Materials that change their state or property in response to external stimuli1 such as heat,2 pressure,3 photo‐irradiation,4 and voltage bias,5 are highly desirable in various applications, such as memory devices,4b, 5c artificial muscles5b and drug delivery systems.5a,5d However, typically those materials are either elaborated polymers having ionic/polar groups1b,1c, 3a, 4a,4b, 5a–5b,5d or inorganic metal oxides that are difficult to be used for biological and soft‐material applications due to insufficient biocompatibility or flexibility.1d, 2, 3b, 4b, 5c

Therefore, the development of new smart materials and new design principles is extremely important for the future of this type of enabling technology. One attractive and successful strategy is to assemble stimuli‐responsive parts and function‐exerting parts in a controlled molecular system.3a, 4a, 5a,5b,5d From this viewpoint, a simple promising method is to use a porous framework in which a potentially external‐stimuli‐responsive functionality is embedded, and to switch the properties by binding a guest molecule to the framework (Figure 1 a).6

Figure 1 Open in figure viewer PowerPoint Electric‐stimuli‐responsive porous materials. a) Schematic illustration of the strategy to develop electric‐stimuli‐responsive materials using porous solid. b) Electric‐stimulus‐induced phase transition of the assembly of hydrocarbon nanoring cycloparaphenylene (CPP) and iodine, leading to turn‐on electronic conductivity and white light emission.

Confinement of a molecule in the porous framework then allows the hybridization of each property of the framework and the guest species, leading to the emergence of new stimuli‐responsive multifunctionality. This seemingly simple strategy has not been realized sufficiently yet.7 As far as electric stimulus is concerned, no electric‐stimulus‐responsive porous framework has been known until very recently.8

Here we report an extremely unique electric‐responsive material that is made just by mixing a simple hydrocarbon nanoring and iodine (Figure 1 b). After the application of electric stimulus, this assembled material takes on two attractive properties: turn‐on electronic conductivity and white light emission (Figure 1 b).

The hydrocarbon nanoring used in this study is [n]cycloparaphenylene ([n]CPP), a nanoring composed of para‐connected benzene rings, where n represents the number of benzene rings (Figure 1 b).9 The groups of Jasti,9c Itami,9d and Yamago9e have established protocols for the ring size‐selective synthesis of [n]CPPs (n=5–16) and some CPPs are now commercially available.9 Unique photophysical, redox, and host–guest properties of CPPs have been intensively investigated and gaining growing interests in various fields.9 Moreover, CPP represents a new carbon‐based soft porous material with unique adsorption behaviors, directly related to the intrinsic hollow nanospace in the solid state.10 CPP‐based porous frameworks can confine only molecules that fit the specific pore size. Precise control of the number and alignment of confined molecules is realized by changing the number of benzene rings of [n]CPP, making it possible to explore the proper combinations to show cooperative changes in structure and properties.

The present study is built upon our discovery that structural distortions of a CPP crystal occur with the application of electric stimulus. CPP, which forms uniform channels in the solid state (see Figure S1 in the Supporting Information),9, 10 showed structural distortions caused by electric stimulus application with the stacking mode of CPP being preserved (Figure S2).

Aiming at developing new CPP‐based stimuli‐responsive materials, we decided to combine CPP with iodine, which shows structural flexibility in response to environmental changes.11 This led to the even more exciting discovery that the thus‐formed [10]CPP‐iodine assembly ([10]CPP‐I) showed turn‐on electronic conductivity and white light luminescence upon voltage bias application. To uncover the ring‐size effect, [9]CPP‐I and [12]CPP‐I were also prepared and investigated.

Single crystals of [9]CPP‐I, [10]CPP‐I, and [12]CPP‐I were prepared by concentrating mixed solution of CPPs and iodine to dryness (Scheme S1). The molecular structures of these assemblies were successfully obtained by X‐ray crystallography (Figure 2 and Table S1), with their electron density maps derived by the maximum entropy method (MEM).12 In the CPP pores, I 2 molecules were periodically accommodated in a commensurate fashion with their crystal lattices (Figure 2 a,b). Two I 2 molecules were encapsulated per [9]CPP and [10]CPP molecule, and four I 2 molecules in each [12]CPP molecule. The electron density of the iodine atoms is more delocalized in [10]CPP and [12]CPP rings than in [9]CPP (Figure 2 c).

Figure 2 Open in figure viewer PowerPoint Crystal structures of [n]CPP‐iodine assembly. a) Crystal structure of [10]CPP‐I viewed from the b‐axis. b) Schematic illustrations of the iodine molecules accommodated in the pores of [9]CPP (left), [10]CPP (center), and [12]CPP (right). Iodine atoms are located in the most probable positions based on the electron density. c) Contour plots of electron densities derived from the MEM for [9]CPP‐I (left), [10]CPP‐I (center), and [12]CPP‐I (right). Isosurface level: 1.0 e Å−3. d) Superposed DFTB/MD trajectories of atom positions of [9]CPP‐I (left), [10]CPP‐I (center), and [12]CPP‐I (right) at 123 K obtained from the MD simulations. In (a), (b), and (d) gray lines and purple spheres represent CPP molecules and iodine atoms, respectively. Hydrogen atoms are omitted for clarity.

Density‐functional tight‐binding molecular dynamics (DFTB/MD) simulations13 indicate that the iodine atoms can move dynamically inside a [10]CPP ring (Figure 2 d and movie S1). In comparison, inside [9]CPP‐I and [12]CPP‐I, movement of the iodine atoms is relatively suppressed (Figure 2 d and movies S2 and S3). These observations from the MD simulations are consistent with the experimentally derived electron density maps. We therefore surmised that the fluctuation in [10]CPP‐I might lead to sensitive responses to external environmental change.

As expected, voltage bias was found to impact a range of physical properties of [9]CPP‐I, [10]CPP‐I, and [12]CPP‐I. Figure 3 shows the electronic conductivity of [10]CPP‐I studied by impedance analysis. Before voltage bias application, two arcs equivalent to 2.2×108 Ω cm and 5.3×107 Ω cm appeared in the Nyquist plot (Figure 3 a, upper), which are assigned to bulk and grain boundary resistances, respectively.

Figure 3 Open in figure viewer PowerPoint Electric resistivity measurements for [10]CPP‐I. a) Nyquist plots scaled for the plot before (upper) and after (lower) the voltage bias was applied to [10]CPP‐I. Blue plots, red plots, and gray lines represent the ones before the application, after 170 minutes of 500 mV application, and their intermediates, respectively. Two pairs of arcs drawn for the blue and the red plots are fitting curves for the Nyquist plots. b) Time course of the DC resistivity. Voltage bias was applied to [10]CPP‐I for 10 minutes (gray colored region in the Figure), and then, it was turned off.

When direct current (DC) voltage bias 500 mV was applied, these arcs became smaller, and the bulk and grain boundary resistivities finally decreased to 5.8×105 Ω cm and 1.7×106 Ω cm, respectively (Figure 3 a, lower). Thus, upon voltage bias application, the bulk resistivity of [10]CPP‐I became approximately 380 times lower. DC resistivity measurement (Figure 3 b) shows that the resistivity surprisingly continued to decrease even after voltage bias was turned off. This cascade conversion is likely due to a domino effect in the phase transition,14 and it indicates that the observed turn‐on conductivity is based on a phase transition from a kinetically generated metastable state. While some organic host molecules accommodate iodine in a charged state and their clathrates exhibit conductivity without electric application,11b,11c,11d electric conductivity of [10]CPP‐I was turned on by electric stimulus to turn I 2 molecules into anionic iodide chains as discussed below.

We also applied voltage bias to [9]CPP‐I and [12]CPP‐I. In these cases, the bulk resistivity ratios in the initial/final states of [9]CPP‐I and [12]CPP‐I were only 0.9 (3.0×1010 Ω cm→3.3×1010 Ω cm) and 29 (9.3×107 Ω cm→3.2×106 Ω cm), respectively (Figure 4 and Figure S3). The change in the resistivity was very small in [9]CPP‐I, in which the iodine atoms are tightly encapsulated in the CPP rings, and relatively small in [12]CPP‐I, in which iodine atoms are not allowed to move dynamically in the CPP rings due to larger number of iodine molecules, even though [12]CPP‐I has larger pore size, showing that the dynamic motion of iodine (Figure 2 c) is crucial to the macroscopic response. Nevertheless, it is clear that the pore size of CPPs can control the response to voltage bias of the CPP‐I assembly.

Figure 4 Open in figure viewer PowerPoint The plots of bulk resistivity versus the time elapsed under voltage bias application. Black squares, black triangles, and red circles represent the data of [9]CPP‐I, [12]CPP‐I, and [10]CPP‐I, respectively. Dotted lines are to guide the eye.

To investigate how the electric conductivity of [10]CPP‐I is turned on by voltage bias, X‐ray absorption near‐edge spectroscopy (XANES), Raman spectroscopy, and fluorescence spectroscopy measurements were conducted. In the XANES spectra of [10]CPP‐I before electric stimulus application, a specific peak was observed at 5187 eV at the L 1 edge of iodine (Figure 5 a), which is interpreted as the 2s→5p transition of an iodine atom.15 This observation is consistent with the results of crystallography that iodine species exist in the form of molecular I 2 , for which the empty antibonding molecular orbital originates mainly from 5p orbitals of iodine atoms. After the application of voltage bias, the peak at 5187 eV decreased and a new peak appeared at 5194 eV (Figure 5 a), which is known to be related to anionic iodine species.15 This result showed that the above‐mentioned antibonding molecular orbitals were being occupied, that is, iodine molecules should receive electrons from the anode or CPP, though its valence value is not clear.

Figure 5 Open in figure viewer PowerPoint Responses of [10]CPP‐I to electric stimulus. a) XANES spectra of [10]CPP‐I before (blue) and after (red) the voltage bias application. b) Raman spectra of [10]CPP‐I in wavenumber region of 80–230 cm−1 (left) and 1550–1640 cm−1 (right), before (blue) and after (red) the voltage bias application. c) Schematic illustration of electric‐stimulus‐induced phase of [10]CPP‐I.

In the Raman spectra, introduction of iodine atoms to [10]CPP caused a peak at 207 cm−1, which is assigned to a stretching vibration mode, ν 1 , of I 2 molecule16 (Figure 5 b). After the voltage bias application, the peak at 207 cm−1 shifted to 205 cm−1, and new peaks appeared at 112 cm−1 and 165 cm−1, which are assigned to polyiodide chains.16 These observations strongly suggest the generation of polyiodide chains. It is known that polymerized iodine can provide a path to electronic conduction.11b–11d Therefore, conduction in [10]CPP‐I is also considered to be electronic, and conductivity is attributed to the chains (Figure 5 c). Here, the one‐dimensional channel structure should be a key factor for the generation of the polyiodide chains, unlike the three‐dimensional pore structures in which fragmented iodide species of I 3 − or I 5 − are observed.11b

The charge transfer from [10]CPP molecules to iodine should be retained to satisfy electric neutrality. Considering that a C−C stretching mode of [10]CPP17 showed only small change from 1587 cm−1 to 1589 cm−1 in the Raman spectra, only a small amount of CPP takes part in the charge transfer,18 resulting in a positive charge. The peak positions in the XRD patterns showed no shift, although the intensities of some peaks significantly changed (Figure S4), which indicates that the herringbone packing structure of the CPP molecules is largely preserved, and the polyiodide chains are located within the CPP pores (Figure 5 c). If polyiodide chains were generated outside the pores, the lattice constants should be changed.

The electronic structure of the CPP molecules was investigated by temporal evolution of fluorescence spectra during voltage bias application (Figure 6 a). Before applying voltage bias to [10]CPP‐I, strong greenish‐blue luminescence from CPP was observed around 475 nm.19 Electrical treatment induced a bathochromic broadening of the emission band, indicating changes of electronic structures in the CPP molecules. The spectral broadening is attributed to the inhomogeneous distribution of electronic structures of CPPs, which is induced according to the formation of the polyiodide chains. It should be noted that after the voltage bias application, the fluorescence spectrum covered the whole visible light range, corresponding to white on the standard chromaticity diagram20 (Figure 6 b). Thus, voltage bias can switch photoluminescence color of [10]CPP‐I from greenish‐blue to white (Figure 6 a, inset). The achievement of white luminescence from a single molecular component or assembly has been the target of significant research in the development of next‐generation illumination systems.21 As white light emission is usually achieved by mixing several components of different colors,21 the electrically treated [10]CPP‐I represents an example of such material made from a single molecular assembly.21

Figure 6 Open in figure viewer PowerPoint a) Fluorescence spectra of [10]CPP‐I before (blue), during (gray), and after (red) the voltage bias application. Their intensities are normalized with the intensity at their peak tops. Photographs of [10]CPP‐I irradiated with 405 nm laser light before (left) and after (right) the voltage bias application are shown in the insets. b) Color change of fluorescence of [10]CPP‐I in the standard chromaticity diagram upon voltage bias application. Each point was converted from the corresponding spectrum in the fluorescent spectra of (a).

Here, we have demonstrated the specific alignment of iodine in [9], [10] and [12]CPPs, investigated the electro‐responsivity of [10]CPP‐I, and confirmed that the formation of polyiodide chains, accompanied by charge transfer from [10]CPP molecules, induces electronic conduction and white light emission. Although the detailed mechanisms responsible for these effects are yet to be investigated, it was proven that CPPs can initiate phase transitions upon application of an electric stimulus. Furthermore, changing the diameters of CPPs can control the response. This approach is expected to be applicable for the other stimuli, such as photo‐irradiation, pressure application, pH change, and so forth, resulting in a generic strategy to develop stimuli‐responsive materials in controllable and predictable fashions.

Acknowledgements This work was supported by the ERATO program from JST (K.I.) (grant number: JPMJER1302). We thank Ms. Xiaolin Li (Shinshu University) for the preliminary investigations, Mr. Kohei Tanaka (Nagoya University) for the synthesis of CPPs, Dr. Shogo Kawaguchi (JASRI/SPring‐8) for the support in the synchrotron experiments, Ms. Haruko Hirukawa (Nagoya University) for the artwork, and Prof. Cathleen M. Crudden (Queen's University and Nagoya University) for fruitful discussion and critical comments. The synchrotron radiation experiments were performed at the BL02B2 of SPring‐8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal number 2015A1490, 2016A1036, 2016B1220, 2016B1272) and the BL5S1 and BL5S2 of Aichi Synchrotron Radiation Center. The impedance measurements were conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of MEXT, Japan. ITbM is supported by the World Premier International Research Center Initiative (WPI), Japan.