A dark ionic liquid for flexible optoelectronics

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A dark ionic liquid for flexible optoelectronics Yonglin He, Xiao-Qi Xu, Shanzhi Lv, Hongguang Liao, and Yapei Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02125 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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A dark ionic liquid for flexible optoelectronics Yonglin He, Xiao-Qi Xu, Shanzhi Lv, Hongguang Liao, and Yapei Wang*

Department of Chemistry, Renmin University of China, Beijing 100872, China

ABSTRACT: Owing to wide visual angle, low aberrations and great depth of focus, flexible optoelectronics have become one subject of intense investigations for rescue equipments and endoscopy tools. Ionic liquids are rising as a kind of fluidic “semiconductor” with advantages of high flexibility and self-healing ability. However, challenges in molecular design of photo-responsive ionic liquids impede the exploration of ionic liquids as intrinsic flexible liquid optoelectronics. This work demonstrated an imidazole-based ionic liquid covalently linked with polypyrrole oligomer by alkyl chains. Such an ionic liquid has a wide absorption from visible light range to near-infrared light range. The imidazole moiety acts as an electrical conductor which is thermally responsive. On the other hand, the polypyrrole segment serving as a light antenna is able to convert light energy into thermal heat. The alkyl linker tailors the energy transfer between polypyrrole and imidazole cation. Negligible molecular aggregation and phase separation are attributed to the preservation of fluidic nature at room temperature. This photo-responsive ionic liquid is successfully exploited as a flexible light detector that is adaptable to special sensing tests at bending states.

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Due to the intrinsic advantages of flexibility and self-healing ability, ionic liquid is rising as an intriguing electronic material for flexible and stretchable electronics.1-5 The acceleration of ion mobility against the increase of environmental temperature renders the possibility to serve the ionic liquids as thermal sensing material.1-2, 6 According to this principle, flexible, stretchable and self-healing thermometers were successfully exploited by loading ionic liquids into elastomeric or self-healing matrix.2 Besides the emphasis of fluidic advantages, concerns about easy preparation and leakage prevention also came into focus in the consideration of the practical applications. For example, the combination of inkjet printing technology enables the easy and scalable preparation of paper-based thermometers.7 Narrowing the micro-channels in which ionic liquids are loaded enhances the capillary effect and hinders the leakage phenomenon.2 Generally, only when the resistance is changed the ionic liquid can function as electrical sensors. In addition to the temperature actuation, the resistance change can be also tailored through the choice of mechanical deformation,8-10 by which ionic liquids were extended to strain sensing applications. Most ionic liquids should obey these laws that their conductivity is influenced by temperature and tensile deformation. However, other sensing tests with the use of ionic liquids are limited because few of them are sensitive to non-temperature and non-strain signals. One of the great potentials for applying ionic liquids into other sensing areas is the development of flexible optoelectronics. The liquid light-sensitive material is highly desirable to fabricate curved optical sensing system with wider visual angle, lower aberrations and greater depth of focus than planar systems, which meets the special requirements by rescue equipments and endoscopy tools.11-13 There are two ways to endow ionic liquids with the 2 ACS Paragon Plus Environment

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light-responsive ability. One is blending photo-responsive agents in ionic liquids, by which the light energy is supposed to induce the change of ionic mobility. Long-term stability is not satisfactory because phase segregation may occur in these complex systems.1 The other strategy is the synthesis of ionic liquids that can intrinsically absorb light energy.14 A few azobenzene-containing ionic liquids exhibited a moderate electrical response to UV light based on the photoisomerization of azobenzene moiety.15-17 However, examples of ionic liquids that can absorb light with longer wavelength are sparse. Principally, larger π-conjugated chromophores are needed to allow lower band-gap electron transition by absorbing light with a longer wavelength. Due to intermolecular π-π interaction, it is challenging to formulate those large chromophores as ionic liquids with a melting point below room temperature.18-21 Herein, we presented a kind of intrinsic light-sensitive ionic liquids that have wide absorption in the ranges of visible light and near-infrared light. The ionic liquid is composed of imidazolium bromide and polypyrrole oligomer. These two components are covalently linked each other by alkyl chains of different length. In terms of low tacticity and wide polydispersity, the polypyrrole oligomers generate little degree of aggregation which preserves the fluidic performance of imidazolium bromide. Polypyrrole possesses the ability of photothermal conversion,22 by which light energy is converted to thermal energy and subsequently causes the conductivity change of ionic species. Based on this principle, this particular class of ionic liquids was successfully used as fluidic light sensing materials to prepare flexible optoelectronic sensors.


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Figure 1. (a) Schematic illustration of the synthesis of poly-[Py-Cn-MIm]Br based on the oxidative polymerization of [Py-Cn-MIm]Br. (b) 1H-NMR spectra of [Py-C6-MIm]Br upon exposure to the atmosphere for different time. (c) ATR-IR spectra of [Py-C6-MIm]Br and Poly-[Py-C6-MIm]Br.

(d)

Time-dependent

polymerization

rates

of

[Py-C4-MIm],

[Py-C6-MIm] and [Py-C8-MIm].

Figure 1a illustrates the synthesis of polypyrrole-containing imidazolium-based ionic liquids. The liquid precursor of [Py-Cn-MIm]Br was synthesized according to a modified Li’s method,20 in which Cn refers to alkyl chain with carbon number of n. Typically, [Py-Cn-MIm]Br was heated to 100 oC under air atmosphere with vigorous stirring (Figure S1 4 ACS Paragon Plus Environment

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and S2), allowing pyrrole group to be oxidized by the oxygen and polymerized into polypyrrole. As a result, [Py-Cn-MIm]Br was turned into poly-[Py-Cn-MIm]Br. Since both [Py-Cn-MIm]Br and poly-[Py-Cn-MIm]Br are soluble in DMSO-d6, the polymerization process could be monitored in real time by 1H-NMR spectroscopy. As shown in Figure 1b, the integral area of hydrogens on α and β carbon of the pyrrole ring in [Py-C6-MIm]Br decreases with the reaction time, indicating both α and β carbons are involved in the polymerization of pyrrole units. This polymerization type is unlike oxidation of pyrrole at low temperature which mainly happens at α carbons.23-24 It is assumed that high temperature diminishes the difference in reactive activity between α and β carbons. However, high temperature may also cause quinone oxidation of aromatic rings. As shown in Figure 1c, attenuated total reflection infrared (ATR-IR) spectroscopy is used to confirm if quinone oxidation occurs in the process of pyrrole polymerization. Characteristic frequencies at 1568, 1462, 1167 and 1091 cm-1 assigned to pyrrole ring, and characteristic frequencies at 1504 and 1281 cm-1 assigned to imidazole ring are all observed (Figure S6 and S7, Table S1). Yet characteristic frequencies assigned to C=O group, usually at 1680-1690 cm-1, does not appear, indicating mild oxidation by atmosphere at 100 oC does not induce high valence oxidation and destroy the aromatic structures. Further confirmation by 1H-NMR spectroscopy reveals that other hydrogens remain unchanged except the decreased number of α and β carbons, convincing that only polymerization as a form of chain growth takes place during the oxidation process (Figure S3, S4 and S5).


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Figure 2. Optical images of [Py-C4-MIm]Br, [Py-C6-MIm]Br and [Py-C8-MIm]Br, (a) before and (b) after polymerization. (c) Normalized UV-Vis-NIR spectra of the poly-[Py-C4-MIm]Br, poly-[Py-C6-MIm]Br and poly-[Py-C8-MIm]Br in DMSO. (d) Cyclic voltammogram curves of [Py-C6-MIm]Br and poly-[Py-C6-MIm]Br at a scan rate of 100 mVs−1 in acetonitrile solution. (e) Normalized DSC curves of Poly-[Py-C4-MIm]Br, poly-[Py-C6-MIm]Br and poly-[Py-C8-MIm]Br, respectively.

The time-dependent polymerization rates of three liquid precursors with alkyl chain length of 4, 6, and 8 were investigated according to 1H-NMR spectroscopy studies (Figure 1d). The polymerization rate refers to the ratio of the integral area of hydrogens on both α and 6 ACS Paragon Plus Environment

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β carbon of the pyrrole ring before and after oxidation for specific time. Obviously, the polymerization

of

[Py-C4-MIm]Br

is

faster

than

that

of

[Py-C6-MIm]Br

and

[Py-C8-MIm]Br. It is considered that [Py-Cn-MIm]Br with longer alkyl chain has lower concentration of active hydrogen (α and β carbon of pyrrole ring) because of the dilution effect by alkyl chain. To ensure the polymerization degrees are similar, the polymerization time of poly-[Py-C4-MIm], poly-[Py-C6-MIm] and poly-[Py-C8-MIm] are kept as 2 hours, 3 hours and 6 hours, respectively, unless otherwise indicated. Insights were further provided into the products of polymerized ionic liquids. As shown in Figure 2a and 2b, the color of three precursors all turns from light yellow to deep dark brown after oxidation, implying the products have intense absorption in visible light. UV spectroscopy confirms the broad absorption in ultraviolet-visible light, even part of near-infrared light (Figure 2c). In comparison, [Py-Cn-MIm]Br has negligible absorption in these light regions. The studies of cyclic voltammogram demonstrate that the band gap as a difference between oxidation peak and reduction peak is decreased after the polymerization of [Py-Cn-MIm]Br into poly-[Py-Cn-MIm]Br (Figure 2d, Figure S9). The decrease of band gap explains the red shift of light absorption of poly-[Py-Cn-MIm]Br in contrast to [Py-Cn-MIm]Br (Table S2). It is worth noting that several oxidation peaks as well as reduction peaks are found in each poly-[Py-Cn-MIm]Br, which should be attributed to wide polydispersity of polymer products. Accordingly, the increased number of band gaps enriches the pathways of electron transition, thus widening the light absorption. In this regard, poly-[Py-C4-MIm]Br has wider polydispersity and more polymerization consumption than


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poly-[Py-C6-MIm]Br and poly-[Py-C8-MIm]Br, and thus it relatively has stronger and broader absorption (Figure 2c). Unlike polymerization of pyrrole at low temperature in which chain growth mainly happens at α carbons, the uncommon polymerization at α and β carbons of pyrrole, as stated above, widens the polydispersity and significantly reduces the tacticity of polypyrrole. The low tacticity and wide polydispersity are assumed to weaken the aggregation of poly-[Py-Cn-MIm]Br. This assumption is fully supported by the physical performance of poly-[Py-C4-MIm]Br and poly-[Py-C6-MIm]Br. They both are liquid-like polymers at room temperature. As shown in Figure 2d, their melting points are -27.8 oC and -26.5 oC, respectively, much lower than room temperature. Yet for poly-[Py-C8-MIm]Br, it possesses two forms, including liquid-like poly-[Py-C8-MIm]Br (l) and solid-like poly-[Py-C8-MIm]Br (s). Temporally, poly-[Py-C8-MIm]Br (l) is a fluidic product after polymerization (Tm ~ -24.7 o

C), which gradually turns solid after sealed storage for two weeks. The solid

poly-[Py-C8-MIm]Br (s) could be melted back to poly-[Py-C8-MIm]Br (l) at a temperature above its melting point (80 oC, Figure S8). A comprehensive assessment was provided in the light sensitivity of three kinds of ionic liquids. The optoelectronic detector was prepared by loading ionic liquids in a circular micro-chamber which is bridged with a thermometer and current tester to monitor the change of conductivity and temperature under light irradiation (Figure S10 and S11). Sunlight was typically used in terms of its wide wavelength. As schematically illustrated in Figure 3a, three processes are involved to transform optical signal to electrical signal, including light absorption, thermal generation and electrical response. Specifically, light is firstly harvested 8 ACS Paragon Plus Environment

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by the polypyrrole segments of ionic liquid. Almost at the same time, the absorbed light energy is partially converted to thermal heat by non-irradiated emission based on internal conversion and vibration relaxation. The thermal energy then raises the local temperature within ionic liquids, which accelerates the ionic mobility and increases the current in the external circuits. Namely, polypyrrole acts as a critical energy mediator in the light sensing process.

Figure 3. (a) The mechanism of light sensing by poly-[Py-Cn-MIm]Br based on photothermal conversion. (b) Electrical responses of different ionic liquids as a function of light intensity. (c) On-off cycles of the light response of different ionic liquids between one standard solar intensity and darkness.

The light sensing performance of poly-[Py-Cn-MIm]Br was evaluated in Figure 3b and 3c. The change of the conductance, ∆/ , is used to represent the light sensibility, in which is the conductance without light irradiation and ∆ is the conductance change between 9 ACS Paragon Plus Environment

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irradiation and darkness. The conductance of three ionic liquids are all increased proportionally with the increase of light intensity. In comparison, the light sensitivity improves

in

an

order

of

poly-[Py-C8-MIm]Br,

poly-[Py-C4-MIm]Br,

and

poly-[Py-C6-MIm]Br. To be specific, the conductance change of poly-[Py-C6-MIm]Br reaches 36.2% under a standard solar radiation, higher than that of poly-[Py-C4-MIm]Br and poly-[Py-C8-MIm]Br which is 24.0% and 18.1%, respectively. Remarkably, the ionic liquid-based light detectors exhibit excellent sensing stability and reversibility. The deviation among different light on-off cycles is less than 1%, offering great promise for practically reliable uses without tedious calibration. Three processes involved in the optoelectronic transformation are mathematically analyzed to investigate factors that influence the light sensitivity. First, the heating power () of the light irradiation ( ) is expressed as equation (1):

= ∙ ∙ # 1 Where is the light absorption of the ionic liquids in micro-chambers at a given wavelength ( ), and is the photothermal coversion efficiency. To simplify the calculation, light intensity is fixed at a standard solar irradiation. The second part in equation (1) is defined as , namely = ∙ , which can be calculated by integrating the

product of and according to light absorption curves in Figure 4b. The exact

values of different ionic liquids are summarized in Table S3. Agreeing with the discussion as stated above, the ionic liquid with shorter alkyl chain possesses stronger light absorption. Poly-[Py-C4-MIm]Br is able to harvest light with a high efficiency of 89.9%, while


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poly-[Py-C8-MIm]Br has a smaller light absorption efficiency of 68.6%. As for , it can be calculated if the value of is identified, which will be discussed in detail as below.

Figure 4. IR images of the micro-chambers filled with poly-[Py-C4-MIm]Br, poly-[Py-C6-MIm]Br and poly-[Py-C8-MIm]Br under a standard solar irradiation. (b) Optical spectrum of standard solar radiation (blue curve) and the absorption spectra of the ionic-liquids containing micro-chambers (red line). (c) Integrated heat energy for ionic liquids with the temperature increased from 20 oC to 50 oC. (d) Thermal sensitivity of ionic liquids as the slope of conductance change against temperature change. The abscissa number (n) represents the carbon number of alkyl chain in poly-[Py-Cn-MIm]Br. (e-g) The temperature

change

of

ionic

liquids,

including

poly-[Py-C4-MIm]Br

(e),

poly-[Py-C6-MIm]Br (f) and poly-[Py-C8-MIm]Br (g), under a standard solar irradiation (dash lines). The temperature increasing part and decreasing part refer to light-on state and light-off state, respectively. The temperature increasing parts are curvedly fitted equations with R-square higher than 0.97. 11 ACS Paragon Plus Environment

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Followed by light absorption, the second process is analyzed to identify . The heating power ( ) can be divided into two parts, including the thermal energy raises the temperature of light detector and the thermal energy is dissipated to surroundings due to the temperature difference (∆) between the detector and environment. As the specific heat capacity, , is almost unchanged at the temperature ranging from 20 oC to 50 oC, is regarded as a constant. In these regards, a relationship between and temperature at a specific time (t) is formulated as equation (2):

= − ℎ∆ # 2

To be speciﬁc, and represent the weight and the temperature of the ionic liquid, respectively, and is the heat dissipating area. To simplify the model, the heat transfer coefficient, ℎ, is regard as a constant, which is validated by temperature curves in Figure 3e, 3f and 3g. The solution of the equation (2) can be stated as another form in equation (3):

∆ =

! ∙% 1 − "#$ & # 3 ℎ

The heat capacity, , can be obtained by experimental measurements. To make a direct comparison, the thermal heat needed for increasing temperature from 20 oC to a specific temperature ( ( ) with regardless to thermal dissipation is demonstrated as )

* + , which is linearly plotted in Figure 4c. Less thermal energy is needed to increase

the temperature of poly-[Py-C6-MIm]Br to a specific value than that of poly-[Py-C4-MIm]Br or poly-[Py-C8-MIm]Br. Such a difference is attributed to two opposite factors, including the density of polypyrrole and the specific heat capacity ( ). The former is supposed to decline as the increase of alkyl chain length while the latter follows an opposite law. Though lowering heat capacity can facilitate the increase of temperature at a given heat energy, the 12 ACS Paragon Plus Environment

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practical ∆ of poly-[Py-C6-MIm]Br is indeed smaller than that of poly-[Py-C4-MIm]Br as presented in Figure 4a, 4e and 4f. Therefore, it is concluded that the energy harvesting by polypyrrole segments is superior to the heat capacity for determining the light sensitivity. The values of heating power ( ) and the product of heat transfer coefficient with heat dissipating area (ℎ) can be identified by fitting the curves as in Figure 4e, 4f and 4g. Particularly, ℎ of three ionic liquid systems is similar because the testing setups are almost the same. For the heat power ( ), the situation is completely different from the light absorption spectra, as poly-[Py-C8-MIm]Br generates almost the same thermal energy in unit time as poly-[Py-C6-MIm]Br. Once the values of are identified, can be calculated according to the equation (1). As summarized in Table S3, the poly-[Py-C6-MIm]Br has the lowest photothermal conversion efficiency among three kinds of ionic liquids, as a result of the lowest heat power. Based on these analysis, it is assumed that improving the polymerization

degree

(poly-[Py-C4-MIm]Br)

or

prolonging

the

alkyl

chain

(poly-[Py-C8-MIm]Br) is favorable for photothermal conversion. Another

key

factor

for

the

outstanding

light

sensing

performance

of

poly-[Py-C6-MIm]Br is its excellent thermal sensitivity. The relationship between the conductivity, ,, and the temperature of ionic liquids follows VTF equation.25-26 Accordingly, we can obtain the relationship between ∆/ and ∆ by equation (4): ∆ , = − 1 = ∆)# − 1# 4 ,

In which the factors of , / and are constant. Since ∆ is relatively small, the function can be expressed as the form of Taylor expansion and only the first coefficient of Maclaurin series needs to be counted. In this regard, the relationship between conductivity change and 13 ACS Paragon Plus Environment

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temperature change is remodified as a linear equation. As compared in Figure 4d, the slopes which stand for thermal sensitivity of ionic liquids reveal that poly-[Py-C6-MIm]Br is more thermal sensitive than the other two ionic liquids. As a brief summary, the light sensing process, including absorption and conversion, heat capacity, and thermal response are three main steps to influence the light sensitivity. These three steps should occur at the same time though we have discussed them separately. Poly-[Py-C6-MIm]Br does not prevail in the first step of energy absorption and conversion, it exhibits better performance in the latter two steps, which comprehensively possesses better light sensitivity than the other two ionic liquids. Therefore, poly-[Py-C6-MIm]Br was used as the liquid light sensing material to fabricate a flexible optoelectronic sensor. As shown in Figure 5a, a PDMS microchannel was templated from a silicon master which was fabricated by photo-lithography. The microchannel was fully filled with poly-[Py-C6-MIm]Br to ensure the formation of liquid conductive pathways. Once bridged in a circuit, this microchip is expected to deliver electrically response to light irradiation. Since this sensor is only formulated by elastomeric matrix and liquid sensing material, it is extremely flexible which can be arbitrarily bent without any mechanical damages (Figure 5b and 5c). The high flexibility enables the sensor to work on flat surfaces and curved surfaces (Figure 5d and 5e). Infrared radiation (IR) camera notably distinguished the temperature change of the sensor at flat or bending states under light radiation. Accordingly, the conductivity is reversibly increased and decreased along with the cyclic light on-off treatments.


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Figure 5. (a) Schematic fabrication of flexible optical sensors by loading ionic liquids in microchannels. (b, c) Optical images of the flexible sensor at bending states. (d, e) IR images and electrical responses of the flexible optical sensor under a standard solar radiation on flat and curved surfaces, respectively.

In conclusion, a particular class of intrinsic light sensing ionic liquids were synthesized based on a method of mild oxidation polymerization. The ionic liquids are formulated by imidazolium cation countered with bromide ion and polypyrrole between which an alkyl chain with different length is bridged. The low tacticity and wide polydispersity ensure the polypyrrole part does not cause evident molecular aggregation that may destroy the fluidic performance. The basic light sensing mechanism was attributed to a combination of photothermal conversion by polypyrrole part and the electrical response by imidazolium ion 15 ACS Paragon Plus Environment

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part. Such a kind of dark ionic liquid exhibits intriguing sensitivity to light with long wavelength, ranging from visible light to near-infrared light. This breakthrough affords powerful supplements to organic optoelectronics which are currently challenging to detect low-energy light owing to poor exciton separation. According to the mathematical analysis, it is believed that the light sensitivity could be further improved by enhancing the light absorption and photothermal conversion efficiency, or reducing the heat capacity. Ionic liquids with other chromophores will be attempted in the consideration of extending the light absorbing wavelength and improving the extinction coefficient.

ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Materials, instruments and characterizations, experimental details and supplementary figures are included here. The following files are available free of charge.

AUTHOR INFORMATION

Corresponding Author * Email: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT


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This work was financially supported by the National Natural Science Foundation of China (21674127, 21422407, and 51373197).

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A dark ionic liquid for flexible optoelectronics

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