Electrochemical impedance spectroscopy (EIS) was performed at the material-electrolyte interface, to determine the flat band potential using Mott-Schottky analyses (Figure ). The positive slope observed in the Mott-Schottky plots confirms that all material are n-type conductors. The flat band potential corresponds to the extrapolated potential for C -2 = 0 F -2 , independent of the applied potential perturbation frequency. In first approximation, the flat band potential can be associated with the conduction band (CB, electron affinity) position. Here we want to stress that prior to flat band potential measurements, knowledge of the isoelectric point (pH IEP ) of the material is required for rigorous determination of its flat band position. This is at odds with some practices in the literature, where a pH IEP close to pH 7 is assumed for CTF materials, which may cause an incorrect alignment of the frontier orbital positions of several tenths of eV, due to the neglected influence of charged surface states (see below). For example for CTF-1 a broad range of pH IEP has been reported (pH 5 -7.2). For the bipyridine based materials, we found a pH IEP around pH 4 ± 0.5, while for CTF-02 the pH IEP is at around pH 8, in line with literature data for CTF-02 material prepared by the ionothermal method (SI, section 4.9, Figure ). The lowest flat band potential at pH IEP in the series is achieved for CTF-02 at approx. -3.72 eV (-1.33 V vs Ag/Ag + ), while CTF-02-Bpy 0.33 to CTF-02-Bpy hold similar flat band potentials around -4.1 eV (~-0.95 V vs Ag/Ag + ). As the flat band potential is associated with the conduction band position, taking the pH dependency of the conduction band edge into account, we expect the highest driving force for HER of ~650 meV for CTF-02 and the lowest for CTF-02-Bpy 0.66 (>460 meV, Figure ). To align the valence band (VB, minus ionization energy) positions, we assumed that the indirect optical band gap is close to the true band gap. Here, we note that the VB position is similar for CTF-02 to CTF-02-Bpy 0.66 and increases sharply in energy for CTF-02-Bpy. Thus, for all materials but CTF-02-Bpy, the VB position is sufficient low in energy to ensure efficient reductive quenching of the corresponding photo-excited states of the CTF by TEOA. In case of CTF-02-Bpy, hardly any thermodynamic driving force is observed (<<100 meV, Figure ). In addition, we note that a completely different picture will arise, if EIS was performed at pH different from pH IEP , and observed differences for the VB position as large as 0.5 eV. In particular, hardly any (<<100 meV, CTF-02-Bpy 0.66 ) or even no driving force for the reductive quenching would be expected for CTF-02-Bpy when the EIS measurements were conducted at pH 7 or pH 10.5 (see also Figure ). These results highlight the importance to perform Mott Schottky analysis at the isoelectric point in order to align frontier orbitals positions precisely and to rationalize catalytic activities by calculating thermodynamic driving forces (vide infra).