Demethylracemosol, a racemosol derivative, was isolated from Bauhinia malabarica and structurally characterized by 1D and 2D NMR spectroscopy. This compound exhibited distinct pH-dependent color changes accompanied by bathochromic shifts in the visible absorption spectra, which were attributed to acid–base transformation and enhanced π-electron delocalization. The color response showed good reversibility under repeated pH cycling. Demethylracemosol also demonstrated good antioxidant activity in DPPH and ABTS assays. Demethylracemosol was immobilized onto cellulose paper to produce colorimetric indicator strips with good thermal stability and controlled release behavior. FTIR analysis confirmed interactions between demethylracemosol and cellulose fibers. The prepared indicator paper showed clear and reversible color transitions, indicating that demethylracemosol represents a new class of nonanthocyanin natural pH indicators with potential applications in paper-based sensors.
Wisetsai et al. (Mon,) studied this question.