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. 2023 Feb 17;3(2):153–160. doi: 10.1021/acsnanoscienceau.2c00049

Electrically Conductive Carbazole and Thienoisoindigo-Based COFs Showing Fast and Stable Electrochromism

Katharina Muggli †, Laura Spies †, Derya Bessinger †, Florian Auras ‡, Thomas Bein †,*
PMCID: PMC10119976  PMID: 37096229

Abstract

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Thienothiophene thienoisoindigo (ttTII)-based covalent organic frameworks (COFs) have been shown to offer low band gaps and intriguing optical and electrochromic properties. So far, only one tetragonal thienothiophene thienoisoindigo-based COF has been reported showing stable and fast electrochromism and good coloration efficiencies. We have developed two novel COFs using this versatile and nearly linear ttTII building block in a tetragonal and a hexagonal framework geometry to demonstrate their attractive features for optoelectronic applications of thienoisoindigo-based COFs. Both COFs exhibit good electrical conductivities, show promising optical absorption features, are redox-active, and exhibit a strong electrochromic behavior when applying an external electrical stimulus, shifting the optical absorption even farther into the NIR region of the electromagnetic spectrum and achieving absorbance changes of up to 2.5 OD. Cycle-stable cyclic voltammograms with distinct oxidation and reduction waves reveal excellent reversibility and electrochromic switching over 200 cycles and confirm the high stability of the frameworks. Furthermore, high coloration efficiencies in the NIR region and fast switching speeds for coloration/decoloration as fast as 0.75 s/0.37 s for the Cz-ttTII COF and 0.61 s/0.29 s for the TAPB-ttTII COF at 550 nm excitation were observed, outperforming many known electrochromic materials, and offering options for a great variety of applications, such as stimuli-responsive coatings, optical information processing, or thermal control.

Keywords: Absorption, Covalent organic frameworks, Electrical conductivity, Electrochromism, Oxidation, Redox reactions, Thin films

Introduction

Covalent organic frameworks (COFs) constructed from organic building blocks represent a highly ordered and porous group of materials offering large internal surface areas and high crystallinity.1−5 So far, COFs have demonstrated intriguing and highly promising characteristics for applications in different fields, including photocatalysis6−8 or gas storage and separation.9−11 After metal organic frameworks (MOFs) had already exhibited good electrochromic properties,12,13 the ever-expanding scope of covalent organic frameworks has now also provided access to their use in this optoelectronic field, with visible and near-infrared (NIR) electrochromic COFs being promising candidates for modern applications, such as in smart windows, in stimuli-response materials, or as a molecular logic gate.14−17 Here, electrochromism describes the phenomenon of an electrically induced reversible color change of a given material.

A high degree of order and continuous electronic connectivity of the π-conjugated COF materials represent ideal prerequisites for a good and stable electrochromism. To date, an increasing number of COFs with high intrinsic electrical conductivities have been reported.18−21

Thienoisoindigo, a thiophene-analogue of isoindigo, represents an electron-deficient moiety commonly used in molecular donor–acceptor designs for promoting enhanced optical, electronic, and photophysical properties of the corresponding polymers.22−25 In this context, thienothiophene thienoisoindigo (ttTII)-based COFs have proven to show intriguing electrochromic properties with high stabilities, remarkable coloration efficiencies, fast switching speeds, and fully reversible vis–NIR absorption changes.14

Taking advantage of the modular concept of reticular chemistry, here we expanded the group of two-dimensional electrochromic ttTII-based COFs and covalently connected the versatile linear building block with promising counterparts: A functionalized biscarbazole-based building block (Cz) was used for the synthesis of the tetragonal Cz-ttTII COF, and the established trigonal 1,3,5-tris(4-aminophenyl)benzene (TAPB) moiety was employed for the generation of the hexagonal TAPB-ttTII COF.

The well-known carbazole moiety has already proven of value in host materials of optoelectronic devices or in organic light emitting diodes (OLEDs),26−29 as well as hole transporting materials.30 Recently, carbazole-based building blocks have been integrated into crystalline COF materials and showed their strong performance in various application fields, e.g., for sensing,31−33 in catalysis,7 or as a supercapacitor.34

Likewise, the trigonal TAPB building block has been incorporated in several COF systems, which were utilized as cathode materials in electrochemical applications35,36 and have been shown to offer good characteristics in electrochemical sensors.37,38

We will show that the newly developed ttTII-based COFs meet important criteria that render these materials suitable for high-performing electrochromism.

COF Synthesis

For the construction of ttTII-based covalent organic frameworks, we designed, on the one hand, a functionalized biscarbazole-based, tetragonal building block (Cz), with carbazole molecules interconnected by a linking phenylene moiety. The single crystal structure data of a biscarbazole-phenylene derivative exhibits a virtual coplanarity of the carbazole units,39 which may be favorable for a close slip-stacked aggregation of a building block with a similar molecular design. Here, a slip-stacked packing was already reported for the tetragonal pyrene counterpart that produced various highly ordered COFs,40 among others the previously published Py-ttTII.14 Therefore, we anticipated that a building block with an analogous stacking behavior, directing the slip-stacked packing of 2D COF layers, would promote crystalline growth. On the other hand, we expanded the tetragonal geometry of existing ttTII-based COFs and developed a hexagonal framework by imine-linking the nearly linear ttTII to the well-established trigonal TAPB node. The hexagonal pore geometry provides larger pores than the tetragonal structure and sufficient space between the extended hexyl chains that reach into the pores.

The two new imine-linked COFs were synthesized under solvothermal conditions, condensing the nearly linear 5,5′-bis(2-formylthienothiophen-5-yl)-N,N′-dihexyl-thienoisoindigo [ttTII(CHO)2] building block with the novel tetradentate 4,4′,4″,4‴-(1,4-phenylenebis(9H-carbazole-9,3,6-triyl)) tetra-aniline [Cz(NH2)4] building block, as well as with the trigonal 1,3,5-tris(4-aminophenyl)benzene (TAPB) unit, forming the tetragonal Cz-ttTII COF and the hexagonal TAPB-ttTII COF, respectively (Figure 1). For experimental details, see the Methods section.

Figure 1.

Figure 1

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Synthesis of the new Cz-ttTII and TAPB-ttTII COFs. The cocondensation of the nearly linear ttTII(CHO)2 building block with the tetragonal carbazole-based precursor and the trigonal TAPB unit, respectively, yields a tetragonal and a hexagonal two-dimensional framework.

Structural and Morphological Investigations

Structural characterization via powder X-ray diffraction (PXRD) reveals high crystallinity of the two-dimensional Cz-ttTII and TAPB-ttTII COFs (Figure 2a,d). The black dots represent the experimentally obtained PXRD patterns and confirm the high crystallinity of the new materials. The simulated structure of the quasisquare Cz-ttTII COF was Pawley-refined based on the monoclinic space group C2/m and that of the hexagonal TAPB-ttTII COF based on the space group P6. The respective refined patterns (red lines) provide a good fit for the experimentally obtained data, with only minor differences, confirmed by the difference plots (blue lines). Refined unit cell parameters are a = 6.63 nm, b = 6.74 nm, c = 0.39 nm, and β = 58° for the Cz-ttTII COF and a = b = 6.20 nm, c = 0.28 nm, and γ = 120° for the TAPB-ttTII COF (Figure 2b,e). Nitrogen sorption at 77 K of the materials (Figure 2c,f) confirms their porosity, yielding a type IVb isotherm for the Cz-ttTII COF, typical for smaller mesopores. The TAPB-ttTII COF isotherm exhibits a type IVa shape, known for larger mesopores. Brunauer–Emmett–Teller (BET) surface areas amount to 614 m2 g–1 for the Cz-ttTII COF and 876 m2 g–1 for the TAPB-ttTII COF, with the respective total pore volumes being 0.41 and 0.99 cm3 g–1. Deviations from the theoretically calculated values could be attributed to building block residues or oligomeric fragments trapped in the porous structure of the COF, despite extensive extraction. Pore size distributions (PSD) were calculated based on a quenched solid density functional theory (QSDFT) equilibrium model for cylindrical pores, showing a bimodal shape with peaks at 2.2 and 3.8 nm for the Cz-ttTII COF and a unimodal shape peaking at 4.8 nm in the case of TAPB-ttTII COF, being in very good agreement with the simulated wall-to-wall distances (Figure S1a,b). The bimodal shape of the Cz-ttTII PSD derives from the modeling based on the QSDFT method, describing a noncylindrical pore shape.

Figure 2.

Figure 2

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Structural analysis of the Cz-ttTII and the TAPB-ttTII COFs. (a) Experimental PXRD pattern of the Cz-ttTII COF powder (black dots). Pawley refinement of the simulated structure model (red line) based on the monoclinic space group C2/m, providing a good fit for the experimental data with only minor differences (blue line). Rwp = 3.63% and Rp = 2.13%. The corresponding Bragg positions are indicated by green ticks. Inset: magnification of the 2θ > 20° region. (b) View onto the crystallographic a–b plane of the Pawley-refined Cz-ttTII COF structure model. The COF exhibits a quasisquare topology with slip-stacked layers and high porosity with a calculated Connolly surface area of 2241 m2 g–1 and a pore volume of 1.33 cm3 g–1. (c) Nitrogen sorption analysis of the Cz-ttTII COF bulk material at 77 K shows a type IVb isotherm. Inset: Pore size distribution (PSD) of the Cz-ttTII COF. The bimodal PSD is derived from a fit of the isotherm based on a QSDFT equilibrium model for cylindrical pores. Peaks are found at 2.2 and 3.8 nm, which is in very good agreement with the wall-to-wall distances of the refined COF structure, confirming the shamrocklike pore shape. (d) Experimental PXRD pattern of the TAPB-ttTII COF powder (black dots). Pawley refinement of the structure model (red line) in the hexagonal space group P6, providing a good fit to the experimental data with only minor differences (blue line). Rwp = 3.24% and Rp = 4.18%. The corresponding Bragg positions are indicated by green ticks. Inset: magnification of the 2θ > 20° region. (e) View onto the crystallographic a–b plane of the Pawley-refined TAPB-ttTII COF structure model. The COF exhibits a hexagonal topology with eclipsed layers and high porosity with a calculated Connolly surface area of 1730 m2 g–1 and a pore volume of 1.42 cm3 g–1. (f) Nitrogen sorption analysis of the TAPB-ttTII COF bulk material at 77 K shows a type IVa isotherm. Inset: Pore size distribution (PSD) of the TAPB-ttTII COF. The unimodal PSD is derived from a fit of the isotherm based on a QSDFT equilibrium model for cylindrical pores. The peak is found at 4.8 nm, which is in very good agreement with the wall-to-wall distances of the refined COF structure model.

High-resolution transmission electron microscopy (TEM) images of the polycrystalline COF powders reveal highly crystalline domains, confirming the tetragonal structure of the Cz-ttTII COF and the hexagonal structure of the TAPB-ttTII COF (Figure S1c,d). The observed periodicities of 4.6 nm for the Cz-ttTII COF and 6.0 nm for the TAPB-ttTII COF are in very good agreement with the according refined structure models.

COF Thin Films

For further investigations of the properties of the newly synthesized COFs, thin films of both structures were prepared in a solvothermal procedure. The crystallinity and oriented growth of thin films were confirmed via grazing-incidence wide-angle X-ray scattering (GIWAXS) analyses. The GIWAXS patterns of the films display a preferential orientation with the COFs’ a–b planes growing parallelly to the surface of the substrate, confirmed by the high intensities near the sample horizon and the lack of semicircular reflections (Figure S6a,d).

Scanning electron microscopy (SEM) cross sections of the thin films reveal an approximate thickness of 240 nm in the case of the Cz-ttTII COF and 790 nm for the TAPB-ttTII COF. SEM top view images confirm a homogeneously grown dense COF film on top of the substrates with Cz-ttTII showing a pillarlike and TAPB-ttTII displaying a weblike morphology (Figure S6c,f).

Electrical Conductivity

For the investigation of the COF films’ vertical electrical conductivity, a COF thin film was grown on a partially etched indium tin oxide (ITO) substrate. Subsequently, gold electrodes were evaporated onto the COF film, and a gold counter electrode was evaporated onto the blank ITO surface (Figure S13). The electrodes were contacted under exclusion of light, and I–V curves for both COFs were measured. Here, the potential was increased from 0 V to 1000 mV in 100 mV s–1 steps, then decreased to −400 mV, and finally again increased to 0 V. The resulting I–V curves were fitted with a linear fit, yielding the respective slope values of 0.0380 A V–1 for the Cz-ttTII COF film and 0.0253 A V–1 for the TAPB-ttTII COF film. The current density plots were calculated based on the electrode area of 3 × 4 mm (Figure S12). The electrical conductivities of the COFs were calculated using the equations described in the Supporting Information, section J, and based on film thicknesses of 240 nm for the Cz-ttTII COF film and 790 nm for the TAPB-ttTII COF film. Good electrical conductivities of 7.60 × 10–4 S m–1 for the Cz-ttTII COF and 1.66 × 10–3 S m–1 for the TAPB-ttTII COF were found, confirming their good electrical connectivity and suitability for electrochromic experiments.

Optoelectronic and Electrochromic Properties

Both COFs are dark green materials and were investigated via UV–vis/NIR absorption spectroscopy. The examined bulk powders exhibit a strong dual-band absorption in the visible spectrum with the respective higher-energy absorption band deriving from π → π* interactions and the respective lower-energy absorption band being assigned to intramolecular charge transfer (ICT) processes (Figure S3a,b).41,42 This absorption behavior has already been observed for the previously reported Py-ttTII COF.14 The Cz-ttTII COF has an absorption onset at approximately 980 nm and the TAPB-ttTII COF an onset at about 955 nm. Comparing the absorption characteristics of the COF materials to the bare ttTII(CHO)2 building block, a red shift of approximately 70 nm is found for the Cz-ttTII COF, while the TAPB-ttTII COF exhibits a respective red shift of up to 55 nm, assigned to the electronic integration of the precursors and hence the expansion of the conjugated π-system in the COFs. Low band gaps of 1.35 eV for the Cz-ttTII COF and 1.40 eV for the TAPB-ttTII COF were calculated using Tauc plots, assuming a direct transition for both materials (Figure S3c,d).

Likewise, the absorption spectra of the respective COF thin films on indium tin oxide (ITO) display an analogous dual-band shape, also confirming the successful growth of the COFs as thin films.

Both COFs were investigated in terms of their electrochromic behavior. Here, the COF thin film acts as the active material of the working electrode, a platinum wire as the counter electrode, a silver wire as the quasireference electrode, and a 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile as the electrolyte. The voltage was cycled in a range between −800 and 800 mV vs the ferrocene couple fc/fc+ in 200 mV steps. UV–vis/NIR absorption spectra were recorded between 350 and 1400 nm at every voltage step, keeping the voltage constant when recording the respective spectrum.

Increasing the applied potential oxidizes the respective COF film, leading to a darkening of the color from dark green to nearly black. Here, a strong red shift of the absorption spectra is found starting at 200 mV vs fc/fc+. The two main absorption bands of the neutral state with peaks at 420 and 650 nm for the Cz-ttTII COF (Figure 3a) and at 445 and 680 nm for the TAPB-ttTII COF (Figure 3d), respectively, are attenuated while new absorption features are found at 535 nm and above 720 nm for the Cz-ttTII COF and at 530 nm and above 755 nm for the TAPB-ttTII COF (Figures 3b,e). For the Cz-ttTII COF, an extraordinary increase of almost 2.5 OD at 895 nm between the neutral state (−400 mV) and the oxidized state (800 mV) can be reported. Likewise, the TAPB-ttTII COF also shows a very strong increase in absorption of even above 2.5 OD at 960 nm between the neutral and the fully oxidized state. When reversing the scanning direction, the absorption characteristics of the COFs exhibit a reversible behavior with only slight differences between the spectra in the forward and the backward scanning directions at a certain voltage, which can be assigned to charging effects that are entirely overcome by further reducing the potential.

Figure 3.

Figure 3

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Investigations on the electrochromic behavior of the Cz-ttTII COF films (a–c) and the TAPB-ttTII COF films (d–f). Voltage-dependent absorption spectra of (a) the Cz-ttTII COF and (d) the TAPB-ttTII COF, obtained by scanning from −400 to 800 mV, subsequently down to −800 mV and back to −400 mV vs fc/fc+ in 200 mV steps, featuring a strong shift in absorption to the NIR region. The respective differential changes in absorption are shown for (b) the Cz-ttTII COF and (e) the TAPB-ttTII COF, illustrating an extraordinary increase in absorption of both materials for wavelengths above approximately 730 nm (Cz-ttTII COF) and 755 nm (TAPB-ttTII COF). Parallel to this absorption increase, an attenuation of the absorption bands at 420 and 650 nm (Cz-ttTII COF) and 445 and 680 nm (TAPB-ttTII COF) of the electronically neutral state is observed, while new absorption bands appear at 535 nm (Cz-ttTII COF) and 530 nm (TAPB-ttTII COF). The solid and dashed lines of the optical spectra indicate the forward and backward scanning direction, respectively. CV curves were recorded for the (c) Cz-ttTII COF and (f) TAPB-ttTII COF, displaying well-defined oxidation and reduction peaks. In order to examine the stability and reversibility, the voltage was cycled 15 times between −800 and 800 mV (Cz-ttTII COF), respectively 900 mV (TAPB-ttTII COF), vs fc/fc+.

To monitor the cycle stability, CV curves for both COFs were recorded by cycling the potential between −800 and 800 mV vs fc/fc+ (Cz-ttTII), respectively 900 mV (TAPB-ttTII) at a scan speed of 100 mV s–1 for 15 times. For the Cz-ttTII COF (Figure 3c), a first distinct oxidation peak is found at approximately 280 mV, and a second oxidation process starts for potentials above 600 mV. To reduce stress on the COF film caused by higher potentials and ensure the chemical stability of the electrolyte solution, the scanning direction was reversed at 800 mV. After reversing the scanning direction, the oxidized COF is gradually reduced with the corresponding reduction peaks at 600 and 120 mV. The COF film shows good stability, with nearly no degradation over the course of 15 cycles. In the case of the TAPB-ttTII COF film (Figure 3f), two oxidation peaks are found at 165 and 405 mV, and a third oxidation process starts at potential values above 525 mV but could also not completely be measured due to stability limitations of the COF film and the electrolyte. The corresponding reduction peaks occur at 615, 285, and 45 mV. The COF also exhibits a high stability after the 15 cycles, with only minor drift between the first and the second cycle.

In addition, the reversibility of the electrochromic color change of both COFs was monitored at two wavelengths, 550 and 900 nm, by switching the applied potentials between −600 and 600 mV vs fc/fc+ with a dwell time of 4 s after each potential step for 200 oxidation/reduction cycles (Figure S8). At a wavelength of 550 nm, the Cz-ttTII COF exhibits a change in absorbance of approximately 1.0 OD, retaining >90% of the initial electrochromic response after 200 cycles. The second absorption feature at 900 nm, which is assignable to different transition processes as described above, exhibits a distinct absorption change of approximately 1.5 OD. The small performance drift of 6% after 200 cycles confirms the exceptional stability of this COF in this potential range. We attribute the deviations of the absorption change between the long-term stability tests and the absorption spectra in this wavelength region to the different measurement conditions of both measurements. The stability tests were conducted under more conservative potential settings not fully applying the maximal possible voltage to the films in order to reduce strain on the frameworks and ensure the best possible trade-off between performance and material protection. While the Cz-ttTII COF can reach the fully oxidized state with the set potential settings of the UV–vis spectra scans, the long-term cycling measurements aim to test the stability under fast switching conditions in the optimally functioning window, still exhibiting considerable absorption changes in the NIR.

For the TAPB-ttTII COF, monitoring the electrochromic switching at 550 nm showed an initial absorbance change of about 0.9 OD, retaining >83% of the initial electrochromic response over the course of 200 cycles. When monitoring at a wavelength of 900 nm with a new thin film, an absorption change of 1.9 OD was found, which only showed a minor drift of approximately 5%, also confirming the good stability of the COF film at this wavelength.

Additionally, the response times of the oxidation and reduction processes, i.e., the electrochromic switching speeds, were determined. Using the data from the electrochromic cycling stability measurements, the averages of the respective first ten cycles for each COF and wavelength were calculated (Figure S9). The switching speeds for the oxidation and reduction processes were subsequently determined between the 10% and 90% boundaries of the respective total absorbance change at excitation wavelengths of 550 and 900 nm. At 550 nm, coloration/decoloration times of 0.75 s/0.37 s were determined for the Cz-ttTII COF, with the respective times for the absorbance change at 900 nm being 1.22 s/0.29 s. The corresponding switching times of the TAPB-ttTII COF are 0.61 s/0.29 s recorded at 550 nm excitation and 1.26 s/0.47 s at 900 nm, with both COFs outperforming many known organic and inorganic electrochromic materials.16,17,43−46 This overall fast and stable electrochromic response of the TAPB-ttTII COF confirms the initial considerations of building an extended hexagonal pore geometry. Our design concept with sufficient space between the alkyl chains reaching into the pore aims for the unimpeded diffusion of counterions which balance the oxidized porous framework.

Finally, the coloration efficiencies were determined according to the equation described in the Supporting Information, section H. Here, the calculated efficiencies at 900 and 550 nm for the single-oxidized Cz-ttTII COF were found to be 807 and 272 cm2 C–1 at a potential of 400 mV. The respective values at 400 mV of the double-oxidized TAPB-ttTII COF are 596 and 114 cm2 C–1, confirming the highly efficient electrochromic performance of the COFs in the NIR region, which exceeds that of known covalent organic and hybrid frameworks.16,17,43

Conclusion

We have developed two novel and highly crystalline thienothiophene thienoisoindigo-based imine-linked 2D COFs—one with a tetragonal geometry using a newly designed tetradentate carbazole-based node as the counterpart, and the other one in a hexagonal geometry fused with the established trigonal TAPB building block. Both COFs, grown as thin films, exhibit good vertical conductivities in the range 10–4–10–3 S m–1, which represent an important factor for excellent electrochromic performance. The COFs show intriguing optical properties with a very broad absorption in the visible range, reaching into the NIR region of the electromagnetic spectrum. To highlight the versatility and the excellent optoelectronic features of the ttTII building block, we investigated the COFs’ fast and stable electrochromic behavior, showing strong absorption changes of up to 2.5 OD in the NIR region. Electrochromic monitoring over 200 cycles showed a high stability of the frameworks. Cyclic voltammetry measurements revealed distinct redox waves, demonstrating high cycle stability and reversibility of the redox processes. Additionally, high coloration efficiencies in the NIR region and response times as fast as 0.75 s/0.37 s for the Cz-ttTII COF and 0.61 s/0.29 s for the TAPB-ttTII COF at 550 nm excitation were established. Despite the slightly faster electrochromic response of Py-ttTII,14 these switching speeds exceed those of many known covalent organic and hybrid frameworks, further demonstrating the high electrochromic performance level of thienoisoindigo-based covalent organic frameworks, and making them highly promising candidates for NIR optoelectronic applications, such as optical information processing or thermal control.

In this context, we have discovered that the ttTII unit not only performs well when embedded in a tetragonal framework geometry but that it can also be incorporated in a hexagonal COF structure while maintaining its excellent electrochromic behavior. It is envisioned that additional thienoisoindigo-based materials with different geometries could be incorporated in stimuli-responsive coatings and high-performing electrochromic devices.

Methods

The COF building blocks were synthesized according to the procedures described in section K of the Supporting Information. All reagents and solvents are commercially available and were obtained in high-purity grades. Unless stated otherwise, solvents were degassed and saturated with argon before use.

COF bulk powder syntheses were performed under an argon atmosphere in PTFE-sealed glass reaction tubes (6 mL volume).

Cz-ttTII COF

Cz(NH2)4 (3.86 mg, 5.0 μmol, 1.0 equiv) and ttTII(CHO)2 (7.75 mg, 10 μmol, 2.0 equiv) were filled into a 6 mL reaction tube. Mesitylene (167 μL), benzyl alcohol (83 μL), and 6 M aqueous acetic acid (25 μL) were added to the tube, which was then sealed and heated to 120 °C for 5 days. After cooling to room temperature, the precipitate was collected by filtration and subsequently extracted with supercritical CO2 for 2 h to remove byproducts, yielding the COF as a dark green powder.

Anal. (Calcd for C260H204N20O8S24, found): C (69.30, 69.64); H (4.56, 4.67); N (6.22, 5.34); S (17.08, 16.04).

TAPB-ttTII COF

TAPB (1.8 mg, 5.0 μmol, 1.0 equiv) and ttTII(CHO)2 (5.8 mg, 7.5 μmol, 1.5 equiv) were filled into a 6 mL reaction tube. Mesitylene (167 μL), benzyl alcohol (83 μL), and 6 M aqueous acetic acid (25 μL) were added to the tube, which was then sealed and heated to 120 °C for 5 days. After cooling to room temperature, the precipitate was collected by filtration and subsequently extracted with supercritical CO2 for 2 h to remove byproducts, yielding the COF as a dark green powder.

Anal. (Calcd for C162H132N12O6S18, found): C (66.64, 66.91); H (4.56, 4.50); N (5.76, 5.42); S (19.76, 19.45).

COF thin films were synthesized in 100 mL autoclaves equipped with a 28 mm diameter glass liner. ITO-coated glass substrates were cleaned in detergent solution (Hellmanex III, 0.5% v/v), water, acetone, and isopropanol and treated with an O2-plasma for 10 min directly before use. For the COF thin film syntheses, substrates were placed horizontally in PTFE sample holders with the plasma-treated surface face-down.

Cz-ttTII COF Film

Cz(NH2)4 (3.86 mg, 5.0 μmol, 1.0 equiv) and ttTII(CHO)2 (7.75 mg, 10 μmol, 2.0 equiv) were filled into an autoclave. Mesitylene (1333 μL) and benzyl alcohol (667 μL) were added, and an ITO/glass substrate was placed in the autoclave with the ITO side facing upside-down. 6 M aqueous acetic acid (200 μL) was added, and the autoclave was sealed and heated to 120 °C for 3 days. After cooling to room temperature, the rear side of the substrate was cleaned with MeCN, and the COF film was dried with a stream of nitrogen.

The COF film exhibits a dark green color with a thickness of approximately 240 nm (determined by cross-sectional SEM).

TAPB-ttTII COF Film

TAPB (1.8 mg, 5.0 μmol, 1.0 equiv) and ttTII(CHO)2 (5.8 mg, 7.5 μmol, 1.5 equiv) were filled into an autoclave. Mesitylene (1333 μL) and benzyl alcohol (667 μL) were added, and an ITO/glass substrate was placed in the autoclave with the ITO side facing down. 6 M aqueous acetic acid (200 μL) was added, and the autoclave was sealed and heated to 120 °C for 3 days. After cooling to room temperature, the rear side of the substrate was cleaned with MeCN, and the COF film was dried with a stream of nitrogen.

The COF film exhibits a dark green color with a thickness of approximately 790 nm (determined by cross-sectional SEM).

Characterization Methods

Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance III HD spectrometer. Chemical shifts of the protons are expressed in parts per million (ppm, δ scale) and are calibrated using residual undeuterated solvent peaks as an internal reference (1H NMR: CDCl3, 7.26; DMSO-d6, 2.50; THF-d8, 3.58). Data for 1H NMR spectra are reported in the following manner: chemical shift (δ ppm) (multiplicity, coupling constant, integration). Multiplicities are reported as follows: s = singlet, d = doublet, t = triplet, m = multiplet, or combinations thereof.

Powder X-ray diffraction (PXRD) patterns were measured on a Bruker D8 Advance diffractometer equipped with a Cu Kα source (0.1 mm divergence slit, knife edge air scatter screen) and a LynxEye detector. Kβ radiation was attenuated with a 0.0125 mm Ni filter.

2D grazing-incidence wide-angle X-ray scattering (GIWAXS) data were recorded on an Anton Paar SAXSpoint 2.0 system equipped with a Primux 100 Micro Cu Kα source and a Dectris EIGER R 1M detector. The COF films were positioned at a sample–detector distance of 140 mm and were measured with an incidence angle of 0.22°.

The structure models of the COFs were simulated using the Accelrys Materials Studio software package. The highest possible symmetry was applied for the COFs. The structure models were optimized using the Forcite module with the Universal force-field. Structure refinements were carried out with the Reflex module using the Pawley method. Pseudo-Voigt peak profiles were used, and peak asymmetry was corrected using the Finger–Cox–Jephcoat method. Connolly surfaces and accessible surfaces were generated using a N2-sized probe (r = 0.184 nm) at a 0.025 nm grid interval.47

Nitrogen sorption analyses were conducted with a Quantachrome Autosorb 1 instrument at 77 K. The samples were first extracted with supercritical CO2 and subsequently degassed at 120 °C under a high vacuum for 5 h prior to the measurements. BET areas were calculated based on the pressure range 0.05 ≤ p/p0 ≤ 0.2. Pore size distributions were determined using the QSDFT equilibrium model with a carbon kernel for cylindrical pores.

Scanning electron microscopy (SEM) images were recorded with an FEI Helios NanoLab G3 UC microscope equipped with a Schottky field-emission electron source operated at 1–30 kV.

Transmission electron microscopy (TEM) images were recorded with an FEI Titan Themis microscope equipped with a field emission gun operated at 300 kV.

Fourier-transform infrared (FTIR) spectra were recorded on a Bruker Vertex 70 FTIR instrument using a liquid nitrogen-cooled MCT detector and a germanium ATR crystal.

UV–vis–NIR spectra were measured with a PerkinElmer Lambda 1050 spectrometer equipped with a 150 mm InGaAs integrating sphere. Diffuse reflectance spectra were recorded with a Harrick Praying Mantis accessory kit and were referenced to barium sulfate powder as the white standard.

Electrochemical measurements were performed with a Metrohm Autolab PGSTAT potentiostat/galvanostat. Cyclic voltammetry (CV) scans were recorded with the respective COF film on ITO/glass as the working electrode, a Pt wire counter electrode and a Ag wire quasi-reference electrode, and referenced to fc/fc+ as the internal standard with a potential of 400 mV. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in MeCN was chosen as the electrolyte. The measurements were conducted with a scan speed of 100 mV s–1.

For the spectroelectrochemical measurements, the COF films were placed inside a fused silica cuvette (10 mm path length) equipped with a Pt wire counter electrode, a Ag wire quasireference electrode, and 0.1 M TBAPF6/MeCN electrolyte under an argon atmosphere. The potential was increased/decreased in 200 mV intervals using a 20 mV s–1 ramp and then held constant for the duration of the UV–vis scan (ca. 6 min per spectrum). For the stability and response time measurements, the applied potential was switched between −600 and +600 mV vs fc/fc+ and held constant for 4 s between each step. In order to reduce the instrument response time, absorption changes were tracked with the Lambda 1050 photomultiplier tube (Vis) and InGaAs (NIR) detectors set to fixed gain mode.

For the conductivity measurements, COF films were grown onto an etched ITO substrate (see Figure S13). The films were fastened in a specifically designed mold; the COF film was removed from the counter electrode opening using acetone, and gold electrodes (40 nm thickness) were evaporated on the COF film/substrate. The conductivity was subsequently determined in the dark using a vertical conductivity measurement setup and a Metrohm Autolab PGSTAT potentiostat/galvanostat, scanning from −400 to +1000 mV.

Acknowledgments

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013/ERC grant agreement 321339). The authors acknowledge funding from the Bavarian Network “Solar Technologies Go Hybrid” and the DFG Excellence Cluster e-conversion (EXC 2089/1-390776260). We thank Markus Döblinger for the TEM and Melisande Kost for the SEM characterization, and Roman Guntermann for the GIWAXS measurements.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.2c00049.

  • Structure analyses, IR spectroscopy, UV–vis spectroscopy, Tauc plots, SEM images and GIWAXS patterns, stability cycling measurements, response times, coloration efficiency, images of the electrochromic COF films, electronic conductivity, building block syntheses, and NMR spectroscopy (PDF)

The authors declare no competing financial interest.

Supplementary Material

ng2c00049_si_001.pdf (17.5MB, pdf)

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ng2c00049_si_001.pdf (17.5MB, pdf)

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