## 3 Results and Discussion
### Triplet Electronic States  
The proposed mechanism occurs from the triplet state of ATZ, populated from inter-system crossing with the singlet excited state. The geometry of the located global minimum for the lower triplet state, \(T_{1}\), can be found in Table 1 for the three functionals (as before, the full coordinates may be found in the Supplementary Information). The minimum is located 293.4, 402.8, and 300 kJ mol\({}^{-1}\) above the \(S_{0}\) for the B3 LYP, M06-HF, and PBE0, respectively. Such values may lead to an overestimation of the energy range for the case of M06-HF. However, such an analysis can only be tested with high-accuracy calculations which is out of the scope of the current work. Main changes of the \(T_{1}\) state arises on the dihedral angles of the phenyl group except for the M06-HF functional. Along the reaction pathway towards pyrimidine, several transitions have been calculated. To obtain such structures, the z-matrix has been constructed to calculate a partial optimised profile in which all degrees of freedom are optimised except the critical angles and/or distances. For example, after populating the triplet electronic state, the system undergoes N\({}_{2}\) photoextrusion in which the N(1)-N(2) and N(3)-N(4) bonds break. Those two variables have been fixed for different values until a critical point is obtained. This initial condition z-matrix is then used to obtain the real, non-constrained transition state. With this methodology, the calculated transition state connects the two minimum along the reaction path. The mechanism can be divided in three main steps, see Figure 5 for the stationary points on the reaction coordinate. Step 1 (a)-(b), denotes the two consecutive reactions that leads to the photoextrusion of N\({}_{2}\) in ATZ, step 2 is the cyclization, and step 3 is the intramolecular proton transfer. Figure 4: UV spectra for the ground-state of ATZ using the TDDFT/VDZ basis set with the Tamm–Dancoff approximation. For comparison the experimental result is also shown. Both experimental data and theoretical results have been obtain considering methanol as the solvent. The photoextrusion of N\({}_{2}\) in ATZ is still a subject of discussion [18]. It can occur through the triplet state or directly from a high-energy singlet excited state, and may occur via N-N simultaneous or concerted bond breaking channels. A scan of the partially relaxed pathways between the two channels has shown that the simultaneous bond-breaking is energetically unfavourable (by more than the double of the energy of TS1). Moreover, starting from several initial guess geometries in the simultaneous reaction coordinate, no transition-state has been found that connects \(T_{1}\) to M1. In fact, the transition-state searches lead to the TS1 structure. In the non-concerted mechanism bond-breaking, two pathways have been studied: Starting with the rupture of N(1)-N(2) or N(3)-N(4) bonds. The energy barrier for the N(1)-N(2) bond-breaking has been found to be 211 kJ mol\({}^{-1}\) higher than the in the case of the N(3)-N(4) bond-breaking. Therefore, the energy profile of the photodegradation has been studied considering the pathway that corresponds to the breaking of the N(3)-N(4) bond in the first place, step 1a, followed by the breaking of the N(1)-N(2) bond, step 1b. This N\({}_{2}\) photoextrusion process can be translated as the passage from a geometry in which all the atoms in the two rings and the oxygen are in approximately the same plane, to one in which the different groups of the molecule are placed in three different planes: One plane for the phenyl, another for the NCON group, and another one for the allyic group. The postulated mechanism states that M1 and possible M2 are biradicals intermediates. Open-shell biradicals can possess a multideterminant (multireference) character, which implies that single determinant methods will fail to describe these molecules in an appropriate way. Reliable descriptions can only be obtained with multireference results.