5-Amino-2-chloropyridine (99% Oakwood Chemical), 12,5-hexanedione (98% Sigma), p-Toluenesulfonic acid (p-TsOH, 98% Alfa Aesar), Zn powder (Zn, 97% Alfa Aesar), tetraethylammonium bromide (Et4NBr, 98% Alfa Aesar), hydroxylamine hydrochloride (NH2OH·HCl, 97% TCI), triethylamine (Et3N, 99% Oakwood Chemical), 2,2’-bipyridyl-5,5’-dicarbaldehyde (CHO-N2, 95–98% Aaron Chemicals), 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (NH2-N3, 95–98% Aaron Chemicals), 4,4’,4’’-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (CHO-N3, 95–98% Ambeed), Rhenium pentacarbonyl chloride (98%, Acros Organics), Mesitylene (99%, Acros Organics), toluene (ACS reagent grade, Sigma), tetrahydrofuran (THF, ACS reagent grade, Sigma), dichloromethane (CH2Cl2, ACS reagent grade, Sigma), sodium bicarbonate (NaHCO3, 99% Sigma), magnesium sulfate (MgSO4, 99% Sigma), hexane (ACS reagent grade, Sigma), ethyl acetate (EtOAc, ACS regent grade, Sigma), aqueous ammonia (25%, Sigma), ethanol (EtOH, ACS reagent grade, Sigma) p-Dioxane (anhydrous, EMD Millipore), glacial acetic acid (ACS reagent grade, Acros Organics), 2,2’-bipyridyl-5,5’-diamine is synthesized via the procedure outlined in the Supporting Information.
Synthesis of f-COF, r-COF, Re-f-COF, and Re-r-COF.
f-COF and Re-f-COF are synthesized according to previously published protocols.4 The synthesis of r-COF is found to result in a lower yield than f-COF. The details of the synthetic procedure of monomers and COFs are discussed in supporting information (SI). The structure is confirmed by Fourier transformed infrared (FTIR) spectroscopy, powder x-ray diffraction (PXRD), Brunauer-Emmett-Teller (BET) analysis, and X-ray absorption fine structure (XAFS) with extended XAFS (EXAFS) fitting.
PXRD patterns are obtained using a Rigaku Miniflex II XRD diffractometer with Cu Kα radiation source. UV-visible absorption and diffuse reflectance data are recorded with a Cary 5000 UV-VIS-NIR spectrophotometer with an internal diffuse reflectance accessory for diffuse reflectance measurements. FTIR spectroscopy is performed with solid samples on a Thermo Fischer Scientific iS5 FTIR spectrometer equipped with an iD3 ATR accessory. Gas adsorption isotherms are collected by using the ASAP-2020 surface area analyzer. N2 gas adsorption isotherms are measured at 77 K using a liquid N2 bath. The ICP-MS measurements are performed using an Agilent 7700x. Samples for ICP-MS analysis are digested by sonicating ~ 0.1–0.5 mg COFs in concentrated HNO3 for 6 hours and then diluting it with a 2% HNO3 solution to achieve a concentration within the calibration range of 0-200 ppb. Calibration solutions with concentrations of 0, 1, 5, 10, 25, 50, 100, 150, and 200 ppb are prepared by diluting the standard Re solutions purchased from Sigma-Aldrich. The R-value of the calibration curves is 0.9999.
fs-Transient Absorption (TA) Spectroscopy.
A regenerative amplified Ti-Sapphire laser (Solstice, 1KHz repetition rate, 800 nm, < 100 fs FWHM, 3.5mJ/pulse) provides the pump and probe pulses for TA measurements. The pump is generated by taking 75% the 800 nm Solstice output and generating the 400 nm second-harmonic in a BBO crystal followed by 1000 Hz beam chopping. The remaining 25% of Solstice output is used to generate white light in a sapphire crystal (400–800 nm) in a Helios ultrafast spectrometer (Ultrafast Systems LLC). The film samples are prepared by sonicating COF in 5% (w/w) Nafion/1-propanol dispersion then drop-casting onto a clean glass substrate. During data capture the sample is translated continuously to avoid sample degradation from the 400 nm pump pulses (0.15 µJ/pulse).
ns-Transient Absorption (TA) Spectroscopy.
Measurements are made on an enVISion transient absorption spectrometer (Magnitude Instruments) using real-time, photoluminescence correction to eliminate emission signals from the TA data. The excitation source is the third harmonic of an Nd:YAG laser, operating at 1 kHz repetition rate with a pulse width of 2.5 ns. The samples are not translated during the measurements, therefore the incident energy density of the 355 nm excitation pulses is kept below 250 µJ/cm2, to eliminate sample degradation and minimize thermal artifacts.
Density Functional Tight-Binding (DFTB) calculations are performed on hexagonal unit cells consisting of a single COF layer. The SCC-DFTB20 method from the DFTB + package21,22 is used for structural optimization. The 3ob-3-1 Slater-Koster parameters23 is used for all atoms, and D3 dispersion24 with BJ damping25 is used to model non-covalent interactions. Suitable convergence is found with a 1x1x3 k-point sampling. For structural optimization, the atomic positions are converged when the force on all atoms is less than 0.01 eV/Å, and the Pulay stress on the unit cell vectors is less than 0.05 eV/Å.
From the optimized unit cell, the molecular structure is decomposed into what we denote an “edge unit” consisting of the COF structure between two tritopic cores. This decomposition is based on previous reports26,27 that show the most important electronic transitions occur independently on each branch around the tritopic core. These edge units represent a single layer of COF within the lattice packed framework, and no further structural optimization is employed. Time-dependent density functional theory28,29 (TDDFT) at the CAM-B3LYP/6-311G** level of theory30–32 is used to model the transitions on the COF edge unit in Gaussian1633 with the full population (Pop = Full) and Gaussian functions (GFINPUT) printed into the output as required by TheoDORE34 which is used for analysis of the transition density matrix (TDM) to characterize the photogenerated exciton.
Rhenium tricarbonyl chloride is incorporated onto the bipyridine portion of the optimized edge unit and optimized using the wB97XD functional.35 In the optimization the nuclear positions of the triazine ring are fixed such that the planarity of the edge unit is conserved. Following a mixed basis set approach, the 6-311 + G** basis set is used for C, N, O, Cl, and H atoms31,32,36 while the LANL2DZ37 (with LANL2DZ ECP) basis from Basis Set Exchange38–40 is used to model Re. An ultrafine grid is used with tight convergence criteria to achieve convergence to a potential minimum that is verified by non-negative frequencies. The first 10 singlet and 10 triplet transitions of these Re-COF model are then calculated by TDDFT on the same ultrafine grid and mixed basis set with wB97XD functional. Transitions are then visualized by their natural transition orbitals41 (NTOs).