Synthesis and characterization of COFs
A progressive substitution of alternate carbons in the central aryl ring of the triphenylaryl platform (Fig. 1a, green dots) by nitrogen atoms leads to a change in the electronic and steric properties of the central ring, that is, N=0 (phenyl), N=1 (pyridyl), N=2 (pyrimidyl) and N=3 (triazine). As a consequence of the substitution of the C–H moiety with nitrogen atoms, a change in dihedral angle between the central aryl ring and the peripheral phenyl rings is expected, which in turn leads to varied degrees of planarity in the platform. This was corroborated by density functional theory (DFT) calculations at the PBE0–D3/Def2–SVP level as evident by the decreasing dihedral angles in the energy-minimized structures of precursor aldehydes Nx–Alds (Table 1). In addition, this results in a progressive decrease in electron density in the central aryl ring of the COF platform (Fig. 1a) as the number of nitrogen atoms increase from 0 to 3.
Encouraged by the theoretical calculations, we decided to synthesize Nx–COFs (x=0, 1, 2 and 3) by an azine formation reaction9,25 of the aldehydes with hydrazine and investigate the translation of chemical and structural variation in the precursors to the overall order and optoelectronic properties of the resulting COFs, with the consequent influence on photocatalytic hydrogen production.
The precursor aldehydes (Nx–Alds) were synthesized as described in the Supplementary Methods. Nx–Alds as well as their precursors were characterized using 1H and 13C spectroscopy (Supplementary Figs 2–15). Nx–COFs were synthesized in quantitative yields by a condensation reaction between the corresponding trialdehydes with hydrazine in the presence of 6 M acetic acid using 1:1 mesitylene/dioxane as solvent at 120 °C for 72 h (Fig. 1b; Supplementary Fig. 1). Fourier transform infrared (FTIR) spectra of the Nx–COFs were compared with the corresponding aldehydes and hydrazine (Supplementary Figs 16–19), and show the disappearance of aldehydic C−H and C=O stretches and the appearance of the azine C=N stretch at 1,622 cm−1. These observations were further corroborated by the complementary Raman spectra (Supplementary Fig. 20), which showed characteristic Raman signals at 1,000–1,010 cm−1 for the ν(N–N) stretch, 1,540–1,560 cm−1 for the νsym(C=N) stretch and at 1,600–1,625 cm−1 for νasym (C=N) stretch26. The composition and local structures of the Nx–COFs were further confirmed by X-ray photoelectron spectroscopy (XPS; Supplementary Fig. 80) and 13C cross-polarization magic angle spinning (CP–MAS) solid-state NMR (ssNMR) spectroscopy. As seen in Fig. 2 and Supplementary Figs 50, 53, 56 and 59, the characteristic aldehyde carbonyl 13C resonance located at ≈190 p.p.m. in the precursor aldehydes disappears with the concomitant appearance of the azine C=N peak at ≈160 p.p.m., thereby attesting the conversion of the precursors into the respective COFs. The molecular structure of the building blocks remains intact during COF formation as evident by the largely unchanged chemical shifts of the peripheral phenyl rings as well as the central aryl ring that shows characteristic peaks in the NMR. Thus, for example, the C3 symmetric carbon in the central triazine ring in the N3–Ald at 171 p.p.m. shows minimal shift and appears at ∼168 p.p.m. in N3–COF. Similarly, the characteristic 13C NMR peak of the central pyridine/pyrimidine ring in N1–Ald and N2–Ald at 119 and 112 p.p.m., respectively, can easily be spotted in the corresponding COFs due to their upfield nature (Fig. 2). The assignment of these peaks was done using 2D heteronuclear multiple quantum coherence (HMQC) NMR of N1 and N2 aldehydes (Supplementary Figs 8 and 11).
Structural and morphological characterization
To gain insight into the structural details and morphology of the COFs, powder X-ray diffraction (PXRD), gas sorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were performed. PXRD of Nx–COFs indicated the formation of crystalline networks with 2D honeycomb-type lattices as evident by the presence of an intense 100 reflection at 2θ=3.52° and reflections at 6.0, 7.1 and 9.5° corresponding to the 110, 200 and 120 reflections, respectively (Fig. 3). The observed reflections match well with the calculated patterns (Supplementary Figs 42–45) obtained from structural simulations performed for an AA eclipsed layer stacking (Supplementary Figs 34–41), using the Materials Studio v6.0.0 program (Accelrys). It should be noted that a slight lateral offset of the layers is expected for the stacked structures27, which however cannot be distinguished from the eclipsed topology by PXRD due to substantial peak broadening. Thus, for practical reasons, the eclipsed structure is used as a simplified working model28. Interestingly, as we traverse the series from N0 to N3–COF, the PXRD peaks become sharper with the appearance of prominent stacking peaks for N2 and N3–COF at 2θ=26° arising from the d-spacing between the 001 lattice planes. Thus, the variation in planarity in the precursor aldehydes as a function of the dihedral angles translates well into the crystallinity of the resulting COFs. The unit-cell parameters were obtained by Pawley refinements for all COFs and the results are included in Supplementary Table 1.
The permanent porosities evaluated by measuring the argon adsorption isotherm at 87 K (Supplementary Fig. 46) reveal that the Brunauer–Emmett–Teller (BET) surface area of symmetrical and unsymmetrical Nx–COFs show a deviation from the trend expected from the increasing level of planarity from N0 to N3–Ald. Thus, while PXRD shows a similar yet lower degree of crystallinity for N0 and N1–COF in comparison to that observed for N2 and N3–COF, a further demarcation is seen in the BET area of the symmetrical and unsymmetrical COFs. Interestingly, the BET surface area of the symmetrical N0–COF was found to be 702 m2 g−1, which is significantly higher than that of the unsymmetrical N1–COF with a BET area of 326 m2 g−1. Likewise, in case of comparatively planar N2 and N3–COF, the symmetrical N3–COF showed a higher BET area of 1,537 m2 g−1 as compared with the unsymmetrical N2–COF with a BET area of 1,046 m2 g−1. Thus, the lowering of symmetry leads to a lower degree of order and hence lower BET surface area. Accordingly, a narrow pore-size distribution for the most crystalline N3–COF was calculated by the non-local DFT method with a peak maximum at ≈24 Å, which is in excellent agreement with the pore size established from the structural analysis and simulations. The maximum at 24 Å is accompanied by a minor peak in the pore size distribution (PSD) at ≈16 Å, which becomes more prominent in the other Nx–COFs (x=0–2; Supplementary Fig. 47), likely resulting from an increasing degree of lateral layer offsets and stacking disorder in the less crystalline materials.
SEM images show a change in morphology from purely ball-like agglomerates in N0–COF to elongated ones in N1–COF followed by transformation to rod-like morphology in N2 and N3–COF (Fig. 4a–d; Supplementary Fig. 21). The structure and morphology of Nx–COFs was further investigated by TEM analysis where hexagonal pores are clearly visible in N2 and N3–COF (Fig. 4e,f; Supplementary Figs 24 and 25). Selected area electron diffraction along the  zone axis of the multilayers of N3–COF is consistent with a nanocrystalline, hexagonally ordered in-plane structure composed of crystalline domains with sizes ∼50–100 nm. TEM analysis of N0 and N1–COF, however, did not show any crystalline domains, possibly due to instability in the electron beam (Supplementary Figs 22 and 23).
The thermal stability of the COFs was investigated by thermogravimetric analysis (TGA), suggesting that the COFs are stable up to ≈350 °C in argon. TGA analysis in air shows total decomposition of all the COFs at temperatures >550 °C, thus indicating the absence of any metal residues (Supplementary Figs 26–33).
Optical properties and photocatalysis
The diffuse reflectance spectrum reveals that all the COFs absorb light in the ultraviolet and blue parts of the visible region and show similar absorption profiles with an absorption edge at ∼465–475 nm (Fig. 5a), thereby suggesting an optical band gap of ≈2.6–2.7 eV as determined by the Kubelka-Munk function. These values are red shifted by 40–60 nm in comparison to the solid-state absorption spectra of the precursor aldehydes and can be attributed to the introduction of the azine group and a higher degree of conjugation resulting from delocalization along as well as across the plane in the extended frameworks29. As seen in Fig. 5a, unsymmetrical N1 and N2–Ald show a slightly red-shifted absorption in comparison to the symmetrical N0 and N3–Ald. DFT calculations performed on the precursor aldehydes also indicate a similar trend in the optical band gaps of symmetrical and unsymmetrical aldehydes (Supplementary Table 4). To understand the differences in absorption spectra of symmetrical and unsymmetrical aldehydes in the solid state, the absorption spectra of the precursors were additionally recorded in dilute (8 μM) dichloromethane solutions. The absorption profiles (Fig. 5b) are marked by a clear difference between the symmetrical N3–Ald with one absorption band and the unsymmetrical N1 and N2–Ald with twin absorption bands arising from the non-planar and hence non-fully conjugated phenyl–aryl ring systems. The two bands in the absorption spectrum of N0–Ald are rationalized by absorptions from the two types of ring systems that are essentially decoupled due to strong out-of-plane torsion of the peripheral phenyl rings. The increase in planarity leading to a higher degree of conjugation (hence red shift) along the series N0 to N3 is partly compensated by the increase in electron-deficient character of the central aryl ring (hence blue shift), thereby resulting in minimal changes in the optical band gap on network formation. The similar optical gap makes the Nx–COF ensemble an ideal model platform for photocatalysis experiments, as their relative activities will not be governed by differences in their light-harvesting capability.
The Nx–COFs were next evaluated as photocatalysts for visible light-induced hydrogen evolution. The hydrogen evolution experiments were performed by taking a suspension of the COFs in PBS at pH 7 and irradiating with visible light (≥420 nm) at 25 °C. Hexachloroplatinic acid was added for the in situ formation of the platinum (Pt) co-catalyst30 to reduce the overpotential for hydrogen evolution, and triethanolamine (TEoA) was used as sacrificial electron donor31,32.
Notably, all COFs evolve hydrogen in the test period of 8 h (Fig. 5c). Interestingly though, the Nx–COFs show about fourfold increase in hydrogen evolution with each substitution of C–H by N in the central aryl ring of the COF platform. Thus, at the end of 8 h the average amount of hydrogen produced by N0, N1, N2 and N3–COF was 23, 90, 438 and 1,703 μmol h−1 g−1, respectively. The amount of hydrogen evolved from the most active N3–COF is competitive with carbon nitride photocatalysts and outperforms benchmark systems such as Pt-modified amorphous melon (720 μmol h−1 g−1)33, ‘g–C3N4’ (840 μmol h−1 g−1)34 or crystalline poly(triazine imide) (864 μmol h−1 g−1)33. Photocatalysis lasting up to 48 h using N3–COF showed that the amount of hydrogen evolved was about four times higher than the amount of hydrogen present in the COF (Supplementary Fig. 62), thus ascertaining that the primary hydrogen source is water rather than decomposition products of the COF. After photocatalysis, the COFs were isolated and checked for stability. The FTIR (Supplementary Figs 48, 51, 54 and 57) and ssNMR (Supplementary Figs 50, 53, 56 and 59) spectra of the Nx–COFs obtained after photocatalysis did not show any significant structural change in the material, thus indicating the retention of molecular connectivity of the framework during photocatalysis. The framework crystallinity was preserved to a large extent in the post-photocatalysis recovered COFs, although a partial loss of the long-range order was observed by PXRD (Supplementary Figs 49, 52, 55 and 58). Such a decrease in long-range order has been attributed to the delamination of framework layers24,35. In addition, the TEM images obtained from the post-photocatalysis sample (Supplementary Fig. 61) clearly demonstrate the retention of hexagonally ordered crystalline domains in addition to the uniform distribution of platinum nanoparticles that are formed in situ during photocatalysis as observed in the SEM images (Supplementary Fig. 60). Photocatalysis experiments performed in the absence of hexachloroplatinic acid did not show measurable amounts of hydrogen, thus underlining the role of platinum as electrocatalyst and microelectrode to mediate the electron transfer process30. Long-term studies performed with ascorbic acid as sacrificial electron donor revealed that the N3–COF is stable in light for over 120 h, showing sustained hydrogen evolution (Supplementary Fig. 63). To quantify the spectral distribution of the photocatalytic activity of the four COFs, the photonic efficiency (PE) was calculated using four different band-pass filters with central wavelengths at 400, 450, 500 and 550 nm (Fig. 5d; Supplementary Table 2; Supplementary Figs 64 and 65). These measurements clearly indicate that N3–COF shows the best PE over the entire spectral range, with the maximum of 0.44% with a 450-nm band-pass filter.
To rationalize the observed trend and to provide insights into the change in band gaps and band positions along the Nx series, Kohn-Sham band gaps were calculated at the PBE0–D3/Def2–SVP level for the precursor aldehydes (Nx–Ald; Supplementary Figs 66 and 67; Supplementary Table 4), model phenylazines (Nx–PhAz; Supplementary Figs 68 and 69; Supplementary Table 5) and two sets of hexagons with different terminations (aldehydes Nx–HxAl and hydrazones Nx–HxHz; Supplementary Figs 70–73; Supplementary Tables 6 and 7), serving as representative semi-extended model systems (Fig. 6). The hexagons (Nx–HxAl and Nx–HxHz) were stacked up to three layers for probing the role of stacking on the band gaps and positions (Supplementary Figs 75 and 76; Supplementary Tables 8 and 9).
Kohn-Sham band gaps calculated for the oligomers (Fig. 7a,b) do not show significant variation on going from the N0− to the N3−model systems, which is in line with the experimentally observed optical spectra of the COFs that show minor differences in their absorption edge (Fig. 5a). Also, as observed by Zwijnenburg and co-workers29, the stacked hexagons of N3–COF both with hydrazone (N3–HxHz) and aldehyde terminations (N3–HxAl) show a decrease in the band gap with increasing numbers of layers (Fig. 7c).
The observed steady decrease in lowest unoccupied molecular orbital (LUMO) energy in these calculations for the Nx–COFs with increasing nitrogen content suggests a decreased thermodynamic driving force for electrons to move to the platinum co-catalyst. This trend, however, does not account for the differences in the observed hydrogen evolution activities. Instead, the lowering of the highest occupied molecular orbital (HOMO) along the Nx series may lead to an increase in the oxidizing power of the hole36. This increase in thermodynamic driving force may therefore facilitate hole removal from the COF by the sacrificial electron donor most efficiently in N3–COF37.
In addition, molecular orbitals for the model systems were extracted and compared for the extended COF systems. Although HOMO–LUMO distributions are often invoked in the literature to rationalize exciton delocalization, charge separation and the location of potential charge-transfer sites38,39,40, it has to be stressed that orbitals are not observables and hence, such model considerations need to be taken with care. Also, since DFTB+ orbitals did not always agree with orbitals from DFT calculations in the case of our 7-hexagon model systems, orbital data from DFTB+ calculations may be of limited use here. Nevertheless, unit cells of the Nx–COFs were optimized on the DFTB+/mio-1-0 level of theory using periodic single-point calculations, and molecular orbitals were subsequently calculated on the same level of theory. Across the different Nx–COFs, the HOMO is localized solely on the azine linker unit (Supplementary Fig. 77), while the LUMO is delocalized across the conjugated -system of the framework (Supplementary Fig. 78). When associating the HOMO with the charge-transfer sites for holes, we may infer that efficient hole quenching is possible through hydrogen-bonding interactions with the sacrificial donor TEoA via the azine moiety for all COFs.
For smaller clusters calculated on a higher level of theory (PBE0–D3/Def2–SVP), we find that the HOMOs of hydrazone-terminated model hexagons are exclusively located at these terminations, while the LUMO is distributed along the hexagon with the maximum delocalization found for the planar N3–HxHz model (Supplementary Figs 72 and 73). To check the reliability of these model systems, even larger molecular clusters with 1,248 atoms were modelled for the N3 system to reduce the influence of terminating groups on the electronic structure of the innermost aryl core. However, the terminal hydrazone moieties still strongly localize the HOMO, whereas the LUMO in the 7-hexagon system is still centred on the innermost hexagon, despite the vicinal hexagons (Supplementary Fig. 74). This suggests an efficient HOMO–LUMO separation for oligomers with electron-rich terminal groups, which could assist the charge separation process on excitation. Note, however, that the HOMO and LUMO do not necessarily correspond to the orbitals participating in the brightest transitions and further calculations are necessary to pinpoint the orbital contributions to the optical absorptions involved in the photocatalytic process.
Since orbital considerations as non-observables are always difficult as discussed above, an alternative approach was pursued in considering the possible reaction intermediates. The stability of reactive intermediates formed during the photocatalytic process appears as a more reliable descriptor to rationalize the observed trend in the photocatalytic activity across the series of COFs. Hence, the energies and relative stabilities of radical anions of hydrazone-terminated Nx–hexagons were calculated as outlined in Fig. 8.
Electron affinities of the Nx-systems in the order of −2 eV were computed (Supplementary Table 10), whereas the ionization potential for N3–HxHz is estimated in the regime of roughly +10 eV in vacuum, thus rendering the formation of a radical cation during the photocatalytic process less likely. Considering the formation of a radical anion as the rate-determining step, which is most favoured for the N3 system and least facile for the N0 system, the observed energetics of the radical anion upholds the observed trend. On one hand, this finding emphasizes the role of the sacrificial electron donor that needs to swiftly remove the hole from the COF leading to the formation of a radical anion. On the other hand, it underlines the importance of the electron-poor character of the triazine building block, which is efficient at stabilizing the negative charge generated on the COF and at transferring it to the nearest platinum site. This trend is fully in line with the observed hydrogen evolution activity and we therefore reason that the formation of the radical anion is crucial for the photocatalytic process, as exemplified by Nx–COFs.
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