Pressure Modulation of Backbone Conformation and Intermolecular

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Pressure Modulation of Backbone Conformation and Intermolecular Distance of Conjugated Polymer Toward Understanding the Dynamism of #-Figuration of their Conjugated System Yuki Noguchi, Akinori Saeki, Takenori Fujiwara, Sho Yamanaka, Masataka Kumano, Tsuneaki Sakurai, Naoto Matsuyama, Motohiro Nakano, Naohisa Hirao, Yasuo Ohishi, and Shu Seki J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5100389 • Publication Date (Web): 07 Jan 2015 Downloaded from http://pubs.acs.org on January 11, 2015

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The Journal of Physical Chemistry

Pressure Modulation of Backbone Conformation and Intermolecular Distance of Conjugated Polymers Toward Understanding the Dynamism of π-Figuration of their Conjugated System Contribution to The Special Issue in The Journal of Physical Chemistry entitled: “John R. Miller and Marshall D. Newton Festschrift”

Yuki Noguchi,† Akinori Saeki, † Takenori Fujiwara,† Sho Yamanaka,† Masataka Kumano,† Tsuneaki Sakurai, † Naoto Matsuyama, † Motohiro Nakano, †,‡ Naohisa Hirao,§ Yasuo Ohishi, § and Shu Seki*,† †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,

Suita, Osaka 565-0871, Japan ‡

Present address: Research Center for Structural Thermodynamics, Graduate School of Science, Osaka

University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan §

Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198,

Japan * [email protected] (S.S.)

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ABSTRACT

Continuous tuning of the backbone conformation and interchain distance of a π-conjugated polymer is an essential prerequisite to unveil the inherent electrical and optical features of organic electronics. To this end, applying pressure in a hydrostatic medium or diamond anvil cell is a facile approach without the need for side chain synthetic engineering. We report the development of highpressure,

time-resolved

microwave

conductivity

(HP-TRMC)

and

evaluation

of

transient

photoconductivity in the regioregular poly(3-hexylthiophene) (P3HT) film and its bulk heterojunction blend with methanofullerene (PCBM). X-ray diffraction experiments under high pressure were performed to detail the pressure dependence of π-stacking and interlamellar distances in P3HT crystallites and PCBM aggregates. The HP-TRMC results were further correlated with high-pressure Raman spectroscopy and density functional theory calculation. The increased HP-TRMC conductivity of P3HT under pressure was found to be relevant to the planarity of the backbone conformation and intramolecular hole mobility. The effects of pressure on the backbone planarity are estimated as ~0.3 kJ mol-1 based on the compressibility derived from the X-ray diffraction under high pressure, suggesting the high enough energy to cause modulation of the planarity in terms of the Landau-de Gennes free energy of isolated P3HT chains as 0.23 kJ mol-1. In contrast, the P3HT:PCBM blend showed a simple decrease in photoconductivity irrespective of the identical compressive behavior of P3HT. A mechanistic insight into the interplay of intra- and intermolecular mobilities is a key to tailoring the dynamic π-figuration associated with electrical properties, which may lead to the use of HP-TRMC in exploring divergent π-conjugated materials at the desired molecular arrangement and conformation.

Keywords: high pressure; charge carrier mobility; conjugated polymer; time-resolved microwave conductivity; Raman spectroscopy; X-ray diffraction


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INTRODUCTION Central to the development of new π-conjugated polymers is the enhancement of their optical and electrical properties. These properties are susceptible to backbone conformation,1-4 molecular ordering,5,6 energetic landscape,7 and thermodynamic behavior.8 In particular, the influence of backbone conformation on the thermochromism9-11 and solvatochromism12-14 of conjugated polymers is well documented. A synthetic approach via side chain modification can tailor the backbone, leading to the modulation of π-conjugation length in the isolated phase15-17 and intermolecular arrangement in condensed matter.18-20 However, synthetic modifications involve tedious procedures, where the steric hindrance and solubility of the substituent also impact the variation of molecular weight and film quality. More recently, modulation of absorption and emission by mechanical stimuli, such as rearrangement of molecular ordering or chemical alternation, to elicit piezo- and mechanochromic effects has intrigued scientists.21-23 The transition occurs between two states, e.g., metastable and stable states,24,25 or broken and connected bonds,26,27 allowing a drastic change in optical and morphological properties. In contrast, applying pressure to a single polymer in a hydrostatic chamber or diamond anvil cell (DAC) is a straightforward and facile way to continuously tune the intrachain conformation and interchain distance without chemical engineering. An increase in pressure has been reported to cause a redshift in the absorption and fluorescence maxima of conjugated polymers.28,29 This is rationalized by the decrease of the dihedral angle of the π-conjugated unit and consequent increase of the conjugation length, and Raman spectroscopy under pressure is often utilized to gain direct access to the polymer planarity.30-34 Further increase of pressure may cause geometrical distortion of the backbone of, for example, polysilane,35 resulting in the reduction of conjugation length (blue-shift of absorption maximum). Continuous adjustment of the free energy change in an electron attachment reaction is another notable example of pressure mediation.36 Moreover, the effects of strain and bending force on flexible organic electronics, including field-effect transistors (FETs)37,38 and organic photovoltaic cells (OPVs),39-41 are of paramount importance to consolidate their forthcoming practical application. For ACS Paragon Plus Environment

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instance, the power conversion efficiencies of polymer:fullerene OPV on flexible substrate are decreased from 6.42% to 5.45% with increasing curvature, which is caused by electrical degradation of both organic layer and inorganic transparent conducting layer.42 Modulation of open circuit voltage under tensile and compressive stresses is reported in polymer:fullerene OPV.43 The residual stress in the respective layer on the order of a few to one hundred MPa is another important factor to secure the mechanical reliability. 44 Despite vigorous research on the optical spectroscopy of conjugated materials under pressure, electrical measurements for these materials has been limited.31,45-47 The latest study of poly(3hexylthiophene) (P3HT) showed that FET mobility is improved by a factor of three at 1 GPa owing to enhanced intermolecular interaction.48 Molecular crystals such as pentacene polycrystalline films49 and rubrene single crystals50,51 have displayed a similar trend in the low-pressure region (98%, head-to-tail regioregularity) and PCBM (>99.5%) were purchased from Aldrich and Frontier Carbon Inc., respectively. These were dissolved in chlorobenzene at a 1:0 or 1:1 weight ratio and drop cast onto a quartz substrate. In the HP-TRMC measurements, a P3HT film on a quarts was used as is, while a P3HT:PCBM film on a quarts was covered by polyvinylalcohol (PVA) purchased from Wako Chemical Industry Co. to prevent dissolution of PCBM in hydrostatic media. PVA layer (ca. 0.6 µm-thick) was cast by spin-coating its water solution. PVA does not absorb the excitation laser, and thus gives no TRMC signal. P3HT and P3HT:PCBM films without PVA coat for HP-XRD were peeled off from the quartz substrate and loaded into a DAC with a hydrostatic medium (methanol/ethanol volume ratio = 4:1) and a small amount of ruby powder. XRD measurements were conducted at the beamline of BL10XU at SPring-8, Japan Synchrotron Radiation Research Institute, using an X-ray wavelength of λ = 0.41326 Å. The inner pressure (P) was monitored from the observed fluorescence peak shift (∆λ) from the ruby powder (λ0 = 694.30 nm at 301 K and 1 atm) excited by an Ar ion laser (λex = 488 and 514.5 nm). It was calculated using the empirical equation, P (in GPa) = 380.8 × {(1 + ∆λ/λ0)5 - 1}. The baseline spectrum of an identical empty DAC was subtracted from the raw signal data. Confocal Raman spectroscopy was performed in a DAC using a Nihon Bunko NRS-4100 spectrometer with a 532-nm Nd:YAG laser. The pressure was monitored in a similar way using the same excitation laser. TRMC Measurements. An overview of the HP-TRMC system is described in the main text. The third harmonic generation (355 nm) of an Nd:YAG laser (Spectra-Physics Inc., Quanta-Ray GCR-130, 5–8 ns pulse duration, 10 Hz) was used as the excitation source. The laser power was fixed at 2.9 × 1016 photons·cm-2·pulse-1. Conductivity was converted to the product of the quantum yield φ and the sum of charge carrier mobilities Σµ using the equation φΣµ = ∆σ (eI0Flight)-1, where e, I0, FLight, and ∆σ are the unit charge of a single electron, incident photon density of excitation laser (photons·m-2), correction (or filling) factor (m-1), and transient photoconductivity, respectively. The change in conductivity is ACS Paragon Plus Environment

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equivalent to ∆Pr/(APr), where ∆Pr, Pr, and A are the change in reflected microwave power, reflected microwave power, and sensitivity factor (m·S-1), respectively. The change in the Q value and other geometry and sensitivity factors were calibrated against the inorganic photoconductor, molybdenum sulfide (MoS2) powder purchased from Aldrich, which is assumed to have a constant photoconductivity under low pressure. Hole Mobility Calculation. The charge transfer integrals for the hole of oligo(3-methylthiopene)-2,5’dimethyl

(alkyl-sexithiophene:6T) were calculated by density functional theory (DFT) using the

fragment orbital approach in the Amsterdam Density Functional (ADF) program.65 Calculations were performed for the parallel π-stacking of two 6T by changing the distance from 3.82 Å (1 atm) to 3.5 Å (5 GPa). The polarized triple-ζ (TZP) basis set was used with the generalized gradient approximation functional PW91.66 The effective charge transfer integral was calculated using the site energy and overlap matrix.67 Reorganization energies were calculated on the basis of the relaxation energy of the radical cation and neutral species,68 with optimized geometries obtained using B3LYP/6-31G(d,p). Charge carrier mobilities were calculated using the Marcus theory69 according to reported procedure.70 DFT-Raman spectra were calculated at the B3LYP/6-31G(d,p) level in a similar fashion using the Gaussian 09 package.71


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RESULTS AND DISCUSSION High-Pressure X-ray Diffraction (HP-XRD). Figure 1a shows the XRD spectra of the P3HT film under pressure from 1 atm to 5 GPa. The spectra are composed of low (2θ = 1.4°) and high (2θ = 6.2°) angle diffractions, which are attributed to the interlamellar (100) distance interdigitated by n-hexyl chains and π-stacking (010) distance between neighboring P3HT backbones, respectively.72,73 A weak and broad amorphous signal overlaps with the higher peak. Upon increase of pressure, both (100) and (010) peaks are systematically shifted to a higher angle (i.e., shorter distance). In contrast, release of pressure shifts these peaks back to their original positions by following the same pathway of pressurization. Each peak was analyzed by a pseudo-Voigt function (fractional linear combination of Gaussian and Lorentzian functions)74,75 to accurately evaluate the peak position and its integration. The π-stacking distance d(010) as a function of pressure is shown in Figure 1b, clearly demonstrating the continuous modulation of d(010) from 3.75 Å at normal pressure to 3.43 Å at 5 GPa. In the same fashion, the interlamellar distance d(100) is shortened from 16.8 to 13.95 Å (Figure 1c). Upon releasing the pressure, d(010) and d(100) follow identical d-P curves toward the pressurized direction without distinguishable hysteresis. The lattice shrinkage found in the present XRD study is in good agreement with previous results for regiorandom polythiophene.76 Compression entails redshifts of absorption, emission, and bandgap, which is explained by the extension of π-conjugation and enhanced intermolecular interaction,28-34 and is occasionally accompanied by an increase in excimer emission.77 The P dependence of the distance (d) can be fitted by an exponential function given by d = (d 0 − d sat )e− cP + d sat ,

(1)

where d0 and dsat are the initial and saturated distances at atmospheric and infinite pressures, respectively.76 The compressibility κ is calculated using the coefficient c, d0, and dsat through


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 d  ∂ ln d = c1 − sat  P → 0 ∂P d0  

κ = lim

(2).

As listed in Table 1, the least-mean-square analyses of the observed d-P curves by eqs 1 and 2 yielded

κ(010) = 0.038 GPa-1 for d(010) and κ(100) = 0.15 GPa-1 for d(100). Remarkably, an anisotropic compression of as much as 4 was obtained for the interlamellar direction compared to the π-stacking one (fitting results are provided in Figure S1 of the Supporting Information). This anisotropy is higher than that of poly(3-octylthiophene) (P3OT) having 75% head-to-tail regioregularity (2.5)76 and DNTT single crystal (~1.5).52 Interestingly, the κ of d(010) precisely coincides with that of P3OT (0.039 GPa-1),76 asserting that compressibility in the π-stacking direction is dominated by electrostatic repulsion between polymer backbones rather than side chain interactions. On the other hand, the d(100) of P3HT is compressed more easily by 50% than P3OT (κ = 0.105 GPa-1),76 although P3OT has longer, flexible alkyl chains. Nonetheless, the ratio of finite to initial distances (compression ratio dsat/d0) of P3HT is similar to that of P3OT in both π-stacking (90% for P3HT and 89% for P3OT) and interlamellar (83% for P3HT and 84% for P3OT) directions. This highlights the fact that the interdigitated region of shorter side chains undergoes faster rearrangement upon mechanical compression than the longer ones, and dsat/d0 is determined by the density of the side chains. Therefore, if the alkyl side chains have a denser arrangement, dsat/d0 should increase toward unity. This is the case with poly(3,3’-dioctyl-2,2’bithiophene) (PDOT2) comprising alternating 3-octylthiophene and unsubstituted thiophene units.76 The alkyl chains are intercalated by filling the free space at the unsubstituted thiophene, leading to a higher density (1.28 g·cm-3 for PDOT2 compared with 1.05 g·cm-3 for poly(octylthiophene) (POT)) and shorter interlamellar distance (14.94 Å for PDOT2 compared with 20.0 Å for POT). The dsat/d0 thereby increased to 91%.76 Therefore, the coexistence of the hard crystalline axis (π-stack) and short soft axis (alkyl interlamella) is a key for high anisotropic compression. The XRD spectra of the polymer:fullerene blend film (P3HT:PCBM weight ratio = 1:1 in chlorobenzene) are displayed in Figure 2a. These consist of three main peaks, two of which are identical ACS Paragon Plus Environment

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to those of the pristine P3HT film (π-stacking and interlamellar diffractions). The third new peak at 2θ = 5.2° is attributed to the diffraction from PCBM aggregates with a d-spacing (dPCBM) of 4.49 Å at 1 atm.78 Figure 2b shows the π-stacking distance of P3HT in the blend, in which d(010) exponentially decreases with pressure in a similar fashion as the pristine P3HT film (Figure 1b). The κ in the πstacking direction obtained from eqs 1 and 2 is 0.060 GPa-1, about 50% larger than that of pristine P3HT, while the d0/dsat of the blend (90%) is exactly the same as that of the pristine polymer (Table 1, with analyses provided in the Supporting Information Figure S2). The compressive behavior of interlamellar spacing is also the same as that of P3HT with respect to both κ (0.15 GPa-1) and d0/dsat (84%), as shown in Figure 2c. Therefore, blending with PCBM leads to the promotion of π-stacking in P3HT crystallites, irrespective of the unchanged interlamellar compression. The κ and d0/dsat of PCBM aggregates are 0.050 GPa-1 and 94%, respectively (Figure 2d), indicating that the compressibility is close to that of the P3HT π-stacking. Another difference between P3HT:PCBM and P3HT films is identified from a comparison of the d0/dsat (Figures 3a and 3b), where a subtle hysteresis appears in both π-stacking and interlamellar

directions of the former upon release of pressure. However, PCBM aggregates do not show such hysteresis, suggesting that these preferentially expand during the pressure release, followed by the delayed recovery of P3HT. In addition, the amorphous mixed phase of P3HT and PCBM, which is inactive during XRD evaluation but important in efficient charge separation in bulk heterojunction OPV,79,80 may be responsible for the appearance of the hysteresis. Guha et al.81 have reported highpressure absorption and emission spectroscopies of a ladder polymer (PhLPPP) or P3HT film blended with PCBM. The authors observed 0-0 absorption and 0-0 photoluminescence peaks of the polymer in the blend and found that the redshifts of the blend are larger than those of the pristine polymer. Notably, it was concluded that incorporation of PCBM promotes the change in the band-edge offset at the heterojunction via enhanced interaction. In other words, the lowest unoccupied molecular orbital (LUMO) level of the polymer is lowered more effectively than in the pristine polymer by virtue of the ACS Paragon Plus Environment

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mechanical interaction with the hard isotropic PCBM aggregates at the interface. Although XRD analyses were not performed for these blends, the polymer domains in the blend are assumed to be efficiently compressed since marked decreases in the LUMO and bandgap are associated with planarization of the polymer backbone and enhanced intermolecular interaction.81 These assumptions are in line with the present XRD results indicating that the κ of the π-stacking distance of P3HT in the blend is 1.5 times higher than that of the pristine polymer (Table 1). The anisotropic compression inherent to soft organic materials is also evident from the pressuredependent diffraction intensities. Figures 3c and 3d show the peak intensities of P3HT and P3HT:PCBM, which were calculated by integrating the extracted spectrum and normalizing at atmospheric pressure. The intensity of the π-stacking distance of P3HT increases until 2 GPa and is nearly saturated at high pressure. Moreover, the width of the peaks broadens at high pressure in both spectra of P3HT and P3HT:PCBM (Supporting Information Figure S3), most likely due to the increased variation in the πstacking distance of P3HT crystallites by inhomogeneous compression. This is also linked to the decrease in crystallite size expressed by the Scherrer equation.82 The observations on pressuremodulated intensity and spacing are in good agreement with that for P3OT.76 Note that the fragmentation of crystallites is a local and elastic event because the peak width recovers its original value after the pressure release, indicating that neither irreversible decomposition, crosslinking, nor growth of crystallites occurs. On the contrary, the intensity of interlamellar diffraction is monotonically reduced by pressure. Remarkably, this is a completely opposite behavior to the growth of lamellar diffraction found for thermal annealing.83 Fragmentation via slippage at the central boundary of doublelayered alkyl chains, which leads to a decrease in the diffraction intensity and increase in the diffraction width, may account for the XRD results. Figures 4a and 4b illustrate the proposed compressive behavior of the P3HT crystalline, with the structure in the absence of pressure reconstructed by following the work of Brinkmann et al.84


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The pressure-dependent diffraction intensity for P3HT:PCBM is analogous to that for the pristine P3HT (Figure 3d). PCBM aggregates also exhibit decreasing intensity with pressure similar to that of the interlamellar distance of P3HT, although the extent of decrease is almost half of the latter. This is consistent with the fact that the smallest change in d was observed for the PCBM aggregates, from 4.49 to 4.20 Å (Table 1). In addition, the peak width of dPCBM did not undergo a significant decrease (16% at 5 GPa). This is in sharp contrast to the >100% increase in the diffraction widths of d(100) and d(010) of P3HT (Supporting Information Figure S3b), reiterating that isotropic PCBM aggregates with a wide distance distribution are harder and more tolerant against pressure than P3HT. The incorporation of such aggregates is expected to facilitate the compression of P3HT in the π-stacking direction (Figure 4c). PCBM aggregates are selectively concentrated in the amorphous region and rim of the lamellar layer of P3HT,80,85 and will not disturb the self-organization of P3HT lamella. During the compression, neither intercalation nor mixing of PCBM into P3HT lamellas is probable, because the interlamellar and π-π stacking distances of P3HT domains are simply decreased. Thus, this agrees with the present XRD results and view of pressure-induced morphology.

High-Pressure Time-Resolved Microwave Conductivity (HP-TRMC). The HP-TRMC system is schematized in Figure 5. Microwave from a signal generator is introduced to the microwave circuit through a coaxial cable, and guided towards an X-band resonant cavity (around 9 GHz, parallelepiped-shaped TE102 mode) installed in a stainless chamber with a hydrostatic medium such as n-heptane and n-hexane. The cavity and waveguide are connected via a sealed coaxial cable, where the coupling between them is controlled by a stub. A typical Q value of the unpressurized resonant cavity filled by a hydrostatic medium is 500–700, slightly varied by the dielectric constant of the medium. An excitation laser pulse from a nanosecond laser is exposed to a sample through a 1-cm thick quartz window. The photo-induced transient change in reflected microwave power is magnified by an amplifier and detected by a diode and an oscilloscope. The pressure is controlled by a 12 ACS Paragon Plus Environment

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manual pressure pump connected to the syringe located underneath the chamber. The maximum pressure achievable is around 0.15 GPa, defined by the geometry of whole setup and hardness of the quartz window. Upon increasing a pressure, resonant frequency (f0) of the cavity filled by n-heptane is shifted downward from ca. 9.2 GHz at 0 GPa to 8.9 GHz at 0.14 GPa (Supporting Information Figures 4a and 4b), along with the decrease of Q value from ca. 550 to 450 (Supporting Information Figures 4c). From the dielectric constant (εr) data of n-hexane and n-octane,86 pressure-modulated εr of n-heptane was calculated to be marginal (1.920 at 0 GPa to 2.043 at 0.15 GPa, Supporting Information Figures 4d). Since the intensity of change in the reflected microwave power is proportional to Q/(f0 εr),87 the small decrease of f0 (ca. 3%) and increase of εr (ca. 7%) at 0.15 GPa almost cancel out. As a consequence, the decreasing Q value is a predominant sensitivity factor during pressurization (Supporting Information Figures 4e). However, we thought that other practical factors such as capacitance change of coaxial cable, distortion of cavity, and resultant variation of interaction between the sample and microwave electric field might cause further variation in the sensitivity. Therefore, we used TRMC transients of MoS2 powder under pressure as the calibration, which is a premier material having a high hole mobility,88 photoconductive response,89 and applicability as solid lubricant under severe condition.90 High pressure studies of MoS2 in a DAC have shown that the lattice parameter of its crystal does not change under a low pressure of HP-TRMC system.91,92 MoS2 indicated high photoconductivity signals almost twice that of P3HT:PCBM as shown in Supporting Information Figure S5. The intense and stable signals of MoS2 gave excellent reproducibility with small fluctuation, and thus used as the calibration curve including all of the pressure-dependent sensitivity factors. Figures 6a and 6b show the TRMC kinetics observed in P3HT and P3HT:PCBM films, respectively, under a pressure range of 0.0–0.14 GPa, where φΣµ is the product of the charge carrier generation yield upon 355-nm laser pulse excitation (φ) and sum of hole and electron mobilities (Σµ =

µh + µe). The maximum value (φΣµmax) of the transients is plotted as a function of pressure in Figures 6c (P3HT) and 6d (P3HT:PCBM). Interestingly, the φΣµmax of P3HT increases by about 30% at relatively ACS Paragon Plus Environment

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low pressure (0.04 GPa) and most likely will become constant. In contrast, P3HT:PCBM shows a monotonic decrease in φΣµmax. HP-XRD demonstrates that the compressive behavior of P3HT crystallites is almost similar between that of the pure polymer and the blend with PCBM, except for the doubled κ in the π-stacking distance of the latter. Assuming that TRMC predominantly probes the intermolecular hole mobility, the φΣµmax of P3HT:PCBM should increase more significantly than, or at least equally to, that of P3HT. However, this scenario is not the case with the present results. Therefore, the observed GHz local mobility (Σµ) is presumably attributed to the intramolecular hole mobility of P3HT. In fact, we have reported the separation of intra- and intermolecular mobilities by mixing an insulating polymer matrix such as poly(styrene) with P3HT93 and fluorene-thiophene copolymers.4 The intramolecular mobility of regioregular P3HT was found to be 0.12–0.6 cm2·V-1·s-1, depending on the casting solvent and thermal annealing, while that of regiorandom P3HT was two orders of magnitude smaller (0.007 cm2·V-1·s-1).94,95 The observed initial saturation in the φΣµmax of P3HT, regardless of the continuous narrowing of intermolecular distance dictated by HP-XRD (Figure 1), also corroborates the fact that the GHz local mobility is due to the intramolecular hole mobility rather than the intermolecular one. This suggests that pressurization has a peculiar impact on the backbone conformation. Conformational change under pressure has been intensively surveyed using optical and Raman spectroscopies,

where

conjugated

polymers

(e.g.,

polythiophene,28,29,31

polyacetylene,30

and

polyfluorene96) undergo backbone planarization that leads to an increase of π-conjugation length and decrease of bandgap. Intermolecular shrinkage in crystalline domains is evident from the current HPXRD experiments (Figures 1-4), although it should be noted that this shrinkage at the pressure level of HP-TRMC (

Pressure Modulation of Backbone Conformation and Intermolecular

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