Temperature Sensing of Thiolate Addition by Phenolate Merocyanine Dyes: Importance of the Quinone Methide Resonance Structure.

Merocyanine (MC) dyes containing an aromatic donor vinyl linked to a cationic acceptor serve as chemosensors for analyte detection. Their electrophilicity permits anion detection through addition reactions that disrupt dye conjugation. Herein, we demonstrate the temperature influence on thiolate addition to MCs containing the N-methylbenzothiazolium (Btz) acceptor. The zwitterionic phenolate dye (PhOBtz) displays impressive temperature sensitivity to thiolate addition, with the brightly colored phenolate favored upon heating and the colorless thiolate adduct favored upon cooling. In contrast, MC dyes containing neutral donors (PhOMeBtz and PhNMe2Btz) display only moderate temperature sensitivity to thiolate capture and release. Extraction of thermodynamic parameters demonstrates a strong enthalpic driving force for thiolate addition to PhOBtz that is absent for PhOMeBtz and PhNMe2Btz. Variable temperature 1H NMR studies demonstrate that PhOBtz adopts the para-quinone methide (p-QM) resonance structure. Thus, thiolate addition to PhOBtz resembles 1,6-conjugate addition to p-QMs which is accompanied by a large increase in the π-stabilization energy upon adduct formation. Manipulation of PhOBtz electrophilicity by attaching chlorine substituents to the phenolate caused the thiolate adducts to dissipate over time for p-QM regeneration. Our work provides new design ideas for the utility of phenolate MC dyes, given that they are carriers of the p-QM electrophile.