A benzofuran[b]-fused BODIPY as an efficient sensitizer for photocatalytic hydrogen production

Abstract

A benzofuran[b]-fused BODIPY-based photosensitizer type 1 was synthesized for the first time by using an unsymmetrical π-conjugated system composed of benzofuro[3,2-b]pyrrole and pyrromethene units with anchoring groups that are bound to TiO₂. Dye 1 has an intense absorption band in the visible to red region with a λ_max value of 605 nm and a molar extinction coefficient (ε) of 1.07 × 10⁵ M⁻¹ cm⁻¹. Benzofuran fusion at the b-band of the pyrrole unit causes stabilization of both the highest occupied molecular orbital (HOMO) energy levels and lowest unoccupied molecular orbital (LUMO) energy levels, endowing it with long-term stability. After deposition on the P25/Pt photocatalyst to produce P25/Pt/1, the photocatalytic activity was investigated in the presence of ascorbic acid as a sacrificial agent in aqueous phosphate buffer solution (pH 3.6) under irradiation with λ > 400 nm using a 300 W Xe lamp at 100 mW cm⁻² light intensity. After light irradiation for 10 h, the system achieved a remarkable catalytic activity for H₂ production with a turnover number (TON) value of 11 700. The dye grafted onto TiO₂ remains almost unchanged even after photoirradiation for 30 h. The electron dynamics between the dye and TiO₂ were investigated by time-resolved fluorescence spectroscopy, photocurrent measurements and electrochemical impedance spectroscopy. The apparent quantum yields (AQYs) were evaluated by monitoring H₂ production for each monochromatic light irradiation. The values of AQY for P25/Pt/1 were 3.80% and 3.72% at λ = 420 nm and 600 nm, respectively, indicating an excellent light harvesting capability as a sensitizer.