Self-Assembled Oligoanilinic Nanosheets: Molecular Structure

Dec 10, 2015 - The products obtained during the early stages of the oxidative polymerization of aniline in the “falling pH” reaction were investig...
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Self-Assembled Oligoanilinic Nanosheets: Molecular Structure Revealed by Solid-State NMR Spectroscopy Zoran Zujovic,*,§,‡,† Amy L. Webber,⊥ Jadranka Travas-Sejdic,‡,† and Steven P. Brown⊥ §

NMR Centre, School of Chemical Sciences, University of Auckland Private Bag 92019, Auckland 1142, New Zealand Polymer Electronics Research Centre, School of Chemical Sciences, University of Auckland Private Bag 92019, Auckland 1142 New Zealand † MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand ⊥ Department of Physics, University of Warwick, Coventry CV4 7AL, U.K. ‡

S Supporting Information *

ABSTRACT: The products obtained during the early stages of the oxidative polymerization of aniline in the “falling pH” reaction were investigated using multinuclear solid-state magicangle spinning (MAS) NMR combined with first-principles NMR chemical shielding calculations using the GIPAW (gaugeincluding projector augmented wave) method. A sample was synthesized starting from a 50:50 mixture of U−13C aniline and 15 N-labeled aniline; two-dimensional 13C refocused INADEQUATE, 15N−13C double CP, 15N PDSD, and 1H−13C/15N refocused INEPT MAS NMR spectra revealed the presence of quinoneimine structural units. Structural models that are consistent with the connectivities revealed by the 13C refocused INADEQUATE and 15N−13C Double CP spectra are proposed. GIPAW chemical shift calculations are performed for model structures based on the proposed oligomeric structures; noting that the model structures do not take into account intermolecular hydrogen bonding and CH−π interactions, agreement and discrepancy to experiment are discussed.

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INTRODUCTION Polyaniline (PANI) is one of the most versatile and widely utilized polymers today, with applications ranging over polymer electronics, chemical and bio- sensors, photoelectrochemical solar cells, supercapacitors, antimicrobial agents, nerve tissue engineering, anticorrosive materials, catalysts, and electromagnetic interference shielding.1−8 The standard oxidative polymerization of PANI is carried out at low pH in the presence of a strong acid (1 M HCl). This yields an electrically conductive polymer with featureless granular morphologies, where the chain structure is largely determined by the constituent benzenoid diamine and quinoid diimine alternating units.9 In contrast, other aniline oxidation reactions can yield products of mainly low molecular weight oligomers with wellstructured morphologies at the micro- and nanoscale. Specifically, in the falling-pH approach,9−13 the aniline monomer and an oxidant are mixed in aqueous solution: the initial pH is usually 6−7, gradually decreasing toward pH 0 due to the release of protons from the chemically oxidized polymerization reaction. Depending on the initial conditions, such as the choice of oxidant and/or acid, the concentration of the starting reagents, temperature and pH, a variety of products that exhibit very well-defined and interesting morphologies, such as nanotubes, nanoparticles, nanoflakes, nanorods and nanospheres have been observed to form.9−15 Because of their © 2015 American Chemical Society

well-defined shapes as well as the high surface area, selfassembled oligomeric structures can be used either directly or indirectly as nanostructured templates for conducting polymers and drug carriers.9−11,13 Moreover, these materials can be potentially applied in the field of micro and nanoelectronics where various preformed nanostructures (e.g., fibers, tubes, and rods) can serve as nucleation centers to promote anisotropic polymerization of PANI into well-formed shapes.9−11 This paper is set in the context of the need for an understanding at the atomic level of how the polymerization process leads to the formation of various oligomeric nanostructures. This knowledge is a prerequisite for enabling the materials chemist to improve properties such as conductivity, mechanical resistance, as well as morphological characteristics of size, length, and shape, with achieving such control currently being a significant challenge. It was proposed that oligoanilinic nanotubes that form early on during the “falling pH” reaction are made of thin, rolled nanosheets.12 Depending on the reaction conditions (e.g., pH, reagent concentration, temperature), these nanosheets may either roll, consequently forming cylindrical morphologies (i.e., Received: October 8, 2015 Revised: November 24, 2015 Published: December 10, 2015 8838

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Figure 1. Solid-state MAS NMR (1H Larmor frequency of 500.1 MHz) spectra of a PANI sample synthesized using alanine and ammonium persulfate by the “falling pH” reaction: One-dimensional (a) 13C CP MAS (20 kHz) and (e) 15N CP MAS (12.5 kHz) spectra. (b) 13C−13C refocused INADEQUATE (20 kHz MAS, 1.0 ms spin−echo duration) spectrum. (c, g) Double CP heteronuclear 13C−15N (12.5 kHz MAS) spectrum recorded with 1H−15N and 15N−13C contact times of 1.0 and 5.0 ms, respectively. In part g, the spectrum has been rotated through 90 deg away from the usual convention, such that the horizontal dimension corresponds to the indirect 15N dimension, so as to allow comparison with the other spectra presented in the right-hand column of the figure. (d, h) Heteronuclear (d) 13C−1H (10 kHz MAS) and (h) 15N−1H (12.5 kHz) refocused INEPT spectra employing eDUMBO22 1H homonuclear decoupling with spin−echo durations of (d) 0.6 and (h) 3.2 ms. (f) Twodimensional 15N−15N proton-driven spin diffusion spectrum with a mixing time of 2.0 s. Spectra a−d and g were recorded on a sample synthesized starting from a 50:50 mixture of U−13C aniline and 15N-labeled aniline, while spectra e, f, h are for a sample synthesized using 15N-labeled aniline.

nanotubes),12 or stack on each other, thus creating thicker, layered sheets called nanoflakes.16 It was shown that when the concentration of nanosheets reaches a critical point, one-layer sheets stack on top of each other due to hydrogen bonding, π−π stacking and van der Waals forces, so as to form layered sheets, or nanoflakes.16 During the later reaction stage, the reaction solution pH falls and regular PANI starts to form, which in turn covers the initially smooth oligomeric nanotubes giving them a granular texture.9 According to this hypothesis, the later formed nanoflake and nanotube morphologies should have the same chemical structure as the initially formed nanosheets.16 Therefore, an analytical characterization of the chemical structure of nanoflakes is also of direct relevance for an understanding of the structure of the nanosheets that however are very delicate to isolate and investigate individually.

A number of spectroscopic methods including FTIR, UV− vis, and Raman spectroscopy have been used in investigations of nanostructured materials obtained by the oxidation of aniline,9 with solution-state NMR also identifying the molecular structure of some early formed oligoanilines.17 Moreover, we have previously presented one-dimensional 13C and 15N solidstate magic-angle spinning (MAS) NMR spectra that show features that are very different to conventional PANI; however, it was only possible to speculate on the repeating molecular motif adopted in the oligomeric structures.9,12 In this paper, we employ, for the first time, advanced two-dimensional solid-state NMR experiments to investigate the structure of these early formed oligomeric morphologies. For a sample synthesized starting from a 50:50 mixture of U−13C aniline and 15N-labeled aniline, such two-dimensional solid-state NMR spectra reveal specific carbon−carbon and carbon−nitrogen one-bond 8839

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Parts c and g of Figure 1 present a heteronuclear 13C−15N spectrum obtained using a double CP pulse sequence, where an initial 1H−15N CP step to create 15N magnetization that evolves during t1 is followed by a 15N−13C CP step, whereby magnetization is transferred between dipolar-coupled pairs of 15 N−13C nuclei. Note that the same spectrum is presented in Figure 1, parts c and g, except for a rotation through 90° in Figure 1g. The use of a long contact time, τCP = 5 ms, for the 15 N−13C CP step means that correlations corresponding to both one-bond connectivities and longer range proximities are observed. The strongest correlation peaks are those at, first, δ(13C) = 139.4 ppm, δ(15N) = 75.7 ppm and, second, at δ(13C) = 148.1 ppm, δ(15N) = 82.1 ppm; both these 15N peaks correspond to protonated nitrogen sites (see below discussion of Figure 1h). While the latter peak can be unambiguously assigned to the d 13C resonance, the peak at 139.4 ppm could be assigned to either or both of the a or a′ 13C resonances. There are also weaker peaks at δ(13C) = 180.2 ppm, δ(15N) = 75.7 ppm and between a third 15N resonance at 90.7 ppm and a 13 C resonance at 146.8 ppm corresponding to the right-hand side of peak d. Note that there are no correlation peaks involving the b′ and c′ 13C resonances of the pendant aromatic group due to the synthesis starting from a 50:50 mixture of U−13C aniline and 15N-labeled aniline. It is also evident that there are no cross peaks observed for the 15N e peak; this is possibly a consequence of poor CP efficiency. Tables S2 and S3 (Supporting Information) list the observed heteronuclear correlations for the distinct 13 C and 15 N resonances, respectively. Figure 1f presents a two-dimensional 15N−15N proton-driven spin diffusion (PDSD) spectrum recorded with a mixing time of 2 s. The observation of cross peaks linking the protonated 15 N resonances at ∼80 ppm to the nonprotonated 15N resonance at ∼250 ppm shows that the corresponding nitrogen atoms are in the same molecular entity; such a cross peak would be expected for a N−N distance up to ∼5 Å, i.e., the resonances do not correspond to different crystallite domains. Finally, parts d and h of Figure 1 show 1H−13C and 1H−15N correlation spectra, respectively, recorded using the refocused INEPT24 sequence, whereby heteronuclear correlation is established via through-bond J couplings and high resolution in the 1H dimension is achieved using eDUMBO22 1H homonuclear decoupling.25,26 In the 1H−13C correlation spectrum in Figure 1d, correlation peaks are only observed for the c′, b′, c, and f 13C resonances. The experiment was performed using a short spin−echo duration of 0.6 ms such that the observed correlation peaks correspond to only onebond connectivities. Thus, only the c′, b′, c, and f 13C resonances correspond to protonated carbons. For the 1H−15N spectrum in Figure 1h, a longer spin−echo duration of 3.2 ms was used, such that cross peaks corresponding to one bond and longer-range couplings are observed. The sites labeled Na (75.7 ppm) and Nd (82.1 ppm) show strong 15N−1H correlation peaks to 1H resonances at 9.3 and 9.5 ppm, respectively, i.e., these are NH moieties. A much weaker peak is observed between the 15N resonance at 250 ppm, labeled Ne, and a 1H resonance at 8.5 ppm, for which correlation peaks are also observed for the b′ and c′ resonances in the 1H−13C correlation spectrum in Figure 1d. Tables S2 and S3 (Supporting Information) list the observed heteronuclear correlations with 1 H for the distinct 13C and 15N resonances, respectively.

connectivities. By comparison to chemical shifts calculated for monomer and extended models, we propose molecular structures that are consistent with the experimental data.

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RESULTS AND DISCUSSION Figure 1 presents solid-state NMR spectra of a “falling pH” 1 h PANI sample (see Supporting Information for full synthesis details). The top row of Figure 1 shows 13C (Figure 1a) and 15 N (Figure 1e) cross-polarization (CP) MAS NMR spectra. Such one-dimensional 13C and 15N spectra have been presented in Figure 9 of ref 12 for an analogous sample prepared starting with natural abundance aniline. As discussed in ref 12, the line widths are significantly narrower than for PANI, with, in addition, there also being some key differences, notably a 13C resonance at ∼180 ppm that is indicative of a carbonyl moiety as well as a 15N resonance for a nonprotonated nitrogen environment with a chemical shift that is ∼80 ppm different from that for conventional PANI. The synthesis starting from a 50:50 mixture of U−13C aniline and 15N-labeled aniline leads to 50% of carbon atoms and 50% of nitrogen atoms being labeled with the spin I = 1/2 nuclei 13C and 15N, respectively. This 50% 13C and 15N labeling enables the carrying out of two-dimensional homonuclear (i.e., 13 C−13C and 15N−15N) and heteronuclear (i.e., 13C−15N) experiments that utilize through-bond J couplings or throughspace dipolar couplings to probe connectivities or proximities. Consider first Figure 1b that presents a 13C−13C double quantum−single quantum (DQ−SQ) correlation spectrum recorded using the refocused INADEQUATE18,19 pulse sequence. In this experiment, DQ coherence (DQC) is created for pairs of J-coupled 13C nuclei, with the use of a short spin− echo evolution period, 1.0 ms, ensuring that correlations are only observed for directly bonded carbon atoms. For each Jcoupled spin pair, the DQ frequency (vertical dimension in Figure 1b) equates to the sum of the SQ frequencies. Hence, correlation peaks due to a DQC between nuclei with different SQ frequencies appear equidistant from the FDQ = 2FSQ diagonal (dashed gray line in Figure 1b). SQ and DQ frequencies observed in Figure 1b are listed in Table S1 (Supporting Information). Such refocused INADEQUATE 13C NMR spectra are invaluable for tracing out the carbon backbone in organic molecules;20−22 two separate sets of linked resonances labeled a to f and a′ to c′, are identified by means of solid gray lines in Figure 1b. Note that the spreading out into two spectral dimensions enables the identification of two distinct resonances, a and a′, at ∼140 ppm. For the first set of six resonances, from a to f, there is a complete loop (a−b, b−c, c− d, d−e, e−f, f−a), i.e., two DQ peaks are observed for each SQ frequency. By contrast, for the second set of only three distinct resonances, a′ to c′, only a′−b′ and b′−c′ connectivities are observed, with there being no c′-a′ pair of correlation peaks. As for the above discussion of the one-dimensional 13C CP MAS spectrum, it is noteworthy that the 13C peaks are narrow in the refocused INADEQUATE spectrum in Figure 1b. The only evident broadening is for the b′ resonance, which, as discussed below, we assign to corresponding to three distinct carbon sites on the same benzene ring. The appearance of a “sloped” twodimensional peak such as that observed for b′ in Figure 1b has been discussed previously for other refocused INADEQUATE spectra.20,23 8840

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Table 1. Comparison of Calculated (GIPAW) 13C and 15N NMR Chemical Shifts for the Proposed Structures A, AX, B, and BX (See Scheme 1 and Figure S2) with Those for the Experimental Spectra in Figure 1

Scheme 1 presents two repeating molecular units (A and B) which are consistent with the observed solid-state NMR Scheme 1. Proposed Monomeric Units (A and B) with Red Dashed Lines Indicating How Oligomerization Would Proceeda

13 13

Cexpt/ppm

a b c d e f a′ b′ b′ b′ c′ c′

a

See Figure S1 (Supporting Information) for a proposed resonance structure of unit A.

spectra. Note that the proposed oligomerization can proceed via Cd−NHd−Cd and CeNe+Ce linkages for A and Ca− NHa−Ca and Cd−NHd−Cd linkages for B. In both cases, the oligomeric backbone is made of quinoneimine units (carbons a to f), with a phenyl group (carbons a′ to c′) attached to each unit, in agreement with the connectivities revealed by the 13C−13C refocused INADEQUATE spectrum in Figure 1b as well as the C−N proximities observed in Figures 1c and 1g. It is to be remembered that the synthesis started from a 50:50 mixture of U−13C aniline and 15N-labeled aniline such that in the heteronuclear 13C−15N spectrum (in Figure 1c and 1g) it is not possible to see a correlation between the phenyl group carbons a′ to c′ and the directly bonded nitrogen. We note that the molecular entity in structure B is part of an oligomeric structure identified by solution-state NMR for initially formed oligomers by Kriz et al. (see Scheme 1),17 though this oligomeric structure proposed by Kriz et al. contains other aromatic units found in conventional PANI. To further investigate the proposed repeating units presented in Scheme 1, NMR chemical shieldings were calculated using the GIPAWa (gauge including projector augmented wave) method27−29 for the monomer (A, B) and the extended model structures (AX, BX, Figure S2, Supporting Information). Note that the NMR chemical shieldings could also be calculated by an alternative quantum chemical method, e.g., B3LYP GIAO/6311G**. The calculated 13C and 15N chemical shifts for all atoms in the monomer structures and for the central unit in the extended structures are presented in Table 1, where they are compared to the experimental values as observed in Figure 1. When considering these calculated chemical shifts, it is important to remember that these monomer and extended models have the following deficiencies as a representation of the solid-state structure. Although the first step in the calculation of NMR parameters is a geometry optimization,30 for the monomer and extended models here, this is effectively a gas phase optimization; the effect of the three-dimensional efficient packing of molecules in a periodic repeating structure cannot be included. Notably, the effect of intermolecular hydrogen bonding and CH−π interactions in the threedimensional structure on the NMR chemical shifts is completely or partially neglected in the monomer and extended models. 1H chemical shifts are particularly sensitive to intermolecular hydrogen bonding as well as ring current

139.4 180.2 97.4 148.1 163.4 96.6 139.4 123.3 123.3 123.3 129.0 129.0

RSSc(13C)

RSSc(15N)

A

B

AXa

BXa

136.7 182.9 98.1 144.2 168.9 101.4 141.6 121.1 121.0 117.0 129.9 129.7

143.0 180.1 96.0 151.4 156.2 85.6 154.8 124.4 122.7 118.9 130.0 128.7

137.3 177.8 106.9 143.1 147.8 98.6 143.1 126.7 125.3 120.6 133.0 131.8

134.9 182.5 104.9 143.0 154.4 94.8 152.3 125.7 125.3 117.6 128.5 128.2

1.8 1.7 15 Ncalc/ppmb

1.7

1.0 15

a d e

Nexpt/ppm 75.7 82.1 248.8

Ccalc/ppmb

A 87.4 36.7 282.6

B 49.9 51.8 304.9

AXa 88.0 76.3 242.4

BXa 53.6 59.9 293.1

19.3

22.9

5.0

18.1

a

For extended structures AX and BX, only chemical shifts of the central monomer are presented here. See Table S4 in the Supporting Information for a full listing. bCalculated chemical shifts are referenced for each structure according to the procedure in ref.30 such that the experimental and calculated mean coincide: 13C: δmean(expt) = 132.7 ppm, σref (13C) = (A) 169.4, (B) 169.1, (AX) 171.6, and (BX) 168.1 ppm. 15N: δmean(expt) = 136.6 ppm, σref (15N) = (A) 200.4, (B) 209.2, (AX) 190.9, and (BX) 161.1 ppm. cAverage root sum squared deviation per site between calculated and experimental chemical shifts for the different structural models.

effects;31 thus, the calculated 1H chemical shifts are not included in Table 1. As noted above, there is a 15N resonance (labeled e) for a nonprotonated nitrogen environment with a chemical shift that is ∼80 ppm different to that for conventional PANI. An inspection of Table 1 reveals that best agreement to experiment is obtained for the GIPAW calculated 15N chemical shifts for model AXthe average deviation per site of 5 ppm is within ∼1% of the 15N chemical shift range, corresponding to the typically observed agreement between experiment and GIPAW calculation.29 For the 13C chemical shifts, for all four models, the average deviation per site of less than 2 ppm is within ∼1% of the 13C chemical shift range. It is, however, evident that while good agreement to experiment is observed for model A (biggest deviations are 4.8 ppm for site f and 6.3 ppm for one of the three b′ sites), this agreement is significantly poorer for model AX, for which there is best agreement for the 15N chemical shifts: notably, there is a deviation of 15.8 ppm for 13C site e, i.e., the carbon next to nitrogen site e that exhibits the significantly different 15N chemical shift. There are further discrepancies between experiment and calculation for model AX. First, consider the heteronuclear 13C−15N spectra in Figure 1, parts c and g. Experimentally, the strongest correlation peaks 8841

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Macromolecules are those at, first, δ(13C) = 139.4 ppm, δ(15N) = 75.7 ppm, and, second, δ(13C) = 148.1 ppm, δ(15N) = 82.1 ppm, whereby δ(13C) of 139.4 ppm could be assigned to either or both of the a or a′ 13 C resonances, while δ( 13 C) 148.1 ppm is unambiguously assigned to the d 13C resonance. However, looking at Table 1, the calculated 15N chemical shifts for the a and d sites are 88.0 and 76.3 ppm, respectively, i.e., the other way around to the experimental observation. Experimentally, there is also a third 15N resonance at 90.7 ppm which has a 13 C−15N correlation peak with a 13C resonance corresponding to the right-hand side of peak d and for which we do not have a structural explanation based on Scheme 1 and the A, B, AX, and BX models. Consider also the 13C−1H and 15N−1H spectra in Figure 1, parts d and h, respectively. In the 15N−1H spectrum in Figure 1h, a weak peak is observed between the 15N resonance at 250 ppm, labeled Ne, and a 1H resonance at 8.5 ppm, for which correlation peaks are also observed for the b′ and c′ resonances in the 1H−13C correlation spectrum in Figure 1d; such a peak is more readily explained by models B and BX. In summary, we have proposed structural schemes that are consistent with the two-dimensional solid-state NMR spectra and for which there is good, if not complete, agreement to experiment for the calculated 13C and 15N chemical shifts. The quinoneimine structures can be explained by an attack of an aniline molecule on a terminal or inner phenylenediamine group in an ortho-position and subsequent oxidation. Of particular importance is that our results show that phenazine units which were conceived as main building blocks for nanotubes10 are not detected. We note that our structural models are not able to take into account intermolecular hydrogen bonding as well as CH−π interactions, for example, the weaker cross peak corresponding to a longer-range N−C proximity between the 15N resonance at 76.8 ppm and only the 13 C b resonance at 180.2 ppm is likely indicative of a NH···O C intermolecular hydrogen bonding interaction. In conclusion, we believe that our work is an important step forward toward an unequivocal identification of the molecular structure adopted in oligoanilinic nanosheets. Looking forward, the wealth of new experimental two-dimensional solid-state NMR spectra presented here for these oligoanilinic nanostructures and the sensitivity of NMR chemical shift to local molecular structure and packing presents a challenge for structural modellers to take up.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS EPSRC is thanked for Ph.D. funding for A.L.W. Calculations were performed on the University of Warwick Centre for Scientific Computing cluster. Helpful discussions with Maria Baias concerning GIPAW calculations are acknowledged. ADDITIONAL NOTE

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REFERENCES

a

DFT GIPAW calculations (magres and pdb) and experimental NMR data for this study are provided as a supporting data set from WRAP, the Warwick Research Archive Portal, at http:// wrap.warwick.ac.uk/74600.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02204. Experimental and computational details, visualizations of the A, B, AX, BX model structures, Tables of experimental solid-state NMR correlation peaks and a full listing of calculated (GIPAW) 13C and 15N chemical shifts for the A, B, AX, BX model structures (PDF). (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Author Contributions

Z.Z. and A.L.W. contributed equally to this research. 8842

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