Impact of Precise Control over Microstructure in Thiophene

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Impact of Precise Control over Microstructure in Thiophene− Selenophene Copolymers Andria Fortney, Chia-Hua Tsai, Manali Banerjee, David Yaron, Tomasz Kowalewski,* and Kevin J. T. Noonan* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

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S Supporting Information *

ABSTRACT: Controlling the sequence of repeat units in a synthetic polymer has been a long-standing topic of interest in chemistry. As methods to regulate sequence become more sophisticated, it is critical to consider how controlling the arrangement of repeat units along the polymer backbone impacts properties. In this work, thiophene−selenophene copolymers (statistical and periodic) were compared to elucidate the impact of periodicity on electronic properties and structural organization in conjugated macromolecules. Polymers were synthesized using catalyst-transfer polycondensation (CTP) enabling control over molecular weight and dispersity. The study revealed that optical bandgaps and redox potentials of periodic and statistical copolymers varied with composition in a predictable manner, regardless of monomer ordering along the chain. While the bandgaps of the two types of copolymers were indistinguishable, X-ray scattering revealed differences in solid-state packing. Both types of copolymers exhibited well-defined morphologies, but larger π-stacking distances and more orientational disorder were evident in the statistical systems. This indicates periodicity is an attribute that should be considered when synthesizing semiconducting materials.

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INTRODUCTION In the past decade, many new methods have emerged to construct more complicated arrangements of repeat units in synthetic macromolecules.1−6 Though this opens many exciting avenues for exploration, regulating polymer sequence is often more synthetically demanding and time-consuming than a simple combination of monomers in a statistical copolymerization. As such, it is critical to consider what benefits arise from building precise periodic copolymers as opposed to statistical distributions of the same monomers along the chain. Researchers have already illustrated that the electronic properties (HOMO−LUMO gaps) of semiconducting macromolecules can be precisely tuned by engineering the sequence of aromatic building blocks within a polymer or oligomer backbone. This is highly relevant in donor−acceptor copolymers7,8 (built using step-growth techniques) and in discrete conjugated oligomers.9−12 In our own work, we have also seen this effect using an oligomer approach to construct periodic copolymers of thiophene (S) and selenophene (Se) (Figure 1, top).13 The bandgap of these periodic materials was found to be a linear combination of the repeat units used in the chain and was tuned by controlling the amount of each building block.13 However, this work raised the question as to what extent this was a function of the periodic sequence or a function of composition since the electronic structure is highly delocalized. If similar bandgap tuning could be achieved using a simple copolymerization of the two monomer units, this would eliminate the need for control of periodicity. Moreover, the energy gap cannot be the sole consideration as nanoscale © XXXX American Chemical Society

Figure 1. Top: periodic copolymers of thiophene−selenophene prepared from discrete oligomers. Bottom: statistical copolymers synthesized by combination of two active monomers.

morphology plays an integral role in the performance of semiconducting polymers in organic electronic applications. Building from our past work, we wanted to evaluate how the electronic structure and solid-state organization of random compositions compare to their periodic analogues (Figure 1). To achieve this, monomers must be incorporated into the Received: July 5, 2018 Revised: October 27, 2018

A

DOI: 10.1021/acs.macromol.8b01434 Macromolecules XXXX, XXX, XXX−XXX

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compensate for the limited solubility of the polymers bearing selenophene. After 15 min, reaction mixtures were quenched and precipitated with 6 M HCl/MeOH solution31 and washed with methanol and acetone. Gel permeation chromatography (GPC) of the crude polymer samples revealed that the polymers were of high molecular weight, with relatively narrow dispersities, as expected (Table 1). The statistical copolymers are designated as stat-TSeXi where Xi corresponds to selenophene content (31, 55, and 66%, as determined by 1H NMR spectroscopy). The periodic copolymers are named by their respective repeat units (TTSe, TSe, and TSeSe, Figure 1) which have similar amounts of selenophene (Xi = 33, 50, and 66%) compared to the statistical materials, providing a suitable starting point for comparison. NMR Spectroscopy. P3HT obtained from Ni(dppp)Cl2initiated polymerizations has been studied extensively using NMR spectroscopy.32 Model compound studies have been used to assign the four configurational triads from the regiochemical possibilities with hexyl chains of adjacent thiophenes in head-to-head (HH), head-to-tail (HT), or tailto-tail (TT) arrangements.33 The regioregularity (HT arrangement of alkyl chains) in the T−Se statistical copolymers was ensured by the deprotonation method used to produce only one active regioisomer for polymerization. The proton signals for the aromatic rings in the polymer are also sensitive to the adjacent heterocycle and nearest-neighbor, so multiple signals are observed in the statistical copolymers. The spectra are consistent with those obtained by Seferos and co-workers with two selenophene-centered dyads (7.19/7.18 and 7.12 ppm) and two thiophene-centered dyads (6.98/6.97 and 6.92 ppm).14 The previously synthesized periodic copolymers were also very helpful for the 1H NMR assignments of the statistical materials as they exhibit only the expected dyad signals in each of the different cases (Figure 2).13 NMR spectroscopy did present an interesting opportunity to explore composition of the statistical materials and compare to what would be expected from a perfectly random distribution when polymerizing a mixture of S:Se (either 2:1, 1:1, or 1:2). We noted in the statistical copolymers that all dyad fractions were within 5% of the expected values if the polymerization were perfectly random with equal probability of inserting either monomer (Figure S19). Cyclic Voltammetry and Absorption Spectroscopy. The band structure of the statistical and periodic copolymers was probed using CV to estimate the HOMO and LUMO levels of the synthesized materials (Figure S18). Optical bandgaps were also calculated from the solid-state absorption edge (νedge, eV) in UV−vis spectroscopy. These two techniques combined provide a general overview of the electronic structure of the two types of conjugated frameworks (Table 2). The quasi-reversible oxidation and quasi-reversible reduction potential onsets (± 0.1 eV) for the related statistical and periodic copolymers were in relatively good agreement, though a perfect trend in the electrochemical bandgaps did not emerge. Measured bandgaps for stat-TSe31 and TSe were significantly higher than those measured for other copolymers (Table 2) and are closest to the values obtained for P3HT. We attribute this difference to the difficulty in selection of onset oxidation and reduction potentials. In some of the cyclic voltammograms, small secondary features arise prior to the large signals for oxidation and reduction (Figure S18). These

chain in a random fashion which is governed by their reactivity ratios. Prior efforts from Seferos,14−16 McNeil,17 and Heeney18 have already demonstrated that statistical copolymers of thiophene and selenophene can be prepared using Kumada− Corriu coupling, with a slight preference for selenophene incorporation based on the reactivity ratios.17 Given this, we were able to synthesize statistical copolymers of thiophene and selenophene with compositions closely matching our periodic copolymers for direct comparison (Figure 1). The copolymers were synthesized using Kumada catalyst-transfer polycondensation (CTP).19−26 The chain-growth nature of this polymerization process ensured high molecular weight polymers with relatively narrow molecular weight distributions (Đ = 1.2−1.3) could be obtained, facilitating direct comparison. Herein, the obtained results revealed that the electronic properties of individual polymer chains for these copolymers are independent of monomer sequence. Owing to the strong coupling between the two heterocycles, the delocalized πsystem is simply an “amalgamation” of the monomers used to construct the polymer backbone; thus, its gross characteristics such as the HOMO−LUMO gap depend primarily on composition. However, three substantial differences were observed regarding the bulk organization of the polymeric materials. First, the periodic polymers have smaller π-stacking distances, suggesting that the periodic polymers pack in register with regards to sequence. Second, there are larger degrees of orientational disorder for statistical copolymers, indicating that these materials may require more efforts in processing to achieve optimal transport behavior. Finally, care must be taken for periodic sequences where the periodicity introduces curvature into the backbone of the polymer. Curvature-inducing sequences are considerably more disordered than those formed from random sequences. Clearly, the largest effects of sequence control are due to solid-state packing, so to take advantage of sequence, one must go beyond consideration of the electronic properties of individual polymer chains.

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RESULTS AND DISCUSSION The synthesis of statistical copolymers was conducted similarly to prior reports with a different monomer activation method Table 1. Polymerization Data for Statistical Copolymers Xi (Se)a

target DP

Mn (g/mol)b

Đ

yield (%)

0 31 55 66 100

100 150 150 150 100

23900 30800 28500 26700 24200

1.18 1.30 1.20 1.23 1.19

54 41 31 50 48

a

Percent incorporation of 3-hexylselenophene was determined by integration of aromatic signals in the 1H NMR spectrum (Supporting Information). bGPC traces were recorded at 40 °C versus polystyrene standards using THF as the eluent.

(Figure 1).14−17 2-Bromo-3-hexylthiophene and 2-bromo-3hexylselenophene were each treated with 2,2,6,6-tetramethylpiperidinylmagnesium chloride−lithium chloride complex (TMPMgCl·LiCl) to prepare the active monomers (forming a single active regioisomer in each case).27−30 The monomer solutions were then combined and polymerizations were initiated using Ni(dppp)Cl2 (Figure 1, bottom). Typically, reactions were conducted in dilute solution (0.01 M) to B

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Figure 2. 1H NMR spectra of the aromatic regions for the periodic and statistical copolymers with assignments for the dyad configurations included above (spectra collected in CHCl3 at 22 °C, 500 MHz). The asterisk symbols (∗) indicate 13C satellite signals for the solvent.

copolymers decreased with increasing selenophene content, indicating a raised HOMO, as expected. The optical properties of the periodic and statistical copolymers were probed both in solution and solid state (Figure 3). The bandgap and excitation energies determined from the absorption spectra revealed consistent trends. Solution and solid-state spectra both exhibited a red-shift in absorbance maxima with respect to increasing selenophene content (Table 2 and Figure 3), regardless of monomer sequence in the chain.18 Therefore, the red-shift in absorption maxima is dependent only on the overall composition of the polymer, not on the monomer ordering (Figure S20). This indicates the delocalized electronic structure is dependent on the composition of the two polymers but is relatively independent of sequence due to strong coupling between the two heterocycles.13

Table 2. Electrochemical and Optical Properties of Statistical and Periodic Copolymers entry

solution λmax (nm)

Bandgap νedge (eV)

Bandgap CV (eV)

P3HT stat-TSe31 TTSe stat-TSe55 TSe stat-TSe66 TSeSe P3HSe

453 465 466 473 473 476 480 493

1.90 1.79 1.81 1.73 1.77 1.71 1.72 1.65

2.25 2.12 2.01 2.05 2.17 1.86 1.88 1.84

ultimately change the onset potentials and produce variability in the electrochemical bandgaps. Electrochemical analysis did reveal that the oxidation onset of both statistical and periodic

Figure 3. Solution and solid-state UV−vis spectroscopy. Periodic copolymers (top panel: TTSe (red), TSe (blue), and TSeSe (black)) and statistical copolymers (bottom panel: stat-TSe31 (dashed red), stat-TSe55 (dashed blue), and stat-TSe66 (dashed black)). C

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Macromolecules Table 3. Thermal Properties for Thiophene and Selenophene Copolymers

Figure 4. Thermogravimetric analysis of periodic (top: red, blue, black) and statistical copolymers (bottom: dashed red, blue, and black) from 300 to 600 °C. Inset: 5% decomposition temperature (Td5%).

entry

Mna

Tm (°C)

Tc (°C)

ΔHc (J/g)

ΔHm (J/g)

ΔHm/ring (kJ/mol)

P3HT P3HT P3HT statTSe31 TTSe statTSe55 TSe statTSe66 TSeSe P3HSe

11500 23900 39300 30800

227 236 239 232

197 205 208 201

24 23 25 18

25 29 22 22

4.1 4.7 3.6 4.0

32100 28500

230 231

199 203

21 16

25 15

4.6 3.4

39800 26700

219 233

161 206

6 19

9 21

1.8 4.2

39000 24200

235 248

204 221

16 19

20 23

3.9 4.9

GPC traces were recorded at 40 °C vs polystyrene standards using THF as the eluent.

a

Figure 6. Top: calculated bond lengths and angles from dodecamer calculations for thiophene and selenophene in the oligomer chain (end groups were excluded). Bottom: energy-minimized TTSe and TSe dodecamer structures. Reproduced with permission from ref 13.

used to quantify the degree of excitonic coupling within polymer aggregates which is related to intrachain order including average conjugation length and crystallinity.34−37

Figure 5. Differential scanning calorimetry cyclization curves (second run) measured at a heating rate of 10 °C/min. Both periodic copolymers (left: TTSe (red), TSe (blue), TSeSe (black)) and statistical copolymers (right: stat-TSe31 (dashed red), stat-TSe55 (dashed blue), and stat-TSe66 (dashed black)) compared with respect to homopolymers P3HT (green) and P3HSe (orange).

A 0 − 0 ijj 1 − 0.24W /Ep yzz zz ≈ jj A 0 − 1 jj 1 + 0.073W /Ep zz (1) k { Comparison of exciton bandwidths determined from the UV−vis spectra of periodic and statistical copolymers using eq 1 is shown in Figure S21. All calculations were performed by fitting the spectra to the Spano model setting the value of Ep to 0.18 eV as in calculations reported for polythiophenes.34−37 Results presented in Figure S21 demonstrate clearly that with the exception of the TSe copolymer periodic polymers exhibited smaller values of W in comparison with the statistical analogues. This suggests the periodic polymers are more ordered and have longer conjugation lengths. Thermal Analysis. Thermal characterization of the periodic and statistical copolymers was performed using DSC and TGA. The thermal decomposition range measured by TGA points to good thermal stability of all copolymers with 5% weight loss temperatures (Td5%) above 370 °C (Figure 4). For P3HT and P3HSe homopolymers, the Td5% temperatures were equal to 412 and 372 °C, respectively. Accordingly, thermal decomposition temperatures for both periodic and 2

Interestingly, examination of the solid-state absorption spectra revealed differences between the vibronic structure for the periodic and statistical copolymers (Figure 3), even though all polymers were spin-cast identically from toluene. For the periodic polymer series, the A0−0 vibronic peaks were well-defined and distinguishable. In contrast, for the statistical series these transitions are less clearly defined. The spectra were analyzed using a model developed by Spano and coworkers to describe the photophysical behavior in P3HT.34−37 According to this model, the ratio of the A0−0 and A0−1 peaks in the absorption spectra is given by eq 1, where W and Ep denote respectively the free-exciton bandwidth and the vibrational energy of the CC symmetric stretch of the ring. The intensity of the peak corresponding to the A0−1 transition can be obtained by fitting the spectrum to the vibronic progression predicted by the Spano model.34−37 The value of the exciton bandwidth obtained from eq 1 can then be D

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Figure 7. AFM phase-shift images of ultrathin films visualizing nanofibril morphologies of statistical copolymers (a) stat-TSe31, (b) stat-TSe55, and (c) stat-TSe66 and sequence copolymers (d) TTSe (e) TSe, and (f) TSeSe (inset: scale bar 400 nm).

evaluated in terms of mole quantities, rather than mass. The enthalpy per ring within the polymer chain (ΔHm/ring) was calculated by multiplying the enthalpy (J/g) and the molecular weight of the effective repeat unit (g/mol) (Supporting Information). The molecular weight of the copolymers must also be considered since ΔHm/ring is molecular weight dependent (illustrated in Table 3 when comparing the three different entries for P3HT).32,38,39 Considering the entire copolymer series had high Mn values (26700−39800), a direct comparison of the systems seemed justified (Table 3). Nearly all the statistical and periodic copolymers exhibited enthalpies per ring comparable to high molecular weight P3HT. Periodic copolymer TSe showed the largest deviation from the expected behavior with an enthalpy per ring below 2 kJ/mol. Drastic reduction in crystallization and melt temperatures points to smaller crystallites and higher structural disorder caused by chain imperfections.32 For the TSe copolymer, backbone curvature of the planar chain predicted by computational modeling13 produces more disorder and negatively impacts its thermal response. The curvature observed in computational analysis of TSe is attributed to the different bond angles along the conjugated backbone from the alternating thiophene and selenophene rings (Figure 6). This bending of the chain with coplanar repeat units is not observed for the other sequences (TTSe and TSeSe). Considering that TSe is the only sequence that is expected to have a curved backbone and the disorder of this material is anomalous, it suggests this is the source of the disorder. Interestingly, this effect is absent with stat-TSe55, with a ΔHm/ ring in the range of all other copolymers. AFM Analysis. Nanoscale morphologies of statistical and periodic copolymers were probed by atomic force microscopy (AFM). Phase-shift and height images were acquired on ultrathin films prepared by drop-casting from dilute chloroform solutions directly onto silicon substrates with solvent vapor annealing (Figure 7). In all cases, nanofibril structures were observed with fibril widths ranging from 21 to 33 nm, reminiscent of those reported for rr-P3ATs.40 Periodic copolymer TSe and statistical copolymer stat-TSe31 both exhibited lower average fibril widths indicating more disorder, limiting chain organization (Figure 7). The most

Figure 8. Depiction of polymer stacks adopting either an edge-on (top), face-on (middle), or mixed (isotropic, bottom) orientation (ϕ) with respect to the substrate surface (x,y-plane) normal (z). πStacking azimuthal range for each orientation highlighted (arrow).

statistical copolymers decreased with increasing content of selenophene. Periodic sequences exhibited higher thermal stability in comparison to their statistical analogues. This can be explained by the presence of longer −Se−Se−Se− runs in the statistical copolymers which skews their thermal stability toward the P3HSe homopolymer. DSC traces of the periodic copolymers were also compared to statistical copolymers and homopolymers as illustrated in Figure 5. It became evident that regardless of monomer ordering, all copolymers exhibited lower crystalline and melting transition temperatures (°C) than the homopolymers. Similar trends were observed by McNeil and co-workers when comparing block, gradient, and statistical copolymers17 where the reduced melting temperature range was attributed to less favorable cocrystallization of combined thiophene and selenophene heterocycles in the polymer backbone. Crystallization and melting properties for both the periodic and statistical copolymer sequences are listed in Table 3. The enthalpy of crystallization (ΔHc) and heat of fusion (ΔHm) were calculated from the integrated peak area. Comparison of the melting enthalpies in thiophene- and selenophene-based polymer backbones required that enthalpy values were E

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Figure 9. GIWAXS 2D patterns of annealed samples from CHCl3. Statistical copolymers (a) stat-TSe31 (b) stat-TSe55, and (c) stat-TSe66 and periodic copolymers (d) TTSe, (e) TSe, and (f) TSeSe. Inset: assigned d-spacing. Scale bar: log scale intensities.

Figure 10. GIWAXS packing and relative orientational ordering of periodic copolymers (TTSe (red), TSe (blue), TSeSe (black)), statistical copolymers (stat-TSe31 (dashed red), stat-TSe55 (dashed blue), and stat-TSe66 (dashed black)) and homopolymers P3HT (green) and P3HSe (orange) with respect to selenophene content (%). Top left: π-stacking distance (Å); top right: lamellar spacing (Å); bottom left: azimuthal FWHM (deg); bottom right: radial FWHM (Å−1) of the lamellar (100) Bragg peak.

drastically hindered species was periodic copolymer TSe, whose nanofibril width was less than half of the fully extended polymer chain length. Once again, this is proposed to stem from backbone curvature of the perfectly alternating −T−Se− T−Se− sequences, a recurring theme in all the data collected for this specific copolymer.13 X-ray Scattering. Atomic-scale packing was studied using grazing incidence wide-angle X-ray scattering (GIWAXS) experiments which were performed at the Synchrotron Radiation X-ray Source (CHESS). GIWAXS patterns of thin films of conjugated polymers which can be considered as

derivatives of rr-P3HT typically reveal the presence of two characteristic types of spacings: (a) the lamellar spacing dictated by the length of alkyl side chains between layered polyaromatics and (b) π-stacking spacing of the conjugated backbones of adjacent polymer chains (Figure 8, depiction of possible orientations). For P3HT drop-cast from chloroform, GIWAXS patterns typically exhibit characteristic anisotropy, which is indicative of an edge-on orientation of conjugated heteroaromatic backbones with respect to the substrate (Figure S17).41 In contrast, patterns observed for P3HSe cast under the same conditions are typically more isotropic, pointing to F

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azimuthal disorder and larger π-stacking distances. Although gross electronic properties such as the HOMO−LUMO gap can be tuned equally well by periodic and random sequences, randomness leads to larger degrees of packing disorder, which may be undesirable in some applications for organic electronics.

mixed organization with π-stacking at all angles with respect to the substrate surface (0° < ϕ < 90°). Upon thermal annealing, the packing of P3HSe evolves toward more anisotropic, edgeon orientation, like the one observed for P3HT (Figure S17). The direct comparison of homopolymers and both types of copolymers revealed several interesting morphological effects (Figure 9). All copolymers exhibited an edge-on orientation (like P3HT) with π-stacking distance linearly increasing from 3.7 to 4 Å as the proportion of selenophene rings increased (Figure 10, top left). This points to the increase of the interchain π-stacking distance due to incorporation of a larger heteroatom (Se vs S). Interestingly, the statistical copolymers exhibited larger π-stacking distances than the periodic analogues. This can be explained by the statistical distribution of selenophene rings along the polymer backbone with longer Se−Se−Se segments, favoring π-stacking closer to P3HSe (3.9 Å). The observed variations of π-stacking distances are highly significant, given the critical dependence of charge transport in conjugated polymers on this structural aspect. The lamellar spacing, defined by the aliphatic side chains of the polymers, decreased from 15.8 to 15.2 Å as selenophene content was increased in both the periodic and statistical copolymers (Figure 10, top right). This decrease may be due to the increase in π-stacking distance, which makes more volume available for side chains between the adjacent π-stacks. Altogether, no clear trend was evident, and the spacings for all the copolymers were within the range spanned by the two homopolymers. Additional insights into the relative disorder in periodic and statistical copolymers were obtained by calculating the azimuthal and radial full width at half-maximum (FWHM) of the lamellar (100) Bragg peak.42−44 For statistical copolymers the FWHM of (100) Bragg peaks was higher in comparison to their periodic counterparts, indicating higher orientational disorder (Figure 10, bottom left). As an exception, periodic copolymer TSe exhibited the highest packing disorder as reflected by the large FWHM of the (100) peak and by the increase of the corresponding d-spacing, (Figure 10, bottom right). This anomaly has been attributed to the curvature of this copolymer’s backbone predicted by computational modeling.13 stat-TSe66 (Figure 9c) revealed mixed orientations, reminiscent of P3HSe (Figure S17), likely due to the presence of longer −Se−Se−Se− runs. Periodic sequences TTSe and TSeSe revealed the highest degree of anisotropy with FWHM demonstrating highly oriented (edge-on) packing with respect to the substrate surface,43 even with relatively large quantities of selenophene along the backbone. This observation suggests that the presence of periodic sequences promotes more ordered packing of polymer chains.

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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.8b01434. Experimental details; Figures S1−S22 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. ORCID

David Yaron: 0000-0001-8485-8685 Tomasz Kowalewski: 0000-0002-3544-554X Kevin J. T. Noonan: 0000-0003-4061-7593 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.J.T.N. is grateful to the ARO (W911NF-16-1-0053) and NSF for a Career Award (CHE-1455136). REFERENCES

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CONCLUSION In summary, we have synthesized and compared conjugated copolymers with statistical and periodic arrangements of 3hexylthiophene and 3-hexylselenophene. Utilization of CTP to prepare these materials ensured relatively good control over molecular weight and dispersity and facilitated direct comparison of the samples. The bandgap energies of the statistical and periodic copolymers depended only on composition and not on monomer ordering. However, the less distinct vibronic structure of the UV−vis for the statistical copolymers hinted at larger degrees of disorder in comparison with periodic sequences. Accordingly, X-ray scattering analysis revealed that statistical copolymers showed higher degrees of G

DOI: 10.1021/acs.macromol.8b01434 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01434 Macromolecules XXXX, XXX, XXX−XXX