Theoretical investigations of electronic structure and tautomerism in ground and excited states of 1-[N-(4-fluorophenyl)]naphthaldimine

Electronic structure of 1-[N-(4-Fluorophenyl)]naphthaldimine phenol-imine (1) and quinoid (2) tautomers in ground and excited states are presented. The density functional calculations predict the ground state of 1 to be 5.4 kJ mol−1 lower in energy owing to stronger O-H⋯N (1.638 Å) interactions relative to O⋯H-N (1.651 Å) interactions in 2. The influence of solvents on the tautomeric stability as revealed from the self-consistent reaction field (SCRF) calculations suggests tautomer 2 to be of lower energy in protic polar solvents. Time-dependent density functional (TDDFT) calculations on the other hand bring about the reversal of relative stabilization and facilitate the tautomer 2 to be 6.6 kJ mol−1 lower in energy in the lowest excited state which results from the extended conjugation owing to its planar geometry.