Understanding Charge Transfer Mechanism on Effective Truxene-Based Porous Polymers–TiO₂ Hybrid Photocatalysts for Hydrogen Evolution

Truxene-based porous polymers synthesized through the simple “solvent knitting” strategy from hexamethyl or tribenzyl truxene-based monomers (TxPP1 and TxPP2), and their corresponding TxPP@TiO2 hybrids are used as photocatalysts for H2 production from water using methanol as sacrificial agent, under UV–vis light. These polymers present higher hydrogen evolution rate (HER) than TiO2 and remarkable thermo- and photostabilities. Hybrids TxPP-TiO2 exhibited intensely enhanced photocatalytic activity compared to TiO2 or TxPPs alone. In the presence of platinum (1 wt %) as co-catalyst, HER from TxPP1@T-10 is significantly boosted, reaching values above 21000 μmol·g–1·h–1, which, to the best of our knowledge, represents the highest HER reported for hybrids based on TiO2 and conjugated porous polymers. Interestingly, small structural differences of the corresponding truxene monomers result in different photocatalytic behavior. We focused here on gaining insight on the charge transfer mechanism and rationalizing the different photocatalytic performances in order to establish clear structure–activity relationships. In fact, photoluminescence and transient absorption spectroscopy demonstrated that the remarkably enhanced photocatalytic activity of the most active hybrids (TxPP1@TiO2) can be attributed to the efficiently photogenerated electron–hole separation by a direct Z-scheme mechanism, while lower performance of TxPP2@TiO2 is probably due to a less efficient heterojunction type II charge transfer mechanism.