Article pubs.acs.org/Macromolecules
Electrochemical Generation and Spectroscopic Characterization of Charge Carriers within Isolated Planar Polythiophene Ryo Shomura,†,‡ Kazunori Sugiyasu,*,† Takeshi Yasuda,§ Akira Sato,⊥ and Masayuki Takeuchi*,†,‡ †
Organic Materials Group, Polymer Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡ Department of Materials Science and Engineering, Graduate School of Pure and Applied Science, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8571, Japan § Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ⊥ Materials Analysis Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *
ABSTRACT: In order to unveil the nature of charge carriers in a doped polythiophene, a sterically isolated polythiophenene, poly(1EDOT), was electrochemically synthesized on electrodes. Generation of charge carriers was induced upon controlled electrochemical doping and investigated through various spectroscopic methods; the charge carriers were identified based on spin concentration (ESR spectra), aromatic character (Raman spectra), and electronic transition (UV− vis−NIR absorption spectra) of the polythiophene. Peculiarity of this study lies in the fact that the electrochemistry of the poly(1EDOT) reflects the p-doping process of a single polythiophene wire because interwire interaction (i.e., π−π stacking) is effectively prevented; therefore, the results should be essential and informative to understand polythiophene-based materials and devices. Upon electrochemical doping, ESR active polarons were generated. Further doping concentrated the polarons, which led to the formation of polaron pairs. Eventually, the polaron pairs merged into bipolarons at the doping level of about 30−35%. Such a transformation of charge carriers under different doping levels has been extrapolated from studies using oligomeric model compounds. To the best of our knowledge, this is the first example addressing the nature of the charge carriers generated in a single polythiophene wire by exploiting the unique structure of the isolated polythiophene. Importantly, the comparison of poly(1EDOT) with common polythiophenes such as poly(3,4-ethylenedioxythiophene) (i.e., polyEDOT) implied that π−π stacking strongly affects the generation and stability of charge carriers. Furthermore, we found that the polaron pair plays an important role in charge hopping transport in the conduction mechanism.
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π−π stacking. In practice, their contributions to the electrical conductivity cannot be assessed independently because the formation of the π−π stacking accompanies the planarization of the backbone; i.e., the intrawire and interwire processes are simultaneously affected by self-assembly.10 Because of these ambiguities behind the charge carrier transport in polymeric systems, experimental3−5,7 and theoretical6 studies have been performed mainly by using simplified model compounds (i.e., oxidized oligothiophenes).11 So far, the characteristics of polythiophenes have been extrapolated, but an investigation of the polythiophene itself has yet to be elaborated. This issue could be addressed through synthesizing unprecedented structures or supramolecular systems of polythiophenes. For example, by isolating a single semiconducting polymer backbone, one would be able to extract and evaluate the
INTRODUCTION Polythiophenes, which are one of the most studied πconjugated polymers, have received an increasing amount of attention as functional materials in a variety of optoelectronic devices, such as light-emitting diodes, field-effect transistors, lightweight batteries, solar cells, and electrochromic devices.1 In spite of the practical applications developed so far, the mechanism of charge carrier conduction through the polymeric materials is still actively debated.2−11 The first point concerns the charge carrying species. For instance, σ-dimers,3 π-dimers,4 and polaron pairs5−7 have been proposed as charge carriers in addition to the simple polaron−bipolaron model (Scheme 1a).8 These charge carriers usually coexist at a given doping level but are often indistinguishable, which makes unraveling the conduction mechanism difficult. Second, there are two conceivable carrier transport pathways in the polymeric materials: namely, intrawire and interwire processes (Scheme 1b).9 The former is governed by the torsional angle of the πconjugated backbone, and the latter is achieved through the © 2012 American Chemical Society
Received: February 22, 2012 Revised: April 11, 2012 Published: April 24, 2012 3759
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We have recently synthesized isolated polythiophenes, the unique structure of which enabled us to determine the intrawire hole mobility along the planar polythiophene wire to be 0.9 cm2 V−1 s−1 through a time-resolved microwave conductivity (TRMC) method.20a Our previous study was related to the issue represented in Scheme 1b, and in this paper, we focus our attention to the nature of the charge carriers (Scheme 1a). One convenient technique for preparing semiconducting polymeric films on electrodes is electrochemical polymerization.29 The polymers deposited on an electrode are useful not only in various applications but also for in-detail investigations of the polymer characteristics by means of electrochemistry. In fact, UV−vis−NIR absorption,30 electron spin resonance (ESR),31 and infrared (IR) and Raman spectroscopic methods,32 when combined with an electrochemical setup, have provided deep insights into the electronic, magnetic, and structural perturbations, respectively, induced upon electrochemical doping. Despite these instrumental benefits, there are few examples of electrochemical polymerization that afford IMWs.23−27 Such systems should be useful for understanding the doping process of a single polythiophene wire by excluding the effects derived from aggregation (π−π stacking). Herein, we report on the synthesis of sterically isolated oligothiophenes and their electrochemical polymerization and characterization. Exploiting the unique polythiophene structure deposited on the electrodes, we could clearly identify the charge carriers generated within a single and planar polythiophene wire under controlled electric potentials.
Scheme 1. (a) Potential Charge Carriers Generated at Various Doping Levels; (b) Intrawire and Interwire Charge Carrier Transport in Semiconducting Polymeric Materials
intrinsic (intrawire) characteristics independent of the collective (interwire) properties.12 In fact, attempts to confine semiconducting polymers (and oligomers) within zeolite,13 mesoporous silica,14 organic nanochannels,15 polymer matrices,16 and so forth have been reported. In this context, the concept of insulated molecular wires (IMWs) is of great significance because the π-conjugated backbones are molecularly isolated according to the sophisticated molecular designs.17−28 For instance, Fréchet et al.18 synthesized oligothiophenes functionalized with dendrons to control over the interchain interactions in the various redox states. Komatsu et al.19 provided X-ray crystallographic analyses of the oxidized states of sterically isolated oligothiophenes; consequently, polaron and bipolaron states were unambiguously visualized.
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RESULTS AND DISCUSSION Synthesis of Monomers and Electrochemical Polymerization. We have recently reported efficient synthesis of a bithiophene derivative (1) in which the conjugated backbone is threaded through its own cyclic side chain (1,8,15,22tetraoxa[6.6]metacyclophane) (Scheme 2).20 The cyclic side chain not only prevents the π-conjugated backbone from aggregation (isolation effect) but also dictates the dihedral rotation to the planar conformation (planarization effect). Thus, the monomer (1) is endowed with a better conjugation and a lower oxidation potential as compared with its precursor
Scheme 2. Synthesis of the Monomers Used in This Study
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polymer film was insoluble in common organic solvent such as chloroform, THF, and DMF; therefore, molecular weight could not be determined. We thus measured MALDI-TOF MS spectrum for the powder sample scratched from the electrode; we observed polymerized species such as 10-mer of 1EDOT (40 thiophenes are connected) and even longer (Figure S2). Likewise, 2EDOT and 3EDOT could also be electrochemically polymerized on the electrode, thus producing magenta and navy blue films, respectively (Figure 2b,c). All the EDOT-based monomers, 1EDOT, 2EDOT, and 3EDOT, showed more or less the same electrochemical polymerization efficiencies as EDOT, as can be seen from their redox current (Figure 3). Interestingly, the CV of poly(1EDOT) showed three distinguishable peaks (see black dots in Figure 3a) unlike the broad CVs of common polymeric films: for reference, compare with the CVs of poly(3EDOT) and poly(EDOT) (Figure 3, parts c and d, respectively). This is an indication of a stepwise charge carrier generation within the isolated and planar polythiophene backbone of poly(1EDOT) (discussed later). Poly(2EDOT) showed a sharp redox process (Figure 3b), which is probably due to the segmented conjugation that arose from its twisted backbone.20b Structural and Electronic Properties of the Monomer and Poly(1EDOT). Regarding the isolation effect, we anticipated that one cyclic side chain per four thiophene repeating units (in 1Th and 1EDOT) would be sufficient to prevent the polythiophene backbone from π−π stacking although the so-called covering ratio is half of that of the previously synthesized insulated polythiophene, poly(1′) (Chart 1).20 In fact, we could not find any π-surface overlap in the X-ray crystal structures of 1Th and 1EDOT (Figure 4a,b), which is clearly in contrast to that of the 3EDOT packing into the herringbone structure like common π-conjugated systems (Figure 4c). In addition, we also evaluated the ability of πdimer formation; quaterthiophene derivatives are known to form the π-dimers.4 Taking advantage of the fact that the oxidation process of 1Th is reversible despite the absence of terminal protecting groups (Figure 1b), we could perform UV− vis−NIR absorption spectral measurements for oxidized 1Th. The addition of SbCl5 (up to 6.0 equiv) to a CH2Cl2 solution of 1Th induced the appearance of longer wavelength transitions of the cationic radical (1Th•+, λmax = 630 nm, 1.97 eV, Figure 5a,b). Importantly, even when the concentrated 1Th•+ solution was cooled (0.3 mM, using liquid nitrogen), we did not observe any significant spectral changes attributable to the π-dimer, (1Th•+)2 (Figure 5c). Similar effects have also been reported for other insulated oligothiophenes.19,21e−g
(2): Eox = 0.62 and 0.69 V for 1 and 2, respectively (note that all the potentials hereafter are referenced to the Fc/Fc+ couple). In the course of the electrochemical measurements, however, we found that this redox process did not yield poly(1) on the electrode; the process was irreversible, but an increase in the current during the electrochemical cycles was not observed (Figure 1a). To decrease the oxidation potential of the
Figure 1. Cyclic voltammograms of (a) 1 and (b) 1Th: [monomers] = 0.5 mM, 100 mV s−1, measured in CH2Cl2 containing 100 mM TBAPF6 as supporting electrolyte. Tenth cycles are shown here.
monomer and facilitate the electrochemical polymerization, we installed thienyl and 3,4-ethylenedioxythienyl (EDOT) groups in the 5- and 5′-positions of 1 through Stille coupling and obtained 1Th and 1EDOT, respectively (Scheme 2). 2EDOT and 3EDOT, which lack the cyclic side chain, were synthesized as control compounds. 3,4-Etheylenedioxythiophene (EDOT), a well-established monomer, was also used for comparison. Figure 1b displays cyclic voltammogram (CV) of 1Th measured under ambient conditions in CH2Cl2 solution containing 100 mM n-Bu4NPF6. 1Th showed reversible twostep one-electron oxidation processes and did not undergo electrochemical polymerization (Eox1 = 0.29 V, Eox2 = 0.78 V). This is probably because the cationic radicals localize on the central segment of 1Th rather than the terminals and are inert against the polymerization (Figure S1).29 In contrast, 1EDOT showed an increase in current during the electrochemical cycles (Figure 2a); the electron-rich EDOT moieties in 1EDOT facilitated the electrochemical polymerization, and a violet film was deposited on the electrode. In fact, the dependence of the scan rate on the electrochemical response in a monomer-free electrolyte solution showed a linear relationship, thus demonstrating that the redox processes originate from electrode-bound species (Figure 3a,e). Therefore, the quasireversible oxidation waves, built up between −0.5 and 0.5 V, were attributed to the electrode-deposited poly(1EDOT). The
Figure 2. Electrochemical polymerization of (a) 1EDOT, (b) 2EDOT, (c) 3EDOT, and (d) EDOT. Results of 10 CV scans are overlaid: [nEDOT] = 0.5 mM, [EDOT] = 1.0 mM, 100 mV s−1, measured in CH2Cl2 containing 100 mM TBAPF6 as supporting electrolyte. 3761
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Figure 3. Scan rate dependences of the electrodeposited films of (a) poly(1EDOT), (b) poly(2EDOT), (c) poly(3EDOT), and (d) poly(EDOT). The scan rates are, from inner to outer, 25, 50, 75, 100, 125, 150, 175, and 200 mV s−1. Plots of the observed current as a function of the scan rate for the (e) poly(1EDOT), (f) poly(2EDOT), (g) poly(3EDOT), and (h) poly(EDOT). Each triangle in (e−h) shows the current changes at the potential indicated correspondingly by the same triangle in (a−d), respectively.
the threshold voltage (Vth), the field-effect hole mobility (FETμ(h+)) can be calculated from the slope of the plot between the square root of drain current and the gate voltage. From the plot in Figure 6b, the FET-μ(h+) was determined to be 5.4 × 10−7 cm2 V−1 s−1 with the Vth of 12 V. This value was poorer by 5 orders of magnitude than that of 2,2′:5′,2″:5″,2‴-quarterthiophene (4T) which shows FET-μ(h+) of 1.2 × 10−2 cm2 V−1 s−1, Vth = −50 V.33 Since they have more or less the same ionization potentials (5.36 and 5.20 V for 1Th and 4T, respectively), and thus, the similar injection barriers, the significantly reduced FET-μ(h+) of 1Th as compared with 4T is attributable to the limited transfer integral among the oligothiophene π-systems sheathed within the cyclic side chain. These observations indicate that the one cyclic side chain per four thiophene repeating unit can sufficiently
Chart 1
Furtheremore, we also evaluated an organic field-effect transistor (OFET) fabricated using 1Th as an organic semiconductor. Figure 6a shows the drain current−voltage relationships in the gate voltage range from 0 to −100 V. Above
Figure 4. Packing structures of (a) 1Th, (b) 1EDOT, (c) and 3EDOT in crystallized form. Dihedral angles are shown on the bottom. 3762
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Figure 5. Absorption spectral changes of (a, b) 1Th induced upon addition of SbCl5 (0−6 equiv/1Th) and (c) 1Th•+ (with 3 equiv of SbCl5) observed upon cooling: [1Th] = 0.3 mM, (solid line) before and (open circle) after cooling.
Figure 7. Normalized absorption spectra of (a) poly(1EDOT), (b) poly(2EDOT), (c) poly(3EDOT), and (d) poly(EDOT) deposited on ITO electrode. The spectra are shown in a stacked mode.
measurements. Polymers were electrochemically deposited on a platinum wire and doped by applying given potentials. The wire was then transferred to an ESR sample tube for the measurement; note that under our measurement operations, the ESR intensities were negligibly affected by the positions of the platinum wire in the ESR tube and the interval time after the doping processes. ESR spectral changes of poly(3EDOT) upon doping were typical of those observed for the polythiophenes and poly(EDOT) (Figure 8b);31 a sharp ESR signal was registered which then gradually broadened and became weaker upon further oxidation. In marked contrast, ESR signals of poly(1EDOT) were sharp throughout the electrochemical doping up to 0.73 V (Figure 8a). The peak-topeak line widths are plotted as a function of the applied potential in Figure 8c. Although ESR signals tend to be broader in solid samples, the distinctly sharp peak of poly(1EDOT) as compared with that of poly(3EDOT) suggests the confinement of the polarons even at the higher doping levels. The relative concentrations of spins, given by the double integral of the ESR signals, are shown in Figure 8d. The spin concentration of poly(1EDOT) increased upon electrochemical doping and reached a maximum when the first redox process was completed (−0.3 V). Then, the spin concentration decreased rapidly, indicating a stepwise transformation in the nature of the charge carriers in poly(1EDOT). The slow decrease in the relative spin concentration of poly(3EDOT), in contrast, is probably due to various redox equilibriums and self-trapping accompanied by both the intrawire and interwire processes. We previously reported the UV−vis−NIR absorption spectral changes of poly(1′) induced by doping with I2 vapor.20a These changes would give an insight into the nature of the doped states of isolated polythiophenes; however, the doping level was unable to be controlled and quantified. In this context, spectroelectrochemistry is a powerful technique that correlates spectroscopic features with electrochemical conditions, i.e., the applied potential or doping level.29 Assuming that the molar extinction coefficient of poly(1EDOT) is close to those of poly(1′) (ε = 11.8 × 103 [thiophene unit]−1 cm−1 at λmax = 532 nm) and 3,4-dioxythiophene decamer (ε = 11.5 ×
Figure 6. (a) Plots of drain-to-source current (IDS) versus drain-tosource voltage (VDS) with different gate voltages (VG) for the FET of 1Th and (b) IDS and (IDS)1/2 versus VG plots for the same device at VDS of −100 V.
electronically isolate the polythiophene backbone of poly(1EDOT) even in the film states. As shown in Figure 7a, the UV−vis absorption spectrum of poly(1EDOT), prepared on an indium tin oxide (ITO) coated glass electrode, is resolved well with absorption peaks at 2.00 and 2.21 eV (620 and 562 nm) and a band gap (Eg: absorption onset) of 1.8 eV. The vibronic fine splitting is a good indication of the rigid planar structure of poly(1EDOT).11b The relatively small band gap of poly(1EDOT) is comparable with that of poly(EDOT) (1.6−1.7 eV, Figure 7d),1e,f thus indicating the well-developed conjugation. In fact, DFT calculations of the 1EDOT dimer, (1EDOT)2, showed that the frontier orbitals were spread well over the π-conjugated backbone (Figure S3).34 This spectroscopic feature is consistent with the planar backbone of 1EDOT monomer confirmed in the crystallized form (Figure 4b). In clear contrast, poly(2EDOT) showed an absorption peak at 2.30 eV (538 nm), reflecting its twisted πconjugation (Figure 7b).20b In general, it is difficult to cope both with isolation and planarization of π-conjugated polymers.35 In this context, these results demonstrate that the poly(1EDOT) is an ideal material to investigate the doping process of a single and planar polythiophene wire. Dope Induced Spectral Changes. In order to gain insight into the nature of the generated charge carriers during the electrochemistry, we performed electron spin resonance (ESR) 3763
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Figure 8. ESR spectra of (a) poly(1EDOT) and (b) poly(3EDOT) measured during electrochemical doping. Plots of the (c) peak-to-peak width and (d) normalized double integral of the ESR signals as a function of the applied potential: (filled circle) poly(1EDOT) and (open circle) poly(3EDOT).
103 [thiophene unit]−1 cm−1 at λmax = 521 nm),7d we estimated the molar amount of poly(1EDOT) on the ITO-coated glass electrode (2.74 × 10−8 mol cm−2 based on ε = 12.0 × 103 [thiophene unit]−1 cm−1 at λmax = 562 nm for poly(1EDOT)). Together with the film area and coulometric titration (Figure S4),36 we calculated and plotted the doping level of poly(1EDOT) (per thiophene unit) as a function of the applied potential (Figure 9).
separation of the two positive polarons.5,6 Equally important is the fact that the molecular geometry of the π-conjugated backbones is sensitive to the doping level (electron−phonon coupling); it has been shown that excess charges concentrate on a limited section of the chain (i.e., formation of bipolaron) so that geometric distortion can be minimized.6 Thus, there is a threshold in the conjugation length at which the Coulomb repulsion and the energy cost required for creating two polaronic deformations compete. This conjugation length threshold has been estimated to be about 6−12-mer.5−7 However, it is often pointed out that the results are still the subject of some controversy. For example, a planar conformation is not always ensured for oligothiophenes (especially for the longer ones), therefore, the characteristics of which deviate from the extrapolation.11 In addition, the possibility of the π-dimers has not been convincingly ruled out in the experiments: note that the π-dimer and polaron pair (in the singlet state) are spinless, as is the bipolaron. Such intermolecular π-dimerization could be predominant especially in the solid states, that is to say, in organic electronic devices. Therefore, it is important to pursue the matter not only from a fundamental point of view but also for practical applications based on π-conjugated polymeric materials. Recently, Nishinaga et al.7c synthesized π-conjugated oligomers composed of pyrrole and thiophene rings (6−9-mers), the unique structure of which stabilized the biradical character and enabled the solidstate characterization (in a KBr pellet). Combined with the theoretical calculations, the vis−NIR absorption spectra suggested that both closed- and open-shell singlet states coexist in 7- and 8-mers2+. Furthermore, Casado et al.7a,b took the advantage of Aso’s insulated oligothiophene that is unable to form a π-dimer.21e Consequently, they elicited conclusions about the transformation of a bipolaron into polaron pairs by increasing the chain length up to the 12-mer2+. Solid-state Raman spectroscopy of the dicationic oligomers showed that there is a turning point in conjugation length (8-mer2+) at which populations of aromatic and quinoidal isomers are inverted. To the best of our knowledge, however, a related study on polythiophenes has yet to be elaborated because the electrochemistry (i.e., doping/dedoping) of polymers, especially in the solid state, is complicated due to the various redox sites and states originating from the backbone conformation and aggregation morphology.9a,37 Given the unique structure of
Figure 9. Plot of the doping level of poly(1EDOT) as a function of the applied potential.
In the studies using oligothiophenes,5−7 the dependence of the chain length on the nature of the charge carrier is associated with the charge carriers transformations in the corresponding polymers under different doping levels; for example, a dicationic state of 12-mer (i.e., 12-mer2+) can be a model of the doped polymer with a doping level of 2/12 (i.e., 16.6%) per thiophene unit. Through such systematic studies using a series of n-mers, the concepts of π-dimers and polaron pairs have been proposed in order to elucidate the nature of the charge carrier in polythiophene. It has been reported that shorter oligomers (e.g., 4-mer2+) result in closed-shell dicationic species; in contrast, longer oligomers (e.g., 12-mer2+) prefer to be open-shell biradical states in their two-electron oxidation states.5−7 The former is a model of the bipolaron and the latter is the polaron pair, which is also referred to as “two polarons on a single chain”. Polaron pairs have been predicted based on theoretical calculations and qualitatively rationalized by the decrease in the Coulomb repulsion with an increase in the 3764
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addition, the vibrational structure of the π−π* transition faded away (Figure 11a,b), thus indicating the structural deformation upon oxidation. In fact, the CαCβ vibrational mode (1381 cm−1) in the Raman spectra of poly(1EDOT) disappeared by applying ca. −0.1 V (Figure 12); this is the point at which aromatic character of the thiophene vanished.7a,b,32 The blueshifted absorption bands of oxidized thiophene derivatives, as observed in stage II, have been rationalized in two ways: namely, by the formation of π-dimers or polaron pairs.4,5,18 The interpretation of the blue-shift has been a matter under debate as mentioned before; on the contrary, the π-dimerization issue is circumvented in our system, and thus, we can attribute the absorption bands in stage II to those of the polaron pair. The magnitude of the blue-shift (0.23 eV) was smaller than that observed for 12-mer2+ (0.33 eV),5 which is presumably due to the reduced effect of the chain ends. Further oxidation led to the unimodal absorption band at 1.0 eV (1240 nm) (stage III) which is assignable to the transition from the HOMO to the LUMO of the bipolaron band. Such clear stepwise transformation of the charge carriers could be observed neither for twisted poly(2EDOT) nor bare poly(3EDOT): see the difference spectroelectrochemistry in Figure S5. In the case of poly(2EDOT), the twisted conformation hampers two polarons from interacting as the polaron pair. Thus, the segmented π-conjugation of poly(2EDOT) behaves like short oligothiophenes; polarons directly transform into the bipolarons.40 On the other hand, in the case of poly(3EDOT), interwire interaction may facilitate the formation of π-dimers and the disproportionation of polarons; therefore, the carrier generation process should involve several redox equilibriums.41 These comparisons clearly indicate that generation and stability of the charge carriers are strongly affected by the planarity of the backbone and the degree of interwire interaction. Considering the correlations between the spectroscopic features (ESR, Raman, and UV−vis−NIR spectra, see Figure 13) and the doping level of poly(1EDOT) (Figure 9), generation and transformation of the charge carrier within a single and planar polythiophene wire can be depicted as shown in Scheme 3. Between the doping levels of 20−40%, the nature of the charge carriers undergoes dramatic changes. Importantly, the results show good agreement with those expected from the oligomer studies. For example, the polaron pair vs the bipolaron rivalry occurs at the doping level of 30−35%, which corresponds to model systems of hepta- to sexthiophene dications (7-mer2+ to 6-mer2+).42 Finally, conductivity of poly(1EDOT) was evaluated by using an interdigitated microelectrode. In this configuration, the device functions in a transistor mode wherein a drain current (IDS) flows between two electrodes (source and drain electrodes) under a small offset potential (VDS, fixed to 10− 50 mV) while gate electrochemical potential (VG) is applied to both the electrodes relative to a reference electrode; in other words, potentials of (VG + VDS) and VG were applied to the source and drain electrodes, respectively. Here, the observed IDS is directly proportional to the polymer’s conductivity.43,44 As shown in Figure 14b, the IDS of poly(1EDOT) increased then decreased throughout the doping process. In contrast, the conductivity profile of poly(3EDOT) did not show such a peak but reached a plateau once it changed into a conducting state (Figure 14e); this tendency is similar to those reported for poly(3,4-alkylenedioxythiophene)s.45a,b The IDSs showed proportional relationship to the VDS: Ohm’s law (Figure S6). Notable difference between poly(1EDOT) and poly(3EDOT)
the poly(1EDOT) connected to the electrochemical setup, we envisaged that spectroelectrochemistry of poly(1EDOT) could identify those charge carriers generated in the polythiophene backbone. Here, the distinctive structural advantages of poly(1EDOT) over the common π-conjugated oligomers investigated thus far are its (1) isolated (no π−π stacking), (2) planar (defined conformation), and (3) lengthy (minimal effects by the chain ends) backbone.38 Figure 10 displays the UV−vis−NIR absorption spectral changes of poly(1EDOT), poly(2EDOT), poly(3EDOT), and
Figure 10. Spectroelectrochemistry of (a) poly(1EDOT), (b) poly(2EDOT), (c) poly(3EDOT), and (d) poly(EDOT) deposited on an ITO electrode.
poly(EDOT), induced upon electrochemical doping (i.e., spectroelectrochemistry). All the spectral changes look similar as is typical of the polaron−bipolaron model; however, careful comparison among them implied that only poly(1EDOT) showed a stepwise carrier generation. To clarify, the process is divided into three stages (Figure 11a−c) and shown with their difference spectral changes (Figure 11a′−c′). It should be noted here that stages I to III nearly correspond to the three redox peaks observed for the poly(1EDOT) film in Figure 3a. At the lowest doping level (stage I), bleaching of the π−π* transition and a growth of dope-induced bands at 0.62 and 1.23 eV (2000 and 1010 nm) are observed. The new subgap absorptions can readily be assigned to the transitions from the highest occupied molecular orbital (HOMO) to the singly occupied polaron level and from this level to the empty polaron band (lowest unoccupied molecular orbital (LUMO)).6a,8 In the next stage of applying potentials (stage II), these polaron bands are slightly blue-shifted to 0.70 and 1.46 eV (1770 and 850 nm). In 3765
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Figure 11. Spectroelectrochemistry of poly(1EDOT) (Figure 10a) was divided into three stages: (a) stage I, (b) stage II, and (c) stage III. Parts a′, b′, and c′ are the difference spectral changes: (a, a′) −0.76 to −0.31 V, (b, b′) −0.43 to −0.16 V, and (c, c′) −0.16 to 0.74 V. Purple: −0.76 V; blue: −0.41 V; green: −0.16 V; red: 0.14 V. Changes in absorption intensity at 562, 825, 1005, and 1240 nm are plotted in Figure 13d.
interwire charge transport should be impeded significantly, as indeed suggested by Figure 6 and our previous experiments.20a Thus, the conductivity profile of poly(1EDOT) extracts the charge hopping process that is independent of π−π stacking, from the other possible charge transporting processes. These results, considered together with the previously reported studies so far,45 indicate that the polaron pair plays an important role in the conduction mechanism of polythiophenes, especially in the charge hopping process. We assume that the electron exchange at this stage is accompanied by small reorganization energy. Based on Figures 9 and 14b, relative mobilities of the charge carriers in poly(1EDOT) were estimated; the film with the doping level of ca. 30% (VG = −0.2 V, i.e., the polaron pair region) was the best (Figure 14c).
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CONCLUSION In conclusion, we have described here generation of charge carriers within an isolated polythiophene induced upon controlled electrochemical oxidation. The results reflect the p-doping process of a single and planar polythiophene, which is the essential phenomenon to understand polythiophene-based materials and devices. In the lightly doped state of poly(1EDOT), the ESR-active polaron is generated, as has been well documented.4−8 Further doping (increase in the concentration of the polarons) leads to the formation of the polaron pairs. As such, the polaron pairs represent the transitional regime from polarons to bipolarons in a dilemma of breaking a double bond or losing aromatization stability (Scheme 3). This dilemma, related to the geometric distortion, is biased by the Coulomb repulsion between the two cationic charges: namely, by the doping level. As the cationic charges are concentrated more upon further oxidation, the polaron pairs merge into bipolarons so that the geometric distortion can be minimized. Such a charge carrier transformation under different
Figure 12. Raman spectra (1064 nm) of poly(1EDOT) measured during electrochemical doping.
was that the insulating-to-conducting transition of poly(3EDOT) was induced at relatively lower doping level, whereas that of poly(1EDOT) occurred only after the polaron pair was formed (Figure 14b,e and Figure S7). Furthermore, interesting is the fact that the conductivity of poly(1EDOT) reached maximum at the VG of −0.15 V; this is the potential at which the absorption of π−π* transition of poly(1EDOT) disappeared (Figure 13d, λabs = 562 nm), and more importantly, the polaron pair vs bipolaron rivalry occurred (see above). It should be reminded that π−π stacking between the poly(1EDOT) backbones is prevented; thereby, the 3766
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Scheme 3. Generation and Transformation of Charge Carriers within the Isolated and Planar Polythiophene Backbone of Poly(1EDOT) with Increasing Doping Level
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EXPERIMENTAL SECTION
Synthesis of the compound 2 has already been reported elesewhere.20 3EDOT was synthesized according the literature.47 Air- and watersensitive synthetic manipulations were performed under an argon atmosphere using standard Schlenk techniques. All chemicals were purchased from Aldrich, Kanto Chemical, or Wako and used as received. NMR spectra were recorded on Bruker Biospin DRX-600 spectrometer, and all chemical shifts are referenced to (CH3)4Si (0 ppm for 1H) or residual CHCl3 (77 ppm for 13C). MALDI-TOF-MS spectra were obtained with a Shimadzu AXIMA-CFR Plus. Electrochemical measurements were conducted with an Eco Chemie AUTOLAB PGSTAT12 potentiostat or BAS electrochemical analyzer 612B, ALS/HCH instruments, using a quasi-internal Ag/Ag+ reference electrode (Ag wire submersed in MeCN solution of 0.01 M AgNO3 and 0.1 M n-Bu3NPF6). ESR spectra were obtained on a JEOL JESFA100. Raman spectra were recorded on a PerkinElmer Spectrum GX NIR FT Raman spectrometer. UV−vis−NMR absorption spectra were obtained on a JASCO V-630 spectrophotometer or a Shimadzu UV3600 UV−vis−NIR spectrophotometer. Organic Field-Effect Transistor of 1Th. We adopted an OFET having a top contact geometry. Onto a cleaned glass substrate, a gold gate electrode was deposited through a shadow mask to form 5 mm wide and 30 nm thick stripes. Poly(chloro-p-xylylene) (Parylene-C) was deposited subsequently by thermal chemical vapor deposition with a thickness ranging from 1000 to 1200 nm to serve as a gate insulator. The thickness was determined with a Sloan Dektak 3 profilometer. Film of 1Th was prepared by the spin-coating method onto the Parylene-C layer at room temperature. Gold source-drain electrodes with an interdigitated configuration were deposited through a shadow mask. The channel length L and width W were 75 μm and 5 mm, respectively. The OFET was temporarily exposed to air prior to characterization by Keithley 2636A system source meter in a vacuum atmosphere. The electronic parameters were estimated using the standard analytic theory for metal-oxide-semiconductor field-effect transistors (MOSFETs) according to the equation
Figure 13. (a) CV of poly(1EDOT). Changes in (b) double integral of ESR signal, (c) relative Raman intensity (1381 cm−1), and (d) absorbance of poly(1EDOT) induced by the electrochemical doping. Electrodes used are (a) Pt button, (b) Pt wire, and (c, d) ITO grass coated electrodes, respectively.
doping levels has been extrapolated from the systematic oligomer studies so far.5−7 To the best of our knowledge, this is the first example unveiling the nature of the charge carriers by applying isolated polythiophene. In fact, in the case of the bare polythiophenes (i.e., without the insulating layer), the results were more elusive, which in turn suggested that interwire interaction strongly affects the generation of charge carriers, as has been pointed out by other experimental data46 and theoretical studies.9 Interestingly, conductivity of poly(1EDOT) reached maximum when the polaron pair vs bipolaron rivalry occurred, indicating that the polaron pair plays an important role in charge hopping transport. The unique structure of poly(1EDOT) provided deeper insights into the conduction mechanism through addressing, in part, the fundamental issue of long years of study (Scheme 1): what is the charge carrier in highly doped polythiophenes?
IDS,sat = (W /2L)C iμ(VG − Vth)2 where IDS,sat is the saturation drain current, Ci is the capacitance per unit area of the insulating layer, Vth is the threshold voltage, VG is the gate voltage, and μ is the field effect mobility. In Situ Conductivity Measurement. In the in situ conductivity measurement (Figure 14b), the observed IDS is proportional to the conductivity as described by the equation
σ = (IDS/VDS)(D/NTL) 3767
(1)
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Figure 14. CVs of (a) poly(1EDOT) and (d) poly(3EDOT) which were electrodeposited on a interdigitated microelectrode. Plots of changes in IDS of (b) poly(1EDOT) and (e) poly(3EDOT) as a function of the VG. (c) Relative carrier mobility of poly(1EDOT) plotted against the VG. where D and N are the distance and number of the gaps of the interdigitated microelectrode, respectively, T is the thickness of the film, and L is the length of the electrode. In addition, conductivity (σ) is proportional to the product of μ and n (charge carrier density) as described by the equation
σ = neμ
concentrated, and the solid residue was purified through column chromatography on silica gel to give 1Th as light yellow powder (77 mg, 78%). 1H NMR (CDCl3, TMS): 1.01 (m, 4H), 1.01 (m, 4H), 1.51 (m, 8H), 3.77 (m, 4H), 4.09 (m, 4H), 6.70 (d, J = 8.4 Hz, 4H), 6.90 (s, 2H), 6.93 (m, 4H), 7.10 (dd, J = 1.2, 4.8 Hz, 2H), 7.41 (t, J = 8.4 Hz, 2H). 13C NMR (CDCl3): 27.48, 30.30, 69.77, 106.91, 116.45, 122.41, 123.48, 126.29, 127.55, 130.51, 132.03, 132.28, 133.88, 138.27, 158.82. MALDI-TOF-MS, 710.59, calcd 710.17 (C40H38O4S4). Elemental analysis: calcd (%) for C40H38O4S4 + 0.1 CH2Cl2: C, 66.94; H, 5.35; found: C, 66.81; H, 5.34. Synthesis of 1EDOT. 1EDOT was synthesized through the same procedure for the synthesis of 1Th, using 2-(tributylstannyl)-3,4ethylenedioxythiophene instead of 2-(tributylstannyl)thiophene. 1H NMR (CDCl3, TMS): 1.01 (m, 4H), 1.01 (m, 4H), 1.51 (m, 8H), 3.77 (m, 4H), 4.09 (m, 4H), 6.70 (d, J = 8.4 Hz, 4H), 6.90 (s, 2H), 6.93 (m, 4H), 7.10 (dd, J = 1.2, 4.8 Hz, 2H), 7.41 (t, J = 8.4 Hz, 2H). 13C NMR ((CDCl2)2): 27.48, 30.30, 69.77, 106.91, 116.45, 122.41, 123.48, 126.29, 127.55, 130.51, 132.03, 132.28, 133.88, 138.27, 158.82. MALDI-TOF-MS, 826.53, calcd 826.18 (C44H42O8S4). Elemental analysis: calcd (%) for C44H42O8S4 + 0.3 CH2Cl2: C, 62.41; H, 5.04; found: C, 62.30; H, 4.95. Synthesis of 2Br. 2Br was synthesized through the same procedure for the synthesis of 1Br, using compound 2 instead of compound 1. Mp: 298−299 °C (decomp). 1H NMR (CDCl3, TMS): 3.54 (s, 12H), 6.36 (d, J = 8.4 Hz, 4H), 6.79 (s, 2H), 7.13 (t, J = 8.4 Hz, 2H). 13C NMR (CDCl3): 55.27, 103.84, 109.67, 112.52, 128.98, 130.90, 133.76, 134.74, 157.79. MALDI-TOF-MS, 596.43, calcd 595.91 (C 24 H 20 Br 2 O 4 S 2 ). Elemental analysis: calcd (%) for C24H20Br2O4S2: C, 48.34; H, 3.38; found: C, 48.19; H, 3.48. Synthesis of 2EDOT. 2EDOT was synthesized through the same procedure for the synthesis of 1Th, using 2-(tributylstannyl)-3,4ethylenedioxythiophene and compound 2 instead of 2(tributylstannyl)thiophene and compound 1. 1H NMR (CDCl3, TMS): 3.50 (s, 12H), 4.22 (m, 4H), 4.28 (m, 4H), 6.15 (s, 2H), 6.29 (d, J = 8.4 Hz, 4H), 7.04 (t, J = 8.4 Hz, 2H), 7.04 (s, 2H). 13C NMR ((CDCl2)2): 55.54, 64.98, 65.20, 96.64, 104.24, 112.67, 113.81,
(2)
Because VDS, D, N, T, and L are constant in Figure 14b, and n is proportional to the doping level in Figure 9, eqs 1 and 2 result in
μ ∝ IDS/(doping level)
(3)
Accordingly, dividing the IDS profile (Figure 14b) by the doping level profile (Figure 9) should show the profile of charge carrier mobility (μ) as a function of the doping level, that is, the relative mobility of each charge carrying species (Figure 14c). In Figure 9, doping level showed linear relationship between the potentials of −0.5 and 0.2 V, as described by the equation (doping level, %) = 48.1 + 91.7(potential), R = 0.998 Synthesis of 1Br. To a solution of compound 1 (200 mg, 0.37 mmol) in CHCl3/AcOH mixture (10 mL/10 mL) at 0 °C, NBS (137 mg, 0.77 mmol) was added. The solution was stirred for 1 h and then diluted with CHCl3. The mixture was washed with water and dried over MgSO4. The solution was concentrated, and a white precipitate was obtained with the addition of MeOH (245 mg, 95%). 1H NMR (CDCl3, TMS): 3.55 (s, 12H), 6.37 (d, J = 8.4 Hz, 4H), 6.79 (s, 2H), 7.13 (t, J = 8.4 Hz, 2H). 13C NMR (CDCl3): 55.27, 103.84, 109.68, 112.51, 128.37. 130.89, 133.77, 134.74, 157.48. MALDI-TOF-MS, 704.42, calcd 704.01 (C32H32Br2O4S2). Elemental analysis: calcd (%) for C32H32Br2O4S2: C, 54.55; H, 4.58; found: C, 54.54; H, 4.58. Synthesis of 1Th. A DMF (5 mL) solution of 1Br (100 mg, 0.14 mmol), 2-(tributylstannyl)thiophene (133 mg, 3.6 mmol), and Pd(PPh3)2Cl2 (10 mg, 10 mol %) was stirred for 12 h at 80 °C under an Ar atmosphere. The solution was diluted with CHCl3, washed with water, and dried over MgSO4. The solution was 3768
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128.27, 128.44, 130.48, 131.46, 133.13, 137.52, 142.11, 157.72. MALDI-TOF-MS, 718.53, calcd 718.08 (C36H30O8S4). Elemental analysis: calcd (%) for C36H30O8S4 + 0.5 CH2Cl2: C, 57.58; H, 4.10; found: C, 57.51; H, 4.27.
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ASSOCIATED CONTENT
S Supporting Information *
Frontier orbitals of 1Th, MALD-TOF MS spectrum of poly(1EDOT), Frontier orbitals of (1EDOT)2, coulometric titration for poly(1EDOT), sepctroelectrochemistry of the polymers, in-situ conductivity measurement for poly(1EDOT) and poly(3EDOT), and .cif files of 1Th, 1EDOT, and 3EDOT. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.S.),
[email protected] (M.T.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by KAKENHI (No. 20750097 and No. 23655108), Shorai Foundation for Science and Technology, and The Association for the Progress of New Chemistry for K.S. The authors thank Nanotechnology Network Project of the Ministry of Education, Culture, Sports, Science, and Technology Japan (MEXT) and Center of Materials Research for Low Carbon Emission.
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REFERENCES
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