Single‐Molecule Conductance Studies of Organometallic Complexes Bearing 3‐Thienyl Contacting Groups

Abstract The compounds and complexes 1,4‐C6H4(C≡C‐cyclo‐3‐C4H3S)2 (2), trans‐[Pt(C≡C‐cyclo‐3‐C4H3S)2(PEt3)2] (3), trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2(dppe)2] (4; dppe=1,2‐bis(diphenylphosphino)ethane) and trans‐[Ru(C≡C‐cyclo‐3‐C4H3S)2{P(OEt)3}4] (5) featuring the 3‐thienyl moiety as a surface contacting group for gold electrodes have been prepared, crystallographically characterised in the case of 3–5 and studied in metal|molecule|metal junctions by using both scanning tunnelling microscope break‐junction (STM‐BJ) and STM‐I(s) methods (measuring the tunnelling current (I) as a function of distance (s)). The compounds exhibit similar conductance profiles, with a low conductance feature being more readily identified by STM‐I(s) methods, and a higher feature by the STM‐BJ method. The lower conductance feature was further characterised by analysis using an unsupervised, automated multi‐parameter vector classification (MPVC) of the conductance traces. The combination of similarly structured HOMOs and non‐resonant tunnelling mechanism accounts for the remarkably similar conductance values across the chemically distinct members of the family 2–5.


Introduction
The development of ar ange of complementary and relatively facile methods for the measurement of the electrical properties of single molecules hass een ar enaissance in the fieldo fm olecular electronics. [1][2][3][4] The continued progress of the area from fundamental science towards technology now rests on an umber of key issues, [5] among which are the reliable contactingo fm olecules within ajunction, [6,7] the reduction in electronic variationbetween individual junctions [1,[8][9][10][11][12] and the optimisationo ft he transport properties of these junctions. [13][14][15][16] To these ends, considerable effort is being madet oe xplore the effectso ft he electrode-molecule contact groups and structure of the contact, [17][18][19][20] the potential applications of non-metallic electrodes to create an "all-carbon" molecular electronic device platform, [21] as well as the backbones tructure of the molecular component on the electrical properties of the junction. [22][23][24] Althought he majority of single-molecule and thin-film junctions studied to date have been based on organic molecules, such as alkanes, [25,26] oligo(arylene)ethynylenes [27][28][29] and polyynes, [30][31][32][33] metal complexesh ave also been recognised as potential components in af uture molecular electronics technology. [34][35][36][37][38] Metal complexes offer ar ange of potential advantages over structurally and electronically simpler organic molecules, including redox activity and aw ider range of readily accessible and systematicallyv ariable spin-states and magnetic properties, [39][40][41][42] diversity of molecular structure and potential for modular constructiont hrough in situo r" on surface" coordination chemistry, [43][44][45][46] bettera lignment of the frontierm olecular orbitals with the Fermi level of the (usually metallic) junction electrodes, [8,[47][48][49][50] as well as high thermoelectric efficiency. [51] In some earlier studies of organometallic complexes in molecular electronics, the complex trans-[Pt(CCC 6 H 4 SAc) 2 (PPh 3 ) 2 ] was assembled within am echanicallyc ontrolled break-junction (MCBJ)b ased molecular junction. From the resulting I/V curves,c ollected over ab ias range of AE 5V ,aresistance of 5-50 GW (i.e., G = 0.2-0.02 nS;2 0-2 10 À6 G 0 )w as estimated at the extremes of the bias range, some three orders of magnitude less conductive than similarly contacted organic oligoarylene systems. [52] This "insulating" behaviour,e ven under such ah igh applied bias, was ascribed to the largely s-type PtÀ C(sp) bonds in the CCÀPtÀCCb ackbone, although it is clear that at this bias voltage, the conductance mechanism is likely to be field emission rather than tunnelling. [53] In contrast, al ater study with af amily of complexes of type trans-[Pt(C CC 6 H 4 SAc) 2 (L) 2 ]( L = PCy 3 ,P Ph 3 ,P (OEt) 3 )i nc rossed-wire junctions at more modest bias (up to 1V)r evealed at wo-to threefold higher conductance than 1,4-(4-AcSC 6 H 4 CC) 2 C 6 H 4 ,w hich was ascribed to the shorters ulfur-sulfur distance in the metal complexes. [54] In seekingt oe nhancet he wire-like response, significant attention was turned to ruthenium bis(alkynyl) complexes, which are generallyt hought to offer more significant d-p orbital mixing in the occupied frontier molecular orbitals. [9,34,50,[55][56][57] The thioacetate complex trans-[Ru(CCC 6 H 4 SAc-4) 2 (dppm) 2 ] (dppm = 1,1-bis(diphenylphosphino)methane)h as been assembled into monolayersa nd studied within as canning tunnelling microscope break junction (STM-BJ), with ac omparison made to the oligo(phenyleneethynylene) (OPE) compound 1,4-(4-AcSC 6 H 4 CC) 2 C 6 H 4 as abenchmark. Extrapolationtosingle-molecule conductances gave values of 19 AE 7nS( ca. 2.5 10 À4 G 0 ) for the ruthenium complex and 3.6 AE 2.0 nS (ca. 4.6 10 À5 G 0 ) for the OPE. The higherc onductance has been attributedt o both the shorter molecular length andt he extensive Ru(d)ÀC C(p)m ixingi nt he metal complex. [56] These conceptsh ave been extendedt oo ther examples of organometallic wires based on group 8m etal centres andt he trans-bis(alkynyl) motif, with topics of interesti ncluding the exploration of surface contacting groups, [9,50] the inclusion of multiple metal centres along the molecular back-bone [35,47,57] and electronic func-tion beyond that of as imple wire, such as charge storagea nd gated transistor-like response. [41,58] One particular advantage of organometallic complexes within the field of molecular electronics lies in the ability to systematically alter the molecular structures of these systems with af air degree of synthetic ease, which permits am odular approacht om olecular designs and as ystematic search for structure-property relationships. In seekingt of urthere xplore the electrical properties of oligophenyleneethynylene (OPE), and trans-bis(alkynyl) complexes of platinum and ruthenium, we have turned to such as ystematic study here. Here, the 3thienyl moiety, [59][60][61] which is readily introduced into both organic and organometallic structures, is used as ac ontacting group for the ready attachment of organic,r uthenium and platinum-based organometallic complexes within Au j molecule j Au junctionsa nd electrical characterisation by using both the I(s) [62] (measuringt he tunnellingc urrent (I)a safunctiono f distance (s)) andS TM-BJ [63] methods. The conductance results are interpreted with the aid of DFT level calculations and junction simulations.
The general structuralf eatures of trans-[Ru(CCR) 2 (dppe) 2 ] complexes have been summarised recently, [65] and compound 4 offers some points worthyo fb rief comment.T he structure is composed of two independent molecules, both of which are situated on crystallographic inversion centres and which differ in the orientation of the thiophene group. The dihedral angles between the thiophene plane and the plane containing the Ru and C(n1) atomsa nd the midpointso ft he two ligand Pa toms are 3.48 for molecule 1, and 96.3 and 77.88 for the two compo-    2 ]i n which the aromatic rings sit close to the "privileged" orientations that allow maximum d-p conjugation along the molecular backbone, and the first in whichb oth conformational isomers are observed for the same chemicalcompound.T he comparable bond lengths within the two molecules are essentially indistinguishable,w ith the possible exception of the most precisely determined RuÀPb ond lengths, which appear to be marginally shorteri nm olecule 2.
Although the mixed ligand vinylidene-acetylidec omplex [Ru(CCPh){C=C(Me)Ph}{P(OEt) 3 } 4 ][CF 3 SO 3 ]h as been structurally characterised, [66] compound 5 appearst ob et he first structurally characterisedb is(acetylide)d erivative of Ru(CCR) 2 {P(OR) 3 } 4 . In the crystal,t he tetrakis(triethylphosphite)d erivative 5 is situated on ac rystallographic " 4a xis so that there is only one unique phosphite group.T he dihedral angle between the two thiophene groups (which are disordered about the crystallographic twofold axis) is therefore 908.T he RuÀP( 2.3149(3) ) and RuÀC(1) (2.0592(15) )d istances in 5 are shorter than in the mixed vinylidene-acetylide derivative( RuÀP2 .341(3)-2.350(2) ;R u ÀC2 .114(8) )r eflecting the increased electron density at Ru in 5 andi ncreased RuÀPa nd RuÀCb ack bonding. Although back-bonding plays only am odest role in the bondingo fm etal-alkynyl complexes, [69] the notion is also supported by the trends in CCb ond lengths in 5 (1.221 (2)  Single-molecule conductance: STM-BJ and I(s) Single-molecule conductance measurements werec arried out by using substrates with al ow surface coverage of the mole-cules of interest on gold substrates. Low surfacec overage was chosen to minimise the formationo fm ulti-molecule junctions and promote formation of single-molecule events.A dsorption of 2-5 at low surface coverage was achieved by immersion of ag old-on-glass substrate in CHCl 3 solutions of the analyte (1 mm)f or about 80 s. After adsorption, the samples were washed in ethanol and then blown dry in as tream of nitrogen. All in situ I(s)a nd STM-BJ measurements were conducted in mesitylene, an on-polar solvent commonly used in STM-based single-molecule electrical measurements because of its high boilingp oint and relativelyl ow vapour pressure. For ag iven set-point current and bias voltage, typically6 000-7000 events were observed in both the STM-BJ and I(s)e xperiments.
Ta king compound 2 as ar epresentative example, without any data selection, it is rather difficult to assign ac onductance value to the data ( Figure 4). Possible reasonsf or this include al ow junctionf ormation probability and short plateau features. The one-dimensional (1D) conductance histogramo ft he whole data set shows only af aint shoulder,t hat is, ac onductance peak partially obscured by the exponentialb ackground ( Figure 4, left). Matching the peak, af aint plateau feature can be seen in the 2D conductance histogram ( Figure 4, right), but there is an eed for data selection for this system to increase signal-to-noise ratio of the data.
Data selectionc an be made manually,byu sing arational criterion,f or example by selecting traces with ac urrent plateau that exceeds 0.1 nm in length,a nd disregarding those without. However,asmanual data selection can never be fully objective, it is of interestt oc ompare the resultsa gainst an automated data selection approach. Here, the unsupervised, automated multi-parameter vector classification (MPVC) has been adopted to verify the conclusions reached from the manually sorted data. [70] By way of example, the data for molecule 2 are analysed in more detail in the following paragraphs, which compare the results of manuala nd automated data selection methods.
For the MPVC, an exponentially decaying current-distance trace was created asareference vector, R (I 0 = 30 nA, b = 0.5 À1 ). Three vectorp roperties (classifiers) were then calculated for each I(s)t race with respecttot he reference:  and I(s)t race; 2) q:t he angle,b etween R and ÀY; 3) h r :t he reduced Hamming distance, with h r being the number of component changes to transform the reduced distance vector, Y,i nto the reduced reference vector, R.I n this context, "reduced" meanst hat every vectore lement is divided by its absolute value, so that the resulting vector consistsonly of 1, 0a nd À1.
The whole data set, consisting of 3838 I(s)t races is thus transformed into 3838vectors in three-dimensional space (see cylinderp lots in the Supporting Information). Here, similar traces, for example, traces with plateaus, are in close proximity to each other.P lain exponential and plateau-containing traces are expected to form distinctc lusters in this representation and fuzzy c-meansc lustering (FCM) was then used to assign the cluster membership. [71,72] Note that the total number of clusters k was selected to be two in this case, to account for plain exponentiald ecays and molecular events, but can be chosen to be ah ighern umber if any clusterc onsists of subclusters (e.g.,t oa ccount for av ariety of different junction geometries).
The results of the unsupervised algorithm approachs how excellent agreement with the data selectedo nt he basis of the 0.1 nm plateaul ength criterion described above in terms of conductances, but with understandable differences in terms of number of selectedt races. With respectt ot he latter,s ome 81 (37.3 %) of the plateau traces in cluster1 were also marked as plateau-containing during the hand-sorting process. Also, 17.6 %o ft he manually selected traces were found by the clustering algorithm. The traces found both by MPVC and hand selection are predominantly long plateaus aroundt he most probable conductance value. In addition, 135 traces were included by the MPVC algorithm but not by hand sorting. These traces contained plateausa tv arious conductance values or unconventional features, meaning deviations from the plain exponentiald ecay other than plateaus. These can possibly originate from different molecular processes during junctionf ormation (or rupture)o rn oise features. In contrast, some 377 traces were only marked as plateau-containing during hand selection and not by MPVC. Mostly,t hose were traces with very short plateau features, or longer plateaus in exponentialt races with large decay coefficients. Such features can arise from changes in the molecular junction geometry during the tip retraction process.
After MPVC analysis, cluster 1e xhibits ac onductancep eak around0 .41 10 À4 G 0 ( Figure 5). Hand sorting gives am ost probablec onductance of 0.42 10 À4 G 0 (Figure 7). This indicates that although there are differences in the curve selection betweenhand sorting and automated sorting, the most probable conductance of both data selection methodss hows excellent agreement.
While the non-contact I(s)t echnique favoursl ow conductance groups, [1] the STM-BJ method generally leads to agreater propensity of higher conductance values. Thesed ifferences can be explained in terms of the way in which the junctions are formed in both methods. In the I(s)m ethod, the (typically gold) STM tip is brought into close proximity of the surface to encourage molecular junction formation, but withouta ny initial contact between the STM tip and substrate. In contrast, in the STM-BJ technique, the STM tip is fused (orc rashed) into   the substrate and withdrawn to give am etallic filament between the tip and the substrate. Molecular junctions form immediately after the Au-Au pointc ontactb reaks. [1] The rough or fractal nature of these cleaved gold contact junctions often leads to av ariety of conductance featuresi nS TM-BJ-based metal j molecule j metal junctions formed from common anchoring groups such as thiol, [1,73] carboxylic acid [74] or pyridine [75] where in each case more than one single molecule conductance value hasb een reported, and attributed to differing contact morphologies between the contacting groups and the gold electrode(s).
Data recorded by using the STM-BJ technique for compound 2 at U tip = 0.6 Vare summarized in Figure 8. As shown, the conductance profile from these STM-BJ data for 2 shows only one conductance group (labelled H, for high conductance group). Its mostp robable conductance ((2.83 AE 0.65) 10 À4 G 0 ,T able 1) is in good agreement with that reported by van der Zant et al. by using the mechanically controlled break-junction (MCBJ) technique (4 10 À4 G 0 ). [59] As shown in Figure 8a nd Table 1, distinct conductance groups were also obtained for the metal complexes 3-5 by using the I(s)( L group, forl ow conductance group) and the STM-BJ (H group) method. Interestingly,t he compounds 2-5 conductance values differ by af actor of about two for the L group, whereas this factor is lower for the Hg roup (Table 1).

Quantumc hemical modelling
In the quest to betteru nderstand the conductance behaviour, the electronic properties of the molecules and electrical behaviour of the junctionsh ave been investigated by using DFTbased methods. Initial studies of the electronic structures of 2-5 were carriedo ut at the B3LYP level of theory [76] with the LANL2DZ basis set used form etal atoms (Ru,P t) [77] and the 6-31G** [78] basis set for all other atoms to explore the influence of the central fragment (C 6 H 4 (2), [Pt(PEt 3 ) 2 ]( 3), [Ru(dppe) 2 ]( 4), [Ru{P(OEt) 3 } 4 ]( 5)) on the distribution andc omposition of the frontier molecular orbitals. Plots of the HOMOs are given in Figure 9, and plots of the LUMOs are given in the Supporting Information.
The organic compound 2 again providesaconvenient point to commence discussion and ab asis for comparison of the metal complexes 3-5.U nsurprisingly,t he lowest energy structure features ac o-planar arrangement of the thienyl and 1,4phenylene rings, with the frontier orbitals distributed almost evenly acrosst he molecular backbone, making al inear, p-type conjugated pathway between the two sulfur atoms. For the platinum complex 3,t he lowest energy identified minimum  featured the thienyl moieties lying perpendicular to the square plane defining the coordination geometry at the metal centre (perp-3). However,asecond minimum, barely 0.8kcal mol À1 higher in energy,i nw hich the thienyl moieties lie in the same plane as the metal coordination sphere (planar-3)w as also identified. Again, the HOMOso ft hese complexes are p-type and delocalized over the molecular backbone, and feature as mall but important metal contribution (perp-3,1 0% Pt; planar-3,1 9% Pt). The LUMOs are more metal in character (perp-3,4 2%;p lanar-3,28%)a nd rather delocalized in the case of planar-3. The ruthenium complexes 4 and 5 offer HOMOst hat are similarly structured to those described for 2 and offer only marginally more metal character than planar-3 (4,3 3% Ru; 5, 24 %R u). The LUMO of 4 is largely of metal/dppec haracter,i n the case of the phosphitea nalogue 5 the LUMO is thienyl-p* in character,w ith the unoccupied metal orbital lying slightly (ca. 0.04 eV) higherine nergy.
To provide further insight into the experimentally observed trends, and to better evaluate the properties and behaviour of these molecular junctions, calculations using ac ombination of DFT (the SIESTAc ode) [79] and an on-equilibrium Green's function formalism were also carried out. For the transport calculations, eight layerso f( 111)-oriented bulk gold with each layer consisting of 6 6a toms and al ayer spacingo f0 .235 nm were used to create the molecular junctions as shown in Figure 6, and described in detail elsewhere. [80] These layers were then furtherr epeated to yield infinitelyl ong current-carrying gold electrodes. Each molecule was attached to two (111)d irected pyramidal gold electrodes. The molecules and first layers of gold atoms within each electrode were then allowed to relax again, to yield the optimal junction geometries shown in Figure 10. From these model junctions, the transmission coefficient, T(E), was calculated by using the GOLLUM code. [80] Akey factor governing the conductance of amolecular junction is the positiono ft he Fermi level of am etal electrode with respectt ot he molecular HOMO and LUMO levels. In turn, this energy alignment is sensitive to not only the chemical nature of the contacting groups that bind the molecule to the electrode, but also the precise configuration of the metal electrode-molecule contact. [15,81] However,i ti sw ell knownt hat the Fermi energy predicted by DFT (E F DFT )i so ften not reliable, [33] and as such the room-temperature electrical conductance G was computed for ar ange of Fermi energies E F .T he calculated conductances G are plotted as functions of E F ÀE F DFT in Figure 11,w hich reveals imilar conductance values over aw ide range of Fermi energies, between À0.4 eV to + 0.4 eV relative to the DFT-predicted value. The predicted conductance  values of all molecules were compared with the experimental values and as ingle common value of E F was chosen, which gave the closest overall agreement. This yielded as mall correction of E F ÀE F DFT = À0.075 eV,w hichh as been used in all of the theoretical results described below.T hygesen and colleagues have discussed similars ituations for C 60 -contacted molecular wires, and have shownt hat critical molecular orbitals can become pinned close to the Fermi level owing to partial charge transfer,l eadingt og ood quantitativea greement between calculated and experimentally determinedc onductance. [82] The experimental data, now interpreted with the aid of Figure 11,i ndicates that in all cases the Fermi level lies close to the centre of the HOMO-LUMO gap, but shifteds lightly towards the HOMO resonance, and therefore aH OMO-mediated hole tunnelling mechanism is anticipated in each case. [47-50, 83, 84] However,i nc ontrast to the studies of Wang, [56] Rigaut [57] and Mayor [52] with organic, ruthenium and platinum bis(alkynyl) compounds and complexesc ontacted into molecular junctions by thiolateg roups, conductance valueso nly differing by af actor of % 2a re obtaineda cross the thienyl-contacted series 2-5.T his lacko fv ariation occurs because althought he HOMO and LUMO transport resonances differ significantly between molecules 2-5,t ransport in the vicinity of the middle of the HOMO-LUMO gap is similar for all molecules (Figure 11).
To further explore the reasonsf or the small differencesi n conductance across the series,t he nature of the moleculegold contact was also examined. Table 2s ummarises the molecule-gold interaction in terms of the number of valence elec-trons (Q I )a ssociated with the molecule, the number calculated on the molecule in the junction (Q MG )a nd hence the number of electrons associated with the thienyl SAu contacts (G)( or "bonds") based on calculated Mulliken charges. Mulliken charges are basis-set-dependent mathematical constructions and therefore only approximately coincide with the physical charge on am olecule. However,i ti sc lear from the data in Ta ble 2 that the value of G,a nd the nature of the contact, is only weakly dependent on the nature of the backbone and auxiliary ligandsi n2-5.O verall, the molecular conductances of these molecules are similar, with minor variations arisingt hrough convolution of the strength of the S!Au bond, and the position of the tail of the HOMO resonance relative to the Fermi level of the electrodes (Table2).

Conclusions
The family of 3-thienylethynyl contactedc ompounds1 ,4-C 6 H 4 (CC-cyclo-3-C 4 H 3 S) 2 (2), trans-[Pt(CC-cyclo-3- The MPVC method has been appliedt ov erify the lowest conductanceg roup in an algorithmically definable fashion. For the 3-thienyl contact employed here, the conductance values obtained by using MPVC and manuald ata selection were very similar, although there were some differences between thecurrent-distance data sets assignedb ye ach method. The MPVC method,w hich allows reproducible and objective analysis of conductance features close to the limit of the current amplifier, is therefore ap romising avenue fort he furthere xploration of low conductance features.I na ddition, with an increasei nt he number of sub-clusters the method should also prove useful in the analysiso faw ider array of junction configurations or in cases where the junction evolves over time or with distance. Further effortst od evelop and exploit the MPVC toola re now underway.Aquantum chemical analysis of the electronic structures of the isolated molecules reveals as imilarly structured HOMO in each case. Within model junctions, the Fermi level lies slightly towards the HOMO resonance in each case, and  Molecule an on-resonant hole tunnelling mechanism mediated by the similarly structured HOMOs is proposed. The positioningo ft he Fermi level well within the HOMO-LUMO gap is proposedt o account for the similarc onductance behavioura cross the series. Our study demonstrates that although for somes ystems, platinum complexes may well be less conductive than purely organic analogues or similarly structured complexes of the group 8m etals,t his is not au niversal situation, andb ya ppropriate use of contacts and ancillary ligandst op osition key molecular orbitals with respect to the Fermi levels of the electrodes, rather efficient molecular wires may be engineered. For the future,i tw ill be of interest to study thermal transport through such wires,a sa lthough they have similar electrical properties, their vibrational properties and phonon thermal conductances are likely to differ significantly.T his ability to tune the latter,w hile preserving electronic conductance is an attractive proposition for thed esign of thermoelectric thin films. [85] Experimental Section Crystal and refinement data The thiophene group on molecule 2i sd isordered over two sites with occupancies constrained to 0.5 after trial refinement. Geometries were restrained to ideal values. Both dichloromethane solvent molecules are disordered about crystallographic inversion centres.

Single-molecule conductance measurements
All single-molecule conductance measurements were recorded at room temperature in mesitylene with an Agilent 5500 SPM microscope. Molecular adlayers were formed on flame-annealed gold on glass samples, purchased from Arrandee, Germany.T hese commercially available substrates were rinsed with acetone and flame-annealed carefully for about 20 sw ith ab utane torch until as light orange glow was obtained. This flame-annealing procedure was performed three times and generally resulted in relatively large area flat Au(111)t erraces. [86] Gold STM tips were fabricated from 0.25 mm Au wire (99.99 %), which was freshly anodically electrochemically etched at + 2.4 Vf or each experiment in am ixture of ethanol (50 %) and HCl (50 %). Single-molecule electrical measurements were performed by using both the in situ break-junction (BJ) and I(s)m ethods. The in situ break-junction method developed by Xu and Ta or elies on the formation and cleavage of metallic break junctions between the STM tip and the underlying gold substrate. [63] Such metallic break junctions are formed by forcing the STM tip ac ertain distance into the gold substrate. The STM tip is then retracted until the gold-gold contact breaks, which leaves an open nanoscale gap into which the molecular targets can adsorb. These molecular bridges then cleave upon further retraction of the STM tip and molecular conductance can be determined by monitoring the current versus distance retraction profiles.
In the I(s)t echnique, ag old STM tip is brought to af ixed distance, determined by the set point conditions, above the gold surface covered with the target molecule under analysis. [62] Direct metalto-metal contact between the STM tip and substrate is avoided. The initial approach distance of the STM tip to the substrate surface is controlled by the bias voltage and set-point current (I 0 ). The measurement involves first locating the STM tip close to the gold substrate at ag iven height by setting the I 0 and V bias values. The feedback loop of the STM is then temporary disabled and the STM tip is rapidly retracted (s = distance) while the tunnelling current (I) is continuously recorded. At the initial set-point conditions, the target molecules can be trapped between the STM tip and the gold substrate as am olecular bridge. In such circumstances, during the retraction of the STM tip, the molecular bridge is then pulled up and stretched in the nanojunction until the molecular junction is cleaved. For both the BJ and the I(s)methods, when the molecular bridge is formed and then cleaved, ac haracteristic current plateau is typically observed, with as tep-like drop in the current reflecting cleavage of the molecular bridge. . P. C. and S.M. are gratefulfor financial assistancefrom Ministerio de Economia yC ompetitividadf rom Spain and fondos FEDER in the framework of projects CTQ2012-33198 and CTQ2013-50187-EXP. S.M.a nd P. C. also acknowledgeD GA and fondos FEDER for fundingt he research group Platón( E-54).