56 research outputs found
Thermodynamic Insights on a Bistable Acid–Base Switchable Molecular Shuttle with Strongly Shifted Co-conformational Equilibria
No full textBistable [2]rotaxanes in which the affinities of the two stations can be reversed form the basis of molecular shuttles. Gaining quantitative information on such rotaxanes in which the ring distribution between the two stations is largely nonsymmetric has proven to be very challenging. Herein, we report on two independent experimental methodologies, based on luminescence lifetime measurements and acid–base titrations, to determine the relative populations of the two co‐conformations of a [2]rotaxane. The assays yield convergent results and are sensitive enough to measure an equilibrium constant (K≈4000) out of reach for NMR spectroscopy. We also estimate the ring distribution constant in the switched (deprotonated) state (K′99.92 %). Finally, our results show that the pKa of the pH‐responsive station depends on the ring affinity of the pH‐insensitive station, an observation that paves the way for the design of new artificial allosteric systems
Metal-Controlled Assembly and Selectivity of a Urea-Based Anion Receptor
No full textThe terdentate ligand 3 (LH, 2-formylpyridine 4-thiosemicarbazone) forms with FeII and NiII 2:1 complexes of octahedral geometry of formula [MII(LH)2]2+. X-ray diffraction studies have shown that in both complexes the thiourea moieties of the coordinated thiosemicarbazones are exposed to the outside and are prone to establish hydrogen-bonding bifurcate interactions with oxoanions. However, spectrophotometric studies in CHCl3 soln. have shown that only the poorly basic NO3- ion is able to form authentic hydrogen-bond complexes with thiourea subunits, whereas all the other investigated anions (CH3COO-, NO2-, F-) induce deprotonation of the N-H fragment. The extreme enhancement of the thiourea acidity is based on the coordinative interaction of the sulfur atom with the metal, which stabilizes the thiolate form, and it is much higher than that exerted by any other covalently linked electron-withdrawing substituent, for example, -NO2
Light-driven molecular machines based on ruthenium(II) polypyridine complexes: Strategies and recent advances
No full textMulticomponent molecular systems that exhibit large amplitude movements controlled by external inputs – namely, molecular machines – are extensively investigated both for their basic science interest and for their potential applications in technology and medicine. Light is a convenient stimulus to operate molecular machines because it can provide both an energy supply to feed their motion and an analytical signal to monitor their state. Research in the past two decades has shown that the unique and highly tunable structural, photophysical, photochemical and redox properties of Ru(II) polypyridine complexes are advantageous tools for implementing a light-induced response in molecular devices and machines. Here we describe the latest progresses in the realization of artificial nanoscale machines that use such metal complexes to process light signals. We will show the level of creativity and sophistication reached in this research area by describing a few selected examples
Redox Driven Intramolecular Anion Translocation Between a Metal Centre and a H bond Donating Compartment
No full textAbstract: Dicationic ligands incorporating
two 2,2’-bipyridine units and two
imidazolium moieties, [1]2+ and [2]2+,
form stable chelate complexes with
CuII and CuI in acetonitrile solution.
Each CuII complex binds two X ions
according to two stepwise equilibria,
the first involving the CuII centre and
the second involving the bis-imidazolium
compartment. CuI complexes are
able to host only one NO3 ion in the
bis-imidazolium cavity, while other
anions induce demetallation. Thus, in
the presence of one equivalent of
NO3, the CuII/CuI redox change
makes the anion translocate quickly
and reversibly from one binding site to
the other within the [CuII,I(1)]4+/3+
system, as demonstrated by cyclic voltammetry
and controlled-potential
electrolysis experiments
A Metal-Based Trisimidazolium Cage that Provides Six C-H Hydrogen-Bond-Donor Fragments and Includes Anions
No full textSheltering under the roof: A trisimidazolium cage is capped with a {FeII(bpy)3}2+ subunit to produce a receptor that can bind small anions (bpy=2,2′-bipyridine). Rodlike “pseudohalide” (N3−, NCO−, and NCS−) and spherical halide (Cl−, Br−, and I−) anions accept hydrogen bonds from CH fragments in the receptor cavity. The N3− ion forms the most stable inclusion compound (see structure; Fe red, C light blue, H white, N dark blue)
Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes
No full textThe interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron-donor and electron-acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers. To this aim, the photoinduced electron transfer processes involving a 1,8-naphthalimide chromophore embedded in several rotaxane-based dyads were investigated by steady-state and time-resolved absorption and luminescence spectroscopic experiments in the 300 fs-10 ns time window. Different rotaxanes built around the dialkylammonium / dibenzo[24]crown-8 ether supramolecular motif were designed and synthesized to decipher the relevance of key structural factors, such as the chemical deactivation of the ammonium-crown ether recognition, the presence of a secondary site for the ring along the axle, and the covalent functionalization of the macrocycle with a phenothiazine electron donor. Indeed, the conformational freedom of these compounds gives rise to a rich dynamic behavior induced by light, and may provide opportunities for investigating and understanding phenomena that take place in complex (bio)molecular architectures
Remote electrochemical modulation of pKa in a rotaxane by co-conformational allostery
No full textAllosteric control, one of Nature’s most effective ways to regulate functions in biomolecular machinery, involves the transfer of information between distant sites. The mechanistic details of such a transfer are still object of intensive investigation and debate, and the idea that the intramolecular communication could be enabled by dynamic processes is gaining attention as a complement to traditional explanations. Mechanically interlocked molecules, owing to the particular kind of connection between their components and the resulting dynamic behavior, are attractive systems to investigate allosteric mechanisms and exploit them to develop functionalities with artificial species. We show that the pKa of an ammonium site located on the axle component of a [2]rotaxane can be reversibly modulated by changing the affinity of a remote recognition site for the interlocked crown ether ring through electrochemical stimulation. The use of a reversible ternary redox switch enables us to set the pKa to three different values, encompassing more than 7 units. Our results demonstrate that in the axle the two sites do not communicate, and that in the rotaxane the transfer of information between them is made possible by the shuttling of the ring, that is, by a dynamic intramolecular process. The investigated coupling of electron- and proton-transfer reactions is reminiscent of the operation of the protein complex I of the respiratory chain
Electrochemically and Chemically Induced Redox Processes in Molecular Machines
No full textInternational audienceThe controlled motion of ions, molecules, or supramolecular entities has inspired scientists for more than 25 years, and the concept remains of great interest for the development of moveable nanoscale objects for applications in electronics, medicine, and materials chemistry. Artificial molecular machines that involve electron exchange processes induced by electrical, chemical, or photochemical energy, have been widely developed. More specifically, machines that are stimulated by electrochemical means have been the focus of particular attention because such systems can allow the change of states to be followed and can enable access to mechanistic pathways through direct monitoring. Furthermore, these devices have the ability to connect the nanoscale machine to the macroscopic world, for instance by surface immobilization. In this review, we report recent examples of redox-based molecular machines in both fluid solution and in organized or controllable environments such as modified electrodes, nanoparticles, polymers, gels, and liquid crystals. Basic concepts of molecular machinery and general applications are also discussed
An Artificial Molecular Transporter
No full textThe transport of substrates is one of the main tasks of biomolecular machines in living organisms. We report a synthetic small‐molecule system designed to catch, displace, and release molecular cargo in solution under external control. The system consists of a bistable rotaxane that behaves as an acid–base controlled molecular shuttle, whose ring component bears a tether ending with a nitrile group. The latter can be coordinated to a ruthenium complex that acts as the load, and dissociated upon irradiation with visible light. The cargo loading/unloading and ring transfer/return processes are reversible and can be controlled independently. The robust coordination bond ensures that the cargo remains attached to the device while the transport takes place
Nouveaux récepteurs cavitaires dérivés de calix[6]arènes :fonctionnalisation sélective, chimie de coordination et reconnaissance moléculaire dans l’eau
No full textL’élaboration de récepteurs artificiels pouvant reconnaître soit des ions métalliques soit des molécules chargées ou neutres avec une haute affinité et une grande sélectivité est l’un des objectifs majeurs de la chimie supramoléculaire. En effet, de tels récepteurs ont de potentielles applications dans de nombreux domaines comme en imagerie médicale ou en analyse environnementale. Afin de pouvoir obtenir des récepteurs sophistiqués, il faut cependant généralement passer par une étape d’ingénierie de la molécule dont la conception concrète nécessite une fonctionnalisation ad hoc. Cela requiert souvent la mise au point de méthodes originales de synthèse accompagnée d’un travail méthodologique important. Par ailleurs, alors que l’eau, de par ses propriétés physico-chimiques, joue un rôle tout à fait particulier dans la reconnaissance moléculaire (effet hydrophobe) et dans les réactions enzymatiques (transfert d’électrons et de protons), la majorité des études décrites dans la littérature ont été faites en solvant organique. Si quelques récepteurs, principalement basés sur des molécules intrinsèquement hydrosolubles (telles que la cyclodextrine ou le cucurbiturile) ont permis d’enrichir la chimie supramoléculaire dans l’eau, l’étude comparée de récepteurs en milieux organique et aqueux, qui apporterait de nombreuses informations est, elle, en revanche peu explorée par la communauté supramoléculaire. Ce travail de thèse s’insère dans ces deux cadres et cherche à répondre en partie à ces problématiques. Il est axé sur :(i) le développement d’une méthodologie unique pour la fonctionnalisation sélective de calix[6]arènes; et (ii) la synthèse et l’étude de nouveaux récepteurs hydrosolubles dérivés du calix[6]tren.La première partie de ce travail décrit le développement d’une stratégie supramoléculaire, qui consiste en l’utilisation des propriétés réceptrices du macrocycle calix[6]arène comme un outil pour sa monofonctionnalisation sélective. Le champ d’application d’une telle approche est notamment étudié à partir de différents récepteurs calix[6]arèniques dont la reconnaissance est basée sur divers types d’interactions (interactions ioniques ou chimie de coordination). Ces travaux s’insèrent dans un cadre plus général de fonctionnalisation sélective de macrocycles via assistance supramoléculaire, qui fait l’objet du chapitre introductif de cette partie. Les propriétés émergentes de ces nouveaux ligands monofonctionnalisés sont également présentées dans ce document.La deuxième partie du travail concerne l’hydrosolubilisation de ligands calixaréniques et l’étude de leurs propriétés de reconnaissance ainsi que celles de leurs complexes (Zn(II), Cu(II)) correspondants dans l’eau. L’étude est pour cela dirigée vers l’hydrosolubilisation d’un récepteur qui s’est révélé particulièrement prometteur en solvant organique, mais qui n’était pas soluble en milieu aqueux :le calix[6]tren.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe
- …
