A Photoresponsive Smart Covalent Organic Framework

Ordered π-columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external-stimuli-responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π-building blocks. The COF is switchable upon photoirradiation to yield a concavo-convex polygon skeleton through the interlayer [4π+4π] cycloaddition of anthracene units stacked in the π-columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external-stimuli-induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π-electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.


Section 1. Methods
1 H NMR spectra were recorded on a JEOL model JNM-LA400 NMR spectrometer, where the chemical shifts (δ in ppm) were determined with a residual proton of the solvent as standard.
Fourier transform Infrared (FT IR) spectra were recorded on a JASCO model FT IR-6100 infrared spectrometer. UV-Vis-IR diffuse reflectance spectrum (Kubelka-Munk spectrum) was recorded on a JASCO model V-670 spectrometer equipped with integration sphere model . Powder X-ray diffraction (PXRD) data were recorded on a Rigaku model RINT Ultima III diffractometer by depositing powder on glass substrate, from 2θ = 1.5° up to 60° with 0.02° increment. Elemental analysis was performed on a Yanako CHN CORDER MT-6 elemental analyzer. TGA measurements were performed on a Mettler-Toledo model TGA/SDTA851 e under N 2 , by heating to 800 °C at a rate of 10 °C min -1 . Field-emission scanning electron microscopy (FE-SEM) was performed on a JEOL model JSM-6700 operating at an accelerating voltage of 5.0 kV. The sample was prepared by drop-casting an acetone suspension onto mica substrate and then coated with gold. High-resolution transmission electron microscopy (HR-TEM) images were obtained on a JEOL model JEM-3200. Photoluminescence spectrum was recorded on a JASCO model FP-6600 spectrofluorometer. The absolute quantum yield was determined by standard procedure with an integral sphere JASCO model ILF-533 mounted on the FP-6600 spectrofluorometer. Time-resolved fluorescence spectroscopy was recorded on Hamamatsu compact fluorescence lifetime spectrometer Quantaurus-Tau model C11367-11. Nitrogen sorption isotherms were measured at 77 K with a micromeritics @ 3Flex model analyzer. Before measurement, the samples were degassed in vacuum at 120 °C for more than 10 h. By using the non-local density functional theory (NLDFT) model, the pore volume was derived from the sorption curve.
Geometry optimization of the unit pore structure was performed at PM3 level by using the Gaussian 03 program package (Revision C.02) S1 to give the pore size of 2.9 nm in diameter.
Molecular modeling and Pawley refinement were carried out using Reflex, a software package for crystal determination from PXRD pattern, implemented in MS modelling ver 4.2 (Accelrys Inc.). S2 Unit cell dimension was first manually determined from the observed PXRD peak positions by using hexagonal arrangement. We performed Pawley refinement to optimise the lattice parameters iteratively until the R WP value converges. The refinement indicates a S3 hexagonal crystal system with a unit cell of a = b = 30.33032 Å and c = 3.36227 Å. The pseudo-Voigt profile function was used for whole profile fitting and Berrar-Baldinozzi function was used for asymmetry correction during the refinement processes. The final R WP and R P values were 8.73 and 5.98%, respectively. Simulated PXRD patterns were calculated from the refined unit cell and compared with the experimentally observed patterns. This structure could have two distinct arrangements: (1) a staggered AB type arrangement with graphite-like packing, where three-connected vertices lie over the center of the six-membered rings of neighbouring layers; (2) an eclipsed AA type arrangement, where all atoms in an each layer of the framework lie exactly over one another. The AA type arrangement was constructed in space group P6/mmm symmetry (space group number 191) and the AB type arrangement was constructed in space group P63/mmc symmetry (space group number 194). The atoms are placed on the special position to form the 2D framework where all bond lengths and angles are taken from the optimised geometrical parameters calculated at B3LYP/6-31G(d) to maintain reasonable values. After comparing each simulated pattern with experimentally observed pattern, only the simulated pattern from the eclipsed AA type arrangement shows good agreement with the observed PXRD pattern.
Preparation of Ph-An-COF thin film on quartz substrates. Before use, the quartz substrate was washed by boiling water, acetone and boiling isopropanol and then dried under vacuum at 120 °C for 10 h. A mixture of 1,3,5-benzenetriboronic acid (3.5 mg, 0.1 mmol) and 2,3,6,7-tetrahydroxyanthracene (6.1 mg, 0.15 mmol) in 1,4 dioxane/mesitylene (25 mL, 1/1 in vol.) in a 50 mL Schlenk tube was degassed through three freeze-pump-thaw cycles. After that, the washed quartz substrate (8 mm × 8mm) was carefully put in the tube. The tube was sealed and heated at 120 °C for 3 days. Cooling to room temperature, the glass substrate was taken out, washed with anhydrous acetone, and dried under vacuum to yield Ph-An-COF as a pale yellow thin film on quartz. A photo of a typical film is shown below and the film thickness is 242 nm as measured by using profilometer (Veeco Dektak 6M).

Photoirradiation and thermal treatments. Light irradiation experiments were conducted under
Ar by using light at 360 nm through band-path filter of a xenon lamp at 22 °C. At designated period of photoirradiation, the films were monitored using electronic absorption and fluorescence spectroscopy. For the reverse thermal induced reactions, the films after photoirradiation were heated at 100 °C under Ar in the dark and monitored using electronic absorption and fluorescence spectroscopy. To evaluate the stimuli-induced porosity change, powder samples were utilized. The Ph-An COF powder was irradiated with the xenon lamp under Ar, and the resulting Ph-An CD COF powder was degassed at room temperature under high vacuum and subjected to nitrogen adsorption measurements at 77 K. For thermal reversibility study, the Ph-An CD COF powder samples were heated at 100 °C under Ar in the dark and subjected to nitrogen adsorption measurements at 77 K.    Figure S1. FT IR spectra of Ph-An-COF (black curve), 1,3,5-benzenetriboronic acid (blue curve), and 2,3,6,7-tetrahydroxyanthracene (red curve).     Figure S8. Detailed data for the gas sorption measurements including BET plots, BET constant C, and Rouquerol plots.