Micromechanical Redox Actuation by Self-Assembled Monolayers of

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Micromechanical Redox Actuation by Self-Assembled Monolayers of Ferrocenylalkanethiolates: Evens Push More Than Odds Eric R. Dionne,†,‡ Christopher Dip,†,‡ Violeta Toader,§,‡ and Antonella Badia*,†,‡ †

Département de chimie, Université de Montréal, C.P. 6128, succursale Centre-ville, Montréal, QC H3C 3J7, Canada Department of Chemistry, McGill University, 801 rue Sherbrooke Ouest, Montréal, QC H3A 2K6, Canada ‡ Quebec Center for Advanced Materials, FRQNT, Canada §

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S Supporting Information *

contraction and a surface stress change to which the cantilever responds by bending. We have previously used SAMs of ferrocenylalkanethiolates (Fc(CH2)nS or FcCnS) of n = 11 or 12 chemisorbed onto goldcoated cantilevers as a model system to investigate the mechanochemistry of a surface-confined Faradaic reaction.10−12 FcCnSAu SAMs present a low driving voltage (E°′ ≈ 0.3−0.4 V vs Ag/AgCl) and two stable redox states.13,14 The electrochemical oxidation of the SAM-bound ferrocene (Fc) to ferrocenium (Fc+) proceeds via single-electron-transfer and ion-pairing reactions: FcSAM + X−(aq) ⇌ (Fc+X−)SAM + e−.15,16 Due to physical crowding of the ferrocenes in the SAM, ionpair formation induces a structural change in which the alkyl chains adopt a more perpendicular orientation with respect to the underlying surface and the ferrocene units rotate.17,18 We have demonstrated that these re-orientational motions can be transformed into mechanical work (Figure 1).10 FcCnSAu

ABSTRACT: Microcantilever transducers can be valuable tools for the investigation of physicochemical processes in organized molecular films. Gold-coated cantilevers are used here to investigate the electrochemomechanics of redox-active self-assembled monolayers (SAMs) of ferrocenylalkanethiolates (Fc(CH2)nS) of different alkyl chain lengths. A significant odd−even effect is observed in the surface stress and cantilever movement generated by the oxidation of the SAMconfined ferrocenes as the number of methylene units n in the SAM backbone is varied. We demonstrate that stronger alkyl chain−chain interactions are at the origin of the larger surface stresses generated by SAMs with an even versus odd n. The findings highlight the impact of subtle structural effects and weak van der Waals interactions on the mechanical actuation produced by redox reactions in self-assembled systems.

M

olecular motions and conformational changes derived from the stimuli-triggered alteration of intra- and intermolecular interactions are the key driving forces of mechanical actuation by polymers and highly ordered organic assemblies.1 Because of the large structural changes induced by charge, attention has been devoted to actuation driven by electrostatic (Coulombic) attraction/repulsion (refs 2−5 are examples). The effect of weaker chemical interactions has been largely ignored. We show here via the odd−even effect that small variances in the stereostructural characteristics of and intermolecular van der Waals dispersion interactions in redoxterminated self-assembled monolayers (SAMs) have a significant influence on their electrochemomechanical behavior. Our findings indicate that van der Waals interactions should be considered when designing efficient actuators and molecular devices based on switchable SAMs. Silicon microcantilevers modified on one side with an appropriate metal and/or organic thin film can transduce a variety of chemical and physical phenomena into a nanometerto micrometer-scale mechanical movement.6,7 Redox reactions are one type of chemical transformation that have been used to actuate the cantilever. Examples include the ion doping/ de‑doping of conducting polymers8,9 and movement of artificial molecular muscles (bistable [3]rotaxanes)4. A voltage applied to the cantilever triggers a change in the redox state of the adsorbate molecules, resulting in a molecular extension or © XXXX American Chemical Society

Figure 1. Schematic illustration of the redox-induced molecular reorientations17,18 proposed to give rise to the cantilever bending.10,12

SAMs are particularly interesting for further study because the charge-density-normalized surface stress generated by these ultrathin nanometer-thick layers is at least 10-fold greater than that produced by the oxidoreduction of 100-fold thicker films of conducting polymers commonly used for electroactuation.8,9,11 Moreover, ion pairing is not per se required to induce a surface stress; in solid-state junctions, electrostatic repulsions between the ferroceniums induce an analogous change in the SAM structure.19,20 The present work focuses on the impact of an odd (nodd or SAModd) versus an even (neven or SAMeven) number of methylene repeat units in the alkyl chain on the redox-induced actuation of cantilevers modified with FcCnSAu SAMs of n = 9−16. The fixed Au−S−C bond angle (∼110°) results in an odd−even effect in the orientation of the ferrocene termini.21 The ferrocene tilt angle with respect to the surface normal is 5 Received: April 16, 2018 Published: August 2, 2018 A

DOI: 10.1021/jacs.8b04054 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

anodic (SCnFc) and cathodic (SCnFc+ClO4−) traces. Consistent with this explanation is the weaker effect of pressure on the electrochemical reduction of the SAM-confined ferrocenium compared to the oxidation of the ferrocene, for which a volume expansion of ∼10−20 cm3 mol−1 is coupled with the electron-transfer and ion-pairing reactions.27 Consequently, σ is not expected to follow the exact same pathway with potential during the anodic and cathodic sweeps. σ−E traces for n = 9−16 are shown in Figure 3a. Surface stress changes of 275−350 mN m−1 are generated across the

± 1° higher in SAModd than in SAMeven for n = 9−15.22 Computational studies reveal that a lesser tilt angle reduces the steric hindrance and improves packing of the molecules, resulting in more favorable interchain interactions and a stiffer SAM.23,24 These seemingly small structural differences have been shown to crucially impact the charge-transport characteristics of FcCnSAu SAM-based tunnel junctions.24 We investigated whether this odd−even difference in the molecular packing of FcCnSAu SAMs influences the mechanical actuation that these generate. The evolution of the surface stress change (σ), determined from the measured cantilever bending, during linear sweeps of the potential (E) between 0 V (reduced form) and 0.65 V (oxidized form) versus Ag/AgCl is shown in Figures 2 and S1

Figure 2. Current i and surface stress change σ vs applied potential E for three consecutive oxidation−reduction cycles of cantilevers modified with FcC11SAu and FcC16SAu in 0.1 M NaClO4(aq). Dotted lines are the σ−E traces of cantilevers modified with CH3C11SAu and CH3C15SAu SAMs. Scan rate is 10 mV s−1.

Figure 3. (a) Surface stress change σ vs applied potential E recorded for FcCnSAu SAM-modified cantilevers. (b) Chain length dependency of the oxidation-induced surface stress change σox and midpoint slope of the anodic σ−E traces determined using a sigmoidal Logistic fit. Each data point is the mean of 10−16 cantilevers. Error bars represent 95% confidence intervals.

for two selected chains, n = 11 and 16, of the series investigated. NaClO4(aq) was used as the electrolyte. The poorly solvated ClO4− anions form contact ion pairs with the SAM-bound ferroceniums that favor reversible redox electrochemistry.15,25,26 Redox peaks centered at ∼0.39 V are observed in 0.1 M NaClO4(aq) (discussion of the voltammetric signature in Supporting Information). The onset of the surface stress change coincides with the generation of a Faradaic current. A compressive surface stressthe cantilever bends away from the gold-coated faceis generated during the anodic (oxidation) sweep as the SAM-bound ferrocene is converted to the ferrocenium. σ tends toward a limiting value at potentials positive of the anodic peak once all the ferrocene is oxidized. The cantilever returns to its initial position at the end of the cathodic (reduction) sweep. The surface stress change and bending are reversible over successive oxidation− reduction cycles. Non-electroactive CH3CnSAu SAMs of comparable chain lengths do not generate appreciable bending over the same potential range (Figure 2). There is generally a hysteresis between the anodic and cathodic segments, which we ascribe to the different states of the SAM at the start of the

potential range of 0−0.65 V. Most of the positive charge of the SAM-bound ferroceniums is compensated by the counteranions (i.e., degree of dissociation between Fc+ and ClO4− of 0.5%).28,29 It is therefore unlikely that Coulombic repulsion between neighboring ferroceniums or between the ferroceniums and positively charged gold surface is a significant contributor to σ. Additionally, the surface stress change corresponding to a ferrocenium coverage of 100% is independent of the anion concentration between 10−3 and 1 M. The surface stress at E = 0.65 V indicates that neven and nodd fall into two distinct sets. There is also an apparent difference in the steepness of the σ−E traces. To validate these observations, the individual anodic traces were analyzed using a sigmoidal Logistic function (Figure S2 and Table S1) to determine the oxidation-induced surface stress change (σox) and slope at the midpoint (σox/2). This model provided a useful empirical method for comparing the data obtained for the different chain lengths. The results are presented in Figure 3b. Both σox and the midpoint slope present odd−even alternations that are statistically significant, except for the pair B

DOI: 10.1021/jacs.8b04054 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society n = 15/16 (Supporting Information). SAMseven generate a larger σox than SAMsodd. The surface stress evolves differently with potential in SAMeven and SAModd (slope of SAMeven > SAModd) and with chain length (there is an overall increase in the slope with increasing n). The largest odd−even variations of σox are observed between n = 11 and 15. A greater degree of chain conformational disorder (gauche defects) for n < 1130 and bending of the alkyl chains to accommodate a larger number of ferrocenes in SAMs of n > 1522 may diminish the odd−even distinction. σox of SAMeven is 68 ± 10 mN m−1 higher than σox of SAModd (n = 11−15). This difference, caused by a single methylene, is considerable, given that surface stress changes of 1−30 mN m−1 are typically produced by interfacial (bio)chemical reactions at SAM-functionalized cantilevers.31 The odd−even difference in σox does not arise from the FcCnS surface coverage densities (Figure S3). To address the origin of the observed odd−even effect, we turn to what is known about the mechanism of redox-induced surface stress generation in FcCnSAu-modified cantilevers. The response of cantilevers functionalized with mixed FcC12SAu/CH3C10SAu SAMs of varying ferrocene surface coverage and degree of phase segregation is consistent with the cantilever bending from the in-plane force exerted by the monolayer expansion resulting from the collective molecular re-orientations induced by the sterically hindered pairing of the anions with the SAM-bound ferroceniums.10,12,27 The electrochemical oxidation of mixed SAMs consisting of isolated and non-interacting FcC12S molecules does not cause any bending (ion pairing is not sterically hindered).10,12 Structural variances that impact the redox-induced molecular re-orientations could affect the force exerted on the cantilever and magnitude of the surface stress generated. Molecular dynamics simulations indicate that the ferrocene−ferrocene and ferrocene−alkyl chain interactions are nearly constant as a function of n, while the alkyl chain−chain interactions increase with increasing n.22,23 The latter dominate the SAM packing structure when n ≥ 5 and exhibit odd−even fluctuations. The computed molecule packing energy of SAMeven is 0.7 ± 0.3 kcal mol−1 lower than that of SAModd for n = 9−14 (supplementary material of ref 23). A recently reported odd−even effect in the dielectric constant of FcCnSAu SAMs corroborates the presence of an odd−even distinction in the SAM packing.32 The larger σox generated by SAMeven can thus be rationalized by the need to overcome stronger intermolecular interactions to decouple the alkyl chains and effect the aforementioned orientation change. The magnitude of the thickness change resulting from the molecular re-orientation is the same for neven and nodd (i.e., 1.9 ± 0.1 Å),32 implying that the SAMs reach the same final state upon ferrocene oxidation. The effect of the stronger intermolecular interactions in SAMeven is to intensify the perturbing action of the ion pairing on the SAM structure. The comparatively weaker intermolecular interactions in SAModd would make it easier for the FcCnS molecules to reorient, thereby resulting in a lower in-plane force and surface stress change. An odd−even difference in σ of 13 mN m−1 is obtained from the computed difference in the alkyl chain packing interactions (Supporting Information). This value is of the same order of magnitude as the odd−even difference in the measured σox, indicating that the variances in the lateral alkyl chain interactions can account for the odd−even effect in the redox-induced cantilever response. σox may contain contribu-

tions from potential-induced anion and solvent penetration of the SAM,33,34 the extent of which could be influenced by odd− even differences in the molecular packing and SAM defect density, as observed for the monolayer capacitance32 and leakage current of FcCnSAu-based diodes24. The reductive desorption of the FcCnS from the gold was used to obtain experimental evidence for an odd−even difference in the strength of the intermolecular interactions.35 The desorption peak potential can be considered a measure of the electrochemical stability of the SAM, which includes an energetic contribution from lateral interactions.35 More negative potentials are generally required for the desorption of thiolates with longer alkyl chains due to an increase in the attractive (stabilizing) interchain interactions and resistance to ion permeation.35,36 Cyclic voltammograms for the reductive desorption and oxidative re-adsorption of selected chain lengths of FcCnS and dependence of the cathodic peak potential on the alkyl chain length are shown in Figures 4 and

Figure 4. Cathodic peak potential Epc vs alkyl chain length for the reductive desorption of FcCnS from gold in 0.5 M KOH(aq). Scan rate is 20 mV s−1. Each data point is the mean of 10−14 SAMs. Error bars represent 95% confidence intervals. Inset: cyclic voltammograms of selected even/odd pairs.

S4. The cathodic peak between −1.0 and −1.1 V, from the desorption of FcCnS bound to Au(111) surface sites,37,38 progressively shifts to more negative potentials with increasing n, as expected. More importantly, the desorption potentials of even/odd neighbors are statistically indistinguishable for n = 10−15. This is likely a manifestation of the computationally predicted odd−even difference in the alkyl chain interactions, whereby the longer-chain SAModd is less stable than its shorterchain SAMeven neighbor, so that its desorption potential remains nearly the same instead of shifting in the cathodic direction. This observed odd−even effect in the SAM desorption potential provides a molecular basis for the odd− even effect observed in the redox-generated surface stress. In summary, cantilever-based surface stress measurements on a homologous series of FcCnSAu SAMs demonstrate the sensitivity of the electrochemically induced micromechanical actuation to small odd−even differences in the molecular organization and associated intermolecular van der Waals interactions. The strength of the alkyl chain−chain interactions determines the in-plane force generated by the oxidationinduced change in orientation of the FcCnS molecules and, ultimately, the magnitude of the surface stress change. These interactions can be maximized by optimizing the chain packing C

DOI: 10.1021/jacs.8b04054 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society in the SAM. The findings provide new stereostructural insights into the transformation and amplification of small molecularscale motions triggered by redox reactions in surface-confined monolayer assemblies into a mechanical deformation. They not only contribute to broader efforts in using surface chemistry-based strategies to increase the surface stress response and sensitivity of micrometer- or nanometer-sized mechanical sensors and actuators,34,39 but also are relevant for molecular electronic and electro-optical devices whose operation features the charge-transfer-induced rearrangement of surface-tethered redox-active molecules.19,40 [Note: The compressive surface stress changes are expressed as negative values in Figures 2 and 3, in accordance with common sign convention.41]

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b04054. Experimental section, discussion of the voltammetric signature of the FcCnSAu SAMs, ferrocene surface coverages, atomic force micrographs of the gold surfaces, sigmoidal Logistic fit example, statistical analyses, surface stress calculation, and additional results, including Tables S1−S6 and Figures S1−S5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Eric R. Dionne: 0000-0002-2509-9110 Antonella Badia: 0000-0002-1026-4136 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada through grants no. RGPIN-03588-2014 and RGPAS-462153-2014. The authors thank Dr. Olga Borozenko and Patricia Moraille (Laboratoire de caractérisation des matériaux) for AFM imaging of the gold surfaces.

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REFERENCES

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DOI: 10.1021/jacs.8b04054 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX