First-principles study of salicylideneaniline molecular crystals: Tautomerization reaction involving intermolecular hydrogen bonds

Abstract

Salicylideneaniline molecular crystals, in which tautomerization reactions involving intermolecular hydrogen bonds take place under photoirradiation, are investigated by using the first-principles calculation based on density-functional theory. Structures and properties of the two crystal phases, enol form with intramolecular hydrogen bonding and trans-keto form with intermolecular hydrogen bonding, have been theoretically examined from the viewpoint of atomic bonding pattern. The present calculation result indicates that the trans-keto form, which is somewhat different from a former X-ray analysis result, appears close to a quinoidal form rather than a zwitterionic form. Analysis of energetics in gas and solid phases and differential electron density indicate that stabilization of the crystalline trans-keto form can be realized by intermolecular hydrogen bonds electrostatically reinforced by charge redistribution that is induced by packing the molecules in solid state. These findings lead to a mechanism in which the quinoidal trans-keto form in crystalline phase may exhibit extraordinary stabilization as experimentally observed. Furthermore, dielectric permittivity tensors of the molecular crystals have been calculated on the basis of density-functional perturbation theory. The calculation result indicates that the photodriven tautomerization reaction induces sufficient change of the refractive indices advantageous to various applications such as photochromic memory or switch.