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Confirmation of the Rectifying Behavior in a Pentacoordinate [NO] Iron(III) Surfactant using a Eutectic Ga-In | LB Monolayer | Au Assembly Marcus Shabazz Johnson, Lanka D. Wickramasinghe, Claudio N Verani, and Robert Melville Metzger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11314 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 2, 2016

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The Journal of Physical Chemistry

Confirmation of the Rectifying Behavior in a Pentacoordinate [N2O2] Iron(III) Surfactant using a Eutectic “Ga-In | LB Monolayer | Au” Assembly

Marcus S. Johnson,‡ Lanka Wickramasinghe,† Claudio Verani,*,† and Robert M. Metzger*,‡ ‡Laboratory for Molecular Electronics, Department of Chemistry, University of Alabama, Box 870336, Tuscaloosa, AL 35487, USA †Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 482302, USA

Emails: [email protected]; [email protected] ABSTRACT Examples of coordination compounds that act as molecular rectifiers are rare. Recently a pentacoordinate [N2O2] Fe(III) surfactant, namely [FeIII(LN2O2)Cl] (1), was studied as a Langmuir-Blodgett (LB) monolayer between two Au electrodes, “Au | LB1 | Au”. Rectification was observed, but only at low currents. In order to verify the current rectification of this species, a new setup is used, where an LB monolayer of 1 is placed between Au and a soft contact of gallium indium eutectic (EGaIn), as the “sandwich” “EGaIn/Ga2O3 | LB1 | Au”. When scanned from 0 to -1.5 V, 90% of the sandwiches remained stable, while scanning from 0 to +1.5 V only 10% remained stable. For the scan range of ± 0.7 V, 90% of the sandwiches were stable on the first scan; about half of them could withstand repeated scans; the rectification ratios (RR) at 0.7 V ranged between 3 and 12. Pushing the bias range to ±1.0 V, the RR increased to between 50 and 150, but the sandwiches lasted for at most 3 full scans.

INTRODUCTION Molecular rectification, first proposed by Aviram and Ratner,1 anticipates the feasibility of directional electron transfer via the frontier orbitals of a molecule, thus emulating a semiconductor p–n junction. Experimentally, such rectification can result from three kinds of contributions: interfacial Schottky barrier dipoles (“S”), asymmetric chromophore placement (“A”), and unimolecular processes (“U”) due directly to the donor-bridge-acceptor (D-σ-A) “chromophore” or “electrophore”.2 The effect

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of Schottky barriers on conductance has been highlighted.3,4,5 More recently, rectification by asymmetric Schottky barriers was proposed,6 and solvent-induced polarization was reported.7 The use of physisorbed Langmuir-Blodgett (LB) monolayers of electroactive amphiphilic molecules2,8-14 with local low symmetry reduces the formation of Schottky barriers and totally prevents solvent polarization, and thus enables the study of the A and U mechanisms that rely on the energetic compatibility between the Fermi levels of the electrodes and the relevant frontier orbitals of the molecule. Molecular rectification has largely favored the use of organic systems. Efforts toward the incorporation of transition metal complexes into “electrode|molecule” junctions have been rather slow in development, likely due to its reliance on symmetrical complexes such as nickel and copper phthalocyanines.5,15-17 An early example of rectification mediated by dorbitals in an asymmetric system was attained when a ruthenium(II) ion was coordinated to a thiophene-functionalized bipyridine ligand, and attributed improved conjugation to the planarity acquired upon metallation.18 More recently the Verani group has determined that the high-spin five-coordinate 3d5 redoxresponsive metallosurfactant [FeIII(LN2O2)Cl] (1) shown in Figure 1, [where (LN2O2)- is the doubly deprotonated form of 6,6'-(1E,1'E)-(4,5-bis(2-methoxyethoxy)-1,2-phenylene)bis(azanylylidene)bis(methanylylidene) bis(2,4-di-tert-butylphenol)] displays current rectification when placed as a LangmuirBlodgett (LB) monolayer between Au electrodes as “Au | LB monolayer of 1 | Au” assemblies.19,20 By replacing the axial meta-tert-butyl methoxy (2) with Cl (1) the onset of rectification starts at a lower threshold.20 In Figure 1 the ionic model with attributed charges as well as the red and blue zones are a simplistic effort to identify electron donor and acceptor regions. The observed rectification ratios RR(V) ≡ -I(V)/I(-V) for 1 ranged between 3.99 and 28.6 at V = 2 Volts, and between 2.04 to 31 at V = 4 Volts.20 As measurements were repeated, the RR values became smaller, while the IV response became symmetric. The beginning of rectification (deviation from ohmic, or linear, IV behavior) appeared at about 0.6 Volts, and the currents were fairly low. Indeed for the closely related metallosurfactant [FeIIILN2O3] (2) with a triply deprotonated tris-phenolate ligand, top Au electrode areas of about 0.3 × 0.8 mm2 = 0.24 mm2 were reported19 and a modest 1.5 nA of enhanced current was seen.19 Such small currents are usually observed for single molecules interrogated by an STM tip, where 1 nA would correspond to 6.3×109 electrons per molecule per second. Considering an approximate molecular area of 100 Å2 for 1,19 a monolayer of area 0.24 mm2 should have about 2.4 × 10-7 / 1 × 10-18 = 2.4 × 1011 molecules, and a current of 1.5 nA corresponds to 1.5 × 10-9 / [1.6 × 10-19 × 2.4 × 1011] = 0.039 electrons per molecule per second. For similar “Au|LB|Au” systems used by the Metzger group for the first well-characterized rectifier based on hexadecylquinolinium tricyanoquinodimethanide, up to 4.5 × 104 electrons per molecule per second were measured.8 This seemingly discrepant result led us to hypothesize that the methods used by the Verani group19,20 encountered unexpectedly large extrinsic series resistances in the measuring circuit, such as a point contact. It is reassuring, however, that reversing the electrodes also reversed the rectification,20 so the large series resistances in the sandwich did not affect the sense of the rectification. In order to address this issue we have studied the rectification of 1 using an alternative combination of eutectic gallium/indium and gold electrodes.21 This assembly is described as


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“EGaIn | Ga2O3 | LB monolayer of 1 | Au”, where “EGaIn | Ga2O3“ is a eutectic GaIn droplet covered by a very thin and disordered adventitious Ga2O3 oxide formed by contact with air. The term EGaIn has been popularized by others.22,23 Hereinafter we will use the term “sandwich”2 to describe the contact made with an EGaIn drop on the same LB monolayer. [For these metal-molecule-metal assemblies, most people use the term “junction’, but “junction” can also be used for a single interface (e.g. metal to semiconductor, or metal to organic molecule). Using “sandwich” makes it clear that we have two interfaces on the two sides of a molecule or monolayer].2 Thus, the “sandwiches” described herein are equivalent to the “devices” described elsewhere.19-20

Figure 1. Langmuir-Blodgett monolayer-forming rectifying transition metal complexes 1 and 2, studied previously using a “cold” Au top electrode,19,20 and studied here (for complex 1) using a eutectic GaIn drop top electrode instead. The direction of large rectifying electron flow under forward bias is shown by the hollow arrow. The electron donor region D (red) and acceptor ferric ion A (blue) are also indicated. EXPERIMENTAL METHODS Synthesis of [FeIII(LN2O2)Cl] (1): The ligand H2LN2O2 has been reported elsewhere [20]. A solution of the ligand H2LN2O2 (0.25 g, 0.36 mmol) containing anhydrous sodium methoxide (0.04 g, 0.73 mmol) in a 1:1 MeOH:CH2Cl mixture was treated with FeCl3.6H2O (0.10 g, 0.36 mmol). The resulting solution was kept



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under mild reflux for 4 h, and then cooled to room temperature. The crude product was filtered off and recrystallized at ambient conditions to yield dark brown crystals. Yield: 90 %. ESI (m/z+) in methanol = 742.3641 (100%) for [C42H58N2O6Fe+]. Anal. Calc. for [C42H58ClFeN2O6]: C, 64.82; H, 7.51; N, 3.60%. Found: C, 65.19; H, 7.48; N, 3.69%. IR (KBr, cm-1) 2819-2956 (νC-H), 1606 (νC=C, aromatic), 1507 (νC=C, aromatic) 1585 (νC=N), 1273 (νC-O-C), 1131 (νC-O-C). Physical Measurements: The measurement protocol used here replicates the protocol published elsewhere,21 which in turn is an adaptation of an earlier protocol.8 A solution of 1 in CDCl3 (1 mg/mL) was placed in a NIMA (now KSV) film balance, and the surface pressure-area (Π-A) isotherm was measured at room temperature. The Pockels-Langmuir film (i.e. the monolayer at the air-water interface) was compressed and expanded at relatively low coverage (large area) to probe the reversibility of the isotherm, and, instead of heading to film collapse, the film pressure was monitored as the solution was slowly added, until small 3-dimensional gold-colored crystals were observed: this is the equilibrium spreading pressure:24 ΠESP =13.5 to 14 mN/m: the area at this pressure AESP was about 100 Å2. The values for the collapse pressure and area are Πc = 40 mN/m and Ac = 71 Å2 molecule-1. Langmuir-Blodgett (LB) monolayers of 1 were transferred on the upstroke onto Si at the ESP (with transfer ratio 1.0), and onto Si wafers covered with a fresh thermally evaporated hydrophilic Au film (Au thickness 150 nm). Variable-frequency ellipsometry (J. A. Woollam VB-250 VASE) for an LB monolayer of 1 on Si gave a monolayer thickness of 17.1 ± 0.1 Å. X-ray photoelectron spectroscopy (Kratos Axis 165) of a monolayer of 1 on Si detected a N signal, but no Fe or Cl signal (because of low atom abundance). The IV measurements on the “EGaIn | Ga2O3 | LB monolayer of 1 | Au” sandwiches proceeded in two phases. In the first, exploratory phase, sandwiches were studied non-repetitively to determine which bias was tolerated before either open circuits or short circuits set in. The second phase studied the same sandwich (if stable) several times until short circuits developed. As previously,21 each approach of a conical EGaIn drop (1 mm diameter, 2 mm long, delivered by a plastic syringe to a hook in a Au wire) to the LB monolayer was monitored at a low bias of 0.0001 V as the droplet was lowered very gently using a mechanical micromanipulator (Parker Daedal Division) in an open custom-built steel Faraday cage8 (mounted on a polyurethane base to reduce ambient vibrations) until electrical contact was made, and the initial contact current (ICC) of 1 nA or less was recorded to chronicle a successfully formed sandwich. Sandwiches where such ICC was not seen were not studied further. All subsequent measurements were done with the Faraday cage closed.8


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RESULTS [FeIII(LN2O2)Cl] (1) has a hydrophilic portion at the methoxy terminations, whereas the [N2O2] environment around the metal ion is predominantly hydrophobic, due to the presence of four tert-butyl groups. The species shows evidence of strong Fe(III) ← PhO- ligand-to-metal charge transfer associated to strongly covalent bonds.20 Figure 2 and Table 1 provide the phase-one evaluation of the sandwiches. Ten sandwiches were scanned from 0 to +1.5 V (Figure 2A) and ten more were scanned from 0 or -1.5 Volts (Figure 2C). The positive-bias runs had frequent short circuits once the current went above an initial ohmic region: 9 out of 10 sandwiches failed when attempting the scans out to +1.5 V. Run #4 showed electrical instability: it exhibited a very high current (close to an electrical short circuit), but failed instead as an open circuit. Seven out of the 9 sandwiches that failed presented short circuits and 2 failed by open circuits. The one sandwich that remained stable at positive bias (run #2) had very low currents (0 at 0.6 Volts; (D) Fowler-Nordheim plot (log10 (I V-2) vs. V-1 exhibiting a “transition voltage” minimum Vtrans = +0.4 ± 0.03 Volts at positive bias.

Looking only below 0.7 V, the RR curve of Figure 5B is essentially the same as the RR curve of Figure 3B, but at 1.0 V RR reaches a maximum of 150. The rectification ratios for the ±1 V scans are promising, although the sandwich does not support a large number of successive scans at this field strength. The log10I vs V curve of Figure 5C shows an increase in slope around 0.6 V. The slope in the negative bias is ~1. In the positive bias there are two slopes; between (0.1 -0.5 V the slope is 2.25, between 0.5 – 1.0 V is 5.2. Figure 5D) yields a transition voltage value Vtrans = 0.40 ± 0.03 V. For some reason, positive bias induces device failure more readily than negative bias: compare Figure 2A with Figure 2C: is the device failure due to asperities formed by Au electromigration, or to local reduction of Ga2O3 to Ga forming asperities of Ga? Such changes in electrodes, interfaces, and even molecular vibrations can be important.28,29 The data can be normalized like Figure 4B to show all of the data overlapping directly.



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DISCUSSION The results shown here mostly agree with the previous report.20 Since the work function of the GaIn electrode is about 4.3 eV,22 while the work function of pure Au is 5.2 eV2 (but lowered considerably by adventitious adsorbates),2 it is reasonable that the onset of rectification in the “EGaIn/Ga2O3 | LB1 | Au” sandwiches used in this study should occur at a lower bias than that observed in the previous “Au | LB1 | Au” study. Alas, the effective area of our sandwiches, i.e. the cross-sectional electrically conducting area of the GaIn drop, is not known. Recently, using alkanethiolate monolayers and comparing currents across carefully shaped EGaIn top electrodes with currents across metal electrodes of known area yielded an estimate that the effective electrically conducting area of such EGaIn top electrode was a small fraction (10-4.0±0.5) of its geometrical area.30 Absent a theory that predicts such curves, deciding at what bias the enhanced rectification current will “start” is somewhat arbitrary: Figures 5A or 5B seem to suggest that this enhancement starts around 0.3 V; the transition voltage in Figure 5D suggests 0.4 V; the change in slope in Figure 5C suggests 0.6 V. Monitoring dI/dV may show changes of slope, which hopefully are due to the onset of resonance with applicable molecular orbitals. At negative bias the charge transport seems to be ohmic. The previously reported rectification process20 seems to start around -1.0 V (see Figure 4 of Ref. 20). The available DFT calculations for the system place the SOMO of molecule 1 about 1.0 eV above the pure Au Fermi level.20 The SOMO is a linear combination of 3dxz + 3dyz molecular orbitals with predominant metal character and these MOs are the lowest lying in a five-coordinate Fe(III) ion with an idealized square pyramidal C4v configuration e(dxz1 + dyz1) b2(dxy1) a1(dz21) b1(dx2+y21). Thus the rectification is ascribed to the population (1-electron reduction) of this SOMO, followed by rapid transfer to the Au electrode. Excellent agreement between both measurements is reached when the rectification onset is considered at 0.3 V, therefore suggesting a variant of the asymmetric rectification, where a metal-based SOMO —rather than the expected LUMO—mediates electron transfer. The highest occupied HOMO is lower in energy and remains uninvolved.

CONCLUSIONS In this study we evaluated the rectification properties of the metallosurfactant [FeIII(LN2O2)Cl] (1) placed as a Langmuir Blodgett film between Au and a soft contact of gallium indium eutectic (EGaIn). The resulting “EGaIn/Ga2O3 | LB1 | Au” assembly showed rectification in excellent agreement with previously reported results.20 The fact that the EGaIn/Ga2O3 displays a lower work function is in good agreement with the population of the Fe(III) doubly degenerate SOMO dxz1 + dyz1. These results confirm the viability of using transition metal ions in current rectification, and suggest that SOMO orbitals can be used for electron transport. ACKNOWLEDGEMENTS The authors thankfully acknowledge support from the National Science Foundation through grants NSFCHE-0848206 (RMM) and NSF-CHE-1012413 and NSF-CHE-1500201 (CNV). This includes financial support for MSJ and LDW.


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Eutectic Ga-In drop electrode Defective Ga2O3

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Direction of rectifying electron flow at large positive bias

Gold electrode

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