Research Paper

Copper-complexed catenanes and rotaxanes in motion: 15 years of molecular machines.

Abstract: In this review, we try to summarise the work performed in our laboratories in the course of the last 15 years, in the field of catenane- and rotaxane-based molecular machines containing copper(I), copper(II) or zinc(II) atoms. We put our work into perspective, showing how the properties of the compounds made have been gradually improved, mostly in terms of motion rate. In parallel, the function of the molecular machines elaborated have been made more and more complex. Instead of discussing all the systems elaborated and studied in our team, we preferred to select a few representative examples and show what were the principles which guided us for improving their performances and how the compounds were experimentally modified to afford new functions and faster-responding molecular machines. Starting from an electrochemically-driven "swinging" [2]catenane, reported in 1994, whose rearrangement was disappointingly slow, we could recently elaborate fast moving pirouetting copper-complexed [2]rotaxanes or molecular shuttles. The rearrangement mechanism of the pirouetting systems, as studied by experimental and computational methods, led us to synthesise interlocking compounds with a very open structure around the copper centre, allowing facile ligand exchange. With such compounds, whose copper centres are highly accessible, the minutes or hours required for the first generation of molecular machines to rearrange were converted to milliseconds or seconds, demonstrating that the rate limiting step in the electrochemically-steered copper catenanes and rotaxanes is probably decoordination of ligands to be replaced by entering ligands leading to the new form of the species. In addition to the electrochemically-piloted systems, we discuss a few compounds, which were not set in motion using an electrochemical signal but rather a chemical stimulus, including porphyrin-containing [2]rotaxanes or a "molecular muscle", based on a [2]rotaxane dimer of the hermaphroditic type. In these two machines, the stimulus is based on metal complexation, decomplexation or exchange (copper(I) being replaced by zinc(II) and vice versa for the "muscle"). Using non-sterically hindering ligands of the 8,8'-diaryl-3,3'-biisoquinoline, it is likely that the chemically driven motions used for contracting or stretching the "muscle" could be replaced by electrochemical signals, which are certainly more promising in terms of future devices.

Pub.: 25 Aug '10, Pinned: 13 Oct '16

Research Paper

Contractile and extensible molecular figures-of-eight.

Abstract: Two large rings, 66- (m-66) and 78-membered (m-78) rings, each one incorporating two pairs of transition-metal-complexing units, have been prepared. The coordinating fragments are alternating bi- and tridentate chelating groups, namely, 2,9-diphenyl-1,10-phenanthroline (dpp) and 2,2',2',6''-terpyridine (terpy) respectively. Both macrocycles form molecular figures-of-eight in the presence of Fe(II) , affording a classical bis-terpy complex as the central core. The larger m-78 ring can accommodate a four-coordinate Cu(I) center with the formation of a {Cu(dpp)2 }(+) central complex and a highly twisted figure-of-eight backbone, whereas m-66 is too small to coordinate Cu(I) . Macrocycle m-78 thus affords stable complexes with both Fe(II) and Cu(I) ; the ligand around the metal changes from (terpy)2 to (dpp)2 . This bimodal coordination situation allows for a large amplitude rearrangement of the organic backbone. When coordinated to preferentially octahedrally coordinated Fe(II) or Cu(II) , the height of the molecule along the coordinating axis of the tridentate terpy ligands is only about 11 Å, whereas the height of the molecule along the same vertical axis is several times as large for the tetrahedral Cu(I) complex. Chemically or electrochemically driven contraction and extension motions along a defined axis make this figure-of-eight particularly promising as a new class of molecular machine prototype for use as a constitutive element in muscle-like dynamic systems.

Pub.: 04 Sep '15, Pinned: 13 Oct '16

Research Paper

Transition-metal-complexed catenanes and rotaxanes: from dynamic systems to functional molecular machines.

Abstract: Transition metal-based catenanes and rotaxanes constitute a specific class of mechanically interlocked molecules whose metal centers are essential both as templates in the construction of the compounds and for their ability to induce large-amplitude motions. In the present chapter we will first present a historical perspective of the field of interlocking compounds in general, in relation to molecular machines, starting with old work dating back to the 1980s and 1990s. Copper was shown many years ago to be the metal of choice for synthesizing the compounds via a template approach and for setting the molecules in motion using a redox signal (Cu(II)/Cu(I)). In a second paragraph, we will discuss various rotaxanes able to undergo a pirouetting motion of the axis within the threaded ring. Two families of such molecules will be mentioned: (1) a porphyrin-containing [2]rotaxane whose pirouetting motion is induced by a chemical reaction and (2) electrochemically driven systems. In this second category of [2]rotaxanes, the rate of motion could be dramatically increased by gradually modifying structural parameters and, in particular, by making the metal center less and less hindered by its surrounding ligands. The third section will be devoted to molecular shuttles and muscles, both families of compounds being reminiscent of linear machines such as biological muscles. By replacing the classical 2,9-diaryl-1,10-phenanthroline chelate (highly shielding and hindering) used by our group since the 1980s by an endocyclic but non-sterically hindering 3,3'-biisoquinoline derivative, the shuttling rate was increased in spectacular fashion, demonstrating the importance of steric factors in transition metal-based molecular machines. The same 3,3'-biisoquinoline motif was also used in the elaboration of a three-station shuttle, leading to long-distance (>20 Å) transport of a ring along the axis on which it is threaded. Finally, porphyrin-containing [3]rotaxanes and [4]rotaxanes, the latter displaying an overall cyclic structure, will be discussed and shown to behave as adjustable and switchable receptors. The synthesis of such compounds is a particularly challenging task in itself. In addition, the new receptors display fascinating properties such as, in particular, their ability to compress various guests and to expel them from their binding site using a chemical signal.

Pub.: 25 Feb '14, Pinned: 13 Oct '16

Research Paper

The beauty of knots at the molecular level.

Abstract: What makes a given object look beautiful to the observer, be it in the macroscopic world or at the molecular level? This very general question will be briefly addressed at the beginning of this essay, in relation to contemporary molecular chemistry and biology, leading to the general statement that, most of the time, beauty is tightly connected to function as well as to the cultural background of the observer. The main topic of the present article will be that of topologically non-trivial molecules or molecular ensembles and the fascination that such species have exerted on molecular or solid state chemists. Molecules with a graph identical to Kuratowski's K₅ or K₃,₃ graphs are indeed highly attractive from an aesthetical viewpoint, but perhaps even more fascinating and beautiful are molecular knots. A general discussion will be devoted to these compounds, which are still considered as exotic species because of the very limited number of efficient synthetic strategies leading to their preparation. Particularly efficient are templated approaches based either on transition metals such as copper(I) or on organic groups able to form hydrogen bonds or acceptor-donor stacks. A particularly noteworthy property of knots, and in particular of the trefoil knot, is their topological chirality. The isolation of both enantiomers of the trefoil knot (3₁) could be achieved and showed that such species have fascinating chiroptical properties. Finally, various routes to more complex and beautiful knots than the trefoil knot, which is the simplest non-trivial knot, will be discussed in line with the remarkable ability of transition metals to gather and orient in a very precise fashion several organic components in their coordination spheres, thus leading to synthetic precursors displaying geometries which are perfectly well adapted to the preparation of the desired knots or links.

Pub.: 26 Oct '11, Pinned: 13 Oct '16

Research Paper

A fast-moving copper-based molecular shuttle: synthesis and dynamic properties.

Abstract: Fast-track changes: The synthesis of a new copper-based molecular shuttle is described, with a coordinating macrocycle based on a nonhindering but endocyclic ligand (see scheme), which makes the ligand exchange easier, and thus the motions of the ring along the thread faster.The present report deals with the synthesis of a two-station [2]rotaxane consisting of a dpbiiq-incorporating macrocycle (dpbiiq: 8,8'-diphenyl-3,3'-biisoquinoline) threaded by a coordinating fragment whose complexing units are a dpp and a terpy ligand (dpp: 2,9-diphenyl-1,10-phenanthroline; terpy: 2,2',6',2"-terpyridine). The [2]rotaxane was prepared in 11 steps from commercially available or easy-to-make molecules, without taking into account the preparation of the dpbiiq-containing 39-membered ring, which was available in our group. The ring-incorporated bidentate chelate is at the same time endocyclic and sterically nonhindering, which is a specific property of the dpbiiq-coordinating unit. This unique feature has a profound influence on the rate of the ring-and-copper translation motion between the two stations of the axle. Based on an analogous multistep strategy, a related molecular shuttle has also been prepared that contains exactly the same axle and stoppers as the first compound but whose threaded ring incorporates the sterically hindering dpp chelate. The translation motions of this other system are several orders of magnitude slower than the corresponding movements of the dpbiiq-based compound. The motion corresponding to the rearrangement of the unstable five-coordinate copper(I) form of the compounds is relatively fast for both shuttles; the half lifetime of the five-coordinate Cu(I) species being below 20 ms for the dpbiiq-containing system and below 1 s for the dpp-based molecule. The reverse motion corresponding to the rearrangement of the four-coordinate copper(II) complexes is much slower, especially for the dpp-based system. It is of the order of several hours for the dpp-based shuttle and only one second or less for the dpbiiq system, under exactly the same conditions. The remarkable difference between the motion rates for the two two-station shuttles demonstrates that the use of a very open chelate such as dpbiiq is extremely beneficial in the context of fast-moving molecular machines.

Pub.: 24 Feb '09, Pinned: 13 Oct '16

Research Paper

Light-driven monodirectional molecular rotor.

Abstract: Attempts to fabricate mechanical devices on the molecular level have yielded analogues of rotors, gears, switches, shuttles, turnstiles and ratchets. Molecular motors, however, have not yet been made, even though they are common in biological systems. Rotary motion as such has been induced in interlocked systems and directly visualized for single molecules, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon-carbon double bond in a chiral, helical alkene, with each 360 degrees rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180 degrees rotation around the carbon-carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for 'molecular machinery' driven by light.

Pub.: 18 Sep '99, Pinned: 13 Oct '16

Research Paper

Unidirectional molecular motor on a gold surface.

Abstract: Molecules capable of mimicking the function of a wide range of mechanical devices have been fabricated, with motors that can induce mechanical movement attracting particular attention. Such molecular motors convert light or chemical energy into directional rotary or linear motion, and are usually prepared and operated in solution. But if they are to be used as nanomachines that can do useful work, it seems essential to construct systems that can function on a surface, like a recently reported linear artificial muscle. Surface-mounted rotors have been realized and limited directionality in their motion predicted. Here we demonstrate that a light-driven molecular motor capable of repetitive unidirectional rotation can be mounted on the surface of gold nanoparticles. The motor design uses a chiral helical alkene with an upper half that serves as a propeller and is connected through a carbon-carbon double bond (the rotation axis) to a lower half that serves as a stator. The stator carries two thiol-functionalized 'legs', which then bind the entire motor molecule to a gold surface. NMR spectroscopy reveals that two photo-induced cis-trans isomerizations of the central double bond, each followed by a thermal helix inversion to prevent reverse rotation, induce a full and unidirectional 360 degrees rotation of the propeller with respect to the surface-mounted lower half of the system.

Pub.: 28 Oct '05, Pinned: 13 Oct '16