Efficient visible-light-driven hydrogen production by Zn–porphyrin based photocatalyst with engineered active donor–acceptor sites

Abstract

Here, we report a fine-tuned donor-π-acceptor concept based zinc-porphyrin-sensitized TiO₂ photocatalyst for the application in photochemical hydrogen evolution. The newly designed system showed unprecedented photocatalytic activity. The molecular structure comprises of 3-ethynyl phenothiazine as a donor moiety, porphyrin as a π-spacer, and either 3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)phenyl)-2-cyanoacrylic acid (LG-22) or 3-(4-(benzo[c][1,2,5]thiadiazol-4-yl)thiophene-2-yl)-2- cyanoacrylic acid (LG-23) as an acceptor attached at meso-position of the zinc-porphyrin. Superior hydrogen generation efficiency has been keenly observed over PCT-LG-23 in correspondence with PCT-LG-22 and that of Pt–TiO₂ through visible light-induced photocatalytic water splitting. The effect of pH over photocatalytic hydrogen generation efficiency has been studied in the presence of triethanolamine (TEOA) which acts as a sacrificial electron donor (SED). LG-22 and LG-23 show H₂ production yields of 8850.9 μmol h⁻¹ and 9793.5 μmol h⁻¹ at pH = 12 and pH = 3, respectively with a high turnover number (TON was 11801.2, 13058 for LG-22 and LG-23, respectively) under light irradiation with apparent quantum yields (AQY) of 7.20% and 7.96% for LG-22 and LG-23, respectively. The presence of thiophene and Benzo[1,2-b:4,5-b′]dithiophene (BDT) facilitated the extended π-conjugation, which gave rise to the excited state electronic properties and basicity of thiophene that enhanced the light absorption capacity of LG-23 in acidic conditions. Thus, this work provides a prime example of more efficient photonic conversion of an already existing, moderately active Zn porphyrin synthetic complex core encompassed precisely positioned functionalities of thiazine.