An electrically actuated molecular toggle switch

Transcription

1 Reeive 17 Aug 216 Aepte 23 Jn 217 Pulishe 9 Mr 217 DI: 1.138/nomms14672 PEN An eletrilly tute moleulr toggle swith Luks Gerhr 1, Kevin Eelmnn 1,2, Jn Homerg 1, Mihl Vlášek 1, f G. Bhoosh 3, My Luks 1, Fin Puly 3, Mrel Myor 1,4,5 & Wulf Wulfhekel 1,2 Moleulr eletronis is onsiere promising pproh for future nnoeletroni evies. In orer tht moleulr juntions n e use s eletril swithes or even memory evies, they nee to e tute etween two istint onutne sttes in ontrolle n reprouile mnner y externl stimuli. Here we present tripol pltform with ntilever rm n nitrile group t its en tht is lifte from the surfe. The formtion of oorintive on etween the nitrile nitrogen n the gol tip of snning tunnelling mirosope n e ontrolle y oth eletril n mehnil mens, n les to hystereti swithing of the onutne of the juntion y more thn two orers of mgnitue. This toggle swith n e tute with high reprouiility so tht the fores involve in the mehnil eformtion of the moleulr ntilever n e etermine preisely with snning tunnelling mirosopy. 1 Institut für Nnotehnologie, Krlsruhe Institute of Tehnology, Eggenstein-Leopolshfen, Germny. 2 Physiklishes Institut, Krlsruhe Institute of Tehnology, Krlsruhe, Germny. 3 Deprtment of Physis, University of Konstnz, Konstnz, Germny. 4 Deprtment of Chemistry, University of Bsel, t Johnns-Ring 19, 456 Bsel, witzerln. 5 Lehn Institute of Funtionl Mterils (LIFM), un Yt-en University, Gungzhou 51275, Chin. Corresponene n requests for mterils shoul e resse to L.G. (emil: luks.gerhr@kit.eu) or to F.P. (emil: fin.puly@uni-konstnz.e) or to M.M. (emil: mrel.myor@unis.h). NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

2 NATURE CMMUNICATIN DI: 1.138/nomms14672 The formtion of moleulr juntion n thus ultimtely the performne of single-moleule evie is etermine y quntum mehnis. Thus, juntions of hemilly ientil moleules shoul ehve ientilly llowing for relile opertion, whih is one of the funmentl vntges of moleulr eletronis 1,2. In the pst, rek-juntion experiments lrgely ontriute to the progress of unerstning eletron trnsport in single-moleule juntions 35. In these mesurements, however, the moleulr sorption n the rrngement of the toms in the metlli eletroes re priori unknown n often mny ifferent nerly egenerte juntion geometries exist. This in turn les to lrge vritions of the eletroni properties of the iniviul juntions n requires sttistil nlysis of the t 6. Furthermore, rek-juntion tehniques inherently lk the possiility of lterl resolution, whih neither llows to iretly ientify the moleule of interest t the eletroe surfe nor to eliertely explore vritions in on onfigurtions. Reprouile performne n well-efine hrteristis of the moleulr juntion, however, re prerequisites for oth the omprehension of moleulr evies n their implementtion into future eletroni evies 7,8. Reently, snning tunnelling mirosope (TM) n tomi fore mirosope (AFM) set-ups 9 emerge s powerful lterntive to rek-juntion methos. In this pproh, the sorption geometry of the moleules n e etermine efore ontting moleule with the tip t speifi positions 114. Fores in the rnge of pionewtons tht result from the onset of on formtion n e erive from the frequeny shift of the ntilever of n AFM osillting y some tens to some hunres of piometres 12,14,15. With view to future pplitions, it is neessry to ontrol the properties of single-moleule juntions y externl mens 16 suh s mehnil fores 17, eletri fiels 182, urrents 21,22 or light 23,24. In this respet, it is helpful to esign the moleule speifilly suh tht it ins to the sustrte eletroe in wellefine geometry n to lift the funtionl group of the moleule from the metlli sustrte. This n e hieve y threeimensionl moleulr rrier pltforms tht llow for wellefine rrngement of moleulr suunits with respet to the sustrte. Tremenous progress in the synthesis of suh moleules hs een me reently In this work, we present low-temperture TM mesurements n ensity funtionl theory (DFT) lultions on tripol spiroifluorene erivtives (spiro), whih re esigne to firmly in to gol surfe vi three sulfur toms t the feet 27. They rry, s funtionl suunit, freely suspene nitrile group tht is well seprte from the metlli sustrte, freely essile y the TM tip n mounte on rigi suunit. In the following, we show tht the high reprouiility of the proess of ontt formtion of the nitrile group to gol eletroe llows us to uil fully ontrollle eletromehnil moleulr toggle swith n to eue the fores tht re neee to streth the moleulr juntion. Results Deposition of spiro A n spiro B moleules. The moleules, entitle spiro A n spiro B, re presente in Fig. 1,, n their esign omprises firmly immoilize pltform, exposing rigily mounte freely suspene ntilever. They were eposite onto len Au(111) surfes y spry eposition metho esrie previously 27, followe y mil nneling in ultr-high vuum (UHV), whih promotes the eprotetion of the sulfur nhor groups n the proper ining to the gol sustrte. Both moleules rrnge in orere islns with wellefine sorption geometry (Fig. 1,). While the synthesis of spiro A is lrey reporte 27, the synthesis of spiro B is provie in the Methos setion. Detils of the sorption geometry re presente in upplementry Fig. 14 n upplementry Note 9. imulte TM imges re isusse in upplementry Figs 12 n 13 n upplementry Note 8. Mehnil opening n losing of the moleulr juntions. To stuy the istne-ontrolle ontt formtion to the nitrile group, we reore the urrent while the TM tip ws pprohe towrs the moleule (lk urves in Fig. 2,) n ws retrte k to its initil position (re urves in Fig. 2,) 3,31. At ertin ritil istne z N, the moleulr ontt loses n the mesure onutne jumps to higher vlue ( 38 n 11 pm in Fig. 2,). Upon retrtion, t lrger istne z FF etween tip n smple ( 18 n 4 pm in Fig. 2,), the ontt reopens n the onutne jumps k to lower vlue. The experimentlly oserve hystereti ehviour results from the short-rnge ining fores etween the tip n the nitrile group of the moleule omine with elsti eformtions of the moleulr rige, whih omprises lso surfe toms of the tip n sustrte. We mnge to extrt the fores involve in this purely elsti eformtion of the moleulr juntion s esrie t the en of the mnusript. To further investigte the istle ehviour foun in the moleulr juntions, we performe DFT lultions of juntion opening n losing proesses (see the Methos setion for etils on the pplie DFT proeures). In prtiulr, we investigte whether the moleule lone is responsile for the hysteresis in onutneisplement urves, s mesure in Fig. 2, or to wht extent the eletroes n moleuleeletroe interfes nee to e involve in the N-to-FF-stte swithing. To perform effiient lultions, we ssume tht single Au tom represents the tip. The toms of spiro B were ssume to e either ompletely fixe in the plne z ¼ or free to move to esrie the role of the sustrte. The Au tom ws then ple B2 Å iretly ove the N tom of fully relxe moleule. usequently, it ws rought out of n into ontt y moving it long the z xis wy in steps of.1 Å until the ontt roke n the moleule i not feel the tip. Afterwrs, the proeure ws reverse n the tip pprohe the moleule until the ontt forme gin n the Au tom rehe its strting position. We ompute the hnge in the AuN on length s funtion of the height of the Au tom ove the z ¼ plne, s isplye in Fig. 2,. In the se tht ll toms re fixe ompletely, we fin prtilly no hysteresis (Fig. 2). But if the toms re llowe to move in the xy plne, the legs gin more flexiility n pronoune hysteresis is oserve (Fig. 2). This shows tht lso the ssume nhoring of the moleule to the sustrte plys ruil role for the juntion istility, or in other wors the mehnil properties of the moleuleeletroe interfes s well s those of the eletroes. Generlly speking, the energy surfe t those tip istnes, where the hysteresis is oserve, must exhiit oule-well potentil sine oth N n FF sttes re stle, of rther similr energy ut seprte y n energy rrier, thus llowing the stte of the system to epen on proess history. This mens tht the elsti energy in the moleulr juntion must e lose to the energy of the AuN on etween the tip n the moleulr nitrile group. At first sight, it n e seen tht the onutne of spiro A is mrkely lower thn tht of spiro B (5 1 6 G in Fig. 2 ompre to G in Fig. 2, where the onutne is given in units of G ¼ 2e 2 /h). To etter unerstn this ifferene, we hve ompute the onutne of spiro A n spiro B with the DFT 32 n DFT þ methos As esrie further in the Methos setion, the onutne n e otine from the 2 NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

3 NATURE CMMUNICATIN DI: 1.138/nomms14672 ARTICLE N N 5 pa 2 pa 4 pa 16 pa 3 pa 12 pa 2 pa 8 pa 1 pa 4 pa pa pa Figure 1 Asorption geometry of the tripol moleules. (,) Chemil struture of spiro A n spiro B s synthesize. (,) TM imge of orere lyers of spiro A n spiro B reore in onstnt height moe t is voltge of 1.9 V n 7 mv with the orresponing moleulr moels superimpose to sle. A re ross inites typil position for ontt formtion. The length of the sle rs is 1 nm. Retrt Retrt Conutne G/G Approh 3 2 z (pm) 1 Conutne G/G Approh z (pm) 5 5 Bon length (pm) Approh Retrt Bon length (pm) Approh Retrt Tip height (nm) Tip height (nm) Figure 2 Contt formtion y pprohing the tip towrs the smple. () Iz urve reore ove the he group of spiro A t is of 3 mv. z vlues re given with respet to the strting position (z ¼ ), n the sweep iretion is inite y rrows. () Iz urve reore ove the he group of spiro B t is of 25 mv. (,) DFT lultions of the opening n losing of moleulr juntions ontining spiro B s result of the tip movement with, ll toms fixe in their positions, n, ll toms free to move in the plne z ¼, proviing n enhne mehnil flexiility. Plotte is the istne etween the nitrogen tom of the moleulr he group n the gol tip tom s funtion of the tip height ove the surfe t z ¼. NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

4 NATURE CMMUNICATIN DI: 1.138/nomms14672 g 1 DFT 1 1 Trnsmission e f 2 2 EE F (ev) e f h 1 DFT 1 1 Trnsmission e f 2 2 EE F (ev) Figure 3 Trnsport through tripol moleules on Au(111) in ifferent onfigurtions. () piro A. (e,f) piro B. (g) Compute trnsmission urves in DFT n (h) DFT þ frmeworks re shown for onfigurtions f. Tle 1 Conutne of the moleulr juntion. e f G DFT G DFT þ Experiment Compute onutne of the onfigurtions shown in Fig. 3 using DFT n DFT þ methos in omprison with experimentl vlues. Conutnes re given in units of G ¼ 2e 2 /h. energy-epenent trnsmission funtion t(e) vi eqution (4), if phse-oherent trnsport is ssume. This reues to GEG t(e F ) t low tempertures with the Fermi energy E F, n the onutne n thus e estimte from energy-epenent trnsmission urves. They re shown for ifferent juntion geometries of spiro A n B in Fig. 3, n vlues of the onutne (t vnishing temperture) re liste in Tle 1. We oserve tht the short moleule spiro B shows lrger onutne thn spiro A, s expete for off-resonnt trnsport. A rottion of the ssume tomilly shrp tip hs silly no influene on trnsport, ut rottion of the phenyl ring t the he, tht my e inue y tipmoleule intertions, les to strong suppression of the onutne of spiro A y more thn one orer of mgnitue for perpeniulr ring orienttion. We oserve tht the onutne vlues etermine from DFT re onsistently ftor of roun 24 lrger thn those of DFT þ. This results from n opening of the highest oupie moleulr oritllowest unoupie moleulr oritl gp tht is lerly visile for t(e) in Fig. 3g,h. DFT þ results hve een reporte to show goo greement with experimentl onutne vlues 3335, sine the DFT þ metho is onstrute to yiel improve qusiprtile energies for the moleule in the metlli juntion. For this reson, we will ompre elow only the DFT þ results to the experiment. Further etils of the DFT þ lultions n e foun in the Methos setion, n upplementry Note 7 n upplementry Fig. 11. To summrize Tle 1 n ompre to the experiment, vlues foun for spiro B for ifferent tip onfigurtions re entre B G, tht is, ftor of 7 ove the experimentl results. As the terminl phenyl ring of spiro A is freely revolving roun the ethynyl onnetion, we lso onsiere the twiste rrngement of the phenyl ring. In its untwiste onfigurtion, the onutne is B G, whih is lrger thn the experimentl vlue y ftor of 13. A lultion for twist ngle of 9 les to vlue of B G whih is, similr to the se of spiro B, within n orer of mgnitue from the mesurements. In full greement with the simulte TM imges presente in the upplementry Figs 12 n 13, this inites tht the terminl phenyl ring twists in the presene of the tip. Typilly, suh ontting mesurements involve plsti eformtions of the eletroes or isplements of the moleule, whih les to wiespre rnge of onutne vlues of the juntion. Roust n reprouile onutne swithing, however, is essentil with regr to possile pplitions n requires suitle nhoring groups s is the se for the moleulr tripos isusse in this work. n one sie, the spiro moleules re firmly tthe to the sustrte vi three sulfur ons, n on 4 NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

5 NATURE CMMUNICATIN DI: 1.138/nomms14672 ARTICLE Retrt y (nm) x (nm) Conutne G/G Conutne G/G Approh z (pm) Conutne G/G 1 3 N FF Counts Conutne G/G N FF z (pm) 5, 1, Counts Figure 4 Reprouiility of ontt formtion. () Conutne mesurements with Iz urves reore t every pixel ove spiro B moleules t is of 7 mv. The olour oe shows the onutne t the losest istne (z ¼ 21 pm) with moleulr struture superimpose to sle. () Ten Iz urves reore on equivlent positions of 1 ifferent moleules mrke y re ots in. () Two-imensionl histogrm of ll 14, pproh n retrtion urves. () ne-imensionl histogrm showing the lerly seprte onutne vlues of the N n the FF stte inluing ll Iz urves. the other sie, nitrile group forms oorintive on to the gol tip tht n e reprouily opene n lose. everl thousn onutne urves were reore on n rry of spiro B moleules n ll moleulr juntions forme resulte in silly the sme vlue of onutne s n e seen in the onutne mp presente in Fig. 4, where ll moleulr he groups show up in ientil olour. Iz urves reore on equivlent positions of ifferent moleules show ientil swithing ehviour (Fig. 4). As n e seen in the orresponing onutne histogrms (see Fig. 4,, inluing ll 14, Iz urves on top of the moleulr he group, ner to it, on the sie of the moleule n ove the gol surfe), the NFF rtio of the swith mounts to out two orers of mgnitue with no overlp of the orresponing onutne peks. The high reprouiility of the nitrilegol ontt in the TM experiment goes well eyon rek-juntion experiments of moleules with similr nhor groups or similr ore strutures 36,37. It llows to unmiguously ttriute the remining vrition of the onutne of the N stte to the ifferene in tomi-sle ontt geometries for ifferent tip positions (see lterl mpping of the hysteresis esrie in etil in upplementry Fig. 15 n upplementry Note 1). Eletril opening n losing of the moleulr juntions. The opening n losing of the moleulr juntion n lso e ontrolle y the pplie is. As shown in Fig. 5,, IV urves reore ove the he group exhiit hystereti swithing etween high- n low-onutne urves. The ritil voltges tht re neessry to open n lose the moleulr juntion strongly epen on the lterl n the vertil position of the tip n rnge from 1 to.5 V n.7 to 2 V for losing n opening the juntion, respetively (for etils, see upplementry Figs 16 n 17 n upplementry Note 11). This ehviour exlues swithing mehnisms tht re relte to prtiulr energy of the eletrons suh s inelsti exittions or tunnelling to speifi oritls 38,39. However, it n e expline y the eletri fiel tht ts on the ipole moment of the moleulr he group s follows: t positive is voltges pplie to the smple, the eletri fiel points from the smple towrs the tip n exerts torque on the moleulr ipole tht in turn pushes the moleulr he group wy from the tip (Figs 5 n 6). When the moleule is strethe, the elsti energy tht ounterts the ining energy in the lose stte inreses n the ontriution of the eletrostti energy eomes ruil. As soon s open n lose onfigurtions of the juntion re similr in energy, the eletri fiel n rek the nitrilegol on n opens the ontt etween tip n moleule, whih les to suen rop in the onutne t voltge of 1 V for spiro A in Fig. 5 (.35 V for spiro B in Fig. 5). imilrly, t negtive is voltges, the eletri fiel points towrs the smple n pulls the he group towrs the tip. This loses the tipmoleule ontt, n the lone pir of the terminl nitrile nitrogen forms oorintive on to the gol tip, whih is reflete in suen inrese in the onutne t.8 V for spiro A in Fig. 5 (.2 V for spiro B in Fig. 5). The effet is ue to purely elsti eformtion in ontrst to the eletri-fiel-inue isomeriztion in zoenzene moleules 18 or the reently esrie oxitionreution proesses of orgnometlli ompouns 2. A series of ontrolle swithing events inue y voltge pulses of þ 2V/ 1.5 V with re-out voltge of.5 V is shown in Fig. 5, reore on spiro A. The tip ws ple iretly over the nitrile group suh tht the re-out voltge lies within NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

6 NATURE CMMUNICATIN DI: 1.138/nomms14672 Current (pa) 2 Closing ontt 2 4 pening ontt mple voltge (V) V 2 V Current (na) mple voltge (V) 2 Conutne G/G mple voltge (V) Height (pm) p Eletri fiel (GV m 1 ) Figure 5 Contt formtion y pplition of eletri fiels. (,) IV urves t the position of the he group of spiro A () n spiro B (). The rrows inite the iretion of the voltge rmp. () Time tres of pplie voltge n resulting urrent uring istle swithing etween the two sttes of spiro A (open n lose juntion) y voltge pulses of 13 ms n þ 2V/ 1.5 V. Reore with n integrtion time of 2 ms. () imulte response of spiro B to n externl eletri fiel. The z omponent of the N tom ove the plne of the three toms t z ¼ is shown s funtion of the pplie fiel. Re irles represent spiro B moleule with ll toms fixe, while the lk squres re for the se, where toms n move freely in the xy plne t z ¼. Definition of the ipole moment p n the eletri fiel e s inite in the moleulr moels. the voltge hysteresis, whih llows istility with n NFF rtio of the onutne swith of more thn two orers of mgnitue. Note tht every voltge pulse of 13 ms les to swithing n the finl stte of the juntion solely epens on the polrity of the pulse, whih mkes this juntion n iel memristor. The short swithing times inite n effiient eletri-fiel-riven swithing mehnism oppose to tht of urrent-inue swithing in spin-rossover memristors 22. Beuse the unerlying swithing mehnism oes not involve plsti eformtion, similr to the mehnil ontrol presente ove, the eletri fiel llows to swith etween two well-efine n istint onutne vlues. To theoretilly exmine these results, we first evlute the ipole moments of enzonitrile s well s of the gs phse relxtions of spiro A n B moleules presente in upplementry Fig. 8 n upplementry Note 4. The solute vlues of the ipole moments re shown in Tle 2, n the ngles F ip etween the surfe norml (z iretion) n the ipole moment in upplementry Tle 1. We fin tht solute vlues of the ipole moment of the spiro moleules re similr to those of enzonitrile 4. The ngle etween surfe norml n the moleulr he group is lose to the ngle etween the surfe norml n the ipole moment (for etils, see upplementry Tle 1), initing tht the ipole moment of the spiro moleules, inee, rises minly from the enzonitrile motive t its he. With regr to the influene of eletri fiels, we performe geometry optimiztions of the spiro B moleule in uniform, stti eletri fiel within DFT. We ssume tht the three sulfur toms of the moleule re lote in the xy plne t z ¼. In Fig. 5, we show the z height of the terminl N tom s funtion of the eletri fiel pplie long the z iretion, similr to the geometry shown on the sies in Fig. 5. Two ses of onstrine relxtion re istinguishe. In one se, the three toms in the legs re ompletely fixe, n in the other they re just 6 NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

7 NATURE CMMUNICATIN DI: 1.138/nomms14672 ARTICLE Current (pa) Current (pa) Bis voltge (mv) Bis voltge (mv) Bis voltge (mv) , 1,2 1,4 1,6 Current (pa) N N /(N FF N N ) ΔE (mev) Bis voltge (mv) N N /(N FF N N ) ΔE (mev) Current (pa) Energy FF ΔE = E FF E N e N p Tip position z (pm) δ δ Figure 6 Energy ifferenes etween the open n lose moleulr ontt. () ttistil onutne swithing of spiro A t is voltge of 2 mv. () Moel of oule-well potentil for N n FF sttes of the moleulr he group. () Moleulr moel for the two sttes inluing efinitions of the ipole moment p, the eletri fiel e n the ngle F. () ttistil swithing s funtion of pplie is voltge n time. In ition to the enlrge view, shown in, tht orrespons to the grey line in the mile, two more voltge intervls mrke in grey re shown in enlrge views (18 n 22 mv). The orresponing popultion of the N stte N N /(N FF þ N N ) is inite y re rosses, the resulting energy ifferene etween N n FF sttes erive from Boltzmnn sttistis is shown y lk rosses n the re line is liner fit to the t. (e) ttisti swithing reore t 6 mv s funtion of time n istne etween tip n smple, orresponing popultion of the N stte (lue rosses) n resulting energy ifferene etween N n FF sttes (lk rosses, liner fit in lue). Tle 2 Clulte ipole moments. Benzonitrile piro A piro A-rotte ring piro B 4.33 D 6.45 D 5.77 D 5.6 D onstrine to z ¼ n n move freely in the xy plne otherwise. While the hnges in height re somewht smller for the moleule with ll toms fixe, s expete ue to the less flexile geometry, the qulittive fetures re the sme for oth: the height of the N tom is reue for positive fiels n inrese for negtive ones. In perfet onsisteny with the experimentl finings, this n e interprete in terms of the NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

8 NATURE CMMUNICATIN DI: 1.138/nomms14672 lignment of the ipole of the nitrile group in the eletri fiel. We lso oserve tht the isplements of the N s ompre to its equilirium position t zero eletri fiel re slightly lrger t lrge negtive eletri fiel strength thn those t omprle ut positive one. This symmetry mens tht it is somewht esier to streth the moleule thn to ompress it. Deuing fores from sttistil onutne swithing. With the tip entrlly ple ove the moleulr he group, formtion n reking of the nitrilegol on les to swithing etween two metstle sttes. However, the short-rnge on energy n thus the energy rrier etween the two sttes re rstilly reue, if the tip is ple lterlly in front of the nitrile group (upplementry Fig. 1; upplementry Note 6). Then, the pplition of eletri fiels llows us to fine-tune the energy lne etween the two sttes of the moleulr juntion suh tht sttistil swithing etween the two sttes is oserve. Figure 6 shows n exmple of urrent tre, where the juntion sttistilly opens n loses. In ition to the rguments given in the esription of the voltge-inue ontt formtion experiments shown in Fig. 5, the very low urrents (o.1 pa in the FF stte) strongly suggest thermlly tivte swithing n exlue inelsti exittions s the origin of the swithing. Furthermore, these low-urrent ensities re unlikely to le to onsierle inrese of the temperture of the moleulr juntion. The therml popultion of the two sttes in the oulewell potentil (Fig. 6) is given y Boltzmnn sttistis: N FF ðeff ENÞ=k ¼e BT ; ð1þ N N with k B the Boltzmnn onstnt, E FF n E N the energy of the FF stte, n the N stte, respetively. At known temperture T, this reltion llows us to lulte the ifferene etween the energy levels E FF E N from the popultion of the two sttes. The energy lnspe of our system n e esrie y oule-well potentil forme y sum of three ontriutions: E tot ¼E tip þ E ipole þ E eform : ð2þ E tip is the effetive potentil for the intertion of the moleulr he group with the tip. The energy of n eletri ipole p in n eletri fiel e is given y E ipole ¼ jej jpj osf with E¼jj¼U=, e where U is the is voltge, the istne etween tip n smple, n F the ngle etween the ipole n the eletri fiel (Fig. 6). Plese note tht we follow the onvention of the ipole moment given y the Interntionl Union of Pure n Applie Chemistry (IUPAC) pointing from the negtive to the positive hrge, while in hemistry often the opposite efinition of the iretion of moleulr ipole is use. The energy for the elsti eformtion of the moleulr juntion n e pproximte to leing orer y E eform ¼.5k(z z ) 2,withk the effetive stiffness n z the height of the relxe moleule. Following this moel, we first nlyse the energy ifferene s funtion of the pplie is voltge, tht is, s funtion of the pplie eletri fiel. Figure 6 shows the time tre of the thermlly tivte sttisti swithing of the onutne, while the pplie voltge ws slowly swept from 5 to 35 mv. Three enlrge views re shown t is voltges roun 18, 2 n 22 mv (Fig. 6,). Clerly, the popultion epens on the is voltge. Averging the isriminte signl over intervls of 7.5 s results in smooth urve representing the voltge-epenent popultion of the N stte (N N /(N FF þ N N )), whih is then trnslte into the energy ifferene etween the N n the FF stte oring to eqution (2), tking T ¼ 5.2 K from the thermometer reing. It n e seen tht this energy ifferene sles linerly with the pplie voltge s is expete for the energy of ipole in n eletri fiel. This lerly shows the role of the eletri fiel n inites tht the trnsition is not use y the tunnelling urrent. Furthermore, the slope of ¼ 26 mev V 1 llows to estimte the hnge in tilt of the ipole moment to B17, when swithing etween the N (F N ¼ 48 ) n FF sttes (F FF ¼ 65 ; for etils see upplementry Note 12). The sme experiment ws performe on spiro B, where hnge in tilt ngle of F FF F N ¼ 7 ws foun (upplementry Fig. 18). Although the preise ngle is expete to epen on the tip position, this ifferene n intuitively e expline y the shorter n therefore more rigi ntilever he group of this vrint. In spite of the very low urrents involve, we nnot exlue tht the moleulr temperture iffers from the thermometer reing, whih woul influene the preise vlue of the tilt of the ipole. eon, we extrte the energy ifferene etween the open n lose moleulr juntion s funtion of the istne etween tip n smple tht llows to eue the involve fore. Figure 6e shows the orresponing time tre of the urrent for istne vrition of 14 pm for spiro A t onstnt voltge. As this isplement is negligile in omprison to the overll tipsmple istne, the eletri fiel n e ssume to e onstnt. imilr to the se of the voltge vrition, we oserve ontinuous trnsition of the popultion from the N stte to the FF stte with inresing istne etween tip n smple. The ounter-intuitive inrese in onutne while strething the moleulr juntion might e expline y relese of mehnil stress n relte hnges in the oritl level lignment tritly speking, this experiment etermines the energy ifferene etween the N n FF onfigurtions of the juntion s funtion of the z position of the tip, whih orrespons, following lssil mehnis, to the fore tht is neee to streth the lose moleulr juntion. The slope of the fitte stright line is.9k B T pm 1. Assuming temperture of T ¼ 5.2 K, this orrespons to.4 mev pm 1 whih is, in other units, fore of 64 pn (42 pn for spiro B). In simultions of the stiffness of the moleule, we foun vlues of the sme orer of mgnitue (9.7 pn pm 1 for spiro A n 18.6 pn pm 1 for spiro B, see upplementry Note 5 n upplementry Fig. 9). This les to hnge of the potentil energy y ±1.5 mev (±2.8 mev for spiro B) when ompressing or strething the moleulr juntion y 7 pm, in greement with our experimentl results. These stiffness vlues result in the experimentlly oserve fores when strething the moleule y 6.6 pm in the se of spiro A (2.3 pm in the se of spiro B). However, the stiffness n the elsti eformtion of tip, smple n in prtiulr the on etween the moleule n the tip were not onsiere in these simultions. The nitrilegol on is expete to lrgely influene the stiffness of the overll moleulr juntion, s it is the preetermine reking point. In ition, the on etween moleule n tip is wekene y the voltge pplie in the experiments. As the thermlly inue opening n losing of the moleulr juntion oes not involve hnge of the mrosopi tipsustrte istne, the fore erive in this wy oes not ontin relte ontriutions from vn er Wls fores tht n mount up to 2 nn (refs 12,14). Disussion In ontrst to typil AFM fore mesurements, only the energy vrition relte to the strething of the moleulr rige is involve in our experiment. Thus, the metho presente here llows to ress exlusively the tomi fores involve in the juntion formtion provie tht urrent-inue heting n e exlue. Given the low therml energy of.4 mev t temperture of 5.2 K, minute fores in the rnge of pionewtons n e extrte with high preision, ut lrger fores re out of reh ue to too few thermlly tivte trnsitions. 8 NATURE CMMUNICATIN 8:14672 DI: 1.138/nomms

9 NATURE CMMUNICATIN DI: 1.138/nomms14672 ARTICLE ur results highlight the importne of tomi-sle ontrol of the juntion geometry to otin relile mehnil n eletril properties in future moleulr evies. The very relile ontrol of the presente moleulr toggle swith llowe us to emonstrte new experimentl methos tht revel the sutle energy vritions uring ontt formtion on the nnosle n tht will e pplile to wie rnge of single-moleule juntions. Methos yntheses of the moel ompouns spiro A n spiro B. ur syntheti strtegy use for the preprtion of the 2,7,3,6 -tetrsustitute 9,9 -spiroifluorene tripol erivtives spiro A n spiro B is outline in upplementry Fig. 1. The first moulr pltform se on rigi 9,9 -spiroifluorene with three etyl protete thiol groups in the positions 2, 3 n 6 exposing pr-ynophenylethynyl ro in the position 7 (spiro A) ws reporte reently 27. ur syntheti pproh is se on metl-hlogen exhnge retion of the 2-iooiphenyl erivtive n its susequent retion with 2,7-isustitute fluoren-9-one to ffor the rinol. Further eletrophili yliztion n seprtion of regioisomers provie the orresponing 2,7,3,6 -tetrsustitute 9,9 -spiroifluorene 1 s the key intermeite. This previously prepre spiroifluorene erivtive 1 (ref. 27) ws lso employe for susequent synthesis of shorter erivtive spiro B. To introue the thir lkylsulfnyl group, spiroifluorene erivtive 1 ws trete with 2-(trimethylsilyl)ethnthiol in the presene of P 2 () 3, Xntphos n Hünig s se to provie the esire tripol spiroifluorene 2 in 75% yiel (upplementry Note 1). usequent yntion of romo erivtive 2 ffore nitrile erivtive 3 in 79% yiel (upplementry Note 2). Finl trnsprotetion of the thiol ws suessfully performe using AgBF 4 n etyl hlorie in ihloromethne to otin the esire tripol thioette spiro B in 95% yiel (upplementry Note 3). The presene of terminl thioette groups llows the trget spiroifluorene moleules spiro A n spiro B to in to gol surfes. The etyl works s lile thiol-proteting group, n n milly n effiiently e leve y therml nneling upon eposition of moleules on gol surfe. All strting mterils n regents were otine from ommeril suppliers n use without further purifition. Thin-lyer hromtogrphy ws performe on ili gel 6 F 254 pltes, spots were etete y fluoresene quenhing uner ultrviolet light t 254 nm n/or stining with pproprite solutions (nislehye, phosphomolyi i, KMn 4 ). Column hromtogrphy ws performe on ili gel 6 (.4.63 mm). All experimentl mnipultions with nhyrous solvents were rrie out in flme-rie glsswre uner inert tmosphere of rgon. Degsse solvents were otine y three yles of the freezepumpthw. Dioxne ws rie n istille from soium/enzophenone uner rgon tmosphere. Dihlormethne ws rie n istille from CH 2 uner rgon tmosphere.,, -[7-(4-Cynophenylethynyl)-9,9 -spiroifluorene- 2,3,6 -triyl] tris(thioette) (spiro A) 27 n 2-romo-7-ioo-3,6 - is[2-(trimethylsilyl)ethylsulfnyl]-9,9 -spiroifluorene (1) were prepre oring to the pulishe proeure 27. All nuler mgneti resonne (NMR) spetr were reore t 25 C incdcl 3. 1 H NMR (5.16 MHz) spetr were referene to the solvent resiul proton signl (CDCl 3, H ¼ 7.24 p.p.m.). 13 C NMR ( MHz) spetr with totl eoupling of protons were referene to the solvent (CDCl 3, C ¼ p.p.m.). For orret ssignment of oth 1 Hn 13 C NMR spetr, the 1 H 1 H orreltion spetrosopy (CY), 13 C DEPT-135, heteronuler single quntum oherene (HQC) n heteronuler multiple on orreltion (HMBC) experiments were performe. Eletron ioniztion mss spetr (EI M) were reore with gs hromtogrphy mss spetrometry (GC/M) instrument (smples were issolve in iethyl ether, hloroform or introue iretly using iret injetion proes (DIP) n iret exposure proes (DEP)), n m/z vlues re given long with their reltive intensities (%) t n ionizing voltge of 7 ev. Infrre spetr were mesure in KBr pellets. Anlytil smples were rie t 41 C uner reue pressure (1 2 mr). Elementl nlyses were otine using n elementl nlyser. NMR spetr of the ompouns 2, 3 n spiro B re isplye in upplementry Figs 27. Eletroni struture lultions. Atomisti simultions re performe to support n help interpret the experimentl oservtions. Eletroni struture lultions n relte geometry optimiztions re rrie out y pplying the quntum hemistry softwre pkge TURBMLE 6.6 (ref. 44). All the lultions were performe within DFT using the exhnge-orreltion funtionl perew urke-ernzerhof (PBE) 4548 n the ef-v(p) sis set 4951, whih is of split-vlene qulity with polriztion funtions on ll non-hyrogen toms. To nlyse the eletron trnsport through the spiro moleules on n Au(111) surfe (Fig. 3), we followe the proeure esrie in refs 52,53. The moleulr geometry ws first optimize on the surfe, keeping ll of the gol toms fixe. usequently, the TM tip ws onnete to the he of the moleule. The Au pyrmi, hosen s the TM tip, n the surfe represent semi-infiniteeletroes.theyreorientelongthe (111) iretion, whih oinies with the trnsport iretion or z xis. The theoretil investigtion of the quntum trnsport properties of omplex moleules remins hllenging euse of the lrge numer of toms involve n the infinite, non-perioi geometry of the system. DFT is one of the few initio eletroni struture methos tht n hnle the hyri metlmoleulemetl ontts. n the other hn, ue to self-intertion errors in the stnr exhnge-orreltion funtionl n missing imge hrge effets, DFT-se methos hve iffiulties to urtely esrie the energy gp n level lignment of moleules t surfes. This n e improve y ing self-energy orretion, resulting in the DFT þ metho The min ifferene in the eletroni struture etween DFT n DFT þ for given ontt geometry is typilly pronoune inrese of the gp etween the highest oupie moleulr oritl n lowest unoupie moleulr oritl y severl evs tht rise from (lmost symmetri) erese of the oupie energy levels n inrese of the unoupie energy levels with respet to the Fermi energy E F of the metl eletroes (see Fig. 3 n its isussion). Detils regring our DFT þ implementtion n e foun in ref. 35, n for more informtion on our prtiulr DFT þ lultions, see upplementry Note 7 n upplementry Fig. 11. Using the eletroni struture of DFT n DFT þ s input, the trnsmission t(e) s funtion of energy is lulte using the Lnuer-Büttiker formlism expresse in terms of nonequilirium Green s funtions 32. In the Lnuer- Büttiker pproh, the sttionry urrent is given s I¼ 2e Z 1 E tðeþðfðe m h L Þ fðe m R ÞÞ: ð3þ 1 For the liner onutne one otins G¼ I Z 1 V ¼ 2e2 E f ÞtðEÞ: ð4þ V¼ h TM experiments. TM mesurements were rrie out in home-uilt TM in UHV t temperture of 5.2 K. The TM tip ws prepre y hemil ething of tungsten wire n y repete ipping into the gol surfe. The Au(111) single-rystl surfe ws lene y severl yles of Ar þ sputtering n nneling to 7 K. piroifluorene moleules were sprye onto the surfe through pulse vlve t pressure of B1 mr. After the eposition, the smple ws trnsferre into the UHV hmer, nnele t 41 K for 1 h n susequently trnsferre to the TM. The voltge is pplie to the smple. Dt vilility. 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Lett. 77, (1996). 49. häfer, A., Horn, H. & Ahlrihs, R. Fully optimize ontrte Gussin sis sets for toms Li to Kr. J. Chem. Phys. 97, (1992). 5. Eihkorn, K., Treutler,., Öhm, H., Häser, M. & Ahlrihs, R. Auxiliry sis sets to pproximte Coulom potentils. Chem. Phys. Lett. 24, (1995). 51. Eihkorn, K., Weigen, F., Treutler,. & Ahlrihs, R. Auxiliry sis sets for min row toms n trnsition metls n their use to pproximte Coulom potentils. Theor. Chem. A. 97, (1997). 52. Weer, H. et l. Eletroni trnsport through single onjugte moleules. Chem. Phys. 281, (22). 53. Bürkle, M. et l. Conution mehnisms in iphenyl-ithiol single-moleule juntions. Phys. Rev. B 85, (212). Aknowlegements We knowlege the support y the Reserh Network Funtionl Nnostrutures of the Ben-Württemerg-tiftung n y the Helmholtz Reserh Progrm TN (iene n Tehnology of Nnosystems)..G.B. n F.P. grtefully knowlege finnil support y the Germn Reserh Fountion (DFG) through the Collortive Reserh Center (FB) 767 n the Crl-Zeiss Fountion. The simultions were supporte y the whpc progrm through the omputtionl resoures wunicluster n the JUTU HPC fility. We thnk Mgnus Pulsson for inspiring isussions n initil omputtionl moelling. We knowlege support y Deutshe Forshungsgemeinshft n pen Aess Pulishing Fun of Krlsruhe Institute of Tehnology. Author ontriutions M.L., M.M., L.G. n W.W. oneive n esigne the experiments. M.V. n M.M. synthesize the moleules. L.G., J.H. n K.E. rrie out TM experiments..g.b. n F.P. performe the moelling n initio simultions. L.G., J.H. n K.E. nlyse the t. L.G., W.W., F.P. n M.M. o-wrote the pper. All uthors isusse the results n ommente on the mnusript. Aitionl informtion upplementry Informtion ompnies this pper t ntureommunitions Competing finnil interests: The uthors elre no ompeting finnil interests. Reprints n permission informtion is ville online t reprintsnpermissions/ How to ite this rtile: Gerhr, L. et l. An eletrilly tute moleulr toggle swith. Nt. Commun. 8, oi: 1.138/nomms14672 (217). 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