Conformational Analysis of Oxygen-Induced Higher Ordered Structure

Article pubs.acs.org/Macromolecules

Conformational Analysis of Oxygen-Induced Higher Ordered Structure of A, B‑Alternating Poly(arylene vinylene) Copolymers by Solid-State NMR and Molecular Dynamics Simulations In-Sun Jung,*,† Young Joo Lee,*,‡ Daun Jeong,† Robert Graf,§ Tae-Lim Choi,∥ Won-Joon Son,† Xavier Bulliard,† and Hans Wolfgang Spiess§ †

Samsung Advanced Institute of Technology, Samsung Electronics Co., Ltd., 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea ‡ Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany § Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ∥ Department of Chemistry, Seoul National University, Seoul, 08826, Republic of Korea S Supporting Information *

ABSTRACT: Two types of poly(arylene vinylene) copolymer derivatives, one containing heptoxy side chains and the other containing octyl side chains, are investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation in order to understand the effect of the chemical structure of the side chains on the molecular packing structure and the charge transport properties. Solid-state NMR study shows that octyl-functionalized polymer has higher side chain mobility, indicating a higher degree of molecular stacking disorder than heptoxy-functionalized polymer. MD simulations suggest that these differences in molecular packing and mobility are associated with the different side chain geometry. Overall, we demonstrate how the new poly(arylene vinylene) copolymer derivatives with a small change in polymer side chain (alkyl vs alkoxy) lead to large changes in the molecular packing arrangement and thereby the improved hole mobility in organic thin-film transistors.

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INTRODUCTION

structure of conjugated polymers profoundly changes the macromolecular structures and affects the electronic properties.12−14 However, many solution-processed organic semiconductor materials have a tendency to form amorphous morphology with short-range ordering only despite similar molecular structures. Hence, understanding the influence of molecular motion, geometry, and structure on the stacking behavior is essential to improve the performance of organic devices. In our previous work,15,16 we showed that poly(arylene vinylene) copolymer derivatives can have outstanding mobility. Although these polymers exhibited amorphous morphology, they showed relatively good π−π stacking, attributed to highly stereoregular vinylene backbone repeating unit.15,16 We, however, reported that their OTFT performance, in particular the mobility, strongly depended on the functional group. The mobility of octyl-functionalized poly(arylene vinylene) copolymer, with 10−4 cm2/(V s) (referred to as P1, Figure 1a), was 2

In the past decade, solution-processed conjugated polymers such as regioregular polythiophene have been actively studied as materials for organic thin-film transistors (OTFTs), solar cells, and organic light-emitting diode (OLED) displays.1−5 They are characterized by an ordered structure including sp2hybridized conjugated backbones and show electronic properties such as the charge transport and the on/off ratio comparable to that of amorphous Si.6,7 Additionally, they can be easily prepared by solution process, enabling the possibility to manufacture wide displays at low cost. Therefore, the need for conjugated polymers has increased to cover the versatile ranges of applications. Charge transport properties in organic semiconductors usually depend on the π-orbitals overlapping of adjacent conjugated molecules and on the charge carrier injection efficiency through the material (p-type (holeconducting) or n-type (electron-conducting)).2 Thus, the control of the degree of structural order and orientation of polymer chains is a crucial parameter for the improvement of the charge mobility.8−11 Various approaches have been suggested in designing polymers with good OTFT performance. For example, it has been reported that the packing © XXXX American Chemical Society

Received: October 11, 2015 Revised: April 9, 2016

A

DOI: 10.1021/acs.macromol.5b02245 Macromolecules XXXX, XXX, XXX−XXX

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fast spinning of the sample. 1H MAS NMR and 13C cross-polarization (CP) MAS NMR experiments were performed on a Bruker AVANCE 700 narrow-bore spectrometer operating at a 1H Larmor frequency of 700.1 MHz with a spinning frequency of 30 kHz. 1H NMR pulse length and repetition time were 2.5 μs and 2 s, respectively. In 13C CP condition, a cross-polarization contact time of 2500 μs and delay times of 2.0 s were used. 1H−1H SQ-DQ correlation NMR experiments were carried out on an 850 MHz NMR spectrometer (AVANCE III). The experiments were performed at a 25 kHz MAS with an excitation and reconversion time of one rotor period length.23 Acquisition of 256 slices in t1 with 16 transients for each rotor synchronized t1 increment leads to a measuring time of approximately 3 h per 2D spectrum21 using States-TPPI detection to record the phase sensitive spectrum in the DQ dimension. The REPT-HDOR experiments were done at a MAS frequency of 25 kHz with a recoupling time of 160 μs corresponding to 4 MAS rotor periods. In order to obtain dipole− dipole sideband patterns, the rotor phase was sampled for one or two rotor periods with short t1 increments of 2 μs, and 400−1024 transients were recorded for each increment. As the evolution with the rotor phase should be strictly periodic, the acquired data for an integer number of rotor periods were numerically periodically concatenated, and after multiplication for an artificial line broadening the dipole− dipole sideband pattern was computed via a numeric cosine Fourier transform. Model Description and Simulation Methods. Molecular modeling and simulation of the polymer are based on an implementation of topologies generated by PRODRG2 Server.27 Only the polar and aromatic hydrogen atoms were treated explicitly, while −CH2 and −CH3 groups of the alkyl chains were modeled as spherical united atoms with appropriate masses. All constituent atoms were identified with the atom types defined in the GROMOS96 53A6 force field parameter,28 thereby determining bonded and nonbonded interactions. The atom types and partial charges are displayed in Figures S4 and S5 in the Supporting Information. All MD simulations were performed with GROMACS MD simulation program.29 We employed the velocity-Verlet algorithm with a time step of 1 fs for integration. The simulation cell was subjected to periodic boundary conditions along all three directions. Long-range electrostatic interactions were calculated by the particle-mesh Ewald summation method. The Lennard-Jones and Coulombic terms were treated with the cutoff distance of 2.5 nm. The P1 and P2 systems consist of 140 chains, each one made up of six A, B, and A, B′ units. For the simulations corresponding to the lamellar phase of P1 and P2, the system was initially prepared by repeating one planar molecule periodically without overlap. As we equilibrate the system at 300 K and 1 atm, the polymer molecules attract each other, and therefore the size of simulation cell gradually decreases under the temperature and pressure control using the Berendsen thermostat and barostat with corresponding relaxation times equal to 1 ps. We carried out equilibration monitoring stationarity of the potential energy and cell size for 40 ns followed by the production run for 10 ns. The calculated average densities using the stationary trajectories for the P1 and P2 systems were found to be 1.02 and 1.06 g/cm3, respectively. As a result of current equilibration method, configurations in a stationary state exhibit neither a complete crystal nor totally disordered structure, but partial interlayer ordering (see Figure S3 in the Supporting Information). We made a comparison of the P1 and P2 systems in terms of the degree of interlayer ordering by analyzing equilibrium structures.

Figure 1. Structures of poly(arylene vinylene) polymer (a) P1 and (b) P2.

orders of magnitude lower than that of the heptoxy-functionalized poly(arylene vinylene) copolymer, with 0.022 cm2/(V s) (referred to as P2, Figure 1b), for the same fabrication procedure. We presumably explained this large difference in mobility by the better π−π overlapping between alkoxy polymers because of their smaller size (−O− vs −CH2−) and electron-richer polymer backbone. Recently, higher hole mobility and red-shift in UV−vis absorption spectra were also reported for conjugated polymers when the alkyl side chains were replaced by oligo(ethylene glycol) side chains.17 This was attributed to the decrease in the π−π stacking distance of polymer backbone resulting from the flexible structure of oligo(ethylene glycol) side chains. These results indicate that the fine-tuning of the structures and functional groups in polymer design is crucial. It is to be mentioned that in triphenylene discotic liquid crystals,18 the influence of side chains has been earlier observed by NMR.19,20 Liquid crystal systems are however more mobile than our material types, and it is worth studying this aspect in solid systems. In this study, we focus on the analysis of the poly(arylene vinylene) copolymers derivatives (P1 and P2) to understand the origin of the difference in hole mobility observed for different side chains. Specifically, we will use 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) at high spinning frequency (25−30 kHz) and 1H−1H and 1H−13C dipole−dipole recoupling techniques, such as single quantum (SQ)−double quantum (DQ) correlation NMR21−23 and recoupled polarization transfer−heteronuclear dipole−dipole order recoupling (REPT-HDOR) NMR24,25 to acquire sitespecific information on dynamics and spatial proximity. Molecular dynamics simulation will be performed to obtain assembly structures from different polymer chains and molecular geometries.26 Based on the combined study of solid-state NMR and molecular dynamics simulations, a detailed explanation accounting for the mobility tuning by the difference in the chain packing structure will be proposed.

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EXPERIMENTAL SECTION

Materials. Both octyl- (P1) and heptoxy-functionalized poly(arylene vinylene) (P2) copolymers were prepared following a typical Horner−Emmons procedure. The detailed synthetic and the characterization procedures are described elsewhere.15,16 NMR Spectroscopy. Solid-state NMR experiments were performed on Bruker Avance consoles, at magnets with 16.4 and 19.9 T B0 field strength corresponding to 1H Larmor frequencies of 700.13 MHz (AVANCE, 16.4T) and 850 MHz (AVANCE III, 19.9 T), respectively. Commercial Bruker double-resonance 2.5 mm MAS probes that allow spinning frequencies up to 35 kHz were used for all experiments. Unless stated otherwise, all experiments were performed under bearing gas at room temperature, which amounts to the effective sample temperature of 320 and 312 K at spinning frequency of 30 and 25 kHz, respectively, due to frictional heating. For variable temperature experiments, probe temperature was controlled with standard Bruker equipment. The temperature was calibrated using Pb(NO3)2 by taking into account the frictional heating caused by the

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RESULTS AND DISCUSSION In our previous observations, we noted that P1 with n-octyl side chains yielded no shoulder in UV−vis spectra, while P2 with alkoxy side chains exhibited a vibronic splitting pattern (Figure S1), which can be attributed to π−π interactions between adjacent polymer chains.15,16 This implies that P1 and P2 form different stacked assembly structures. The interchain interactions observed in P2 can result in lamellar structures, which contribute to well-ordered molecular arrangements in B


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Macromolecules the film. However, the significant impact of such a minor change in the chemical structure on the assembly is not well understood yet. It remains challenging to get information on the local molecular assembly of these polymers due to their low crystallinity. In contrast, solid-state NMR spectroscopy is a suitable technique to probe molecular structure and local ordering in macromolecular systems even when long-range ordering is lacking. Recently, it has been demonstrated that advanced solid state NMR spectroscopy under fast magic angle spinning (MAS) conditions can provide valuable information on 3-dimensional packing arrangements, molecular motion, and static disorder as well as molecular structures of various soft matters such as polymers, supramolecular compounds, and biomolecules.30−37 The degree of structural order and orientation of polymer chains can be determined by 1H−1H DQ NMR spectroscopy under fast MAS in solids.23,24,38−43 Moreover, rotor-encoded double resonance schemes such as REPT-HDOR25,44 and REREDOR45−47 have been used to measure site-selective molecular dynamics via sideband pattern analysis.48−51 Herein, the structure and dynamics of P1 and P2 are investigated by analyzing 1H chemical shift values, high temperature behavior, 1 H−1H dipole−dipole coupling, and 1H−13C coupling sideband patterns. Subsequently, MD simulation results are discussed, and the different π−π stacking structures of these polymers are explained based on the NMR and MD simulation study. Variable Temperature 1H MAS NMR Measurements. 1 H MAS NMR experiments were performed in the temperature range of 320−371 K to investigate molecular packing and molecular motion of P1 and P2 at different temperatures. At 320 K, the lowest temperature recorded, the 1H MAS NMR spectrum of P1 shows a broad resonance at 1.1 ppm corresponding to the aliphatic side chains and a weak signal at 6.7 ppm corresponding to the aromatic ring and vinylene unit (Figure 2a). The resonance related to CH2 linker group, which is expected at 2.7 ppm, is not visible, probably due to the overlap of broad aliphatic signals. For P2, an additional resonance at 3.6 ppm resulting from −OCH2− linking unit is observed together with the signals from the aliphatic side chains and conjugated backbone at 1.0 and 6.7 ppm, respectively

(Figure 2b). No significant difference is noted between the two samples except for the slight narrowing of both aromatic and aliphatic protons for P1. In contrast to low temperature conditions, the behavior of two samples is markedly different at elevated temperatures. On sample heating, P1 exhibits substantial line narrowing of all signals, and nonequivalent segments of aliphatic chains start to be resolved above 338 K. At 371 K, a peak at 2.2 ppm as indicated by the gray arrow in Figure 2a is clearly seen, which can be attributed to CH2 unit linking the aromatic backbone to the aliphatic side chains. Moreover, three distinctive signals for vinylene, thiophene, and aromatic units are clearly discernible between 6.5 and 6.8 ppm. In contrast, only a slight line narrowing is observed for P2 with increasing temperature. This indicates an increase in molecular mobility of P1 at elevated temperatures, while the molecular mobility of P2 is highly restricted even at the highest temperature. A significant line narrowing of signals assigned to aliphatic side chains at elevated temperature is often observed due to nearly isotropic molecular fluctuations averaging the strong 1H dipole−dipole coupling as well as local heterogeneities. However, the molecular origin of the highly restricted mobility in P2 is not clear yet, and better suited NMR experiments are needed to selectively probe the local molecular mobility. 1 H−1H DQ NMR Experiments. In order to gain insight into the spatial proximity of protons, 1H−1H DQ correlation NMR experiments using the back-to-back (BaBa) recoupling scheme were performed at room temperature. Since the intensity of DQ signal depends on the dipole−dipole coupling between close 1H spins, the internuclear distances between 1H sites as well as the molecular mobility can be obtained from DQ experiment; the stronger the DQ signal intensity is at short excitation times, the stronger is the coupling between the spins in the systems. Thus, the 1H−1H SQ−DQ correlation experiment can be seen as the coherent analogue to NOESY type experiments known from liquid state NMR spectroscopy. In two-dimensional SQ−DQ correlation NMR spectra, the DQ coherences (DQCs) between like spins with the same chemical shift appear along the diagonal (autopeak), and DQCs of nonequivalent spins give rise to two symmetric peaks on both sides of the diagonal (cross-peak). The measured frequency in DQ dimension corresponds to the sum of the chemical shift values of the spins involved in the double-quantum coherence. As expected from the chemical structure, both P1 and P2 show autopeaks for aliphatic and aromatic protons as well as cross-peaks between aliphatic and aromatic protons (Figure 3). For P2, additional cross-peaks between −OCH2− and aromatic protons and between −OCH2− and aliphatic groups are clearly seen. However, intriguing differences between the two samples are observed. The signal intensity of DQC is stronger for P2 than for P1. In particular, the DQ signal arising from the aliphatic side chains is significantly reduced for P1. This further confirms the higher side chain mobility of P1 and the more rigid interchain stacking of P2 already observed in the variable 1H MAS NMR measurements. Dynamics Study from REPT-HDOR Sideband Pattern Analysis. First, 13C CP MAS NMR spectra were acquired as shown in Figure 4. For P1, phenyl carbons backbone signals are found at 137 (C1 and C4), 133 (C3 and C6), and 126 ppm (C2, C5 and alkene unit), and thiophene units are located at 142 (C7 and C10) and 122 ppm (C8 and C9). Compared to P1, the respective peaks of P2 backbone corresponding to the

Figure 2. 1H MAS NMR spectra of (a) P1 and (b) P2 at variable temperatures. The spectra were obtained at a 1H frequency of 700.13 MHz and a spinning frequency of 30 kHz. The calibrated temperatures taking into account the frictional heat caused by fast spinning are denoted in the figures. C


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Figure 4. 13C CP MAS NMR spectra of P1 and P2. The spectra were acquired at the 13C Larmor frequency of 176.06 MHz and the spinning frequency of 30 kHz.

motionally reduced dipole−dipole coupling constant and the magic angle spinning frequency, respectively. Thus, the amplitude of the recoupled signal for a 1H−13C spin pair is modulated along t1 according to the equation24,25,46,47

Figure 3. 1H−1H double-quantum BABA NMR spectra of (a) P1 and (b) P2. The autopeaks due to aliphatic protons and aromatic protons are denoted as ∗ and +, respectively. The cross-peaks between aliphatic and aromatic protons are denoted as red lines. The cross-peaks between −OCH2− and aromatic protons and between −OCH2− and aliphatic groups are denoted as green and blue lines, respectively. The spectra were recorded at a 1H Larmor frequency of 850.27 MHz and a MAS spinning rate of 25 kHz. All spectra are plotted on the same intensity scale.

S(t1) ∝ ⟨sin(Nexcϕ0) sin(Nrecϕt1)⟩

(1)

where the brackets ⟨ ⟩ denote the powder average. Nexc and Nrec are the numbers of rotor periods (τR=1/ωR) for the excitation and reconversion periods, respectively. Therefore, the excitation and reconversion times become τexc = NexcτR and τrec = NrecτR, respectively. The phase angles before and after the t1 periods, ϕ0 and ϕt1 contain the Euler angles (β, γ) defining the internuclear C−H vector with respect to the rotor axis and the dipole−dipole coupling constant Dij, as described in eqs 2 −Dij ϕ0 = 2 2 sin(2βij) sin(γij) ωR (2a)

phenyl rings are shifted to 151 (C1 and C4) and 108 ppm (C2 and C5). The resonances associated with aliphatic side chains range from 14 to 32 ppm for both samples, and −OCH2− group in the side chain of P2 is seen at 68 ppm. Using the rotor-encoded TEDOR scheme, referred to as REPT-HDOR, heteronuclear dipole−dipole coupling (DCH) between covalently bound 13C and 1H sites can be estimated, probing the local segmental dynamics.24,25,46,47 In the 2D spectrum of the REPT-HDOR experiment, the 13C chemical shift evolution is recorded along the f 2 axis, while dipole− dipole sideband patterns are generated by the rotor-encoding of the recoupled dipolar interaction between 13C and 1H nuclei along the f1 dimension. Such rotor encoding is achieved by incrementing the t1 evolution time in fractions of the rotor period so that the initial rotor phase of the second recoupling block (reconversion period) is shifted relative to the initial rotor phase of the first recoupling block (excitation period). Hence, the average Hamiltonians for the recoupling periods before and after the t1 increment are different by a factor proportional to Dij̅ cos(ωR t1), where D̅ ij and ωR are the

ϕt1 = Dij =

−Dij ωR

2 2 sin(2βij) sin(ωR t1 + γij)

(2b)

μ0 ℏ γγ i j 4π rij 3

(2c)

where μ0 denotes the magnetic constant, γi,j the magnetogyric ratios of the nuclei, and rij the internuclear distance. Fourier transformation of S(t1) yields a spinning sideband patterns along the f1 frequency axis. It is noteworthy that the distribution of the intensity over the sideband pattern is governed by the recoupling time as well as the dipole−dipole coupling; longer excitation and reconversion times yield higher order sidebands. An intriguing feature is that the sideband pattern generated by the REPT-HDOR method consists of only odd order D


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Figure 5. Residual 1H−13C dipole−dipole couplings of the different backbone units of P1 (a, b) and P2 (c, d) polymers measured by REPT-HDOR spinning sideband patterns. Computed (blue lines) and experimental (red lines) patterns for alkene (a, c) and thiophene (C8 and C9) carbons (b, d) are given in the figures. The experiments were performed at 25 kHz MAS spinning frequency and 160 μs REDOR recoupling for excitation and reconversion, corresponding to four rotor periods REDOR recoupling.

would yield ∼13 kHz residual dipole−dipole coupling (S = 0.63), while a full ring rotation would result in ∼2.6 kHz of dipole−dipole coupling (S = 0.13). Here, we focus on the local dynamics of the polymer backbone to distinguish between the structure and the dynamic influence on the π−π stacking structure. In particular, alkene and thiophene units are considered since they exhibit strong signal intensities. When fitting REPT-HDOR experimental sideband pattern to the simulated data, the first-order sidebands need to be excluded since the intensity of their experimental result is usually too high due to the coupling to remote protons via strong 1H−1H dipole coupling.23 As shown in Figure 5, dipole−dipole coupling of approximately 18 kHz (Shet = 0.85) is obtained for alkene unit of both P1 and P2. P1 shows slightly reduced dipole−dipole couplings for the thiophene moiety (DCH ∼ 16 kHz, S = 0.77) compared to P2 (DCH ∼ 18 kHz, S = 0.85), indicating more pronounced local molecular fluctuations of the P1 backbone. NMR interactions can be averaged by angular fluctuations in the fast limit. We have used the NMRWeb Lab to assess the influence of angular fluctuations on the anisotropic NMR parameters for 13C−1H dipolar coupling tensor (D, η) (see Figure S6 in the Supporting Information for details).55 Assuming a Gaussian distribution for the molecular deviations with variance σ, the difference of the angular distributions of aromatic group (out-of-plane fluctuation) can be estimated. The width of the Gaussian distribution of P1 (σ = 28°) is in fact slightly larger than that of P2 (σ = 22°).56 Thus, the residual dipole−dipole couplings of 16−18 kHz, which are similar for the equivalent sites of P1 and P2, indicate that the backbones of both polymers are relatively rigid and that the difference in backbone mobility between polymers P1 and P2 is not significant. Under the experimental condition of τrcpl = 4 rotor periods, the signal of −OCH2− linker group of P2 disappears completely, whereas that of −CH2− linker group

sidebands, in contrast to the REREDOR experiment, generating both even and odd order sidebands. Since the signal is concentrated over small number of sidebands, the overall signal-to-noise ratio of the REPT-HDOR experiment is enhanced. However, the smaller number of sideband peaks can lead to the reduced sensitivity to weak dipole−dipole couplings. Therefore, the REPT-HDOR experiment is better suited to measure moderate or even rigid moieties with substantial dipole−dipole couplings, except for rigid CH2 group. Under TEDOR recoupling conditions, the 13C signal of rigid CH2 group vanishes for recoupling times longer than two rotor periods (τrcpl > 60 μs) due to the interference of the two C−H couplings at the particular geometry of CH2 group.52 However, in mobile CH2 groups, the motional averaging of C− H dipole−dipole coupling tensor changes this geometry, resulting in the reappearance of 13C signals for τrcpl > 100 μs. Therefore, TEDOR experiments provide semiquantitative information on the mobility of individual CH2 groups. At the same time, due to this signal cancellation for rigid CH2 group in the TEDOR transfer scheme, REPT-HDOR experiment is not fully adapted to measure dipole−dipole coupling of rigid CH2 groups. The dipole−dipole coupling constant DCH can be extracted individually for each spectrally resolved 13C site via sideband pattern analysis.46,47 The dipole−dipole coupling (DCH,stat) of rigid CH, CH2, and aromatic sites is approximately ∼21 kHz, and the effective dipole−dipole coupling values DCH,eff decrease with increasing local mobility.53,54 The local order parameter (S) indicating the degree of mobility is determined from the ratio of respective DCH values to the rigid coupling value (S = DCH,eff/DCH,stat). The S parameter is not only an indicator for the presence of local molecular fluctuations, but also a measure to get insight into the geometric details of the local dynamic processes. For example, a 180° ring flip motion of phenyl group E


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Figure 6. Partial radial distribution functions for (a) S−S, (b) C1−C1, and (c) S−C1 atoms. (d) A typical interlayer structure of backbone of P1, where stacked, reversed, and staggered arrangements of two thiophene rings are highlighted.

chains. As shown in Figure 6a, gSS(r) for P1 exhibits only a shoulder at r = 0.38 nm, while P2 shows a peak at the same distance, which is associated with π−π stacking of the thiophene rings. This indicates an enhanced stacking in the structure of P2. In Figure 6a, for both P1 and P2, we also observe two other peaks at approximately r = 0.60 and 0.80 nm in gSS(r) corresponding to the reversed and a staggered arrangement of two thiophene rings, respectively (see Figure 6d). We can also see the predominance of a staggered arrangement in the P2 system. Similar results are obtained for gCC(r) and gSC(r) displayed in Figures 6b and 6c, respectively, demonstrating that the π−π stacking of thiophene is correlated with the arrangement of phenyl rings. For example, the radial distribution function for C1−C1 of the P2 system shows a strong peak at r = ∼0.42 nm, whereas the P1 systems yields a broader and weaker peak of gCC(r) at r = 0.60 nm, suggesting that the distance between C1 atoms in neighboring chains of the P1 system is larger than that of the P2 system. This difference in interchain distances can be explained by the distribution of the torsional angle, denoted by P(Φ), associated with four atoms, C−C1−CH2−CH2 in the P1 system and C−C1−OA−CH2 in the P2 system (see Figure 7). According to P(Φ) displayed in Figure 7, the CH2 atom is out of the plane of the phenyl group in the P1 system. Two neighboring phenyl rings would thus have steric hindrance resulting in reduced π−π stacking of thiophene rings. In the P2 system, in contrast, the CH2 atom bonded to OA atom is generally in the plane of the phenyl group, and therefore the interlayer structure formation is facilitated. A similar behavior was observed in a recent quantum chemical study on the torsional potential of ethylbenzene and ethoxybenzene.61 Ethylbenzene reaches energy minimum when ethyl chain lies perpendicular to the aromatic rings, whereas ethoxybenezene shows optimum conformation when both ring and ethyl group are in the same plane. The difference in torsional barriers of P1 and P2 can be effectively understood in terms of hyper-

directly bound to the aromatic group in the P1 polymer is observed with reduced intensity. This suggests that −OCH2− group of P2 is relatively rigid, and the side chain mobility of P1 is higher than that of P2, which is consistent with the observation from the VT and 1H−1H DQ experiments. The remaining −CH2− groups in the side chains yield only firstorder sideband pattern (DCH < 10 kHz) and therefore cannot be evaluated quantitatively. Nevertheless, the data support the more rigid behavior of the alkoxy side chains of sample P2 compared to the n-octyl side chains of P1 and indicate the expected higher chain mobility toward chain ends. From the different NMR experiments, we can conclude that the alkoxy-functionalized materials possess higher degrees of interchain interaction and a more compact interlayer structure, resulting from the difference in the side chain geometry as compared to the alkyl chain. The next step is now to determine the major contribution to the stronger interchain coupling of P2. In order to answer this question, MD simulation was performed. MD Simulation Results for the Interlayer Structure. The small difference in molecular structures causes different supramolecular structures57 and dynamics mechanisms.58,59 In our systems, we have also observed different patterns of the interlayer structures between P1 and P2 polymers. Several possible reasons can be considered to explain this phenomenon, i.e., carbon−oxygen bonding distance, oxygen dipole moment effect, differences in torsional angle of the side chains, substituent effect in π-staking interactions, etc.60 The difference between the interlayer structures in the P1 and P2 systems can be probed by calculating the partial radial distribution functions for S−S atoms in thiophene ring, C1−C1 atoms in phenyl rings, and S−C1 atoms, which are denoted by gSS(r), gCC(r), and gSC(r), respectively. The atom C1 denotes a phenyl carbon where the side chain is connected (see Figure 6). When computing the partial radial distribution functions, we only consider atom pairs so that each atom belongs to different F


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CONCLUSION Variable temperature MAS NMR in combination with homoand heteronuclear dipole−dipole recoupling NMR methods allowed us to make a site specific analysis of the local structure and molecular dynamics in A, B-alternating copolymers for OTFT. At elevated temperatures, higher mobility of the side chains was observed for octyl-functionalized P1 compared to the heptoxy-functionalized P2. In addition, lower 1H−1H DQ intensity was observed for the octyl-functionalized polymer. However, for the backbones of both polymers, similar 1H−13C residual dipole−dipole couplings corresponding to order parameters of 0.85 were obtained from the sideband pattern analysis of REPT-HDOR experiments. This indicated fluctuations of the polymer backbone with an amplitude of about ±20°. These results suggest that the heptoxy-functionalized polymer possesses a higher degree of intermolecular stacking or interdigitated packing structures resulting from significant differences in side chain mobility compared to the n-octylfunctionalized polymer. MD simulation suggested that the alkoxy-functionalized polymer have a better interlayer stacking resulting from different preferred geometries of the side chains relative to the aromatic backbone. In the alkyl-functionalized polymer, the preferred orientation of the side chain is out of the phenyl group plane, while the alkoxy side chains show a general trend to reside in the plane of the phenyl group. Therefore, two neighboring phenyl rings in the alkyl-functionalized polymer would experience a more pronounced steric hindrance resulting in reduced π−π stacking of the thiophene rings. In contrast, the steric hindrance is reduced in the alkoxy-functionalized polymer, facilitating the formation of an interlayer stacking structure between the neighboring polymer chains. This is attributed to different side chain bonding conformations to the polymer backbone, namely out-of-plane for the octyl and inplane for the heptoxy. In-plane conformation has been earlier observed by NMR for the triphenylene discotic liquid crystals with alkoxy side chains. It is rewarding to note that the same conformation is also present in polymer materials, where molecular motion is restricted. In conclusion, we proved that the oxygen bond in the alkoxy side chain changes its orientation and induce the alignment of ordered layered structures, which is essential for a high hole mobility. The results of this study can be exploited for the molecular design of new solution-processed polymers and for the development of new organic semiconductors systems.

Figure 7. Distribution of the torsional angle associated with C−C1− CH2−CH2 in the P1 system and C−C1−OA−CH2 in the P2 system.

conjugation, through which lone pair electrons of the oxygen in P2 cannot be stabilized. In detailed computational studies, competitions among hyperconjugation, electrostatics, and steric (van der Waals) interactions have been suggested.61,62 We note in addition that the partial charges assigned by the PRODRG server have been discussed in view of the compatibility with GROMOS force field.63 Various force fields and sets of point charges have been used in modeling studies of oligothiophene systems and are still under development focusing on different phases and thermodynamic properties.64,65 In this regard, we consider the effect of partial charges simply by setting all atoms neutral. As a result, the interlayer arrangement is more pronounced in the P2 system than in the P1 system, as it is in the case of partial charges. The simulation results for P1 and P2 systems with neutral atoms are shown in the Supporting Information (Figure S2). Our simulation results reveal that molecular geometry of the side chain and phenyl group affects the effective interaction between phenyl rings and thereby changes the inter-ring arrangement. The conformations of the side chain play an important role in the molecular packing of many supramolecular systems as was highlighted in the recent review articles.66,67 The features in the interlayer structures can be related to the different molecular packing deduced from the experimental results. We point out that our simulation method was intended to demonstrate the differential degrees of interlayer structure resulting from the presence of oxygen atom in the side chain. Our model might not reproduce all features of P1 and P2 systems, e.g., strikingly different results of UV−vis spectrum for P1 and P2 system in Figure S1, which probes lamellar structures for the entire system. The different molecular mobility of side chains in the two systems was also observed in the time scale of ∼kHz via the NMR experiments. However, the side chain dynamics measured at the frequency of ∼kHz is not possibly studied via MD simulations because of the limitation of the time and length scale. Therefore, the interlayer structures obtained from MD simulations may not represent the global structures that can be related to the side chain dynamics. Nevertheless, microscopic structures derived from MD simulation can give insight into the dynamic phenomena observed by NMR.

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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.5b02245. UV−vis spectrum of P1 and P2, partial radial distribution functions with neutral atoms, equilibrium structures force field parameter from P1 and P2 simulation results, averaging of 13C−1H dipolar tensor of P1 and P2 by angular fluctuation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(I.-S.J.) E-mail: [email protected]. *(Y.J.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS The authors thank Dr. Anass Benayad and Dr. Chan Hee Lee for carefully proofreading the manuscript and for the discussions on simulation part. We acknowledge also Dr. Hyouk Soo Han and Dr. Eun Jeong Jeong for providing a very productive working environment. T.-L. Choi acknowledges the financial supports from Basic Science Research and NanoMaterial Technology Development through the National Research Foundation of Korea.

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