Photocurrent and Gelation Properties of Polyphenol-Modified Titanium-Oxo Compounds

Organic-inorganic hybrid metal-polyphenols as stable structural modules have gained extensive interest due to their diverse applications. However, titanium-oxo compounds (TOCs) with large molecular polyphenols have been less explored, and they were expected to be different from small polyphenols with isolated metal ions. Herein, 4-methyl-esculetin (Mesc), a catechol derivative, was selected to construct three TOCs, namely, [Ti₁₇O₂₄(Mesc)₄(OⁱPr)₁₆] (1), [Ti₁₂O₁₄(OⁱPr)₁₈][Ti₁₆O₁₄(Mesc)₁₂(OⁱPr)₁₄] (2), and [Ti₃O(Mesc)₂(OAc)₂(OⁱPr)₄] (3). These compounds were structurally characterized. Photocurrent responses were evaluated using the compound-sensitized TiO₂ electrodes. It was found that the current densities of 1-3 electrodes are in the order of 1 ≫ 3 > 2, which relates to the ligand-to-TiO core and ligand-to-ligand charge transfers (LMCT and LLCT, respectively). Density functional theory calculations showed that the lowest band gap of 1 originates from its LLCT. Compound 1 reacted with polyphenol tannin (TA) to form a fully transparent and robust gel (1-TA), and the gelation properties were investigated. Using the gel as a nano-TiO₂ fixing agent, solar cell electrodes were prepared by a low-temperature wet method. The photocurrent responsive behavior of the 1-TA/TiO₂ electrode was compared with that of the 1-sensitized traditional high-temperature-treated TiO₂ electrode. Although the current density of the former is somewhat lower than that of the traditional electrode, the low-temperature wet preparation of the 1-TA/TiO₂ electrode is more energy-efficient and sustainable.