Da-Hui Qu

Novel and efficient templates for assembly of rotaxanes and catenanes

Efficient templates play an important role in the construction of mechanically interlocked molecules such as catenanes and rotaxanes. This minireview presents a retrospective introduction on the traditional templates and highlights recent significant accomplishments in developing novel and efficient templates, especially active metal templates and radical templates, employed in the construction of rotaxanes and catenanes. The current status of this field is summarized and the scope and future prospects are also discussed in this minireview.

A ferrocene-functionalized [2]rotaxane with two fluorophores as stoppers.

In the past few decades, bistable [2]rotaxanes have been extensively studied because of their applications in the fields of functional molecules and molecular machines. In this paper, a di-ferrocene-functionalized [2]rotaxane with two fluorophores as stoppers was designed, prepared, and studied. In this bistable [2]rotaxane, a dibenzo-24-crown-8 macrocycle functionalized with two ferrocene moieties as electron donors can reversibly shuttle between two distinct stations, namely, a dialkylammonium recognition site and a N-methyltriazolium recognition site, by external acid-base stimuli, which has been demonstrated using (1)H NMR spectroscopy. It has been shown that, by introducing two ferrocene units into the macrocycle component, the fluorescence of two fluorescent stoppers, namely, the anthracene fluorophore and the 4-morpholin-naphthalimide fluorophore, can be changed in an alternate mode by an adjustable, distance-dependent photoinduced electron transfer process that occurs between the ferrocene electron donors and each of the two fluorophores.

Altering intercomponent interactions in a photochromic multi-state [2]rotaxane

Rotaxanes have attracted much attention because of their challenging constructions and potential applications. In this paper, a multi-state [2]rotaxane, in which a dithienylethene-functionalized dibenzo-24-crown-8 macrocycle was interlocked onto a thread component bearing a 4-morpholin-naphthalimide fluorescent stopper and two distinct recognition sites, namely, dibenzylammonium and N-methyltriazolium recognition sites, was prepared and studied. By introducing a dithienylethene photochrome into the macrocycle component, multi-mode alteration of the intercomponent interactions, such as energy transfer, electron transfer, and charge transfer interaction between the photochrome and the fluorescent naphthalimide stopper could be altered in this multi-state rotaxane system in response to the combination of chemical and photochemical stimuli.

Dual‐Mode Controlled Self‐Assembly of TiO2 Nanoparticles Through a Cucurbit[8]uril‐Enhanced Radical Cation Dimerization Interaction

The realization of controllable multicomponent self-assembly through reversible supramolecular interactions is a challenging goal, and is an important strategy for the fabrication of switchable nanomaterials. Herein we show that the self-assembly of TiO2 nanoparticles (NP) functionalized with methyl viologen can be controlled both by light irradiation and chemical reduction through cucurbit[8]uril-enhanced radical cation dimerization interactions. Moreover, the controlled assembly and disassembly of this system are accompanied by switchable photocatalytic activity of the TiO2  NPs, which shows potential application as a novel smart and recyclable photocatalyst.

A switchable ferrocene-based [1]rotaxane with an electrochemical signal output.

A [1]rotaxane, in which a linear rod is attached to one cyclopentadienyl (Cp) ring of a ferrocene unit and threaded into a dibenzo-24-crown-8 connected to the other Cp ring, was prepared. The mechanical motion of the rod-like part relative to the macrocycle has been demonstrated using (1)H NMR spectroscopy. Cyclic voltammetry (CV) showed that the system can be chemically switched between electrochemically reversible and irreversible states, depending on the inclusion and exclusion of the ammonium/amine group from the macrocycle.

A Switchable bis-Branched [1]Rotaxane featuring Dual-Mode Molecular Motions and Tunable Molecular Aggregation

A multifunctional bis-branched [1]rotaxane containing a perylene bisimide (PBI) core and two identical bistable[1]rotaxane arms terminated with ferrocene units was prepared and characterized by (1)H NMR, (13)C NMR, and 2D ROESY NMR spectroscopies and by HR-ESI spectrometry. The system is shown to possess several key features: (1) In acetone solution, external acid-base stimuli can result in relative mechanical movements of its ring and thread, which can induce extension and contraction movements of the whole system accompanied by a rotational movement of the ferrocene units, thus realizing dual-mode molecular motions, and the optimized conformations at different states are obtained through molecular dynamics simulations employing the general Amber force field. (2) The introduction of PBI enables the system fluorescence encoding through distance-dependent photoinduced electron transfer process from the ferrocene units to the PBI fluorophore. (3) The addition of Zn(2+) can increase the degree of aggregation of the system, while adding base hinders aggregation because of the movement of the macrocycle. The tunable aggregated nanostructural morphologies of [1]rotaxane were examined by scanning electron microscopy. These results can pave the way to achieve precise control of integrated and coupling nanomechanical motions at a single-molecule level and provide more insight into controlling the aggregate behavior of switchable mechanically interlocked molecules.

Fluorescence Modulation in Tribranched Switchable [4]Rotaxanes

Two novel tribranched [4]rotaxanes with a 1,3,5-triphenylene core and three rotaxane arms have been designed, synthesized, and characterized by (1)H and (13)C NMR spectroscopies and HR-ESI mass spectrometry. [4]Rotaxanes 1 and 2 each possess the same three-armed skeleton. Each arm incorporates two distinguishable binding sites for a dibenzo[24]crown-8 ring, namely a dibenzylammonium site and an N-methyltriazolium site, and is terminated by a 4-morpholino-naphthalimide fluorophore as a stopper. [4]Rotaxane 1 has three di-ferrocene-functionalized dibenzo[24]crown-8 rings whereas 2 has three simple dibenzo[24]crown-8 rings interlocked with the thread component. Uniform shuttling motions of the three macrocycles in both 1 and 2 can be driven by external acid-base stimuli, which were confirmed by (1)H NMR spectroscopy. However, [4]rotaxanes 1 and 2 show distinct modes of fluorescence modulation in response to external acid-base stimuli. [4]Rotaxane 1 exhibits a remarkable fluorescence decrease in response to the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base, which can displace the ferrocene-functionalized macrocycle from the dibenzylammonium station to the N-methyltriazolium station. In contrast, the fluorescence intensity of [4]rotaxane 2 showed an enhancement with the addition of DBU. Time-resolved fluorescence measurements have been performed. The different photoinduced electron-transfer processes responsible for the fluorescence changes in the two molecular systems are discussed. Topological structures of this kind have significant potential for the design and construction of large and complex assemblies with controllable functions.

Two Switchable Star-Shaped [1](n)Rotaxanes with Different Multibranched Cores

Two novel star-shaped [1](n)rotaxanes with three and four identical [1]rotaxane arms but different multibranched cores were designed, synthesized, and well-characterized. In the two systems, external base-acid stimuli result in the uniform relative mechanical movement of the macrocyclic rings and threads of their [1]rotaxane arms. The energy-minimized structures of the two rotaxanes in different states were obtained using molecular dynamics simulations in acetone solution, suggesting the construction of more sophisticated molecular machines mimicking the extension and contraction motions.

A Musclelike [2](2)Rotaxane: Synthesis, Performance, and Molecular Dynamics Simulations

A novel bistable symmetric [2](2)rotaxane was prepared by a threading-followed-by-stoppering strategy and characterized with (1)H, (13)C, and 2D ROESY NMR spectroscopy and HR-ESI spectrometry. The symmetric [2](2)rotaxane system consists of an anthracene-based bis(crown ether) as macrocycles, and each of the two dibenzo[24]crown-8 (DB24C8) rings is threaded by the pendant substituents of a symmetrically substituted central rotatable ferrocene subunit that possesses two distinguishable recognition sites for the DB24C8 ring: namely, a dibenzylammonium site and an N-methyltriazolium site. The uniform shuttling motion of the thread relative to the two DB24C8 rings in [2](2)rotaxane can be driven by external acid-base stimuli, which was evidenced by (1)H and 2D ROESY NMR spectroscopy. Furthermore, molecular dynamics simulations of the [2](2)rotaxane were carried out both in protonated (stretched) and in neutral (contracted) forms. The calculated percentage change in molecular length of the [2](2)rotaxane between the two end-capping bis(methoxyl)phenyl groups is about 48% in the two different states (in acetone), which is much larger than the percentage change (∼27%) in human muscle. Moreover, in the two states, the C*-Cp-Cp-C* dihedral angles are computed as -177° in the stretched state and -112° in the contracted state, indicating a correlation between the translational and rotational motions of the [2](2)rotaxane.

A fluorescent hyperbranched supramolecular polymer based on triple hydrogen bonding interactions

A fluorescent hyperbranched supramolecular polymer was constructed through triple hydrogen bonding interactions between a three-arm monomer (PTDA) which possesses one melamine moiety in each arm and a perylene bisimide derivative (PBI 1) in dichloromethane.