Sorting of Molecular Building Blocks from Solution to Surface

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Sorting of Molecular Building Blocks from Solution to Surface Hodaya Keisar, Graham de Ruiter, Aldrik H. Velders, Petr Milko, Antonino Gulino, Guennadi Evmenenko, Linda J.W. Shimon, Yael Diskin-Posner, Michal Lahav, and Milko E. van der Boom J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02968 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Journal of the American Chemical Society

Sorting of Molecular Building Blocks from Solution to Surface Hodaya Keisar,† Graham de Ruiter,† Aldrik H. Velders,§ Petr Milko,‡ Antonino Gulino,‖ Guennadi Evmenenko,# Linda J. W. Shimon,‡ Yael Diskin-Posner,‡ Michal Lahav, †✻ and Milko E. van der Boom†✻ †

Department of Organic Chemistry, The Weizmann Institute of Science, 7610001 Rehovot, Israel. Chemical Research Support, The Weizmann Institute of Science, 7610001 Rehovot, Israel. § Laboratory of BioNanoTechnology, Wageningen University, 6708 WG Wageningen, The Netherlands. ‖‖ Dipartimento di Scienze Chimiche, Università di Catania, and INSTM UdR of Catania, Catania 95125, Italy. # Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112, United States; and Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208-3108, United States. ‡

ABSTRACT: We demonstrate that molecular gradients on organic monolayers are formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometerthick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon–or metal–oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nanometers. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences, causes the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer, significantly contributes to the observed differences in chemical composition on the surface.

INTRODUCTION The spontaneous organization of molecular components into supramolecular assemblies enables the design and construction of a wide variety of well-defined nanoscale architectures.1,2 Within these supramolecular assemblies, the overall spatial organization is dictated by the geometrical constraints imposed by the molecular building blocks, where non-covalent interactions facilitate the formation of large extended networks.3,4 The interplay between molecular geometry and non-covalent interactions is important and enables the formation of highly diverse molecular assemblies (MAs) both in solution and on the surface.5,6 From the various available assembly strategies, Layer–by–Layer (LbL) deposition from solution is a versatile method to control the molecular composition and/or the physicochemical properties of the surface-confined MAs.7–9 For instance, Wöll and Fischer demonstrated that with LbL assembly unique surface-confined metal organic frameworks can be constructed.10,11 Similar to LbL assembly, spatial organization in two or three dimensions is also possible via photolithography,12 micro-contact printing,13,14 or dip-pen nanolithography.15 These techniques generate MAs with clear boundaries between the molecular components or between the molecular components and the surface. In contrast, the fabrication of continuous films with varying chemical compositions (i.e., gradients) is challenging and can have some benefits over conventionally segregated MAs. For instance, MAs with two-dimensional gradients – lateral to the surface plane – have been used for

(i) molecular reactivity mapping on surfaces,16 (ii) size-selective protein sorting,17 (iii) studying gene expression and cell-adhesion,18,19 or (iv) studying molecular motion on surfaces.20–22 As a result, MAs containing molecular gradients exhibit physicochemical properties that are highly sought after within the fields of molecular self-assembly and nanomaterials. We recently demonstrated that the sequence by which polypyridyl complexes (e.g., those of Fe, Ru, and Os) are assembled is critical for controlling the direction of electron transfer.7,8 These MAs consist of spatially separated polypyridyl complexes organized in well-defined regions with clear boundaries between each molecular component. However, when equimolar mixtures of ruthenium and osmium complexes are used, the composition of the MAs does not reflect the composition of the solution from which the complexes were deposited. Although fascinating, this effect has remained unclear and further research is needed.7 Here we demonstrate that the chemical composition of MAs can be controlled as a function of the physicochemical properties of the used monolayer. This template layer (TL), having a thickness of only several ångströms, induces a sorting mechanism that propagates throughout the MA, ranging over several tens of nanometers. X-ray analysis as well as spectroscopic and computational studies, reveals that the observed ratios in the molecular assembly result from the nature of TL in combination with the observed preferential packing of the ruthenium complexes. Our combined data emphasize the unique position TLs can occupy within the larger assembly strategy, and our observations aid in providing a better understanding of molecular gradient formation with metal complexes.

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Scheme 1. Schematic Representation of the Formation and Composition of Molecular Assemblies (MA1–5) Generated with Different Template Layers (TL1–TL5).a

Layer-by-Layer Assembly

MA1: Molecular Gradient

MA2-5: Equimolar Distribution

(i)

x

Sorting

Sorting

(ii)

PdCl2

n

or

or

TL1

ITO, Si, quartz

or

TL2

1

TL3

2

or

TL4

3-5

TL5

3 (M=Co) 4 (M=Ru) 5 (M=Os)

PdCl2

a

Compounds 1–5 were used to generate the corresponding template layers TL1–TL5, which are covalently attached to the substrate surface. Molecular assemblies (MA1–MA5) were generated by alternating Layer-by-Layer (LbL) deposition using PdCl2(PhCN)2 (1.0 mM in THF) and a mixture of complexes 4 and 5 (0.1 mM each, THF:DMF, 9:1 (v/v). For characterization details, see Figure 1 and the Supporting Information. The charges (+) of TL1-TL5, and of the cobalt (3), ruthenium (4), and osmium (5) complexes have been omitted from the cartoons as well as the PF6− and Cl− counter anions. The structure of MA1 is drawn to visualize the gradient, whereas the actual molecular distribution in a plane is homogeneous. There is no evidence of physical clustering of complexes 4 and 5 when using TL1.

RESULTS AND DISCUSSION In our efforts to elucidate the factors governing the observed gradient formation, we first turned our attention to the template layer (TL). The assembly formation (Scheme 1) requires the functionalization of an oxide surface with a monolayer of organic or inorganic molecules (i.e., TL). Siloxane-based monolayers consisting of terminal benzyl-chloride groups on Indium-Tin Oxide (ITO) coated glass, Si, and quartz are treated with a solution of compounds 1–5 in order to form a dense layer of pyridinium salts (Scheme S1). These TLs serve as connections between the oxide surface and the first layer of the MA. The orientation of those molecules comprising the template layer can influence the direction of growth relative to the surface normal.23 We also reasoned in addition to the geometrical contraints of the template layer, its physicochemical properties might also influence the molecular composition of the MAs. In order to test this hypothesis, five different template layers (TL1– TL5) were prepared including 1,3,5-tris(4-pyridylethenyl)benzene (1), 1,3,5-tris(4-pyridyl-ethyl)benzene (2), [Co(mbpy-py)3][PF6]2 (3), [Ru(mbpy-py)3][PF6]2 (4), and [Os(mbpy-py)3][PF6]2 (5). These template layers (TL1–TL5) display a rich diversity of contrasting molecular parameters that include (i) conjugated vs. non-conjugated, (ii)

organic vs. inorganic, and (iii) mono-cationic (TL1,TL2) vs. tri-cationic (TL3–TL5). We investigated their influence on assembly formation with polypyridyl complexes of ruthenium (4) and osmium (5). For template layers TL4 and TL5, i.e. those that contain ruthenium (4) and osmium (5), we referred to the supporting information (Figure S1). The MAs were formed by iteratively immersing TL1–TL5 in a 1.0 mM solution of trans-bis(benzonitrile)palladium(II)chloride (Pd(PhCN)2Cl2) followed by immersing these functionalized substrates in an equimolar solution of complexes 4 and 5 (0.1 mM each, THF:DMF, 9:1 (v/v). In between these reactions, the substrates were sonicated in common organic solvents to remove any physisorbed materials. Within the outlined assembly strategy in Scheme 1, one deposition step is defined as the deposition of PdCl2(PhCN)2, followed by the deposition of complexes 4 and 5. The palladium salt is used to generate a network on the surface by 1-coordination of the pyridine moieties of the molecular components. All compounds (1–5) and molecular assemblies (MA1–MA5) thereof were characterized using a variety of physical methods that include NMR spectroscopy, UV-vis spectroscopy, Xray photoelectron (XPS) spectroscopy, synchrotron X-ray reflectivity (XRR), and electrochemistry.


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Figure 1. Characterization chart for MA1 (left), MA2 (middle), and MA3 (right). (A) UV/vis spectra. (B and C) cyclic voltammograms (CVs) of MA1–MA3 on ITO after the first (B) and eighth (C) deposition steps from an equimolar mixture of complexes 4 and 5 (see Scheme 1 and the supporting information for details). CVs were recorded at a scan rate of 100 mV/s in 0.1 M TBAPF 6 in acetonitrile. (D) Oxidative currents obtained from cyclic voltammetry (CV), and (E) normalized charges (Qox in %) showing the relative amounts of the ruthenium (4) and osmium (5) complexes.



assembly (Figure 2C,D).29 The molecular footprint is quite different among the template layers. For TL1 and TL2, XRR data estimate a footprint of approximately 3 molecules/nm2, whereas for the larger metal complexes TL4 and TL5, a footprint of 1-2 molecules/nm2 was estimated. Although the spectroscopic data show similar trends among the different MAs, the UV/vis, XRR, and spectroscopic ellipsometry data of MA1–MA5 do not convey the actual distribution of complexes 4 and 5 within the assembly. In other words, although MA1–MA5 have similar spectroscopic properties, their composition can be radically different. A

B

0

10

MA1

0

10

-1

R/RF

10

R/RF

MA2

-2

10

-2

10

-4

-3

10

0.0

0.2

0.4

0.6

10

0.8

0.0

0.2

0.4

0.6

0.8

qz(Å-1)

C

D

0.8

0.8

MA2

MA1 0.6

r (eÅ-3)

When the growth of MA1–MA3 is ex-situ monitored by UV/vis spectroscopy, the high intensity of the singlet metal-to-ligand charge-transfer (1MLCT) band of complexes 4 and 5 is quite informative (Figure 1). Plotting the absorbance of the 1MLCT band at λmax ≈ 500 nm, vs. the number of deposition steps reveals that all MAs exhibit an exponential increase in their optical properties (Figure S2). The π–π* transition at λ ≈ 320 nm can also be used as a benchmark for monitoring thin film growth. Similar results are obtained when complexes 4 or 5 are used as template layers (Figures S1, S2). Spectroscopic ellipsometry also revealed exponential growth (Figure S3). During the exponential growth of the MAs, regular deposition of the two metal complexes was observed, as indicated by the linear interdependence between the film thickness and their optical properties (Figure S4). Exponential growth has been observed by us previously with polypyridyl complexes of Os, Ru, Fe, and Co.24–26 The growth can be controlled by varying several parameters including the deposition time of the metal salts and the nature of the group 10 metal salts used (e.g., Pd(PhCN)2Cl2 vs. Pt(PhCN)2Cl2).27 During the film growth an excess of palladium is captured inside the MAs and is used to bind more than a single layer of metal complexes to the surface during each deposition step. The XRR-derived thicknesses of representative samples are in good agreement with those obtained by spectroscopic ellipsometry (Figure S3). The XRR-derived surface roughness varies from 4% to 20% of the film thickness, which is typical for such assemblies.28 These measurements on representative samples confirm the uniformity within each MA. The presence of Kiessig fringes in the XRR spectra are characteristic of the destructive interference of reflections between the substrate and MA, and between MA and air interfaces (Figure 2A, B). These Kiessig fringes could also indicate the presence of uniform electron density profiles consisting of complexes 4 and 5. Irrespective of the template layer, the XRR-derived electron density profiles for MA1, MA2, MA4, and MA5 are nearly identical and remain constant (σ ≈ 0.5 e Å-3) throughout molecular

r (eÅ-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.4 0.2

0.2 0.0

0.6

0

100

200

300

400

0.0

0

100

200

300

400

z(Å)

z(Å)

Figure 2. Representative X-ray reflectivity (XRR) spectra and fits of MA1 (A) and MA2 (B) after four deposition steps. (C, D) Corresponding electron density profiles of MA1 and MA2 up to eight deposition steps of complexes 4 and 5. The local minima at ca. 5 Å is due to the template layer (TL1, TL2).

Table 1. Ratio of complexes 4 (Ru) and 5 (Os) in molecular assemblies MA1–MA3 after each deposition step (1–8). The elemental composition was determined by XPS spectroscopy and cyclic voltammetry (see the supporting information for experimental details). MA1

MA2

MA3

XPS

CV

XPS

CV

XPS

CV

Entry

Os:Ru

Os:Ru

Os:Ru

Os:Ru

Os:Ru

Os:Ru

[Os|Ru]1

1:5

1:10

2:3

1:2

1:1

3:5

[Os|Ru]2

1:7

1:5

5:4

5:6

4:3

3:5

[Os|Ru]3

2:7

1:4

6:5

6:7

6:4

3:4

[Os|Ru]4

2:9

1:4

6:5

9:10

5:3

9:10

[Os|Ru]5

3:7

1:3

1:1

9:10

6:4

9:10

[Os|Ru]6

1:3

1:3

6:5

1:1

6:5

9:10

[Os|Ru]7

1:2

2:5

6:7

1:1

6:5

1:1

[Os|Ru]8

3:8

2:5

7:4

9:10

7:6

4:5


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The composition of MA1–MA5 was investigated both by cyclic voltammetry (Figures 1B–E, Figure S1B–E) and XPS spectroscopy (Table 1). Cyclic voltammograms (CVs) of MA1–MA5 confirmed that molecular gradients are formed on TL1. Typical CVs are shown in Figure 1B, C and display two reversible redoxwaves at E½ ≈ 0.8 V and at E½ ≈ 1.2 V (vs. Ag/Ag+) which are attributed to the Os2+/3+ and the Ru2+/3+ redox couples, respectively. The large separation between the half-wave potentials (ΔE½ ≈ 400 mV) enabled us to accurately determine the amount of each metal complex by integrating the area underneath the oxidative redox-waves (see the supporting information for experimental details). For comparison, the CVs of MA1–MA3 are shown after the first (Figure 1B) and eighth deposition steps (Figure 1C). Even by visual inspection, it is clear that the CVs obtained for MA1 and those of MA2–MA5 greatly contrast with each other. However, for MA2, MA3, and MA2-5 the distribution of complexes 4 and 5 is approximately equal (Figure 1B–E; Table 1). Note that for MA4 and MA5 the CV data also include the contribution of redox-active TL4 and TL5 (Figure S1B–C). For MA1 the ratio between complexes 4 and 5 changes upon increasing the film thickness from 1:10 to 1:2.5. A detailed analysis of the electrochemical response of the individual molecular components shows that for MA2–MA5 the currents associated with osmium and ruthenium metal centers are nearly identical, whereas for MA1 clear differences are observed (Figure 1D, E, and Figure S1B, C). Even varying the assembly thicknesses of MA2–MA5 does not change the ratio between the metal complexes. In contrast, for MA1 large differences are observed (Figure 1D, E, and Figure S1B, C). These differences indicate that in the early stages of assembly formation, complex 4 is the major species in MA1 (Figure 1B). However, as the molecular assembly continues to grow, the concentration of complex 4 diminishes and at higher thicknesses, the ratio between complexes 5 and 4 reaches 1:2.5. These data confirm that the sorting mechanism is only observed when compound 1 is used as the template layer. By cyclic voltammetry, the observed ratio between 5 and 4 is cumulative. In other words, after each deposition step the electrochemical properties of the entire assembly are surveyed and not only the properties of the material that was assembled during the last deposition step. In order to calculate the ratio of complexes 4 and 5 during each deposition step, the values of the last two deposition steps were subtracted from each other. For example, the ratio between 4 and 5 is calculated by subtracting the charge (Q), associated with each individual metal complex after the first deposition step, from that obtained after the second deposition step (Figure S5). XPS analysis of MA1-MA3 revealed similar trends (Table 1). Because of the the penetration depth of the X-ray beams (ca. 6.0 nm at 65°), XPS spectroscopy is an excellent technique to determine the experimental Os:Ru ratios after each deposition step of the molecular assembly. For instance, for MA1, the ratio between complexes 5 and 4 changes from 1:5, after the first deposition step, to about 1:2.5 after the seventh and eighth deposition steps. Moreover, the osmium content – relative to that of ruthenium – appears to increase upon increasing the number of deposition steps. Evidently, the continuous change in the ratio of complexes 4 and 5 is consistent with the formation of a molecular gradient within MA1. In contrast, XPS measurements on MAs formed on TL2 and TL3 reveal a different trend. In these MAs, the ratio between the two metal complexes remains nearly constant and does not change upon increasing the thickness of MA. Furthermore, changing the

cobalt metal center in TL3 to osmium or ruthenium (TL4 and TL5) does not change the observed ratio of complexes 4 and 5. A clear distinction is thus observed between MAs constructed on TL1 and those constructed on TL2–TL5. Such control over the chemical composition is highly unusual. Both cyclic voltammetry and XPS spectroscopy unequivocally indicate that a gradient is formed only when TL1 is used for assembly formation. Moreover, the data also indicate that the observed template layer effect propagates throughout the assembly as the ratio between the metal complexes continues to change upon increasing the assembly thickness. The template layer, TL1, however, might not be the only driving force for formation of a gradient. Geometrical differences between complexes 4 and 5 might also be responsible. It is known that polypyridyl complexes with asymmetric ligands can form a mixture of facial and meridional isomers. A different distribution of these isomers between complexes 4 and 5 might favor the deposition of one metal complex over the other. To quantify the ratios of facial and meridional isomers, we used highfield (950 MHz) 1H and 13C{1H} NMR spectroscopy (Figure 3, Figures S6–17). The hydrogen atoms adjacent to the nitrogen donors (H6 or H12) are especially suitable for assigning the different isomers (see the supporting information for experimental details). The 1 H NMR spectrum of the aromatic region of 5 is shown in Figure 3, and the resonances were assigned by 2D NMR techniques (Figures S10-11). For the facial isomer (C3 symmetric), all three bipyridine-based ligands are equivalent and a single doublet is observed at 7.82 and 8.06 ppm, respectively (Figure 3; inset). In contrast, for the meridional isomer (Cs symmetric) all ligands are inequivalent and a series of three doublets are observed between 7.8 and 8.1 ppm (Figure 3; inset). The resonance at 7.90 ppm, in particular, shows that all resonances associated with H6 or H12 are overlapping doublets instead of the observed apparent triplet. Evidently, complex 5 is a mixture of facial and meridional isomers. 1H NMR was used to determine the ratio of these isomers. Since all resonances integrate in a 1:1:1:1 fashion, the ratio of the two isomers in solution is 1:3. The 1H NMR spectrum of complex 4 reveals an identical ratio (Figure S13).

18 3

9

17 6

12

14 15 5 11

Figure 3. 1H NMR (950 MHz) spectra of complex 5 between 7.1 and 9.2 ppm. The inset displays the region between 7.7 and 8.1 ppm showing the doublets for hydrogen atoms H6 and H12. The assignment is based on 2D NMR techniques and by enrichment of the mother liquor with the meridional isomer upon selectively crystallizing 5-facial from acetone:toluene by layering (see the supporting information).

In contrast to the composition of the facial and meridional isomers in solution, as shown by 1H NMR spectroscopy, the



crystal structures of 4 and 5 display a different composition when crystallized under identical conditions. For complex 4, both isomers were observed in the crystal structure (fac:mer = 1:5), whereas only a single isomer was obtained for complex 5 (Figure 4, 5–mer). Crystals of 5–fac were obtained using different solvents. These observations could be relevant for explaining the formation of the gradient. For complex 4 there is seemingly a lower barrier for incorporating both the facial and meridional isomers into solid-state structures. The preferred incorporation of both isomers of complex 4 increases its effective concentration and therefore, it will be preferentially incorporated over complex 5. The incorporation of one type of metal complex over the other, while having identical geometries, has also been observed in the crystal structures of other polypyridyl complexes.30 Indeed different packing is also observed on the surface. Reaction of only complex 4 or complex 5 with TL1 for 15 min on ITO resulted in a less dense film (50%) for the osmium complex (5), implying that different packing arrangements are playing also a role (Figure S18).

Ru:

4-fac and 4-mer

P-1 Os: 5-mer

Os: 5-fac

P-1

P21/c

Figure 4. Crystal structures of meridional and the facial isomers of complexes 4 and 5. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and outer sphere counter ions are not shown for clarity. Blue: N, gray: C, green: metal ion.

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To further investigate potential differences between complexes 4 and 5, their electronic properties were investigated by 1H–15N HMBC NMR spectroscopy and by density functional theory (DFT). High-Field NMR studies (see the supporting information for further details) revealed only small electronic differences, reflected in the 1H–15N HMBC of complexes 4 and 5 (Figure 5A,B). The 15N resonances – associated with the bipyridine nitrogen atoms – shift from 221 and 228 ppm in complex 5, to 242 and 249 ppm in complex 4, reflecting the expected higher electron density of the osmium metal center. Only a minor change is observed for 15N resonance associated with terminal pyridine moieties (Figure 5A; 4: 314 ppm vs. 5: 319 ppm; Figure 5B). Although the Os and Ru metal centers are electronically different, the differences are not significantly reflected in the vinylpyridine units. Consequently, the basicity of the remote pyridine nitrogen atoms in complex 4 (Ru) is only marginally higher than that in complex 5 (Os). DFT studies support these findings. The electron density maps of the four isomers are shown in Figure 5C–F, and the partial atomic charges are summarized in Table 2. Besides the obvious geometrical differences between the facial and meridional isomers, no significant differences exist in the electronic structure between the organic ligands of each isomer, irrespective of the nature of the metal ion (Os or Ru). Not surprisingly, the highest electron density is found on the outer pyridine moieties: Np = –0.42 (4) and –0.42 (5). The outlying pyridyl functionalities are more electron rich relative to the bipyridine moieties, which are directly bound to the osmium and ruthenium metal centers. Between the metal complexes, however, there are a few differences. The calculated partial atomic charge on Os (0.51) is lower than that for Ru (0.70). As a result, the bipyridine-based ligands in complex 5 bear a slightly higher positive charge compared with their counterparts in complex 4, thus the observed 15N resonances are slightly shifted (vide supra). Although the electronic and geometrical differences between complexes 4 and 5 seem small, our experimental observations indicate that some of these differences might be expressed at the solution-surface interface when using TL1. These findings are further substantiated by (i) in MA2-MA5 the ratio between complexes 4 and 5 approaches unity (Figure 1), and (ii) the addition of a solution of PdCl2 (26 mmol) in THF to a mixture of an excess of 4 and 5 (1.0 equiv.) in a mixture of THF:DMF (9:1, v/v) results in the formation of an insoluble polymer, with a 1:1 osmium to ruthenium ratio, as judged by XPS spectroscopy. This experiment indicates a similar reactivity for both complexes (4,5). Foster and Faulkner have shown that with related osmium and ruthenium complexes, the composition of binary monolayers on platinum microelectrodes reflects the ratio of the complexes in solution. Absorption occurs via pyridine-platinum coordination chemistry, indicating that the reactivity of these complexes is similar.31


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A)

B) 220

220

240

280

260 280

300

{7.63,313.54}

300 {7.58,319.73}

320 9.0

8.8

8.6

8.4

C)

8.2 8.0 f2 (ppm)

7.8

7.6

7.4

7.2

D)

9.0

8.8

E)

4-fac

8.6

8.4 8.2 8.0 f2 (ppm)

7.8

320

7.6

7.4

7.2

F)

4-mer

5-fac

5-mer

Figure 5. 1H 15N HMBC NMR plot (96 and 950 MHz) of (A) complex 4 and (B) complex 5. The cross-peaks at 7.6, 313.5, and 7.6, 319.8 ppm reveal the non-coordinated pyridine nitrogen atoms. (C-F) Electron density map of the DFT-optimized structures for the facial and meridional isomers of complexes 4 and 5. Associated CM5-calculated partial charges are shown in Table 2.

Table 2. The CM5 partial charges of complexes 4 and 5 in THF. bpy-based ligand 1 Entry

bpy-based ligand 2

bpy-based ligand 3

Metal Core

-CH3

(py)-et-

Core

-CH3

(py)-et-

Core

-CH3

(py)-et-

mer-Ru (4)

0.70

0.24

0.10

0.09

0.24

0.10

0.09

0.24

0.10

0.09

fac-Ru (4)

0.70

0.24

0.10

0.09

0.24

0.10

0.09

0.25

0.10

0.09

mer-Os (5)

0.51

0.31

0.10

0.08

0.31

0.10

0.08

0.31

0.10

0.08

fac-Os (5)

0.51

0.31

0.10

0.08

0.31

0.10

0.08

0.31

0.10

0.08

To further demonstrate the importance of the template layer, we assembled a single layer of only compounds 3, 4, or 5 on top of TL1 (Scheme 2, right, gray). By doing so, the template layer is shielded from the reaction mixture. Subsequently, these bilayers were exposed to solutions of the palladium salt and an equimolar mixture of complexes 4 and 5. The CVs reveal that the role of the template layer on the composition is significantly diminished by at least a factor of 2.5 (Os:Ru = 1:5 vs. 1:2) (Table 3). Apparently, the deposition of the unimolecular layers consisting of complexes 3, 4, or 5 (Scheme 2, right, gray) resulted in masking TL1 from the reaction mixture. Consequently, the ratio of complexes 4 and 5 is different when only TL1 is used. Comparing the CV data of MA1 after two deposition cycles (Scheme 2, left) with a MA having unimolecular layers of complex 3, 4 or 5 on top of TL1 (Scheme 2,

right) reveals that the presence of a layer consisting of both complexes 4 and 5 contributes to the sorting effect observed for MA1 (Scheme 2, left).30 Scheme 2. Shielding of TL1 by a Layer of Complexes 3-5.a

a

4+5

4+5

4+5

3

TL1

TL1

or 4 or 5 ‘blocking layer’

Structures of the MAs after two deposition steps on TL1 of: (Left) an equimolecular mixture of complexes 4 and 5. (Right) a


f1 (ppm)

260

240

f1 (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Journal of the American Chemical Society single layer of compounds 3, 4, or 5, followed by deposition of an equimolecular mixture of complexes 4 and 5. Table 3. Total charge (Q) associated with the oxidative Os 2+/3+ and Ru2+/3+ redox-waves in Molecular Assemblies on TL1. Total oxidative charge (C).b [Os|Ru]2

[Co]1[Os|Ru]1

[Ru]1[Os|Ru]1

[Os]1[Os|Ru]1

4

10.65

13.37

7.96

11.52

5

2.20

7.32

6.13

8.30

Os:Ru

1:5

1:2

3:4

3:4

a

CVs of MAs were recorded after deposition of compounds 3-Co, 4Ru, or 5-Os on top of TL1. b All values are ×10-6.

Time-dependent CV experiments for the deposition of both complexes 4 and 5 on TL1 were performed to evaluate their different reactivities. The CVs showed that the ratios between the surface-bound complexes 4 and 5 are not constant (Figures 6, S19). After 1 min reaction time, similar amounts of both complexes are bound to the surface. The amount of complex 5 on the surface remains constant, whereas the amount of complex 4 continued to increase during the first 5 min. These observations suggest that the initial reactivity of 4 and 5 with TL1 is similar, but in time, complex 4 binds more efficiently to the surface. The assembly process is completed within 5-10 min. Time-dependent CV experiments performed on TL3 revealed that the film containing both complexes 4 and 5 is already fully formed after only 1 min reaction time. This sorting behavior might indicate that during the assembly process an intermediate structure consisting of both ruthenium (4) and osmium (5) complexes is formed that only permits surface binding of complex 4.

Ru2+/3+

20

Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

Os2+/3+

0

-10 0.4 0.6 0.8 1.0 1.2 1.4+ 1.6 Potential (V; vs Ag/Ag )

Figure 6: Cyclic voltammograms (CVs) of TL1 at different exposure times to a solution containing an equimolar mixture of complexes 4 and 5: after 1 min (black curve) and after 5 min (red curve). CVs were recorded at a scan rate of 100 mV/s in 0.1 M TBAPF 6 in acetonitrile.

SUMMARY AND CONCLUSIONS Here we have demonstrated how the composition of molecular assemblies (MAs) can be controlled by carefully designing a template layer that is only 1-2 nanometers thick. When compound 1 is used for the formation of the template layer, a gradient of ruthenium (4) and osmium (5) polypyridyl complexes is observed within the resulting MAs. However, no such gradients were formed when molecularly related, but structurally different template layers were used. In an identical chemical environment, the two isostructural

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ruthenium and osmium complexes (4,5) and their fac- and mer-isomers form different crystals. The differences in the molecular composition of the films can be rationalized by the different ratios of the facial and meridional isomers within the crystal structures of 4 and 5. Since for complex 4 both fac- and mer-isomers are present in the crystal structure, there is a clear advantage of binding to a surface. A favorable packing of the ruthenium complex (4) on the surface is only expressed in combination with TL1. Besides the similar footprint of TL1 and TL2, the rigidity of the molecular components (1 and 2) is different, allowing complex 4 to more readily form higher-ordered structures on the surface. Both these factors can result in a different surface packing of complexes 4 and 5 on TL1. Our ability to control the composition of molecular assemblies simply by changing the template layer can be the next step in the molecular control of nanoscale assemblies based on metallo-organic chemistry.32–48

EXPERIMENTAL General procedures Compounds 1–5 and PdCl2(PhCN)2 were prepared according to published procedures.25,26,49 4-Chloromethyl-phenyltrichlorosilane was purchased from Gelest, Inc. Solvents (AR grade) were purchased from Bio-Lab (Jerusalem), Frutarom (Haifa), or Mallinckrodt Baker (Phillipsburg, NJ). Toluene was dried and purified using a M. Braun solvent purification system, and degassed before being introduced into a M. Braun glovebox. Single-crystal silicon wafers were purchased from Wafernet (San Jose, CA) and Indium Tin Oxide (ITO)-coated glass substrates (7 mm × 50 mm × 0.5 mm) were purchased from Delta Technologies (Loveland, CO). ITO, silicon, and quartz substrates were cleaned using standard protocols. ITO and silicon substrates were cleaned by sonication for 8 min in dichloromethane (DCM), followed by hexane, acetone, and ethanol. The substrates were subsequently dried under a nitrogen stream and cleaned for 30 min with a UVOCS cleaning system (Montgomery, PA). Quartz substrates (10 mm × 20 mm × 1.0 mm) were purchased from Chemglass, Inc. and were cleaned by immersing the substrates in a “piranha” solution (7:3 (v/v) H2SO4/30% H2O2) for 1 h. Caution: piranha solution is an extremely dangerous oxidizing agent and should be disposed of in a proper manner and under no circumstances should it be combined with organic materials. When handling piranha, appropriate personal protection should be worn. After the piranha treatment, the substrates were rinsed with deionized (DI) water, followed by the RCA cleaning protocol: 1:5:1 (v/v) NH4OH/H2O/30% H2O2 at 80 °C for 45 min. The substrates were washed several times with deionized (DI) water, then with isopropanol and dried under a nitrogen stream. All substrates were dried at 130 °C for 2 h, before being introduced into the glovebox. Siloxane-based chemistry was carried out in a N2-filled M. Braun glovebox with oxygen and water levels below 2.0 ppm. Template layers of compounds 1 and 4–5 on silicon, ITO, and quartz substrates were formulated according to published procedures.25,26,50 UV-Vis spectra were recorded on a Cary 100 spectrophotometer. Spectroscopic ellipsometry measurements were performed with an M-2000 V (J. A. Woollam Co., Inc.) instrument with VASE32 software. The thicknesses of the assemblies on ITO were estimated using the spectroscopic ellipsometry of assemblies grown simultaneously


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on silicon substrates. One deposition step is defined as the deposition of PdCl2 (PhCN)2 and the deposition of one type of metal complex or two metal complexes in a 1:1 ratio (Scheme 1). All experiments were carried out at room temperature, unless stated otherwise. Physical Methods NMR Spectroscopy. The 1H, 13C{1H}, and 15N NMR spectra were recorded at 950, 239, and at 96 MHz, respectively, on a Bruker 22.3 Tesla NMR spectrometer with an AVANCE III HD console equipped with a TCI cryoprobe. All 1D and 2D experiments were executed using the standard pulse sequences available in Topspin 3.1 software. All chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hz. The 1H and 13C{1H} NMR spectra were referenced using residual H-impurities in deuterated solvent. CO(CD3)2 was purchased from Cambridge Isotope Laboratories. X-ray Reflectivity (XRR). X-ray reflectivity (XRR) was measured on the 12-BM-B beamline at the Advanced Photon Source (APS) at the Argonne National Laboratory (Argonne, IL, USA). A fourcircle Huber diffractometer was used in the specular reflection mode (i.e., the incident angle θin was equal to the exit angle θex and the wave vector transfer |q| = qz = (4π/λ ) sin θ is along the surface normal). X-rays of the energy of E = 10.0 keV (λ= 1.24 Å) were used for these measurements. The beam size was 0.40 mm vertically and 0.60 mm horizontally. The samples (MAs on silicon substrates) were held under a helium atmosphere during the measurements to reduce radiation damage and background scattering from the ambient gas. The off-specular background was measured and subtracted from the specular counts. XRR measurements were performed at ambient laboratory temperatures, which ranged between 20 and 25 °C. XRR analysis used Motofit51 with a multiple-slab model that included a silicon substrate and variable electron densities and thicknesses of template interlayer and ruthenium metal complexes. X-ray photoelectron spectroscopy (XPS) measurements. XPS measurements were performed on a PHI 5600 Multi Technique System (the base pressure of the main chamber was 2×10–10 Torr) at a take-off angles relative to the surface plane of 45°. The acceptance angle of the analyzer and the precision of the sample holder concerning the take-off angle are ±3° and ±1°, respectively. The substrates (MAs on quartz slides) were mounted on Mo stubs and irradiated using monochromatized Al Kα radiation. Highresolution spectra of C(1s), O(1s), Si(2p), N(1s), Pd(3d), Cl(2p), Os(4f), and Ru(3p3) were collected with 5.85 eV pass energy and a resolution of 0.45 eV. The XPS peak intensities were obtained after Shirley background removal, and Gaussian line shapes were used for the curve fitting in the data analysis. The C(1s) line at 285.0 eV was used for calibration. Additional XPS measurements were carried out with the Kratos AXIS ULTRA system using a monochromatized Al K X-ray source (h = 1486.6 eV) at 75W and detection pass energies ranging between 20 and 80 eV. A low-energy electron flood gun (eFG) was applied for charge neutralization. Curve fitting analysis was based on linear or Shirley background subtraction and application of Gaussian- Lorentzian line shape. X-ray Crystallography. For [Ru(mbpy-py)3][PF6]2 (2), fac[Os(mbpy-py)3][PF6]2 (3), and mer-[Os(mbpy-py)3][PF6]2 (3’), low-

temperature (100 K) diffraction data ( -scans) were collected on a Rigaku XtaLab diffractometer coupled to a Pilatus 200K detector with MicroMax003 graphite monochromated MoK ( = 0.71073 Å) or with CuKα(λ = 1.54178 Å) radiation. All diffractometer manipulations, including data collection, integration, absorption corrections, and scaling were carried out using Rigaku CryAlisPro software.52 Structures were solved by direct methods using SHELXT and refined against F2 on all data by full-matrix least squares with SHELXL using established refinement techniques.53 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model. (Tables S1S6). Electrochemical Measurements. Cyclic voltammograms (CVs) were recorded with a CHI 660A potentiostat. All measurements were performed with a three-electrode cell configuration. ITO electrodes functionalized with our assemblies were used as the working electrode, whereas Pt- and Ag-wires were used as counter and reference electrodes, respectively. Solutions of Bu4NPF6 (0.1 M) in dry acetonitrile were used as the electrolyte. All electrochemical measurements were performed at RT in air. Computational Details All calculations were performed with Gaussian09 Revision C.01. The PBEPBE54 functional, including the second version of Grimme’s empirical dispersion correction,55 was used to optimize structures. Optimizations were carried out with the B1 basis set, which corresponds to a combination of the Huzinaga-Dunning double- basis set (D95(d,p))56 on the lighter atoms and the Stuttgart-Dresden (SDD)57 basis set in conjunction with a relativistic-effective-core potential on the transition metals. Density fitting basis sets (DFBS)58,59 were generated by the automatic algorithm implemented in Gaussian09 and applied to reduce the computational cost of the calculations. The optimized structures were checked for the presence of imaginary frequencies at the same level of theory as the geometry optimization. The optimized structures were confirmed as local minima by the absence of imaginary frequencies. Partial atomic charges were obtained using CM5PAC, which is a charge model 5, yielding class IV atomic charges derived from the Hirschfeld population analysis.60–62 Synthetic procedures Template Layer Formation (Scheme S1). Template layers of compounds 1, 4, and 5 were prepared according to published procedures.25,26,50 Template layers consisting of compound 2 were prepared by loading quartz (2 cm × 1 cm), ITO (0.7 cm × 5 cm, Rs = 8–12 Ω), and silicon (single-crystal (100), 2 cm × 1 cm) substrates – functionalized with 4-chloromethylbenzyl terminated monolayers – into a glass pressure vessel containing a solution of 2 (0.5 mM) in dry toluene. The reactor was sealed and heated to 95 °C for 3 days. After 3 days, the formed template layers were sonicated for 8 min in dichloromethane (× 2) and in THF to remove any physisorbed materials. Next, the slides were dried under a stream of N2 and stored in the dark. The formed template layers (TL2) were analyzed by UV-vis and XPS spectroscopy as well as spectroscopy ellipsometry (Figure S21). Template layers consisting of complex 3 were prepared similarly. The 4-chloromethylbenzyl terminated monolayers were loaded into a glass



pressure vessel containing a solution of 3 (0.2 mM) in dry toluene/acetonitrile (1:1, v:v). The reactor was sealed and heated to 95 °C for 4 days. After 4 days, the formed template layers were sonicated for 8 min in acetonitrile (× 2) and in acetone to remove any physisorbed materials. Next, the slides were dried under a stream of N2 and stored in the dark. Formation of Bimolecular Assemblies (MA1–MA5) with Ruthenium and Osmium complexes 4 and 5, and PdCl2. Molecular assemblies with TL1 and complexes 4 and 5 were prepared as previously reported.7 Molecular assemblies with TL2–5 were prepared by immersing quartz (2 cm × 1 cm), ITO (0.7 cm × 5 cm, Rs = 8-12 Ω), and silicon (single-crystal (100), 2 cm × 1 cm) substrates, functionalized with template layers TL2–TL5, for 15 min in a solution of PdCl2(PhCN)2 (1.0 mM) in THF. Thereafter, the samples were sonicated for 3 min in THF (2×) and acetone (1×). The substrates were subsequently immersed for 15 min in an equimolar THF:DMF (9:1, v/v) solution of complexes 4 and 5 (0.1 mM each). Next, the samples were sonicated for 5 min in THF (2×) and acetone (1×). This procedure was repeated for eight successive deposition steps, where one deposition step is defined as the deposition of the palladium salt, followed by the deposition of a mixture of complexes 4 and 5. The slides were dried under a stream of N2 prior to spectroscopic and electrochemical analyses. Formation of Interstitial Films of Compounds 3–5 between the Template Layer and the Molecular Assembly. ITO substrates, modified with a 1-based monolayer (TL1), were immersed for 15 min in a solution of PdCl2(PhCN)2 (1.0 mM) in THF at room temperature. Next, the samples were sonicated for 3 min in THF (2×) and acetone (1×), and subsequently immersed for 15 min in a THF:DMF (9:1, v/v) solution of complex 3 (0.2 mM). Then, the samples were sonicated for 5 min in THF (2×) and acetone (1×), and again immersed for 15 min in a solution of PdCl2(PhCN)2 (1.0 mM) in THF. After sonication for 3 min in THF (2×) and acetone (1×), the samples were immersed in an equimolar solution of complexes 4 and 5 (0.1 mM each) in THF: DMF (9:1, v/v). After 15 min, the samples were sonicated for 5 min in THF (2×) and acetone (1×), and subsequently dried under a stream of N2 prior to electrochemical analysis. Identical procedures were used when compounds 4 and 5 were deposited as interstitial layers.

Irving and Cherna Moskowitz Center for Nano and Bio-Imaging, and the Gerhardt Schmidt Minerva Center on Supramolecular Architectures. M.E.vdB. is the incumbent of the Bruce A. Pearlman Professional Chair in Synthetic Organic Chemistry. G.E. gratefully acknowledges support from the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figures S1-S21, Tables S1-S6; (PDF) Crystallographic data: (CIF)

REFERENCES 1. 2. 3.

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5.

6.

7.

Formation of Bimolecular Ru|Os networks in solution. A mixture of complex 4 (46.5 mg, 26 mmol), and complex 5 (50.0 mg, 26 mmol) in THF/DMF (10 mg; 9:1, v/v) was added to a solution of PdCl2(PhCN)2 (14.8 mg, 26 mmol) in THF (5 mL) at room temperature. After 15 min, the mixture was centrifuged for 5 min at 4000 rpm and the mother liquor was decanted to obtain a black precipitate. Next, the precipitate was rinsed with THF and centrifuged for another 5 min at 4000 rpm. This procedure was repeated three times, after the precipitate was rinsed and centrifuged for 10 min (4000 rpm) with acetone until the mother liquor became colorless. The resulting precipitate was collected and dried under vacuum, yielding a black powder, which was analyzed by XPS.

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ACKNOWLEDGMENTS

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This research was supported by the Helen and Martin Kimmel Center for Molecular Design, the Israel Science Foundation (ISF), the

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TOC Figure


Sorting of Molecular Building Blocks from Solution to Surface

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