Side-chain engineering of green color electrochromic polymer materials: toward adaptive camouflage application

The syntheses of adaptive camouflage devices based on novel side-chain engineered organic electrochromic materials have been demonstrated. Herein we report a molecule engineering approach for the tuning and syntheses of green-brown switchable electrochromic materials and also demonstrate their applications in chameleonic fabric devices. We have also successfully demonstrated the fabrication of chameleonic fabric devices.

This journal is © The Royal Society of Chemistry 2016 structure (short, straight and branched) to deliberately modify their electrochromic properties for the envisaged intelligent camouflage applications.
The monomer structures of G1, G2 and G3 are shown in Fig. 1a and the specific synthetic routes are exhibited in Fig. S1 (ESI †).Conceivably, the side chains of the polymer function primarily as solubilizing groups, and also have substantial impacts on polymer packing motifs, film morphologies and optical properties.We chose three specifically designed molecules to fine-tune the optical properties.The designed monomer G1 has a rigid-body model with short side chain methoxyl substitutes.The monomer G2 contains the straight long C12 alkoxyl side chain which is tilted relative to the quinoxaline unit backbone.The G3 monomer was constructed with the more bulky double-alkoxyl side chains which have large spaces between alkyl side chains.This may hinder the interchain p-p stacking twisted relative to the quinoxaline moiety of conjugated polymers.All the designed molecules are studied to fine-tune the molecule structures and hence optical properties.The appearance of the synthetic G1, G2 and G3 are shown in Fig. 1b-d, respectively.The state and color of the samples changes from solid to liquid and light-orange to brown, which suggests that the alkoxyl side chains of quinoxaline have a great effect on the interaction of monomer molecules.However, these monomers all have a high solubility in some common solvent (Table S1, ESI †), which is beneficial for further polymerization by chemical or electrochemical methods.
In order to elucidate the polymer properties, electrochemical polymerization of monomers was applied by cyclic voltammetry on a platinum button electrode (area: 0.02 cm 2 ) or indium tin oxide (ITO) coated glasses by oxidative electropolymerization in a dichloromethane (DCM) solution containing a 0.01 M monomer and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ).The representative electrochemical growth processes revealing electroactivities of monomers G1, G2 and G3 and formation of corresponding polymers are given in Fig. S2 (ESI †).The oxidation of G1, G2 and G3 on a bare electrode starts at 0.74 V, 0.80 V and 0.85 V versus Ag wire pseudo-reference electrode in TBAPF 6 /DCM system, respectively.The increase of oxidation potential can be attributed to the improvement of acceptor capacity of the quinoxaline unit as the change of the side-chains.Redox couples for both monomers (G1 and G2) rapidly grow at relatively low potentials (at À0.25 V, À0.01 V for G1, and À0.01 V, 0.10 V for G2 vs. the same reference electrode), which indicates the formation of highly electroactive polymers except G3 (Fig. S2a-c, ESI †).It can be observed that polymer G1 (PG1) and G2 (PG2) could be deposited on a Pt or ITO glass slide, and the deposition rate of PG1 is faster than that of PG2.Moreover, some insoluble solids were formed near the electrodes in the process of electrochemical polymerization of G2, which suggests that PG1 and PG2 both have poor solubility in DCM and PG1 is more stable.To decrease the solubility of PG3 and increase the polymer forming performance of PG3, electrochemical polymerization of G3 was achieved potentiodynamically in a mixture of DCM and acetonitrile (ACN) (50/50, v/v) solution containing 0.01 M monomer and 0.1 M TBAPF 6 , because ACN was a more efficient medium to produce adhesive films. 9It should be noted that the redox couples rapidly grow at relatively high potentials (0.75 V, 0.65 V) compared to PG1 and PG2 (Fig. S2d, ESI †).It was attributed to a decrease in conductivity and poor hydrophilic character resulting from the vast alkyl-substituent, which prevents the polymerization process of G3 and provides a worse adherence to the electrodes.These results above imply that the presence of alkyl side chains not only allows an easier processing of electroactive polymers but also affects the electronic properties of the conjugated main chain. 18The alkoxyl side-chain greatly increased the solubility of corresponding polymers, leading to an excellent solubility of PG3 with the bulky double-alkoxyl in common solvents.
To further verify the properties of polymers, PG1, PG2 and PG3 were obtained by conventional chemical polymerization method as reported. 27As expected, the obtained PG1 and PG2 both are insoluble and PG3 has an excellent solubility in common solvents, such as DCM and TOL.As a polymeric electrochromic material, solution process of PG3 is convenient for the large-scale production of electrochromic devices using spin-coating or spray-coating methods.To compare the electrochemical behaviors of different alkoxysubstituted polymers, the cyclic voltammetry studies were performed.Fig. 2a shows a stable cyclic voltammograms of PG1, PG2 and PG3 (obtained from electrochemical polymerization) films coated on a Pt electrode at scan rates of 100 mV s À1 in 0.1 M TBAPF 6 /ACN and very well-defined redox processes are observed, in consistency with the redox processes reported for the similar polymers with other alkyl substituted. 14,15It is surprising to note that, alkoxy side chain on pendant phenyl rings greatly affect the potentials for the oxidation of monomer and the redox couple of corresponding polymer.As observed for the redox potentials of different films, the value of E 1/2 is increasing for PG1 with short methoxyl side chains, PG2 with straight long C 12 alkoxyl side chains, to PG3 with more bulky double-alkoxyl side chains.This can be attributed to the fact that the substituent may distort the polymer backbone and decrease the degree of conjugation, leading to a lower conductivity.The charge injected/ ejected during the CV was calculated by integrating the current density passing through the system and the value also decreased gradually, which suggests that the capacitive behavior of these materials decreases with an increase of alkyl-substituent sidechains.We can attribute this to a drop in electronic conductivity as interchain interactions decrease with the change of polymer side chains. 18The scan rate dependence of the anodic and cathodic peak currents is illustrated in Fig. 2b for PG3 and Fig. S3a and b (ESI †) for PG1 and PG2 films.A linear dependence demonstrates that these films are well adhered and the electrochemical processes are reversible and non-diffusion-controlled. 18pectroelectrochemistry experiments were performed to probe the optical changes at different applied potentials.Polymer films were deposited on ITO glass slides through electrochemical polymerization with a thickness of around 600 nm (Fig. S4, ESI †).The spectroscopic changes were investigated using a UV-Vis-NIR spectrophotometer in a 0.l M TBAPF 6 /ACN solution without monomers, as the increasing applied potentials.Fig. 3 reveals the spectroelectrochemistry and corresponding colors of PG1, PG2 and PG3.All polymers have two distinct absorption bands in the visible region (Fig. 3a-c), which is essential for neutralstate green conducting polymers.Although both of absorption bands are necessary, the values of maximum absorption wavelengths are critical for neutral-state green polymers.The absorption maxima of PG1, PG2 and PG3 are centered at 415, 420, 435 and 730, 714, 715 nm, which are essential values for a green color to be observed in the neutral state.It should be noted that, the potential of their fully neutral state (green color) greatly decreases as the increase of polymer alkoxyl side chains (À0.5 V for PG1, À0.3 V for PG2, and À0.1 V for PG3), leading to a more saturated green color.Meanwhile, their full oxidation potential (brown color) increases slowly (+0.8 V for PG1, +0.9 V for PG2, and +1.0 V for PG3) and gradually to produce a brown color analogous to the color of sands at the same applied potential of +0.8 V.The alkoxy groups affect the potentials not only for the oxidation of monomer and the redox couples of polymers, but also for their neutral and oxidized states.This behavior is attributed to the different electronic structures of the polymers, and to the different acceptor capacities of the quinoxaline derivatives, which leads to unique donor-acceptor matches with EDOT moieties.Compared to PG1 and PG2, PG3 exhibited unique vegetable green neutral and sand-tone oxidized color, and the corresponding (Y; x; y) values were shown in Fig. 3d.Representation of the hue and saturation for these three polymers is given in Fig. S5 (ESI †).Although an extremely transmissive colorless oxidized state is essential for the realization of polymer electrochromic-based display devices, PG3 is unique in the literature with its highly saturated green color in the neutral state and exceptional sand-tone color in the oxidized state (Fig. 3d).In addition, the absorption spectrum of all polymers in the NIR (Near Infrared Ray) and IR regions increases greatly during the bleaching of the visible absorption as the applied potential increases.It can act as an infrared electrochromic process for potential applications in modulation of thermal transmission and absorption.Regarding these superior properties of PG3, the green-colored in the neutral state and brown-colored in the oxidized state and its absorbing infrared exactly meet the demands of chameleonic materials in military applications.The electrochromic switching time and long-term stability of PG3 films are important for their potential applications in intelligent camouflage application.As illustrated in Fig. 4a, PG3 coated on a Pt electrode by spin-coating method was switched by stepping the potential between À0.1 V and +0.8 V with a switching interval of 5 s in a 0.1 M TBAPF 6 /ACN electrolytesolvent system.PG3 showed a fast switching time of 1.0 s at fully neutral state and 1.5 s at fully oxidized state (defined as the time required to reach 95% of the full response).To investigate their stability, PG3 films were cycled between their fully neutral state (À0.1 V) and fully oxidized state (0.8 V) 5000 times in 0.1 M TBAPF 6 /ACN electrolyte-solvent system.PG3 showed a decrease by lower than 5% for the anodic peak current (i pa ) and a decrease by lower than 4% for the cathodic (i pc ) one (Fig. 4b), which highlights the robustness of the polymers upon switching between the neutral and oxidized states.Additionally, PG3 exhibited a super-hydrophobic property due to the bulky and long alkoxyl side chains (Fig. 4c and Movie S1, ESI †).As contact angles portrayed in Fig. 4c, the hydrophlic ITO (nearly 01) and rough paper have been turned into a hydrophobic surface with much larger contact angles (about 1201 and 1351).
These findings indicate that PG3 is an excellent candidate for electrochromic devices with a low driving voltage and may find important applications in future camouflage devices.
To elucidate the effect of PG3 films in the ECDs, a simple sandwich-type PG3 EC cell was fabricated based on the reported method with modification, 28 the letters ''PKU'' formed by PG3 films were prepared on an ITO-coated glass slide by spraycoating the 5 mg mL À1 solution of PG3/toluene on a mask.The appearance of PG3 and the corresponding solution (such as DCM or TOL) for spray-coating were shown in Fig. 5a.Switching tests were done in air using a two-electrode potentiostat (reference and counter electrode shorted together).Fig. 5b and Movie S2 (ESI †) shows the reversible changing process of letters between green color in the fully neutral state by applying a potential of À1.5 V and sand-color in the fully oxidized state by applying a potential of +1.5 V.
Aiming to fabricate the camouflage clothing, a chameleonic fabric device was built by spray-coating the 5 mg mL À1 solution of PG3/toluene onto the surface of a conductive fabric as the working electrode.Another conductive fabric without treatment was used as the counter electrode.A fiberglass separator soaked with a gel electrolyte (weight ratio of ACN : PC : PMMA : LiClO 4 was 70 : 20 : 7 : 3) was added to provide ions and prevent short circuit.The structure of the chameleonic fabric device is shown in Fig. 5c.
As demonstrated in Fig. 5d-f, Movies S3 and S4 in the (ESI †), the whole fabric device was composed by one or more piece working electrodes and a whole piece counter electrode on the other side of diaphragm.The potential was applied on the interval piece working electrodes (1, 2, 3, 4 or 5, 6, 7) simultaneously (Fig. 5d).The whole device can reversibly change between green color and sand-tone color at an applied potential just like a camouflage clothing.Moreover, the chameleonic ECDs could maintain a given redox state (green or brown color) when taken to open circuit.If desired, the color can be refreshed just by a short potential pulse without a continuous supply of electrical energy to operate.

Conclusions
Three donor-acceptor-type polymers have been synthesized and can be switched between green (neutral state) and brown (oxidized state) colors.Compared with other alkoxy side chain polymers PG1 and PG2, the novel PG3 with bulky double-alkoxyl side chains revealed a highly vegetable-green color at a lower applied voltage and a soil-brown color in oxidized-state, strong absorption in the NIR and IR regions, excellent solubility in common organic solvents, super-hydrophobic, very fast switching times, and high stability.Chameleonic fabric has also been fabricated, and the ability to reversibly change between green and sand colors is essential for adaptive camouflage to the conditions of forest and sands.

Fig. 4
Fig. 4 (a) Chronoamperometry for PG3 with a 5 s delay for each potential (À0.1 V and +0.8 V).(b) Stability of a 600 nm thick PG3 film cycled 5000 times with a scan rate of 100 mV s À1 in 0.1 M TBAPF 6 /ACN: the change in the anodic (&) and cathodic (J) peak currents as a function of number of cycles.(c) Photographs of the water droplet volume of 10 mL on the surface of ITO, PG3/ITO and PG3/rough paper, respectively.Fig. 5 (a) The appearance of PG3, and in a DCM or TOL solution.Images of the PG3 camouflage fabric with space between each piece, (b) example of the patterned device in neutral and oxidized states, (c) fabric chameleonic electrochromic device cross-section for a typical device used in this study, (d) all pieces 1-7 as the working electrode in their neutral, (e) 1, 2, 3, 4 in neutral and 5, 6, 7 in oxidized, (f) in oxidized states.