Light-melt adhesive based on dynamic carbon frameworks in a columnar liquid-crystal phase

Liquid crystal (LC) provides a suitable platform to exploit structural motions of molecules in a condensed phase. Amplification of the structural changes enables a variety of technologies not only in LC displays but also in other applications. Until very recently, however, a practical use of LCs for removable adhesives has not been explored, although a spontaneous disorganization of LC materials can be easily triggered by light-induced isomerization of photoactive components. The difficulty of such application derives from the requirements for simultaneous implementation of sufficient bonding strength and its rapid disappearance by photoirradiation. Here we report a dynamic molecular LC material that meets these requirements. Columnar-stacked V-shaped carbon frameworks display sufficient bonding strength even during heating conditions, while its bonding ability is immediately lost by a light-induced self-melting function. The light-melt adhesive is reusable and its fluorescence colour reversibly changes during the cycle, visualizing the bonding/nonbonding phases of the adhesive.


Supplementary Methods
All reagents and solvents were obtained from commercial suppliers and used as received without further purification unless otherwise stated. Tetrahydrofuran (THF) and CH 2 Cl 2 were dried using Glass Contour solvent purification system. Polymethyl instrument. 1 H and 13 C NMR spectra were recorded on a JEOL AL-400 (400 MHz for 1 H, and 100 MHz for 13 C NMR) spectrometer or a JEOL JNM-A600 spectrometer equipped with UltraCOOL probe (600 MHz for 1 H, and 150 MHz for 13 C NMR). The NMR spectra were measured in CDCl 3 , CD 2 Cl 2 or C 2 D 2 Cl 4 (1,1,2,2-tetrachloroethane-d 2 ) at room temperature unless otherwise stated. Chemical shifts were referenced to the residual solvent protons in the 1 H NMR (7.26 ppm in CDCl 3 , 5.32 ppm in CD 2 Cl 2 , and 5.98 ppm in C 2 D 2 Cl 4 ) and the solvent carbons in the 13 C NMR (77.0 ppm in CDCl 3 , 53.1 ppm in CD 2 Cl 2 , and 73.7 ppm in C 2 D 2 Cl 4 ). Mass spectrometry (MS) was recorded on a Bruker micrOTOF Focus using the atmospheric pressure chemical ionization (APCI) time of flight (TOF) method in the positive-ion mode in toluene and on a Bruker ultraflex III using the matrix-assisted laser desorption/ionization (MALDI) TOF method. Single crystal X-ray diffraction (XRD) measurement was performed on a Rigaku X-ray diffractometer equipped with a molybdenum MicroMax-007 HF rotating anode X-ray generator (Mo K, 0.7107 Å), VariMax-Mo optics, and a Saturn 70 CCD detector. XRD measurements of bulk liquid crystal (LC) samples using a transmission method were performed on a Rigaku X-ray diffractometer equipped with a copper FR-E rotating anode X-ray generator (Cu K, 1.5418 Å), VariMax-Cu optics, and an R-AXIS IV 2D imaging plate detector. The bulk sample was packed in a Lindemann glass capillary (Hilgenberg GmbH) with a diameter of 1.0 mm. Grazing incidence (GI) XRD measurement of a thin film was performed using the same X-ray diffractometer equipped with a hot stage accessory. Ozone treatment of the thin film was performed on a vacuum excimer UV chamber system H0017 (USHIO). Differential scanning calorimeter (DSC) was performed on an SII Exstar 6000 DSC 6200, calibrated using an Al 2 O 3 standard sample in a sealed Al pan. Polarized optical microscopy (POM) was performed on a Leica DM2500 P. Thickness of more than 1 m-thick film was measured using a digital micrometer QuantuMike Series 293 (Mitutoyo). Ultimate shear strength was measured with a push-pull type digital force gauge RZ-10 (AIKOH engineering). Tempax borosilicate glass plates (SANRITSU, 2.0 mm thickness) were used for sandwiching the adhesive material. Preparation of uniform 130-μm-thick films was conducted using a polytetrafluoroethylene (PTFE) silicone tape AGF-100 FR (CHUKOH) as a spacer.
Temperature of the sample was controlled by a hot plate CHP-170DN (AS ONE).
Accurate temperature on the hot plate was monitored using an infrared thermometer IT-545 (HORIBA). Quartz glass plates (Hiraoka Special Glass, 1.0 mm thickness) were used for the evaluation of the light transmittance through spin-coated films. These thin films with less than 1 m thickness were prepared using a spin coater 1H-D7 (MIKASA), whose thickness was determined with an atomic force microscope (AFM), SII Nanopics 2100. As a light source, a UV-400 series UV-LED (Keyence, UV-50H type, 365 nm) equipped with a UV-L3 lens unit or a hand-held UV lamp LUV-16 (AS ONE, 365 nm) was used. Irradiance from these light sources on the sample was measured using an ultraviolet irradiance meter UIT-150 (USHIO). Thermography was performed using TH9100WR (NEC Avio Infrared Technologies). UV-visible absorption spectra of solution and spin-coated films were recorded on Shimadzu UV-3150 and Agilent 8453 spectrometers, respectively. Fluorescence spectra in solution were recorded on a Hitachi F-4500 spectrometer. Variable-temperature fluorescence spectra were recorded on a JASCO FP-6500 spectrophotometer equipped with HPC-503 high temperature cell for film samples.
After that, 3,4-Bis(dodecyloxy)benzyl bromide (960 mg, 1.78 mmol) was further added to the mixture and then stirred at 60 °C for 2 days. After cooling to room temperature, the reaction was quenched with water and the reaction mixture was extracted with CH 2 Cl 2 .
The extract was washed with brine and dried over anhydrous Na 2 SO 4 . After removal of the solvent under reduced pressure, the residue was purified by column chromatography over silica gel using CH 2 Cl 2 /hexane (2/1) to afford bis [3,4-

Shear experiment of the LC film of 1
Liquid crystalline nature of 1 in the mesophase (65-140 °C) was confirmed by a shear experiment (Supplementary Fig. 1). A 3-µm-thick film of 1 was prepared between glass plates. At 100 °C, the glass plate covered on the LC film was slid in a single direction with the bottom glass plate fixed. The sample was cooled to room temperature and observed by polarized optical microscopy (POM) under the crossed Nicols. When the analyzer and polarizer were both rotated by 45°, the contrast of the POM image clearly changed. Moreover, the POM image turned back to an initial state when the analyzer and polarizer were further rotated by 45° in the same direction, indicating the anisotropy of the molecular alignment in the shear direction.

XRD measurement of the bulk LC sample of 1
Compound 1 was melted at 160 °C and poured into a glass capillary under vacuum.
The XRD measurement of the bulk sample using Cu K radiation ( = 1.5418 Å) was performed at 100 °C in the LC phase ( Supplementary Fig. 2). Miller indices (hkl) of the observed diffraction peaks were determined based on the standard peaks at 2 = 2.60 and 3.98°, which was assigned as (200) and (220), respectively. Structural parameters are discussed in Methods of the main text. In consideration of the molecular size and the sample density, the number of molecules in the repeating cell unit of the liquid crystal phase was estimated to be Z = 4, which suggests a side-by-side packing of the columnar array structures as demonstrated in Supplementary Fig. 3. The XRD measurement was also performed at 25 °C in the solid phase, in which the bulk sample was prepared by cooling the liquid crystalline sample at the rate of 5 °C min -1 (Supplementary Fig. 4). The result suggested that the solid phase is a LC glass state, whose packing structure is comparable with that in the LC phase.

Single crystal X-ray structure analysis of 2
Structural parameters are described in Fig. 2b and Methods of the manuscript. Mo K radiation ( = 0.7107 Å) was used for the measurement. The oscillation angle and camera distance were 0.5° and 75 mm, respectively. The X-ray exposure time per frame was 160 sec. Data sets of 1440 frames were integrated and scaled with the CrystalClear-SM Expert 2.0 r4 program (Rigaku, 2009). The structures were solved by direct method (SHELXS-2013) and refined by least-squares calculations on F 2 for all independent reflections (SHELXL-2013) 6 . All non-hydrogen atoms were refined with anisotropic displacement parameters, and hydrogen atoms were placed in idealized positions and refined by applying riding models with the relative isotropic displacement parameters.
The deposition number of the Cambridge Crystallographic Data Centre: 1054572.

Thermography analysis after the photoirradiation on the LC film of 1
A 5-μm-thick LC film of 1 was prepared between glass plates, where the temperature was kept at 100 °C on a hot stage. The LC film turned into liquid by the UV-LED irradiation (160 mW cm -2 ) in 10 sec. Before and after the UV irradiation, thermography analysis was conducted to confirm the negligible temperature increase ( Supplementary   Fig. 7).

Analyses of the photoproducts after the photoirradiation on the LC film of 1
The 1-μm-thick film of 1 between glass plates was irradiated at 100 °C with a handheld UV lamp (365 nm, 3.2 mW cm -2 ) for 100 sec. The resulting products were analyzed by MALDI-TOF MS and subjected to recycling GPC (Supplementary Fig. 8). The residue mainly consisted of the unreacted monomer 1 and its photodimer, although a small amount of oligomeric products were included in the isotropic mixture. The photoproducts ratio among the monomer/dimer/oligomers of 1 is dependent on the total irradiation dose as well as the film thickness. When the 365-nm UV irradiation with a total dose of 320 mJ cm -2 was performed on a 1-m-thick LC film at 100°C, half of the monomer 1 was consumed for the dimerization.
The isotropic mixture after the photoirradiation was subjected to GPC, and then the photodimer of 1 (compound 8) was isolated as a main product except for the unreacted monomer 1. The less symmetric structure of the photodimer 8 was determined by the 1 H and 13 C NMR analyses ( Supplementary Figs 9 and 10).
The thermal back reaction of the isolated photodimer 8 into 1 was also demonstrated in the 1 H NMR analysis. Upon heating the solution of 8 in C 2 D 2 Cl 4 at 130 °C, the monomer 1 was obtained after 9 h (Supplementary Fig. 11).
The thermal recovery of the LC film of 1 from the photoirradiated fluid mixture was supported by the POM observation ( Supplementary Fig. 12).
UV-visible absorption and fluorescence spectra of the isolated photodimer 8 were measured in CH 2 Cl 2 ( Supplementary Fig. 13). The blue fluorescence of 8 is consistent with the result of the fluorescence color change during the photoirradiation on the LC film of 1 (Fig. 4), which elucidated the processes of the in situ generation of 8 and the following thermal back reaction at 160 °C ( Supplementary Fig. 14).

GI-XRD measurement of the spin-coated film of 1
A spin-coated film of 1 with the thickness of ca. 450 nm was prepared on a quartz glass plate. However, the high-temperature GI-XRD measurement using this film failed because of a dewetting behavior during the temperature control. In order to avoid the dewetting, the spin-coated film was exposed to ozone atmosphere for 120 sec, and then a 1 wt% aqueous solution of polyvinyl alcohol (PVA) was spin-coated over the sample at 1000 rpm for 60 sec. The ozone treatment was performed to form a hydrophilic surface for coating the film with a PVA layer. This ozone treatment only leads to a surface modification rather than an internal exposure, which was confirmed by the fact that the absorption spectrum of the spin-coated film did not change upon the ozone treatment.
Thus the surface modification has little influence on the following observations. The spin-coated film was initially annealed at 160 °C for 15 min and gradually cooled to 100 °C, and then the GI-XRD measurement was conducted (Supplementary Fig.   15). Incident angles of X-ray beam to the films were set at 0.18-0.22° using pulse controllers. In this experiment, the 2D diffractions of the sample were observed as an almost angle-independent image, although a specular reflection and a weak diffuse scattering were observed. This result support that the columnar LC arrays are randomly oriented in the thin film. To avoid counting a specular reflection observed in the out-ofplane direction ( = 90°), the 2D diffractions were integrated in the range of  = 30-60°.
After UV light irradiation at 30 mW cm -2 for 5 sec at the same temperature (100 °C), a change in the diffraction pattern was observed. After that, the sample was heated at 160 °C for 20 min and gradually cooled to 100 °C, and then the recovery of the diffraction pattern was confirmed. This result supported the photoinduced reaction of 1 and its thermal back reaction in the spin-coated film.

Film preparation between two glass plates
Powder sample of 1 was sandwiched between glass plates. The sample was melted by heating the glass plates at 160 °C for 15 min on a hot plate. The glass plates were gradually cooled to room temperature, affording the film of the pure compound 1 between the glass plates. The thickness was measured using a digital micrometer.
Particularly in the experiment of the shear strength measurement, 130-μm-thick films of 1 were uniformly prepared between glass plates (Supplementary Fig. 16). A 130μm-thick PTFE silicone tape with a 6 mmϕ punched hole was placed on a glass plate as a spacer. Powder sample of 1 was placed in the hole surrounded by the PTFE silicone tape, and the sample was heated above the melting point of 140 °C. The sample was covered with the other glass plate and a 0.5 kg weight was placed on the glass plates at the same temperature. Then, the sample was cooled for 30 min to prepare a 130-μm-thick film. In this procedure, the temperature was controlled on a hot plate and monitored by an infrared thermometer.

General notes for the cohesive force and the adhesion force
In general, adhesive strength is discussed with cohesive force and adhesion force ( Supplementary Fig. 17) 7 . The cohesive force F cohesion derives from the internal strength of the adhesive material, which is closely related to the intermolecular interaction particularly in the case of small molecules. On the other hand, the adhesion force F adhesion is the interaction between the adhesive material and the substrate surface. If F cohesion > F adhesion , the bonding strength is determined by the adhesion force and thus largely depends on the surface conditions of the substrates such as hydrophilicity. When F cohesion < F adhesion , in contrast, the bonding strength is determined by the cohesive force regardless of the surface conditions. The exhibition mechanism of the adhesion force has also been widely studied both in molecular to macroscopic scales in relation to the rheology of the materials at interface 8 .

Preparation of the spin-coated films of 1 for the light transmittance measurement
The spin-coated films of 1 were prepared with various thicknesses (See Methods in the main text). UV-visible absorption measurement of these thin films was conducted ( Supplementary Fig. 18) and the transmittance at 365-nm wavelength was plotted (Fig.   4a). The film thickness was determined by the AFM analysis. After the thin film was scratched with a spatula, the difference in the height between scratched and undamaged area was measured. According to the height profile along the direction perpendicular to the scratched line, the thickness of the spin-coated film was estimated.

Light transmittance of the spin-coated films of 1
The logarithmic values of the light transmittance T (= I/I 0 ) at 365-nm wavelength were plotted against the corresponding film thicknesses L (Fig. 4a). In this figure, the fitting curve was delineated according to the following equation (Supplementary   Equation 1). In consideration of a transmittance loss, the Lambert-Beer law was described below: The absorbance of the samples at the 365-nm wavelength showed a linear fitting against the film thickness with the coefficient of determination R 2 = 0.997 (Supplementary Fig.   19). On the basis of the fitting parameters obtained here, the light transmittance T (= I/I 0 ) was delineated as the exponential function of the film thickness (Fig. 4a).

Demonstration of the small adhesive residue on the UV irradiated glass plate
Relative amounts of the adhesive residues on the glass plates were compared after the photoseparation at 100 °C ( Supplementary Fig. 20). First, two films of 1 with the same thickness (20 μm) were prepared. As for one sample, the 365-nm light was irradiated using a hand-held UV lamp (3.2 mW cm -2 ) from the face of a glass plate attached to the weight until the glass plates were separated in ca. 100 sec ( Supplementary   Fig. 20a). As for the other sample, the UV irradiation was performed from the face of a hanged glass plate (Supplementary Fig. 20b). Then, the residues adhering to the dropped glass plate and to the hanged glass plate were individually dissolved in the same amount of CH 2 Cl 2 (500 mL), and then the absorption spectra of these solutions were compared.
When the light was irradiated from the face of the covered glass plate, the absorbance of the residue solution obtained from the dropped glass plate was significantly smaller than that from the hanged glass substrate (Supplementary Fig. 20a).
On the other hand, the tendency was reversed when the light irradiation was performed from the opposite direction ( Supplementary Fig. 20b). These results demonstrated that the photoinduced melting of the adhesive film took place near the interface within a few micrometers range from the irradiated surface.

Theoretical calculations
The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of compound 3 were performed at the PBE0/def-SV(P) and TD-PBE0/def-SV(P) levels of theory. Ground and excited state geometries for 3 have been fully optimized, and a relaxed potential energy scan was performed from the Franck-Condon geometries to the minima on the lowest excited singlet state. The energy levels of the ground state S 0 and excited states S a , S b , S c , and S d with fixed bent angle  of the central 8-membered ring are listed in Supplementary Fig. 21. Kohn-Sham molecular orbitals of 3 and their corresponding orbital energies for different bent angles are shown in Supplementary Fig. 22. The effect of antiaromaticity of the planar 8-membered ring (cyclooctatetraene) on their orbital energies has been previously reported. 9