Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks

doi:10.21203/rs.3.rs-1554507/v1

PDF

Article

Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks

https://doi.org/10.21203/rs.3.rs-1554507/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted 20 Apr, 2022

You are reading this latest preprint version

We investigate the correlation between CO₂ reduction performance in twin COFs with reversed imine connectivity and reveal that imine-linkage imparts intramolecular charge transfer (ICT) directionality – an effect we term an “ICT Tesla Valve.” This reversal also affects the coordination of Re molecular catalyst (MC) and leads to a slowdown in light driven CO₂→CO photoreduction. ICT towards Re MC in Re-f-COF facilitates efficient CO₂ photoreduction while performance is reduced when ICT is direct away from Re MC in Re-r-COF. Combining theory with experimental data shows that ICT directionality affects the charge separation mechanism which is particularly evident in the much longer-lived excited state of Re-f-COF compared to Re-r-COF. These findings are unprecedented, revealing not only that the linker chemistry should be recognized as a key factor to consider in the development of COF photocatalysts but also provides a unique approach to tailor photocatalysts by controlling the direction of ICT.

Powering hydrogen fuel cells through water splitting (H₂ and O₂) or providing syn gas (H₂ and CO) via CO₂ reduction represent some of the best strategies to renewably address global energy demands.^1,2 Performing either of these splitting/redox reactions efficiently requires the development of an advanced photocatalytic architecture that can integrate both light absorption and a catalyst.^3,4 Covalent organic frameworks (COFs), a novel class of crystalline porous polymers constructed from organic moieties linked by covalent bonds, have emerged as exceptional photocatalytic materials due to their inherent porosity, large surface area, high thermal and mechanical stability, and structural flexibility.^5–13 Among them, 2D COFs are of particular interest because the in-plane π-conjugation allows charge delocalization and the interlayer π-stacking may enable significant electronic overlap, facilitating out-of-plane migration of charge carriers. Indeed, 2D COFs have been successfully used as photocatalytic materials in several systems, including those that incorporate metal catalysts through post-synthesis modification,¹⁴ Z-scheme type catalysts using covalently bound semiconductor/COF heterojunctions,¹⁵ COFs with photosensitizer/co-catalyst assemblies,^7,16,17 and COFs synthesized as metal-free photocatalysts.¹⁸ Most recently, we have also demonstrated the incorporation of Re(CO)₃(bpy)Cl (bpy=2,2’-bipyridyl) and Mn(CO)₃(bpy)Br molecular catalyst (MC) into 2D COFs based on a triazine core and bipyridine bridge, which can catalyze CO₂ to form CO with significantly enhanced efficiency and stability with respect to their corresponding homogenous counterparts.^4,19

These prior studies demonstrate that COFs offer a promising platform for creating the next generation of photocatalytic materials. However, a knowledge gap exists in how COF monomer selection and linker chemistry impact their respective photophysical and photocatalytic properties, preventing accurate prediction and informed design. In this work, we fill these needs and report fundamental theoretical and experimental studies on two topologically and constitutionally isomeric COFs with reversed imine connectivity, which reveals that the direction of the imine linker plays an outsized role in controlling coordination of Re MC, photophysical properties, and photocatalytic performance towards CO₂→CO reduction. We find that reversing the direction of the imine-linkage between a tritopic monomer and ditopic monomer from the recently reported COF with forward imine direction (f-COF, Fig. 1a) reveals completely different behavior of bipyridine, which is switched from an electron acceptor in f-COF to an electron donor in its constitutional twin with reversed imine-linkage (r-COF, Fig. 1b). This implies that the imine direction imparts directionality to intramolecular charge transfer (ICT) that impacts the uptake of Re/bipyridine orbital interactions which drastically changes the excited state (ES) and charge separation (CS) pathways. These factors lead to distinct catalytic functions between the two COFs, where Re-f-COF functions as an efficient and selective catalyst for light driven CO₂→CO reduction, in contrast to comparatively sluggish catalytic function in Re-r-COF. This interesting phenomenon suggests that linker directed ICT should be a key consideration when designing COFs for artificial photosynthesis.

Synthesis and Characterization of COF and Re-COF.

The synthesis and characterization of f-COF and Re-f-COF (Fig. 1a) have been reported previously.⁴ The newly designed r-COF with reversed imine direction (Fig. 1b) is synthesized by the condensation of 2,2’-bipyridyl-5,5’-diamine (NH₂-bpy) and 4,4’,4’’-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde (CHO-TTA) in a 10mL N₂ purged Pyrex pressure tube with 4:1 1,4-dioxane:mesitylene solvent system and 0.1 mL glacial acetic acid catalyst sealed and heated at 120°C for 3 days. As shown in Fig. 2a, the FTIR spectrum of r-COF shows an imine (C = N) stretching band at 1620 cm^-1, which is accompanied by attenuation of -NH₂ and -C = O bands that were present in NH₂-bpy in CHO-TTA, respectively, indicating the imine bond is formed by condensation of -NH₂ and -CHO. PXRD patterns (Fig. 2b) of r-COF show noticeable diffraction intensity at 2.3° 2ϴ, suggesting its crystalline nature. The DFTB optimized unit cell is used as a lattice model (inset of Fig. 2b) to cross-validate the experimental and computational structures. In addition, a comparison of AA and AB stacking modes is made with AA appearing the most likely. These patterns resemble that of f-COF (Figure S1), suggesting similar crystallinity and topology between r-COF and f-COF.

Refluxing r-COF with Re(CO)₅Cl in toluene results in the formation of Re-r-COF (Fig. 1b). Compared to r-COF, the PXRD patterns (Fig. 2b) show more prominent diffraction intensities that suggest retainment, and possible enhancement of crystallinity upon incorporation of fac-Re(bpy)(CO)₃Cl. FTIR (Fig. 2a) and X-ray absorption spectroscopy (XAS, Fig. 2c) collected at Re L3 edge confirmed that Re(CO)₅Cl is incorporated into r-COF through NH₂-bpy ligand to form fac-Re(bpy)(CO)₃Cl.^42–45 As shown in Fig. 2a, the FTIR spectrum of Re-r-COF exhibits three bands at 1878 cm^-1, 1910 cm^-1, and 2018 cm^-1. These bands respectively correspond to the A’(2), A’’, and A’(1) stretching modes of C ≡ O in the bound Re complex,⁴⁵ and are in good agreement with the trend in analytic frequencies from DFT (Table S1). Furthermore, fitting the extended X-ray absorption fine structure (EXAFS) R-space spectrum of Re-r-COF indicates that Re center is coordinated by three C (C ≡ O), two N (bpy), and one Cl ligand (Fig. 2c), which unambiguously confirm the presence of fac-Re(bpy)(CO)₃Cl in Re-r-COF. BET analysis of the pseudo-type IV N₂ isotherm (Fig. 2d) obtained from Re-r-COF yields 739 m²/g BET surface area and pore size distributed around the 27.3 Å modal average.

Like f-COF and Re-f-COF,⁴ the Kubelka-Munk transformed diffuse reflectance spectra of r-COF and Re-r-COFs (Fig. 2e) show broad absorption in the ultraviolet and visible regions (> 400 nm). Because the monomers show negligible absorption in the visible region (> 400 nm), we attribute the absorption in the visible region for r-COF and Re-r-COF to the ICT band, which is consistent with our previous report⁴ and further supports the formation of r-COF.

The impact of imine direction on electronic structures of COFs

To gain insight on the role imine direction plays in the electronic structure and photophysical properties of COFs, we performed TDDFT on an edge unit (Figure 3a) followed by TDM analysis to quantitatively unravel the electronic correlation between fragments that contribute to ICT. Selection of fragments is crucial to TDM analysis and as such we carefully selected four fragments - triazine, phenylene, imine linker, and bipyridine bridge. The population of matrix elements shown in the electron correlation plot (Figure 3b) indicates ICT seemingly favors some degree of electron transfer from phenylene to both imine and bipyridine in f-COF. However, reversing the direction of the imine linker in the edge unit leads to completely different behavior of the bipyridine bridge, switching it from an electron acceptor to an electron donor, thus reversing the ICT direction. This interesting phenomenon finds analogy to one-directional fluidic valves like the “Tesla Valve,” suggesting the key role imine direction plays on ICT in imine-linked COFs.

The impact of imine direction on electronic structures of Re-COFs

Reversal of the imine linker not only affects ICT but also the energetics and electronic structure of Re-COFs. Natural transition orbitals (NTOs, Fig. 4a) of the optically allowed S₄ transition predicted by TDDFT shows that Re-f-COF possesses COF-centered hole density, and electron density with mixed Re/f-COF orbital contributions. Observing this suggests that some degree of ligand-to-metal charge transfer (LMCT) may take place, which is consistent with our previous X-ray transient absorption (XTA) spectroscopic studies on Re-f-COF that show Re gains electron density after photoexcitation.⁴ In contrast, Re-r-COF NTOs (Fig. 4a, bottom) shows hole density with mixed Re/r-COF orbital contributions with significant Re participation, in line with some degree of metal-to-ligand charge transfer (MLCT) typically associated with Re(bpy)(CO)₃Cl.⁴³ Thus, theory is in alignment suggesting a lack of forward ET from r-COF to Re MC in Re-r-COF is linked to directionality of ICT predicted by TDM analysis on r-COF above. Further insight about the lack of forward ET in Re-r-COF comes from orbital energies of the predominant contributions to the S₄ transition (Fig. 4b). In Re-r-COF HOMO-3 and LUMO are the primary MOs involved in the S₄ transition and they are respectively 0.18 eV lower and 0.39 eV higher than those of Re-f-COF (also HOMO-3/LUMO). This disparity results from a relatively electron-rich bipyridine in ground state of Re-r-COF, causing energy lowering that prevents Re(dπ) and bipyridine(π*) orbitals from mixing. The frontier orbital energies (Fig. 4b) provide a clear picture of the poor Re/bipyridine orbital mixing in Re-r-COF by showing that the occupied MOs that localize Re electron density are energetically separate from r-COF centered MO while Re localized MOs in Re-f-COF are energetically much closer to f-COF localized MOs.

Theoretical predictions agree well with our experimental results. As shown in Fig. 4c, quantitative analysis of the EXAFS spectrum in R-space using Feff calculation based on a reported Re(bpy)(CO)₃Cl crystal structure (CCDC Deposition #649892) reveals significant reduction of the Re-C (-C ≡ O) bond (1.83 Å equatorial and 1.84 Å axial) in Re-r-COF with respect to Re-f-COF (1.91 Å equatorial and 1.88 Å axial). These Re-C bond reductions in Re-r-COF correspond with a similar increase to the Re-N (2.24 Å) and Re-Cl (2.47 Å) bonds relative to those in Re-f-COF – 2.17 Å and 2.45 Å for Re-N and Re-Cl, respectively. In contrast, Re→C→O forward scatterings only show slight change, which is 3.06 Å in Re-r-COF and 3.07 Å in Re-f-COF, leading to 0.07 Å and 0.03 Å net elongation of the equatorial and axial C ≡ O, respectively. These EXAFS results are explained by π back-bonding from Re d-orbitals (π symmetry) to anti-bonding molecular orbitals of C ≡ O (π*), which strengthens and shortens the Re-C bond while weakening and lengthening the C ≡ O bond. π back-bonding to equatorial C ≡ O is likely more significant as indicated by the larger C ≡ O net elongation relative to axial C ≡ O. These findings are supported by the lower frequency C ≡ O stretching modes in the FTIR spectra of Re-r-COF (1878 cm^-1, 1912 cm^-1 and 2018 cm^-1) in relation to Re-f-COF (1890 cm^-1, 1910 cm^-1 and 2021 cm^-1) due to increased Re(dπ)→C ≡ O (π*) back bonding in the former (Fig. 4d). These results together suggest that imine direction directly controls bipyridine π-acceptor characteristics in both f-COF and r-COF thus manipulating Re(dπ)→bipyridine(π*) back-bonding that is inversely related with back-bonding into the equatorial C ≡ O counterparts. The stronger Re(dπ)→C ≡ O (π*) back bonding in Re-r-COF is associated with its weaker Re(dπ)→bipyridine(π*) back-bonding, consistent with the weaker Re/bipyridine orbital mixing (Fig. 4b) and C ≡ O stretching frequencies (Table S1, S2) predicted by DFT calculation.

The impact of imine direction on the ES and CS dynamics of Re-COFs

Transient absorption (TA) spectroscopy is used to examine the exited state (ES) and CS dynamics of Re-f-COF and Re-r-COF. Both femtosecond (fs-TA) and nanosecond TA (ns-TA) spectroscopy are used to probe the early time and later time dynamics of these Re-COFs. Figure 5a compares the fs-TA spectra of Re-f-COF and Re-r-COF following 400 nm excitation. The fs-TA spectra of Re-f-COF (top panel) possess a ground state bleach (GSB) at ~ 470 nm, a broad ES absorption (ESA) centered at 675 nm, and an isosbestic point at 500 nm. It is interesting to note that the whole spectrum red shifts accompanied by ~ 25 nm at later time (> 25 ps), suggesting that the system relaxes to form a new intermediate state. The fs-TA spectra of Re-r-COF has 640 nm centered ESA across the entire spectral window with missing GSB. Like that of Re-f-COF, noticeable red shift of the whole spectral window was observed at later time (> 25 ps), suggesting the formation of a similar intermediate state to Re-r-COF. This is supported by their relatively similar decay profiles of their ESA kinetic traces (Inset of Fig. 5c), suggesting both Re-COFs undergo similar ES relaxation dynamics within the 5 ns time window of the fs-TA instrument.

While the ES decay dynamics in fs-TA spectra of both Re-COFs appear to be similar, drastic differences were observed between their ns-TA. As shown in Fig. 5b, the ns-TA spectra of Re-r-COF (bottom panel) has similar spectral features to its fs-TA spectra and decays to ground state within 4 µs. In contrast, the ns-TA spectra of Re-f-COF exhibits a prominent spectral evolution where the whole spectrum shows significant blue shift (Fig. 5b). Qualitatively, the center frequency of the broad ESA appears to be centered around 580 nm suggesting the formation of an additional intermediate species in Re-f-COF. Moreover, Re-f-COF has ES population persisting well after the 15 µs time window of the ns-TA measurement, evidence of an exceptionally long-lived ES. This is more clearly represented by the kinetic traces at 600 nm (Fig. 5c), which involves a rising component at early time and shows negligible decay within 14 ms. In fact, an attempt to find the end of this lifetime (with controls in place preventing charge build-up) was unsuccessful as significant signal remained after 4 ms, which is in stark contrast to the ESA of Re-r-COF which completely decays within 4 µs – representing a more than 1000-fold difference. Putting these results together, we propose an ES relaxation dynamics model that can well explain the observed experimental data (Fig. 5d). Following photoexcitation, both Re-COF systems are promoted to their higher energy ES which possibly has local excitation (LE) character. LE state quickly relaxes down to a lower energy level which is likely associated with ICT, corresponding to the spectral evolution observed in fs-TA at > 25 ps. Suggested by ns-TA data, Re-r-COF returns to GS from its CT, whereas Re-f-COF populates an additional CS state before slowly returning to GS due to ET from f-COF to Re MC.

Photocatalytic CO₂ reduction reaction and structure-function relationship

With fundamental insight about the distinct electronic structure and ICT properties between Re-f-COF and Re-r-COF due to imine reversal, we next proceed to examine their catalytic performance for light driven CO₂→CO reduction. The photocatalytic reactions are performed in a 11 mL reaction vial filed with 3 mL CH₃CN, 0.2 mL triethanolamine (TEOA), and 1 mg Re-COF. The reaction vial is purged with CO₂ for 15 minutes then exposed to a focused broadband Xe lamp with a cut off filter (> 420 nm) and a one-foot water filter to remove IR light. As shown in Fig. 6a, under the same experimental conditions, Re-f-COF can generate 6.3 ± 1.2 mmol CO per gram Re-f-COF catalyst within 8 hours whereas Re-r-COF with reversed imine direction can only produce 2.1 ± 0.8 mmol CO per gram Re-r-COF catalyst within the same period. These findings are meaningful and suggest that reversed imine direction in Re-COF reduces the catalytic activity of Re-r-COF for CO₂→CO reduction.

Taking all results together, we attribute differences in photocatalytic CO₂ reduction by Re-COF to electronic structure effects (ICT, CS, energetics) caused by reversing the direction of the imine linker. TDDFT and TDM analysis suggests that the imine direction in f-COF is a suitable choice for CO₂ reduction as electron density is directed toward the bipyridine ligand upon photoexcitation which is expected to facilitate CS through ET from f-COF to Re MC. In contrast, upon photoexcitation the reversed imine direction in r-COF drives electron density away from the bipyridine ligand which inhibits ET from r-COF to Re MC. These results are reflected by the NTOs of the optically allowed S₄ transition where LMCT is favored in Re-f-COF while MLCT is in Re-r-COF. Theoretical data are supported by experimental results like the exceptionally long-lived CS state observed by ns-TA in Re-f-COF that is absent from Re-r-COF. In addition, FTIR and EXFAS revealed enhanced Re(dπ)→C ≡ O(π*) back bonding due to weaker Re(dπ)→bpy(π*) back-bonding in Re-r-COF. Weaker Re(dπ)→bpy(π*) back-bonding in Re-r-COF is indicative of bipyridine possessing weaker π-acceptor characteristics than Re-f-COF which is unified with the theoretical model that shows bipyridine acting as a poor π-electron acceptor, or perhaps even an electron donor in r-COF.

The formation of an additional intermediate species with dramatically prolonged lifetime in Re-f-COF with respect to that of Re-r-COF, together with the experimental observation of the reduction of Re moiety by XTA in Re-f-COF,¹⁷ leads us to believe that this long-lived species (> 4 ms) is associated with the formation of CS state due to ET from f-COF to Re moiety. In turn, the absence of such CS in Re-r-COF suggests the lack of such ET process in Re-r-COF, which agrees well with directional ICT from bipyridine towards TTA and poor Re/bipyridine orbital mixing predicted by TDDFT and TDM analysis. While the identity of this CS state is unclear, its persistence suggests a possible chemical change like Cl^- dissociation from Re may take place in Re-f-COF but not Re-r-COF.⁴⁶ As the formation of a long-lived CS is essential for the catalytic cycle, the lack of the long-lived CS in Re-r-COF explains the slow-down of its photocatalytic activity for CO₂ reduction, which in turn supports the validity of the proposed ES dynamics mechanism (Fig. 5d).

Finally, an important geometric factor to consider might be the surface area, where the surface area of Re-r-COF (739 m²/g, 27.3Å pore size) is about 2 times less than that of Re-f-COF (1367 m²/g, 29.5 Å pore size) BET surface area (Fig. 2d). Yet, this factor alone likely cannot explain the difference in photocatalytic CO₂ reduction especially when taking into consideration the similar pore size distributions and increased percent mass of Re in Re-r-COF from ICP-MS (6.2% Re in Re-f-COF, 7.4% Re in Re-r-COF, Figure S5).

In summary, we report the design, synthesis, photophysical properties, and photocatalytic performance of two constitutional twin COFs with reversed imine connectivity, that demonstrate distinct relationships between their structure, properties, and function. Using theoretical predictions to lead our investigation by advanced spectroscopic methods, we show that the imine-linker acts as an electron mediator and imparts directionality to ICT. Re-f-COF constructed from CHO-bpy and NH₂-TTA can drive ICT from TTA to bipyridine, which facilitates CS through ET to Re MC, and thus functions as an efficient and selective CO₂ photoreduction catalyst. In contrast, its sibling Re-r-COF, which is constructed from CHO-TTA and NH₂-bpy (reverse imine direction), favors ICT away from bipyridine which inhibits CS and significantly slows down its photocatalytic activity. This finding – that the imine linker plays a key role in ICT by mediating the direction of electron donation – is unprecedented and provides a unique approach to design photocatalysts that might be capable of performing both reductive and oxidative half reactions in artificial photosynthesis. For example, f-COF can direct electron density toward the bipyridine ligand that is directly linked to a CO₂ reduction or H₂ evolution MC while of r-COF may have the potential, in other systems, to drive hole density toward bpy and to the desired MC to perform the OER half-reaction.

Chemicals.

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.

Standard Characterization.

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.

Computational Modeling.

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 + package^21,22 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 reports^26,27 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 theory^28,29 (TDDFT) at the CAM-B3LYP/6-311G** level of theory^30–32 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 atoms^31,32,36 while the LANL2DZ³⁷ (with LANL2DZ ECP) basis from Basis Set Exchange^38–40 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).

Acknowledgements

This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0020122. This work used high-performance computational facilities supported by National Science Foundation grants number ACI-1548562 and CNS-1828649. Use of the Advanced Photon Source in Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-AC02-06CH11357. We also acknowledge support from National Science Foundation award CNS-1828649 “MRI: Acquisition of iMARC: High Performance Computing for STEM Research and Education in Southeast Wisconsin”.

Author Contributions

The manuscript was written through contributions of all authors. D.S. conceptualized the reversed imine idea and performed theoretical calculations, along with collection and analysis of experimental data. D.W. synthesized the COFs and Re-COFs, C.F. performed collection of the N₂ adsorption isotherms and BET analysis. E.K. performed collection of the ns-TA data and was helpful in the analysis. D.L. performed collection of ICP-MS data. K.K. oversaw the computational study. L.L. oversaw the collection of ICP-MS data. J.H. led this research and is the corresponding author for this work.

Competing Interests

The collection of ns-TA data by E.K. at Magnitude Instruments was performed as a demonstration of their ns-TA instrument which was ultimately not purchased.

Materials & Correspondence

*Jier Huang ([email protected])

Ashford, D. L. et al. Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications. Chemical Reviews 115, 13006–13049 (2015).
Lewis, N. S. Introduction: Solar Energy Conversion. Chemical Reviews 115, 12631–12632 (2015).
Pan, Q. et al. Ultrafast charge transfer dynamics in 2D covalent organic frameworks/Re-complex hybrid photocatalyst. Nature Communications 13, 845 (2022).
Yang, S. et al. 2D Covalent Organic Frameworks as Intrinsic Photocatalysts for Visible Light-Driven CO2 Reduction. J Am Chem Soc 140, 14614–14618 (2018).
Diercks, C. S. & Yaghi, O. M. The atom, the molecule, and the covalent organic framework. Science (1979) 355, eaal1585 (2017).
Huang, N., Wang, P. & Jiang, D. Covalent organic frameworks: a materials platform for structural and functional designs. Nature Reviews Materials 1, 16068 (2016).
Vyas, V. S. et al. A tunable azine covalent organic framework platform for visible light-induced hydrogen generation. Nature Communications 6, 8508 (2015).
Bunck, D. N. & Dichtel, W. R. Postsynthetic functionalization of 3D covalent organic frameworks. Chemical Communications 49, 2457–2459 (2013).
Waller, P. J. et al. Chemical Conversion of Linkages in Covalent Organic Frameworks. J Am Chem Soc 138, 15519–15522 (2016).
Kandambeth, S., Dey, K. & Banerjee, R. Covalent Organic Frameworks: Chemistry beyond the Structure. J Am Chem Soc 141, 1807–1822 (2019).
Zhao, W., Xia, L. & Liu, X. Covalent organic frameworks (COFs): perspectives of industrialization. CrystEngComm 20, 1613–1634 (2018).
Vazquez-Molina, D. A. et al. Mechanically Shaped Two-Dimensional Covalent Organic Frameworks Reveal Crystallographic Alignment and Fast Li-Ion Conductivity. J Am Chem Soc 138, 9767–9770 (2016).
Li, X. et al. Facile transformation of imine covalent organic frameworks into ultrastable crystalline porous aromatic frameworks. Nature Communications 9, 2998 (2018).
Fu, Z. et al. A stable covalent organic framework for photocatalytic carbon dioxide reduction. Chemical Science 11, 543–550 (2020).
Zhang, M. et al. Semiconductor/Covalent-Organic-Framework Z-Scheme Heterojunctions for Artificial Photosynthesis. Angewandte Chemie International Edition n/a, (2020).
Stegbauer, L., Schwinghammer, K. & Lotsch, B. V. A hydrazone-based covalent organic framework for photocatalytic hydrogen production. Chemical Science 5, 2789–2793 (2014).
Pachfule, P. et al. Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation. J Am Chem Soc 140, 1423–1427 (2018).
Bhunia, S. et al. Electrochemical Stimuli-Driven Facile Metal-Free Hydrogen Evolution from Pyrene-Porphyrin-Based Crystalline Covalent Organic Framework. ACS Applied Materials & Interfaces 9, 23843–23851 (2017).
Wang, D., Streater, D., Peng, Y. & Huang, J. 2D Covalent Organic Frameworks with an Incorporated Manganese Complex for Light Driven Carbon Dioxide Reduction. ChemPhotoChem 5, 1119–1123 (2021).
Elstner, M. et al. Self-consistent-charge density-functional tight-binding method for simulations of complex materials properties. Physical Review B 58, 7260–7268 (1998).
Aradi, B., Hourahine, B. & Frauenheim, Th. DFTB+, a Sparse Matrix-Based Implementation of the DFTB Method. The Journal of Physical Chemistry A 111, 5678–5684 (2007).
Hourahine, B. et al. DFTB+, a software package for efficient approximate density functional theory based atomistic simulations. The Journal of Chemical Physics 152, 124101 (2020).
Gaus, M., Lu, X., Elstner, M. & Cui, Q. Parameterization of DFTB3/3OB for Sulfur and Phosphorus for Chemical and Biological Applications. Journal of Chemical Theory and Computation 10, 1518–1537 (2014).
Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. The Journal of Chemical Physics 132, 154104 (2010).
Grimme, S., Ehrlich, S. & Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. Journal of Computational Chemistry 32, 1456–1465 (2011).
Thomas, S. et al. Electronic Structure of Two-Dimensional π-Conjugated Covalent Organic Frameworks. Chemistry of Materials 31, 3051–3065 (2019).
Feng, T. et al. Tuning Photoexcited Charge Transfer in Imine-Linked Two-Dimensional Covalent Organic Frameworks. The Journal of Physical Chemistry Letters 13, 1398–1405 (2022).
Bauernschmitt, R. & Ahlrichs, R. Treatment of electronic excitations within the adiabatic approximation of time dependent density functional theory. Chemical Physics Letters 256, 454–464 (1996).
Casida, M. E., Jamorski, C., Casida, K. C. & Salahub, D. R. Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: Characterization and correction of the time-dependent local density approximation ionization threshold. The Journal of Chemical Physics 108, 4439–4449 (1998).
Yanai, T., Tew, D. P. & Handy, N. C. A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chemical Physics Letters 393, 51–57 (2004).
Ditchfield, R., Hehre, W. J. & Pople, J. A. Self-Consistent Molecular‐Orbital Methods. IX. An Extended Gaussian‐Type Basis for Molecular‐Orbital Studies of Organic Molecules. The Journal of Chemical Physics 54, 724–728 (1971).
Krishnan, R., Binkley, J. S., Seeger, R. & Pople, J. A. Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions. The Journal of Chemical Physics 72, 650–654 (1980).
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V., and D. J. F. Gaussian 16 Revision C.01. (2016).
Plasser, F. TheoDORE: A toolbox for a detailed and automated analysis of electronic excited state computations. The Journal of Chemical Physics 152, 84108 (2020).
Chai, J.-D. & Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections. Physical Chemistry Chemical Physics 10, 6615–6620 (2008).
Clark, T., Chandrasekhar, J., Spitznagel, G. W. & Schleyer, P. V. R. Efficient diffuse function-augmented basis sets for anion calculations. III. The 3–21 + G basis set for first-row elements, Li–F. Journal of Computational Chemistry 4, 294–301 (1983).
Hay, P. J. & Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. The Journal of Chemical Physics 82, 299–310 (1985).
Schuchardt, K. L. et al. Basis Set Exchange: A Community Database for Computational Sciences. Journal of Chemical Information and Modeling 47, 1045–1052 (2007).
Feller, D. The role of databases in support of computational chemistry calculations. Journal of Computational Chemistry 17, 1571–1586 (1996).
Pritchard, B. P., Altarawy, D., Didier, B., Gibson, T. D. & Windus, T. L. New Basis Set Exchange: An Open, Up-to-Date Resource for the Molecular Sciences Community. Journal of Chemical Information and Modeling 59, 4814–4820 (2019).
Martin, R. L. Natural transition orbitals. The Journal of Chemical Physics 118, 4775–4777 (2003).
Kurz, P., Probst, B., Spingler, B. & Alberto, R. Ligand Variations in [ReX(diimine)(CO)3] Complexes: Effects on Photocatalytic CO2 Reduction. European Journal of Inorganic Chemistry 2006, 2966–2974 (2006).
Benson, E. E. et al. The Electronic States of Rhenium Bipyridyl Electrocatalysts for CO2 Reduction as Revealed by X-ray Absorption Spectroscopy and Computational Quantum Chemistry. Angewandte Chemie International Edition 52, 4841–4844 (2013).
Oppelt, K. T., Sevéry, L., Utters, M., Tilley, S. D. & Hamm, P. Flexible to rigid: IR spectroscopic investigation of a rhenium-tricarbonyl-complex at a buried interface. Physical Chemistry Chemical Physics 23, 4311–4316 (2021).
Asbury, J. B., Wang, Y. & Lian, T. Time-Dependent Vibration Stokes Shift during Solvation: Experiment and Theory. Bull Chem Soc Jpn 75, 973–983 (2002).
Takeda, H., Koike, K., Inoue, H. & Ishitani, O. Development of an Efficient Photocatalytic System for CO2 Reduction Using Rhenium(I) Complexes Based on Mechanistic Studies. J Am Chem Soc 130, 2023–2031 (2008).

There is NO Competing Interest.

ReCOFSI.docx
Imine Reversal Mediates Charge Separation and CO Photoreduction in Covalent Organic Frameworks

PDF

Version 1

posted 20 Apr, 2022

You are reading this latest preprint version

Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks

Status:

Version 1

Abstract

Figures

Introduction

Results

Synthesis and Characterization of COF and Re-COF.

The impact of imine direction on electronic structures of COFs

The impact of imine direction on electronic structures of Re-COFs

The impact of imine direction on the ES and CS dynamics of Re-COFs

Photocatalytic CO₂ reduction reaction and structure-function relationship

Discussion

Methods

Declarations

References

Competing Interests

Supplementary Files

Status:

Version 1

Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks

Status:

Version 1

Abstract

Figures

Introduction

Results

Synthesis and Characterization of COF and Re-COF.

The impact of imine direction on electronic structures of COFs

The impact of imine direction on electronic structures of Re-COFs

The impact of imine direction on the ES and CS dynamics of Re-COFs

Photocatalytic CO2 reduction reaction and structure-function relationship

Discussion

Methods

Declarations

References

Competing Interests

Supplementary Files

Status:

Version 1

Photocatalytic CO₂ reduction reaction and structure-function relationship