A smart photosensitive fluorescent probe for sensing Co²⁺ in extremely alkaline aqueous solution

A novel pH-, viscosity-, and photo-sensitive polymorphic fluorescence probe NHP for sensing Co²⁺ has been developed. The hydrazone-based NHP can be synthesized by only one step of reflux reaction and purified by washing with poor solvents. Three single crystals of NHP-1, NHP-2, and NHP-3 with different conformations were resolved. As a photo acid generator (PAG), the hydrogen atom on the imino group of the probe NHP can be shed with 365 nm ultraviolet (UV) light illumination or in alkaline conditions. Due to the above two conditions, the negatively charged ligand obtained after dehydrogenation of NHP can accelerate its chelation with Co²⁺. When irradiated with 365 nm UV light, the product (NHP₂-Co²⁺ (I)) of NHP chelating with Co²⁺ appears yellowish in aqueous solution. In a strong alkali aqueous solution, the chelate product (NHP₂-Co²⁺ (II)) of NHP and Co²⁺ showed bright blue-green fluorescence. The formed divalent Co(II) complex NHP₂-Co²⁺ can be oxidized to trivalent Co(III) complex NHP₃-Co³⁺, as confirmed by the resolution of single crystals of NHP₃-Co³⁺.