Photoswitchable Imines Drive Dynamic Covalent Systems to Nonequilibrium Steady States.

Coupling a photochemical reaction to a thermal exchange process can drive the latter to a nonequilibrium steady state (NESS) under photoirradiation. Typically, systems use separate motifs for photoresponse and equilibrium-related processes. Here, we show that photoswitchable imines can fulfill both roles simultaneously, autonomously driving a dynamic covalent system into a NESS under continuous light irradiation. We demonstrate this using transimination reactions, where E-to-Z photoisomerism generates a more kinetically labile species. At the NESS, energy is stored both in the metastable Z-isomer of the imine and in the system's nonequilibrium constitution; when the light is switched off, this stored energy is released as the system reverts to its equilibrium state. The system operates autonomously under continuous light irradiation and exhibits characteristics of a light-driven information ratchet. This is enabled by the dual-role of the imine linkage as both the photochromic and dynamic covalent bond. This work highlights the ability and application of these imines to drive systems to NESSs, thus offering a novel approach in the field of systems chemistry.

Light-Driven Hexagonal-to-Cubic Phase Switching in Arylazopyrazole Lyotropic Liquid Crystals.

Photoinduced manipulation of the nanoscale molecular structure and organization of soft materials can drive changes in the macroscale properties. Here we demonstrate the first example of a light-induced one- to three-dimensional mesophase transition at room temperature in lyotropic liquid crystals constructed from arylazopyrazole photosurfactants in water. We exploit this characteristic to use light to selectively control the rate of gas (CO2) diffusion across a prototype lyotropic liquid crystal membrane. Such control of phase organization, dimensionality, and permeability unlocks the potential for stimuli-responsive analogues in technologies for controlled delivery.

Self-Assembly of Double-Helical Metallopolymers.

Metal-containing polymers, or metallopolymers, have diverse applications in the fields of sensors, catalysis, information storage, optoelectronics, and neuromorphic computing, among other areas. The approach of metal-templated subcomponent self-assembly using dynamic covalent linkages allows complex architectures to be formed with relative synthetic ease. The dynamic nature of the linkages between subunits in these systems facilitates error checking during the assembly process and also provides a route to disassemble the structure, rendering these materials recyclable. This Account summarizes a class of double-helical metallopolymers. These metallopolymers are formed via subcomponent self-assembly and consist of two conjugated helical strands wrapping a linear array of CuI centers. Starting from discrete model helicates, we discuss how, through the judicious design of subcomponents, long helical metallopolymers can be obtained and detail their subsequent assembly into nanometer-scale aggregates. Two approaches to generate these helical metallopolymers are compared. We describe methods to govern (i) the length of the metallopolymers, (ii) the relative orientations (head-to-head vs head-to-tail) of the two organic strands, and (iii) the screw-sense of the double helix. Achieving structural control allowed the growth behavior of these systems to be probed. The structure influenced properties in ways that are relevant to specific applications; for example, the length of the metallopolymer determines the color of the light it emits in solution. In the solid state, the ionic nature of these helices renders them useful as both emitters and ionic additives in light-emitting electrochemical cells. Moreover, recent experimental work has clarified the role of the linear array of Cu ions in the transport of charge through these materials. The conductivity displayed by a film of metallopolymer depends upon its history of applied voltage and current, behavior characteristic of a memristor. In addition to the prospective applications already identified, others may be on the horizon, potentially combing stimuli-responsive electronic behavior with the chirality of the helical twist.

Photocontrolled Energy Storage in Azobispyrazoles with Exceptionally Large Light Penetration Depths.

Azobispyrazole, 4pzMe-5pzH, derivatives with small terminal substituents (Me, Et, i-Pr, and n-Pr) are reported to undergo facile reversible photoswitching in condensed phases at room temperature, exhibiting unprecedentedly large effective light penetration depths (1400 µm of UV at 365 nm and 1400 µm of visible light at 530 nm). These small photoswitches exhibit crystal-to-liquid phase transitions upon UV irradiation, which increases the overall energy storage density of this material beyond 300 J/g that is similar to the specific energy of commercial Na-ion batteries. The impact of heteroarene design, the presence of ortho methyl substituents, and the terminal functional groups is explored for both condensed-phase switching and energy storage. The design principles elucidated in this work will help to develop a wide variety of molecular solar thermal energy storage materials that operate in condensed phases.

Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides.

Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of π-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a "toolbox" for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic-hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular π wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin-orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.

Efficient Electrocatalytic Switching of Azoheteroarenes in the Condensed Phases.

Azo-based photoswitches have shown promise as molecular solar-thermal (MOST) materials due to their ability to store energy in their metastable Z isomeric form. The energy is then released, in the form of heat, upon photoisomerization to the thermodynamically stable E form. However, obtaining a high energy density and recovering the stored energy with high efficiency requires the materials to be employed in the condensed phase and display a high degree of Z to E switching, both of which are challenging to engineer. Here, we show that arylazopyrazole motifs undergo efficient redox-induced Z to E switching in both the solution and the condensed phase to a higher completeness of switching than achieved photochemically. This redox-initiated pathway lowers the barrier of Z to E isomerization by 27 kJ/mol, while in the condensed phase, the efficiency of electrochemical switching is improved by over an order of magnitude relative to that in the solution state. The influence of the photoswitch's phase, electrical conductivity, and viscosity on the electrochemical switching in the condensed phase is reported, culminating in a set of design rules to facilitate further investigations. We anticipate the use of an alternative stimulus to light will facilitate the application of MOST materials in situations where phototriggered heat release is unachievable or inefficient, e.g., indoor or at night. Furthermore, exploiting the electrocatalytic mechanism, whereby a catalytic amount of charge triggers Z to E switching via a redox process, bypasses the need for fine tuning of the photoswitching chromophore to achieve complete Z to E switching, thus providing an alternative approach to photoswitch molecular design.

Glucose Binding Drives Reconfiguration of a Dynamic Library of Urea-Containing Metal-Organic Assemblies.

A bis-urea-functionalized ditopic subcomponent assembled with 2-formylpyridine and FeII , resulting in a dynamic library of metal-organic assemblies: an irregular FeII4 L6 structure and three FeII2 L3 stereoisomers: left- and right-handed helicates and a meso-structure. This library reconfigured in response to the addition of monosaccharide derivatives, which served as guests for specific library members, and the rate of saccharide mutarotation was also enhanced by the library. The (P) enantiomer of the FeII2 L3 helical structure bound ß-D-glucose selectively over α-D-glucose. As a consequence, the library collapsed into the (P)-FeII2 L3 helicate following glucose addition. The α-D-glucose was likewise transformed into the ß-D-anomer during equilibration and binding. Thus, ß-D-glucose and (P)-3 amplified each other in the product mixture, as metal-organic and saccharide libraries geared together into a single equilibrating system.

Photochemical Probe Identification of a Small-Molecule Inhibitor Binding Site in Hedgehog Acyltransferase (HHAT)*.

The mammalian membrane-bound O-acyltransferase (MBOAT) superfamily is involved in biological processes including growth, development and appetite sensing. MBOATs are attractive drug targets in cancer and obesity; however, information on the binding site and molecular mechanisms underlying small-molecule inhibition is elusive. This study reports rational development of a photochemical probe to interrogate a novel small-molecule inhibitor binding site in the human MBOAT Hedgehog acyltransferase (HHAT). Structure-activity relationship investigation identified single enantiomer IMP-1575, the most potent HHAT inhibitor reported to-date, and guided design of photocrosslinking probes that maintained HHAT-inhibitory potency. Photocrosslinking and proteomic sequencing of HHAT delivered identification of the first small-molecule binding site in a mammalian MBOAT. Topology and homology data suggested a potential mechanism for HHAT inhibition which was confirmed by kinetic analysis. Our results provide an optimal HHAT tool inhibitor IMP-1575 (Ki =38 nM) and a strategy for mapping small molecule interaction sites in MBOATs.

LaIII and ZnII Cooperatively Template a Metal-Organic Capsule.

An organic subcomponent was designed with 2-formyl-8-aminoquinoline and triazole-pyridine ends. The relative orientations and geometries of these two ends enabled this subcomponent to assemble together with ZnII and LaIII cations to generate a heterobimetallic tetrahedral capsule. The LaIII cations each template three imine bonds that hold together a 3-fold-symmetric metallo-ligand, defining the center of each tetrahedron face. The ZnII cations occupy the other ends of these C3 axes, defining the vertices of the tetrahedron. This is the first example where subcomponent self-assembly brought into being the faces of a polyhedron, as opposed to the vertices. Host-guest studies show positively cooperative binding toward ReO4-, the encapsulation of which also resulted in the quenching of capsule fluorescence.

Hydrogen-Bond-Assisted Symmetry Breaking in a Network of Chiral Metal-Organic Assemblies.

Herein we elucidate the interplay of chiral, chelate, solvent, and hydrogen-bonding information in the self-assembly of a series of new three-dimensional metal-organic architectures. Enantiopure ligands, each containing H-bond donors and acceptors, form different structures, depending on the ratio in which they are combined: enantiopure components form M4L4 assemblies, whereas racemic mixtures form M3L3 stacks. Chiral amplification within M3L3 enantiomers was observed when a 2:1 ratio of R and S subcomponent enantiomers was employed. Simply switching the solvent (from MeCN to MeOH) or chelating unit (from bidentate to tridentate) increased the diversity of structures that can be generated from these building blocks, leading to the selective formation of novel M2L2 and M3L2 assemblies. The addition of achiral ligand building blocks resulted in the formation of further structures: When an achiral subcomponent was combined with its R and S chiral congeners, a three-layer heteroleptic architecture was generated, with the achiral unit sitting at the top of the stack. When combined with the S enantiomer only, however, the achiral unit assembled in the center of the structure, thus demonstrating the selective placement of achiral units within chiral systems. Further sorting experiments revealed that combining R and S stereocenters within a single ligand led to diastereoselective product generation. These results show how geometric complementarity between different ligands impacts upon the degree of hydrogen-bonding within the assembly, stabilizing specific low-symmetry architectures from among many possible structural outcomes.

FeII4L4 Tetrahedron Binds to Nonpaired DNA Bases.

A water-soluble self-assembled supramolecular FeII4L4 tetrahedron binds to single stranded DNA, mismatched DNA base pairs, and three-way DNA junctions. Binding of the coordination cage quenches fluorescent labels on the DNA strand, which provides an optical means to detect the interaction and allows the position of the binding site to be gauged with respect to the fluorescent label. Utilizing the quenching and binding properties of the coordination cage, we developed a simple and rapid detection method based on fluorescence quenching to detect unpaired bases in double-stranded DNA.

Enantiopure [Cs+/Xe⊂Cryptophane]⊂FeII4L4 Hierarchical Superstructures.

Hierarchically nested hosts offer new opportunities to control the guest binding of the inner host, functionalize the cavity of the outer host, and investigate communication between different layers. Here we report a self-assembled triazatruxene-based FeII4L4 capsule, which was able to encapsulate a covalent cage, cryptophane-111 (CRY). The resulting cage-in-cage complex was capable of accommodating a cesium cation or xenon atom with altered guest binding behavior compared to the CRY alone. A crystal structure of the Russian doll complex [Cs+⊂CRY]⊂FeII4L4 unambiguously demonstrated the unusual encapsulation of a cation within a capsule bearing a 8+ charge. Moreover, the binding of enantiopure CRY occurred with high enantioselectivity (530-fold) between the two enantiomers of the tetrahedron. This discrimination resulted in stereochemical information transfer from the inner covalent cage to the outer self-assembled capsule, leading to the formation of enantiopure [guest⊂cage]⊂cage complexes. The stereochemistry of the tetrahedron persisted even after displacement of CRY with an achiral guest.

Unraveling Mechanisms of Chiral Induction in Double-Helical Metallopolymers.

Self-assembled helical polymers hold great promise as new functional materials, where helical handedness controls useful properties such as circularly polarized light emission or electron spin. The technique of subcomponent self-assembly can generate helical polymers from readily prepared monomers. Here we present three distinct strategies for chiral induction in double-helical metallopolymers prepared via subcomponent self-assembly: (1) employing an enantiopure monomer, (2) polymerization in a chiral solvent, (3) using an enantiopure initiating group. Kinetic and thermodynamic models were developed to describe the polymer growth mechanisms and quantify the strength of chiral induction, respectively. We found the degree of chiral induction to vary as a function of polymer length. Ordered, rod-like aggregates more than 70 nm long were also observed in the solid state. Our findings provide a basis to choose the most suitable method of chiral induction based on length, regiochemical, and stereochemical requirements, allowing stereochemical control to be established in easily accessible ways.

Depleting Depletion: Maintaining Single-Walled Carbon Nanotube Dispersions after Graft-To Polymer Functionalization.

Grafting polymers onto single-walled carbon nanotubes (SWCNTs) usefully alters properties but does not typically yield stable, solvated species directly. Despite the expectation of steric stabilization, a damaging (re)dispersion step is usually necessary. Here, poly(vinyl acetate)s (PVAc's) of varying molecular weights are grafted to individualized, reduced SWCNTs at different concentrations to examine the extent of reaction and degree of solvation. The use of higher polymer concentrations leads to an increase in grafting ratio (weight fraction of grafted polymer relative to the SWCNT framework), approaching the limit of random sequentially adsorbed Flory "mushrooms" on the surface. However, at higher polymer concentrations, a larger percentage of SWCNTs precipitate during the reaction; an effect which is more significant for larger weight polymers. The precipitation is attributed to depletion interactions generated by ungrafted homopolymer overcoming Coulombic repulsion of adjacent like-charged SWCNTs; a simple model is proposed. Larger polymers and greater degrees of functionalization favor stable solvation, but larger and more concentrated homopolymers increase depletion aggregation. By using low concentrations (25 µM) of larger molecular weight PVAc (10 kDa), up to 65% of grafted SWCNTs were retained in solution (at 65 µg mL-1) directly after the reaction.

Self-Assembly of Conjugated Metallopolymers with Tunable Length and Controlled Regiochemistry.

Self-assembled materials can be designed to express useful optoelectronic properties; however, achieving structural control is a necessary precondition for the optimization of desired properties. Here we report a simple, metal-templated polymerization process that generates helical metallopolymer strands over 75 repeat units long (28 kDa) from a single bifunctional monomer and CuI . The resulting polymer consists of a double helix of two identical conjugated organic strands enclosing a central column of metal ions. The length of this metallopolymer can be controlled by adding monofunctional subcomponents to end-cap the conjugated ligands. The use of ditopic and bulky monotopic subcomponents, respectively, allows a head-to-head or head-to-tail double helix to be generated. Spectroscopic measurements of different polymer lengths demonstrate how control over polymer length leads to control over the electronic and luminescent properties of the resulting material, thereby enabling tunable white-light emission.

Post-grafting amination of alkyl halide-functionalized silica for applications in catalysis, adsorption, and 15N NMR spectroscopy.

An anhydrous synthesis of aminosilica materials from alkyl halide-functionalized mesoporous SBA-15 silica by post-grafting amination is introduced for applications in CO2 adsorption, cooperative catalysis, and (15)N solid-state NMR spectroscopy. The synthesis is demonstrated to convert terminal alkyl halide-functionalized silica materials containing Cl, Br, and I to primary alkylamines using anhydrous ammonia in a high-pressure reactor. The benefits of the post-grafting amination procedure include (i) use of anhydrous isotopically labeled ammonia, (15)NH3, to create aminosilica materials that can be investigated using (15)N solid-state NMR to elucidate potential intermediates and surface species in CO2 adsorption processes and catalysis, (ii) similar CO2 uptake in experiments extracting CO2 from dry simulated air experiments, and (iii) improved activity in acid-base bifunctional catalysis compared to traditional amine-grafted materials. The effects of the type of halide, the initial halide loading, and the total reaction time on the conversion of the halides to primary amines are explored. Physical and chemical characterizations of the materials show that the textural properties of the silica are unaffected by the reaction conditions and that quantitative conversion to primary amines is achieved even at short reaction times and high initial alkyl halide loadings. Additionally, preliminary (15)N solid-state NMR experiments indicate formation of nitrogen-containing species and demonstrate that the synthesis can be used to create materials useful for investigating surface species by NMR spectroscopy. The differences between the materials prepared via post-grafting amination vs traditional aminosilane grafting are attributed to the slightly increased spacing of the amines synthesized by amination because the alkylhalosilanes are initially better spaced on the silica surface after grafting, whereas the aminosilanes likely cluster to a greater extent when grafted on the silica surface. A slight increase in amine spacing allows for more effective amine-silanol interactions in cooperative catalysis without reducing the amine efficiency in CO2 uptake under the conditions used here.

Photoswitchable imines: aryliminopyrazoles quantitatively convert to long-lived Z-isomers with visible light.

Arylimines offer promise in dynamic-covalent materials due to their recyclability and ease of synthesis. However, their light-triggered E/Z isomerism has received little attention. This is attributed to challenges that include low thermal stability of their metastable state (<60 s at 20 °C), incomplete photoswitching (<50% to the metastable state), and the need for UV light (≤365 nm). We overcome these limitations with a novel class of imine photoswitch, the aryliminopyrazoles (AIPs). These AIPs can be switched using visible light (470 nm), attain photostationary states with over 95% of the Z-isomer, exhibit great resistance to fatigue, and have thermal half-lives up to 19.2 hours at room temperature. Additionally, they display T-type and negative photochromism under visible light irradiation-a useful property. The photochromic properties, quantitative assembly and accessibility of precursors set these photoswitches apart from their azo-based analogues. These findings open avenues for next-generation photoresponsive dynamic-covalent materials driven solely by these new photochromic linkages and further exploration of photocontrolled dynamic combinatorial chemistry.

Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host-Guest Complexes.

A series of arylazopyrazole-loaded metal-organic frameworks were synthesized with the general formula Zn2(BDC)2(DABCO)(AAP)x (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo-[2.2.2]octane; AAP = arylazopyrazole guest). The empty framework adopts a large pore tetragonal structure. Upon occlusion of the E-AAP guests, the frameworks contract to form narrow pore tetragonal structures. The extent of framework contraction is dependent on guest shapes and pendant groups and ranges between 1.5 and 5.8%. When irradiated with 365 nm light, the framework expands due to the photoisomerization of E-AAP to Z-AAP. The proportion of Z-isomer at the photostationary state varies between 19 and 57% for the AAP guests studied and appears to be limited by the framework which inhibits further isomerization once fully expanded. Interestingly, confinement within the framework significantly extends the thermal half-life of the Z-AAP isomers to a maximum of approximately 56 years. This finding provides scope for the design of photoresponsive host-guest complexes with high stability of the metastable isomer for long-duration information or energy storage applications.

Emerging properties from mechanical tethering within a post-synthetically functionalised catenane scaffold.

Maintaining close spatial proximity of functional moieties within molecular systems can result in fascinating emergent properties. Whilst much work has been done on covalent tethering of functional units for myriad applications, investigations into mechanically linked systems are relatively rare. Formation of the mechanical bond is usually the final step in the synthesis of interlocked molecules, placing limits on the throughput of functionalised architectures. Herein we present the synthesis of a bis-azide [2]catenane scaffold that can be post-synthetically modified using CuAAC 'click' chemistry. In this manner we have been able to access functionalised catenanes from a common precursor and study the properties of electrochemically active, emissive and photodimerisable units within the mechanically interlocked system in comparison to non-interlocked analogues. Our data demonstrates that the greater (co-)conformational flexibility that can be obtained with mechanically interlocked systems compared to traditional covalent tethers paves the way for developing new functional molecules with exciting properties.

Light Responsiveness and Assembly of Arylazopyrazole-Based Surfactants in Neat and Mixed CTAB Micelles.

The self-assembly of an arylazopyrazole-based photosurfactant (PS), based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle formation with CTAB in aqueous solution was investigated by small angle neutron and X-ray scattering (SANS/SAXS) and UV-vis absorption spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into smaller, spherical micelles with larger shell thickness. Doping PS with CTAB resulted in mixed micelle formation at all stoichiometries and conditions investigated; employing selectively deuterated PS, a monotonic variation in scattering length density and dimensions of the micellar core and shell is observed for all contrasts. The concentration- and irradiance-dependence of the E to Z configurational transition was established in both neat and mixed micelles. A liposome dye release assay establishes the enhanced efficacy of photosurfactants at membrane disruption, with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure CTAB. Our findings pave the way for external triggering and modulation of the wide range of CTAB-based biomedical and material applications.