Covalent Immobilization of Redox-ActiveFe(κ2‑dppe)(η5‑C5Me5)‑Based π‑Conjugated Wires on Oxide-FreeHydrogen-Terminated Silicon SurfacesKaty Green, Nicolas Gauthier, Hiba Sahnoune, Jean-Francois Halet,* Frederic Paul,* and Bruno Fabre*

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex,France

*S Supporting Information

ABSTRACT: Several redox-active Fe(κ2-dppe)(η5-C5Me5) arylacetylide complexes (dppe = 1,2-bis(diphenylphosphino)ethane)featuring a pendant ethynyl (1b−d and 2) or ethenyl (3) group have been grafted on oxide-free hydrogen-terminated silicon(Si−H) surfaces through a covalent interfacial Si−C bond. They form densely packed redox-active monolayers. The charge-transfer process between the terminal redox center and the underlying silicon interface was subsequently studied by cyclicvoltammetry. The latter turned out to be strongly dependent on the nature of the spacer linking the organometallic end groupsto the silicon surface, the highest charge-transfer rates being obtained for monolayers anchored through conjugated andunsaturated spacers. Although the rates measured were among the highest values obtained for redox-active systems grafted to Si−H surfaces, this study nevertheless suggests that the electron tunnelling is not entirely controlling the interfacial charge-transferprocess for the shorter linkers tested. In this respect, strategies to improve further the charge-transfer kinetics of the producedredox-active films are briefly discussed.

■ INTRODUCTION

The modification of conducting surfaces at the molecular levelwith redox-active “building blocks” constitutes a powerfulapproach to design integrated systems devoted to informationstorage or transfer.1−3 Indeed, functional molecular materialsare playing an ever-increasing role in the fabrication of smartintegrated devices that can perform inter alia logic oper-ations.4−6 Thus, several families of redox-active carbon-richgroup 8 σ-alkynyl complexes have been identified as interestingcandidates for reversible charge storage at the molecular level7

and were shown to present promising prospects for theelaboration of molecular-based electro-switchable devices.8 Forinstance, after the seminal publications of Fehlner1 and Blum,3

several researchers have now independently established thatboth homo- and heteropolynuclear representatives of this classof molecules can be anchored on (semi)conducting supportssuch as halogenated silicon1 or on gold9 surfaces and shownthat a better interfacial conduction takes place throughorganometallic junctions than through purely organic ones ofsimilar length. Semiconducting surfaces, such as doped silicon,are particularly attractive for the elaboration of molecular-scale

devices, since “π-conjugation” between the redox tag and thesurface can in principle be maintained when the grafting isconducted from an alkyne-terminated molecular precursor andoxide-free hydrogen-terminated silicon (Si−H) surfaces.10,11

Such a feature might prove crucial for developing electro-chemically switchable devices exhibiting fast clocking rates,4 ifthe electron-transfer process through these particular hybridinterfaces is the determining factor for the macroscopic“response time” of any device made from such junctions.However, to our knowledge, while silicon-based interfacesderivatized with various types of redox-active centers, such asferrocene13−15 and metal-complexed porphyrins,2,4,6,16 haverecently attracted sustained attention, the interfacial electron-transfer kinetics have not yet been thoroughly investigated withmetal alkynyl based monolayers.1,14,17 Accordingly, in order tolearn how the nature and length of the spacer influence theinterfacial electron-exchange process, we have now decided tomeasure the kinetics of electron transfer between the “Fe(κ2-

Received: June 24, 2013

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dppe)(η 5 -C5Me5) ” end group (dppe = 1 ,2 -b i s -(diphenylphosphino)ethane) anchored through various typesof spacers and the underlying Si surface. Toward this aim, usinga photochemical grafting protocol previously communicated,17

we have now isolated the corresponding monolayers on Si−H(Chart 1), starting from a series of mononuclear Fe(κ2-dppe)(η5-C5Me5){[CC(1,4-C6H4)]nCCH} (1a−d; n =0−3), Fe(κ2-dppe)(η5-C5Me5)[CC(1,3-C6H4)CCH] (2),and Fe(κ2-dppe)(η5-C5Me5)[CC(1,4-C6H4)CHCH2] (3)complexes previously synthesized.18 From such functionalinterfaces, electrochemical studies on 1a−d-modified Si−Hsurfaces (Si-1a−d) should reveal the incidence of thephenylethynyl spacer extension on the interfacial electron-transfer characteristics, while comparison among Si-1b, Si-2,and Si-3 should bring complementary information on thetopological and structural features which are determining forthis property with this kind of unsaturated spacer.We therefore report (i) the preparation and characterization

of the desired organometallic hybrid molecule/Si interfaces Si-1a−d, Si-2, and Si-3 from the corresponding organometallicprecursors 1a−d, 2, and 3, (ii) measurements of the interfacialelectron-transfer kinetics, and (iii) a discussion of these keydata, which clearly establish the central role of the organiclinker on the electro-switching process, in connection withavailable data on related analogous ferrocene-based interfacesand DFT modelization. The synthesis of the organometallicFe(II) precursors 1b−d, 2, and 3 has been previouslyreported.18−20

■ RESULTS

Preparation and Characterization of the Redox-ActiveFe(II) Complex Terminated Si−H Surfaces. The alkyne-terminated complexes 1a−d and 2 as well as the ethenylcomplex 3,18−20 diluted in toluene, were photochemicallyreacted at 254 nm for ca. 3−4 h in the presence of a p-typeSi(111)−H surface (denoted Si−H afterward, Scheme 1),according to a workup previously adapted by some of us17 fromthe photochemical grafting reaction initially developed byChidsey and co-workers for phenylacetylene or styrene.21,22

Attempts at thermal grafting (100 °C overnight) were alsoperformed but were found to yield redox-active films with alower surface coverage and a more pronounced oxidation of theunderlying silicon. In contrast, the UV-activated hydrosilylationreaction yielded the covalent grafting of redox-active 1b−d, 2,and 3 (sub-) monolayers on Si−H, which were thencharacterized by X-ray photoelectron spectroscopy (XPS),

atomic force microscopy (AFM), FTIR spectroscopy, andcyclic voltammetry (CV). As expected, complex 1a, possessingthe shortest chain, could not be anchored onto Si−H, in spiteof repeated attempts. This particular behavior might beattributed to the steric hindrance of the coordination spherearound the Fe(II) atom, which certainly prevents the terminalalkyne moiety from interacting with Si−H, an interpretationstrongly supported by the structural data available for 1a.20 Forlonger homologues such as 1b−d and 2, the terminal ynemoiety must be more easily accessible, as indicated by thecrystal structures of 1b−c and 2.18 Such an observation can betaken as indirect evidence that the grafting of these compoundsactually involves the terminal alkyne moiety of these complexes,in line with recent theoretical calculations on modelcompounds.23

The XPS analysis of the organometallic monolayer-modifiedsurfaces showed a major and well-resolved component for theFe 2p3/2 level at 707.7 ± 0.2 eV, which is a typical value forFe(II) complexes (Figure 1a).24 However, we also notice thepresence of another unresolved component as a shoulder at ca.709.5 eV. A priori, this might correspond to a differentlycoordinated Fe(II) and/or a Fe(III) species. From itsapproximate area, its contribution can be estimated at less

Chart 1. Targeted Organoiron(II)-based Monolayers Si-1a−d, Si-2, and Si-3

Scheme 1. Photochemically Activated Grafting of 1b−d, 2,and 3 on Si(111)−H Surfaces

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than 30% of total Fe. We believe that it corresponds to thepresence of Fe(III) centers which originate from the oxidationof some of the electron-rich Fe(II) centers of the superficialorganometallic monolayer by adventitious traces of oxygenduring the grafting step. Furthermore, the Si 2p spectra at thegrazing angle revealed a very slight oxidation of the underlyingsilicon surface, indicated by the presence of a peak related toSiOx at ca. 102−104 eV (Figure 1b). As expected, the intensityof both iron and silicon signals monitored for the surfaces Si-1b−d was found to decrease upon the lengthening of thegrafted molecular chains.The AFM analysis also confirmed the presence of the

organometallic layers on silicon characterized by denselypacked globular features (Figure 2), as exemplified for the

surfaces Si-1b−d showing increasing root mean square (rms)roughness values of 12, 15, and 20 ± 2 Å. From cross-sectionprofiles, the mean height of these particles was found toincrease from 12 to 25 Å with an increase in the number ofphenyleneethynylene bridges from n = 1 to 3. These values areperfectly in agreement with the calculated theoretical lengths ofthe molecules. The presence of some brighter (and larger)spots which might correspond to local formation of aggregatesor “bundles” of wires on the surface are also observed, but theserepresent a minor fraction in comparison to other spots. On thebasis of steric considerations, the immobilization of 1a onSi(111)−H was unsuccessful, as shown by the AFM image ofthe surface characterized by atomically flat terraces ofunderlying Si(111) as is commonly observed for cleanSi(111)−H.25Finally, transmission FTIR spectroscopy performed with

modified porous Si(100) surfaces was used to probe theattachment of the organoiron derivatives on silicon, because thechemistry of porous silicon is usually similar to that of flatsilicon crystals.26,27

Comparison of spectra of organoiron-modified porousSi(100) surfaces with that of hydrogen-terminated porousSi(100) evidenced νCCFe vibration bands characteristic of theimmobilized organometallic center at 2054, 2048, and 2046cm−1 for Si-1b−d, respectively (Figure 3a), which are

consistent with the values and trends observed for thecomplexes (Figure 3b).18 In the νCC stretching vibrationregion (i.e., between 1600 and 1400 cm−1), the presence of aband at 1589 ± 2 cm−1 in the spectrum of Si-1b−d can beascribed to the 1,4-phenylene spacer, while the CHCH unitbound to the silicon surface at 1570 cm−1 are not always plainlydetected but show up as a shoulder to the previous peaks.28

This is somewhat comparable to the value near 1620 cm−1

attributed to the νCHCH2mode for 3.18 As has been noted by

others21,29 and some of us,12,17,30 the spectral signature for theinterfacial vinyl bond is usually weak. However, the absence ofsuch a band in the spectrum of Si-3, for which an interfacialethyl bond is expected, is indirect proof that the films producedfrom ethyne-terminated complexes are covalently bound to Sithrough vinyl bonds. The peak near 1630 cm−1 might be due towater coadsorbed in the pores.27

Electrochemistry of Redox-Active Fe(II) ComplexTerminated Si−H Surfaces. As mentioned above, with theexception of Si-1a, all the resulting interfaces are redox-activeand show a perfectly reversible one-electron system that was

Figure 1. High-resolution XP Fe 2p3/2 (a) and Si 2p (b) spectra of Si-1b (magenta line) and Si-1d (blue line) surfaces. The black line in (a)corresponds to the reference signal of 1b in powder form.

Figure 2. Contact-mode 2 × 2 μm2 AFM images of Si-1a (a), Si-1b(b), Si-1c (c), and Si-1d (d) surfaces and corresponding cross-sectionprofiles taken in the middle of the images (e). The mean height of theobserved particles was estimated at 12, 17, and 25 Å for Si-1b−d,respectively.

Figure 3. (a) Transmission FTIR spectra of porous Si-1b (magentaline), Si-1c (black line), and Si-1d (blue line) and (b) IR spectra (KBrpellets) of the starting complexes 1b (magenta line), 1c (black line),and 1d (blue line). The νPh, νCCFe, and internal νCC vibrationmodes are highlighted by asterisks. For the unbound complexes 1b−d(b), the terminal νCCH mode is also barely visible around 2100 cm−1,to the right of the intense νCCFe peak.

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attributed to the oxidation of the terminal iron center (Figure4). Indeed, the measured open circuit potential (OCP) of p-

doped Si(111)−H surfaces under identical conditions waspreviously found to be significantly more negative (−0.51 V vsSCE)31 than the E°′ values determined for the correspondingmolecules in solution (Table 1), indicating that the underlyingSi−H surface should not spontaneously oxidize the Fe(II)centers of the organometallic monolayers, in line with the XPSresults. Actually, the E°′ redox potential values correspondingto the monolayers bound to silicium are relatively low, around−0.10 V vs SCE, and remain quite close to those determinedfor the corresponding complexes in solution, although theredox process takes place at slightly higher potentials. It mustbe noted that all the electrochemical measurements of thisstudy have been performed in the dark to avoid complicatedcharge transfer phenomena related to photoelectrogeneratedelectron−hole pairs in p-doped silicon.32−34 As expected forsurface-confined redox species,35 the anodic and cathodic peakcurrent densities jpa and jpc are proportional to the potentialscan rate v within the 0.0 to ca. 1.0 V s−1 range (Figure 4c). Thetotal amount of different complexes attached to the surface wasdetermined by CV from the area under either the anodic orcathodic voltammetric peak. Thus, the surface coverage ofattached 1b−d was estimated at ca. 2.2 × 10−10 mol cm−2

(Table 1), which corresponds to a surface coverage of 0.16−0.17 organoiron per surface silicon atom, considering that theatomic density of Si(111) is 7.8 × 1014 atoms cm−2.36 Thisgives a specific area of about 75 Å2 per grafted molecule. On thebasis of steric considerations, the value presently found for theSi−CHCH-linked linear organoiron films indicates a densepacking of the chains on the silicon surface. Indeed, from theavailable structural data, the effective specific area covered bycomplexes featuring the Fe(κ2-dppe)(η5-C5Me5) end group canbe estimated as between 65 (±5) and 105 (±5) Å2 dependingon whether the dppe aryl groups can interdigitate or not. Incontrast, the surface coverage calculated for Si-2 and Si-3,namely around 1.5 × 10−10 mol cm−2, indicates less denselypacked films. The value found for Si-2 is consistent with anincreased steric hindrance of the grafted molecule owing to themeta position of the alkyne bond on the phenyl ring. Moreover,the smaller surface coverage of Si-3 with respect to Si-1b is alsoin agreement with some recent results highlighting that thealkyne-derived monolayers bound to Si are usually both moredensely packed and ordered than alkene-derived monolayerswith a higher surface coverage.11

Apparent rate constants for charge transfer of the boundFe(II)/Fe(III) system, kapp, were calculated for all modifiedsurfaces using Laviron’s formalism (eq 1, where α is the charge

transfer coefficient and va is the intersect of the two linearregions in the graph Epa vs log v obtained at low and high scanrates)37 based on the classical Butler−Volmer theory. Uponincreasing v, the anodic and cathodic peak potentialscorresponding to the redox couple, Epa and Epc, shift in positiveand negative directions relative to E°′, which reflects control ofthe voltammetry by the rate of electron transfer of theorganoiron units (Figure 4d). The slope of the linear Epa vs logv part at high v is equal to 2.3 RT/(1 − α)nF and enables α tobe determined. As exemplified for Si-1b (Figure 4d), theobserved symmetry between the anodic and cathodic branchessuggests that α is close to 0.5, as proved by the slope value ofca. 100 mV per decade of v determined at high v.The calculated kapp values are gathered in Table 1. A careful

examination of these values reveals that the charge-transferkinetics increases when going from Si-1b to Si-1d (i.e., whenextending the conjugated linker) and also from Si-3 to Si-1bamong interfaces featuring only one phenyl group in thecarbon-rich linker. It shows also that charge transfer proceeds

Figure 4. Cyclic voltammograms recorded at 0.2 V.s−1 of Si-1a−d (a)and Si-2 and Si-3 (b) in CH3CN + 0.1 M Bu4NClO4 andrepresentative jp−v (c) and Ep−log v (d) plots extracted from theCV of Si-1b.

Table 1. Electrochemical Data for Selected Fe(κ2-dppe)(η5-C5Me5)(R) Complexes in Solution and Grafted on Si(111)Surfaces,a and Related Surface Coveragesb

Fe(II)precursor R

E°′ [ΔEp](V)c,d

organometallicinterface

E°′bound(V)c,e

surface coverage(10−10 mol cm−2)f

atom per surface (Siatom) kapp (s

−1)g

1b CC(1,4-C6H4)CCH −0.11 [0.08] Si-1b −0.10 2.2 ± 0.2 0.17 ± 0.02 350 ± 1501c {CC(1,4-C6H4)}2CCH −0.11 [0.07] Si-1c −0.09 2.1 ± 0.2 0.16 ± 0.02 410 ± 2001d {CC(1,4-C6H4)}3CCH −0.11 [0.06] Si-1d −0.08 2.2 ± 0.2 0.17 ± 0.02 600 ± 2002 CC(1,3-C6H4)CCH −0.12 [0.07] Si-2 −0.12 1.5 ± 0.1 0.11 ± 0.01 140 ± 603 CC(1,4-C6H4)CHCH2 −0.15 [0.07] Si-3 −0.07 1.6 ± 0.1 0.12 ± 0.01 55 ± 20

aThree samples at least of each grafted surface have been prepared. bConditions: CH3CN + 0.1 M Bu4NClO4.cAll E°′ and ΔEp (peak-to-peak

separation) values are given in V vs SCE; E°′ values are taken as the average of the anodic and cathodic peak potentials (±5 mV). dPt electrode; theferrocene/ferrocenium (FcH/FcH+) couple was used as an internal reference for electrochemical measurements. The ratio between the anodic andcathodic peak current intensities was unity in each case (Ipa/Ipc = 1). eFor surface-bound species; uncertainty ±5 mV. fDetermined electrochemically(see text). gApparent charge transfer rate constant for the bound Fe(II)/Fe(III) couple (see text).

α= −k nFv RT(1 ) /app a (1)

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more quickly when the complex is grafted at the para position(Si-1b) than when it is grafted at the meta position (Si-2). Suchan acceleration phenomenon is not expected if only thedistance between the grafted redox-active center and theunderlying surface determines the charge-transfer kinetics.38

Actually, the significant meta vs para influence on the rate ofelectron-exchange clearly suggests that the charge transfer doesnot proceed “through space” but rather “through bonds”, sincethe redox-active Fe(II) end group for which a higher rate isobserved certainly lies closer to the surface in Si-2 than in Si-1b. This ordering recalls the trend observed for propertieswhere mesomeric effects are operative and further suggests thatpreservation of the π conjugation between the redox-active endgroup and the surface is an important factor for the electron-transfer process. Additional evidence for this statement isprovided by the even smaller kapp value found for Si-3, forwhich a saturated interfacial linkage interconnects in principlethe organoiron end groups to the surface (Scheme 1). We willcome back to this point later on (see the Discussion).DFT Calculations. In order to study the electronic effect of

grafting such molecules on Si(111)−H surfaces, DFTcalculations were carried out on the model cluster [Fe(κ2-dpe)(η5-C5H5){CC(1,4-C6H4)CHCH}]−Si22H33 (Si-1b′,Figure 5), as well as on the molecular model Fe(κ2-dpe)(η5-

C5H5){CC(1,4-C6H4)CHCH2 (3′) and the cluster modelSi22H34 (used to mimic the Si(111)−H semiconductingsurface) for comparison (dpe = 1,2-diphosphinoethane).Models were chosen to reduce the computational effort (seethe Experimental Section for computational details). Indeed, acomparison between 3′ and 3 previously computed18 revealscomparable electronic structures (see Figure 6). Only anarrowing of the HOMO−LUMO gap, as expected for themodel complex 3, is observed, showing that the Fe(κ2-dpe)(η5-C5H5) end group is less electron-rich than Fe(κ2-dppe)(η5-C5Me5), present in the real complexes. The ordering and natureof the frontier MOs remain the same in 3′ and 3. The HOMOsare π in character and are rather delocalized over the metal−ethynyl−aryl backbone, with a heavy contribution from the Fe

center and the adjacent ethynyl group and, to a lesser extent,from the phenyl ring.Results indicate that Si-1b′ compares well with 3′, with a

slight decrease of the HOMO−LUMO gap of ca. 0.2 eV for thegrafted species (Figure 6). We note that some MOs in theHOMO region show both molecule and surface contributions,whereas Si−H-based MOs are found in the LUMO region(Figure 7). This may explain why the oxidation potential valuesof the grafted species are overall weakly affected relative tothose of the redox potentials in solution.Note, however, that the present modeling achieved on a

discrete Si−H structure fails to reproduce typical solid-statefeatures of nearly infinite periodic Si−H arrays such as pdoping. For instance, on the basis of this MO scheme, partialoxidation of the Si−H monolayer should eventually result inFe(III) sites. Moreover, when operative, such a process wouldbe detrimental to the conductivity of the silicon chip, since thehole density at the silicon surface is of the same order ofmagnitude as the electrochemically determined surface cover-age of the redox-active chains.39

Moreover, the electrostatic influence of the medium and thepresence of adventitious counterions associated with dopingmight locally perturb the ideal MO scheme shown in Figure 6.As a result, a dramatic decrease of the silicon chip conductionwas experimentally not observed. Most likely, the partialpositive charge of the superficial layer remains trapped onsurface defects and does not contribute to oxidize the Fe(II)sites of the organometallic monolayer, in line with the morenegative open circuit potential (OCP) found for p-dopedSi(111)−H surfaces relative to the Fe(II)/Fe(III) oxidationpotentials of Si-1b−d, Si-2, and Si-3 (Table 1).31

■ DISCUSSIONRedox-Active Fe(II) Complex Functionalized Si(111)−

H Surfaces. On the basis of our FTIR and electrochemicaldata, we have presently established that the covalent anchoringof our organometallic species 1b−d and 2 on Si−H occursthrough the formation of an ethenyl linkage, in line with recenttheoretical (DFT) calculations.23 As a result, the grafting of thevery sterically congested complex 1a was unsuccessful undertypical conditions, owing to the lack of accessibility of theethynyl end group. This confirms that anchoring to the surfacedoes not take place to any other part of these redox-activecompounds than on the exposed side of the (poly)arylethynylchain. In line with DFT computations performed on thesimplified Si-1b′ model (Figure 5), we then evidenced that theFe(II) centers could be oxidized at potentials lower than that ofthe Si−H surface. Although the surface was presently simulatedby a neutral Si22H34 cluster, our modeling remains apparentlyalso correct for the true species anchored on p-doped Si(111)−H surfaces. In addition, we show that the E°′ potential valuescorresponding to the Fe(III/II) oxidation of the grafted speciesare only marginally different from these of the free species insolution. According to these values, the environment of thegrafted species seems therefore to act as a perturbationresembling that observed when a slightly electron withdrawinggroup is appended to the first phenyl ring. The observed shiftmight thus be related to the presence of the partial positivecharge originating from the p-doping process. Moreover, weshow that the surface coverages obtained with the variouscomplexes match well with figures that can be derivedconsidering the superficial extension of the compounds onthe surface, when bound normal to the surface. Finally, the

Figure 5. Optimized geometry of the cluster model Si-1b′.

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determining influence/nature of the linker during the graftingprocess to the surface with Si-1b−d is also evidenced by thesignificantly slower charge-transfer process taking place with the

analogous compound 3, in line with expectations based on theformation of a saturated linker vs an unsaturated one (Scheme1).

Comparison with Related Redox-Active InterfacesFunctionalized with Organometallics. Among all theredox-active species that were grafted on oxide-free hydrogen-terminated silicon surfaces so far, redox-active organometallicend groups remain rare. Thus, Si−H surfaces were mostlyfunctionalized with ferrocene-terminated monolayers such as4,2,6 5, or 6 (Chart 2).13,40 More recently, Fehlner and co-workers have also evidenced that more elaborated organo-metallic molecules such as 71b or 81a (Chart 2) could be graftedon chlorinated silicon surfaces and that the redox state of theanchored molecules could be controlled by application of apotential to the silicon chip, in a way similar to what has beendone previously with purely ferrocene monolayers.13 Incomparison, however, the charge-transfer event proceedsmuch more quickly with silicon chips derivatized withmolecules such as 1b−d (Table 1). Thus, the kapp valuesreported for ferrocene-terminated and 7-derived monolayersare lower than 100 and equal to 5 s−1, respectively.13,1 Further,we have also clearly shown here that organoiron-basedinterfaces of 1b−d, 2, or 3 are oxidized at significantly lowerpotentials. As a result, faster switching/clocking rates can beexpected for a device built with these organometallics onsilicon.In all the previous studies on redox-active interfaces

produced from complexes 4−8, the interfacial bond withsilicon was always a saturated alkane of fixed length so that, toour knowledge, the dependence of the interfacial electron-transfer rate on the spacer nature (or length) has never beeninvestigated with Si-based interfaces. Presently, our dataindicate a determining influence of the unsaturated andconjugated spacer between the redox-active metal center andthe surface on the electron-exchange process,41 as significantlydecreased kapp values are found for the meta-substituted

Figure 6. DFT molecular orbital diagrams of the [Fe(κ2-dppe)(η5-C5Me5){CC(1,4-C6H4)CHCH2}] para complex 3, the Fe(κ2-dpe)(η5-C5H5){CC(1,4-C6H4)CHCH2} model 3′, and the cluster models [Fe(κ2-dpe)(η5-C5H5){CC(1,4-C6H4)CHCH}]−Si22H33 (Si-1b′, seeFigure 5) and Si22H34. The Fe/carbon chain (3 and 3′) and Fe/carbon chain/Si (Si-1b′) percentage contributions are given in italics.

Figure 7. Plots of the highest occupied molecular orbitals (HOMO,HOMO-1) and the lowest unoccupied molecular orbitals (LUMO,LUMO+1) for Si-1b′. Contour values are ±0.03 (e/bohr3)1/2. Theenergies are given in parentheses.

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complex in Si-2 and even lower ones for Si-3, for which anonconjugated linker connects the redox-active end group tothe surface.Then, among the interfaces Si-1b−d containing a conjugated

and unsaturated spacer with the silicon surface, the kapp valuesobtained do not exhibit an inverse exponential dependencewith increasing distance, as was most often observed when theelectron exchange process between the redox end groups andthe surface is dominated by electron tunneling through thespacer in a nonadiabatic regime.42 Instead, a slight increase withthe increasing length of the oligo(1,4-phenyleneethynylene)moiety is observed. However, among all the possibilities thatmight explain a nonexponential dependence of the electron-tunneling process,43 none can accommodate an increase. Thus,we think that the apparent charge-transfer rates presentlymeasured are at least partially controlled by anotherphenomenon. Considering the semiconducting nature of thesilicon surface, this rate-determining phenomenon might likelybe related to the presence of an electrical barrier at thesemiconductor−molecule interface, regardless of the fact thatthe measurements were done in the dark and that anodicoxidation of the bound Fe(II) center took place at rather closepotential values for Si-1b−d. Also, the conformational flexibilityof oligo(phenyleneethynylene) (OPE) bridges as a result of theintrinsically low barrier to rotation of the phenylene rings inthese bridges could constitute another potential explanation.44

Such a conformational flexibility could result in the improve-ment of the overall order of the monolayer structure (andconsequently in the increase of kapp) as the number ofphenyleneethynylene units increases, in line with reported high-resolution STM observations.45

Finally, the charge-transfer characteristics observed for Si-1b−d can also be compared to those reported for ferrocene-terminated conjugated wires attached to gold surfaces throughOPE bridges.44 For the latter systems, the kapp valuesdetermined for spacers of comparable lengths were muchlarger, while the distance dependence of charge-transfer rateconstants with increasing spacer length was shown to beexponential.38,44,46 Recalling that all the electrochemicalmeasurements of this study were performed in the dark(under conditions where only a majority of charge carriers, i.e.holes, is available for charge transfer),32,33 this difference ininterfacial charge transfer kinetics might also be ascribed to thesemiconducting nature of silicon. For this reason, a relativelyslower charge transfer process between the underlying siliconsurface and the terminal organometallic center is not surprising.Alternatively, the occurrence of charge transfer via surface states(e.g., patches of silicon oxide) might also account for thesmaller rate constants observed with all our functionalizedsurfaces. Indeed, the derivatization of Si−H surfaces is thoughtto introduce surface states that probably favor someunavoidable oxidation of Si−H, as revealed by the XPS analysisof the modified surfaces (Figure 1b), also resulting in thelimited temporal stability of these interfaces.47 It must be kept

in mind that a large amount of Si−H sites remain aftercompletion of the monolayers and thus are susceptible tooxidation if water or oxygen can penetrate through themolecular layer via defects and pinholes. Owing to the sterichindrance of the electroactive units, the films produced in thisstudy are not so highly packed as to forbid access to the surfaceto small molecules. Consequently, they do not provide anefficient hydrophobic environment to prevent water and/oroxygen from diffusing and interacting with the remaining Si−Hsites.

■ CONCLUSIONS

We have reported here the preparation and characterization offive densely packed monolayers (Si-1b−d, Si-2, and Si-3),obtained from the corresponding Fe(II) arylacetylide com-plexes 1b−d and 2 featuring a pendant ethynyl group or fromcomplex 3 with a pendant ethenyl group. These redox-activemonolayers were all grafted on Si−H surfaces through acovalent interfacial Si−C bond, using a photochemical protocoloperating under rather “mild” conditions. Their Fe(III/II)oxidation potentials are close to those of the starting unboundprecursors in solution. Moreover, these organometallicmonolayers allow relatively fast charge transfer to take placebetween the terminal redox centers and the underlying siliconsurface. The rate constants were accurately determined and areamong the highest values obtained for redox systems grafted toSi−H surfaces. They provide experimental insight on the role ofthe spacer in the charge-transfer process. Thus, the highestvalues were obtained for monolayers for which a conjugatedand unsaturated spacer links the redox-active end groups to thesilicon surfaces, underpinning the importance of unsaturationand conjugation in the linker for such silicon-based redox-activeorganometallic interfaces. Also, for the first time, an unexpectedincrease in the electron-exchange rate with increasing spacerlength has been experimentally evidenced. At odds with ourinitial hopes, this suggests that the electron tunneling is notentirely controlling the interfacial charge transfer process, atleast for the shortest linkers. The latter trend was presentlytentatively rationalized considering kinetic limitations due tothe presence of an electrical barrier at the semiconductor−molecule interface, although changes in the monolayerstructure as the number of phenylene ethynylene unitsincreases might also contribute to the observed trend.Importantly, this observation shows that the tunnelling processis not yet the limiting factor which controls the interfacialcharge transfer with Si-1d and leaves some room for furtherimprovements. In this respect, a successful strategy to improveboth the quality and the packing density of the produced redox-active films could be the dilution of the redox-center-terminated chains with electrochemically inert organic chains(e.g., derived from phenylacetylene). This should also enableminimizing the oxidation of silicon and improve the charge-transfer characteristics, as previously demonstrated forferrocene- and metalloporphyrin-terminated monolayers

Chart 2. Previously Reported Examples of Redox-Active Organometallics Deposited on Conducting Si Surfaces

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dx.doi.org/10.1021/om4006017 | Organometallics XXXX, XXX, XXX−XXXG

bound to Si−H.6 Work directed toward these stimulatingprospects is currently underway.

■ EXPERIMENTAL SECTIONGeneral Considerations. All reactions and workup procedures

were carried out under dry, high-purity argon using standard Schlenktechniques.48 All solvents were freshly distilled and purged with argonbefore use. Acetone (MOS electronic grade, Erbatron from CarloErba) and anhydrous ethanol (RSE electronic grade, Erbatron fromCarlo Erba) were used without further purification. The chemicalsused for cleaning and etching of silicon wafer pieces (30% H2O2, 96−97% H2SO4, 40% NH4F and 50% HF aqueous solutions) were of VLSIsemiconductor grade (Riedel-de-Haen).Caution! Proper precautions must be used when handling hydrogen

f luoride. Hydrogen f luoride is extremely corrosive to human tissue, withcontact resulting in painful, slow-healing burns. Laboratory work with HFshould be conducted only in an ef f icient hood, with the operator wearing afull-face shield and protective clothing.The complexes Fe(κ2-dppe)(η5-C5Me5){[CC(1,4-C6H4)]nC

CH} (1a−d; n = 0−3), Fe(κ2-dppe)(η5-C5Me5)[CC(1,3-C6H4)CCH] (2), and Fe(κ2-dppe)(η5-C5Me5)[CC(1,4-C6H4)CHCH2] (3) were obtained following published proce-dures.18−20 Other chemicals were purchased from commercialsuppliers and used as received.Preparation of the Redox-Active Fe(II) Complex Terminated

Si−H Surfaces. A single-side-polished silicon(111) shard (1−5 Ω cm,p-type, boron doped, thickness 525 ± 25 μm, Siltronix) was cut into a1.5 × 1.5 cm2 piece and sonicated for 10 min successively in acetone,ethanol, and ultrapure 18.2 MΩ cm water (Veolia Water STI). It wasthen cleaned in 3/1 v/v concentrated H2SO4/30% H2O2 at 100 °C for30 min, followed by copious rinsing with ultrapure water.Caution: The concentrated H2SO4/H2O2 (aq) piranha solution is very

dangerous, particularly in contact with organic materials, and should behandled extremely carefully.The surface was etched with ppb grade 40% aqueous argon-

deaerated NH4F for 20 min to obtain atomically flat Si(111)−H. Itwas then dipped in argon-deaerated ultrapure water for severalseconds, dried under an argon stream, and transferred immediatelyinto a quartz Schlenk tube containing ca. 8−10 mM of the Fe(II)complex in ca. 10 mL of deoxygenated anhydrous toluene. Thesolution was thoroughly purged with argon for 30 min, and theSchlenk tube was sealed with paraffin film (Parafilm M) and thenirradiated for 4 h in a Rayonet photochemical reactor (254 nm). Themodified surface was then rinsed copiously with anhydrous tolueneand methylene chloride and dried under an argon stream.Characterization of the Redox-Active Fe(II) Complex

Terminated Si−H Surfaces. Electrochemical Characterizations.Cyclic voltammetry measurements were performed with an Autolabelectrochemical analyzer (PGSTAT 30 potentiostat/galvanostat fromEco Chemie BV) equipped with the GPES software in a homemadethree-electrode Teflon cell. For the electrochemical analysis ofcomplexes in solution (at 1 mM concentration), the working electrodewas a Pt disk (Ø = 1 mm), the counter electrode a Pt wire, and thereference electrode a saturated calomel electrode (SCE). The FeCp2

0/+

couple (E1/2 = 0.40 V vs SCE, ΔEp = 0.07 V; Ipa/Ipc = 1) was used asan internal calibrant for the potential measurements.49 For modifiedsilicon surfaces, all of the electrochemical measurements have beenperformed in the dark to avoid photoinduced electron transferprocesses. The working electrode, modified Si(111), was pressedagainst an opening in the cell bottom using a fluoroelastomer withspecial tetrafluoroethylene additives (FETFE, from Aldrich) O-ringseal. An ohmic contact was made on the previously polished rear sideof the sample by applying a drop of an In−Ga eutectic (Alfa-Aesar,99.99%). The electrochemically active area of the Si(111) surface was0.1 cm2. The counter electrode was a platinum grid, and 10−2 M Ag+/Ag in acetonitrile was used as the reference electrode (+0.29 V vsaqueous SCE). Tetra-n-butylammonium perchlorate [Bu4N][ClO4]was purchased from Fluka (puriss, electrochemical grade) and wasused as received, at 0.1 mol L−1 concentration as the supporting

electrolyte in acetonitrile (>99.5%, puriss, over molecular sieves,Sigma-Aldrich). The electrolytic medium was dried over activated,neutral alumina (Merck) for 30 min, with stirring under an argonatmosphere. All electrochemical measurements were carried out insidea homemade Faraday cage, at room temperature (20 ± 2 °C) andunder a constant argon flow. The resistance of the electrolytic cell wascompensated by positive feedback.

X-ray Photoelectron Spectroscopy (XPS) Analysis. After a fewminutes of exposure to ambient atmosphere, the modified surfaceswere introduced in the UHV chamber and kept at 1 × 10−9 mbar forseveral hours before XPS analysis. XPS measurements were performedwith a Mg Kα (hν = 1254 eV) X-ray source, using a VSW HA100photoelectron spectrometer with a hemispherical photoelectronanalyzer, working at an energy pass of 22 eV. The experimentalresolution was then 1.0 eV. Spectral analysis included a Shirleybackground subtraction and peak separation using mixed Gaussian−Lorentzian functions. Note that the Si 2p signal measured before andafter grafting was averaged over several emission angles around normalemission in order to eliminate photoelectron diffraction effects.

FTIR Characterization of the Modified Porous Si(100) Surfaces.FTIR spectra were acquired using a Bruker Optics Vertex 70 FT-IRspectrometer in the transmission mode (100 scans, 2 cm−1 resolutionand automatic gain) using a DTGS detector. The porous silicon wasmounted on a homemade Teflon sample holder. A flat Si(100)−Hsurface was prepared by immersing a freshly Piranha-cleaned double-side-polished Si(100) (1−5 Ω cm, p-type, boron doped, thickness 250± 25 μm, Siltronix) sample in ca. 5% HF for 2 min and drying underan argon stream without rinsing. It was pressed against an opening inthe cell bottom using a FETFE O-ring seal, and an ohmic contact wasmade on the polished rear side of the sample with the steel bottom cap(care was taken to avoid surface contamination for subsequent FTIRinvestigation). A platinum counter electrode was used. The hydrogen-terminated porous Si surface was produced by applying a currentdensity of 20 mA cm−2 for 5 min in 50% HF/ethanol/ultrapure 18.2MΩ cm water (2/2/1 v/v/v). The surface was then rinsed withethanol and dried under an argon stream. After the FTIR spectrum ofthe freshly hydrogenated porous silicon was monitored, the samplewas then dipped in ca. 5% HF for 2 min and dried under an argonstream without rinsing. The reaction conditions of porous Si(100)−Hwith the Fe(II) complex were identical with those described for the flatSi(111) surface.

Atomic Force Microscopy (AFM). AFM topography images wererecorded under ambient conditions in contact mode with a PicoSPMII microscope from Molecular Imaging using nitride silicon tips (k ≈0.06 N m−1) from ScienTec-Nanosensors.

Computational Details. DFT calculations using the Amsterdamdensity functional (ADF) program50 were carried out on theoptimized cluster models Si22H34 (used to mimic the H−Si(111)surface) and [Fe(κ2-dpe)(η5-C5H5){CC(1,4-C6H4)CHCH}]−Si22H33 (Si-1b′) to reduce computational effort, as well as on themolecular model Fe(κ2-dpe)(η5-C5H5){CC(1,4-C6H4)CHCH2(3′) for comparison. Electron correlation was treated within thelocal density approximation (LDA) in the Vosko−Wilk−Nusairparametrization.51 Nonlocal corrections were added to the exchangeand correlation energies using the Perdew−Wang 1991 (PW91)52

functional. Calculations were performed using the standard ADFtriple-ζ quality basis set for the atom valence shells augmented with a2p polarization function for H, a 3d polarization function for C and P,and a 4p function for Fe. Orbitals up to 1s, 2p, and 3p were keptfrozen for C, P, and Fe, respectively. Full geometry optimizations(assuming C1 symmetry) were carried out on each model, using theanalytical gradient method implemented by Versluis and Ziegler.53

■ ASSOCIATED CONTENT*S Supporting InformationCartesian coordinates for all calculated geometries (Table S1)and a file of all computed molecule Cartesian coordinates in aformat for convenient visualization. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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dx.doi.org/10.1021/om4006017 | Organometallics XXXX, XXX, XXX−XXXH

■ AUTHOR INFORMATIONCorresponding Authors*J.-F.H.: e-mail, Jean-francois.halet@univ-rennes1.fr.*F.P.: tel, 33 (0)2 23 23 59 62; fax, 33 (0)2 23 23 56 37; e-mail,Frederic.paul@univ-rennes1.fr.*B.F.: tel, 33 (0)2 23 23 65 50; fax, 33 (0)2 23 23 67 32; e-mail, Bruno.fabre@univ-rennes1.fr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSF.P. and N.G. thank Region Bretagne for financial support. H.S.acknowledges a Ph.D. fellowship from the PROFAS French-Algerian Programme. The Agence Nationale de la Recherche(ANR 09-BLAN-0109) and Centre National de la RechercheScientifique (CNRS) are fully acknowledged for financialsupport. S. Ababou-Girard is fully acknowledged for XPSmeasurements.

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sensitive to oxidation or water contamination than flat silicon. In thepresent work, transmission FTIR spectroscopy has been used toextract qualitative information on the chemical composition of theproduced films. It was not our purpose to determine quantitatively themolecular surface concentrations of different chemical bonds.(28) Harada, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J.J. Phys. Chem. C 2013, 117, 7497−7505.(29) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37,3257−3260.(30) Yzambart, G.; Fabre, B.; Lorcy, D. Langmuir 2012, 28, 3453−3459.(31) Fabre, B.; Hauquier, F.; Allongue, P. J. Electroanal. Chem. 2009,629, 63−68.(32) Electrochemical impedance spectroscopy measurements per-formed with other modified p-doped silicon surfaces have shown thatthe flatband potential Efb of the silicon surface (i.e., the electrodepotential for which there is no space-charge region in thesemiconductor) is close to 0.0 V vs SCE in acetonitrile medium.33

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= ×−⎛

⎝⎜⎞⎠⎟n N

e E EkT

exp( )

p afb

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Organometallics Article

dx.doi.org/10.1021/om4006017 | Organometallics XXXX, XXX, XXX−XXXJ