Donor–acceptor covalent organic frameworks for visible light induced free radical polymerization

Crystalline and porous covalent organic frameworks (COFs) with donor-acceptor moieties in their backbone are utilized as initiators for visible light induced radical polymerization. The COFs are efficient photoinitiators, maintaining their structural integrity for several cycles.


Table of Contents
Section S1. Materials  Section S1. Materials and methods:

General
All inert reactions and manipulations were carried out in an argon atmosphere using standard Schlenk techniques or in an MBRAUN (MB-120 BG) inert atmosphere glove box containing an atmosphere of argon. Unless otherwise specified, reagents were used as received without further purification.

NMR measurements
Liquid-state 1 H NMR and 13 C NMR were recorded on a Bruker Avance II 200 and Bruker Avance 400 MHz spectrometer in the given solvent. 13 C{ 1 H} cross polarized magic angle spinning (CP MAS) NMR measurements were carried out using a Bruker range Avance 400 MHz Solid State spectrometer operating at 100.6 MHz for 13 C and a Bruker 4 mm double resonance probe-head operating at a spinning rate of 10 kHz.

Physisorption measurements
Nitrogen (N 2 ) sorption analyses were conducted at 77 K using an Autosorb-iQ-MP from Quantachrome. The pore size distributions were calculated from the adsorption isotherms by Quenched Solid State Functional Theory (QSDFT) using N 2 sorption data collected at 77 K. We used the carbon slit/cylindrical/spherical pore model for analyzing the distribution. Before analysis, samples were degassed at 150 °C for 24 h. BET and Langmuir surface areas were determined over a P/P 0 range of 0.1-0.25.

Thermogravimetric analysis
TGA measurements were carried out under nitrogen atmosphere on a Mettler Toledo TGA 1 Stare thermal instrument with a heating rate of 10 K min -1 .

X-ray photoelectron spectroscopy
X-Ray photoelectron spectra were measured on a K-Alpha™ + X-ray Photoelectron Spectrometer System (Thermo Scientific) with Hemispheric 180° dual-focus analyzer with 128-channel detector. Micro focused Al-Kα radiation was used as X-ray monochromator.

Other Characterizations:
Powder X-ray diffraction (PXRD) data was collected on a Bruker D8 Advance diffractometer in reflection geometry operating with a Cu Kα anode (λ = 1.54178 Å) operating at 40 kV and 40 mA. Samples were ground and mounted as loose powders onto a Si sample holder. PXRD patterns were collected from 2 to 60 2θ degrees with a step size of 0.02 degrees and an exposure time of 2 seconds per step. The Fourier transform infrared spectroscopy (FT-IR) analyses of the samples were carried on Varian 640IR spectrometer equipped with an ATR cell. Solid-state diffuse reflectance Ultraviolet-visible spectroscopy (UV-vis) spectra of the as synthesized COF powders have been collected on Varian Cary 300 UV-Vis Spectrophotometer. The scanning electron microscope (SEM) analyses of COF samples were performed on S-2700 scanning electron microscope (Hitachi, Tokyo, Japan).

Electron paramagnetic resonance (EPR) spectroscopy studies:
EPR measurements in X-band (microwave frequency ≈ 9.8 GHz) were performed at 300 K by a Bruker EMX CWmicro spectrometer equipped with an ER 4119HS-WI high-sensitivity optical resonator with a grid in the front side.  The radical polymerization reaction of MMA to PMMA, initiated using COFs as initiator and triethylamine (TEA) as co-initiator, was performed using glass reactor as shown in Figure S1. The addition of MMA, TEA and COF initiator into glass reactor has been done in the glove box, operating under Argon atmosphere. The temperature of reaction mixture was maintained at 25 °C using external water circulator, whereas the mixture was stirred using an external magnetic stirrer. The Lumatec Superlite 400 (500 W mercury lamp) equipped with various filters and variable light intensity has been used as light source for performing the radical polymerization reaction. The wavelength (λ) of 400-700 nm was used for reaction, with 75% light intensity.

Section S2. Synthesis of COFs and details about the radical polymerization reaction:
Synthesis of TTT-DTDA and TTT-BTDA COFs: Figure   for 10-12 minutes in order to get a homogenous dispersion. The tube was then flash frozen at 77 K (liquid N 2 bath) and degassed by three freeze-pump-thaw cycles. The tube was sealed off under vacuum and then heated at 120 °C for 3 days. A red colored precipitate was collected by centrifugation, washed with anhydrous methanol and finally with anhydrous acetone. The powder collected was then purified by soxhlet treatment with THF (12 h) at 90 °C, and then dried at 100 C for 2 hours to give a dark-red colored pure TTT-BTDA powder. was added into the reaction mixture and the reactor was closed with rubber septum for isolation from air.
Afterwards, the content was irradiated using a 500 W mercury lamp equipped with a cutoff filter (400-700 nm), 75% intensity and a water cooling system for 12h ( Figure S1). At the end of irradiation, the bulk solid was dissolved in THF (10-15 mL), filtered and precipitated in methanol (250 mL). The PMMA polymers were collected by filtration and then dried in air for 24 h. The recovered COF initiator was washed multiple times with THF and then subjected to the soxhlet treatment for 2h to get rid of the PMMA polymer trapped in COF pores.
The similar protocol was followed for using the using the TTT-DTDA and TTT-BTDA initiators. For the reaction without initiator, except COF, all other parameters were kept constant for polymerization reaction. The optimization of conditions for polymerization reaction:
[d] Reaction using the TTT-DTDA Polymer prepared following the Schiff-base reaction, using atmospheric conditions.
[e] Using the non-donor acceptor (non-DA) type COF IISERP-COF4 synthesized using reported procedure.      The structure models were generated using the Material Studio Modelling 5.0 package. 6 The geometry was optimized using the Forcite module and UFF forcefield. 7 Full profile pattern fitting (Pawley) was performed against the experimental powder pattern using the Reflex module.  In order to reveal the structures of these COFs and to calculate the unit cell parameters, possible 2dimensional (2D) models were optimized using Density Functional Tight-Binding method ( Figure S5 and S6).
Several stacking possibilities were considered for calculating the model structures. 8 We found that, the experimental PXRD patterns are nicely matching with the simulated patterns of some near-eclipsed (AA) stacking models ( Figure S11). Due to aforementioned reasons and comparing the experimental and simulated PXRD patterns, we propose the structures of TTT-DTDA and TTT-BTDA close to hexagonal space group (P6/m).
Refinements of PXRD pattern were done using Reflex module of Material studio. 6

Section S8. Synthesis and Characterizations of IISERP-COF4:
The synthesis of IISERP-COF4 was performed by following the reported procedure. 3 Typically, 17 mg of terephthalaldehyde (0.127mmol) was dissolved in 3 mL ethanol in a Pyrex tube. To   (b) The radical polymerization activity was carried out in presence of ethyl α-bromophenylacetate.