Effects of the symmetry and carboxyl anchoring group of zinc phthalocyanine derivatives on g-C3N4 for photosensitized H2 production

Zinc phthalocyanine (ZnPc) derivatives containing bulky 2,6-diphenylphenoxy peripheral substituents with different structures and symmetries are used as sensitizers of graphitic carbon nitride (g-C3N4) for photocatalytic H-2 production. It is found that an A(2)BC type asymmetrical ZnPc derivative (Zn-di-PcNcTh- 1) containing four 2,6-diphenylphenoxy substituents, a thiophene unit and a fused benzene ring bearing one carboxylic group shows an obvious red-shift in the Q-band absorption with a broader absorption spectrum as compared to its symmetrical analogue (Zn-tetrad-Pc-1) containing eight 2,6-diphenylphenoxy substituents. The asymmetrical Zn-di-PcNcTh-1 with an additional carboxyl group exhibits a higher dye-loaded amount and stable grafting on g-C3N4 than the symmetrical Zn-tetrad-Pc-1, and thus causing more efficient interfacial electron transfer in Zn-di-PcNcTh-1/g-C3N4. Especially, an impressively high apparent quantum yield (3.05%) can be obtained under 730 nm monochromatic light irradiation, higher than that (1.14%) of Zn-tetrad-Pc-1 without a carboxyl group. The present results not only provide a significant advance in the molecular engineering aspect of ZnPc derivatives for effectively utilizing the red/near-IR light of sunlight, but also exhibits a promising strategy for improving the solar-to-hydrogen conversion efficiency.