Dual Stimuli-Responsive [2]Rotaxanes with Tunable Vibration-Induced Emission and Switchable Circularly Polarized Luminescence

Aiming at the construction of novel stimuli-responsive fluorescent system with precisely tunable emissions, the typical 9,14-diphenyl-9,14-dihydrodibenzo[a, c]phenazine (DPAC) luminogen with attractive vibration-induced emission (VIE) behavior has been introduced into [2]rotaxane as a stopper. Taking advantage of their unique dual stimuli-responsiveness towards solvent and anion, the resultant [2]rotaxanes reveal both tunable VIE and switchable circularly polarized luminescence (CPL). Attributed to the formation of mechanical bonds, DPAC-functionalized [2]rotaxanes display interesting VIE behaviors including white-light emission upon the addition of viscous solvent, as evaluated in detail by femtosecond transient absorption (TA) spectra. In addition, ascribed to the regulation of chirality information transmission through anion-induced motions of chiral wheel, the resolved chiral [2]rotaxanes reveal unique switchable CPL upon the addition of anion, leading to significant increase in the dissymmetry factors (glum ) values with excellent reversibility. Interestingly, upon doping the chiral [2]rotaxanes in stretchable polymer, the blend films reveal remarkable emission change from white light to light blue with significant 6.5-fold increase in glum values up to -0.035 under external tensile stresses. This work provides not only a new design strategy for developing molecular systems with fluorescent tunability but also a novel platform for the construction of smart chiral luminescent materials for practical use.

Noncovalently bound and mechanically interlocked systems using pillar[n]arenes

Pillar[n]arenes are pillar-shaped macrocyclic compounds owing to the methylene bridges linking the para-positions of the units. Owing to their unique pillar-shaped structures, these compounds exhibit various excellent properties compared with other cyclic host molecules, such as versatile functionality using various organic synthesis techniques, substituent-dependent solubility, cavity-size-dependent host-guest properties in organic media, and unit rotation along with planar chiral inversion. These advantages have enabled the high-yield synthesis and rational design of pillar[n]arene-based mechanically interlocked molecules (MIMs). In particular, new types of pillar[n]arene-based MIMs that can dynamically convert between interlocked and unlocked states through unit rotation have been produced. The highly symmetrical pillar-shaped structures of pillar[n]arenes result in simple NMR spectra, which are useful for studying the motion of pillar[n]arene wheels in MIMs and creating sophisticated MIMs with higher-order structures. The creation and application of polymeric MIMs based on pillar[n]arenes is also discussed.

Rotaxane-branched radical dendrimers with TEMPO termini

The precise synthesis of novel rotaxane-branched radical dendrimers Gn-TEMPO (n = 1-3) with up to 24 TEMPO radicals as termini was successfully achieved, from which nanoparticles with a good longitudinal relaxivity were further prepared, thus making them potential candidates as promising contrast agents for magnetic resonance imaging.

Dendritic polyphenylene AIEgens: fluorescence detection of explosives and stimulus-responsive luminescence

Dendritic AIEgens with polyphenylene peripherals and TPE luminescent core had shown detection capability to picric acid and tunable response to external stimulus.

Rotaxane Dendrimers: Alliance between Giants

During recent decades, the blossoming of the field of mechanically interlocked molecules (MIMs), i.e., molecules containing mechanical or topological bonds such as rotaxanes, catenanes, and knots, has been reported in the literature. Taking advantage of the rapid development of diverse synthetic strategies, the precise control of both the architectures and topologies of MIMs has become realizable, which thus enables the construction of MIMs with specially desired functions. By mimicking biomolecular machines, a variety of MIM-based artificial molecular machines such as molecular shuttles, molecular muscles, molecular motors, and molecular assemblers have been constructed and operated by relying on the unique interlocked structures and controllable intramolecular movements. Two pioneers in this field, J. Fraser Stoddart and Jean-Pierre Sauvage, were awarded the 2016 Nobel Prize in Chemistry, thereby marking a golden age of MIMs. Along with the burgeoning of MIMs, the engineering of mechanical bonds into macromolecular scaffolds such as polymers or dendrimers has become an attractive topic since the targeted novel mechanically bonded macromolecules would feature interesting processable and mechanical properties, making them excellent candidates for practical applications such as device fabrication or smart materials. In particular, rotaxane dendrimers, attributed to the combination of the advantageous features of both rotaxanes (controllable dynamic motions) and dendrimers (nanoscale highly branched architectures), have evolved as versatile platforms for extensive applications such as gene delivery, light harvesting, and molecular nanoreactors. However, compared with the widely investigated polyrotaxanes and polycatenanes, in-depth investigations on rotaxane dendrimers have rarely been explored mainly because of the synthetic challenge that makes the preparation of diverse rotaxane dendrimers, especially high-generation ones, extremely difficult. During recent years, through the rational design and synthesis of organometallic rotaxane units as key building blocks, the employment of a controllable divergent approach led to the successful synthesis of a variety of rotaxane dendrimers with precise arrangements of rotaxane units as well as stimuli-responsive sites and functional groups. More importantly, on the basis of the synthetic accessibility to diverse rotaxane dendrimers, rotaxane dendrimers have been proven to hold great promise for extensive applications in diverse fields such as light harvesting, photocatalysis, and soft actuators. In this Account, we summarize our expedition in rotaxane dendrimers, including addressing the synthetic challenges, investigating their stimuli-responsive properties, expanding their potential applications, and inventing higher-order daisy chain dendrimers. We believe that this Account will inspire scientists from various disciplines to explore these appealing and versatile higher-order mechanically bonded macromolecules.

Artificial Light-Harvesting Systems Based on AIEgen-branched Rotaxane Dendrimers for Efficient Photocatalysis

Aiming at the construction of novel platform for efficient light harvesting, the precise synthesis of a new family of AIEgen-branched rotaxane dendrimers was successful realized from an AIEgen-functionalized [2]rotaxane through a controllable divergent approach. In the resultant AIE macromolecules, up to twenty-one AIEgens located at the tails of each branches, thus making them the first successful example of AIEgen-branched dendrimers. Attributed to the solvent-induced switching feature of the rotaxane branches, the integrated rotaxane dendrimers displayed interesting dynamic feature upon the aggregation-induced emission (AIE) process. Moreover, novel artificial light-harvesting systems were further constructed based on these AIEgen-branched rotaxane dendrimers, which revealed impressive generation-dependent photocatalytic performances for both photooxidation reaction and aerobic cross-dehydrogenative coupling (CDC) reaction.

An orthogonal photoresponsive tristable [3]rotaxane with non-destructive readout

An orthogonal photoresponsive [3]rotaxane is constructed by introducing two orthogonal photoswitchable azobenzene binding sites, and it features reversible photoregulated tristate absorption spectral changes with non-destructive readout capability.

Synthesis, structure elucidation and functionalization of sulfonamide [2]catenanes

A pyrene-functionalized [2]catenane with switchable optical output was constructed through a novel sulfonamide [2]catenane synthesized by a self-templation approach.

Coordination assembly and host–guest chemistry of a triply interlocked [2]catenane

Triply catenated systems composed of two or more discrete coordination-metal cages through mechanical bonds exhibit excellent host–guest behaviors, which can be potentially applied in drug delivery systems.

Diversiform Nanostructures Constructed from Tetraphenylethene and Pyrene-Based Acid/Base Controllable Molecular Switching Amphiphilic [2]Rotaxanes with Tunable Aggregation-Induced Static Excimers

Dual-emissive tetraphenylethene (TPE) and pyrene-containing amphiphilic molecules are of great interest because they can be integrated to form stimuli responsive materials with various biological applications. Herein, we report the study of mechanically interlocked molecules (MIMs) with aggregation-induced static excimer emission (AISEE) property through a series of TPE and pyrene-based amphiphilic [2]rotaxanes, where t-butylcalix[4]arene with hydrophobic nature was used as the macrocycle. Evidently, by adorning TPE and pyrene units in [2]rotaxanes P1, P2, P1-b, and P2-b, they display remarkable emission bands in 70% of water fraction (f_w) in tetrahydrofuran (THF)/water mixture, which could be attributed to the restricted intramolecular rotation of phenyl groups, whereas prominent blue-shifted excimer emission of pyrene started to appear as f_w reached 80% for P1 and 90% for P1-b, P2, and P2-b, which was ascribed to the favorable π-π stacking and hydrophobic interactions of the pyrene rings that enabled their static excimer formation. The well-defined distinct amphiphilic nanostructures of [2]rotaxanes including hollowspheres, mesoporous nanostructures, spheres, and network linkages can be driven smoothly depending on the molecular structures and their aggregated states in THF/water mixture. These fascinating diversiform nanostructures were mainly controlled by the skillful manner of reversible molecular shuttling of t-butylcalix[4]arene macrocycle and also the interplay of multinoncovalent interactions. To further understand the aggregation capabilities of [2]rotaxanes, the human lung fibroblasts (MRC-5) living cell incubated with either P1, P2, P1-b, or P2-b was studied and monitored by confocal laser scanning microscopy. The AISEE property was achieved at an astonishing level by integrating TPE and pyrene to MIM-based reversible molecular switching [2]rotaxanes; furthermore, distinct nanostructures, especially hollowspheres and mesoporous nanostructures, were observed, which are rarely reported in the literature but are highly desirable for future applications.

Daisy Chain Dendrimers: Integrated Mechanically Interlocked Molecules with Stimuli-Induced Dimension Modulation Feature

The precise construction of the high-order mechanically interlocked molecules (MIMs) with well-defined topological arrangements of multiple mechanically interlocked units has been a great challenge. Herein, we present the first successful preparation of a new family of daisy chain dendrimers, in which the individual [c2]daisy chain rotaxane units serve as the branches of dendrimer skeleton. In particular, the third-generation daisy chain dendrimer with 21 [c2]daisy chain rotaxane moieties was realized, which might be among the most complicated discrete high-order MIMs comprised of multiple [c2]daisy chain rotaxane units. Interestingly, such unique topological arrangements of multiple stimuli-responsive [c2]daisy chain rotaxanes endowed the resultant daisy chain dendrimers controllable and reversible nanoscale dimension modulation through the collective and amplified extension/contraction of each [c2]daisy chain rotaxane branch upon the addition of acetate anions or DMSO molecules as external stimulus. Furthermore, on the basis of such an intriguing size switching feature of daisy chain dendrimers, dynamic composite polymer films were constructed through the incorporation of daisy chain dendrimers into polymer films, which could undergo fast, reversible, and controllable shape transformations when DMSO molecules were employed as stimulus. The successful merging of [c2]daisy chain rotaxanes and dendrimers described herein provides not only a brand-new type of high-order mechanically interlocked systems with well-defined topological arrangements of [c2]daisy chain rotaxanes, but also a successful and practical approach toward the construction of supramolecular dynamic materials.

Recent advances in higher order rotaxane architectures

Despite dramatic advances in the template-directed synthesis of archetypal [2]rotaxanes, higher order rotaxanes with multiple molecular components (rings or dumbbells) are relatively daunting subjects owing to their synthetic challenges. With unique interlocked architectures, higher order rotaxanes have found applications in artificial molecular machines. In this feature article, we will focus on the recent advances in higher order rotaxanes with well-defined structures. Different types of rotaxane architectures will be described, and their synthetic approaches will be highlighted. Moreover, the stimuli-responsive molecular motion with increasing complexity in these diverse architectures will also be discussed.

Construction of Type III-C Rotaxane-Branched Dendrimers and Their Anion-Induced Dimension Modulation Feature

Starting from a novel rotaxane building block with dendrimer growth sites being located at both the wheel and axle component, we realized the successful construction of a new family of rotaxane-branched dendrimers, i.e., Type III-C rotaxane-branched dendrimers, up to fourth generation as a highly branched [46]rotaxane through a controllable divergent approach. In the resultant rotaxane-branched dendrimers, the wheel components of the rotaxane units are located on the branches as well as at the branching points, making them excellent candidates to mimic the amplified collective molecular motions. Thus, taking advantage of the urea moiety inserted into the axle components of the rotaxane units as the binding sites, the addition or removal of acetate anion as stimulus endows the individual rotaxane unit a switchable feature that lead to a collective expansion-contraction motion of the integrated rotaxane-branched dendrimers, thus allowing for the remarkable and reversible size modulation. Such a three-dimensional size switching feature makes Type III-C rotaxane-branched dendrimers a very promising platform toward the fabrication of novel dynamic smart materials.

Recent advances in the development of rotaxanes and pseudorotaxanes based on pillar[n]arenes: from construction to application

This article summarizes recent advances in the development of rotaxanes and pseudorotaxanes based on pillar[n]arenes: from construction to application.