Photochemical anti–syn isomerization around the –N–N[double bond, length as m-dash] bond in heterocyclic imines

EPR and NMR experiments on a quinazolinone-based Schiff's base in DMSO solution showed that irradiation with UV light (365 nm) leads to photochemically-induced isomerization from the anti- to the higher-energy syn-form around the –N–N[double bond, length as m-dash] linkage. The anti- to syn-isomerization was relatively fast, and the maximum amount of conversion detected (25%) was reached within 10 min; thermodynamic equilibrium re-established itself in about 15 min. DFT calculations were performed on the investigated compound and small model systems, and reproduced the experimental fact of the anti-conformer being lower in energy than the syn. Theoretical analysis of excited states, including visualisation of natural transition orbitals, identified possible pathways for syn–anti isomerisation, although the details vary with π-system size, making the use of small models of limited utility. The investigated compound probably isomerises through the third singlet excited state (S(3)), a π–π* excitation, relaxing through S(2), also a π–π* state.