Tandem High-Pressure Crystallography-Optical Spectroscopy Unpacks Noncovalent Interactions of Piezochromic Fluorescent Molecular Rotors.

To develop luminescent molecular materials with predictable and stimuli-responsive emission, it is necessary to correlate changes in their geometries, packing structures, and noncovalent interactions with the associated changes in their optical properties. Here, we demonstrate that high-pressure single-crystal X-ray diffraction can be combined with high-pressure UV-visible absorption and fluorescence emission spectroscopies to elucidate how subtle changes in structure influence optical outputs. A piezochromic aggregation-induced emitter, sym-heptaphenylcycloheptatriene (Ph7C7H), displays bathochromic shifts in its absorption and emission spectra at high pressure. Parallel X-ray measurements identify the pressure-induced changes in specific phenyl-phenyl interactions responsible for the piezochromism. Pairs of phenyl rings from neighboring molecules approach the geometry of a stable benzene dimer, while conformational changes alter intramolecular phenyl-phenyl interactions correlated with a relaxed excited state. This tandem crystallographic and spectroscopic analysis provides insights into how subtle structural changes relate to the photophysical properties of Ph7C7H and could be applied to a library of similar compounds to provide general structure-property relationships in fluorescent organic molecules with rotor-like geometries.

Extended Conjugation Attenuates the Quenching of Aggregation-Induced Emitters by Photocyclization Pathways.

Herein, we expose how the antagonistic relationship between solid-state luminescence and photocyclization of oligoaryl alkene chromophores is modulated by the conjugation length of their alkenyl backbones. Heptaaryl cycloheptatriene molecular rotors exhibit aggregation-induced emission characteristics. We show that their emission is turned off upon breaking the conjugation of the cycloheptatriene by epoxide formation. While this modification is deleterious to photoluminescence, it enables formation of extended polycyclic frameworks by Mallory reactions. We exploit this dichotomy (i) to manipulate emission properties in a controlled manner and (ii) as a synthetic tool to link together pairs of phenyl rings in a specific sequence. This method to alter the tendency of oligoaryl alkenes to undergo photocyclization can inform the design of solid-state emitters that avoid this quenching mechanism, while also allowing selective cyclization in syntheses of polycyclic aromatic hydrocarbons.

Control of Porphyrin Planarity and Aggregation by Covalent Capping: Bissilyloxy Porphyrin Silanes.

Porphyrins are cornerstone functional materials that are useful in a wide variety of settings, ranging from molecular electronics to biology and medicine. Their applications are often hindered, however, by poor solubilities that result from their extended, solvophobic aromatic surfaces. Attempts to counteract this problem by functionalizing their peripheries have been met with only limited success. Here, we demonstrate a versatile strategy to tune the physical and electronic properties of porphyrins using an axial functionalization approach. Porphyrin silanes (PorSils) and bissilyloxy PorSils (SOPS) are prepared from porphyrins by operationally simple κ4N-silylation protocols, introducing bulky silyloxy "caps" that are central and perpendicular to the planar porphyrin. While porphyrins typically form either J- or H-aggregates, SOPS do not self-associate in the same manner: the silyloxy axial substituents dramatically improve the solubility by inhibiting aggregation. Moreover, axial porphyrin functionalization offers convenient handles through which optical, electronic, and structural properties of the porphyrin core can be modulated. We observe that the identity of the silyloxy substituent impacts the degree of planarity of the porphyrin in the solid state as well as the redox potentials.

Electrochemically addressable trisradical rotaxanes organized within a metal-organic framework.

The organization of trisradical rotaxanes within the channels of a Zr6-based metal-organic framework (NU-1000) has been achieved postsynthetically by solvent-assisted ligand incorporation. Robust Zr(IV)-carboxylate bonds are forged between the Zr clusters of NU-1000 and carboxylic acid groups of rotaxane precursors (semirotaxanes) as part of this building block replacement strategy. Ultraviolet-visible-near-infrared (UV-Vis-NIR), electron paramagnetic resonance (EPR), and 1H nuclear magnetic resonance (NMR) spectroscopies all confirm the capture of redox-active rotaxanes within the mesoscale hexagonal channels of NU-1000. Cyclic voltammetry measurements performed on electroactive thin films of the resulting material indicate that redox-active viologen subunits located on the rotaxane components can be accessed electrochemically in the solid state. In contradistinction to previous methods, this strategy for the incorporation of mechanically interlocked molecules within porous materials circumvents the need for de novo synthesis of a metal-organic framework, making it a particularly convenient approach for the design and creation of solid-state molecular switches and machines. The results presented here provide proof-of-concept for the application of postsynthetic transformations in the integration of dynamic molecular machines with robust porous frameworks.

Excited-State Aromatic Interactions in the Aggregation-Induced Emission of Molecular Rotors.

Small, apolar aromatic groups, such as phenyl rings, are commonly included in the structures of fluorophores to impart hindered intramolecular rotations, leading to desirable solid-state luminescence properties. However, they are not normally considered to take part in through-space interactions that influence the fluorescent output. Here, we report on the photoluminescence properties of a series of phenyl-ring molecular rotors bearing three, five, six, and seven phenyl groups. The fluorescent emissions from two of the rotors are found to originate, not from the localized excited state as one might expect, but from unanticipated through-space aromatic-dimer states. We demonstrate that these relaxed dimer states can form as a result of intra- or intermolecular interactions across a range of environments in solution and solid samples, including conditions that promote aggregation-induced emission. Computational modeling also suggests that the formation of aromatic-dimer excited states may account for the photophysical properties of a previously reported luminogen. These results imply, therefore, that this is a general phenomenon that should be taken into account when designing and interpreting the fluorescent outputs of luminescent probes and optoelectronic devices based on fluorescent molecular rotors.

Triply Threaded [4]Rotaxanes.

[4]Rotaxanes featuring three axles threaded through a single ring have been prepared through active metal template synthesis. Nickel-catalyzed sp(3)-sp(3) homocouplings of alkyl bromide "half-threads" through 37- and 38-membered 2,2':6',2″-terpyridyl macrocycles generate triply threaded [4]rotaxanes in up to 11% yield. An analogous 39-membered macrocycle produced no rotaxane products under similar conditions. The constitutions of the [4]rotaxanes were determined by NMR spectroscopy and mass spectrometry. Doubly threaded [3]rotaxanes were also obtained from the reactions but no [2]rotaxanes were isolated, suggesting that upon demetalation the axle of a singly threaded rotaxane can slip through a macrocycle that is sufficiently large to accommodate three threads.

Influence of Constitution and Charge on Radical Pairing Interactions in Tris-radical Tricationic Complexes.

The results of a systematic investigation of trisradical tricationic complexes formed between cyclobis(paraquat-p-phenylene) bisradical dicationic (CBPQT(2(•+))) rings and a series of 18 dumbbells, containing centrally located 4,4'-bipyridinium radical cationic (BIPY(•+)) units within oligomethylene chains terminated for the most part by charged 3,5-dimethylpyridinium (PY(+)) and/or neutral 3,5-dimethylphenyl (PH) groups, are reported. The complexes were obtained by treating equimolar amounts of the CBPQT(4+) ring and the dumbbells containing BIPY(2+) units with zinc dust in acetonitrile solutions. Whereas UV-Vis-NIR spectra revealed absorption bands centered on ca. 1100 nm with quite different intensities for the 1:1 complexes depending on the constitutions and charges on the dumbbells, titration experiments showed that the association constants (Ka) for complex formation vary over a wide range, from 800 M(-1) for the weakest to 180 000 M(-1) for the strongest. While Coulombic repulsions emanating from PY(+) groups located at the ends of some of the dumbbells undoubtedly contribute to the destabilization of the trisradical tricationic complexes, solid-state superstructures support the contention that those dumbbells with neutral PH groups at the ends of flexible and appropriately constituted links to the BIPY(•+) units stand to gain some additional stabilization from C-H···π interactions between the CBPQT(2(•+)) rings and the PH termini on the dumbbells. The findings reported in this Article demonstrate how structural changes implemented remotely from the BIPY(•+) units influence their non-covalent bonding interactions with CBPQT(2(•+)) rings. Different secondary effects (Coulombic repulsions versus C-H···π interactions) are uncovered, and their contributions to both binding strengths associated with trisradical interactions and the kinetics of associations and dissociations are discussed at some length, supported by extensive DFT calculations at the M06-D3 level. A fundamental understanding of molecular recognition in radical complexes has relevance when it comes to the design and synthesis of non-equilibrium systems.

Hydroacenes Made Easy by Gold(I) Catalysis.

A novel strategy for the synthesis of partially saturated acene derivatives has been developed based on a Au(I) -catalyzed cyclization of 1,7-enynes. This method provides straightforward access to stable polycyclic products featuring the backbone of the acene series, up to nonacene.

Gold(I) carbenes by retro-Buchner reaction: generation and fate.

The fate of the aryl gold(I) carbenes generated by retro-Buchner reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent on the constitution of the ortho substituent. Indenes and fluorenes are obtained by intramolecular reaction of highly electrophilic gold(I) carbenes with alkenes and arenes. According to density functional theory calculations, the gold-catalyzed retro-Buchner process occurs stepwise, although the two carbon-carbon cleavages occur on a rather flat potential energy surface.

Energetically demanding transport in a supramolecular assembly.

A challenge in contemporary chemistry is the realization of artificial molecular machines that can perform work in solution on their environments. Here, we report on the design and production of a supramolecular flashing energy ratchet capable of processing chemical fuel generated by redox changes to drive a ring in one direction relative to a dumbbell toward an energetically uphill state. The kinetics of the reaction pathway juxtapose a low energy [2]pseudorotaxane that forms under equilibrium conditions with a high energy, metastable [2]pseudorotaxane which resides away from equilibrium.

Solid-state characterization and photoinduced intramolecular electron transfer in a nanoconfined octacationic homo[2]catenane.

An octacationic homo[2]catenane comprised of two mechanically interlocked cyclobis(paraquat-p-phenylene) rings has been obtained from the oxidation of the septacationic monoradical with nitrosonium hexafluoroantimonate. The nanoconfinement of normally repulsive bipyridinium units results in the enforced π-overlap of eight positively charged pyridinium rings in a volume of <1.25 nm(3). In the solid state, the torsional angles around the C-C bonds between the four pairs of pyridinium rings range between 16 and 30°, while the π-π stacking distances between the bipyridinium units are extended for the inside pair and contracted for the pairs on the outside--a consequence of Coulombic repulsion between the inner bipyridinium subunits. In solution, irradiation of the [2]catenane at 275 nm results in electron transfer from one of the paraphenylene rings to the inner bipyridinium dimer, leading to the generation of a temporary mixed-valence state within the rigid and robust homo[2]catenane.

Gold(I) as an artificial cyclase: short stereodivergent syntheses of (-)-epiglobulol and (-)-4ß,7α- and (-)-4α,7α-aromadendranediols.

Three natural aromadendrane sesquiterpenes, (-)-epiglobulol, (-)-4ß,7α-aromadendranediol, and (-)-4α,7α-aromadendranediol, have been synthesized in only seven steps in 12, 15, and 17 % overall yields, respectively, from (E,E)-farnesol by a stereodivergent gold(I)-catalyzed cascade reaction which forms the tricyclic aromadendrane core in a single step. These are the shortest total syntheses of these natural compounds.

Electron delocalization in a rigid cofacial naphthalene-1,8:4,5-bis(dicarboximide) dimer.

Investigating through-space electronic communication between discrete cofacially oriented aromatic π-systems is fundamental to understanding assemblies as diverse as double-stranded DNA, organic photovoltaics and thin-film transistors. A detailed understanding of the electronic interactions involved rests on making the appropriate molecular compounds with rigid covalent scaffolds and π-π distances in the range of ca. 3.5 Å. Reported herein is an enantiomeric pair of doubly-bridged naphthalene-1,8:4,5-bis(dicarboximide) (NDI) cyclophanes and the characterization of four of their electronic states, namely 1) the ground state, 2) the exciton coupled singlet excited state, 3) the radical anion with strong through-space interactions between the redox-active NDI molecules, and 4) the diamagnetic diradical dianion using UV/Vis/NIR, EPR and ENDOR spectroscopies in addition to X-ray crystallography. Despite the unfavorable Coulombic repulsion, the singlet diradical dianion dimer of NDI shows a more pronounced intramolecular π-π stacking interaction when compared with its neutral analog.

Pillar[5]arene as a co-factor in templating rotaxane formation.

After the manner in which coenzymes often participate in the binding of substrates in the active sites of enzymes, pillar[5]arene, a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges, modifies the binding properties of cucurbit[6]uril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbit[6]uril, a macrocycle composed of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillar[5]arene and cucurbit[6]uril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two [4]rotaxanes and one [5]rotaxane in >90% yields inside 2 h at 55 °C in acetonitrile. Since the hydroxyl groups on pillar[5]arene and the carbonyl groups on cucurbit[6]uril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillar[5]arene can be trapped on the dumbbell components of the [4]rotaxanes. In the case of the [5]rotaxane, it is possible to isolate a compound containing two pillar[5]arene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillar[5]arene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.

Relative unidirectional translation in an artificial molecular assembly fueled by light.

Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability not only to control relative movements of components in molecules but also to drive their components preferentially in one direction relative to each other using versatile stimuli is one of the keys to future technological applications. Herein, we describe a wholly synthetic small-molecule system that, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by the two termini--one a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unit--of a constitutionally asymmetric dumbbell, which can impair the threading/dethreading of a [2]pseudorotaxane, and (ii) controls the ring's affinity for a 1,5-dioxynaphthalene binding site located in the dumbbell's central core. The formation and subsequent dissociation of the [2]pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction relatively defined has been demonstrated by (i) spectroscopic ((1)H NMR and UV/vis) means and cyclic voltammetry as well as with (ii) DFT calculations and by (iii) comparison with control compounds in the shape of constitutionally symmetrical [2]pseudorotaxanes, one with two positively charged ends and the other with two neutral ends. The operation of the system relies solely on reversible, yet stable, noncovalent bonding interactions. Moreover, in the presence of a photosensitizer, visible-light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.

Rupturing aromaticity by periphery overcrowding.

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of π-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic π-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as 'Dewar tropylium'. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity.

Control of dynamic sp3-C stereochemistry.

Stereogenic sp3-hybridized carbon centres are fundamental building blocks of chiral molecules. Unlike dynamic stereogenic motifs, such as sp3-nitrogen centres or atropisomeric biaryls, sp3-carbon centres are usually fixed, requiring intermolecular reactions to undergo configurational changes. Here we report the internal enantiomerization of fluxional carbon cages and the consequences of their adaptive configurations for the transmission of stereochemical information. The sp3-carbon stereochemistry of the rigid tricyclic cages is inverted through strain-assisted Cope rearrangements, emulating the low-barrier configurational dynamics typical for sp3-nitrogen inversion or conformational isomerism. This dynamic enantiomerization can be stopped, restarted or slowed by external reagents, while the configuration of the cage is controlled by neighbouring, fixed stereogenic centres. As part of a phosphoramidite-olefin ligand, the fluxional cage acts as a conduit to transmit stereochemical information from the ligand while also transferring its dynamic properties to chiral-at-metal coordination environments, influencing catalysis, ion pairing and ligand exchange energetics.

Aggregation-Induced Emission (AIE), Life and Health.

Light has profoundly impacted modern medicine and healthcare, with numerous luminescent agents and imaging techniques currently being used to assess health and treat diseases. As an emerging concept in luminescence, aggregation-induced emission (AIE) has shown great potential in biological applications due to its advantages in terms of brightness, biocompatibility, photostability, and positive correlation with concentration. This review provides a comprehensive summary of AIE luminogens applied in imaging of biological structure and dynamic physiological processes, disease diagnosis and treatment, and detection and monitoring of specific analytes, followed by representative works. Discussions on critical issues and perspectives on future directions are also included. This review aims to stimulate the interest of researchers from different fields, including chemistry, biology, materials science, medicine, etc., thus promoting the development of AIE in the fields of life and health.

Gold for the generation and control of fluxional barbaralyl cations.

The frog prince with his two identities pales in comparison with the shape-shifting barbaralyl cation, which exists as a mixture of 181,400 degenerate forms. Gold-catalyzed cycloisomerizations of 7-alkynyl cyclohepta-1,3,5-trienes were found to proceed via fluxional barbaralyl intermediates. The evolution of the intermediates into 1- or 2-substituted indenes could be controlled by the choice of gold complex.

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring.

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.