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Electrochemical studies of ferrocene and maleimide containing alternating copolymers
Journal of Electroanalytical Chemistry 786 (2017) 129–134
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochemical studies of ferrocene and maleimide containing alternating copolymers Ahmed Alzharani, Charles Ault, Esam Allehyani, Chris S. Hance, Raymond B. Westby, Benjamin O. Tayo, Charles J. Neef ⁎ Department of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 9 January 2017 Accepted 10 January 2017 Available online 12 January 2017 Keywords: Ferrocenophane Maleimide Copolymer Electrochemistry
a b s t r a c t Copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane with N-ethyl and N-phenylmaleimide were electrochemically characterized. The deposition method of the polymer onto the electrode by oxidative deposition or cast film was studied and showed that films produced by cast film exhibited a greater electrochemical response. The oxidation potentials of these materials were dependent on supporting electrolyte when using NaClO4, NaNO3 or phosphate buffered saline, varying from 0.46 to 0.53 for oxidative deposited films and 0.35 to 0.43 V for cast films. Also, multiple redox waves were observed in the cyclic voltammograms of these materials at a pH of 1. Molecular modeling showed a low energy conformation with the ferrocenyl moiety in close proximity to the maleimide. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Redox active polymers have received considerable attention within the past 25 years. The potential applications of redox polymers include electrochemical sensors [1], batteries [2], biosensors [3], photovoltaics [4], and biofuel cells [5]. To meet the requirements of these applications, redox polymers must be electrochemically stable and possess a high degree of redox material. Within redox polymers, a variety of polymer backbones and redox active materials have been utilized to meet these requirements. Poly(vinylpyridine) [3], poly(N-vinylimidazole) [6], poly(allylamine) [7], and poly(ethylenimine) [8] are included in the polymers reported for support of the redox mediator. Among the most popular redox mediators reported include ferrocene [9], osmium [10], and ruthenium [11]. Ferrocene polymers have received considerable attention due to their well-behaved reversible oxidation and redox stability. Due to these properties, a renewed interest in ferrocene has been observed in recent literature [12]. Strained ferrocenophanes have been extensively studied and polymerized by ring opening polymerization [13]. The versatility of ferrocene polymers can be seen in the tunability of their electronic, magnetic, and optical properties [14]. This tunability has also been observed pendant ferrocene polymers which change hydrophilicity upon oxidation [15]. With each approach, ferrocene has been shown to be an effective mediator for electron transfer and extremely versatile for a variety of applications. ⁎ Corresponding author. E-mail address: [email protected] (C.J. Neef).
http://dx.doi.org/10.1016/j.jelechem.2017.01.024 1572-6657/© 2017 Elsevier B.V. All rights reserved.
Although ferrocene polymers have been shown to be effective redox mediators, one of their limitations is low molecular weight, particularly in free radical polymerization. In addition to typical termination mechanisms, ferrocene monomers have an additional termination mechanism. Ferrocene can transfer an electron to the radical at the end of the growing chain, creating a zwitterion and stopping polymerization. To circumvent this problem, we recently reported the synthesis of copolymers from 3-phenyl[5]ferrocenophane-1,5-dimethylene or vinylferrocene with various N-substituted maleimides [16]. These copolymers were high molecular weight and amorphous films could be solution cast from typical solvents such as THF or CHCl3. Initial electrochemical studies on these materials revealed one oxidation potential. Compared to the ferrocenophane homopolymer [16] which showed two redox waves due to electronic communication between neighboring ferrocenyl moieties, one oxidation potential for the copolymers indicated an alternating copolymer which isolates the ferrocenyl moieties. The cyclic voltammograms in CH2Cl2 showed a greater Ipc than Ipa and Ipa scaled linearly with the scan rate which is characteristic of adsorption of the oxidized polymer to the electrode surface. In addition, oxidative electrodeposition from CH2Cl2 gave films with good redox activity in an aqueous NaCl solution making these materials good candidates as chemically modified electrodes (CMEs). In this paper, we report the electrochemical characterization of copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane-1,5dimethylene with N-ethyl and N-phenyl maleimide (Fig. 1). Previous results with these materials showed good electrochemical response in aqueous solutions, as well as, ease in preparation of the modified electrodes. In addition, the alternating distribution of the electron rich
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Fig. 1. Copolymers of vinylferrocene or 3-phenyl[5]ferrocenophane-1,5-dimethylene.
ferrocenyl moiety and the electron deficient maleimide creates an unusual motif along the polymer backbone. Due to these results, further characterization of these materials was warranted to determine their use in electrochemical applications. The electrolyte and pH effects on the materials, as well as, UV–Vis spectroscopy and molecular modeling were investigated. 2. Materials and methods All materials were commercially available and used as received unless otherwise stated. Vinylferrocene and 3-phenyl[5]ferrocenophane1,5-dimethylene were synthesized by a Wittig reaction according to a literature procedure [17]. Copolymers (1:1 M ratio) were also synthesized by a known procedure [15]. Polyvinyl ferrocene (PVFc) was prepared by free radical polymerization of vinylferrocene using AIBN as the initiator. Electrochemical measurements were carried out using a Gamry Interface 1000 potentiostat using a platinum working and counter electrodes with a Ag/AgCl reference electrode from Pine Research Instrumentation. Cyclic voltammetry experiments were carried out for three scans, unless otherwise stated, with the third scan shown in each figure to ensure reproducibility. Supporting electrolyte concentration were 100 mM, in all experiments. UV–Vis spectra were obtained with a Shimazu UV-1201 spectrophotometer with thin film UV spectra recorded on an ITO plate with a PET backing. All calculations were performed using the GAUSSIAN 09 software package [18]. Computational resources were provided by the National Energy Research Supercomputing Center [19]. Molecular geometries of the polymers were created using the Build Polymer tool in the Maestro program [20]. Optimized geometries were analyzed in Maestro and AVOGADRO [21]. The geometry was optimized at the B3LYP level of theory using the 6-31G + LANL2DZ mixed basis set that utilizes effective core potential on the Fe atom and the 6-31G double zeta Pople type basis set on all other atoms. The combination of the B3LYP hybrid-GGA (generalized gradient approximation) functional with the 631G + LANL2DZ mixed basis set provides a reasonable basis set size for performing geometry optimizations especially for systems with large number of atoms like polymers. The optimized geometries of all different conformations were calculated and the lowest energy geometry was used. Vibrational frequencies were computed for the optimized geometry to ensure that the converged geometry was indeed a local minimum.
tetrabutylammonium hexfluorophosphate (0.1 M) as the supporting electrolyte. The potential was stepped from 0.0 V to 1.0 V and maintained for 1 min. Since the polymers are not CH2Cl2 soluble in their oxidized state, they precipitated onto the surface of the electrode. The chemically modified electrodes (CMEs) were removed and allowed to air dry for 1–2 min. The CMEs were then placed into an aqueous solution of electrolyte (NaClO4, NaNO3, or PBS) followed by applying a potential of 0.0 V to reduce the polymer to its neutral form. For comparison, chemically modified electrodes were also prepared by solution cast films. A chloroform solution (2 μL) of the polymer (2 mg/mL) was directly cast onto the electrode followed by air drying for ca. 30 min. Cyclic voltammograms (CVs) of a CME prepared by oxidative deposition of PVFc-co-EMI with NaClO4, NaNO3, or PBS as the supporting electrolyte are shown in Fig. 2. Similar results were observed for the remaining polymers. The oxidation potentials (E1/2) of PVFc-co-EMI were 0.51, 0.53, and 0.46 V using NaClO4, NaNO3, or PBS, respectively, as the supporting electrolyte. The CVs using NaClO4 or NaNO3 showed a much greater Ipc than Ipa. In addition, the CV with PBS showed a greater separation between Ipc than Ipa than expected for an electrode bound species. These results indicate poor electrochemical response by copolymer deposited oxidatively. CVs of PVFc-co-EMI deposited by film casting are shown in Fig. 3. The oxidation potentials (E1/2) of PVFc-co-EMI were 0.35, 0.43, and 0.42 V using NaClO4, NaNO3, or PBS, respectively, as the supporting electrolyte and were lower than the potentials observed for oxidatively deposited films. With each supporting electrolyte, the polymer showed a good current response. Compared to films oxidatively deposited, the CVs with cast films showed peak currents for oxidation and reduction that were closer in magnitude and the separation of the peaks were more consistent with an electrode bound
3. Results and discussion 3.1. Electrolyte effects Deposition of copolymers onto a Pt electrode was carried out by two methods: oxidative deposition and cast film. Oxidative deposition was performed from CH2Cl2 solutions of the polymer (1.0 mmol) using
Fig. 2. Cyclic voltammograms of chemically modified electrode prepared by oxidative deposition of PVFc-co-EMI with aqueous solutions (0.1 M) of NaNO3, NaClO4, or phosphate buffered saline at a scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode.
A. Alzharani et al. / Journal of Electroanalytical Chemistry 786 (2017) 129–134
Fig. 3. Cyclic voltammagrams of chemically modified electrode prepared from a cast film of PVFc-co-EMI with aqueous solutions (0.1 M) of NaNO3, NaClO4, or phosphate buffered saline (PBS) as the supporting electrolyte at scan rate of 100 mV/s using a Pt counter electrodes and a Ag/AgCl reference electrode.
material. Even though this behavior has not been reported, changes in oxidation potentials have been observed in ferrocene polymers with various electrolytes. Polyvinylferrocene has exhibited a significant shift in oxidation potential from 0.25 to 0.53 V by changing the supporting electrolyte from NaClO4 to Na2SO4 [22]. This behavior has also been observed for self-assembled monolayers of ferrocene-terminated alkanethiol systems on a gold electrode [23]. These literature reports indicate that electrochemical conditions can have a significant effect and that the observed changes in oxidation potential in these materials could result due to changes in casting technique. Due to the more open morphology of the cast films, all subsequent studies were performed these films. 3.2. pH effects The effect of pH on the electrochemical response of the polymers was studied with NaClO4 as the supporting electrolyte and pH was varied to values of 1, 3, 5, and 7 using HClO4. As pH varied from 3 to 7, CMEs prepared via cast films showed a single redox potential. However, at a pH of 1, CMEs from PVFc-co-PMI, PPhFcP-co-EMI, and PPhFcP-co-PMI showed a second redox wave (Fig. 4). Multiple redox waves have been reported at a pH of 2.4 for copolymers from vinylferrocene and 4-vinylpyridine due to protonation of the pyridinyl moiety [24]. When the pyridinyl moiety is protonated, it becomes a strong electron acceptor and interacts with the electron rich ferrocene, disfavoring the
Fig. 4. Cyclic voltammagrams at a pH of 1 with chemically modified electrode prepared from a cast film of PVFc-co-EMI, PVFc-co-PMI, PPhFcP-co-EMI, or PPhFcP-co-PMI with aqueous NaClO4 as the supporting electrolyte at scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode.
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formation of the ferrocenium cation. By analogy, the maleimide moiety of the vinylferrocene-maleimide copolymers was protonated and affecting the redox potential of neighboring ferrocenyl moieties. These results were unexpected since the pKa of N-ethylsuccinimide and Nphenylsuccinimide are −1.4 and −0.3, respectively and these moieties should not become protonated at a pH of 1. The relative ease of protonation of the maleimide moiety suggests an interaction with the electron rich ferrocene resulting in a higher pKa than expected. Comparing the current in the CVs of the N-ethylmaleimide copolymers to the Nphenylmaleimide copolymers showed a greater current for the second redox wave of the for the N-phenylmaleimide copolymers. A greater current response was consistent with the higher pKa of Nphenylmaleimide facilitating more protonation and therefore a greater effect on neighboring ferrocenyl moieties. In contrast, the CV from PVFc-co-EMI showed minimal or lack of a second redox wave. While these results are not well understood, they may suggest that the polymer morphology does not favor protonation as observed for the previously described polymers. 3.3. Electrochemical stability In order to determine the potential of these materials in electrochemical applications, stability studies were performed. CMEs were prepared by solution cast films on to a Pt electrode as previously described. The CME was then placed in PBS and cycled from 0.0 to 0.8 V for 100 cycles. The cyclic voltammograms from 0.0 to 0.8 V for 100 cycles using PVFc-co-EMI on to a Pt electrode via film casting are shown in Fig. 5. Similar results were observed for the remaining polymers. For scan 1, the polymer showed an oxidation potential (E1/2) of 0.43 V with a good current response. With subsequent scans, a decrease in current response was observed with a broadening of the oxidation wave. In addition to the broadening of the oxidation wave, a significant decrease in the reduction potential from 0.38 to 0.12 V was observed. Part of the changes in the oxidation and reduction waves may result from the increase in hydration of the polymer which facilitates ion mobility. However, the significant decrease in reduction potential was not completely explained by an increase in hydration of the polymer, since the difference in oxidation and reduction peaks, 0.41 V, was much greater than anticipated for an immobilized film. The significant difference in oxidation and reduction peaks may indicate a weak charge transfer complex between the electron rich ferrocene and the electron deficient maleimide. When the polymer is in its neutral form, a weak charge transfer complex would raise the oxidation potential of the ferrocene and upon
Fig. 5. Cyclic voltammograms of chemically modified electrode prepared from a cast film of PVFc-co-EMI with a phosphate buffered saline (PBS) solution as the supporting electrolyte at a scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode. Scans 1, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 are shown.
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oxidation the charge transfer complex dissipates. The increase in hydration with subsequent scans may allow for limited mobility of the ferrocene within the film which allows the resulting ferricinium cation to rotate away from the maleimide moiety. Therefore, the reduction potential returns to that anticipated for a ferrocene moiety without interactions. For comparison, electrochemical stability studies were also conducted on a cast film of Poly(vinylferrocene) (PVFc) on Pt using PBS as the supporting electrolyte, Fig. 6. In this first CV scan, PVFc exhibited an oxidation potential of 0.17 V which was considerably less than the 0.46 V oxidation potential observed for PVFc-co-EMI. Subsequent scans with the CME from PVFc showed a decrease in current response indicating a loss of active material from the surface of the electrode. However, a shift in oxidation potential was not observed as was observed for PVFc-co-EMI. Comparing the stability studies of PVFc-co-EMI, which did not show a loss in current, to PVFc indicated that the maleimide moiety was having a significant effect on the polymer. The maleimide moiety was enhancing the electrochemical stability, as well as, having a significant effect on the oxidation potential.
3.4. UV–Vis To better understand the interactions between the electron rich ferrocene and the electron deficient maleimide, UV–Vis spectra were taken of the copolymers in solution, CH2Cl2, and as a thin film at a neutral pH and at a pH of 1. UV–Vis spectra with PPhFcP-co-PMI are shown in Fig. 7 and similar results were observed for the remaining polymers. In solution, PPhFcP-co-PMI showed an absorbance at 440 nm. This absorption was consistent with the homopolymer of 3-phenyl[5]ferrocenophane1,5-dimethylene and indicates the formation of a [3]ferrocenophane during polymerization [16]. As a thin film in water at a neutral pH, PPhFcP-co-PMI also exhibited an absorption at 440 nm. In addition to the absorption at 440 nm, the thin film exhibited a very broad, weak absorbance extending well into the visible region. A similar absorption has been observed for solutions of ferrocene and bis(arene)iron(II) dications which was attributed to a charge transfer complex [25]. For the thin film of PPhFcP-co-PMI at pH of 1, a slight shift in λmax to 449 nm was observed. In addition, the appearance of an additional absorbance was observed beginning at ca. 750 nm and extending well into the near IR region. A broad weak absorbance has been observed for the partially oxidized homopolymer of 3phenyl[5]ferrocenophane-1,5-dimethylene that was attributed to π and π* interactions. By analogy, the absorbance extending into the
Fig. 6. Cyclic voltammograms of chemically modified electrode prepared from a cast film of PVFc with a phosphate buffered saline (PBS) solution as the supporting electrolyte at scan rate of 100 mV/s using Pt working and counter electrodes and a Ag/AgCl reference electrode. Scans 1, 5, 10, and 20 are shown.
Fig. 7. UV–Vis Spectra of PPhFcP-co-PMI in solution (CHCl3), as a thin film in water at a neutral pH, and thin film at a pH of 1.
near-IR by PPhFcP-co-PMI can be attributed interactions between the electron rich ferrocene and the electron poor maleimide.
3.5. Molecular modeling Recently, density function theory (DFT) has become a powerful tool for predicting molecular properties such as equilibrium geometries, energy levels, heats of formation, ionization potentials, harmonic frequencies, bond energies, dipole moments and absorption spectra. While DFT is very successful for predicting molecular properties of small molecules very accurately, performing DFT on polymers is still very challenging due to the large number of atoms present. For polymers like PPhFcPco-PMI, the problem of reliably and accurately predicting molecular properties becomes even more complicated due to the presence of transition metal atoms with 3d electrons. In order to gain useful insights into the properties of these polymers, the oligomeric model was employed. This model which replaces a polymer with a finite length oligomer has proven to be very successful as calculated properties have been found to saturate with polymer length [26]. Furthermore to reduce computational time, instead of using an all-electron basis set, a mixed basis set that uses an all-electron basis set for non-transition metals and an effective core potential on the transition metal was utilized. Such an approach has proven to be successful in accurately predicting the molecular properties of systems containing transition metals at a reasonable computational resource expense [27]. In these studies, a trimer model of each polymer (n = 3) was used and as a case study, PVFc-co-PMI and PPhFcP-co-PMI were considered. All geometries were optimized to a local minimum and vibrational analysis was performed to ensure the absence of imaginary frequencies. For the PPHFcP-co-PMI, only one low energy conformation was observed upon optimization. For this conformation, the maleimide and ferrocenyl moieties were in close proximity, however, they were not completely aligned face-to-face (Fig. 8). Complete face-to-face alignment was prevented since the maleimide is unable to rotate orthogonal to the polymer backbone. However, the closest distance between theses moieties was 3.2 Å. This distance places the maleimide and ferrocenyl moieties in close proximity, suggesting that π-π interactions between the moieties were possible. This close proximity also supports the electrochemical and UV–Vis spectral results observed for this material. In contrast, two low energy conformations were observed for PVFc-co-PMI. One conformation places the ferrocenyl and maleimide moieties in close proximity, while the other conformation showed the ferrocenyl moiety rotated away from the maleimide moiety. Even though the distribution between the two conformations is unknown, the electrochemical and UV–Vis spectral results suggested that a sufficient number of
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Fig. 8. Ball and stick model of optimized geometry for PPhFcP-co-PMI using GAUSSIAN 09 software.
ferrocenyl and maleimide moieties are in close proximity to observe interactions between these moieties. 4. Conclusion Copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane with N-ethyl and N-phenylmaleimide have been electrochemically characterized for their use in electrochemical applications. The oxidation potential of these materials using NaClO4, NaNO3 or phosphate buffered saline as the supporting electrolyte showed a dependency on supporting electrolyte and this behavior for ferrocene polymers has been previously observed. The effects of pH on the oxidation potential were minimal at values from 3 to 7. However, at a pH of 1 multiple redox waves were observed, indicating electronic interactions between the ferrocenyl moiety and the maleimide moiety. These interactions were also observed in electrochemical stability studies and in the UV– Vis spectra. Electrochemical stability studies showed a significant shift in Epc with subsequent scans and in the UV–Vis spectra a broad absorbance was observed extending well into the visible region. Both of these studies were consistent with a weak charge transfer interaction between the ferrocene and maleimide moieties. Molecular modeling of these materials showed that the ferrocene and maleimide moieties are in close proximity, supporting the electrochemical and UV–Vis spectral results for the materials. Acknowledgements Gratitude is extended to Pittsburg State University for financial support of this project. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.01.024. References [1] J.W. Gallaway, S.A.C. Barton, Kinetics of redox polymer-mediated enzyme electrodes, J. Am. Chem. Soc. 130 (2008) 8527. [2] K. Tamura, N. Akutagawa, M. Satoh, J. Wada, T. Masuda, Charge/discharge properties of organometallic batteries fabricated with ferrocene–containing polymers, Macromol. Rapid Commun. 29 (2008) 1944. [3] P.P. Joshi, S.A. Merchant, Y. Wang, D.W. Schmidtke, Amperometric biosensors based on redox polymer-carbon nanotube-enzyme composites, Anal. Chem. 77 (2005) 3188.
[4] P.W. Cyr, M. Tzolov, I. Manners, E.H. Sargent, Photooxidation and photoconductivity of polyferrocenylsilane thin films, Macromol. Chem. Phys. 204 (2003) 915. [5] F. Giroud, C. Gondran, K. Gorgy, V. Vivier, S. Cosnier, An enzymatic biofuel cell based on electrically wired polyphenol oxidase and glucose oxidase operating under physiological conditions, Electrochim. Acta 85 (2012) 278. [6] Z. Gao, G. Binyamin, H.-H. Kim, S.C. Barton, Y. Zhang, A. Heller, Electrodeposition of redox polymers and Co-electrodeposition of enzymes by coordinative crosslinking, Angew. Chem. Int. Ed. 41 (2002) 810. [7] S. Zhang, Y. Fu, C. Sun, Fabrication of poly(allylamine)ferrocene monolayer based on electrostatic interaction and its electrocatalytic oxidation of ascorbic acid, Electroanalysis 15 (2003) 739. [8] S.A. Merchant, D.T. Glatzhofer, D.W. Schmidtke, Effects of electrolyte and pH on the behavior of cross-linked films of ferrocene-modified poly(ethylenimine), Langmuir 23 (2007) 11295. [9] V.S. Tripathi, V.B. Kandimalla, H. Ju, Amperometric biosensor for hydrogen peroxide based on ferrocene-bovine serum albumin and multiwall carbon nanotube modified ormosil composite, Biosens. Bioelectron. 21 (2006) 1529. [10] Q. Gao, Y. Guo, J. Liu, X. Yuan, H. Qi, C. Zhang, A biosensor prepared by co-entrapment of a glucose oxidase and a carbon nanotube within an electrochemically deposited redox polymer multilayer, Bioelectrochemistry 81 (2011) 109. [11] B. Xu, W.D. Zhang, Modification of vertically aligned carbon nanotubes with RuO2 for a solid-state pH sensor, Electrochim. Acta 55 (2010) 2859. [12] M. Gallei, The Renaissance of side-chain ferrocene-containing polymers: scope and limitations of vinylferrocene and ferrocenyl methacrylates, Macromol. Chem. Phys. 215 (2014) 699–704. [13] A.D. Russell, R.A. Musgrave, L.K. Stoll, P. Choi, H. Qiu, I. Manners, Recent developments with strained metallocenophanes, J. Organomet. Chem. 784 (2015) 24–30. [14] A.S. Abd-El-Aziz, C. Agatemor, N. Etkin, Sandwich complex-containing macromolecules: property tunability through versatile synthesis, Macromol. Rapid Commun. 35 (2014) 513–559. [15] J. Elbert, M. Gallei, C. Rüttiger, A. Brunsen, H. Didzoleit, B. Stühn, M. Rehahn, Ferrocene polymers for switchable surface wettability, Organometallics 32 (2013) 5873–5878. [16] J. Carberry, J.I. Irvin, D.T. Glatzhofer, K.M. Nicholas, C.J. Neef, High molecular weight copolymers of vinylferrocene and 3-phenyl[5]ferrocenophane-1,5-dimethylene with various N-substituted maleimides, React. Funct. Polym. 73 (2013) 730. [17] C.J. Neef, D.T. Glatzhofer, K.M. Nicholas, Cyclopolymerization of 3phenyl[5]ferrocenophane-1,5-dimethylene: synthesis and electronic properties of a poly(ferrocenophane), J. Polym. Sci. A Polym. Chem. 35 (1997) 3365. [18] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., GAUSSIAN 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2009. [19] This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 [20] Maestro, Version 10.0, Schrödinger, LLC, New York, NY, 2014. [21] M.D. Hanwell, D.E. Curtis, D.C. Lonie, T. Vandermeersch, E. Zurek, G.R. Hutchison, Avogadro: an advanced semantic chemical editor, visualization, and analysis platform, J. Cheminform. (2012). [22] G. Inzelt, L. Szabo, The effect of the nature and the concentration of counter anions on the electrochemistry of poly(vinylferrocene) polymer film electrodes, Electrochim. Acta 31 (1986) 1381. [23] G.K. Rowe, S.E. Creager, Redox and ion-pairing thermodynamics in self–assembled monolayers, Langmuir 7 (1991) 2307. [24] N. Kobayashi, F.C. Anson, Effects of adsorption and association with multiply charged counterions on the electrochemical responses of an electroactive polyelectrolyte, J. Electroanal. Chem. 421 (1997) 99.
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[25] R.E. Lehmann, J.K. Kochi, Structures and photoactivation of the charge-transfer complexes of bis(arene)iron(II) dications with ferrocene and arene donors, J. Am. Chem. Soc. 113 (1991) 501. [26] T.M. McCormick, C.R. Bridges, E.I. Carrera, P.M. DiCarmine, G.L. Gibson, J. Hollinger, L.M. Kozycz, D.S. Seferos, Conjugated polymers: evaluating DFT methods for more accurate orbital energy modeling, Macromolecules 46 (2013) 3879.
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochemical studies of ferrocene and maleimide containing alternating copolymers Ahmed Alzharani, Charles Ault, Esam Allehyani, Chris S. Hance, Raymond B. Westby, Benjamin O. Tayo, Charles J. Neef ⁎ Department of Chemistry, Pittsburg State University, Pittsburg, KS 66762, USA
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 9 January 2017 Accepted 10 January 2017 Available online 12 January 2017 Keywords: Ferrocenophane Maleimide Copolymer Electrochemistry
a b s t r a c t Copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane with N-ethyl and N-phenylmaleimide were electrochemically characterized. The deposition method of the polymer onto the electrode by oxidative deposition or cast film was studied and showed that films produced by cast film exhibited a greater electrochemical response. The oxidation potentials of these materials were dependent on supporting electrolyte when using NaClO4, NaNO3 or phosphate buffered saline, varying from 0.46 to 0.53 for oxidative deposited films and 0.35 to 0.43 V for cast films. Also, multiple redox waves were observed in the cyclic voltammograms of these materials at a pH of 1. Molecular modeling showed a low energy conformation with the ferrocenyl moiety in close proximity to the maleimide. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Redox active polymers have received considerable attention within the past 25 years. The potential applications of redox polymers include electrochemical sensors [1], batteries [2], biosensors [3], photovoltaics [4], and biofuel cells [5]. To meet the requirements of these applications, redox polymers must be electrochemically stable and possess a high degree of redox material. Within redox polymers, a variety of polymer backbones and redox active materials have been utilized to meet these requirements. Poly(vinylpyridine) [3], poly(N-vinylimidazole) [6], poly(allylamine) [7], and poly(ethylenimine) [8] are included in the polymers reported for support of the redox mediator. Among the most popular redox mediators reported include ferrocene [9], osmium [10], and ruthenium [11]. Ferrocene polymers have received considerable attention due to their well-behaved reversible oxidation and redox stability. Due to these properties, a renewed interest in ferrocene has been observed in recent literature [12]. Strained ferrocenophanes have been extensively studied and polymerized by ring opening polymerization [13]. The versatility of ferrocene polymers can be seen in the tunability of their electronic, magnetic, and optical properties [14]. This tunability has also been observed pendant ferrocene polymers which change hydrophilicity upon oxidation [15]. With each approach, ferrocene has been shown to be an effective mediator for electron transfer and extremely versatile for a variety of applications. ⁎ Corresponding author. E-mail address: [email protected] (C.J. Neef).
http://dx.doi.org/10.1016/j.jelechem.2017.01.024 1572-6657/© 2017 Elsevier B.V. All rights reserved.
Although ferrocene polymers have been shown to be effective redox mediators, one of their limitations is low molecular weight, particularly in free radical polymerization. In addition to typical termination mechanisms, ferrocene monomers have an additional termination mechanism. Ferrocene can transfer an electron to the radical at the end of the growing chain, creating a zwitterion and stopping polymerization. To circumvent this problem, we recently reported the synthesis of copolymers from 3-phenyl[5]ferrocenophane-1,5-dimethylene or vinylferrocene with various N-substituted maleimides [16]. These copolymers were high molecular weight and amorphous films could be solution cast from typical solvents such as THF or CHCl3. Initial electrochemical studies on these materials revealed one oxidation potential. Compared to the ferrocenophane homopolymer [16] which showed two redox waves due to electronic communication between neighboring ferrocenyl moieties, one oxidation potential for the copolymers indicated an alternating copolymer which isolates the ferrocenyl moieties. The cyclic voltammograms in CH2Cl2 showed a greater Ipc than Ipa and Ipa scaled linearly with the scan rate which is characteristic of adsorption of the oxidized polymer to the electrode surface. In addition, oxidative electrodeposition from CH2Cl2 gave films with good redox activity in an aqueous NaCl solution making these materials good candidates as chemically modified electrodes (CMEs). In this paper, we report the electrochemical characterization of copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane-1,5dimethylene with N-ethyl and N-phenyl maleimide (Fig. 1). Previous results with these materials showed good electrochemical response in aqueous solutions, as well as, ease in preparation of the modified electrodes. In addition, the alternating distribution of the electron rich
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Fig. 1. Copolymers of vinylferrocene or 3-phenyl[5]ferrocenophane-1,5-dimethylene.
ferrocenyl moiety and the electron deficient maleimide creates an unusual motif along the polymer backbone. Due to these results, further characterization of these materials was warranted to determine their use in electrochemical applications. The electrolyte and pH effects on the materials, as well as, UV–Vis spectroscopy and molecular modeling were investigated. 2. Materials and methods All materials were commercially available and used as received unless otherwise stated. Vinylferrocene and 3-phenyl[5]ferrocenophane1,5-dimethylene were synthesized by a Wittig reaction according to a literature procedure [17]. Copolymers (1:1 M ratio) were also synthesized by a known procedure [15]. Polyvinyl ferrocene (PVFc) was prepared by free radical polymerization of vinylferrocene using AIBN as the initiator. Electrochemical measurements were carried out using a Gamry Interface 1000 potentiostat using a platinum working and counter electrodes with a Ag/AgCl reference electrode from Pine Research Instrumentation. Cyclic voltammetry experiments were carried out for three scans, unless otherwise stated, with the third scan shown in each figure to ensure reproducibility. Supporting electrolyte concentration were 100 mM, in all experiments. UV–Vis spectra were obtained with a Shimazu UV-1201 spectrophotometer with thin film UV spectra recorded on an ITO plate with a PET backing. All calculations were performed using the GAUSSIAN 09 software package [18]. Computational resources were provided by the National Energy Research Supercomputing Center [19]. Molecular geometries of the polymers were created using the Build Polymer tool in the Maestro program [20]. Optimized geometries were analyzed in Maestro and AVOGADRO [21]. The geometry was optimized at the B3LYP level of theory using the 6-31G + LANL2DZ mixed basis set that utilizes effective core potential on the Fe atom and the 6-31G double zeta Pople type basis set on all other atoms. The combination of the B3LYP hybrid-GGA (generalized gradient approximation) functional with the 631G + LANL2DZ mixed basis set provides a reasonable basis set size for performing geometry optimizations especially for systems with large number of atoms like polymers. The optimized geometries of all different conformations were calculated and the lowest energy geometry was used. Vibrational frequencies were computed for the optimized geometry to ensure that the converged geometry was indeed a local minimum.
tetrabutylammonium hexfluorophosphate (0.1 M) as the supporting electrolyte. The potential was stepped from 0.0 V to 1.0 V and maintained for 1 min. Since the polymers are not CH2Cl2 soluble in their oxidized state, they precipitated onto the surface of the electrode. The chemically modified electrodes (CMEs) were removed and allowed to air dry for 1–2 min. The CMEs were then placed into an aqueous solution of electrolyte (NaClO4, NaNO3, or PBS) followed by applying a potential of 0.0 V to reduce the polymer to its neutral form. For comparison, chemically modified electrodes were also prepared by solution cast films. A chloroform solution (2 μL) of the polymer (2 mg/mL) was directly cast onto the electrode followed by air drying for ca. 30 min. Cyclic voltammograms (CVs) of a CME prepared by oxidative deposition of PVFc-co-EMI with NaClO4, NaNO3, or PBS as the supporting electrolyte are shown in Fig. 2. Similar results were observed for the remaining polymers. The oxidation potentials (E1/2) of PVFc-co-EMI were 0.51, 0.53, and 0.46 V using NaClO4, NaNO3, or PBS, respectively, as the supporting electrolyte. The CVs using NaClO4 or NaNO3 showed a much greater Ipc than Ipa. In addition, the CV with PBS showed a greater separation between Ipc than Ipa than expected for an electrode bound species. These results indicate poor electrochemical response by copolymer deposited oxidatively. CVs of PVFc-co-EMI deposited by film casting are shown in Fig. 3. The oxidation potentials (E1/2) of PVFc-co-EMI were 0.35, 0.43, and 0.42 V using NaClO4, NaNO3, or PBS, respectively, as the supporting electrolyte and were lower than the potentials observed for oxidatively deposited films. With each supporting electrolyte, the polymer showed a good current response. Compared to films oxidatively deposited, the CVs with cast films showed peak currents for oxidation and reduction that were closer in magnitude and the separation of the peaks were more consistent with an electrode bound
3. Results and discussion 3.1. Electrolyte effects Deposition of copolymers onto a Pt electrode was carried out by two methods: oxidative deposition and cast film. Oxidative deposition was performed from CH2Cl2 solutions of the polymer (1.0 mmol) using
Fig. 2. Cyclic voltammograms of chemically modified electrode prepared by oxidative deposition of PVFc-co-EMI with aqueous solutions (0.1 M) of NaNO3, NaClO4, or phosphate buffered saline at a scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode.
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Fig. 3. Cyclic voltammagrams of chemically modified electrode prepared from a cast film of PVFc-co-EMI with aqueous solutions (0.1 M) of NaNO3, NaClO4, or phosphate buffered saline (PBS) as the supporting electrolyte at scan rate of 100 mV/s using a Pt counter electrodes and a Ag/AgCl reference electrode.
material. Even though this behavior has not been reported, changes in oxidation potentials have been observed in ferrocene polymers with various electrolytes. Polyvinylferrocene has exhibited a significant shift in oxidation potential from 0.25 to 0.53 V by changing the supporting electrolyte from NaClO4 to Na2SO4 [22]. This behavior has also been observed for self-assembled monolayers of ferrocene-terminated alkanethiol systems on a gold electrode [23]. These literature reports indicate that electrochemical conditions can have a significant effect and that the observed changes in oxidation potential in these materials could result due to changes in casting technique. Due to the more open morphology of the cast films, all subsequent studies were performed these films. 3.2. pH effects The effect of pH on the electrochemical response of the polymers was studied with NaClO4 as the supporting electrolyte and pH was varied to values of 1, 3, 5, and 7 using HClO4. As pH varied from 3 to 7, CMEs prepared via cast films showed a single redox potential. However, at a pH of 1, CMEs from PVFc-co-PMI, PPhFcP-co-EMI, and PPhFcP-co-PMI showed a second redox wave (Fig. 4). Multiple redox waves have been reported at a pH of 2.4 for copolymers from vinylferrocene and 4-vinylpyridine due to protonation of the pyridinyl moiety [24]. When the pyridinyl moiety is protonated, it becomes a strong electron acceptor and interacts with the electron rich ferrocene, disfavoring the
Fig. 4. Cyclic voltammagrams at a pH of 1 with chemically modified electrode prepared from a cast film of PVFc-co-EMI, PVFc-co-PMI, PPhFcP-co-EMI, or PPhFcP-co-PMI with aqueous NaClO4 as the supporting electrolyte at scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode.
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formation of the ferrocenium cation. By analogy, the maleimide moiety of the vinylferrocene-maleimide copolymers was protonated and affecting the redox potential of neighboring ferrocenyl moieties. These results were unexpected since the pKa of N-ethylsuccinimide and Nphenylsuccinimide are −1.4 and −0.3, respectively and these moieties should not become protonated at a pH of 1. The relative ease of protonation of the maleimide moiety suggests an interaction with the electron rich ferrocene resulting in a higher pKa than expected. Comparing the current in the CVs of the N-ethylmaleimide copolymers to the Nphenylmaleimide copolymers showed a greater current for the second redox wave of the for the N-phenylmaleimide copolymers. A greater current response was consistent with the higher pKa of Nphenylmaleimide facilitating more protonation and therefore a greater effect on neighboring ferrocenyl moieties. In contrast, the CV from PVFc-co-EMI showed minimal or lack of a second redox wave. While these results are not well understood, they may suggest that the polymer morphology does not favor protonation as observed for the previously described polymers. 3.3. Electrochemical stability In order to determine the potential of these materials in electrochemical applications, stability studies were performed. CMEs were prepared by solution cast films on to a Pt electrode as previously described. The CME was then placed in PBS and cycled from 0.0 to 0.8 V for 100 cycles. The cyclic voltammograms from 0.0 to 0.8 V for 100 cycles using PVFc-co-EMI on to a Pt electrode via film casting are shown in Fig. 5. Similar results were observed for the remaining polymers. For scan 1, the polymer showed an oxidation potential (E1/2) of 0.43 V with a good current response. With subsequent scans, a decrease in current response was observed with a broadening of the oxidation wave. In addition to the broadening of the oxidation wave, a significant decrease in the reduction potential from 0.38 to 0.12 V was observed. Part of the changes in the oxidation and reduction waves may result from the increase in hydration of the polymer which facilitates ion mobility. However, the significant decrease in reduction potential was not completely explained by an increase in hydration of the polymer, since the difference in oxidation and reduction peaks, 0.41 V, was much greater than anticipated for an immobilized film. The significant difference in oxidation and reduction peaks may indicate a weak charge transfer complex between the electron rich ferrocene and the electron deficient maleimide. When the polymer is in its neutral form, a weak charge transfer complex would raise the oxidation potential of the ferrocene and upon
Fig. 5. Cyclic voltammograms of chemically modified electrode prepared from a cast film of PVFc-co-EMI with a phosphate buffered saline (PBS) solution as the supporting electrolyte at a scan rate of 100 mV/s using a Pt counter electrode and a Ag/AgCl reference electrode. Scans 1, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 are shown.
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oxidation the charge transfer complex dissipates. The increase in hydration with subsequent scans may allow for limited mobility of the ferrocene within the film which allows the resulting ferricinium cation to rotate away from the maleimide moiety. Therefore, the reduction potential returns to that anticipated for a ferrocene moiety without interactions. For comparison, electrochemical stability studies were also conducted on a cast film of Poly(vinylferrocene) (PVFc) on Pt using PBS as the supporting electrolyte, Fig. 6. In this first CV scan, PVFc exhibited an oxidation potential of 0.17 V which was considerably less than the 0.46 V oxidation potential observed for PVFc-co-EMI. Subsequent scans with the CME from PVFc showed a decrease in current response indicating a loss of active material from the surface of the electrode. However, a shift in oxidation potential was not observed as was observed for PVFc-co-EMI. Comparing the stability studies of PVFc-co-EMI, which did not show a loss in current, to PVFc indicated that the maleimide moiety was having a significant effect on the polymer. The maleimide moiety was enhancing the electrochemical stability, as well as, having a significant effect on the oxidation potential.
3.4. UV–Vis To better understand the interactions between the electron rich ferrocene and the electron deficient maleimide, UV–Vis spectra were taken of the copolymers in solution, CH2Cl2, and as a thin film at a neutral pH and at a pH of 1. UV–Vis spectra with PPhFcP-co-PMI are shown in Fig. 7 and similar results were observed for the remaining polymers. In solution, PPhFcP-co-PMI showed an absorbance at 440 nm. This absorption was consistent with the homopolymer of 3-phenyl[5]ferrocenophane1,5-dimethylene and indicates the formation of a [3]ferrocenophane during polymerization [16]. As a thin film in water at a neutral pH, PPhFcP-co-PMI also exhibited an absorption at 440 nm. In addition to the absorption at 440 nm, the thin film exhibited a very broad, weak absorbance extending well into the visible region. A similar absorption has been observed for solutions of ferrocene and bis(arene)iron(II) dications which was attributed to a charge transfer complex [25]. For the thin film of PPhFcP-co-PMI at pH of 1, a slight shift in λmax to 449 nm was observed. In addition, the appearance of an additional absorbance was observed beginning at ca. 750 nm and extending well into the near IR region. A broad weak absorbance has been observed for the partially oxidized homopolymer of 3phenyl[5]ferrocenophane-1,5-dimethylene that was attributed to π and π* interactions. By analogy, the absorbance extending into the
Fig. 6. Cyclic voltammograms of chemically modified electrode prepared from a cast film of PVFc with a phosphate buffered saline (PBS) solution as the supporting electrolyte at scan rate of 100 mV/s using Pt working and counter electrodes and a Ag/AgCl reference electrode. Scans 1, 5, 10, and 20 are shown.
Fig. 7. UV–Vis Spectra of PPhFcP-co-PMI in solution (CHCl3), as a thin film in water at a neutral pH, and thin film at a pH of 1.
near-IR by PPhFcP-co-PMI can be attributed interactions between the electron rich ferrocene and the electron poor maleimide.
3.5. Molecular modeling Recently, density function theory (DFT) has become a powerful tool for predicting molecular properties such as equilibrium geometries, energy levels, heats of formation, ionization potentials, harmonic frequencies, bond energies, dipole moments and absorption spectra. While DFT is very successful for predicting molecular properties of small molecules very accurately, performing DFT on polymers is still very challenging due to the large number of atoms present. For polymers like PPhFcPco-PMI, the problem of reliably and accurately predicting molecular properties becomes even more complicated due to the presence of transition metal atoms with 3d electrons. In order to gain useful insights into the properties of these polymers, the oligomeric model was employed. This model which replaces a polymer with a finite length oligomer has proven to be very successful as calculated properties have been found to saturate with polymer length [26]. Furthermore to reduce computational time, instead of using an all-electron basis set, a mixed basis set that uses an all-electron basis set for non-transition metals and an effective core potential on the transition metal was utilized. Such an approach has proven to be successful in accurately predicting the molecular properties of systems containing transition metals at a reasonable computational resource expense [27]. In these studies, a trimer model of each polymer (n = 3) was used and as a case study, PVFc-co-PMI and PPhFcP-co-PMI were considered. All geometries were optimized to a local minimum and vibrational analysis was performed to ensure the absence of imaginary frequencies. For the PPHFcP-co-PMI, only one low energy conformation was observed upon optimization. For this conformation, the maleimide and ferrocenyl moieties were in close proximity, however, they were not completely aligned face-to-face (Fig. 8). Complete face-to-face alignment was prevented since the maleimide is unable to rotate orthogonal to the polymer backbone. However, the closest distance between theses moieties was 3.2 Å. This distance places the maleimide and ferrocenyl moieties in close proximity, suggesting that π-π interactions between the moieties were possible. This close proximity also supports the electrochemical and UV–Vis spectral results observed for this material. In contrast, two low energy conformations were observed for PVFc-co-PMI. One conformation places the ferrocenyl and maleimide moieties in close proximity, while the other conformation showed the ferrocenyl moiety rotated away from the maleimide moiety. Even though the distribution between the two conformations is unknown, the electrochemical and UV–Vis spectral results suggested that a sufficient number of
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Fig. 8. Ball and stick model of optimized geometry for PPhFcP-co-PMI using GAUSSIAN 09 software.
ferrocenyl and maleimide moieties are in close proximity to observe interactions between these moieties. 4. Conclusion Copolymers from vinylferrocene and 3-phenyl[5]ferrocenophane with N-ethyl and N-phenylmaleimide have been electrochemically characterized for their use in electrochemical applications. The oxidation potential of these materials using NaClO4, NaNO3 or phosphate buffered saline as the supporting electrolyte showed a dependency on supporting electrolyte and this behavior for ferrocene polymers has been previously observed. The effects of pH on the oxidation potential were minimal at values from 3 to 7. However, at a pH of 1 multiple redox waves were observed, indicating electronic interactions between the ferrocenyl moiety and the maleimide moiety. These interactions were also observed in electrochemical stability studies and in the UV– Vis spectra. Electrochemical stability studies showed a significant shift in Epc with subsequent scans and in the UV–Vis spectra a broad absorbance was observed extending well into the visible region. Both of these studies were consistent with a weak charge transfer interaction between the ferrocene and maleimide moieties. Molecular modeling of these materials showed that the ferrocene and maleimide moieties are in close proximity, supporting the electrochemical and UV–Vis spectral results for the materials. Acknowledgements Gratitude is extended to Pittsburg State University for financial support of this project. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.01.024. References [1] J.W. Gallaway, S.A.C. Barton, Kinetics of redox polymer-mediated enzyme electrodes, J. Am. Chem. Soc. 130 (2008) 8527. [2] K. Tamura, N. Akutagawa, M. Satoh, J. Wada, T. Masuda, Charge/discharge properties of organometallic batteries fabricated with ferrocene–containing polymers, Macromol. Rapid Commun. 29 (2008) 1944. [3] P.P. Joshi, S.A. Merchant, Y. Wang, D.W. Schmidtke, Amperometric biosensors based on redox polymer-carbon nanotube-enzyme composites, Anal. Chem. 77 (2005) 3188.
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