The analysis of photochemical anti-syn isomerization process across the –N–N= bond in heterocyclic imines

: Many biologically active molecules experience 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 focused 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.


Introduction
 some chemical substances are photoactive when subjected to UV radiation  can form many reactive species  play a crucial role in essential biochemical processes  exhibit structural changes and biological activities upon radiation  act as the first step in a number of light-induced biological processes  photochemical nature makes them applicable in many fields Compounds  Schiff-base possessing a quinazolinone moiety (S1)  High-resolution 600 MHz 1 H NMR spectrum of S1 in DMSO at 25C before irradiation (a) and after 10 minutes of UV irradiation (λ max = 365 nm) (b). Smaller signals (marked with subscript s) belong to the syn-isomer, which formed upon irradiation; assignments without indices belong to the main form (anti-isomer).

Results and discussion
9  temperature coefficients of the -N 3 -N=C(H) protons and OH protons (Table 1)  coefficients for the OH protons for 1 -7 varied between -5.36 and -6.11 ppb/K  OH groups are involved in intramolecular H-bonds with different strengths depending upon the structure and conformation  the same behaviour also for the NH protons  stability of syn-isomers due to the formation of intramolecular hydrogen bonds

Results and discussion
10  The relative energies between the anti-and syn-forms for M1-M4 and S1 are listed in Table  2 and the selected geometry parameters are in  A significant effect is also seen after addition of the aromatic ring C to the carbon atom C 10 ; there is a visible zigzag double-bond formation in the C 4 -N 3 -N 9 -C 10 linkage, where the N 3 -N 9 bond was shortened and the N 9 -C 10 bond was prolonged ( DFT-optimised geometries of compound S1: anti-isomer (left) , syn-isomer (right) N 3 -N 9 -C 10 -C 1'' ----−179.5 −178.0 --179.9 −178.1 N 9 -C 10 -C 1" -C 6
Hricovíni, M.; Asher, J.; Hricovíni, M. RSC Adv. 2020, 10, 5540.  first excited state has its highest energy at 180°and its lowest energy at 0° The first state arises from an n-π* excitation  the dominant excitation is HOMO → LUMO

Results and discussion
15  M3 and S1 -the situation is a little different  the first two have similar energy profiles to the ground state; third is inverted  the conical intersection between 2 nd and 3 rd excited state  deexcitation mechanism is similar for both systems Energy curves for ground state (GS) and excited states (ES1-ES4) of M3 (left) and S1 (right) with proposed mechanisms of excitation and deexcitation processes (E -excitation, 1structural relaxation, 2 -internal conversion, 3 -emission). Relative energies (kJ/mol) of the curves have been adjusted for clarity.