Molecular Doping of a High Mobility ... - ACS Publications

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Molecular Doping of a High Mobility Diketopyrrolopyrrole− Dithienylthieno[3,2‑b]thiophene Donor−Acceptor Copolymer with F6TCNNQ Yevhen Karpov,†,‡ Tim Erdmann,†,‡ Manfred Stamm,† Uwe Lappan,† Olga Guskova,† Mikhail Malanin,† Ivan Raguzin,† Tetyana Beryozkina,∥ Vasiliy Bakulev,∥ Florian Günther,‡,⊥ Sibylle Gemming,‡,⊥,# Gotthard Seifert,‡,§ Mike Hambsch,‡ Stefan Mannsfeld,‡ Brigitte Voit,†,‡ and Anton Kiriy*,†,‡ †

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany Center for Advancing Electronics Dresden (cfaed) and §Theoretical Chemistry, Technische Universität Dresden, 01062 Dresden, Germany ∥ Ural Federal University, Mira str., 28, 620002 Yekaterinburg, Russia ⊥ Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany # Institut für Physik, Technische Universität, 09107 Chemnitz, Germany ‡

S Supporting Information *

ABSTRACT: Herein we present a molecular doping of a high mobility diketopyrrolopyrrole−dithienylthieno[3,2-b]thiophene donor−acceptor copolymer poly[3,6-(dithiophene-2-yl)-2,5-di(6-dodecyloctadecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophene], PDPP(6DO)2TT, with the electron-deficient compound hexafluorotetracyanonaphthoquinodimethane (F6TCNNQ). Despite a slightly negative HOMOdonor−LUMOacceptor offset of −0.12 eV which may suggest a reduced driving force for the charge transfer (CT), a partial charge CT was experimentally observed in PDPP(6-DO)2TT:F6TCNNQ by absorption, vibrational, and electron paramagnetic resonance spectroscopies and predicted by density functional theory calculations. Despite the modest CT, PDPP(6-DO)2TT:F6TCNNQ films possess unexpectedly high conductivities up to 2 S/cm (comparable with the conductivities of the benchmark doped polymer system P3HT:F4TCNQ having a large positive offset). The observation of the high conductivity in doped PDPP(6-DO)2TT films can be explained by a high hole mobility in PDPP(6-DO)2TT blends which compensates a lowered (relatively to P3HT:F4TCNQ) concentration of free charge carriers. We also show that F6TCNNQ-doped P3HT, the system which has not been reported so far to the best of our knowledge, exhibits a conductivity up to 7 S/cm, which exceeds the conductivity of the benchmark P3HT:F4TCNQ system.

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INTRODUCTION Most of the π-conjugated polymers and small-molecule organic semiconductors in their pristine state possess rather low electrical conductivity because of a relatively large offset between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (large band gaps); however, the conductivity may be increased by many orders of magnitude upon addition of conductivity dopants.1 It was particularly demonstrated that admixing of organic semiconductors with strong oxidizing agents leads to the generation of free charge carriers (holes) due to the transfer of electrons from the semiconductor to the oxidizer.1 This process, called p-doping, is widely used for improvement of performance of optoelectronic devices. For example, the introduction of heavily doped organic layers facilitates charge injection, thus greatly improving the operation of organic lightemitting diodes (OLEDs)2 (e.g., lowers their operating voltage and increases the lifetime) up to a level acceptable by industry, © XXXX American Chemical Society

and this concept is used for fabrication of AMOLED displays in smartphones. It was also demonstrated that controlled doping of organic semiconductors increases charge-carrier mobility in transistors.3 In addition, doped layers are essential components in inversion and vertical transistors.4 To date, the most impressive progress in application of doped organic semiconductors was achieved with small molecules processed by vacuum evaporation.2 However, further progress in organic electronics is frequently associated with the use of solutionprocessed conjugated polymers which can enable cheap fabrication of large-area and flexible electronic devices.8 That is why the investigation of the doping process of solutionprocessable polymers is an actual task. Although various inorganic oxidants such as I2,5 FeCl3,6 SbCl5,7 ReO3,8 and Received: November 22, 2016 Revised: January 12, 2017

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DOI: 10.1021/acs.macromol.6b02452 Macromolecules XXXX, XXX, XXX−XXX

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conductivity of 10−2 S/cm was achieved for PCPDTTBT,23 which is, however, 2 orders of magnitude lower than the conductivity of the doped P3HT. Recently, dithienyldiketopyrrolopyrrole (DPP)-based copolymers were identified as a very promising class of DA copolymers due to exceptional environmental stability and very high charge carrier mobility exceeding the mobility of P3HT by 2−3 orders of magnitude.26 As the electrical conductivity is directly proportional to the charge carrier mobility, the use of DPP polymers would be beneficial for the preparation of highly conductive films if high enough charge carrier concentrations could be provided. However, DPP-based copolymers have relatively low-lying HOMO energies (below −5.4 eV, as determined by CV), which may complicate their doping by commercially available dopant F4TCNQ having insufficiently deep LUMO levels of −5.24 eV. Very recently, we introduced a new and highly potent hexacyano-substituted [3]-radialene-based p-dopant CN6-CP having reduction potential of +0.78 vs Fc/Fc+ and a LUMO of −5.87 eV,27 which is 0.65 eV lower than the LUMO of F4TCNQ. We further found that CN6-CP efficiently dopes DPP-based polymers, such as poly[3,6-(dithiophene-2-yl)-2,5di(6-dodecyloctadecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-altthieno[3,2-b]thiophene] (PDPP(6-DO)2TT) (HOMO level of −5.49 eV), leading to doped films with electrical conductivities up to 70 S/cm.27 Absorption spectroscopy and DFT calculations confirmed efficient electron transfer from PDPP(6-DO)2TT to CN6-CP-dopant which is consistent with the large positive HOMOdonor−LUMOacceptor offset of +0.38 eV.27 Another p-dopant, F6TCNNQ, has slightly deeper LUMO level (−5.37 eV) than that of F4TCNQ (−5.24 eV) but significantly higher than that of CN6-CP (−5.87 eV). In a previous work, we demonstrated that evaporation of ultrathin (2 nm) film of F6TCNNQ atop of poly[3,6-(dithiophene-2-yl)2,5-di(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione-altthieno[3,2-b]thiophene], PDPP(2-OD)2TT, field-effect transistors has a beneficial effect on their performance, and this effect was attributed to a passivation of traps.28 At the same time, it was found that the deposition of F6TCNNQ significantly increases the off-current, suggesting that an interfacial doping of PDPP(2-OD)2TT occurred in that case. Herein we process F6TCNNQ from solutions and investigate the bulk doping of a DPP-based polymer. To avoid possible processability problems (i.e., solubility of polymer:dopant blend solutions), in the present study we use the DPP-based polymer PDPP(6-DO)TT having relatively bulky C30H61-solubilizing side groups (Chart 1). We found that F6TCNNQ-doped PDPP(6-DO)2TT films shows relatively high conductivity up to 2 S/cm. This result is somewhat unexpected because the LUMO of F6TCNNQ13−15,28 is positioned somewhat higher than the HOMO of the polymer PDPP(6-DO)2TT. As such, we demonstrate here that the system with a slightly negative HOMOdonor−LUMOacceptor offset of −0.12 eV (−5.49 − (−5.37)) displays similar conductivity to the one of polymer/dopant blend with large positive offsets, such as F4TCNQ-doped P3HT (HOMOdonor− LUMOacceptor offset of −4.9 − (−5.24) = +0.34 eV). Absorption and vibrational spectroscopy and electron paramagnetic resonance (EPR) studies reveal that the CT process in F6TCNNQ-doped PDPP(6-DO)2TT proceeds to a moderate extent (much less than in the systems with large offsets). We propose that the relatively high conductivities in F6TCNNQdoped PDPP(6-DO)2TT are due to the high hole mobility of

WO39 are strong dopants, they are rarely used in organic devices due to their corrosive activity and diffusivity at high electric fields, which significantly lower device efficiency and lifetime. Alternatively, electron-deficient organic compounds (molecular p-dopants) such as tetrafluorotetracyanoquinodimethane (F4TCNQ),10 its derivatives,11,12 and, more recently, 2,2-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ)13−15 have been widely used for the generation of highly conductive interlayers in organic electronic devices. Particularly, it was demonstrated that doping of soluble thiophene-based semiconducting polymers such as poly(3-hexylthiophene) (P3HT)10 and PBTTT16 by F4TCNQ leads to a dramatic increase in the conductivity from 10−6 S/cm for neat polymers up to 1−2 S/cm for solution-processed doped polymers and even up to 250 S/cm for the vacuumdeposited dopant.17 Although F6TCNNQ was applied for the doping of vacuum-processed hole-conductive materials in OLEDs,14,15 we are not aware of their use for the doping of high-performance solution-processable π-conjugated polymers, especially donor−acceptor copolymers. It is commonly accepted that a sufficient energetic offset between a semiconductor HOMO and a dopant LUMO levels is needed for efficient p-doping.1,16,18 This criterion is met in highly conductive systems, e.g., F4TCNQ-doped P3HT and PBTTT, because the HOMO of these polymer donors determined by cyclic voltammetry (CV) falls in a −4.8 to −5.1 eV range, whereas the LUMO of F4TCNQ is −5.24 eV.10−12 Although the method for determination of frontier orbital energies by CV sometimes lacks in accuracy, it is a useful approach for a quick estimation of feasibility of redox reactions, and frequently it is in good agreement with experimental observations. Semiconductor/dopant pairs with large positive HOMOdonor−LUMOacceptor offsets were described by a simple model which assumes a quantitative charge transfer from the donor to the acceptor (integer-type CT) and neglects hybridization of orbitals of the donor and acceptor. It should be however noted that even in such cases a predominant fraction of thus-formed charges remain in a tightly bound state, as demonstrated by Pingel and Neher for F4TCNQ-doped P3HT.18 Another model assumes hybridization of the semiconductor’s HOMO and the dopant’s LUMO which leads to the formation of a ground-state charge-transfer complex with a reduced energy gap between an occupied bonding and an unoccupied antibonding hybrid orbital and a partial charge transfer dependent on relative strengths of the semiconductor and the acceptor.19,20 This model describes experimentally observed interactions of donor/acceptor pairs with a small or even negative HOMOdonor−LUMOacceptor offsets. In these cases, the hybridization of frontier orbitals for the donor and acceptor rather than the charge transfer acts as the driving force for the complex formation which can be experimentally observed by substantial changes in UV−vis and IR spectra. However, one can intuitively suggest that the doping efficiency in the systems with small or negative HOMOdonor−LUMOacceptor offsets is weaker than that in systems with large positive offsets. Donor−acceptor (DA) copolymers received much attention in past decade21,22 due to their outstanding semiconducting properties; however, examples of the p-doping DA copolymers remain scarce in the literature.23,24 DiNuzzo et al. studied interaction of PCPDT-BT with F4TCNQ; however, conductivity values of F4TCNQ-doped PCPDT-BT films were not discussed.25 In another work, p-doping of a series of DA copolymers was studied by Koch et al., and the highest B


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ratio, MDR = [dopant]/[repeat unit of the polymer]. According to our definition, MDR equal, for example, to 0.5 corresponds to 1 molecule of the dopant per 2 repeat units. The mixing of PDPP(6-DO)2TT with the dopant does not result in the formation of insoluble aggregates, and the resulting blend solutions easily passed through nanoporous filters with pore diameter of 200 nm. The addition of the dopant to the polymer solution of the PDPP(6-DO)2TT caused no visual color changes. This is in a sharp contrast to the behavior of other doped semiconducting polymers such as P3HT and PBTTT which immediately precipitate and changed color in the presence of the dopant F4TCNQ due to the formation of insoluble charge-transfer complexes.14 Likewise, we observed a similar colloidal instability of solutions of P3HT doped by F6TCNNQ in our experiments. It should be however taken into account that although the observed high solubility of the doped PDPP(6-DO)2TT solutions is a convenient technological feature as it facilitates the casting of uniform films, it may reflect much weaker interactions between the polymer and the dopant which may cause a reduced doping efficiency. Film Preparation. In this work, we investigated the conductivity of F6TCNNQ-doped PDPP(6-DO)2TT films by using two methods: (1) by a standard four-probe technique with a device having distances between the electrodes in a subcentimeter range and (2) by using bottom-contact gold electrodes with 200 and 300 μm distances and 4.5 and 11 mm length, respectively (the thickness of the doped polymer layer of about 50 nm). For the former type of measurements, relatively thick (micrometer-range) doped films were prepared by drop casting onto freshly cleaned glass slides at a slow evaporation of the solvent (the substrates were covered by a Petri dish). This simple method led to visually smooth and

Chart 1. Chemical Structures of the Polymers and the Dopantsa

a

Energy diagram shows frontier molecular orbital energies based on cyclic voltammetry measurements.27

PDPP(6-DO)2TT which compensates for the effect of rather low charge carrier concentration.

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RESULTS AND DISCUSSION Doping. Blend solutions for casting doped polymers were prepared by combining appropriate amounts of freshly prepared solutions of PDPP(6-DO)2TT (weight-average molecular weight, Mw = 248.0 kg/mol, and dispersity, Đ = 7.97; Chart 1) in chloroform or chlorobenzene and solutions of F6TCNNQ in CH2Cl2 to achieve the desired molar doping

Figure 2. Representative AFM topography images of the PDPP(6-DO)2TT):F6TCNNQ blend films at different MDRs spin-coated on Si wafer from 4 g/L solutions in chlorobenzene. C


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Macromolecules uniform films over areas of several square centimeters. The mean thicknesses of the doped films were determined by AFM measurements of scratched films, and for a given film the measurements were repeated many times in different places of the samples. Thus-obtained thickness values of ∼1 μm are in good agreement with expected thicknesses calculated from the known concentration of the components, volume of the solution, and the film area, assuming that films are completely uniform. It is noteworthy that the uniform morphology at the macroscopic level is a rather unusual phenomenon for films of doped polymers. For example, Chabinyc et al. previously reported that casting of uniform films from stable, processable solutions of blends of F4TCNQ, and thiophene-based polymer PBTTT-C14 required significant care due to the formation of aggregates of charge-transfer complexes in solution which preclude the formation of uniform films.16 To overcome this obstacle, Chabinyc et al. developed a procedure for casting from hot solutions. In the present work, we also faced problems in the preparation of uniform F6TCNNQ-doped P3HT films because the P3HT:F6TCNNQ blend solutions underwent a precipitation within several minutes after admixing of the components. Nevertheless, it was possible for us to cast films of satisfactory uniformity from P3HT:F6TCNNQ solutions by rapid deposition right after the solution preparation. For the conductivity measurements on the bottom contact gold electrodes, the PDPP(6-DO)2TT:F6TCNNQ films were prepared by spin-coating of the blend solution in chlorobenzene, and this procedure resulted into uniform ∼50 nm thick layers. The mean thicknesses of the doped films were determined by AFM measurements of scratched films and for a given film the measurements were repeated 6−8 times in different places. Morphology. Thin-film morphology of pristine and doped PDPP(6-DO)2TT thin films was investigated by atomic force microscopy (AFM) on films prepared by spin-coating onto silicon wafers with 300 nm SiO2 and thermally evaporated gold electrodes, which have also been used for the conductivity measurements. Pristine PDPP(6-DO)2TT films contain randomly oriented nanowire-like features, a typical morphology for this class of polymers. Similar features are also seen in doped PDPP(6-DO)2TT:F6-TCNNQ blend films at MDRs varied from 0 to 0.33 (Figure 2). These films exhibit a root-meansquare roughness (Rrms) in 1−1.5 nm range and no signs of the phase separation. The phase separation was observed only for the doped films with very high MDRs above MDR 1, as seen from appearance of large crystallites presumably of F6TCNNQ. Conductivity Measurements. The conductivity data for the F6TCNNQ-doped PDPP(6-DO)2TT and P3HT films for various molar doping ratios, MDR, ranging from 0.01 (100:1) to 1 (1:1) are summarized in Figure 3. As seen from the data for ∼1 μm thick PDPP(6-DO)2TT films, the conductivity increases steeply from ∼10−7 S/cm for undoped PDPP(6DO)2TT to maximum 0.2 S/cm for the F6TCNNQ-doped PDPP(6-DO)2TT at MDR = 1. Interestingly, thin, ∼50 nm PDPP(6-DO)2TT:F6TCNNQ films prepared by spin-coating from high boiling solvent chlorobenzene exhibited 1 order of magnitude higher conductivities than the drop-casted thick films. Furthermore, the conductivity of doped thin films compared to thick films increases much more steeply with the MDR rise and saturates at the level above ∼2 S/cm already at MDR = 0.1 (for exact values see Supporting Information, Tables S1 and S2). I−V characteristics for all MDRs show ohmic behavior (linear dependencies), suggesting the absence

Figure 3. Conductivity of ∼50 nm thick spin-coated (black squares) and ∼1 μm thick drop-casted (red circles) films of PDPP(6-DO)2TT: F6TCNNQ at different MDRs as well as ∼1 μm thick drop-casted (blue triangles) films of P3HT:F6TCNNQ.

of contact problems as expected for highly doped materials (Supporting Information Figure S1). The stability of the conductivity values over time for the doped PDPP(6-DO)2TT films was also tested. To this end, blend solutions were prepared in air, and the doped films were left at ambient conditions for a certain time. It was found that thick films did not change substantially their conductivities during 3 days after the preparation. In contrast, the electrical properties of the thin PDPP(6-DO)2TT:F6TCNNQ films with MDR = 1 decreased from initially 1.5 to 0.35 S/cm after 3 days (Figure 4, also see Table S2), presumably due to a chemical

Figure 4. Conductivity of ∼50 nm thick doped films PDPP(6DO)2TT:F6TCNNQ at different MDRs stored in air over time.

reduction of the dopant by organic residues present in the atmosphere. We suggest that thick doped films exhibit a higher stability because the topmost polymer layer serves as a protective film preventing penetration of detrimental impurities into the film bulk. Conductivities of F6TCNNQ-doped P3HT films were measured for a comparison (Figure 3). While undoped P3HT exhibits a conductivity of ∼10−5 S/cm, addition of the dopant steeply increases the conductivity by at maximum ∼6 D


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Macromolecules orders of magnitude and the maximum conductivity saturates at ∼7 S/cm for MDR = 0.2. In general, the 3 times higher conductivity of the F6TCNNQ-doped P3HT compared to the previously reported conductivities of F4TCNQ-doped P3HT (in the 1−2 S/cm range)10 may be attributed either to morphological differences in correspondingly doped films resulting in a difference in charge carrier mobility or to a higher doping strength of F6TCNNQ (LUMO levels −5.37 eV for F6TCNNQ vs −5.24 eV for F4TCNQ, according to cyclic voltammetry data), leading to a higher charge carrier concentration in the F6TCNNQ-doped P3HT. However, because of rather small difference in the electron affinity of the dopants (−0.06 eV) and since both the dopants exhibit exceedingly high electron affinity relative to the P3HT oxidative potential, both the dopants should derive the integer-type CT, so that the difference in CT degrees is unlikely the factor to explain the difference in the conductivity. Instead, a somewhat higher colloidal stability of P3HT:F6TCNNQ blend solutions compared to the P3HT:F4TCNQ solutions may result into their better film-forming properties and explain higher conductivities. Although the observed conductivity in PDPP(6DO)2TT:F6TCNNQ films does not exceed the conductivity of doped P3HT and it is ∼50 times lower than that for previously reported CN6-CP-doped PDPP(6-DO)2TT, the conductivity result obtained in the present work is surprising in a view of a negative driving force for the CT process in the in PDPP(6-DO)2TT:F6TCNNQ system. To shed a light on the CT process, absorption spectroscopy, electron paramagnetic resonance (EPR), and FTIR investigations were undertaken. Absorption Spectroscopy. UV−vis−near-IR spectroscopy is a powerful tool for monitoring the doping process.11,12,16,18 It was previously shown that addition of F4TCNQ to electronrich polymers such as P3HT results in a bleaching of absorption bands of the neutral polymer and the appearance of signals in the IR region assignable to absorption of the oxidized polymer and reduced form of the dopant.18 Similar changes in the UV− vis spectra occur in the doped PDPP(6-DO)2TT films (Figure 5a). A near-quantitative bleaching of the neutral P3HT absorption occurs between MRD = 0.1−0.2, similarly to the F4TCNQ case. This MRD ratio range coincides with the saturation of the conductivity as discussed earlier. As seen from Figure 5b, the addition of F6TCNNQ to PDPP(6-DO)TT solution in chloroform gradually reduces the intensity of the major absorption band of the neutral polymer with λmax = 832 nm (A) and leads to appearance of new signals in the IR region: the absorbance with λmax = 1450 nm (B) assignable to one of the PDPP(6-DO)2TT:F6TCNNQ CT complex states and the absorbance with λmax = 1140 nm (C) of the anion-radical form of F6TCNNQ. Furthermore, the addition of the dopant results in a new peak with λmax = 480 nm (D) assignable to unreacted F6TCNNQ which grows with the increase of the concentration of the dopant. A modest decrease of the band A intensity even at the highest MDR = 1 (the band A reduces to about 85% of its initial value as MDR changes from 0 to 1) indicates that only the maximum amount of 15% of the polymer repeat units undergo the CT already in solution. It should be noted that degrees of the CT in PDPP(6DO)2TT:F6TCNNQ are much smaller than that in a previously reported case of PDPP(6-DO)2TT doped by CN6-CP where about 60% repeat units participated in the formation of the CT complex at MDR = 1.27 Qualitatively, this

Figure 5. UV−vis−near-IR spectra of P3HT:F6TCNNQ thin films (a), PDPP(6-DO)2TT:F6TCNNQ in solution (b), and thin film (c).

is consistent with the difference in the doping strengths of F6TCNNQ vs CN6-CP. The absorption measurements for the PDPP(6DO)2TT:F6TCNNQ blend films (Figure 5c) reveal that the CT process proceeds to a somewhat higher degree in the solid state. In this case, the band A of the neutral polymer reaches ∼81% at MRD = 1. As expected, the peak of the single-electron reduction product of F6TCNNQ is observed at 1150 nm, and its intensity continues to increase in the MDR interval between 0 and 1. A difference between the absorption spectra in solution E


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that each molecule of CN6-CP•−N+Bu4 is EPR-active, the CT degrees in PDPP(6-DO)2TT:CN6-CP blends were estimated for different MDRs. It was found that in the MDR range from 0.03 to 0.2 the CT occurs with near 100% efficiency, and it gradually decreases down to ∼66% for MDR = 1. 27 Unfortunately, we were not able to independently prepare a stable single-electron reduction product of F6TCNNQ to use it as calibration standard for PDPP(6-DO)2TT:F6TCNNQ blends. However, we compare EPR intensity in PDPP(6DO)2TT:F6TCNNQ blends and EPR intensity of the CN6CP•−N+Bu4 standard.27 As seen from Figure 6b, EPR intensities in PDPP(6-DO)2TT:F6TCNNQ lay significantly below those of CN6-CP•−N+Bu4 when normalized to the same molar concentrations of respective radical anions. Assuming that anion radicals F6TCNNQ•− and CN6-CP•− of the same concentration contribute to the same EPR signal intensity, we estimate that the CT degree in PDPP(6DO)2TT:F6TCNNQ blends does not exceed 15%, which excellently collaborates with the UV−vis−near-IR data. Vibrational Spectroscopy. Vibrational spectroscopy was proven to be a powerful method to study interactions between electron-rich and electron-deficient compounds and probing the ground-state CT process.30 Stretching modes of the CN triple bonds are particularly sensitive to the CT because an excess electron transferred from the donor to the dopant is localized preferentially on cyano groups.31 A combination of both computational and experimental approaches was applied in this work. According to DFT calculations at the B3LYP/6-31G(d) level (for details see Experimental Section), pristine F6TCNNQ showed the totally symmetric and three (Figure 7a) infrared active CN stretching modes (Table S3). But in fact, the present symmetry group C2h allows characterization of vibration modes as either ag or bu (Figure 7, for explanations see Supporting Information). When going from the neutral F6TCNNQ to its anion radical form F6TCNNQ•−, the calculated frequencies of the CN group stretching modes undergo shifts from 2236 to 2199 cm−1 (Δν = 37 cm−1) for one band and from 2216 to 2169 cm−1 (Δν = 47 cm−1) for another band. In addition, as follows from DFT calculations, the signals corresponding to the anion radical form of the dopant have significantly higher intensity (2 orders of magnitude, see Table S3) than the signals from the neutral dopant. The calculations discussed so far were performed for the F6TCNNQ•− anion radical. Its formation corresponds to the integer-type CT, and this may be relevant for the P3HT:F6TCNNQ system. However, because the CT efficiency in the PDPP(6-DO)2TT:F6TCNNQ system is lower, we also investigated the cyano group stretching modes in complexes with incomplete CT. To this end, IR spectra in the characteristic cyano group stretching region for the complex of F6TCNNQ and polymer fragments DPP-TT-DPP and TTDPP-TT were also calculated. First, the geometries of the DPPTT-DPP:F6TCNNQ and TT-DPP-TT:F6TCNNQ complexes were optimized by the B3LYP/6-31G(d) method (details below and in the Supporting Information), and then frequencies of the CN group stretching modes were calculated (Figure S2). These calculations predict that all vibrational peaks of the CN group undergo red-shifts, similarly to the case of the isolated F6TCNNQ•− anion radical; however, the most intense peak in this case is appearing at 2187

and solid state is that the new weak absorption band (E) observed in the latter case at 2000−2600 nm, which also can be assigned to one of the CT complex. Importantly, this absorption band has much lower intensity compared to the case of CN6-CP-doped PDPP(6-DO)2TT. Electron Paramagnetic Resonance (EPR). To shed more light on the doping process, EPR measurements were conducted which allow direct observation of radicals formed upon electron transfer. EPR spectroscopy measurements of oxygen-free blend solutions of PDPP(6-DO)2TT:F6TCNNQ were performed at room temperature for different MDRs, and they revealed resolved spectra consisting of at least 19 lines centered at g = 2.0032 and a line spacing of about 1 G (Figure 6a). With decreasing MDR the spectra become less resolved.

Figure 6. EPR spectra of PDPP(6-DO)2TT:F6TCNNQ blend solutions at different MDRs (a); normalized double integrated intensity DIInorm as a function of the concentration c of F6TCNNQ and CN6-CP•−N+Bu4, respectively (b).

We assign the signal to the anion radical F6TCNNQ•− formed upon one-electron reduction of F6TCNNQ. The intensity of the EPR signal increases almost linearly with the increase of the dopant concentration (at constant polymer concentration) up to MDR = 1, as shown in Figure 6b. For comparison, Figure 6b shows previously reported data for the doping of the same polymer PDPP(6-DO)TT by exceptionally strong dopant CN6-CP.27 In that work, we compared the EPR intensities of CN6-CP•− anion radical formed in PDPP(6-DO)2TT:CN6-CP blend solutions with the intensities of independently prepared anion radicals CN6CP•−N+Bu4 in solutions with known concentrations. Assuming F


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Figure 7. (a) Stretching modes of CN in the F6TCNNQ molecule. (b) Simulated IR spectra of the CN stretching modes for the isolated F6TCNNQ molecule, the F6TCNNQ•− anion radical in the gas phase, and the F6TCNNQ in two CT complexes, optimized at B3LYP/6-31G(d) level. A full width at half-maximum of 4.5 cm−1 is used in the simulations. The intensity of the neutral F6TCNNQ was multiplied by a factor of 30. The molecular geometry changes upon charging are illustrated in Figures S3 and S4.

Figure 8. FTIR ATR spectra of (a) F6TCNNQ dopant (orange line) and F6TCNNQ•− radical-anion form obtained as a complex with ferrocene (Fc) (pink line) and (b) P3HT films doped with F6TCNNQ at different MDRs are shown.

cm−1, whereas the second one has much smaller intensity (see Table S3). We now compare the theoretically predicted vibrational spectroscopy shifts with experimental values. We first recorded FTIR spectra of F6TCNNQ in the presence of P3HT, the system for which near integer-type CT and the formation of the F6TCNNQ•− anion radical are expected. This expectation is based on the fact that the integer type of the CT was documented for the parent P3HT:F4TCNQ system and because F6TCNNQ possesses even stronger electron affinity compared to F4TCNQ.13−15 Experimental FTIR data reveal a somewhat more complicated spectrum than the theoretically predicted one (Figure 8a). The main peak of the cyano group of neutral F6TCNNQ lies at 2213 cm−1 and has two shoulders at 2222 and 2204 cm−1 with a tail extending up to 2100 cm−1. The presence of additional shoulders as a result of the band splitting as well as frequency deviations from the calculated positions is associated with the fact that in a real crystal a mixture of states and various intermolecular group interactions take place, which is obviously absent in the case of isolated molecules in DFT calculations. The spectrum of F6TCNNQ•−

anion radical is in a good agreement with the theoretical prediction providing the remarkable red band shift in comparison with neutral F6TCNNQ as expected. Mixtures of F6TCNNQ with P3HT at different doping ratios have similar spectra with the most intense band appearing around 2188 cm−1 (Figure 8b; for more values see Figure S5). The FTIR spectrum of F6TCNNQ mixed with ferrocene (Fc) at 1:1 molar ratio was also recorded for comparison. According to our cyclic voltammetry data, the half-wave of the Fc oxidation is by 0.3 eV more electronegative relative to the F6TCNNQ/F6TCNNQ•− half-wave so that spontaneous CT from Fc is expected. However, Fc is electropositive by 0.1−0.2 eV to P3HT so that Fc may generate slightly weaker CT than P3HT. Our FTIR measurements reveal the main peak for the F6TCNNQ/Fc complex at 2194 cm−1, corresponding to Δν = 25 cm−1 relative to the neutral F6TCNNQ. As it was previously demonstrated,19,20,30,31 the Δν shift of the CN stretching peak serves as a good measure of the CT degree. For example, the largest difference of Δν = 44 cm−1 in CN stretching peak is observed for fully reduced (versus pristine) form of a dopant TCNQ reduced by alkali metals.20 So, the Δν shift of 37 cm−1 G


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configurations (for details see Experimental Section). To this end, finite fragments of the PDPP(Me)TT polymer chains were studied. Two different structures were considered (see Figure 10). The DPP-TT-DPP model structure describes the donor

observed in the P3HT:F6TCNNQ blend is rather close to the shift in fully reduced dopant; therefore, it represents a near integer type CT. On the other hand, somewhat smaller Δν shift of 25 cm−1 is found in the Fc:F6TCNNQ complex. In addition, F6TCNNQ/Fc has a peak centered at 2155 cm−1, which is not observed in the P3HT:F6TCNNQ complex. We suggest that the appearance of this peak together with smaller Δν values reflects a lowered CT degree in F6TCNNQ/Fc compared to F6TCNNQ:P3HT. In the next step, we considered the transformation of F6TCNNQ FTIR spectra occurring in the presence of PDPP(6-DO)2TT. It should be pointed out that PDPP(6DO)TT has a much stronger IR absorbance in the region of CN stretching vibrations than P3HT, especially in its doped form, and this causes severe problems in observation of the CT process. As long as the dopant molecule is the minor component in the polymer blend films, the polymer absorption is relatively weak. However, significant care should be taken to record FTIR spectra of satisfactory quality in the CN stretching region for blends with higher dopant content due to a significantly increased infrared absorption of the doped polymer. In this case, the doped film thickness must be decreased from 1 to 2 μm for poorly doped films to tens of nanometers for highly doped ones. Figure 9 shows FTIR spectra of the doped polymer blends with different MDRs (more values see in Figure S6). It reveals

Figure 10. Considered models for PDPP(Me)TT: two different fragments of the whole chain, one having the TT unit in the middle surrounded by two DPP units and the other vice versa.

unit (TT) between the surrounding acceptor DPP units, whereas the TT-DPP-TT model describes the opposite situationthe acceptor unit between two donor units. First, we performed calculations of F6TCNNQ and isolated fragments which model the polymer by using the B3LYP functional and a DZP basis (Figure 11). The LUMO energy of F6TCNNQ was found to be −6.66 eV. With the same numerical setting, a marginally lower calculated LUMO energy value (−6.71 eV) has been previously reported for the superstrong dopant CN6-CP.27 Since under experimental conditions CN6-CP has much higher electron affinity than F6TCNNQ (the difference is 0.5 eV, as determined by cyclic voltammetry), the results of the present DFT calculations should be taken with caution. Nevertheless, qualitatively these calculations reproduce the energetic order of the considered dopant molecules. For the HOMO energies of the TT-DPPTT and DPP-TT-DPP, our calculations predict −5.66 and −5.96 eV, respectively. For all considered models, the HOMO is higher than the LUMO of the dopant models, such that from the purely theoretical point of view the electron CT from PDPP(Me)TT to CN6-CP or F6TCNNQ, respectively, is favored for both dopants. This contradicts the experimental observation which shows that HOMO level of PDPP(6DO)2TT (−5.49 eV) is positioned between the LUMO levels of F6TCNNQ (−5.37 eV) and CN6-CP (−5.87 eV) so that the CT should be favored only for CN6-CP. To that extent solvent induced shifts in the CV measurements cause this mismatch is unclear in the present status. We considered charge transfer complexes in which the dopant molecules were positioned on top of the DA models in a distance of 3 Å in different rotational orientations (Figure 12). For the E LUMO −E HOMO energy gaps of the DPP-TTDPP:F6TCNNQ and the TT-DPP-TT:F6TCNNQ charge transfer complexes, we got 0.47 up to 1.19 eV (corresponded to wavelength of ∼2700 to 1050 nm) and 0.45 up to 0.62 eV (∼2700 to 2000 nm). According to the most stable of these configurations, our calculations allow attributing experimentally observed subgap absorptions at 900−1500 nm and about 2500 nm (Figure 5) to different geometrical structures of the CT complexes. Moreover, we calculated the complex formation energy and the CT in more detail. In order to save computational resources, we utilized the PBE functional and the DZP basis (for details see Experimental Section). To study the CT process in detail, we considered the Mulliken,32 Hirshfeld,33 and

Figure 9. FTIR ATR spectra of PDPP(6-DO)2TT films doped with F6TCNNQ at different MDRs.

three major peaks with variable intensity: the first one is positioned closely to the position of the neutral dopant (2215 cm−1), whereas the second one (2196 cm−1) and the third one (2155 cm−1) coincide with the positions of the peaks in the Fc:F6TCNNQ complex. These facts may indicate that only a partial CT occurs in PDPP(6-DO)2TT:F6TCNNQ blends, in contrast to the P3HT:F6TCNNQ case. Furthermore, the presence of the peak at 2215 cm−1 assignable to the neutral form of the dopant indicates that a significant portion of dopant molecules is not ionized, in contrast with the Fc:F6TCNNQ and P3HT:F6TCNNQ systems in which the signals from the neutral F6TCNNQ are negligible. DFT Calculations. We performed density functional theory (DFT) calculations in order to model the charge transfer (CT) process as well as the interaction energy between the πconjugated polymer and F6TCNNQ for various stacking H


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Macromolecules

Figure 11. Energy diagram for different dopants and polymer fragments.

Figure 12. Top: calculated charge transfer and complex formation energy obtained from PBE/DZP single point calculations for the CT complex DPP-TT-DPP:F6TCNNQ at a distance of 3 Å. The rotation angle indicates the orientation of the F6TCNNQ starting from the configuration where the long axis of the dopant is parallel to the polymer backbone. The colored curves specify the CT obtained by the different population analyses (left scale), and the energies of the complex relative to the individual molecules are given in black (right scale). Here the energy barriers indicated by the dotted line while the stable configurations are shown as bars. Bottom: energy of the HOMO, LUMO, and LUMO+1 levels obtained by B3LYP/DZP single point calculations for the CT complex DPP-TT-DPP:F6-TCNNQ.

Voronoi34 approaches to calculate the atomic charges. For the considered configurations, the degree of the charge transfer from the polymer fragments to the dopant ranges up to 0.56e (Figure 12). We note that for the F6TCNNQ dopant positioned above the DPP unit no CT is observed. This is in line with the observations of the CT behavior of the previously studied PCPDT-BT:F4TCNQ system25 but in contrast to our observations of CN6-CP dopant where we observed a CT also in this configuration. Starting from the configurations, where the dopant is positioned above the center of the DA model, we finally shift the dopant along the polymer direction. We observe that when the dopant is approaching the acceptor, the CT decreases.

Concerning the complex formation energy, our results suggest that the configuration with the dopant next to the acceptor is more favored than configurations with the dopant located closer to the donor units where the charge transfer is higher. The calculated interaction energy gain of the complex formation lies in 60−90 kJ/mol for different configurations, and the gain is about of 30 kJ/mol smaller for configurations with F6TCNNQ placed above the acceptor DPP unit. A similar result was obtained for the CN6-CP-doped system.27 Thus, we conclude that favorable interactions between the highly polar groups of the DPP unit and the dopant molecules enhance the stability of these CT complexes. As such, although DFT calculations overestimate the driving force of the CT, i.e., the I


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Macromolecules

(relatively to P3HT:F4TCNQ) concentration of free charge carriers. This may further suggest that increase of the doping strength and, hence, charge carrier concentration while preserving the structural order and, hence, charge carrier mobility in the doped PDPP(6-DO)2TT should lead to even higher conductivities which was experimentally demonstrated in our recent work. We also show that F6TCNNQ-doped P3HT, the system which has not been described so far to the best of our knowledge, exhibits the conductivity up to 7 S/cm, which exceeds the conductivity in the benchmark system F4TCNQ-doped P3HT.

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EXPERIMENTAL SECTION

In this work as the donor semiconductor, we used previously characterized PDPP(6-DO)2TT (weight-average molecular weight, Mw = 248.0 kg/mol, and dispersity, Đ = 7.97; GPC 150 °C, 1,2,4trichlorobenzene; Chart 1) having relatively bulky C30H61-solubilizing side groups. The large size of solubilizers was chosen because of the better film-forming properties. P3HT (weight-average molecular weight, Mw = 70.1 kg/mol, and dispersity, Đ =2.31, GPC 40 °C, CHCl3) was used as a model system for comparison as well. F6TCNNQ (Novaled Inc., Dresden, Germany) was used in this work as the dopant. Because its LUMO is positioned somewhat above the HOMO of the PDPP(6-DO) 2 TT (−5.37 vs −5.49 eV as demonstrated cyclic voltammetry measurements), PDPP(6DO)2TT:F6TCNNQ is a suitable system to study CT in pairs with negative HOMOdonor−LUMOacceptor offsets. In contrast, the HOMO level of P3HT lies above the LUMO of the dopant which ensures a positive offset and energetically favored spontaneous electron transfer. Chart 1 gives the chemical structures of the polymers and dopants used as well as their orbital energy levels. Details of CV measurements and results were previously reported.27 Electrical Measurements. Electrical conductivity of the doped films was measured by a standard method, using a commercially available Loresta-GP MCP-T610 device (Mitsubishi Chemical Analytech) with standard ESP 4-pin in a line probe (measuring range 10−3−107 Ω/sq). The distance between the current electrodes is 15 mm and between the potential electrodes is 5 mm. At least six measurements of square resistivity (ρs) were performed for each sample in different positions on the film surface. Conductivity σ was calculated as σ = 1/ρs·t, where t is the film thickness in cm. To perform measurements of ρs > 107 Ω/sq for the undoped polymer, a Hiresta-UP MCP-HT450 device with URS standard probe was used. Bottom-contact devices. Highly doped silicon wafers with 300 nm SiO2 were used as substrates. For the electrodes 2 nm Cr and 50 nm Au were thermally evaporated through a shadow mask at a vacuum of ∼10−7 mbar. The electrodes have a width of 4.5 and 11 mm, and the distance between two electrodes is 200 and 300 μm, respectively (see Figure S9). For the current−voltage (IV) measurement a manual probe station (Cascade Microtech GmbH) and a Keysight B1500A Semiconductor Device Parameter Analyzer were used. IV sweeps for voltages of 0−10 V were performed for each substrate with the different molar doping ratios (three sweeps for 300 μm distance and three sweeps for 200 μm distance). The linear current−voltage dependencies were extracted, and the resistance of each sample was calculated due to the Ohm’s law

Figure 13. Calculated charge transfer and energy obtained from PBE/ DZ single point calculations for the CT complex DPP-TTDPP:F6TCNNQ. The distance in the z-direction was fixed to 3 Å. The shift dx indicates the relative distance of the centers of masses of the two molecules in backbone direction. The colored curves specify the CT obtained by the different population analyses (left scale), and the energies of the complex relative to the individual molecules are given in black (right scale). Here the energy barriers are indicated by the dotted line, while the energies of the stable configurations are represented by bars.

relative position of the HOMO of the donor located above the F6TCNNQ LUMO (which contradicts our cyclic voltammetry data), our calculations reveal a much less favored CT from oligomer fragments to F6TCNNQ compared to the oligomer: CN6-CP system, as follows from the following facts. (1) For F6TCNNQ being positioned above the donor part of the oligomer fragment, the CT degree is ∼0.4e, which is smaller than the CT degree in the case of CN6-CP (∼0.6e). (2) Almost no CT is observed when F6TCNNQ is located above the acceptor part of the oligomer, in contrast to the CN6-CP case; if the dopant is distributed statistically along the oligomer molecule, the population of states which result in the CT is less in the case of F6TCNNQ than in the case of CN6-CP for which all configurations result in a CT. (3) The most energetically favored configuration (for both dopants) is such when the dopant is located next to the DPP unit; together with the fact that in the case of F6TCNNQ no CT is observed in this configuration suggests that at a thermodynamic equilibrium F6TCNNQ will be attracted in the CT-free configurations. So, all these facts together suggest an effectively reduced CT in PDPP(6-DO)TT:F6TCNNQ compared to the PDPP(6-DO)TT:CN6-CP, in agreement with our experimental observations.

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CONCLUSIONS In conclusion, in this work we investigated a molecular doping of a high mobility diketopyrrolopyrrole−dithienylthieno[3,2b]thiophene donor−acceptor copolymer with F6TCNNQ. Despite a negative HOMOdonor−LUMOacceptor offset of −0.13 eV, ground-state charge transfer occurs in this system however, only a partial one, as revealed by absorption, vibrational, and EPR spectroscopies as well as DFT calculations. Despite the modest CT, blend PDPP(6DO)2TT:F6TCNNQ films possess unexpectedly high conductivities up to 2 S/cm (as high as the benchmark doped polymer system P3HT:F4TCNQ having a positive offset). Rather high conductivity in F6TCNNQ-doped PDPP(6DO)2TT films can be explained by a high hole mobility in PDPP(6-DO)2TT27 blends which compensates for a lowered

R=

V I

For low conductivity samples (resistance over 107 ohm) IV curves did not correspond to Ohm’s law, and these samples were not considered. From the resistance the conductivity of the film was calculated based on Pouillet’s law σ=

1 l l = = ρ RA Rtw

where ρ is the resistivity, l is the distance between two electrodes, t is the thickness of the doped film, and w is the width of the electrodes. J


Article

Macromolecules The exact distances between two electrodes were measured for every sample with optical microscopy. UV−Vis−Near-IR Absorption Spectroscopy. A Cary 5000 UV− vis−NIR (Agilent Technologies) at a scan rate of 20 nm s−1 was used to record UV−vis−NIR absorption spectra of solutions in a quartz cuvette (path length 1 mm) or of thin polymer films prepared by drop casting/spin coating on glass. Atomic Force Microscopy (AFM). The microscope (Bruker, Dimension Icon) was operated in tapping mode using silicon SPM sensors (Budget Sensors, Bulgaria) with spring constant of ca. 40 N/m and resonance frequency of ca. 300 kHz. Thickness of the polymer layers was measured using a scratch test technique. EPR Spectroscopy. Continuous wave (CW) EPR spectra were recorded on an EMX-plus spectrometer (Bruker Biospin) operating at X-band, equipped with the high-sensitivity resonator ER 4119 HS-W1 and the variable temperature unit ER4141VT. Acquisition parameters were microwave power of 0.1 mW, modulation frequency of 100 kHz, modulation amplitude of 0.5 G, sweep width of 30 G, time constant of 10.24 ms, conversion time of 40.96 ms, four scans, and 2048 data points. The temperature was controlled within ±1 K. The liquid samples were loaded into glass capillaries (100 μL) which were flamesealed and put into quartz tubes with inner diameter (i.d.) = 3 mm. The double integrated intensity DII of the spectra has been normalized to account for receiver gain, modulation amplitude, conversion time, and number of scans. Infrared Spectroscopy. To characterize structural changes in polymer based conductive nanolayers under consideration, ATR (attenuated total reflection)−FTIR (Fourier transform infrared) spectroscopy was used. Conventional single ATR mode is known to be useful for both micro- and submicroscales down to several hundred nanometers using Ge as an ATR crystal. In the case of thinner films multiple ATR is recommended.35 In this work to obtain the ATR spectra on scales around 50 nm, the advanced multiple ATR approach was utilized to increase device sensitivity and to break the sample thickness limit. Polymer-dopant compositions as well initial components were deposited onto Float-Zone Si-wafer functioning both as a film substrate and as the ATR crystal.36,37,40,41 The ATR spectra were acquired using an evacuated FTIR spectrometer Vertex80v (Bruker) equipped with both ATR Si wafer 40 mm unit (Bruker) and mercury cadmium telluride detector (InfraRed Associates). The total number of reflections was 46. The spectroscopic range was restricted to nitrile band region 2250−2050 cm−1 with 2 cm−1 spectral resolution. 300 scans were coadded to every spectrum. To compare the results properly, spectral baseline was corrected as well as spectra were normalized to a band of stretching vibration of nitrile group around 2190 cm−1 acting as an internal reference.38,42 DFT Calculations. We performed density functional theory (DFT) calculations as implemented in SCM ADF 1.3,39 using the B3LYP40 and the PBE41 functionals and a double-ζ basis set in order to determine the HOMO and LUMO energies of the model compounds and to calculate the charge transfer (CT) as well as the interaction energy of DPP(Me)TT and F6TCNNQ for various stacking configurations. In this work, the conjugated fragments were modeled by substituting methyl side groups for the C30 alkyl chains. Furthermore, finite fragments of PDPP(Me)TT rather than full polymer chains were considered because the computation of whole chains requires too large computing efforts. These simplifications should not strongly affect the calculation results because the charge transfer is a local effect which is not affected by those parts of the chain, which are several units distant from the stacking region. However, to ensure a correct modeling of the influence of the adjacent units, two PDPP(Me)TT fragments were modeled: the one having DPP as the middle group which is surrounded by two TT units and the other one having the TT unit surrounded by two DPP units. Working Procedure. First, we considered the isolated model compounds. In order to obtain the HOMO and LUMO energies, we optimized the molecular structures using the hybrid B3LYP functional and a DZP basis. This version of DFT is well accepted for giving HOMO−LUMO gaps with better accuracy than standard GGA functionals.42

Second, we performed calculations of CT complexes by considering π−π stacks. For this, we utilized the PBE functional since the derived quantities are not affected by the unoccupied states. For cases where we calculated both B3LYP and PBE functionals the CT obtained by PBE is 0.1e up to 0.2e greater than for B3LYP. Qualitatively, however, they are in good agreement. For considering dispersion interaction, the Grimme dispersion correction43 has been used. At first, the dopant molecule was placed on top of the DPP-TT model at a distance of 3 Å and then systematically rotated around the stacking axis or shifted along the polymer backbone. For each of those stacks single point calculations have been performed to evaluate the total energy as well as the charge transfer. For the latter one, we considered the Mulliken,32 Hirshfeld,33 and Voronoi34 approaches to calculate the atomic charges. Vibrational Spectra Computations. The full geometry optimizations and the corresponding harmonic vibrational frequency computations of isolated neutral F6TCNNQ and the isolated F6TCNNQ•− anion radical and CT complexes were carried out using the DFT method with B3LYP functional and 6-31G(d) basis set as implemented in Gaussian 09.44 The calculated harmonic vibrational frequencies and intensities were scaled by a factor of 0.9614.45,46 The electrostatic potentials with an isovalue of electron density of 0.02 au were calculated using the Merz−Singh−Kollman procedure.47

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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.6b02452. Conductivity data tables, calculated vibrational frequencies for the stretching modes of the CN group, changes of the bond lengths of the dopant and CT complexes, electrostatic potential surfaces of F6TCNNQ and its anion radical, full IR spectrum of F6TCNNQ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(A.K.) E-mail: [email protected]. ORCID

Uwe Lappan: 0000-0002-7464-9449 Anton Kiriy: 0000-0002-4263-9377 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from DFG (grant KI-1094/ 9) and the German Excellence Initiative via the Cluster of Excellence EXC 1056 “Center for Advancing Electronics Dresden” (CFAED). Y.K. thankfully acknowledges financial support by the German Academic Exchange Service (DAAD) within the frame of the DAAD-STIBET Program. We thank Dr. K.-J. Eichhorn for fruitful discussions concerning infrared spectroscopy experimental data.

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REFERENCES

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