Artificial Supramolecular Pumps Powered by Light

Nature nanotechnology, 2015

Biomolecular motors convert energy into directed motion and operate away from thermal equilibrium. The development of dynamic chemical systems that exploit dissipative (non-equilibrium) processes is a challenge in supramolecular chemistry and a premise for the realization of artificial nanoscale motors. Here, we report the relative unidirectional transit of a non-symmetric molecular axle through a macrocycle powered solely by light. The molecular machine rectifies Brownian fluctuations by energy and information ratchet mechanisms and can repeat its working cycle under photostationary conditions. The system epitomizes the conceptual and practical elements forming the basis of autonomous light-powered directed motion with a minimalist molecular design.

The Journal of Organic Chemistry, 2020

The synthesis and characterization of a series of light-driven third-generation molecular motors featuring various structural modifications at the central aromatic core are presented. We explore a number of substitution patterns, such as 1,2dimethoxybenzene, naphthyl, 1,2-dichlorobenzene, 1,1′:2′,1″terphenyl, 4,4″-dimethoxy-1,1':2′,1″-terphenyl, and 1,2-dicarbomethoxybenzene, considered essential for designing future responsive systems. In many cases, the synthetic routes for both synthetic intermediates and motors reported here are modular, allowing for their post-functionalization. The structural modifications introduced in the core of the motors result in improved solubility and a bathochromic shift of the absorption maxima. These features, in combination with a structural design that presents remote functionalization of the stator with respect to the fluorene rotors, make these novel motors particularly promising as light-responsive actuators in covalent and supramolecular materials.

The Journal of Chemical Physics

Journal of the American Chemical Society, 2002

Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360°rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t1/2 θ 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.

Chemistry - A European Journal, 2014

Pseudorotaxanes are the simplest prototypes for the construction of molecular machines based on threaded species. Investigation on molecular motions in these model systems is a necessary action for an efficient design of working molecular machines and motors. Herein we report on photoactive pseudorotaxanes based on the interaction between bipyridinium and cucurbit[7]uril (CB7). The molecular axle is composed of a central bipyridinium unit and two azobenzene moieties at the extremities. CB7 can form two different complexes with this molecule: a [2]pseudorotaxane, in which the macrocycle shuttles fast along the length of the axle, and a [3]pseudorotaxane, in which two CB7 s are confined at the extremities of the axle. Upon trans to cis isomerization of the azobenzene moieties, the [3]pseudorotaxane is destabilized, and only one CB7 resides on the axle, surrounding the bipyridinium unit. The system was successfully inserted into the core of liposomes, and preliminary investigations confirmed that it maintains its switching ability.

Journal of the American Chemical Society, 2013

Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability not only to control relative movements of components in molecules but also to drive their components preferentially in one direction relative to each other using versatile stimuli is one of the keys to future technological applications. Herein, we describe a wholly synthetic small-molecule system that, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion. Altering the redox state of a cyclobis(paraquat-p-phenylene) ring simultaneously (i) inverts the relative heights of kinetic barriers presented by the two terminione a neutral 2-isopropylphenyl group and the other a positively charged 3,5-dimethylpyridinium unitof a constitutionally asymmetric dumbbell, which can impair the threading/ dethreading of a [2]pseudorotaxane, and (ii) controls the ring's affinity for a 1,5-dioxynaphthalene binding site located in the dumbbell's central core. The formation and subsequent dissociation of the [2]pseudorotaxane by passage of the ring over the neutral and positively charged termini of the dumbbell component in one, and only one, direction relatively defined has been demonstrated by (i) spectroscopic (1 H NMR and UV/vis) means and cyclic voltammetry as well as with (ii) DFT calculations and by (iii) comparison with control compounds in the shape of constitutionally symmetrical [2]pseudorotaxanes, one with two positively charged ends and the other with two neutral ends. The operation of the system relies solely on reversible, yet stable, noncovalent bonding interactions. Moreover, in the presence of a photosensitizer, visible-light energy is the only fuel source that is needed to drive the unidirectional molecular translation, making it feasible to repeat the operation numerous times without the buildup of byproducts.

Chemistry-a European Journal, 1997

Photochemical control of a self-assembled supramolecular 1:1 pseudorotaxane (formed between a tetracationic cyclophane, namely the tetrachloride salt of cyclobis(paraquat-p-phenylene), and 1,5-bis[2-(2-(2-hydroxy)ethoxy)ethoxy]naphthalene) has been achieved in aqueous solution. The photochemical one-electron reduction of the cyclophane to the radical trication weakens the noncovalent bonding interactions between the cyclophane and the naphthalene guest—π-π interactions between the π-electron-rich and π-electron-poor aromatic systems, and hydrogen-bonding interactions between the acidic α-bipyridinium hydrogen atoms of the cyclophane and the polyether oxygen atoms of the naphthalene derivative—sufficiently to allow the guest to dethread from the cavity; the process can be monitored by the appearance of naphthalene fluorescence. The radical tricationic cyclophane can be oxidized back to the tetracation in the dark by allowing oxygen gas into the system. This reversible process is marked by the disappearance of naphthalene fluorescence as the molecule is recomplexed by the tetracationic cyclophane. This supramolecular system can be chemically modified such that the π-electron-rich unit, either a naphthalene derivative or a hydroquinone ring, and the tetracationic cyclophane are covalently linked. We have demonstrated that the π-electron-rich residue in this system is totally “self-complexed” by the cyclophane to which it is covalently attached. Additionally, the self-complexation can be switched “off” and “on” by electrochemical two-electron reductions and oxidations, respectively, of the tetracationic cyclophane component. Thus, we have achieved the construction of two switches at the nanoscale level, one driven by photons and the other by electrons.

SPIE Proceedings, 2010

The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not involving the limitations associated with using photochemical change, nor the high intensities required to produce and manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the generation of continuous rotary motion, the ultimate aim of molecular motor design.

Physical chemistry chemical physics : PCCP, 2017

A fundamental requirement for achieving photoinduced unidirectional rotary motion about an olefinic bond in a molecular motor is that the potential energy surface of the excited state is asymmetric with respect to clockwise and counterclockwise rotations. In most available light-driven rotary molecular motors, such asymmetry is guaranteed by the presence of a stereocenter. Here, we present non-adiabatic molecular dynamics simulations based on multiconfigurational quantum chemistry to demonstrate that this chiral feature is not essential for inducing unidirectional rotary motion in molecules that incorporate a cyclohexenylidene moiety into a protonated Schiff-base framework. Rather, the simulations show that it is possible to exploit the intrinsic asymmetry of the puckered cyclohexenylidene to control the direction of photoinduced rotation.

The Journal of Organic Chemistry, 2010

Controlling the unidirectional rotary process of second-generation molecular motors demands access to these motors in their enantiomerically pure form. In this paper, we describe an enantioselective route to three new second-generation light-driven molecular motors. Their synthesis starts with the preparation of an optically active alpha-methoxy-substituted upper-half ketone involving an enzymatic resolution. The subsequent conversion of this ketone to the corresponding hydrazone by treatment with hydrazine led to full racemization. However, conversion to a TBDMS-protected hydrazone by treatment with bis-TBDMS hydrazine, prepared according to a new procedure, proceeds with nearly full retention of the stereochemical integrity. Oxidation of the TBDMS-protected hydrazone and subsequent coupling to a lower-half thioketone followed by recrystallization provided the molecular motors with >99% ee. As these are the first molecular motors that have a methoxy substituent at the stereogenic center, the photochemical and thermal isomerization steps involved in the rotary cycle of one of these new molecules were studied in detail with various spectroscopic techniques.