Discrete DonorâAcceptor Conjugated Systems in Neutral and

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Discrete Donor−Acceptor Conjugated Systems in Neutral and Oxidized States: Implications toward Molecular Design for High Contrast Electrochromics Natasha B. Teran and John R. Reynolds* School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Donor−acceptor systems are ubiquitous redox-active materials in electrochromic devices, making the study of their neutral and charged state characteristics expedient for the design of materials with improved properties. In this paper, we explore the absorption properties of the neutral and oxidized states of two dioxythiophene- and benzothiadiazole-containing penta- and hepta-heterocycles (EPBPE, EPPBPPE) having a monodisperse, well-defined π-conjugated structure, using electrochemistry, optical absorption and electron paramagnetic resonance (EPR) spectroscopy, spectroelectrochemistry, and microscopy. The molecules and their precursors were obtained via a direct (hetero)arylation coupling strategy that exploits stoichiometric control to obtain well-defined ter- and penta-heterocycles from bifunctional heteroarenes. Both molecules show intense and narrow dual-band absorptions in the visible region, reflecting the discrete nature of their πsystems, leading to strongly colored neutral states. The electron-rich dioxythiophene units enable access to their radical cation and dication states at potentials below 5 mV and 260 mV (vs ferrocene/ferrocenium), respectively, and give rise to stability toward repeated oxidative switching (voltammetric cycling). EPR and absorption spectroscopy of their chemically and electrochemically derived oxidized states showed them to be dominated by polaronic, π-dimeric, and, in the case of EPPBPPE, bipolaronic charge carriers. These species exhibited transitions with maxima in the near-IR region, leading to highly transmissive oxidized states and promising structures for high contrast electrochromics. A polymer (Poly-EPBPE) that maintains a discrete conjugated segment along the backbone was also designed using EPBPE as the multi-ring heterocycle linked together with an aliphatic n-decyl chain, to obtain a mechanically robust yet solution processable material. Poly-EPBPE showed narrow optical transitions and well-resolved oxidation waves in solution that correlated strongly with the properties of EPBPE. However, strong intermolecular interactions were observed in the absorption spectroscopy and electrochemistry of its film state. The oxidized state absorption properties of Poly-EPBPE reflected these interactions, with absorption properties dominated by π-dimers and higher order aggregates, leading to irreversibility in its film spectroelectrochemistry. The coupled structural, optical, electrochemical, magnetic, and microscopic studies enabled us to propose potential resonance structures of the charge carriers in these discrete conjugated systems and inform the design of high contrast electrochromic materials.

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absorption for solar cells27,30,34,36 and large polarizabilities for nonlinear optics.21,29,35 It has also been used to generate materials with a dual band absorption composed of a low energy band arising from the charge-transfer interaction and a higher energy band that tends to correlate with the extent of conjugation in the π-system.25,32 This dual band absorption has been used to obtain a variety of intensely colored polymers for electrochromics,16,17,37 including green38−44 and broadly absorbing polymers.24,45,46 In many applications, conjugated materials perform their function not only as neutral molecules but also in their charged

INTRODUCTION Conjugated molecules and polymers with semiconducting properties find application as optoelectronic and photonic materials in devices including transistors,1−3 solar cells,4−7 light emitting diodes,8−10 optical switches,11−14 and electrochromics,15−18 among others. One of the key advantages of these materials is the ease with which their properties can be modulated for specific functions by tailoring their structures using the myriad techniques of synthetic organic chemistry. A well-established method is the donor−acceptor approach, in which electron-rich and electron-poor heterocycles are covalently coupled together to form multiring molecules or copolymerized such that an intramolecular charge-transfer interaction is induced.19−35 This concept has been implemented successfully to obtain low energy gap materials for light © XXXX American Chemical Society

Received: November 11, 2016 Revised: December 20, 2016

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DOI: 10.1021/acs.chemmater.6b04725 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Scheme 1

propose a route to translate these small molecules into polymeric materials that retain their well-defined properties, with ultimate implications for high contrast electrochromism.

states. In electrochromics, the difference in the absorption/ transmission characteristics of both the neutral and charged states accessed via the application of a voltage is exploited, and with sufficient contrast, they can find applications in displays and smart windows. In this, it is important to design the conjugated system such that the absorption properties of both the neutral and the oxidized state can be situated within specific regions of the electromagnetic spectrum. For example, for colored-to-transmissive applications, one charge state must absorb within the visible region, whereas the other state must absorb outside of this region in order to maximize optical contrast. Numerous studies have been reported which focus on properties of charge carriers in molecules, oligomers, and polymers containing electron-rich heteroarenes.47−58 On the other hand, relatively little has been reported regarding charge carrier species in multiring molecules containing electron-poor units.59−61 Here, we report on the neutral and oxidized state absorption and spin properties for both neutral and oxidized states of a new family of donor−acceptor−donor (D−A−D) molecules and a polymer with a discrete D−A−D conjugated system in its repeat unit. These systems are based on electronrich dioxythiophenes and electron-poor 2,1,3-benzothiadiazole (BTD) and heterocycles that are important building blocks of many electrochromic materials16,17,24,25,37,44,45 as well as holetransporting donor polymers used in bulk-heterojunction solar cells.62,63 These results allow us to propose potential structures of their oxidized state charge carriers and provide insights into the steric and electronic structural elements that contribute to determining these. Using these insights, we report on the design and synthesis of a D−A−D molecule that has a highly transmissive oxidized state. Furthermore, we are able to

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EXPERIMENTAL SECTION

Materials and Methods. All reactions were carried out under an Ar atmosphere. Reagents were purchased from commercial sources and used as received without further purification, unless otherwise noted. 164 and 565 were synthesized according to published procedures. Toluene, tetrahydrofuran, dichloromethane, and acetonitrile were obtained from a solvent purification system and stored over activated 4 Å molecular sieves under argon a few hours prior to use (if not used inside a glovebox). NMR spectra were recorded on a Varian Mercury 300 MHz spectrometer. Low resolution mass spectra were recorded on a Micromass Quattro LC Triple Quadrupole HPLC/MS/ MS mass spectrometer, while high resolution mass spectra were recorded on a Waters Autospec M Three Sector Tandem mass spectrometer. Elemental analysis was carried out by Atlantic Microlab, Inc. Gel-permeation chromatography (GPC) of Poly-EPBPE was performed at 35 °C in THF on a Waters column (4.6 mm × 300 mm; Styragel HR 5E), a Waters HPLC pump 1515, using Refractive Index Detector 2414. The GPC was calibrated to narrow molecular weight polystyrene standards. The polymer was dissolved to a concentration of 1 mg mL−1 in THF and filtered through a 0.45 μm PTFE filter, and 20 μL of the polymer solution was injected for analysis. Molecular weights were calculated using Waters Breeze II software. Synthesis. Procedures for the preparation of all compounds and their characterization are described in the Supporting Information. Electrochemistry. Solution and film electrochemical experiments were done on a EG&G Princeton Applied Research 273 A potentiostat/galvanostat using a three-electrode cell inside an argonfilled glovebox with a 0.02 cm2 platinum button working electrode (for solution and Pt-button film experiments), a 1 × 1 cm2 platinum mesh electrode (for solution OTTLE experiments), or a 7 × 50 × 0.7 mm3 ITO-coated glass electrode (purchased from Delta Technologies, Ltd.; B


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Chemistry of Materials Scheme 2

sheet resistance Rs 8−12 Ω sq−1), a platinum wire counter electrode, and a Ag/AgNO3 reference electrode, at a scan rate of 25 mV s−1, unless otherwise indicated. All electrochemistry experiments were done with 0.1 M (in CH2Cl2 solution) or 0.5 M (in acetonitrile) tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The Ag/Ag+ reference electrode was calibrated against a ferrocene/ferrocenium standard, the E1/2,ox for which is taken to be 5.12 eV below vacuum level.66 Solution electrochemistry experiments were done at 1 mM concentrations of analyte, unless otherwise noted (e.g., for OTTLE experiments). Polymer film experiments on platinum button were done on films drop cast from 1 mg mL−1 chloroform solutions. Polymer film experiments on ITO electrode were done on films spray cast from 50 mg mL−1 toluene solutions (or other solvent mixtures, as indicated) of the polymer, filtered through 0.45 μm PTFE syringe filters. Spray casting was done using Anest Iwata airbrushes with nitrogen carrier gas at 25 psi. The airbrush was disassembled and cleaned with copious amounts of toluene and acetone until all traces of color were removed. The ITO-coated glass electrodes were cleaned on both conducting and nonconducting sides by rinsing and wiping with isopropyl alcohol, acetonitrile, acetone, and toluene sequentially and dried with a stream of nitrogen gas from the airbrush. Optical absorption spectra were collected at room temperature using an Agilent Cary 5000 UV−vis−NIR spectrophotometer, using 10 mm path length quartz cells for solution chemical doping measurements, an OTTLE cuvette for electrochemical doping experiments, and an ITO slide for film experiments. A constant gentle flow of argon was delivered to the spectrometer throughout all the experiments. Chemical Doping. All chemical doping experiments were performed in CH2Cl2 solutions. The solvent was obtained from a solvent purification system, degassed with at least three freeze− pump−thaw cycles, and stored in the glovebox under light protection. The solvent used was never more than a week old. Chemical oxidants used were dispensed exclusively in the glovebox and protected from light. Stock solutions of the π-conjugated molecules and chemical oxidant were prepared at various known concentrations in CH2Cl2 in the glovebox. The oxidant and π-conjugated molecule solutions were then transferred to two separate amber vials kept under a blanket of argon and protected from light throughout the experiment. A known volume of the molecule solution (2.00 mL) was transferred via syringe (previously rinsed with CH2Cl2 three to five times and purged with argon) to an argon filled cuvette with a septum-lined screw-cap. The same volume of CH2Cl2 was transferred to two other cuvettes to serve as reference solutions. While not inside the spectrophotometer, the vials were kept under light protection. The oxidant solution was then gradually titrated into the molecule solution and one reference solution, at 0.01 mL increments, with a syringe (rinsed and purged as previously described). All syringes were kept plugged into a septumcapped vial full of argon when not in use. Blank spectra were obtained with the pure solvent solution and the doped solvent (without the analyte molecule) for every titration point. Results were reported with the absorption values corrected for volume changes with titration. Electron paramagnetic resonance (EPR) spectroscopy was also performed on neutral and chemically doped solutions, with a Bruker X-Band instrument at room temperature, and 9.848 GHz. EPR sample tubes (Wilmad 715-PQ-250M) were purged with argon prior to use. Solutions of π-conjugated small molecules (1 mM) were doped with

AgPF6 in CH2Cl2. The volumes (∼500 μL) were kept constant through each solution studied.

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RESULTS AND DISCUSSION Design and Synthesis. Two discrete conjugated donor− acceptor penta- and hepta-heterocyclic molecules based on dioxythiophenes (DOTs) and BTD were synthesized (Scheme 1). DOTs were selected as the donor unit to take advantage of electron donation by the two β-alkoxy units to give low oxidation potentials and high stability of the oxidized state.67−71 Furthermore, the sulfur−oxygen interactions between adjacent rings in DOTs allow for enhanced planarization and conjugation.57,65,72,73 The 3,4-propylenedioxythiophene (ProDOT) unit (1), moreover, combines these desirable properties with the ability to attach solubilizing groups at the 2-position of the propylene bridge, without steric repulsions substantially hindering planarization along the conjugated segment. BTD was selected as the acceptor unit due to its strong electron accepting properties enabling access to low-energy chargetransfer transitions,18,29 and its pervasiveness in organic electronics, especially in electrochromic polymers.24,25,37,44,46 With the β-positions in dioxythiophenes blocked, direct (hetero)arylation was selected as the cross coupling strategy.74,75 The internal donor−acceptor ter-heterocycle 3 was synthesized in one step from bifunctional (H−Ar−H and X− Ar′−X) heterocycles by careful control of the stoichiometry of the reactants and the Pd-catalyst loading (Table S1), allowing the ter-heterocycle to be produced in high yields, rather than the alternating copolymer. The high catalyst loading promotes oxidative addition to a greater number of the C−Br bonds of 2, whereas the large excess of 1 ensures that every unit of 2 that has undergone oxidative addition reacts with 1, rather than a dimer or other oligomers. Longer oligomers were observed as poorly eluted spots on thin-layer chromatography, but their formation was arrested and minimized by quenching the reaction as soon as 2 and the monocoupling product of 2 and 1 are completely consumed (as determined by TLC). This onestep ter-heterocycle formation avoids the need for an additional step to asymmetrically derivatize the ProDOT ring to yield ProDOT-X (X = SnR3, Bpin, Br, etc.), which can involve toxic reagents and tricky purification steps. Furthermore, the excess H−Ar−H starting material (12 equiv) is recovered after column chromatography, leading to high synthetic efficiency. The ter-heterocycle 3 was then symmetrically brominated, followed by another direct (hetero)arylation coupling with an α-n-hexyl-substituted 3,4-ethylenedioxy-thiophene (EDOT) to yield EPBPE, or 1 to yield 6. The latter is then symmetrically brominated and then end-capped with α-n-hexyl EDOT to yield EPPBPPE. Both donor−acceptor oligomers were thus obtained in moderate to good yields. The branched R-groups on the ProDOT units and the n-hexyl chains on the EDOT C


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Chemistry of Materials

and efficient conjugation between donor segments and BTD. Comparing EEBEE,29 EPBPE, and PPBPP,24 however, shows that the red-shift is more moderate when adding ProDOT as opposed to EDOT, indicating that the propylene bridge and the branched side chains in the ProDOT unit introduce some steric torsion reducing π-overlap.78 The lack of vibrational fine structure in either spectrum can also be attributed to some degree of interaryl torsion in PBP units,79 as well as steric repulsion from the branched side chains preventing significant intermolecular interactions. Poly-EPBPE also shows two absorption peaks assignable to a D−A charge transfer and a π−π* transition (Figure 1b). In solution, its absorption spectrum is comparable to that of EPBPE, with minimal shifts in the maxima of both peaks, indicating that the conjugated units in the polymer chain are able to retain their discrete character with little effect on optical properties from the aliphatic linker. The solid state absorption spectra, however, show overall broadening of both peaks, and scattering is observed beyond 750 nm, likely due to a fibrillar morphology of the spray-cast film (vide inf ra) manifested by the rod−coil nature of the polymer structure. Electrochemistry. Solution electrochemistry of the discrete molecules (1 mM) and polymer (5 mg mL−1) were performed under an Ar atmosphere in a CH2Cl2 solution with 0.1 M tetran-butylammonium hexafluorophosphate (TBAPF6) as the electrolyte (Figure 2). The oxidation potentials from cyclic (CV) and differential pulse voltammetry (DPV) and derived ionization potential (IP) and electron affinity (EA) values are summarized in Table 1. Both molecules show electrochemically reversible first and second oxidation steps, indicating that the radical cation and dication states of both molecules are accessible under inert atmosphere. Both molecules also withstood repeated cycling, maintaining the same CV shape over 100 cycles (Figure S1A,B). The radical cation and dication states were obtained at low potentials, due to electron donation from the oxygen units around the donor rings. Extending the oligomer structure by two ProDOT rings in EPPBPPE lowers the first (Eox,1) and second (Eox,2) oxidation potentials by 300− 400 mV relative to EPBPE. It also leads to a decrease in the peak-to-peak separation (ΔE1−2) between the first and second oxidation waves from ∼250 mV for EPBPE to ∼120 mV for EPPBPPE. The same decrease in ΔE1−2 has been observed with increasing lengths of EDOT oligomers.52 This decrease indicates that the energy required to introduce a second positive charge to the oligomer decreases, i.e., that there is lower Coloumbic repulsion toward introduction of the second positive charge.80 This can be attributed to the longer donor segments in EPPBPPE being able to better separate the two positive charge centers. The CV of Poly-EPBPE (Figure 2) also shows two oxidation peaks, with the first wave overlapping well with that of EPBPE. The second oxidation wave, on the other hand, is broader, with an onset at a lower potential (∼0.13 V vs ∼0.18 V for EPBPE), a peak at a higher potential, and a higher current. DPV shows the first oxidation step as a small shoulder to the second step. The polymer also shows less than 10% loss in current over 50 CV cycles (Figure S1C). Electrochemistry of the polymer can also be studied in the solid state under conditions where the ions formed do not dissolve in the electrolyte solution. A film of the polymer was deposited from a 1 mg mL−1 solution in CHCl3 onto a Pt working electrode, with a 0.5 M TBAPF6 in CH3CN serving as electrolyte solution. In the solid polymer, both first and second redox processes are shifted to higher

units conferred high solubility to the oligomers, allowing characterization and processing in CH3CN, CH2Cl2, ethyl acetate, and toluene, among other solvents. A polymer with the same donor−acceptor conjugated segment as EPBPE was also synthesized (Scheme 2) to impart solution processability and mechanical integrity, while maintaining the discrete nature of the conjugated segment. An ndecyl chain was selected as an aliphatic linker between two EDOT units to afford enhanced flexibility and processability to the resulting polymer.76 Direct (hetero)arylation copolymerization of monomers 4 and 8 then afforded Poly-EPBPE. The polymer was end-capped with n-hexyl terminated EDOT units. The CHCl3 fraction from Soxhlet extraction gave Mn = 23 kDa, Mw = 54 kDa, and Đ = 2.33, which was then used for all characterizations. Neutral State Absorption Properties. The absorption spectra of the neutral penta- and hepta-heterocyclic molecules in CH2Cl2 solution (Figure 1a) show two strong peaks in the

Figure 1. Absorption spectra of D−A oligomers in CH2Cl2 solution (A) and EPBPE molecule and polymer in CH2Cl2 solution and film (B).

visible region: a low-energy absorption attributed to a D−A charge transfer excitation and a high-energy absorption attributed to a π−π* transition.20,25 Table 1 compiles their optical properties, along with related oligomers reported in the literature. Compared with EBE,77 each addition of a ProDOT unit on either side of the acceptor unit leads to a significant bathochromic shift in both peaks, which, along with the large molar extinction coefficients (ε ∼ 104 M−1 cm−1) observed, indicates the strong electron-donating character of ProDOT D


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Chemistry of Materials Table 1. Summary of Optoelectronic Properties of the Discrete-Conjugation Donor−Acceptor Systems optical λmax (nm) (ε × 104 M−1 cm−1) D−A EPBPE EPPBPPE Poly-EPBPE Soln Film EBE EEBEE PBPh PPBPPh PPPBPPPh

π−π*

electrochemical a

Eg,opt (eV)

b

Eox,1 (V)

Eox,2b (V)

ΔE1−2 (V)

e

e

IPc (eV)

EAd (eV)

570 (4.0) 588 (5.5)

384 (5.2) 415 (7.3)

1.78 1.84

0.004 (0.31 ) −0.28

0.26 (0.41 ) −0.16

0.256 (0.10 ) 0.12

−5.1 −4.8

−3.3 −3.0

574 585 481 585g (2.7) 462 542 576

383 386 320 396g 317 371 406

1.81 1.69 2.25 1.89g

∼0.02 0.14f 0.33 −0.08

0.36 0.44

0.34

−5.1

−3.3

−5.4 −5.02

−3.3 −3.42

e

ref

77 29 24 24 24

a

Taken from the onset of the D−A charge transfer peak. bDPV peak potential; values referenced to ferrocene (E1/2 = 0.085 V vs Ag/Ag+ in CH2Cl2). Values reported relative to vacuum (−5.1 eV). dCalculated from the sum of IP and Eg,opt. eCV half-wave potentials from OTTLE cell vs Ag/Ag+ in CH2Cl2. fPotential from shoulder of main DPV peak. gValues reported from CHCl3 solution. hValues reported from toluene solution. c

The evolution of the spectra with oxidation of a 25 μM solution of EPBPE is shown in Figure 3, and the transitions are summarized in Table 2. Addition of calculated equivalents of AgPF6 results in the gradual depletion of the neutral state transitions at 384 and 570 nm, while concurrently generating two strong and well-defined transitions in the near-IR at 920 and 1640 nm (Figure 3A, difference spectra Figure S2). The oxidized state peaks also show high energy shoulders at 810 nm and 1140 and 1370 nm, respectively. An isosbestic point is present at ∼650 nm. These charged state transitions are more narrow and well-defined when compared with those from fully conjugated polymers with similar repeat unit structures,24,44 as expected from the discrete conjugated system in EPBPE. The same spectral evolution is observed when the oxidized states are generated with NOBF4 as the dopant (Figure S3), and similar transitions are also observed from OTTLE spectroelectrochemistry (Figure 3B), though the high energy shoulders at 820 and 1320 nm are more pronounced than in the chemical doping cases. These spectral changes lead to an overall change from purple to transmissive between neutral and oxidized states of EPBPE (Figure 3B and D), with an optical contrast at 385 nm of 47% and at 569 nm of 38%. The oxidized state’s dual transitions in the near-IR may be attributed to polaronic midgap states created as the molecule accommodates the positive charge in a quinoidal geometry.82−85 The oscillator strengths of the two transitions are also significantly higher than the neutral transitions, which can be attributed to better delocalization in the π-system from a quinoidal geometry in the charge carrier. The polaronic peaks gradually grow as more dopant is added, but surprisingly, after more than one equivalent of the dopant is added (Figure 3A, blue traces), their absorbances continue to increase with the amount of dopant, and no new transitions ascribable to a bipolaronic type of charge carrier are observed. At the first equivalence point (1:1 EPBPE:AgPF6, bold blue trace Figure 3A), the neutral state transitions are only about half-depleted, suggesting the presence of neutral molecules at equilibrium with the polaronic species. Since there is a clear isosbestic point at 650 nm, only these two species are likely present in solution, implying the following reaction: EPBPE + yAg+ → EPBPEy+ + yAg(s). Using the analysis reported by Cao and Curtis,59 the stoichiometric coefficient y can be determined from the change in absorbance of the neutral and oxidized state transitions with addition of dopant (see Supporting Information Figure S4,

Figure 2. Cyclic (solid lines, 25 mV/s scan rate) and differential pulse (dashed lines) voltammograms of the discrete-conjugation molecules and the polymer in 0.1 M TBAPF6 in CH2Cl2 solution.

potentials (Figure S2A, Table 1) likely in order to drive the PF6− counterions to intercalate between interacting chains. The two oxidation steps can only be resolved when CV is done on a pristine film at especially slow scan rates (5 mV s−1) (Figure S1D). With successive scans, the two peaks coalesce into one broad oxidation wave. The polymer film is also stable to repeated cycling, with minimal change in the CV trace over 50 cycles (Figure S1E). Oxidized State Properties. Chemical oxidation was performed on CH2Cl2 solutions of the molecules and polymer at room temperature while minimizing exposure to ambient oxygen with an Ar blanket. The solutions of known concentration and volume were gradually titrated with a soluble dopant. The UV−vis−NIR spectra were obtained against a reference solution similarly titrated with the dopant. Silver hexafluorophosphate (E0 = 0.65 V vs Fc in CH2Cl2) and nitrosonium tetrafluoroborate (E0 = 1.00 V vs Fc in CH2Cl2), with formal potentials at least 200 mV more positive than the second oxidation potential of the small molecules and polymer, are expected to be able to generate their dications.81 Spectroelectrochemistry was also performed with an optically transparent thin-layer electrode (OTTLE) with 0.1 M TBAPF6 in CH2Cl2 electrolyte solution to study the absorption properties of the electrochemically generated oxidized states. E


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Figure 3. Evolution of absorption properties of EPBPE with oxidation. (A) Chemical doping of a 25 μM solution with AgPF6 in CH2Cl2. (B) Electrochemical oxidation of a 30 μM solution in an OTTLE with 0.1 M TBAPF6 in CH2Cl2 (Inset: cyclic voltammogram in OTTLE cell marking the first (blue) and second (red) oxidation steps shown in the absorption spectra). (C) Photographs of neutral and dication solutions from chemical doping. (D) Photographs of OTTLE cell solutions under specified applied potentials.

Table 2. Absorption Maxima of the Oxidized States of the Small Molecules and Polymer Obtained from Chemical and Electrochemical Oxidation chemical doping EPBPE EPPBPPE Poly-EPBPE

polaron

π-dimer

920, 1640 1020, >2000 950, 1650

820, 1370 880, 1680 810, 1240

electrochemical doping bipolaron

polaron 920, 1640

1140 910, ∼1700

π-dimer

bipolaron

820, 1320 945, 1700 730, 1240

1130

for EPBPE (Figure S5B), leading to a decrease in the ΔE1−2 to 100 mV and Kd = 0.02. Thus, in the OTTLE electrochemical cell, disproportionation consumes the radical cations to generate the dication and the neutral molecule,57 leading to the similar oxidation behavior observed for both chemical and electrochemical doping. Comparing EPBPE solutions of different concentration (Figure S6), chemical and electrochemical doping at higher concentration leads to more pronounced absorptions of the higher energy shoulder peaks at 810−820 nm and 1320−1370 nm, especially at oxidation levels beyond the first equivalence point or oxidation wave. The relative contribution from the higher energy shoulders is shown to be much higher than the main polaronic transitions at 920 and 1640 nm when comparing electrochemically generated oxidized species versus chemically generated ones (Figure S7). These observations point to the possibility that the transitions at 820/1370 nm and 810/1320 nm are due to π-aggregates, likely dimers, which result in blue-shifted transitions87 relative to the main polaronic absorptions. The higher concentrations promote interactions between generated oxidized species and can be further enhanced by the presence of PF6− counterions,88,89 which are at much higher concentrations in electrochemical doping (100 mM) versus chemical doping (100 μM). The PF6− counterions

Table S2). An average y value of 1.90 is obtained from AgPF6 and 1.97 from NOBF4. The resulting oxidation equation is thus EPBPE + 2Ag+ → EPBPE2+ + 2Ag(s), indicating that the neutral molecule is converted to the dication state. This may be brought about by the disproportionation reaction 2EPBPE+ → EPBPE2+ + EPBPE, such that upon addition of one equivalent of AgPF6, a mixture of the neutral and dication states is present in solution, leading to the observed transitions at 380/580 nm and 920/1640 nm. The similarity in observed spectra for both chemical and electrochemical doping suggests that the same redox reaction occurs in the OTTLE. The ΔE1−2 can be used to estimate the equilibrium constant for disproportionation from the equation Kd = exp(−nFΔE1−2/RT), where n is the number of electrons and F is Faraday’s constant. While the ΔE1−2 = 250 mV (Kd = 6.0 × 10−5 at room temperature) measured for EPBPE from a standard bulk electrolysis setup does not seem to predict a onestep two-electron oxidation of the molecule, Figure S4A shows that the voltammogram obtained from the OTTLE is different. The OTTLE (Figure S5B) has a different geometry in which there is a significant distance between the working electrode and the counter and reference electrodes that can lead to substantial resistive effects.86 These give rise to an increase in the applied potential necessary to induce the first anodic wave F


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With the results from EPBPE showing that positive charge carriers are predominantly localized in the donor segments, chemical and electrochemical doping of EPPBPPE were also conducted to explore the effect of a more extended donor segment on the optical properties of the oxidized state. The spectral changes accompanying chemical and electrochemical doping are shown in Figure 4 and summarized in Table 2. As with EPBPE, the neutral state transitions at 415 and 588 nm are gradually depleted with addition of AgPF6, while two new transitions appear in the near-IR, one at 1020 nm and another at wavelengths greater than 2000 nm (Figure 4B). There is also a clear isosbestic point at 660 nm (Figure 4B) up to the first equivalence point. The polaronic peaks are red-shifted by at least 100 nm or have energies 0.17 eV below those of EPBPE, due to the added ProDOT ring. These same differences in transition energies of the polaron peaks were observed between bi-EDOT and ter-EDOT by Apperloo et al.52 Thereafter, gradual addition of a second equivalent of AgPF6 leads to the growth of a new peak at 1140 nm, and a new isosbestic point appears at 1415 nm (Figure 4C). The new transition occurs at energies between the two polaronic transitions and can be attributed to a bipolaronic type of charge carrier. These absorption changes with chemical doping lead to an overall change from a dull cyan neutral state to a highly transmissive oxidized state (Figure 4A, inset), with negligible absorptions remaining in the visible region (Figure 4B and C) and an optical contrast at 590 nm of 20% and at 416 nm of 32%. Electrochemical doping in the OTTLE shown in Figure 4D displays the same trends, with dual transitions at 945 nm and ∼1700 nm observed during the first oxidation wave, and a set of transitions centered at 1130 nm during the second wave. The significant blue-shift of the polaronic transitions observed in electrochemical doping relative to chemical doping can again be attributed to π-dimerization, due to the large concentration of PF6− counterions in the electrolyte solution. The small ΔE1−2 (∼100 mV) again indicates a Kd ∼ 0.02 and the likelihood of disproportionation from the neutral molecule to the dication. EPR spectroscopy was also performed on 1 mM solutions of EPPBPPE in the neutral and AgPF6-doped states (Figure S8). In the neutral state, no signal is observed, whereas in the doped states, low intensity signals are observed relative to those for EPBPE at the same concentration. These may be attributed to the dominance of π-dimer species for the radical cation state and the EPR-silent bipolaron in the dication state. The g factors are 2.004−2.005, similar to EPBPE and other dioxythiophenes.52,57,59,73 Scheme 4 shows two of the possible contributing resonance structures of the charge carriers generated from chemical and electrochemical oxidation of EPPBPPE. In the polaron pair structure, the positive charges are predominantly localized in the donor segment, as in EPBPE. The addition of a ProDOT ring leads to the observed stabilization of the polaronic transitions by ∼0.17 eV, similar to the stabilization of the polaronic transitions observed for bi-EDOT and ter-EDOT.52 However, upon further oxidation, the bipolaronic transition observed with its high oscillator strength suggests the extension of the deformation across the entire π-system. The discrete conjugated D−A penta- and hepta-heterocycles discussed above show narrow and well-defined transitions in both the neutral and the oxidized states that are promising for high contrast electrochromics. In order to translate these properties into materials that can be cast from solution as thin films that maintain their mechanical integrity during electro-

are thought to stabilize the dimer interactions by screening Coulombic repulsions between positively charged species.88 To gain further insight into the nature of the oxidized state, EPR spectroscopy was performed on 1 mM solutions of EPBPE in CH2Cl2 in the neutral state, and doped with one and two equivalents of AgPF6 (Figure S8). No signal is observed from the neutral molecule solution, as expected from its closed-shell configuration, while broad signals with minimal fine structure are observed for the doped solutions. The signals are centered at g factors of 2.004−2.006, a value close to many other EDOT-52,59,73 and ProDOT-57 containing oligomers. The elevated g factor value found in dioxythiophenes (relative to the free electron, g = 2.002) has been attributed to the delocalization of the radical species on the oxygen substituents,90 which is likely the case for EPBPE as well. The lack of hyperfine coupling can be attributed to the delocalization of the radical cation between EDOT and ProDOT units and the radical sampling the two different environments of the two heterocycles. A marked decrease in the intensity of the EPR signal is observed when two equivalents of the dopant are added, but it is clear that there is still a significant amount of paramagnetic species in solution. Since the EPR samples are at a concentration 100 times that used for the absorption studies above, it can be expected that π-dimer formation becomes significant. Thus, for the second equivalence point, the decrease in EPR intensity is attributed to the formation of diamagnetic π-dimers, from the interaction of a larger concentration of polaronic species. The dual optical transitions and the singlet character of the oxidized state of EPBPE suggests a dicationic diradical species. In oligothiophenes50,58,91 and oligoProDOTs,57 a polaron pair type of charge carrier has been invoked to explain these observations. Scheme 3 shows such a possible contributing Scheme 3

resonance structure for EPBPE, which allows for the positive charges to be localized in the electron-rich dioxythiophene segments, with the electron-poor BTD ring serving as a conjugation break that prevents extension of the geometric distortion leading to bipolaron formation.60,92 This proposed structure is likely made favorable by S−O interactions that enhance conjugation between dioxythiophene rings65,72,73,93,94 and the higher twist angle between ProDOT and BTD due to steric torsion between the seven-membered ring CH2 units and the phenyl CH and CN ortho substituents46,95,96 that disrupt the conjugation between the donor rings and BTD. G


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Figure 4. Evolution of absorption properties of EPPBPPE with oxidation. (A) Chemical doping with AgPF6 in CH2Cl2 (Inset: photographs of neutral and dication solutions from chemical doping). (B) Difference spectra after subtraction of neutral state absorption, showing polaronic transitions. (C) Difference spectra after subtraction of absorption spectra at the first equivalence point, showing bipolaronic transition. (D) Electrochemical oxidation in an OTTLE with 0.1 M TBAPF6 in CH2Cl2 (Inset: cyclic voltammogram in OTTLE cell marking first (blue) and second (red) oxidation steps shown in absorption spectra).

Scheme 4

H


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The evolution of the polymer’s absorption properties with electrochemical doping was studied on a film of the polymer spray-cast from toluene onto a conductive ITO substrate in 0.5 M TBAPF6 in CH3CN (Figure 5B). The pristine polymer film shows some scattering as evidenced by the nonzero absorption at wavelengths above 750 nm due to the formation of fibrillar structures as determined from microscopy (Figure S10). Upon initial CV cycling (“break-in”) to oxidize the polymer and allow the electrolyte solution to intercalate in the polymer film, irreversible spectral changes occur, with a strong and broad transition evolving at 880 nm. At 300 mV (vs Ag/Ag+), transitions at 910 and 1700 nm (Figure S11A) are prominent and can be attributed to polarons. Increasing the potential to 600 mV causes a blue-shift of these transitions to 730 and 1240 nm (Figure S11B), which may be due to π-dimers. The transitions are significantly broader than those observed in solution, likely due to different stacking interactions leading to different environments for the charge carriers. After full oxidation of the polymer film, the neutral absorptions are not depleted, leading to a blue to gray-blue color change in the polymer film. This irreversibility leads to an inability to repeatedly switch this material between color states, and was similarly observed in the film CV of the polymer, in which two oxidation waves can only be observed in the first CV scan performed at very slow scan rates (Figure S1D). This can be attributed to trapping of holes or a structural relaxation in the polymer film,97 likely promoted by PF6−-stabilized dimer structures.

chromic device operation, a polymer that retains the discrete nature of the conjugated segment was designed. By using an aliphatic n-decyl linker, the EPBPE D−A penta-heterocycle chromophore unit remains isolated along the Poly-EPBPE backbone, thereby generating a mechanically robust polymer with well-defined optoelectronic properties, as shown in Figures 1B and 2. The oxidized state properties of Poly-EPBPE were also studied via chemical doping in CH2Cl2 solution (Figure 5A)

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CONCLUSIONS AND PERSPECTIVE Donor−acceptor small molecules and polymers based on DOTs and an electron-poor heterocycle, BTD, were conveniently synthesized through direct (hetero)arylation methods. Stoichiometric control in this cross coupling method allowed access to D−A−D ter- and penta-heterocycles in high yields from bifunctional heterocycles, eliminating the need for boronic or stannic derivatives that are often difficult to purify and require toxic precursors. The three-, five-, and sevenheterocycle conjugated molecules and monomers were thus obtained in four or fewer steps. All materials gave well-defined dual-band absorptions in the neutral state, due to the D−A charge-transfer interaction, and the π−π* high-energy excitation. The electron donation from the β-oxygens in these also allowed access to the mono- and dication states at low applied voltages. The D−A nature of the materials resulted in low ionization potentials, high electron affinities, and overall low energy gaps. The well-defined π-system of the small molecules allowed analysis of the potential structures of charge carriers in DOTand BTD-containing D−A systems. Oxidized states generated via chemical and electrochemical doping gave rise to structures in which the positive charge is localized in the donor segments, with the BTD ring serving as a conjugation break. In EPBPE, this led to the polaron pair and its π-dimer being the dominant charge carriers at both low and high oxidation states. Both charge carriers had transitions peaking outside the visible range, leading to a purple to transmissive color change. In EPPBPPE, the longer donor segment produced polaron pairs that have significantly red-shifted absorptions, peaking well outside of the visible region. The extended structure also allowed bipolarons to be formed at higher doping levels. Both of the charge carriers observed had absorptions with maxima in the near-IR, leading to a dull cyan to highly transmissive color change. Thus, D−A

Figure 5. Evolution of absorption properties of Poly-EPBPE with oxidation. (A) Chemical doping with AgPF6 in CH2Cl2. (B) Electrochemical oxidation of polymer thin film spray-cast onto a conductive ITO substrate in 0.5 M TBAPF6 in CH3CN (spectra taken at 50 mV steps).

and electrochemical doping in the solid state (Figure 5B), and the transitions are summarized in Table 2. As with EPBPE, the neutral state transitions at 383 and 574 nm gradually decrease, while two new transitions at 950 and 1650 nm grow in at low doping levels. Subsequently, these transitions are superseded by the growth of new transitions at 810 and 1240 nm. Comparison of these transitions with those for the chemical doping of EPBPE (Figure S9) indicate that the transitions at 950 and 1650 nm are due to polaron pairs, whereas those at 810 and 1240 nm are due to their π-dimers. In the polymer, the πdimers are the dominant charge carriers, being prominent even at low doping levels. This can be attributed to the strong tendency of the polymer to π-stack, which is promoted by its rod−coil structure where the flexible linkers can help bring the chromophores into proximity. Overall, these changes result in a transition from a purple neutral state to a transmissive green oxidized state. I


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Chemistry of Materials π-conjugated structures have been identified that can give rise to high contrast electrochromic materials. In order to study the applicability of these D−A molecules as electrochromic materials, a polymer that retains the discrete nature of the π-system by linking these with aliphatic chains was synthesized. Poly-EPBPE was found to have well-defined and narrow transitions in its absorption spectra and resolved oxidation waves, relative to fully conjugated polymers. However, in higher oxidation states, the effect of interchain π−π interactions becomes pronounced. Thus, the polymer oxidized state absorption spectra were dominated by π-dimer transitions. In developing polymeric materials from discrete chromophores, the linkers must thus be designed such that undesired interactions are obstructed, such as by sterically bulky aliphatic moieties. These moieties may also serve to improve the reversibility of electrochemical switching of the polymer film, by preventing trapping of holes. The polymer showed improved mechanical integrity, yielding solution processable films that remained intact in the electrolyte solution in its oxidized state. However, the rod−coil structure of the conjugated segments linked with aliphatic chains gave rise to fibrillar networks that caused scattering. These interactions must be disrupted to increase the amorphous nature and electrochemical switching reversibility of the resulting film.

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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.chemmater.6b04725. Details of synthesis, electrochemistry, spectroscopy, and microscopy (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(J.R.R.) E-mail: [email protected]. ORCID

John R. Reynolds: 0000-0002-7417-4869 Funding

We appreciate funding of this work from the Air Force Office of Scientific Research FA9550-14-1-0271. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Caroline Grand and Keith Johnson for their assistance with the AFM and film spectroelectrochemistry measurements.

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