Communication pubs.acs.org/cm
Transmissive-to-Black Electrochromic Devices Based on CrossLinkable Tetraphenylethene-Diphenylamine Derivatives Silja Abraham,† Sreejith Mangalath,†,‡ Deepika Sasikumar,† and Joshy Joseph*,†,‡ †
Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, Kerala, India ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST Campus, Thiruvananthapuram 695019, Kerala, India S Supporting Information *
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mostly due to the complexities associated with the tuning of the absorption properties of the redox states to cover the entire visible spectral region.14,15,28,29 Recently, McGehee et al. reported a small molecule, p-aminotriphenylamine, assembled on the surface of mesoporous tin-doped indium oxide to have transmissive-to-black electrochromic behavior, albeit with a low switching stability over 50 cycles.30 Liou and co-workers reported a copolymer of tetraphenylbenzidine-based and tetraphenyl-p-phenylenediamine-based aramides that exhibited a nearly black (L*, 58.2; a*, 3.1; b*, 3.2) dication, along with other NIR and multicolored oxidized states.31 Other efforts in this direction mostly rely on device engineering incorporating multichromic materials to achieve the desired neutral color state.32 In spite of these developments, the design of transmissive-to-black switching electrochromic materials and devices with reasonable switching stability and optical contrast remains elusive. In this context, to develop color tunable, organic electrochromic materials with enhanced electrochromic switching stabilities, we have adopted a donor-π-donor (D-π-D) design strategy in which diphenylamine groups have been used as the donor moieties and various aromatic acceptors as the π-core units. Following this strategy, we have recently demonstrated multicolor electrochromism of diphenylamine-fluorene derivatives with excellent color contrast and electrochromic switching stabilities.33 Thermally cross-linkable styryl moieties were incorporated in these small molecules as pendent groups to ensure firm attachment to the electrode surface via the formation of stable, solvent resistant cross-linked films during ECD fabrication. Tetraphenylethene (TPE) based molecules are known for aggregation-induced emission (AIE) effect due to their propeller-like conformation, central alkene plane and intersection angle between four phenyl rings.34 Moreover, TPE and its ring-substituted derivatives are bifunctional electron donors or acceptors as a result of the aromatic and olefinic units that can act as either electron-rich or electron-deficient centers. Also, TPE analogues can undergo one electron oxidation or reduction and further the disproportionations of the (oneelectron) oxidation/reduction product to form dications/ dianions. The propeller shape of tetraphenylethylenes and their conformation are responsible for their remarkable redox properties.35
lectrochromic (EC) materials, which exhibit reversible change in their optical properties by electrochemical redox reactions, have been extensively investigated due to their potential applications in multicolor displays, smart windows, camouflage materials, E-paper, sunglasses, switchable rear-view mirrors etc.1−4 Most studied EC materials include transition metal oxides,5−7 inorganic coordination complexes,8,9 conjugated polymers,3,10 discrete oligomers11,12 and organic small molecules.13,14 Among these classes of materials, organic EC materials possess several advantages such as high coloration efficiencies, low switching time, color tunability via substitution, excellent electrochromic switching stability, open circuit memory and solution processability. 15,16 The requisite optoelectronic properties of electrochromic devices, including switching potentials, exhibited colors and response times, may vary for different applications, which open up the necessity of tuning of these properties. A major challenge in the design of electrochromic materials for automotive and building applications is their color tuning to achieve aesthetically pleasing color states. Though multicolor electrochromism is desirable for display applications, neutral shades such as gray and black are favored for smart window, rear view mirror, ophthalmic and signage applications.17−19 Obtaining gray/black electrochromic coloration either in the neutral or in the redox states of organic EC materials with a highly transmissive complementary state is a challenging problem. In general, two strategies have been adopted in the literature for the fabrication of gray/black electrochromic devices (ECDs). Gray/black-to-transmissive switching ECD design is based on materials that exhibit gray/black colored neutral states that upon redox activation produce colorless, transmissive states.20 Reynolds and co-workers have made pioneering contributions in the design of black-to-transmissive organic EC polymers with excellent electrochromic switching properties, where they have used copolymerization of carefully selected multicolor electrochromic subunits or blends and bilayers of several polymer electrochromes with complementary spectral absorption for achieving gray/black color at the neutral state.17,21,22 Similarly, significant efforts to achieve gray/black color in the neutral state have been made by different research groups either by mixing or by copolymerization of different monomer units with suitable absorption.23−27 Despite the involved synthetic complexities, black-to-transmissive ECDs were shown to have potential in various applications.19 The second and more challenging approach, the design of transmissive-to-gray/black EC materials, is rather unexplored, © 2017 American Chemical Society
Received: August 7, 2017 Revised: November 15, 2017 Published: November 15, 2017 9877
DOI: 10.1021/acs.chemmater.7b03319 Chem. Mater. 2017, 29, 9877−9881
Communication
Chemistry of Materials
become more uniform, disrupting any aggregation/clustering occurred during the spin coating process, which can enhance their absorption intensities. Thin films of TPOSt on quartz showed a strong, yellowish green emission with an emission maximum of 540 nm. The cross-linked TPOSt (TPOSt-X) showed a similar emission profile with an emission maximum of 540 nm and slightly decreased emission intensity (Figure S3B). Further evidence for complete thermal cross-linking of TPOSt was obtained from FT-IR, DSC and wide-angle X-ray diffraction (WAXD) analyses (Figure S4, S5). As reported in the case of diphenylamine-fluorene derivatives, the absence of vinylic C−H out of plane bending frequency in the IR spectra for TPOSt-X, the appearance of the exothermic transition after the melting of TPOSt in the DSC thermogram and the crystalline to amorphous phase transition inferred from the WAXD pattern of TPOSt and TPOSt-X, indicate the formation of cross-linked amorphous polymer.33 Surface morphology analysis conducted using Atomic Force Microscopy revealed the formation of uniform films of TPOSt (average roughness, Ra = 0.428 nm) and TPOSt-X (Ra = 0.42 nm) on ITO under the experimental conditions (Figure S6). Further, the thermal stability of TPOMe and TPOSt were investigated by thermogravimetric analysis (TGA). Both compounds, exhibited good thermal stability without significant weight loss up to 300 °C and showed a thermal decomposition temperature, Td (5% mass loss) of 384 and 336 °C for TPOMe and TPOSt, respectively (Figure S7). Electrochemical analyses of TPOMe and TPOSt using cyclic voltammetry and differential pulse voltammetry were conducted in DMF solution with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte, platinum disc as working electrode, platinum wire as counter electrode, Ag/AgCl as reference electrode and ferrocene as an internal standard (Figure 1A, S8). The CV of TPOMe and TPOSt in DMF showed reversible peaks with half wave potentials around 0.69 and 0.70 V (vs Fc/Fc+), respectively. As previously reported for tetraphenylethene systems, both compounds undergo two sequential one-electron oxidation processes and further disproportionations of the (one-electron)
Herein, we report a diphenylamine-tetraphenylethene (TPE) derivative (TPOSt, Chart 1) as a novel example for a crossChart 1. Structures of TPOMe and TPOSt
linkable, small molecule exhibiting transmissive-to-black switching electrochromic behavior with high optical contrast and excellent switching stability over 1500 cycles. Moreover, the constructed ECDs also switch between a highly fluorescent, yellow-green neutral state and a strongly quenched dark oxidative state demonstrating the dual electrochromic and electrofluorochromic behavior of these devices. The tetraphenylethene core group is chosen for the current EC design based on its favorable spectroelectrochemistry36 and AIE properties,34 which are being exploited in the current study. The diphenylamine-TPE derivatives with and without crosslinkable styryl moieties, TPOSt and TPOMe,36 respectively (Chart 1) were synthesized by modified Buchward−Hardwig coupling reactions and characterized by FT-IR, 1H NMR, 13C NMR and ESI-MS techniques.37 These derivatives are found to be soluble in most common organic solvents such as toluene, chlorobenzene, THF, DMF, ethyl acetate, acetone, dichloromethane etc. Detailed synthetic procedures and characterization data are provided in the Supporting Information. A typical coupling reaction after purification would yield a mixture of E and Z isomers that, without further separation, was used for optoelectronic studies and device fabrication. Solution state absorption spectra of TPOSt and TPOMe, in toluene, chlorobenzene and DMF showed two absorption bands with maxima around 279 and 369 nm and tail endabsorptions extending up to 450 nm (Figure S1, Table S1). Optically dilute solutions of TPOSt and TPOMe in THF were nonemissive, whereas addition of water above 50% (v/v) drastically enhanced the fluorescence of the solution, characteristic to AIE behavior of TPE-based fluorophores (Figure S2).36 The absorption spectra of thin films of TPOSt on quartz showed broad absorption bands with maxima around 303 and 381 nm. On thermal cross-linking of TPOSt above 215 °C, the long wavelength absorption peak showed a hypsochromic shift to 368 nm with good solvent resistive properties (Figure S3A). For example, though the monomer TPOSt was readily soluble in chlorobenzene, the corresponding, cross-linked polymer (TPOSt-X) remained insoluble. The UV−visible spectra of TPOSt and TPOSt-X show significant differences in their absorbance intensities, which could be attributed to the characteristic change of the underlying morphology after cross-linking. During the cross-linking process, the films
Figure 1. (A) Cyclic voltammogram of TPOMe (black), TPOSt (red) in DMF. (B) CV of TPOSt-X film on ITO (red) in acetonitrile, supporting electrolyte 0.1 M TBAPF6, scan rate 50 mV s−1. (C) Spectroelectrochemical response of TPOSt-X film (thickness ≈ 155 ± 5 nm) on ITO. (D) Electrochromic switching of TPOSt-X film on ITO substrate. 9878
DOI: 10.1021/acs.chemmater.7b03319 Chem. Mater. 2017, 29, 9877−9881
Communication
Chemistry of Materials oxidation product, leads to the formation of dications (Figure 1A).35,36 Square wave analyses of both TPOMe and TPOSt showed broad peaks having potentials around 0.71 V (vs Fc/ Fc+). Film state CV analysis of TPOSt-X on ITO coated glass working electrode in acetonitrile also showed similar, reversible oxidation peak with a half-wave potential around 0.68 V (Figure 1B). We have compared the cyclic voltammetry traces of TPOMe, TPOSt and TPOSt-X as thin films to understand the role of cross-linking on current density properties of the films (Figure S9). The current density value of cross-linked film (TPOSt-X) was much higher than the TPOMe and monomer films (TPOSt). After oxidation, TPOMe and TPOSt films became readily soluble in acetonitrile compared to the TPOSt-X film, which showed high solvent resistance. The TPOSt-X films on ITO were electrochemically oxidized from 0 to +1.2 V vs Ag/AgCl and their spectro-electrochemical behavior was analyzed (Figure 1C). Upon gradual oxidation of the polymer film, the absorption bands below 400 nm depleted, with a concomitant enhancement in the absorption throughout the visible region extending up to 1100 nm with broad absorption maxima around 490 and 680 nm. Consequently, the color of the TPOSt-X film changed from light yellow to dark gray. The electrochromic response of TPOSt-X was further investigated by double-potential-step chronoamperometry and chronoabsorptometry (Figure 1D, S10). The absorption changes at different wavelengths were monitored as a function of time by switching between ±1.2 V. At +1.2 V, during the oxidation, the response time for the contrast ratio to reach over 90% of its maximum was 7.6 s at 490 nm and 7.2 s at 680 nm. On the other hand, a faster response time of 3.6 s at 490 nm and 3.7 s at 680 nm was observed for bleaching at −1.2 V (Figure 1D). Electrochromic devices with an FTO/Electrolyte/EC layer/ FTO architecture (Figure S11) were fabricated with spin-cast films of TPOSt-X on FTO as the EC layer. The photographic images of the device at 0 and 2.8 V under ambient light and UV light conditions are shown in Figure 2A,B. The corresponding changes in the transmittance spectra of the device under different applied potentials (0−2.8 V) are shown in Figure 2C. Upon increasing the potential from 0 to 2.8 V, the transmittance spectra showed a steady decrease in the transmittance throughout the visible region, with a high %ΔT value of ∼50 at 490 and 700 nm. Accordingly, the color of the device changed from its transmissive light yellow colored state (L* = 97, a* = −5, b* = 16) to black colored state (L* = 41, a* = 2, b* = 8). Under this potential sweep, the fluorescence emission of the TPOSt-X ECDs also switched between a highly fluorescent yellowish green neutral state with an emission maximum around 524 nm and a strongly quenched dark oxidized state (Figure 2B,D). Kinetic studies to estimate the optical switching times and stability of the device toward different electrochromic switching potentials are summarized in Figure 3. In addition, separate analysis of the transmittance spectra and corresponding chronoamperometric data of the device at different pulse widths are shown in Figure S12, S13. The ECDs showed transmittance changes (%ΔT) between 50% and 40% with different pulse widths of 30, 20, 10 and 5 s at representative wavelengths, recording only a slight loss in transmittance (5− 7%) upon decreasing the switching times from 30 to 10 s. The calculated electrochromic response times for the coloration and bleaching monitored at 490 nm were found to be 7.9 and 4.2 s,
Figure 2. Images of TPOSt-X device at potentials of 0 and 2.8 V under (A) normal light and (B) UV light. (C) Transmittance spectra of TPOSt-X (thickness ≈ 155 ± 5 nm) device at different applied potentials (0−2.8 V) (baseline with air). (D) Fluorescence spectra of the TPOSt-X device at applied potentials of 0 V (black) and 2.8 V (red). Dimension of the devices and surface resistivity of FTO are 2 cm × 1.5 cm and 8Ω/sq, respectively. Poly(ethylene glycol) diacrylate/LiTFSI/propylene carbonate was used as gel electrolyte and Delo-Katiobond epoxy as sealant.
Figure 3. Transmittance spectra TPOSt-X device with pulse width of 30, 20, 10 and 5 s at (A) 490 nm and (B) 700 nm. Switching cycle TPOSt-X device at (C) 490 nm and (D) 700 nm.
respectively (Figure S14A). Similarly, the switching times required for coloration and bleaching monitored at 700 nm were 8.1 and 4.2 s, respectively (Figure S14B). The electrochromic switching stability, being an important parameter considering practical applications, was analyzed in the actual ECD configurations by monitoring the EC contrast at 490 and 700 nm while switching between ±2.8 V with 30 s pulse width. As shown in Figure 3C,D, S15, the TPOSt-X devices exhibited good electrochromic switching stabilities over 1500 switching cycles with only a decrease of 6% in the EC contrast after 1500 cycles. The cross-linking will effectively prevent the delamination of the material from the electrode surface upon repeated redox cycling. This will enhance the long-term stability of the material. Further, the open circuit memory of the TPOSt-X device was demonstrated (Figure 9879
DOI: 10.1021/acs.chemmater.7b03319 Chem. Mater. 2017, 29, 9877−9881
Communication
Chemistry of Materials
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S16) by monitoring the transmittance changes corresponding to the colored oxidized state of a representative device at 490 and 700 nm over 2.5 h, after removing the external voltage of +2.8 V. These devices remained in the colored oxidized state for several days retaining more than 80% of the EC contrast under open circuit conditions. In conclusion, we have synthesized a cross-linkable, symmetric diphenylamine-TPE derivative (TPOSt) that exhibits transmissive-to-black electrochromic and fluorescentto-dark electrofluorochromic dual behavior and demonstrated its optoelectronic and electrochromic device properties. The thermal-cross-linking of these small molecules via styryl end groups ensure the formation of flawless films on electrode substrates with long-term stability. Electrochromic devices of TPOSt-X showed an electrochromic switching between a highly transmissive, light yellow colored neutral state (L* = 97, a* = −5, b* = 16) and a black colored oxidized state (L* = 41, a* = 2, b* = 8) with high optical contrasts (up to 50%ΔT) and reasonably low switching times (
