Influence laws of air gap structure manipulation of covalent organic frameworks on dielectric properties and exciton effects for photopolymerization

Boosting the dissociation of excitons is essential to enhance the photocatalytic efficiency. However, the relationship between the structure of the catalyst and the exciton effect on the photocatalytic activity is still unclear as the main problem. Here, it is proposed that as a descriptive factor, an experimentally measurable dielectric constant (εr) is available to quantitatively describe its relationship with exciton binding energy (Eb) and photocatalytic activity. With tuning the linker of covalent organic frameworks (COFs), the “air gap” structure is oriented to shrink, leading to an increased εr of COFs and a lower Eb to facilitate exciton dissociation. Meanwhile, taking “water-/oxygen-fueled” photo-induced electron transfer reversible addition–fragmentation chain transfer (PET-RAFT) polymerization as a demonstration platform, it can be seen that COFs with a small “air gap” structure have relatively superior photocatalytic activity. This provides important implications for the evolution of efficient photocatalysts.

.   Figure S11). Table S2. Results of the effect on the reaction solvent for "oxygen-/waterfueled" PET-RAFT polymerization with Py-TPA-COF.  Figure S11).  Figure S11).  Figure S11).  Figure S11).  Figure S11). The XRD pattern of TFP-1Ben presents an intense peak at 4.6°, corresponding to the (100) plane ( Figure S15a). Slight peaks appear at 8.0° and 26.5°, which can be attributed to the (110) and (001) planes, respectively. 1 Similarly, the PXRD pattern of TFP-2Ben displays diffraction peaks at 5.8° and 27.4° that can be assigned to the (110) and (001) planes ( Figure S15b). Moreover, in the FT-IR spectra ( Figure S15c) of the two TFP COFs, the peaks at the 3300 cm -1 and 1628 cm -1 disappear, accounting for the disappearance of N-H stretching peak and the aldehyde stretching peak in their correspondent building blocks, respectively. In parallel, the presence of the C=O stretching peak at 1570 cm -1 and the C=C stretching peak at 1608 cm -1 substantiates the condensation between amines and aldehydes and the formation of enol to keto tautomerism. 2 As per the solid-state 13 C CP-MAS NMR spectrum ( Figure S15d), two TFP COFs show a clear signal around 184 ppm, emerging from the carbonyl carbons. Further evidence for enol to ketone tautomer formation is provided by the two peaks around 148 ppm and 106 ppm attributed to the C=C carbon. 3 The above results indicate that the successful preparation of target COFs.  respectively. According to the nonlocal density functional theory (DFT) model, the pore size distributions were estimated to be 1.19 and 1.31 nm, respectively, indicating that TFP-1Ben has a relatively smaller air gap structure. SEM and TEM images demonstrate that the COF exhibits a barlike morphology ( Figure S17).  Temperature-dependent photoluminescence (PL) spectra were measured to examine the E b . As seen from the inset of Figure S19a

Synthesis of TFP COFs
The synthesis of TFP COFs follows the previous method. 3

Characterization method
The powder X-ray diffraction (PXRD) spectra were recorded on DY1602/Empyrean X-ray diffractometer. Diffraction intensity data for 2 theta were collected at the scanning speed of 10 theta/min with 2 theta step increment of 0.02 theta. Fourier transform infrared (FT-IR) spectra of building blocks and COFs were recorded by Thermo Scientific Nicolet i5. (αhν) 1/n = A (hν-E g ) Where α represents absorption coefficient, A represents the absorption constant, and n is equal to 1/2 (direct band gap semiconductor). E g of the samples was estimated from the intercept of the tangent in the plots of (αhν) 2 versus photon energy (hν).

Photoluminescence spectra
All electrochemical related tests were made on CHI 660e electrochemical workstation which has standard three-electrode electrochemical cell with working electrode, platinum plate as counter electrode, saturated Ag/AgCl electrode as reference electrode and sodium sulfate solution (0.2 M) as electrolyte. A 300 W Xenon lamp with a 420 nm cut-off filter was used as the light source during the measurement with 10 s -1 switching frequency.
The prepared working electrode: 5 mg of COF-based bionic enzymes powder was blended with 2.0 mL of ethanol and 20 μL of Nafio solution and sonicated for 30 min. Afterwards, 10 μL of the mixture was dropwise placed on the surface of an FTO glass plate with an area of 1 × 1 cm 2 and placed in air to dry.

Mott-Schottky plot measurements
Mott-Schottky plot was measured in a potential range from -1.0 to 1.0 V (vs. Ag/AgCl) at a frequency of 500, 1000 and 2000 Hz without illumination. The flat band potential was obtained according to Equation where C is the space charge capacitance in the semiconductor, N D is the electron carrier density, e is the elemental charge, ε 0 is the permittivity of a vacuum, ε is the relative permittivity of the semiconductor, E is the applied potential, E fb is the flat band potential, T is the temperature, and k is the Boltzmann constant.
The measured potentials vs. Ag/AgCl can be converted to normal hydrogen electrode (NHE) scale using Equation S4 9, 10 :

Nyquist plot measurements
Electrochemical impedance spectroscopy (EIS) was carried out on CHI 660e in a frequency range from 10 5 Hz to 10 -2 Hz. Nyquist plot at high frequency represents charge-transfer process and the diameter of capacitance arc reflects the charge-transfer resistance (R ct ).

Cyclic voltammogram measurements
Cyclic voltammogram was carried out on CHI 660e at scan rate of 0.05 V/s from 0 to -2.0 V to + 2.0 V. Using photoluminescence maximum E 0,0 and E ox / E red , the excited state reduction potential was estimated for COFs