Influence of Poly(Diketopyrrolopyrrole) Chain Length and Chemical Structure on Photocatalytic Hydrogen Evolution in Composites With TiO₂

For this study, we have synthesized two different poly(diketopyrrolopyrrole) copolymers with different chain lengths. The diketopyrrolopyrrole (DPP) core is substituted with oligoethylene glycol side chains to increase its compatibility with water and copolymerized with either fluorene or carbazole moieties. The polymers form a composite photocatalyst with anatase TiO2. Detailed characterization such as NMR spectroscopy, UV?Vis, DRIFT, and UPS is used to analyze the structural and optical properties as well as the frontier orbital energy levels of the components and the composite materials. The optical properties of the polymers are tunable with respect to the copolymer used, opening up the possibility of optimizing the photocatalytic activity. These composite materials (without the addition of a co-catalyst) provide up to an eightfold enhancement of the hydrogen evolution reaction (HER) compared to pristine TiO2. The polymers also exhibit stability in the reaction medium as shown by solid-state NMR, DRIFT, and UV?Vis spectroscopy. A significant influence of the chain length of the polymers on HER is found as well. As the chain length increases, the activity toward hydrogen evolution increases. We show a correlation between hydrogen evolution and PDPP chain length whereby the active site of the photocatalytic process remains the inorganic semiconductor.