Excited state proton transfer in 2-(oxazol-2-yl)-3-hydroxychromone

Abstract

The energy profiles and the dynamics of proton transfer (PT) in the excited states of the low-energy conformers of 2-(oxazol-2-yl)-3-hydroxychromone (OHC) are investigated. OHC has three low-energy conformers arising from the OH torsion and rotation about the CC bond bridging the chromone and oxazole rings. Two of them, OHC-A and B, have OH⋯O H-bonding within the chromone ring, while OHC-C has OH⋯N H-bonding between the chromone and oxazole rings. At the B3LYP/cc-pVDZ level, OHC-B and C are found to be similar in energy and OHC-A is slightly higher. Using TDA-B3LYP, the energy profiles computed on the lowest bright state indicate lower PT barriers for OHC-A and B compared to OHC-C. Fewest-switches surface hopping simulations initialized on both S₁ and S₂ for each conformer reveal different propensities for intra- and inter-ring PT. Through these simulations, we have attempted to understand the role of the excited states in the dynamics, propensities for PT, and fluorescence behaviour of the normal and PT forms of the conformers.