Theoretical Insights Into the Excited State Double Proton Transfer Mechanism of Deep Red Pigment Alkannin

As the most important component of deep red pigments, alkannin is investigated theoretically in detail based on time-dependent density functional theory (TDDFT) method. Exploring the dual intramolecular hydrogen bonds (O1-H2···O3 and O4-H5···O6) of alkannin, we confirm the O1-H2···O3 may play a more important role in the first excited state than the O4-H5···O6 one. Infrared (IR) vibrational analyses and subsequent charge redistribution also support this viewpoint. Via constructing the S₁-state potential energy surface (PES) and searching transition state (TS) structures, we illuminate the excited state double proton transfer (ESDPT) mechanism of alkannin is the stepwise process that can be first launched by the O1-H2···O3 hydrogen bond wire in gas state, acetonitrile (CH₃CN) and cyclohexane (CYH) solvents. We present a novel mechanism that polar aprotic solvents can contribute to the first-step proton transfer (PT) process in the S₁ state, and nonpolar solvents play important roles in lowering the potential energy barrier of the second-step PT reaction.