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Density Functional Theory Investigations of Ferrocene-Terminated Self-Assembled Monolayers: Electronic State Changes Induced by Electric Dipole Field of Coadsorbed Species Yasuyuki Yokota,*,† Sumito Akiyama,† Yukio Kaneda,† Akihito Imanishi,† Kouji Inagaki,‡ Yoshitada Morikawa,*,‡,§ and Ken-ichi Fukui*,† †

Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan ‡ Department of Precision Science and Technology, Graduate School of Engineering, and §Research Center for Ultra-Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: Ferrocene-terminated self-assembled monolayers (SAMs) have been widely studied in the past quarter century to reveal the electrochemical properties of chemically modified electrodes. It has been well-known that the formal potential of the system strongly depends on the local environment of the ferrocene moiety. Although electronic states of ferroceneterminated SAMs should directly affect the electrochemical properties, knowledge concerning electronic structures with respect to different local environment is very limited. In this study, we performed density functional theory calculations of ferrocene-terminated SAMs with different coadsorbed species to reveal the relationship between the electronic structures and the local environment of ferrocene moieties. Depending on the local electrostatic potential, density of states derived from the highest-occupied molecular orbital (HOMO) and its vicinities (HOMO−1 and HOMO−2) of the ferrocene moiety were found to largely shift with respect to the electrode Fermi level up to 0.75 eV. This result leads to a novel strategy for designing sophisticated chemically modified electrodes where the electronic properties are electrostatically regulated by coadsorbed inert molecules.

1. INTRODUCTION Electrochemical properties of redox molecules that are chemically bound to electrodes have attracted much attention due to promising electrochemical devices, such as dyesensitized solar cells1 and electrochemically gated molecular circuits. 2 Electron transfer at the electrolyte−electrode interfaces is of particular importance under the electrochemical environment and has been systematically studied for a long time. For example, it is known that not only the redox-active species but also coadsorbed redox inactive molecules drastically change the performance of dye-sensitized solar cells because the electrochemical properties of chemically modified electrodes are quite sensitive to the microscopic local environment of the redox-active molecule.3,4 Among them, ferrocene (Fc)-terminated self-assembled monolayers (SAMs) on gold electrodes have been studied as a prototypical system for the past quarter century to determine the relationship between electrochemical properties and microscopic structures (Figure 1).5,6 Since the pioneering work of Chidsey,7,8 a tremendous number of studies have been reported in the literature.9−16 An advantage for using Fcterminated SAMs is that the local environment of the redox© 2016 American Chemical Society

Figure 1. (a) Thiol derivatives and ferrocene molecule (Fc) used in this study. X = OH, CN, and Br. (b) Typical geometry of C5Fc and C4X molecules coadsorbed on Au(111). Orange, yellow, light blue, white, and green spheres represent positions of gold, sulfur, carbon, hydrogen, and iron atoms, respectively. Purple spheres indicate positions of the hydrogen atom (C4) or X functional groups (C4X).

active Fc moiety can be easily controlled compared to other systems. Creager and Rowe have revealed that the formal Received: January 25, 2016 Revised: April 6, 2016 Published: April 7, 2016 8684

DOI: 10.1021/acs.jpcc.6b00812 J. Phys. Chem. C 2016, 120, 8684−8692

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The Journal of Physical Chemistry C

adsorbed thiolates to describe the SAMs. Each slab was separated by vacuum regions of more than 2.0 nm thickness. For the structural optimization, the adsorbates and the top two gold layers were allowed to relax. The surface Brillouin zone was sampled with (3 × 2) and (4 × 4) shifted k-point meshes for the structural optimization and energy calculation, respectively.52 To compensate for the work function difference between the two sides of a slab, the effective screening medium method (ESM) was utilized to compensate the difference of work functions on both sides.53−56 The gas phase structures of molecules were optimized using only the Γ-point in large unit cells with dimension of 26.5 × 26.5 × 26.5 Å3. The images of SAMs in this article were created with the molecular graphics program VMD.57 The calculated electronic density of states (DOS) and projected density of states (PDOS) were convoluted with the use of Gaussian broadening with the full width at half-maximum (fwhm) of 0.64 and 0.085 eV, respectively.

potentials of the Fc-terminated SAMs change with coadsorbed alkanethiol molecules possessing different terminal groups which alter the local environment of the Fc moiety such as local hydrophilicity, permittivity, and availability for making efficient ion-pairing with counterions.17−19 Although these studies have provided significant insight into the electron-transfer processes of Fc-terminated SAMs, knowledge concerning electronic states of these systems, which is directly related to the electrochemical properties, is very limited.20−24 Using ultraviolet photoelectron spectroscopy (UPS), Sikes et al. investigated the energy difference between the electronic states composed of highest-occupied molecular orbital (HOMO) and its vicinities (presumably HOMO−1 and HOMO−2 states; we call these states as HOMO-related states hereafter) of the Fc moiety and the electrode Fermi level for applications in molecular electronics.20 We also performed UPS measurements for three types of Fc-terminated SAMs possessing different electron-donating abilities and revealed a linear relation with a slope of ∼0.7 between the HOMO-related states and the formal potential determined by electrochemical measurements.23,24 De Leo et al.21 and Hirata et al.22 have reported UPS results of mixed SAMs of ferrocenylalkanethiol and n-alkanethiol, and the latter group revealed that when increasing the surface density of Fc derivatives, the binding energy of HOMO-related states is shifted to a lower value. Whereas the valence band of Fc-terminated SAMs has also been studied through density functional theory (DFT),25 the dependence of the electronic state on the local environment of the Fc moiety has not been studied so far. In this study, we performed DFT calculations of Fcterminated SAMs composed of 5-ferrocenyl-1-heptanethiol (C5Fc) and various matrix thiols possessing different terminal functional groups (C4 and C4X) to reveal the relationship between the HOMO-related states and the local environment of the Fc moiety (Figure 1). The functional groups of the C4X thiols are −OH (C4OH), −CN (C4CN), and −Br (C4Br) groups. Corresponding Fc-terminated SAM systems including the case of the −CH3 group have been thoroughly studied by Creager and Rowe using electrochemical measurements; the C6 family was used in their experiments. It is known that the structures and the electronic states of Fc molecule (Figure 1a)26−33 and n-alkanethiol SAMs34−45 are well-described by DFT calculations. Here we present a systematic study of work functions and local electrostatic potentials felt by the central iron atom as well as the structures and electronic states of Fcterminated SAMs.

3. RESULTS AND DISCUSSION Prior to the investigations of Fc-terminated SAMs, the structures and electronic properties of the molecules were thoroughly characterized to demonstrate the accuracy of our method. The molecular length (dmol) and the equilibrium bond distances of alkanethiol moiety (r(S−C)) and the Fc moiety (r(Fe−C) and r(C−H)) are summarized in Table 1 along with Table 1. Geometry Parameters of Optimized Molecular Structures (in Å) molecules

dmola

r(S−C)b

C4 C4OH C4CN C4Br C5Fc Fc Fc (calcd)

6.24 7.26 7.65 7.01 11.26

1.84 1.84 1.84 1.83 1.84

Fc (expt)

r(Fe−C)c

r(C−H)d

2.04 2.05 2.05e 2.07f 2.06g 2.06h

1.09 1.08 1.09e 1.08f 1.10g 1.08h

a

The molecular length dmol is defined as the length between the sulfur atom and the farthest atom in the molecule. bThe length r(S−C) is defined as the length of the sulfur and carbon bond in the molecule. c The length r(Fe−C) is defined as the average length between the iron atom and ten carbon atoms in the Fc moiety. dThe length r(C−H) is defined as the average length of the carbon and hydrogen bond in the Fc moiety. eFrom ref 32. B3LYP/6-31+G*. fFrom ref 30. PBE/TZVP. g From ref 59. Determined by gas-phase electron diffraction. hFrom ref 60. Determined by single-crystal X-ray diffraction.

2. EXPERIMENTAL SECTION All calculations in this work were carried out by using DFT as implemented in the STATE (Simulation Tool for Atom TEchnology) code, which has been successfully applied to semiconductor as well as metal surfaces.46,47 Ultrasoft and norm conserving pseudopotentials were used to represent the interaction between electrons and ion cores.48,49 Wave functions and the augmentation charge were expanded by a plane wave basis set with the cutoff energies of 25 and 225 Ry, respectively. The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) was used for the exchangecorrelation energy functional.50 The dispersion correction proposed by Grimme was used to describe the dispersion interaction.51 The unit cell used in this work is shown in Figure 1b. We used (3 × 2√3) unit cells with 12 gold atoms per layer and 4

previous theoretical and experimental data. The values of r(S− C) for thiol derivatives are ∼1.84 Å, slightly longer than the calculated value of dimethyl disulfide under the similar calculation condition (1.82 Å)34 and the crystallographic values of dithiol with a long alkyl chain (∼1.80 Å).58 The structural parameters of the Fc moiety in C5Fc (r(Fe−C) and r(C−H)) are in good agreement with those of the Fc molecule calculated in this study and the previous theoretical and experimental data.26−33,59,60 It is known that the Fc molecule has energetically closed eclipsed (D5h) and staggered (D5d) structures in gas phase and solid phase, respectively.59,60 Because the structure of the Fc moiety of ferrocenylalkanethiol 8685


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this peak is composed of almost degenerate two peaks, in consistent with Kera et al.61 The electronic state of the C5Fc molecule is characterized as the superposition of DOS for C4 and Fc molecules, and the PDOS onto the iron atom indicates that the DOS peak observed at around 1 eV is attributed to the HOMO-related states (HOMO, HOMO−1, and HOMO−2 states) localized at the iron atom. This can be clearly visualized by the molecular orbitals shown in Figure S1. The similarity of the DOS, PDOS, and molecular orbitals of C5Fc and Fc molecules indicates that Fc and alkanethiol moieties in the C5Fc molecule are electronically isolated. Although the theoretical evaluations of structural and electronic properties of Fc and related molecules have been still actively investigated,62 the above properties obtained in this study are in consistent with the recent DFT calculations,26−33 indicating that our method is validated for the studies of Fc-terminated SAMs. After eliminating the hydrogen atom in the −SH functional group, we optimized adsorption structures of the C4 or C4X molecule on Au(111) with and without the C5Fc substitution.63 Although it has been known that the Au(111) surface is reconstructed during the SAM formation,39,42,64,65 we did not intentionally create gold vacancies and adatoms before structural optimizations, as in the cases of previous studies.21,34−38,40,41,43−45,66 In addition, as for the initial structure, sulfur atoms of thiol derivatives were positioned at the bridge sites slightly shifted to the face-centered cubic (fcc)-hollow site.34−37,42,43,45 Then, the adsorbates and the top two layers of gold atoms were allowed to relax. Optimized SAM structures with and without the C5Fc substitution are shown in Figure 3, and structural parameters are summarized in Table 2 (see Table S1 for more details). The film thicknesses (dSAM) of C4 and C4X matrix SAMs are correlated to corresponding dmol values in Table 1, and the average height of sulfur atoms with respect to the ideal bulk terminated Au(111) surface (hS) is ∼2.19 Å. The values of dSAM − hS are lower than dmol, indicating that the tilted adsorption structures of C4 and C4X SAMs by 20°−30°.63 Other parameters, r(S−C), θ(S−C), and r(Au−S), are not dependent on the terminal functional groups, being consistent with the previous studies of n-alkanethiol SAMs.35,39,41,42,45 Asterisks in Figure 3 show the portions of specific structural changes upon the C5Fc substitution in the matrix SAMs. As expected, the substitution of the C5Fc molecule does not alter the structures of matrix SAMs except for the outermost surface.

SAMs are not known to the best of our knowledge, we used energetically favorable eclipsed structure for C5Fc in the following calculations. DOS and PDOS curves of the equilibrium molecules are shown in Figure 2. The energy zero is set at the localized −SH

Figure 2. DOS (thick lines) and PDOS (thin lines) curves for molecules used in this study. Colored thin lines represent the PDOS for the sulfur atom. Black thin lines of C4OH, C4CN, and C4Br indicate the PDOS for oxygen, nitrogen, and bromine atoms, respectively. Dotted thin lines of C5Fc and Fc show the PDOS for the iron atom. The energy zero is set at the localized −SH state of thiol derivatives; the energy of Fc is aligned so that the first peak top of the DOS curve is coincident with that of C5Fc.

state of thiol derivatives; the energy of the Fc molecule is aligned so that the first peak top of the DOS curve is coincident with that of C5Fc. From DOS and PDOS curves of the C4 molecule, the electronic states between −7 and − 2 eV are derived from the mixture of the alkyl chain and −SH states. The results of C4X molecules were quite similar to the C4 molecule except for their characteristic states originated from X functional groups (see thin black lines; these PDOS curves were projected onto oxygen, nitrogen, and bromine atoms for C4OH, C4CN, and C4Br, respectively). Sharp PDOS peaks observed at −1 eV for C4OH and C4Br molecules are localized at the X functional groups. In the case of the C4Br molecule,

Figure 3. Computed geometries for (a) C4, (b) C4OH, (c) C4CN, and (d) C4Br SAMs on Au(111). Corresponding geometries after substituting C5Fc are superimposed in each panel, and dominant structural changes are indicated by asterisks. Red, blue, and pink spheres represent positions of oxygen, nitrogen, and bromine atoms, respectively. Other color representations are the same as in Figure 1. 8686


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The Journal of Physical Chemistry C Table 2. Geometry Parameters of Optimized SAM Structures SAMs

dSAMa (Å)

hSb (Å)

r(S−C) (Å)

θ(S−C)c (deg)

r(Au−S)d (Å)

r(Fe−C) (Å)

r(C−H) (Å)

C4 C4OH C4CN C4Br C5Fc/C4 C5Fc/C4OH C5Fc/C4CN C5Fc/C4Br

7.97 9.05 8.55 8.52 12.75 12.81 12.79 12.80

2.19 2.19 2.18 2.19 2.19 2.18 2.20 2.19

1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84

55.0 55.0 55.9 55.2 55.2 55.5 55.8 55.6

2.48 2.48 2.48 2.48 2.48 2.48 2.49 2.49

2.04 2.04 2.04 2.04

1.08 1.09 1.09 1.09

a

The SAM thickness dSAM is defined as the height of the farthest atom from the ideal bulk terminated Au(111) surface. bThe height of sulfur atoms hS is defined as the average height of the sulfur atoms from the ideal bulk terminated Au(111) surface. cThe angle θ(S−C) is defined as the average angle of S−C bond with respect to the surface normal. dThe length r(Au−S) is defined as the average length of the sulfur atom and two gold atoms that constitute the bridge site; the position of sulfur atom is slightly shifted to the fcc-hollow site. eThe spacing of Au layers; h4 represents the average height of the topmost Au layers.

Figure 4. DOS (thick lines) and PDOS (thin lines) curves for SAMs with different terminal groups (a) before and (b) after substituting the C5Fc molecule. Dotted lines in (a) and (b) show DOS for clean Au(111) and C4 SAM, respectively, for comparison. Colored thin lines represent the PDOS for the sulfur atom. Black thin lines of C4OH, C4CN, and C4Br systems indicate the PDOS for oxygen, nitrogen, and bromine atoms, respectively. The energy zero is taken to be the Fermi level of the system.

The electronic states localized at −X functional groups are found to be modulated by the close-packed adsorption in C4X SAMs; the order of their energy is changed from C4CN → C4Br → C4OH for gas phase molecules to C4CN → C4OH → C4Br for the matrix SAMs, as indicated by PDOS onto oxygen, nitrogen, and bromine atoms (black thin lines). However, chemical interactions upon adsorption are small, if any, because the PDOS peaks of these localized states are much sharper than those of S−Au states. Figure 4b shows the DOS (thick lines) and PDOS (thin lines) curves after substitution of the C5Fc molecule into matrix SAMs. The S−Au states for these systems (−1.8 to −0.8 eV) are indistinguishable from those for matrix SAMs. This result is consistent with the negligible structural changes upon the substitution of the C5Fc molecule and the electronically isolated Fc and alkanethiol moieties. On the other hand, the electronic states localized at −X functional groups are shifted and notably split, indicating that the local environment of the −X functional groups is altered by the C5Fc substitution. Increases of DOS at around −0.6 eV are observed after the substitution of the C5Fc molecule into the matrix C4 SAM (cf. thick solid and dotted lines), and this new feature is observed for all the C5Fc systems at the different energy ranges. These

The values of dSAM are almost the same irrespective of the functional groups, and structural parameters such as hS, r(S− C), θ(S−C), and r(Au−S) are comparable to the matrix SAMs. In addition, structures of Fc moieties (i.e., r(Fe−C) and r(C− H)) are kept the same as the gas phase structure (Table 1). This result indicates the negligible electron transfer from the Fc moiety to the gold electrode because it is known that the r(Fe− C) increases with the oxidation of the Fc moiety.67,68 Figure 4a shows the valence-region DOS curves of the matrix SAMs (thick solid lines) and the clean Au(111) (thick dotted line), in which the binding energy scale is referenced to the Fermi level (EF). In the case of Au(111), the energy ranges close to the Fermi level and deeper than −1.5 eV are dominated by the sp-band and d-band of the gold substrate, respectively. Comparison between C4 SAM and Au(111) DOS curves indicates that the broad peak at −1.8 to −0.8 eV is derived from chemically bonded S−Au states, which is validated by the corresponding PDOS onto the sulfur atom (thin line). DOS and PDOS of S−Au states for other matrix SAMs have almost the same features. This is consistent with the previous report by Sun et al., where the S−Au states are peaked at −0.96 eV irrespective of thiol species (e.g., conjugated or saturated).36 8687


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The Journal of Physical Chemistry C subtle changes can be clearly visualized by the differences of DOS curves before and after the C5Fc substitution, as shown in Figure 5 (thick lines). Comparison to PDOS onto the iron

in the lower panel in (a). The plane-averaged electron densities for C5Fc-substituted SAMs at the position of Fc moieties (upper panel in (b), see arrow) are the same irrespective of the terminal functional groups of the matrix SAMs, as expected from the similar Fc orientation and the negligible electron transfer discussed above. The lower panel in (b) shows that the differences of the electrostatic potential energies at the vacuum region (>13 Å) decrease with keeping the same order as the parent matrix SAMs. Work functions (W) of clean Au(111) and SAMs calculated by the energy difference between the electrostatic potential energy at the vacuum region and the Fermi level are summarized in Table 3. The value of W for clean Au(111) surface is in good agreement with previous calculated (5.2−5.4 eV)37,69,70 and experimental values (5.2−5.4 eV).71−74 While relatively large deviations of W values for n-alkanethiol SAMs were found in the literatures (calcd 3.9−4.3 eV37,38,40,42−44 and expt 3.5−4.4 eV75,76), our value for the C4 matrix SAM (3.80 eV) is within the uncertainty. Compared to the above systems, knowledge concerning W values for SAMs with various terminal groups is very limited.77 Alloway et al. have reported that W values for 1-dodecanethiol (C12) and 12-bromododecanethiol (C12Br) SAMs are 3.9 and 4.8 eV, respectively,78 indicating that our calculations give correct tendency (cf. 3.80 and 5.28 eV for C4 and C4Br SAMs, respectively). The W value for the 8-cyano-1-octanethiol (C8CN) SAM has been calculated by Heimel et al. as 6.25 eV,79 in good agreement with our value for the C4CN SAM (6.44 eV). De Leo et al. have reported the W values for Fc-terminated SAMs by UPS measurements and DFT calculations.21 Their experimental values for 1-hexanethiol (C6) and C6Fc SAMs were ∼4 eV in both cases, and calculations also predicted the similar values (4.16 and 4.26 eV for C6 and C6Fc SAMs, respectively). This tendency can be also reproduced by our calculations for C4 SAMs with and without the C5Fc substitution (∼3.8 eV). For comparison, using Kelvin probe measurements, Watcharinyanon have found that surface potential of 1-decanthiol (C10) SAM is lower than 11ferrocenyl-1-undecanethiol (C11Fc) SAM by ∼0.4 V, and mixed SAMs of C10 and C11Fc give intermediate values depending on the surface compositions.80 Interestingly, whereas the W values for C4X SAMs are ranged between 3.80 and 6.44 eV, the C5Fc substitution decreases the diversity (3.41− 4.55 eV), indicating that the total dipole moment perpendicular to the surface shrink to the values close to the pure and C5Fcsubstituted C4 SAM systems due to the decrease of the

Figure 5. DOS differences upon C5Fc substitution for SAMs with different terminal groups. The first peak near the Fermi level consists of HOMO, HOMO−1, and HOMO−2 orbitals of the C5Fc molecule. Thin lines represent the PDOS for the iron atom after C5Fc substitution.

atom (thin lines) suggests that peaks close to the Fermi level are attributed to HOMO-related states. While these peaks have almost the same widths, the peak positions are shifted depending on the matrix SAMs, even though the same C5Fc molecules are adsorbed with the similar structural parameters (Table 2). The peak positions are summarized as HOMOSAM in Table 3. To reveal the dependence of HOMOSAM levels on the matrix SAM, we calculated the plane-averaged electron density (upper panels) and electrostatic potential energy (bottom panels) of matrix and C5Fc-substituted SAMs (Figure 6).56 The curves of the plane-averaged electron density for matrix SAMs (upper panel in (a)) are indistinguishable except at the outermost surface, the position of terminal functional groups (see inset). This leads to the similar electrostatic potential energies at the position range less than 7 Å, while those at the vacuum region (>10 Å) are quite different depending on the terminal functional groups (C4OH < C4 < C4Br < C4CN), as shown Table 3. Electronic Properties of SAMs (in eV) SAMs Au(111) C4 C4OH C4CN C4Br C5Fc/C4 C5Fc/C4OH C5Fc/C4CN C5Fc/C4Br

HOMOSAMa

−0.57 −0.84 −0.09 −0.11

ΔHOMOSAMb

work function (W)

ΔWb

ΔVES(Fe)b,c

ΔIpb,d

0.00 −0.27 0.48 0.46

5.16 3.80 3.18 6.44 5.28 3.81 3.41 4.55 4.34

1.35 −0.01 −0.63 2.63 1.47 0.00 −0.40 0.74 0.53

0.00 −0.39 0.43 0.39

0.00 −0.13 −0.08 −0.02

The peak top energy of the first peak in Figure 5. bRelative value with respect to the C5Fc/C4 SAM. cElectrostatic potential energy at the position of the iron atom. dIonization energy Ip is defined as the difference between the vacuum level and HOMOSAM. a

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Figure 6. Plane-averaged electron density (upper panels) and electrostatic potential energy (bottom panels) of SAMs with different terminal groups (a) before and (b) after substituting the C5Fc molecule. Corresponding curves for clean Au(111) are also shown. The electrostatic potential is presented with respect to the common vacuum value at the bare side of each SAM. Inset shows the geometry of the C4 molecule on Au(111) for comparison. Arrows in (b) indicate the positions of the iron atom in C5Fc systems.

electrostatic potentials felt by the central iron atom in the Fc moiety. Next, we compare the ΔW values with ΔHOMOSAM (blue circles in Figure 7); the difference of ΔVES(Fe) and ΔW is originated from the positions estimating the electrostatic potential energy (cf. at the iron atom and at the vacuum region far from the surface, respectively). Whereas ΔW is positively correlated with ΔHOMOSAM, we found that only the C5Fc-substituted C4CN system increases the overall deviations from a dotted line compared to the ΔVES(Fe) plot. In the case of the C5Fc-substituted C4CN system, terminal −CN groups are inclined with respect to the surface normal (Figure 3c), and thus the average distance between the iron atom and the nitrogen atom (5.80 Å) is comparable to that between the iron atom and the carbon atom in the −CN group (5.87 Å). Hence, the contribution of negative charge of the nitrogen atom to modulate the local electrostatic potential is compensated by the positive charge of the carbon atom. On the other hand, the fact that the average height of the nitrogen atom is higher than that of the carbon atom by 0.63 Å results in the large increase of the W value.36−38,40,42−44 These characteristics are the reason for the large deviation of the C5Fc-substituted C4CN system in the ΔW vs ΔHOMOSAM plot. We note that although the common C5Fc molecule is used, the ionization energy Ip defined as the difference between the vacuum level and HOMOSAM (Table 3 and orange rectangles in Figure 7) is not constant mainly due to the deviations of the ΔW vs ΔHOMOSAM plot from a dotted line. Different ionization energies with the same molecular component have been also reported for the organic semiconductors such as pentacene and oligothiophene molecules deposited on solid substrates.81,82 Heimel and Koch et al. revealed that the difference of the molecular orientation leads to the large changes of Ip up to 0.6 eV due to the fact that the vacuum level is highly modulated by interface dipole differences originated from anisotropic atomic and electronic distributions of the molecules. In contrast, orientations of the Fc moiety in our systems are not dependent on the matrix SAMs (Figure 3),

concentration of polar functional groups of matrix thiols (Table S2). Local electrostatic potential energies at the iron atom in the Fc moiety (ΔVES(Fe)) are summarized in Table 3. These values are changed with the coadsorbed matrix molecules (±0.4 eV with respect to the C5Fc-substituted C4 system), while corresponding average values at the sulfur atoms (VES(S)) are not affected by the difference of terminal functional groups (Table S2). Figure 7 shows ΔVES(Fe) vs ΔHOMOSAM plots (red diamonds, the dotted line shows the unit slope for comparison), revealing that the HOMOSAM values have excellent correlation with VES(Fe). This analysis indicates that the energy shifts of the HOMO-related state observed in Figures 5 and 6 are practically determined by the local

Figure 7. Comparisons between electronic properties and HOMOSAM of C5Fc-substituted SAMs with various coadsorbed molecules. All the data are relative values with respect to the C5Fc-substituted C4 SAM. Red diamonds, blue circles, and orange rectangles represent the electrostatic potential energy (ΔVES (Fe)), the work function (ΔW), and the ionization energy (ΔIp), respectively. Oxidation potentials experimentally determined by Creager and Rowe using similar SAM systems are also plotted (green triangles). The dotted line indicates a slope of 1 for comparison. 8689



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indicating that the electronic states of the functional molecule can be independently modulated by the dipole field of inert matrix molecules. In other words, VES(Fe) and the vacuum level are not necessarily interlocked for our systems. We also compare the above electronic properties with the formal potential of similar Fc-terminated SAM systems determined by Creager and Rowe (green triangles in Figure 7).19 Whereas the electron transfer occurs from the Fc moiety to gold electrode in electrochemical measurements, there is no correlation between the formal potential and ΔHOMOSAM. Rather, the formal potential and Ip have good correlation except for the C5Fc-substituted C4CN system. The origin of these relationships is still an open question but the solvation of the Fc moiety,17−19,31−33 and the ion-pair formation after the oxidation10,11,13−15,17 would be significant for determining the microscopic differences between the electronic states (e.g., HOMOSAM) and the formal potentials. Finally, we discuss the difference between the experimentally and theoretically obtained energies of HOMO-related states. As shown in Figure 5, the HOMOSAM values calculated in this study are shallower than −1 eV irrespective of the matrix SAMs, while the peak top of HOMO-related states by UPS measurements is much deeper irrespective of the presence of nalkanethiol coadsorbates (about −1.6 eV).21−24 This tendency has also been reported by De Leo et al. and Lima et al.21,25 The former group has pointed out that the possible reasons for the discrepancy between the experimental and calculated values might be originated from uncertainties of the work function, molecular disorder, and the surface coverage. Here we add one more possible reason that the self-interaction error, which inevitably appeared in the GGA approximation, underestimates the binding energy of the HOMO-related states.83−85 Although many of the previous studies of SAM systems have been performed by the GGA approximation, systematic studies using various exchange-correlation functionals including hybrid functionals or other sophisticated ones are required to reveal the above discrepancy for more details.86

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.Y.). *E-mail [email protected] (K.F.). *E-mail [email protected] (Y.M.). Present Address

Y.Y.: Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Funding Program for Next Generation World-Leading Researchers (GR071) from the Japan Society for the Promotion of Science (JSPS) and by JSPS KAKENHI Grants 23750013 and 26105010. The computation in this work has been done in part using the facilities of the Supercomputer Center, the Institute for Solid State Physics, The University of Tokyo.

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REFERENCES

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4. CONCLUSIONS Structural and electronic properties of Fc-terminated SAMs composed of C5Fc and various matrix thiols possessing different terminal functional groups (C4 and C4X) were systematically investigated by DFT calculations to reveal the relationship between the HOMO-related states and the local environment of the Fc moiety. While the structures of these systems are practically indistinguishable, HOMO-related states were found to largely shift with respect to the electrode Fermi level up to 0.75 eV due to the difference of the local electrostatic potential at the central iron atom in the Fc moiety. This result indicates the possible strategy for designing sophisticated chemically modified electrodes where the electronic properties are electrostatically regulated by coadsorbed inert molecules.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00812. Molecular orbitals of Fc and C5Fc molecules, complete sets of geometry parameters, and electronic properties of SAMs (PDF) 8690


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