From nonconjugation to conjugation: novel meso-OH substituted dipyrromethanes as fluorescence turn-on Zn2+ probes

Most reported Zn2+ probes suffer from the interference of background fluorescence originated from the conjugated structures of commonly utilized fluorophores. In this work, three novel meso-hydroxyl group substituted dipyrromethanes DPMOH1-DPMOH3 were synthesized and found to be colourless and nonfluorescent due to the interruption of the conjugated p system by an sp(3) carbon between the two pyrrolic units. Interestingly, only the addition of Zn2+ to the solutions of DPMOH1-DPMOH3 promoted their oxidation to dipyrrin forms, and bright fluorescence "turn on" was observed due to the formation of corresponding dipyrrin complexes with the dipyrrin : zinc stoichiometry of 2 :1. Zn2+ detection mechanism was investigated by UV-Vis, fluorescence, H-1 NMR and HRMS analyses, which can be ascribed to the CHEF type fluorescence enhancement, resulting from good rigidity of the dipyrrin complexes. Hence, DPMOH1-DPMOH3 can be used as fluorescence turn-on Zn2+ probes with the advantage of no background fluorescence.