Synthesis, photophysics and photosensitizing hydrogen evolution of a near-infrared heavy-atom-free sensitizer†

Although halogenated boron-dipyrromethene (BODIPY) dyes have been investigated as synthetic metal-free photosensitizers in noble-metal-free homogeneous photocatalytic systems for H₂ generation, they typically absorb the high-energy part of sunlight (in the green-light region: 530–550 nm) with a moderate absorption capacity of 5 × 10⁴–8 × 10⁴ M⁻¹ cm⁻¹, and are relatively short-lived (4–5 hours) during the photocatalytic H₂-evolving process, leaving room for improvement. Herein, by fusing thiophene groups into the BODIPY skeleton, we successfully synthesized a thienopyrrole-expanded BODIPY, B1, which was structurally characterized by single-crystal X-ray diffraction (SC-XRD) analysis and NMR spectroscopy. The photophysical properties of B1 were fully investigated with a combination of absorption and emission spectroscopy, ultrafast time-resolved spectroscopy and theoretical calculations. Compared with the thiophene-free BODIPY (B2), B1 boasts an impressive ε of 1.58 × 10⁵ M⁻¹ cm⁻¹ and a moderate fluorescence quantum efficiency in the red/near-infrared region with high photostability. The redox properties of B1 were also studied by electrochemical methods. Based on the photophysical and electrochemical results, the thienopyrrole-expanded BODIPY B1 was used as a heavy-atom-free organic sensitizer for solar-driven hydrogen production with a molecular nickel catalyst and triethylamine as a sacrificial electron donor, exhibiting higher stability under prolonged irradiation than the previous halogenated BODIPYs. The photosensitizing mechanism for H₂ production was analyzed through an electrochemical method, fluorescence titration and ultrafast time-resolved spectroscopy. The present work demonstrates that thienopyrrole-fused BODIPYs can serve as near-infrared heavy-atom-free alternatives to green-absorbing halogenated BODIPYs for solar fuel generation.