Controlling growth of self-propagating molecular assemblies

Abstract

A series of six self-propagating molecular assemblies (SPMAs) were generated by alternative solution-deposition of ruthenium polypyridyl complexes and d⁸palladium and platinum salts on glass and silicon substrates. The d⁶ polypyridyl complexes have three pyridine units available for forming networks by coordination to the metal salts. This two-step film growth process is fast (15 min/step) and can be carried out conveniently under ambient conditions in air. The reactivity of the common metal salts (ML₂X₂: M = Pd, X = Cl, L = PhCN, ½ 1,5-cyclooctadiene (COD), SMe₂ and M = Pt, X = Cl, Br, I, L = PhCN) is a dominant factor in the film growth. Although the assembly structures are comparable, their exponential growth can be controlled by varying the metals salts. The co-ligands, halides, and metal centers can be used to control the film thicknesses and light absorption intensities of the metal-to-ligand charge transfer (MLCT) bands by a factor of ∼3.5 for 13 deposition steps, whereas the surface morphologies and molecular densities of the SPMAs are similar. The surface-confined assemblies have been characterized using a combination of optical (UV/Vis, ellipsometry) spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray reflectivity (XRR).

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Introduction

The formation of coordination metal–organic assemblies is an intriguing but often complex process that involves many variables. Numerous fascinating structures have been reported in solution and as solid-state materials with possible applications ranging from sensing and catalysis to energy storage.^1–4 For example, Nitschke et al. reported the stabilization of white phosphorus within a self-assembled tetrahedral capsule.⁴ The design of such materials with precise control over their structure and properties is a difficult task. To understand the underlying formation mechanism for a given metal–organic assembly and to be able to predict structures and function with a high level of precision, one must vary the experimental reaction parameters systematically. Such an experimental approach allows one to identify those factors that play dominant roles and to possibly determine the synergy between them.

Several methods have been developed to form metal–organic solids and films, including the stepwise solution-based deposition of ligands and metal ions on various substrates.^5–8 The formation of such coordination-based multilayers has been studied in much detail for more than two decades and has resulted in diverse materials. Alternating solution-based deposition has also afforded metal–organic frameworks (MOFs) with unique structural features not attainable by other routes.^1,9 We have shown that chromophores 1–6 (Scheme 1) and a palladium salt can be used for the stepwise formation of solid-state metal–organic structures, including 3D-ordered assemblies.^10,11 Metal-mediated layer-by-layer (LbL) growth of all-organic films is relatively rare.¹²

Scheme 1 Molecular structures for the stepwise generation of coordination-based films with PdCl₂(PhCN)₂. The use of compounds 1–6 resulted in linear growth (a),^10,11 whereas self-propagating molecular assemblies (SPMAs) were obtained with complexes 7–11 (b).^13–17

Regardless of the wide variety of metal ions and organic ligands used, the aforementioned surface-confined assemblies are formed by linear processes.^5,6,9–11 Physicochemical properties, including material thickness and light absorption intensities, are linearly correlated with the number of deposition steps. The surfaces of these linearly growing metal–organic assemblies are efficient binding sites for incoming metal ions or ligands from solution, and they can direct the supramolecular structure of the incoming layer. The bulk of the assembly, however, does not play a significant role during the growth process.

Exponentially growing assemblies are intriguing because the material being formed is involved in its own formation. A rare example is Sagiv et al.'s self-replicating organosilane-multilayers.¹⁸Polyelectrolyte-based films may also exhibit nonlinear growth.^19–22 We have shown that the alternative deposition of polypyridyl ruthenium, osmium, and cobalt complexes (Scheme 1; 7–11) as well as a d⁸palladium salt, PdCl₂(PhCN)₂, from solution results in exponentially growing metal–organic assemblies.^13–17 The overall process is a three-component process in which the self-propagating molecular assemblies (SPMAs) store palladium. This excess of palladium is used to bind the polypyridyl complexes to the surface. This d⁸ metal is an important structural synthon for forming metal–organic networks. Pyridine and derivatives thereof readily coordinate to PdCl₂ in a 2 : 1 ligand-to-metal ratio forming stable 16-electron complexes having a square-planar geometry around the metal center.¹⁰ However, the role of the metal complex precursor in SPMA formation has not been evaluated. Until now, all SPMAs have been generated from Pd(PhCN)₂Cl₂.^13–17 We have shown that the growth of the SPMAs can be systematically controlled by changing the structure of the molecular component (Scheme 1) and by varying the reaction conditions. Linear growth is observed with polypyridyl complexes (8) when the surface-bound assemblies are only briefly exposed to the palladium salt.¹⁴ Combining organic chromophores (4) with polypyridyl complexes (11)¹⁴ in one assembly results in enhanced exponential growth.¹³ The redox-active SPMAs have been used (i) to study electron-transfer and electrochromism,¹⁶ (ii) to fabricate inverted solar cells,¹⁵ and (iii) to demonstrate Boolean and sequential logic operations.²³

In the present study we show that the choice of the metal-ion source significantly affects the formation, optical properties, and thickness of the SPMAs. Six SPMAs were generated on silicon and glass substrates with a ruthenium polypyridyl complex (11)^14,15 and a series of metal salts of the formula: ML₂X₂ with M = Pd, X = Cl, L = PhCN, ½ 1,5-cyclooctadiene (COD), SMe₂ and M = Pt, X = Cl, Br, I, L = PhCN). These transition metal ions, halides, and co-ligands play not only a key structural role by coordination to the polypyridyl complexes to generate the surface-confined networks but are also important for efficient film assembly. The reactivity of the metal salts is expressed in the SPMAs' properties. We demonstrate here that both absorption intensities and thicknesses of the SPMAs can be varied by a factor of ∼3.5, while a similar surface morphology and molecular density is maintained.

Results and discussion

p-Chlorobenzyl-based monolayers²⁴ covalently attached to glass and silicon substrates were used to generate template layers (TLs) consisting of the pyridinium salt of complex 11 (Scheme 2a).¹⁵

Scheme 2 (a) Glass and silicon substrates functionalized with a p-chlorobenzyl-terminated monolayer²⁴ were reacted at elevated temperatures with a dry acetonitrile/toluene solution of complex 11 for 4 days under an inert atmosphere with the exclusion of light to generate a template layer (TL = deposition step 1).^14,15X-ray photoelectron spectroscopy (XPS) studies have shown that the analogous complexes 7 and 10 (Scheme 1) are only bound via one pyridine moiety.^14,17 (b) Schematic presentation of the formation of SPMAs with complex 11 and PdCl₂(PhCN)₂.^13–17

Subsequently, the six SPMAs were formed by iterative immersion of the functionalized substrates for 15 min into a THF solution of one of the Pd or Pt precursors and a THF/DMF (9 : 1, v/v) solution of complex 11. The SPMAs were thoroughly washed and sonicated in organic solvents between each deposition step (for details, see Materials and methods).

The SPMAs were characterized by UV/Vis spectrometry, atomic force microscopy (AFM), ellipsometry, X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray reflectivity (XRR) measurements. The optical absorption intensities of the metal-to-ligand charge transfer (MLCT) bands at λ = 495 nm and the thickness of the SPMAs versus the number of deposition steps show that exponential growth occurs (Fig. 1, 3 and 4). The half-width and the maximum position of the absorption bands are not affected during the growth process, as shown for SPMA|Pd(PhCN)₂Cl₂ in Fig. 1, suggesting no strong inter- and intramolecular interactions. Semicontact AFM measurements show that the SPMAs have comparable film surfaces after 13 deposition steps with a low root-mean-square surface roughness (R_rms) of 0.4–1.1 nm for scan areas of 0.5 nm × 0.5 nm, with no apparent grain boundaries (Fig. 2 and Fig. S2†). These surface morphologies are common for related molecule-based multilayers that exhibit linear growthversus the number of deposition steps.^10,11 The optical and AFM data of all SPMAs are shown in Fig. S1 and S2, respectively.†

Fig. 1 Formation of SPMA|Pd(PhCN)₂Cl₂: Representative UV/Vis absorption spectra on glass substrates. These ex-situspectra were recorded after exposing the SPMA to a THF/DMF (9 : 1, v/v) solution containing complex 11 (0.2 mM) for 15 min. The red spectrum is from the template layer (TL). The baseline is shown in black.

Fig. 2 Representative semicontact AFM image of SPMA|Pd(PhCN)₂Cl₂ after 13 deposition steps of complex 11 on silicon substrates (Scheme 2). Step 1 is the template layer (TL). A scan area of 500 nm × 500 nm is shown with a roughness, R_rms, of 0.4 nm. UV/Vis spectra and AFM images of all the SPMAs are available in the ESI (Fig. S1 and S2).†

The growth of the SPMAs is clearly affected by the nature of the metal complex precursor, as can been seen by plotting the absorption intensities and film thickness versus the number of deposition steps (Fig. 3 and 4). Steep exponential growth is achieved with Pd(PhCN)₂Cl₂, which is in contrast with the build-up observed with Pt(PhCN)₂Cl₂ (Fig. 3). Thus, varying the metal ion of the metal complex precursors results in significant absorption intensity enhancement (3.5×) and a difference of 11 nm in the final SPMA thickness (Table 1, entries 1 and 4). Pd(PhCN)₂Cl₂ is the more reactive precursor. The halide also plays an important role. The exponential growth increases in the order: Pt(PhCN)₂I₂ > Pt(PhCN)₂Br₂ ≥ Pt(PhCN)₂Cl₂ (Fig. 3; Table 1, entries 1–3). Whereas the difference between the Br and Cl derivatives is minor, the use of Pt(PhCN)₂I₂ resulted in exponential growth similar to the one observed with Pd(PhCN)₂Cl₂. The ‘bulkiness’ of halogen atoms is known to accelerate ligand exchange process.²⁵ Our observations might suggest that the reactivity of the metal complex precursors plays a dominant role.^26–28 This hypothesis is fully supported by the SPMA growth upon varying the organic ligands (PdL₂Cl₂; L = PhCN, ½ COD and SMe₂). The SPMA growth follows the following order: Pd(PhCN)₂Cl₂ > Pd(COD)Cl₂ > Pd(SMe₂)₂Cl₂ (Fig. 4). Also here the differences are significant. The growth is enhanced by ∼2.8× for 13 deposition steps by varying these co-ligands (Table 1, entries 4–6). The SPMA growth is directed related to the relative reactivity/stability of these three palladium salts. Pd(PhCN)₂Cl₂ reacts readily with COD in solution to form Pd(COD)Cl₂, which is more stable by 13 kcal mol⁻¹.²⁸ In addition, Pd(COD)Cl₂ and Pd(PhCN)₂Cl₂²⁶ react with two equivalents of SMe₂ to form Pd(SMe₂)₂Cl₂.

Fig. 3 Exponential dependence of (a) the intensities of the metal–ligand charge transfer (MLCT) bands at λ ≈ 495 nm on glass substrates, and (b) the film thicknesses on silicon for (■) SPMA|Pt(PhCN)₂Cl₂, () SPMA|Pt(PhCN)₂Br₂, () SPMA|Pt(PhCN)₂I₂, and () SPMA|Pd(PhCN)₂Cl₂versus the number of deposition steps (R² = 0.99). Step 1 is the template layer (TL). The film thicknesses were obtained by spectroscopic ellipsometry.

Fig. 4 Exponential dependence of (a) the intensities of the metal–ligand charge transfer (MLCT) bands at λ ≈ 495 nm, and (b) the film thicknesses on silicon for (■) SPMA|Pd(SMe₂)₂Cl₂, () SPMA|Pd(COD)Cl₂, and () SPMA|Pd(PhCN)₂Cl₂versus the number of deposition steps (R² = 0.99). Step 1 is the template layer (TL). The film thicknesses were obtained by spectroscopic ellipsometry.

Table 1 Characterization data (light absorption intensity, thickness and elemental ratio) of the six SPMAs after 13 deposition steps

Entry	M	L	X	T/nm	A ^a	E ^b	M/N^c	M/Ru^c	N/Ru
a Absorption at λ ≈ 495 nm ×10^−2. b E = Enhancement of the growth derived from the thickness (T) and the optical data (A) (normalized to entry 1). c M = Pt or Pd; the ratios were obtained by XPS. d The S/Pd ratio is 0.28. The XPS spectra were recorded at take-off angles of θ = 0° and 45.0° and show similar elemental ratios.
1	Pt	PhCN	Cl	5.5	3.9	1.0	0.20	2.7	13
2	Pt	PhCN	Br	6.6	4.7	1.2	0.17	1.8	11
3	Pt	PhCN	I	17.0	12	3.1	0.19	1.9	10
4	Pd	PhCN	Cl	18.1	15	3.6	0.26	2.8	11
5	Pd	½COD	Cl	11.2	9.2	2.2	0.16	1.5	9
6	Pd	SMe2^d	Cl	6.4	5.8	1.3	0.14	1.5	11

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The structural regularity of the SPMAs is evident from the good linear dependence of the absorption intensities versus the film thicknesses (Fig. 5, Table S1†; R² ≥ 0.98), indicating that the molecular density and the relative roughness remain nearly constant during the growth process. The average density of complex 11 for the fully formed SPMAs is roughly estimated from the optical and thickness data to be 1.1 (Pt) and 1.3 (Pd) molecules nm⁻³ and is very similar for each deposition step of complex 11 (Table S1†). Such high values have been reported for single-crystal packing densities of polypyridyl complexes.²⁹ Moreover, the similarity between the slopes of the Pd (8.1–10 × 10⁻⁴) and the Pt (6.4–7.1 × 10⁻⁴) SPMAs indicate that the structures are comparable (Table S1†), but are affected by the different metal. The palladium salts might seem to form slightly more densely packed SPMAs.

Fig. 5 The SPMA absorption intensity at λ ≈ 495 nm (Fig. 3a and 4a) versus thickness (Fig. 3b and 4b). The 6 lines are linear fits with R² ≥ 0.98 for (■) SPMA|Pt(PhCN)₂Cl₂, () SPMA|Pt(PhCN)₂Br₂, () SPMA|Pt(PhCN)₂I₂, () SPMA|Pd(PhCN)₂Cl₂, () SPMA|Pd(SMe₂)₂Cl₂, and (△) SPMA|Pd(COD)Cl₂. The slopes are listed in Table S1.†

The SPMA formation was further confirmed by XRR measurements of SPMA|Pt(PhCN)₂Br₂ and SPMA|Pd(COD)Cl₂ after 13 deposition steps (Fig. 6 and S3†). We observed film thicknesses of 6 and 1.1 nm, respectively, with a relative roughness of ∼20%. These thickness values are in agreement with the ellipsometry data (Table 1, entries 2 and 5). The electron density, ρ, for the SPMA|Pd(COD)Cl₂ is ∼11% higher (Fig. 6b), which is in agreement with the above conclusions. We have previously shown by XRR measurements that ρ is constant during the build-up of a SPMA formed with osmium complex 10 and Pd(PhCN)₂Cl₂ (Scheme 1).¹⁷

Fig. 6 (a) Representative synchrotron X-ray reflectivity (XRR) spectrum of SPMA|Pt(PhCN)₂Br₂ on a silicon substrate (see Fig. S3† for the spectrum of SPMA|Pd(COD)Cl₂). The blue trace is a fit of the experimental data (black dots).³⁰ (b) Electron density plots for SPMA|Pt(PhCN)₂Br₂ (blue line) and SPMA|Pd(COD)Cl₂ (red line) after 13 deposition steps with step 1 being the template layer (TL).

The XPS-derived elemental ratios are centred around the theoretical values expected for a fully formed network where two pyridinium groups are coordinated to a palladium or platinum centre: M/N = 0.17; M/Ru = 1.5 and N/Ru = 9 (M = Pt or Pd). No clear trend is apparent (Table 1; entries 1–6). Angle-dependent XPS measurements indicate the formation of homogeneous structures. Some of the SPMAs contain some of the d⁸ metal salts and/or free ligands. For instance, SPMA|Pd(SMe₂)₂Cl₂ contains sulfur as a minor component (S/Pd = 0.28).

Summary and conclusions

Regardless of the metal salt used, all SPMAs form regularly growing assemblies on glass and silicon substrates. Interestingly, the different metal salts do not seem to drastically affect the surface morphology of the SPMAs, the elemental ratios, molecular density, and the half-width as well as the maximum position of the absorption bands. The nature of the metal complex precursors significantly affects the SPMA growth, but our observations indicate that the overall structure of the SPMAs is similar. The following parameters were noted for promoting efficient film growth: (i) Pd > Pt, (ii) I > Br ≥ Cl, and (iii) PhCN > COD > SMe₂. These trends can be explained by the reactivity of these metal salts with the surface-bound polypyridyl complexes.^26,27 Ligand substitution reactions with Pd tend to be faster in comparison with the corresponding Pt complexes. The metal–ligand bond strength of the precursors also affects the growth. A higher reactivity results in more efficient growth because more Pd or Pt is accessible for the system. The different dimensions of the metal salts studied here seem to play a minor role. These findings are in good agreement with our mechanistic interpretation of the film growth where the SPMAs are reservoirs of metal salts.^14,17 This model is to some extent similar to the one operating with exponentially growing polyelectrolyte-based materials.^19,31 However, the film roughness might also play a role. This issue is currently under debate.³² At present, we cannot exclude that the surface morphology affects the SPMA growth. Fine tuning of the SPMAs' composition without compromising the efficiency of the film growth can be achieved by combining parameters having opposite effects. For instance, much more efficient growth was observed with Pd(PhCN)₂Cl₂ than with Pt(PhCN)₂Cl₂. However, Pd(PhCN)₂Cl₂ and Pt(PhCN)₂I₂ are equally efficient. In order to grow SPMAs with different metals (Pd or Pt) having similar properties (i.e., light absorption, thickness, and roughness), one has to use different halides as well. These parameters (i.e., Pt, I) may affect the molecular density, which might be interesting for size-selective sensing. A few other studies have addressed the use of different metal complex precursors with linear growing metal–organic multilayers and oligomers.^6,8 Of course, there remain many other parameters to be explored, to gain a better and more thorough picture of the exponential growth. Solvents,³³ temperature,²⁰ and the anions²¹ of the polypyridyl complexes might also affect the growth and the SPMA structures. However, the metal salts are certainly of prime importance for the exponential growth and they can be used to control it.

Materials and methods

PtCl₂, PtBr₂, and PtI₂ were purchased from Aldrich and used as received. Solvents (AR grade) were purchased from Bio-Lab (Jerusalem) and Aldrich. trans-Pt(PhCN)₂X₂, (X = Cl, Br, I),³⁴trans-Pd(PhCN)₂Cl₂,³⁵Pd(COD)Cl₂,³⁶trans-Pd(SMe₂)₂Cl₂,³⁷ and complex 11 were prepared according to procedures in the literature.¹⁴ Single-crystal silicon (100) substrates were purchased from Wafernet (San Jose, CA) and were cleaned by sonication in hexane followed by acetone, then ethanol, and dried under an N₂ stream. Subsequently, they were cleaned for 20 min with UV and ozone in a UVOCS cleaning system (Montgomery, PA). Float glass (Corning, NY) was cleaned by immersion 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 handled with care using appropriate personal protection. Subsequently, the substrates were rinsed with deionized (DI) water followed by the RCA cleaning protocol: 1 : 5 : 1 (v/v) NH₄OH/H₂O/30% H₂O₂ at room temperature for 45 min. The substrates were washed with ample amounts of DI water and were dried under an N₂ stream. All substrates were then dried in an oven for 2 h at 130 °C. UV/Vis spectra were recorded on a Cary 100 spectrophotometer. Spectroscopic ellipsometry was recorded on an M 2000V (J. A. Wollam Co., Inc.) instrument with VASE32 software. AFM images were recorded using a Solver P47 (NT-MDT, Russia) operated in semicontact mode. All measurements were carried out at 295 K unless otherwise mentioned. X-ray reflectivity (XRR) measurements were performed at beamline X18A of the synchrotron National Synchrotron Light Source, Brookhaven (USA) using a four-circle Huber diffractometer in the specular reflection mode (i.e., incident angle was equal to the exit angle). An X-ray beam with an energy of E = 10.0 keV (λ = 1.240 Å) was used. The beam size was 0.50 mm vertically and 1.0 mm horizontally. The samples were held under a helium atmosphere during the measurements to reduce radiation damage and the background scattering from the ambient gas. The off-specular background was measured and subtracted from the specular counts. X-ray photoelectron spectroscopy (XPS) measurements were carried out on glass substrates with a Kratos AXIS ULTRA system using a monochromatized Al Kα X-ray source (hν = 1486.6 eV) at 75 W and detection pass energies ranging between 10 and 80 eV. Angular resolved spectra were recorded at θ = 0° and 45.0°, where θ is the take-off angle with respect to the surface normal. A low energy flood gun was used to neutralize charging while measuring glass substrates.

Template layer (TL) formation

Glass and silicon substrates (0.8 cm × 2.5 cm) functionalized with a p-chlorobenzyl-terminated monolayer²⁴ were loaded into a glass pressure vessel and immersed in a dry acetonitrile/toluene (1 : 1 v/v) solution of complex 11¹⁴ (0.2 mM) and heated for 96 h at 95 °C with the exclusion of light (Scheme 2a). The TLs were then rinsed with acetonitrile and sonicated for 5 min each in acetonitrile, acetone and isopropanol. The samples were then carefully wiped with a task wipe and dried under a stream of N₂. The TL is deposition step 1 of the SPMA formation.

SPMA formation

Glass and silicon substrates (0.8 cm × 2.5 cm) functionalized with the 11-based TLs (= deposition step 1; Scheme 2a) were loaded onto a Teflon holder and immersed for 15 min in a 1 mM solution of a metal salt (ML₂X₂: M = Pd, X = Cl, L = PhCN, ½ 1,5-cyclooctadiene (COD), SMe₂ and M = Pt, X = Cl, Br, I, L = PhCN) in THF. The samples were then sonicated twice in THF and once in acetone for 2 min each. Subsequently, the samples were immersed for 15 min in a 0.2 mM solution of compound 11 in THF/DMF (9 : 1, v/v). The samples were then sonicated twice in THF and once in acetone for 2 min each. This procedure was repeated 6 times (=12 deposition steps). Finally, the samples were rinsed in ethanol and dried under a stream of N₂. This procedure was carried out in air at room temperature.

Acknowledgements

This research was supported by the Helen and Martin Kimmel Center for Molecular Design, the Israel Science Foundation (289/09), MINERVA, and by the U.S. National Science Foundation under Grant No. DMR-1006432. The X-ray reflectivity measurements were performed at Beamline X18A of the National Synchrotron Light Source, supported by the U.S. Department of Energy under contract no. DE-AC02-98CH10886. X-ray photoelectron spectroscopy (XPS) measurements were performed by Drs T. Bendikov and H. Cohen (Department of Chemical Research Support, The Weizmann Institute of Science).

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Footnote

† Electronic supplementary information (ESI) available: UV/Vis absorption spectra (Fig. S1) and AFM images (Fig. S2) of the 6 SPMAs. Synchrotron X-ray reflectivity data of SPMA|Pd(COD)Cl₂ (Fig. S3), linear fitting data and molecular densities (Tables S1 and S2). See DOI: 10.1039/c1sc00318f

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