Many biologically active molecules experience photoisomerization processes after light-induced excitation (e.g. rhodopsin). Important photochemical properties showed particularly compounds possessing aromatic systems in the presence of conjugated heteroatoms. Such systems often constitute a part of natural biomolecules and play a crucial role in essential biochemical and biological processes. Due to photochemical nature of these aromatic compounds, such as Schiff bases, they are often studied for pharmacological applications and used in biochemistry and medicine. For this reason, we focussed our study on photochemical processes of quinazolinone-based Schiff bases. The isomerization from the energetically more favourable anti-isomer to the syn-isomer by UV/vis excitation has been found namely in the systems possessing a double bond systems. Presented analysis deals with NMR spectroscopy and theoretical DFT analysis of photochemical processes of the Schiff base possessing a quinazolinone moiety with a series of model compounds to investigate the photochemical behaviour of the –N–N= linkage. The NMR experiments in solution showed that irradiation at 365 nm leads to photochemically-induced isomerization from the anti- to the higher-energy syn-form around the –N–N= linkage.