Porphyrin-based donor–acceptor COFs as efficient and reusable photocatalysts for PET-RAFT polymerization under broad spectrum excitation

Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λmax from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.


Materials and Methods
All chemicals were purchased from commercial sources and used without further purification unless specific treatment mentioned. were synthesized according to literature procedures.

General instrumentation
Nuclear Magnetic Resonance (NMR) spectra was obtained on an NMR Bruker 500 MHZ μm diameter X-rays were shot on the sample. The XPS survey scan spectra in the 1100-0 eV binding energy range were recorded in 0.5 eV steps with a pass energy of 140 eV. Highresolution scan spectra were recorded in 0.1 eV steps with a pass energy of 26 eV. Low energy electrons and Ar + ions were conducted for specimen neutralization in each measurement.
General procedure for the synthesis of RICE-1 and RICE-2 TTAP (0.01 mmol) and ETTA or BDTA (0.01 mmol) were weighed and dissolved in a mixture of 0.33 mL of benzyl alcohol and 0.67 mL of mesitylene in a Pyrex tube. Before the tube was sealed, 0.1 ml 6M acetic acid was added, and the solution was sonicated for 5 min. The sealed tubes were then transferred into an oven and heated at 120 °C for 7 days.
All of the products were separated and washed thoroughly using THF, DMF and acetone.

General procedure for RICE COFs photocatalyzed PET-RAFT polymerization
To a 4 ml vial, 0.5 mg RICE COF, 0.25 ml solvent, 0.25 ml monomer and the corresponding amount of RAFT agents (BTPA or CPADB) were mixed together in the glove box. For Table 1, the degree of polymerization (DP) was 190. For the kinetic study ( Figure 4d-f), DP was 100. The reaction vial was then sealed and sonicated for around 10 min until COF was well dispersed in the solution. The reaction vial was constantly stirred at ambient temperature under irradiation of a household LED lamp (λ max = 460 nm for the blue LED, λ max = 535 nm for the green LED, λ max = 635 nm for the red LED). The reaction vial was placed approx. 0.15 cm from the lamp, where the light intensity was 15 mW/cm 2 , unless otherwise specified. The whole setup was covered with aluminum foil to block exposure to any other light sources. The monomer conversion was determined by 1 H NMR.

General procedure for first order kinetic study
To study the reaction kinetics, polymerization was carried out under 15 mW/cm 2 blue LED in the glove box. To a 4 ml vial, 1 mg RICE-2, 0.25 ml DMF, 0.25 ml MA and 6.55 mg BTPA (DP=100) were mixed together in the glove box. The reaction vial was then sealed and sonicated for around 10 min until COF was well dispersed in the solution. The mixture was constantly stirred at ambient temperature. At specific time points, 0.01 ml mixture was taken and dissolved in 0.7 ml deuterated chloroform for 1 H NMR. The NMR solution was further concentrated and dissolved in THF for GPC measurement.

General procedure for chain extension
Yielding PMA was precipitated in MeOH, filtered and redissolved in THF. This process was repeated two more times to make sure no residual initiators inside. The purified PMA (6 mg), momomer (0.15 mL), 1 mg RICE-2 and 0.25 ml DMF was deaerated by three freeze-pump-thaw cycles, backfilled with argon, and stirred at ambient temperature under irradiation of a household blue LED floodlight for 6 h.

General procedure for COF recycling
Polymerization mixtures were diluted with 2 ml THF and centrifuged at 5000 rpm. The supernatant was removed and 5 ml THF was added following by centrifugation. This procedure was repeated three times to ensure all the impurities were washed out. For The valence band maximum (VBM) and the conduction band minimum (CBM) of the COFs were determined from the calculated and the experimentally measured bandgap ( ), using Eqns. S1-S2 16 : In this work, the computed potentials are referenced to standard hydrogen electrode (SHE), i.e., 4.44 V (vs. vacuum potential). 17 The vacuum potential of the simulation cell is determined by the electrostatic potential at the center of the internal pore of the COFs. 18,19 Supporting data                   Table S2.