Investigation of heavy atom effects and pH sensitivity on the photocatalytic cross-dehydrogenative coupling reaction of halogenated fluorescein dyes

Fluorescein halogenated dyes are excellent photocatalysts for cross-dehydrogenation-coupled (CDC) reaction. In this paper, seven different halo-fluorescein dyes were selected to investigate the influence of heavy halogen atoms and proton concentrations on halo-fluoresceins for photocatalytic CDC reaction. The results showed that the heavy atoms directly affected intersystem crossing (ISC), which could act as a gatekeeper in the photoredox cycle and also affect photoinduced electron transfer (PET) process. Thus, the photocatalytic performance clearly increased in the following order: fluorescein FL (0.11 mol L⁻¹•h⁻¹)＜dibromo-fluorescein FL-2Br (0.19 mol L⁻¹•h⁻¹)＜diiodo-fluorescein FL-2I (0.29 mol L⁻¹•h⁻¹)＜tetrabromo-fluorescein FL-4Br (0.375 mol L⁻¹•h⁻¹)＜tetrabromo-tetrachloro-fluorescein FL-4Br–4Cl (0.53 mol L⁻¹•h⁻¹)＜tetraiodo-fluorescein FL-4I (0.58 mol L⁻¹•h⁻¹)＜ tetraiodo-tetrachloro-fluorescein FL-4I–4Cl (0.74 mol L⁻¹•h⁻¹). Furthermore, the visible light absorption of halogenated fluorescein dyes varied significantly with pH, thereby affecting their photocatalytic activity of CDC reaction. The dianionic FL-4I–4Cl could fully absorb light energy and exhibit excellent photocatalytic performance under mildly alkaline conditions (pH = 8), achieving a CDC reaction conversion rate of 99.92 %, which was 2.6 times higher than that of the lactone form of FL-4I–4Cl under acidic conditions (pH = 2).