Dyad Sensitizer of Chlorophyll with Indoline Dye for Panchromatic Photocatalytic Hydrogen Evolution

Photocatalytic H-2 evolution under a wide visible spectral range is a challenging issue. In this work, we report the synthesis of a novel organic dyad consisting of an indoline rhodanine pi-conjugation with a strong absorption in the green region and a chlorophyll derivative with intense absorbance in the purple and red regions. This synthetic dyad (Dyad) was employed as a photosensitizer in the TiO2-based system for photocatalytic H-2 evolution with Pt as the assisting catalyst and ascorbic acid (AA) as the sacrificial reagent. The Dyad-sensitized Pt/TiO2 photocatalyst exhibited a maximum turnover number (TON) of 1044 after 6 h of continuous light irradiation (lambda > 400 nm). In comparison, under the same conditions, the TONs of photocatalytic systems based on sole counterpart chlorophyll (Chl) or indoline dye (Ind) were merely 267 or 301, respectively. Moreover, the apparent quantum yield of Dyad/Pt/TiO2 (1.27%) is much higher than those of Chl/Pt/TiO2 (0.37%) and Ind/Pt/TiO2 (0.20%) under 420 nm monochromatic light irradiation. The interfacial charge transfer and recombination processes between TiO2 and Dyad, Chl, or Ind were evaluated with photocurrent responses and electrochemical impedance spectroscopy. The DFT calculations were in accordance with the observed charge recombination processes. The high photocatalytic activity of Dyad was attributed to not only an excellent light absorption ability over the whole visible range, but also an efficient electron transfer and balanced charge recombination processes between TiO2 and Dyad. In addition, the sustained activities of the H-2 evolution systems were 78%, 84%, and 38% for Dyad, Chl, and Ind, respectively, after three 6 h illumination periods, indicating a similarly high reusability of both Dyad and Chl dyes as compared to Ind dye. This study simultaneously solves the problems of insufficient visible spectral response, poor stability, and high cost in photocatalytic water-splitting H-2 evolution.