Stimuli-responsive mechanically interlocked molecules constructed from cucurbit[n]uril homologues and derivatives

Cucurbit[n]uril supramolecular chemistry has developed rapidly since 2001 when different cucurbit[n]uril homologues (Q[n]) were successfully separated in pure form. The combination of Q[n] cavity size and various types of external stimuli has given birth to numerous types of Q[n]-based mechanically interlocked molecules (MIMs), including (pseudo)rotaxanes, catenanes, dendrimers and poly(pseudo)rotaxanes. In this review article, the important advances in the field of Q[n]-based MIMs over the past two decades are highlighted. This review also describes examples of heterowheel (pseudo)rotaxanes and poly(pseudo)rotaxanes involving Q[n]s, and reflects on the opportunities and challenges of constructing Q[n]-based stimuli-responsive MIMs.

Achieving Enhanced Photochromic Properties of Diarylethene through Host-Guest Interaction in Aqueous Solution

Diarylethene (DTE) has been widely used in fluorescence probes, molecular logic gates, optical data-storage devices owing to the excellent photochromic property, while constructing high-performance photochromic DTE in aqueous media remains a big challenge. Herein we present several host-guest systems formed between cucurbit[n]uril (CB[n], n=7, 8, 10) and two water-soluble DTE derivatives 1 and 2. It was found that host-guest interactions not only affect the photophysical properties of photochromic guests, but also make great differences on the photoreaction process. Different host-guest binding behaviors also lead to different effects on the photochromic properties of guests. In the presence of CB[n], both 1 and 2 showed enhanced emission and higher fluorescence quenching ratio at photostationary state. Besides, CB[10]⋅1 exhibited faster response rate in cyclization reaction and better photofatigue resistance than free 1 in aqueous solution, while the supramolecular assembly of (CB[8])_n ⋅(2)_n showed slower response rate in both directions of the reversible photoreaction. Besides, the photofatigue resistance of 2 can be greatly improved through binding with CB[7]. Our results suggest that host-guest interactions could be an efficient way to improve photochromic properties of DTE in aqueous solution.

Threading-gated photochromism in [2]pseudorotaxanes

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Photochromic rotaxanes and pseudorotaxanes

Among stimulus-responsive molecular ring-on-thread rotaxanes and pseudorotaxanes, variants incorporating photochromic sub-units are attracting considerable attention as their properties and structure can be remotely and precisely controlled, additionally without producing chemical waste. The focus herein is on photoswitching-driven assembly/disassembly and modulation of properties resulting from light-activated isomerization or changes in electronic properties.

Synthesis of rotaxanes and catenanes using an imine clipping reaction

Supramolecular chemistry and self-assembly provide a valuable chance to understand the complicated topological structures on a molecular level. Two types of classical mechanically interlocked molecules, rotaxanes and catenanes, possess non-covalent mechanical bonds and have attracted more attention not only in supramolecular chemistry but also in the fields of materials science, nanotechnology and bioscience. In the past decades, the template-directed clipping reaction based on imine chemistry has become one of the most efficient methods for the construction of functionalized rotaxanes and catenanes. In this review, we outlined the main progress of rotaxanes and catenanes using the template-directed clipping approach of imine chemistry. The review contains the novel topological structures of rotaxanes and catenanes, functions and applications.

Application of functionalized ether in lithium ion batteries

Functionalized ethers have been explored to improve the electrochemical properties of lithium ion batteries (LIBs).

Design and synthesis of reversible solid-state photochromic pyrazolones by introducing a pyridine ring

We demonstrate a new strategy for designing reversibility, fatigue resistance and fluorescence switching materials, which are based on pyrazolone derivatives by introducing a pyridine ring. The reversible "on" and "off" modulation of fluorescence emission was up to 95% in the solid state.

Bodipy-C60 triple hydrogen bonding assemblies as heavy atom-free triplet photosensitizers: preparation and study of the singlet/triplet energy transfer

Supramolecular triplet photosensitizers based on hydrogen bonding-mediated molecular assemblies were prepared. Three thymine-containing visible light-harvesting Bodipy derivatives (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, respectively) were used as H-bonding modules, and 1,6-diaminopyridine-appended C60 was used as the complementary hydrogen bonding module (C-1), in which the C60 part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepared as references (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C60 module (C-1). The photophysical properties of the H-bonding assemblies and the reference non-hydrogen bond-forming mixtures were studied using steady state UV/vis absorption spectroscopy, fluorescence emission spectroscopy, electrochemical characterization, and nanosecond transient absorption spectroscopy. Singlet energy transfer from the Bodipy antenna to the C60 module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C60 module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). Intra-assembly forward-backward (ping-pong) singlet/triplet energy transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was observed for the non-hydrogen bonding mixtures. As a proof of concept, these supramolecular assemblies were used as triplet photosensitizers for triplet-triplet annihilation upconversion.

Cyanine-based dithienylethenes: synthesis, characterization, photochromism and biological imaging in living cells

Photochromic materials have been widely used in many fields such as electro-optical functional materials and novel bio-materials.

Construction of photoswitchable rotaxanes and catenanes containing dithienylethene fragments

Mechanically interlocked structures such as rotaxanes and catenanes provide a novel backbone for constructing functional materials with unique structural characteristics.

Tetraphenylethene modified [n]rotaxanes: synthesis, characterization and aggregation-induced emission behavior

A series of novel [n]rotaxanes based on a tetraphenylethene (TPE) backbone were constructed by a template-directed clipping approach and their structures were well-characterized.

Aggregation-induced emission behavior of a pH-controlled molecular shuttle based on a tetraphenylethene moiety

A TPE-based molecular shuttle having amide and amine units has been synthesized. The shuttling motion of the macrocycle component can adjust its AIE behaviour.

Synthesis of [3]rotaxanes that utilize the catalytic activity of a macrocyclic phenanthroline-Cu Complex: remarkable effect of the length of the axle precursor

[3]Rotaxanes, which consist of one macrocyclic phenanthroline compound and two axle components, were prepared by the oxidative dimerization of an alkyne compound with bulky tris[4'-cyclohexyl-(1,1'-biphenyl)-4-yl]methyl blocking group. The catalytic activity of a macrocyclic phenanthroline-Cu complex was utilized to thread the two axle components inside the ring. The alkyne compound with chain of 15 or 20 methylene groups gave [3]rotaxanes in high yields, whereas the axle with a chain of six methylene groups afforded a [3]rotaxane in very poor yield. We also examined the effect of the ring size on the synthesis of [3]rotaxanes. [3]Rotaxanes were not isolated when a macrocyclic phenanthroline compound with a smaller ring size was used.

Photoswitching of the triplet excited state of diiodobodipy-dithienylethene triads and application in photo-controllable triplet-triplet annihilation upconversion

Dithienylethene (DTE)-2,6-diiodoBodipy triads were prepared with the aim to photoswitch the triplet excited state of the 2,6-diiodoBodipy moiety. Bodipy was selected due to its low T1 state energy level to avoid sensitized photocyclization of DTE, which is very often encountered in DTE photoswitches, so that the photochemistry of DTE and the organic chromophore can be addressed independently. This is the first time that DTE was covalently connected with an organic triplet photosensitizer. For the triad with DTE-o structure, selective photoexcitation into the diiodoBodipy part did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET), with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O2-independent, but can be greatly accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as triplet-triplet annihilation upconversion.