Novel organic dibranched photosensitizers featuring spiro-fluorene for efficient hydrogen generation from water

Four new metal-free organic photosensitizers featuring either a mono-anchoring donor–π–acceptor (D–π–A) or di-anchoring donor–(π–acceptor)₂ (D(–π–A)₂) architecture containing a spirobifluorene or spiro[fluorene-9,9′-xanthene] central donor core was prepared. These photosensitizers were subsequently tested for their efficiency in the photocatalytic production of H₂, in combination with a Pt/TiO₂ catalyst. Additionally, electrochemical, photophysical, and computational studies were conducted. The computational studies were specifically carried out to ascertain the electronic distribution within the photosensitizers. The influence of the molecular design on the efficiency of H₂ production was examined. The di-anchoring photosensitizers notably amplified the spectral response in the visible light region, reduce aggregation on TiO₂ and enhanced charge transfer. Among the fabricated photocatalytic systems, the di-anchoring photosensitizers exhibited superior photocatalytic efficiency compared to their mono-anchoring counterparts. Among all the photosensitizers, the spiro[fluorene-9,9′-xanthene]-based di-anchoring photosensitizer X2 possessing photocatalytic system demonstrated remarkable performance, producing 406.3 μmol (10.07 mL) of hydrogen over a period of 47 h under blue light irradiation. This performance corresponds to a turnover number (TON) of 6674, a turnover frequency (TOF_i) of 445 h⁻¹, an initial hydrogen production activity (activity_i) of 278140 and an apparent quantum yield (AQY_i%) of 4.762. The bulky spiro[fluorene-9,9′-xanthene] component endowed the photosensitizers with photostability, as confirmed by their high hydrogen production efficiency over 152 h of extended irradiation, yielding 12.69 mL of hydrogen. This study revealed, for the first time, that the bulky spiro-fluorene structure contributes positive synergistic effects to hydrogen production. It demonstrates that dibranched photosensitizers containing spiro-fluorene are promising candidates for achieving high and stable hydrogen production activity.