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Effect of 4′-Substituents (H, CN, NO2) on the Acid–Base properties and kinetic signatures of flavylium compounds
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Resumo(s)
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 NO2) at the 4′-position on the acid–base equilibria and kinetic behavior of three 7-diethylamino flavylium derivatives. pKa values were obtained from equilibrium spectra recorded by direct pH-jump experiments and were found to systematically decrease upon CN and NO2 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.
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Publisher Copyright: © 2025 The Author(s)
Palavras-chave
Equilibria multistate Flavylium dyes Kinetic signatures pH-dependent network Stopped-flow Thermodynamic constants General Chemical Engineering Process Chemistry and Technology