Highly Stable Indacenodithieno[3,2-b]thiophene-Based Donor–Acceptor Copolymers for Hybrid Electrochromic and Energy Storage Applications

Stable doping of indacenodithieno[3,2-b]thiophene (IDTT) structures enables easy color tuning and significant improvement in the charge storage capacity of electrochromic polymers, making use of their full potential as electrochromic supercapacitors and in other emerging hybrid applications. Here, the IDTT structure is copolymerized with four different donor–acceptor–donor (DAD) units, with subtle changes in their electron-donating and electron-withdrawing characters, so as to obtain four different donor–acceptor copolymers. The polymers attain important form factor requirements for electrochromic supercapacitors: desired switching between achromatic black and transparent states (L*a*b* 45.9, −3.1, −4.2/86.7, −2.2, and −2.7 for PIDTT–TBT), high optical contrast (72% for PIDTT–TBzT), and excellent electrochemical redox stability (Ired/Ioxca. 1.0 for PIDTT–EBE). Poly[indacenodithieno[3,2-b]thiophene-2,8-diyl-alt-4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole-7,7′-diyl] (PIDTT–EBzE) stands out as delivering simultaneously a high contrast (69%) and doping level (>100%) and specific capacitance (260 F g–1). This work introduces IDTT-based polymers as bifunctional electro-optical materials for potential use in color-tailored, color-indicating, and self-regulating smart energy systems.

3,6-Dibromobenzene-1,2-diamine (1): 4,7-Dibromobenzo[c] [1,2,5]thiadiazole (5.867 g, 20.00 mmol) was dissolved in acetic acid (250 mL) and the mixture was heated to 80 °C. Zinc powder (26.152 g, 400.00 mmol) was added in three portions and the mixture was stirred vigorously for 1 h. After cooling, the mixture was filtered to remove the gray solids. The filtrate was poured over water and extracted with DEE (4 × 200 mL), and the combined organic phase was dried over anhydrous MgSO 4 . The solvent was removed until ca. 100 mL of acetic acid was left. This mixture, containing the diamine product, was used directly to the next reaction without further purification.

4,7-Dibromo-1H-benzo[d][1,2,3]triazole (2):
To the mixture of compound 1 in ca. 100 mL acetic acid was added water (100 mL) and THF (200 mL) until all starting material dissolved. The mixture was cooled to 0 °C on an ice bath. NaNO 2 (1.518 g, 22.00 mmol) was dissolved in water (8 mL) and added dropwise to the mixture. After stirring at 0 °C for 15 min, the mixture was heated to 80 °C for 1 h. After cooling, the mixture was poured over water and extracted with DCM (3 × 200 mL). The combined organic phase was washed with water (2 × 200 mL), saturated sodium bicarbonate solution (2 × 200 mL) and again with water (2 × 200 mL), and then dried over anhydrous MgSO 4 . After removing the solvent in vacuum the product was collected as pale brownish solid (4.50 g, 81%). 1   allowed to warm to room temperature overnight. After 20 h, the reaction mixture was concentrated, filtered and purified with column chromatography over silica gel, using hexane as the eluent. The solvent was removed in vacuum and the product was collected as colorless oil (19.28 g, 73%). 1
The tube was sealed and heated at 115 °C on an oil bath. After 38 h, the mixture was cooled, poured over water and extracted with DEE (3 × 100 mL). The combined organic phase was washed with water (3 × 100 mL) and dried over anhydrous MgSO 4 . The solvent was removed and the crude product was purified with column chromatography over silica gel, by gradually increasing the eluent polarity from hexane to  To start the polymerization, Pd 2 (dba) 3 (0.0073 g, 0.008 mmol) and tri(o-tolyl)phosphine (0.0097 g, 0.032 mmol) were dissolved in dry toluene (2 mL) and added to the flask.

Density Functional Theory Calculations
Density functional theory (DFT) calculations were carried out using the Gaussian 16 program (revision B.01). 2 Graphical visualization of the molecular orbitals was done with the Visual Molecular Dynamics 1.9.3 (VMD) software. 3 The geometry optimizations were performed in gas phase using the long-range corrected ωB97XD functional 4 and the 6-31G(d,p) basis set. 5 This method has been demonstrated to accurately describe the doping of π-conjugated systems like oligothiophenes and PEDOT. 6

XPS was conducted on a SPECS XPS instrument (Berlin, Germany). X-ray emission was conducted with
Mg Kα line (12 kV -200 W) anode from an UHV no-monochromatic source. High-resolution scans at a pass energy of 10 eV were recorded after a survey scan for characterizing the chemical states. The excitation energy was 1253.6 eV. 11,12 Electrochemistry was conducted on the same instrument described thiophene-sulphur. 16 The S(2p 3/2 ) peak of neutral PIDTT-TBzT is found at a slightly higher binding energy compared to those of the other three polymers, presumably due to some level of oxidation in its neutral state. This polymer was the least stable candidate in the electrochemical studies. The fitted O(1s) and C(1s) peaks are listed in Table S1. There are no peaks assigned to the fitted O(1s) spectra due to the S28 presence of trapped solvent as well as moisture, which complicates significantly the peak assignment for oxygen in the polymers. The C(1s) spectra show up to five peaks each. A peak at 284.3-284.5 eV is identified as sp 2

Electrochromic Characterization and Electrochemical Impedance Spectroscopy
The electrochromic spectra were measured by combining Agilent Cary 60 UV-Vis spectrophotometer and CH-Instruments 650A Electrochemical Workstation. Multi-potential step electrochemical measurements were performed with a three-electrode setup identical to that described for CV. The polymer films were spin-coated from 10-20 mg mL -1 polymer in chlorobenzene solution and spray-coated from 1-2 mg mL - Ag/Ag + RE was calibrated against the Fc/Fc + redox couple, identically to the CV measurements. The color coordinates were derived from the electrochromic spectra (Figure 4 and S14) using the colormatching functions of the CIE standard illuminant D65, 2-deg observer and 1976 L * a * b * color space in the wavelength range of 360-740 nm ( Figure S15 and Table S3). 21 Color chroma (C * ab ) represents the overall hue of the color, i.e., magnitude of the vector in the (a * b * ) plane. 10 The corresponding transmission spectra in the visible spectral region and pseudo-color of the spectra in RGB color space are shown in Figure S16 and S17. The photographs of the films in Figure S16 were taken in situ under ambient light, using a Nikon D5300 camera with an AF-S DX Micro Nikkor 85 mm f/3.5G ED VR lens.
Coloration efficiency (η, cm 2 C -1 ) was calculated as the charge density consumed per unit of absorption change using Eqn (S2): 22 Where is the change of absorbance of the polymer between its colored and bleached states, and is the redox charge density (mC cm -2 ).

S32
The redox charge density was calculated from cyclic voltammogram of the polymer on ITO-glass using Eqn (S3): 23 Where ∫ ( ) is the integral of the CV trace, is the scan rate (V s -1 ) and is the area of the polymer film (cm 2 ). Two CV scans were applied for each sample and Q was calculated as an average of the anodic charge and cathodic discharge parts of the second CV redox cycle. A scan rate of 0.1 V s -1 was used, since IDTT-based polymers were previously shown to undergo full charge and discharge switching within this range. 10 The volumetric charge (Q v , mC cm -3 ) and capacitance (C v , mF cm -3 ) were calculated using Eqn (S4) and (S5): 24,25 Where is the film thickness (cm) and is the potential range of the CV scan. The potential range was the same as in the kinetic measurements in Figure 5   Before  PIDTT-EBzE films before (filled symbols) and after the long-term kinetics (empty symbols   This work a Not reported. b Estimated from the plot. c Calculated as an average of the reported oxidation (197 cm 2 C -1 ) and reduction (232 cm 2 C -1 ) at the maximum contrast.  Table S4, as referenced therein.