## 1 Introduction  
Tetrazoles are synthetic molecules containing a cyclic system with high aromaticity formed by four nitrogen atoms and one carbon atom [1; 2]. Tetrazoles are being used in medicinal chemistry as bioisostere for carboxylic acids, as they originate from similar receptor-ligand interactions [3], as well as drugs in the treatment of several diseases, due the diversity of physical-chemical properties [4; 5; 6; 7]. The energy materials industry also uses tetrazolic compounds as they are rich in nitrogen [8], have a high value of enthalpy of formation, thermal stability, explosive performance, and low sensitivity [9]. These compounds are used with different applications, from aerospace to the mining industry [10]. Tetrazoles are considered a more ecological variant since they produce N\({}_{2}\) in combustion processes [11]. In chemical synthesis, they are widely used in the preparation of heterocyclic compounds with nitrogen atoms. Synthetic routes are described for various classes of compounds [12], using tetrazoles or derivatives, such as \(9H\)-pyrimindo(4,5-b)indoles [13], diaziridinones [14], iminoaziridines [15], carbodiimides [16], oxazines [17], benzimidazoles [16], and pyrimidinomes [18; 19; 20]. Compounds with the tetrazol-5-one group (tetrazolone) are characterised by forming a carbonyl group with the ring carbon. The photochemistry of tetrazolic compounds focuses on ring cleavage, and the most frequently obtained products are azides, isocyanates, or aziridines. However, the presence of labile hydrogen atoms in the ring or substituting groups can lead to alternative reaction pathways, which occur simultaneously with other possible photochemical pathways. These processes can be favoured or hindered by the chemical nature and the conformational flexibility of the substituents linked to the tetrazolic ring, changing the relative quantities of various photoproducts [15; 16; 21; 22; 23; 24; 25]. Molecular nitrogen photoextrusion is common in tetrazolones photochemistry, and can occur with a concerted process, or through the formation of radical or zwitterionic species [18; 22]. The photochemistry of tetrazolic compounds has been studied in solution and by matrix isolation. The matrix isolation approach limits the study to confined processes, eliminating alternative reaction pathways that could arise through molecular diffusion (except for small species produced), or that depend on conformational rearrangements [18; 22; 23; 26]. Solution photoreactions have more associated variables, since it is known that the solvent and other reaction conditions have a direct influence on the yield and selectivity of the photodegradation of tetrazolic compounds [12]. The photochemistry of 1-phenyl-4-allyl-tetrazol-5-one (ATZ) has been studied with excitation at 254 nm, and several reaction pathways have been proposed with the products characterised by various analytical techniques [17]. The photoproducts are strongly dependent of the solvent: Pyrimidinone was identified as the main product, in the case of protic solvents, and in aprotic solvents, isocyanates, and anilines. Based on the experimental observations, the photochemical mechanism of ATZ has been postulated for this reaction pathway, and it involves the formation of a triplet biradical intermediary after the photoextrusion of molecular nitrogen [18]. Pyrimidinone is formed after this intermediate undergoes a conformational rearrangement through the allylic group and a proton transfer. This proton transfer has been found to be intramolecular, as its occurrence is detected in both protic and aprotic solvents [18]. Based on the experimental detection of photoproducts, a mechanism has been postulated ad-hoc [18; 26], which assumes the relevance of the conformational rearrangements of the triplet intermediaries.