To obtain a detailed and quantitative insight into the composition of the CTFs, C ComPmultiCP MAS NMR spectra were recorded. The signal around 170 ppm can be attributed to the triazine core carbon atom, confirming the successful condensation and oxidation of amidine and aldehydes to the triazine core in all materials (Figure ). The absence of any signal in the C NMR spectra at about 190 ppm and 163 ppm highlights the complete transformation of aldehyde and amidine moieties in all materials (see Figure ). In the spectrum of the biphenyl based material CTF-02, signals at 143.0, 135.1 and 128.0 ppm in an integrated ratio of 1 : 1 : 4 of the biphenyl moiety occur, while in the spectrum of CTF-02-Bpy, five different resonances at 156.9, 149.6, 136.9, 130.5 and 121.6 ppm appear with equal intensities (Figure ). In the spectra of the mixed CTFs, containing both biphenyl and bipyridine based monomers, all those signals are present, their integrated ratio in line with a 1to-2 and 2-to-1 composition (see Table and SI section 4.3 for further discussion). This perfect stoichiometric control is further evidenced by elemental analysis. The nitrogen content of these materials increases linearly from 12.8% for CTF-02 to 22.1% for CTF-02-Bpy, while the molar ratio of carbon to nitrogen is in line with the theoretical composition (Table ). The signal of the triazine carbon atom shifts from 170.6 ppm in CTF-02 to 169.1 ppm in CTF-02-Bpy 0.66 and CTF-Bpy. This shielding effect reflects the increase in electron density on the triazine moiety with increasing number of bipyridine ligands, reflecting the enhanced donor capability of the n-doped building block. A further increase from 66% bipyridine to 100% bipyridine did not result in a further shielding effect on the triazine moiety acting as electron acceptor. All materials show a permanent porosity towards nitrogen at 77 K, together with the characteristic swelling behavior for CTFs prepared by this methodology (Figure ). The hysteresis between absorption and desorption branch of the isotherms increases with increasing bipyridine content, indicative for a more pronounced swelling of the flexible network structure in the CTF-02-Bpy series. The purely biphenyl-based CTF-02 has the highest apparent surface area within the series, achieving 570 m 2 /g. As for other CTFs prepared by the condensation approach, the apparent surface area of CTF-02 is significantly lower than the apparent surface areas obtained under ionothermal conditions (1880 -2480 m 2 /g), but comparable to the apparent surface area of materials prepared by trifluoromethanesulfonic acid (TfOH) mediated polycondensation (560 m 2 /g). The observed lower surface area might be explained by avoiding the partial carbonization that occurs during ionothermal synthesis in molten ZnCl 2 , which may increase the surface area. The ZnCl 2 may also act as a porogene. With increasing bipyridine content, and thus with increasing surface polarity (vide infra), the apparent surface area decreases to approx. 400, 370 and 240 m 2 /g, for CTF-02-Bpy 0.33 , CTF-02-Bpy 0.66 and CTF-02-Bpy, respectively (Table ). A possible explanation for the decrease in apparent surface area might include a non-favorable interaction of N 2 with the more polar surface of the bipyridine containing materials or a denser state of the flexible bipyridinecontaining CTFs after activation and cooling to 77 K. Especially in heterogeneous catalysis, a high specific surface area in an open material is a key feature to assure accessibility of the active sites. As we aim for aqueous liquid phase catalysis, not only the apparent surface area, but also the interaction and porosity towards a given solvent has to be considered. Water vapor physisorption isotherms revealed a high water vapor uptake at high partial pressure for all materials (Figure ). The solid-solvent interaction and thus the wettability of the surface, reflected by the material's Henry constant (K H ), depends on the amount of bipyridine moieties. With increasing amount of bipyridine, K H increases linearly from 1.5•10 -6 mol/g/Pa for CTF-02, providing the weakest solid-solvent interaction, to 5.2•10 -6 mol/g/Pa for CTF-02-Bpy (Table , Figure ). In the same way, the degree of pore filling (Ξ), increases from 0.7 to 3.1 (Figure ). Thus, in all bipyridine-CTFs, the pores are completely filled with water, a beneficial characteristic for the intended application in aqueous HER. In case of CTF-02-Bpy 0.66 and CTF-02-Bpy, the H 2 O-accessible pore volume even strongly increased compared to N 2 accessible pore volume, highlighting the pronounced swelling of the material and therefore good accessibility of active sites even in the center of the material. An important number to evaluate the hydrophilicity is the relative partial pressure α at which the capacity for water uptake is reached to 50 %; the lower α is, the more hydrophilic is the material. In this series, the highest hydrophilicity is observed for CTF-02-Bpy 0.66 (α = 0.63), the highest hydrophobicity for CTF-02 (α = 0.85, Table , Figure ). ).