J. Phys. Chem. C 2007, 111, 12817-12821
12817
Influence of Film Architecture on the Charge-Transfer Reactions of Metallophthalocyanine Layer-by-Layer Films Wagner S. Alencar,† Frank N. Crespilho,‡ Maria Rita M. C. Santos,† Valtencir Zucolotto,‡ Osvaldo N. Oliveira, Jr.,‡ and Welter C. Silva*,† Departamento de Quı´mica, Centro de Cieˆ ncias da Natureza, UniVersidade Federal do Piauı´, CEP. 64049-550, Teresina-PI, Brazil, and Instituto de Fı´sica de Sa˜ o Carlos, UniVersidade de Sa˜ o Paulo, Cx. Postal 369, CEP. 13560-970, Sa˜ o Carlos-SP, Brazil ReceiVed: January 26, 2007; In Final Form: June 14, 2007
A judicious combination of materials and molecular architectures has led to enhanced properties of layerby-layer (LbL) films, in which control at the molecular level can be achieved. In this paper we provide one such example by showing that supramolecular effects in electroactive LbL films comprising tetrasulfonated metallophthalocyanines (NiTsPc or FeTsPc) alternated with poly(allylamine hydrochloride) (PAH) may depend on both the choice of material and the film-forming technique. Indeed, though some properties such as film growth were common to both types of LbL film, those containing NiTsPc displayed unique features. PAH/ NiTsPc films assembled onto ITO (indium tin oxide) showed two redox processes, with E1/2 at 0.54 and 0.80 V (vs SCE) attributed to the phthalocyanine unit ([TsPc]6-/[TsPc]5-) and Ni2+/Ni3+ redox couple, respectively. In contrast, only one redox process was observed for PAH/FeTsPc films, with E1/2 at 0.45 V assigned to the [TsPc]6-/[TsPc]-5 couple. For both systems, the anodic peak current versus scan rate increased up to 500 mV‚s-1, indicating that the electrochemical response of NiTsPc and FeTsPc in LbL films is governed by charge-transport mechanism. Interestingly, the second redox process for PAH/NiTsPc became totally reversible at high scan rates, showing fast charge transfer. The influence from the film-forming technique was proven by comparison with results from an electrodeposited film of NiTsPc on ITO, which was less stable than its LbL counterpart. It is envisaged that the high electrochemical stability, reversibility, and unique features arising from the supramolecular structure of PAH/NiTsPc LbL films may be exploited in applications such as electrochromic and sensing devices.
1. Introduction The properties of solid-state supramolecular architectures can be controlled by tuning hydrogen bonding and electrostatic interactions in coordination compounds containing metallic centers and distinct coordination spheres.1-5 An interesting strategy to self-assemble highly organized nanostructures is the use of coordination chemistry strategies,4,6 where the molecules employed play a key role in the development of supramolecular systems.4,6-8 For example, nickel(II) and iron(II) metallic ions, in 3d8 and 3d6 low-spin electronic configurations, respectively, may be present in metallophthalocyanines (MPc, where M ) metallic ion), when coordinated to the phthalocyanine ligand (Pc). In water solution, this adduct exhibits high kinetic and thermal stability regarding substitution in the equatorial plane,9-11 in addition to being an effective probe for molecular recognition.1 With the central metal ion being the reactive and catalytic site,9-11 metallophthalocyanines may be used as sensor for toxic gases,12 electrocatalysis,13 and electrochemical sensors.7,8,14 Moreover, metallophthalocyanines are interesting for electrochemistry studies, in addition to nonlinear optics, optical memories, organic solar cells, and secondary batteries.15,16 Molecular-level control of materials properties can also be reached in nanostructured layered films built from noncovalent interactions,1,3,17 which allows the development of inorganic * Corresponding author. E-mail:
[email protected]. † Universidade Federal do Piauı´. ‡ Universidade de Sa ˜ o Paulo.
nanostructures tailored with a suitable choice of constituents.3,5,17,18 An efficient method for producing organized films is the layer-by-layer (LbL) technique based on the deposition of oppositely charged molecules.3,5,7,8,18 The advantages of the LbL method are simplicity, versatility, low cost, and control of film thickness and molecular architecture.3,7,8,18 Over the past few years, LbL films have been fabricated with phthalocyanines that were rendered water soluble through incorporation of carboxylic and sulfonic groups.19,20 With varied metallophthalocyanines and the LbL method, one has therefore two instruments with which it is possible to control properties at the molecular level. The electroactivity of LbL films from metallophthalocyanines, for instance, has been reported as a oneelectron reversible electrochemical process in a large range of pH values, attributed to the redox couple of the metallic center.7-11,18 Metallophthalocyanines solutions usually exhibit redox processes in which the potentials may change as a function of pH.9-11 In addition, the potentials in which such processes occur depend on several factors including the nature of the complex, metallic center, axial ligands, solvent, and polymerization.10,11 Because an interaction between the adsorbed molecule and the electrode surface occurs, a difference in the redox potentials from the MTsPc in homogeneous solution or in nanostructured films is expected.7-11,18 In this study we investigated the electrochemical properties of LbL films containing nickel and iron tetrasulfonated phthalocyanines (MTsPc, M ) Ni or Fe) alternated with poly(allylamine hydrochloride) (PAH). Although the films displayed
10.1021/jp070695r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007
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Figure 1. Schematic diagram illustrating the fabrication of a 1-bilayer LbL film. In detail: the salt-bridge between PAH and MTsPc molecules at the supramolecular level.
well-defined electroactivity, significant differences were observed upon varying film architecture, more specifically the manner in which the Pc molecules were adsorbed on the substrate. 2. Experimental Section 2.1. Chemicals. Hydrochloride acid (Reagen Co.) was used as electrolyte for controlling ionic strength and pH. Nickel and iron tetrasulfonated phthalocyanine complexes (NiTsPc and FeTsPc) and PAH were purchased from Aldrich Co. All other chemical reagents were of analytical grade purity and used as received. Doubly distilled water was used in the experiments. 2.2. Film Assembly and Characterization. PAH and nickel (or iron) tetrasulfonated phthalocyanine (MTsPc, where M ) Ni or Fe) were employed as cationic and anionic electrolytes, respectively, at concentrations of 0.5 g‚L-1 and pH 2.5. PAH/ MTsPc multilayers were deposited on hydrophilic glass and ITO (indium tin oxide)-covered glass substrates by dipping the substrate alternately into the PAH and MTsPc solutions for 5 min (Figure 1). After each layer deposition, the substrate was rinsed with HCl solution at pH ) 2.5 and then dried under a N2 flow.21 The growth of the multilayers was monitored via cyclic voltammetry in an Autolab electrochemical analyzer (Eco Chemie) using films containing 5, 10, 15, and 20 PAH/MTsPc bilayers deposited onto ITO substrates. The three-electrode electrochemical cell had a saturated calomel glass (SCE) as reference electrode, Pt wire (A ) 0.65 cm2) as counter electrode, and ITO (A ) 0.35 cm2) or ITO covered with LbL film as working electrode. Measurements were taken under a controlled N2 flow, at room temperature. The cyclic voltammograms (CVs) were recorded immediately after inserting the electrodes into the cell containing pH 1.0 HCl solution as supporting electrolyte. Film buildup was also studied by measuring the UV-vis spectra using a Hitachi U3501 spectrophotometer. 3. Results and Discussion 3.1. Multilayer Deposition. The suitability of a molecule for the fabrication of LbL films depends on thermodynamic parameters such as the pKa. For metallophthalocyanines, in particular, the back-bonding capability of metal centers can be estimated by analyzing the pKa of the free ligands and coordination compounds, from which the ionization process at a given pH of groups bound to the compound can be inferred.22-24 Lever and co-workers10,11 reported a pKa for the sulfonic group in TsPc ligand as being close to that for benzene sulfonic acid (pKa ) 0.7). Since Ni2+ and Fe2+ ions are
Figure 2. Cyclic voltammograms for (a) PAH/NiTsPc and (b) PAH/ FeTsPc for various numbers of bilayers. Electrolyte: HCl solution 0.1 mol‚L-1, scan rate: 50 mV‚s-1, 25 °C.
coordinated to the TsPc ligand, their pKa values should be higher due to the small back-bonding capability between metal centers and ligands.10,11,22-24 For NiTsPc and FeTsPc complexes used in the present work, the four sulfonic groups are expected to be ionized, since the solutions employed had pH ) 2.5, at least one unit above the pKa.3,7,8,18 Such conditions ensure that all SO3- groups may interact with NH3+ groups from PAH with salt-bridge formation (Figure 1).7,8,18 Furthermore, the negative charge of the phthalocyanine rings should prevent inter- or intramolecular aggregation,9-11 indicating that these molecules are amenable to use in LbL films. These expectations were confirmed with the growth of PAH/NiTsPc and PAH/FeTsPc films via electrostatic interaction between polycationic PAH and anionic MPc species in aqueous solution, as indicated in Figure 2 for PAH/NiTsPc. For both systems, the peak currents increased with the number of bilayers, confirming the deposition of multilayers.7,8,18,21 Adsorption was also monitored by UVvis spectroscopy, as shown in Figure 3, where the absorbance
Metallophthalocyanine LbL Films
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12819
Figure 3. Electronic spectra for PAH/NiTsPc films onto a glass substrate with various numbers of bilayers. Inset: absorbance vs number of PAH/NiTsPc bilayers.
of the Q-band at 627 nm increased linearly with the number of PAH/NiTsPc bilayers (shown in the inset).7,8,18 Adsorption of PAH/FeTsPc LbL films also increased linearly with the number of bilayers (not shown), but deposition was less efficient.7 The latter is in agreement with previous studies concerning MTsPc immobilization in LbL films.3,7,8,18 3.2. Unusual Kinetics Dependence of NiTsPc on the Supramolecular Structure. A key feature of film-forming techniques such as the LbL method has been the ability to produce supramolecular structures whose properties may differ entirely from bulk properties of the same materials used to make the films.3,7,18 For NiTsPc investigated here, this has been found to be the case. First, as mentioned above, deposition of PAH/ NiTsPc films was more efficient than that of PAH/FeTsPc, which may be related to the structural organization of PAH and NiTsPc groups within the inorganic structure, probably allowing a better packing of individual layers.1,3,7,18 Similar results were obtained for chitosan/MTsPc pairs (M ) Ni or Fe), where adsorption of chitosan/NiTsPc was more efficient than that for chitosan/FeTsPc films.18 In fact, with both chitosan and PAH, molecular interactions with the phthalocyanines may occur via the metallic ion, hydrogen bonding, electrostatic interactions, or π-π stacking, allowing the formation of supramolecular assemblies.1,2 Most importantly, the electrochemical properties of NiTsPc LbL films are also different from those of FeTsPc LbL films and even from those of another type of film made with NiTsPc, as we show below. Figure 4a shows CVs for a 15-bilayer NiTsPc film at various scan rates, exhibiting two redox electrochemical processes with E(1/2)1 and E(1/2)2 at 0.54 and 0.80 V (vs SCE). These redox processes were attributed to the phthalocyanine ring and Ni2+/ Ni3+ couple, respectively. In contrast, under the same experimental conditions only one redox pair was observed for a 15bilayer PAH/FeTsPc film, with E(1/2) at 0.48 V (Figure 2) assigned to the TsPc-6/TsPc-5 unit.7-11,19 The E1/2 values for PAH/NiTsPc and PAH/FeTsP are consistent with the redox processes reported for similar systems (Table 1). It is worth noting that internal metal redox processes affect strongly the potentials for ring redox.9-11 However, the determination of the initial site for electrooxidation in nickel(II) and iron(II) porphyrins and related macrocycles is still under debate.25 Wolberg and Manassen26 proposed oxidation in [M(II)TPP] complexes, where M ) Fe or Ni and TPP ) tetraphenylporphyrin, to occur first at the central metal atom, with subsequent oxidation of
Figure 4. (a) Cyclic voltammograms for a 15-bilayer PAH/NiFtPc film at different scan rates: 10, 25, 50, 100, 200, 300, 400, and 500 mV‚s-1. Electrolyte: HCl solution 0.1 mol‚L-1, T ) 25 °C. (b) Anodic peak current (Ipa) vs scan rate.
the ligand TPP. In contrast, Kadish et al.25 observed two reversible one-electron oxidations for neutral metalloporphyrins with two types of electrode mechanisms: (1) formation of metal(II) porphyrin π cation radical followed by generation of the M(III) species; (2) initial electrogeneration of M(III) porphyrin complexes followed by π cation radical formation.25 To check the reversibility of the electrochemical processes for the LbL films, we explore the well-known concepts of electrochemical reversibility.27 A more careful analysis from Figure 4 shows that a 15-bilayer PAH/NiTsPc film exhibits an oxidation peak current (Ipa1) that shifts toward more positive potentials upon increasing the scan rate, indicating irreversibility. However, the second electrochemical process appears as a typical fast charge-transfer reaction, with Ipa2 becoming totally reversible at high scan rates.14,18,28,29 The electrochemical response at potentials from 0 to 0.7 V and 0.7 to 1.0 V (not shown) confirmed that both processes are independent and highly influenced by the kinetics of Ni2+/Ni3+ couple in the charge-transfer reactions within the multilayers. Surprisingly, for the PAH/FeTsPc LbL film the redox process from Fe2+/ Fe3+ does not appear even at scan rates up to 500 mV‚s-1 (result not shown here). Since PAH is not electroactive, the electrochemical processes reported for the PAH/MTsPc LbL films come from the metallophthalocyanines species.7,8,18 The electrochemical performance of PAH/NiTsPc and PAH/FeTsPc LbL films differed from that of MPc-modified electrodes.13,30 According to Nyokong and Vilakazi13 and Oni and Nyokong,30 redox couples for MPc-modified electrodes are expected at the metallic center in MnPc, FePc, and CoPc compounds, but ringbased processes should occur in ZnPc, NiPc, and CuPc species. Furthermore, MPc compounds with metal-based oxidation should be more electrocatalytic than ring-based MPc complexes.13,30 As shown in Figure 4b, the peak currents (Ipa1 and Ipa2) for PAH/NiTsPc increase linearly with the scan rate, indicating that
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TABLE 1: Electrochemical Data for LbL Films Containing Iron and Nickel Phthalocyanines
LbL film PAH/FeTsPc DAR/FeTsPc chitosan/NiTsPc chitosan/FeTsPc PAH/FeTsPc PAH/NiTsPc
ring-centered process
metal-centered process M(II)/M(III), V vs SCE 0.94 0.04 0.80 0.78
0.48 0.54
the electrochemical response is governed by charge-transfer mechanism and not by charge diffusion.7,18,28,29 This observation is confirmed in Figure 4b, where a linear increase with the scan rate is observed for the two processes, which are intimately related to the charge-transfer process between metallophthalocyanines species.7,18 A similar behavior was reported by Lvov et al.,28 where PDDA/CoIITsPc-4 LbL films (PDDA ) polydimethyldiallylammonium chloride) showed a characteristic unsymmetrical diffusional CV peak shape indicating a diffusional limitation of charge transport. In a previous study, we observed that the voltammetric profiles are almost the same for different MTsPc bilayers immobilized onto ITO.7 In addition, ITO-PAH/NiTsPc and ITO-PAH/FeTsPc systems display similar characteristics as redox polymer systems, where charge transport within the multilayers is expect to occur via electron hopping, in agreement with findings from Laurent and Schlenoff.31 The PAH/NiTsPc and PAH/FeTsPc films exhibited a high redox stability, illustrated in Figure 5, with the same voltammograms obtained during several cycles. This points to highly organized structures with phthalocyanines sites available for redox reactions, allowing an intense charge-change reaction.7,14,18 On the basis of the electrochemical behavior of LbL PAH/ NiTsPc films, it was possible to propose a mechanism of charge transfer within the multilayers. We assume that the first oxidation process occurs at the phthalocyanine unit (Epa1) with the second oxidation occurring at the Ni2+ metallic center (Epa2). Moreover, both processes are chemically independent, as already inferred from the CV results. Because the increasing scan rate
Figure 5. Cyclic voltammograms (15 cycles) for (a) 5-bilayers PAH/ NiTsPc LbL film and (b) 25-bilayers PAH/FeTsPc LbL film onto ITO electrode. Electrolyte: HCl (pH 1.0). Scan rate: 50 mV‚s-1.
0.80
electrolyte
ref
acetonitrile phosphate HCl HCl HCl HCl
7 14 18 18 this work this work
causes the current peak in the Epa1 process to decrease relatively to the current peak in Epa2, the second redox couple involves a fast charge-transfer reaction. The latter was not observed for PAH/FeTsPc films, where the second process was attributed to the Fe2+/Fe3+ couple, occurring at higher potentials than for the PAH/NiTsPc LbL film.7,18
[TsPc]6- f [TsPc]5- + e-
(Epa)1 (I, slow step)
[Ni(II)TsPc]4- f [Ni(III)TsPc]3- + e-
(Epa)2 (II, fast step)
[Ni(III)TsPc]3- + e- f [Ni(II)TsPc]4-
(Epc)2 (III, fast step)
[TsPc]5- + e- f [TsPc]6-
(Epc)1 (IV, slow step)
In order to verify the influence of film architecturesmore specifically, the manner the Pc molecules are immobilized on the substrateson the electrochemical properties of the film, an ITO/NiTsPc (without PAH polyelectrolyte) modified electrode was prepared. An ITO substrate was immersed in 2.0 × 10-3 mol‚L-1 of NiTsPc electrolytic solution (0.1 mol‚L-1 HCl), and the potential was cycled from 0 to 1.0 V. Growth of NiTsPcelectrodeposited film was not observed; however, there was evidence of NiTsPc adsorption on the ITO substrate, consistent with the results of Tre´vin et al.,29 in which electropolymerized Ni porphyrin films could only be formed in aqueous 0.1 mol‚L-1 NaOH solution. The ITO/NiTsPc-modified electrode was used as working electrode in an electrochemical cell containing 0.1 mol‚L-1 HCl, where the voltammograms were immediately
Figure 6. Cyclic voltammograms for an ITO electrode coated with adsorbed NiTsPc in 0.1 mol‚L-1 HCl aqueous solution. Scan rates: 25, 50, 100, 200, 300, 400, and 500 mV‚s-1.
Metallophthalocyanine LbL Films recorded. The adsorbed NiTsPc complexes on ITO showed two defined redox peaks with E1/2 at 0.29 and 0.7 V (Figure 6), which can be attributed to the redox process from Pc unit and ion nickel(II), respectively. These redox processes are shifted to negative potentials in comparison with those for the NiTsPc LbL film. Also, a different electrochemical behavior was observed for the second redox couple (Epa2) compared to the Epa2 from PAH/NiTsPc LbL films, suggesting that for NiTsPc the supramolecular nature of the film is only revealed in organized LbL multilayers.3,7,18 The profile of voltammograms of ITO/NiTsPc-modified electrode is consistent with adsorbed species onto the electrode surface.32 Remarkably, the current peak from the second redox process in the PAH/NiTsPc LbL films did not increase more than the first peak at high scan rates, indicating a charge-transport mechanism for reversible systems. The latter is strong evidence that PAH/NiTsPc LbL films exhibit distinct electrochemical properties at the supramolecular scale, which might be of interest in catalysis, since the electrochemical properties can be tuned. 4. Conclusion Nickel and iron phthalocyanines were immobilized in nanostructured LbL films with conventional polyelectrolytes. Both systems containing NiTsPc or FeTsPc displayed high electrochemical stability. Interestingly, the charge-transfer mechanism for ITO-PAH/NiTsPc electrodes was found to occur in two steps, which did not apply for the PAH/FeTsPc-modified electrodes, indicating that only a ring-based process occurs in the latter system. For comparison, when NiTsPc molecules are adsorbed on ITO in an electrochemical cell, the redox processes are similar to those for PAH/NiTsPc LbL films but with more negative E1/2 values, suggesting a lower stability than the LbL films. We also observed that the second redox process only occurs effectively at high scan rates when NiTsPc is immobilized in LbL films, pointing to a strong influence of the supramolecular effects on the charge-transport mechanism. It is envisaged that this procedure to obtain PAH/NiTsPc LbL films may find applications in polymeric composites for enzyme immobilization, heterogeneous catalysis, catalysis within metal clusters, and electrochromic and sensing devices. Acknowledgment. The authors acknowledge the financial support from FAPEPI, CNPq, CAPES, FAPESP, and Instituto do Mileˆnio (Brazil). Also, the authors are indebted to Professor A.B.P. Lever for his useful suggestions. Dedicated in memoriam to our colleague and friend Francisco C. Nart. References and Notes (1) Lehn, J. M. Rep. Prog. Phys. 2004, 67, 249-265.
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