturbation theory at the DLPNO-CCSD(T)/def2-TZVPP and sSAPT0/6-311G(d,p) levels of theory. Additionally, we applied a double-mutant cycle to experimentally study the effects of bulky substituents on the equilibria. We determined London dispersion as the key interaction that shifts the equilibria towards the syn-syn conformers. This preference is likely a factor why such thiourea derivatives can be poor catalysts.

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Beyond Steric Crowding: Dispersion Energy Donor Effects in Large Hydrocarbon Ligands

Interactions between sterically crowded hydrocarbon-substituted ligands are widely considered to be repulsive because of the intrusion of the electron clouds of the ligand atoms into each other's space, which results in Pauli repulsion. Nonetheless, there is another interaction between the ligands which is less widely publicized but is always present. This is the London dispersion (LD) interaction which can occur between atoms or molecules in which dipoles can be induced instantaneously, for example, between the H atoms from the ligand C-H groups.These LD interactions are always attractive, but their effects are not as widely recognized as those of the Pauli repulsion despite their central role in the formation of condensed matter. Their relatively poor recognition is probably due to the relative weakness (ca. 1 kcal mol^-1) of individual H···H interactions owing to their especially strong distance dependence. In contrast, where there are numerous H···H interactions, a collective LD energy equaling several tens of kcal mol^-1 may ensue. As a result, in some molecules the latent importance of the LD attraction energies emerges and assumes a prominence that can overshadow the Pauli effects (e.g., in the stabilization of high-oxidation-state transition-metal alkyls, inducing disproportionation reactions, or in the stabilization of otherwise unstable bonds).Despite being known for over a century, the accurate quantification of individual H···H LD effects in molecular species is a relatively recent phenomenon and at present is based mainly on modified DFT calculations. A few leading reviews summarized these earlier studies of the C-H···H-C LD interactions in organic molecules, and their effects on the structures and stabilities were described. LD effects in sterically crowded inorganic and organometallic molecules have been recognized.The author's interest in these LD effects arose fortuitously over a decade ago during research on sterically crowded heavier main-group element carbene analogues and two-coordinate, open-shell (d¹-d⁹) transition-metal complexes where counterintuitive steric effects were observed. More detailed explanations of these effects were provided by dispersion-corrected DFT calculations in collaboration with the groups of Tuononen and Nagase (see below).This Account describes our development of these initial results for other inorganic molecular classes. More recently, the work has led us to move to the planned inclusion of dispersion effects in ligands to stabilize new molecular types with theoretical input from the groups of Vasko and Grimme (see below). Our approach sought to use what Grimme has described as dispersion effect donor (DED) groups (i.e., spatially close-lying, densely packed substituents either as ligands (e.g., -C₆H₂-2,4,6-Cy₃, Cy = cyclohexyl) or as parts of ligands (e.g., a Cy substituent) that produce relatively large dispersion energies to stabilize these new compounds.We predict that the future design of sterically crowding hydrocarbon ligands will include the consideration and incorporation of LD effects as a standard methodology for directed use in the attainment of new synthetic targets.

An Incremental System To Predict the Effect of Different London Dispersion Donors in All‐ meta ‐Substituted Azobenzenes

Predictive models based on incremental systems exist for many chemical phenomena, thus allowing easy estimates. Despite their low magnitude in isolated systems London dispersion interactions are ubiquitous in manifold situations ranging from solvation to catalysis or in biological systems. Based on our azobenzene system, we systematically determined the London dispersion donor strength of the alkyl substituents Me, Et, iPr up to tBu. Based on this data, we were able to implement an incremental system for London dispersion for the azobenzene scheme. We propose an equation that allows the prediction of the effect of change of substituents on London dispersion interactions in azobenzenes, which has to be validated in similar molecular arrangements in the future.

Noncovalent CH-π and π-π Interactions in Phosphoramidite Palladium(II) Complexes with Strong Conformational Preference

The weak noncovalent interactions and flexibility of ligands play a key role in enantioselective metal-catalyzed reactions. In transition metal complexes and their catalytic applications, the experimental assessment and the design of key interactions is as difficult as the prediction of the enantioselectivities, especially for flexible, privileged ligands such as chiral phosphoramidites. Therefore, the interligand interactions in cis-PdII L2 Cl2 phosphoramidite complexes were investigated by NMR spectroscopy and computations. We were able to induce a strong conformational preference by breaking the symmetry of the C2 -symmetric side chain of one of the ligands, and shift the equilibrium between hetero- and homocomplexes towards heterocomplexes because of interligand interactions in the cis-complexes. The modulation of aryl substituents was exploited, along with the solvent effect. The combined CH-π and π-π interactions reveal design patterns for binding and folding of chiral ligands and catalysts.

Rational Remodeling of Atypical Scaffolds for the Design of Photoswitchable Cannabinoid Receptor Tools

Azobenzene-embedded photoswitchable ligands are the widely used chemical tools in photopharmacological studies. Current approaches to azobenzene introduction rely mainly on the isosteric replacement of typical azologable groups. However, atypical scaffolds may offer more opportunities for photoswitch remodeling, which are chemically in an overwhelming majority. Herein, we investigate the rational remodeling of atypical scaffolds for azobenzene introduction, as exemplified in the development of photoswitchable ligands for the cannabinoid receptor 2 (CB2). Based on the analysis of residue-type clusters surrounding the binding pocket, we conclude that among the three representative atypical arms of the CB2 antagonist, AM10257, the adamantyl arm is the most appropriate for azobenzene remodeling. The optimizing spacer length and attachment position revealed AzoLig 9 with excellent thermal bistability, decent photopharmacological switchability between its two configurations, and high subtype selectivity. This structure-guided approach gave new impetus in the extension of new chemical spaces for tool customization for increasingly diversified photo-pharmacological studies and beyond.

The origin of supramolecular chirality in 1-ferrocenyl amino acids

π-Conjugated amino acids are widely applied in chiroptical materials, in which chiroptical activities are believed to originate from supramolecular packing. However, the intramolecular contribution has been largely ignored. In this work, we report that intramolecular chirality transfer behaviors in ferrocene-conjugated amino acids depend on the substituent groups, which influence the modality of multiple intramolecular interactions, as well as the molecular geometry. The structural basis and structure-property relationships of chirality and chiroptical activities were unveiled in this work. Based on single crystal structure and density functional theory calculations, we demonstrate that intramolecular weak forces, including hydrogen bonds, CHπ interactions and van der Waals interactions, affect the molecular geometry and contribute to diverse Cotton effects. This work provides evidence for the ignored intramolecular factors in self-assembled systems and paves the way for the fabrication of functional chiroptical systems.

London Dispersion Interactions Rather than Steric Hindrance Determine the Enantioselectivity of the Corey-Bakshi-Shibata Reduction

The well-known Corey-Bakshi-Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivities. While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination. We exemplify this through a combination of detailed computational and experimental studies for a series of modified CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates. Our results demonstrate that attractive LD interactions between the catalyst and the substrate, rather than steric repulsion, determine the selectivity. As a key outcome of our study, we were able to improve the catalyst design for some challenging CBS reductions.

London Dispersion in Alkane Solvents

The importance of London dispersion interactions in solution is an ongoing debate. Although the significance of dispersion for structure and stability is widely accepted, the degree of its attenuation in solution is still not properly understood. Quantitative evaluations are derived mostly from computations. Experimental data provide guidelines to include London dispersion in solution phase design. Herein, dispersive interactions were examined with an azobenzene probe. Alkyl substituents in meta positions of the azobenzene core were systematically varied and the effect on the half-lives for the thermally induced Z to E isomerization in several alkane solvents was determined. The results show that intramolecular dispersion is only marginally influenced. In solvents with low surface tension, reduced destabilizing solvent-solvent interactions increase the half-life up to 20 %. Specific individual interactions between alkyl chains on the azobenzene and those of the solvent lead to additional fluctuations of the half-lives. These presumably result from structural changes of the conformer ensemble.

Rational Design of Azothiophenes-Substitution Effects on the Switching Properties

A series of substituted azothiophenes was prepared and investigated toward their isomerization behavior. Compared to azobenzene (AB), the presented compounds showed red-shifted absorption and almost quantitative photoisomerization to their (Z) states. Furthermore, it was found that electron-withdrawing substitution on the phenyl moiety increases, while electron-donating substitution decreases the thermal half-lives of the (Z)-isomers due to higher or lower stabilization by a lone pair-π interaction. Additionally, computational analysis of the isomerization revealed that a pure singlet state transition state is unlikely in azothiophenes. A pathway via intersystem crossing to a triplet energy surface of lower energy than the singlet surface provided a better fit with experimental data of the (Z)→(E) isomerization. The insights gained in this study provide the necessary guidelines to design effective thiophenylazo-photoswitches for applications in photopharmacology, material sciences, or solar energy harvesting applications.

Aromatic Nanosandwich Obtained by σ-Dimerization of a Nanographenoid π-Radical

A 139-π-electron nanographenoid radical was obtained by expanding the periphery of a naphthalimide–azacoronene hybrid with a methine bridge. The radical was isolated in the form of its σ-dimer, which was shown to possess a conformationally restricted two-layer structure both in the solid state and in solution. The dimer is cleaved into its parent radicals when exposed to ultraviolet or visible radiation in toluene solutions but is resistant to thermally induced dissociation. Under inert conditions, the radicals recombine quantitatively into the σ-dimer with observable kinetics, but they are oxidized into a ketone derivative in the presence of atmospheric oxygen. Combined structural, spectroscopic, and theoretical evidence shows that the σ-dimer contains a weak C(sp³)–C(sp³) bond, but is stabilized against thermal dissociation by a very strong dispersive interaction between the overlapping π surfaces.

Exploring London Dispersion and Solvent Interactions at Alkyl-Alkyl Interfaces Using Azobenzene Switches

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene-based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n-decane was determined. The stability of the Z-isomer increased with longer chains and reached a maximum for n-butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including 1 H NOESY NMR spectroscopy. The results indicate that there are additional long-range interactions between n-alkyl chains with the opposite phenyl core in the Z-state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra- as well as an intermolecular setting.