Effect of 4′-Substituents (H, CN, NO2) on the Acid–Base properties and kinetic signatures of flavylium compounds

Flavylium-based dyes are multistate chemical systems that exhibit complex pH-dependent equilibria, kinetics and color outputs. In the present work, we studied the influence of electron-withdrawing substituents (CN and NO 2 ) at the 4′-position on the acid–base equilibria and kinetic behavior of three 7-diethylamino flavylium derivatives. p K a values were obtained from equilibrium spectra recorded by direct pH-jump experiments and were found to systematically decrease upon CN and NO 2 substitution. Fast kinetic events were monitored by stopped-flow UV–Vis spectroscopy through both direct and reverse pH jumps, allowing the characteristic kinetic fingerprints of each compound to be determined. The results demonstrate that electron-withdrawing substituents accelerate B4 formation, decelerate B2 protonation, and leave the Cc → Ct isomerization as the substituent-independent rate-determining step. Overall, these findings show that substituent effects control the pH response and kinetic pathways of flavylium dyes, demonstrating their suitability for pH-driven optical switching applications. • New diethylamino flavylium pigments were designed without hydroxyl groups. • Electron-withdrawing groups (4’-CN, NO 2 ) strongly affected chromatic behavior. • A wide pH range was identified where the trans -chalcone is the only species present. • CN, NO 2 induced redshifts (∼27 nm), especially in trans -chalcone species.