pubs.acs.org/Langmuir © 2009 American Chemical Society
Luminescent Metal Complexes within Polyelectrolyte Layers: Tuning Electron and Energy Transfer† Lynn Dennany,*,‡,§ Gordon G. Wallace,‡ and Robert J. Forster§ ‡
Intelligent Polymer Research Institute and ARC Centre of Excellence for Electromaterials Science, AIIM Facility, Innovation Campus, University of Wollongong, Fairy Meadow, NSW 2519, Australia, and §Biomedical Diagnostics Institute, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland Received May 10, 2009. Revised Manuscript Received June 23, 2009 The electrochemical and photophysical properties of a luminescent metal center, [Os(bpy)3]2þ, are significantly modified by encapsulation within a conducting polymer composite film. Cyclic voltammetry reveals that the encapsulation in an inherently conducting polymer, polyaniline (Pani) or polypyrrole (PPy), can dramatically influence the charge-transfer rates between the metal centers. The increased electron transport, most likely mediated through the conducting polymer backbone, significantly enhances the electrochemiluminescence (ECL) efficiency. The increased communication between adjacent metal centers can also result in other interesting properties, such as photoinduced electron-transfer processes. In situ electron spin resonance (ESR) spectroscopy has been used to probe the photooxidation of an osmium metal center encapsulated in a PPy composite film. The irradiation of PPy in the presence of the osmium metal center resulted in the photo-oxidation of the Os2þ to Os3þ state and the consequent reduction of the PPy polyelectrolyte. The degree of communication between luminescent metal centers allows the composite properties to be tuned for various applications including ECL sensor devices and light-switching and light-harvesting systems.
Introduction Polyelectrolytes used to encapsulate luminescent metal complexes1-3 are attractive for spectroscopic-based sensors because of their synthetic flexibility, processability, high absorption coefficients, and relatively high fluorescence quantum yields. Polypyrroles and polyanilines containing coordinated metal complexes have been shown to form interfacial metallopolymer films with the π-conjugated backbone providing a rapid electron-transfer pathway between the metal complex and the electrode.4-7 Polyelectrolytes that show these unique luminescence properties are confined to those that contain π-conjugated backbones or have luminophore substituents. In contrast to the “light-producing” transduction strategies, these subclasses of polymers have recently received attention as components in high-performance luminescence quenching detection schemes.8,9 In this approach, one exploits the luminescent polyelectrolyte’s high absorption coefficients in combination with relatively high quantum yields of emission and the extraordinarily efficient quenching of their luminescence emission by small-molecule electron and energy acceptors. In these “superquenching” or even “hyperquenching” † Part of the “Langmuir 25th Year: Self-assembled polyelectrolyte multilayers: structure and function” special issue. *Corresponding author. Phone: þ61 2 4298 1428. Fax: þ61 2 4298 1499. E-mail:
[email protected].
(1) Forster, R. J.; Vos, J. G. J. Chem. Soc., Faraday Trans. 1991, 87, 1863–1867. (2) Doherty, A. P.; Forster, R. J.; Smyth, M. R.; Vos, J. G. Anal. Chim. Acta 1991, 255, 45–52. (3) Lyons, C. H.; Abbas, E. D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem. Soc. 1998, 120, 12100. (4) Ochmanska, J.; Pickup, P. G. J. Electroanal. Chem. 1989, 271, 83–105. (5) Pickup, P. G. J. Mater. Chem. 1999, 9, 1641–1653. (6) Cameron, C. G.; Pickup, P. G. Chem. Commun. 1997, 3, 303–304. (7) Cameron, C. G.; Pickup, P. G. J. Am. Chem. Soc. 1999, 121, 7710–7711. (8) Li, C.-Y.; Zhang, X.-B.; Jin, Z.; Han, R.; Shen, G.-L.; Yu, R.-Q. Anal. Chim. Acta 2006, 580, 143–148. (9) Fleming, C. N.; Maxwell, K. A.; DeSimone, J. M.; Meyer, T. J.; Papanikolas, J. M. J. Am. Chem. Soc. 2001, 123, 10336–10347.
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approaches, a single target molecule can quench more than one luminescent center.10 Apart from the analytical applications, the demands of achieving artificial photosynthesis have also inspired several studies of the photochemical electron and energy transfer of excited-state ruthenium centers in molecular assemblies. Many of these studies highlight the energy migration processes involved in the light-harvesting ruthenium “antenna” polymer. Electrochemiluminescence, ECL, represents a powerful analytical approach that combines simple equipment with inherent sensitivity, selectivity, and a wide linear dynamic range for aminecontaining analytes such as alkylamines, NADH, hydrazine, amino acids, biomolecules, and a variety of pharmaceutical compounds.11-15 ECL usually involves the reaction of electrogenerated species that react to form excited states, usually via an energetic redox reaction.15 Thus, ECL can also be utilized to probe electron- and energy-transfer processes at electrified interfaces.16,17 Consequently, increasing and improving the ECL efficiency could advance the sensitivity ranges and expand the dynamic range of current ECL systems. The encapsulation of the metal centers within a matrix that provides improved rates of electron transfer between adjacent centers and is protected from quenchers such as molecular oxygen can be advantageous. Another possible application that can exploit this unique property is within light-switching and light-harvesting devices. Photoionization and photoelectron transfer in heterogeneous systems have been extensively studied with respect to light-energy (10) Kwon, S.; Carson, J. H. Anal. Biochem. 1998, 264, 133–140. (11) Noffsinger, J. B.; Danielson, N. D. Anal. Chem. 1987, 59, 865–868. (12) Brune, S. N.; Bobbitt, D. R. Anal. Chem. 1992, 64, 166–170. (13) Knight, A. W.; Greenway, G. M. Analyst 1996, 121, 101R–106R. (14) Lee, W.-Y. Mikrochim. Acta 1997, 127, 19–39. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (16) Faulkner, L. R.; Bard, A. J. Electroanalytical Chemistry; Marcel Dekker: New York, 1977. (17) Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879–890.
Published on Web 07/14/2009
DOI: 10.1021/la901661v
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Article
storage systems and as photorewritable images utilizing polyaniline.18,19 Numerous systems involving ruthenium tris(bipyridyl), [Ru(bpy)3]2þ/3þ, complexes have been investigated for photoinduced charge-transfer reactions. The combination of conjugated polymers with these types of systems presents some intriguing possibilities. The presence of the metal center affects the polymer backbone properties via its redox state and a combination of steric and inductive effects and vice versa.20 The characterization of the electronic interactions between adjacent metal centers has been of particular interest. Recently, significant communication between the metal centers and the π-conjugated backbone of the conducting polymer containing a ruthenium moiety has been demonstrated.21 It has also been demonstrated that these metal centers can function as “electronic gates”, allowing charge to be inserted into the polymer at reduced potentials.22 In this article, we report on the two possible applications of an osmium metal center encapsulated within either a Pani or PPy composite film. The communication between the metal centers within the polyelectrolyte layers is improved by an order of magnitude over that of conventional metallopolymer systems. The impact of this improved charge transfer on possible applications is examined within this contribution. The effect of the Pani conducting polymer on the ECL response, charge transport, and photochemical properties of an osmium metal center are reported in this article. We also report on the photoinduced electron transfer between PPy and an osmium metal center within a surface-confined structure. Electrochemical and photochemical techniques and electron spin resonance spectroscopy (ESR) have been utilized to elucidate the mechanism of electron transfer between the metal center and the conducting polymer. The reversible photoswitching and enhanced ECL responses observed here for these composites highlight the considerable promise of these materials for applications in areas such as chemical sensors and light-switching and light-harvesting devices.22
Experimental Section Materials and Reagents. [Os(bpy)3]2þ was synthesized, pur-
ified, and characterized per a literature method.23,24 Metallopolymer [Os(bpy)2(PVP)10]2þ was synthesized and characterized as previously described, where bpy is 2,20 -bipyridyl and PVP is poly(4-vinylpyridine).23,25 Aniline and pyrrole monomers were purchased from Sigma-Aldrich and used as received. All other reagents used were of analytical grade, and all solutions were prepared in Milli-Q water (18 mΩ cm). Composite Synthesis. Encapsulation of the metal center into the polyelectrolyte composite film was performed by galvanostatic growth at a current density of 2 mA cm-2. Electrosynthesis was performed in 0.1 M HCl electrolyte that contained the monomer for PPy or Pani at 0.1 or 0.01 M , respectively, and 2 mM [Os(bpy)3]2þ at nearly neutral pH for PPy and at acidic pH for Pani. Films were electrodeposited onto Pt, glassy carbon, or ITO working electrodes. These modified electrodes were then washed with Milli-Q water and allowed to dry overnight prior to (18) Kobayashi, N.; Fukuda, N.; Kim, Y. J. Electroanal. Chem. 2001, 498, 216– 222. (19) Kim, Y.; Teshima, K.; Kobayashi, N. Electrochim. Acta 2000, 45, 1549– 1553. (20) Lafolet, F.; Genoud, F.; Divisia-Blohorn, B.; Aronica, C.; Guillerez, S. J. Phys. Chem. B 2005, 109, 12755–12761. (21) Dennany, L.; O’Reilly, E. J.; Innis, P. C.; Wallace, G. G.; Forster, R. J. Electrochim. Acta 2008, 53, 4599–4605. (22) Guillerez, S.; Kalaji, M.; Lafolet, F.; Novaes Tito, D. J. Electroanal. Chem. 2004, 563, 161–169. (23) Forster, R. J.; Vos, J. G. Macromolecules 1990, 23, 4372–4377. (24) Richter, M. M.; Brewer, K. J. Inorg. Chim. Acta 1991, 180, 125–131. (25) Dennany, L.; Forster, R. J.; White, B.; Smyth, M.; Rusling, J. F. J. Am. Chem. Soc. 2004, 126, 8835–8841.
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Dennany et al. analysis. Post-synthesis characterization was performed in 0.1 M H2SO4 solution unless otherwise stated. Surface coverages of the composite films (Γ) were determined by graphical integration of background-corrected cyclic voltammograms (
