Regioregular Polymer Analogous Thionation of Naphthalene Diimide

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Regioregular Polymer Analogous Thionation of Naphthalene Diimide−Bithiophene Copolymers Young-hun Shin,† Adam Welford,‡ Hartmut Komber,§ Rukiya Matsidik,† Thomas Thurn-Albrecht,∥ Christopher R. McNeill,‡ and Michael Sommer*,† †

Institut für Chemie, Technische Universität Chemnitz, Straße der Nationen 62, 09111 Chemnitz, Germany Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, Victoria 3800, Australia § Leibniz Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany ∥ Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz 3, 06099 Halle, Germany ‡

S Supporting Information *

ABSTRACT: Polymer analogous thionation of the n-type conjugated polymer PNDIT2 is investigated using Lawesson’s reagent (LR). Detailed high-temperature NMR spectroscopic investigations show that due to the copolymer structure, two out of the four available carbonyl groups present in the naphthalene diimide (NDI) comonomer are sterically less hindered and react preferentially. This leads to regioselective thionation in the trans-configuration even for a large excess of LR. For high degrees of O/S conversion, signals of minor intensity show up in addition pointing to undesired side reactions. These signals could not be eliminated despite further optimized reaction conditions including different aromatic solvents and reaction temperatures. Compared to PNDIT2, the resulting 2S-trans-PNDIT2 features strong aggregation, lower solubility, an 80 nm bathochromic shift of the charge-transfer band, a by 0.22 eV lower LUMO energy level, a lower thermal stability, and higher melting temperatures (Tm). As the combination of the lower thermal stability and higher melting points renders the characterization of thermal transitions challenging, fast scanning calorimetry (flash-DSC) is successfully used to determine Tm. With increasing O/S conversion, Tm first increases but then decreases, which is ascribed to a combined effect of stronger main chain interactions and increasing chemical defects. Microstructural order and field-effect electron mobilities decrease with increasing O/S conversion compared to PNDIT2.

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INTRODUCTION Organic semiconductors have great potential to realize organic electronic devices which can be designed to be flexible as well as lightweight and be made at low cost and with high throughput.1−3 Compared to p-type organic semiconducting materials, n-type organic semiconducting materials are less developed.1,4,5 Despite n-type materials are catching up, the number of available structures and their electrical performance are still behind those of p-type copolymers. Doping of organic semiconductors, which is a stoichiometric redox process, renders conjugated polymers conductivea property useful for antistatic coatings, injection layers, or thermoelectric applications.6 Doping of n-type conjugated polymers, i.e., their reduction, requires efficient electron donors such as pyronin B,7−9 1H-benzoimidazol-2-yl derivatives,10−13 and 4(4-methoxyphenyl)-1-methyl-2,6-diphenyl-1,4-dihydropyridine.14 However, the reoxidization of reduced n-type organic semiconducting materials (i.e., the oxidation of radical anions) by oxygen has been recognized as one of the main obstacles for achieving air-stable materials. Hence, one factor governing air stability of radical anions is the LUMO level position, with a LUMO energy level more negative than −4.0 eV being required.15 For this reason, designing polymers with consid© XXXX American Chemical Society

erably low LUMO energy levels is of central importance for developing dopable, efficient, and air-stable n-type materials.4,5,16,17 Recently, among the n-type polymers available, poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}, here abbreviated as PNDIT2, has been investigated extensively.18,19 This copolymer, which comprises alternating naphthalene diimide (NDI) and bithiophene (T2) units, has been recognized as an outstanding n-type copolymer due to its good solubility, high stability, high crystallinity, relatively low LUMO energy level (−3.7 eV), and high electron mobility.18,20,21 However, the LUMO energy level of n-type copolymers with NDI in the main chain is in most cases not deep enough to prevent oxidation of its charge carriers by oxygen or water.22,23 Efforts have been made to exchange the imide oxygens of small molecule NDIs by mono- or dithioimide groups using Lawesson’s reagent (LR), which leads to more negative LUMO energy levels.24−27 However, when using NDI as substrate, the Received: November 7, 2017 Revised: January 5, 2018

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DOI: 10.1021/acs.macromol.7b02355 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules reaction with LR leads to a mixture of mono-, di- (cis, trans), tri-, and tetra-thionated NDIs. Compared to non-thionated NDIs, all thionated NDIs showed enhanced crystallinities and electron mobilities, reduced optical band gaps, lower-lying LUMO energy levels, and decreased thermal stabilities. Welterlich et al. investigated polymer analogous thionation of diketopyrrolopyrrole (DPP)-based donor−acceptor copolymers. Compared to the non-thionated copolymers, the thionated DPP-based copolymers showed smaller optical band gaps and lower LUMO energy levels but also significantly reduced photostabilities.28 Here, PNDIT2 is subjected to polymer analogous thionation with LR. During the finalization of this paper, Pahlavanlu et al. disclosed the microwave-assisted thionation of NDI-based monomers and also presented the first results on the thionation of NDI copolymers.29 However, with characterization of the thionated NDI copolymers being accomplished by IR spectroscopy,29 a detailed NMR spectroscopic characterization coupled to robust synthesis protocols remains to be done to fully characterize thionated PNDIT2, which is hereafter referred to as 2S-trans-PNDIT2. We prepared PNDIT2 via direct arylation polymerization (DAP)21 with relative numberaverage molecular weights Mn,SEC ∼ 12−13 kg/mol (corresponding to absolute values Mn,NMR ∼ 8.6 kg/mol) and performed the thionation reaction under several conditions (Table 1). A central result is that the oxygen/sulfur exchange of PNDIT2 proceeds in a regioregular manner for steric reasons, with the sterically less hindered carbonyl groups pointing away from the neighbored T2 comonomer being reactive. The reaction conditions are optimized to control conversion of O/S by usage of different solvents, different temperatures, and different amounts of LR. High-temperature 1H NMR spectroscopy enables to estimate O/S conversion reliably. While this polymer analogous thionation is 2S-trans-selective in principle, full conversion is only obtained for a large excess of LR, for which small amounts of unidentified main chain defects are seen in addition. These defects most likely cause a significant reduction of the melting temperature of 2S-trans-PNDIT2, microstructural order, and field-effect mobilities.

Table 1. Summary of Thionation Reaction Conditions of PNDIT2 entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

parameter changed amount of Lawesson’s reagent

solvent

amount of Lawesson’s reagent

reaction temperature

LRb

convc (%)

T (°C)

PX

1.6

77

130

14/31

PX PX PX PX PX CB DCB CN TCB CB

2.6 3.2 4.8 6.4 7.8 1.6 1.6 1.6 1.6 2.6

89 92 94 96 99 85 79 74 78 94

130 130 130 130 130 130 130 130 130 130

15/96 15/175 15/305 15/566 15/1036 9/55 9/36 10/28 10/33 7/154

CB CB CB CB CB CB CB

3.2 3.8 4.8 5.8 6.4 7.8 3.8

96 97 97 98 99 99 45

130 130 130 130 130 130 100

8/355 8/431 9/726 9/945 13/1761 11/3724 12/25

CB CB CB CB

3.8 3.8 3.8 3.8

69 91 99 99

110 120 140 150

12/24 12/60 18/2097 16/9044

solventa

Mn,SEC/Mw,SECd (kg/mol)

a

PX, CB, DCB, TCB, and CN are p-xylene, chlorobenzene, 1,2dichlorobenzene, 1,2,4-trichlorobenzene, and 1-chloronaphthalene. b Mol equiv of Lawesson’s reagent per NDI unit. cConversion of imide to thioimide determined from 1H signal integrals of H2′ and H2. d From SEC in CHCl3. The yield of the reaction was 94−99%. Number-average molecular weights Mn (2S-trans-PNDIT2) are ∼9 kg/mol for all entries.

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imide group (see also Figures S1−S3). No NCH2 signal was discernible at about 5.3 ppm characteristic for dithionation of the imide ring.24 Furthermore, the signal of the NDI proton next to the CO group shifts from 8.9 ppm (H1′) to 9.25 ppm (H1). Keeping in mind that only monothionation of an imide group occurs, complete disappearance of the 8.9 ppm signal indicates complete 2-trans dithionation of the NDI units. Any 2-cis dithionation retains a proton neighboring a CO group and thus signal intensity at 8.9 ppm. The almost complete disappearance of this signal indicates that thionation occurs at the sterically less hindered carbonyl groups, whereas the carbonyl group next to T2 remains intact, even for a large excess of LR. The thionation of carbonyl groups by LR starts with the dissociation of LR into two dithiophosphine ylide units. This dithiophosphine ylide forms a four-membered cyclic intermediate with the carbonyl group, giving a thiocarbonyl group and an oxathiophosphine ylide through cycloreversion.30,31 Obviously, as dithionation occurs in a 2S-trans fashion, the T2 comonomer but also a tolyl end group prevent the formation of the cyclic intermediate at the neighboring carbonyl group. PNDIT2 subjected to a large excess of LR, to higher temperatures, or to prolonged reaction times shows an almost complete 2S-trans structure but additionally exhibits broad signals of very low intensity in the 7.9−6.9 ppm region as well

RESULTS AND DISCUSSION The educt polymer PNDIT2 was synthesized in high yield via direct arylation polycondensation (DAP) following the protocol of Matsidik et al.21 A moderate relative numberaverage molecular weight Mn,SEC ∼ 12−13 kg/mol was chosen to ensure good solubility of the product and was used for all entries. Thionation of PNDIT2 with LR was done in various common aromatic solvents such as p-xylene (PX), chlorobenzene (CB), 1,2-dichlorobenzene (DCB), 1,2,4-trichlorobenzene (TCB), and 1-chloronaphthalene (CN) at 100−130 °C for 24 h (Scheme 1 and Table 1). During the reaction the color of the solution changed from blue to green, which could be used as a rough indicator for good conversion. After 24 h, the solution was cooled, and the polymer precipitated into methanol followed by Soxhlet extraction with methanol overnight and finally collected with chloroform. First insight into the thionation process can be obtained from high-temperature 1H NMR spectroscopy. On comparing the spectrum of a partially thionated PNDIT2 (Figure 1a) with that of the parent PNDIT2 (Figure 1b), characteristic changes become apparent. The signal of the NCH2 group of PNDIT2 at 4.19 ppm diminishes (H2′), and a corresponding signal at 4.78 ppm appears (H2), which results from monothionation of the B


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Macromolecules Scheme 1. Thionation of PNDIT2 Using Lawesson’s Reagent (LR)a

a

The 2S-trans-PNDIT2 as the main product is shown for simplicity. However, monothionated NDI units are also present for conversion 4.8 equiv of LR.

Figure 3. 1H NMR spectra of thionated PNDIT2 of different conversion of the imide group (solvent: C2D2Cl4 at 120 °C). The polymers were synthesized in p-xylene at 130 °C using increasing molar equivalents of LR per NDI unit (entries 1−6). D


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Macromolecules Finally, the effect of temperature on the thionation reaction was elucidated using CB and 3.8 equiv of LR (Table 1, entries 13, 18−22). Conversion strongly increased to ∼97% when reaction temperature increased from 100 to 130 °C and then further increased slightly to 99% at 140 and 150 °C (Figure 4). UV−vis spectroscopy of 2S-trans-PNDIT2 was performed on solutions in toluene and chloronaphthalene. For PNDIT2, toluene and chloronaphthalene are known to yield aggregates and unimolecularly dissolved chains, respectively.32 Compared to PNDIT2, 2S-trans-PNDIT2 exhibited additional bands at ∼500 nm, which stem from the 2S-trans-NDI structure as seen by the comparison with the small molecule 2S-trans-NDI (Figure 5). For both solvents, 2S-trans-PNDIT2 showed a red-

Table 2. Summary of OFET Performance Metrics for PNDIT2 and 2S-trans-PNDIT2 polymer PDNIT2 2S-trans-PNDIT285% 2S-trans-PNDIT293% 2S-trans-PNDIT295%

sat. mobility (cm2/(V s))

threshold voltage (V)

on/off ratio

0.16 ± 0.03 0.041 ± 0.004

3.0 ± 0.5 14 ± 1

105 105

0.024 ± 0.002

20 ± 1

104

0.018 ± 0.002

21 ± 1

105

Figure 5. UV−vis absorption spectra of PNDIT2, 2S-trans-PNDIT2 (99% conversion), and the small molecule 2S-trans-NDI in toluene (Tol) and in chloronaphthalene (CN).

shifted charge-transfer (CT) absorption band with a bathochromic shift of ∼80 nm. In chloronaphthalene, the CT band is broad and unstructured for both PNDIT2 and 2S-transPNDIT2, while in toluene additional vibronic bands show up that are overall lower in energy. The lower intensity of the CT band of 2S-trans-PNDIT2 might hint at a higher torsional angle between the comonomers 2S-trans-NDI and T2, which in turn can arise from the larger sterical demand of sulfur. An increasing conversion of the thionation reaction can also be followed by UV−vis spectroscopy, where the red-shift of the CT band is clearly increasing with increasing conversion (see Figures S6 and S7). The estimated optical band gaps from the onset wavelength of the solution spectra showed a clear trend of smaller band gaps for samples with higher conversion as compiled in Table 2. Optical band gaps decreased by ∼0.2 eV upon thionation showing Eg,opt = 1.42 eV for conversion = 99%. To determine the LUMO energy level of 2S-trans-PNDIT2, cyclic voltammetry was measured on various films with conversion >75% (Figure 6). Regardless of conversion, a typical two redox peak behavior was observed for PNDIT2 as well as for all 2S-trans-PNDIT2 samples. 2S-trans-PNDIT2 exhibited two reductions at −0.4 and −0.6 V versus ferrocene/ ferrocenium. With the first reduction taking place at ∼0.22 V, a deeper LUMO energy level of −4.0 eV can be determined for 2S-trans-PNDIT2 compared to PNDIT2 (−3.7 eV). This corresponds to the trend also seen with small molecule

Figure 6. Cyclic voltammetry of films of 2S-trans-PNDIT2 as a function of conversion compared to PNDIT2. The curves are offset along the y-axis for clarity.

NDIs.24,26,27,35 The determined reduction potentials and estimated LUMO energy levels were almost constant for conversions between 77% and 99%. Thermogravimetric analysis (TGA) measurements were conducted to probe thermal stability of 2S-trans-PNDIT2. Thionation of PNDIT2 reduces the excellent thermal stability of PNDIT2 (T95 = 449.8 °C) to values between T95 = 356 and 331 °C, whereby higher conversions caused slightly lower thermal stabilities (Figure S8). With the reduced thermal stability of 2S-trans-PNDIT2 compared to PNDIT2, measuring thermal transitions by conventional differential scanning calorimetry (DSC) appeared difficult, as possible transitions such as main chain melting may show up at regions close to degradation. To this end, “flash” DSC was carried out enabling fast heating and cooling rates. Under these conditions, by minimizing the dwell time at high temperature, it is often possible to probe transitions at or even above the TGA degradation temperature.36 To check if this approach could also be used for the samples investigated here, repeated heating and E


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Macromolecules cooling scans across the melting temperature were performed at a rather high rate of 500 K/s. The results were similar indicating that degradation during the scans was negligible. Figure 7 shows the melting points Tm of the second heating

Figure 7. Flash-DSC measurements (heating and cooling rate 500 K/ s) of 2S-trans-PNDIT2 with various O/S conversion. Melting and crystallization temperatures were taken from the second heating and fifth cooling cycle.

cycle as a function of conversion (for entire DSC scans and all values see Figures S9, S10 and Table S1). Interestingly, Tm first increases from Tm = 282 °C (PNDIT2) to Tm = 322 °C (2Strans-PNDIT2 with 82% conversion) but afterward continuously decreases with further increasing conversion of imide groups (Figure 7). The same trend was found for the crystallization temperature Tc. The trend of an increasing and afterward decreasing transition temperature for continuously increasing conversion of imide groups is unexpected at first glance. However, considering the chemical defects, such behavior can tentatively be ascribed to a combined effect of stronger aggregation in 2S-trans-PNDIT2 compared to PNDIT2 coupled to increasingly occurring defects with increasing conversion. Defect signals of minor intensity were observed in the 1H NMR spectra, and it is well conceivable that a few percent of chemical defects, which are excluded from the crystal lattice, lead to a reduction of the melting temperature of the observed order of magnitude. To assess the charge transport properties of 2S-transPNDIT2, organic field effect transistors (OFETs) were prepared. A top-gate bottom-contact configuration was adopted with CYTOP used as the gate dielectric. Figure 8 presents the output and transfer characteristics of transistors based on 2Strans-PNDIT2 with O/S conversion of 85%, 93%, and 95%, referred to as 2S-trans-PNDIT2-85%, 2S-trans-PNDIT2-93%, and 2S-trans-PNDIT2-95%, respectively. Also shown for comparison are the characteristics of PNDIT2 as a reference.37 Films were spin-coated from dichlorobenzene and annealed at 110 °C following protocols that optimize the performance of PNDIT2 transistors. The average saturation mobility for the PNDIT2 transistor was 0.16 cm2/(V s), which is consistent with previous values of PNDIT2 with this molecular weight processed from dichlorobenzene.37 For the 2S-trans-PNDIT2 transistors, lower average mobilities were measured with values of 0.041, 0.024, and 0.018 cm2/(V s) for 2S-trans-PNDIT285%, 2S-trans-PNDIT2-93%, and 2S-trans-PNDIT2-95%, respectively (Table 2). Significantly higher threshold voltages were also recorded for 2S-trans-PNDIT2 compared to PNDIT2. Thus, unlike for small molecule NDI materials,24

Figure 8. Output (left) and transfer (right) characteristics of OFETs based on PNDIT2 (a, b), 2S-trans-PNDIT2-85% (c, d), 2S-transPNDIT2-93% (e, f), and 2S-trans-PNDIT2-95% (g, h).

thionation does not appear to improve the mobility of PNDIT2, with mobility systematically decreasing with increasing thionation conversion. In order to understand the lower mobilities of 2S-transPNDIT2, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed on thin films prepared using the same condition as for OFETs. Figure 9 presents the two-dimensional scattering patterns, with onedimensional traces taken along the in-plane and out-of-plane directions presented in Figure S11. Compared to PNDIT2, the 2S-trans-PNDIT2 films show reduced crystalline order. Although the thionated samples have very similar lamellar stacking and backbone d-spacings, these peaks are less pronounced. Furthermore, the thionated samples do not show any prominent π−π stacking peak (see profiles in Figure S11), suggesting that π−π stacking is disrupted in these samples. The main chain−side chain separation peak at ∼2.5 Å−1 however is still prominent in the thionated samples, but second-order reflections expected at ∼5.0 Å−1 are not discernible. While the 2S-trans-PNDIT2 chains show a stronger tendency for aggregation (vide supra), apparently this does not F


Macromolecules

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02355. Details of synthesis, measurements, and experimental procedures; additional NMR, GPC, and UV−vis spectroscopy data; and thermal analysis data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.). ORCID

Thomas Thurn-Albrecht: 0000-0002-7618-0218 Christopher R. McNeill: 0000-0001-5221-878X Michael Sommer: 0000-0002-2377-5998 Notes

The authors declare no competing financial interest.

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Figure 9. Two-dimensional GIWAXS patterns of (a) PNDIT2, (b) 2Strans-PNDIT2-85%, (c) 2S-trans-PNDIT2-93%, and (d) 2S-transPNDIT2-95%.

ACKNOWLEDGMENTS Funding by the Deutsche Forschungsgemeinschaft (Project SO 1213/8-1) is greatly acknowledged. We thank M. Hagios (University of Freiburg) for SEC measurements, A. Warmbold (University of Freiburg) for TGA measurements, and K. Herfurt (University of Halle) for flash-DSC measurements. Part of this research was performed at the SAXS/ WAXS beamline at the Australian Synchrotron, part of ANSTO. This work was also perfromed in part at the Melbourne Centre for Nanofabrication (MCN) at the Victorian Node of the Australian National Fabrication Facility (ANFF).

lead to a higher degree of bulk and thin film crystallinity. The reduced microcrystalline order for O/S conversions between 85 and 99% compared to PNDIT2, and in particular the reduced π−π stacking order, is consistent with the reduction in Tm seen in flash-DSC experiments. The chemical defects of unknown nature seen in the 1H NMR spectra are one possibility to explain the lower degree of order in thionated samples, which in turn can reduce OFET performance.

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REFERENCES

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CONCLUSION

Using detailed high-temperature NMR spectroscopy, we have shown that the reaction of PNDIT2 with Lawesson’s reagent (LR) occurs in a regioregular fashion with only two out of four available imide carbonyl groups being replaced by thiocarbonyl groups in a trans arrangement. The reason for the observed regioselectivity is sterical hindrance brought about the bithiophene comonomer T2. This allows an excess of LR being used without observing triple thionation, which is different from small molecule NDIs. The resulting material 2S-trans-PNDIT2 is sufficiently stable for all necessary characterization. The replacement of two imide carbonyl groups by thiocarbonyl groups has a strong effect on all properties. Defects of minor extent are seen by high temperature NMR spectroscopy for PNDIT2 samples subjected to a large excess of LR or higher reaction temperatures (corresponding to conversion approaching 100%). While the nature of these defects remains unclear at present and is subject of ongoing investigations, they are likely responsible for the decrease in melting temperature and thin film microstructural order, and the combination of all these effects leads to reduced OFET electron mobility compared to PNDIT2. Nevertheless, a by ∼0.2 eV lowered LUMO energy level makes 2S-transPNDIT2 a promising candidate for doping applications. G


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Regioregular Polymer Analogous Thionation of Naphthalene Diimide

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