Photo-driven molecular devices

Abstract

In this critical review, we discuss switching of the light-powered bistable rotaxanes and catenanes and highlight the practical applications of some of these systems. Photoactive molecular and supramolecular machines are comprised of two parts—1) a switching element, based on noncovalent interactions within the recognition units, which is responsible for executing mechanical movement, and 2) a light-harvesting unit which utilizes light to control the competitive interactions between the recognition sites. We also survey another class of molecular devices, namely molecular rotary motors—i.e., those that behave like their macroscopic counterparts—in which photochemically and thermally induced mechanical movement relies on isomerizations of a pivotal C=C bond, leading to a rotation of the top propeller part with respect to the stationary bottom part of the helical shaped chiral molecule. (146 references.)

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Sourav Saha

Sourav Saha received an MSc degree in Chemistry from the Indian Institute of Technology in Kanpur in 2000 and a PhD degree from the University of California, Los Angeles (UCLA) in 2005 working on Light and Electron Powered Molecular and Supramolecular Machines. Presently, he is a postdoctoral researcher in Professor Fraser Stoddart's Supramolecular Chemistry Laboratory at UCLA. He is working towards developing bistable rotaxane-based molecular logic-gates in crossbar electrode architectures as well as new generations of photo-driven molecular shuttles as prototypes of molecular devices and machines.

J. Fraser Stoddart

J. Fraser Stoddart was born in Edinburgh, Scotland, in 1942. He received all (BSc, PhD, and DSc) of his degrees from the University of Edinburgh, U.K. Presently, he holds the Fred Kavli Chair in NanoSystems Sciences at UCLA and is the Director of the California NanoSystems Institute. His current research interests are concerned with transporting well-established concepts in biology from the life sciences into materials and medicinal chemistry.

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1. Introduction

Machines need power supplies for them to undergo mechanical movements and hence do work. Just as macroscopic machines require macroscopic energy sources, natural^1–6 and artificial molecular machinery^7–37 require nanoscale power supplies to undergo directed molecular motions. While macroscopic machines do not move until energy is supplied to them for a specific function, nanoscale machines down at the molecular level are in perpetual Brownian motion at ambient temperatures. Thus, for machine-like functions to be expressed by molecular systems, they should be able to execute specific and directional mechanical movements—when stimulated by external inputs—that are otherwise only of a haphazard nature on account of Brownian motion . The intrinsic random motions of molecular machines make the detection of the stimuli-induced, relative movements of their components a nontrivial phenomenon, as compared to the easily detectable, obvious motions executed by macroscopic machines. Three types of external stimuli—that is chemical,^38–45 electrochemical,^46–54 and photochemical^54–64—have been employed to induce well-defined mechanical movements within molecular machines. Several spectroscopic and electrochemical techniques have been introduced to distinguish the stimuli-induced nanomechanical motions from the random Brownian ones.

Mechanically interlocked molecules, such as bistable [2]catenanes^65–67 and [2]rotaxanes^68–76 are some of the most suitable candidates to serve as nanoscale mechanical switches in the rapidly evolving field of nanoelectronics^77–88 and nanoelectromechanical systems^89–100 (NEMS). The advantages of mechanically interlocked molecules for potential applications in the realm of molecular electronics stem from their abilities to undergo^101–128 controllable nanoelectromechanical motions. In bistable catenanes and rotaxanes , changing the affinity of the ring for the two distinct recognition sites on the dumbbell component allows the ring to be translated between the sites and thus relative mechanical movement is produced. A minimum condition for generating such a translational motion within a bistable [2]rotaxane is a large difference in the binding affinities for the ring by the two different recognition sites on the dumbbell component in the ground state . Thereafter, it is important that these relative binding affinities can be reversed by an external stimulus. A free energy difference of greater than 1.2 kcal mol^–1 between the two translational isomers ensures⁵¹ that, at room temperature, 90% of the ring is positioned around one recognition site in preference to the other one, a situation which gives rise to a highly preferred ground state co-conformation (GSCC). Modification of the binding affinity of the primary recognition site by an external stimulus^38–64 turns OFF the stabilizing interactions in the GSCC and triggers the translational motion of the ring component to the alternative recognition site which provides temporarily better stabilizing interactions. The ring's location around the second recognition site, even after the removal of the external stimulus and the re-instatement of the GSCC, results in an isoelectronic translational isomer of the bistable rotaxane , which is defined as the metastable state co-conformation (MSCC). Relaxation of the MSCC to reproduce the GSCC, triggered by either the ambient thermal energy or another stimulus, completes the full cycle of the machine-like movement of bistable rotaxanes , making them one of the most accessible molecular machines.

2. Light-powered linear motion within a metal ion-based rotaxane

The transition metal based [2]rotaxane 1^+/2+ is composed¹²⁹ (Fig. 1) of a 30-membered ring (M30) containing a bidentate 2,9-diphenyl-1,10-phenanthroline (phen) ligand, which is interlocked onto a dumbbell-shaped component, equipped with another 2,9-disubstituted phen ligand in addition to a terdentate 2,2′,6′,2′′-terpyridine (terpy) ligand. The ligands on the ring and dumbbell components chelate differently with Cu^I and Cu^II ions, depending on the oxidation state of the cation. The ring can be switched between the phen and terpy ligands on the dumbbell component by controlling the oxidation states of the complexed Cu ions and hence, their coordination environments. The Cu^I₍₄₎ state favors a tetrahedral geometry, a requirement which is satisfied when the bidentate phen ligands from both the ring and the dumbbell are involved in chelation. By contrast, the pentacoordination of the Cu^II₍₅₎ state is satisfied by the involvement of the phen ligand from the ring and the terpy ligand from the dumbbell component. Photochemical or electrochemical oxidation of the complexed Cu^I₍₄₎ to Cu^II₍₅₎ drives the ring from the bidentate phen site to the terdentate terpy site along the dumbbell component and vice versa. Irradiation of the [2]rotaxane 1⁺-Cu^I with 464 nm light in the presence of an electron acceptor (p-nitrobenzyl bromide) generates 1²⁺-Cu^II₍₄₎, which relaxes to the stable 1²⁺-Cu^II₍₅₎ on account of the mechanical movement of the ring to the terpy site. Addition of a reductant (ascorbic acid) reduces the Cu^II₍₅₎ to the Cu^I₍₄₎ state, which, in turn, triggers the return of the ring to the phen site on the dumbbell. As a result, the movement of the ring can be powered by hybrid photochemical–chemical stimuli. The light-driven switching of the rotaxane 1^+/2+ was found to be completely reversible in the presence of tetrabutylammonium tetrafluoroborate (TBABF₄). A direct electrochemical redox process involving the Cu^I₍₄₎/Cu^II₍₅₎ couple also induces a similar translational motion in the bistable [2]rotaxane.

Fig. 1 Photochemically induced switching of a metal complex-based bistable [2]rotaxane 1⁺/1²⁺. Photo-induced oxidation of the Cu(I) center to Cu(II) drives the bidentate macrocycle M30 to the terdentate terpy unit of the dumbbell component, whereas a chemical reduction of Cu(II) to Cu(I) with ascorbic acid returns macrocycle M30 to the original bidentate phen unit, completing a cyclic process which relies on changes in the Cu(I/II) coordination geometry. (Redrawn with permission from ref. 129. Copyright (1999) American Chemical Society.)

3. Light-driven rotational motion of a metal ion-based catenane

Circumrotary motion in a photoactive Ru(II)-complexing [2]catenane 2²⁺ (Fig. 2), consisting¹³⁰ of a 42-membered ring (M42) which contains a 2,2′-bipyridine (bipy) ligand, interlocked with a 63-membered ring (M63) containing two 1,10-phenanthroline (phen) ligands, can be powered by light and heat. In the ground state of the catenane 2²⁺, the interlocked M42 and M63 rings are oriented in such a way that all three bidentate ligands in the two rings participate in complexation with the Ru(II) ion to satisfy its octahedral coordination environment, producing the characteristic red color of the Ru(II) complex. Upon irradiation of 2²⁺ with a 250 W halogen lamp (λ > 300 nm) either in MeCN or in CH₂Cl₂ in the presence of tetraethylammonium chloride, the color of the solution turns from red (λ_max = 458 nm) to purple (λ_max = 561 nm) with a clean isosbestic point at 484 nm. This spectroscopic change, together with the ¹H NMR chemical shift¹³⁰ of the diagnostic peaks, suggests the photochemical decoordination of the M42 ring from the Ru(II) center, while two Cl⁻ ions or MeCN molecules complex with the Ru(II) ion to generate 2′a or 2′b²⁺, respectively. Heating of the solution of 2′a or 2′b²⁺ transforms it back into the original isomer 2²⁺. Based on the absorption and ¹H NMR spectra, the photochemical decoordination and thermal recomplexation occur with >95% efficiency. Thus, the bistable catenane 2²⁺ represents a prototype light/heat-driven artificial molecular machine, which performs rotary motion.

Fig. 2 Photochemically induced switching of a metal complex-based bistable [2]catenane 2²⁺. Irradiation of the catenane 2²⁺ at λ ≥ 300 nm causes MLCT to the phen ligand of the macrocycle M42 and induces its circumrotation along the larger macrocycle M63 in the presence of (a) Cl^– or (b) MeCN ligands which now occupy the empty coordination sites of the Ru(II) ion to produce 2′a or 2′b²⁺, respectively. Heating of these systems regenerates the original state 2²⁺. (Redrawn with permission from ref. 130. Copyright (2004) Wiley-VCH.)

4. Photo-driven shuttling in two constitutionally isomeric bistable rotaxanes

A multicomponent [2]rotaxane 3⁶⁺ has been designed (Fig. 3a) to work⁵⁶ as a visible light-driven molecular abacus. The bistable rotaxane of the Mark I generation is composed of six molecular modules, namely (i) a bis-para-phenylene[34]crown-10 (BPP34C10) electron-rich macrocycle M and a dumbbell-shaped component which contains (ii) a Ru(II)-bipyridine based light-harvesting unit P²⁺ as one of its stoppers, (iii) a p-terphenyl-type ring system as a rigid spacer S which separates the P²⁺ unit from the mechanical switching moiety comprising (iv) a 4,4′-bipyridinium (A₁²⁺) as a strong primary π-electron accepting unit and (v) 3,3′-dimethyl-4,4′-bipyridinium (A₂²⁺) as a weak secondary π-electron accepting unit that can play the role of stations for the macrocycle M, and (vi) a tetraarylmethane group T as the second stopper. In the Mark Irotaxane 3⁶⁺, the A₁ station is situated farthest away from the P²⁺ unit and is separated by the A₂²⁺ station. In the GSCC, the M encircles the stronger π-accepting A₁²⁺ station. The BPP34C10 macrocycle can be switched mechanically between the A₁²⁺ and A₁²⁺ stations with light in the presence of 1) an external reductant/oxidant—sacrificial mechanism⁵⁴ (Fig. 3b), and 2) a dual-action redox reagent—autonomous mechanism⁶¹ (Fig. 3c).

Fig. 3 (a) Structural formula of a photoactive molecular abacus 3⁶⁺. (b) Photo-driven switching process of 3⁶⁺ in the presence of triethanolamine (TEOA) as an external fuel—a sacrificial mechanism: 1) photosensitization of the Ru(II) complex, 2) PET , 3) intrinsic decay, 4) forward movement of M, 5) BET —does not occur in the presence of TEOA, 6) charge recombination , 7) return of M to the A₁²⁺ unit. (c) Photo-driven switching process of 3⁶⁺ in the presence of phenothiazine (ptz) as a donor–acceptor couple—an autonomous mechanism: 8) reduction of the photogenerated P³⁺ unit by ptz, 4) forward movement of BPP34C10, 5) no BET in the presence of ptz, 9) and 10) oxidation of the A₁⁺radical cation back to A₁²⁺ by ptz⁺. (Redrawn with permission from ref. 63. Copyright (2006) National Academy of Sciences USA.)

Sacrificial mechanism: Photosensitization (step 1) of the Ru(bipy)²⁺ complex with 532 nm light generates⁵⁶ its metal-to-ligand charge-transfer (MLCT) excited state *P²⁺, which undergoes a photo-induced electron transfer (PET , step 2) to the stronger π-electron accepting A₁²⁺ unit, reducing it to a A₁⁺radical cation (Fig. 3b). In the presence of an external electron donor (D), triethanolamine (TEOA), which reduces the photo-oxidized P³⁺ unit to prevent the back electron transfer (BET ) process (step 5), the A₁⁺radical cation enjoys enough of a lifetime to repel (step 4) the π-electron rich macrocycle M to the weaker π-electron accepting A₂²⁺ unit. Oxidation of the A₁⁺radical cation back to the A₁²⁺ unit by dioxygen generates the MSCC (Fig. 3b, D), which is represented by the BPP34C10 macrocycle's location around the A₂²⁺ unit. Finally, thermally assisted Brownian motions allow M to return to its original A₁²⁺ unit (step 7), completing the mechanical switching cycle in the presence of the sacrificial donor TEOA, which degrades into by-products. Thus, in order to cycle the photochemical switching process of 3⁶⁺ repetitively, a stoichiometric amount of TEOA is essential for every single cycle.

Autonomous mechanism: The very same series of photochemically induced mechanical switching in 3⁶⁺ was achieved⁶³ (Fig. 3c) using phenothiazine (PTZ) as a dual-action redox reagent, which directly reduces the P³⁺ unit, allowing M enough time to move to the A₂²⁺ station when A₁²⁺ is photo-reduced to A₁⁺. The reduction potential of PTZ^+/0 matches very well with the oxidation potential of the A₁^+/2+ station, allowing a dark-phase electronic recombination to regenerate A₁²⁺. At this point, the M returns to the A₁²⁺ station, completing an autonomous switching cycle, a process in which no additional redox reagent is needed⁶³ for the subsequent photochemical cycles.

A photoactive molecular abacus 4⁶⁺ of the Mark II generation⁶⁴ (Fig. 4) has been synthesized which retains all six modules used in 3⁶⁺. The locations, however, of the A₁²⁺ and A₂²⁺ electron acceptors on its dumbbell component have been swapped.⁶⁴ In this isomer, the macrocycle M still encircles the stronger π-electron acceptor A₁²⁺ in the GSCC which is now located closer to the light-harvesting P²⁺ moiety than it was in the Mark I version. Closer proximity between the P²⁺ and A₁²⁺ units not only accelerates the photo-induced electron transfer (PET ) process from the *P²⁺ unit to the A₁²⁺ unit, but also has a similar effect on the back electron transfer (BET ) process between the photogenerated P³⁺ and the A₁⁺radical cation . Therefore, the lifetime of the A₁⁺radical cation is much shorter⁶² in 4⁶⁺ than in 3⁶⁺, a feature that prevents M from moving back to the A₂²⁺ station. This issue, however, could be circumvented⁶⁴ by employing TEOA as a sacrificial electron donor to reduce P³⁺ before BET takes place, and thus, allowing M to move to the A₂²⁺ station. Upon oxidation of the A₁⁺radical cation back to the A₁²⁺ state by dioxygen, M returns to the A₁²⁺ by thermally assisted Brownian motion s to complete the cycle.

Fig. 4 (a) Structural formula of a photoactive molecular abacus 4⁶⁺ which is a constitutional isomer of the rotaxane 3⁶⁺. (b) Photo-driven switching process of 4⁶⁺ in the presence of TEOA—sacrificial mechanism: 1) photosensitization of the Ru(II) complex, 2) PET , 3) intrinsic decay, 4) forward movement of M, 5) BET —does not occur in the presence of TEOA, 6) charge recombination , 7) return of M to the A₁²⁺ unit, 8) reduction of the photogenerated P³⁺ unit by TEOA, 9) oxidation of the A₁⁺radical cation back to A₁²⁺ by O₂. (Redrawn with permission from ref. 64. Copyright (2006) CSIRO Publishing, http://www.publish.csiro.au/journals/ajc.)

Since the sacrificial donor prevents BET in both Mark II and I versions of these bistable rotaxanes , it has been observed^63,64 that the fuel-assisted photo-driven shuttling is more efficient in the Mark Irotaxane than in the Mark II generation on account of the faster PET in the former. The only other difference in the fuel-assisted, light-driven mechanical shuttling in these two rotaxanes is the opposite directions of the macrocycles' movements—while in the Mark Irotaxane , M moves towards the P²⁺ unit, it moves away from the P²⁺ in the Mark IIrotaxane . In both cases, however, M moves to the secondary electron accepting A₂²⁺ station, when A₁²⁺ is photo-reduced to the A₁⁺radical cation .

5. A Lockable light-driven molecular shuttle

One of the simplest examples of photoactive molecular switches relies on light-activated cis–trans isomerization to effect switching. The bistable [2]rotaxane (E)-5 (Fig. 5) composed¹³¹ of an α-cyclodextrin (α-CD) ring, which is interlocked onto a dumbbell-shaped component containing two recognition sites—a stronger binding stilbene (S) unit and a weaker biphenyl (P) one—and terminated with two different stoppers—a 4-amino-3,6-disulfonic-1,8-naphthalimide disodium salt (N)-based stopper attached to the P unit and an isophthalic acid (I) stopper attached to the stilbene unit, has been synthesized via a Pd-catalyzed Suzuki coupling to produce the biphenyl unit in the presence of α-CD. In the ground state when the stilbene unit is predominantly in the (E)-configuration, the α-CD ring encircles the S unit—a translational isomer which is also stabilized by intramolecular hydrogen-bonding between the carboxylic acid groups on the stopper I and the secondary alcohol groups on the α-CD—preventing the light-induced E–Zphotoisomerization of the S unit. In the presence of an excess of base (Na₂CO₃), the isophthalic acid is deprotonated to produce (E)-5^2–, a process that unlocks the hydrogen-bonding interactions and allows the α-CD to move to the P unit upon photoisomerization of the S unit to the (Z)-isomer at 335 nm. The movement, as well as precise location of the α-CD driven by E–Zphotoisomerization of the stilbene unit, were investigated by ¹H NMR, absorption, and emission spectroscopies, allowing a clean detection of the light-driven mechanical process. Z→E Photoisomerization at 280 nm brings the α-CD back to the S unit, completing the cycle. In the (E)-configuration of stilbene unit, the rotaxane 5^2– shows a weak fluorescence band at 530 nm. The emission intensity, however, rises in the (Z)-isomer when the α-CD moves back to the P unit, a process which can be activated instantaneously by simply switching the excitation wavelengths between 335 and 280 nm.

Fig. 5 Photo-induced mechanical switching of a α-CD-based lockable [2]rotaxane (E/Z)-5 in the presence of Na₂CO₃.

6. Photochemically and thermally driven azobenzene–α-CD rotaxane

The bistable [2]rotaxane (E/Z)-6⁴⁺ (Fig. 6)—composed¹³² of an α-CD ring interlocked onto a dumbbell component consisting a photoisomerizable azobenzene unit as a primary station with two bismethylene units playing the roles of secondary stations for the α-CD and two bipyridinium units acting as polar stoppers for the hydrophobic cavity of the α-CD with two 2,4-dinitrophenyl units as steric stoppers for the α-CD ring—constitutes a light and thermally driven molecular shuttle. In the ground state , the α-CD ring encircles the trans-configurated azobenzene unit, which can be photoisomerized to the cis-configuration by UV-irradiation (360 nm), at which point α-CD moves mechanically away from it to one of the adjacent bismethylene stations. A cis- to trans- photoisomerization with visible light (430 nm) or by thermal isomerization of the azobenzene station brings the α-CD back to its original position. The photo-driven mechanical movement of (E/Z)-6⁴⁺ in H₂O and Me₂SO has been investigated¹³² by ¹H NMR and absorption spectroscopies as well as induced circular dichroism (ICD).

Fig. 6 Mechanical switching of α-CD ring by photo-isomerization of the encircled (E/Z)-azobenzene-based recognition unit in the rotaxane 6⁴⁺.

7. Mechanical switching induced by photoisomerization

The [2]rotaxane (E/Z)-7-H₂²⁺ (Fig. 7) incorporates¹³³ a fumaramide–maleamide unit—a photoisomerizable alkene diamide that also plays the role of a strong hydrogen-bond-acceptor in its (E)-form (fumaramide) to the surrounding hydrogen-bond donating tetramide macrocycle in (E)-7-H₂²⁺. A second hydrogen-bond accepting glycylglycine (Gly-Gly) unit and a nearby anthracene unit to act as a fluorescent label are also present in the dumbbell component of the rotaxane . The tetramide-based macrocycle provides four hydrogen-bond donating centers. Two pyridinium units in the macrocycle are known to quench the anthracene-based fluorescence through electron transfer when they are in close proximity. Irradiation of the (E)-7-H₂²⁺ isomer with 312 nm light converts the (E)-fumaramide into its (Z)-isomer, maleamide which has a significantly lower hydrogen-bond accepting ability. This change in the binding affinity allows the macrocycle to move to the intermediate hydrogen-bond accepting station Gly-Gly, generating the (Z)-7-H₂²⁺ translational isomer. Heating of the (Z)-7-H₂²⁺ isomer at 115 °C regenerates the fumaramide unit and the macrocycle returns to its original binding site in (E)-7-H₂²⁺. The longer distance between the anthracene fluorescent tag and the macrocycle containing the two pyridinium units in the (E)-7-H₂²⁺ isomer allows this isomer to fluoresce (λ_exc = 365 nm), whereas the anthracene fluorescence in the (Z)-7-H₂²⁺rotaxane is quenched by close proximity of the pyridinium ions.

Fig. 7 Photochemically and thermally controllable switching in the [2]rotaxane (E/Z)-7-H₂²⁺. Larger distance between the anthracene unit and the tetramide macrocycle prevents the fluorescence quenching of the anthracene unit, allowing it to fluoresce in the (E)-7-H₂²⁺ isomer. Photo-isomerization of the succinamide (E) unit to fumaramide (Z) drives the macrocycle to the Gly-Gly unit, quenching the anthracene fluorescence.

8. Macroscopic transport by light-driven molecular machines

The photoisomerizable bistable [2]rotaxane¹³⁴ (E/Z)-8-H₂²⁺ has been prepared (Fig. 8a). It consists of a hydrophilic fumaramide (F) unit and a hydrophobic tetrafluorosuccinamide (S) unit on its dumbbell component which play the roles of two stations for the switchable tetramide-based macrocycle (M) by virtue of tunable hydrogen-bonding interactions. The macrocycle is interlocked onto the dumbbell by two terminal diphenylmethane stoppers at both ends. In the GSCC of the rotaxane (E)-8-H₂²⁺, the hydrophilic fumaramide station provides stronger hydrogen-bonding interactions with the surrounding chair-shaped macrocycle, exposing the hydrophobic S station. Irradiation of the (E)-8-H₂²⁺ isomer with 254 nm light causes E→Zphotoisomerization , producing the (Z)-8-H₂²⁺ isomer, in which the macrocycle M moves from the maleamide station to the S station, shielding the hydrophobic section of the dumbbell component. By converting the maleamide back to fumaramide thermally (115 °C) in the presence of piperidine as a base, which disrupts the hydrogen-bonding interactions between the M and S units, the macrocycle moves back to its original F station to regenerate the (E)-8-H₂²⁺ state.

Fig. 8 (a) Photochemically and thermally induced switching of the [2]rotaxane (E/Z)-8-H₂²⁺ containing a polarophobic and a polarophilic units on its dumbbell component. (b) and (c) Photoresponsive behavior of Au{111}-surfaces functionalized with 11-MUA and molecular shuttle (E/Z)-8-H₂²⁺. (d1–d4) Lateral photographs of photo-driven transport of a 1.25 µL diiodomethane drop on a (E/Z)-8-H₂²⁺.11-MUA.Au{111} substrate on mica up a 12° incline. (Redrawn with permission from ref. 134. Copyright (2005) Nature Publishing Group.)

The polarity of Au{111} surfaces functionalized with physisorbed monolayers of (E/Z)-9-H₂²⁺ and 11-mercaptoundecanoic acid (MUA) can be switched¹³⁴ between being polarophobic (Fig. 8b) in the ground state and polarophilic (Fig. 8c) in the photo-excited state . The polarity switching depends upon the macrocycle's location on the dumbbell component. Consequently, the contact angles (θ) of small droplets (0.5–5 µL) of low-volatile polar solvents—such as, water, diiodomethane, formamide and ethylene glycol—on the (E/Z)-8-H₂²⁺-functionalized surface can be altered¹³⁴ significantly (8–22°) by irradiation. For example, the diiodomethanecontact angle of 35° before UV-irradiation of the polarophobic surface (exposed S station) decreases to 13° upon UV-irradiation, a process which transforms the surface to a polarophilic one by shielding the hydrophobic S station with the macrocycle M. Similarly, θ changes for water from 55° to 45°, for formamide from 40° to 31°, and for ethylene glycol from 48° to 40°. It was observed that the (E/Z)-8-H₂²⁺ molecules are not self-assembled on the Au{111} surface in any particular orientation, and in the photostationary state (PSS), the E/Z-ratio is 50 : 50. Therefore, it can be argued that the change in surface polarity does not depend on the location of the macrocycle in every single rotaxane molecule, but on the ensemble of them, in which the overall shielding or exposure of the hydrophobic S station by the macrocycle dictates the surface polarity in a given area.

This unique light-sensitive property of the rotaxane -functionalized surface has been exploited to transport¹³⁴ (Fig. 8d1–d4) a 1.25 µL diiodomethane (ρ = 3.325 g mL^–1) droplet from one point to another by 1.38 mm against gravity on an α = 12° slope. In this experiment, the surface directly under the leading edge of the droplet is irradiated, a process which reduces the contact angle by θ = 22° spreading the droplet in the forward direction. When the (E/Z)-8-H₂²⁺rotaxane reaches its PSS, the irradiated section of the surface becomes more polarophilic on account of the shielding of the hydrophobic S station by the macrocycle. At the same time, the un-irradiated part of the surface remains polarophobic as a consequence of the predominance of the E-8-H₂²⁺ isomer. The difference in surface polarity in two adjacent areas moves the polar solvent diiodomethane to the more polar region against gravity at 12° gradient. Therefore, the work done against gravity by the ensemble of molecular machines is 1.2 × 10^–8 J. Based on the molecular footprint of ∼3 nm² per molecule, there are ∼2 × 10¹² molecules under the elongated drop just before its transport. Considering that 40% of the rotaxane molecules on the surface have been photoisomerized at the PSS during the droplet transport, the work done by each molecule against gravity is ∼1.5 × 10^–12 J, that is ∼9 kJ mol^–1. The macroscopic liquid transportation by ensembles of nanoscale molecular machines may be useful for delivering reactants in lab-on-chip environments by bringing individual drops containing different reagents to perform high-precision reactions or to develop microfluidic devices.

9. PET -driven switching of a hydrogen-bonded molecular motor

The [2]rotaxane 9 is composed⁵⁷ (Fig. 9) of a hydrogen-bond-donating benzylic amide macrocycle, mechanically interlocked with a dumbbell component, which is equipped with a strong hydrogen-bond-accepting unit—succinamide (succ) and a poor hydrogen-bond-accepting 3,6-di-tert-butyl-1,8-naphthalimide (ni) unit. The chair-shaped macrocycle encircles the succ unit in the neutral GSCC of the rotaxane . Photoreduction of the ni station to ni˙^–radical anion by irradiation at 355 nm in the presence of an electron donor (D) 1,4-diazabicyclo[2.2.2]octane (DABCO) makes it a stronger hydrogen-bond-acceptor than the succ unit. This modification in the hydrogen-bonding abilities of the recognition sites provides a driving force for the ring to move to the ni˙^–radical anion along the C-12 alkyl spacer. Charge recombination (CR) between the ni˙^–radical anion and the DABCO˙⁺radical cation generates the neutral MSCC, which relaxes back thermally to the GSCC as the ring returns to the original succ unit. Hence, the light-induced movement of the macrocycle resembles a “power stroke” and the CR-induced return of the ring is similar to the “recovery stroke” of a piston's mechanism. This reversible photo-induced movement of components in the [2]rotaxane 9 generates⁵⁷ a mechanical power of ca. 10^–15 W molecule^–1.

Fig. 9 Photo-induced switching in the [2]rotaxane 9 in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) as an external electron donor –acceptor reagent.

10. A light-driven mechanically interlocked molecular rotor

Leigh and coworkers²⁸ have synthesized the [3]catenane 10 (Fig. 10a) in which two identical hydrogen bond donating benzylic amide macrocycles (blue and purple rings) are mechanically interlocked on to a larger macrocycle containing four hydrogen bond accepting units—namely A, B, C, and D in the sequence of decreasing hydrogen-bonding ability. In the ground state of the catenane 10, the blue and purple rings encircle the two strongest recognition units A and B, respectively, which are also designed to be photoisomerizable at uniquely different wavelengths. The hydrogen bond donating units in the large macrocycle possess gradually decreasing affinity to the smaller interlocked rings—A: a secondary amide fumaramide, B: a tertiary amide fumaramide, C: a succinic amide ester , and D: an isolated amide group which makes fewer hydrogen bonding contacts than the previous three units. A benzophenone unit was attached directly to the strongest binding fumaramide unit A to ensure its selective photoisomerization to the corresponding (Z)-olefin maleamide unit at 350 nm, whereas the tertiary fumaramide unit B can be isomerized separately to its (Z)-form at 254 nm. While the (E)-olefin fumaramide units A and B have strong affinities for the blue and purple rings, the corresponding (Z)-olefins A′ and B′ offer significantly lower affinities, allowing the interlocked rings to move away from themselves upon photoisomerization . The second of the smaller interlocked rings on the larger macrocycle also serves the purpose of a guide to the first ring by occupying the second most preferred recognition unit at any given instance, forcing the first ring to move in one overall direction—a circumrotation which is illustrated in Fig. 10b. The first photoisomerization of the A unit at 350 nm forces the blue ring to move counter-clockwise from the resulting A′ unit to the third strongest hydrogen bond donating C unit because the B unit is already occupied by the purple ring. The second photoisomerization of the B unit to generate B′ moves the purple ring counter-clockwise from the second maleamide unit B′ to the next best hydrogen bond donor D because A′ is now weaker than D, and C is already occupied by the blue ring. Thermal isomerizations (100 °C) of A′ and B′ units back to their respective (E)-olefin fumaramide forms enable the blue ring to move from the C unit to the adjacent B unit counter-clockwise and purple ring from D to the adjacent A unit clockwise—a process which introduces an impediment into the unidirectionality of the rotational motion in the [3]catenane 10 under photochemical/thermal conditions. Furthermore, this sequence of motions leaves the blue ring on the B unit and purple ring on A unit, producing a constitutional isomer of the initial state. A second series of photochemical/thermal process is required to bring the blue ring back to its original A unit and purple ring to its original B unit. In the second series, however, the blue ring moves overall clockwise and the purple ring moves overall counter-clockwise—both are in opposite directions with respect to the first series of motions. This process has been monitored by ¹H NMR spectroscopy in solution only.

Fig. 10 (a) Structural formula of the four-station [3]catenane 10. (b) (1) 350 nm, CH₂Cl₂, 5 min, 67%; (2) 254 nm, CH₂Cl₂, 20 min, 50%; (3) 100 °C, C₂H₂Cl₄, 24 h, ∼100%. (Redrawn with permission from ref. 28. Copyright (2003) Nature Publishing Group.)

11. Light-driven supramolecular machines

Another class of nanoscale machines is based⁵⁵ on a pseudorotaxane (Fig. 11) which is composed of a π-electron-accepting cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) cyclophane encircling a π-electron donating 1,5-bis-[(2-hydroxyethoxy)ethoxy]-naphthalene (BHEEN) thread. Absence of any bulky stopper at the ends of the thread component allows the macrocycle to slip on and off the thread under equilibrium conditions. However, a strong π–π charge transfer (CT) interaction, assisted by [C–H⋯O] and [C–H⋯π] interactions between the BHEEN thread and CBPQT⁴⁺ ring forms a stable BHEEN⊂CBPQT⁴⁺ pseudorotaxane in 80% yield (K_a = 2.53 × 10⁴ M^–1 in MeCN, 298 K)¹³⁵ at equilibrium. Photo-induced electron transfer to the CBPQT⁴⁺ ring from an external photo-active electron donor or electrochemical reduction of the ring component destabilizes the CT interactions, inducing the decomplexation of the pseudorotaxane molecules—a process which can be monitored by measuring the BHEEN-based fluorescence intensity which is higher in the dethreaded form than it is in the pseudorotaxane.

Fig. 11 Light-induced dethreading of a BHEEN⊂CBPQT⁴⁺-based pseudorotaxane using 9-anthracenecarboxylic acid as a photosensitizer .

The first generation of this series of supramolecular machines has been driven⁵⁵ (Fig. 11) photochemically using 9-anthracenecarboxylic (ACA) acid as a photosensitizer (P). It donates an electron to the CBPQT⁴⁺ ring in the presence of the sacrificial reagent TEOA, thus preventing the BET process. In the absence of TEOA, BET is faster than the nuclear motion—that is, the dethreading of the ring from the thread. However, TEOA quenches the photooxidized P⁺ unit allowing enough time for the photoreduced CBPQT²˙^/2+ to slip off the BHEEN thread, thus increasing the BHEEN-based fluorescent intensity in the 320–370 nm region. Based on the change in fluorescent intensity, 5% of the 4.8 × 10^–5 M pseudorotaxane could be dethreaded after 25 min of irradiation in a deoxygenated solution in the presence of 5 × 10^–6 M of P and 0.01 M TEOA. Introduction of air into the system regenerated the CBPQT⁴⁺ ring resulting in a recomplexation, a process which reproduced the original fluorescence spectrum, albeit with lower intensity.

The [2]pseudorotaxane-based supramolecular machines have been powered⁸⁹ by photosensitizers, not only in solution, but also when they have been trapped physically in a rigid nanoporous sol –gel silica framework or attached covalently to silica surfaces. Irradiation of a condensed silica sol –gel matrix containing the BHEEN⊂CBPQT⁴⁺ pseudorotaxane, ACA, and a sacrificial donor , ethylenediaminetetraacetate⁸⁹ (EDTA), with a 365 nm Hg-lamp (100 W) bleaches the original pink color (520 nm) of the sample arising from the CT interactions between the π-electron rich BHEEN thread and the surrounding π-accepting CBPQT⁴⁺ ring and replaces it with a pale blue color for the reduced bipyridinium units. This process is concomitant with the rise in the BHEEN-based fluorescent intensity at 320–370 nm region. Prolonged exposure of the sample to the air in the dark reproduces the original spectra, thus establishing a reversible process. These spectroscopic changes indicate that photo-induced dethreading of the pseudorotaxane occurs also in condensed phases, in which the process is slower by at least an order of magnitude than in the solution phase on account of spatial confinement within the nanopores.

In another device, the BHEEN homolog (BHEEEN) with an extra ethylene glycol chain has been attached⁸⁹ covalently to a silica surface. Dipping of the BHEEEN-functionalized surface into an aqueous solution of the CBPQT⁴⁺ ring forms the corresponding pseudorotaxane, confined onto the surface, as monitored by reduced fluorescence intensity. A second immersion of the pseudorotaxane-coated surface into an ACA and EDTA solution, followed by an irradiation at 365 nm wavelength increased the naphthalene-based fluorescence intensity. Similar experiments also suggest⁸⁹ that the pseudorotaxanes, attached covalently to the silica surfaces, undergo the same photo-induced dethreading in the presence of a photosensitizer and sacrificial donor .

12. Supramolecular machines powered by a light-harvesting molecular triad

A molecular triad 11 (Fig. 12)—composed of three unique electroactive components, namely, (i) an electron-donating tetrathiafulvalene (TTF) unit, (ii) a chromophoric porphyrin (P) unit, and (iii) an electron-accepting C₆₀ unit—has been developed^136,137 to harness light into electrical energy. A disulfide-based anchoring group was tagged to the TTF end of the triad in order to allow its self-assembly onto gold surfaces. When irradiated near the absorption maximum (λ_max) of the chromophore P at 413 nm (40 mW cm^–2 Kr-ion laser), the triad 11 undergoes a PET from the excited *P to the electron-accepting C₆₀ unit, followed by a charge shift to the better electron donating TTF unit to generate the final charge-separated state TTF˙⁺–P–C₆₀˙^–. This charge-separated state is responsible for generating^136,137 photocurrents in a closed electronic circuit on account of a unidirectional electron flow from the working cathode through the photoactive triad , through the electrolyte solution, to the counter electrode, and through the outer electronic circuit where the current is measured (ΔI ∼1 µA cm^–2, Φ_photocurrent ∼ 1%). The triad 11 has been utilized^136,137 as a nanoscale power supply to drive the dethreading of the BHEEN⊂CBPQT⁴⁺ pseudorotaxane in the presence of 413 nm light at an applied potential (V_ap = 0 V) that is much lower than is required for a direct electrochemical reduction (E^1/2 = –300 mV) of the CBPQT⁴⁺ ring. In accordance with the PET mechanism (Fig. 12) at V_ap = 0 V, the charge-separated state of the triad affords a C₆₀˙^– unit on the triad -functionalized Au-working electrode, resulting in an effective terminal potential of –550 mV which is the reduction potential of the C₆₀^0/–˙ unit. This potential is high enough to reduce^55,138 the CBPQT⁴⁺ ring and induce its dethreading from the BHEEN thread—a process which has been monitored by detecting the BHEEN-based fluorescence intensity. Based on the increase in the fluorescence intensity (320–370 nm), 6.7% of the 0.37 mM pseudorotaxane in MeCN could be dethreaded¹³⁷ in the presence of triad -excitation over 2900 s, an estimation which is commensurate with the triad's ability to photoreduce 7% of the CBPQT⁴⁺ ring by generating 1.1 µA cm^–2 photocurrent during that time. In this case, triad molecules are self-assembled onto an Au-working electrode, and so are present at a much lower concentration compared to that of the pseudorotaxanes in the medium. Furthermore, the experimental setup requires a non-air-free system contributing to the lower dethreading percentage compared to the previous system, in which the supramolecular machine, photosensitizer , and the sacrificial fuel are all present in a deoxygenated solution.

Fig. 12 Powering of a supramolecular machine BHEEN⊂CBPQT⁴⁺ with photocurrent generated by the molecular triad 11. (Redrawn with permission from ref. 137. Copyright (2005) Wiley-VCH.)

13. Light-driven unidirectional rotary motor which rotates microscale objects

Feringa and his coworkers^{24,27,31,35–37,139–141} have developed molecular rotary motors based on the sterically crowded, intrinsically helical-shaped molecule 12 (Fig. 13) which has a photoisomerizable elongated C=C bond acting as an axle, and a stereogenic center bearing a tert-butyl substituent which adopts a pseudoaxial orientation on account of steric hindrance . The photoinduced rotary motion of such a helically chiral molecule resembles^142,143 that of the ATP-synthase. The upper half of the molecule acts¹⁴⁴ essentially as a propeller which rotates unidirectionally with respect to the bottom stator part as a result of the isomerization of the C=C axle. Upon irradiation of 12 with 366 nm light, energetically uphill cis/trans isomerization of the C=C bond occurs with inversion of helicity and conformational change in the cyclopentane rings directly attached to the C=C bond, forcing the tert-butyl group to assume the less favored pseudoequatorial orientation. In order to regain the more favored pseudoaxial orientation, the naphthalene upper part now slips past the lower part of the molecule as a consequence of a thermal isomerization, resulting in helix inversion . A significant amount of energy is required for the thermal helical inversion to maintain a continuous unidirectional rotary motion. The unidirectional rotation of the top part of the molecule with respect to the bottom part was monitored by detecting the reversible chemical shift of the CH at the stereogenic center, which shifts up-field from 4.35 ppm (pseudoequatorial) to 4.09 ppm (pseudoaxial) upon irradiation of the stable isomer (P)-(R)-trans-12 isomer as it is converted to the unstable (M)-(R)-cis-12 isomer. Heating of the sample causes conformational changes in the five-membered ring to generate (P)-(R)-cis-12 isomer, changes which are reflected in the chemical shift of the OMe-singlet signal. Similarly, the stable cis-isomer can be converted into the original trans-isomer (P)-(R)-trans-12 by irradiating with 366 nm light, followed by a thermal flipping of the cyclopentane ring.

Fig. 13 Unidirectional rotation of the helical molecular rotor 12 under photochemical and thermal conditions. (Redrawn with permission from ref. 141. Copyright (2006) American Chemical Society.)

Doping¹⁴⁴ (1 wt.%) of a similar right-handed helical alkene 13 (Fig. 14) containing a phenyl group at the stereogenic center into a liquid crystal (LC) film renders the film a polygonal texture—a characteristic of a cholesteric LC having a helix axis parallel to the surface. Irradiation of the doped LC film with 365 nm light causes a photochemical isomerization of the C=C bond to occur, resulting in an inversion of helicity from the initial right-handed to the subsequent left-handed. A second inversion activated by a thermal process, followed by a second photochemical-thermal cycle, completes the full 360° rotation. Therefore, in the presence of the light the LC texture rotates clockwise whereas in the dark it is counterclockwise—a process which comes to a halt after about 10 min. The controlled texture reorganization of a molecular motor-doped LC film has been utilized¹⁴⁴ to rotate a microscopic (5 × 28 µm) glass rod placed on the surface at an average speed of 0.67 r.p.m. clockwise photochemically and 0.22 r.p.m. counterclockwise thermally. Thus, the collective unidirectional rotary motion of an ensemble of molecular motors can be harnessed to produce mechanical movement of microscopic, and perhaps, macroscopic objects.

Fig. 14 (a) Structural formula of the molecular rotor 13. (b) Polygonal texture of the liquid crystal (LC) doped with 1% of the compound 13. (c) Clockwise rotations (28°, 141°, 226°) of the glass rod (5 × 28 µm) placed on the 13-doped LC film. (d) Atomic force microscopy image (15 µm²) of the LC film. (Redrawn from ref. 144. Copyright (2006) Nature Publishing Group.)

Another photoisomerizable molecular rotor 14 (Fig. 15) has been developed¹⁴⁵ which also has a pseudoaxial methyl group at the stereogenic center. Compound (2′R)-(M)-14 can be attached to Au-surfaces via two alkylthiol pendants attached to the stator part of the molecule. The surface mounting allows the stator part to be fixed onto a stationary substrate and the propeller part can move freely on account of photochemical isomerization and thermal helix inversion to complete unidirectional 360° rotation. This process has been monitored by circular dichroism and ¹H NMR spectroscopy.

Fig. 15 (a) Stepwise unidirectional rotation of the Au-nanoparticles functionalized with the corresponding dithiol 14 under alternating photochemical and thermal conditions. (b) Cyclic change in the CD intensity at 290 and 320 nm at each photochemical (λ > 280 nm and λ > 365 nm) and thermal (353 K) step indicating unidirectional rotation of 14. (Redrawn with permission from ref. 145. Copyright (2005) Nature Publishing Group.)

14. Conclusions and outlook

In this Chapter, we have described how different noncovalent bonding interactions and controllable molecular recognition events within the mechanically interlocked bistable molecules can be executed by light as a “clean” external input. Light induces either electronic reorganizations or conformational changes in the recognition sites—a process which, in turn, alters the binding affinities of the sites with the surrounding macrocycle—triggering its relative mechanical movement within the molecule. Some of these light-driven molecular switches have been incorporated into device environments in which they switch in a manner similar to the way they do in the solution phase. When self-assembled onto surfaces, some of these bistable molecules can switch surface polarity in the presence or absence of light, a property which could be exploited in the development of microfluidic devices. Covalent attachment of bistable molecules onto the mesoporous silica surface and their light-driven mechanical switching properties have already been utilized to fabricate molecular nanovalves,¹⁴⁶ in which the bistable rotaxane molecules act as gatekeepers. This idea could be extended to the use of these compounds for controlled release of reagents in vitro reactions or for delivery of drugs in vivo conditions. Evolution of such switchable molecules through structure–function feedback loops will help the emergence of functional molecular nanotechnology. In addition to the mechanically interlocked photo-driven molecular switches, we have discussed briefly one of the most intriguing examples of the molecular motors which exploit photoisomerization for their unidirectional rotary motion, a process that mimics the rotational motion of the ATP-synthase. When embedded into a LC film such molecular rotors can even move microscopic objects by controlling the polygonal texture of the surface.

Acknowledgements

We thank all researchers—including our collaborators, colleagues, and co-workers, especially Professors Vincenzo Balzani and Alberto Credi—whose work has been presented in this Chapter for their creative contributions to the field of artificial molecular machines and devices. Our own research in this area has been supported by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), and the Office of Naval Research (ONR).

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