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Metallo-supramolecular polyelectrolyte multilayers with cobalt(II): preparation and properties
Colloids and Surfaces A: Physicochemical and Engineering Aspects 198– 200 (2002) 633– 643 www.elsevier.com/locate/colsurfa
Metallo-supramolecular polyelectrolyte multilayers with cobalt(II): preparation and properties Dirk G. Kurth *, Markus Schu¨tte, Jin Wen Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam, Germany Received 30 August 2000; accepted 7 May 2001
Abstract Metal ion mediated self-assembly of the ditopic ligand 1,4-bis(2,2%:6%,2%%-terpyridine-4%-yl)benzene and Co(II) results in a metallo-supramolecular coordination polyelectrolyte (Co-MEPE), which is analyzed by spectroscopic methods, elemental analysis and cyclovoltammetry. The Co-MEPE shows a reversible one-electron redox transition in the potential range from −0.15 to 0.35 V with a half-wave potential of 87 mV. Thin films of Co-MEPE are fabricated by layer-by-layer self-assembly together with poly(styrenesulfonate) (PSS) on planar substrates and are analyzed by UV/vis spectroscopy, microgravimetry, and electrochemistry. The Co-ions in the multilayer can be removed and displaced by Fe-ions. The electrochemical activity of the Co-ions is fully maintained in the films. Charge transport through the multilayer is consistent with electron self-exchange reactions. The permeability of the multilayers is investigated by electrochemistry using K3[Fe(CN)6] as electroactive probe. Compared to poly(allylamine hydrochloride) (PAH)/PSS films, the electroactive probe does not permeate through PSS/Co-MEPE multilayers. Fluorescence measurements with pyrene as polarity probe indicate that the Co-MEPE films are more hydrophobic than PAH/PSS multilayers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Metal ion coordination; Layer-by-layer self-assembly; Thin films; Polyelectrolytes; Electrochemistry
1. Introduction Currently, polypyridine ligands attract considerable attention as synthon in metallosupramolecular chemistry because of their interesting photo- and electrochemical properties [1]. Bipyridine ligands in combination with second and third transition row metal ions, in particular Ru(II) and Os(II), have favorable excited-state * Corresponding author. Tel.: + 49-331-567-9211; fax: + 49-331-567-9202. E-mail address: [email protected] (D.G. Kurth).
redox properties [2], but first transition row metal ions, such as Fe(II), also show exciting properties due to thermal and optically induced spin-transitions [3]. However, in terms of structure, bipyridine is not an ideal synthon because bidentate ligands in an octahedral coordination geometry give rise to stereo and geometric isomers. In case of polynuclear assemblies, this approach results in randomly coiled macromolecules [4]. The terpyridine ligand is much more convenient as structural synthon because coordination results in stereochemically defined octahedral complexes.
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Also, substitution in the 4%-position does not cause isomerism and offers a possibility to align the substituents in a clearly defined direction. Based on the structural advantage of the terpyridine group, we started to investigate polynuclear metallo-supramolecular assemblies utilizing the ditopic ligand 1,4-bis(2,2%:6%,2%%-terpyridine-4%yl)benzene. The rigid design of the ligand with back-to-back metal ion receptors and the high directionality of the terpyridine metal ion interaction induce a linear, rod-like structure. The high binding constant of terpyridine and transition metal ions results in formation of extended, linear macromolecules (Scheme 1). In case of first transition row elements, metal ion coordination occurs spontaneously in solution at ambient conditions. We refer to these assemblies as metallosupramolecular coordination polyelectrolytes (MEPE) because of the overall positive charge. With Fe(II) as central metal ion, we showed recently that Fe-MEPE can be incorporated into thin multilayer films by using layer-by-layer selfassembly [5] on planar [6] and colloidal [7] substrates. In this process, we take advantage of the positive charge of MEPE because electrostatic interactions of the oppositely charged polyelectrolytes are thought to play a primary role in the
Fig. 1. UV/vis absorption spectra of (me-ph-terpy)2Co(OAc)2 (dotted line), and Co-MEPE (solid lines) in methanol and water.
deposition. The thickness of a single layer can be controlled within the nanometer range. In the current study, we extend this approach to Co(II) as coordinating center. Due to its suitable redox potential, it is possible, in contrast to Fe-MEPE, to investigate the electrochemical response in solution and in multilayers. Compared to Fe(II), Co(II) ions bind more weakly to terpyridine, so that we can study the displacement and exchange of metal ions in multilayers. Finally, we present results from permeability measurements using an electroactive probe, as well as polarity studies based on pyrene fluorescence.
2. Results and discussion
2.1. UV –6is spectroscopy
Scheme 1. Metal ion mediated self-assembly of ditopic ligand 1 and cobalt(II) results in formation of a metallo-supramolecular coordination polyelectrolyte. Multilayers on arbitrary substrates are fabricated by layer-by-layer self-assembly of the positively charged Co-MEPE and negatively charged PSS.
Neat 1,4-bis(2,2%:6%,2%%-terpyridine-4%-yl)benzene (1) dissolved in methanol, shows a strong, broad absorption band at around 290 nm associated with p–p* transitions (Fig. 1). In addition, shoulders at 320 and 260 nm are observed. Metal ion mediated self-assembly of 1 and cobalt acetate in water or methanol results in a red–brown coloration of the solution, indicating metal ion coordination. The color is associated with unresolved metal centered d–d transitions in the range from 450 to 550 nm; in addition, there are two strong, but unresolved p–p* transitions of the aromatic moieties centered at 292 and 355 nm. In aqueous solution, peak maxima are discernible at 290, 300,
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Fig. 2. Left: UV/vis spectra of (PSS/Co-MEPE)n multilayers with n =1 – 10 on a PEI modified quartz substrate. Right: Absorption of individual band maxima as a function of the number of adsorbed layers. The dashed lines represent linear regressions.
347, and 360 nm. In methanol as solvent, we observe small shifts in the peak maxima and changes in the relative intensities of the p– p* transitions, as well as tailing of the d– d transitions towards longer wavelength. Apparently the absorbance of Co-MEPE is a sensitive function of the solvent polarity. The mononuclear complex, (me-ph-terpy = 4%-p(me-ph-terpy)2Co(OAc)2 tolyl-2,2%,6%,2%%-terpyridine), which represents the repeat unit of Co-MEPE, has a very similar absorption spectrum with peak maxima at 282, 292, 300, 346, and 358 nm and weak d– d transitions in the range from 450 to 550 nm. Fig. 1 displays the spectra of Co-MEPE in water and methanol as well as the spectrum of (me-ph-terpy)2Co(OAc)2. It is well known that terpyridine complexes of Co(II) exhibit an octahedral coordination geometry [8]. The similarity of the (me-phterpy)2Co(OAc)2 and Co-MEPE absorption spectra supports the assumption that Co-MEPE also has an octahedral coordination geometry.
2.2. Multilayer assembly Multilayer fabrication is readily achieved by repeated immersion of the substrate in solutions containing PSS and Co-MEPE with intermittent washing steps. Due to the characteristic absorption bands of Co-MEPE, UV/vis spectroscopy is the method of choice to investigate multilayer build-up. The left hand side of Fig. 2 shows UV/vis spectra of (PSS/Co-MEPE)n multilayers on a PEI modified quartz substrate (n = 1–10).
Absorption bands at 196 and 225 nm indicate the presence of PSS, while absorption bands at 293, 305, 347 and 367 nm prove the presence of CoMEPE in the films. The band contours of CoMEPE in films and in solution are somewhat different with respect to the relative intensities, widths, and positions. In terms of the relative intensities of the major bands at around 300 and 360 nm, the multilayer spectra are comparable to the spectrum recorded in methanol, as inspection of Figs. 1 and 2 reveals. If we consider that the dielectric constant decreases from 79 for water to 33 for methanol, we can conclude that the polarity within the multilayers is reduced substantially, that is at least to the level corresponding to alcohol. However, due to the limited solubility of Co-MEPE in less polar solvents, it is not possible to give a more accurate account of the polarity in the multilayers with this method. Another approach will be discussed below. The right side of Fig. 2 shows the absorption maxima at 196, 225, 293, 347 and 450 nm as a function of the number of adsorbed layers of (PSS/Co-MEPE) as well as corresponding linear curve fits. The plots show that film build-up is very regular and linear. The adsorption of CoMEPE increases linearly from the first layer onward since the linear regressions of the data points for peak maxima at 293, 347, and 450 nm go through the origin. The curve fits for the PSS bands at 195 and 225 nm cross the ordinate above the origin. We conclude that more PSS adsorbs on the PEI precursor layer than on Co-MEPE
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layers in consecutive deposition steps. This difference may be associated with a different charge density and/or hydrophobicity of the PEI interface compared to Co-MEPE. From the absorption of the film, we can calculate the surface coverage, Y, according to Lambert–Beer’s law Y = (AuN)/2mu, where mu is the isotropic molar extinction coefficient (M − 1 cm − 1), N is Avogadro’s number, and Au is the absorbance. Using the bulk extinction coefficient in this formula is problematic because, as stated above, the band contour depends on the polarity of the surrounding medium. The computed surface coverage is, therefore, only an estimate. We choose to take the extinction coefficients determined in methanol because in this case the spectra are more similar. The slope of the linear regression of the absorbance maxima at 293 (347) nm is 0.028 (0.025) (films on both sides of the substrate). The estimated surface coverage amounts to 2.2 (2.3) chromophores per nm2 or 0.45 (0.44) nm2 per chromophore. With a metal ion– metal ion separation of approximately 1.55 nm and an approximate width of 1.2 nm of the ligand, the area of a chromophore is approximately 1.86 nm2 [9]. Assuming that the chromophores are oriented with the long axis parallel to the substrate, the absorbance corresponds to approximately 4 molecular layers. If we also take into account an anisotropic extinction coefficient, the absorbance corresponds to approximately 2.6 layers.1 Within the experimental errors, these values are in agreement with previously reported results based on X-ray reflectance and surface plasmon resonance spectroscopy of structurally similar Fe-MEPE films, which have a thickness of approximately 2 molecular layers [10].
a monolayer of mercaptopropionic acid (MPA), which acts as an adhesion promoter for the subsequently deposited PAH layer. The QCM is coated on both sides with a PAH/(PSS/Co-MEPE)n multilayer ( n= 1–6). The change in the resonance frequency after each deposition is measured ex situ. Each adsorption step is associated with a decrease of the resonance frequency, indicating an increase of the total mass on the electrodes. The adsorbed amount per layer is nearly constant, which is consistent with the results from UV/vis spectroscopy. The mean frequency decrease per adsorbed layer pair (PSS/Co-MEPE) is 230 Hz, which corresponds to 627 ng cm − 2 (one side of QCM). Assuming that there is no water in the film and densities of 1.2 g cm − 3 for PSS [11] and 1.5 g cm − 3 for Co-MEPE,2 the mass corresponds to a thickness of 4.2 nm per (PSS/Co-MEPE) layer pair. Assuming a layer thickness for PSS of approximately 2 nm [10], the Co-MEPE thickness is approximately 2.2 nm, which would correspond to approximately 2 molecular layers.
2.4. Exchange of metal ions
Microgravimetry allows direct weighing of the adsorbate with a sensitivity in the ng-scale and, therefore, complements UV/vis spectroscopic measurements. The gold electrodes of the quartz crystal microbalance (QCM) are first coated with
In contrast to covalent polymers, the non-covalent nature of the metal ion–ligand bond offers an opportunity to chemically modify the multilayers after deposition. Direct exchange of Co(II) with Fe(II) in Co-MEPE multilayers does not occur and, therefore, we take advantage of the fact that Co(II) binds strongly to ethylenediamine (en) [12]. The Co-MEPE films are first subjected to en to complex and to remove the Co(II) ions from the multilayer. Afterwards, the vacant metal ion receptors are filled with Fe(II). The Fe(II)–terpyridine complex has a characteristic absorption band at approximately 590 nm, which can be easily detected. The UV/vis spectrum of the reaction of CoMEPE with en and subsequently with Fe(II) is shown in Fig. 3 (left). The solid line shows the spectrum of an aqueous solution of Co-MEPE. To this solution en was added and acetic acid to adjust the pH to 7. After addition of Fe(II),
1 In case of an in-plane orientation of chromophores, the anisotropic extinction coefficient, mani, is given by mani =3/2m.
2 The density of Co-MEPE was determined in aqueous solution with a densitometer (DMA 602, PAAR).
2.3. Microgra6imetry
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spectra were recorded after 2, 6 and 60 min. An absorption band appears around 600 nm, which increases in intensity with time. This band is characteristic of the MLCT band of Fe-MEPE, indicating the exchange of Co(II) for Fe(II). It is interesting to note that the maximum of the MLCT band shifts with increasing time from 574 nm to longer wavelength to 588 nm. This shift is attributed to a successive aggregation of Fe(II) in MEPE indicating dipolar coupling between metal centers. Investigations with oligomeric RuMEPEs show a similar shift of the MLCT band to longer wavelength with increasing number of connected units [13]. The right side of Fig. 3 displays the UV/vis spectrum of a (PSS/Co-MEPE)3 multilayer on a PEI modified quartz substrate (solid line). After immersion in an aqueous solution of en for 1 h (dashed line) we observe a decrease of absorption bands in the region of 300– 450 nm, as a result of decomplexation of the Co(II) ions in the CoMEPE multilayer. In particular, the characteristic band at 355 nm, which occurs upon metal ion coordination (vide supra), decreases considerably. The presence of the band at 290 nm associated with uncoordinated ligand, demonstrates that the ligand remains in the film. It should also be noted that the bands that are due to PSS do not change in intensity, which indicates that the layer is not decomposed. The complex Co(en)23 + shows ab-
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sorption bands at 356 (m= 28.100 cm2 mmol − 1) and 480 nm (m= 20.800 cm2 mmol − 1) [14]. Since the spectrum after en addition shows no significant absorption bands at these positions, we conclude that Co(en)23 + diffuses out of the multilayer. Finally, the film was dipped in an acidic solution containing Fe(II). The appearance of the band at 350 nm as well as the characteristic MLCT-transition at 591 nm (inset) indicates metal ion coordination and formation of Fe-MEPE. A single layer of Fe-MEPE shows an absorbance of the MLCT band of approximately 0.02. For a complete exchange of the Co-ions in all three layers, we would expect an absorbance of 0.06. The absorbance of about 0.03 shows that approximately every second Co(II)-ion in the film is removed and replaced by Fe(II).
2.5. Electrochemistry The electrochemical behavior of Co-MEPE was investigated both in solution and in films. In the following, all experiments were carried out with MPA modified electrodes in order to be able to make a direct comparison of the experimental results. In the investigated potential region, MPA coated electrode shows a very weak current maximum, presumably due to the redox activity of the thiol groups. Fig. 4 shows the cyclovoltammograms (CV) of a MPA modified gold electrode
Fig. 3. Metal ion exchange in solution (left) and in films (right) followed by UV/vis spectroscopy. Left. Solid line: Co-MEPE. Dashed lines: Spectra after addition of Fe(II) after 2 (i), 6 (ii), and 60 (iii) min showing the characteristic MLCT band of Fe-MEPE. Right. Solid line: (PSS/Co-MEPE)3 multilayer. Dashed line: After dipping the film in a solution containing en to remove Co-ions. The band at 355 nm decreases in intensity indicating loss of Co-ions. Dotted line: After dipping the film in a solution of (NH4)2Fe(SO4)2 in acetic acid to add Fe-ions to the multilayer. Inset: Characteristic MLCT-transition, demonstrating metal ion coordination and formation of Fe-MEPE.
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Fig. 4. Left: CV of a MPA modified gold electrode in a solution of Co-MEPE as a function of the scan rate, 6 (6: 20, 50, 100, 150, 200 mV s − 1). Right: Dependence of the current, I, on 6 (Ip,a =anodic current, Ip,c =cathodic current).
in a solution of Co-MEPE (c= 0.1 mmol l − 1) with sodium acetate as supporting electrolyte (c= 0.1 mol l − 1) in the potential range from −0.15 to 0.35 V as a function of the scan rate. The CoMEPE shows a pronounced pair of current waves with a redox half-wave potential of E1/2 = 1/ 2(Ea +Ec)= +87 mV (Ea and Ec are the cathodic and anodic peak potentials, respectively), which is independent of the scan rate. The half-wave potential is comparable to other mononuclear Coterpyridine complexes [15]. The appearance of only one pair of current waves indicates electrochemically isolated subunits in Co-MEPE [16]. The anodic and cathodic currents are proportional to the scan velocity, which is illustrated in Fig. 4 (right) for scan velocities up to 800 mV s − 1. The proportionality between the current and the scan velocity reflects the reversibility of the redox process. The potential difference between the two current peaks DEp is 74 mV and remains constant during the variation of the scan velocity, indicating fast electron transfer. The CVs of a (PSS/Co-MEPE) multilayer as a function of the scan rate are shown in Fig. 5 (left). The redox potential E1/2 is slightly shifted to 128 mV. In this case, the anodic and cathodic current peaks are proportional to the square root of the scan velocity (Fig. 5 (right)). This behavior is consistent with electron self-exchange reactions within the electrochemical sites in the film, in which a semi-infinite electrochemical charge diffusion condition prevails [17]. The current peak difference DEp increased to approximately 123
mV in the film, which may reflect a higher ohmic resistance of the multilayer, presumably caused by the (inactive) intermittent PSS layers. The peak current, ip, is equal to 2.69×105 1/2 6 AD 1/2C, where 6 is the scan rate, A is the electrode area, D the charge transport diffusion coefficient and C the concentration [18]. Taking the results from UV/vis spectroscopy and QCM measurements, the concentration of redox active species in the film is approximately 10 − 3 mol cm − 3. The electrode area is 7.9× 10 − 3 cm2 and the slope of the ip versus 6 1/2 plots for the anodic and cathodic currents is 2 nA (mV s − 1) − 1/2. Therefore, D amounts to approximately 10 − 15 cm2 s − 1. This value is considerably smaller than diffusion constants reported in the literature [19], presumably because the distance of redox active sites is rather large due to intermittent PSS layers and the mobility of the redox active sites within the film may be reduced due to the rigid backbone of MEPE. Finally, the counter ion mobility within these layers may also affect the charge transport diffusion coefficient.
2.6. Permeability Electroactive species have frequently been used as probes to investigate the nature and extent of structural defects in thin films [20]. In the following study, we used K3[Fe(CN)6] as electroactive species, which undergoes a reversible one electron redox reaction. Fig. 6 summarizes the electrochemical response of Au/MPA/(PAH/PSS)n and
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Au/MPA/(PAH/PSS)/(Co-MEPE/PSS)n multilayers as a function of the number of layers. For comparison, the CV of the Au-electrodes (dotted line) as well as the MPA modified electrodes (solid line) are shown in the top of Fig. 6. The MPA monolayer on the gold electrode causes an increase of the current peak separations, presumably a result of the electrostatic repulsion between the charged surface and redox species. Deposition of two PAH/PSS layers on the electrode reduces the current by an order of magnitude because the ohmic resistance of the interface is increased. While deposition of another PAH/ PSS layer reduces the current, additional layers do not alter the CV any further. The electrochemical signature of the Fe(CN)36 − /Fe(CN)46 − couple remains discernible in all samples, indicating that the redox active probe diffuses through the film and undergoes electron transfer reactions at the underlying Au-electrode. Similarly, the Co-MEPE samples for 1, 2, and 3 layer pairs show a reduction in the peak current and a predominantly radial diffusion. However, the sample with 5 and more Co-MEPE/PSS layers shows only a capacitive current and no electroactivity of the probe, indicating that the electrode is completely blocked at this stage. In both cases, deposition of up to 12 layer pairs did not result in further electrochemical changes. The electrochemical behavior of the Fe(CN)36 − /Fe(CN)46 − couple in the two samples indicates a difference in the film properties, such as pore size, defects, hydrophobic and hydrophilic properties, as well as charge distribution. To ad-
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dress this point in more detail we investigated the polarity of the multilayers.
2.7. Polarity determination Pyrene (py) fluorescence has gained an outstanding role in the characterization of the local environment in bulk liquids, micelles [21], Langmuir–Blodgett monolayers [22], polymers [23], or hydrogels [24]. The extensive use of pyrene as an environmentally sensitive probe is due to the dependence of vibronic band intensities on solvent polarity. In particular, the py emission spectrum exhibits five vibronic bands in the region between 372 and 400 nm, usually labeled I–V. The intensity of the 0–0 vibrational band (I) is enhanced by increasing the polarity of the environment, while the intensity of the third band (III) remains almost constant. Thus, the ratio of the emission intensities of the bands I and III (I/III, also called py-value) is employed as a measure for solvent polarity. Here, we use pyrene fluorescence to estimate the polarity of Co-MEPE films, allowing a better understanding of the electrochemistry results. The emission spectrum obtained from the pyrene doped PEI/(PSS/Co-MEPE)4 multilayer, normalized to peak I, is shown in Fig. 7. The small signal at 450 nm is attributed to Co-MEPE because it is not observed in Ni-MEPE or PAH/ PSS multilayers. The film py-value for the PSS/ Co-MEPE multilayer is 1.48. However, the py spectrum shows a background due to Co-MEPE
Fig. 5. Left: CV of a MPA modified gold electrode with the composition MPA(PAH/PSS)(Co-MEPE/PSS)10 in a solution of sodium acetate (6: 10, 20, 50, 100, 200, 400, 800 mV s − 1). Right: Dependence of the current on 6 0.5.
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Fig. 6. Electrochemical response of multilayer coated electrodes for the K4[Fe(CN)6]/K3[Fe(CN)6] redox couple. Left: MPA(PAH/ PSS)n (n =0, 2, 4, 6, 8). Right: MPA(PAH/PSS)(Co-MEPE/PSS)n (n =0, 1, 3, 5, 7). Dashed lines: Au-electrode. Solid lines: MPA coated electrodes.
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Fig. 7. Fluorescence spectrum of a PEI/(PSS/Co-MEPE)4 multilayer doped with pyrene.
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cobalt acetate dihydrate for 1 h at reflux and 24 h at room temperature. The solution was filtered and the solvent evaporated under reduced pressure to yield a red–brown solid. Elemental analysis: found C, 52.8; H, 5.5; N, 8.8; calculated for C40H30N6O4Co·11H2O: C, 52.5; H, 5.7; N, 9.2; UV/vis (methanol): umax (m in cm2 mmol − 1)=288 (38.428), 302 (37.777), 348 (36.735), 362 (39.080), UV/vis (H2O): umax (m in cm2 mmol − 1)=290 (44.981), 300 (44.465), 347 (49.509), 360 (53.494), IR (KBr) (w in cm − 1): 734, 749, 793, 837, 1014, 1163, 1246, 1401, 1432, 1472, 1558, 1570, 1608.
3.3. Film assembly fluorescence. If the background is carefully substracted, the py-value amounts to 1.29, which corresponds to the polarity of chloroform [25,26]. Due to the interfering background fluorescence, it is currently not possible to make a more accurate assignment of the polarity in these multilayers. However, both values are lower than in multilayers of strong polyelectrolytes [27]. This result supports the conclusion drawn from UV/vis spectroscopy that the polarity of the multilayers is reduced. A possible reason for the electrochemical blockade of PSS/Co-MEPE multilayers could, therefore, be the more hydrophobic nature of the film compared to strong polyelectrolyte layers, which would strongly alter the diffusion of ions through the Co-MEPE multilayers.
Quartz slides from Hellma Optik (Jena, Germany) were used and cleaned according to literature procedure (RCA-cleaning) [29]. Multilayers were prepared by the method developed by Decher et al. [30]. The following solutions were used: poly(ethyleneimine) (PEI) (c =10 − 2 mol l − 1 in water), poly(styrenesulfonate) (PSS) (c= 10 − 3 mol l − 1, cNaCl = 1 mol l − 1 in water), poly(allylamine hydrochloride) (PAH) (c= 10 − 3 mol l − 1, cNaCl = 1 mol l − 1 in water) and CoMEPE (c= 0.1 mg g − 1 in methanol).
3.4. UV/6is-spectroscopy The UV/vis-absorption-measurements were recorded with a Varian Cary 50. Uncoated quartz slides were used as reference spectrum.
3. Experimental
3.5. Microgra6imetry 3.1. Chemicals All chemicals were of analytical grade and were used without further purification. The ligand 1,4bis(2,2%:6%,2%%-terpyridine-4-yl)benzene (1) was synthesized according to a literature procedure [28]. Gold electrodes were coated with a monolayer of MCA according to previously published procedures [10].
3.2. Co-MEPE Ligand 1 was crushed in a mortar and stirred under Argon in a degassed methanolic solution of
The measurements were carried out with a custom build electronic driver circuit and a HPE3620A power supply. The quartz crystals were equipped on both sides with gold electrodes (evaporated, polished gold). Frequencies were recorded with a frequency counter (Hewlett Packard HP 5313 A) connected to a computer. Measurements were carried out ex-situ in a climatic room at a temperature of 22 °C. Quartz crystals were rinsed several times with ethanol and water before dipping in solutions of MPA, PAH, PSS and Co-MEPE (20 min). After each deposition step the crystals were rinsed several times with
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Acknowledgements
water and dried in a stream of air. The crystal was left to equilibrate for 10 min, after which the resonance frequency was measured every second over a period of 3 min. After averaging, the mean deviation is within 1 Hz. The adsorbed amount of polyelectrolyte was calculated according to the equation of Sauerbrey [31].
Valuable discussions with Helmuth Mo¨ hwald are gratefully acknowledged. The authors thank Tina Tedeschi for her help with the polarity measurements.
3.6. Cyclic 6oltammetry
References
Electrochemical measurements were performed in a three-electrode glass cell with gold disc electrode (d= 2 mm) as working electrode, Pt foil as counter electrode and Ag/AgCl+/KCl (c= 3 mol l − 1) as reference electrode (Autolab Potentiostat PGSTAT 30). Before modification with MPA, the gold disc electrode was first polished successively with 5, 1 and 0.3 mm aluminum oxide (Leco, USA) slurry on polishing pads (Leco, USA). Afterwards, the electrode was electrochemically cleaned by scanning in H2SO4 (c =1 mol l − 1). The clean electrode was then incubated in an ethanolic MPA-solution (c =10 mmol l − 1) for 1 h. The concentration of the K3[Fe(CN)6] solution was 10 mmol l − 1. The adsorption of polyelectrolytes, e.g. PAH, PSS and Co-MEPE on the electrode was performed as described above.
3.7. Fluorescence spectroscopy The multilayers were immersed into a methanol solution containing pyrene (py, 0.1 mmol l − 1) for 20 min. After rinsing with water to remove excess py from the film surface, the samples were dried with an argon stream. Steady-state fluorescence measurements were performed with a Spex Fluorolog-2 (model FL-2T2) spectrofluorometer. Emission spectra were measured in the front face arrangement using a solid sample holder. The excitation wavelength was 340 nm. The excitation and emission bandwidths were both 0.8 nm, while wavelength increments of 0.5 nm and integration times of 2 s were employed. Polyelectrolyte films on quartz substrate were measured before exposure to pyrene solutions and served as reference spectrum for substraction.
[1] V. Balzani, F. Scandola, Supramolecular Photochemistry, Ellis Horwood, West Sussex, 1991. [2] R. Ziessel, M. Hissler, A. El-ghayoury, A. Harriman, Coord. Chem. Rev. 178-180 (1998) 1251. [3] P. Gu¨ tlich, A. Hauser, H. Spiering, Angew. Chem., Int. Ed. 33 (1994) 2024. [4] R. Knapp, A. Schott, M. Rehahn, Macromolecules 29 (1996) 479. [5] G. Decher, Science 277 (1997) 1232. [6] M. Schu¨ tte, D.G. Kurth, M.R. Linford, H. Co¨ lfen, H. Mo¨ hwald, Angew. Chem., Int. Ed. 37 (1998) 2891. [7] D.G. Kurth, F. Caruso, C. Schu¨ ler, Chem. Commun. (1999) 1579. [8] (a) B.N. Figgis, E.S. Kucharski, A.H. White, Aust. J. Chem. 36 (1983) 1527; (b) B.N. Figgis, E.S. Kucharski, A.H. White, Aust. J. Chem. 36 (1983) 1537. [9] P. Lehmann, D.G. Kurth, G. Brezesinski, C. Symietz, Chem. Eur. J., 8 (2001) 1646. [10] D.G. Kurth, R. Osterhout, Langmuir 15 (1999) 4842. [11] Y. Lvov, K. Ariga, M. Onda, I. Ichinose, T. Kunitake, Coll. Surf. A 146 (1999) 337. [12] D.H. Wilkins, G.F. Smith, Anal. Chim. Acta 9 (1953) 338. [13] T.E. Janini, J.L. Fattore, J. Organometall. Chem. 578 (1999) 260. [14] G.L. Roberts, F.H. Field, J. Am. Chem. Soc. 72 (1950) 4232. [15] (a) J.M. Rao, M.C. Hughes, D.J. Macero, Inorg. Chim. Acta 16 (1976) 231; (b) M. Maskus, H.D. Abruna, Langmuir 12 (1996) 4455. [16] (a) J.P. Collins, P. Laine, J.-P. Launay, J.-P. Sauvage, A. Sour, Chem. Commun. (1993) 434; (b) E.C. Constable, A.M.W.C. Thompson, J. Chem. Soc., Dalton Trans. (1995) 1615. [17] R.W. Murray, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 13, Marcel Dekker, New York, 1984, p. 191. [18] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 1980. [19] G. Inzelt, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 18, Marcel Dekker, New York, 1994, p. 89. [20] O. Chailapakul, R.M. Crooks, Langmuir 11 (1995) 1329. [21] M. Wilhelm, C.L. Zhao, Y. Wang, R. Xu, M.A. Winnik, Macromolecules 24 (1991) 1033.
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[28] E.C. Constable, A.M.W.C. Thompson, J. Chem. Soc., Dalton Trans. (1992) 3467. [29] (a) W. Kern, RCA Eng. 28 (1983) 99; (b) B.F. Phillips, J. Vac. Sci. Technol. A 1 (1983) 646; (c) W. Kern, Semicond. Intern. (1984) 94. [30] (a) G. Decher, Y. Lvov, J. Schmitt, Thin Solid Films 244 (1994) 772; (b) G. Decher, J.D. Hong, J. Schmitt, Thin Solid Films 210/211 (1992) 831; (c) Y. Lvov, F. Essler, G. Decher, J. Phys. Chem. 97 (1993) 13773. [31] G. Sauerbrey, Z. Phys. 155 (1959) 206.
Metallo-supramolecular polyelectrolyte multilayers with cobalt(II): preparation and properties Dirk G. Kurth *, Markus Schu¨tte, Jin Wen Max-Planck-Institute of Colloids and Interfaces, 14424 Potsdam, Germany Received 30 August 2000; accepted 7 May 2001
Abstract Metal ion mediated self-assembly of the ditopic ligand 1,4-bis(2,2%:6%,2%%-terpyridine-4%-yl)benzene and Co(II) results in a metallo-supramolecular coordination polyelectrolyte (Co-MEPE), which is analyzed by spectroscopic methods, elemental analysis and cyclovoltammetry. The Co-MEPE shows a reversible one-electron redox transition in the potential range from −0.15 to 0.35 V with a half-wave potential of 87 mV. Thin films of Co-MEPE are fabricated by layer-by-layer self-assembly together with poly(styrenesulfonate) (PSS) on planar substrates and are analyzed by UV/vis spectroscopy, microgravimetry, and electrochemistry. The Co-ions in the multilayer can be removed and displaced by Fe-ions. The electrochemical activity of the Co-ions is fully maintained in the films. Charge transport through the multilayer is consistent with electron self-exchange reactions. The permeability of the multilayers is investigated by electrochemistry using K3[Fe(CN)6] as electroactive probe. Compared to poly(allylamine hydrochloride) (PAH)/PSS films, the electroactive probe does not permeate through PSS/Co-MEPE multilayers. Fluorescence measurements with pyrene as polarity probe indicate that the Co-MEPE films are more hydrophobic than PAH/PSS multilayers. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Metal ion coordination; Layer-by-layer self-assembly; Thin films; Polyelectrolytes; Electrochemistry
1. Introduction Currently, polypyridine ligands attract considerable attention as synthon in metallosupramolecular chemistry because of their interesting photo- and electrochemical properties [1]. Bipyridine ligands in combination with second and third transition row metal ions, in particular Ru(II) and Os(II), have favorable excited-state * Corresponding author. Tel.: + 49-331-567-9211; fax: + 49-331-567-9202. E-mail address: [email protected] (D.G. Kurth).
redox properties [2], but first transition row metal ions, such as Fe(II), also show exciting properties due to thermal and optically induced spin-transitions [3]. However, in terms of structure, bipyridine is not an ideal synthon because bidentate ligands in an octahedral coordination geometry give rise to stereo and geometric isomers. In case of polynuclear assemblies, this approach results in randomly coiled macromolecules [4]. The terpyridine ligand is much more convenient as structural synthon because coordination results in stereochemically defined octahedral complexes.
0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 1 ) 0 0 9 8 1 - 5
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Also, substitution in the 4%-position does not cause isomerism and offers a possibility to align the substituents in a clearly defined direction. Based on the structural advantage of the terpyridine group, we started to investigate polynuclear metallo-supramolecular assemblies utilizing the ditopic ligand 1,4-bis(2,2%:6%,2%%-terpyridine-4%yl)benzene. The rigid design of the ligand with back-to-back metal ion receptors and the high directionality of the terpyridine metal ion interaction induce a linear, rod-like structure. The high binding constant of terpyridine and transition metal ions results in formation of extended, linear macromolecules (Scheme 1). In case of first transition row elements, metal ion coordination occurs spontaneously in solution at ambient conditions. We refer to these assemblies as metallosupramolecular coordination polyelectrolytes (MEPE) because of the overall positive charge. With Fe(II) as central metal ion, we showed recently that Fe-MEPE can be incorporated into thin multilayer films by using layer-by-layer selfassembly [5] on planar [6] and colloidal [7] substrates. In this process, we take advantage of the positive charge of MEPE because electrostatic interactions of the oppositely charged polyelectrolytes are thought to play a primary role in the
Fig. 1. UV/vis absorption spectra of (me-ph-terpy)2Co(OAc)2 (dotted line), and Co-MEPE (solid lines) in methanol and water.
deposition. The thickness of a single layer can be controlled within the nanometer range. In the current study, we extend this approach to Co(II) as coordinating center. Due to its suitable redox potential, it is possible, in contrast to Fe-MEPE, to investigate the electrochemical response in solution and in multilayers. Compared to Fe(II), Co(II) ions bind more weakly to terpyridine, so that we can study the displacement and exchange of metal ions in multilayers. Finally, we present results from permeability measurements using an electroactive probe, as well as polarity studies based on pyrene fluorescence.
2. Results and discussion
2.1. UV –6is spectroscopy
Scheme 1. Metal ion mediated self-assembly of ditopic ligand 1 and cobalt(II) results in formation of a metallo-supramolecular coordination polyelectrolyte. Multilayers on arbitrary substrates are fabricated by layer-by-layer self-assembly of the positively charged Co-MEPE and negatively charged PSS.
Neat 1,4-bis(2,2%:6%,2%%-terpyridine-4%-yl)benzene (1) dissolved in methanol, shows a strong, broad absorption band at around 290 nm associated with p–p* transitions (Fig. 1). In addition, shoulders at 320 and 260 nm are observed. Metal ion mediated self-assembly of 1 and cobalt acetate in water or methanol results in a red–brown coloration of the solution, indicating metal ion coordination. The color is associated with unresolved metal centered d–d transitions in the range from 450 to 550 nm; in addition, there are two strong, but unresolved p–p* transitions of the aromatic moieties centered at 292 and 355 nm. In aqueous solution, peak maxima are discernible at 290, 300,
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Fig. 2. Left: UV/vis spectra of (PSS/Co-MEPE)n multilayers with n =1 – 10 on a PEI modified quartz substrate. Right: Absorption of individual band maxima as a function of the number of adsorbed layers. The dashed lines represent linear regressions.
347, and 360 nm. In methanol as solvent, we observe small shifts in the peak maxima and changes in the relative intensities of the p– p* transitions, as well as tailing of the d– d transitions towards longer wavelength. Apparently the absorbance of Co-MEPE is a sensitive function of the solvent polarity. The mononuclear complex, (me-ph-terpy = 4%-p(me-ph-terpy)2Co(OAc)2 tolyl-2,2%,6%,2%%-terpyridine), which represents the repeat unit of Co-MEPE, has a very similar absorption spectrum with peak maxima at 282, 292, 300, 346, and 358 nm and weak d– d transitions in the range from 450 to 550 nm. Fig. 1 displays the spectra of Co-MEPE in water and methanol as well as the spectrum of (me-ph-terpy)2Co(OAc)2. It is well known that terpyridine complexes of Co(II) exhibit an octahedral coordination geometry [8]. The similarity of the (me-phterpy)2Co(OAc)2 and Co-MEPE absorption spectra supports the assumption that Co-MEPE also has an octahedral coordination geometry.
2.2. Multilayer assembly Multilayer fabrication is readily achieved by repeated immersion of the substrate in solutions containing PSS and Co-MEPE with intermittent washing steps. Due to the characteristic absorption bands of Co-MEPE, UV/vis spectroscopy is the method of choice to investigate multilayer build-up. The left hand side of Fig. 2 shows UV/vis spectra of (PSS/Co-MEPE)n multilayers on a PEI modified quartz substrate (n = 1–10).
Absorption bands at 196 and 225 nm indicate the presence of PSS, while absorption bands at 293, 305, 347 and 367 nm prove the presence of CoMEPE in the films. The band contours of CoMEPE in films and in solution are somewhat different with respect to the relative intensities, widths, and positions. In terms of the relative intensities of the major bands at around 300 and 360 nm, the multilayer spectra are comparable to the spectrum recorded in methanol, as inspection of Figs. 1 and 2 reveals. If we consider that the dielectric constant decreases from 79 for water to 33 for methanol, we can conclude that the polarity within the multilayers is reduced substantially, that is at least to the level corresponding to alcohol. However, due to the limited solubility of Co-MEPE in less polar solvents, it is not possible to give a more accurate account of the polarity in the multilayers with this method. Another approach will be discussed below. The right side of Fig. 2 shows the absorption maxima at 196, 225, 293, 347 and 450 nm as a function of the number of adsorbed layers of (PSS/Co-MEPE) as well as corresponding linear curve fits. The plots show that film build-up is very regular and linear. The adsorption of CoMEPE increases linearly from the first layer onward since the linear regressions of the data points for peak maxima at 293, 347, and 450 nm go through the origin. The curve fits for the PSS bands at 195 and 225 nm cross the ordinate above the origin. We conclude that more PSS adsorbs on the PEI precursor layer than on Co-MEPE
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layers in consecutive deposition steps. This difference may be associated with a different charge density and/or hydrophobicity of the PEI interface compared to Co-MEPE. From the absorption of the film, we can calculate the surface coverage, Y, according to Lambert–Beer’s law Y = (AuN)/2mu, where mu is the isotropic molar extinction coefficient (M − 1 cm − 1), N is Avogadro’s number, and Au is the absorbance. Using the bulk extinction coefficient in this formula is problematic because, as stated above, the band contour depends on the polarity of the surrounding medium. The computed surface coverage is, therefore, only an estimate. We choose to take the extinction coefficients determined in methanol because in this case the spectra are more similar. The slope of the linear regression of the absorbance maxima at 293 (347) nm is 0.028 (0.025) (films on both sides of the substrate). The estimated surface coverage amounts to 2.2 (2.3) chromophores per nm2 or 0.45 (0.44) nm2 per chromophore. With a metal ion– metal ion separation of approximately 1.55 nm and an approximate width of 1.2 nm of the ligand, the area of a chromophore is approximately 1.86 nm2 [9]. Assuming that the chromophores are oriented with the long axis parallel to the substrate, the absorbance corresponds to approximately 4 molecular layers. If we also take into account an anisotropic extinction coefficient, the absorbance corresponds to approximately 2.6 layers.1 Within the experimental errors, these values are in agreement with previously reported results based on X-ray reflectance and surface plasmon resonance spectroscopy of structurally similar Fe-MEPE films, which have a thickness of approximately 2 molecular layers [10].
a monolayer of mercaptopropionic acid (MPA), which acts as an adhesion promoter for the subsequently deposited PAH layer. The QCM is coated on both sides with a PAH/(PSS/Co-MEPE)n multilayer ( n= 1–6). The change in the resonance frequency after each deposition is measured ex situ. Each adsorption step is associated with a decrease of the resonance frequency, indicating an increase of the total mass on the electrodes. The adsorbed amount per layer is nearly constant, which is consistent with the results from UV/vis spectroscopy. The mean frequency decrease per adsorbed layer pair (PSS/Co-MEPE) is 230 Hz, which corresponds to 627 ng cm − 2 (one side of QCM). Assuming that there is no water in the film and densities of 1.2 g cm − 3 for PSS [11] and 1.5 g cm − 3 for Co-MEPE,2 the mass corresponds to a thickness of 4.2 nm per (PSS/Co-MEPE) layer pair. Assuming a layer thickness for PSS of approximately 2 nm [10], the Co-MEPE thickness is approximately 2.2 nm, which would correspond to approximately 2 molecular layers.
2.4. Exchange of metal ions
Microgravimetry allows direct weighing of the adsorbate with a sensitivity in the ng-scale and, therefore, complements UV/vis spectroscopic measurements. The gold electrodes of the quartz crystal microbalance (QCM) are first coated with
In contrast to covalent polymers, the non-covalent nature of the metal ion–ligand bond offers an opportunity to chemically modify the multilayers after deposition. Direct exchange of Co(II) with Fe(II) in Co-MEPE multilayers does not occur and, therefore, we take advantage of the fact that Co(II) binds strongly to ethylenediamine (en) [12]. The Co-MEPE films are first subjected to en to complex and to remove the Co(II) ions from the multilayer. Afterwards, the vacant metal ion receptors are filled with Fe(II). The Fe(II)–terpyridine complex has a characteristic absorption band at approximately 590 nm, which can be easily detected. The UV/vis spectrum of the reaction of CoMEPE with en and subsequently with Fe(II) is shown in Fig. 3 (left). The solid line shows the spectrum of an aqueous solution of Co-MEPE. To this solution en was added and acetic acid to adjust the pH to 7. After addition of Fe(II),
1 In case of an in-plane orientation of chromophores, the anisotropic extinction coefficient, mani, is given by mani =3/2m.
2 The density of Co-MEPE was determined in aqueous solution with a densitometer (DMA 602, PAAR).
2.3. Microgra6imetry
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spectra were recorded after 2, 6 and 60 min. An absorption band appears around 600 nm, which increases in intensity with time. This band is characteristic of the MLCT band of Fe-MEPE, indicating the exchange of Co(II) for Fe(II). It is interesting to note that the maximum of the MLCT band shifts with increasing time from 574 nm to longer wavelength to 588 nm. This shift is attributed to a successive aggregation of Fe(II) in MEPE indicating dipolar coupling between metal centers. Investigations with oligomeric RuMEPEs show a similar shift of the MLCT band to longer wavelength with increasing number of connected units [13]. The right side of Fig. 3 displays the UV/vis spectrum of a (PSS/Co-MEPE)3 multilayer on a PEI modified quartz substrate (solid line). After immersion in an aqueous solution of en for 1 h (dashed line) we observe a decrease of absorption bands in the region of 300– 450 nm, as a result of decomplexation of the Co(II) ions in the CoMEPE multilayer. In particular, the characteristic band at 355 nm, which occurs upon metal ion coordination (vide supra), decreases considerably. The presence of the band at 290 nm associated with uncoordinated ligand, demonstrates that the ligand remains in the film. It should also be noted that the bands that are due to PSS do not change in intensity, which indicates that the layer is not decomposed. The complex Co(en)23 + shows ab-
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sorption bands at 356 (m= 28.100 cm2 mmol − 1) and 480 nm (m= 20.800 cm2 mmol − 1) [14]. Since the spectrum after en addition shows no significant absorption bands at these positions, we conclude that Co(en)23 + diffuses out of the multilayer. Finally, the film was dipped in an acidic solution containing Fe(II). The appearance of the band at 350 nm as well as the characteristic MLCT-transition at 591 nm (inset) indicates metal ion coordination and formation of Fe-MEPE. A single layer of Fe-MEPE shows an absorbance of the MLCT band of approximately 0.02. For a complete exchange of the Co-ions in all three layers, we would expect an absorbance of 0.06. The absorbance of about 0.03 shows that approximately every second Co(II)-ion in the film is removed and replaced by Fe(II).
2.5. Electrochemistry The electrochemical behavior of Co-MEPE was investigated both in solution and in films. In the following, all experiments were carried out with MPA modified electrodes in order to be able to make a direct comparison of the experimental results. In the investigated potential region, MPA coated electrode shows a very weak current maximum, presumably due to the redox activity of the thiol groups. Fig. 4 shows the cyclovoltammograms (CV) of a MPA modified gold electrode
Fig. 3. Metal ion exchange in solution (left) and in films (right) followed by UV/vis spectroscopy. Left. Solid line: Co-MEPE. Dashed lines: Spectra after addition of Fe(II) after 2 (i), 6 (ii), and 60 (iii) min showing the characteristic MLCT band of Fe-MEPE. Right. Solid line: (PSS/Co-MEPE)3 multilayer. Dashed line: After dipping the film in a solution containing en to remove Co-ions. The band at 355 nm decreases in intensity indicating loss of Co-ions. Dotted line: After dipping the film in a solution of (NH4)2Fe(SO4)2 in acetic acid to add Fe-ions to the multilayer. Inset: Characteristic MLCT-transition, demonstrating metal ion coordination and formation of Fe-MEPE.
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Fig. 4. Left: CV of a MPA modified gold electrode in a solution of Co-MEPE as a function of the scan rate, 6 (6: 20, 50, 100, 150, 200 mV s − 1). Right: Dependence of the current, I, on 6 (Ip,a =anodic current, Ip,c =cathodic current).
in a solution of Co-MEPE (c= 0.1 mmol l − 1) with sodium acetate as supporting electrolyte (c= 0.1 mol l − 1) in the potential range from −0.15 to 0.35 V as a function of the scan rate. The CoMEPE shows a pronounced pair of current waves with a redox half-wave potential of E1/2 = 1/ 2(Ea +Ec)= +87 mV (Ea and Ec are the cathodic and anodic peak potentials, respectively), which is independent of the scan rate. The half-wave potential is comparable to other mononuclear Coterpyridine complexes [15]. The appearance of only one pair of current waves indicates electrochemically isolated subunits in Co-MEPE [16]. The anodic and cathodic currents are proportional to the scan velocity, which is illustrated in Fig. 4 (right) for scan velocities up to 800 mV s − 1. The proportionality between the current and the scan velocity reflects the reversibility of the redox process. The potential difference between the two current peaks DEp is 74 mV and remains constant during the variation of the scan velocity, indicating fast electron transfer. The CVs of a (PSS/Co-MEPE) multilayer as a function of the scan rate are shown in Fig. 5 (left). The redox potential E1/2 is slightly shifted to 128 mV. In this case, the anodic and cathodic current peaks are proportional to the square root of the scan velocity (Fig. 5 (right)). This behavior is consistent with electron self-exchange reactions within the electrochemical sites in the film, in which a semi-infinite electrochemical charge diffusion condition prevails [17]. The current peak difference DEp increased to approximately 123
mV in the film, which may reflect a higher ohmic resistance of the multilayer, presumably caused by the (inactive) intermittent PSS layers. The peak current, ip, is equal to 2.69×105 1/2 6 AD 1/2C, where 6 is the scan rate, A is the electrode area, D the charge transport diffusion coefficient and C the concentration [18]. Taking the results from UV/vis spectroscopy and QCM measurements, the concentration of redox active species in the film is approximately 10 − 3 mol cm − 3. The electrode area is 7.9× 10 − 3 cm2 and the slope of the ip versus 6 1/2 plots for the anodic and cathodic currents is 2 nA (mV s − 1) − 1/2. Therefore, D amounts to approximately 10 − 15 cm2 s − 1. This value is considerably smaller than diffusion constants reported in the literature [19], presumably because the distance of redox active sites is rather large due to intermittent PSS layers and the mobility of the redox active sites within the film may be reduced due to the rigid backbone of MEPE. Finally, the counter ion mobility within these layers may also affect the charge transport diffusion coefficient.
2.6. Permeability Electroactive species have frequently been used as probes to investigate the nature and extent of structural defects in thin films [20]. In the following study, we used K3[Fe(CN)6] as electroactive species, which undergoes a reversible one electron redox reaction. Fig. 6 summarizes the electrochemical response of Au/MPA/(PAH/PSS)n and
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Au/MPA/(PAH/PSS)/(Co-MEPE/PSS)n multilayers as a function of the number of layers. For comparison, the CV of the Au-electrodes (dotted line) as well as the MPA modified electrodes (solid line) are shown in the top of Fig. 6. The MPA monolayer on the gold electrode causes an increase of the current peak separations, presumably a result of the electrostatic repulsion between the charged surface and redox species. Deposition of two PAH/PSS layers on the electrode reduces the current by an order of magnitude because the ohmic resistance of the interface is increased. While deposition of another PAH/ PSS layer reduces the current, additional layers do not alter the CV any further. The electrochemical signature of the Fe(CN)36 − /Fe(CN)46 − couple remains discernible in all samples, indicating that the redox active probe diffuses through the film and undergoes electron transfer reactions at the underlying Au-electrode. Similarly, the Co-MEPE samples for 1, 2, and 3 layer pairs show a reduction in the peak current and a predominantly radial diffusion. However, the sample with 5 and more Co-MEPE/PSS layers shows only a capacitive current and no electroactivity of the probe, indicating that the electrode is completely blocked at this stage. In both cases, deposition of up to 12 layer pairs did not result in further electrochemical changes. The electrochemical behavior of the Fe(CN)36 − /Fe(CN)46 − couple in the two samples indicates a difference in the film properties, such as pore size, defects, hydrophobic and hydrophilic properties, as well as charge distribution. To ad-
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dress this point in more detail we investigated the polarity of the multilayers.
2.7. Polarity determination Pyrene (py) fluorescence has gained an outstanding role in the characterization of the local environment in bulk liquids, micelles [21], Langmuir–Blodgett monolayers [22], polymers [23], or hydrogels [24]. The extensive use of pyrene as an environmentally sensitive probe is due to the dependence of vibronic band intensities on solvent polarity. In particular, the py emission spectrum exhibits five vibronic bands in the region between 372 and 400 nm, usually labeled I–V. The intensity of the 0–0 vibrational band (I) is enhanced by increasing the polarity of the environment, while the intensity of the third band (III) remains almost constant. Thus, the ratio of the emission intensities of the bands I and III (I/III, also called py-value) is employed as a measure for solvent polarity. Here, we use pyrene fluorescence to estimate the polarity of Co-MEPE films, allowing a better understanding of the electrochemistry results. The emission spectrum obtained from the pyrene doped PEI/(PSS/Co-MEPE)4 multilayer, normalized to peak I, is shown in Fig. 7. The small signal at 450 nm is attributed to Co-MEPE because it is not observed in Ni-MEPE or PAH/ PSS multilayers. The film py-value for the PSS/ Co-MEPE multilayer is 1.48. However, the py spectrum shows a background due to Co-MEPE
Fig. 5. Left: CV of a MPA modified gold electrode with the composition MPA(PAH/PSS)(Co-MEPE/PSS)10 in a solution of sodium acetate (6: 10, 20, 50, 100, 200, 400, 800 mV s − 1). Right: Dependence of the current on 6 0.5.
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Fig. 6. Electrochemical response of multilayer coated electrodes for the K4[Fe(CN)6]/K3[Fe(CN)6] redox couple. Left: MPA(PAH/ PSS)n (n =0, 2, 4, 6, 8). Right: MPA(PAH/PSS)(Co-MEPE/PSS)n (n =0, 1, 3, 5, 7). Dashed lines: Au-electrode. Solid lines: MPA coated electrodes.
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Fig. 7. Fluorescence spectrum of a PEI/(PSS/Co-MEPE)4 multilayer doped with pyrene.
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cobalt acetate dihydrate for 1 h at reflux and 24 h at room temperature. The solution was filtered and the solvent evaporated under reduced pressure to yield a red–brown solid. Elemental analysis: found C, 52.8; H, 5.5; N, 8.8; calculated for C40H30N6O4Co·11H2O: C, 52.5; H, 5.7; N, 9.2; UV/vis (methanol): umax (m in cm2 mmol − 1)=288 (38.428), 302 (37.777), 348 (36.735), 362 (39.080), UV/vis (H2O): umax (m in cm2 mmol − 1)=290 (44.981), 300 (44.465), 347 (49.509), 360 (53.494), IR (KBr) (w in cm − 1): 734, 749, 793, 837, 1014, 1163, 1246, 1401, 1432, 1472, 1558, 1570, 1608.
3.3. Film assembly fluorescence. If the background is carefully substracted, the py-value amounts to 1.29, which corresponds to the polarity of chloroform [25,26]. Due to the interfering background fluorescence, it is currently not possible to make a more accurate assignment of the polarity in these multilayers. However, both values are lower than in multilayers of strong polyelectrolytes [27]. This result supports the conclusion drawn from UV/vis spectroscopy that the polarity of the multilayers is reduced. A possible reason for the electrochemical blockade of PSS/Co-MEPE multilayers could, therefore, be the more hydrophobic nature of the film compared to strong polyelectrolyte layers, which would strongly alter the diffusion of ions through the Co-MEPE multilayers.
Quartz slides from Hellma Optik (Jena, Germany) were used and cleaned according to literature procedure (RCA-cleaning) [29]. Multilayers were prepared by the method developed by Decher et al. [30]. The following solutions were used: poly(ethyleneimine) (PEI) (c =10 − 2 mol l − 1 in water), poly(styrenesulfonate) (PSS) (c= 10 − 3 mol l − 1, cNaCl = 1 mol l − 1 in water), poly(allylamine hydrochloride) (PAH) (c= 10 − 3 mol l − 1, cNaCl = 1 mol l − 1 in water) and CoMEPE (c= 0.1 mg g − 1 in methanol).
3.4. UV/6is-spectroscopy The UV/vis-absorption-measurements were recorded with a Varian Cary 50. Uncoated quartz slides were used as reference spectrum.
3. Experimental
3.5. Microgra6imetry 3.1. Chemicals All chemicals were of analytical grade and were used without further purification. The ligand 1,4bis(2,2%:6%,2%%-terpyridine-4-yl)benzene (1) was synthesized according to a literature procedure [28]. Gold electrodes were coated with a monolayer of MCA according to previously published procedures [10].
3.2. Co-MEPE Ligand 1 was crushed in a mortar and stirred under Argon in a degassed methanolic solution of
The measurements were carried out with a custom build electronic driver circuit and a HPE3620A power supply. The quartz crystals were equipped on both sides with gold electrodes (evaporated, polished gold). Frequencies were recorded with a frequency counter (Hewlett Packard HP 5313 A) connected to a computer. Measurements were carried out ex-situ in a climatic room at a temperature of 22 °C. Quartz crystals were rinsed several times with ethanol and water before dipping in solutions of MPA, PAH, PSS and Co-MEPE (20 min). After each deposition step the crystals were rinsed several times with
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Acknowledgements
water and dried in a stream of air. The crystal was left to equilibrate for 10 min, after which the resonance frequency was measured every second over a period of 3 min. After averaging, the mean deviation is within 1 Hz. The adsorbed amount of polyelectrolyte was calculated according to the equation of Sauerbrey [31].
Valuable discussions with Helmuth Mo¨ hwald are gratefully acknowledged. The authors thank Tina Tedeschi for her help with the polarity measurements.
3.6. Cyclic 6oltammetry
References
Electrochemical measurements were performed in a three-electrode glass cell with gold disc electrode (d= 2 mm) as working electrode, Pt foil as counter electrode and Ag/AgCl+/KCl (c= 3 mol l − 1) as reference electrode (Autolab Potentiostat PGSTAT 30). Before modification with MPA, the gold disc electrode was first polished successively with 5, 1 and 0.3 mm aluminum oxide (Leco, USA) slurry on polishing pads (Leco, USA). Afterwards, the electrode was electrochemically cleaned by scanning in H2SO4 (c =1 mol l − 1). The clean electrode was then incubated in an ethanolic MPA-solution (c =10 mmol l − 1) for 1 h. The concentration of the K3[Fe(CN)6] solution was 10 mmol l − 1. The adsorption of polyelectrolytes, e.g. PAH, PSS and Co-MEPE on the electrode was performed as described above.
3.7. Fluorescence spectroscopy The multilayers were immersed into a methanol solution containing pyrene (py, 0.1 mmol l − 1) for 20 min. After rinsing with water to remove excess py from the film surface, the samples were dried with an argon stream. Steady-state fluorescence measurements were performed with a Spex Fluorolog-2 (model FL-2T2) spectrofluorometer. Emission spectra were measured in the front face arrangement using a solid sample holder. The excitation wavelength was 340 nm. The excitation and emission bandwidths were both 0.8 nm, while wavelength increments of 0.5 nm and integration times of 2 s were employed. Polyelectrolyte films on quartz substrate were measured before exposure to pyrene solutions and served as reference spectrum for substraction.
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