StructureâFunction Relationships of High-Electron Mobility

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Structure-function relationships of high-electron mobility naphthalene diimide copolymers prepared by direct arylation Alessandro Luzio, Daniele Fazzi, Fritz Nübling, Rukiya Matsidik, Alexander Straub, Hartmut Komber, Ester Giussani, Scott E. Watkins, Mario Barbatti, Walter Thiel, Eliot H. Gann, Lars Thomsen, Christopher R. McNeill, Mario Caironi, and Michael Sommer Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 10 Oct 2014 Downloaded from http://pubs.acs.org on October 10, 2014

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Chemistry of Materials

Structure-function relationships of high-electron mobility naphthalene diimide copolymers prepared by direct arylation Alessandro Luzio1, Daniele Fazzi2, Fritz Nübling3, Rukiya Matsidik3, Alexander Straub3, Hartmut Komber4, Ester Giussani1, Scott E. Watkins5, Mario Barbatti2, Walther Thiel2, Eliot Gann6,7, Lars Thomsen7, Christopher R. McNeill6, Mario Caironi1, Michael Sommer3,8 1

Center for Nanoscience and Technology @PoliMi, Instituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy 2

Max-Planck-Institut für Kohlenforschung, (MPI-KOFO), Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany

3

Universität Freiburg, Makromolekulare Chemie, Stefan-Meier-Str. 31, 79104 Freiburg, Germany

4 5

Leibniz Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany

CSIRO Future Manufacturing Flagship, Bayview Avenue, Clayton, Victoria, 3169, Australia

6

Department of Materials Engineering, Monash University, Wellington Road, Clayton, Australia

7

Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria, 3168, Australia

8

Freiburger Materialforschungszentrum, Stefan-Meier-Str. 21, Universität Freiburg, 79104 Freiburg, Germany

ABSTRACT: Direct arylation (DA) is emerging as a highly promising method to construct inexpensive conjugated materials for large area electronics from simple and environmentally benign building blocks. Here we show that exclusive α-C-H selectivity is feasible in the DA of π-extended monomers having unsubstituted thiophene or furan units, leading to fully linear materials. Two new naphthalene diimide-based conjugated copolymers P(FuNDIFuF4) and P(ThNDIThF4) comprising naphthalene diimide (NDI) , furan (Fu) or thiophene (Th), and tetrafluorobenzene (F4), are synthesized. Insight into structure-function relationships is given by DFT calculations and variety of experimental techniques, whereby the effect of the heteroatom on the optical, structural and electronic properties is investigated. The use of furan (Fu) allows for enhanced solubilities, a smaller dihedral angle between NDI and Fu as a result of the smaller size of Fu, and a smaller π-πstacking distance in the solid state. P(FuNDIFuF4) also exhibits a more edge-on orientation compared to P(ThNDIThF4). Despite these advantageous properties of P(FuNDIFuF4), P(ThNDIThF4) exhibits the highest electron mobilities of ~1.3 cm2/Vs, which are greater by a factor of ~3 compared to P(FuNDIFuF4). The enhanced OFET performance of P(ThNDIThF4) is explained by reduced orientational disorder and the formation of a terrace-like thin film morphology.

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1.

Introduction

The manifold advantages of direct arylation (DA) in comparison to conventional transition metal-catalyzed crosscoupling techniques such as the widely used Stille reaction have been fascinating scientists for many years.1-3 The use of DA in material and polymer science is a lately discovered field with clear advantages regarding simplicity, cost, environmentally friendly building blocks, scalability, and purity.4-9 As DA polycondensation (DAP) does not rely on the use of organometallic building blocks, inexpensive, stable, and non-toxic monomers can be used that guarantee upscaling and purer materials at the same time.10 These unrivaled benefits are partially compromised by the issue of C-H selectivity in monomers with several C-H bonds11, and the fact that multi-component systems including carboxylates as additives are required which increase the parameter space to be screened. While a few percent yield of unselective C-H arylation may not cause major problems in DA of small molecules where purification is simple, such side reactions are detrimental to conjugated polymer synthesis as they result in defective backbone structures, which strongly alter electronic properties and hinder structure formation.12-14 Meaningful results from materials made by DAP require knowledge on the extent C-H selectivity to correlate differences in structure-function relationships with potential backbone defects. If monomers with different C-H bonds are used, branching or even cross-linking is expected to occur at varying degrees15-18, which not only reduces yield but most importantly leads to ill-defined electronic and structural properties. Ways to circumvent unselective C-H arylation are blocking the C-H bonds of interest or decreasing temperature to increase discrepancy between dissimilar bonds.15,19-21 However, especially the former approach is tedious and additionally leads to increased dihedral angles in the conjugated backbone, which strongly alters opto-electronic performance.19,20,22 Hence, the use of unsubstituted building blocks that allow efficient backbone planarization is an important design principle for conjugated polymers with high electronic performance. The demonstration that DAP can be used to deliver well-defined and high performance materials based on a wide range of monomers is an important step in realizing the full potential of this promising polymerization technique. Here, we show that the π-extended monomers FuNDIFu 1 and ThNDITh 2 with an electron-deficient naphthalene diimide (NDI) core unit and two flanking, unsubstituted furan (Fu) or thiophene (Th) units are ideal C-H monomers in that they are easy to prepare and exhibit selective α-arylation when performing direct arylation polycondensation with 1,4-dibromo-2,3,5,6-tetrafluorobenzene 3 (F4) (Scheme 1), giving 4 and 5. Furthermore, in a comparative study, we present detailed structure-function relationships and explain how the replacement of oxygen in P(FuNDIFuF4) by sulfur in P(ThNDIThF4) changes the optical, structural, and electronic properties of the material. Both new copolymers exhibit high electron mobilities up to 1.3 cm2/Vs, which is comparable to the best electron mobilities of NDI copolymers reported23-26, but orders of magnitude higher compared to NDI copolymers made by DAP.27 Considering the moderate molecular weight of the

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new materials, the already high charge carrier mobilities may be improved even further. O

R N

F

O

F

Br X X O

N R

O

3 F

O

R N

F

O

F

Br X F

Pd(OAc) 2 KOPiv DMAc/ solvent µW

X= O: FuNDIFu 1 X= S: ThNDITh 2a,b

F

X O

N R

n F

O

X= O: P(FuNDIFuF 4) 4 X= S: P(ThNDIThF 4) 5a,b

Scheme 1. DAP of furan- (X=O) and thiophene- (X=S) flanked naphthalene diimide and 1,4-dibromo-2,3,5,6tetrafluorobenzene. R is 2-octyldodecyl for 1, 4, 2a and 5a, and 2-decyltetradecyl for 2b and 5b. Solvent: THF, toluene, chlorobenzene. 2.

Results and Discussion

The rational design of the new materials is based on excellent electron mobilities of NDI copolymers26 and on the electron-withdrawing character of the tetrafluorobenzene unit F4, which lowers the energy of the LUMO.28 Flanking NDIs with two Th or Fu units enables facile purification of 1 and 2 by recrystallization29,30 and introduces reactive αprotons for DA. The rationale behind the use of Fu in comparison to Th is given by improved solubilities of furan-based conjugated polymers31; thus larger molecular weights (MWs) and/or shorter side chains become possible. Equally important, the dihedral angle between NDI and Fu is expected to be smaller than that between NDI and Th as a result of reduced steric hindrance. Here, the question arises of how smaller angles influence the optoelectronic properties through enhanced donor-acceptor interactions. The copolymerizations of 1 and 3 were performed starting from an inexpensive and simple phosphine-free protocol with Pd(OAc)2 and KOPiv in 1:1 DMAc:co-solvent mixtures, whereby THF, toluene and chlorobenzene were screened as co-solvent. Phosphine-free conditions are highly attractive due to their simplicity, low cost, and enhanced reaction rates.32 The parameters that gave the highest MW of 4 were 8 mol-% Pd(OAc)2, 100 °C, and a reaction time of 3 h. The highest molar masses were obtained for toluene and chlorobenzene as co-solvents. Lower and higher reaction temperatures, lower catalyst concentrations, and other co-solvents such as THF all furnished lower MWs. Materials 5 made from the thiophenebased comonomers 2 generally gave lower MWs than 4. Therefore the longer side chain in 2b was used giving 5b. Molecular weight determination by size exclusion chromatography (SEC) was most reliable in trichlorobenzene at 150 °C. Initially measured samples in THF or CHCl3 at RT gave all higher molar masses most likely due to aggregation, which is often observed for NDI copolymers (Figure S1).24 Table 1 shows the SEC data of 4, 5a and 5b, which exhibit Mn,SEC= 6.6, 7.2 and 7.8 kg/mol, respectively, and dispersities Đ= 1.45, 1,43, and 1.67, respectively. For comparison, the absolute number average degrees of polymerization from NMR were DPn,NMR= 6, 4 and 5, respectively, corresponding to number average molecular weights Mn =6.8, 4.9 and 6.3 kg/mol, respectively (see also section on NMR). This comparison clearly shows that NMR yields


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different results and enables a better comparison of molecular weights in this MW region by providing absolute values. As the chain length can be considered as the most important parameter that governs charge carrier mobility in this MW region, we use 4 and 5b with DPn,NMR of 6 and 5, respectively, in this comparative study. We note that these molecular weights are moderate, however the very similar chain lengths still justify a reliable comparison. Below we will point out reasons for chain termination which lead to reduced molar masses. Unselective C-H arylation of thiophenes or furans is frequently discussed as a detrimental side reaction in DA, whereby C-H selectivity between the 2-and 3-protons is slightly decreased in furan compared to thiophene.11 Hence, monomers with multiple C-H bonds can in principle be prone to undergo unselective C-H arylation, and the reaction conditions will decide whether the proton in 2position with higher reactivity reacts exclusively (also referred to as α-arylation). While certain observations such as altered UV-vis spectra, lower degrees of crystallinity or decreased yields are taken as indirect indicators for the occurrence of backbone kinks or branches induced by unselective arylation additionally in 3-position (referred to as β-arylation), direct spectroscopic evidence is rare.14,21 Here, 1 H and 19F NMR spectroscopy were performed to unambiguously prove selective α-arylation. NMR spectra of 4 and 5b were taken at 120 °C in C2D2Cl4 to reduce line broadening, which is essential to reliable signal assignment. For the NMR spectrum of 5a see Figure S2a.

In the 1H NMR spectra of both 4 and 5b, all signals including end groups were assigned, further corroborating selective α-arylation (Fig. 1a,b). Signal assignment was made on the basis of several two-dimensional NMR experiments and the comparison with the monomer spectra (Fig. S2b,c). Comparing the chemical shifts of H2’ and H2 it is obvious that α-arylation by F4 results in a significant deshielding effect of ~0.6 ppm and ~0.3 ppm for the proton in 3-position of Fu and Th, respectively. It can be assumed that this effect is similar on the proton in 2-position in case of 3-substitution.33 Thus, any β-arylation should lead to a new Fu or Th signal for the corresponding 2proton with chemical shifts above 7.9 ppm for both 4 and 5b. This region in the 1H NMR spectra is not covered by other signals, and hence shows the absence of β-arylation. Also the 19F NMR spectra of 4 and 5b only exhibited one backbone signal and two sets of end groups arising from F4-H and -F4-Br chain ends (Fig. 1c,d), and there is no hint for an asymmetrically substituted F4 unit located between the 2- and 3-position of Fu or Th. Therefore, these NMR data show the absence of β-arylation within the limits of the method, and hence verify the linear structures of 4 and 5b. End group analyses further show Fu- or Th- terminated chains (Fig. 1a,b, signals 1´, 2´, and 3´), and F4-terminated chains with either F4-H or F4-Br terminals (Fig. 1c,d, signals 6°, 7°, and 6*, 7*), as expected for a polycondensation reaction. Considering the 1H NMR signal intensities of all end groups also allow determining number average molecular weights Mn more precisely than SEC.

Figure 1. 1H NMR spectra (region of aromatic protons) (a,b) and 19F NMR spectra (c,d) of P(FuNDIFuF4) 4 (a,c) and P(ThNDIThF4) 5b (b,d) with assignment of main chain and end group signals (# signal from -C6F4- unit next to an end group). Spectra were recorded at 120°C in C2D2Cl4.



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The Mn values were 6.3 and 6.8 kg/mol for 4 and 5b, respectively, corresponding to number average degrees of polymerization DPn of 6 and 5, respectively (Table 1). It is also interesting to note that next to F4-Br terminals F4-H end caps are seen with considerable intensity, which arise from debromination. While this side reaction has been observed before in DAP21 and also in Suzuki polycondensation34, its mechanistic origin under the DAP conditions used is unclear.35 Obviously, debromination leads to inactive chain ends and is therefore one reason for the limited molar masses obtained. What is remarkable at this point is that expected side reactions such as unselective C-H arylation do not play a role here, while other side reactions such as dehalogenation are prevalent. A reason for the absence of β-arylation could be the used temperature of 100 °C, which is lower than those used in reports with similar DAP conditions.7-9 Also, it is possible that the presence of the acceptor unit NDI increases the difference in C-H selectivity of the α- and β-positions, even though this has not been investigated in great detail.11,21 Therefore, a profound understanding of mechanistic features of DAP is important, and should be addressed by future studies. In the case of the DAP leading to 4 and 5, the use of 19F NMR will be most instructive to follow the process of debromination under different reaction conditions. The structural and optical properties of 4 and 5b were investigated by UV-Vis spectroscopy and DFT calculations (Figure 2). Figure 2a shows absorption spectra which display marked differences. In solution, the broad low energy absorption charge transfer (CT) band of 4 is red-shifted compared to 5b (see Table S1 for details) and the intensity ratio between the CT and the high energy band is higher for 4 than for 5b. In the solid state (film thickness ~50 nm), the CT band of 5b is considerably red-shifted with respect to solution. For 4, the red-shift of the CT band from solution to solid-state is smaller, but the intensity increases strongly. Additionally, a shoulder already present in solution (591 nm) gives rise to a new peak at 617 nm. Lowenergy shoulders in the CT band have been observed for similar copolymers36-38, which are assigned to the absorption of aggregates. Their intensities depend on local polymer chain packing, thus being sensitive to annealing and morphological changes.39 UV-vis analysis on thermally annealed films showed little changes for 5b, but significant changes in the relative intensity of the band at 617 nm of 4 with increasing annealing temperature (Figure S3,4). Further experiments that allow investigating the relative orientation of two stacked chains40 are in progress. From the absorption spectra of thin films, we extracted optical band gaps Eg,opt of 1.90 eV and 1.70 eV for 4 and 5b, respectively (Table 1). The HOMO and LUMO values were estimated from their ionization potentials (IP) obtained by photoelectron spectroscopy in air (PESA) and cyclic voltammetry of films (CV), respectively (Figure S5). For 4 and 5b, LUMO values of -3.75 and -3.85 eV, respectively, and HOMO values of -5.75 and -5.50 eV, respectively, were obtained (Table 1). Hence, the presence of Th or Fu only weakly affects the LUMO, a trend commonly observed in NDI copolymers. To understand the structural and electronic properties, DFT calculations of both the repeat units and the tetramers of 4 and 5 were performed (Figure 2b-c and Figures S6,7). A detailed DFT analysis is found in the Supporting

Information. Figure 1b shows B3LYP/6-31G** relaxed potential energy profiles for the repeat units of 4 and 5 along the dihedral coordinate connecting Fu or Th with NDI. 4 has two stable non-equivalent minimum conformations at 25° (syn) and 140° (anti), with syn being more stable by ~1.2 kcal/mol, and a high syn-anti rotational barrier in the range of 4 kcal/mol. 5 shows a more distorted structure, with minima at 45° (syn) and 125° (anti) with a syn-anti rotational barrier of ~1 kcal/mol. Interestingly, at τ=0° (i.e. the coplanar geometry) ΔE is only 0.3 kcal/mol for 4 while it is 2.7 kcal/mol for 5. This strongly decreased value in the Fu-based system is caused by the reduced steric hindrance due to the smaller size of Fu, while the strongly increased potential-energy barrier at τ=90° mirrors the lower aromaticity of Fu. Therefore, the Fu-based tetramer is much more planar than the Th-based one (Figure 2c), a characteristic which may affect the π-π stacking in the solid state. The more coplanar backbone of 4 also explains the more intense and red-shifted CT band in the solution absorption spectrum (Figure 2a), which is also supported by longrange-corrected TDDFT calculations (Figure S8). In addition, a straighter backbone of 5 compared to 4 is seen, which results from the increased dihedral angle between NDI and the larger Th. Conversely, the flat NDI-Fu structure in 4 translates the full C-O-C angle of furan into backbone curvature. The increased backbone curvature of 4 also explains its higher solubility and reduced long range order41, as will be shown further below. The insertion of the electron withdrawing F4 unit between the Fu/Th units confers planarity to the donor blocks, which is favored by stabilizing interactions between oxygen(sulfur) and fluorine.42 Average O(S)-F distances computed on tetramers are 2.55 Å for 4 and 2.68 Å for 5. At the same time, the F4 unit does not break electron delocalization along the donor rings. The distributed HOMO density over three conjugated units should be beneficial for hole transport, while the LUMO remains highly localized on the acceptor NDI unit (Figure S9). To get insight into the electron transport properties, intramolecular electron reorganization energies (λelec and λhole) were computed for the monomers of 4 and 5 (Table 1, Table S2). The repeat units of both 4 and 5 show λelec= 0.27 eV (B3LYP/6-31G**), suggesting that the replacement of Th with Fu does not affect the structural and energetic reorganization of the extra electron of a repeat unit. This value reflects an n-type character of the two copolymers43, and, in a first approximation, similar electron mobilities of the two materials. Moreover, the computed λelec are equal to those of P(NDI2OD-T2), which is considered as one of the best n-type polymeric semiconductors.23,26 λhole is 0.1 eV lower for both 4 and 5 compared to P(NDI2OD-T2). This is directly correlated to the higher delocalization of the HOMO compared to P(NDI2OD-T2), which is achieved through the insertion of the F4 unit. These are very promising results especially for 4 to be used in field effect transistors (FETs), where one may expect a more planar structure with smaller π-π stacking. The charge transport properties were experimentally probed in ~50 thin films using FETs with top-gate, bottomcontact geometry. In Figure 3a the transfer characteristics of both copolymers in the regime of electron accumulation are shown.


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Table 1. Summary of physical, thermal, optical, electrical, and structural properties of P(FuNDIFuF4) and P(ThNDIThF4).

P(FuNDIFuF4) 4 P(ThNDIThF4) 5a P(ThNDIThF4) 5b

Mw,SEC/ a) Mn,SEC/ Mn,NMR

Eg,opt [eV]

HOMOc)/ LUMOd) [eV]

λelec/λh

9.5/6.6/ 6.8

1.90

-5.75/ -3.75

11.0/7.2/ 4.9

n.d.

12.7/7.8/ 6.3

1.70

b)

µe,sat [cm2 /Vs]

Ton [°C]

Tm/Tc [°C]

(100) spacing [nm]g)

(100) coherence length [nm]g)

(100) orientational FWHM [˚]g)

(010) spacing [nm]g)

0.27/ 0.27

0.4

380

303/ 291

2.60

32

13.0

0.36

n.d.

0.27/ 0.31

0.6

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

-5.50/ -3.85

0.27/ 0.31

1.3

400

-- f) /239

2.43

20

5.5

0.41

ole

[eV]e)

a)

measured by SEC in trichlorobenzene at 150 °C, b) from solid state UV-Vis, c)from the ionization potential measured by PESA, d)from CV of thin films, e)electron and hole intra-molecular reorganization energy from DFT calculations using B3LYP/6-31G**, f)not extracted from DSC, g)from GIWAXS of films annealed at optimum OFET conditions. n.d. not determined.

Figure 2. a) UV-vis spectra in solution and thin film, b) Torsion potential energies between NDI and Fu (X=O) or Th (X=S) of the monomer repeat units NDIFuF4 and NDIThF4. c) DFT-optimized geometries of the tetramers of 4 and 5, top view (left) and lateral view (right). All devices showed ideal n-type behavior, with saturation electron mobilities (µe,sat) strongly depending on the temperature employed for thermal annealing of the devices (Figure 3b; for output curves see Figs. S10,11). The transport properties of both copolymers improved upon annealing up to temperatures close to the melting points of 4 and 5b (~300°C and~250 °C, respectively, see Fig. S13 for differential scanning calorimetry curves). Higher temperatures led to reduced electron mobilities in both cases, whereby thermal degradation could be neglected (see Fig. S14 for thermal analysis). Charge density (i.e. gate voltage Vg) dependent saturation mobilities were obtained (for a

representative plot of µe vs. Vg see Figure S12), a phenomenon commonly encountered in semiconducting polymers with high mobility.44 Depending on the thermal treatment, µe ranged from 0.008 cm2/Vs (100 °C) to ~1.3 cm2/Vs (250 °C) for 5b (average mobility 0.97±0.27 cm2/Vs). A similar trend could be observed for 4, here a maximum electron mobility of ~0.4 cm2/Vs (average mobility 0.40±0.02 cm2/Vs). was obtained for devices annealed at 300 °C. Strikingly, to the best of our knowledge, these values are comparable to the highest electron mobilities of NDI copolymers reported so far25, but several orders of magnitude higher when compared to NDI co-


Chemistry of Materials polymers made by simple DAP.27 Given the moderate molecular weight of both 4 and 5b, their performances appear especially interesting with larger molar masses potentially rivaling the herein measured µe,sat=1.3 cm2/Vs. The maximum electron mobility that could be obtained from 5a was 0.6 cm2/Vs (average mobility 0.58±0.05 cm2/Vs, Table 1, Fig. S15). Thus, mobilities similar to 4 can be obtained from 5a already for smaller degrees of polymerization (DPn,NMR=4 vs DPn,NMR=6), which suggests a slightly better performance for the thiophene copolymer. Further correlations of electronic performance with molecular weight are underway to corroborate this dependency on the heteroatom. Poor device characteristics were observed in the hole accumulation regime, likely affected by a high injection barrier and a high threshold voltage. The low effective hole mobilities are reflected by the limited current at negative Vg values in Figure 3a.

|Id| [ A ]

(a)

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

10

4

5b

100 °C 150 °C 200 °C 250 °C 300 °C 350 °C

100 °C 150 °C 200 °C 250 °C 300 °C

-10 -11

10 -25

incidence wide-angle X-ray scattering (GIWAXS), nearedge X-ray absorption fine-structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM) were performed on thin films prepared in the same way as those used in FETs. Figure 4 presents the 2D GIWAXS scattering patterns, angle-resolved NEXAFS spectra, and AFM height images of 5b annealed to 250 ˚C and 4 to 300 ˚C corresponding to optimum device conditions. GIWAXS and NEXAFS data sets for other annealing temperatures are found in the Supporting Information (Figs. S16-S22). Both materials show a pronounced edge-on character in thin films with the lamellar stacking direction (h00) oriented out of plane and the π-π stacking direction (0k0) oriented in-plane (Figure 4a,b).

0

25

50

-25

0

25

50

Vg [ V ]

(b) 0

2

[cm /Vs]

10

-1

sat

10

µ

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5b 4

-2

10

100

150

200

250

300

350

Tannealing [ °C ] Figure 3. a) N-channel transfer characteristics in saturation regime (Vd=60 V) of the FETs with 5b and 4 subjected to different thermal treatments. b) Saturation mobility values (µsat) extracted at Vd=60V versus film annealing temperature. To explore the potential influence of film microstructure on FET performance, synchrotron-based grazing-

Figure 4. GIWAXS (a,b), NEXAFS, (c,d), and AFM (height mode, e,f) data of P(ThNDIThF4), (a,c,e), and P(FuNDIFuF4), (b,d,f), films annealed at 250 and 300 °C respectively. Scale bars in e) and f) are 2 µm. For both materials up to 5 orders of (h00) are observed oriented along qz, with (100) coherence lengths of ~ 22 nm for the optimum 5b film and ~ 32 nm for the optimum 4 (Table 1). Interestingly, despite 5b having the longer 2decyltetradecyl side-chain, a larger (100) spacing was observed for 4 with R= 2-octyldodecyl (2.43 nm vs 2.60 nm), see also cross-sectional traces in Figs. S17 and Table 1. The larger (100) spacing of 4 despite its shorter side-chain


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length suggests that the side-chains are oriented more vertically which is consistent with the NEXAFS results (vida infra). Clear π-π stacking peaks are also observed for both polymers, with a significantly lower π-π staking distance for 4 (0.36 nm) compared to 5b (0.41 nm). The lower π-π staking distance of 4 indicate a tighter in-plane packing of this polymer, and is consistent with a more planar backbone geometry as predicted by DFT torsional potential energy profiles and optimized structures for the tetramers (Figure 2b-c). The smaller van-der-Waals radius of oxygen compared to sulphur can additionally contribute to this behavior.45 Both polymers also exhibit mixed index peaks indicative of three-dimensional ordering. NEXAFS spectroscopy measurements (Figure 4c,d and also Figures S21 and S22) provide complementary information to the GIWAXS data, with NEXAFS measurements being sensitive to molecular orientation (rather than crystallographic orientation) and having a higher surface sensitivity (commensurate with the thickness of the accumulation layer at the top of the film). Both polymers are found to exhibit a preferential edge-on orientation of the conjugated backbone at the film surface (dichroism associated with the C1s-π* peaks located at 284 to 286 eV), with 4 showing a substantially more edge-on orientation than 5b. Specifically, the conjugated units in 4 are determined to be tilted on average only 12.5 ± 0.5˚ from the surface normal (corresponding to an average tilt angle of C1s - π* transition dipole moments of 77.5 ± 0.5˚), while the conjugated units in 5b are tilted on average 29.5 ± 0.5˚ from the surface normal. The more edge-on orientation of the backbone for 4 compared to 5b is likely to enable a tighter in-plane π-π stacking as observed by GIWAXS. Furthermore significantly more dichroism is also observed for the C1s-σ* peaks located at 292 to 310 eV of 4 predominantly associated with the alkyl side-chains. This enhanced dichroism observed for the C1s-σ* peaks of 4 compared to 5b indicates that the side chains are more vertically oriented again consistent with the larger (100) spacing of 4. The GIWAXS and NEXAFS data taken together indicate that the two polymers have substantially different packing geometries, with 4 exhibiting a more edge-on orientation of the conjugated units within the unit cell, a larger (100) spacing and smaller π-π stacking distance. The decreased π-π stacking distance of 4 is in perfect agreement with the lower torsion angle between NDI/Fu compared to NDI/Th and the resulting more planar geometry of the backbone. Interestingly, a more pronounced edge-on character, smaller π-π stacking distances and higher molar masses are conventionally associated with superior charge transport properties.46,47 However, this is not reflected in the FET data with 5b exhibiting higher mobilities. Two observations explain the lower than expected mobility of 4. Firstly, we note that from the GIWAXS measurement 4 has a lower orientational order than 5b, potentially caused by the increased backbone curvature (Table 1).41 Secondly, films of 4 are rougher than 5b as indicated by higher scatter at very low angles in the 2D GIWAXS images. Thus 4 may possess superior charge

transport properties on the molecular scale but these are not reflected in the long-range transport measurements where orientational disorder and film roughness impact.48,49 This explaation is also in line with AFM images which indicate highly ordered, terrace-like, and poorly ordered, nodular morphologies of 5b and 4, respectively (Figure 4e,f). AFM roughnesses of 0.45 and 1.25 nm for 5b and 4, respectively, also correspond to the trends in roughness seen by GIWAXS. The AFM terrace step-height of 5b is 2.4 nm (Figure S23), again in perfect agreement with the (100) spacing identified by GIWAXS (Table 1). Finally, microstructural measurements performed on films annealed at different temperatures (see Figures S1622) enable an understanding of the optimization of FET performance. Annealing in both cases is found to lead to increased crystallinity and orientational order. For 4 a dramatic increase in crystallinity and orientational order is observed with annealing at 300 ˚C, with crystallinity dropping after melt annealing, explaining the lower performance of the 4 film annealed to 350 ˚C. For 5b, crystallinity and orientational order more steadily improve, with the melt-annealed film having an even higher crystallinity and orientational order than the optimum 250 ˚C film. However, the in plane coherence of 5b strongly decreases at 300 °C, which explains the decreasing mobility of 5b at this temperature. In conclusion, we have demonstrated that direct arylation polycondensation of large, π-extended monomers is well-suited for the preparation of well-defined, highperformance conjugated materials with excellent electron mobilities. α-C-H arylation of the two monomers is selective as proven by detailed high-temperature NMR measurements, which significantly broadens monomer scope for direct arylation polycondensation and additionally emphasizes that blocking groups can be avoided. Despite their oligomeric character, 4 and 5b exhibit high electron mobilities ~1 cm2/Vs, which have previously not yet been achieved with materials made by DAP. The detailed comparison of 4 and 5b highlights how backbone planarization, backbone curvature, π-π stacking distance, and long range order are intertwined. While much more mechanistic work remains to be done to uncover chain end degradation processes such as dehalogenation as one reason responsible for the limited MWs, this study shows that direct arylation polycondensation catches up with conventionally employed polymerization techniques in that high performance materials can now be made from simplified and non-toxic building blocks.

ASSOCIATED CONTENT Supporting Information. All synthesis procedures of building blocks, instrumentation, device measurements, scattering, and details of DFT calculations are given in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.



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Okamoto, K.; Housekeeper, J. B.; Michael, F. E.; Luscombe, C. K. Polym. Chem. 2013, 4, 3499. (19) Mercier, L. G.; Aïch, B. R.; Najari, A.; Beaupré, S.; Berrouard, P.; Pron, A.; Robitaille, A.; Tao, Y.; Leclerc, M. Polym. Chem. 2013, 4, 5252. (20) Nakabayashi, K.; Mori, H. Chem. Lett. 2013, 42, 717. (21) Lombeck, F.; Komber, H.; Gorelsky, S. I.; Sommer, M. ACS Macro Lett. 2014, 819. (22) Oberhumer, P. M.; Huang, Y.-S.; Massip, S.; James, D. T.; Tu, G.; Albert-Seifried, S.; Beljonne, D.; Cornil,

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AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The IRTG 1642 Soft Matter Science, the Fonds der chemischen Industrie and the Innovationsfond Forschung of the University of Freiburg are greatly acknowledged for financial support. The authors thank M. Hagios for SEC measurements and A. Hasenhindl for NMR measurements on monomers. Parts of this research were undertaken on the soft X-ray 50 51 beamline and the SAXS/WAXS beamline of the Australian Synchrotron, Australia. CRM acknowledges support from the Australian Research Council (FT100100275, DP130102616) and thanks Dr. Nigel Kirby of the Australian Synchrotron for technical support. DF thanks the Alexander von Humboldt foundation for a fellowship.

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Page 10 of 10

Figure for TOC O

R N

F

O X

X O

N R

O

F

Br

O

R N

F

F

DAP

F

O

Br

F

X F

X O

N R

O

n F

X= O,S


simplified reaction

selective α-arylation

µe ~ 1cm2/Vs

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StructureâFunction Relationships of High-Electron Mobility

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