Synthesis and characterization of Azo doped ICs. The long alkyl chain length-based imidazolium ionic crystals (C12VIM[X]) were synthesized through anion exchange reaction with bromide procure 24 and their structures were shown in Fig. 1a. Azo was mixed with six varieties of C12VIM[X], each containing an anion [X] of different size, shape, and symmetry: bromide, tetrafluoroborate, hexafluorophosphate, carbonate, thiocyanate, and perchlorate. The resulting building blocks (Azo-C12VIM[X]) are referred to hereafter as Azo-C12VIM[Br], Azo-C12VIM[BF4], Azo-C12VIM[PF6], Azo-[C12VIM]2[CO3], Azo-C12VIM[SCN] and Azo-C12VIM[ClO4], respectively. Azo-C12VIM[X], containing predominately trans-Azo, is defined as trans-Azo-C12VIM[X], as opposed to cis-Azo-C12VIM[X]. The 5 wt% Azo was the optimized dose according to photoresponse and the phase compatibility of 5 wt% Azo-C12VIM[X]. For neat C12VIM[X], the thermoinduced first-order solid-to-liquid transition upon heating
process was confirmed by DSC curves (Supplementary Fig. 1) and phase transition temperatures were listed in Supplementary Table 1. After an addition of Azo, crystalline structures of trans-Azo-C12VIM[X] were less altered as indicated by Power-XRD results (Supplementary Fig. 2) and mostly mixtures remained the thermoinduced solid-to-liquid transitions under POM observation (Supplementary Fig. 3). Unexpectedly, trans-Azo-C12VIM[Br] and trans-Azo-C12VIM[SCN] demonstrated a mesophase, liquid crystal phases before entering isotropic liquid state (Supplementary Figs. 4 and 5) resembled supramolecular character of Azo and ICs.
Photoinduced solid-liquid transitions. Photoinduced liquefaction of trans-Azo-C12VIM[X] was then investigated. Taken Azo-C12VIM[Br] as an example, Azo-C12VIM[Br] was illuminated by alternating 365 nm UV light and 530 nm Vis light (Fig. 1b). Under UV irradiation, the yellow powder sample of trans-Azo-C12VIM[Br] directly melted to give a red-orange IL within 60 s at 25°C, as shown in Fig. 1b. Also, the adjacent drops fused into single huge drops. Both indicated that UV illumination could induce flow of the cis-Azo-C12VIM[Br]. In contrast, Vis light irradiation frozen the fluid cis-Azo-C12VIM[Br] in a rather rapid speed of 50 s. By pressing the solidified trans-Azo-C12VIM[Br], powder cracks were observed as typical character of solids. The light switchable solid-liquid transitions were observed for non-photoresponsive C12VIM[Br] by doping a small fraction of Azo.
Additional experiments were performed to study the photoinduced solid-liquid transitions. First, the surface temperature of Azo-C12VIM[Br] under UV irradiation (180 mW/cm2) measured by an infrared thermometer, was arranged from 25 ~ 30°C (Fig. 1c), which was lower than the melting point of trans-Azo-C12VIM[Br] (54°C). Also, Azo-C12VIM[Br] showed a noticeable temperature discontinuity for liquefiability upon heating process. It can be liquified by UV light from 15 to 35°C, but not at a rather high temperature, such as at 38°C, no UV induced liquefication were observed, even by prolonging UV irradiation time (Supplementary Fig. 6). In general, the photoinduced solid-liquid transitions are originated from the low melting points of the cis-isomers.25 In such a case, the cis-isomers should maintain its liquid state under UV irradiation as long as the temperature is above melting points of the cis-isomers. Therefore, either photothermal effect or phototunable melting points of trans-/cis-isomers were insufficient to liquefy Azo-C12VIM[Br], and the solid-liquid transitions of Azo-C12VIM[Br] mainly belong to photoinduced isothermal phase transitions.
Photoinduced ordering structure changes Photoinduced structural changes during the solid-liquid transitions were then investigated. As shown in Fig. 2a, trans-Azo-C12VIM[Br] exhibited crystalline birefringence at 25°C under polarized optical microscope (POM) observations, in consistent with sharp peaks appeared in powder X-ray diffraction (XRD) patterns. The layer spacing d of trans-Azo-C12VIM[Br] was almost double for the single chain length of C12VIM[Br] based on the (001) reflection, and such double layer assembled order character is similar to the structure of the neat C12VIM[Br] compound.24,26 Therefore, Azo well assembled in ICs and barely altered layer arrangements of C12VIM[Br]. Under UV irradiation, the crystalline birefringence disappeared quickly along with an absence of peaks in XRD patterns within 11 s. It suggested that UV light disturbed the ordering assembly of C12VIM[Br] with a result of the UV induced solid-to-liquid transition. Meanwhile, switching photo-irradiation from UV to Vis light caused a reversible crystallization from isotropic liquid to solid within 5 s (Supplementary Movie 1), as suggested by the reappearance of crystalline birefringence and the sharp peaks in the powder-XRD patterns in Fig. 2a. Such exceptional photoresponsive speed surpassed the previously reported solid-liquid transitions of azobenzene compounds.5,10,14 Moreover, these data confirmed that the photoinduced solid-liquid transitions of Azo-C12VIM[Br] was related to photoswitchable ordering structures. It was plausible to induce that photoresponsive order parameter determined the solid-liquid transitions for these Azo-doped ICs.
In addition, trans-Azo-C12VIM[Br] exhibited reversible photoinduced transitions from the liquid crystal state to the isotropic liquid state as confirmed by XRD measurements and POM observations (Supplementary Fig. 7 and Movie 2). Such liquid crystal-isotropic liquid transitions were similar to the reported azobenzene containing liquid crystal systems.20
Photoisomerization of Azo UV–vis absorption spectroscopy showed that Azo-C12VIM[Br] exhibited reversible photoisomerization during solid-liquid transitions (Fig. 2b). The experiments here were based on pristine photoisomerization of Azo in solid during solid-liquid transitions. Azo-C12VIM[Br] exhibited a strong π-π* band at 320 nm. Following the UV irradiation, the π-π* band of trans-isomers decreased and the n-π* band of the cis-isomers at 450 nm increased. The cis-ratio was about 53% as calculated from the intensity changes.27 Subsequent visible light irradiation on cis-Azo-C12VIM[Br] switched it back to the trans-Azo-C12VIM[Br] (Fig. 2c). Those data suggested that UV induced trans-to-cis isomerization of Azo was able to be occurred in C12VIM[Br] crystals and under Vis light irradiation, the cis isomers reversed back to trans-isomers. Apparently, nanoscale isomerization of Azo determined macroscale solid-liquid transitions of the non-photoresponsive crystals.
It should point out that successful photoisomerization of Azo in solid phase is indispensable prerequisite for solid-liquid transitions. Photoisomerization was mostly occurred in solvent or liquid-like state, such as photoisomerization in liquid crystal mesophase.28,29 Noticeably, photoisomerization of Azo here is quite different from the above, not due to liquid-like environment. In Azo-C12VIM[Br] building blocks, it was assumed that solid host C12VIM[Br] might have local free volume for the photoisomerization in microscale assembled crystalline environment.30,31 Proper loosely order packed C12VIM[Br] offered enough space for successful Azo isomerization. Also, Azo and imidazolium ionic crystals has high compatibility. C12VIM[Br] might play a role of soft phase to dissolve the azobenzene phase, which might contribute to Azo isomerization in solid state.
Ionic structure effect on photoinduced solid-liquid transitions To study the necessary structure constituents for the photoinduced solid-liquid transitions, structure effect of cations and anions on photoresponse was then investigated. Several typical ICs with different cations have been investigated as shown in Fig. 3 and supplementary Table 2. C2VIM[Br] and C12IM[Br] are imidazolium-based cations. C12Py[Cl] and C12Amin[Br] have pyridine cations and amino cations with long alkyl chains, respectively. Both Azo-C2VIM[Br] and Azo-C12IM[Br] demonstrated UV-induced solid-to-liquid transitions under POM observations at various temperatures (Fig. 3a and b). However, Azo-C12Py[Cl] and Azo-C12Amin[Br] did not induce the flow of the powder as shown in Fig. 3c and d. UV irradiated Azo-C12Py[Cl] was further investigated by UV-vis spectra measurements and there was no cis-isomer appeared (Supplementary Fig. 8). It seemed that imidazolium cations are essential for photoisomerization of Azo and photoresponsive solid-liquid transitions.
By altering anions, reversible photoinduced solid-liquid transitions were observed for Azo-C12VIM[X] building blocks (Azo-C12VIM[BF4], Azo-C12VIM[PF6], Azo-[C12VIM]2[CO3], Azo-C12VIM[SCN] and Azo-C12VIM[ClO4]) at various temperatures (Supplementary Fig. 9). The liquefied temperature ranges of Azo-C12VIM[X] are depicted in Fig. 4 according to POM observations under UV and Vis light illuminations. The photoinduced solid-liquid transition temperatures were just below the melting points of the corresponding ionic crystals and greatly dependent on anions of ICs. It partially proved that ionic crystals could provide free volume space for photoisomerization of Azo. Azo-C12VIM[SCN] offered the lowest liquefied temperatures below 10°C while Azo-C12VIM[PF6] could be liquefied above 50°C. Azo-C12VIM[Br] showed the widest photoresponsive solid-liquid temperature window, ca. 20°C. Thus, the method in doping a tiny amount of Azo to achieve photoinduced solid-liquid transitions is particularly useful for non-photoresponsive imidazolium-based ICs.
Comparison with other photoliquefiable azobenzene containing compounds It is well-studied that azobenzene containing molecules or polymers can reversibly convert the compounds from the solids to isotropic liquids. One possible mechanism is different melting temperatures or glass transition temperatures dependent on configurations of trans or cis isomers. Wu et al reported that azobenzene-containing polymers having two quite different glass transition temperatures, facile for process fabrication.18 Also, some azobenzene molecules can reversibly convert the compounds from the solids to isotropic liquids because of ordering changes. For example, by using X-ray crystal structure analysis, Hoshino et al. showed that azobenzene-containing crystal demonstrated photoinduced crystal-melt transition.11
However, the photoinduced solid-liquid transitions is different from reported ones, that is, the photoinduced solid-liquid transitions were targeted on non-photoresponsive compounds, rather than on azobenzene compounds themselves. Such photoinduced isothermal phase transitions were mostly similar with the reported azobenzene-doped liquid crystal materials, where Azo photoisomerization could induce liquid crystal-isotropic liquid transitions for the host non-photoresponsive liquid crystals.20 It strongly suggest that photoinduced order changes determine the solid-liquid transitions for these Azo-doped ICs.
Modulation of solid-liquid transitions. Molecular dynamics simulations in a Gromacs software were conducted in order to gain insight into solid-liquid transition mechanism. Different colors for Azo were set up, at the same time, Corey-Pauling-Koltun and ball-and-stick models were used to represent azobenzene and ionic liquids, respectively to better distinguish each other. In the trans-Azo-C12VIM[BF4] system (Fig. 5a and 5b), the imidazolium chain distribution was more regular, while in the cis-Azo-C12VIM[BF4] system (Fig. 5c and 5d), the chains were distributed irregularly and the interface was curved (concave and convex). The simulation results showed that both trans- and cis-Azo were assembled around imidazolium cations and anions, which was mainly caused by van der Waals and electrostatic interaction (Supplementary Fig. 10). Upon geometric change from in-plane trans-Azo to out-of-plane cis-Azo, the ordered layer structures were perturbed with showing curved edges, ultimately resulting in an isotropic liquid state. It seemed that photoisomerization of Azo produces intermolecular ‘‘communication’’ between the responsive machines and non-responsive subassemblies, thereby, phase states determined by the wholly assembled structure ordering degree amplified the nanoscale operations of Azo. FT-IR experiments were also conducted to confirm that Azo was confined nearly the ionic regions, in which UV induced vibrational changes in the imidazolium cation and in the anion (Supplementary Fig. 11). The proposed building block of Azo-C12VIM[X] not only offers a universal methodology for light-induced solid-liquid transitions of the non-photoresponsive C12VIM[X], but also makes a leap to demonstrate how Azo communicates with ionic crystals and deliver macroscale response of ICs.32
Applications based on photoinduced solid-liquid transitions. Due to distinguished solid-liquid transitions, ionic function could be significantly tuned by light. In crystalline state, the ions are immobilized as confined in crystalline lattices while in liquid state, the ions are flowing to demonstrate various functions of ionic liquids as illustrated in Fig. 6a. The immobilization and mobilization of ions means the switching off and on of certain ionic functions, thus more integrated functions and sophisticated opto-iontronics device of Azo-C12VIM[X] would be expected to be explored.
Light regulated insulator-conduction transitions. Based on the switchable solid-liquid transitions, light-controlled ionic conductivities of Azo-C12VIM[Br] were next measured by electrochemical impedance spectroscopy (EIS) to demonstrate macroscale property changes derived from nanoscale motion of molecular machines. Figure 6b shows a full frequency range of the Bode plots under Vis and UV irradiation at 15°C. For trans-Azo-C12VIM[Br], the impedance magnitude (|Z|) was very high, and the phase angle (–ϕ) was close to 90 °, indicating highly resistive and capacitive responses. These behaviors strongly suggest that trans-Azo-C12VIM[Br] behaves as an insulator as ions are frozen in the crystal lattice. Under UV irradiation, |Z| greatly decreased and –ϕ approaches 0 ° as the frequency increased, which is typical for an ionic conductor. The
differences became more apparent when |Z| and -ϕ values were compared at high frequencies (10-1-102 kHz) (Supplementary Figs. 12 and 13), indicating a UV induced sharp transition from an ionic insulator to an ionic conductor. Changes in |Z| and –ϕ were repeatable (Supplementary Fig. 14 and Fig. 15). The bulk impedance (Rb) was obtained from the Nyquist plot as shown in Fig. 6c. Upon the solid-liquid transition, trans-Rb, 2.7 x 108 Ω, was reduced to cis-Rb, 25 Ω, corresponding to a change in ionic conductivity σ from 6.1 x 10− 11 S/cm to 6.6 x 10− 4 S/cm (Fig. 6d), respectively. In short, Azo could switch ionic insulator-conductor transitions of the C12VIM[Br].
Light-controlled capacitors. In addition, light-controlled charge and discharge processes were demonstrated in Azo-C12VIM[Br] based electric double layer capacitors as illustrated in Fig. 6e. An Azo-C12VIM[Br] electrolyte was sandwiched by indium tin oxides (ITO) electrodes to fabricate a capacitor. Current density of the trans-Azo-C12VIM[Br] based capacitor was close to zero at various scan rates because of frozen ions (Fig. 6f and Supplementary Fig. 16), while a cis-Azo-C12VIM[Br]-based one showed stable electrochemical performance in cyclic voltammetry and galvanostatic charge–discharge tests (Fig. 6f and Supplementary Fig. 17) with a capacitance around 25 µF/cm2. The rectangular-shaped CV curves (Fig. 6f) and the increasing current with an increase in the scan rates were of typical capacitance response. Thus, a light controlled capacitor concept was proposed: the capacitor could be charged in liquid state and the energy could be stored in solid state as illustrated in Fig. 6d. As shown in Fig. 6g, the open-circuit voltage (VOC) accumulated by charging cis-Azo-C12VIM[Br] was maintained at 0.23 V after immediately switching to Vis irradiation, this finding provides evidence that frozen ions can store energy. The charged state was able to be maintained by in solid trans-Azo-C12VIM[Br]. As expected, the voltage showed a rapid decline to 0 V under UV irradiation as an energy release process. Light-switched solid-to-liquid and liquid-to-solid transitions endowed charge and discharge process of the capacitors to be controlled, which might shed light on designing advanced capacitors.
UV sensors As an example of UV detector application, we monitored the ionic signals of Azo-C12VIM[Br] under various UV intensities (Fig. 6h) in a circuits with a blue LED light. By tuning various UV intensities, different ionic signals could be output with good reversibility as shown in Fig. 6h. LED light showed distinguished light intense based on various UV intensities. These findings indicate Azo-C12VIM[Br] with light switchable ionic resistance can output recognizable ionic signals, potentially working as a robust UV detector with rapid response. The UV sensors makes full use of each component: Azo as a molecular trigger to sense UV light, solid-liquid transitions to transfer and amplify ionic signals.