Photoacidity and ESIPT in luteolin: acidity-vibrational frequency relationships across electronic states

The photochemical antioxidant properties of small organic molecules in electronically excited states have become an important topic in modern photochemistry and redox biology. Photoexcitation can trigger ultrafast proton and hydrogen transfer processes, leading to substantial changes in acidity and redox behavior. This study systematically explores the excited-state proton/hydrogen transfer pathways, acidity, and photophysical characteristics of luteolin (LU), a compound containing multiple potential deprotonation sites, using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Validation against previously reported theoretical and experimental data demonstrates that the employed DFT and TD-DFT/B3LYP/aug-cc-pVDZ methodology provides a reliable and consistent description of the molecular structure, energetics, and photophysical behavior of LU in both the ground (S₀) and excited (S₁) states. To evaluate acidity, the pK _a and values are determined using three distinct yet complementary computational models: gas phase (isolated LU molecule), an implicit solvation model, and an explicit solvation model (LU-H₂O hydrogen-bonded complexes). The close agreement among these models confirms that they can be used in a complementary manner to reliably investigate systems with multiple deprotonation sites, offering a comprehensive picture of acid-base properties across different environments. The DFT and TD-DFT calculations reveal that the intramolecular hydrogen bond and the associated excited-state proton transfer (ESIPT) involving the benzopyran-ring O-H group (LU-C5) directly influence the vibrational properties and acidity of the most acidic catecholic O-H group (LU-C4'). Additionally, vibrational analysis reveals that near-linear correlations between O-H stretching frequencies and both pK _a and persist in all three models. To the best of our knowledge, these results provide the first evidence that such vibrational descriptors remain valid across different solvation environments and electronic states, offering a practical approach for predicting photoacidity. Overall, this study provides detailed insights into the excited-state deprotonation behavior and photophysical properties of LU while establishing transferable computational strategies for investigating excited-state proton transfer in complex photoactive systems.