Swift Electrofluorochromism of DonorâAcceptor Conjugated

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Swift Electrofluorochromism of DonorAcceptor Conjugated Polytriphenylamines Jingwei Sun, and Ziqi Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05661 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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ACS Applied Materials & Interfaces

Swift Electrofluorochromism of Donor−Acceptor Conjugated Polytriphenylamines Jingwei Sun and Ziqi Liang* Department of Materials Science, Fudan University, Shanghai 200433, China ABSTRACT: Electrofluorochromic (EFC) materials, which exhibit electrochemically controllable fluorescence, hold great promise in optoelectronic devices and biological analysis. Here we design such donor−acceptor conjugated polymersP(TPACO) and P(TCEC) that contain the same electron−rich and oxidizable polytriphenylamine (PTPA) as π−backbone, yet with different electron−deficient ketone and cyano units as pendant groups, respectively. They both exhibit solvatochromic effects due to intrinsic characteristics of intramolecular charge transfer (ICT). Compared to P(TPACO), P(TCEC) shows stronger ICT, which leads to higher electrochemical oxidation potential and lower ion diffusion coefficient. Moreover, both polymers present simultaneous electrochromic (EC) and EFC behaviors with multistate display and remarkably rapid fluorescence response. The response time of P(TPACO) is as short as 0.19 s, nearly 4-fold faster than that of P(TCEC) (0.92 s). Such rapid response is found to be determined by the ion diffusion coefficient which is associated with the ICT nature. Finally, the EFC display device based on P(TPACO) is successfully demonstrated, which shows green fluorescence ON/OFF switching upon applied potentials. This work has successfully demonstrated that swift

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EFCs can be achieved by rational modulation of the ICT effect in such D−A conjugated polymers.

KEYWORDS:

electrofluorochromism,

electrochemical

redox,

conjugated

polymer,

intramolecular charge transfer, rapid response

1. INTRODUCTION Electrofluorochromic

(EFC)

materials,

which

exhibit

electrochemically

controllable

fluorescence, are receiving increasing attention for potential applications in display, optical memory, sensor and biological analysis.1−3 Their fluorescence are generally switched between ON/OFF or different colors in relation to the electrically driven redox reactions.4−6 Such redox states dependent fluorescence alternately lead to direct observation of the progress of electrochemical reaction. Several examples of transition metal complexes, quantum dots, polyoxometalates, organic small molecules and polymers have successfully demonstrated EFC behaviors.7−12 Among them, the polymeric ones that enable solution-processing of large−area flexible EFC devices are promising candidates for EFC applications. Polytriphenylamine (PTPA) along with its derivatives, owing to their good hole-transporting and light-emitting properties, have been widely applied in organic light-emitting diodes and solar cells.13 Upon applied voltage, PTPAs undergo electrochemical oxidation reaction and generate stable cationic radicals, thus holding promise as anode material for lithium ion battery and supercapacitor applications.14,15 As a kind of classical electrochromic (EC) materials, they have been recently explored as EFC materials.10,16,17 However, desirable EFC materials with fast response and high contrast have yet to be largely explored. To realize such high−performance EFCs, more effort should be devoted


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to the investigations on structure−property relationships of EFC polymers, which will afford guidelines for rational molecular design. It is particularly necessary to evaluate the effects of both molecular configuration and electronic structure on the EFC performance. The donor-acceptor (D−A) π-conjugated structures enable fine-tuning of energy levels and optical bandgap and thus have been thoroughly studied in EC area.18 Different than EC materials, EFCs are mainly focused on fluorescence switching. As a major class of fluorescent units, the D−A conjugated structures allow for photoinduced intramolecular charge transfer (ICT).19−22 Yet the EFC behavior arising from electroactive D−A conjugated polymers is scarcely studied regarding the role of ICT in EFC processes. On the one hand, the ICT effect, which largely determines the fluorescent properties, is appreciably sensitive to external stimuli such as photons, electrons and protons.23−26 It is therefore anticipated that D−A pairs will respond intensely to applied bias, thereby displaying obvious fluorescence response. Besides, multiple emission colors could be realized by integrating suitable donors and acceptors.27−29 On the other hand, the ICT effect would impact strongly both the electrochemical redox and ions insertion/extraction process, thereby influencing the charge/discharge rate of materials. Therefore, the interplay between ICT and EFC behavior is an important issue that needs to be clarified. In this work, we design D−A conjugated polymers−P(TPACO) and P(TCEC), which are comprised of electron−rich PTPA as the π−backbone and electron−deficient ketone (C=O) and or cyano (−CN) based units as pendant groups. Note that in P(TPACO) the TPA, CO represent triphenylamine unit and carbonyl group, respectively, while in P(TCEC) the T, both C, and E represent triphenylamine, cyano, and ethenyl groups, respectively. Bearing the same electroactive TPA unit, P(TPACO) in ground and excited state as well as P(TCEC) show an increasing trend of ICT character, which allows one to study the influence of ICT on



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electrochemical and EFC properties. As a result, P(TPACO) shows much higher ion diffusion coefficient than P(TCEC). Moreover, both P(TPACO) and P(TCEC) exhibit simultaneous EC and EFC behaviors with multistate display and remarkably rapid fluorescence response. P(TPACO) shows a significantly short response time of 0.19 s, nearly 4-fold shorter than that of P(TCEC). Such rapid response is found to be determined by the ion diffusion coefficient which is associated with the ICT character. Lastly, an EFC display device based on P(TPACO) is successfully constructed, which shows remarkable green fluorescence ON/OFF switching upon applied potentials.

2. EXPERIMENTAL SECTION Materials. 4-(Diphenylamino)phenylboronic acid (98%), 4-bromobenzophenone (98%), tetrakis (triphenylphosphine)palladium (Pd(PPh3)4, 99%), 4-bromophenylacetonitrilelithium (99%), 4-cyanobenzaldehyde (98%) and perchlorate (LiClO4, 99%) were purchased from Energy Chemical Reagent Co. and used as received. All solvents and other reagents (analytical grade) were used without further purification, unless otherwise claimed. Fluorine-doped tin oxide (FTO) glass substrate (Kaivo Optoelectronic Technology Co., Ltd., Rs ≤ 15 Ω □ −1) as working electrode was cleaned by ultrasonic in distilled water, ethanol, toluene and acetone solutions, successively. Characterizations. The 1H and

13

C NMR spectra were recorded on a Bruker DMX-500

spectrometer using chloroform-d (CDCl3) as the solvent and tetramethylsilane (TMS) as an internal standard. Mass spectroscopy was recorded using a Waters GCT Premier MS spectrometer. Fourier transform infrared spectrometer (FTIR) was recorded on a Nicolet 6700 (Thermo Fisher Nicolet, USA) with KBr pellets. Gel permeation chromatographic (GPC)


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analysis was carried out on a Viscotek GPCmax integrated with a TDA 305 triple detector array using CHCl3 as the eluent and was calibrated with polystyrene standards at 40 oC. The UV-vis absorption spectrum was recorded on an Agilent 8453 UV-vis spectrophotometer (Agilent, USA). Fluorescent measurements were recorded on a Jobin Yvon FluoroMax-4 photoluminescence (PL) spectrophotometer. Fluorescence quantum yield φF was determined using a calibrated integrating sphere. The electrochemical property measurement was performed in a three-electrode cell on a CHI 660E electrochemical workstation (CH Instruments, China). The electrochemical measurements including the impedance spectroscopy were performed in a conventional threeelectrode cell contained 0.1 M LiClO4/(CH3CN or PC) solution as the supporting electrolyte. The counter and reference electrodes in the three-electrode cell are Pt and Ag/Ag+ electrodes, respectively. UV-vis and PL spectroelectrochemical tests were carried out on the CHI660E electrochemical analyser integrated with an Agilent 8453 UV-vis spectrophotometer or a FluoroMax-4 PL spectrophotometer, respectively. Synthesis. TPACO Monomer. Under a nitrogen atmosphere, a mixture of (4(diphenylamino)phenyl)boronic acid (1.74 g, 6 mmol), 4-bromobenzophenone (1.30 g, 5 mmol), Pd(PPh3)4 (0.065 g, 0.06 mmol), Na2CO3 (2.0 M, 3.0 mL), and toluene (50 mL)/THF (30 mL) was stirred at 90οC for 24 h. After it was cooled to room temperature, 100 mL of CHCl3 was added to the mixture. The organic portion was separated and washed with brine before drying over anhydrous MgSO4. The solvent was evaporated off, and the solid residues were purified by column chromatography to afford 1.49 g of green powder TPACO with a yield of 70.1%. 1H NMR (500 MHz, CDCl3, 25 °C): δ (ppm) 7.90 (d, J = 8.0 Hz, 2H); 7.85 (d, J = 7.0 Hz, 2H); 7.70 (d, J = 8.0 Hz, 2H); 7.62 (t, J = 7.5 Hz, 1H); 7.55 (d, J = 8.5 Hz, 2H); 7.52 (t, J = 7.5 Hz, 2H); 7.31 (t, J = 8.0 Hz, 4H); 7.17 (d, J = 8.5 Hz, 6H); 7.08 (t, J = 7.5 Hz, 2H). 13C NMR (125 MHz,



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CDCl3): δ (ppm) 196.26, 148.16, 147.46, 144.72, 137.95, 135.65, 133.28, 132.24, 130.81, 129.95, 129.38, 128.27, 127.94, 126.26, 124.79, 123.35. MS(EI): m/z 425.3. FTIR(KBr): ν = 1655 (C=O), ν = 1593 (C=C), ν = 1485 (C−C), ν = 1285 (C–N). TCEC Monomer. TCEC was synthesized as previously reported.30 1H NMR (500 MHz, CDCl3, 25 °C): δ (ppm) 7.98 (d, J = 8.0 Hz, 2H); 7.76 (d, J = 8.5 Hz, 2H); 7.75 (d, J = 8.0 Hz, 2H); 7.68 (d, J = 8.5 Hz, 2H); 7.56 (s, 1H); 7.51 (d, J = 8.5 Hz, 2H); 7.29 (t, J = 8.0 Hz, 4H); 7.15 (d, J = 8.5 Hz, 6H); 7.06 (t, J = 7.5 Hz, 2H). 13C NMR (125 MHz, CDCl3): δ (ppm) 148.09, 147.45, 141.00, 138.46, 132.65, 132.33, 130.11, 129.57, 129.37, 129.26, 127.66, 127.17, 126.65, 124.74, 123.43, 123.34, 118.12, 117.73. MS(EI): m/z 473.2. P(TPACO) and P(TCEC) Polymers. The P(TPACO) and P(TCEC) polymersb were prepared in the same manner via chemical oxidative polymerization of respective monomer TPACO and TCEC (1.2 mmol) in 30 mL of chloroform by using ferric chloride (FeCl3, 4.2 mmol) as oxidant. The solutions were stirred 24 h at 40 oC under N2. After completion of polymerization reactions, the mixtures were poured into methanol to deposit the product, which were then isolated by filtration and washed with methanol several times to afford green and orange powders, respectively, with yields no less than 70%. Device Fabrication. Polymer films were prepared by spin-coating chloroform solution of P(TPACO) (20 mg/mL) onto FTO glass substrates. The films were dried under vacuum. A gel electrolyte was prepared with poly(methyl methacrylate) (PMMA, 3 g) and LiClO4 (0.2 M) dissolving in propylene carbonate (PC, 5 g) to form a highly transparent and conductive gel. The gel electrolyte was spread on the polymer−coated side of the electrode, and then two electrodes were sandwiched. Finally, epoxy resin was used to seal the device.


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3. RESULTS AND DISCUSSION Photophysical Property. The D−A homopolymers P(TPACO) and P(TCEC) as shown in Figure 1a were synthesized through chemical oxidative polymerization of TPACO and TCEC respectively, and subsequently characterized with GPC, FTIR and XPS. The FTIR spectra of P(TPACO) in comparison to TPACO (Figure S1 in the Supporting Information, SI) shows that the intensity of C−H bending from 1,4−disubstituted benzene at 827.4 cm−1 is noticeably increased while that from monosubstituted benzene at 756.1 cm−1 is notably decreased. Similar phenomenon is observed in P(TCEC). These results indicate that the monomers have been successfully polymerized on the para positions of phenyl groups in TPA units. The weightaverage molecular weights (Mw) of P(TPACO) and P(TCEC) are 31000 and 28000 with the polydispersity indexes (PDI) of 2.6 and 2.1, respectively. Both polymers are soluble in common organic solvents such as toluene (Tol) and dichloromethane (DCM). The XPS spectrum of P(TPACO) (Figure S2 in SI) shows the absence of Fe element and very little Cl which presumably comes from glass substrate. This indicates that the obtained polymers have been well purified to the neutral state without residual Fe cations and Cl anions.



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Figure 1. (a) Chemical structures of P(TPACO) and P(TCEC). UV-Vis and PL spectra of (b) P(TPACO) and (c) P(TCEC) in various solvents. (d) Solvatochromic Lippert–Mataga models of P(TPACO) and P(TCEC). The dots represent the Stokes’ shift in different solvents and the lines are the fitted linear curve.

Given that P(TPACO) and P(TCEC) are comprised of an electron−rich PTPA backbone and an electron−deficient pendant group (C=O and −CN, respectively), both of them are expected to show ICT properties. Thus, the absorption and PL spectra of these two polymers in varying solvents were characterized to investigate their solvatochromic effect. As depicted in Figure 1b, P(TPACO) exhibits two characteristic absorption peaks at 255 and 384 nm, which are ascribed to the π−π* and ICT transitions, respectively. Both absorption bands change little with variation in the solvent polarity. However, the PL spectra are dramatically red shifted from 480 to 564 nm as the solvent varies from nonpolar toluene (Tol) to moderately polar THF, DCM and chloroform (CF) and to highly polar acetonitrile (Ace) and DMF. These results indicate that P(TPACO) has a small dipole moment in the ground state, whereas a large dipole moment forms in the excited state, which is associated with ICT due to the substantial charge redistribution. As shown in


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Figure 1c, P(TCEC) also shows two absorption bands at 329 nm and around 400−417 nm, which are derived from π−π* and ICT transitions respectively. In contrast to P(TPACO), the latter band is blue shifted from 417 to 400 nm with an increase of the solvent polarity, suggesting that P(TCEC) has a clear ICT character even in the ground state owing to the strong electronwithdrawing ability of −CN. Moreover, a more significant bathochromic luminescence from 552−647 nm is observed in P(TCEC) with increasing solvent polarity. In addition, the optical bandgaps of P(TPACO) and P(TCEC) are estimated 2.7 and 2.4 eV, respectively, from the onset absorptions in acetone. Such larger red-shifted wavelength and narrower optical band gap of P(TCEC) reveal its much stronger ICT character than P(TPACO). The solvatochromic behaviors were further examined using Lippert-Mataga equation31 which correlates the solvent polarity parameters f(ε, n) and the Stokes shifts (νa−νf) of P(TPACO) and P(TCEC) in varying solvents (Figure 1d). The detailed Stokes shifts and f(ε, n) are listed in Table S1. Clearly, P(TCEC) exhibits a good linear dependence of (νa−νf) on f(ε, n) with a comparatively higher slope of the plot, indicating that its large dipole moment remains unchanged regardless of the solvent polarity. However, the relationship between f(ε, n) and Stokes shifts of P(TPACO) deviates a lot from linearity, which implies the presence of different excited states. The possible excited states might be the locally excited (LE) and ICT excited states.32 Normally, the emission at short wavelength region in nonpolar solvents can be predominantly attributed to the LE component. In polar solvents, the polymer rotates to twisted conformation to equilibrate with surrounding solvent molecules, thus forming ICT state at which there is a total charge separation between D and A segments. The absorption and PL spectra of P(TPACO) and P(TCEC) films are shown in Figure S3 in SI. The P(TPACO) film displays an intensified green emission with a remarkable blue-shift (31



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nm) of the maximum wavelength compared to that of its acetone solution. The fluorescent quantum yield (φF) of P(TPACO) solid reaches 28%. Such substantially enhanced and blueshifted fluorescence is attributed to the limited ICT transition in solid state.33,34 For P(TCEC), the acetone solution shows weak emission with its PL spectrum approaching the base line, which is overlapped with the PL spectra of the mixture solution containing 10 vol% and 30 vol% of water, respectively. However, significantly increased PL intensity and little shift of PL spectra are observed when P(TCEC) gradually aggregates from the acetone solution with an increasing fraction of added water. P(TCEC) film shows the PL spectrum that nearly resembles that of its suspension solution containing 90 vol% of water (Figure S3). The φF of P(TCEC) is determined as 2.1 %. These results suggest that P(TCEC) possesses a certain ICT character in solid state. In short, both P(TPACO) and P(TCEC) present remarkable ICT properties. The ICT effect of P(TPACO) is limited in solid state or nonpolar solvents, which will be largely reinforced when P(TPACO) is in excited state in polar solvents. By contrast, P(TCEC) shows comparatively stronger ICT in all ground, excited states, and solution or solid states. Electrochemical Property. To examine the ICT effect on electrochemical property, the cyclic voltammograms (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectra (EIS) of P(TPACO) and P(TCEC) were measured. As shown in Figure 2a, the CV trace of P(TPACO) under room light shows two reversible redox couples with the oxidation peaks at 1.01 and 1.13 V and corresponding reduction peaks at 0.74 and 0.85 V, respectively. In addition, the DPV curve (Figure S4) shows more precise redox potential values of P(TPACO) at 0.83 and 0.97 V, respectively. Note that the larger redox voltages from CV results relative to those of DPV are attributed to the rapid scanning rate. The two−step redox of P(TPACO) in above CV and DPV curves is assigned to the sequential conversion of TPA moiety to its cationic


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radical (polaron) and dication (bipolaron) species.6 The approximately symmetrical areas between the oxidation and reduction waves suggest a good reversibility of these redox reactions. Interestingly, for the UV light excited P(TPACO) with a large dipole moment, the onset oxidation potential along with the potential separation between the oxidation and reduction peaks is obviously increased. A further increased onset oxidation potential is observed in P(TCEC) having an even larger dipole moment, which is coincident with the DPV result. Since P(TPACO) and P(TCEC) have the same electroactive TPA unit and electrochemical redox process, the increased oxidation potential is presumably assigned to the enhanced ICT from TPA to the electron-withdrawing groups of C=O or −CN, which decrease the electron cloud density in the central nitrogen atom of TPA and make the redox reactions difficult. In addition, P(TCEC) exhibits similar potential separation of 0.29 V between the oxidation and reduction peaks as P(TPACO) in ground state (0.28 V), which indicates that they might have comparable chargetransfer kinetics in thin film. This issue is further confirmed by the EIS results as discussed below.



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Figure 2. (a) CV traces of P(TCEC) film, P(TPACO) film under room light and UV light excitation at a scanning rate of 100 mV/s. (b) EIS spectra and (c) the relationship curves between Zre and ω−0.5 at low frequencies for P(TCEC) and P(TPACO) films. The EIS spectra of P(TPACO) and P(TCEC) as shown in Figure 2b consist of an initial intercept at the Z' axis in high frequency, a semicircle and an inclined line at low frequencies, which correspond to the resistance of the electrolyte (Re), the charge-transfer resistance across the film (Rct) and the Warburg impedance (Rw).35−37 The Rct are almost the same for P(TPACO) and P(TCEC) as anode materials, which reveal that these two polymers have similar chargetransfer resistance and electrical conductivity. Notably, the Warburg impedance of P(TPACO) and P(TCEC), which reflect the diffusion of doping ions into the bulk of electrode material, are significantly distinct. The diffusion coefficient (D) of ions doping into P(TPACO) and P(TCEC) can be obtained by Eq. (1):

= 0.5

(1)

where R is the gas constant (8.314 J mol−1 K−1), T is the temperature (298 K), A is the area of the electrode surface (1.5 cm × 2 cm), F is the Faraday’s constant (96500 C mol−1) and C is the molar concentration of doping ions (0.1 M). The Warburg coefficient σw can be obtained by Eq. (2): = + + σ ω.

(2)

where ω is the angular frequency in the low frequency region. The σw is the slope for the plot of Zre vs. ω-0.5 as shown in Figure 2c, which are 1395.06 and 3047.83 Ω cm2 s−0.5 for P(TPACO) and P(TCEC), respectively. Therefore, P(TPACO) shows much higher ion diffusion coefficient


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of 2.02 × 10−14 cm2 s−1 than P(TCEC) (4.23 × 10−15 cm2 s−1), which result in faster EFC response as discussed below. Such distinct polymer permeability to doping ions is presumably attributed to the varied interactions between ions and the polar molecules with different ICT characteristics. Note that these ion diffusion coefficients are relatively small because they were measured at open-circuit potentials. In order to evaluate the polymers in oxidized state, diffusion coefficient can also be extracted from the change of CV curves at the limiting scan rate. Owing to the instrumental limit, however, no linear relationship is found between the square roots of scanning rate and the peak current in Figure S5, from which the D value could be obtained. Electrofluorochromic Property. It is interesting to note that both EC and EFC behaviors can be simultaneously observed from P(TPACO) and P(TCEC) films. As shown in Figure 3a, the color of P(TPACO) changes under room light from faint yellow to orange at 1.0 V, brown at 1.1 V and subsequently dark blue at 1.2 V. Note that these applied potentials are close to the oxidation potentials in CV curve as discussed before. Upon excitation at 365 nm, the green fluorescence is gradually weakened by increasing the voltage and finally quenched above 1.1 V (Figure 3b). To better understand these phenomena, spectroelectrochemical spectra of P(TPACO) film were characterized to quantify its potential dependent optical properties. As shown in Figure 3c, in the neutral state at 0 V, P(TPACO) exhibits the maximum absorption at 389 nm. Upon increasing the potential to 1.0 V, a shoulder peak at 501 nm appears concomitantly with the weakened absorption band at 389 nm. When the voltage is raised to 1.1 V, a broad band around 833 nm arises and eventually becomes the maximum absorption at 1.2 V. The formation of these two new peaks at 501 and 833 nm are assigned to the evolution of polaron and bipolaron bands, respectively.38 In the meantime, the intensity of maximum PL peak at 521 nm of neutral P(TPACO) drops significantly and continuously with increasing applied potential, as displayed



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in Figure 3d. When the voltage is up to 1.2 V, the fluorescence almost vanishes with the PL spectrum approaching the base line. In comparison to P(TPACO), the color of P(TCEC) changes from orange to dark blue at 1.2 V, and its red fluorescence is quenched by increasing the voltage above 1.2 V (Figure S6 in SI). The different switching colors and fluorescence of P(TPACO) and P(TCEC) are attributed to their varied optical band gaps and ICT excited state.

Figure 3. The photographs under (a) room light and (b) 365 nm UV light, (c) UV–vis and (d) PL spectra of P(TPACO) film upon different applied potentials.

The changes in PL intensity of P(TPACO) and P(TCEC) films during CV scanning are presented in Figure 4 for in depth understanding of the EFC process. At the beginning of potential sweep from 0 V in the positive direction, the fluorescence of P(TPACO) remains the same until the oxidative current starts to increase at 0.75 V. Since then, the PL intensity drops dramatically to the lowest at about 1.2 V. This implies that the oxidized TPA quenches the fluorescence. In reverse scan from 1.3 V, during which the TPA radical cations are reduced to the neutral state, the PL intensity is increased at the same speed as that of the oxidation process. However, when the potential is decreased to 0 V, the PL intensity is not completely recovered to the initial state, which is attributed to a little charge trapping in the film. In the negative potential


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region, the PL intensity remains increasing until finally to the initial value as the trapped charge is released. Such a closed cycle indicates the good reversibility of P(TPACO) as EFC material. Similar phenomenon is observed from P(TCEC), which shows a comparatively slow change rate of PL intensity. Notably, for both polymers, the change rates of PL intensity in the positive and negative potential regions are distinct. Meanwhile, the fluorescence quenching is much faster than the full recovery process. It is evident that the redox of TPA moiety in both polymers induces reversible fluorescence ON/OFF switching. However, the change rate of PL intensity is not only related to the electrochemical redox reactions and the ions insertion/extraction, but also strongly affected by the traps, a very small amount of which could completely quench the fluorescence via energy transfer. The study of fluorescence switching kinetics is still a big challenge in the EFC field.

Figure 4. The changes in PL intensity of P(TPACO) and P(TCEC) films during CV scan at a rate of 100 mV/s.

Since the optical changes in response to electrochemical potential are reversible, repetitive fluorescence switching of P(TPACO) and P(TCEC) were examined upon cyclic potential-step between −1.3 and 1.3 V. The response time which is estimated at 95% of the full change of PL intensity is also compared between the two polymers. As shown in Figure 5a, both P(TPACO)



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and P(TCEC) have good reversibility without a decrease of maximum contrast ratio (namely, Ion/Ioff), which is the contrast between the intensities of fluorescence ON and OFF states. P(TPACO) has a larger contrast ratio of 21.8 in comparison to that of P(TCEC) (11.3), due to its high φF in solid state. Figure 5b depicts the response times of P(TPACO) and P(TCEC). Notably, P(TPACO) exhibits a significantly rapid switching with a fading time of 0.19 s, nearly 4-fold faster than that of P(TCEC) (0.92 s). This is attributed to remarkably high ion diffusion coefficient of P(TPACO), which is also nearly 4-fold larger than that of P(TCEC). The lighting times derived from these PL switching are 3.84 and 3.24 s for P(TPACO) and P(TCEC), respectively. Since no visible plateau of PL intensity is obtained within 5 s, a longer time is needed to reach the steady state in lighting process. It is well known that in conventional EC polymers, the response time is controlled by their electrical conductivity and ion diffusion coefficient. However, in EFC polymers, as we discussed before, the fluorescence switching kinetics is also influenced by the small amount of traps, which even become the most decisive factor during the fluorescence recovery process. According to the results of EIS, P(TPACO) and P(TCEC) have similar electrical conductivity, yet distinct ion diffusion coefficient. Thus in this PTPA system, the fading time is mainly determined by the ion diffusion coefficient, while the lighting time correlates to the traps. Although the complicated situations of the traps in polymers are unknown, the shorter lighting time of P(TCEC) indicates that an increase in the ICT characteristic might lead to an easy release of trapped charges.


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Figure 5. (a) Repetitive fluorescence switching and (b) response time of P(TPACO) and P(TCEC) films upon potential step between −1.3 and 1.3 V with a residence time of 5 s.

The long-term switching stability of EFC materials is one of the most critical parameters for their practical applications. Figure S7 exhibits the fluorescence switching durability curves of P(TPACO) and P(TCEC), respectively. P(TPACO) shows good stability with little loss of PL intensity after 15 cycles, while P(TCEC) shows poor stability with a significant decrease of PL intensity within 10 cycles. Different than conventional EC materials, poor stabilities are usually reported for EFC materials, in particular the polymeric ones, because a very small amount of trapped charges could significantly quench the fluorescence via energy transfer. Therefore, apart from good electrochemical and thermal stabilities, high electronic and ionic conductivities are also required to obtain stable EFC property in polymers. Other factors such as the polymer morphology and fluorescence lifetime may also influence strongly the EFC stability.



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Electrofluorochromic Device. Finally, to demonstrate the EFC application, we fabricated a solid−state display device in size of ca. 3 cm × 2.5 cm based on P(TPACO). As shown in Figure 6, the P(TPACO) film on patterned ITO-coated glass substrate is transparent under room light. The size of the ITO-coated stripe is ca. 10 mm × 3 mm. Upon excitation at 365 nm light, it shows bright green fluorescence. When 1.3 V is applied to the ITO electrode, the fluorescence of P(TPACO) on ITO is completely quenched. Such remarkable fluorescence ON/OFF switching in this prototype device (as presented in video in the SI) is in accord with the above experimental results. Further work on the long-term stability of the EFC device is underwent in our lab.

Figure 6. Photographs of fluorescence switching in EFC device based on P(TPACO).

We further investigated the electrochemical and EFC behaviors of this device by CV, EIS and PL spectroelectrochemical experiments as shown in Figure 7. In comparison to the CV trace of P(TPACO) film in three-electrode system (Figure 2a), P(TPACO) device reveals larger oxidation and reduction voltage peaks at 1.20 and 0.83 V, respectively (Figure 7a). The increased redox potentials are presumably attributed to the two-electrode cell structure and gel electrolyte, which is confirmed by the EIS results as shown in Figure 7b. Obviously, the charge-transfer resistance Rct is increased by more than one order of magnitude, indicating that the wettability of electrolyte on the surface of EFC film strongly affects the electrochemical reaction rate. As a result, longer response times of 4.42 s for lighting and 0.25 s for turn-off respectively are observed in the device, respectively. These results suggest that developing electrolyte materials for EFCs is another important issue to be addressed in the future. The device displays repeatable


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EFC behavior with a small contrast ratio of 1.48 due to the partially switched area of P(TPACO) film on patterned ITO-coated glass substrate. However, after several cycles of switch, the device shows slower responses and lower contrasts. Studies on long-term stability of these EFC devices are under way.

Figure 7. Properties of P(TPACO) EFC device. (a) CV trace at a scanning rate of 100 mV/s, (b) EIS spectrum, (c) Repetitive fluorescence switching and (d) its response time upon potential step between −1.3 and 1.3 V with a residence time of 5 s.

4. CONCLUSIONS In summary, we have prepared D−A structured conjugated polymers, P(TPACO) and P(TCEC), that contain the same electron−rich PTPA as the π−backbone and different electron−deficient ketone or cyano unit as pendant group. Both polymers show simultaneous EC and EFC behaviors, in which P(TPACO) exhibits four-color interconversion and a remarkably rapid fluorescence response of 0.19 s compared to P(TCEC) (0.92 s). Such different response times are attributed to their distinctive ion diffusion coefficients, which correlate intimately with their



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different ICT characteristics. Finally, we have successfully constructed an EFC display device based on P(TPACO), which shows remarkably green fluorescence ON/OFF switching upon applied potentials. This work demonstrates that the D−A π-conjugated structure with a certain ICT character represent promising candidates for swift EFC applications. ASSOCIATED CONTENT Supporting Information. The structural characterization such as FTIR and XPS and detailed photophysical data, along with a video of EFC device. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is funded by National Natural Science Foundation of China (NSFC) under grant No. 51473036 and Chinese Postdoctoral Science Foundation under grant No. 2015M581524. REFERENCES (1) Audebert, P.; Miomandre, F., Electrofluorochromism: from Molecular Systems to Setup and Display. Chem. Sci. 2013, 4, 575−584. (2) Sun, J.; Chen, Y.; Liang, Z., Electroluminochromic Materials and Devices. Adv. Funct. Mater. 2016, 26, 2783−2799.


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Swift Electrofluorochromism of DonorâAcceptor Conjugated

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