Structure–Dependent Halochromism in Azine-Derived Molecules: Optical Response in Solution and Films

Azines are interesting molecules with a wide range of properties and applications, such as photophysical and liquid crystal properties, and applicability in both catalysis and sensing. However, an important factor that can affect their intrinsic properties is the presence of different functional groups in their structure. In this paper, we explored the optical, and even halochromic, response induced by hydrochloric acid in solution and solid state (Drop Casting films) of a benzalazine not yet reported in the literature ( BZZ ). The properties of this molecule when exposed to acid vapor were also analyzed through computational studies (DFT). For comparison, two structurally similar molecules, but containing distinct central groups, were also synthesized and analyzed, one a hydrazide derivative ( HDZ ) and the other a dihydrazide derivative ( DHZ ). Although they have similar structural characteristics, these two molecules exhibit centers with less basic nitrogens due to the electronic influence of adjacent carbonyls. We show that BZZ exhibits intense halochromism and an easily observable optical response, while the HDZ and DHZ molecules exhibit only small or no colorimetric and spectral alterations, highlighting the importance of the electronic nature of the central group.