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 (TOFi) of 445 h−1, an initial hydrogen production activity (activityi) of 278140 and an apparent quantum yield (AQYi%) 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. [Display omitted] •Four organic photosensitizers featuring a spirobifluorene or spiro[fluorene-9,9′-xanthene] central donor core was prepared.•They were applied as photosensitizers for photocatalytic hydrogen generation from water.•A hydrogen turnover number (TON) up to 6674 was demonstrated.•The bulky spiro-fluorene structure contributes positive synergistic effects to hydrogen production.