Note pubs.acs.org/Macromolecules
Densely Functionalized Pendant Oligoaniline Bearing Poly(oxanorbornenes): Synthesis and Electronic Properties Danming Chao,*,†,‡ Shutao Wang,‡ Bryan T. Tuten,† Justin P. Cole,† and Erik B. Berda*,† †
Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, United States ‡ Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *
O
ligoanilines1 are an interesting class of compounds, attractive as a means to impart electroactivity and stimuli responsiveness into a wide array of polymeric scaffolds. As model compounds of polyaniline (PANI), oligoaniline containing copolymers feature superior solubility, processability, and tunability when compared to their parent homopolymer, while retaining much of PANI’s functional capabilities.2 A recent area of effort in our laboratories has focused on polymers carrying aniline tetramer (AT) pendants.2c,3 By exploiting the characteristics inherent in the AT and enhancing these abilities through synergistic interplay with other functional groups, we have synthesized and tested materials promising for applications in multicolor electrochromic devices2c,3c,e and fluorescent sensors.3a,b,f Although the results presented in these reports are encouraging, there remains room for improvement in the performance of these materials. In all cases, the AT incorporation was at best 50 wt %. We surmised that utilizing a scaffold in which we could substantially increase the amount of AT present would yield materials with superior performance. Scheme 1 outlines the synthetic strategy we adopted to these ends. Ring-opening metathesis polymerization (ROMP) of a
leucoemeradine base (LEB)1a to yield mono-AT functionalized monomer 2. This was then reacted with a second equivalent of AT under DIC coupling conditions to afford the difunctionalized monomer 3. Attempts to ROMP this monomer directly were unsuccessful. We suspect this is due to the interaction of the nitrogen lone pairs of the AT in LEB with the ruthenium center of the polymerization catalyst. To alleviate this, we reacted 3 with BOC anhydride, yielding the octa-BOC protected monomer 4. Polymerization of 4 proceeded smoothly to high molecular weight when initiated using Grubbs’ catalyst (second generation). Deprotection with TFA results in the final AT functionalized poly(oxanorbornene) 5 (Mw = 47 600 kDa, PDI = 1.38). An overlay of NMR spectra showing the progression from monomer 2 through polymer 5 is shown in Figure 1. Polymer 5 is soluble in a variety of common polar solvents including THF, NMP, DMAc, and DMSO. Further, it shows good thermal stability (degradation beginning at 300 °C, 5% mass loss at 369 °C by TGA, see Supporting Information). Simple synthesis, high solubility, and good thermal stability suggest a class of materials based on this motif could be attractive in a number of applications. We next evaluated the spectral properties of 5 in DMAc solution. Trace amounts of (NH4)2S2O8 were added to the solution, and the oxidation of the polymer was monitored by UV−vis spectra collected at appropriate intervals (Figure 2). First, one absorption peak at 320 nm is observed, which is assigned to the π−π* transitions in the benzoid rings.5 As oxidation occurs, the absorption peak at 320 nm undergoes a blue-shift (from 320 to 306 nm) while decreasing in intensity. This is concomitant with the appearance of a new absorption peak at 580 nm, attributed to the exciton-type transition between the HOMO orbital of the benzoid ring and the LUMO orbital of the quinoid ring.5 The absorption peak of the exciton-type transition increased in intensity until reaching a maximum, indicating the oligoaniline segment was in the emeraldine base (EB), with the AT segment containing one quinoid ring. With further oxidation, both absorption peaks decreased in intensity. The peak at 580 nm proceeded to redshift to a maximum of 645 nm, after which it remained unchanged indicating full oxidation to the pernigraniline base (PNB) with the AT segment containing two quinoid rings.
Scheme 1. Polymer Synthesisa
a
Reagents and conditions: (i) aniline tetramer/THF; (ii) aniline tetramer, DIC, DMAP/THF; (iii) BOC anhydride, DMAP/THF; (iv) a, Grubbs’ catalyst (2nd gen)/THF; b, TFA.
difunctionalized oxanorbornene derivative was an obvious choice; ROMP of (oxa)norbornenes proceeds rapidly and efficiently even in the case of highly functionalized or sterically demanding monomers.4 This permits the incorporation of two aniline tetramers into every repeating unit. First, the exo-oxanorbornene anhydride 1 was functionalized via simple nucleophilic substitution using AT in the © XXXX American Chemical Society
Received: July 1, 2015 Revised: July 6, 2015
A
DOI: 10.1021/acs.macromol.5b01446 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules
Figure 2. UV−vis spectra monitoring the chemical oxidation of polymer 5 (A) from the leucoemeraldine base to the emeraldine base and (B) from the emeraldine base to the pernigraniline base.
transmittance change with an optical contrast value of 55% (85% (at 0 V) ↔ 30% (at 0.8 V)) at 700 nm. The color of the film changed drastically from gray (at 0.2 V), to green-yellow (at 0.4 V), to aquamarine blue (at 0.6 V), and finally to dark blue (at ca. 0.8 V). Electrochromic switching studies were performed by spectrochronoamperometry to monitor changes in the optical contrast at 700 nm during repeated potential stepping between reduced state (0 V) and oxidized state at 0.8 V with a residence time of 60 s. Results from the first seven cycles are presented in Figures 3D and 3E. Some important evaluation parameters, such as switching time and coloration efficiency, were deduced from these data. Switching time in this case is the time required to bring 5 to its most reduced state from its most oxidized state or vice versa. It is defined here as the time required for reaching 90% of the full change in the coloring/bleaching process. The film revealed a switching time of 12 s at 0.8 V for the coloring process at 700 nm and 9 s for bleaching (0 V). Coloration efficiency CE (η = ΔOD/Q) is generally used to measure the power requirements of an electrochromic material. It was calculated by monitoring the amount of ejected charge (Q) as a function of the change in optical density (ΔOD) of the polymer film. Polymer 5 exhibited a high CE up to 95.6 cm2/C (at 700 nm). In summary, we have demonstrated that ROMP is a facile route to poly(oxanorbornenes) densely functionalized with pendant aniline tetramers. The spectral properties of this polymer are consistent with other copolymers bearing AT; it is thermally stable and soluble in a variety of solvents and exhibits
Figure 1. NMR overlay showing the progression from monomer 2 to polymer 5 (see Supporting Information for full peak assignments).
The electrochemical behavior of 5 was studied by cyclic voltammetry (CV); results are shown in Figure 3A. A film of 5 was spin-coated onto an ITO electrode from a DMAc solution; CV was performed in 1.0 M H2SO4 at different potential scan rates from 10 to 100 mV s−1. Under these conditions, the CV exhibited two pairs of redox current peaks at 380 mV/300 mV and 600 mV/540 mV, which were respectively assigned to the leucoemeraldine base (LEB)/emeraldine base (EB) transition and the emeraldine base (EB)/pernigraniline base (PNB) transition (Figure 3B). A linear dependence of the peak currents as a function of scan rate in the region of 10−100 mV/ s (inset of Figure 3A) confirmed both a surface controlled process and a well-adhered electroactive polymer film. Furthermore, the film was very stable with virtually no change in its cyclic voltammogram after 100 cycles between 0 and 1000 mV. Spectroelectrochemical measurements were performed on an ITO electrode coated with 5 in 0.5 M H2SO4 with applied potentials of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 V (vs Ag/AgCl), in conjunction with the acquisition of UV−vis spectral data (Figure 3C). The optical contrast value (%ΔT) was obtained from the transmittance difference at 700 nm between its colored (oxidized) and bleached (reduced) states. The polymer 5/ITO electrode exhibited good contrast and a high optical B
DOI: 10.1021/acs.macromol.5b01446 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules
Figure 3. (A) Cyclic voltammograms of the polymer 5 electrode in 1.0 M H2SO4 at different potential scan rates 10−100 mV s−1. Inset shows the relationships between the oxidation peaks and reduction current vs potential scan rate. (B) Molecular structures of AT pendants at various oxidation states. (C) Optical transmittance of polymer 5/ITO electrodes (0.6 × 3 cm2) in 0.5 M H2SO4 at different potentials. Inset shows photographs of polymer 5/ITO electrode at different potentials. (D) Current consumption and (E) absorbance changes monitored at 700 nm of polymer 5/ITO electrode for the first seven cycles.
■
good film forming qualities. The electrochromic performance of this material reveals good optical contrast, reasonable switching times, good coloration efficiency, and access to multiple color states. Although not a suitable candidate for modern electrochromic devices, this material shows that protected oligoanilines can be readily incorporated at high functional group densities into high polymers via ROMP, then subsequently deprotected, and can retain their solution and bulk electroactive properties. These attributes could be useful for a number of applications; our laboratories are currently investigating the stimuli-responsive aspects of these materials for possible sensing capabilities.
■
ACKNOWLEDGMENTS We graciously acknowledge the University of New Hampshire for financial support, NSF for support through award NSF EEC 0832785, Army Research Office for support through award W911NF-14-1-0177, and NIST for support through award 70NANB15H060. Danming Chao thanks the National Natural Science Foundation of China for funding (grant No. 21104024 and 21274052) as well as National 973 Project (No. S2009061009).
■
(1) (a) Zhang, W. J.; Feng, J.; MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1997, 84, 119. (b) Wei, Z.; Laitinen, T.; Smarsly, B.; Ikkala, O.; Faul, C. F. J. Angew. Chem., Int. Ed. 2005, 44, 751. (c) Wei, Z.; Faul, C. F. J. Macromol. Rapid Commun. 2008, 29, 280. (2) (a) Gao, J.; Liu, D. G.; Sansiñena, J. M.; Wang, H. L. Adv. Funct. Mater. 2004, 14, 537. (b) Chao, D.; Lu, X.; Chen, J.; Zhao, X.; Wang, L.; Zhang, W.; Wei, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 477. (c) Chao, D.; Jia, X.; Liu, H.; He, L.; Cui, L.; Wang, C.; Berda, E. B. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1605. (d) Chen, R.; Benicewicz, B. C. ACS Symp. Ser. 2003, 843, 126. (e) Chen, R.; Benicewicz, B. C. Macromolecules 2003, 36, 6333. (3) (a) Chao, D.; Jia, X.; Bai, F.; Liu, H.; Cui, L.; Berda, E. B.; Wang, C. J. Mater. Chem. 2012, 22, 3028. (b) Chao, D.; Yang, R.; Jia, X.; Liu, H.; Wang, S.; Wang, C.; Berda, E. B. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 2330. (c) Chao, D.; Zheng, T.; Liu, H.; Yang, R.; Jia, X.; Wang, S.; Berda, E. B.; Wang, C. Electrochim. Acta 2012, 60, 253.
ASSOCIATED CONTENT
S Supporting Information *
Full experimental details, supporting figures, and spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01446.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected] (E.B.B.). *E-mail
[email protected] (D.C.). Notes
The authors declare no competing financial interest. C
DOI: 10.1021/acs.macromol.5b01446 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules (d) Jia, X.; Chao, D.; Berda, E. B.; Pei, S.; Liu, H.; Zheng, T.; Wang, C. J. Mater. Chem. 2011, 21, 18317. (e) Wang, S.; Chao, D.; Berda, E. B.; Jia, X.; Yang, R.; Wang, C. Electrochim. Acta 2013, 89, 594. (f) Wang, S.; Chao, D.; Berda, E. B.; Jia, X.; Yang, R.; Wang, X.; Jiang, T.; Wang, C. RSC Adv. 2013, 3, 4059. (g) Chao, D.; Jia, X.; Tuten, B.; Wang, C.; Berda, E. B. Chem. Commun. 2013, 49, 4178. (4) (a) Schaefer, M.; Hanik, N.; Kilbinger, A. F. M. Macromolecules 2012, 45, 6807. (b) Theryo, G.; Jing, F.; Pitet, L. M.; Hillmyer, M. A. Macromolecules 2010, 43, 7394. (c) Johnson, J. A.; Lu, Y. Y.; Burts, A. O.; Xia, Y.; Durrell, A. C.; Tirrell, D. A.; Grubbs, R. H. Macromolecules 2010, 43, 10326. (d) Johnson, J. A.; Lu, Y. Y.; Burts, A. O.; Lim, Y.-H.; Finn, M. G.; Koberstein, J. T.; Turro, N. J.; Tirrell, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 559. (e) Xia, Y.; Olsen, B. D.; Kornfield, J. A.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 18525. (f) Xia, Y.; Kornfield, J. A.; Grubbs, R. H. Macromolecules 2009, 42, 3761. (g) Matson, J. B.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 6731. (h) Matson, J. B.; Grubbs, R. H. Macromolecules 2008, 41, 5626. (5) (a) Shacklette, L. W.; Wolf, J. F.; Gould, S.; Baughman, R. H. J. Chem. Phys. 1988, 88, 3955. (b) Furukawa, Y.; Ueda, F.; Hyodo, Y.; Harada, I.; Nakajima, T.; Kawagoe, T. Macromolecules 1988, 21, 1297.
D
DOI: 10.1021/acs.macromol.5b01446 Macromolecules XXXX, XXX, XXX−XXX
