Phase transition thermochromism based on C–H acidity of 4-alkylflavylium compounds in Pluronic F-127

Abstract

In moderately acidic water/ethanol solutions, 7-(naphthalen-2-ylmethyl)-5,6-dihydrobenzo[c]xanthen-12-ium perchlorate shows C–H proton acidity giving rise to an ethylenic base species, instead of the hydration products of the typical flavylium network of chemical reactions, hemiketal, cis-chalcone and trans-chalcone. The kinetics of this acid–base process is much slower, k_obs(s⁻¹) = 3.5 × 10⁻³ + 0.16[H⁺], than those commonly observed for O–H deprotonation. In the presence of the triblock copolymer Pluronic F-127, the ethylenic base is dramatically stabilized shifting the pK_a from 1.35 (in ethanol : water (1 : 1) to −0.35. Taking profit from the well-described temperature dependence of this polymer, a phase transition thermochromic system was achieved. The system was cycled between 5 and 35 °C with negligible changes in absorbance after 20 cycles.