Letter pubs.acs.org/JPCL
Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials David C. Bobela,† Barbara K. Hughes,† Wade A. Braunecker,† Travis W. Kemper,‡ Ross E. Larsen,‡ and Thomas Gennett*,† †
Chemical and Materials Science Center and ‡Computational Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *
ABSTRACT: The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly(2,2,6,6tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. Below 60% coverage, the polymer rearranges to accommodate larger radical−radical spacing. These findings are consistent with theoretical calculations and help explain some experimentally determined electron-transport properties.
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structural integrity and chemical stability.1,8 The chargetransport mechanism allowing these rapid transfer rates is not yet fully understood, but most likely, the facile transport is related to both the absence of bond breaking, and the extent of polymer structural reorganization during the redox processes. Therefore, this study investigates the three-dimensional molecular packing and resultant radical environment, as it is central to advance the understanding of electron transport through the polymer both in solution and solid state. The focus of this work was to elucidate possible structure− property relationships that can account for the rapid electrontransport in PTMA. Through our past theoretical work on PTMA, we demonstrated that in the solid state, the radicalpolymer forms an amorphous network with regions of localized radicals.9 In that work, we conducted an extensive experimental and theoretical investigation of the radical environment in a series of PTMA polymers.9,10 This past theoretical work focused on electron coupling between radicals within PTMA films and found that close contact between radicals on different polymer chains dominated the total electron diffusivity in the materials matrix.9 Through the theoretically computed film structures, it was predicted that the nitroxyl radical groups, regardless of whether they are on different or the same polymer, tend to approach each other in a “head-on” or “parallel” nitrogen−oxygen bond stack configuration at length
onconjugated, organic polymers with stable pendant radical groups have emerged as a unique class of electroactive materials for use in chemical or electronic devices traditionally dominated by conjugated conducting polymers or inorganic semiconductors.1,2 Of particular technological importance is the stable and reversible redox properties that allow the otherwise neutral radical polymers to easily convert to cation (positive) and anion (negative) rich materials in a way that is analogous to doping a semiconductor p or n-type. Unlike π-conjugated polymer or semiconductor counter-parts, these stable noncounjugated radical polymers contain highly localized unpaired electrons and rely on electron-hopping transport for moving charge through the polymer. The charge transport kinetics in these materials are surprisingly rapid and robust, even though the materials are not conductive. For example: the electron self-exchange rates of approximately 108 M−1 s1 have been reported for TEMPO/TEMPO+ (2,2,6,6-tetramethylpiperidine-N-oxyl) molecules in acetonitrile;3 the heterogeneous electron transfer rate at a TEMPO/platinum electrode-interface was found to be greater than 10−1 cm s−1 and the self-exchange rates are on the order 105 M−1 s−1 for thin films of cross-linked, TEMPO-substituted polynorbornene.4,5 Overall, these rates are comparable to transition metal ion redox reactions commonly employed in energy storage applications, thus allowing the exciting opportunity to implement these types of organic radical materials in electronic devices.6,7 Specifically, recent literature illustrates their significance in energy storage, that is, battery devices and supercapacitor-battery hybrids, where rapid charge and discharge cycles are achieved while maintaining © 2015 American Chemical Society
Received: February 6, 2015 Accepted: March 31, 2015 Published: March 31, 2015 1414
DOI: 10.1021/acs.jpclett.5b00259 J. Phys. Chem. Lett. 2015, 6, 1414−1419
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) Structures of PTMA-X, where X = 100n/(m + n). (b) Room temperature, X-band EPR spectra of PTMA-X and the stable radical 4oxo-TEMPO, in toluene. All solutions contain 5 mM of radical.
scales of approximately 5−6 Å, with rare close approaches of 4.5 Å. The relative orientation of the TEMPO groups as a function of separation was dictated by the inherent steric hindrance provided by corresponding methyl groups.9 Subsequently, recent space charge limited measurements aimed at measuring hole mobility were reported for PTMA;11 however, the thickness scaling and voltage dependence made it difficult to ascertain whether the device achieved space charge limited conditions.11 Nonetheless, the estimated hole mobilities showed only a weak dependence over a temperature range of 125−325 K, where one would expect some influence from either thermal motion of the polymer or the TEMPO moeity.12,13 This observation indicates that thermally induced changes to the polymer network, if any, negligibly impact the transport mechanism. Taken together, these results suggest that the electron environment is defined by a three dimensional and isotropic network, whereby the electron can be transferred from neutral radical site to an adjacent cation site during the generation of the oxoammonium cation. In this work, through the utilization of well-known electron paramagnetic resonance (EPR) methods previously developed for studying TEMPO “spin labeled” systems,14we have focused our efforts to definitively determine structural information about the radical environment of isolated oligomers. This investigation is centered on random co-oligomer systems having a fraction X of monomer units containing TEMPO group with the remaining fraction being ordinary methyl methacrylate groups. We call these oligomers PTMA-X systems and they can be considered as a “highly” spin labeled PMMA, where we attempt to alter the radical environment in order to establish the type/extent of radical−radical interactions by varying X (X = 100n/(m + n), see Figure 1a). In these experiments, we varied the percentage of TEMPO spin labeled
methyl-methacrylate sites from 10% to 100%. Hence, in our notation PTMA-100 represents the well-known case where all pendant sites contain a TEMPO radical, whereasfor instancein PTMA-20 a TEMPO radical occupies only one in five available sites. From the literature of sparsely spin labeled PMMA, the EPR spectrum from the TEMPO group exhibits hyperfine triplet structure from the radical-nitrogen coupling, and in some cases, resolved hyperfine interactions between the radical and surrounding methyl hydrogens.12 As the value of X increases, radical−radical interactions should increase due to the decreased average distance between radicals and, therefore, measurably change the EPR spectra. In order to obtain EPR spectra containing negligible contributions from interpolymer interactions, we prepared dilute solutions of PTMA-X samples in degassed toluene. All solutions and solid-state EPR samples were prepared from PTMA-X powders inside a helium filled inert atmosphere chamber (drybox) (see section S1 of Supporting Information). Representative spectra taken at room temperature for 5 mM (radical moiety concentration) solutions of radical polymers X = 100, 60, and 20 are shown in Figure 1a. Solutions were further diluted up to 100-fold, with no changes observed to the EPR spectra that could be attributed to interoligomer interactions.15 For reference, a spectrum for the 4-oxoTEMPO radical, at the same 5 mM concentration, is shown. The trend in these spectra as X increases is quite clear; the intensity of the TEMPO-like triplet decreases rapidly as the intensity of a Lorentzian-like component (seen most clearly in PTMA-60 and PTMA-100) increases. The vast majority of radicals in PTMA-100 and PTMA-60 contribute to the Lorentzian-component. The small TEMPO-like component in PTMA-100 ( 60, the radical−radical exchange coupling is maximal and insensitive to dispersion of the polymer in either toluene or acetonitrile. Moreover, this coupling does not change below 293 K and corresponds to a distance scale of about 5.7 Å in PTMA-100, where theoretically predicted electron−electron coupling is strong. The structural aspects obtained here help validate structural aspects of previous theoretical work aimed at explaining electron transport and provide a basis for explaining measurements of hole mobility. Perhaps most importantly for device applications, these findings challenge the paradigm that fully spin-labeled radical polymers are optimal with respect to transport properties because the radical network appears to be unchanged from 60% to 100%.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (303) 384-6628. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC36-08GO28308. Research was performed using resources sponsored by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, located at the National Renewable Energy Laboratory, and also facilities located at Colorado School of Mines. We thank Rex Rideout, Reuben Collins, and Craig Taylor for assistance with the EPR measurements.
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REFERENCES
(1) Oyaizu, K.; Nishide, H. Radical Polymers for Organic Electronic Devices: A Radical Departure from Conjugated Polymers? Adv. Mater. 2009, 21, 2339−2344. (2) Tomlinson, E. P.; Hay, M. E.; Boudouris, B. W. Radical Polymers and Their Application to Organic Electronic Devices. Macromolecules 2014, 47, 6145−6158. (3) Grampp, G.; Rasmussen, K. Solvent Dynamical Effects on the Electron Self-Exchange Rate of the TEMPO*/TEMPO+ Couple (TEMPO = 2,2,6,6-Tetramethyl-1-piperidinyloxy radical). Phys. Chem. Chem. Phys. 2002, 4, 5546−5549. (4) Oyaizu, K.; Ando, Y.; Konishi, H.; Nishide, H. Nernstian Adsorbate-Like Bulk Layer of Organic Radical Polymers for HighDensity Charge Storage Purposes. J. Am. Chem. Soc. 2008, 130, 14459. (5) Sullivan, M. G.; Murray, R. W. Solid-State Electron Self-Exchange Dynamics in Mixed-Valent Poly(Vinylferrocene) Films. J. Phys. Chem. 1994, 98, 4343−4351. (6) Suga, T.; Pu, Y. J.; Oyaizu, K.; Nishide, H. Electron-Transfer Kinetics of Nitroxide Radicals As an Electrode-Active Material. Bull. Chem. Soc. Jpn. 2004, 77, 2203−2204. (7) Satoh, M.; Nakahara, K.; Iriyama, J.; Iwasa, S.; Suguro, M. High Power Organic Radical Battery for Information Systems. Trans. Inst. Electron., Inf. Commun. Eng., Sect. E 2004, E87c, 2076−2080. (8) Vlad, A.; Singh, N.; Rolland, J.; Melinte, S.; Ajayan, P. M.; Gohy, J. F. Hybrid Supercapacitor-Battery Materials for Fast Electrochemical Charge Storage. Sci. Rep. 2014, 4, 4315. (9) Kemper, T. W.; Larsen, R. E.; Gennett, T. Relationship between Molecular Structure and Electron Transfer in a Polymeric NitroxylRadical Energy Storage Material. J. Phys. Chem. C 2014, 118, 17213− 17220. (10) Hughes, B. K.; Braunecker, W. A.; Ferguson, A. J.; Kemper, T. W.; Larsen, R. E.; Gennett, T. Quenching of the perylene fluorophore by stable nitroxide radical-containing macromolecules. J. Phys. Chem. B 2014, 118, 12541−8. (11) Baradwaj, A. G.; Rostro, L.; Alam, M. A.; Boudouris, B. W. Quantification of the Solid-State Charge Mobility in a Model Radical Polymer. Appl. Phys. Lett. 2014, 104, 213306. (12) Bullock, A. T.; Cameron, G. G.; Krajewski, V. Electron-Spin Resonance Studies of Spin-Labeled Polymers 0.11. Segmental and End-Group Mobility of Some Acrylic Ester Polymers. J. Phys. Chem. 1976, 80, 1792−1797. (13) Shiotani, M.; Sohma, J.; Freed, J. H. Anisotropic MolecularMotion of Spin-Labeled Poly(Methyl Methacrylate) Detected by Electron-Spin-Resonance. Macromolecules 1983, 16, 1495−1499. (14) Biological Magnetic Resonance; Plenum Press: New York, 1989; Vol. 8.
ASSOCIATED CONTENT
S Supporting Information *
Synthesis of samples and EPR sample preparation. Experimental conditions, analysis, and simulation of EPR spectra. Comparison of first-derivative EPR spectra of PTMA-100 with weight fractionated samples. Molecular dynamics simulations 1418
DOI: 10.1021/acs.jpclett.5b00259 J. Phys. Chem. Lett. 2015, 6, 1414−1419
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The Journal of Physical Chemistry Letters (15) Miller, T. A.; R. Spin-Spin and Electron-Exchange Rates of Radicals and Radical Ions. J. Am. Chem. Soc. 1966, 88, 5713−5714. (16) Bosman, A. W.; Janssen, R. A. J.; Miejer, E. W. Five Generations of Nitroxyl-Functionalized Dendrimers. Macromolecules 1997, 30, 3606−3611. (17) Chechik, V.; Wellsted, H. J.; Korte, A.; Gilbert, B. C.; Caldararu, H.; Ionita, P.; Caragheorgheopol, A. Spin-Labelled Au Nanoparticles. Faraday Discuss. R. Chem. Soc. 2004, 125. (18) Lloveras, V.; Badetti, E.; Chechik, V.; Vidal-Gancedo, J. Magnetic Interactions in Spin-Labeled Au Nanoparticles. J. Phys. Chem. B 2014, 118, 21622−21629. (19) Luckhurst, G. Alternating Linewidths. A Novel Relaxation Process in the Electron Resonance of Biradicals. Mol. Phys. 1966, 10, 543−550. (20) Parmon, V. Z.; G. Calculation of the E.S.R. Spectrum Shape of the Dynamic Biradical System. Mol. Phys. 1974, 27, 367−375. (21) Physical Properties of Polymers Handbook; American Institute of Physics: Woodbury, NY, 1996. (22) Eaton, S. S.; More, K. M.; Sawant, B. M.; Eaton, G. R. Use of the Electron-Paramagnetic-Res Half-Field Transition to Determine the Interspin Distance and the Orientation of the Interspin Vector in Systems with 2 Unpaired Electrons. J. Am. Chem. Soc. 1983, 105, 6560−6567. (23) Coffman, R. E.; Pezeshk, A. Analytical Considerations of Eatons Formula for the Interspin Distance between UnpairedE lectrons in Electron-Spin-Resonance. J. Magn. Reson. 1986, 70, 21−33.
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DOI: 10.1021/acs.jpclett.5b00259 J. Phys. Chem. Lett. 2015, 6, 1414−1419
