Size-Dependent and Step-Modulated Supramolecular

Article pubs.acs.org/Langmuir

Size-Dependent and Step-Modulated Supramolecular Electrochemical Properties of Catechol-Derived Adlayers at Pt(hkl) Surfaces Margarita Rodríguez-López,† Enrique Herrero,‡ Víctor Climent,‡,∥ Antonio Rodes,‡,∥ Antonio Aldaz,‡ Juan M. Feliu,*,‡ and Arnaldo Carrasquillo, Jr.*,§ †

Pontifical Catholic University of Puerto Rico, Ponce 00717, Puerto Rico University of Alicante, Alicante E03080, Spain § University of Puerto Rico, Mayagüez 00681-9019, Puerto Rico ‡

S Supporting Information *

ABSTRACT: The electrochemical reactivity of catechol-derived adlayers is reported at platinum (Pt) single-crystal electrodes. Pt(111) and stepped vicinal surfaces are used as model surfaces possessing well-ordered nanometer-sized Pt(111) terraces ranging from 0.4 to 12 nm. The electrochemical experiments were designed to probe how the control of monatomic step-density and of atomic-level step structure can be used to modulate molecule−molecule interactions during self-assembly of aromatic-derived organic monolayers at metallic single-crystal electrode surfaces. A hard sphere model of surfaces and a simplified band formation model are used as a theoretical framework for interpretation of experimental results. The experimental results reveal (i) that supramolecular electrochemical effects may be confined, propagated, or modulated by the choice of atomic level crystallographic features (i.e.monatomic steps), deliberately introduced at metallic substrate surfaces, suggesting (ii) that substrate-defect engineering may be used to tune the macroscopic electronic properties of aromatic molecular adlayers and of smaller molecular aggregates.

1. INTRODUCTION New physical and chemical macroscopic properties can emerge from collective microscopic interactions between aromatic molecules in close spatial proximity. Such collective interactions and the emergence of supramolecular electronic properties are invoked to interpret complex phenomena such as (i) the structure−activity relations responsible for molecular recognition at neurotransmitter receptors,1 (ii) the photophysical properties of dyes and J aggregates,2 (iii) the photoelectrochemical energy transduction schemes that have evolved in nature,3 or (iv) those incorporated into biomimetic photosystems.4,5 Their study is important to an in-depth understanding of nature.6 An evolving technological trend has been the use of aromatic organic molecules in organic electronic devices7 where the envisioned devices range from field effect transistors to solar cells. The theoretical description of fundamental aspects of these technologies relies extensively on a combination of supramolecular and electrochemical concepts, which include the proposed models of organic semiconductor doping,8 of charge delocalization and transport,9 and of electrical charge-injection across aromatic/metal contacts in the devices.10 A more detailed description of supramolecular interactions and of their effects over electrontransfer processes at aromatic/metal interfaces would help advance the design of such organic electronic devices.11 © 2013 American Chemical Society

Additionally, improved atomic-level control and understanding of supramolecular effects will soon be needed to achieve the ultrahigh spatial densities12 of next-generation technologies13 of molecular-electronic (moletronic) elements,14 where a single molecule or a small aggregate of molecules, may constitute a functional moletronic element.15 In the closely related field of electrochemical surface science (ESS),16 the study of aromatic molecular adsorption has been a central topic. ESS merges the inherent surface sensitivity of classical electrochemistry methods17 with surface science techniques and concepts18 to develop an atomic-level understanding of electron-transfer processes at heterogeneous electrode−electrolyte interfaces, including supramolecular effects and their role during molecular self-assembly, directedassembly, or 2D phase transition (2D PT) phenomena. A growing trend, arising from ESS19−21 is that microscopic adsorbate−adsorbate interactions and collective supramolecular effects are ubiquitous and play an important, at times controlling, role. Because they can influence the dynamics of molecular adsorption22 as well as the final equilibrium adlayer structures,23,24 supramolecular effects can determine the overall Received: July 17, 2013 Revised: September 23, 2013 Published: October 11, 2013 13102

dx.doi.org/10.1021/la402627e | Langmuir 2013, 29, 13102−13110

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window of potentials (dE → 0) near the formal potential of the 2D PT (E1/2 = 0.078 V), leading to the growth of a full monolayer atop Pt(111) single crystals. Consequently, the faradaic pseudo capacitance (Ĉ = dQ/dE) at the peak trends toward infinity, Ĉ peak → ∞ and the fwhm → 0. The presence of the 2D PT and of a persistent hysteresis between the anodic and cathodic maxima, even at near equilibrium conditions (limυ→0), imply that the simple relations between surface coverage (Γ), concentration, and electrode potential (E) of classical adsorption models do not apply.38−40 2D PT behavior has been noted during the electrosorption, or "condensation", of many organic discotic molecules at liquid mercury electrodes.19 Understanding the structural and molecular electronic features leading to 2D PT behavior has been a long-standing question in ESS. A fundamental prerequisite, needed to understand the role of supramolecular effects during such electron-transfer phenomena, is that “(scientists) must be able to reason intelligently about the (supramolecular) electronic structure of the compounds they make in order to understand how these properties and structures may be tuned.”41 At present, there is no general satisfactory theory for adsorbate−adsorbate interactions at electrochemical interfaces, and consequently none for related electrochemical 2D PT phenomena.42 By assuming reversible nernstian behavior, many classical models of molecular adsorption have described CV peak morphology, and consequently fwhm measurements, as macroscopic manifestations of entropic and enthalpic contributions, which are influenced by molecule−molecule interactions and reflect the underlying statistical mechanics of the adsorption process. Under Langmuir isotherm conditions, for example, normal (nearly Gaussian) CV peak morphologies are expected17 with fwhm = (90.6/number of electrons) mV. This entropically determined value is derived assuming that adsorption has identical probability at any unoccupied surface site and that the mean-field experienced by the molecules is independent of surface coverage, implying the (net) absence of adsorbate− adsorbate interactions. The observation of peaks narrower than (90.6/number of electrons) mV is ascribed to the presence of (net) attractive interactions within the adlayer, while broader peaks are ascribed to (net) repulsive interactions.38−40 The mean-field affecting such coadsorbed molecules is assumed to vary continuously as a function of surface coverage, in accord with specific superposition principles characteristic of each model. Classical models do not predict the electrochemical responses reported below for o-H2Q at Pt(hkl) electrode surfaces, but they suggest that the pronounced difference, between the surface electrochemistry of o-H2Q and p-H2Q (Figure S0, Supporting Information) at Pt(111) surfaces, could originate from (net) attractive adsorbate-adsorbate interactions in the o-H2Q/o-Q system, highlighting the existence of a complex interplay between molecular orbital topology, adsorbate-to-surface coordination,28 intermolecular donor− acceptor (DA) interactions,22 and molecule aggregation status.43 In ESS, improved theoretical frameworks and experimental approaches are still needed to describe and elucidate such effects. This Article showcases (i) the use of catechol, as a model electroactive aromatic molecular surface probe,22 (ii) the use of deliberately stepped platinum single-crystal electrode surfaces (Pt(hkl)), as model nanostructured metallic substrates,44,45 and (iii) the use of CV, as a surface-sensitive electroanalytical technique, to unveil the role played by monatomic steps in

macroscopic electrochemical response that is measured experimentally during the course of molecular adsorption reactions at electrode/solution interfaces. To date, the state-ofthe-art in electrocatalysis pertaining aromatic molecular catalysts25 lacks a theoretical framework for consideration of the role of electrode surface composition and structure. This has been a long-standing problem since the discovery of aromatic condensation at liquid mercury electrodes. Understanding precisely how do microscopic molecule−molecule interactions influence the emergent macroscopic electrochemical reactivity detected at extended 2D interfaces would improve current understanding of supramolecular effects in other chemical, biological and technological contexts. The electrochemical properties of quinodal redox couples26 have been studied extensively. Quinones are involved in biological systems as neurotransmitters, antioxidants, and energy transducers. A prototypical example is the pbenzoquinone/p-hydroquinone (p-Q/p-H2Q) couple, which in aqueous media reacts in a reversible two electron, 2e−, plus two protons, 2H+, redox process. p-H2Q denotes the parasubstitution of the benzene ring in the 1,4-benzenediol molecule. In ESS studies, quinoidal molecules have also served as prototypical redox-active, aromatic chemisorbates.22−24,27−33 During the early development of ESS as a field of study, one of the first reports of molecular self-assembly dealt with the molecular adsorption of p-hydroquinone monolayers at Pt electrode surfaces.34 Few molecular ESS probes have been studied in such detail. Recently, a surface-selective redox process (eq 1) has been reported for p-H2Q in aqueous sulphuric media:29−32 Q (ads) Pt(111)

Pt(111)

+ (z + 2)e− + (z + 2)H+ ←⎯⎯⎯⎯→

z H(ads) Pt(111)

+ H 2Q (aq) (1)

where z represents the number of protons reduced to adsorbed hydrogen adatoms (Hads) at well-ordered Pt(111) surface domains, when an adsorbed quinone molecule, Q(ads), is reduced to H2Q(aq). The high surface specificity and selectivity of the redox process, occurring only at well-ordered Pt(111) domains, was recently employed analytically to quantify (111) terraces at model Pt(hkl) stepped surfaces and to confirm the presence of ordered (111) facets at preferentially oriented Pt nanoparticles32 using cyclic voltammetry (CV). The study demonstrated (i) that the hard sphere model (hsm), taken as a guide for analysis of electroanalytical measurements, accurately describes aromatic molecular adsorption at model Pt(hkl) stepped electrode surfaces; and (ii) that metal surface reconstructions,35 which may be promoted by some surface analysis methods,36 are avoided when mild electrochemical conditions are used for measurement. At Pt(111), the CV response of the ortho isomer (o-H2Q), that is, the 1,2-benzenediol or catechol molecule, does not conform to the behavior observed for p-H2Q (Figure S0, Supporting Information). o-H2Q is characterized by exceedingly narrow peaks with reported full width at half-maximum (fwhm) values as small as 1 mV.22 Characterization of the reactants and products, using in situ FTIR, revealed that the o-H2Q process takes place according to an electrochemically reversible, compositional 2D phase-transition controlled by collisionnucleation-growth (cng) phenomena22,37 conforming to eq 1. Accordingly, after nucleation under quasi-static conditions at slow electrode potential sweep rates (υ = dE/dt), all the faradaic charge (QT) is transferred within a very narrow 13103


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Figure 1. (a) CVs of well ordered Pt(S)[(n − 1)(111) × (110)] stepped surfaces electrodes collected in 2 mM o-H2Q + 0.5 M H2SO4. Scan rate = 50 mV s−1. T = 25 °C. (b) fwhm as a function of the surface step density, for the anodic branch of the CVs in panel (a); Pt(221), Pt(331), and Pt(110) are excluded. Dotted line connecting experimental measurements does not represent a model and is drawn as a guide. (c) Solid black squares; experimental charge-density as a function of the surface step density, for the anodic branch of CVs in (a) (Pt(110) is excluded) with best-fit line connecting experimental measurements. Open green squares and best-fit line; theoretical charge-density as a function of the surface step density for a surface-limited, domain-selective process assuming one electron (1e−) per Pt(111) terrace atom (see HSM in the Supporting Information). (d) Hard sphere model representation of the Pt(n, n, n − 2) crystallographic planes. The surfaces can be referred as belonging to the Pt(S)[(n)(111) × (111)] vicinal stepped series. However, as the junction of a (111) terrace site and a (111) step site define a (110) step site, the series may be alternatively described as Pt(S)[(n − 1)(111) × (110)], which better describes the electrochemical behavior of these electrodes.

formation41,43 to describe adlayer electronic structure. A rich supramolecular electronic description, of the quinoidal surface aggregates (see Figure 3), of the electrochemical 2D phase transition, and of the step-density dependence at (111) vicinal surfaces, emerges when band formation is considered as a descriptor of the molecular interactions and of the CV experimental results. The results identify important differences between the redox reactivity of aggregated quinoidal aromatic molecules and their isolated counterparts, and identify an emerging theoretical framework from which electrochemical structure−property relations of the aggregates could be derived. Biological, technological and analytical implications are outlined.

modulating supramolecular interactions at well-defined aromatic/metal interfaces during a 2D PT. Well-ordered Pt(111) and related vicinal crystallographic planes were selected as electrode surfaces because the size and density of well-ordered Pt(111) facets, required for the molecular 2D PT and present as nanometer-sized terraces, could be restricted and controlled experimentally32,44,46 by the crystallographic introduction of monatomic steps. At the Pt(111) terraces/facets, nanometersized aromatic-derived adlayers of o-Q(ads) were synthesized and studied electrochemically via the 2D PT described by eq 1. This study shows the feasibility of fully suppressing the characteristic 2D PT of the o-H2Q/o-Q system at well-ordered Pt(111) terraces by introducing monatomic platinum steps, possessing a saddle-type geometric structure, that behave electrochemically as Pt(110) steps. In their presence, fwhm measurements show a systematic increase above a threshold step density (Figure 1). In the text, these Pt(n, n, n −2) crystallographic planes are described as Pt(S)[(n − 1)(111) × (110)] stepped surfaces. Surprisingly, not all monatomic steps are capable of fully suppressing the 2D PT. Results from a second vicinal series, that is, Pt(n + 1, n − 1, n − 1) crystallographic planes, consisting of Pt(S)[(n)(111) × (100)] stepped surfaces, demonstrate this effect. When the geometry of the steps is changed to a square-type geometric structure, that is, Pt(100) symmetry, the magnitude of the fwhm measurements shows a complex modulation pattern as a function of step density (Figure 2): (i) initially increasing, (ii) later decreasing above a threshold value of surface step densities, (iii) to finally disappear abruptly. These seemingly complex structure−property relations are successfully interpreted by adopting basic principles of band-

2. RESULTS After electrochemical characterization (Supporting Information Figures S1 and S2), each clean and ordered Pt(hkl) single crystal electrode was transferred into a 0.5 M H2SO4 solution containing 2 mM o-H2Q(aq) to produce Pt(hkl) electrodes fully coated with o-Q(ads). The oxidative chemisorption of catechol is spontaneous at Pt(hkl) surfaces.22,27,47 After equilibration, each electrode was cycled voltammetrically until a steady-state CV was achieved, as shown in Figures 1 and 2. The disappearance of the (bi)sulfate and hydrogen adsorption, characteristic of the clean Pt(hkl) electrodes (Supporting Information Figures S1 and S2), reveals the formation of compact o-Q(ads) layers on the Pt(hkl) surfaces. The redox pair at ca. 0.08 V, consistent with eq 1,22 is the process of interest in this study. All 24 CV experiments involved identical electrode composition and identical experimental pretreatments. Thus, the pronounced differences observed in Figures 1 and 2 result from the 13104


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Figure 2. (a) CVs of well ordered Pt(S)[(n)(111) × (100)] stepped surfaces electrodes collected in 2 mM o-H2Q + 0.5 M H2SO4. Scan rate 50 mV s−1. Temp. 25 °C. (b) fwhm as a function of the surface step density, for the anodic branch of the CVs in panel (a); Pt(311) and Pt(100) are excluded. Dotted line connecting experimental measurements does not represent a model and is drawn as a guide. (c) Solid black squares; experimental charge-density as a function of surface step density, for the anodic branch of CVs in (a) (Pt(311) and Pt(100) excluded). Open (i) red squares or (ii) green circles and best-fit lines; theoretical charge-density as a function of the surface step density for surface-limited processes assuming one electron per (111) terrace atom (i) with or (ii) without a charge density contribution of 1e− per Pt(100) step. Dotted line connecting experimental measurements does not represent a model and is drawn as a guide. (d) Hard sphere model representation of the Pt(n + 1, n − 1, n − 1) crystallographic planes. These surfaces are more conveniently described as the stepped vicinal series Pt(S)[(n)(111) × (100)].

crystallographically controlled, atomic-level structural differences at the single-crystal Pt(hkl) surfaces. Figure 1a shows independent CV experiments performed at individual Pt(S)[(n − 1)(111) × (110)] electrodes. For reference, the electrochemically controlled 2D phase transition behavior at well-ordered Pt(111) is also shown. It is important that the redox process at ca. 0.08 V has only been reported, at well-ordered Pt(111) domains22 and is not observed at other basal planes, i.e. Pt(110) (Figure 1a) or Pt(100) (Figure 2a). The peak current density, indicative of the dynamics of the faradaic reaction, decreases (nonlinearly) as a function of the step density. The integrated charge density (Figure 1c), which is indicative of the extent of the faradaic reaction, decreases continuously as a function of step density. Note that, at Pt(110) steps, equation 1 is not indicated. During the CV experiments, Pt(110) steps remain coated due to strong irreversible molecular coordination (Supporting Information Figure S3). The theoretical charge density, predicted from the hard sphere model for a domain selective process taking place selectively at well-ordered Pt(111) terrace atoms, was calculated assuming one electron per terrace atom, as detailed in the Supporting Information, and is shown for reference as the green solid trendline in Figure 1c. The agreement between experimental measurements and theoretical charge density are consistent with a surface redox process precluded at Pt(110) steps. When the step density increases above a threshold value of 1.4211 × 106 cm−1 (i.e., corresponding to n = 30 and to a step-to-step separation of ca. 7 nm), Figure 1b reveals an increase in the fwhm ranging from the initial value of 5.5 mV up to 42.8 mV. For noninteracting redox centers, a constant value of (90.6/ number of electrons) mV would have been expected.17

Accordingly, above this threshold, all experimental observables indicate the disappearance of the reversible narrow peaks characteristic of the catechol 2D phase-transition at the Pt(111) basal plane in accord with a simple superposition effect due to Pt(110) steps, which remain coated with irreversibly adsorbed molecules. As will be discussed latter, in the context of this experimental design, the Pt(110) monatomic steps have served as an effective potential energy barrier to the propagation of the 2D PT; that is, the 2D PT is confined to the Pt(111) terrace sites due to the presence of irreversibly adsorbed molecules at Pt(110) steps. Note that the height of the potential energy barrier (and its physical width) can be controlled by crystallographic selection of a different atomic geometry at the monatomic step as shown next. Figure 2a shows independent CV measurements for each of the Pt(S)[(n)(111) × (100)] electrodes for the Pt(n + 1, n − 1, n − 1) planes. The CV response at the Pt(111) basal plane is also shown for reference. In Figure 2b and c, the anodic fwhm and peak charge density are plotted, respectively, as a function of the (100) step density. At low step densities the dependence is similar to the behavior in Figure 1; that is, the peak current densities decrease with concomitant increases in fwhm. Surprisingly, the trend is reversed above a threshold value of step density in the presence of Pt(100) square-type steps. Above this new threshold, (i) the peak current densities increase and (ii) the fwhm decrease as a function of step density. Such reversals are highly unusual and are not expected from simple superposition principles used to describe the effects of systematically introduced surface defects, such as steps, at single-crystal surfaces. In Figure 2c, the (i) red and (ii) green solid best-fit lines are derived from anodic charge 13105


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Figure 3. Schematic representation of energy (E) for frontier energy levels of hypothetical molecular aggregates at Pt(111) terraces. Open circles represent aggregate LUMO energy levels and closed circles represent aggregate HOMO energy levels as a function of surface aggregate size (N). Donor−acceptor interactions, charge-transfer, and coordination effects are not considered except for the discontinuity in the y-axis. No specific mode of coordination or hapticity has been experimentally established. The molecular orientation has been arbitrarily selected.

the time of this writing, ESS suffers from an absence of theoretical objects that may serve as supramolecular descriptors of electronic structure. To fill this void, the conceptual simplicity of band formation, between aromatic molecules self-assembling on the surface of the well-ordered metallic substrate, provides a convenient descriptor that can be related to the classical concept of adsorbate−adsorbate lateral interactions, for example, via the quantum mechanical concept of the interaction parameter or electronic coupling. Band formation principles are already used as descriptors of small 1D oligoacene assemblies43 and of larger 3D9 aromatic organic semiconductor crystals and polymers. Based on the topology of the molecular orbitals, orthoquinone can be viewed as a small oxoacene oligomer, fitting in the broader context of acene supramolecular chemistry. Figure 3 summarizes the electrochemically relevant electronic structural features of the frontier orbitals expected as a result of band formation due to π−π orbital overlap in a model molecular surface-aggregate formed by a number (N) of neutral adsorbed o-H2Q molecules. Figure 3 is derived48,49 (assuming (i) aggregation along a complex one dimentional (1D) reaction coordinate, (ii) a constant equilibrium distance to yield an interaction parameter (δ) between molecular orbitals (MOs), (iii) whose energy (Ek) varies according to Ek = αMO + 2δ cos(kπ/N + 1), where k = 1, 2,..., N and αMO is the energy of each MO in the absence of aggregation) and is used as a qualitative model. For simplicity: (iv) the interaction parameters (δ) were assumed to be identical for both HOMO and LUMO levels, and (v) only their relative energy levels (αHOMO, αLUMO) was varied. (vi) Donor− acceptor interactions, charge-transfer, and coordination effects are not considered except for the discontinuity in the y-axis. Open circles represent aggregate LUMO energy levels (E1) and closed circles represent aggregate HOMO energy levels (EN) as a function of aggregate size or number of molecules (N). As depicted in Figure 3, electrified metal−solution interfaces are characterized by the existence of a potential difference, associated to the difference of Fermi level between species in

densities theoretically calculated by assuming one electron per Pt(111) terrace atom (i) with or (ii) without (red and green, respectively) charge-density contribution of one electron per Pt(100) step atom. The experimental charge-density measurements for the anodic process, plotted in Figure 2c, do not conform to the theoretical charge density expected from a process precluded at Pt(100) steps. The positive deviations in the experimental charge densities become substantial at high step densities, consistent with a process taking place at both terrace and step sites. The positive deviation in charge density abruptly disappears when the narrowest Pt(111) terrace, that is, when the 1D limit, is reached (see Figure 2a).

3. DISCUSSION The experimental results in Figures 1 and 2 are revealing because they are consistent with expectations from modern band-formation models, such as the simple supramolecular electronic-structure model depicted in Figure 3. This description of catechol-derived surface aggregates rests on the hypothesis that the molecular orbitals of the repeating molecular units will combine linearly and thus afford band structures, providing a quantum framework for interpretation of electrochemical reactivity involving aromatic molecular adsorption phenomena. The assumption is made that π−π intermolecular interactions, resulting from the overlap between orbitals of adjacent molecules, would lead to the creation of narrow π-bands in the ground state of the neutral organic surface aggregates or molecular assemblies. The possible formation of catechol-derived aggregates at well ordered Pt(111) is not an ex nihilo assumption. In the past, detailed structural studies of related systems23,24 have documented the formation of hydroquinone-derived molecular aggregates on well-ordered Pt(111) and Rh(111) surfaces. As is the case in organic crystals, the magnitude of these π−π intermolecular interactions, that is, the electronic coupling, would be determined by molecular orbital topology, spatial orientation, and intermolecular distances within the surface aggregates. At 13106


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predicted by the band-formation model explain the increases in fwhm, experimentally observed in Figure 1b, as Pt(110) stepdensity increases. A fundamental assumption is that terrace-toterrace interactions (i.e., interterrace band formation) be precluded by the presence of Pt(110) steps. This assumption is consistent with the highly anisotropic electronic properties measured experimentally at 3D solid-state aromatic crystals,50 which are sensitive to subatomic level displacements in the periodic orientation of the molecules in the 3D crystals and to small disruptions in the organic crystal momentum space, even for 2D systems.51 Pt(110) steps introduce a significant disruption in momentum space with respect to the (111) basal plane periodicity. According to the hsm in Figure 1d, the terrace-to-terrace distance (3.20 Å) introduced by a Pt(110) step is four thirds (4/3) the normal interatomic row distance IRD = (d(√3/2)) on the Pt(111) crystal plane, where d is the Pt metallic diameter. The terrace-to-terrace distance is important because π−π interactions are expected to decay exponentially with distance.49,50 This terrace-to-terrace distance (3.20 Å) is larger than the interplanar separations of known quinonoid π dimers (2.9 Å).52 Due to their geometry (Figure 1d), Pt(110) steps also exhibit chemical coordination properties28 which differ from the Pt(111) crystal plane (Supporting Information Figure S3). Pt(110) surface sites, being occupied by irreversibly adsorbed redox-inactive molecules (Figure S3), prevent the MO overlap topology characteristic at the Pt(111) crystal plane, interfering with band spreading and energy degeneracy, hence explaining the observed increases in fwhm and the trend toward 0D electrochemical behavior. This is comparable to the well-documented effects of thermal energy over the spectroscopic behavior of J-aggregates,2 where the statistical mechanics relating fwhm to thermally induced disruptions in the intermolecular interactions within the ensembles have already been described.53 Deliberately Stepped Pt(S)[(n)(111) × (100)] Electrodes. A static hsm and Figure 3 may be used to interpret the reemergence of the 2DPT and the associated modulations of fwhm reported in Figure 2c. These imply a re-establishment of the π−π MO overlap leading to π-band and surface molecular aggregate formation as step-density increases. The hsm predicts that a terrace-to-terrace distance of 1.60 Å is introduced by square-type Pt(100) steps, corresponding to two-thirds (2/3) the IRD of the Pt(111) surface (Figure 2d). The presence of a Pt(100) step represents a small compressive disruption (i) with respect to the atomic IRD and periodicity at the ideal (111) terrace, or (ii) with respect to the van der Waals diameter of the carbon atoms, ca. 3.4 Å, in the molecules. Additionally, as demonstrated experimentally in Figure S4 (Supporting Information), molecules interacting with Pt(100) steps are labile at high step-density surfaces implying that terrace-toterrace band formation is possible and must be considered in that limit. At those surfaces, the behavior of molecules during the growth stage of the 2D PT, and specifically their oxidative chemisorption, will be determined by the change in surface tension (energy per unit surface area) upon incorporation of a molecule into the surface aggregate (Eaggregation). This energy term is dependent on the value of (dEHOMO/dNsize), which is a (quantized) function of aggregate size and is represented in Figure 3 by the slopes between the points on the lower curve. Accordingly, the quantum of energy gained from incorporation of additional aggregate molecules upon oxidative chemisorption, that is, after electron transfer, varies with (dEHOMO/

the electrolyte and electrons at the electrode metal surface atoms, that can promote electron transfer between the Fermi level of the electrode (Ef metal) and the electroactive species. Such electron transfer processes can be conveniently studied by using classical electrochemical techniques such as cyclic voltammetry (CV). Typically, electron removal from the highest occupied MO (HOMO) of the molecule or, alternatively, electron transfer to the lowest unoccupied MO (LUMO) takes place. Correspondingly, only the frontier HOMO and LUMO energy levels of the molecular aggregates are represented in Figure 3: (i) starting at the left side, for the isolated (0D) molecular species and (ii) progressing toward the right, for aggregates with increasing size and dimensionality along the reaction coordinate, that is, increasing number of molecules (N). For molecular surface aggregates in the semiconducting regime, the HOMO levels would correspond to the top of the filled valence band while the LUMO levels correspond to the bottom of the empty conduction band. As depicted, π−π interactions initially lead to a systematic increase (decrease) in the HOMO (LUMO) energy levels of the molecular surface aggregate, as a result of band spreading. As the aggregate size (N) increases, a limiting energy value is quickly reached and from there on the degeneracy of the levels increases sharply therein from this threshold. As a proof-ofconcept, the oxidative chemisorption of o-H2Q is considered below assuming near equilibrium conditions. Pt(111). According to this model, the characteristic 2D phase transition for the oxidative-chemisorption of catechol at Pt(111) takes place if a critical aggregate size is reached, (Ncritical) in Figure 3. At higher (N) values along the surfaceaggregate-size reaction coordinate, electron-transfer, from the aggregate to the fermi level (Ef1) of the Pt(111) metal electrode, takes place spontaneously (depicted by the blue arrow). At this electrode potential, spontaneous electrontransfer promotes increases of aggregate size (growth) while disfavoring new nucleation events. The conditions imply continuous, site-selective electrochemical reactivity if a perfect single-crystal surface can be assumed. Hence, under quasiequilibrium conditions nearly all charge (QT) would be transferred in a very narrow window of potentials for a narrow up-running band. The band-formation model hence provides a rationale for the collision-nucleation-growth dynamics experimentally determined for the catechol system at the wellordered Pt(111) electrodes.22 It is important that the electronic structure of a 2D (supramolecular) aggregate is markedly different from that of an isolated (0D) molecule. It is intuitive from this model that disordering the electrode surface, which serves as a template for molecular aggregate alignment, would prevent the long-range intermolecular π−π orbital overlap and interfere with the supramolecular electronic structural effects. Strong irreversible molecular coordination to the electrode surface would have similar consequences. Deliberately Stepped Pt(S)[(n − 1)(111) × (110)] Electrodes. According to Figure 3, decreasing or limiting the size of molecular aggregates decreases the energy degeneracy and reverts band-spreading, lowering the HOMO energy level and promoting molecular (0D) behavior. Hence, limiting the size of Pt(111) terraces, a consequence of introducing irreversibly occupied Pt(110) steps, would limit the dimensionality of intraterrace 2D supramolecular aggregates, and interfere with band-formation, promoting 0D molecular reactivity and behavior, for example, as step-density increases an increased number of nucleation sites are required. The size effects 13107


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or site-dependent electrochemical properties of this important class of biomolecules.

dNsize), which is largest for smaller aggregates and negligible for large aggregates. Depending on the relative size of the aggregation energy contribution compared to the surface coordination energy (Ecoord), molecules interacting with Pt(100) steps in the vicinity of the propagating 2D PT may exhibit either of two limiting behaviors as a result of energy minimization: (i) irreversible coordination at the step if Ecoord ≫ Eaggregation (i.e., exhibiting a smaller tendency to aggregate, disfavoring band formation) or (ii) strong aggregation if Ecoord ≪ Eaggregation, favoring band formation, (i.e., exhibiting weaker, more labile coordination at the step). As per Figure 3, at high step-density and small terrace sizes, that is, small aggregate size, the energy difference due to aggregation becomes largest, relative to the coordination energy at Pt(100) steps. At the high step-density limit, molecules (weakly) coordinated at Pt(100) steps no longer serve as effective potential energy barriers with respect to propagation of the 2D PT among Pt(111) terraces. In this limit, terrace-to-terrace aggregation effects dominate energy minimization, favoring the incorporation of molecules near the step to the growing surface aggregate, leading to interterrace band formation and to the “unexpected” decreases in fwhm due to the re-establishment of the freely propagating 2D PT predominant at extended well-ordered Pt(111) domains. This trend is reinforced by the reversal of the Smoluchowski effect54 arising from dipole−dipole interactions between steps at high surface step-densities, expected to be largest at Pt(100) steps due to their relative dipole spatial orientation as portrayed in Figures 1d and 2d.55 Finally, at the turning point of the vicinal series, that is, Pt(311), the highest (100) step density is reached and the 2D phase transition vanishes as expected from Figure 3 for a system that does not reach the critical dimensionality or as expected, from the Peierls theorem, for a 1D system.41

5. EXPERIMENTAL SECTION Aqueous 0.5 M H2SO4 solutions were used as supporting electrolyte throughout the voltammetric study. They were prepared from concentrated sulfuric acid (Merck Suprapur or Aldrich Teflon grade) and Purelab Ultra (Elga-Vivendi) water (18 MΩ cm). This electrolyte is convenient because the adsorption states at Pt(hkl) surfaces have been thoroughly studied and are well-defined voltammetrically. Pt(hkl) single crystal electrode surfaces were prepared from single-crystal beads using the procedures developed by Clavilier et al.60 All Pt(hkl) single crystal surfaces have been cooled in an atmosphere of argon/ hydrogen. Working electrode diameter were around 2 mm for the voltammetric experiments. Hanging meniscus configuration was used throughout. All experiments were conducted at room temperature, 25 °C (±2 °C). o-H2Q and p-H2Q were obtained from Aldrich and used as received. High purity gases (5N) were used. An EG&G PAR Model 175 Universal Programmer, an AMEL 551 potentiostat, a Soltec XY recorder, and an eCorder401 (eDAQ, Australia) were used in the voltammetric experiments for the Pt(hkl) electrodes. Platinum counter electrodes were used and all potentials were measured and are reported versus the reversible hydrogen electrode (RHE) with the same supporting electrolyte solution. These were contained in a separate compartment from the working electrode with contacts made through a Luggin capillary.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

́ ́ *(J.M.F.) Mailing address: Departamento de Quimica Fisica, ́ Instituto Universitario de Electroquimica, Universidad de Alicante, Apartado 99, E03080, Alicante, Spain. E-mail: juan. [email protected]. Tel: + 34 965 903 535. Fax: + 34 965 903 537/ 3464. *(A.C.) Mailing address: Department of Chemistry, University of Puerto Rico, Mayagüez, PR, 00681-9019. E-mail: arnaldo. [email protected]. Tel: 787-832-4040 x2386. Fax: 787-2653849.

4. CONCLUSIONS The results in Figures 1 and 2 reveal structural limits to the spatial density of molecular features before manifestation/ modulation/confinement of long-range supramolecular electronic effects are observed at aromatic−metal interfaces. According to these results, achievable spatial density limits of molecular features will depend on controlling both the (sub)atomic-level distances and the structure and composition at the intervening spatial regions56 separating aromatic molecular elements at substrates, with implications to organic electronic technology, moletronic devices, and electrochemical sensor designs. Such effects will be important when heterostructural materials are considered, suggesting that substrate-defect engineering may be used to fine-tune the macroscopic electronic properties of aromatic molecular adlayers and of smaller molecular aggregates. The construction of chemical knowledge is facilitated by qualitative guiding rules (“thumb rules”) with predictive/ explanatory character. In the absence of supramolecular descriptors, Figure 3 suggests a useful framework from which electrochemical structure−property relations may be derived for ortho-quinone analogues in aggregated states, with potential impact on structure−activity relationships of bioactive molecules,1 of solid-state hybrid materials,57 and of photoelectrochemical sensitizers.58,59 The wealth of biological processes involving catechol-related biomolecules warrant special notice to the important role that π−π interactions, including self-aggregation, can play in determining site-selective

Author Contributions ∥

V.C. and A. R.contributed equally to the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ACJ and MRL thank support from PCUPR, UPRM, and the Institute of Electrochemistry at University of Alicante through project MICINN (Spain) CTQ2010-16271 and Generalitat Valenciana (project PROMETEO/2009/045, Feder). ACJ thanks Benoit Braida for important bibliographic contributions.

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