From the Synthesis of Nanovehicles to Participation in the First Nanocar Race-View from the French Team

From the Synthesis of Nanovehicles to Participation in the First Nanocar Race-View from the French Team Henri-Pierre Jacquot de Rouville, Claire Kammerer, Gwénaël Rapenne

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Henri-Pierre Jacquot de Rouville, Claire Kammerer, Gwénaël Rapenne. From the Synthesis of Nanove- hicles to Participation in the First Nanocar Race-View from the French Team. Molecules, MDPI, 2018, 23 (3), pp.612. 10.3390/molecules23030612. hal-01850648

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Review From the Synthesis of Nanovehicles to Participation in the First Nanocar Race—View from the French Team

Henri-Pierre Jacquot de Rouville 1, Claire Kammerer 1 ID and Gwénaël Rapenne 1,2,* ID

1 CEMES, Université de Toulouse, CNRS, 31055 Toulouse, France; [email protected] (H.-P.J.d.R.); [email protected] (C.K.) 2 Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara, Japan * Correspondence: [email protected]; Tel.: +33-562-257-841

Received: 29 January 2018; Accepted: 28 February 2018; Published: 8 March 2018

Abstract: This review article presents our accomplished work on the synthesis of molecular triptycene wheels and their introduction into nanovehicles such as wheelbarrows and nanocars, equipped with two and four wheels, respectively. The architecture of nanovehicles is based on polycyclic aromatic hydrocarbons, which provide a potential cargo zone. Our strategy allowed us to obtain planar or curved nanocars, exhibiting different mobilities on metallic surfaces. Our curved nanocar participated in the ﬁrst nanocar race organized in Toulouse (France) on 28 and 29 April 2017.

Keywords: nanowheel; wheelbarrow; nanocar; polyaromatic hydrocarbon; triptycene; subphthalocyanine; single molecule; STM; nanocar race

1. Introduction In 2013, in a perspective article on “molecule concept nanocars” [1], Christian Joachim and Gwénaël Rapenne proposed “a yearly competition whereby all nanocar designers would meet together for a few days, similar to the microrobotics contests held around the world”. It took almost four years to find a way to organize such a race, find sponsors, leave the time necessary for competitors to design and synthesize their prototypes, and develop a four-tip scanning tunneling microscope (STM) suitable to host the race. In 2017, six competitors from Austria, Germany, Japan, Switzerland, the United States, and France finally met in Toulouse (France) to compare their designs and strategies to speed up the movement of a single nanocar on a metallic surface. On the 28 April 2017, the starting signal was sounded at exactly 11 a.m. by Dr. Jean-Pierre Launay, emeritus Professor at the University Paul Sabatier (Toulouse) and race commissioner. The Nanocar Race [2] was seen as a major communication event by the general public. In this review, we will describe why it was a highly challenging scientific event, which was selected to be a part of “Research of the Year 2017” by Chemical Engineering News [3], the scientific journal of the American Chemical Society, as well as the leading French newspaper Le Monde [4]. The last 15 years of research in our group will be detailed, from the design and synthesis of the first molecular wheelbarrow (a platform with two wheels), to the different prototypes of nanocars developed for the nanocompetition. Nanovehicles are molecular machines able to move efficiently and with some kind of control and directionality at the nanoscale. The first prototypes were technomimetic [5], with a chassis connected to wheels. As pioneers in the field, we proposed using triptycenes [6] to act as rolling elements [7] and synthesized the first nanovehicle with two wheels—a wheelbarrow [8]. Two years after us, Tour’s group reported a family of nanovehicles consisting of a molecular-scale chassis with four [60]-fullerene wheels [9]. Various nanocars were subsequently synthesized by the same group using carboranes [10],

Molecules 2018, 23, 612; doi:10.3390/molecules23030612 www.mdpi.com/journal/molecules Molecules 2018, 23, 612 2 of 11 ruthenium organometallic complexes [11], and adamantanes [12] as alternative wheels. During that time, we also proposed another wheel candidate, subphthalocyanin [13], which gave very good STM images with direct information about the rolling motion on a metallic surface [14], but unfortunately too unstable to be incorporated in nanovehicles. In the technomimetic family, Feringa et al. proposed a nanovehicle with four motors as wheels [15] and Masson and Hla designed and synthesized the largest car of about 3.5 nm in length. Officially called the Bobcat Nano-Wagon, it had a pseudorotaxane H-shaped frame with four cucurbituril molecules as wheels [16]. It must be noted that in the last example, there was no covalent bond linking the wheels and the axle, but instead a supramolecular charge-transfer interaction. Like physics, which follows different laws at the nanoscale (quantum physics) as compared to the macroscopic world (Newtonian physics), the design of nanovehicles will not follow the same rules developed for vehicles that surround us in our everyday life. In this way, a new kind of nanovehicle design evolved at the nanoscale. Some nanocars were developed without wheels or motors, and were able to transform an electronic excitation into a controlled movement. Indeed, the teams from Germany, Japan, and Switzerland competed during the Nanocar Race with such nanovehicles. The Japanese team synthesized a molecule similar to a butterfly, consisting of a bisbinaphthyldurene, with two binaphthyl groups (the wings) connected to a central durene spacer [17]. The German team and the Swiss team selected commercially available molecules: 4-acetylbiphenyl [18] and 40-(p-tolyl)-2,20:60,200-terpyridine [19], respectively. It must be noted that the German nanocar (“the Nano-Windmill”) was composed of four 4-acetylbiphenyl units self-assembled on the surface and stabilized by hydrogen bonds. We present here our contribution to the field, from the synthesis of a molecular wheelbarrow, the first direct proof of the rolling motion of the wheels and their introduction into four-wheel nanovehicles, to our participation in the first Nanocar Race.

2. The First Two-Wheeled and Two-Legged Vehicle: The Molecular Wheelbarrow The Toulouse team published the design for the ﬁrst molecule-vehicle in 2002 [7], and it was synthesized in 2003. It was equipped with a chassis, two wheels at the front, two legs, and two handles at the rear, structurally similar to a wheelbarrow [8]. In the case of a macroscopic wheelbarrow, pushing results in the rotation of the wheels. The purpose of this ﬁrst vehicle was thus to observe the rotational movement of the two front wheels while pushing on the handles and keeping the rear part of its chassis far from the surface, thanks to the two rear molecular legs.

2.1. Design Wheelbarrow 1 consists of two legs (3,5-di-tert-butylphenyl groups, shown in green in Figure1) and two three-cogged wheels (triptycene groups, shown in red in Figure1), which can freely rotate around the axle thanks to the presence of ethynyl spacers [20]. As shown in Figure1, the skeleton consists of polycyclic aromatic hydrocarbons (PAHs), which due to their rigidity make manipulation by the STM tip easy. As anticipated, the two 3,5-di-tert-butylphenyl legs were shown to be held in a conformation in which the phenyl groups are nearly perpendicular to the main aromatic board. This was exploited to lift the chassis, and thus to minimize its interaction with the surface. Moreover, tert-butyl groups connected to PAHs are used to increase organic solubility and are easily observed by STM techniques, inducing good contrast in the image. The two 4-tert-butylphenyl groups (in blue) play the role of handles for subsequent manipulation using the tip of the microscope.

2.2. Synthesis The synthesis of the molecular wheelbarrow 1 was achieved in twelve steps, with an overall yield of 2% [21]. As shown in Scheme1, our strategy used the repetition of a “double Knœvenagel condensation—Diels–Alder reaction” sequence on an α-diketo fragment. The ﬁrst sequence allowed the connection of the two 3,5-di-tert-butylphenyl legs (Step a) and the handles (Step b), while the second one provided the precursor for the connection of the wheels (Steps d and e). Thus, 1,3-di(4- Molecules 2018, 23, 612 3 of 11

Molecules 2018, 23, x FOR PEER REVIEW 3 of 11 Molecules 2018, 23, x FOR PEER REVIEW 3 of 11 iodophenyl)propan-2-oneiodophenyl)propan-2-one was was selected selected as partneras partner in this in lastthis double last double Knoevenagel Knoevenagel reaction reaction to introduce to iodophenyl)propan-2-one was selected as partner in this last double Knoevenagel reaction to theintroduce iodine centers the iodine necessary centers for necessary the ﬁnal for double the final Sonogashira double Sonogashira coupling, coupling, which yielded which theyielded molecular the introduce the iodine centers necessary for the final double Sonogashira coupling, which yielded the wheelbarrowmolecular wheelbarrow (Step f). (Step f). molecular wheelbarrow (Step f).

Figure 1. Macroscopic wheelbarrow (left), chemical structure of the molecular analogue 1 (center), FigureFigure 1. 1.Macroscopic Macroscopic wheelbarrow wheelbarrow ((leftleft), chemical structure structure of of the the molecular molecular analogue analogue 1 (1center(center), ), and space-filling model showing its minimum energy conformation (right). andand space-ﬁlling space-filling model model showing showing its its minimum minimum energyenergy conformation ( (rightright).).

Scheme 1. Synthesis of molecular wheelbarrow 1: reagents and conditions: (a) KOH, EtOH, 20 h, Ar, SchemeScheme 1. 1.Synthesis Synthesis of of molecular molecular wheelbarrowwheelbarrow 1: reagents reagents and and conditions: conditions: (a ()a KOH,) KOH, EtOH, EtOH, 20 20h, Ar, h, Ar, 20 °C; (b) di(4-tert-butylphenyl)acetylene, diphenylether, 16 h, Ar, reflux; (c) (C6H5SeO)2O, chlorobenzene, 2020◦C; °C; (b ()b di(4-) di(4-terttert-butylphenyl)acetylene,-butylphenyl)acetylene, diphenylether,diphenylether, 16 h, h, Ar, Ar, reflux; reflux; ( (cc) )(C (C6HH5SeO)SeO)2O, O,chlorobenzene, chlorobenzene, 62 h, Ar, reflux; (d) 1,3-bis(4-iodophenyl)propan-2-one, KOH, EtOH, Ar,6 reflux;5 2 (e) di(4-tolyl) 6262 h, Ar,h, Ar, reflux; reflux; (d) 1,3-bis(4-iodophenyl)propan-2-one,(d) 1,3-bis(4-iodophenyl)propan-2-one, KOH, KOH, EtOH, EtOH, Ar, reflux; Ar, reflux; (e) di(4-tolyl)acetylene, (e) di(4-tolyl) acetylene, diphenylether, 16 h, Ar, reflux; (f) 9-ethynyltriptycene, Pd(PPh3)4 10 mol %, CuI 20 mol %, acetylene, diphenylether, 16 h, Ar, reflux; (f) 9-ethynyltriptycene, Pd(PPh3)4 10 mol %, CuI 20 mol %, diphenylether,piperidine-THF 16 h,(1:1), Ar, reflux;24 h, Ar, (f) 20 9-ethynyltriptycene, °C. Pd(PPh3)4 10 mol %, CuI 20 mol %, piperidine-THF (1:1),piperidine-THF 24 h, Ar, 20 ◦ C.(1:1), 24 h, Ar, 20 °C. The starting cyclopentadienone 4 was obtained first via a double Knœvenagel reaction of 1,3- The starting cyclopentadienone 4 was obtained first via a double Knœvenagel reaction of 1,3- bis(3,5-di-tert-butylphenyl)propan-2-one 2 with diketopyracene 3 (Step a). The Diels–Alder reaction bis(3,5-di-The startingtert-butylphenyl)propan-2-one cyclopentadienone 4 was 2 with obtained diketopyracene ﬁrst via a 3 double (Step a). Knœvenagel The Diels–Alder reaction reaction of 1,3- of cyclopentadienone 4 with di-(4-tert-butylphenyl)acetylene (Step b) provided the ethane-bridged bis(3,5-di-of cyclopentadienonetert-butylphenyl)propan-2-one 4 with di-(4-tert-butylphenyl)acetylene2 with diketopyracene (Step3 (Step b) provided a). The the Diels–Alder ethane-bridged reaction precursor 5 in 97% yield. The 1H-NMR spectrum clearly showed the expected 2:1 ratio between the ofprecursor cyclopentadienone 5 in 97% yield.4 with The di-(4- 1H-NMRtert-butylphenyl)acetylene spectrum clearly showed (Step the b)expected provided 2:1 theratio ethane-bridged between the different types of tert-butyl groups,1 belonging to the legs and to the handles, respectively. Oxidation precursordifferent5 typesin 97% of yield.tert-butyl The groups,H-NMR belonging spectrum to the clearly legs and showed to the the handles, expected respectively. 2:1 ratio betweenOxidation the of the ethane bridge of the pyracene fragment with benzeneseleninic anhydride (Step c) yielded the differentof the ethane types ofbridgetert-butyl of the groups,pyracene belonging fragment towi theth benzeneseleninic legs and to the handles,anhydride respectively. (Step c) yielded Oxidation the α-diketo fragment 6 necessary for the connection of the second axle. This is the key step in our ofα the-diketo ethane fragment bridge of6 necessary the pyracene for the fragment connection with of benzeneseleninic the second axle. anhydride This is the (Step key c)step yielded in our the strategy. Aryl halides are introduced at this stage for subsequent cross-couplings, in order to connect α-diketostrategy. fragment Aryl halides6 necessary are introduced for the at connectionthis stage for of subsequent the second cross-couplings, axle. This is in the order key to step connect in our the triptycene wheels. The double Knoevenagel condensation with 1,3-di(4-iodophenyl)propan-2- strategy.the triptycene Aryl halides wheels. are The introduced double Knoevenagel at this stage forcondensation subsequent with cross-couplings, 1,3-di(4-iodophenyl)propan-2- in order to connect one (Step d) followed by another Diels–Alder reaction with di(4-tolyl)acetylene provided the diiodo theone triptycene (Step d) wheels.followed The by another double KnoevenagelDiels–Alder reaction condensation with di(4-tolyl)acetylene with 1,3-di(4-iodophenyl)propan-2-one provided the diiodo intermediate 8 in 30% yield (Step e). The two wheels were then simultaneously covalently attached (Stepintermediate d) followed 8 in by30% another yield (Step Diels–Alder e). The two reaction wheels withwere di(4-tolyl)acetylenethen simultaneously covalently provided theattached diiodo

Molecules 2018, 23, 612 4 of 11

Molecules 2018, 23, x FOR PEER REVIEW 4 of 11 intermediate 8 in 30% yield (Step e). The two wheels were then simultaneously covalently attached to theto the axle axle via avia double a double coupling coupling of 9-ethynyltriptycene of 9-ethynyltriptycene under classicalunder classical Sonogashira Sonogashira conditions conditions (Step f), affording(Step f), affording the molecular the molecular wheelbarrow wheelbarrow1 in 55% 1 yield. in 55% yield.

2.3. Single Molecule ScanningScanning Tunneling MicroscopyMicroscopy ImagesImages andand ManipulationManipulation Such a molecular-scalemolecular-scale machine inevitably displays a high level of complexity due to the incorporation ofof severalseveral functionalities functionalities within within the th samee same molecule. molecule. This This complexity complexity was was reflected reflected in the in relativelythe relatively large large molecular molecular weight weig of theht wheelbarrow of the wheelbarrow (1802 g·mol (1802−1), whichg·mol− can1), which often leadcan tooften problems lead into theproblems deposition in the due deposition to the possibility due to the of thermalpossibility fragmentations of thermal fragmentations during the sublimation during the step sublimation [22]. In the presentstep [22]. case, In the the present required case, linear the wheel required axle linear included wheel two axle thermally-sensitive included two thermally-sensitive triple bonds. However, triple abonds. few intact However, molecular a few wheelbarrows intact molecular were successfully wheelbarrows imaged were on a Cu(100)successfully surface, imaged as shown on ina FigureCu(100)2. Itsurface, leads to as a shown rather in complicated Figure 2. It STM leads image, to a rath dominateder complicated by three STM intense image, maxima dominated separated by three by variousintense weakermaxima patterns, separated as by expected various for weaker such apatterns, complex as chemical expected structure. for such a The complex characteristic chemical dimensions, structure. theThe equivalency characteristic of differentdimensions, molecules the equivalency in the STM images,of different and comparisonmolecules in with the images STM calculatedimages, and by electroncomparison scattering with quantumimages calculated chemistry by (ESQC) electron method scattering [23] indicated quantum that chemistry intact molecular (ESQC) wheelbarrows method [23] hadindicated been imaged.that intact Manipulation molecular wheelbarrows with the STM had tip atbeen 5 K imaged. resulted Manipulation in a conformational with the change STM intip the at molecule,5 K resulted which in unambiguouslya conformational confirmed change thein the molecularity molecule, of which the observed unambiguously object: the confirmed spots obtained the weremolecularity not an assembly of the observed of fragments. object: the spots obtained were not an assembly of fragments.

(a) (b)(c)

Figure 2. Experimental scanning tunnelingtunneling microscopemicroscope (STM)(STM) imageimage (3(3× × 4 nm2,, V V = = 0.5 0.5 V, V, I = 0.3 0.3 nA) nA) on Cu(100) at 5 KK ((aa)) andand thethe molecularmolecular conformationconformation ((cc)) correspondingcorresponding toto thethe electronelectron scatteringscattering quantum chemistry (ESQC)(ESQC) calculatedcalculated imageimage ((bb).).

Unfortunately, laterallateral motionmotion ofof thethe wheelbarrowwheelbarrow couldcould notnot bebe achieved.achieved. This could be explained by the the fact fact that that two two wheels wheels may may not notbe enough be enough to allow to allow the motion the motion on the surface on the of surface such a of large such molecule, a large molecule,due to strong due coupling. to strong Increasing coupling. the Increasingnumber of wheels the number was proposed of wheels to enhance was proposed the molecular to enhance mobility the molecularof nanovehicles, mobility and of as nanovehicles,a result the design and asof nanovehicles a result the design with four of nanovehicleswheels was envisioned. with four wheels was envisioned. 3. The First Rotation of Wheels on a Surface 3. TheWheels First Rotationare of course of Wheels one of on the a Surface key mechanical elements of a vehicle, even on a molecular chassis.Wheels At the are macroscopic of course one scale, of the the key molecula mechanicalr wheels elements move ofthe a frame vehicle, slightly even onaway a molecular from the chassis.supporting At surface, the macroscopic in order to scale, lower the the molecularlateral diffusion wheels barrier move of the the frame molecule-vehicle slightly away by fromreducing the supportingthe frame–surface surface, electronic in order tointeractions. lower the lateralSuch decoupling diffusion barrier might ofbe theachieved molecule-vehicle using simple by molecular reducing thelegs, frame–surface although the electronicmovement interactions. of the molecule Such on decoupling the surface might would be then achieved cost more using energy, simple as molecular it would legs,require although a tilting the movement movement of of each the moleculemolecular on leg. the As surface in the would macroscopic then cost world, more rotation energy, asaround it would an requireaxis is thus a tilting more movement energetically of eachfavorable. molecular leg. As in the macroscopic world, rotation around an axis isIt thuswas moreshown energetically that the mechanical favorable. motion of a single molecule on a surface can be triggered by the tipIt wasapex shown of the STM that the[24]. mechanical Pushing a molecule motion of wi ath single an STM molecule tip was on a a convenient surface can way be triggeredto probe the by thetriptycene tip apex fragment of the STM to act [24 ].as Pushing a wheel a at molecule the nano withscale. an For STM that tip purpose, was a convenient a prototype way composed to probe the of triptycenetwo wheels fragment linked to to an act axle as awas wheel designed. at the nanoscale. For that purpose, a prototype composed of two wheels linked to an axle was designed.

Molecules 2018, 23, 612 5 of 11 MoleculesMolecules 2018 2018, ,23 23, ,x x FOR FOR PEER PEER REVIEW REVIEW 55 ofof 1111

3.1.3.1. SynthesisSynthesis andand DepositionDeposition ofof a a Bis(ethynyltriptycene) Bis(ethynyltriptycene) as as Prototype Prototype of of a a Wheel Wheel Dimer Dimer Copper(II)-mediatedCopper(II)-mediated Glaser Glaser homocouplinghomocoupling ofof 9-et9-et 9-ethynyltriptycenehynyltriptycenehynyltriptycene fragments—thefragments—the wheelswheels alreadyalready usedused in in the the molecular molecular wheelbarrow—gave wheelbarrow—gave thethe correspcorresp correspondingondingonding dimerdimer ofof wheels wheels linked linked via via a a butadiyne butadiyne spacer.spacer. TheTheThe latter latterlatter was waswas a good aa goodgood candidate, candidate,candidate, since itsincesince allowed itit allowedallowed almost free almostalmost rotation freefree of therotationrotation attached ofof fragmentsthethe attachedattached [20] fragmentsinfragments combination [20] [20] in in with combinationcombination a linear geometry. withwith a a linearlinear The ethynyltriptycenegeom geometry.etry. The The ethynyltriptyceneethynyltriptycene dimer has subsequently dimerdimer hashas been subsequentlysubsequently studied as a beenprototypebeen studiedstudied wheel asas aa dimer. prototypeprototype wheelwheel dimer.dimer. ItsIts deposition deposition on on a a Cu(110) Cu(110)Cu(110) surface surface was was achieved achieved very very cleanly cleanly by by sublimation sublimationsublimation under under high high vacuumvacuum [25].[25]. [25]. Figure Figure Figure 3 33 showsshows shows thethe the imageimage image obtained;obtained; obtained; eacheach each brightbright bright lobelobe lobe corresponds corresponds corresponds toto oneone to triptycene onetriptycene triptycene unit,unit, andunit,and thethe and observedobserved the observed pairpair ofof pair lobeslobes of lobes constitutesconstitutes constitutes thethe imageimage the image ofof aa dimer,dimer, of a dimer, asas confirmedconfirmed as conﬁrmed byby calculations.calculations. by calculations. NoNo fragmentationNofragmentation fragmentation oror decompositiondecomposition or decomposition waswas was observed.observed. observed.

((aa)) ((bb)) ((cc))

2 FigureFigure 3. 3. ((aa)) Experimental Experimental STMSTM imageimage ofof a a wheel wheel dimer dimer on onon a aa Cu(110) Cu(110)Cu(110) surface surfacesurface (30 (30(30 ×× 303030 ÅÅ Å2,2, ,VV V == = 11 1 V,V, V, II = = 0.20.20.2 nA) nA)nA) and andand (c )((c thec)) thethe molecular molecularmolecular conﬁguration configurationconfiguration corresponding correspondingcorresponding to (b) to theto (( calculatedbb)) thethe calculatedcalculated STM image. STMSTM Adapted image.image. AdaptedfromAdapted [22 ], fromfrom copyright [22],[22], copyrightcopyright 2006, with 2006,2006, permission withwith permissionpermission from Elsevier. fromfrom Elsevier.Elsevier.

3.2. Unidirectional Rotation of the Wheel 3.2.3.2. Unidirectional Unidirectional Rotation Rotation of of the the Wheel Wheel For the first time, it was possible to observe the rotation of a wheel on a surface, by working at ForFor the the first ﬁrst time, time, it it was was possible possible to to observe observe the the ro rotationtation of of a a wheel wheel on on a a surface, surface, by by working working at at 5 K under ultra-high vacuum. This was achieved by inducing the translation of the molecule upon 55 K K under under ultra-high ultra-high vacuum. vacuum. This This was was achieved achieved by by inducing inducing the the translation translation of of the the molecule molecule upon upon pushing it with the STM tip [25]. In order to understand the motion of this molecule in detail, it was pushingpushing it it with with the the STM STM tip tip [25]. [25]. In In order order to to understand understand the the motion motion of of this this molecule molecule in in detail, detail, it it was was particularly important to study the manipulation signal (i.e., the tunneling current recorded at particularlyparticularly importantimportant toto study study the the manipulation manipulation signal signal (i.e., the(i.e., tunneling the tunneling current current recorded recorded at constant at constant tip height during the motion), because the STM images before and after manipulation did constanttip height tip during height the during motion), the motion), because thebecause STM the images STM beforeimages and before after and manipulation after manipulation did not givedid not give information about the type of movement between measurements (translational or rotational). notinformation give information about the about type the of movementtype of movement between betw measurementseen measurements (translational (translational or rotational). or rotational). The tip was moved across the molecular axle at a constant height, while the tunneling current TheThe tip tip was was moved moved across across the the molecular molecular axle axle at at a a constant constant height, height, while while the the tunneling tunneling current current was recorded. In most cases, it was possible to rotate one wheel, and in a few cases, the rolling of both waswas recorded. recorded. In In most most cases, cases, it it was was possible possible to to rotate rotate one one wheel, wheel, and and in in a a few few cases, cases, the the rolling rolling of of both both wheels was observed (as shown in Figure 4). This behavior seems to depend on the precise shape of wheelswheels was was observed observed (as (as shown shown in in Figure Figure 4).4). This This be behaviorhavior seems seems to to depend depend on on the the precise precise shape shape of the tip apex, which in some cases allowed us to address both wheels during the manipulation. This theof thetip tipapex, apex, which which in some in some cases cases allowed allowed us to us ad todress address both bothwheels wheels during during the manipulation. the manipulation. This triptycene dimer was the first molecule designed to yield a controlled rotation upon pushing the triptyceneThis triptycene dimer dimer was wasthe first the ﬁrstmolecule molecule designed designed to yield to yield a controlled a controlled rotation rotation upon upon pushing pushing the the wheels using the STM tip. With this rolling motion proven, four triptycene units were then integrated wheelswheels using using the the STM STM tip. tip. With With this this rolling rolling motion motion pr proven,oven, four four triptycene triptycene unit unitss were were then then integrated integrated into the next generation of nanovehicles, with the aim of increasing mobility on the surface. intointo the the next next generation generation of of nanovehicles, nanovehicles, with with the the aim aim of of increasing increasing mobility mobility on on the surface.

FigureFigure 4.4. RotationalRotational motionmotion ofof triptycenetriptycene wheels.wheels. StepStep 1:1: thethe STMSTM tiptip approachesapproaches thethe molecule;molecule; Figure 4. Rotational motion of triptycene wheels. Step 1: the STM tip approaches the molecule; Step 2: StepStep 2:2: thethe rotationrotation ofof thethe wheelwheel starts;starts; StepStep 3:3: ththee STMSTM tiptip lieslies onon thethe otherother sideside ofof thethe wheelwheel afterafter aa the rotation of the wheel starts; Step 3: the STM tip lies on the other side of the wheel after a 120◦ 120° rotation. Adapted from [25], copyright 2007, with permission from Springer Nature. 120°rotation. rotation. Adapted Adapted from from [25], [25], copyright copyright 2007, 2007, with with permission permission from from Springer Springer Nature. Nature.

4.4. AA PlanarPlanar NanocarNanocar ForFor thethe preparationpreparation ofof nanovehiclesnanovehicles containingcontaining fourfour wheelswheels (i.e.,(i.e., nanocars),nanocars), itit waswas necessarynecessary toto developdevelop the the synthesis synthesis of of a a larger larger chassis chassis based based on on a a perylene perylene unit. unit. This This was was to to sterically sterically accommodate accommodate

Molecules 2018, 23, 612 6 of 11

4. A Planar Nanocar For the preparation of nanovehicles containing four wheels (i.e., nanocars), it was necessary to developMolecules the 2018 synthesis, 23, x FOR of PEER a larger REVIEW chassis based on a perylene unit. This was to sterically accommodate6 of 11 all the wheels on the same molecular structure [26], as the chassis used for molecular wheelbarrow 1 wasall too the short.wheels on Therefore, the same asmolecular shown structure in Scheme [26],2, as our the strategy chassis used started for molecular with the wheelbarrow synthesis of 1 di(4- iodophenyl)cyclopentadienonewas too short. Therefore, as11 shownvia a doublein Scheme Knœvenagel 2, our strategy reaction started between with acenaphthenequinone the synthesis of 9 anddi(4-iodophenyl)cyclopentadienone 1,3-di(4-iodophenyl)propan-2-one 1110 undervia basica double conditions Knœvenagel (Step a). Diels–Alderreaction reactionbetween with acenaphthenequinone 9 and 1,3-di(4-iodophenyl)propan-2-one 10 under basic conditions (Step a). 1,2-di(3,5-di-tert-butylphenyl)ethyne gave—after aromatization—the half-chassis 12 along with evolution Diels–Alder reaction with 1,2-di(3,5-di-tert-butylphenyl)ethyne gave—after aromatization—the half- of carbon monoxide (Step b). The half-chassis 12 was subsequently dimerized by using a Scholl-type chassis 12 along with evolution of carbon monoxide (Step b). The half-chassis 12 was subsequently oxidative coupling with FeCl as oxidant (Step c). In the last step, the resulting planar chassis 13 was dimerized by using a Scholl-type3 oxidative coupling with FeCl3 as oxidant (Step c). In the last step, functionalizedthe resulting with planar four ethynyltriptycenylchassis 13 was functionalized wheels via Sonogashirawith four ethynyltriptycenyl cross-couplings towheels yield ourvia first prototypeSonogashira nanocar cross-couplings14 in 9% overall to yield yield. our first prototype nanocar 14 in 9% overall yield.

SchemeScheme 2. Synthesis 2. Synthesis of of the the planar planar nanocar 1414. (.(a) KOH,a) KOH, MeOH, MeOH, 2 h, Ar, 2 h, room Ar, temperature room temperature (RT); (b) (RT); 1,2-bis(3,5-di-tert-butylphenyl)ethyne, diphenylether, 45 min, 250 °C, microwave◦ (300 W); (c) FeCl3 25 (b) 1,2-bis(3,5-di-tert-butylphenyl)ethyne, diphenylether, 45 min, 250 C, microwave (300 W); (c) FeCl3 eq, CH2Cl2–CH3NO2 (9:1), 15 min, Ar, RT; (d) 9-ethynyltriptycene 10 eq, Pd(PPh3)4 10 mol%, CuI 20 25 eq, CH Cl –CH NO (9:1), 15 min, Ar, RT; (d) 9-ethynyltriptycene 10 eq, Pd(PPh ) 10 mol%, mol%,2 piperidine-THF2 3 2 (1:1), 6 h, Ar, 80 °C. 3 4 CuI 20 mol%, piperidine-THF (1:1), 6 h, Ar, 80 ◦C. This nanovehicle was deposited on Au(111). However, due to its planarity and the resulting high Thisinteraction nanovehicle of the polyaromatic was deposited chassis on with Au(111). the surface, However, it was not due possible to its to planarity move the andvehicle. the Thus, resulting highwe interaction envisioned of thethe polyaromaticdesign of an alternative chassis with molecule-car the surface, with it wasa modified not possible chassis, to which move would the vehicle. Thus,undergo we envisioned less surface the coupling. design of an alternative molecule-car with a modiﬁed chassis, which would undergo less surface coupling. 5. A Curved Nanocar 5. A CurvedTo decrease Nanocar the interaction of the polyaromatic chassis with the surface, a highly curved chassis was prepared. The synthetic strategy remained identical, but the 3,5-di-tert-butylphenyl fragments Topositioned decrease at theboth interaction ends were substituted of the polyaromatic with 4-tert-butylphenyl chassis with groups the surface, [26]. In a this highly case, curved planarity chassis was prepared.of contiguous The phenyl synthetic rings strategywas more remained easily achieved identical, and dimerizatio but the 3,5-di-n of thetert half-chassis-butylphenyl 16 under fragments positionedScholl atoxidative both ends conditions were substituted led to an overcyclized with 4-tert -butylphenylperylene platform groups bearing [26]. six In thisadditional case, planarityC-C of contiguousbonds, as shown phenyl in ringsblue (highlighted was more easilyin bold) achieved in Scheme and 3. dimerization of the half-chassis 16 under Scholl oxidative conditions led to an overcyclized perylene platform bearing six additional C-C bonds, as shown in blue (highlighted in bold) in Scheme3.

Molecules 2018, 23, 612 7 of 11 Molecules 2018, 23, x FOR PEER REVIEW 7 of 11

Scheme 3.3. Synthesis of of a a new new curved curved polyaromatic polyaromatic platform. platform. (a) Diphenylether, (a) Diphenylether, 45 min, 45 260 min,◦C, MW260 °C, (300 MW W); ((300b) FeCl W);3 (25b) eq,FeCl CH3 252Cl eq,2/CH CH32NOCl2/CH2 (9:1),3NO 152 (9:1), min, Ar,15 min, RT; ( cAr,) 9-ethynyltriptycene RT; (c) 9-ethynyltriptycene 8 eq, Pd(PPh 8 eq,3)4 Pd(PPh10 mol %,3)4 CuI10 mol 20 mol%, CuI %, piperidine/THF 20 mol %, piperidine/THF (1:1), 6 h, Ar, (1:1), 80 ◦C. 6 h, The Ar, additional 80 °C. The carbon-carbon additional carbon-carbon bonds obtained bonds in the oxidativeobtained couplingin the oxidative are in blue coupling and highlighted are in blue in bold,and andhighlighted the cyclopentadiene in bold, and fragments the cyclopentadiene inducing the curvaturefragments are inducing shown inthe red. curvature are shown in red.

The lastlast stepstep consistedconsisted ofof connectingconnecting thethe fourfour triptycenetriptycene wheelswheels viavia aa quadruplequadruple SonogashiraSonogashira coupling to to give give the the corresponding corresponding nanovehicle nanovehicle 17 in17 34%in 34%yield. yield. Geometric Geometric optimization optimization of this second- of this second-generationgeneration nanovehicle nanovehicle showed showedthat this that overcyclized this overcyclized chassis has chassis a curved has a shape curved at shape both ends at both (Figure ends (Figure5b). As 5withb). As fullerenes, with fullerenes, this geometry this geometry can be expl canained be explained by the presence by the presence of alternating of alternating five- and five- six- andmembered six-membered rings in ringsthe polycyclic in the polycyclic aromatic aromatic hydrocarbon hydrocarbon platform. platform. The two five-membered The two five-membered rings are ringsshown are in shown red in inScheme red in 3. Scheme This specificity3. This specificity is of great is of interest, great interest, since it since provides it provides an appropriate an appropriate shape shapeto act as to an act efficient as an efficient cargo zone. cargo zone.

(a) (b)

Figure 5. (a) CPK (Corey–Pauling–Koltun) modelmodel ofof thethe nanocarnanocar 14 with a planar chassis and (b)) side view of the optimizedoptimized geometrygeometry ofof thethe nanocarnanocar 1717 withwith itsits highly-curvedhighly-curved chassis.chassis.

The planar and curved nanovehicles have very largelarge differences in their solubilitiessolubilities and their spectroscopic properties.properties. ForFor instance,instance, the the color color of of the the planar planar molecule molecule is is pink, pink, while while the the curved curved one one is darkis dark green; green; hence, hence, it wasit was called called “the “the green green buggy”. buggy”.

6. Participation in the First Nanocar Race: The View from the French TeamTeam On 28 28 and and 29 29 April April 2017, 2017, six six teams teams from from three three cont continentsinents metmet in Toulouse in Toulouse to compete to compete with withtheir theirmolecular molecular racers. racers. Both families Both families of nanocars—the of nanocars—the technomimetic technomimetic models models incorporating incorporating wheels wheels (from (fromAustria, Austria, France Franceand the and USA) the and USA) the andsmaller the and smaller lighter and models lighter lacking models wheels lacking (from wheels Germany, (from Germany,Japan and JapanSwitzerland)—entered and Switzerland)—entered the competition the competition [27]. Since the [27 ultra-high-vacuum-low]. Since the ultra-high-vacuum-low temperature (UHV-LT) STM instrument at Centre d’Elaboration de Matériaux et d'Etudes Structurales (CEMES- CNRS) in Toulouse employs only four tips, four nanocars competed using this microscope. Thus, the

Molecules 2018, 23, 612 8 of 11 temperature (UHV-LT) STM instrument at Centre d’Elaboration de Matériaux et d'Etudes Structurales (CEMES-CNRS)Molecules 2018, 23, x FOR in Toulouse PEER REVIEW employs only four tips, four nanocars competed using this microscope.8 of 11 Thus, the Ohio team and the Rice–Graz team were sitting in the same room as the other teams but wereOhio remotelyteam and piloting the Rice–Graz their own team nanocar, were sitting deposited in the in thesame STM room located as the at other their hometeams university.but were Thisremotely presented piloting a further their own challenge nanoca tor, control deposited a nanocar in the at STM the atomiclocated scale at their from home half theuniversity. world away. This presentedExcept a forfurther the Rice–Grazchallenge to team control who a competed nanocar at on the a silver atomic track, scale the from other half nanocars the world were away. on gold. The latterExcept surface for the was Rice–Graz originally team designated who competed as the common on a silver surface track, for the the other race, nanocars and this were was on why gold. the ofﬁcialThe latter ranking surface contains was originally two winners, designated one on goldas the and common one on surface silver. Each for the track race, was and 100 this nm was long why and includedthe official at ranking least two contains turns. two winners, one on gold and one on silver. Each track was 100 nm long and included at least two turns. 6.1. Deposition and Imaging of the French Nanocar 6.1. DepositionThe French and nanocar Imaging had of the the French advantage Nanocar of being very robust. It was deposited by sublimation, as withThe the French other nanocar nanocars, had the and advantage was identiﬁed of being on very the robust. surface It aswas a longdeposited 3 nm by molecule. sublimation, As shownas with inthe Figure other 6nanocars,, the triptycene and was wheels identified appear on the as surface very bright as a long spots 3 nm when molecule. compared As shown with thein Figure central 6, polyaromaticthe triptycene core.wheels appear as very bright spots when compared with the central polyaromatic core.

Figure 6. Image of the curved nanocar 17 on Au(111). The four bright spots correspond to the four triptycene wheels (U == 1.1 V, II == 5 pA). Scale bar: 1 nm.

6.2. The Race and the OfficialOfficial Ranking On 29 April at 5:00 p.m., after two days and one nigh nightt of intense efforts, the first ever international nanocar race ended. As mentioned, mentioned, two teams were ranked first: the the Swiss Swiss team, team, racing racing on on Au(111), Au(111), as statedstated in in the the original original rules rules [28 ][28] and and the Rice–Grazthe Rice–Graz team, team, racing racing on Ag(111), on Ag(111), since their since nanocar their appearednanocar toappeared move uncontrollably to move uncontrollably fast on gold. fast On Au(111)on gold. with On theAu(111) lightest with nanocar the lightest of the competition nanocar of (only the 42competition atoms), the (only Swiss 42 team atoms), arrived the first,Swiss with team a distance arrivedof first, 100 with nm covered a distance in six of hours 100 nm [29 ].covered The US in team six (Ohio)hours [29]. arrived The second, US team with (Ohio) a distance arrived of second, 43 nm covered. with a distance Their nanocar—the of 43 nm covered. Bobcat Their Nano-wagon—was nanocar—the theBobcat largest Nano-wagon—was to participate, with the about largest 650 atoms. to participate, Finally, the with Nano-Windmill about 650 pilotedatoms. byFinally, the German the Nano- team (Dresden)Windmill piloted covered by a distancethe German of 11 team nm. (Dresden) On Ag(111), cove thered dipolar a distance racer of (which 11 nm. consists On Ag(111), of one the axle dipolar with tworacer wheels) (which was consists very of fast, one with axle a distancewith two of wheels) 1000 nm was covered very fast, in only with 29 a h distance [30] but withoutof 1000 nm competitors, covered sincein only the 29 others h [30] competed but without on Au(111).competitors, The characteristicssince the others of eachcompeted team areon givenAu(111). in Table The 1characteristics. of eachWe team targeted are given two strategies in Table to1. move our nanocar directionally; first, we tried pulling the molecule on theWe surface, targeted as two pushing strategies was forbiddento move our in nanocar the rules. directionally; The second first, strategy we tried was pulling to find the a way molecule for an on the surface, as pushing was forbidden in the rules. The second strategy was to find a way for an electronic excitation to induce the movement. Unfortunately, due to the limited time available both in electronic excitation to induce the movement. Unfortunately, due to the limited time available both training and during the race, this propulsion mode failed to drive our nanocar. Only our first strategy in training and during the race, this propulsion mode failed to drive our nanocar. Only our first was effective, and pulling the molecule with the STM tip allowed us to cover an impressive distance strategy was effective, and pulling the molecule with the STM tip allowed us to cover an impressive of 25 nm in 3 s. However, we were disqualified from the race as the jury decided not to recognize distance of 25 nm in 3 s. However, we were disqualified from the race as the jury decided not to recognize movement by pulling. We are very satisfied to have participated in the first nanocar race; remembering movement by pulling. We are very satisfied to have participated in the first nanocar race; remembering the motto from the creator of the modern Olympic games, Pierre de Coubertin, which emphasizes that the motto from the creator of the modern Olympic games, Pierre de Coubertin, which emphasizes “the importance is to participate”. Moreover, for the first time, we were able to climb a step edge. that “the importance is to participate”. Moreover, for the first time, we were able to climb a step edge.

Molecules 2018, 23, 612 9 of 11

MoleculesMoleculesMolecules Molecules2018Molecules 2018Molecules, 23 2018, ,x2018 23 FOR ,2018 ,23 x ,2018 ,23 FOR xPEER, ,23FOR x, ,23FOR PEERx REVIEW,FORPEER x FORPEER REVIEW PEER REVIEW PEER REVIEW REVIEW REVIEW 9 of 911 of9 of911 of911 of 911 of 11 11 Table 1. The six registered participants and their respective nanocars, with their characteristic parameters: country,Table teamTable Table1. leader, TheTable 1.Table 1.ThesixTable The1. registered landing six1.The sixThe1. registered six Theregistered six registered six participants surface,registered registered participants participants participants andparticipants and participants numberandtheir and theirand respecti theirand theirandrespecti of theirrespecti vetheir atomsrespecti nanocars,respective respective nanocars, ve ofnanocars,ve nanocars, their with venanocars, nanocars, with their nanovehiclewith withtheir characteristic withtheir with theircharacteristic theircharacteristic their characteristic characteristic (with characteristicparameters: parameters: the parameters: parameters: corresponding parameters: parameters: country, team leader, landing surface, and number of atoms of their nanovehicle (with the corresponding chemicalcountry, formula).country,country, teamcountry, teamcountry, leader, Inteam red,leader, team leader, landingteam the leader,landing leader,landing parameters surface, landing surface, landing surface, and surface, and number outsidesurface, and number numberand ofandof numberatoms of number the ofatoms atomsof original of their ofatoms oftheiratoms nanovehicl their of rulesnanovehicl of theirnanovehicl their [nanovehicle28 (with nanovehicl]e but(withe the(with accepted e correspondingthe (with ethe (withcorresponding corresponding the the bycorresponding corresponding the jury. chemicalchemicalchemicalchemical formula).chemicalchemical formula). formula). formula). Informula). red,formula). In In red, the Inred, Inparametersred,the Inthered, parameters red,the parameters the parameters the parameters outside parameters outside outside ofoutside outsidethe of outside oforiginalthe ofthe original ofthe original ofthe originalrules the original rulesoriginal [28]rules rules [28] butrules [28] rules butaccepted[28] but[28] accepted but[28] accepted but acceptedbut by accepted the acceptedby by jury.the by the byjury. the jury.by the jury. the jury. jury.

MANA MANA MANA NIMSNIMS MANA MANA NIMS MANA NIMS NIMS Nano NIMS Nano NIMS Nano Dragster Nano NanoDragster Dragster Nano Dragster Nano Dragster Dragster Nano-WagonDragster Nano-Wagon Nano-Wagon Nano-Wagon Nano-Wagon Nano-Wagon Nano-Wagon Dipolar Dipolar Dipolar DipolarRacer DipolarDipolar RacerDipolar Racer Racer Nano-Windmill Racer Racer Nano-Windmill Nano-Windmill Nano-Windmill Nano-Windmill Nano-Windmill Nano-Windmill Green Green GreenBuggy Green Buggy Green Buggy Green Buggy Buggy Buggy Buggy Country CountryJapan Japan Switzerland Switzerland USA USA Austria & Austria USA & USAGermany Germany France France CountryCountryCountryCountry Country Japan Japan Japan Japan Japan Switzerland Switzerland Switzerland Switzerland Switzerland USA USA USA USA Austria Austria Austria Austria& Austria&USA USA & & USA USA& USAGermany Germany GermanyGermany Germany France France France France TeamTeam TeamTeam Team TeamWaka Waka WakaWaka Waka Waka Rémy Rémy Rémy Rémy Rémy RémySaw SawWai Saw WaiHlaSaw WaiSaw &HlaWaiSaw HlaWai & HlaWaiLeonhard & Hla & HlaLeonhard &Leonhard &Leonhard GrillLeonhard LeonhardGrill Grill Grill GrillFrancesca Grill FrancescaFrancesca Francesca Francesca Francesca Gwénaël Gwénaël Gwénaël GwénaëlGwénaël Gwénaël leader WakaNakanichi RémyPawlack SawEric Wai Masson Hla & Leonhard& James Tour Grill MorescoFrancesca RapenneGw énaël Team leaderleaderleader leader leader NakanichileaderNakanichi Nakanichi NakanichiNakanichi Pawlack Pawlack Pawlack Pawlack Pawlack Eric EricMasson Eric Masson MassonEric Eric Masson &Masson James& &James TourJames& Tour James& Tour James TourMoresco Tour Moresco Moresco Moresco Moresco Rapenne Rapenne Rapenne Rapenne Rapenne SurfaceSurfaceSurface SurfaceSurface NakanichiSurface Au(111) Au(111) Au(111) Au(111)Au(111) Au(111) Au(111) Pawlack Au(111) Au(111) Au(111) Au(111) Au(111) Au(111) Eric Au(111) Au(111) Masson Au(111) Au(111) Au(111) Ag(111) Ag(111)&Ag(111) James Ag(111)Ag(111) Ag(111) Tour Au(111) Au(111) Au(111) Au(111)Moresco Au(111) Au(111) Au(111) Au(111) Au(111) Au(111) Au(111) Rapenne Au(111) SurfaceNumberNumberNumber NumberNumber Number88 Au(111) atoms 88 88 atoms 88atoms 88atoms 88atoms atoms 42 atoms Au(111) 4242 atoms 42atoms 42atoms 42atoms atoms 642 atoms642 642 Au(111) atoms642 atoms642 atoms642 atoms atoms 72 atoms72 72 atoms Ag(111)72atoms 72atoms 72atoms atoms 108 atoms108108 atoms108 atoms108Au(111) atoms108 atoms atoms300 atoms300 300 atoms300 atoms300 atoms300 Au(111)atoms atoms 50 34 4 22 17 3 244 222 120 56 32 36 2 2 56 48 4 184 116 Numberof atomsof ofatoms atomsof ofatoms of(Catoms88 50atomsH atoms(C34 (C50OH 450(C)34 H O(C34H4O)50(C H4 )O50 34 HO(C) 344 22)O H 424(C)17 (C22 N atomsH322(C)17H N(C17H3N)22(C H3 )N(C22 17 HN244) 173 (CHN) 222 (C3244) NH642244 (C120222H(C0N222 atoms56H244(C120)N H2440120N56222H0) N56(C222)0120 N320H)120(C 5636)0(C32 N56H)232(C O36H72N2(C)36 H2N atomsO32(CH22ON)32 36H2N)O 362(CNO) 256)O H(C2)48 (C56OH456(C)48 H108O(C48H4O)56(C atomsH4)O56 48HO(C) 484)184O 4(CH) 116(C184)H184 (C116H(C)116 H300184(C) H184)116atoms H)116 ) of atoms (C50H34O4) (C22H17N3) (C244H222N120056) (C32H36N2O2) (C56H48O4) (C184H116) We WereceivedWe receivedWe receivedWe receivedWe the received received “prizethe the “prize the “prize the of “prizethe elegance” “prize of “prizeof elegance” ofelegance” of elegance” of elegance”due elegance” due to due the dueto dueto verythe duetothe veryto the smart verytothe very thesmart very smart STM very smart smartSTM imagesSTMsmart STM images STM images STM ofimages imagesour of images of ourGreen ofour ofGreenour ofGreenour Buggy Greenour GreenBuggy GreenBuggy recorded Buggy Buggy recorded Buggy recorded recorded recorded recorded duringduringduring theduringduring therace.during the race.the The race.the race.the The race.Japanese The race. TheJapanese TheJapanese TheJapanese teamJapanese Japanese team teamwas team wasteam not was team was notranked wasnot wasrankednot rankednot rankedeinot ther,ranked eiranked ther,ei as ther,ei ther,eitheir as ther,ei as theirther, asnanovehicletheir as their nanovehicleas their nanovehicle their nanovehicle nanovehicle nanovehicle was was unable was wasunable wasunable was tounable unablerecover to unable to recover torecover to recover torecover recover We received the “prize of elegance” due to the very smart STM images of our Green Buggy fromfrom froma softwarefrom afrom softwareafrom softwarea softwarea crash,softwarea software crash, crash, just crash, crash,just one crash,just justone hour justone justonehour onehourinto onehour into hourthe into hour into therace. intothe into race.the Thisrace.the race.the This race. teamThis race. This team This teamwas This team wasteam awarded was team wasawarded wasawarded was awarded awardedthe awarded the“fair the “fairthe play”“fairthe “fairthe play” “fair play” prize.“fair play” play”prize. prize.play” prize. prize. prize. recorded during the race. The Japanese team was not ranked either, as their nanovehicle was unable to recover from7. Conclusion7. 7.Conclusion a 7.Conclusion software 7.Conclusion 7.Conclusion Conclusion and and Perspectives crash,and andPerspectives andPerspectives andPerspectives just Perspectives Perspectives one hour into the race. This team was awarded the “fair play” prize. In summary,InIn summary, Insummary,In summary, Insummary, we summary, havewe we have we presentedhave we have we presented have presented have presented presented the presented thedesign the design the design the and designthe design and sydesign andnthesis andsy andsynthesis andnthesissy syofnthesis synthesispolyaromatic ofnthesis of polyaromatic ofpolyaromatic of polyaromatic ofpolyaromatic polyaromatic hydrocarbons hydrocarbons hydrocarbons hydrocarbons hydrocarbons hydrocarbons conceived conceived conceived conceived conceived conceived 7. Conclusionsby analogybyby analogyby analogybyand analogy bywith analogy analogy with Perspectives macroscopicwith with macroscopic with macroscopic with macroscopic macroscopic macroscopic wheels, wheels, wheels, wheels, wheelbarrows, wheels, wheels,wheelbarrows, wheelbarrows, wheelbarrows, wheelbarrows, wheelbarrows, and and nanocars. and andnanocars. andnanocars. andnanocars. nanocars. Concerning nanocars. Concerning Concerning Concerning Concerning Concerning the thenanocar the nanocarthe nanocarthe nanocarthe race, nanocar nanocar race, werace, race, we race, we race, we we we hadhad confirmationhad hadconfirmation hadconfirmation hadconfirmation confirmation confirmation that that the that thatthesmallest thatthe thatsmallestthe smallestthe smallestthe molecules smallest smallest molecules molecules molecules molecules completed molecules completed completed completed completed thecompleted thetrack the trackthe trackdistancethe trackthe distancetrack distancetrack distance firstdistance distance first (the first first(the Swiss first(the first (theSwiss (theSwiss molecule- (theSwiss Swissmolecule- Swissmolecule- molecule- molecule- molecule- In summary,car carhadcar hadcar only hadcar hadcaronly we had42only had only haveatoms 42only 42 onlyatoms 42 atoms presented42and atoms 42 atoms and is atomsand commercially andis and iscommercially andiscommercially the iscommercially iscommercially design commercially available). available). and available). available). available). synthesis Thisavailable). This isThis partlyThis is This is of partlyThis ispartly polyaromatic because ispartly ispartly because partlybecause because smaller because becausesmaller smaller hydrocarbonssmallerwas smaller smallerwas betterwas wasbetter wasbetter forwas better betterfor betterfor conceived for for for by analogyeasilyeasily witheasily andeasily easilyand rapidly easilyand macroscopic andrapidly andrapidly and rapidlymoving rapidly movingrapidly moving moving on moving wheels, movingtheon on thesurface. on the on surface.the on surface.wheelbarrows,the surface.the surface. surface. and nanocars. Concerning the nanocar race, we had confirmationTheThe firstThe Thefirst nanocar Thefirst The firstnanocar first thatnanocar first nanocar race nanocar thenanocarrace was race race smallestwas arace was real race wasa wasreala success, wasreal a realasuccess, molecules realsuccess,a realsuccess, with success, success,with awith worldwide with a with completedworldwidea with worldwidea worldwidea worldwidea worldwideaudience audience audience the audience audiencefollowing audience track following following following following distancethe following theevent the eventthe event theon event firstthe theeventon onevent the on (thethe on the on the Swissthe internet,internet, broadcasted broadcasted live, night live, and night day, and with day, mo withre than mo re100,000 than 100,000viewers viewers at peak attimes. peak Wetimes. now We now molecule-carinternet,internet, hadinternet,internet, broadcasted only broadcasted broadcasted 42 broadcasted atoms live, live, night andlive, night live, nightand is andnight commercially day, andday, and withday, with day, mowith mowithrere available).mothan thanmore re100,000than 100,000 than 100,000 100,000viewers This viewers viewers is viewersat partlyat peak peak at attimes.peak because times.peak times. We times.We now smallerWenow We now now was havehave haveexperiencehave experiencehave experiencehave experience experience inexperience organizing in in organizing inorganizing in organizing inorganizing organizingsuch such ansuch such event,an such an suchevent, an event, anwhich event, an event, which event,which may which which may which havemay may have may havea may secondhave ahave seconda have seconda secondeditiona seconda editionsecond edition editionaround edition aroundedition around around2021. around 2021.around 2021.All 2021. Allthe2021. All2021. theAll theAll theAll the the better forcompetitors easilycompetitorscompetitorscompetitorscompetitors andcompetitors went rapidlywent wenthome went homewent home wentwith home moving homewith newhomewith with new with ideasnew with on new ideas new ideas thethat new ideas thatideas willsurface. thatideas thatwill ma thatwill that willkema willma them ke willmake mathemke themmamoreke themke morethem morecompetitivethem more competitivemore competitivemore competitive competitive competitive next next time,next next time, next time,and next time, andtime, we and time, andwecan andwe canandwe can we canwe can can Theassume firstassumeassume nanocar assumethatassume assumethat new that thatnew participantsthatnew race that new participants new participants wasnew participants participants participants awill real will enter. will success, willenter. willenter. One willenter. enter.One couldenter.One withOne could One could imagineOne coulda couldimagineworldwide couldimagine imagine organizing imagine imagine organizing organizing organizing organizing audience organizingsemi-finals, semi-finals, semi-finals, semi-finals, semi-finals, followingsemi-finals, test test selections test testselections testselections test theselections selections to selectionsevent to to to onto to the internet,qualify broadcastedqualifyqualify qualifyforqualify forthequalify for thefinalfor thefor live,final thefor orfinalthe finaltheto or nightfinal orrun tofinal orto runorthe torun or andto runtherace torunthe runracethe day, at racethe arace theat higherrace at witha race athighera at highera teathighera more mperature, highera te higher temperature, mperature,te than mperature,te mperature,te mperature,in 100,000 order in in order inorder to in order havein viewersorderto toorder have tohavea to fasterhave toahave fastera athave faster aevent. peakfastera fasterevent.a fasterevent. It event. times. hasevent. It event. It has hasIt Ithas We Ithas has now alreadyalready been shown been shownthat control that controlof motion of motionis possible is possible at higher at highertemperatures temperatures on some on semi-some semi- have experiencealreadyalreadyalready inalreadybeen been organizing beenshown shownbeen shown thatshown that such control that controlthat an control event,controlof of motion motionof whichofmotion motionis ispossible possible mayis ispossible possible haveat at higher higher aat secondathigher temperatureshigher temperatures temperatures editiontemperatures on aroundon some someon on somesemi- 2021.somesemi- semi- semi- All the conductiveconductiveconductiveconductiveconductive conductivesurfaces, surfaces, surfaces, surfaces, suchsurfaces, surfaces, such assuch suchSmSi as such as SmSisuch as SmSi[31,32] as SmSi as [31,32]SmSi [31,32]SmSi or[31,32] [31,32]SiB or [31,32] or SiB[ 33]. orSiB or [SiB33]. orFurthermore, [SiB33]. [SiB 33].Furthermore, [ 33].Furthermore, [ 33].Furthermore, Furthermore, Furthermore, the thequestion the questionthe questionthe questionthe ofquestion questionwhether of of whether ofwhether of whether ofwewhether whether weneed we needwe needwe needwe need need competitorsto createtoto wentcreate tocreate to differentcreate tocreate homedifferent createdifferent different differentcategories withdifferent categories categories categories new categories willcategories ideaswill bewill willaddressed,be willbe that addressed, willbe addressed, be addressed, willbe addressed, addressed,in make orderin in order inorder to themin order in giveorderto orderto give more toeverybodygive to give everybody togive everybody give competitive everybody everybody everybodythe thesame the samethe samethechance next thesame samechance chancesame time, chance(e.g., chance (e.g.,chance (e.g., and (e.g., (e.g., we(e.g., can assumedividing thatdividingdividing newdividing thedividingdividing participants thenanovehicles the nanovehiclesthe nanovehiclesthe nanovehiclesthe nanovehicles nanovehicles by will the by by thenu enter.by the mberby nuthe by nuthember nuthe mberOneof number atoms nuofmber of couldmberatoms ofatoms orof atoms ofbyatoms or imagine atoms orthe by orby themolecularor by the or by molecularthe by molecularthe organizing molecularthe molecular mass). molecular mass). mass). We mass). mass). semi-finals, Wecould mass). We couldWe couldWe also couldWe couldalso organize alsocould also testorganize alsoorganize also organize selections organize organize to qualify fora differenta the differenta differenta finaldifferenta race,differenta different race, or forrace, race, to example,for race, for runrace, example,for example,for theexample,for example,moving example, race moving moving moving atthe moving amoving thenanocar higherthe nanocarthe nanocarthe nanocarthe wi nanocar temperature, thnanocar wi cargowith with cargo wi thcargo onwith cargo th itscargoon oncargo backbone.its on in its onbackbone. its orderbackbone.on its backbone. its backbone. This backbone. to This havewouldThis This would This would Thisa bewould fasterwould similarbe would be similar be similar be event. similar be similar similar It has to the “waiter’sto the “waiter’s race” (“course race” (“course de garçons de garçons de café” de in café” French) in French) created createdin Paris in (France), Paris (France), which testswhich tests already beentoto the shown the to“waiter’s to“waiter’sthe the that“waiter’s “waiter’s race”control race” race”(“course (“courserace” of (“course motion(“course de de garçons garçons de isde garçonspossible garçonsde de café” café” de de incafé” at in café”French) higher French) in inFrench) French)createdtemperatures created created createdin in Paris Paris in in(France),Paris on(France),Paris some(France), (France), which which semi-conductive which tests which tests tests tests the thespeedthe speedthe speedthe of speedthe a speedof waiter speedof a ofwaitera of waitera carryingofwaitera waitera carrying waiter carrying carrying acarrying loaded carrying a loadeda loadeda trayloadeda loadeda trayloaded without tray traywithout traywithout tray without tipping without withouttipping tipping tippingit tippingover ittipping itover [34].overit itover [34]. over it [34]. over [34]. [34]. [34]. surfaces, such as SmSi [31,32] or SiB [33]. Furthermore, the question of whether we need to create differentAcknowledgments: categoriesAcknowledgments:Acknowledgments:Acknowledgments:Acknowledgments:Acknowledgments: will This be This work addressed,This Thiswork This work was Thiswork was work supported waswork wassupported in wassupported was ordersupported supportedby supported theby toby theCNRS, giveby theby CNRS,the by CNRS,the everybody theCNRS,the CNRS, theUniversité CNRS,the Universitéthe Universitéthe Universitéthe Université thePaul Université Paul same SabatierPaul Paul SabatierPaul Sabatier chancePaul Sabatier(Toulouse), Sabatier Sabatier(Toulouse), (Toulouse), (e.g.,(Toulouse), (Toulouse), the(Toulouse), dividing the the the the the the EuropeanEuropean UnionEuropean and Union the Union ANRand andthe (ACTION ANR the ANR (ACTION project (ACTION n8 project ANR project n815- ANRCE29-0005). n8 ANR 15-CE29-0005). 15- AllCE29-0005). the official All the All sponsors official the official sponsors of the sponsors Nanocar of the of Nanocar the Nanocar nanovehiclesEuropeanEuropean byEuropean the Union Union number Unionand and the theand ANR of ANR the atoms (ACTION ANR (ACTION (ACTION or project by project the project n8 n8 ANR molecular ANR n8 15- ANR 15-CE29-0005).CE29-0005). 15- mass).CE29-0005). All All the We the All official couldofficial the official sponsors sponsors also sponsors of organize ofthe the Nanocarof Nanocar the Nanocar a different racerace arerace racealsoare raceare also raceaknowledged,are also are also aknowledged,are alsoaknowledged, alsoaknowledged, aknowledged, aknowledged, the fullthe the fulllist the full the appears listfull the listfull appears listfull appears list appears onlist appears our onappears on ourwebsite on our on websiteour on websiteour http://nanocar-race.cnrs.fr/.websiteour website http://nanocar-race.cnrs.fr/.website http://nanocar-race.cnrs.fr/. http://nanocar-race.cnrs.fr/. http://nanocar-race.cnrs.fr/. http://nanocar-race.cnrs.fr/. We Weare We also areWe areWe also gratefulareWe also are also gratefulare alsograteful alsograteful grateful grateful race, forto example,theto toresearchersthe tothe researcherstothe researcherstothe researchersthe moving researchers who researchers who participated who whoparticipated the whoparticipated who participated nanocarparticipated participatedin all in the inall inall withworkthe in allthe in all workthe all workdithe cargo scussed workthe diwork discussed work scusseddi onscusseddihere scusseddi here itsscussedand here here backbone. andwhose hereand here andwhose andwhose names andwhose whosenames whosenames appear Thisnames namesappear namesappear in wouldappear theappear in appear inreferences,the inthe references,inbethe references,inthe references,similarthe references, references, to the “waiter’sand race” andinand particular andin (“courseand inparticular andinparticular inparticular inparticularthe particular thetalented de the talentedthe garçons talentedthe talentedthePh.D. talented talentedPh.D. Ph.D.and de Ph.D. andPh.D.Post-doctoral caf andPh.D. andPost-doctoralé ”andPost-doctoral inandPost-doctoral Post-doctoral French) Post-doctoral students students students students createdwho students studentswho achievedwho whoachieved in whoachieved who Parisachieved theachieved achieved thesynthesis the (France), synthesisthe synthesisthe synthesisthe ofsynthesis synthesisthese ofwhich ofthese ofthesesystems: ofthese systems:ofthese testssystems: these systems: systems: the systems: speed RomainRomainRomain RomainGarbage,Romain RomainGarbage, Garbage, Garbage, AgnèsGarbage, Garbage, Agnès Agnès Sirven, Agnès AgnèsSirven, AgnèsSirven, andSirven, Sirven, andSirven,Gorka and andGorka andGorka Jimenez-Bu andGorka GorkaJimenez-Bu GorkaJimenez-Bu Jimenez-Bu Jimenez-Bueno, Jimenez-Bueno, aseno, welleno,as eno,as well eno, aswell as Francescawell as as wellas Francesca well asFrancesca as Francesca as FrancescaMoresco, Francesca Moresco, Moresco, Moresco, LeonhardMoresco, Moresco, Leonhard Leonhard Leonhard LeonhardGrill, Leonhard Grill, Grill, Grill, Grill, Grill, of a waiterSébastienSébastien carryingSébastienSébastien SébastienGauthier,Sébastien Gauthier, Gauthier, a Gauthier, loaded Gauthier,Corentin Gauthier, Corentin Corentin Corentin tray Durand,Corentin Corentin Durand, withoutDurand, Durand, OlivierDurand, Durand,Olivier Olivier tippingGuillermetOlivier Olivier GuillermetOlivier Guillermet Guillermet Guillermet it andGuillermet over andWe-Hyo and and [We-Hyo 34 andWe-Hyo ].andWe-Hyo SoeWe-Hyo We-Hyo Soefor Soe SoeforSTM forSoe STMSoefor studies,STMfor STM for studies,STM studies, STM studies,and studies, andstudies,Christian and andChristian andChristian andChristian Christian Christian JoachimJoachimJoachim JoachimandJoachim andJoachimXavier and Xavierand Xavierand Bouju Xavierand Xavier Bouju Xavier Boujufor Bouju calculations. forBouju forBouju calculations. for calculations. for calculations. for calculations. calculations.Colin Colin ColinMartin Colin Martin Colin Martin Colin is Martin also Martin is Martin isalso warmly alsois iswarmlyalso warmlyalsois alsoaknowledgedwarmly warmly aknowledged warmly aknowledged aknowledged aknowledged aknowledged for hisfor for carefulhis for his forcareful his carefulfor hisreading careful his careful reading careful reading andreading reading and reading and and and and Acknowledgments:improvingimprovingimprovingimprovingimproving ofimproving our Thisof of ourmanuscript. ofour work ofmanuscript.our manuscript.ofour manuscript.our wasmanuscript. manuscript. supported by the CNRS, the Université Paul Sabatier (Toulouse), the European Union and the ANR (ACTION project n8 ANR 15-CE29-0005). All the official sponsors of the Nanocar race are ConflictsConflictsConflictsConflicts ofConflicts Interest:Conflicts of of Interest: ofInterest: of Interest: The ofInterest: Interest: Theauthors The authorsThe authorsThe declareauthorsThe authors declare authors declare no declare declareconflict no declare no conflict no conflict no of conflict no interest.conflict of conflict ofinterest. interest.of of interest.Th ofinterest.e Thinterest.founding The foundingThe foundingThe foundingThe sponsors foundinge founding sponsors sponsors sponsors had sponsors sponsors hadno had role nohad nohad rolein nohad role the no inrole no inroledesignthe therolein design inthe design in the design the design design also aknowledged, the full list appears on our website http://nanocar-race.cnrs.fr/. We are also grateful to the of theof ofthestudy; ofthe ofstudy;the study;of thein study;the the study;in study; inthecollection, inthe incollection,the collection,inthe collection,the collection,analys collection, analys analyses, analys ores,analys es,interpretationanalys ores, or interpretationes, orinterpretationes, or interpretation orinterpretation interpretation of data; of ofdata; ofindata; ofdata;the in ofdata; in writingthedata; inthe inwritingthe writinginthe ofwritingthe writingthe ofwriting ofmanuscript,the ofthe ofmanuscript,the manuscript,ofthe manuscript,the manuscript, manuscript,and andin and the andin and inthe andinthe inthe inthe the researchersdecisiondecision whodecision decisionto participateddecision publish decisionto topublish topublish tothepublish topublish results.the publish the inresults. the results. allthe results. the theresults. results. work discussed here and whose names appear in the references, and in particular the talented Ph.D. and Post-doctoral students who achieved the synthesis of these systems: Romain

Garbage, Agnès Sirven, and Gorka Jimenez-Bueno, as well as Francesca Moresco, Leonhard Grill, Sébastien Gauthier, Corentin Durand, Olivier Guillermet and We-Hyo Soe for STM studies, and Christian Joachim and Xavier Bouju for calculations. Colin Martin is also warmly aknowledged for his careful reading and improving of our manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Molecules 2018, 23, 612 10 of 11

References and Notes

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