α‑Monoacylated and α,α′- and α,β′-Diacylated Dipyrrins as Highly Sensitive Fluorescence “Turn-on” Zn²⁺ Probes

With the purpose of developing readily synthesized CHEF (chelation-enhanced fluorescence) type Zn²⁺ probes with relatively simple molecular structures and excellent sensing behavior, p-anisoyl chloride was used for the acylation of 5-(penta­fluoro­phenyl)­dipyrro­methane. Interestingly, the α,β′-diacylated product PS2 with a unique substitution mode was obtained in high yield in addition to the normal α-substituted mono- and diacylated products PS1 and PS3. Further oxidation of PS1–PS3 afforded dipyrrins S1–S3. Crystal structure and ¹H NMR measurements of S2 demonstrate the existence of a pure tautomer, which is consistent with DFT calculations. S1–S3 show highly Zn²⁺ selective “turn-on” fluorescence based on a CHEF mechanism by the formation of 2:1 (probe:metal) Zn²⁺ complexes. The emission colors can be easily tuned from green to red by changing the dipyrrin substitution modes. Furthermore, these probes demonstrate fast responses and wide applicable pH ranges. Among them, S2 shows the highest Zn²⁺ sensitivity, with a detection limit of 4.4 × 10^–8 M.