Natural\nSunlight-Driven\nPhotocatalytic Overall Water\nSplitting with 5.5% Quantum Yield Promoted by Porphyrin-Sensitized\nZn Poly(heptazine imide)

<i>Meso-</i>tetrakis(4-carboxyphenyl)porphyrin\n(H<sub>4</sub>TCPP) has been loaded on a partially exchanged Zn<sup>2+</sup> poly(heptazine imide) (PHI), changing the light harvesting\nproperties\nof the system, without altering the PHI structure. At the optimal\nloading (20 wt %), the photosensitized (Zn/K)-PHI is able to produce\n1.06 mmol<sub>H2</sub>/g and 0.46 mmol<sub>O2</sub>/g after 12 h of\nreaction irradiation of Milli-Q water under visible light by a 100\nmW/cm<sup>2</sup> white LED. The apparent quantum yield for the overall\nwater splitting reaction was 5.5% at 400 nm and 2% at 700 nm. Outdoor\nwater splitting irradiation with natural sunlight shows the feasibility\nof the process. The photocatalytic performance of TCPP20%@(Zn/K)-PHI\nis considerably higher than that of analyzed reference samples such\nas graphitic carbon nitride, poly(triazine imide), and potassium PHI\nwith H<sub>4</sub>TCPP photosensitization. These relative photocatalytic\nactivities point out the relevance of the PHI structure and the presence\nof Zn<sup>2+</sup>. It is proposed that Zn<sup>2+</sup> simultaneously\nbinds PHI and H<sub>4</sub>TCPP. Transient absorption spectroscopy\nsupports the occurrence of photoinduced electron transfer in which\nelectrons are located at the H<sub>4</sub>TCPP and holes at the PHI\nmoiety. Transient photocurrent measurements show a higher charge separation\nefficiency on TCPP20%@-(Zn/K)-PHI compared to (Zn/K)-PHI, and measurement\nof the frontier orbitals indicates an adequate energy alignment of\nthe HOMO/LUMO levels of TCPP<sup>4–</sup> with respect to (Zn/K)-PHI.\nThe results show the possibility of developing efficient noble metal-free\nphotocatalytic systems based on PHI dye sensitization.