Singlet Exciton Delocalization in Gold Nanoparticle-Tethered Poly(3

Oct 18, 2017 - We fabricated hybrid poly(3-hexylthiophene) nanofibers (P3HT NFs) with rigid backbone organization through the self-assembly of P3HT ...
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Singlet Exciton Delocalization in Gold Nanoparticle-Tethered Poly(3hexylthiophene) Nanofibers with Enhanced Intrachain Ordering Dongki Lee,† Dong Hun Sin,† Sang Woo Kim,‡,§ Hansol Lee,† Hye Ryung Byun,‡,§ Jungho Mun,† Woong Sung,† Boseok Kang,† Dae Gun Kim,† Hyomin Ko,† Sung Won Song,† Mun Seok Jeong,‡,§ Junsuk Rho,†,∥ and Kilwon Cho*,† †

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea Center for Integrated Nanostructure Physics, Institute for Basic Science, and §Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea ∥ Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea ‡

S Supporting Information *

ABSTRACT: We fabricated hybrid poly(3-hexylthiophene) nanofibers (P3HT NFs) with rigid backbone organization through the self-assembly of P3HT tethered to gold NPs (P3HT-Au NPs) in an azeotropic mixture of tetrahydrofuran and chloroform. We found that the rigidity of the P3HT chains derives from the tethering of the P3HT chains to the Au NPs and the control of the solubility of P3HT in the solvent. This unique nanostructure of hybrid P3HT NFs self-assembled in an azeotropic mixture exhibits significantly increased delocalization of singlet (S1) excitons compared to those of pristine and hybrid P3HT NFs selfassembled in a poor solvent for P3HT. This strategy for the self-assembly of P3HT-Au NPs that generate long-lived S1 excitons can also be applied to other crystalline conjugated polymers and NPs in various solvents and thus enables improvements in the efficiency of optoelectronic devices.

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INTRODUCTION In organic electronic devices such as organic field effect transistors1−5 and organic photovoltaic cells,6−8 the nanosized morphology of the conjugated polymer has a strong influence on device characteristics, so improving their performances requires the development of methods for the fabrication of conjugated polymer chains with the desired orientation and packing at the molecular level. Among the numerous aggregate systems of crystalline conjugated polymers, films and nanofibers (NFs) of aggregated regioregular poly(3-hexylthiophene) (RRP3HT) with three typical orientations of edge on, face on, and vertical have been widely investigated.9−12 One-dimensional (1D) nanostructures of P3HT NFs have unique 1D chargecarrier transport channels that can improve the performance of optoelectronic devices. Various aspects of these structures have been intensively studied, such as nanostructure fabrication,13−17 device applications,18−22 and their photophysics investigation focusing on how the activation and deactivation pathways of singlet (S1) and triplet (T1) excitons and polarons depend on the two aggregate types of P3HT: the J-aggregate exciton coupling induced by the intrachain head-to-tail interactions and the H-aggregate exciton coupling induced by the interchain head-to-head interactions. Especially, since the conformational behaviors of P3HT chains (intra- or interchain aggregation) have significant effects on the dynamics of charge carriers (S1 excitons and polarons) associated with device performances, © XXXX American Chemical Society

diverse studies on aggregates P3HT chains have been reported.4,23−25 Inorganic nanoparticles (NPs) such as quantum dots, TiO2, and the noble metals gold (Au) and silver (Ag) have been introduced into optoelectronic devices containing P3HT because of their unique and easily tunable physical properties.26−30 In particular, Au and Ag NPs exhibit surface plasmon resonances (SPRs) of the collective oscillation of the conduction electrons on their surfaces, which can interact with the excited states of P3HT, so they have been widely used as charge-trapping sites and to increase the efficiency of light absorption in the optoelectronic devices.31−33 However, studies of the formation mechanisms and excited-state dynamics of hybrid aggregate P3HTs containing inorganic NPs are rare. In this study, we used the “grafting to” approach to introduce Au NPs into the RR-P3HT matrix to achieve stable nanocomposites of inorganic Au NPs and the organic P3HT matrix. We then fabricated hybrid P3HT NFs through the selfassembly of P3HT-Au NPs in two different solvents, i.e., cyclohexanone (CHN) and a 1:1 (v/v) mixed solvent of tetrahydrofuran (THF) and chloroform (CF) known as an azeotropic mixture. And we investigated the effects of the Au Received: July 6, 2017 Revised: September 27, 2017

A

DOI: 10.1021/acs.macromol.7b01416 Macromolecules XXXX, XXX, XXX−XXX

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comparison to a sucrose standard (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, β = 102.938°). Transient Absorption Spectra (TAS) and Time-Resolved Photoluminescence (TRPL) Kinetics. For the femtosecond TAS measurements, an Ultrafast Systems HELIOS femtosecond transient absorption spectrometer and a Libra 1 kHz femtosecond Ti:sapphire regenerative amplifier system with a center wavelength of 780 nm and an 80 fs pulse duration were used. The laser beam was divided by using a beam splitter. About 95% of the laser beam was driven to a Coherent TOPAS Prime optical parametric amplifier (OPA) to be used as a tunable pump beam with a pulse duration of 200 fs. The other 5% was focused on a 5 mm sapphire crystal to generate a whitelight continuum to be used as a probe beam. The Libra 1 kHz femtosecond Ti:sapphire regenerative amplifier system was also used to generate excitation pulses for the measurement of picosecond TRPL kinetics. The selected emission wavelengths were detected by using a Hamamatsu C9300 streak camera with a time resolution of 10 ps.

NPs and solvents on the NF formation mechanism. We also investigated the excited-state dynamics of the hybrid P3HT NFs with transient absorption (TA) spectroscopy. The tethering of the P3HT chains to the Au NPs and the increased polarity of the solvent result in enhanced inter- and intrachain aggregation of P3HT chains in the azeotropic mixture, and these hybrid P3HT NFs were found to have more crystalline nanostructures than the pristine and hybrid P3HT NFs selfassembled in a poor solvent for P3HT. This change in the aggregated structure results in a significant increase in the delocalization of S1 excitons in the hybrid P3HT NFs selfassembled in the azeotropic mixture.

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EXPERIMENTAL SECTION

Synthesis. Propylthiol-terminated poly(3-hexylthiophene) (P3HTSH) with 98% regioregularity (M̅ n = 10 169 g mol−1, PDI = 1.28, MALDI-MS m/z = 6831.2) was synthesized using Grignard metathesis reaction and characterized, as described in detail elsewhere.34 The degree of polymerization (DP) was estimated to be 40 by using 1H NMR. We have prepared all samples used in this paper with this P3HT-SH. P3HT-stabilized gold nanoparticles (P3HT-Au NPs) were synthesized via a “grafting to” approach, as described in detail elsewhere.35 To obtain the pure nanocomposites of P3HT-Au NPs with only covalently attached P3HT chains to the Au NPs, produced P3HT-Au NPs were repurified by centrifugation. The dried P3HT-Au NPs were completely dissolved in THF (HPLC, J.T. Baker), and a few milliliters of ethanol (200 proof, anhydrous, ≥99.5%, Sigma-Aldrich) was added to the P3HT-Au NPs THF solution. The inhomogeneous mixture was centrifuged for 30 min at 4500 rpm to selectively precipitate the P3HT chains attached to Au NPs; the supernatant containing free P3HT-SH chains was discarded. This centrifugation process was repeated about 10 times until the supernatant became optically clearly. The final precipitated P3HT-Au NPs were then dried under vacuum for 24 h at 40 °C. The gravimetric ratio of inorganic Au NPs to organic P3HT in the pure nanocomposites was calculated from the Beer−Lambert law and by comparing the optical density of the pure P3HT-Au NPs in chloroform (CF, anhydrous, ≥99%, SigmaAldrich) to that of pristine P3HT in CF. The content of Au NPs in the nanocomposites was 75%. Consequentially, the grafting density of P3HT chains tethered on Au NP was determined on the assumption that the Au NPs are spherical: 9 P3HT chains/1 Au NP; 0.23 chains/ nm2. Pristine and hybrid P3HT NFs were prepared in a poor solvent of CHN (≥99.0%, Sigma-Aldrich) by a self-assembly method as described in detail elsewhere.36 For the fabrication of hybrid P3HT NFs in the azeotropic mixture, 2.0 mg of P3HT-Au NPs was dissolved in a 1:1 (v/v) mixed solvent of THF (3 mL) and CF (3 mL) in a 20 mL glass vial. The colloid was heated slowly by using a hot water bath to produce a transparent orange solution. When naturally cooled to room temperature, the mixture turned to the clearly violet-colored solution containing the hybrid P3HT NFs. The produced hybrid P3HT NFs in the azeotropic mixture were directly used without further purification. Characterization. UV−vis absorption spectra were measured using a PerkinElmer Lambda 1050 spectrometer, and photoluminescence (PL) spectra were measured using a Horiba Jobin Yvon NanoLog spectrofluorometer. Transmission electron microscopy (TEM) images were obtained by using a JEOL JEM-1011. Highresolution (HR) and high-angle annular dark-field (HAADF) TEM images were obtained by using a JEOL JEM-2200FS equipped with Image Cs corrector; a TEM sample was prepared by evaporating and drying a colloidal droplet on a carbon-coated copper grid under N2 in a glovebox at room temperature. 2D grazing incidence X-ray diffraction (GIXD) experiments were performed by using the synchrotron source at the Pohang Accelerator Laboratory (PAL). 2D GIXD patterns were recorded using a Rayonix SX165 2D CCD detector, and X-ray irradiation time was 1−10 s depending on the saturation level of the detector. Diffraction angles were calibrated by

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RESULTS AND DISCUSSION Nanostructure of Hybrid P3HT Nanofibers (NFs). To synthesize the P3HT-Au NPs, we used the “grafting to” method to prevent the macrophase separation of the organic phase of the P3HT matrix from the inorganic phase of Au NPs; as a result, we obtained stable nanocomposites with covalent bonds between the thiol groups of P3HT-SH and the surfaces of the Au NPs (Figure S1). In a previous paper, it was demonstrated that free P3HT-SH chains that do not react with the gold precursor during the “grafting to” procedure remain in the synthesized P3HT-Au NPs as an impurity, so the effects of the tethering of P3HT chains to the Au NPs on the formation of the hybrid P3HT NFs could not be fully assessed.36 Thus, in this study, we conducted a further purification procedure that removes the majority of the free P3HT-SH chains and clearly suggested that the tethering effect drives the high inter- and intrachain aggregation of the P3HT chains during the formation of the hybrid P3HT NFs. Figure 1a shows that the absorption spectra of hybrid P3HT NFs self-assembled in CHN (green) and in a 1:1 (v/v) mixed solvent of THF and CF (red) are red-shifted from that of pristine P3HT in CHN (blue). And the 0−2, 0−1, and 0−0 transitions around 520, 560, and 610 nm, respectively, of aggregated P3HT chains of hybrid P3HT NFs self-assembled in CHN and a 1:1 (v/v) mixed solvent of THF and CF are obviously more pronounced than those of pristine P3HT NFs self-assembled in CHN. Pristine P3HT NFs self-assembled in CHN have a more amorphous nanostructure (Figure S2) than that of hybrid P3HT NFs self-assembled in CHN (Figure 1b); this difference indicates that the effect of tethering of P3HT chains to the Au NPs induces high inter- and intrachain ordering of the P3HT chains during the formation of the P3HT NFs. The absorption spectrum (Figure S3) of hybrid P3HT NFs formed in a 1:1 (v/v) mixed solvent of THF and CF is greatly red-shifted with respect to that of the nonaggregated P3HT-Au NPs in CF. Moreover, the peaks of the 0−2, 0−1, and 0−0 transitions of the aggregated P3HT chains are more distinct than those of a film of RR-P3HT with 98% regioregularity spin-coated onto a quartz plate from CF, even though the former spectrum was measured in solution. These results suggest that the P3HT chains covalently attached to the surfaces of the Au NPs form P3HT NFs with a nanostructure that is more highly crystalline than that of free P3HT chains. The PL spectra of the solution-state pristine and hybrid P3HT NFs contain strong peaks around 650 and 700 nm, which are due to the 0−0 and 0−1 transitions, respectively (Figure B

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ization is in good agreement with the PL spectra in Figure 1a. Note also that the HAADF TEM image (Figure 1e) of the hybrid P3HT NFs self-assembled in a 1:1 (v/v) mixed solvent of THF and CF shows that the Au NPs are better incorporated into the organic P3HT matrix than the hybrid P3HT NFs selfassembled in CHN (Figure 1d). These results imply that the 1:1 (v/v) mixed solvent of THF and CF strongly affects the orientation along the chain and the packing of the P3HT chains during the formation of the P3HT NFs. Effects of Increasing the Polarity of the Solvent. To prove the solvent effects, we measured the PL spectra (Figure S4) of the P3HT-Au NPs and the absorption spectra (Figure S5) of RR-P3HT samples self-assembled in solvents with various THF to CF volume ratios. Although THF and CF have boiling points of 65.6 and 61.2 °C, respectively, the boiling point of the mixed solvent increases to 72.5 °C when THF:CF (v/v) is exactly 1:1 because it becomes an azeotropic mixture.39 As the THF:CF volume ratio gradually approaches the azeotropic mixture ratio, the peak around 650 nm associated with the 0−0 transition of aggregated P3HT was well observed in the PL spectra of the P3HT-Au NPs. This phenomenon is also present in the absorption spectra of RR-P3HT shown in Figure S5; the absorption spectra of RR-P3HT self-assembled in the azeotropic mixture have three distinct peaks, i.e., the 0− 2, 0−1, and 0−0 transitions of the aggregated P3HT chains. If the solubility effect is the major factor in the formation of NFs, when the content of THF in the mixed solvent is increased, the more crystalline P3HT NFs should be formed; the solubility values of THF and CF for P3HT are 1.1 and 14.1 mg/mL, respectively.40 However, since THF and CF are both good solvents for P3HT, the RR-P3HT chains have not been selfassembled in a homo solvent system of THF and CF (Figure S5). Therefore, the unique phenomenon observed in Figures S4 and S5 could not be ascribed to the solubility effect of the mixed solvent. Instead, we consider this result originate from the increased polarity of the azeotropic mixture of THF and CF. That is the interaction between the solvent molecules results in the increase in the polarity of the medium, which enhanced the self-assembly of P3HT chains into NFs in this mixed solvent as shown in Figures S4 and S5. The increased polarity of the azeotropic mixture can also be evidenced by its increased boiling point. Recently, various studies on simple methods to tune nanostructure containing RR-P3HT by altering the polarity of the medium have been reported.41−45 Thus, we suggest that the increase of polarity of the azeotropic mixture of THF and CF is a critical force to drive the aggregation of P3HT chains during the formation of hybrid P3HT NFs. In addition, as shown in Figure 1a and Figure S5, the solvent effect on the crystallinity of P3HT NFs is more predominant than the effect of tethering of P3HT chains; the absorption spectrum of pristine P3HT NFs in the azeotropic mixture of Figure S5 (red curve) has three more district peaks associated with aggregated P3HT chains than those of pristine and hybrid P3HT NFs in CHN (blue and green curves of Figure 1a). Thus, it is considered that the solvent effect has more influence on the crystallinity of P3HT NFs than the structural tethering effect. Consequently, the tethering effect of P3HT chains and the increase in the polarity of the medium induce high inter- and intrachain aggregation of the P3HT chains in the azeotropic mixture, so the nanostructure of hybrid P3HT NFs that forms in the azeotropic mixture is unique and more crystalline than that of the pristine P3HT NFs and hybrid P3HT NFs formed in CHN.

Figure 1. (a) Absorption (left, solid lines) and PL (right, dashed lines) spectra of pristine P3HT NFs in CHN (blue), hybrid P3HT NFs in CHN (green), and hybrid P3HT NFs in a mixed solvent of THF and CF (red). Samples were excited at 530 nm for the PL spectra. TEM images of (b) hybrid P3HT NFs in CHN and (c) hybrid P3HT NFs in a mixed solvent of THF and CF. High-angle annular dark-field (HAADF) TEM images of (d) hybrid P3HT NFs in CHN and (e) hybrid P3HT NFs in a mixed solvent of THF and CF.

1a).37,38 Notably, in the PL spectrum of the hybrid P3HT NFs self-assembled in a 1:1 (v/v) mixed solvent of THF and CF, the 0−0 transition around 650 nm due to the intrachain head-totail interactions is much stronger than the 0−1 vibronic transition around 700 nm due to the interchain head-to-head interactions. However, this tendency that clearly appears in the PL spectrum of hybrid P3HT NFs self-assembled in a 1:1 (v/v) mixed solvent of THF and CF is gradually diminished from the PL spectrum of hybrid P3HT NFs self-assembled in CHN to that of pristine P3HT NFs self-assembled in CHN. This result indicates that in addition to the effect of the tethering of the P3HT chains to the Au NPs the solvent effect has a large influence on the orientation and packing of the P3HT chains in the formation of the P3HT NFs. The morphological difference between the hybrid P3HT NFs with a thickness of 37 nm formed in a 1:1 (v/v) mixed solvent of THF and CF known as an azeotropic mixture and those with a thickness of 20 nm formed in CHN also shows the solvent effect (Figure 1b,c). The hybrid P3HT NFs formed in CHN have a cobweb-like nanostructure with strong contrast in TEM images (Figure 1b), whereas those formed in the azeotropic mixture have a diffuse appearance with low contrast (Figure 1c). The P3HT NFs with a wide thickness and a diffuse nanostructure have been reported as J- aggregate P3HT NFs having a high intrachain order.25 This morphological characterC

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driving forces, the tethering of the P3HT chains to the Au NPs and the increase in the polarity of the medium, strongly induce the high inter- and intrachain aggregation of the P3HT chains during the formation of the P3HT NFs. Mechanism of the Formation of the Highly Crystalline Hybrid P3HT NFs. To gain deeper insight into the unique orientations and packing of the P3HT chains covalently attached to Au NPs in the P3HT NFs, we obtained the twodimensional grazing-incidence X-ray diffraction (2D GIXD) patterns of films drop-coated onto Si substrates (Figure 3). The 2D GIXD image of the drop-coated film of pristine P3HT NFs self-assembled in CHN is in good agreement with that of RRP3HT films with an edge-on orientation: the X-ray reflections from the (100) crystal plane along the qz-axis correspond to an intermolecular backbone layer with a d-spacing of 15.52 Å, and the (010) crystal planes along the qxy-axis correspond to intermolecular π−π stacking with a d-spacing of 3.8 Å.49 However, the featured patterns of edge-on orientation of P3HT chains are gradually weakened from the 2D-GIXD image of Figure 3a to that of Figure 3c. In particular, the strong patterns newly emerged in the q range of 1.0−2.0 Å−1 of the 2D GIXD image of hybrid P3HT NFs self-assembled in the azeotropic mixture (Figure 3c). Especially, the strong patterns newly emerged in the q range of 1.0−2.0 Å−1 of the 2D GIXD image of hybrid P3HT NFs selfassembled in the azeotropic mixture (Figure 3c). The (002) reflection induced by the rigidity of the main chains of RRP3HT has been reported in the qxy range of 1.5−2.0 Å−1 of the 2D GIXD image.10,11,50 This rigid backbone organization (high intrachain ordering) of P3HT chains observed in the azeotropic mixed solvent would be induced by two driving forces: the tethering effect of P3HT chains and the increase of solvent polarity. Figure 3d shows the HR (inset) and HAADF TEM images of hybrid P3HT NFs self-assembled in the azeotropic mixture and schematic nanostructure of linked P3HT-Au NPs having a rigid backbone organization. The HRTEM image definitely indicates the stable Au NPs within the assembled hybrid P3HT NFs, and the HAADF image looks like the milky way nanostructure due to the good incorporation of Au NPs into the P3HT NFs matrix; comparing the HAADF images of Figures 1d and 1e, the Au NPs are better located within the hybrid NFs self-assembled in the azeotropic mixture than the hybrid NFs self-assembled in CHN. In addition, overall, the Au NPs are irregularly located in the HAADF image of Figure 1d. As mentioned above, we conducted a repurification procedure to completely remove the free P3HT chains that are not covalently attached to the Au NPs (see the Experimental Section). However, due to experimental limitations, it is possible that some free P3HT chains still remain in the final precipitated P3HT-Au NPs; such free P3HT-SH chains are expected have an influence on the formation of the hybrid P3HT NFs. We now explain the formation mechanism of the hybrid P3HT NFs, which is largely influenced by the solvent effect. Since CHN is a poor solvent for P3HT, the free P3HT chains and the tethered P3HT chains compete to form hybrid P3HT NFs due to the gradual decrease in the solvent solubility with decreases in the temperature. Thus, the 2D GIXD image of hybrid P3HT NFs self-assembled in CHN (Figure 3b) have the featured patterns of edge-on orientation of P3HT chains and the newly emerged patterns in the qxy range of 1.0−2.0 Å−1. In addition, the density of the Au NPs within the hybrid P3HT NFs self-assembled in CHN is lower than that within the hybrid P3HT NFs self-assembled in

To quantify the crystallinity of solution-state P3HT NFs, the aggregated portions (red lines) have been obtained by subtracting the amorphous portions (blue lines) from the measured absorption spectra of the solution-state pristine and hybrid P3HT NFs46 (Figure 2) and were deconvoluted into the

Figure 2. Absorption spectra (black solid lines) of (a) pristine P3HT NFs in CHN, (b) hybrid P3HT NFs in CHN, and (c) hybrid P3HT NFs in a mixed solvent of THF and CF. The dashed and red dasheddotted lines represent nonaggregated and aggregated portions of each absorption spectrum, respectively. The theoretical absorption spectrum (gray lines) in each panel was obtained by simulating the H-aggregate-modified Franck−Condon fit.

different vibrational levels of the excited S1 state (gray lines) by using the H-aggregate-modified Franck−Condon fit of eq 147,48 ⎛ e − s S m ⎞⎛ W e −s ⎟⎜⎜1 − Abs(E) ∝ ∑ ⎜ ⎝ m ! ⎠⎝ 2Ep m Γ(E − (E0 ← 0 + mEp))

∑ n≠m

⎞2 Sn ⎟ n! (n − m) ⎟⎠ (1)

where W is the exciton bandwidth, Ep is the phonon energy of the main oscillator coupled to the electronic transition, s is the Huang−Rhys factor, and m represents the vibrational level.47,48 The calculated crystallinity values were 47% for pristine P3HT NFs self-assembled in CHN, 53% for hybrid P3HT NFs selfassembled in CHN, and 63% for hybrid P3HT NFs selfassembled in the azeotropic mixture. The crystallinity of the hybrid P3HT NFs self-assembled in the azeotropic mixture is similar to that of RR-P3HT with >98% regioregularity film spin-coated onto a quartz plate from chlorobenzene.47 This theoretical result supports the above conclusion that two D

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Figure 3. 2D GIXD patterns of drop-coated films on the Si substrates of (a) pristine P3HT NFs self-assembled in CHN (b) hybrid P3HT NFs selfassembled in CHN and (c) hybrid P3HT NFs self-assembled in a mixed solvent of THF and CF. (d) HR (inset) and HAADF TEM images of hybrid P3HT NFs in the azeotropic mixture and schematic nanostructure of linked P3HT-Au NPs having a rigid backbone organization.

the azeotropic mixture. On the other hand, in the case of the azeotropic solvent mixture, when it is naturally cooled its polarity increases rapidly near room temperature, which reduces the solubility of P3HT.39 Thus, the tethering effect is the dominant driving force of the aggregation of P3HT chains in the azeotropic mixture. Consequently, the linked P3HT-Au NPs having high intrachain ordering of P3HT chains are gently formed in the azeotropic mixture (Figure 3d). In addition, the Au NPs are well incorporated within the organic P3HT NFs. Thus, we propose that the hybrid P3HT NFs with rigid backbone organization (high intrachain ordering) are facilely fabricated by the tethering of P3HT chains to Au NPs and the control of solubility. Singlet (S1) Exciton Delocalization in Highly Crystalline Hybrid P3HT NFs. Figure 4 shows the near-infrared (NIR) transient absorption spectra (TAS) of the various P3HT NFs. Figure S6 shows also the NIR TAS of the P3HT NFs observed in time windows of 1 ps. Each TA kinetic profile of P3HT NFs in Figure S7 was obtained in a time window of 500 ps and deconvoluted into three decay components, and the extracted decay time constants are listed in Table S1. As shown in Figure 4a and Figure S6a, the TAS of pristine P3HT NFs self-assembled in CHN have general two broad photoinduced absorption (PIA) bands around 1100 and 1300 nm that are associated with localized polarons and S1 excitons, respectively.51−53 The observed kinetic dynamics is well matched with the reported dynamics of general RR-P3HT aggregate systems; the competitive dynamics between the vibrational relaxation process to the lowest S1 excitons and the fast formation process of polarons from the quasi-continuous band of higher S1 exciton states occurs within 1 ps (Figure S6). In addition, while the PIA band of S1 excitons around 1300 nm almost disappears within 300 ps, that of the localized polarons around 1100 nm is still present in the picosecond time range (Figure 4a).53 On the other hand, the PIA species observed in the NIR range for the hybrid P3HT NFs exhibit unusual kinetic processes and different PIA band positions compared to those of the pristine P3HT NFs self-assembled in CHN. Figure S6b,c

Figure 4. Transient absorption spectra of (a) pristine P3HT NFs in CHN, (b) hybrid P3HT NFs in CHN, and (c) hybrid P3HT NFs in a mixed solvent of THF and CF. Samples were excited at 530 nm. Time delays after excitation are indicated inside in units of picoseconds.

clearly demonstrates that the ratio of the absorbance of the localized polarons to that of the nonrelaxed S1 excitons of the hybrid P3HT NFs is larger than that of the pristine P3HT NFs self-assembled in CHN in time range of 1 ps: ultrafast E

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Macromolecules formation processes of localized polarons around 1100 nm for hybrid P3HT NFs in CHN and 1000 nm for hybrid P3HT NFs in the azeotropic mixture. We suggest that there are two possible explanations of this ultrafast polaron formation phenomenon in hybrid P3HT NFs: (1) the energy or charge transfer from the S1 excitons of P3HT to the surface plasmon resonance (SPR) state of Au NPs (exciton quenching) and (2) the unstable quasi-continuous band of higher S1 exciton states induced by the rigid backbone organization of the P3HT chains covalently attached to the Au NPs.36 We will discuss later in detail about this ultrafast phenomenon with the TAS observed in the visible wavelength of Figure 6. Figure 4b,c clearly shows the featured PIA bands around 1100 nm, which still remain for hundreds of picoseconds. We have assigned these PIA bands to the relaxed S1 excitons because they have similar decay time constants to those obtained from the time-resolved photoluminescence (TRPL) kinetics (Table 1 and Table S1).52,53

Figure 5. PL kinetic profiles of pristine P3HT NFs in CHN (blue circles), hybrid P3HT NFs in CHN (green squares), and hybrid P3HT NFs in a mixed solvent of THF and CF (red triangles). Samples were excited at 530 nm and monitored from 620 to 750 nm. Black lines are best-fitted curves to extract decay constants.

Table 1. PL Decay Constants, PL Quantum Yield (QY) and Radiative (kr) and Nonradiative (knr) Rate Constants of NFs Dispersed in CHN and a Mixed Solvent of THF and CF sample pristine P3HT NFs in CHN hybrid P3HT NFs in CHN hybrid P3HT NFs in THF/CF

decay time/psa 61 (87%) + 300 (13%) 41 (89%) + 1000 (11%) 50 (60%) + 1100 (40%)

PLQY %b

kr/ns−1

knr/ns−1

1.19

0.130

10.73

0.58

0.040

6.787

0.11

0.002

2.125

the shorter decay components of the P3HT NFs have similar decay time constants of ∼50 ps; we assign this process to intraor interchain energy transfer.54 On the other hand, the longer decay components of the hybrid P3HT NFs (∼1 ns) are much slower than that of the pristine P3HT NFs self-assembled in CHN (300 ps). Note that there is no effect of the rigid backbone organization of the P3HT chains on the increase in the delocalization of the S1 excitons of the pristine P3HT NFs self-assembled in CHN.54 Thus, we assign the longer decay component of 300 ps for the pristine P3HT NFs self-assembled in CHN to the biexciton recombination process originating from the unstable quasi-continuous band of the higher S1 exciton state,53 and the longer decay components of ∼1 ns for the hybrid P3HT NFs are assigned to the delocalized S1 exciton recombination induced by the rigid backbone organization of the P3HT chains covalently attached to the Au NPs.25,55,56 In addition, the fraction of the longer decay component for the hybrid P3HT NFs in the azeotropic mixture (40%) is much larger than those for the pristine and hybrid P3HT NFs in CHN (13 and 11%, respectively), and the nonradiative rate constant (knr) of the hybrid P3HT NFs in the azeotropic mixture is 3 orders of magnitude greater than the radiative rate constant (kr) but only 2 orders of magnitude greater than that of the pristine P3HT NFs in CHN. These results strongly support the conclusion that the tethering of the P3HT chains to the Au NPs and the increase in the polarity of the azeotropic mixture have strong influences on the formation of the rigid backbone organization of the P3HT chains, which significantly increases the delocalization of the S1 excitons compared to those in the pristine and hybrid P3HT NFs self-assembled in CHN. Diffusion Coefficient and Diffusion Length of S1 Excitons in the P3HT NFs. To quantitatively characterize the dynamics of delocalized S1 excitons in the solution-state P3HT NFs, we used a singlet−singlet exciton annihilation (SSA) model, as described in detail elsewhere.47 The timedependent exciton density N(t) can be described with eq 2

a Excited at 530 nm and monitored from 620 to 750 nm. bMeasured by comparison of NFs dispersed in CHN and a mixed solvent of THF and CF with a standard Rhodamine B dissolved in ethanol following excitation at 530 nm.

This result indicates that there is a significant increase in the delocalization of the S1 excitons in the hybrid P3HT NFs due to the existence of the rigid backbone organization of the P3HT chains. In particular, the PIA band originating from the longlived S1 excitons in the hybrid P3HT NFs self-assembled in the azeotropic mixture is the most clearly observed of those of the three P3HT NFs samples in the hundreds of picoseconds time range (Figure 4c). Thus, we conclude that the rigid backbone organization (high intrachain order) of the P3HT chains is induced by the tethering effect of the P3HT chains to the Au NPs and the control of change of the solubility of P3HT in the solvent, which results in a significant increase in the delocalization of S1 excitons of the hybrid P3HT NFs in the azeotropic mixture when compared to those of the pristine and hybrid P3HT NFs formed in CHN. As was the case for the TAS results, the relaxed S1 exciton decay profiles for the pristine and hybrid P3HT NFs in Figure 5 show a significant increase in the delocalization of the S1 excitons for the hybrid P3HT NFs self-assembled in the azeotropic mixture. All of the observed decay and calculated rate constants and photoluminescence quantum yields (PLQY) of the P3HT NFs obtained from the analysis of PL kinetic profiles of Figure 5 are listed in Table 1. Each PL kinetic profile in Figure 5 has been deconvoluted into two decay components: 61 ps (87%) and 300 ps (13%) for the pristine P3HT NFs self-assembled in CHN, 41 ps (89%) and 1000 ps (11%) for the hybrid P3HT NFs self-assembled in CHN, and 50 ps (60%) and 1100 ps (40%) for the hybrid P3HT NFs self-assembled in the azeotropic mixture. Overall,

d N (t ) N (t ) =− − fγ(t )N (t )2 dt τ

(2)

where τ is the PL decay time, γ(t) is the SSA rate coefficient, and f is the annihilation factor. And, the three-dimensional (3D) annihilation rate coefficient is given by eq 3 F

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Macromolecules ⎛ γ3D(t ) = 8πDR ⎜1 + ⎝

R ⎞ ⎟ 2πDt ⎠

(3)

where R is the exciton annihilation radius and D is the diffusion coefficient. The detailed fitting procedures are well described in previous papers.47 The fitting parameters used in our analysis are as follows: a pump-spot radius of ∼0.1 mm, a pump-beam path length of approximately 800 nm, which was obtained by performing dynamic light scattering measurements of the pristine P3HT NFs in CHN, and a pump beam energy of 1.41 μJ/pulse. This 3D exciton diffusion model can be used to fit the S1 exciton decay of the pristine P3HT NFs in CHN (Figure S8, black circles), with D = 5.9 × 10−3 cm2 s−1 and a diffusion length given by Dτ = 7.1 nm. These values are in good agreement with the values reported for low-crystallinity RR and regiorandom P3HT 3D S1 exciton diffusion systems;57 the crystallinity of our pristine P3HT NFs in CHN was found to be 47% (Figure 2). However, the relaxed S1 excitons in the solution-state hybrid P3HT NFs decay unusually due to the rigid backbone organization of the P3HT-SH chains covalently attached to Au NPs compared to those of the pristine P3HT aggregate system, so the D and Dτ values of the solution-state hybrid P3HT NFs could not be obtained from the delocalized S1 exciton decays by using the SSA model. Analysis of the Excited-State Dynamics of the Hybrid P3HT NFs. To gain a more information on the delocalized S1 excitons of hybrid P3HT NFs, we have obtained TAS in the visible range in Figure 6. The two negative peaks below 620 nm in each panel are due to ground-state bleaching (GSB) and are well matched with the steady-state absorption spectra in each panel (the black dashed curves).58 The large PIA bands around 660 nm in each panel were assigned to polaron pairs;52,53,58 their center peak positions are blue-shifted by increases in the crystallinity of the P3HT NFs: 680 nm for the pristine P3HT NFs in CHN, 670 nm for the hybrid P3HT NFs in CHN, and 650 nm for the hybrid P3HT NFs in the azeotropic mixture, which suggests that the more unstable higher excited states were well formed in hybrid P3HT NFs than pristine P3HT NFs.52,53 In addition, charge carriers such as polaron pairs and polarons are generated from the unstable quasi-continuous band of the higher S1 exciton states induced by the high intraand interchain aggregation, so the initial absorbance ratio of polaron pairs to GSB in hybrid P3HT NFs is larger than that in pristine P3HT NFs in CHN. These results indicate that the generation of the unstable quasi-continuous band of higher S1 exciton states by the hybrid P3HT NFs is more effective than that by the pristine P3HT NFs self-assembled in CHN due to the rigid backbone organization of the P3HT-SH chains covalently attached to the Au NPs. Furthermore, the ultrafast formation processes of the localized polarons in the hybrid P3HT NFs mentioned in the discussion of Figure 4 can also be clearly explained by these results. Since the absorption bands of the large polaron pairs around 660 nm are clearly evident in Figure 6b,c, energy or charge transfer from the S1 excitons of P3HT to the SPR state of the Au NPs (exciton quenching) rarely occurs in the hybrid P3HT NFs. Thus, since the unstable quasi-continuous band of higher S1 exciton states is better generated in the hybrid P3HT NFs than in the pristine P3HT NFs, localized polarons arise more easily in the hybrid P3HT NFs than in the pristine P3HT NFs (Figure S6). Each TA kinetic profile of GSB at 610 nm, polaron pairs absorption at 660 nm, and delocalized polarons absorption at 740 nm of

Figure 6. Transient absorption spectra of (a) pristine P3HT NFs in CHN, (b) hybrid P3HT NFs in CHN, and (c) hybrid P3HT NFs in a mixed solvent of THF and CF. Time delays after excitation are indicated inside in units of picoseconds. The black dashed and dasheddotted curves represent the steady-state absorption and PL spectra, respectively, of NFs corresponding to the transient absorption spectra of each panel. Samples were excited at 530 nm for the transient absorption and PL spectra.

P3HT NFs in Figure S9 observed in time windows of 40 ps were deconvoluted into three kinetic components, and the extracted kinetic time constants are listed in Table S2. At a time 30 ps after excitation, the negative GSB peaks are blue-shifted to 630 nm in Figures 6b,c and closely approach the steady-state PL spectra (dashed-dotted curves) in Figures 6b,c due to the stimulated emission (SE) originating from the delocalized S1 excitons in the hybrid P3HT NFs. In contrast, this phenomenon is only weakly evident for the pristine P3HT NFs self-assembled in CHN (Figure 6a) because the relaxed S1 excitons have almost completely decayed within 300 ps via the biexciton recombination process. The longest GSB recovery times related to the relaxed S1 excitons of the P3HT NFs are well matched with the decay times of the PIA band around 1200−1300 nm (Tables S1 and S2). Consequentially, we conclude from our combined analysis of the TAS and the TRPL decay kinetics that the tethering of the P3HT chains to the Au NPs and the control of the change in the solubility of P3HT strongly induce the rigid backbone organization of the P3HT chains covalently attached to the Au NPs, which results in the formation of a new type of charge-carrier transport channel that significantly increases the delocalization of the S1 excitons of the hybrid P3HT NFs self-assembled in the G

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Macromolecules

Center for Advanced Soft Electronics under the Global Frontier Research Program of the ministry of science and ICT, Korea.

azeotropic mixture compared to those of the pristine and hybrid P3HT NFs self-assembled in CHN.

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CONCLUSIONS In summary, we have fabricated hybrid P3HT NFs through the self-assembly of P3HT-Au NPs in CHN and in an azeotropic mixture of THF and CF and compared their properties with those of pristine P3HT NFs self-assembled in CHN by using 2D GIXD and TAS to investigate the effects of the Au NPs and the solvents on the formation mechanism and excited-state dynamics of the hybrid P3HT NFs. Especially, the formation of the hybrid P3HT NFs in the azeotropic mixture is predominantly governed by the tethering effect of the P3HT chains to the Au NPs due to the rapid change of the P3HT solubility. Thus, this phenomenon results in the rigid backbone organization of the P3HT chains that significantly increases the delocalization of the S1 excitons compared to those of the pristine and hybrid P3HT NFs self-assembled in CHN. We believe that our fabrication strategy and analysis of the excitedstate dynamics of the hybrid RR-P3HT and Au NPs system provide useful guidelines for the fabrication of nanostructures and the investigation of their photophysical properties. In addition, our proposed hybrid system obtained by the selfassembly of the crystalline conjugated-polymer stabilized NPs generates long-lived S1 excitons and so is expected to contribute to the improvement of the performances of optoelectronic devices.

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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.7b01416. TEM and HRTEM images of P3HT-Au NPs, TEM image of pristine P3HT NFs, absorption spectra, PL spectra of P3HT-Au NPs in mixed solvents, absorption spectra of RR-P3HT in mixed solvents, NIR femtosecond TAS, TA kinetic profiles of PIA of localized polarons and S1 excitons, S1 exciton decay fitted with the 3D exciton diffusion model, and TA kinetic profiles of GSB recovery and PIA of polaron pairs and delocalized polarons (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel +82 54 279 2270; e-mail [email protected] (K.C.). ORCID

Jungho Mun: 0000-0002-7290-6253 Mun Seok Jeong: 0000-0002-7019-8089 Kilwon Cho: 0000-0003-0321-3629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Pohang Accelerator Laboratory (PAL) for providing the synchrotron radiation sources at 3C, 5A, and 9A beamlines and the National Institute for Nanomaterials Technology (NINT) at POSTECH for the HR and HADDF TEM experiments in used in this study. This work was supported by a grant (Code No. 2011-0031628) from the H

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