A spiropyran-acylhydrazone dyad: Uncovering another way of cleaving the Cspiro-O bond of the classical photochromic molecule

Spiropyran, as famous molecular sensor, can transform from its closed forms into the corresponding opened merocyanine through Cspiro-O bond cleavage when an analyte attacks the spirocarbon or oxygen atom in the spiropyran. Herein, a spiropyran-acylhydrazone dyad 4 is synthesized followed by spiropyran-like 5 and non-spiropyran diacylhydrazone 6 obtained simultaneously, and its reactivity to nucleophiles is investigated. Thus a third attacking position, located at the carbon atom opposite to the oxygen connected to Cspiro, is found, which involves Cspiro-O bond cleavage and recombination. Consequently, a nucleophilic addition mechanism in spiropyran is proposed. 4 and 5 both can discriminate Mg2+ from Ca2+ with different response time and sensitivity, while non-spiropyran 6 does not respond to the two metal ions. This work not only discovers an outstanding sensor for discriminating Mg2+ from Ca2+, but also provides new insight for transforming and modifying spiropyrans.