Locking down the Electronic Structure of (Monopyrrolo)tetrathiafulvalene in [2]Rotaxanes

Polysilicon Microchips Functionalized with Bipyridinium-Based Cyclophanes for a Highly Efficient Cytotoxicity in Cancerous Cells

The use of micrometric-sized vehicles could greatly improve selectivity of cytotoxic compounds as their lack of self-diffusion could maximize their retention in tissues. We have used polysilicon microparticles (SiμP) to conjugate bipyridinium-based compounds, able to induce cytotoxicity under regular intracellular conditions. Homogeneous functionalization in suspension was achieved, where the open-chain structure exhibits a more dense packing than cyclic analogues. The microparticles internalized induce high cytotoxicity per particle in cancerous HeLa cells, and the less densely packed functionalization using cyclophanes promotes higher cytotoxicity per bipy than with open-chain analogues. The self-renewing ability of the particles and their proximity to cell membranes may account for increased lipid peroxidation, achieving toxicity at much lower concentrations than that in solution and in less time, inducing highly efficient cytotoxicity in cancerous cells.

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.

Efficient access to conjugated 4,4'-bipyridinium oligomers using the Zincke reaction: synthesis, spectroscopic and electrochemical properties

The cyclocondensation reaction between rigid, electron-rich aromatic diamines and 1,1'-bis(2,4-dinitrophenyl)-4,4'-bipyridinium (Zincke) salts has been harnessed to produce a series of conjugated oligomers containing up to twelve aromatic/heterocyclic residues. These oligomers exhibit discrete, multiple redox processes accompanied by dramatic changes in electronic absorption spectra.

A new fluorescence molecular switch incorporating TTF and tetraphenylporphyrin units

A donor-σ-acceptor compound incorporating tetrathiafulvalene and tetraphenylporphyrin units was synthesized. The structures of the target compound and its intermediates have been characterized by 1 H NMR, ESI MS, UV-vis, IR and mp. The results of UV-vis and cyclic voltammetry of the compound indicated negligible intramolecular charge transfer interaction in its ground state. The fluorescence and fluorescence lifetime of the compound were reduced, compared to tetraphenylporphyrin, which evidently indicated that photoinduced electron transfer occurred from the tetrathiafulvalene (TTF) unit to the tetraphenylporphyrin unit in the excited state. The fluorescence intensity of the compound could be reversibly modulated by sequential oxidation and reduction of the TTF unit using electrochemical and chemical methods. Therefore, a new fluorescence molecular switch based on TTF and porphyrin units was established.

A tristable [2]pseudo[2]rotaxane

A strategy towards increasing the lifetime of the metastable state of a [2]rotaxane incorporating tetrathiafulvalene, 1,5-dioxynaphthalene and bipyridinium (BIPY(2+)) is presented. Incorporation of BIPY(2+) served multiple roles as an electrostatic barrier to relaxation, a supramolecular recognition site for bis-1,5-dioxynaphthalene[38]crown-10 macrocycle, and upon reduction a recognition site for the mechanically bonded cyclobis(paraquat-p-phenylene) ring.

Beta-substituted terthiophene [2]rotaxanes

Two kinds of beta-substituted terthiophene [2]rotaxanes were synthesized using the host-guest pairs of the electron-deficient cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the electron-rich terthiophenes with diethyleneglycol chains at the beta-position. One is made from the alpha-position non-substituted terthiophene (3 T-beta-Rx) and the other is made from the alpha-dibromo-substituted terthiophene (3 TBr-beta-Rx). The binding constants of the beta-substituted terthiophene threads were confirmed to be smaller than that of the alpha-substituted terthiophene analogue. By UV/Vis absorption measurements, we confirmed the charge-transfer (CT) band in the visible region with an extinction coefficient of approximately 10(2) (M(-1) cm(-1)). Strong, but not quantitative, quenching of the terthiophene fluorescence was confirmed for the [2]rotaxanes. Although the beta-substituted terthiophene thread was electrochemically polymerizable, the [2]rotaxane 3 T-beta-Rx was not polymerizable. This result indicates that the interlocked CBPQT(4+) macrocycle effectively suppresses the electrochemical polymerization of the terthiophene unit because electrostatic repulsive and steric effects of CBPQT(4+) hinder the dimerization of the terthiophene radical cations. In the electrochemical measurement, we confirmed the shift of the first reduction peak towards less negative potential compared to free CBPQT(4+) and the splitting of the second reduction peak. These electrochemical behaviors are similar to those observed for the highly-constrained [2]rotaxanes. The beta-substituted terthiophene [2]rotaxanes reported herein are important key compounds to prepare polythiophene polyrotaxanes.

Blue-Colored Donor−Acceptor [2]Rotaxane

[reaction: see text] A guest molecule-a bis-N-tetraethyleneglycol-substituted 3,3'-difluorobenzidine derivative-has been synthesized, and its complexation with the host, cyclobis(paraquat-p-phenylene), has been investigated. This host-guest complex was then employed in the template-directed synthesis of a blue-colored [2]rotaxane. The color of this [2]rotaxane arises from the charge-transfer absorption band between the HOMO of the guest and the LUMO of the host. This host-guest complex, and the derived [2]rotaxane, completes the donor-acceptor-based RGB (red/green/blue) color complex set.

A Clicked Bistable [2]Rotaxane

[structure: see text]. A universal diazide-terminated polyether, incorporating tetrathiafulvalene (TTF, green) and 1,5-dioxynaphthalene (DNP, red) units, was prepared and subsequently employed in the template-directed synthesis of a switchable donor/acceptor [2]rotaxane. The triazole rings (magenta), which are introduced into the rotaxane during requisite click reactions, do not present themselves as competing recognition sites for the tetracationic cyclophane (blue) as it is induced to switch between the TTF unit, when it becomes dicationic (green adorned with yellow extremities), and the DNP unit.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Binding Studies between Triethylene Glycol-Substituted Monopyrrolotetrathiafulvalene Derivatives and Cyclobis(paraquat-p-phenylene)

The synthesis of several pi-electron-donating monopyrrolotetrathiafulvalene (MPTTF) derivatives, which conceptually can be divided into three classes containing none, one, or two triethylene glycol (TEG) substituents, is described. In all cases, the complexation between the pi-electron donating MPTTF unit and the pi-electron-deficient tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) has been investigated using UV-vis dilution techniques. The results reveal that the strength of the binding between MPTTF derivatives and CBPQT(4+) is directly correlated to the pi-electron donating properties of the MPTTF derivatives. However, the pi-electron-donating properties of the MPTTF derivatives is not the only factor of importance. The results enclosed in the present studies demonstrate that the TEG substituents assist the complexation process most likely on account of their capacity to participate in [C-H...O] hydrogen bonding interactions with some of the alpha-CH protons in the bipyridinium units of CBPQT(4+) and the stabilizing effect that attachment of one or two TEG substituents to the MPTTF unit exerts upon complexation with CBPQT(4+) has been quantified to approximately 0.3 and 0.5 kcal mol-1, respectively. These results serve to lay an extended foundation for the understanding of which buttons to push when it comes to improve the design of bistable molecular switches based on (MP)TTF and CBPQT(4+).

Toward Electrochemically Controllable Tristable Three-Station [2]Catenanes

Encouraged by the prospect of producing an electrochemical, color-switchable red-green-blue (RGB) dye compound, we have designed, synthesized, and characterized two three-station [2]catenanes. Both are composed of macrocyclic polyethers containing three pi-electron-rich stations, which act as recognition sites for a pi-electron-deficient tetracationic cyclophane. The molecular structures of the two three-station [2]catenanes were characterized fully by mass spectrometry and 1H NMR spectroscopy. To anticipate the relative occupancies of the three stations in each [2]catenane by the cyclophane, model compounds with the same constitutions in the vicinity of the stations were synthesized. The relative ground-state populations of the three stations occupied in both [2]catenanes were estimated from the thermodynamic parameters for 1:1 complexes between all these model compounds and the cyclophane, obtained from isothermal titration calorimetry (ITC). The electrochemical and electromechanical properties of the three-station [2]catenanes were analyzed by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and spectroelectrochemistry (SEC). The first three-station [2]catenane was found to behave like a bistable system, whereas the second can be described as a quasi-tristable system.