We report the results of quantum-chemical calculations, which show that the keto form of 2-hydroxynaphtylidene-(8-aminoquinoline) (HNAQ) is slightly more stable than the enol form both in the ground and first excited ππ* electronic states. The barrier for proton transfer between the enol and the ketone in the ground state is ca. 3300 cm(-1) (HF), and 770 cm(-1) (B3LYP), indicating a very fast (ps scale) exchange of protons between the two tautomeric forms. This barrier decreases slightly in the first excited ππ* electronic state (2500 cm(-1) - CIS), making proton exchange even faster. We show that the ππ* state of the ketone tautomer is prone to radiationless transition to a state with nearly perpendicular orientation of the two ring systems, similarly to other Schiff bases that are photochromes (for instance salicydeneaniline). This state arises when an electron from the highest occupied molecular orbital (HOMO) of the ketone ring system is transferred to a LUMO localized on the CHNH group of the bridge connecting the two ring systems of the molecule. The energy minimum of this "perpendicular" state lies only ca. 0.09 eV from the ground state potential-energy surface, thus it is prone to extremely rapid radiationless decay. Further relaxation on this surface leads to a metastable conformation that lies ca. 4440 cm(-1) above the planar, hydrogen-bonded, ketone conformation. Unfortunately, photochromism of this metastable conformation does not occur, since its absorption spectrum overlaps the spectrum of the stable species (with the predicted absorption around 438 nm vs calculated 440.6 nm in the stable ketone).
© 2011 American Chemical Society