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Study on #-# Interaction in H- and J-Aggregates of Poly(3-hexylthiophene) Nanowires by Multi-Techniques Yuan Yuan, Jie Shu, Ping Liu, Yinping Zhang, Yongxin Duan, and Jianming Zhang J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 11 Jun 2015 Downloaded from http://pubs.acs.org on June 12, 2015

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The Journal of Physical Chemistry

Study on π-π Interaction in H- and J-Aggregates of Poly(3-hexylthiophene) Nanowires by Multi-Techniques Yuan Yuan, 1 ,† Jie Shu, 2 ,† Ping Liu,1 Yinping Zhang,1 Yongxin Duan,1 and Jianming Zhang1, * 1

Key Laboratory of Rubber-Plastics, Ministry of Education / Shandong Provincial

Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China 2

Analysis and Testing Center, Suzhou University, Renai Road 199, 215123 Suzhou,

China

†

These authors contribute equally to this work.

* To whom all correspondence should be addressed.

Fax: +86-532-84022791

E-mail: [email protected] (J. Z.)

ACS Paragon Plus Environment


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Abstract

There has been intense interest in understanding the self-assembled structure of conductive polymer nanowires due to the potential application in photovoltaics area. In the present study, free-standing poly(3-hexylthiophene) (P3HT) thin films consisting of H- and recently reported J-aggregated nanowires (NWs) were respectively prepared by vacuum filtration method. The microstructure difference of both samples were studied in details by combining the multiple techniques, i.e., synchrotron radiation WAXD, solid-state NMR, and IR spectroscopy. It is found that there is stronger π-π interaction ascribed to the slightly closer interchain stacking in J-aggregates than that in H-aggregates, although both aggregates take the same form I crystal modification. The subtle structural difference in the interchain stacking distance (∆d020 = 0.04 Å) is further manifested in the thermal behavior of both aggregates as indicated by the DSC curves, in which J-aggregates exhibits much higher melting point than that of H-aggregates (∆T = 14 °C). Meanwhile, in situ synchrotron radiation WAXD study suggests that the low-temperature thermal transitions in both aggregates are associated with the shrinkage of the interchain distance. It is expected that this work will be helpful for understanding the structure-properties relationship of varying P3HT aggregate types.

Keywords: Conductive Polymer, Polymer Crystallization, Nanostructure, Thermal Behavior 1. Introduction


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Nanowires (NWs) of conductive polymers have recently attracted increasing attentions for use as building materials for high-efficiency nanostructured organic photovoltaics.1-4 NWs show great promise as they have a high degree of crystalline order and a large aspect ratio which favour long-range charge transport along the nanowire axis.5-7 For the typical organic semiconductor poly(3-hexylthiophene) (P3HT), it has been reported that self-assembled one-dimensional (1D) nanostructures lead to increased carrier mobility in field-effect transistors and improved solar conversion efficiency in solar cells, as compared with the two-dimensional (2D) thin film counterparts.1, 8-10 Therefore, much effort has been devoted to the preparation of P3HT NWs with high yield by choosing suitable solvents and controlling the crystallization temperature.11-13

There is a general recognition that continuous network morphology in thin films fabricated by highly crystalline NWs is beneficial for P3HT to improve charge transport.5-13 Furthermore, the intrinsic microstructure, i.e., the main-chain conformation and the π-π stacking determine the major pathways of intrachain and interchain transport.14 However, unlike much efforts of the structural control on P3HT 2D films,14, 15 the attempts to control the microstructure of P3HT NWs have been rarely reported until recently. Grey and coworkers firstly found that the P3HT NWs with high molecular weight exhibit single-chain J-aggregation under the condition of the self-assembly in toluene.16 This finding is intriguing since P3HT chains are generally considered to organize into NWs as H-aggregation.17-20 Moreover, the advantages of utilizing high-Mw P3HT have been confirmed in references. For



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example, Zen et. al.21 found that the backbone conformation is more planar at increasing molecular weight. Kline et. al.22 proposed the small ordered domains could be interconnected by long chains so that the mobility of the field-effect transistors made from high-Mw P3HT increases accordingly23. Therefore, the newly reported P3HT J-aggregation with high molecular weight was further studied from different perspectives in recent researches.24-27

H- and J-aggregation are identified by their different inter- and intrachain coupling, as proposed by Spano.28 The distinct photophysical characters attracted intense attentions for understanding the microstructure difference between P3HT J- and H-aggregates. Grey et al.16 addressed that the J-aggregated chains adopt a high-degree of main-chain planarization, whereas the thiophene units of H-aggregates are non-coplanar. Subsequently, the same side-chain packing distance in both aggregates is revealed by Moulé’s group29 based on the observation that the (100) reflections of the samples lie at the same angle in the X-ray profiles. However, there is no unambiguous conclusion on the structural difference of the π-π stacking, which is known to provides the main driving force for the NWs growth, in both aggregates. Spano et al.30 reported the competition between intrachain and interchain coupling, by proposing J-aggregated behavior is induced by the long range intrachain order which coincides with weak interchain interactions, while short range intrachain order and the resulting stronger interchain coupling induces appearance of H-aggregates. However, Moulé et al.29 reported the increase of both, the intra- and interchain order in J-aggregates.


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In the present study, by using a vacuum filtration method, we prepared highly oriented films consisting of high-Mw P3HT NWs with H- and J-aggregates, respectively. These samples take the edge-on orientation in the as-prepared thin films so that their structural difference along the π-π staking direction can be revealed directly by infrared spectroscopy and synchrotron radiation WAXD techniques in transmission mode. Meanwhile, the π-π interaction of both aggregates has also been investigated by solid-state NMR spectroscopy. Our results show that H- and J-aggregates exhibit subtle structure differences not only in local chain conformation but also in π-π interaction. This finding would enhance our understanding on the origin of their various thermal behaviors, although both P3HT aggregates essentially have the same crystal modification of form I.

2. Experimental Section

2.1 Material and Preparation of Nanowires High-Mw regioregular poly(3-hexylthiophene) (P3HT) (Mn ≈54-75 kDa, Mw/Mn < 2, regioregularity > 98%) was purchased from Sigma-Aldrich and used as received. P3HT NWs with H- and J-aggregates were prepared from 0.1 % m/v anisole and toluene solutions, respectively. Solutions were heated in a hot water bath at 80 °C until complete dissolution and then cooled slowly (cooling rate: ca. 0.5 °C/min) to room temperature. A gradual color change from orange to purple occurs after the formation of NWs. The NWs in anisole formed immediately when the solution cooled to room temperature, whereas approximately 24 h was needed for the formation of



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NWs in toluene. In order to prepare solid films for WAXD, FTIR, NMR, and DSC measurements, the as-prepared solutions of NWs were filtered through a 0.45 µm teflon membrane by vacuum filtration13. Then, the free-standing NWs films were peeled off from the filter for measurements. The thickness of the vacuum-filtered films is ca.10 µm.

2.2 Characterization

UV–Vis spectra of the NWs solutions were recorded with a UV-Visible spectrophotometer (Shimadzu UV-2550).

AFM measurement was performed by using MultiMode AFM Nanoscope V (Bruker Instrument) operated in the tapping mode. AFM imaging was performed using a silicon cantilever having a spring constant of 0.4 N/m and a resonant frequency of 50-90 kHz. The dimensions of the NWs were determined from the analysis of their area in a 5 µm scan image by using the nanoscope software. The NWs films for AFM measurements were prepared by spin-coating at 2000 rpm on a silicon substrate and the residual solvents were removed by vacuum-drying.

2D-WAXD measurements for the vacuum-filtered films along different directions were performed on a RIGAKUR-axis VII X-ray diffractometer in transmission mode with a graphite-monochromatized Mo-Kα X-ray beam (λ = 0.71 Å) and flat imaging plate as detector. The camera length is 260 mm. The measuring direction along z-axis is named through mode as shown in Figure 2b, and along x-axis is named side mode. The X-ray exposure time was 3600 s for through mode and 600 s for side mode.


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In situ WAXD measurement in transmission mode was performed at the BL40B2 beam station at Sping-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). The wavelength of the incident X-ray beam was 0.1 nm. A flat-panel detector (C9728DK-10, Hamamatsu Photonics) was used. In order to increase the diffraction intensity and signal/noise ratio, the free-standing films were folded for several times and then packed into a DSC aluminum pan (Metlar Toledo FP90). The folded films were parallel to the bottom surface of the DSC pan. Such setup will keep the orientation of the free-standing film. X-ray exposure time was 3s for every WAXD measurement and the heating rate is 10 °C/min. In the heating process from 0 to 120 °C, the WAXD data were collected every 1 °C. WAXD patterns measured were corrected for the background scattering.

FTIR spectra were collected on a Bruker VERTEX 70 spectrometer equipped with a MCT detector. To study the thermal behavior of the aggregates, the free-standing NWs films was set in a Linkam hot stage (FTIR-600, Linkam Scientific Instrument Ltd., Surrey, UK) equipped with a liquid N2 cooling unit. The sample was then heated at 2 °C/min under nitrogen atmosphere. FTIR spectra were recorded in transmission mode by co-adding 32 scans at a 2 cm-1 resolution at 1 min intervals from 20 to 280 °C. Solid-state NMR (SSNMR) experiments were performed on a Bruker Avance III HD 400 spectrometer operating at a Larmor frequency of 400.25 MHz for 1H and 100.64 MHz for

13

C, and equipped with a H/F/X triple-resonance magic-angle

spinning (MAS) probe, supporting MAS rotors of 3.2 mm outer diameter. The



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rf-nutation frequencies for 1H and 13C were 78.1 kHz, corresponding to 3.2 µs and for 90o pulse. 1D

13

C cross polarization/ magic-angle spinning (CP/MAS) spectra were

recorded using a CP time of 1 ms, a recycle delay of 5 s, and 1024 scans with SPINAL-64

decoupling

applied

during

acquisition.31

The

2D

13 32

C

Frequency-Switched Lee-Goldburg Heteronuclear Correlation (FSLG- HECTOR)33 experiments used a MAS frequency of 10.0 kHz, a recycle delay of 5 s, a cross polarization (CP) time of 0.2 ms, and 128 scans for a total of 80 t1 increments. Each t1 increment had a span of two basic FSLG blocks (48.34 µs)34, 35 and high-power 1H SPINAL-64 decoupling was used during acquisition. The CP step utilized Lee-Goldburg-CP (LG-CP) conditions36 to suppress 1H spin diffusion during CP. The Hartmann-Hahn matching condition was pre-optimized on L-alanine. A scaling factor of 0.570 was determined by recording a 2D spectrum for adamantane using the identical experimental conditions and used to rescale the indirect dimension for the investigated samples.

DSC measurement was performed on a TA Q20 calorimeter system under flowing nitrogen gas at a heating rate of 10 °C/min. About 3mg NWs films was sealed in aluminum pan. In order to make sure that the baseline is stable, the temperature is equilibrated at -50 °C prior to measurement.

3. Results and Discussion

3.1 Preparation of P3HT H- and J-Aggregated Nanowires


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Figure 1. UV-Vis absorption spectra of P3HT solutions formed in toluene and anisole in the range of 400-700 nm. The spectra are normalized by the 0-2 absorption for comparison.

It has been reported by several groups that high-Mw P3HT would facilitate to form H-aggregates in anisole solution and to form J-aggregates in slowly cooled toluene solution, respectively.16, 29, 37, 38 UV-Vis spectroscopy was generally used to determine and distinguish both types of aggregates according to the relative intensity of the absorption peaks at larger wavelengths.16, 24, 29, 37, 38 Following reported methods, we prepared J- and H-aggregated samples. As shown in Figure 1, both spectra exhibit the characteristic absorptions of the crystalline and amorphous bands. A broad absorption centered at ca. 470 nm is related to the non-aggregated fraction of P3HT molecules39, and three absorption peaks located at higher wavelengths are induced by aggregates40. In the model developed by Spano, H- and J-aggregation can be identified based on the intensity ratio of A0-0/A0-1.30, 41 J-aggregation exhibits the A0-0/A0-1 slightly larger than 1, while traditional H-aggregation typically shows A0-0/A0-1 in the region of ~0.5 to



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0.8.16 Performing the spectrum deconvolution (the detailed deconvolution process and the calculation of the intensity ratio is shown in the supporting information), A0-0/A0-1 is calculated to be 1.06 for the P3HT/toluene solution, and 0.61 for the P3HT/anisole solution. Accordingly, the UV-Vis data confirms that we have successfully prepared P3HT samples with H- and J-aggregates, respectively. Moreover, the yields of both aggregates are estimated from the deconvolution results as shown in Table S1, S2 in the supporting information. The yield of J-aggregates reaches 46.85%, while H-aggregates have lower yield of 37.17%.

Figure 2. (a, b) AFM topography images (upper) and their typical cross sections (lower) of J-aggregated (a) and H-aggregated (b) NWs. The range of the cross sections is guided by the dotted lines in AFM images. The samples were prepared by


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spin-coating on SiO2/Si substrates from the P3HT solutions with the concentration of 0.01 % m/v. (c, d) Height histograms of P3HT J- (c) and H-aggregated (d) NWs determined from the AFM topography images. The automatic binning was performed. The bins of the histograms are 0.6 nm for J-aggregates and 0.3 nm for H-aggregates.

The morphologies of the J- and H-aggregates were further investigated. In order to obtain the size of the single particle, the samples for AFM morphology measurement were prepared by spin-coating of the 0.01 % m/v solutions. As shown in Figure 2a, P3HT film with J-aggregates demonstrates well-dispersed NWs distributed on the substrate. Similar morphology could be identified in H-aggregated film as shown in Figure 2b. The heights of the NWs were determined from AFM images. As shown in Figure 2c, the height distribution of the J-aggregated NWs exhibits the peak in the 3-6 nm range, whereas the H-aggregated ones show the peak in the 5.5-7.5 nm range (see Figure 2d). The average height of the J- and H-aggregated NWs were respectively counted to be 4.12 and 6.40 nm. Moreover, both aggregates have the NWs in size dimension of several micrometers in length, and 10-15 nm in width. Considering the similarity of the morphology, the distinct absorption characteristic of various P3HT aggregates should originate from the inner structure difference in the intrachain and/or interchain order.

3.2 Microstructure Characterization of P3HT H- and J-Aggregates



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Figure 3. (a) The image of the vacuum-filtered P3HT film. (b) Schematic representation of the WAXD measurement under through mode and side mode. (c, d) WAXD patterns of P3HT J-aggregated sample. (e, f) WAXD patterns of H-aggregated sample. (g) The schematic illustration of the edge-on orientation.

2D-WAXD Measurements. In order to investigate the microstructure of the P3HT samples, isolated aggregates were separated from the dissolved fraction in solutions by vacuum filtration technique which generally induces the ordered nanoparticle self-assembly under flow field.13 A representative photograph of the vacuum filtered film is presented in Figure 3a. Orientation of the NWs in such films was firstly examined by 2D-WAXD measurements in transmission mode along two measuring


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directions (named through mode and side mode, see schematic representation in Figure 3b). As indicated in Figure 3c and 3d, narrow arcs of the indexed (100) and (200) reflections along the a-axis accompanied by weak arc of (020) reflection along the b-axis are observed in J-aggregated film in side mode, whereas only the (020) circular ring appears in through mode. According to this result, it is easy to conclude that the (100) reflection plane is along the film surface (parallel to film surface) and the (020) plane is perpendicular to this direction. It means P3HT chains take mainly edge-on orientation (as shown in Figure 3g) in J-aggregates with the side chains perpendicular to and the π-π stacking parallel to the film surface. The same molecular orientation in film consisting of H-aggregates is observed in Figure 3e, f. As comparing the 2D patterns in Figure 3c and 3e, it is found that the characteristic azimuthal breadths of H-aggregates are broader than that of J-aggregates, which suggests the relatively lower orientation in P3HT H-aggregates. Anyhow, such edge-on orientated P3HT films are very suitable for us to study the π-π interaction by using WAXD technique and IR spectroscopy in transmission mode.

Figure 4. (a) 1D WAXD profiles extracted from the 2D patterns of Figure 3c and 3e under side mode. (b) WAXD profiles of P3HT films in the q region of 9-22 nm-1



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measured by synchrotron radiation technique. The peaks are normalized using the intensity maximum of the reflections for comparison.

More detailed information on the chain stacking along a-axis is further obtained from the integrated 1D WAXD profiles derived from the 2D patterns as shown in Figures 3c and 3e. WAXD profiles of the two aggregates measured in side mode are displayed in Figure 4a. Both H- and J-aggregated samples exhibit the strong (100) reflections located at the same q = 3.82 nm-1 (d100 = 16.44 Å), which confirms that there is the same interlayer distance of the adjacent backbones along a-axis direction with that of form I modification (d100 = 16.2 Å)42. This conclusion is in agreement with Roehling’s report.29

Synchrotron Radiation WAXD Measurements. Due to the weak (020) signals induced by the limited thickness of the P3HT films, the difference of the (020) reflections is hard to be distinguished in the integrated 1D WAXD profiles derived from 2D patterns in Figure 3d and 3f, which are measured with the conventional WAXD setup. Therefore, the measurements using synchrotron radiation in transmission mode were performed. Figure 4b shows the 1D WAXD profiles of P3HT samples collected by synchrotron radiation technique, in which the peak located in the q region of 16-18 nm-1 is indexed to the (020) reflection43, 44. The (020) peak of H-aggregates located at q = 16.75 nm-1 corresponds to the interchain stacking distance along b axis of 3.75 Å, which is consistent with the reported value of P3HT form I.42-45 However, the (020) peak of J-aggregates has a subtle but clear shift to the higher q value of 16.92 nm-1, leading to the shorter d020 of 3.71 Å.


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Scheme 1. Schematic drawing of P3HT titled stacking structure along b axis. It has been accepted that P3HT main chains appear as the tilted stacking in its thermodynamically stable structure.46,

47

The angle ϕ between the plane of the

conjugated thiophene rings and a axis as defined in Scheme 1 lies in the region of 26-30 °. Under such case, π-π stacking distance (dπ-π) is shorter than the stacking distance of d020. The relationship between these parameters can be expressed as follows:

d π-π = d 020 cosφ

(1)

Based on Equation (1), dπ-π is affected not only by d020 but also by the tilting degree. It means we couldn’t assess the difference of the π-π stacking distance or π-π interaction directly according to d020 indicated by WAXD. Therefore, infrared and solid-state NMR spectroscopies which are sensitive to the local molecular environment were performed to investigate the π-π interaction difference of H- and J-aggregates.



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Figure 5. (a) FTIR spectra of P3HT NWs films in the region of 3200-800 cm-1. Chemical structure of P3HT and the directions of the Cβ-H vibration discussed are shown as inset. Backbone-related spectra of δ(Cβ-H), υas(C=C), and υ(Cβ-H) modes, as well as their corresponding second derivative curves are shown in (b-d).

Infrared Spectroscopy Measurements. The structural difference including the main-chain conformation and the interchain interaction for various P3HT aggregates is further revealed by infrared spectroscopy. Considering that H- and J-aggregates have the same side-chain packing distance29, the analysis of the IR spectra is focused on the backbone-related vibrations as displayed in Figure 5a. Enlarged spectral differences in several characteristic wavenumber regions are compared in Figure 5b-d.


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The bands in the 850-800 cm-1 region as shown in Figure 5b are dominated by the Cβ-H out-of-plane deformation mode (δ(Cβ-H)).48, 49 In our previous work on P3HT42 and poly(3-butylthiophene) (P3BT)50, it is found that the frequency of the δ(Cβ-H) mode is sensitive to the π-π stacking modes or interaction. That is, random π-π stacking in amorphous phase corresponds to the δ(Cβ-H) mode at high wavenumber, whereas ordered π-π stacking in crystalline phase of P3ATs presents this mode at lower wavenumber. For our P3HT aggregates, J-aggregates exhibit the low-frequency band shift by ca. 1 cm-1 compared to H-aggregates from the 2nd derivative curves of the original spectra, although the shifting itself is not so prominent. This observation might also indicate that there is stronger π-π interaction in J-aggregates. By studying various oligothiophenes, Navarrete et al.51 found that the peak position of the C=C antisymmetric stretching vibration υas(C=C) shifts to lower wavenumber with increasing the effective conjugation length (ECL). Zerbi et al.52 also concluded that the dispersion of this vibration is associated with the backbone ECL. As presented in Figure 5c, the υas(C=C) modes of both H-aggregates and J-aggregates locates at 1509 cm-1. This observation indicates the ECL in both P3HT aggregates is nearly equal and there is no obvious difference. Red shift of the Cβ-H stretching vibration (υ(Cβ-H)) for J-aggregates by 2 cm-1 in the 3090-3030 cm-1 region is clearly shown in Figure 5d. In our previous work, we observed a continuous blue shift of this vibration during the heating process of a crystalline P3HT film from 30 to 250 °C.42 The peak position of the υ(Cβ-H) mode will be correlated with the local main-chain conformation in the later discussion.



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Figure 6. (a) Structural scheme of P3HT with alphabet labels for assignment. (b) Solid-state 13C {1H}3 CP/MAS spectra of P3HT H- and J aggregates.

Solid-State NMR (SSNMR) Measurements. SSNMR chemical shift is sensitive to the local molecular arrangement, which thus is a reliable tool for probing the molecular assembly structure. Using SSNMR, π-π stacking structure in P3HT form I crystal has been revealed successfully by both 1H and

13

C spectra.53 Therefore, this

technique was also performed to investigate the difference of the π-π interaction for H- and J-aggregates. Figure 6b shows the thiophene region truncated

13

C {1H}

CP/MAS spectra of two P3HT samples. Peaks assignments are labeled by using alphabet related to the structural scheme in Figure 6a. Four dominant peaks positions for thiophene unit are listed in Table 1. Compared to H-aggregates, all the four main-chain carbons in J-aggregates show higher field chemical shift position with slightly deviation from 0.02 ppm to 0.23 ppm as indicated in Table 1. It is known that P3HT main-chain chemical shifts characteristic of the π-π stacking in the form I crystalline regions exhibit the higher field shift by 0.96 ppm than that (if any) in the amorphous regions.53 Similarly, the SSNMR spectra of our P3HT samples reveal again that the π-π interaction between P3HT main chains in J-aggregates is stronger


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than that in H-aggregates. Apart from four dominant peaks, two Cb shoulder peaks located around 126.26 ppm and 127.32 ppm as well as one Cc signal around 138.69 ppm are detected in J-aggregates. The lower field shoulders for Cb and Cc are inferred to be associated with the unstacked and non-planar main-chain structure. More efforts are required to perform further microstructural analysis on this point. Table 1. Main-chain 13C Chemical Shifts of P3HT H- and J-aggregates

Ca (ppm)

Cb (ppm)

Cc (ppm)

Cd (ppm)

J aggregates

130.26

125.06

136.17

132.82

H aggregates

130.42

125.29

136.23

132.84

∆δ (ppm)

0.16

0.23

0.06

0.02

3.3 Thermal Behavior of P3HT H- and J-Aggregates.

Figure 7. DSC heating traces of P3HT H- and J-aggregated samples. The curves are displayed from 0 to 270 °C. It is believed that the microstructural difference of P3HT H- and J-aggregates might result in the distinct thermal behavior, and the structural characteristics could



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be revealed further by monitoring the structural evolution during heating. Figure 7 shows the first heating traces of the P3HT NWs films. The apparent endothermic behavior in the region of 200-270 °C should be associated with the melting process of P3HT crystal.42, 48 It is interesting to note that J-aggregates exhibit a much higher melting point of 256 °C than H-aggregates (∆T = 14 °C). In the temperature region below 100 °C, a subtle endothermic peak is identified for both aggregates as visualized by the dashed lines. In the following sections, in situ FTIR spectroscopy combined with WAXD technique were performed to clarify the structural origin and microstructure evolutions corresponding to the distinct thermal behavior of H- and J-aggregates.

3.3.1 Structural Origin of the High-Temperature Thermal Behavior

According to traditional crystallization theory, the melting temperature of the bulk polymer materials with the same crystal modification depends on the fold length of polymer chain or lamellar crystal thickness.54 Although P3HT is a comb-like polymer with rigid conjugated backbones, it shows similar crystallization behavior like flexible polyethylene.55 Studies based on TEM have indicated the structural evolution of P3HT from extended-chain to folded-chain crystals occurs as molecular weight reaches 25-35 kDa.56 Therefore, for our high-Mw P3HT, the higher melting point of J-aggregates as compared to H-aggregates is firstly considered to arise from its larger initial or subsequently increased folded-chain length during heating.


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Figure 8. Temperature-dependent IR spectra of P3HT J-aggregated (a) and H-aggregated (b) films in the region of 1530-1410 cm-1. The spectra are displayed from 20 to 280 °C with 20 °C intervals. (c) Peak positions of the υas(C=C) band as a function of temperature. The peak positions of the υas(C=C) mode as a function of temperature for P3HT cast film with middle molecular weight is inset.

In Situ Infrared Spectroscopy Measurements. According to the proposed model of P3HT NWs,55, 57 the folded-chain length or lamella thickness in P3HT NWs is equal to the effective conjugation length (ECL). Therefore, peak position of the υas(C=C) mode, which has been reported to be sensitive to the ECL of P3ATs main chains, could be used to monitor the change of folded-chain length in P3HT NWs. By comparing the peak positions of the υas(C=C) vibrations, it is found that the



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conjugation lengths of H- and J-aggregates are almost equal at room temperature. With heating, the υas(C=C) modes of both aggregates show similar low-wavenumber shifting as presented in Figure 8a, b. Quantitative results of the peak positions as a function of temperature are plotted in Figure 8c, in which H- and J-aggregates display very similar trends of evolution during the entire heating process. According to these observations, the higher melting point of J-aggregates might not be ascribed to the longer ECL.

Interestingly, as compared to middle-Mw (Mw = 45.6 kDa) counterpart (see the inset figure in Figure 8c), it is noticed that both aggregates with high-Mw P3HT exhibit much more change with temperature, while for middle-Mw P3HT, the change of the peak positions is confined between 1510 and 1508 cm-1. Moreover, the middle-Mw P3HT shows a slight high wavenumber shifting above 180 °C which is closed to the Tm of P3HT (220 °C determined by DSC42). In contrast, it is found that to a greater extent, the frequency of H- and J-aggregates continuously shift to the low wavenumbers until the sample melts. The peak positions locate around 1504 cm-1 as the temperature reaches 280 °C. It indicates that the ECL of H- and J-aggregates prepared from high-Mw P3HT are increased in the high temperature region. This observation suggests the occurring of chain unfolding in both aggregates with high-Mw at temperature closes to the melting temperature region, which is similar with the lamella thickening phenomenon for traditional semi-crystalline polymer. That is, the crystal thickness tends to increase at elevated temperatures aided by increased molecular mobility58. For middle-Mw sample, it is extended-chain rather than


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folded-chain that exists. Therefore, no increase in ECL could be observed during the heating process.

Considering the texture of the NWs, P3HT crystals are nanoconfined along the c axis as well as along the a axis. That means besides the lamellar thickness, the changes in the dimensions of the crystals along the [100] crystallographic direction would also impact on the melting point. Based on the previous morphology characterization (Figure 2), it is known that the J-aggregated NWs have the comparable height distribution with the H-aggregated ones, both corresponding to 2-4 layers of the backbone stacking along the a axis. Moreover, the average height of J-aggregated NWs (4.12 nm) is less than that of the H-aggregated ones (6.40 nm). Apparently, the smaller crystal dimensions along the [100] crystallographic direction couldn’t explain for the higher melting point of J-aggregates as well.



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Figure 9. Temperature-dependent IR spectra of P3HT J-aggregated (a) and H-aggregated (b) films in the region of 850-800 cm-1. The spectra are displayed from 20 to 280 °C with 20 °C intervals. (c) Intensity changes of the main δ(Cβ-H) band as a function of temperature for P3HT samples.

As discussed above, the high melting point of J-aggregates is not related to the lamellar thickness nor the crystal dimensions along the a axis. Then, is it possible to assign the structural origin of the high-temperature thermal behavior of P3HT aggregates to the difference in π-π interaction? Figure 9a, b presents temperature-dependent IR spectra of the δ(Cβ-H) mode for P3HT films. The spectral profile of the δ(Cβ-H) mode has been confirmed to be sensitive to the π-π interaction as mentioned previously. At room temperature, the crystalline peaks of J-aggregates and H-aggregates are located at 819 and 820 cm-1, respectively. Peak intensities of


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both aggregates decrease abruptly at high temperature higher than 240 °C, as shown in Figure 9c. The peak intensity decrease of J-aggregates occurs at temperature which is 16 °C higher than H-aggregates. The temperature difference of the spectral changes is in agreement with that of the Tm, as shown in Figure 7. Based on previous structural analysis, it can be explained that the higher melting point of J-aggregates is correlated with enhanced ground state (van der Waals) interactions due to the reduced π-π stacking distance. Moreover, it is indicated from WAXD and DSC that the H-aggregated NWs present lower crystallinity than J-aggregated ones. The less structural quality which may exist in the former would also affect the melting point.

It is noticed that, as temperature rises to 280 °C at which the samples are molten based on the DSC data, the crystalline peak for each aggregate is still dominant whereas the amorphous peak43 located at 835 cm-1 shows only weak intensity (see Figure 9a, b). It clearly illustrates the ordered π-π stacking of H- and J-aggregates is still preserved to some extent during the melting process. Combined with the observation of increased conjugation length for P3HT aggregates in the high temperature region, it indicates a nematic phase with two dimensional ordering. A nematic order in P3HT close to the sample melting has also been suggested based on experiments or theories in references.49,

59-62

Moreover, the intensities of the

crystalline peaks for both aggregates show slight increase in the region of 50-90 °C, where a small endothermic peak is observed in the DSC traces. In the next part, it will be demonstrated that this slight intensity increase is correlated with the change of the π-π interaction.



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3.3.2 Structural Change Correlated with the Low-Temperature Thermal Behavior

Figure 10. Two-dimensional map of the temperature-dependent WAXD data for P3HT J-aggregated (a) and H-aggregated (b) samples from 30 to 120 °C. Intensity is represented by color gradation from blue (lower) to white (higher). (c) Temperature dependence of the d020 spacing upon heating. In Situ Synchrotron Radiation WAXD Measurements. The subtle endothermic peak located below 100 °C as presented in Figure 7 has been observed in the heating process of P3HT samples taken in form I modification. The origin of this endothermic behavior was ascribed to the disordering of the side-chain packing59 or twisting of the


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main chains45. P3HT H- and J-aggregates are also in form I modification. By monitoring the structural evolution during heating process using synchrotron radiation WAXD technique, a new insight into the origin of this endothermic peak was gained.

Figure 10a, b shows the two-dimensional (2D) map of the in situ WAXD profiles from 30 to 120 °C, in which the q region of 16-18 nm-1 is dominated by the (020) reflection. In 2D maps, it is observed that the peak positions of the (020) reflection shift to higher q side in both samples in the region of 50-90 °C, leading to the weak but clear shrinkage of the main-chain stacking in π-π direction by ca. 0.04 Å (as indicated by Figure 10c). The slight shrinkage of the P3HT d020 spacing was also observed by other groups.63, 64 Thus, the improved π-π interaction arising from the decreased stacking distance may lead to the low-temperature transition and the crystalline IR peaks increase in intensity. However, it should be mentioned that the changes of the (100) diffraction peaks with temperature have not be monitored because of the edge-on chain orientation and the transmission mode of the measurement.

Therefore,

the effect of

the

side-chain

“melting”

low-temperature thermal behavior could not be ruled out at current stage.


on the


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Figure 11. Temperature-dependent IR spectra of P3HT J-aggregated (a) and H-aggregated (b) films in the region of 3090-3030 cm-1. The spectra are displayed from 20 to 280 °C with 20 °C intervals. (c) Peak positions of the υ(Cβ-H) band as a function of temperature.

In Situ Infrared Spectroscopy Measurements. Since the twisting of the P3HT main chains may occur as temperature rises45, local main-chain conformational adjustments of P3HT NWs films during heating are also investigated by monitoring the spectral changes of the υ(Cβ-H) mode, as shown in Figure 11. Since the vibrational direction (see Figure 5a) is parallel to the thiophene-ring plane, it is reasonable to conclude that the peak position of the υ(Cβ-H) mode could be affected by the local conformation of main-chain. At room temperature, J-aggregated P3HT chains adopt a high-degree main-chain planarity.16 In the subsequent heating process,


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the peak position shifts to higher wavenumber gradually as displayed in Figure 11a. Blue shifting of the conformation-sensitive vibration of the polythiophene is correlated with the destruction of the main-chain planarity.52,

58, 65

Similarly, the

high-frequency shifting of the υ(Cβ-H) mode in J-aggregates indicates the structural disordering of the thiophene main chain induced by the torsional motion around the ring-ring linkages as proposed by Tashiro et al.45 By investigating the molecular dynamics of P3HT, Yazawa et al.66 suggested that the thiophene ring undergoes twist motion above 300 K. The quantitative calculation of the peak positions as a function of temperature (shown in Figure 11c) indicates the high-wavenumber shifting of J-aggregates occurs once the sample is heated, mainly concentrated in the 20-90 °C region. Accordingly, besides the shrinkage of the π-π stacking distance, the locally conformational adjustment also contributes to the low-temperature thermal transition of P3HT J-aggregates.

Compared to J-aggregates, no peak shifting with temperature could be discerned in the original spectra of H-aggregates as presented in Figure 11b. In the quantitative result of Figure 11c, the frequency of the υ(Cβ-H) mode for H-aggregates decreases firstly and then increases afterwards. It is noticed that the shifting range is less than 1 cm-1. Moreover, the peak position at 280 °C locates at the same wavenumber as that at room temperature. Accordingly, it is concluded that the main chains in P3HT H-aggregates take the twisted conformation in the initial state, whereas J-aggregated P3HT chains adopt a high-degree of planar main-chain conformation.

4. Conclusion



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The microstructure and the thermal behavior of vacuum-filtered P3HT films with H-aggregated and J-aggregated nanowires are investigated by multi-techniques. The difference in π-π stacking and its effect on the thermal behavior occurred in both aggregates are discussed. Synchrotron radiation WAXD measurement revealed that J-aggregates have a shorter interchain stacking distance compared to the H-aggregates. The shifting of the characteristic δ(Cβ-H) band to the low wavenumber in IR spectra of J-aggregates indicates the stronger π-π interaction, which is further confirmed by the SSNMR spectra with the observation of high-field shifting of the main-chain carbons. The microstructural difference results in not only the distinct optical characteristics of these two aggregates, but also the thermal behavior. The higher melting point (Tm = 256 °C) of J-aggregates is derived from the higher ordering of the π-π stacking, according to the analysis of temperature-dependent IR spectroscopy. The structural origin of the low-temperature thermal behavior for P3HT H-aggregates could be related to the slight decreasing of the interchain stacking distance. However, besides the shrinkage of the stacking distance, it is found that the local conformational adjustment also contributes to the low-temperature transition of J-aggregates. Our work demonstrates that very subtle structural difference in crystals will pose significant effect on the properties of the conductive polymer. Acknowledgements

Y. Yuan and J. M. Zhang thank Prof. Christoph Bubeck in Max-Planck Institute for Polymer Research (MPIP) for his critical reading, helpful suggestions and revision of


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this manuscript. The financial supports from Natural Science Foundation of China (21274071), Taishan Mountain Scholar Constructive Engineering Foundation (TS20081120, tshw20110510), and Natural Science Fund for Distinguished Young Scholars of Shandong Province (JQ200905) are greatly appreciated. J. Shu thanks the financial support of Natural Science Foundation of China (21303111).

Supporting Information Available: The supporting information includes: 1) deconvolution process of UV-Vis spectra, 2) the calculation of the intensity ratio of A0-0/A0-1, and 3) temperature-dependent WAXD data for P3HT aggregates in the q region of 0-30 nm-1. This information is available free of charge via the Internet at http://pubs.acs.org



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For Table of Contents use only

Study on π-π Interaction in H- and J-Aggregates of Poly(3-hexylthiophene) Nanowires by Multi-Techniques Yuan Yuan, 1 ,† Jie Shu, 2 ,† Ping Liu,1 Yinping Zhang,1 Yongxin Duan,1 and Jianming Zhang1, *


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