Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay

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Low Bandgap Conjugated Polymers Based on a Nature-Inspired BayAnnulated Indigo (BAI) Acceptor as Stable Electrochromic Materials Bo He, Wei Teng Neo, Teresa L. Chen, Liana M Klivansky, Hongxia Wang, Tianwei Tan, Simon J. Teat, Jianwei Xu, and Yi Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00303 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 6, 2016

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ACS Sustainable Chemistry & Engineering

Low Bandgap Conjugated Polymers Based on a Nature-Inspired

Bay-Annulated

Indigo

(BAI)

Acceptor as Stable Electrochromic Materials Bo He,† Wei Teng Neo,‡,# Teresa L. Chen,† Liana M. Klivansky,† Hongxia Wang,†,$ Tianwei Tan,$ Simon J. Teat,§ Jianwei Xu, ‡,¢* and Yi Liu†* †

The Molecular Foundry and §Advanced Light Source, Lawrence Berkeley National Laboratory,

One Cyclotron Road, Berkeley, CA 94720, USA. Email: [email protected] ‡

Institute of Materials Research and Engineering, 2 Fusionopolis Way, Innovis, #08-03,

Singapore 138634, #NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456, and ¢Department of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543, Republic of Singapore. Email: [email protected] $

Beijing Key Lab of Bioprocess, College of Life Science and Technology, Beijing University of

Chemical Technology, Beijing 100029, China KEYWORDS. Bay-annulated indigo, conjugated polymer, electrochromic, electron acceptor, low bandgap

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ABSTRACT. The donor-acceptor (D–A) approach, which is to incorporate alternating electronrich (donor) and electron-deficient (acceptor) units along the conjugated polymer mainchain, has become an effective method to provide an informed search for high performance electrochromic low bandgap polymers. Herein a potent electron acceptor, namely, a much more soluble version of the nature-inspired bay-annulated indigo (BAI), was employed in the synthesis of two solution-processable donor-acceptor polymers for efficient electrochromic devices (ECDs). The devices fabricated from spin-coated polymer thin films can switch reversibly between deep blue and transmissive light green hues, with high optical contrasts in the visible and near-infrared (NIR) regions, good coloration efficiency and promising ambient stability. In particular, electrochromic devices based on the copolymer containing a carbazole donor unit exhibit optical contrasts of 41% and 59% in the visible and NIR regions, respectively, and a long term stability of more than 7500 cycles under ambient conditions with limited reduction in optical contrasts. Such longer term ambient stability underlines the great potential of BAI-derived electron acceptors for the development of practical EC materials.

INTRODUCTION Conjugated polymers have emerged as a promising class of materials for flexible electronics1,2 on account of their tunable optical, electrochemical and electronic properties. Most conjugated polymers have intense neutral-state colors, corresponding to optical absorptions that fall within the visible and near infrared (NIR) spectrum. In response to an electrical stimulus, the polymers may exhibit distinctive color changes at different redox states, forming the basis of electrochromism.3,4 These polymers hence become particularly intriguing for electrochromic (EC) applications such as smart windows and electrochromic displays.5,6

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The optical properties and redox chemistry of conjugated polymers can be readily tuned by judicious choice of monomeric units. The donor-acceptor (D–A) approach, which is to incorporate alternating electron-rich (donor) and electron-deficient (acceptor) units along the conjugated polymer mainchain,7-9 has become a very effective method to generate low bandgap polymers with tunable colored-to-transmissive electrochromic characteristics.3,4 Along the lines of materials discovery, the search for appropriate pairs of electron donor and acceptor units is one of the most critical steps for better control of key electrochromic parameters including color contrast,10 coloration efficiency,11,12 response time,13,14 and cycle life.15 The most widely used electron donors in EC polymers come from the family of 3,4-alkylene-dioxythiophenes (DOTs).3 In terms of electron acceptors, a range of them, such as benzothiodiazole (BTD),16-19 benzotriazole,20-24 isoindigo,25-30 diketopyrrolopyrrole (DPP),31-38 pyrroloazine dione,39,40 and chalcogenodiazolpyridine41,42 has been utilized to copolymerize with electron donors to give polymers with tunable EC properties. While great strides have been made to achieve high performing EC polymers, a material that can satisfy all the characteristics, particularly long cycle life under ambient conditions, still remains a roadblock for practical EC applications, which necessitates further development of new materials chemistry. We have recently demonstrated bay-annulated indigo (BAI)43 as a potent electron acceptor, which was derived from the naturally occurring indigo dye. The BAI unit has a coplanar diketopiperidinopiperidine core and exhibits deeper lowest unoccupied molecular orbital (LUMO) energy level than some well-known acceptors such as isoindigo and DPP. Such characteristics render BAI a promising sustainable electron acceptor for high performance ambipolar materials with tunable bandgaps. Its use as EC materials, however, has not yet been explored. Here we report the synthesis of two D–A polymers based on a modified BAI electron

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acceptor and their electrochromic characteristics. The two soluble polymers, which employ carbazole and fluorene aromatic systems as the respective electron donor units, display a deep blue hue in their neutral and reduced states, and a light green hue in the oxidized state. The ECDs based on corresponding spun-cast thin films show high optical contrasts in both the visible and NIR regions, and high coloration efficiency. Remarkably, ECDs based on the carbazole copolymer exhibit superior stability under ambient conditions without encapsulation, which is able to remain stable over 7500 cycles with limited loss in optical contrast. EXPERIMENTAL SECTION Materials and methods. Reagents were purchased from Aldrich or synthesized as described. Dry solvents were collected from solvent purification system. Thin-layer chromatography (TLC) was carried out using aluminum sheets, precoated with silica gel 60F (Merck 5554). The plates were inspected by UV-light. Proton and carbon nuclear magnetic resonance spectra (1H-NMR and 13C-NMR) spectra were recorded on a Bruker Avance500 II, using the deuterated solvent as lock and tetramethylsilane as internal standard. All chemical shifts are quoted using the δ scale, and all coupling constants (J) are expressed in Hertz (Hz). Matrix-assisted laser desorption ionization (MALDI) mass spectra were measured on 4800 MALDI TOF/TOF analyzer from Applied Biosystems. UV-Vis-NIR spectra were recorded using a Shimadzu UV3600 UV-visNIR spectrophotometer. Thermal properties were recorded by using Q5000 Thermal Gravity Analysis (TGA). Film Thickness was measured using a KLA Tencor P16 surface profilometer. Cyclic voltammetry was performed using a 273A potentiostat (Princeton Applied Research), wherein glassy carbon, platinum and a silver wire act as the working electrode, the counter electrode and the pseudo-reference electrode, respectively. Samples were prepared in CHCl3 solution with tetrabutylammonium hexafluorophosphate (0.1 M) as the electrolyte at a scan rate 4


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of 100 mV s-1, using ferrocene/ferronium (Fc/Fc+) redox couple as an internal standard. All electrochromic

studies

were

performed

in-situ,

using

both

the

potentiostat

and

spectrophotometer. The 2-electrode configuration was employed, with the reference and counter electrodes shorted. The HOMO and LUMO levels of compounds are calculated from the difference between the first oxidation potential (Eoxi) or reduction potential (Ered) of the compounds and the oxidation potential of ferrocene (ELUMO = -(E + 4.8) eV).44 Material synthesis. 2-Ethylhexyl bromide, 2-thienylacetyl chloride, 1, 6 and 7 were purchased from Sigma-Alidrich and used without purification. 245 was synthesized according to literature procedures. 3, 4, 5, PF-EHBAI and PCz-EHBAI were synthesized as described in the following synthetic procedures. 5-((2-ethylhexyl)oxy)-2-nitrobenzaldehyde (2). A mixture of 1 (15.0 g, 89.8 mmol, 1.0 equiv.), 2-ethylhexyl bromide (26.0 g, 135 mmol, 1.5 equiv.), K2CO3 (37.2 g, 269 mmol, 3.0 equiv.) and 18-crown-6 (0.2 g, catalytic amount) in DMF (200 mL) was stirred at 80 oC for 24 hrs. After the reaction was cooled down to room temperature, excess K2CO3 was filtered off and the solvent was removed by rotary evaporation under reduced pressure. The residue was subjected to silica gel chromatography column using CHCl3/hexanes (1:2) as the eluent to give 2 as a liquid (22.6 g, 80.9 mmol, yield: 91%). 1H NMR (CDCl3, 500 MHz): δ = 10.52 (s, 1H), 8.18 (d, J = 9.0 Hz, 2H), 7.34 (d, J = 2.8 Hz, 1H), 7.16 (dd, J = 9.0 Hz, 2.8 Hz, 1H), 4.0 (m, 2H), 1.80 (m, 1H), 1.431.53 (m, 4H), 1.35 (m.4H), 0.97 (m, 6H). 13C NMR (CDCl3, 125 MHz): δ = 188.73, 163.90, 141.97, 134.39, 127.27, 118.88, 113.76, 71.80, 39.17, 30.34, 28.99, 23.71, 22.98, 14.06, 11.06. MS (MALDI-TOF) for C15H21NO4: [M+Na]+, calcd: 302.14, found: 302.17.

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(E)-5,5’-bis((2-ethylhexyl)oxy)-[2,2’-bisindolinylidene]-3,3’-dione (3). To a solution of 2 (5.00 g, 17.9 mmol, 1.0 equiv.) in acetone (25 mL) and H2O (25 mL) was added a solution of NaOH (787 mg, 19.7 mmol, 1.1 equiv.) in acetone (5 mL) and H2O (5 mL) at 60 oC with stirring. The color of the reaction mixture first turned to red and then green. The reaction was cooled down after 4 hrs, and the blue precipitate was collected by filtration, followed by rinsing with water. Indigo 3 was obtained as a blue solid (1.40 g, 2.80 mmol, 30%). 1H NMR (CDCl3, 500 MHz): δ = 8.74 (s, 2H), 7.21 (d, J = 2.5 Hz, 2H), 7.15 (dd, J = 8.7 Hz, 2.5 Hz, 2H), 6.98 (d, J = 8.7 Hz, 2H), 3.86 (m, 4H), 1.75 (m, 2H), 1.42-1.53 (m, 8H), 1.35 (m, 8H), 0.96 (m, 12H). 13C NMR (CDCl3, 125 MHz): δ = 188.62, 154.37, 146.81, 125.85, 122.49, 120.23, 113.11, 106.58, 71.38, 39.36, 30.51, 29.07, 23.85, 23.07, 14.11, 11.10. MS (MALDI-TOF) for C32H42N2O4: [M]+, calcd: 518.31, found: 518.43. 2,9-bis((2-ethylhexyl)oxy)-7,14-di(thiophen-2-yl)diindolo-

[3,2,1-de:3’,2’,1’-

ij][1,5]naphthyridine-6,13-dione (4). To a refluxing xylene (40 mL) solution of 4 (2.00 g, 3.86 mmol, 1.0 equiv.) was added 2-thienylacetyl chloride (2.48 g, 1.91 mL, 15.4 mmol, 4.0 equiv.) dropwise under N2 flow. The mixture was refluxed for 1 hr, cooled down to room temperature and concentrated by rotary evaporation under reduced pressure. The residue was precipitated in acetone (50 mL) to give a precipitate, which was collected by filtration and washed with acetone to give compound 4 as a red solid (1.40 g, 1.92 mmol, 46%). 1H NMR (CDCl3, 500 MHz): δ = 8.45 (d, J = 8.9 Hz, 2H), 7.78 (dd, J = 3.6 Hz, 1.0 Hz, 2H), 7.72 (dd, J = 5.1 Hz, 1.0 Hz, 2H), 7.69 (d, J = 2.5 Hz, 2H), 7.13-7.30 (m, 2H), 7.12 (d, J = 2.5 Hz, 2H), 3.86 (d, J = 5.6 Hz, 4H), 1.75 (m, 2H), 1.42-1.53 (m, 8H), 1.35 (m, 8H), 0.96 (m, 12H). 13C NMR (CDCl3, 125 MHz): δ = 158.41, 157.89, 137.92, 134.74, 130.19, 130.14, 129.95, 127.04, 126.28, 124.96, 122.39,

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118.28, 118.22, 110.92. MS (MALDI-TOF) for C44H46N2O4S2: [M]+, calcd: 730.29, found: 730.45. 7,14-bis(5-bromothiophen-2-yl)-2,9-bis((2-ethylhexyl)

oxy)-diindolo[3,2,1-de:3’,2’,1’-

ij][1,5]naphthyridine-6,13-dione (5). 4 (600 mg, 0.82 mmol, 1.0 equiv.) was mixed with NBS (307 mg, 1.72 mmol, 2.1 equiv.) in CHCl3 (50 mL) at room temperature. After the reaction was done, as monitored by thin layer chromatography, CHCl3 was removed by rotary evaporation to give a residue, which was precipitated and rinsed with acetone. Pure monomer 5 was obtained as a purple solid after recrystallization from cyclohexane (505 mg, 0.68 mmol, 84%). 1H NMR (CDCl3, 500 MHz): δ = 8.35 (d, J = 8.9 Hz, 2H), 7.69 (d, J = 2.5 Hz, 2H), 7.56 (d, J = 3.9 Hz, 2H), 7.23 (d, J = 4.0 Hz, 2H), 7.092 (dd, J = 8.9 Hz, 2.5 Hz, 2H), 3.86 (d, J = 5.6 Hz, 4H), 1.77 (m, 2H), 1.42-1.53 (m, 8H), 1.35 (m, 8H), 0.95 (m, 12H). 13C NMR (CDCl3, 125 MHz): δ = 158.02, 157.88, 137.52, 136.50, 129.87, 129.07, 128.93, 126.71, 124.26, 122.28, 118.62, 118.19, 118.13, 110.93, 71.45, 39.28, 30.45, 29.08, 23.79, 23.08, 14.14, 11.12. MS (MALDI-TOF) for C44H44Br2N2O4S2: [M]+, calcd: 886.11, found: 886.33. General procedures for polymerization. A deoxygenated mixture of bromide 5 (200 mg, 176 μmol, 1.0 equiv.), carbazole or fluorene boronic acid ester (176 μmol, 1.0 equiv), K2CO3 (415 mg, 3.00 mmol, 1.0 M in H2O, 17 equiv.) and Pd(PPh3)4 (4.08 mg, 3.53 μmol, 0.02 equiv.) in toluene (10 mL) was vigorously stirred at 85 oC. After 72 hrs, bromobenzene (4 μL, 0.03 mmol) was added into the reaction mixture. After another three hours, phenylboronic acid (4.3 mg, 0.03 mmol) was added and the reaction mixture was refluxed overnight to complete the end-capping reaction. After the reaction mixture was cooled down to room temperature, the mixture was partitioned between water and CHCl3. Combined and concentrated CHCl3 fractions were concentrated to a smaller volume, to which methanol was added to give a precipitate. The 7



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precipitate was collected by vacuum filtration, and was subjected to consecutive Soxhlet extraction with acetone, hexanes and chloroform, respectively. The chloroform fraction was concentrated under reduced pressure and purified by passing through a silica gel column with chloroform as eluent. The received solution was concentrated under reduced pressure and precipitated in acetone to give the desired polymers. The polymer collected from filtration was dried and stored in desiccators under vacuum. PF-EHBAI: Mn=5000, Mw=13800, PDI=2.76; PCz-EHBAI: Mn=8800, Mw=25800, PDI=2.92. Despite good solution in CDCl3, the 1H NMR spectra of these polymers are featureless due to strong aggregation. Electrochromic Device Fabrication. ITO-coated glass substrates (15 Ω/sq, 35×30×1.1mm) were purchased from Xinyan Technology Ltd. ITO/glass substrates were cleaned by successive ultrasonication in acetone, isopropyl alcohol and distilled water, and blown dry with N2 prior to use. Polymer solutions (10 mg/mL in 1:3 (v/v) chloroform:chlorobenzene) were filtered and spin-coated onto the ITO substrates at 300 rpm for 60 s, followed by 1000 rpm for 15 s to yield film thicknesses of around 165 and 155 nm for PCz-EHBAI and PF-EHBAI respectively. Excessive polymer edges were removed by swabbing with chloroform using a cotton bud to obtain an active area of 2×2 cm2. On a second piece of ITO substrate, an area of 2×2 cm2 was blocked out using parafilm. The total thickness of the parafilm spacer and barrier was kept constant at 0.01”. 250 μL of the gel electrolyte (0.512 g of lithium perchlorate and 2.8 g of poly(methyl methacrylate) (MW = 120 000 g/mol) in 6.65 ml of propylene carbonate and 28 ml of dry acetonitrile) was pipetted within the 2×2 cm2 area and left to dry for 5 minutes. The device was fabricated by assembling the two ITO/glass substrates together with the polymer film and gel electrolyte in contact.

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RESULTS AND DISCUSSION Scheme 1. Synthesis of the EHBAI acceptor and the related donor-acceptor polymers

OR RBr K 2CO3

OH

OR

oC

O 2N

1

CHO

80 91% O 2N

2

O

NaOH / Acetone RO

CHO

Reflux 30%

O H N

Cl

N

OR

3 H O

S

Xylene Reflux

O

B O

O

C8H17

N S O

O S

N

n C8H17

C8H17

PF-EHBAI

OR

O S

84%

O

4 (EHBAI)

RO

6

K 2CO3 / Pd(PPh 3) 4 Toluene / H 2O / 85 oC

RO

S

C8H17

O B O

S

N

RO

N

NBS / CHCl3

N

46%

R=

Br

O

S

Br

N

5 OR

O

B O

O B O

N C8H17

RO

7 N

C8H17 S

K 2CO3 / Pd(PPh 3) 4 Toluene / H 2O / 85 oC

O

O S N

N C8H17 OR

n C8H17

PCz-EHBAI

Materials Synthesis. The original unsubstituted BAI core has a coplanar core and two flanking thiophene units, and has limited solubility that impedes the direct use of its bromide derivatives for polymerization. In order to enhance the solubility, we have modified the structure by introducing two 2-ethylhexyloxy substituents onto the 5,5’-position of the phenyl groups of indigo derivative 3 (Scheme 1), which was synthesized in two steps from readily available precursors. 4 (EHBAI) was obtained from the condensation reaction between the substituted indigo 3 and 2-thienylacetyl chloride in refluxing xylene.46,47 The resulting EHBAI was then

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treated with N-bromosuccinimide (NBS) in CHCl3 to give the corresponding dibromide 5. Both 4 and 5 are highly soluble in common organic solvents. The planar structure of the BAI core was revealed for the first time in the single crystal X-ray structure of bromo-EHBAI (5) (Figure 1).48 Two of the flanking thiophene units were slightly twisted out of the plane of the central BAI unit (Figure 1b), while the two C-Br bonds were arranged anti-parallel with a torsion angle of 180°. These molecules stacked into columns with slipped stack geometry (Figure 1c), which were further organized into a herringbone-like intercolumnar arrangement. In each individual column, an interplanar distance of 3.30 Å could be identified between BAI planes in keeping with strong π-π stacking interactions. Interestingly, another π-π stacking with a slightly larger interplanar distance of 3.61 Å could be identified between thiophene units from adjacent columns. The two types of π-stacking interactions, together with the antiparallel arrangement of bromothiophenes, bodes well for the facile incorporation of this BAI building block in copolymers with preferred chain geometry and interchain interactions. Indeed, subsequent copolymerization with fluorene or carbazole based diboronate ester gave the desired donor-acceptor polymers PF-EHBAI and PCz-EHBAI, respectively. The resulting polymers were readily soluble in common organic solvents such as chloroform and chlorobenzene. Both polymers exhibited high thermal stability, with decomposition temperatures around 410 °C at 5% weight loss (Figure S1 in Supporting Information, SI).

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Figure 1. Stick representation of single crystal X-ray structure of 5. (a) top view, (b) side view, and (c) view of slipped columnar packing, highlighting BAI-BAI and thiophene-thiophene π-π stacking interactions. Carbon, gray; Oxygen, red; Nitrogen, blue. Bromothiophene units are colored in gold. Disorder in one of the side chains and the side chains in (c) were omitted for clarity. Optical and Electrochemical Properties. Optical absorption properties of EHBAI and the polymers were evaluated both in dilute solutions and as thin films (Figure 2), and the relevant data were summarized in Table 1. Similar to the unsubstituted BAI,43 EHBAI showed two characteristic absorption peaks at 547 and 584 nm, the first corresponding to π-π* transition and the latter to intramolecular charge-transfer (ICT). The double-peak feature disappeared in the spectra of the polymers, and instead merged into a broad peak centered at 685 nm and 691 nm for PF-EHBAI and PCz-EHBAI, respectively. In the spun-cast thin films, the absorption peaks 11



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experienced bathochromic shift to 692 nm and 699 nm for PF-EHBAI and PCz-EHBAI respectively. The small red shifts compared to solution absorption are indicative of weak aggregation and chain packing in the thin film, which can be attributed to non-planarity of polymer main chains caused by the long branching side chains attached to carbazole and fluorene units in the polymers. From the lower energy absorption edges of the thin films, optical bandgaps of 1.54 and 1.52 eV were estimated for PF-EHBAI and PCz-EHBAI, respectively (Table 1).

Figure 2. Normalized UV-vis absorption spectra of EHBAI, PF-EHBAI and PCz-EHBAI in CHCl3 solution (solid lines) and as thin films (dashed lines).

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Table 1. Summary of optical and electrochemical parameters of EHBAI and related polymers UV-vis solutiona

Cyclic voltammetry film

Compounds

b

λmax λonset Eg

c

λmax

ε

λonset Eg

(nm)

(M-1cm-1)

(nm) (eV)

(nm) (nm) (eV)

EHBAI

584

182500

622 1.99

PF-EHBAI

685

279100

PCz-EHBAI 691

244100

EHOMO ELUMO Egelec,d (eV)

(eV)

(eV)

621 689 1.80

-5.40

-3.58

1.82

786 1.58

699 806 1.54

-5.31

-3.57

1.74

791 1.57

692 814 1.52

-5.21

-3.57

1.64

a

in CHCl3. bsolution optical bandgap. cthin film optical bandgap. dsolution electrochemical bandgap. The electrochemical properties of BAI polymers were investigated (Figure S2 in SI) by cyclic voltammetry (CV) using conventional three-electrode setup and ferrocene/ferrocenium (Fc/Fc+) redox couple as the internal reference, from which frontier orbital energies were estimated. The LUMO levels of both polymers and EHBAI are very similar, all situated at around -3.57 eV. The highest occupied molecular orbitals (HOMO) energy level of PF-EHBAI is -5.31 eV, which is about 0.10 eV lower than that of PCz-EHBAI, correlating with the less electron rich nature of the fluorene unit compared to carbazole (see Table 1). Electrochromic Device Performance. The electrochromic behavior of the polymers was studied by recording the UV-visible-NIR absorption spectra upon progressive oxidation from their neutral states to 2.2 V. The spectroelectrochemical spectra of the polymers are illustrated in Figure 3. At the operating potential of approximately 1.1 and 1.3 V for PCz-EHBAI and PFEHBAI respectively, the absorption peaks corresponding to π-π* transition start to deplete with

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0.9

(a)

0.0V

0.8

0.0V 0.8V 1.0V 1.1V 1.2V 1.3V 1.4V 1.5V 1.6V 1.7V 1.8V 1.9V 2.0V 2.1V 2.2V

Absorbance

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400

600

800

1000

1200

1400

1600

1800

1.5V

1.8V

2.0V

Wavelength (nm) 0.7

(b)

0.0V

0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0V 0.8V 1.0V 1.1V 1.2V 1.3V 1.4V 1.5V 1.6V 1.7V 1.8V 1.9V 2.0V 2.1V 2.2V

0.5 0.4 0.3 0.2 0.1 0.0 400

600

800

1000

1200

1400

1600

1800

Wavelength (nm)

1.5V

1.8V

2.0V

Figure 3. Spectroelectrochemical graphs and colors of (a) PCz-EHBAI and (b) PF-EHBAI devices at various applied potentials. a concomitant formation of a broad band in the NIR region (Figure 3). The lower potential onset for PCz-EHBAI suggests that electrochemical oxidation occurs more readily for the polymer in

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comparison to PF-EHBAI, which agrees with the shallower HOMO of PCz-EHBAI. For both polymers, the polaron absorption occurs at around 1060 to 1100 nm and peaks at 1.8 V. Upon further oxidation, the polaronic band begins to drop in intensity, together with slight hypsochromic shifts. Interestingly, above the threshold potential of around 1.7 V, increased absorption intensity coupled with a slight hypsochromic shift can be observed for the higher energy absorption peaks at 375 and 369 nm for PCz-EHBAI and PF-EHBAI respectively. These spectroscopic changes could be ascribed to the formation of bipolarons. The residual absorptions in the visible region lead to a light green hue for both polymers in their fully oxidized states (Figure 3). Square-wave potential step absorptiometry was utilized to characterize the degree of transmittance changes of the polymers in both the visible and NIR regions. This measurement recorded in-situ transmittance changes of the polymers during redox switching between +1.8 and -1.8 V. The optical contrasts refer to the absolute transmittance difference between the oxidized and reduced states, while switching time is the time it takes to reach 95 % of the full switch. In the visible region, the bleaching of the polymer corresponds to the oxidation process while the coloration corresponds to the reduction process . In the NIR region, the processes are reversed. The results are shown in Figure 4 and summarized in Table 2. In general, PCz-EHBAI reveals higher optical contrasts of about 40.8 and 58.7 % in the visible and NIR regions respectively. Moreover, faster oxidation speeds are also observed for PCz-EHBAI. This may perhaps be related to its increased susceptibility towards oxidation, as compared to PF-EHBAI. Coloration efficiencies (CE) were also measured at 95% of the full switch in both the visible and NIR regions, as an approximation for the power consumption for the operation of such electrochromic devices. CEs were calculated based on the equation:

15



CE = [log (Tox/Tred)]/(q/A)

(1)

where Tox, Tred, q and A refer to the transmittance of the polymer in its oxidized state, transmittance of the polymer in its reduced state, charge consumed, and area of the active layer, respectively. Generally, PCz-EHBAI exhibits higher CEs compared to PF-EHBAI, which suggests lower power consumption.

(c) 100

95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10

cycles 1-5 cycles 996-1000

90

PCz-EHBAI 668nm PCz-EHBAI 1100nm

Transmittance (%)

Transmittance (%)

(a)

80 70 60 50 40 30

0

50

100

150

0

200

50

95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10

(d)

105

150

200

cycles 1-5 cycles 796-800

100

Transmittance (%)

(b)

100

Time (s)

Time (s)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PF-EHBAI 654nm PF-EHBAI 1060nm

95 90 85 80 75 70 65 60

0

50

100

150

0

200

Time (s)

50

100

150

200

Time (s)

Figure 4. Switching cycles of (a) PCz-EHBAI and (b) PF-EHBAI devices in the visible (λmax) and NIR regions between +1.8 and -1.8 V, and ambient stability of (c) PCz-EHBAI and (d) PFEHBAI devices switched at 18 s cycles between +1.8 and -1.8 V in the NIR region at 1100 and 1060 nm, respectively.

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Table 2. Summary of Electrochromic Performance of ECDs. Visible (λmax) Polymer

NIR

Contrast%

τb/sa

τc/sb

CE/cm2 C-1 c

Contrast%

τb/sa

τc/sb

CE/cm2 C-1 c

PCzEHBAI

40.8

52.54

0.76

521

58.7

1.01

14.05

451

PF-EHBAI

30.2

69.16

0.46

494

52.0

0.58

34.88

279

a

Bleaching time where bleaching refers to the process in which the percent transmittance changes from a lower value to a higher value; bColoration time where coloration refers to the process in which the percent transmittance changes from a higher value to a lower value; c Coloration efficiency. Another parameter of practical importance for polymer-based ECDs is their cycle life. In this work, the stabilities of the devices were investigated by subjecting the cells to repeated redox cycling between +1.8 and –1.8 V with a residence time of 18 s while monitoring the changes in transmittance (Figure 4c and 4d). PCz-EHBAI was found to be able to sustain around 1000 redox cycles before a loss of 20 % in optical contrast. On the other hand, PF-EHBAI displays a cycle life of around 800 cycles after a ‘break-in’ period of 40 cycles was allowed, before exhibiting a loss of 15% in optical contrast. The high redox stability of PCz-EHBAI prompted a long-term stability testing. The device was cycled between +1.8 and –1.8 V at 1100 nm at a residence time of 10 s, over the course of approximately 2 weeks. An equilibrium period of 100 cycles was allowed for the device. Over the entire duration, the device was stored under ambient conditions without any additional encapsulation and no extra efforts were made to exclude oxygen or moisture from the air. The degradation profile of the device over 10000 deep potential steps is illustrated in Figure 5. It can be observed that the optical contrast remains relatively consistent for more than 7500 repeated cycles. Beyond the threshold of approximately 7600 cycles, the device begins to degrade significantly. While the exact nature of the degradation

17



behavior is beyond the scope of this study, we note that similar degradation behavior has been reported by Reynold and coworkers,49 and has been attributed to the loss of physical contract between the substrate and the polymer. In the reported case, sudden degradation occurs after

Transmittance (%)

around 2300 cycles.

110

110

100

100

90

90

80

80

70

70

60

Tox

60

50

Tred

50

40

Tox-Tred

40

30

30

20

20

10

10

0 0

Δ %Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1000 2000 3000 4000 5000 6000 7000 8000 900010000

Cycle No. Figure 5. Long-term ambient stability testing of PCz-EHBAI device switched at 10 s cycles between +1.8 and -1.8 V at 1100 nm for 10000 cycles after equilibrium (equilibrium: 100 cycles). The good stabilities in both EHBAI polymers suggest that BAI derivatives are viable building blocks for highly stable EC materials. In particular, PCz-EHBAI displays even more superior performance than PF-EHBAI. The general stability of EHBAI polymers, and the stability difference between PCz-EHBAI and PF-EHBAI could be understood from the

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respective polaron/bipolaron structures in Scheme 2 (only polaron structures were shown here). In both polymers, the EHBAI unit can stabilize the generated polarons (from one electron oxidation) by keeping a conjugated structure through resonating with the quinoidal structures of thiophene, carbazole or fluorene. Moreover, the nitrogen atom in carbazole unit can accommodate partial positive charges through electron delocalization, thus effectively further stabilizing the polaron (Scheme 2a). In contrast, there is no such extra stabilization from the fluorene copolymer (Scheme 2b). Scheme 2. Simplified illustrations of polaron formation and resonating structures in segments of (a) PCz-EHBAI and (b) PF-EHBAI.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RO

(a)

RO O

N

S

S

S O

N –e

N

N C8H17

O

N

N

C8H17

C8H17 OR

RO

RO O

N

S

S

S

N

N

O

N

S

S

S

O

N

N

C8H17

C8H17

C8H17

OR

C8H17

OR RO

RO (b) N

N

O

S

S

S O

S

S

Oxidation

C8H17

O

S

OR

O

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N

–e Oxidation

C8H17

C8H17

O

S

S

S O

N

C8H17

C8H17

OR

OR RO

N

O

S

S

S O

N

C8H17

C8H17

OR

CONCLUSIONS In summary, low bandgap polymers based on a soluble BAI electron acceptor have been synthesized. The resulting polymers have very good solubility in common organic solvents, and absorb strongly in the visible region with bandgaps between 1.5 and 1.8 eV. The EC devices based on these polymers can switch reversibly between deep blue and light green hues, with high optical contrasts in both the visible and NIR regions, high coloration efficiency and excellent ambient stability. In particular, ECDs based on the copolymer containing a carbazole donor unit exhibits optical contrasts of 41% in the visible and 59% in the NIR region, together with a long

20


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term stability of more than 7500 cycles under ambient conditions with limited reduction in optical contrasts. It should be noted that with different device components and testing conditions (potential range, potential duration, film thickness etc.) employed across research groups, direct comparison of the stability may be difficult.4 In most cases, reported lifetimes range between hundreds to several thousands. By using different device fabrication methods and optimized operation conditions, the EC lifetime in these polymer-based devices may greatly exceed the measured value here. Moreover, there has been no report on indigo and similar derivatives incorporated into polymers as electrochromic materials for direct comparison. The current study clearly suggests that BAI is a potential electron-accepting moiety for the preparation of low bandgap polymers with high EC performance, especially in terms of long cycle life. Future modification of the polymer structures by introducing DOT based derivatives are anticipated to further optimize the optical properties and electrochromic parameters, thus holding great promises for the development of EC materials with practical applications. ASSOCIATED CONTENT Supporting Information. 1H and 13C NMR spectroscopy. Single crystal X-ray information and crystal information file. TGA, CV and long term stability data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors [email protected], [email protected]. Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources US Department of Energy, Basic Energy Sciences ACKNOWLEDGMENT The materials synthesis and characterization was performed at the Molecular Foundry and Advanced Light Source, and was partly supported by Self-Assembly of Organic/Inorganic Nanocomposite Materials program, all supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. JX and WTN were supported by the Agency for Science, Technology and Research (A* STAR) and Ministry of National Development (MND) Green Building Joint Grant (No. 1321760011), Singapore. REFERENCES (1) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Recent Progress in Organic Electronics: Materials, Devices, and Processes. Chem. Mater. 2004, 16, 4413-4422. (2) Facchetti, A. Organic Semiconductors: Made to Order. Nat. Mater. 2013, 12, 598-600. (3) Beaujuge, P. M.; Ellinger, S.; Reynolds, J. R. The Donor-Acceptor Approach Allows a Black-to-Transmissive Switching Polymeric Electrochrome. Nat. Mater. 2008, 7, 795-799.

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(45) Jones, G. H.; Venuti, M. C.; Alvarez, R.; Bruno, J. J.; Berks, A. H.; Prince, A. Inhibitors of Cyclic Amp Phosphodiesterase. 1. Analogs of Cilostamide and Anagrelide. J. Med. Chem. 1987, 30, 295-303. (46) Similar substituted BAI was recently reported by Bronstein and coworkers using a different synthetic approach. See reference 43. (47) Fallon, K. J.; Wijeyasinghe, N.; Yaacobi-Gross, N.; Ashraf, R. S.; Freeman, D. M. E.; Palgrave, R. G.; Al-Hashimi, M.; Marks, T. J.; McCulloch, I.; Anthopoulos, T. D.; Bronstein, H. A Nature-Inspired Conjugated Polymer for High Performance Transistors and Solar Cells. Macromolecules 2015, 48, 5148-5154. (48) The single crystals suitable for X-ray analysis were grown by diffusing Acetone to the corresponding chloroform solution. (49) Beaujuge, P. M.; Vasilyeva, S. V.; Ellinger, S.; McCarley, T. D.; Reynolds, J. R. Unsaturated Linkages in Dioxythiophene−Benzothiadiazole Donor−Acceptor Electrochromic Polymers: The Key Role of Conformational Freedom. Macromolecules 2009, 42, 3694-3706.

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For Table of Contents Use Only

Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable Electrochromic Materials Bo He, Wei Teng Neo, Teresa L. Chen, Liana M. Klivansky, Hongxia Wang, Tianwei Tan, Simon J. Teat, Jianwei Xu, * and Yi Liu* Synopsis: Naturally occurring indigo derives potent electron accepting building blocks for highly stable electrochromic materials

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TOC 99x44mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1 137x86mm (300 x 300 DPI)


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Figure 1 176x140mm (300 x 300 DPI)



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Scheme 2 314x302mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 2 173x141mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig 3 160x192mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 265x215mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5 225x176mm (300 x 300 DPI)


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Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay

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