Photophysical properties and conformational effects on the circular dichroism of an azobenzene-cyclodextrin [1]rotaxane and its molecular components

Topological-skeleton controlled chirality expression of supramolecular hyperbranched and linear polymers

The topology of polymers plays an essential role in their chemical, physical and biological properties. However, their effects on chirality-related functions remain unclear. Here, we reported the topology-controlled chirality expression in the chiral supramolecular system for the first time. Two topological supramolecular polymers, hyperbranched (HP) and linear (LP) supramolecular polymers produced by the host-guest interactions of branched and linear monomers, respectively, exhibited completely different chirality expressions despite the same molecular chirality of their monomers. Significantly, due to the branch points and strong steric hindrance existing in HP, cis-HP showed an enhanced and sign-inverted Cotton effect in the n-π* bands compared with cis-LP, as a result that the distinctive chirality induction and transfer were controlled by the topological skeletons. This topology-controlled chirality induction and transfer in the photoswitchable supramolecular polymers not only enables us to elucidate the in-depth effects of topology on the chiral expression in biopolymers but also inspires the design of chiroptical and bioinspired materials.

Quantifying the barrier for the movement of cyclobis(paraquat-p-phenylene) over the dication of monopyrrolotetrathiafulvalene

A bistable [2]pseudorotaxane 1⊂CBPQT·4PF₆ and a bistable [2]rotaxane 2·4PF₆ have been synthesised to measure the height of an electrostatic barrier produced by double molecular oxidation (0 to +2). Both systems have monopyrrolotetrathiafulvalene (MPTTF) and oxyphenylene (OP) as stations for cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺). They have a large stopper at one end while the second stopper in 2⁴⁺ is composed of a thioethyl (SEt) group and a thiodiethyleneglycol (TDEG) substituent, whereas in 1⊂CBPQT⁴⁺, the SEt group has been replaced with a less bulky thiomethyl (SMe) group. This seemingly small difference in the substituents on the MPTTF unit leads to profound changes when comparing the physical properties of the two systems allowing for the first measurement of the deslipping of the CBPQT⁴⁺ ring over an MPTTF²⁺ unit in the [2]pseudorotaxane. Cyclic voltammetry and ¹H NMR spectroscopy were used to investigate the switching mechanism for 1⊂CBPQT·MPTTF⁴⁺ and 2·MPTTF⁴⁺, and it was found that CBPQT⁴⁺ moves first to the OP station producing 1⊂CBPQT·OP⁶⁺ and 2·OP⁶⁺, respectively, upon oxidation of the MPTTF unit. The kinetics of the complexation/decomplexation process occurring in 1⊂CBPQT·MPTTF⁴⁺ and in 1⊂CBPQT·OP⁶⁺ were studied, allowing the free energy of the transition state when CBPQT⁴⁺ moves across a neutral MPTTF unit (17.0 kcal mol^-1) or a di-oxidised MPTTF²⁺ unit (24.0 kcal mol^-1) to be determined. These results demonstrate that oxidation of the MPTTF unit to MPTTF²⁺ increases the energy barrier that the CBPQT⁴⁺ ring must overcome for decomplexation to occur by 7.0 kcal mol^-1.

Pseudorotaxane formation affected by stereo-electronic effects. A theoretical and experimental study

We report a theoretical and experimental study on different complexes of pseudorotaxanes possessing pyridine axles. In order to evaluate the stereo-electronic effects of the methyl substituents in the pyridine ring, complexes with different substitution patterns were synthesized. In this way, it was possible to analyze the different behaviors of these complexes according to the positions of their methyl substituents. Combined techniques of molecular dynamics and quantum mechanical calculations with the help of molecular electrostatic potentials for a simpler visualization of the electronic effects were employed. We have sought experimental support of NMR spectroscopy analysis to corroborate the conclusions obtained from the molecular simulations. Our results not only clearly demonstrate that both electronic and steric effects play key roles in the feasibility of the formation of such complexes, but also the simulations reported here might predict the degree of difficulty of their formation. The combination of computational techniques employed here seems to be an excellent approach to be able to predict whether or not a complex can be formed and with what degree of difficulty. In addition, our experimental and theoretical results have allowed us to visualize the formation of external complexes in the rotaxanes reported here. In this case, the use of bolaforms with trimethylammonium groups at both ends was very useful to evaluate in detail the formation of the so-called external complexes in these systems.

Helic[6]arene-Based Chiral Pseudo[1]rotaxanes and [1]Rotaxanes

Chiral pseudo[1]rotaxanes and [1]rotaxanes constructed from macrocyclic arenes still remain a big challenge mainly owing to the lack of such chiral macrocycles. In this work, a new system of chiral pseudo[1]rotaxanes formed by self-inclusion of helic[6]arene containing amide linked with the terminal tertiary amines was first discovered. Based on an atom-economic stopping strategy, a pair of chiral [1]rotaxanes were conveniently obtained in almost quantitative yields by blocking the pseudo[1]rotaxanes with monobenzyl bromide of tetraphenylethene. The structures of pseudo[1]rotaxanes and [1]rotaxanes were characterized by 2D NMR spectra in solution, combined with DFT calculations. The photophysical properties further revealed the efficient chirality transfer of helic[6]arene to the tetraphenylethene moiety, compared to their unthreaded chiral isomers. The discovery of the chiral pseudo[1]rotaxanes allows for a wide and available synthesis of chiral [1]rotaxanes, and also opening a new avenue to the design of chiral supramolecular materials.

Salts accelerate the switching kinetics of a cyclobis(paraquat-p-phenylene) [2]rotaxane

The rate at which the macrocyclic cyclobis(paraquat-p-phenylene) ring of a bistable [2]rotaxane moves from a tetrathiafulvalene station to an oxyphenylene station upon oxidation of the tetrathiafulvalene station is found to be increased in the presence of added salts. Compared to the salt-free case, 0.1 M solutions of a series of tetraalkylammonium hexafluorophosphate salts (R4N·PF6, R = H, Me, Et or n-Bu) and of tetrabutylammonium perchlorate (n-Bu4N·ClO4) all afford an increased switching rate, which is largest in the case of n-Bu4N·ClO4 with smaller anions. Variation in the size of the ammonium cation has no significant effect. These results indicate that the addition of excess ions can be used as an accelerator to speed up shuttling processes in rotaxanes and catenanes based on the mobile cyclobis(paraquat-p-phenylene) ring, and that the choice of anion offers a convenient means of controlling the extent of this effect.

Light-driven exchange between extended and contracted lasso-like isomers of a bistable [1]rotaxane

A photoactive hydrogen-bonded lasso having an amide-based [1]rotaxane structure has been constructed from acyclic precursors through a self-templating approach. The stability, structural integrity and switching are described.

Radically promoted formation of a molecular lasso

Two potential viologen-based molecular lasso precursors-both composed of a 4,4'-bipyridinium (BIPY2+) unit as part of a rope appended to a cyclobis(paraquat-p-phenylene) (CBPQT4+) loop-that have been designed to mimic the threading/unthreading motion of lasso peptides, have been synthesised and characterised. Solution and solid-state experiments reveal that, when the BIPY2+ unit in the rope and the CBPQT4+ loop are connected by a bulky linker, no lasso-like conformational transformation is observed between the different redox states on account of steric effects. In sharp contrast, when the linker size is small, the molecule can be switched between (i) a free rope-like conformation in its fully oxidised state and (ii) a self-entangled lasso-like conformation under reducing conditions employing either chemical or electrochemical stimuli: the BIPY˙+ unit in the rope resides inside the cavity of the CBPQT2(˙+) loop, forming a pseudo[1]rotaxane. The switching process is reversible and stereochemically unambiguous. This research shows how tiny structural differences can induce significantly different self-complexing properties and sheds light on designing functional artificial actuators.

pH-dependent complexation of hydroxypropyl-beta-cyclodextrin with chlorin e6: effect on solubility and aggregation in relation to photodynamic efficacy

Objectives

The activity of chlorin e6 (Ce6) in photodynamic therapy of cancers is significantly reduced by its propensity to form aggregates. It was postulated that disaggregation of Ce6 could be achieved with the use of hydroxypropyl-beta-cyclodextrin (HP-β-CD) through solubility enhancement.

Methods

An initial phase solubility study of Ce6 was conducted with various concentrations of HP-β-CD at three different pH conditions, i.e. pH 3, pH 5 and pH 7. Solubility-induced disaggregation of Ce6 was illustrated by fluorescence spectroscopy and singlet oxygen generation studies. Interaction between Ce6 and HP-β-CD was further demonstrated by solid-state characterization techniques. Inclusion complex formulations were tested for improved efficacy on squamous cancer cell lines.

Key findings

Increase in Ce6 solubility was observed, especially at pH 7, indicating the formation of inclusion complex between Ce6 and HP-β-CD. This resulted in disaggregation of Ce6 aggregates illustrated by fluorescence spectroscopy. The mode of binding was predominated by H-bonding supported by temperature-dependent binding studies and molecular simulation work. The inclusion complex demonstrated improved photodynamic efficacy through enhanced singlet oxygen generation and phototoxicity on human oral squamous carcinoma cells.

Conclusions

pH-dependent complexation between Ce6- and HP-β-CD-induced disaggregation of Ce6 aggregates and the resultant formulations facilitated improved PDT efficacy on tested cancer cell lines.

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.

INHIBIT logic operations based on light-driven β-cyclodextrin pseudo[1]rotaxane with room temperature phosphorescence addresses

INHIBIT logic gates based on light-driven β-cyclodextrin pseudo[1]rotaxane were fabricated conveniently utilizing reversible ICD and RTP as output addresses respectively.