Irreversible phototautomerization of o-phthalaldehyde through electronic relocation.

The potential energy surface for the intramolecular excited state hydrogen transfer (IESHT) in ortho-phthalaldehyde (OPA), which generates an enol ketene, has been studied with ab initio calculations (MS-CASPT2//CASSCF). The goal of our study is to establish the mechanistic factors that make the primary phototautomerization step irreversible. Similar to what we recently described for ortho-nitrobenzaldehyde (NBA) (Migani et al., Chem. Commun., 2011, 47, 6383-6385), the IESHT in OPA is characterized by the relocation of two electrons from the in-plane to the out-of-plane orbital system. Consistent with this, OPA has the same IESHT mechanism as NBA. The first step of ketene formation is the hydrogen transfer, which starts on an (n, π*) state. The reaction coordinate goes through a conical intersection with the ground state and leads to a biradical intermediate with a bent ketene moiety. The second step is the linearization of the ketene moiety, which is associated to a change in the electronic configuration from biradical to ketene. Because of the electron relocation, the reverse transfer is similar to a Woodward-Hoffmann forbidden process with a sizeable barrier. This makes the tautomerization irreversible and allows the ketene to react further to biphthalide and benzaldehyde. Together with our previous NBA study, we establish the electronic relocation mechanism as a new mechanism for IESHT. This mechanism explains the different reactivity of OPA and NBA compared to organic photoprotectors, where the IESHT is reversed on a very short time scale.