Signaling Recognition Events with Fluorescent Sensors and Switches.

The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3773%2820020503%2941%3A9%3C1460%3A%3AAID-ANIE11111460%3E3.0.CO%3B2-N

Dynamic covalent chemistry.

The nature and characteristics of the CH/π interaction are discussed by comparison with other weak molecular forces such as the CH/O and OH/π interaction. The CH/π interaction is a kind of hydrogen bond operating between a soft acid CH and a soft base π-system (double and triple bonds, C6 and C5 aromatic rings, heteroaromatics, convex surfaces of fullerenes and nanotubes). The consequences of CH/π hydrogen bonds in supramolecular chemistry are reviewed on grounds of recent crystallographic findings and database analyses. The topics include intramolecular interactions, crystal packing (organic and organometallic compounds), host/guest complexes (cavity-type inclusion compounds of cyclodextrins and synthetic macrocyclic hosts such as calixarenes, catenanes, rotaxanes and pseudorotaxanes), lattice-inclusion type clathrates (including liquid crystals, porphyrin derivatives, cyclopentadienyl compounds and C60 fullerenes), enantioselective clathrate formation, catalytic enantioface discriminating reactions and solid-state photoreaction. The implications of the CH/π concept for crystal engineering and drug design are evident.

CH/π hydrogen bonds in crystals

Crown ethers: sensors for ions and molecular scaffolds for materials and biological models.

Molecular meccano. 1. [2]Rotaxanes and a [2]catenane made to order

Synthese d'une serie d'anthraquinones substituees en −1 (ou Ar) par le substituant E=−CH 2 CH 2 - avec formation d'Ar-OCH 3 , Ar-OEOEOCH 3 , Ar-OEOEOEOCH 3 , Ar-OEOEOEOEOCH 3 , Ar-OEOEOEOEOEOCH 3 , Ar-OEOEOEO-Ar et d'Ar-O-CH 2 -15-crown-5. Mise en evidence de l'existence de couples redox a un ou 2 electrons (voltammetrie cyclique) dans le cas des differents systemes lors de l'addition d'ions Li + , Na + et K + . Le comportement electrochimique est complexe et est en accord avec une combinaison de facteurs electroniques et steriques

Electrochemical switching in anthraquinone-substituted carbon-pivot lariat ethers and podands: chain length effects in geometric and electronic cooperativity.

Contrasting one- and two-cation binding behavior in syn- and anti-anthraquinone bibracchial podand (BiP) mono- and dianions assessed by cyclic voltammetry and electron paramagnetic resonance spectroscopy

Structural studies on [M(NCS)-(SQC-HQDME)] (M=Li, Na) as well as free (QC-HQDME and [M(NCS)((QC-HQDME)] (M=Na, K) (where SQC-HQDME is 15,17-dimethyl-1(,18-dimethoxy-3,6,9,12-tetraoxabicyclo[12.3.1]octadeca(1,14,16)triene, and (QC-HQDME is 15,17-dimethyl-16,18-dimethoxy-3,6,9,12,15-pentaoxabicyclo[15.3.1]heneico (1,14,1()triene) show that in all cases the metal ion binds to the anisole oxygen atom in the 1-position.

Redox-active crown ethers. Electrochemical and electron paramagnetic resonance studies on alkali metal complexes of quinone crown ethers

Considerations sur les proprietes de transport des ions Li + , des radicaux anioniques obtenus par reduction electrochimique de (tetraoxa-«3,6,9,12»-tridecyloxy-1) anthraquinone

Milagros. Delgado

Papers

Molecular meccano. 1. [2]Rotaxanes and a [2]catenane made to order

Electrochemical switching in anthraquinone-substituted carbon-pivot lariat ethers and podands: chain length effects in geometric and electronic cooperativity.

Contrasting one- and two-cation binding behavior in syn- and anti-anthraquinone bibracchial podand (BiP) mono- and dianions assessed by cyclic voltammetry and electron paramagnetic resonance spectroscopy

Redox-active crown ethers. Electrochemical and electron paramagnetic resonance studies on alkali metal complexes of quinone crown ethers

Enhanced transport of Li+ through an organic model membrane by an electrochemically reduced anthraquinone podand