Enhancement of Optical and Electrochemical Properties via Bottom

Article pubs.acs.org/JPCC

Enhancement of Optical and Electrochemical Properties via BottomUp Assembly of Binary Oligomer System Prakash Chandra Mondal,*,†,‡,⊥ Megha Chhatwal,† Yekkoni Lakshmanan Jeyachandran,§ and Michael Zharnikov*,§ †

Department of Chemistry, University of Delhi, Delhi, 110007, India Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel § Applied Physical Chemistry, Heidelberg University, Heidelberg, 69120, Germany ‡

S Supporting Information *

ABSTRACT: Heterometallic, coordination-based, binary oligomer films were fabricated on SiOxbased solid substrates using successive layer-by-layer assembly of optically rich and redox-active polypyridyl complexes, Ru(pytpy)2·2PF6 (Ru-PT) and Os(pytpy)2·2PF6 (Os-PT) (where pytpy = 4′-pyridyl-2,2′:6′,2″-terpyridyl). The individual oligomer chains comprised alternating Ru-PT and Os-PT units connected via Cu2+, Pd2+, Ag+, Fe2+, Co2+, or Zn2+ metallo-linkers. The growth and properties of the oligomer films were monitored in detail by UV−vis spectroscopy and cyclic voltammetry. The films exhibited a linear growth upon addition of the successive building blocks, with a joint grafting density of 3.9−5.0 × 1014 metallo-ligands/cm2 for the final oligomer films (10 layers), corresponding to a characteristic area of 2.0−2.5 nm2/oligomer. The only exception was the Pd2+-linked film on glass that showed an exponential growth, which, however, could also be changed to the linear mode by the introduction of a conductive substrate. The combination of two different functional molecular units in the oligomer chains resulted in enhancement of the optical window and in an increase in the number of the available redox states as compared to the analogous single component assemblies.

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INTRODUCTION

tures as well as with enhanced optical and electrochemical properties.19 The oligomer films prepared from metallo-ligand/metal ions have some advantages as compared to the films composed of multidentate ligand/metal ion combination; e.g., they are believed to be more robust. Among different metallo-ligands, polypyridyl-based transition-metal complexes have attracted a particular attention owing to their unique and tunable optical, photochemical, and redox properties.20,21 Such complexes are well-known for their versatility in supramolecular chemistry, applications in nanoscale materials, etc.22,23 They have been immobilized on various substrates for different purposes. For instance, van der Boom et al.,24−26 Forster et al.,27−29 and others30,31 fabricated a variety of robust monomolecular layers composed of such complexes. Recently, we have also assembled several representative heterometallo-ligand-terminated homooligomer films and heterogeneous molecular dyads comprising functionalized terpyridyl complexes on SiOx substrates and studied their optical, electrochemical, molecular recognition, and data storage properties.32−34 The results for the heterogeneous dyads were especially promising since a combination of two different metallo−organic complexes at a single platform can lead to significant enlargement of the optical window, which can be of interest for potential

A paradigm shift from traditional disciplinary research to interdisciplinary fields motivates chemists to get engaged at the interface between chemistry and other disciplines, such as physics, material science, biology, nanofabrication, and engineering.1−4 In particular, recent advances in developing molecular-based thin films are very impressive, leading to potential applications in catalysis,5 sensors,6,7 photovoltaics,8,9 nanoscale electronic devices,10 and electrochromic materials,11,12 to name a few. An important issue in this context is modification and functionalization of solid substrates, such as glass, quartz, indium−tin oxide (ITO)-coated glass, silicon, and gold, with optically rich and redox-active molecular building blocks, accompanied by molecular level design of the respective systems.13 Several successful methodologies within the general “bottom-up” and “top-down” frameworks were developed to fabricate such molecular assemblies.14 Among them, the bottom-up, layer-by-layer (LBL) technique, described also as the stepwise coordination method, emerged as a facile, inexpensive, environment friendly, and well-controllable approach.15−18 This approach is widely used to grow metal complex wires composed either of metal ions and bridging metal ions or of organo−metal complexes and bridging ligands or bridging metal ions.16,17 In addition, LBL attachment of functional molecular units to preorganized molecular templates makes possible the preparation of surface-confined inorganic− organic hybrid materials (SURIOHMs) with layered architec© 2014 American Chemical Society

Received: March 3, 2014 Revised: April 10, 2014 Published: April 14, 2014 9578

dx.doi.org/10.1021/jp502166k | J. Phys. Chem. C 2014, 118, 9578−9587

The Journal of Physical Chemistry C

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Scheme 1. Schematic Representation of the Wet Chemical LBL Deposition of the Target Oligomer Assemblya

a (i) Chemisorption of 3-iodo-n-propyltrimethoxy-silane on the substrate (uncoated or ITO-coated glass) to form a “coupling layer”; (ii) covalent assembly of Ru-PT to form “template layer”; (iii) coordination of either Cu2+ or Pd2+ with free pyridyl group of the template layer; (iv) deposition of Os-PT, followed by repetition of the steps (iii) and (ii)−(iv) to form SURCO-1 (Cu2+ coupling) or SURCO-2 (Pd2+ coupling) assembly. The counteranions and bound solvents are omitted for simplicity. The template layer and the layer after the step (iv) represent examples of an odd and even layer, respectively (see below). The oligomer films were also prepared with the Ag+, Co2+, Fe2+, and Zn2+ metallo-linkers using the same methodology.

applications.34 The optical and electrochemical response of a heterogeneous system can be, however, enhanced significantly if the individual building blocks are arranged not as a dyad monolayer but in an oligomer fashion. In this context, we report here the fabrication of surface-confined, coordinationbased diblock oligomer (SURCOs) films via wet chemical LBL assembly of two different, Ru- and Os-based terpyridyl complexes alternatively connected via different metallo-linkers (Cu2+, Pd2+, Ag+, Co2+, Fe2+, and Zn2+) at a single platform. Note that the assembly of these oligomers is a nontrivial task since the terpyridyl complexes in bulk succumb generally in the form of polymer chains through a solvothermal process.35 Note also that the fabrication of heterogeneous metallo−organic oligomers is the actual issue within the general framework of metallo−organic assembly on solid supports.17,18 Significantly, assembly of differing metal complexes in alternating fashion, as presented in our case, has some advantages over other types of heterogeneous assembly.18

hexane, acetone, and 2-propanol, followed by drying under a N2 stream. The glass substrates were cleaned by immersion in a “piranha” solution (conc. H2SO4/30% H2O2, 7:3 (v/v)) over 1 h (Caution! “Piranha” is a tremendously dangerous oxidizing agent and should be handled cautiously with appropriate personal protection). The glass substrates were rinsed thoroughly with deionized water and then subjected to “RCA” cleaning reagent (NH3·H2O/H2O/30% H2O2, 1:5:1 (v/v)) for 45 min, followed by rinsing with plenty of double distilled water and subsequent drying under a N2 stream. All the substrates were then dried in an oven (2 h) before functionalization. Preparation of the Template Layer. The glass, ITOcoated glass, and Si(100) substrates were functionalized with 3iodo-n-propyltrimethoxy-silane under a N2 atmosphere using the Schlenk line technique (see Scheme 1).39 The substrates were treated with a dry n-pentane solution of 3-iodo-npropyltrimethoxy-silane (200:1, v/v) for 30 min under a N2 stream. Then, the substrates were washed with dry n-pentane and sonicated for 3 min with dichloromethane and isopropanol to remove any physisorbed materials. The resulting coupling layers were dried under a stream of N2, followed by drying in an oven at 120 °C for 15 min. Consequently, the functionalized substrates were loaded into a Teflon-lined autoclave and immersed in a dry acetonitrile/toluene (1:1, v/v) solution of Ru-PT (0.5 mM) and placed in a programmable oven at 80 °C for 56 h (see the next section). Consequently, the substrates were cooled to room temperature. Finally, they were rinsed

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EXPERIMENTAL SECTION Synthesis and characterization data for 4′-pyridyl terpyridyl36 and its metal complexes, such as Ru(pytpy)2·2PF6 (Ru-PT), Os(pytpy)2·2PF6 (Os-PT)37 (where pytpy = 4′-pyridyl-2,2′: 6′,2″-terpyridyl), and PdCl2(PhCN)2,38 are presented in detail in the Supporting Information (see Scheme S1, Figures S1 and S2). Activation of the Substrates. Si(100) and ITO-coated glass substrates were cleaned by subsequent sonication in n9579



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were measured in 1 mM solution of the complexes in dry acetonitrile with tetra-n-butylammonium hexafluorophosphate (TBAPF6, 0.1 M) as the supporting electrolyte using a glassy carbon as working electrode, a Pt wire as counter electrode, and Ag/AgCl (1 M KCl) as reference electrode. In the case of the template layer and oligomer films, ITO-coated glass (1.5 cm × 0.8 cm) was used as working electrode (WE), Pt wire as counter electrode (CE), and Ag/AgCl as reference electrode. TBAPF6 (20 mM) in dry acetonitrile was used as supporting electrolyte. The electrolyte solution was degassed for 5 min before cyclic voltammetric measurement.

with acetonitrile and sonicated for 3 min with acetonitrile and isopropanol, followed by drying under a nitrogen stream. Preparation of the Oligomer Film. The fabrication of coordination-based diblock oligomer films using the Ru-PT and Os-PT complexes as the building units was performed by a stepwise metal ion/metallo-ligand coordination reaction on the well-defined freshly prepared Ru-PT template layer that was grafted on the SiOx-based substrate, such as transparent glass, smooth Si(100), or conducting ITO-coated glass (see Scheme 1). Starting from the template layer, films of surface-confined, coordination-based diblock oligomers (SURCOs) were fabricated by the assembly of the Ru-PT and Os-PT units in alternating fashion up to the 10th layer. The units were connected by either a Cu2+ or a Pd2+ metallo-linker, resulting in the SURCO-1 or SURCO-2 assembly, respectively (see the Supporting Information). In addition, the oligomer films were also prepared using the Ag+, Co2+, Fe2+, and Zn2+ metallolinkers. The above metallo-linkers were selected due to (i) a suitable binding affinity for coordination with the pyridyl moieties, (ii) discrete coordination number, and (iii) the ability to promote intermolecular electronic communications.40 The use of different metallo-linkers gives a larger flexibility in the design of the oligomer films, depending on a particular application. Characterization of the Template Layer and Oligomer Films. Template layer and oligomer films were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), UV−vis spectroscopy, and cyclic voltammetry. The latter two techniques were also used to monitor the progressive growth of the oligomer films. AFM images were recorded using a Dimension 3100 (Veeco Digital Instruments) device equipped with a Nanoscope IIIa controller (Veeco) and operating in tapping mode in air. For these experiments, we used aluminum-coated cantilevers with silicon nitride tips having a radius of less than 10 nm. The water contact angle of the molecular films (on silicon) was measured with a Rame-Hart goniometer using drops of double distilled water (2 μL). The thickness of the films was measured using a J. A. Woollam model M-2000 V spectroscopic ellipsometer in a range of 400−800 nm. VASE32 software was used. The fitting parameters A, B, and C of the Cauchy model were set to 1.4, 0.02, and 0.01 nm2, respectively, resulting in the mean squared error less than 10 for the fit of the experimental data. The SiO2 layer was estimated at ∼15 Å. XPS measurements were performed with a MAX200 spectrometer (Leybold-Heraeus) that was equipped with a Mg Kα X-ray source (1253.6 eV; 200 W) and a hemispherical analyzer. The measurements were carried out in the normal emission geometry. The recorded spectra were corrected for the spectrometer transmission; the binding energy scale was referenced to the Si 2p3/2,1/2 doublet at 99.15 eV. The spectra were fitted by symmetric Voigt functions and a Shirley-type background. UV−vis spectra were recorded at room temperature with a JASCO (model No. V-670) spectrophotometer in the range of 200−800 nm. The functionalized glass substrates were fixed in a custom-made Teflon holder (1.5 cm × 0.75 cm window), and an identical bare glass substrate (without monolayer) was used to compensate for the background absorption. The electrochemical experiments were performed with a CH Instruments potentiostat (model 660D). In the case of reference measurements in solution, cyclic voltammograms

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RESULTS AND DISCUSSION Preparation Procedure. The reaction time necessary for the efficient coupling of the Ru-PT units to the coupling layer (to form the template layer) was optimized using UV−vis spectroscopy. For a short reaction time (less than 10 h), UV− vis spectra showed only a small extent of the coupling. For longer reaction time, progressive increase in the Ru-PT coverage was observed, with a leveling off behavior after 3 days. Consequently, a reaction time of 56 h was selected, as mentioned in the previous section. The resulting Ru-PT template layer showed good mechanical stability, since neither washing with common organic solvents nor mechanical abrasion could remove the molecules from the substrates. In addition, the template layer showed excellent thermal, temporal, and electrochemical stability. For instance, the modified substrates were found to be stable for months under dark and ambient conditions as judged by UV−vis spectroscopy. Also, according to the UV−vis spectroscopy data, the template layer exhibited excellent thermal stability up to 200 °C and temporal stability (see Figure S3, Supporting Information). Further, the template layer showed good redox stability/reversibility and did not deteriorate upon large numbers (>103 times) of “read−write” cycles (see Figure S4, Supporting Information). AFM. The tapping mode AFM image of the Ru-PT template layer on a Si(100) substrate showed a relatively smooth film surface with no apparent features, such as islands, grains, pinholes, or defects (Figure 1a). The Rrms measured for a 1000

Figure 1. Representative tapping mode AFM images of (a) Ru-PTbased template layer (Rrms = 0.45 nm) and (b) Ru-PT/Cu/Os-PT bilayer (Rrms = 0.72 nm) on Si(100) substrate. The scan area was 1.0 μm × 1.0 μm.

× 1000 nm2 scan area was found to be ∼0.45 nm, confirming the formation of a uniform template. However, the Rrms value increased to some extent (∼0.72 nm) after addition of Cu2+ and Os-PT to the template layer (Figure 1b), reflecting presumably some distribution in the orientation of the individual diblock moieties. 9580



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CA. The water contact angle of the Ru-PT template layer was estimated at ∼78°, corresponding to a hydrophobic surface. Similar contact angle values were obtained for the Ru-PT- and Os-PT-terminated oligomer films. In contrast, the metallolinker (Cu2+)-terminated oligomer films exhibited lower values of the contact angle (∼57°). This behavior is reasonable, since the Cu2+ center is hydrophilic in nature. Ellipsometry. The thickness of the Ru-PT template layer was estimated at ∼18.5 Å. According to the Chem3D Pro energy minimization model, the optimized length of the Ru-PT moiety, including the coupling layer (i.e., 3-iodo-n-propyltrimethoxy-silane), is ∼24.1 Å, which is longer by 5.6 Å than the thickness measured by ellipsometry. This indicates that the RuPT is tilted by ∼30° with respect to the surface normal. The thickness of the second layer was measured to be ∼30 Å, showing a similar tilt (∼30°) of the Os-PT unit. XPS. A representative SURCO-1 oligomer film, viz. the RuPT/Cu/Os-PT dyad layer, was characterized by XPS. The XP spectra of this film exhibit characteristic Si 2p peaks from the substrate as well as C 1s, Ru 3d, N 1s, Os 4f, and Cu 2p emissions. The Si 2p spectrum exhibits Si 2p doublets at 99.15 and 102.6 eV representative of the nonoxidized and oxidized silicon, respectively (see Figure S5, Supporting Information). The C 1s spectrum exhibits a somewhat asymmetric peak at 284.7 eV attributed to the pyridine moieties (Figure 2a); the

PT template layer was estimated as I(N)/I(N+) = 6.8:1, indicating quaternization of only “bottom” pyridine nitrogen interacting with the aliphatic coupling layer. In addition, there is a weak feature at 406.1 eV assigned to the NO3− nitrogen. This peak is associated with Cu(NO3)2, which was employed as the metallolinker for the fabrication of the molecular assembly. The Os 4f spectrum in Figure 2c exhibited the characteristic Os 4f7/2,5/2 doublet at 51.35 eV (Os 4f7/2) and at 53.95 eV (Os 4f5/2), manifesting the expected presence of this metal ion. Similarly, the Cu 2p spectrum in Figure 2d shows the characteristic Cu 2p3/2,1/2 doublet at 932.4 eV (Cu 2p3/2) and 951.8 eV (Cu 2p1/2), confirming the presence of Cu2+ in the molecular assembly. UV−vis Spectra. The UV−vis spectrum of the Ru-PT template layer on a glass substrate showed, in accordance with the previous studies,33,34 an intense metal-to-ligand chargetransfer (MLCT) band at λmax = 508 nm, which was red-shifted by Δλ = 18 nm as compared to the spectrum of Ru-PT in acetonitrile solution (Figure 3a). The red shift of the MLCT band is attributed to the quaternization of the pendant pyridine group with the iodo-terminated coupling layer. The average molecular density in the template layer was roughly estimated at 4.5 × 1013 metallo-ligands/cm2 (i.e., ∼2.2 nm2/metalloligand) from the UV−vis data assuming the same molecular extinction coefficient (ε ≈ 29 800 cm−1 M−1) as for the MLCT band (λmax = 490 nm) in acetonitrile solution. This value is in good agreement with the previously reported density values for the Ru(II)−polypyridyl complex and pyridyl-ligand-based covalent assembled monolayers.24,42 Note that this density is below a limit of ∼1.1 × 1014 metallo-ligands/cm2 (i.e., ∼0.9 nm2/metallo-ligand) given by the cross section of the Ru-PT metallo-ligand so that an even closer packing is, in principle, possible. The full width at half-maximum (fwhm) for the MLCT band of the Ru-PT template layer was estimated at ∼62 nm, which was slightly higher (by 9 nm) than the analogous value obtained for Ru-PT in acetonitrile solution. The broadening of the band can be attributed to (i) constraints in molecular motions, (ii) dense packing, and (iii) possible intermolecular π−π interactions between the surface-confined species.43 In addition, by extrapolating the MLCT band in the UV−vis spectra,44 the optical band gap (Eg) for the Ru-PT template layer on glass was estimated at 2.04 eV, which is lower by 0.2 eV than the analogous value for Ru-PT in acetonitrile solution. Cyclic Voltammetry. Comparative cyclic voltammograms of Ru-PT in acetonitrile solution and the Ru-PT template layer on ITO-coated glass substrate are shown in Figure 3b. The RuPT template layer exhibited single electron oxidation at +1.47 V and reduction at +1.44 V (vs Ag/AgCl). The above oxidation potential of the template layer is higher by ∼50 mV as compared to that of Ru-PT in acetonitrile solution. The halfwave redox potential of the Ru-PT template layer, E1/2, was +1.45 V vs Ag/AgCl, which is comparable with the analogous value for a similar Ru−terpyridyl complex-based monolayer.45 However; this value was higher by 90 mV as compared to that of Ru-PT in acetonitrile solution. This result most likely corresponds to the quaternization of the pendant pyridyl group of the Ru-PT unit by 3-iodo-n-propyltrimethoxy-silane. The shape and intensity of the oxidation and reduction peaks correspond to the typical behavior of surface bound species. The peak-to-peak separation between the anodic and cathodic waves, ΔEp, was in a range of +20 to 40 mV for a scan rate of 100−1000 mV/s. These relatively low ΔEp values are a further

Figure 2. C 1s/Ru 3d (a), N 1s (b), Os 4f (c), and Cu 2p (d) XP spectra of a Ru-PT/Cu/Os-PT layer (SURCO-1) on Si(100). Some spectra are decomposed into individual peaks. The characteristic emissions are marked.

asymmetry is related to the binding energy shift between the carbon atoms in the meta, para, and ortho positions of the pyridine ring.41 In addition, the spectrum shows the characteristic Ru 3d5/2 emission at 281.3 eV, which confirms the presence of the Ru center in the molecular assembly on the silicon substrate. The second peak comprising the Ru 3d5/2,3/2 doublet, viz. Ru 3d3/2 emission at 284.7 eV, overlaps with the intense C 1s feature and is, therefore, indistinguishable in the spectrum. The N 1s spectrum in Figure 2b is dominated by a strong emission at 399.9 eV assigned to the nitrogen atoms in the terpyridine moieties coordinated with the metal centers. The emission is accompanied by a low intense shoulder at 402.5 eV (marked as N*) assigned to the quaternized nitrogen in the pyridinium unit. The respective intensity ratio in the Ru9581



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Figure 3. (a) UV−vis spectra of Ru-PT in acetonitrile solution (red line) and for the Ru-PT-based template layer on glass (blue line, scaled up by a factor of 80). (b) Cyclic voltammogram of Ru-PT in acetonitrile solution (red line) and for the Ru-PT template layer (blue line) on ITO-coated glass (vs Ag/AgCl) at a scan rate 200 mV/s. (c) Current densities of the anodic and cathodic peaks (Ipa and Ipc) as functions of the square root of the scan rate (ν1/2) for Ru-PT in acetonitrile solution (red lines; top scale) and as functions of the scan rate (ν) for the Ru-PT template layer on ITOcoated glass (blue lines; bottom scale).

Figure 4. UV−vis spectra of SURCO-1 (a) and SURCO-2 (d) on glass substrates. The odd layers (1st, 3rd, 5th, 7th, and 9th) are terminated by the Ru-PT units, whereas the even layers (2nd, 4th, 6th, 8th, and 10th) are terminated by the Os-PT units linked to the underlying Ru-PT units by either Cu2+ (SURCO-1) or Pd2+ (SURCO-2) moieties (the UV−vis spectra of the metal ion (Cu2+ or Pd2+)-terminated layers are not shown for simplicity). The absorbance at λmax = 508−501 nm for SURCO-1 (b) and at λmax = 508−503 nm for SURCO-2 (e) as functions of the oligomer chain length, given as the amount of the assembled metallo-ligands; the curves are fitted by either linear (b) or exponential (e) functions. The relations between the absorbance and density of the metallo-ligands (metallo-ligands/cm2) for SURCO-1 (c) and SURCO-2 (f) along with the corresponding linear fits (R2 > 0.98). The density is presented as x axis to have a common y axis with other panels in the same row.

relation occurs in the given case, suggesting that there is strong interaction between the oxidative and reductive sites. At the same time, the current density of the Faradaic peak for the RuPT template layer on ITO-coated glass exhibited an excellent linear dependence (R2 = 0.99) on the scan rate (ν), corresponding to a diffusionless process (see Figure 3c) and indicating a reversible redox process for the adsorbed species. In contrast, the current densities for Ru-PT in acetonitrile solution are proportional to the square root of the scan rate (ν1/2), corresponding to a diffusion-controlled electrochemical process (see Figure 3c). Significantly, the ratio of the current densities of the anodic and cathodic peaks (Ipa/Ipc) for the RuPT template layer was close to 1 for the entire range of the scan rates (ν), also indicating a reversible redox process. UV−vis Spectroscopy of Oligomer Films. The growth of the binary oligomer films on glass substrates was monitored by UV−vis spectroscopy (λ = 400−800 nm) after each metal ion/ metallo-ligand coordination step (Figure 4). The spectra were acquired after the assembly of either a Ru-PT unit (assigned as

confirmation for the grafting of the molecules on a conductive substrate. The scan rate dependence of the E1/2 and ΔEp values is summarized in Table S1 (Supporting Information). The full width at half-maximum of the oxidation peak, ΔEpa,1/2, which is a measure of the interaction of the surface bound species, was found to be in a range of +140 to 200 mV for the Ru-PT template layer. The above value deviates from an ideal Nernstian electrochemical relation46 ΔEpa,1/2 = 90.6/n mV

(1)

where n is the number of electrons. This deviation could be related to the interaction between the redox sites and/or their heterogeneity.47,48 Generally, the value of ΔEpa,1/2 depends on the parameter g = αO + αR − 2αOR, where αO, αR, and αOR are the interaction parameters for oxidative site−oxidative site, reductive site−reductive site, and oxidative site−reductive site, respectively.46 It is then assumed that α > 0 for an attractive interaction, whereas α < 0 for a repulsive one. Thus, ΔEpa,1/2 = 90.6/n mV if g = 0, but ΔEpa,1/2 > 90.6/n mV if g < 0. The latter 9582



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formation of the oligomer film is based on the facile squareplanar geometry of Pd2+ in the lattice along with the trans positioning of the pyridyl group from two different metalloligands and two chloride ions, which are covalently bonded with a d8 metal ion. Presumably, the metallo-ligands store palladium in excess and use it to extend the molecular-based assembly in exponential fashion (Figure 4e). We suggest that the excess palladium remained intact in the layer even after the washing and sonication procedures. This outcome may be due to loss of facile benzonitrile (PhCN), a coordinating solvent from the palladium center. In accordance with the behavior of the absorbance, the density of the Ru-PT and Os-PT units increases in an irregular fashion in the course of the oligomer assembly (Figure 4f). The joint density of these units in the final oligomer film (10 layers) was estimated at 7 × 1014 metallo-ligands/cm2 for SURCO-2, corresponding to ∼1.4 nm2/oligomer (Figure 4f). Note that, in the given case, the above value cannot be considered as the true density of the oligomer chains but, after comparison to the analogous value for SURCO-1, rather reflects a “branching” of these chains in the course of assembly. The full width at half-maxima (fwhm) of the MLCT band for the oligomer films increased slightly in the course of the oligomer assembly as compared to that for the template layer. The fwhm of the most intense MLCT band for the top (10th) layer of the SURCO-1 assembly was estimated at 70 nm, whereas, for the SURCO-2 film, it was 68 nm, which was higher by 8 and 6 nm, respectively, than the fwhm for the corresponding template layer. The peak broadening could be due to the merging of the MLCT bands of the Ru-PT and OsPT units and possible π−π interactions between the individual oligomer chains (vide supra). Remarkably, the oligomer film growth could be inhibited at any stage by quaternizing the free pyridyl group with either diluted HCl or CH3I, as reported in our previous article.33 The optical band gap for the oligomer film (10 layers) was estimated at Eg = 1.64 eV, which is lower than the analogous value for the Ru-PT template layer. This relation indicates an enhancement of the electronic conductivity in the oligomer films as compared to the template layer. Free pyridyl groups of the metallo-ligands have a strong tendency to react with different transition-metal ions. To see the role of the metallo-linker in the molecular assembly, we performed oligomer film growth with several different metal ions, such as Ag+, Fe2+, Co2+, and Zn2+ (see spectra in Figure S6, Supporting Information). In all the above cases, a linear increase (R2 = 0.98−0.99) of the characteristic UV−vis intensity for the most intense MLCT band was found, manifesting a linear growth of the oligomer films in the course of assembly (Figure 5a−d). The joint average densities of the assembled metallo-ligands in the final oligomer films (10 layers) were estimated at 4.52, 4.74, 4.66, and 5.02 (× 1014) for Ag+, Fe2+, Co2+, and Zn2+, respectively (Figure 5e−h). This corresponds to the oligomer density of (4.5−5.0) × 1013 (2.2− 2.5 nm2/oligomer). The above values are similar to those in the Cu2+ case (4.2 × 1014 metallo-ligands/cm2 or ∼2.4 nm2/ oligomer), suggesting that the density of the oligomer chains grown in a linear mode does not depend noticeably on the character of the metal ion. Note once more that the optical properties of the newly fabricated, coordination-based oligomer films are only weakly affected by the presence of the metal ions. The blue shift of the MLCT band was only observed for the films containing either

an odd layer: 1st, 3rd, 5th, 7th, and 9th) or a successive Os-PT unit (assigned as an even layer: 2nd, 4th, 6th, 8th, and 10th) over either a Cu2+ or a Pd2+ metallo-linker; the films were then terminated by either a Ru-PT or a Os-PT metallo-ligand. Note that the spectrum of the Ru-PT template layer exhibits a single, quite intense MLCT band at 508 nm, whereas the spectrum of the analogous Os-PT layer reveals two MLCT bands, viz. an intense band at 502 nm and a less intense band at 687 nm.34 Since the positions of the most intense MLCT bands for the Ru-PT and Os-PT units are quite close to one another, the merging of these bands occurred, resulting in a joint band at λmax = 501 and 503 nm for SURCO-1 and SURCO-2, respectively, accompanied by the Os-PT related band at 694 nm. Consequently, the optical window of the oligomer films composed of the Ru-PT and Os-PT units covers the entire visible region. Since the most intense MLCT bands for the Ru-PT and OsPT units are quite close to each other, we could use the intensity of the merged band to monitor the density of these metallo-ligands in the oligomer films. For SURCO-1, this intensity was found to increase linearly (R2 = 0.98) in the course of the metal ion/metallo-ligand deposition steps, indicating the formation of uniform and dense layers as well as proper incorporation of the metallo-linker and metalloligands into the assembly. Notably, the absorbance of the MLCT band did not increase significantly after deposition of the metallo-linkers (Cu2+, Pd2+); during the growth of the oligomer films the increase was mostly associated with the attachment of the Ru-PT or Os-PT units. Also, the metallolinkers did not alter the positions of the MLCT bands during the fabrication of the oligomer films, suggesting that they do not hamper the optical properties of the films. The most intense MLCT band of SURCO-1 showed a blue shift by 7 nm as compared to that of the Ru-PT template layer (Figure 4a). The coordination of the Os-PT unit in the molecular assembly was further confirmed by the appearance of a spin-forbidden triplet MLCT band at λmax = 694 nm. It is noteworthy to mention that Cu2+ has a square pyramidal geometry40 with trans positioning of two pendant pyridyl groups. The growth of the layers in the oligomer film was consistent with the increase in the absorbance value by ΔA ≈ 0.0041 (Figure 4b) for each metal ion/metallo-ligand unit, which is well comparable with the absorbance value for the template layer (0.0045). The linear increase in absorbance could be associated with a linear increase in the density of the Ru-PT and Os-PT units in the course of the oligomer assembly. The joint density of these units in the final oligomer film (10 layers) was estimated at 4.2 × 1014 metallo-ligands/cm2 for SURCO-1, corresponding, in view of the fact that each oligomer chain is composed of 10 metalloligands, to 2.4 nm2/oligomer (Figure 4c). The latter value correlates perfectly with the packing density of the template layer (∼2.2 nm2/metallo-ligand; see above), which is one more evidence for the linear growth mode. In contrast to SURCO-1, SURCO-2 showed not a constant, but a successively increasing gain in the absorbance value after each coordination step, along with a blue shift of the most intense MLCT band by 5 nm as compared to that of the Ru-PT template layer. This behavior suggests an exponential growth of this film in the course of the coordination steps (Figure 4d). Such an exponential growth of the oligomer film could be due to in-and-out movement of Pd2+ in the lattice during the stepwise assembly and not affected by either Ru2+ or Os2+ centers, as reported earlier by van der Boom et al.12 The 9583



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whereas the Ru center deviated from the ideal reversibility. For example, an oxidation peak at +1.04 V vs Ag/AgCl, in addition to the redox peak of the Ru center, indicates successful assembly of the Os-PT units as the second layer of the SURCO-2. Notably, the appearance of the redox peak at lower potential (from +0.2 to +0.5 V) could be due to the formation of a mixed-valence redox species. The oxidation peak potential for the Os center in the oligomer film (10 layers) was anodic shifted by 40 and 60 mV as compared to that for the Os-PT template layer34 and Os-PT in acetonitrile solution, respectively. On the other hand, the oxidation potential of the Ru center (10 layers) was cathodic shifted by 20 mV as compared to that of the Ru-PT template layer. The difference in the oxidation potential between Ru2+/3+ and Os2+/3+ was estimated at 350 mV in solution, while it was 420 mV in the case of the Ru-PT template layer and 390 mV for the Os-PT/Ru-PT dyad layer (2nd layer). This observation suggests a transfer of the electron density from the Os to the Ru center in the oligomer film. The values of fwhm of the ΔEpa,1/2 for the Os2+/3+ redox process were found to be in the range of +150 to 205 mV. Such high values imply a strong repulsive interaction between the surface bound species. Important information about the electrochemical behavior of a target system can be obtained from the relationship between the peak current density and the scan rate. Accordingly, cyclic voltammograms at various scan rates (100−1000 mV/s) were recorded for the oligomer layers of different thicknesses. The current densities of the anodic and cathodic peaks (Ipa and Ipc, respectively) for the metal centers exhibited a linear dependence on the scan rate, typical of substrate-confined components. Further, the peak-to-peak separation values for the Os center were observed in a range of +50 to 70 mV. Such low values indicate a diffusionless redox process. A linear increase in the anodic current density of the Os center as a function of the oligomer chain length (Figure 6b) suggests a linear growth of the respective oligomer film. The absence of any significant peak shift over a wide range of scan rates (from 0.1 to 1 V/s) is a further indication of a fast electron transfer within the electrochemical experiments. This inspection further established the nonzero peak separation outcome from the lateral interaction of the redox species.47 The current density for the SURCO-2 film on ITO-coated glass increased linearly with successive deposition steps (Figure 6b), in good agreement with the UV−vis data. This suggests a linear growth mode for this system, which is in contrast to the behavior of SURCO-2 on glass (see above). On the basis of the CV data at a scan rate of 300 mV/s, the density of the metallo-ligands for

Figure 5. (a−d) Plots of the UV−vis absorbance at the position of the most intense MLCT band as a function of the oligomer chain length, given as the amount of the assembled metallo-ligands, show linear dependence (R2 = 0.98−0.99) in the cases of the Ag+, Fe2+, Co2+, and Zn2+ linkers. (e−h) Relations between the UV−vis absorbance and density of the metallo-ligands (metallo-ligands/cm2) in the oligomer films. The density is presented as x axis to have a common y axis with the corresponding left panels. The average density of the assembled metallo-ligands in the final oligomer films (10 layers) was estimated at 4.52, 4.74, 4.66, and 5.02 (× 1014) for Ag+, Fe2+, Co2+, and Zn2+, respectively (e−h), which corresponds to the oligomer density of (4.5−5.0) × 1013 (2.0−2.5 nm2/oligomer).

Cu2+ or Pd2+ ions, but not for the other transition-metal ions. The same observations were made during the solution studies. For instance, when a solution of AgNO3 in dry acetonitrile was added to the solution of Ru-PT in the same solvent, no substantial shift in the position of the MLCT band was observed even after addition of excess metallo-linker. Similar results were obtained with Fe2+, Zn2+, and Co2+. Interestingly, Ru-PT exhibited a blue shift in the wavelength of the MLCT band when it was allowed to react with Cu(NO3)2 in acetonitrile solution. Therefore, the metallo-linkers play identical roles both in solution as well as in oligomer films. Cyclic Voltammogram of Oligomer Films. The growth of the oligomer films on ITO-coated glass was also monitored by the cyclic voltammetry (CV) by the example of the SURCO2 system. The voltammograms showed a clear appearance of two reversible electrochemical waves due to the presence of the Ru-PT and Os-PT moieties (Figure 6a). The Os center showed full reversibility throughout the electrochemical process,

Figure 6. (a) Cyclic voltammograms acquired after the formation of the 2nd layer of SURCO-2 vs Ag/AgCl at scan rates ranging from 0.1 to 1 V s−1. (b) Anodic current density (for Os2+) as a function of the oligomer chain length, given as the amount of the assembled metallo-ligands, along with the corresponding linear fit with R2 = 0.98. (c) Relation between the anodic current density (for Os2+) and density of the assembled metallo-ligands (metallo-ligands/cm2), along with the corresponding linear fit with R2 = 0.96. The density is presented as x axis to have a common y axis with the other panels. The scan rates for (b) and (c) were 0.2 and 0.3 V s−1, respectively. 9584



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such as “00”, “10”, and “11”, can be obtained. For instance, the “00” memory state can be related to the neutral state of both metal centers (nonoxidized, at 0 V or above), whereas the “10” memory state is equivalent to the oxidation of one metal center at lower oxidation potential, and the “11” memory state corresponds to the oxidation of both metal centers. In contrast, by applying the reduction potential, the system can be reconfigured into the original oxidation state of the metal centers, viz. Ru2+ and Os2+. After the CV measurements, the electrolyte solution was cross-checked by optical (UV−vis) and electrochemical means, but no signal associated with the oligomers or their building blocks could be recorded. These observations indicate that there was no desorption or decomposition of the assembled oligomer chains to the electrolyte solution during the electrochemical experiments in the measured potential range (up to +2 V). This suggests high stability of the fabricated oligomer films under the condition of our electrochemical experiments and, presumably, in an electrochemical cell in general.

each even layer was estimated (Figure 6c) according to the equation48 Ip = n2F 2νA Γ/4RT

(2)

where n represents the number of electrons involved in the redox reaction, A is the surface area of the ITO-coated glass working electrode (1.20 cm2), Γ is the surface coverage (mol/ cm2), and other symbols have their usual meaning. In accordance with eq 2 and Figure 6, surface coverage increased linearly in the course of the oligomer assembly. The density of the metallo-ligands corresponding to one deposition step (layer) was estimated at 0.39 × 1014 metallo-ligands/cm2. This value can be considered as the density of the oligomer chains on ITO-coated glass. It is close to the analogous value for SURCO-1 on glass (0.42 × 1014 oligomers/cm2; see above), which is reasonable since SURCO-2 on ITO-coated glass has the same, linear growth mode. At the same time, it is noticeably less than the density of the Os-PT units in the 10th layer of SURCO-2 on a glass substrate (7 × 1014 metallo-ligands/cm2; vide supra), which is understandable since the latter value cannot be considered as the density of the oligomer chains, but rather reflects a “branching” of these chains in the course of assembly. Obviously, such a branching did not occur in the case of SURCO-2 on ITO-coated glass. Presumably, the conductive substrate prevented the accumulation of the excess palladium by the metallo-ligands. This might be also a result of the interfacial potential distribution across the film. There is always a change in the potential during the electrochemical measurements, making a difference in the polarity of the oxidized and reduced states. Consequently, the oligomer film is likely to be more hydrophilic when both Os2+ and Ru2+ are oxidized, because the electroactive species are charged, whereas the reduced film is rather hydrophobic as the species are neutral. The cyclic voltammetry data were also used to evaluate the ionization potential of the metal centers using the oxidation potential of each. For instance, the ionization potential value of Ru2+ was estimated at −5.87 eV, whereas, for Os2+, it was −5.48 eV, suggesting that the former center is less prone to the oxidation. This can be expected since Ru2+ is more inert electrochemically than Os2+. Note that the ionization potential was estimated using the formula, IP = −(Eox + 4.44) eV, where Eox was the corresponding oxidation potential of the metal center vs Ag/AgCl. The electron transfer rate constant, KET, which is an important kinetic parameter, was determined for both Os and Ru centers in the dyad layer. Cyclic voltammetry data were employed to determine the KET, between the Ru-PT, Os-PT, and the ITO-coated glass substrate with the help of the method of Laviron.49 To determine the KET, the oxidation and reduction potentials (Epa, Epc) were plotted as a function of log(ν) as shown in Figure S7 (Supporting Information). As the result, KET was estimated at 3.5 and 5.2 s−1 for the Os center and Ru center, respectively. These relatively low KET values arise due to the low concentration (20 mM) of the electrolyte.50 The separation of the peak positions between the metal centers was found to be almost constant in the course of the oligomer assembly. The well-defined redox peaks are obtained for all layers due to the penetration of the electrolyte solution to the ITO-coated glass working electrode, which indicates the porous structure of the film. Note that the distinct redox states in the oligomer film can be considered as a bit of information, being either “0” or “1”. In the case of two successive reversible reactions, three different states of memory,

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CONCLUSIONS

Heterometallic, coordination-based oligomer assemblies were prepared on SiOx-based solid supports using a stepwise coordination method. The oligomers were composed of the Ru-PT and Os-PT metallo-ligands assembled in an alternating fashion over the Cu2+, Pd2+, Ag+, Fe2+, Co2+, and Zn2+ metal ion linkers. The growth mode of the oligomer films depended on the identity of the linker and the character of the substrate. In most cases, a linear growth mode with a joint grafting density of 3.9−5.0 × 1014 metallo-ligands/cm2 for the final oligomer films (10 layers) was observed, corresponding to a characteristic area of 2−2.5 nm2/oligomer. The only exception was Pd2+ that mediated a nonlinear growth mode for the oligomer films on a glass substrate, which, however, changed to the standard, linear growth mode on conductive, ITO-coated glass substrate. The combination of the two different metallo-ligands with the same binding motif at a single platform resulted in a significant enlargement of the optical window, stemming from the combination of the respective MLCT bands in the UV−vis spectra. The “electrochemical window” was enlarged as well since both metal centers in the oligomer films can be oxidized and reduced selectively. This makes the individual oligomer chains potentially useful as ternary (“00”, “10”, “11”) memory units34,51 and gives an opportunity to use them as molecular switches52 or molecular logic gates.53,54 The comparably small band gap of the oligomer films (Eg = 1.64 eV) makes them potentially useful for photovoltaic applications. This band gap is smaller than that of the Ru-PT template layer (2.04 eV), indicating an enhancement of the electronic conductivity in the binary oligomer films as compared to the template layer. The results of this study underline the efficiency of the wet chemical stepwise coordination method to form complex molecular assemblies on SiOx-type solid substrates. Note that the coordination between the solvent molecules and the metallo-linkers can be removed at elevated temperature, which creates an unsaturation site of the linker ions. Consequently, the presented SURCO systems can have potential in host− guest chemistry.55 9585



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Assembly of Metal Complexes. Coord. Chem. Rev. 2007, 251, 2688− 2701. (12) Motiei, L.; Altman, M.; Gupta, T.; Lupo, F.; Gulino, A.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. Self-Propagating Assembly of a Molecular-Based Multilayer. J. Am. Chem. Soc. 2008, 130, 8913−8915. (13) Qi, H.; Gupta, A.; Noll, B. C.; Snider, G. L.; Lu, Y.; Lent, C. S.; Fehlner, T. P. Dependence of Field Switched Ordered Arrays of Dinuclear Mixed-Valence Complexes on the Distance between the Redox Centers and the Size of the Counterions. J. Am. Chem. Soc. 2005, 127, 15218−15227. (14) Liu, B.; Ma, M.; Zacher, D.; Betard, A.; Yusenko, K.; MetzlerNolte, N.; Wöll, C.; Fischer, R. A. Chemistry of SURMOFs: LayerSelective Installation of Functional Groups and Post-Synthetic Covalent Modification Probed by Fluorescence Microscopy. J. Am. Chem. Soc. 2011, 133, 1734−1737. (15) Tuccitto, N.; Ferri, V.; Cavazzin, M.; Quici, S.; Zhavnerko, G.; Licciardello, A.; Rampi, M. A. Highly Conductive ∼ 40-nm-Long Molecular Wires Assembled by Stepwise Incorporation of Metal Centres. Nat. Mater. 2009, 8, 41−46. (16) Maeda, H.; Sakamoto, R.; Nishimori, Y.; Sendo, J.; Toshimitsu, F.; Yamanoi, Y.; Nishihara, H. Bottom-Up Fabrication of Redox-Active Metal Complex Oligomer Wires on an H-Terminated Si(111) Surface. Chem. Commun. 2011, 47, 8644−8646. (17) Maeda, H.; Sakamoto, R.; Nishihara, H. Metal Complex Oligomer and Polymer Wires on Electrodes: Tactical Constructions and Versatile Functionalities. Polymer 2013, 54, 4383−4403. (18) de Ruiter, G.; Lahav, M.; Keiser, H.; van der Boom, M. E. Sequence-Dependent Assembly to Control Molecular Interface Properties. Angew. Chem., Int. Ed. 2013, 52, 704−709. (19) Zacher, D.; Shekhah, O.; Wöll, C.; Fischer, R. A. Thin Films of Metal−Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1418−1429. (20) Sauvage, J. P.; Collin, J. P.; Chambron, C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Decola, L.; Flamigni, L. Ruthenium(II) and Osmium(II) Bis(terpyridine) Complexes in Covalently-Linked Multicomponent Systems: Synthesis, Electrochemical Behavior, Absorption Spectra, and Photochemical and Photophysical Properties. Chem. Rev. 1994, 94, 993−1019. (21) Happ, B.; Winter, A.; Hager, M. D.; Schubert, U. S. Photogenerated Avenues in Macromolecules Containing Re(I), Ru(II), Os(II), and Ir(III) Metal Complexes of Pyridine-Based Ligands. Chem. Soc. Rev. 2012, 41, 2222−2255. (22) Lehn, J. M. Supramolecular Chemistry. Science 1993, 260, 1762−1765. (23) Constable, E. C. 2,2′:6′,2″-Terpyridines: From Chemical Obscurity to Common Supramolecular Motifs. Chem. Soc. Rev. 2007, 36, 246−253. (24) Gupta, T.; Tartakovsky, E.; Iron, M. A.; van der Boom, M. E. A Monolayer-Based Setup for Optical Amplification. ACS Appl. Mater. Interfaces 2010, 2, 7−10. (25) de Ruiter, G.; Motiei, L.; Choudhury, J.; Oded, N.; van der Boom, M. E. Electrically Addressable Multistate Volatile Memory with Flip-Flop and Flip-Flap-Flop Logic Circuits on a Solid Support. Angew. Chem., Int. Ed. 2010, 49, 4780−4783. (26) de Ruiter, G.; Tartakovsky, E.; Oded, N.; van der Boom, M. E. Sequential Logic Operations with Surface-Confined Polypyridyl Complexes Displaying Molecular Random Access Memory Features. Angew. Chem., Int. Ed. 2010, 49, 169−172. (27) Forster, R. J.; Keyes, T. E. Photonic Interfacial Supramolecular Assemblies Incorporating Transition Metals. Coord. Chem. Rev. 2009, 253, 1833−1853. (28) Walsh, J. J.; Mallon, C. T.; Bond, A. M.; Keyes, T. E.; Forster, R. J. Enhanced Photocurrent Production from Thin Films of Ru(II) Metallopolymer/Dawson Polyoxotungstate Adducts under Visible Irradiation. Chem. Commun. 2012, 48, 3593−3595. (29) Forster, R. J.; Figgemeier, E.; Loughman, P.; Lees, A.; Hjelm, A. J.; Vos, J. G. Conjugated vs Nonconjugated Bridges: Heterogeneous Electron Transfer Dynamics of Osmium Polypyridyl Monolayers. Langmuir 2000, 16, 7871−7875.

ASSOCIATED CONTENT

S Supporting Information *

Synthetic details; characterization data of 4′-pyridyl terpyridyl (PT), Ru-PT, and Os-PT; formation of template layer; thermal stability test; the plots of the peak potential as a function of log(ν), and the UV−vis spectra of the oligomer films using the Ag+, Fe2+, Co2+, and Zn2+ metallo-linkers (Figures S1−S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +972 54 797 8617 (P.C.M.). *E-mail: [email protected]. Phone: +49 6221 544921 (M.Z.). Present Address ⊥

Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Department of Science and Technology, Nano-Mission (SR/NM/NS-12/ 2010), New Delhi, India, and the German Research Society (ZH 63/9-3 and ZH 63/14-2). P.C.M. thanks the Council of Scientific and Industrial Research (CSIR, New Delhi) for a Senior Research Fellowship.

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DEDICATION Dedicated to late Prof. Tarkeshwar Gupta, Department of Chemistry, University of Delhi, New Delhi, India. REFERENCES

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