Polarons, Compressed Polarons, and Bipolarons in Conjugated

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Polarons, Compressed Polarons, and Bipolarons in Conjugated Polymers Jin Bakalis,‡,† Andrew R. Cook,† Sadayuki Asaoka,§ Michael Forster,∥ Ulrich Scherf,∥ and John R. Miller*,† †

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Physics Department, City College of the CUNY, New York, New York 10031, United States § Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ∥ Macromolecular Chemistry Group, Bergische Universität Wuppertal, Gauss-Str. 20, D-42119 Wuppertal, Germany ‡

S Supporting Information *

ABSTRACT: Extensive reductive chemical doping in four conjugated polymers showed evolution of optical spectra for negative polarons and more reduced species. Delocalization lengths of the polarons, which ranged from 2 to 6 nm, were determined from measurements of bleaching of the neutrals, with the extinction coefficients measured by pulse radiolysis. A particular advantage of reductive doping is the ability to encapsulate the Na+ counterions in the C222 cryptand to control the interaction of charges with counterions. For lightly doped chains C222 had little effect on the delocalization lengths or spectra of the two polaron transitions P1 and P2, perhaps because most were completely dissociated to free ions. C222 did strongly alter the spectra when many electrons were added to a chain. For the shortest polarons, 2 nm in poly(phenylene-vinylene) (PPV), energies of the P1 and P2 transitions increased with the extent of reduction. The effect on the P1 transition was greater in the absence of C222 indicating ion-pairing equilibria for the short PPV polarons. Highly reduced ions formed upon injection of multiple electrons included polarons compressed by factors of four or more from their normal lengths to ∼1 charge/nm: a highly reduced 60 nm long chain contained ∼60 electrons. For compressed polarons the transitions shifted with increasing reduction indicating sensitivity to counterions: ion pairing is an important determinant of the behavior upon multiple reductions.

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INTRODUCTION

The presence of counterions can modify the charge distribution and affect the extent of charge delocalization in doped conjugated systems.11,12 Numerous computations stress that ion pairing can be responsible for charge localization of bipolarons in highly doped conjugated oligomers.13,14 For lightly doped chains Alkan and Salzner calculated that electron polarons in oligothiophene with n = 13 (T13) occupy 7 units in the presence of their counterions but are delocalized over the entire chain in the absence of the counterions.15 Hole polarons in T19 with the counterions spread over segments of 11 units.16 Such pairing effects were extensively discussed in the literature with various computational methods but very little knowledge that has been determined by experiments. One object of this paper is therefore to learn the effect of ion pairing on the nature of reduced charges in doped conjugated polymers. Chemical doping produces ion pairs between charges and their counterions. One advantage of studying reduced species is that the use of C222 cryptands to encapsulate positive counterions gives control over Coulomb interaction with reduced charges.

Free charge generation in organic photovoltaics stems from electron transfer from donors to acceptors. In organic heterojunction solar cells,1,2 redox reactions at the donor− acceptor interface generate free polarons from bound electron− hole pairs, followed by charge hopping within or between polymer chains. The rate of charge hopping in the conjugated system can be optimized by the delocalization of polarons. Studies3−6 on inverse relation of total reorganization energies (λ) for oligocene anions with their delocalization lengths (n = 2−5) show higher charge mobility in the solid state for the delocalized anions.7,8 These reports further support the idea that delocalized polarons in conjugated polymers can maximize the rate of charge transfer in accord with Marcus theory,9 which describes the dependence of electron transfer rates on reorganization energies of the molecules and medium. Long polarons are therefore expected to be more efficient in charge transport because they have smaller geometric distortion and thus maximize the electronic coupling,10 which is one of the important transport parameters in organic photovoltaics. Investigation of the nature of charges, especially properties such as their delocalization, is thus an important subject to better understand the photovoltaic devices. © 2013 American Chemical Society

Received: September 5, 2013 Revised: November 11, 2013 Published: November 12, 2013 114

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Chart 1. Structures of Polymers Used in This Work

an inert atmosphere glovebox. Polymers used in this work are shown in Chart 1. Synthesis of poly-2,7-(9,9-dihexylfluorene) (pF30 and pF79) and poly-2,7-(9,9-dibutyloctylfluorene) (pBuoF) using a method similar to that of Klaerner and Miller26 was described previously27 as is their separation into fractions with a narrowed range of lengths; pF30 refers to a polymer having an average length of 30 fluorene units. Synthesis of the ladder polymer, Me-Ladder PPP, was reported previously.28,29 Poly[2-(2′,5′-bis(2″-ethyl-hexyloxy)phenyl)1,4-phenylene-vinylene] (PPV) was purchased from Aldrich, as was the 2.2.2 cryptand, 4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane (C222), and sodium tetraphenylborate (NaBPh4). NaBPh4 was recrystallized. For each polymer, the reductions were carried out without and with 15−20 mM C222 to study the effect of sodium ion pairing. Concentrations of polymers will be given in polymer repeat units (PRUs). Pulse Radiolysis: Measurement of Extinction Coefficients. The extinction coefficients of polymer anions were determined by the electron pulse radiolysis at the Laser Electron Accelerator Facility (LEAF)30 at the Brookhaven National Laboratory. Measurements were made in THF containing 10 mM NaBPh4. Biphenyl, benzophenone, and all the polymers listed above were prepared at 1−6 mM PRU concentrations. For the transient absorption of the polymer anions, quartz cells with 2 cm path length were used. The mechanism for formation of the radical anions is as follows. In reaction 1, electron pulses with a pulse width 2.3 × 10−5, >60% of pF•− ions are free of Na+. Polarons at this stage of reduction occupy 4.5 ± 0.8 units, and their lengths can possibly fluctuate with kBT. Since these dissociation fractions were estimated from the lower limit Kd = 2.3 × 10−5, most pF30•− ions are free of their counterions in reduction 1. At n ∼ 0.3 the visible P1 band is shifted blue by ∼0.02 eV. On the basis of this negligible spectral shift, we could assume that ≥90% of pF•− are free at n ∼ 0.3. Because we do not know the spectrum of a polaron paired with Na+, we cannot be confident that this assumption is correct. However, if it is, then Kd is estimated to be >5 × 10−4, 25 times greater than Kd for a perylene anion paired with a Na+. B. Substantial Neutral Absorption at the End of Reduction I. At n ∼ 0.3 in Figure 1(a) where the P2 band reaches the maximum absorption, 30% of the neutral band remains, but that band is noticeably broadened to almost twice its spectral width and blue-shifted from 3.27 to 3.45 eV. It is not clear whether the absorption of this band is completely due to the neutrals or some other species, but the most plausible explanation may be vestigial neutral (VN) bands of remaining neutral units not covered by polarons, introduced by Zaikowski20 in F4−F10 reductions. Their absorption peaks are similar to the pure neutrals but much less intense, noticeably broadened, and blue-shifted. On the basis of these oligomer results, the broad and shifted band seen at n ∼ 0.3 could be a coupling of different vestiges of shorter neutrals with different repeat units created when several polarons are formed on the chain. While plausible one could ask why there are any neutral

(6)

where P denotes neutral polymers and n is the number of electrons in the polymer chain. Table 3 reports that complete reduction of the polymers to the redox level of Bip•− squeezes one electron to an average size as small as ∼1 nm. Reductions of pF30 and pBuoF. A. Free Polarons in Reduction I. Addition of C222 causes little difference in reduction I for both pF30 and pBuoF. This indicates either that ion pairing has little effect on the spectra of negative polarons or that most polarons are free of their Na + counterions. Although steric congestion from the bulky butyloctyl side chains can reduce ion pairing in pBuoF reductions, a negligible spectral shift in both pF30 and pBuoF supports the formation of free polarons. Slates and Szwarc reported the dissociation constant Kd = 2.3 × 10−5 for a perylene anion paired with a Na+ in THF and observed Kd for aromatic anions to increase with the anion size.54 Since the size of pF•− is over ∼4 PRU and greater than the perylene anion, Kd for pF•− is probably at least as large as this value. On the basis of the assumption that Kd > 2.3 × 10−5 121

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segments at all at n ∼ 0.3 (3.3 repeat units/polaron) if polarons prefer lengths of 4−5 units? A possible answer is fluctuation of polaron wave function in its ground state. Such a change in polaron delocalization can be caused by the thermal energy that can alter bond lengths and dihedral angles of the ground-state configuration. This length fluctuation can partially open up some space occupied by polarons and consequently may yield more absorption of the neutrals. Another is that Coulomb repulsion might force polarons apart, even compressing them to leave neutral space between them. Still another possibility is a weak P3 polaron transition located close to the neutral absorption. The FBC model17 predicts three possible optical transitions for polarons, P1 in the NIR, P2 in the visible, and P3 in the ultraviolet region, but the P3 is usually forbidden. Cornil et al.46 calculated the third absorption peak, P3, for cations of oligo(phenylenevinylene) (OPV). Deussen and Bässler55 reported a P3 band in the absorption spectra of OPV3•−, but this peak is difficult to isolate for the anion spectra of longer OPV and PPV due to the overlap with the neutral band.56 Although the anion spectra of F2•−−F10•− show no clear evidence for the third absorption,20 a possible interpretation is that absorption in the neutral region of pF30 at the end of reduction I is partly due to a weak P3 band. Such a P3 band might be concealed by the neutral absorption at early reductions but has become prominent toward the end as the neutral was being removed. If this is true, the rate of neutral decrease observed in eq 5 is a combination of pure neutral bleaching and P3 polaron growth, thereby requiring the removal of this P3 contribution. A corrected delocalization length of pF•− polaron based on this assumption is 5.3 RU, 18% greater than the value reported in Table 2. If the P3 band existed, intermediate spectra like that at n ∼ 0.21 would be linear combinations of the neutral band (n = 0) and P3 bands. A fit, based on the assumption, to the spectrum at n ∼ 0.21 is shown in Figure S2 in the Supporting Information. The fit is not of good quality, militating against a substantial contribution from a P3 band. Although there is no definite knowledge on the VN band or P3 band in the spectra, the broad band seen at the end of reduction I can also be from both weak P3 and VN bands. C. Compressed and Ion-Paired Polarons in Reduction II. The P2 bands in spectra of pF30 anions change markedly from n ∼ 0.4 to n ∼ 0.8. This reduction range begins with spectra of polarons. For n > 0.4 further reduction causes a decrease of the P2 band as some other species having a higher energy transition grow in. Subtraction of the P2 polaron band at a later stage of reduction I from subsequent reduction II spectra is presented in Figure S3 in the Supporting Information. The spectrum of an unknown species estimated in this way in the absence of C222 has a definite transition at 2.26 eV in the range of n ∼ 0.47 to n ∼ 0.53 but shifts 0.35 eV further blue at n ∼ 0.63. The peak at 2.26 eV is a shift of ∼0.2 eV from the P2 polaron peak at 2.09 eV. The spectrum of the unknown species in the presence of C222 also has the same transition at 2.26 eV, but there is an extra sideband at 2.47 eV, an additional shift of 0.2 eV from 2.26 eV. The rapid decrease of the P2 transition in reduction II, after reaching the maximum, is an evident sign that polarons are converted to a new species. Identification of the unknown species is difficult. Figure 5 d presents a picture of an array of compressed polarons paired with Na+ ions as a possible candidate. As observed by Zaikowski and co-workers, compressed polarons have strong NIR bands that resemble the P1

bands of polarons. They also found compressed polarons to have visible bands that resemble P2 polaron bands but are weaker, broader, and shifted blue in F52−−F102− spectra. F22− and F32− spectra display classical bipolaron spectra having a single band located between two polaron transitions.20 They also found that the transition from polarons to bipolarons is not sudden. Instead species of intermediate character are seen, for example in F42−, both by spectroscopy and by TDDFT computation.20 Direct comparison with pF30 and pBuoF (polyfluorene with butyl-octyl chains) in reduction II shows pF ions display absorption spectra very similar to those of compressed polarons like F52−−F102−. On the basis of the oligomer reductions, it is therefore possible to conclude that the unknown species could be compressed polarons with enhanced ion pairing which display polaron-like behavior. At n ∼ 0.63 the average spacing between the centers of adjacent polarons is ∼1.6 nm, requiring polarons to be shorter than their normal >3 nm lengths as described in Figure 5d. Now a Na+ would feel Coulomb attraction to two (or more) polarons. The dissociation constant Kd for Na+ with two polarons, 1.6 nm apart, can be estimated to be ∼10−9, meaning at least every other compressed polaron would be paired with a Na+ counterion, even though there might have been almost no ion pairing at low doping. Figure S5 (Supporting Information) describes a simple estimate of increased ion pairing. Such compressed polarons paired with more Na+’s may be responsible for changes in the spectra as observed in Figure S3 (Supporting Information). In such a chain of closely packed polarons, structures with alternating ion pairing might have considerable stability. Once each “even-numbered” polaron is paired, Kd for dissociation of counterions from the “odd” polarons would be decades larger, approaching Kd for isolated polarons. A number of polarons in a chain have been considered to form a “polaron lattice”.57−62 If so it is plausible that counterions form a superlattice upon such a polaron lattice. D. Effect of C222 in Reduction II. The addition of C222 clearly changes the pF30 anion spectra in reduction II. The visible band appears to be sharper and better resolved. On the other hand, C222 seems to have little effect on pBuoF anion spectra in reduction II. The evolution of both the NIR and visible bands is similar regardless of the presence of C222. A key difference between pF30 and pBuoF reductions arises from comparison of their visible band spectra at n ∼ 0.7, right before the end of titrations. The visible transition in pBuoF looks more like the P2 polaron band and has greater absorption than pF30. This remarkable observation could arise from steric hindrance by bulky butyl-octyl side chains. While ion pairing may be nearly as extensive in pBuoF as in pF, the steric repulsion in pBuoF may reduce contact with Na+ counterions. Reductions of LPPP. A. Dramatic Change of the Neutral Spectra in Reduction I. The absorption spectrum of neutral LPPP shows more pronounced vibronic structure45,63 than other polymers in this study. Two single bonds connecting adjacent LPPP units enforce a planar structure that causes large changes of dihedral angles.64 Other polymers possess flexible single bonds about which rotation is facile by thermal fluctuation. The dihedral angle fluctuation driven by kBT can result in different ground-state configurations, and they overlap to form a smooth contour in the spectra. The presence of negative polarons seems to dramatically alter the shapes of the neutral spectra of LPPP. Both with and without the addition of C222 absorbance in the neutral region decreases and shifts to higher energy. The most abrupt change 122

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occurs from n = 0 to n ∼ 0.33 in the absence of C222 and n = 0 to n ∼ 0.24 in the presence of C222. The spectrum for n = 0.18 in Figure 3b can be approximately reproduced by a linear combination of spectra at n = 0 (the neutral) and n = 0.24 (see Figure S4 in Supporting Information). This observation could be understood by formation of a weak P3 band of the polaron, or as a “vestigial neutral” (VN) band,20 that originates in neutral, not charged, regions of the chain. Upon further reduction the integrated intensity in the neutral region continues to decrease, and the spectra are no longer linear combinations of two species (see Figure S4, Supporting Information, and associated discussion). These observations support continuously shifting VN bands as neutral regions are compressed, with little contribution from a P3 band. B. Compressed Polaron Array in Reduction II. The spectral patterns of LPPP anions follow trends similar to those of pF anions in reduction II. The intense NIR band of LPPP anions grows with little change of shape, but the weak visible band evolves with a larger shift and change of shape. The spectral similarity of LPPP anions to pF anions in late reductions may suggest the formation of a compressed polaron array when multiple electrons are added to an LPPP chain. The oxidation of LPPP with antimony pentachloride yields different species at high doping levels as reported by Scherf et al.65 They observe a characteristic bipolaron band between two polaron transitions with a complete disappearance of the visible polaron band. Reductions of PPV. A. Effects of Ion Pairing in Reduction I. PPV departs from the behavior of the pFs and LPPP in that counterion pairing seems to play a key role in reduction I as well as in reduction II. Spectral shifts of both P1 and P2 bands are observed in reduction I for PPV, although both bands increase without shifts in other polymers. These shifts signal that the negative polarons are paired with Na+ counterions, that the pairing alters the spectra, and that the fraction paired increases during the titration. In the absence of C222 the NIR P1 band measured at n ∼ 0.02 is located at 0.5 eV and then shifts to 0.82 eV at the end of reduction I. The same band shifts only to 0.71 eV in the presence of C222 indicating less effect of ion pairing. The shift of the NIR P1 band starts approximately after n ∼ 0.1 where one electron is injected every 10 PRU; before that shifts are not obvious. The polaron spacing is ∼7 nm at n = 0.1. Two polarons closer than the Onsager radius66 (7.5 nm in THF) can combine to attract Na+ ions to increase the probability of ion-pair formation. The shift continues through n ∼ 0.27, injection of one electron per 3.7 PRU. This is an indication that PPV polarons seem to be compressed close to their delocalization length of 3.0 ± 0.7 PRU. The possible reason for more shift of the P1 band than other polymers may be due to the smaller size of the PPV polarons. On the basis of Slates and Szwarc’s report54 on increase of Kd with the size of aromatic anions, the small PPV polarons with a 2 nm length are expected to have a smaller Kd than longer polarons in other polymers, their sizes typically being 3−6 nm. Therefore, small PPV polarons can be bound to more Na+ counterions than other long polarons. The minimum value of Kd for n ∼ 0.1 is roughly estimated to be 3.8 × 10−4 with an assumption that 90% are free ions. B. Third Optical Transition for PPV Polarons. Two neutral absorption bands at 3.1 and 4.1 eV evolve in an opposite manner throughout reduction I. The low-energy band (3.1 eV) bleaches with reducing equivalents added, but the high-energy band (4.1 eV) grows. The high-energy band at 4.1 eV is therefore associated with the radical anions because (i) it

increases with more electrons added and (ii) it disappears with a decrease of the P2 polaron band. The 4.1 eV band therefore appears to be a P3 transition discussed in the FBC model.17 In this model the P3 transition is forbidden so its transition might be weaker than the other two. The oscillator strengths for the P1 and P2 transitions in PPV are estimated to be 0.14 and 0.25, respectively, smaller than those in other polymers by a factor of ∼3. These weak transition strengths can sufficiently yield the P3 transition to grow. Deussen and Bässler observe the third absorption band with a complete bleach of oligo(phenylenevinylene) neutrals (OPVs) in the spectra of OPV•− anions.55 The third absorption peak becomes difficult to isolate for longer OPV and PPV due to the overlap with the neutral band.46,56 C. Effects of Ion Pairing in Reduction II of PPV. New absorption peaks at n ∼ 0.55 appear at ∼1.9 eV overlapped with a sideband at 2.1 eV with gradual disappearance of the visible anion band at 1.7 eV. This is a sign that the anions are converting into a new species. Some of these transitions are sharp in the presence of C222, but ion pairing broadens them when C222 is absent. The intense NIR band keeps shifting blue until the Bip•− anion band at 3.1 eV appears at the end of titration. The PPV ions seem to possess several sharp transitions in the NIR region in the presence of C222. These are less distinct without C222, perhaps because broadening due to effects of ion pairing conceals these transitions. A single band between the polaron transitions is a classical picture of bipolaron formation which is detected in reduction of tert-butyl-substituted OPV67 and confirmed by the calculation.46 Our result finding two bands (the NIR and visible region) at high doping levels is consistent with other results in the photoinduced absorption experiments68 in both oxidation55,69 and reduction56 of OPV and PPV. Although two band features in our PPV reduction can be due to π-dimers or compressed polarons that exhibit two subgap absorptions, there is no method to distinguish them at the present time. PPV ions in reduction II are quite different from those in pFs and LPPP in that a new visible absorption band gradually replaces the P2 polaron band.

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CONCLUSIONS The evolution of negative polarons and more reduced charges was examined in conjugated polymers in titrations where the extent of reduction could be known. Encapsulation of Na+ counterions into the C222 cavity reduced the intimacy of pairing with the counterions. Delocalization lengths of polarons, ranging from 2 to 6 nm, were measured by removal of neutral absorption. Counterion pairing had little effect on delocalization lengths. The delocalization lengths were determined from early stages of reduction where polarons interfere less with each other. In this range of doping most Na+ ions may have escaped to form free ions exhibiting very little spectral shift during reduction. Polaron formation was sensitive to water, which could not be completely excluded. Water probably irreversibly converts one polaron to a carbanion with consumption of some electrons. This introduced errors to the accurate determination of polarons formed per electron added. Delocalization lengths of polarons were also affected but were less sensitive. A doping method with a coarse titration yielded titrations that were less affected by water. The polaron lengths were possibly affected by new species formation or additional possible transition of polarons where neutrals absorbed, but due to a lack of 123

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(5) Coropceanu, V.; Andre, J. M.; Malagoli, M.; Bredas, J. L. The Role of Vibronic Interactions on Intramolecular and Intermolecular Electron Transfer in Pi-Conjugated Oligomers. Theor. Chem. Acc. 2003, 110, 59−69. (6) Gruhn, N. E.; da Silva, D. A.; Bill, T. G.; Malagoli, M.; Coropceanu, V.; Kahn, A.; Bredas, J. L. The Vibrational Reorganization Energy in Pentacene: Molecular Influences on Charge Transport. J. Am. Chem. Soc. 2002, 124, 7918−7919. (7) Nelson, S. F.; Lin, Y. Y.; Gundlach, D. J.; Jackson, T. N. Temperature-Independent Transport in High-Mobility Pentacene Transistors. Appl. Phys. Lett. 1998, 72, 1854−1856. (8) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals. Science 2004, 303, 1644−1646. (9) Marcus, R. A. Electron-Transfer Reactions in Chemistry - Theory and Experiment. Rev. Mod. Phys 1993, 65, 599−610. (10) Grozema, F. C.; van Duijnen, P. T.; Berlin, Y. A.; Ratner, M. A.; Siebbeles, L. D. A. Intramolecular Charge Transport Along Isolated Chains of Conjugated Polymers: Effect of Torsional Disorder and Polymerization Defects. J. Phys. Chem. B 2002, 106, 7791−7795. (11) Irle, S.; Lischka, H. An Ab-Initio Investigation of the ChargeTransfer Complexes of Alkali Atoms with Oligo (Alpha,Alpha’) Thiophenes and Oligoparaphenylenes - a Model Calculation on Polaronic and Bipolaronic Defect Structures. J. Chem. Phys. 1995, 103, 1508−1522. (12) Bredas, J. L.; Themans, B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Highly Conducting Polyparaphenylene, Polypyrrole, and Polythiophene Chains - an Ab initio Study of the Geometry and Electronic-Structure Modifications Upon Doping. Phys. Rev. B 1984, 29, 6761−6773. (13) Zamoshchik, N.; Salzner, U.; Bendikov, M. Nature of Charge Carriers in Long Doped Oligothiophenes: The Effect of Counterions. J. Phys. Chem. C 2008, 112, 8408−8418. (14) Zade, S. S.; Zamoshchik, N.; Bendikov, M. From Short Conjugated Oligomers to Conjugated Polymers. Lessons from Studies on Long Conjugated Oligomers. Acc. Chem. Res. 2011, 44, 14−24. (15) Alkan, F.; Salzner, U. Theoretical Investigation of Excited States of Oligothiophene Anions. J. Phys. Chem. A 2008, 112, 6053−6058. (16) Salzner, U. Theoretical Investigation of Excited States of Oligothiophenes and of Their Monocations. J. Chem. Theory Comput. 2007, 3, 1143−1157. (17) Fesser, K.; Bishop, A. R.; Campbell, D. K. Optical-Absorption from Polarons in a Model of Polyacetylene. Phys. Rev. B 1983, 27, 4804−4825. (18) Kivelson, S.; Heeger, A. J. 1st-Order Transition to a Metallic State in Polyacetylene - a Strong-Coupling Polaronic Metal. Phys. Rev. Lett. 1985, 55, 308−311. (19) Furukawa, Y. Electronic Absorption and Vibrational Spectroscopies of Conjugated Conducting Polymers. J. Phys. Chem. 1996, 100, 15644−15653. (20) Zaikowski, L.; Kaur, P.; Gelfond, C.; Selvaggio, E.; Asaoka, S.; Wu, Q.; Chen, H. C.; Takeda, N.; Cook, A. R.; Yang, A.; et al. Polarons, Bipolarons, and Side-by-Side Polarons in Reduction of Oligofluorenes. J. Am. Chem. Soc. 2012, 134, 10852−10863. (21) van Haare, J.; Havinga, E. E.; van Dongen, J. L. J.; Janssen, R. A. J.; Cornil, J.; Bredas, J. L. Redox States of Long Oligothiophenes: Two Polarons on a Single Chain. Chem.Eur. J. 1998, 4, 1509−1522. (22) Takeda, N.; Asaoka, S.; Miller, J. R. Nature and Energies of Electrons and Holes in a Conjugated Polymer, Polyfluorene. J. Am. Chem. Soc. 2006, 128, 16073−16082. (23) Miller, L. L.; Mann, K. R. Pi-Dimers and Pi-Stacks in Solution and in Conducting Polymers. Acc. Chem. Res. 1996, 29, 417−423. (24) Kurata, T.; Mohri, T.; Takimiya, K.; Otsubo, T. Conductive, Magnetic, and Optical Properties of Sterically Hindered Dodecithiophenes. Evidence for the Coexistence of Bipolaron and Pi-Dimer. Bull. Chem. Soc. Jpn. 2007, 80, 1799−1807. (25) Apperloo, J. J.; Groenendaal, L.; Verheyen, H.; Jayakannan, M.; Janssen, R. A. J.; Dkhissi, A.; Beljonne, D.; Lazzaroni, R.; Bredas, J. L.

quantitative knowledge these factors were excluded from reported uncertainties. The energies of the P1 and P2 transitions for longer polarons were insensitive to ion pairing in early doping levels, perhaps because most Na+ counterions were dissociated from the ion pairs. An exception was a PPV, which had the shortest polarons with a 2 nm length. The energies of the P1, to a greater extent, and P2 transitions increased with the extent of doping. These shifts may show that ion pairing occurs and that these transitions are sensitive to the ion pairing. Highly reduced ions for all polymers at high doping levels were all influenced by counterions. For pFs and LPPP multiply reduced ions behaved like compressed polarons displaying a strong NIR band and weak visible band with a shift. PF ions were possibly compressed by a factor of 4 or more from their normal length of 4 PRU. In PPV, ion pairing concealed sharp transitions for highly reduced ions. Thus, the ion pairing was an important parameter to study the behavior of multiply reduced ions in all polymers.

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ASSOCIATED CONTENT

S Supporting Information *

UV−visible−NIR spectra of pF30 chemically doped with Na+Bip•, a difference spectrum of pF30 anions in reduction II, a fit for n ∼ 0.28 in pF30 reduction I, a linear combination of pure neutrals and pure anions to fit an intermediate spectra at n = 0.28 in reduction I of pF30, a linear combination of n = 0 and n ∼ 0.24 in reduction I of LPPP, and a pictorial description of a single pF30 polaron paired with a Na+ and three pF30 polarons paired with a Na+ at n = 0.3. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant #DE-AC02-98-CH10886, and use of the LEAF Facility of the BNL Accelerator Center for Energy Research. We thank Dr. Seogjoo Jang for collaborative encouragement, and J.B. thanks National Science Foundation Grant #0652963 titled Queens Borough Bridge to QC and QCC CUNY, and the FAST Program at BNL for partial support.

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REFERENCES

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