An extended conical intersection seam associated with a manifold of decay paths: excited-state intramolecular proton transfer in O-hydroxybenzaldehyde.

O-Hydroxybenzaldehyde (OHBA) is a prototypical photoprotector exhibiting excited-state intramolecular proton transfer (ESIPT). Here we report how its photostability depends on an extended conical intersection seam associated with a manifold of decay paths. Thus, the photoreactivity of OHBA derives from a flat excited-state potential energy surface with barriers of only tenths of electronvolts between the reactant and several conical intersection structures that lead to different products: two isomers of a hydrogen-bonded intersection (HBI) that lead back to the enol reactant or to the tautomerized keto form in its Z conformation; an intersection (ZEI) that mediates the Z-E isomerization of the keto tautomer; and a twisted-pyramidalized one (TPI) that leads to an oxetene adduct. The intersection structures are connected to each other, forming a continuous seam, and the competition between the products depends on where the seam is accessed after the initial excitation. The overall picture must be also valid for the methyl salicylate and salicylic acid analogues of OHBA since it reflects the characteristics reported previously for MS and SA.