Connecting Solution-Phase to Single-Molecule Properties of Ni

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen) Yi C. Zhang, Bhaskar Chilukuri, Tanner B. Hanson, Zachariah M. Heiden, and David Y. Lee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01381 • Publication Date (Web): 05 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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The Journal of Physical Chemistry Letters

Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen) Yi C. Zhang,† Bhaskar Chilukuri,‡ Tanner B. Hanson,‡ Zachariah M. Heiden,∗,‡ and David Y. Lee∗,† †Department of Chemistry and Materials Science & Engineering Program, Washington State University, Pullman, WA 99164 ‡Department of Chemistry, Washington State University, Pullman, WA 99164 E-mail: [email protected]; [email protected]

Abstract

Graphical TOC Entry

We present a strong correlation of Ni(salophen) structure and properties measured in singlemolecule vs. bulk quantities and in ultra-highvacuum vs. solution phase. Under a scanning tunneling microscope (STM), Ni(salophen) forms a self-assembled monolayer (SAM) on Au(111) at 23 °C with molecular structure identical to the X-ray crystallographic measurement. The HOMO and LUMO levels are determined using elastic tunneling spectroscopy at the single-molecule level with confirmation by monolayer-quantity ultraviolet photoelectron spectroscopy (UPS) and by cyclic voltammetry (CV) measurements. These STMdetermined HOMO-LUMO gap of 3.28 eV and (HOMO-1)-HOMO gap of 0.36 eV form a new foundation for the selection of hybrid functionals with a simple basis set to be effective in accurately calculating single-molecule Ni(salophen) frontier MO levels. Our results suggest that microscopy-based experiments on surface, along with free-molecule gas-phase calculations, can provide useful insights into physical properties of the metal(salen) complexes, especially when such direct measurement is not available in solution.

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The diverse functionality of metal-salen complexes (where salen is the N,N’-bis(salicylidene) ethylenediamine motif; Figure 1) have inspired the use of these molecules in numerous disciplines including inorganic, 1–4 organic, 5,6 biological 7–10 and materials 11,12 chemistry. For example, metal-salen complexes have been employed as conducting agents in lithium-ion batteries, 13 as molecular sensors in bioimaging applications, 4 in medicinal applications, 4,14 in optical applications, 15 as catalysts in the epoxidation of olefins 5 or in the production of polycarbonates, 1 to name a few. The attractiveness of implementing salen ligands in molecular reactions, besides their ease of synthesis, 16 is their ability to provide a relatively rigid tetradentate environment (similar to porphyrins 17,18 ) that can be utilized to influence the steric and electronic properties of their respective metal complexes in a predictable manner. In general for molecules with unique properties, their chemical reactivities have been widely studied using solution-based or computational methods. 19–22 In particular for metal-salen, intermediate species (such as metal–hydrides, metal– oxides, and metal–nitrides) have been proposed to exist in solution-phase reaction mechanisms, 23–25 but their short lifetimes have made it experimentally unfeasible to probe these intermediate species in solution.

to the mechanisms proposed in a solution environment, depends heavily on the initial states and structures of the molecules before reaction. In other words, one cannot simply assume solution-phase properties can be directly transposed onto a surface environment for the same molecule. 33 In this work, using a combination of experimental and computational techniques, we demonstrate that the structures and physical properties of Ni(salophen) can be comprehensively correlated in the solution phase vs. in a self-assembled monolayer (SAM) on top of a Au(111) surface in ultra-high-vacuum (UHV). Detailed experimental and computational procedures are provided in the Supporting Information (SI), and all the experiments are performed at 23 °C for the purpose of effective comparison between the different reaction environments.

Figure 1: Comparison of the general metalsalen motif to the metal-salophen motif investigated in this study.

Figure 2: (a) A 1200 ˚ A × 1200 ˚ A STM image of partial Ni(salophen) SAM on Au(111) with its chemical structure drawn in the lower left corner. STM parameters: -300 mV and 350 pA. (b) A 150 ˚ A × 150 ˚ A closer view of the SAM. Colored features are discussed in text. STM parameters: -30 mV and 350 pA. (c) A 80 ˚ A× ˚ 80 A image showing the SAM unit cell. STM parameters: -10 mV 350 pA. (d) Comparison of molecular geometries measured using STM vs. X-ray crystallography. 34 All STM images are collected in UHV at 23 °C.

The ability to place metal complexes on surfaces followed by localized microscopy/spectroscopy measurements of stepwise gas-surface or solution-surface interactions at the single-molecule level 26–32 holds great promise to provide experimental insights into reaction mechanisms not obtainable in solutionphase measurements. However, the validity of reaction steps probed on a surface, as compared


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Structures. Although it has been reported that a halogenated Co(salophen) complex can be isolated and imaged in UHV at a temperature of a few Kelvin using a scanning tunneling microscope (STM), 35 we demonstrate within the white dashed boundary in Figure 2(a) that a SAM of Ni(salophen) (molecular structures illustrated in the lower left corner) can be stably formed on top Au(111) at 23 °C. The Ni(salophen) SAM is shown in Figure 2(b) to be a highly ordered striped pattern with individual phenyl rings and the central Ni atom resolved for each molecule. A significant feature of this SAM is that there are two “rows” of Ni(salophen) molecules within each stripe with the row directions alternating between adjacent stripes. A semi-transparent red arrow is superimposed on the molecular structure in Figure 2(a) with similar arrows drawn in panel (b) to illustrate the alternating molecular surface pattern. In addition, one of the gaps between two adjacent stripes is highlighted with a bold red line in both Figure 2(a) and (b) as visual aids. The angle between the SAM stripes and the Au(111) herringbone shown in Figure 2(a) is 44.8°; however, other angles (between 37.0° and 43.1°) have been observed on various samples, which imply that there is no strict relationship between the SAM and Au surface structures. Due to the above-mentioned alternating pattern of the Ni(salophen) SAM, the unit cell spans across two stripes with its dimensions drawn and indicated in blue in Figure 2(c). Each unit cell contains twelve phenyls and four Ni atoms corresponding to a total of four Ni(salophen) molecules. The direct comparison of the STM measured single-molecule structure on Au(111) vs. the X-ray crystallography (XRC) determined molecular geometry 34 is illustrated in Figure 2(d) for Ni(salophen) and revealed little distortion upon forming a stable SAM on Au(111) at 23 °C. Although STM is powerful to resolve submolecular structures, computational analysis is needed to provide insights into the Ni(salophen)–Au interaction. Since calculations using non-van der Waals (vdW) DFT functionals severely underestimate molecular

Figure 3: (a) top and (b) side view of DFT optimized physisorption configuration of a single Ni(salophen) molecule on Au(111). adsorption energies on metal surfaces, 36,37 periodic calculations employing vdW DFT functionals are used to determine Ni(salophen) adsorption energy on Au(111). Computational analysis reveals -2.74 eV adsorption energy, which is about 2/3 of the adsorption energy of a metal porphyrin molecule on the same surfaces. 38,39 The DFT optimized Ni(salophen)– Au geometries are shown in Figure 3. From the perspective that either the adsorption energy of a metal(salen) or a metal(porphine) complex is an accumulation of vdW interactions of the constituent atoms within each molecule with the gold surface (i.e. physisorption without bond formation to the gold), the computed adsorption energy agrees with the fact that a Ni(salophen) complex can be approximated as 3/4 of a Ni(porphyrin) in chemical structure. Since the computed adsorption energy ratio of 2/3 is less than this structural ratio of 3/4, the adsorption for Ni(salophen) on Au(111) is even weaker than this simple cumulative vdW picture. The DFT optimized physisorption distance between Ni(salophen) and Au(111) is 2.6±0.1 ˚ A, which is comparable to previously reported ∼2.9 ˚ A for Co-octaethyl porphyrin (CoOEP) physisorption on the same substrate. 36 Physical Properties. Based on reports describing the physisorption of metal(porphyrins) and metal(phthalocyanines) on a Au(111) surface, 40–44 molecular orbital (MO) levels of the adsorbed species in the ±2 eV range relative to the Au(111) Fermi energy (E f ) can be directly measured at the single-molecule level using STM-based orbital-mediated tunneling spectroscopy (STM-OMTS) with a tip–


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geometry, in addition to the computed adsorption energy being comparable to that of a metal(porphyrin), the Ni(salophen) frontier MO levels are expected to be unambiguously determined using these same experimental techniques as those of metal(porphyrin) complexes. The combined STM-OMTS, UPS and cyclic voltammetry (CV) results for Ni(salophen) are shown in Figure 4 with the origin of the absolute energy scale set at the vacuum level. The black spectrum in Figure 4(a) shows UHV STM-OMTS measurement for Ni(salophen) on Au(111) at 23 °C scanned between -2 eV and +2 eV relative to the Au(111) E f (the bottom abscissa). Overall, the “U” shaped spectrum clearly indicates a substantial band gap, and the flatness in the middle of the gap further indicates that Ni charge transfer (especially for the Ni(II) → Ni(III) + e− oxidation) does not occur in this ±2 eV range. 48 For UPS, although various Fermi energies have been reported for Au(111) between 5.20 and 5.43 eV, 40,49–51 we add the value of 5.20 eV (which was previously determined using the same UPS/XPS machine here at WSU 40 ) to the UPS binding energy (the top abscissa) for direct comparison to the STM-OMTS result. The difference in UPS data for Ni(salophen) (red ) and for a clean Au(111) substrate (red n) clearly shows the HOMO level at ∼6.65 eV for the Ni(salophen) monolayer on Au(111) (red arrow), and this energy level is readily observed as a shoulder in the STM-OMTS spectrum at ∼6.66 eV. The solution-phase CV result in Figure 4(b) shows that the first reduction level is at -1.38 eV and the first oxidation level is at +1.23 eV (2nd oxidation at +1.38 eV) vs. SCE. When 4.7 eV is added, the reduction level (1.38 + 4.7 = 3.32 eV) agrees well with the STM-OMTS LUMO peak observed at 3.37 eV as indicated in Figure 4(a). However the CV oxidation level of 1.23 + 4.7 = 5.93 eV lies shallower than the STM-OMTS/UPS measured HOMO level of ∼6.66 eV. Armstrong and coworkers 52 have determined a correction factor of 1.71±0.15 that works well to correct this discrepancy for metal(phthalocyanine) and metal(porphyrin) complexes. 40,42,43 In our case

a

b Figure 4: (a) Single-molecule STM-OMTS (shown in black) and ensemble UPS measurements of Ni(salophen) on Au(111) shown in red

(vs. bare Au(111) shown in red n) with origin of the absolute energy scale set at the vacuum level. (b) Cyclic voltammogram of a 1 mM solution of Ni(salophen) in dichloromethane. Scan rate = 100 mV/s, electrolyte = nBu4 NPF6 (0.1M). sample separation of >500 MΩ. Furthermore, the STM-OMTS measured occupied MO levels can be confirmed using ultraviolet photoemission spectroscopy (UPS) measured for an ensemble quantity of ∼1 surface monolayer (∼1012 molecules). The unoccupied MO levels can also be confirmed with electrochemical measurements in a solution environment that exhibit the offset value of 4.7 eV relative to the saturated calomel electrode (SCE); i.e. E1/2 (vacuum) = E1/2 (SCE) + 4.7 eV. 45–47 Due to the similarity between the STM-probed molecular structure on Au(111) and the XRC


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SI). Since the chemical reactivity of a molecular complex is largely dependent on the location of its frontier MO energy levels, our results suggest that surface STM measurements and gas-phase calculations can be used to provide insights at the single-molecule level into future homogeneous and heterogeneous reactions for metal(salen) complexes. Experimental Methods A more detailed description of experimental methods is provided in SI. Essentially, Gold (111) films were deposited on mica in a cryo-pumped chamber with a base pressure of 2 × 10−9 Torr. Ni(salophen) was prepared by the procedure reported previously reported by Teixeira and coworkers, 58 which matched the characterization data reported by Kleij and coworkers. 59 A 25 µL of 5×10−4 M Ni(salophen) in chloroform droplet was dropcast onto a freshly annealed Au(111) film before introduction into the UHV STM chamber. The sample was further annealed in UHV at 92° C for 1 hour, and a subsequent anneal at 130° C for 1 more additional hour. The UHV STM (model PanScan) and controller (model R9) were purchased from RHK technologies . All images were acquired in constant current mode at room temperature using chemically etched Pt0.8 /Ir0.2 tips. STM-OMTS measurements were performed by placing the tip on top of a Ni(salophen) molecule in the middle of the monolayer followed by measuring dI/dV as a function of bias voltage at a fixed sample-tip separation of 2V and 800pA. UPS measurements were conducted by following the previously reported procedure 40 using a home built machine with a He lamp producing two resonances lines via cold cathode capillary discharge at a working pressure of < 10−9 torr. CV experiments were performed at room temperature in nitrogen-purged dichloromethane solutions using a CH Instruments Model CHI600E electrochemical workstation with a glassy carbon working electrode (3.0 mm diameter). The working electrode surface was polished routinely with 0.05 µm alumina-water slurry on a felt surface immediately before use and in between runs. The counter electrode was a carbon rod and the pseudo reference electrode was a Ag

for Ni(salophen), we can also find exact match between the solution-phase CV measurement and the UHV surface STM-OMTS/UPS data by simply multiplying the CV-measured oxidation level by this correction factor (1.23 × 1.594 + 4.7 = 6.66 eV). We can further assign the STM-OMTS peak in Figure 4(a) at 7.01 eV to be HOMO-1. This peak, together with the previously discussed HOMO and LUMO levels, indicate an experimentally determined HOMO - LUMO gap of 3.28 eV and a (HOMO-1) - HOMO gap of 0.36 eV. These UHV single-molecule measurements, in comparison to the recent solutionphase absorption spectroscopy and computation study, 53 provide a more direct and solventfree determination of frontier MO energy gaps. Since each Ni(salophen) molecule experiences little distortion or interaction with Au(111), we further expect that these STM-determined MO gaps to be readily accounted for using gas-phase free-molecule calculations. In fact, metal(salen) complexes have been intensely investigated computationally, with several reports describing that one computational method is better than the others. 20,53–57 Our STM-OMTS measurement thus provides a new foundation to reexamine these computational methods. Table 1: Comparison of single-molecule gasphase calculations vs. single-molecule surface measurements for Ni(salophen). Energy values are shown in eV. Functional B3LYP B3P86 B97-1 PBE1PBE (PBE0) Experiment

Basis Set 6-31G(d,p) 6-31G(d,p) 6-31G(d,p) 6-31G(d,p) STM-OMTS

HOMO-1 to HOMO HOMO to LUMO 0.3901 3.0542 0.4332 3.0384 0.3717 3.1244 0.3562 3.4292 0.36

3.28

The agreement between gas-phase singlemolecule computations using hybrid functionals and the STM-based single-molecule measurement is presented in Table 1. Calculations using pure functionals are included in SI. In general, we find hybrid functionals reproduce experimental values better than the pure ones and the simple 6-31G(d,p) level sufficient with no further improvements using the more timeconsuming coupled-cluster basis sets (also see


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wire. The concentration of the sample and supporting electrolyte, tetrabutylammonium hexafluorophosphate, were 1.0 mM and 0.1 M, respectively. Simulations for Ni(salophen) molecules on Au(111) were performed using the Vienna Ab-initio Simulation Package (VASP) version 5.4. 60–62 Periodic calculations were performed using plane-wave density functional theory (PW-DFT) within the projector augmented wave (PAW) method 63,64 to describe the core electrons and valence–core interactions. All calculations were performed with the dispersion corrected vdW-DF functional that considers the non-local nature of electron correlation. 65,66 The functional used was optB88-vdW GGA. 67 For gas-phase Ni(salophen) free molecules, all structures were fully optimized without symmetry constraints using nine exchange-correlation functionals (four pure functionals: BP86, 68,69 OLYP, 70–72 PW91PW91, 73,74 and PBEPBE 75 and five hybrid functionals: PBE0, 76 B97-1, 77 B3P86, 68 B3LYP, 78 and M06-2X 79 ) as implemented in Gaussian 09 80 using the 6-31G(d,p) basis set.

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Acknowledgement Funding for this project comes from the Washington State University new faculty start-up support. We thank the Murdock Charitable Trust for its financial support in upgrading our XPS/UPS instrument. We also thank Dr. L. Scudiero’s assistance in collecting the UPS/XPS data. The computational work for molecule–surface interactions was performed using resources from the Center for Institutional Research Computing at WSU. Supporting Information Available Materials used, experimental/computational procedures, additional STM images and computational parameters.

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(80) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision D. 01, 2013, Gaussian. Inc., Wallingford CT 2013,


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Connecting Solution-Phase to Single-Molecule Properties of Ni

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