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Stereoisomerism, stemming from the spatial orientation of components in molecular structures, plays a decisive role in nature. While the unconventional bonding found in mechanically interlocked molecules gives rise to unique expressions of stereochemistry, the exploration of their stereoisomers is still in its infancy. Sequence isomerism, characterized by variations in the ordering of mechanically interlocked components in catenanes and rotaxanes, mirrors the sequence variations found in biological macromolecules. Herein, we report the use of artificial molecular pumps for the precise and simple production of sequentially isomeric hetero[3]rotaxanes. Utilizing redox-driven pumping cassettes with different rings, we have synthesized two hetero[3]rotaxane isomers in high isolated yields from two [2]rotaxanes. This research represents a significant advance in sequential molecular assembly, paving the way for the development of sophisticated, functionalized, mechanically interlocked materials.

In reticular chemistry, molecular building blocks are designed to create crystalline open frameworks. A key principle of reticular chemistry is that the most symmetrical networks are the likely outcomes of reactions, particularly when highly symmetrical building blocks are involved. The strategy of synthesizing low-dimensional networks aims to reduce explicitly the symmetry of the molecular building blocks. Here we report the spontaneous formation of hydrogen-bonded fibrous structures from trigonal prismatic building blocks, which were designed to form three-dimensional crystalline networks on account of their highly symmetrical structures. Utilizing different microscopic and spectroscopic techniques, we identify the structures at the early stages of the assembly process in order to and understand the growth mechanism. The symmetrical molecular building blocks are incorporated preferentially in the longitudinal direction, giving rise to anisotropic hydrogen-bonded porous organic nanotubes. Entropy-driven anisotropic growth provides micrometer-scale unidirectional nanotubes with high porosity. By combining experimental evidence and theoretical modeling, we have obtained a deep understanding of the nucleation and growth processes. Our findings offer fundamental insight into the molecular design of tubular structures. The nanotubes evolve further in the transverse directions to provide extended higher-order fibrous structures [nano- and microfibers], ultimately leading to large-scale interconnected hydrogen-bonded fiber-like structures with twists and turns. Our work provides fundamental understanding and paves the way for innovative molecular designs in low-dimensional networks.

The development of architecturally unique molecular nanocarbons by bottom-up organic synthesis is essential for accessing functional organic materials awaiting technological developments in fields such as energy, electronics, and biomedicine. Herein, we describe the design and synthesis of a triptycene-based three-dimensional (3D) nanocarbon, GFN-1 , with geometrical flexibility on account of its three peripheral π-panels being capable of interconverting between two curved conformations. An effective through-space electronic communication among the three π-panels of GFN-1 has been observed in its monocationic radical form, which exhibits an extensively delocalized spin density over the entire 3D π-system as revealed by electron paramagnetic resonance and UV-vis-NIR spectroscopies. The flexible 3D molecular architecture of GFN-1 , along with its densely packed superstructures in the presence of fullerenes, is revealed by microcrystal electron diffraction and single-crystal X-ray diffraction, which establish the coexistence of both propeller and tweezer conformations in the solid state. GFN-1 exhibits strong binding affinities for fullerenes, leading to host-guest complexes that display rapid photoinduced electron transfer within a picosecond. The outcomes of this research could pave the way for the utilization of shape and electronically complementary nanocarbons in the construction of functional coassemblies.

The sequence-controlled assembly of nucleic acids and amino acids into well-defined superstructures constitutes one of the most revolutionary technologies in modern science. The elaboration of such superstructures from carbohydrates, however, remains elusive and largely unexplored on account of their intrinsic constitutional and configurational complexity, not to mention their inherent conformational flexibility. Here, we report the bottom-up assembly of two classes of hierarchical superstructures that are formed from a highly flexible cyclo-oligosaccharide─namely, cyclofructan-6 (CF-6). The formation of coordinative bonds between the oxygen atoms of CF-6 and alkali metal cations (i) locks a myriad of flexible conformations of CF-6 into a few rigid conformations, (ii) bridges adjacent CF-6 ligands, and (iii) gives rise to the multiple-level assembly of three extended frameworks. The hierarchical superstructures present in these frameworks have been shown to modulate their nanomechanical properties. This research highlights the unique opportunities of constructing convoluted superstructures from carbohydrates and should encourage future endeavors in this underinvestigated field of science.

Enzymes are natural catalysts for a wide range of metabolic chemical transformations, including selective hydrolysis, oxidation, and phosphorylation. Herein, we demonstrate a strategy for the encapsulation of enzymes within a highly stable zirconium-based metal-organic framework. UiO-66-F 4 was synthesized under mild conditions using an enzyme-compatible amino acid modulator, serine, at a modest temperature in an aqueous solution. Enzyme@UiO-66-F 4 biocomposites were then formed by an in situ encapsulation route in which UiO-66-F 4 grows around the enzymes and, consequently, provides protection for the enzymes. A range of enzymes, namely, lysozyme, horseradish peroxidase, and amano lipase, were successfully encapsulated within UiO-66-F 4 . We further demonstrate that the resulting biocomposites are stable under conditions that could denature many enzymes. Horseradish peroxidase encapsulated within UiO-66-F 4 maintained its biological activity even after being treated with the proteolytic enzyme pepsin and heated at 60 °C. This strategy expands the toolbox of potential metal-organic frameworks with different topologies or functionalities that can be used as enzyme encapsulation hosts. We also demonstrate that this versatile process of in situ encapsulation of enzymes under mild conditions (i.e., submerged in water and at a modest temperature) can be generalized to encapsulate enzymes of various sizes within UiO-66-F 4 while protecting them from harsh conditions (i.e., high temperatures, contact with denaturants or organic solvents).

Stimuli-responsive molecular crystals have attracted considerable attention as promising smart materials with applications in various fields such as sensing, actuation, and optoelectronics. Understanding the structure-mechanical property relationships, however, remains largely unexplored when it comes to functionalizing these organic crystals. Here, we report three polymorphic crystals ( Forms A , B , and C ) formed by the non-threaded complexation of a dibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammonium cation, each exhibiting distinct thermal phase transitions, photoinduced deformations, and mechanical behavior. Structural changes on going from Form A to Form B and from Form C to Form B during heating and cooling, respectively, are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibits isotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness under photoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulus demonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of the azobenzene unit. This work represents a significant advance in the understanding of the correlation between structure-thermomechanical and structure-photomechanical properties necessary for the development of multi-stimulus-responsive materials with tailored properties.

Supramolecular electronics provide an opportunity to introduce molecular assemblies into electronic devices through a combination of noncovalent interactions such as [π···π] and hydrogen-bonding interactions. The fidelity and dynamics of noncovalent interactions hold considerable promise when it comes to building devices with controllable and reproducible switching functions. Here, we demonstrate a strategy for building electronically robust switches by harnessing two different noncovalent interactions between a couple of pyridine derivatives. The single-supermolecule switch is turned ON when compressing the junction enabling [π···π] interactions to dominate the transport, while the switch is turned OFF by stretching the junction to form hydrogen-bonded dimers, leading to a dramatic decrease in conductance. The robustness and reproducibility of these single-supermolecule switches were achieved by modulating the junction with Ångström precision at frequencies of up to 190 Hz while obtaining high ON/OFF ratios of ∼600. The research presented herein opens up an avenue for designing robust bistable mechanoresponsive devices which will find applications in the building of integrated circuits for microelectromechanical systems.

Organic trisradicals featuring threefold symmetry have attracted significant interest because of their unique magnetic properties associated with spin frustration. Herein, we describe the synthesis and characterization of a triangular prism-shaped organic cage for which we have coined the name PrismCage 6+ and its trisradical trication─ TR 3(•+) . PrismCage 6+ is composed of three 4,4'-bipyridinium dications and two 1,3,5-phenylene units bridged by six methylene groups. In the solid state, PrismCage 6+ adopts a highly twisted conformation with close to C 3 symmetry as a result of encapsulating one PF 6 - anion as a guest. PrismCage 6+ undergoes stepwise reduction to its mono-, di-, and trisradical cations in MeCN on account of strong electronic communication between its 4,4'-bipyridinium units. TR 3(•+) , which is obtained by the reduction of PrismCage 6+ employing CoCp 2 , adopts a triangular prism-shaped conformation with close to C 2 v symmetry in the solid state. Temperature-dependent continuous-wave and nutation-frequency-selective electron paramagnetic resonance spectra of TR 3(•+) in frozen N , N -dimethylformamide indicate its doublet ground state. The doublet-quartet energy gap of TR 3(•+) is estimated to be -0.08 kcal mol -1 , and the critical temperature of spin-state conversion is found to be ca. 50 K, suggesting that it displays pronounced spin frustration at the molecular level. To the best of our knowledge, this example is the first organic radical cage to exhibit spin frustration. The trisradical trication of PrismCage 6+ opens up new possibilities for fundamental investigations and potential applications in the fields of both organic cages and spin chemistry.

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Quantum sensing affords the possibility of using quantum entanglement to probe electromagnetic fields with exquisite sensitivity. In this work, we show that a photogenerated spin-correlated radical ion pair (SCRP) can be used to sense an electric field change created at one radical ion of the pair using molecular recognition. The SCRP is generated within a covalent donor-chromophore-acceptor system PXX-PMI-NDI, 1 , where PXX = peri -xanthenoxanthene, PMI = 1,6-bis( p - t -butylphenoxy)perylene-3,4-dicarboximide, and NDI = naphthalene-1,8:4,5-bis(dicarboximide). The electron-rich PXX donor in 1 acts as a guest molecule that can be encapsulated selectively by a tetracationic cyclophane ExBox 4+ host to give a supramolecular complex 1 ⊂ ExBox 4+ . Selective photoexcitation of the PMI chromophore results in ultrafast generation of the PXX •+ -PMI-NDI •- SCRP. When PXX is encapsulated by ExBox 4+ , the cyclophane generates an electric field that repels the positive charge on PXX •+ within PXX •+ -PMI-NDI •- , reducing the SCRP distance, i.e., the distance between the centers-of-charge on the donor and acceptor. Pulse-EPR measurements are used to measure the coherent oscillations created primarily by the electron-electron dipolar coupling in the SCRP, which yields the distance between the two charges (spins) of PXX •+ -PMI-NDI •- . The experimental results show that the distance between PXX •+ and NDI •- decreases when ExBox 4+ encapsulates PXX •+ , which demonstrates that the SCRP can function as a quantum sensor to detect electric field changes in the vicinity of the radical ions.

Single-crystal-to-single-crystal (SCSC) polymerization offers an effective protocol for the environmentally friendly preparation of polymer single crystals (PSCs) with extremely high crystallinity and very large molecular weights. Single-crystal X-ray diffraction (SCXRD) serves as a powerful technique for the in-depth characterization of their structures at a molecular level. Hence, a fundamental understanding of the structure-property relationships of PSCs is within our reach. Most of the reported PSCs, however, suffer from poor solubility, a property which hampers their post-functionalization and solution processability when it comes to practical applications. Here, we report soluble and processable PSCs with rigid polycationic backbones by taking advantage of an ultraviolet-induced topochemical polymerization from an elaborately designed monomer that results in a multitude of photoinduced [2 + 2] cycloadditions. The high crystallinity and excellent solubility of the resulting polymeric crystals enable their characterization both in the solid state by X-ray crystallography and electron microscopy and in the solution phase by NMR spectroscopy. The topochemical polymerization follows first-order reaction kinetics to a first approximation. Post-functionalization of the PSCs by anion exchange renders them super-hydrophobic materials for water purification. Solution processability endows PSCs with excellent gel-like rheological properties. This research represents a major step towards the controlled synthesis and full characterization of soluble single-crystalline polymers, which may find application in the fabrication of PSCs with many different functions.

Triplet-triplet annihilation-based molecular photon upconversion (TTA-UC) is a photophysical phenomenon that can yield high-energy emitting photons from low-energy incident light. TTA-UC is believed to fuse two triplet excitons into a singlet exciton through several consecutive energy-conversion processes. When organic aromatic dyes─i.e., sensitizers and annihilators─are used in TTA-UC, intermolecular distances, as well as relative orientations between the two chromophores, are important in an attempt to attain high upconversion efficiencies. Herein, we demonstrate a host-guest strategy─e.g., a cage-like molecular container incorporating two porphyrinic sensitizers and encapsulating two perylene emitters inside its cavity─to harness photon upconversion. Central to this design is tailoring the cavity size (9.6-10.4 Å) of the molecular container so that it can host two annihilators with a suitable [π···π] distance (3.2-3.5 Å). The formation of a complex with a host:guest ratio of 1:2 between a porphyrinic molecular container and perylene was confirmed by NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry (ITC) as well as by DFT calculations. We have obtained TTA-UC yielding blue emission at 470 nm when the complex is excited with low-energy photons. This proof-of-concept demonstrates that TTA-UC can take place in one supermolecule by bringing together the sensitizers and annihilators. Our investigations open up some new opportunities for addressing several issues associated with supramolecular photon upconversion, such as sample concentrations, molecular aggregation, and penetration depths, which have relevance to biological imaging applications.

Near-infrared (NIR) light is known to have outstanding optical penetration in biological tissues and to be non-invasive to cells compared with visible light. These characteristics make NIR-specific light optimal for numerous biological applications, such as the sensing of biomolecules or in theranostics. Over the years, significant progress has been achieved in the synthesis of fluorescent cyclophanes for sensing, bioimaging, and making optoelectronic materials. The preparation of NIR-emissive porphyrin-free cyclophanes is, however, still challenging. In an attempt for fluorescence emissions to reach into the NIR spectral region, employing organic tetracationic cyclophanes, we have inserted two 9,10-divinylanthracene units between two of the pyridinium units in cyclobis(paraquat- p -phenylene). Steady-state absorption, fluorescence, and transient-absorption spectroscopies reveal the deep-red and NIR photoluminescence of this cyclophane. This tetracationic cyclophane is highly soluble in water and has been employed successfully as a probe for live-cell imaging in a breast cancer cell line (MCF-7).

Designing and controlling charge transfer (CT) pathways in organic semiconductors are important for solar energy applications. To be useful, a photogenerated, Coulombically bound CT exciton must further separate into free charge carriers; direct observations of the detailed CT relaxation pathways, however, are lacking. Here, photoinduced CT and relaxation dynamics in three host-guest complexes, where a perylene ( Per ) electron donor guest is incorporated into two symmetric and one asymmetric extended viologen cyclophane acceptor hosts, are presented. The central ring in the extended viologen is either p -phenylene ( ExV 2+ ) or electron-rich 2,5-dimethoxy- p -phenylene ( ExMeOV 2+ ), resulting in two symmetric cyclophanes with unsubstituted or methoxy-substituted central rings, ExBox 4+ and ExMeOBox 4+ , respectively, and an asymmetric cyclophane with one of the central viologen rings being methoxylated ExMeOVBox 4+ . Upon photoexcitation, the asymmetric host-guest ExMeOVBox 4+ ⊃ Per complex exhibits directional CT toward the energetically unfavorable methoxylated side due to structural restrictions that facilitate strong interactions between the Per donor and the ExMeOV 2+ side. The CT state relaxation pathways are probed using ultrafast optical spectroscopy by focusing on coherent vibronic wavepackets, which are used to identify CT relaxations along charge localization and vibronic decoherence coordinates. Specific low- and high-frequency nuclear motions are direct indicators of a delocalized CT state and the degree of CT character. Our results show that the CT pathway can be controlled by subtle chemical modifications of the acceptor host in addition to illustrating how coherent vibronic wavepackets can be used to probe the nature and time evolution of the CT states.

Cocrystal engineering, involving the assembly of two or more components into a highly ordered solid-state superstructure, has emerged as a popular strategy for tuning the photophysical properties of crystalline materials. The reversible co-assembly and disassembly of multicomponent cocrystals and their reciprocal transformation in the solid state remain challenging objectives. Herein, we report a color-tunable upconversion-emission switch based on the interconversion between two cocrystals. One red- and one yellow-emissive cocrystal, composed of an electron-deficient naphthalenediimide-based triangular macrocycle and different electron donors, have been obtained. The red- and yellow-emissive cocrystals undergo reversible transformations on exchanging the electron donors. Benefiting from intermolecular charge transfer interactions, the two cocrystals display superior two-photon excited upconversion emission. Accompanying the interconversion of the two cocrystals, their luminescent color changes between red and yellow, forming a dual-color upconversion-emission switch. This research provides a rare yet critical example involving precise control of cocrystal-to-cocrystal transformation and affords a reference for fabricating color-tunable nonlinear optical materials in the solid state.

Motion-induced change in emission (MICE) is a phenomenon that can be employed to develop various types of probes, including temperature and viscosity sensors. Although MICE, arising from the conformational motion in particular compounds, has been studied extensively, this phenomenon has not been investigated in depth in mechanically interlocked molecules (MIMs) undergoing coconformational changes. Herein, we report the investigation of a thermoresponsive dynamic homo[2]catenane incorporating pyrene units and displaying relative circumrotational motions of its cyclophanes as evidenced by variable-temperature 1 H NMR spectroscopy and supported by its visualization through molecular dynamics simulations and quantum mechanics calculations. The relative coconformational motions induce a significant change in the fluorescence emission of the homo[2]catenane upon changes in temperature compared with its component cyclophanes. This variation in the exciplex emission of the homo[2]catenane is reversible as demonstrated by four complete cooling and heating cycles. This research opens up possibilities of using the coconformational changes in MIMs-based chromophores for probing fluctuations in temperature which could lead to applications in biomedicine or materials science.

Molecular recognition, based on noncovalent bonding interactions, plays a central role in directing supramolecular phenomena in both chemical and biological environments. The identification and investigation of weakly associated recognition motifs, however, remains a major challenge, especially when the motifs are interlinked with and obscured by other robust binding modes in complicated systems. For example, although the host-guest recognition between the radical cations of both cyclobis(paraquat- p -phenylene) (CBPQT) and 4,4'-bipyridinium (BIPY) salts has been thoroughly investigated, the question of whether other binding modes exist between these two positively charged entities is the subject of some debate because of the complexity and dynamic nature of this supramolecular system. In order to address this conundrum, we have synthesized a [2]catenane─formed by mechanical interlocking between CBPQT and another BIPY-containing ring─which enhances the weak interactions between components and reduces significantly the complexity of the system for easier characterization. By employing this [2]catenane as a model compound, we have performed a full-spectrum investigation of radical interactions and revealed unambiguously a total of three possible binding modes between CBPQT and BIPY─to be specific, a bisradical tetracationic, a trisradical tricationic, and a bisradical dicationic association─as demonstrated by various methods of characterization including UV/vis/NIR, EPR, and NMR spectroscopies, electrochemical measurements and X-ray crystallography. The two newly discovered bisradical binding modes have potential applications in the construction of self-assembled materials and in mediating supramolecular catalysis. The mechanical bond-assisted approach used in this research is broadly applicable to investigating noncovalent bonding interactions.

Tröger's base (TB) and its derivatives have been studied extensively due to their unique concave shape stemming from the endomethylene strap. However, the strap-clipped TB chemistry has been largely overlooked in metal-organic framework (MOF) solids, leading to a gap in our knowledge within this field. In this work, we report the in situ strap elimination of a carboxylate-carrying TB in the presence of formic acid, both in solution and in Zr(IV)-based MOFs. In the solution system, the methanodiazocine nucleus can be exclusively transformed into an N , N' -diformyl-decorated phenhomazine derivative, regardless of the solvent used (DMF, DMA, or DEF), as unambiguously uncovered by single crystal X-ray crystallography. In contrast, while in the MOF synthetic system, the degree of derivatization reaction can be effectively controlled to give either the secondary diamine or formyl-decorated diamine, depending on the solvent used (DMF or DEF), resulting in the formation of two Zr-MOFs with 8-connected bcu (NU-1900) and 12-connected fcu (NU-407) topologies, respectively. The derivatization mechanism is proposed to be topology-guided and dependent on the local acid concentration during the MOF formation processes. Moreover, we discovered a novel post-synthetically water-induced in situ linker formylation process in NU-1900 through sequential formic acid elimination, migration, and condensation processes, affording an isostructural framework with the same linker as in NU-407, which further corroborates our proposed mechanism. Additionally, the highly defective NU-1900 with abundant accessible Zr sites was demonstrated to be an outstanding catalyst for the detoxification of a nerve agent simulant with a half-life of less than 1 min.

Despite the advances in host-guest chemistry, macrocyclic hosts with deep cavities are far from abundant among the large number of wholly synthetic hosts described in the literature. Herein, we describe the design and synthesis of two new tubular hosts, namely, corral[4]arene and corral[5]arene. The former has been isolated and characterized as two conformational diastereoisomers, one is centrosymmetric and the other asymmetric. The latter, a fivefold symmetrical and flexible host, has also been investigated in detail. It is composed of five 4,4'-dimethoxybiphenyl units bridged by ethynylene linkers at their 2,2'-positions and adopts a pentagonal conformation with a tubular-shaped cavity in the presence of guests. This structure endows corral[5]arene not only with a conjugated backbone, capable of bright fluorescent emission (quantum yield, 56%), but also a deep π-electron-rich aromatic cavity with remarkable conformational flexibility. The adaptive cavity of corral[5]arene allows it to accommodate a wide range of neutral and positively charged electron-deficient guests with different molecular sizes and shapes. Binding constants between this host and these guests in three different nonpolar organic solvents lie in the range of 10 3 to 10 7 M -1 . Moreover, corral[5]arene exhibits dynamic chirality on account of the axes of chirality associated with each of the five biphenyl units and displays first-order transformation as exhibited by circular dichroism in response to the addition of chiral guests. All these stereochemical features render corral[5]arene an attractive host for a variety of supramolecular and nanotechnological applications.

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para -terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Traditionally, the synthesis of polyrotaxanes has been limited by synthetic methods that rely on an innate affinity between the rings and the polymer chains. The use of rotaxane-forming molecular pumps allows this limitation to be circumvented in the production of non-equilibrium polyrotaxanes in which rings are trapped on polymer chains for which they have little or no affinity. Pumping cassettes, each composed of a bipyridinium unit linked (i) by a bismethylene bridge to a terminal 2,6-dimethylpyridinium cationic unit and (ii) by a methylene group to an isopropylphenylene steric barrier, were attached using copper-catalyzed azide-alkyne cycloadditions to the ends of a polypropylene glycol (PPG) chain of number-average molecular weight M n ≈ 2200. Using a one-pot electrosynthetic protocol, a series of PPG-based polyrotaxanes with cyclobis(paraquat- p -phenylene) as the rings were synthesized. Despite the steric bulk of the PPG backbone, it was found to be a suitable collecting chain for threading up to 10 rings. The pumping of two rings is sufficient to render these hydrophobic polymers soluble in aqueous solution. Their hydrodynamic diameters and diffusion constants vary according to the number of pumped rings. The non-equilibrium nature of these polyrotaxanes is manifested in their gradual degradation and dethreading at elevated temperatures.

Mechanochemical syntheses of rotaxanes have attracted considerable attention of late because of the superior reaction rates and higher yields associated with their production compared with analogous reactions carried out in solution. Previous investigators, however, have focused on the demonstration of the mechanochemical syntheses of rotaxanes per se, rather than on studying the solid-phase host-guest molecular interplay related to their rapid formation and high yields. In this investigation, we attribute the lower yields of rotaxanes prepared in solution to the limited concentration and a desolvation energy penalty that must be compensated for by host-guest interactions during complexation that precedes the templation leading to rotaxane formation. It follows that, if the desolvation energy can be removed and higher concentrations can be attained, even weak host-guest interactions can drive the complexation of host and guest molecules efficiently. In order to test this hypothesis, we chose two host-guest pairs of permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) for the simple reason that they exhibit extremely low binding constants (2.7 ± 0.4 M -1 when 1,6-diaminohexane is the guest and <0.1 M -1 when 2,2'-(ethylenedioxy)bis(ethylamine) is the guest in CDCl 3 ; i.e., ostensibly no pseudorotaxane formation is observed). We argue that the amount of pseudorotaxanes formed in the solid state is responsive to mechanical treatments or otherwise and changes in temperature during stoppering reactions. Compared to the amount of pseudorotaxanes that can be obtained in solution, large quantities of pseudorotaxanes are formed in the solid state because of concentration and desolvation effects. This mechanochemical enhancement of pseudorotaxane formation is referred to as a self-correction in the current investigation. Rotaxanes based on permethylated pillar[5]arene/1,6-diaminohexane and permethylated pillar[5]arene/2,2'-(ethylenedioxy)bis(ethylamine) have been synthesized in much higher yields compared to those obtained in solution, aided and abetted by self-correction effects during mechanical treatments and heating at a mild temperature of 50 °C.

Industrial-scale thermal separation processes have contributed greatly to the rise in carbon dioxide emissions. Porous materials, such as metal-organic frameworks (MOFs), can potentially reduce these emissions by achieving nonthermal chemical separations through the physical adsorption of targeted species with high selectivity. Here, we report the synthesis of the channel-based MOFs NU-2000 and NU-2001 , which are constructed from three-dimensional (3D) linkers, to separate the industrially relevant xylene isomers under ambient conditions by leveraging sub-Ångstrom differences in the sizes of each isomer. While the rotation of two-dimensional (2D) linkers in MOFs often affords changes in pore apertures and pore sizes that are substantial enough to hinder separation efficiency, increasing the linker dimensionality from 2D to three-dimensional (3D) enables precise control of the MOF pore size and aperture regardless of the linker orientation, establishing this design principle as a broadly applicable strategy.

Three achiral polycyclic aromatic fluorophores─namely, 1-pyrenecarboxylic acid, 9-anthracenecarboxylic acid, and perylene-3,9-dicarboxylic acid─were chosen based on their desired properties before being incorporated into the construction of a K + -carrying gamma-cyclodextrin (γ-CD)-based metal-organic framework (CD-MOF-1) and γ-CD-containing hybrid frameworks (CD-HFs). Among these fluorophores, only the pyrene-carrying one shows significant noncovalent bonding interactions with γ-CD in solution. This fluorophore is encapsulated in a CD-HF with a trigonal superstructure instead of the common cubic CD-MOF-1 found in the case of the other two fluorophores. Single-crystal X-ray diffraction analysis of the trigonal CD-HF reveals a π-stacked chiral positioning of the pyrene-carrying fluorophore inside the (γ-CD) 2 tunnels and held uniformly around an enantiomorphous 3 2 screw axis along the c direction in the solid-state structure. This helix-like structure demonstrates an additional level of chirality over and above the point-chiral stereogenic centers of γ-CD and the axial chirality associated with the self-assembled π-stacked fluorophores. These arrangements result in specifically generated photophysical and chiroptical properties, such as the controlled emergence of circularly polarized luminescence (CPL) emission. In this manner, a complete understanding of the mechanism of chirality transfer from a chiral host (CD-HF) to an encapsulated achiral fluorophore has been achieved, an attribute which is often missing in the development of materials with CPL.

Artificial molecular pumps (AMPs), inspired by the active cellular transport exhibited in biological systems, enable cargoes to undergo unidirectional motion, courtesy of molecular ratchet mechanisms in the presence of energy sources. Significant progress has been achieved, using alternatively radical interactions and Coulombic repulsive forces to create working AMPs. In an attempt to widen the range of these AMPs, we have explored the effect of molecular pumping on the photophysical properties of a collecting chain on a dumbbell incorporating a centrally located pyrene fluorophore and two terminal pumping cassettes. The AMP discussed here sequesters two tetracationic cyclophanes from the solution, generating a [3]rotaxane in which the fluorescence of the dumbbell is quenched. The research reported in this Article demonstrates that the use of pumping cassettes allows us to generate the [3]rotaxane in which the photophysical properties of fluorophores can be modified in a manner that cannot be achieved with a mixture of the dumbbell and ring components of the rotaxane on account of their weak binding in solution.

The development of synthetic receptors that recognize carbohydrates in water with high selectivity and specificity is challenging on account of their structural complexity and strong hydrophilicity. Here, we report on the design and synthesis of two pyrene-based, temple-shaped receptors for the recognition of a range of common sugars in water. These receptors rely on the use of two parallel pyrene panels, which serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates. In addition, eight polarized pyridinium C-H bonds, projecting from the roofs and floors of the temple receptors toward the binding cavities, form [C-H···O] hydrogen bonds, with the equatorially oriented OH groups on the sugars located inside the hydrophobic cavities. Four para -xylylene pillars play a crucial role in controlling the distance between the roof and floor. These temple receptors are highly selective for the binding of glucose and its derivatives. Furthermore, they show enhanced fluorescence upon binding with glucose in water, a property which is useful for glucose-sensing in aqueous solution.

For the most part, enzymes contain one active site wherein they catalyze in a serial manner chemical reactions between substrates both efficiently and rapidly. Imagine if a situation could be created within a chiral porous crystal containing trillions of active sites where substrates can reside in vast numbers before being converted in parallel into products. Here, we report how it is possible to incorporate 1-anthracenecarboxylate ( 1-AC - ) as a substrate into a γ-cyclodextrin-containing metal-organic framework ( CD-MOF-1 ), where the metals are K + cations, prior to carrying out [4+4] photodimerizations between pairs of substrate molecules, affording selectively one of four possible regioisomers. One of the high-yielding regioisomers exhibits optical activity as a result of the presence of an 8:1 ratio of the two enantiomers following separation by high-performance liquid chromatography. The solid-state superstructure of 1-anthracenecarboxylate potassium salt ( 1-ACK ), which is co-crystallized with γ-cyclodextrin, reveals that pairs of substrate molecules are not only packed inside tunnels between spherical cavities present in CD-MOF-1 , but also stabilized-in addition to hydrogen-bonding to the C-2 and C-3 hydroxyl groups on the d-glucopyranosyl residues present in the γ-cyclodextrin tori-by combinations of hydrophobic and electrostatic interactions between the carboxyl groups in 1-AC - and four K + cations on the waistline between the two γ-cyclodextrin tori in the tunnels. These non-covalent bonding interactions result in preferred co-conformations that account for the highly regio- and enantioselective [4+4] cycloaddition during photoirradiation. Theoretical calculations, in conjunction with crystallography, support the regio- and stereochemical outcome of the photodimerization.

Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.

Macrocycles that assemble into nanotubes exhibit emergent properties stemming from their low dimensionality, structural regularity, and distinct interior environments. We report a versatile strategy to synthesize diverse nanotube structures in a single, efficient reaction by using a conserved building block bearing a pyridine ring. Imine condensation of a 2,4,6-triphenylpyridine-based diamine with various aromatic dialdehydes yields chemically distinct pentagonal [5 + 5], hexagonal [3 + 3], and diamond-shaped [2 + 2] macrocycles depending on the substitution pattern of the aromatic dialdehyde monomer. Atomic force microscopy and in solvo X-ray diffraction demonstrate that protonation of the macrocycles under the mild conditions used for their synthesis drives assembly into high-aspect ratio nanotubes. Each of the pyridine-containing nanotube assemblies exhibited measurable proton conductivity by electrochemical impedance spectroscopy, with values as high as 10 -3 S m -1 (90% R.H., 25 °C) that we attribute to differences in their internal pore sizes. This synthetic strategy represents a general method to access robust nanotube assemblies from a universal pyridine-containing monomer, which will enable systematic investigations of their emergent properties.

Colored charge-transfer complexes can be formed by the association between electron-rich donor and electron-deficient acceptor molecules, bringing about the narrowing of HOMO-LUMO energy gaps so that they become capable of harnessing visible light. In an effort to facilitate the use of these widespread, but nonetheless weak, interactions for visible light photocatalysis, it is important to render the interactions strong and robust. Herein, we employ a well-known donor-acceptor [2]catenane-formed by the mechanical interlocking of cyclobis(paraquat- p -phenylene) and 1,5-dinaphtho[38]crown-10-in which the charge-transfer interactions between two 4,4'-bipyridinium and two 1,5-dioxynaphthalene units are enhanced by mechanical bonding, leading to increased absorption of visible light, even at low concentrations in solution. As a result, since this [2]catenane can generate persistent bipyridinium radical cations under continuous visible-light irradiation without the need for additional photosensitizers, it can display good catalytic activity in both photo-reductions and -oxidations, as demonstrated by hydrogen production-in the presence of platinum nanoparticles-and aerobic oxidation of organic sulfides, such as l-methionine, respectively. This research, which highlights the usefulness of nanoconfinement present in mechanically interlocked molecules for the reinforcement of weak interactions, can not only expand the potential of charge-transfer interactions in solar energy conversion and synthetic photocatalysis but also open up new possibilities for the development of active artificial molecular shuttles, switches, and machines.

Pumps and motors are essential components of the world as we know it. From the complex proteins that sustain our cells, to the mechanical marvels that power industries, much we take for granted is only possible because of pumps and motors. Although molecular pumps and motors have supported life for eons, it is only recently that chemists have made progress toward designing and building artificial forms of the microscopic machinery present in nature. The advent of artificial molecular machines has granted scientists an unprecedented level of control over the relative motion of components of molecules through the development of kinetically controlled, away-from-thermodynamic equilibrium chemistry. We outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective. A key insight connecting biomolecular and artificial molecular machines is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium at every instant. The operation of molecular motors and pumps can be described by trajectory thermodynamics, a theory based on the work of Onsager, which is grounded on the firm foundation of the principle of microscopic reversibility. Free energy derived from thermodynamically non-equilibrium reactions kinetically favors some reaction pathways over others. By designing molecules with kinetic asymmetry, one can engineer potential landscapes to harness external energy to drive the formation and maintenance of geometries of component parts of molecules away-from-equilibrium, that would be impossible to achieve by standard synthetic approaches.

The interaction of low-energy light with matter that leads to the production of high-energy light is known as photon upconversion. This phenomenon is of importance because of its potential applications in optoelectronics, energy harvesting, and the biomedical arena. Herein, we report a pillared-paddlewheel metal-organic framework (MOF), constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC). Single-crystal X-ray diffraction studies reveal that the Zn-metalated sensitizers are coordinated to Zn 2 nodes in a paddlewheel fashion, forming 2D sheets, to which are linked annihilators, such that each sensitizer is connected to five of them. The precise arrangements of sensitizers with respect to annihilators, and the high annihilator-to-sensitizer ratio, facilitate Dexter energy transfer. This level of organization in an extended structure leads to a high TTA-UC efficiency of 1.95% (theoretical maximum = 50%) at an excitation power density of 25 mW cm -2 .

The charge transport in single-molecule junctions depends critically on the chemical identity of the anchor groups that are used to connect the molecular wires to the electrodes. In this research, we report a new anchoring strategy, called the electrostatic anchor, formed through the efficient Coulombic interaction between the gold electrodes and the positively charged pyridinium terminal groups. Our results show that these pyridinium groups serve as efficient electrostatic anchors forming robust gold-molecule-gold junctions. We have also observed binary switching in dicationic viologen molecular junctions, demonstrating an electron injection-induced redox switching in single-molecule junctions. We attribute the difference in low- and high-conductance states to a dicationic ground state and a radical cationic metastable state, respectively. Overall, this anchoring strategy and redox-switching mechanism could constitute the basis for a new class of redox-activated single-molecule switches.

We report the molecular recognition of the Au(CN) 2 - anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN) 2 ⊂α-cyclodextrin, KAu(CN) 2 ⊂(α-cyclodextrin) 2 , and KAg(CN) 2 ⊂(α-cyclodextrin) 2 -demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN) 2 - and Ag(CN) 2 - with linear geometries, while the K + ions fulfill the role of linking α-cyclodextrin tori together as a result of [K + ···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN) 2 - and α-cyclodextrin in aqueous solution, revealed by 1 H NMR titrations, has produced binding constants in the order of 10 4 M -1 . Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN) 2 ⊂α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN) 2 - is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN) 2 can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN) 2 - in the presence of Ag(CN) 2 - , which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.

The solid-state properties of organic radicals depend on radical-radical interactions that are influenced by the superstructure of the crystalline phase. Here, we report the synthesis and characterization of a substituted tetracationic cyclophane, cyclobis(paraquat- p -1,4-dimethoxyphenylene), which associates in its bisradical dicationic redox state with the methyl viologen radical cation ( MV •+ ) to give a 1:1 inclusion complex. The (super)structures of the reduced cyclophane and this 1:1 complex in the solid state deviate from the analogous (super)structures observed for the reduced state of cyclobis(paraquat- p -phenylene) and that of its trisradical tricationic complex. Titration experiments reveal that the methoxy substituents on the p -phenylene linkers do not influence binding of the cyclophane toward small neutral guests-such as dimethoxybenzene and tetrathiafulvalene-whereas binding of larger radical cationic guests such as MV •+ by the reduced cyclophane decreases 10-fold. X-ray diffraction analysis reveals that the solid-state superstructure of the 1:1 complex constitutes a discrete entity with weak intermolecular orbital overlap between neighboring complexes. Transient nutation EPR experiments and DFT calculations confirm that the complex has a doublet spin configuration in the ground state as a result of the strong orbital overlap, while the quartet-state spin configuration is higher in energy and inaccessible at ambient temperature. Superconducting quantum interference device (SQUID) measurements reveal that the trisradical tricationic complexes interact antiferromagnetically and form a one-dimensional Heisenberg antiferromagnetic chain along the a -axis of the crystal. These results offer insights into the design and synthesis of organic magnetic materials based on host-guest complexes.

Donor-acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-of-equilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used single-molecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen units-π-acceptors commonly employed in mechanically interlocked molecules-and several π-donors afforded a characteristic force-distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed in situ demonstrated the sensitivity of the technique. This simple design could be exploited in quantifying the force of a large range of weak noncovalent bonding interactions as well as the potential work that molecular machines can generate at the single-molecule level.

Suitanes are a class of mechanically interlocked molecules (MIMs) that consist of two components: a body with limbs protruding outward and a suit that fits appropriately around it, so that there is no easy way for the suit to be removed from the body. Herein, we report the synthesis and characterization of a suit[3]ane, which contains a benzotrithiophene derivative ( THBTT ) with three protruding hexyl chains as the body and a 3-fold symmetric, extended pyridinium-based cage, namely, HexaCage 6+ , as the suit. Central to its realization is effective templation, provided by THBTT during cage formation, an observation that has been supported by the strong binding constant between benzotrithiophene ( BTT ) and the empty cage. The solid-state structure of the suit[3]ane reveals that the body is confined within the suit's cavity with its alkyl chains protruding outward through the orifices in the cage. Notably, such a seemingly unstable molecule, having three flexible alkyl chains as its only protruding limbs, does not dissociate after prolonged heating in CD 3 CN at 100 °C under pressure for 7 days. No evidence for guest exchange with the host was observed at this temperature in a 2:1 mixture of THBTT and HexaCage 6+ in CD 3 CN. The results indicate that flexible protruding limbs are sufficient for a suit[3]ane to remain mechanically stable even at high temperatures in solution.

Designing new materials for the effective detoxification of chemical warfare agents (CWAs) is of current interest given the recent use of CWAs. Although halogenated boron-dipyrromethene derivatives (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene or BDP or BODIPY) at the 2 and 6 positions have been extensively explored as efficient photosensitizers for generating singlet oxygen ( 1 O 2 ) in homogeneous media, their utilization in the design of porous organic polymers (POPs) has remained elusive due to the difficulty of controlling polymerization processes through cross-coupling synthesis pathways. Our approach to overcome these difficulties and prepare halogenated BODIPY-based porous organic polymers ( X-BDP-POP where X = Br or I) represents an attractive alternative through post-synthesis modification (PSM) of the parent hydrogenated polymer. Upon synthesis of both the parent polymer, H-BDP-POP , and its post-synthetically modified derivatives, Br-BDP-POP and I-BDP-POP , the BET surface areas of all POPs have been measured and found to be 640, 430, and 400 m 2 · g -1 , respectively. In addition, the insertion of heavy halogen atoms at the 2 and 6 positions of the BODIPY unit leads to the quenching of fluorescence (both polymer and solution-phase monomer forms) and the enhancement of phosphorescence (particularly for the iodo versions of the polymers and monomers), as a result of efficient intersystem crossing. The heterogeneous photocatalytic activities of both the parent POP and its derivatives for the detoxification of the sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES), have been examined; the results show a significant enhancement in the generation of singlet oxygen ( 1 O 2 ). Both the bromination and iodination of H-BDP-POP served to shorten by 5-fold of the time needed for the selective and catalytic photo-oxidation of CEES to 2-chloroethyl ethyl sulfoxide (CEESO).

One ring threaded by two other rings to form a non-intertwined ternary ring-in-rings motif is a challenging task in noncovalent synthesis. Constructing multicolor photoluminescence systems with tunable properties is also a fundamental research goal, which can lead to applications in multidimensional biological imaging, visual displays, and encryption materials. Herein, we describe the design and synthesis of binary and ternary ring-in-ring(s) complexes, based on an extended tetracationic cyclophane and cucurbit[8]uril. The formation of these complexes is accompanied by tunable multicolor fluorescence outputs. On mixing equimolar amounts of the cyclophane and cucurbit[8]uril, a 1:1 ring-in-ring complex is formed as a result of hydrophobic interactions associated with a favorable change in entropy. With the addition of another equivalent of cucurbit[8]uril, a 1:2 ring-in-rings complex is formed, facilitated by additional ion-dipole interactions involving the pyridinium units in the cyclophane and the carbonyl groups in cucurbit[8]uril. Because of the narrowing in the energy gaps of the cyclophane within the rigid hydrophobic cavities of cucurbit[8]urils, the binary and ternary ring-in-ring(s) complexes emit green and bright yellow fluorescence, respectively. A series of color-tunable emissions, such as sky blue, cyan, green, and yellow with increased fluorescence lifetimes, can be achieved by simply adding cucurbit[8]uril to an aqueous solution of the cyclophane. Notably, the smaller cyclobis(paraquat- p -phenylene), which contains the same p -xylylene linkers as the extended tetracationic cyclophane, does not form ring-in-ring(s) complexes with cucurbit[8]uril. The encapsulation of this extended tetracationic cyclophane by both one and two cucurbit[8]urils provides an incentive to design and synthesize more advanced supramolecular systems, as well as opening up a feasible approach toward achieving tunable multicolor photoluminescence with single chromophores.

The phenomenon of photon upconversion, in which a system absorbs two or more photons of lower energy and emits a photon of higher energy, has been used in numerous applications, including non-destructive bioimaging, deep-penetrating photodynamic therapy, catalysis, and photovoltaic devices. To date, photon upconversion has been observed typically in inorganic nanocrystals, nanoparticles, metal-organic frameworks, supramolecular assemblies, and organic dyads. Herein, we demonstrate a new strategy for harnessing photon upconversion-supramolecular upconversion-based on host-guest chemistry. We have identified a box-like fluorescent tetracationic host incorporating a thiazolothiazole emitter, which can accommodate a guest-sensitizer, 5,15-diphenylporphyrin, inside its cavity, and demonstrated that the host-guest inclusion complex displays triplet-fusion upconversion when the guest is excited with low-energy light. The strategy of supramolecular upconversion has been employed successfully in two other host-guest complexes-with hosts comprised of anthracene emitters and a 5,15-diphenylporphyrin guest-corroborating the fact that this strategy is a general one and can be applied to the design of a new family of host-guest complexes for photon upconversion. More importantly, supramolecular upconversion is accessible in solution under dilute conditions (μM) compared to most of the existing approaches that require significantly higher concentrations (mM) of emitters and/or sensitizers. Transient absorption spectroscopy and density functional theory have been employed in order to confirm a triplet-fusion upconversion mechanism. Host-guest complexation-mediated supramolecular photon upconversion eliminates multiple issues in the existing systems related to high working concentrations, high incident laser power, and low optical penetration depths.

The ability to control the relative motions of component parts in molecules is essential for the development of molecular nanotechnology. The advent of mechanically interlocked molecules (MIMs) has enhanced significantly the opportunities for chemists to harness such motions in artificial molecular machines (AMMs). Recently, we have developed artificial molecular pumps (AMPs) capable of producing highly energetic oligo- and polyrotaxanes with high precision. Here, we report the design, synthesis, and operation of an AMP incorporating a photocleavable stopper that allows for the use of orthogonal stimuli. Our approach employs a ratchet mechanism to pump a ring onto a collecting chain, forming an intermediate [2]rotaxane. At a subsequent time, application of light triggers the release of the ring back into the bulk solution with temporal control. This process is monitored by the quenching of the fluorescence of a naphthalene-based fluorophore. This design may find application in the fabrication of molecular transporting systems with on-demand functions.

We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely, an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state coconformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat- p -phenylene) (CBPQT 4+ ) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF 2+ dication, the CBPQT 4+ ring is forced toward the molecular rotor, leading to an increased energy barrier for the excited state to rotate the rotor into the state with a high nonradiative rate constant, resulting in an overall 3.4-fold fluorescence enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge-transfer state between the BODIPY rotor and the CBPQT 4+ ring, movement of the ring toward the BODIPY rotor produces an unexpectedly strong fluorescence signal decrease as the result of photoinduced electron transfer from the BODIPY rotor to the CBPQT 4+ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories, and molecular logic gates.

The assembly of a kinetically trapped daisy chain polymer under redox control has been achieved with a self-complementary monomer using an energy ratchet mechanism. The monomer is composed of a molecular pump at one end and a cyclobis(paraquat- p -phenylene) (CBPQT 4+ ) ring at the other end. The pump and ring are linked together by a long collecting chain. When the monomer is reduced to its radical state, it self-assembles into a supramolecular daisy chain polymer on account of radical-pairing interactions. When all of the bipyridinium radical cations are quickly oxidized to dications, the CBPQT 4+ rings are forced to thread onto the collecting chains, forming an out-of-equilibrium, kinetically trapped daisy chain polymer. This polymer can be switched reversibly back to the supramolecular polymer by reduction, followed by depolymerization to afford the monomer as a result of slow oxidation. This proof-of-concept investigation opens up opportunities for synthesizing mechanically interlocked polymers using molecular machines.

Suitanes are two-component mechanically interlocked molecules in which one (torso) of the components, with several protruding limbs, is encompassed by another (suit) all-in-one component. This kind of molecular ship-in-a-bottle architecture remains a challenging one to make. Herein, we report a porphyrin-based suit[4]ane, which is composed of a porphyrin with four protruding ligands (two phenyl groups and two bromine atoms) called the limbs that are encompassed by a tricyclic octacationic cyclophane. The suit[4]ane can be obtained by slippage, a mechanism by which the cyclophane is able to open up two of its entrances in order to take up the porphyrin at high temperature and close them down at low temperature, locking the porphyrin inside its binding cavity. Dynamic 1 H NMR spectroscopy reveals that the trapped porphyrin inside the cyclophane rocks back and forth in the suit[4]ane about 1000 times a second.

We report the encapsulation of free-base and zinc porphyrins by a tricyclic cyclophane receptor with subnanomolar binding affinities in water. The high affinities are sustained by the hydrophobic effect and multiple [CH···π] interactions covering large [π···π] stacking surfaces between the substrate porphyrins and the receptor. We discovered two co-conformational isomers of the 1:1 complex, where the porphyrin is orientated differently inside the binding cavity of the receptor on account of its tricyclic nature. The photophysical properties and chemical reactivities of the encapsulated porphyrins are modulated to a considerable extent by the receptor. Improved fluorescence quantum yields, red-shifted absorptions and emissions, and nearly quantitative energy transfer processes highlight the emergent photophysical enhancements. The encapsulated porphyrins enjoy unprecedented chemical stabilities, where their D/H exchange, protonation, and solvolysis under extremely acidic conditions are completely blocked. We anticipate that the ultrahigh stabilities and improved optical properties of these encapsulated porphyrins will find applications in single-molecule materials, artificial photodevices, and biomedical appliances.

Collisional intermolecular interactions between excited states form short-lived dimers and complexes that lead to the emergence of excimer/exciplex emission of lower energy, a phenomenon which must be differentiated from the photoluminescence (PL) arising from the monomeric molecules. Although the utilization of noncovalent bonding interactions, leading to the generation of excimer/exciplex PL, has been investigated extensively, precise control of the aggregates and their persistence at very low concentrations remains a rare phenomenon. In the search for a fresh approach, we sought to obtain exciplex PL from permanent structures by incorporating anthracene moieties into pyridinium-containing mechanically interlocked molecules. Beyond the optical properties of the anthracene moieties, their π-extended nature enforces [π···π] stacking that can overcome the Coulombic repulsion between the pyridinium units, affording an efficient synthesis of an octacationic homo[2]catenane. Notably, upon increasing the ionic strength by adding tetrabutylammonium hexafluorophosphate, the catenane yield increases significantly as a result of the decrease in Coulombic repulsions between the pyridinium units. Although the ground-state photophysical properties of the free cyclophane and the catenane are similar and show a charge-transfer band at ∼455 nm, their PL characters are distinct, denoting different excited states. The cyclophane emits at ∼562 nm (quantum yield ϕ F = 3.6%, emission lifetime τ s = 3 ns in MeCN), which is characteristic of a disubstituted anthracene-pyridinium linker. By contrast, the catenane displays an exciplex PL at low concentration (10 -8 M) with an emission band centered on 650 nm (ϕ F = 0.5%, τ s = 14 ns) in MeCN and at 675 nm in aqueous solution. Live-cell imaging performed in MIAPaCa-2 prostate cancer cells confirmed that the catenane exciplex emission can be detected at micromolar concentrations.

Two new highly charged [2]catenanes-namely, m He[2]C ·6PF 6 and m Ho[2]C ·6PF 6 -were synthesized by exploiting radical host-guest templation between derivatives containing BIPY •+ radical cations and the meta analogue of cyclobis(paraquat- p -phenylene). In contrast to related [2]catenanes that have been isolated as air-stable monoradicals, both m He[2]C ·6PF 6 and m Ho[2]C ·6PF 6 exist as air-stable singlet bisradicals, as evidenced by both X-ray crystallography in the solid state and EPR spectroscopy in solution. Electrochemical studies indicate that the first two reduction peaks of these two [2]catenanes are shifted significantly to more positive potentials, a feature which is responsible for their extraordinary stability in air. The mixed-valence nature of the mono- and bisradical states endows them with unique NIR absorption properties, e.g., NIR absorption bands for the mono- and bisradical states observed at ∼1800 and ∼1450 nm, respectively. These [2]catenanes are potentially useful in applications that include NIR photothermal conversion, UV-vis-NIR multiple-state electrochromic materials, and multiple-state memory devices. Our findings highlight the principle of "mechanical-bond-induced stabilization" as an efficient strategy for designing persistent organic radicals.

The efficient preparation of single-crystalline ionic polymers and fundamental understanding of their structure-property relationships at the molecular level remains a challenge in chemistry and materials science. Here, we describe the single-crystal structure of a highly ordered polycationic polymer (polyelectrolyte) and its proton conductivity. The polyelectrolyte single crystals can be prepared on a gram-scale in quantitative yield, by taking advantage of an ultraviolet/sunlight-induced topochemical polymerization, from a tricationic monomer-a self-complementary building block possessing a preorganized conformation. A single-crystal-to-single-crystal photopolymerization was revealed unambiguously by in situ single-crystal X-ray diffraction analysis, which was also employed to follow the progression of molecular structure from the monomer, to a partially polymerized intermediate, and, finally, to the polymer itself. Collinear polymer chains are held together tightly by multiple Coulombic interactions involving counterions to form two-dimensional lamellar sheets (1 nm in height) with sub-nanometer pores (5 Å). The polymer is extremely stable under 254 nm light irradiation and high temperature (above 500 K). The extraordinary mechanical strength and environmental stability-in combination with its impressive proton conductivity (∼3 × 10 -4 S cm -1 )-endow the polymer with potential applications as a robust proton-conducting material. By marrying supramolecular chemistry with macromolecular science, the outcome represents a major step toward the controlled synthesis of single-crystalline polyelectrolyte materials with perfect tacticity.

Synthetic macrocycles capable of undergoing allosteric regulation by responding to versatile external stimuli are the subject of increasing attention in supramolecular science. Herein, we report a structurally transformative tetracationic cyclophane containing two 3,6-bis(4-pyridyl)-l,2,4,5-tetrazine (4-bptz) units, which are linked together by two p -xylylene bridges. The cyclophane, which possesses modular redox states and structural post-modifications, can undergo two reversibly consecutive two-electron reductions, affording first its bisradical dicationic counterpart, and then subsequently the fully reduced species. Furthermore, one single-parent cyclophane can afford effectively three other new analogs through box-to-box cascade transformations, taking advantage of either reductions or an inverse electron-demand Diels-Alder (IEDDA) reaction. While all four new tetracationic cyclophanes adopt rigid and symmetric box-like conformations, their geometries in relation to size, shape, electronic properties, and binding affinities toward polycyclic aromatic hydrocarbons can be readily regulated. This structurally transformative tetracationic cyclophane performs a variety of new tasks as a result of structural post-modifications, thus serving as a toolbox for probing the radical properties and generating rapidly a range of structurally diverse cyclophanes by efficient divergent syntheses. This research lays a solid foundation for the introduction of the structurally transformative tetracationic cyclophane into the realm of mechanically interlocked molecules and will provide a toolbox to construct and operate intelligent molecular machines.