Natural Sunlight-Driven Photocatalytic Overall Water Splitting with 5.5% Quantum Yield Promoted by Porphyrin-Sensitized Zn Poly(heptazine imide)

<i>Meso-</i>tetrakis(4-carboxyphenyl)porphyrin (H₄TCPP) has been loaded on a partially exchanged Zn²⁺ poly(heptazine imide) (PHI), changing the light harvesting properties of the system, without altering the PHI structure. At the optimal loading (20 wt %), the photosensitized (Zn/K)-PHI is able to produce 1.06 mmol_H2/g and 0.46 mmol_O2/g after 12 h of reaction irradiation of Milli-Q water under visible light by a 100 mW/cm² white LED. The apparent quantum yield for the overall water splitting reaction was 5.5% at 400 nm and 2% at 700 nm. Outdoor water splitting irradiation with natural sunlight shows the feasibility of the process. The photocatalytic performance of TCPP20%@(Zn/K)-PHI is considerably higher than that of analyzed reference samples such as graphitic carbon nitride, poly(triazine imide), and potassium PHI with H₄TCPP photosensitization. These relative photocatalytic activities point out the relevance of the PHI structure and the presence of Zn²⁺. It is proposed that Zn²⁺ simultaneously binds PHI and H₄TCPP. Transient absorption spectroscopy supports the occurrence of photoinduced electron transfer in which electrons are located at the H₄TCPP and holes at the PHI moiety. Transient photocurrent measurements show a higher charge separation efficiency on TCPP20%@-(Zn/K)-PHI compared to (Zn/K)-PHI, and measurement of the frontier orbitals indicates an adequate energy alignment of the HOMO/LUMO levels of TCPP^4- with respect to (Zn/K)-PHI. The results show the possibility of developing efficient noble metal-free photocatalytic systems based on PHI dye sensitization.