As part of our ongoing projecton the reactivity of halogenated compounds, we are exploring halogenatedsystems such as isophthalonitrile (IPN) and hydropyridine (HYP) derivatives, figure1. We are interested in these compounds since pyridines emerge as one of themost prevalent structural units in pharmaceutical and agrochemical targets, aswell as in material science [1], while chlorinated isophthalonitrilederivatives have a potencial usage as fungicides[2]. Our study aims to analizeand comprehend the reactivity after irradiation (l = 254nm), theidentity of the photoproducts generated under different enviroments (presenceor absence of O₂) and the knowledge of all possible reaction pathsof the excited state, scheme 1 i-v.

We present the analysis of thedisappearance rate constants for the compounds of figure 1, under different conditions.All  photolysis reactions had a lineardecay with time, so it can be concluded that the process is of first order. ForIPN derivatives, even though at the wavelength of photolysis the sampleshave a considerable molar absorptivity coefficient (e), the k_rat which some of them are being photolized are rather small and they resultedto be higher in the absence of oxygen in the medium. For HYPderivatives in contrast, no significant differences were observed in terms ofthe presence of oxygen but the k_rwere significantly higher. These facts indicate that the excited state is takingdifferent paths for IPNs and HYPs. The presence of O₂ plays asignificant role in the photodegradation mechanism of IPNs. A possible role couldbe the formation of ¹O₂ via an energy transfer process whichhas been probed with an indirect approach. It is well known that L-cysteine inthe presence of ¹O₂ forms L-cystine through a softoxidation. Therefore, a solution of reagents/L-cysteine was irradiated in thepresence of O₂ and the formation of a white solid identified ascystine was observed confirming the occurrence of 1iv with high quantum yield. Otherpossible path that was tested, is fluorescence. Therefore, we measured thefluorescence spectra but the quantum yields determined were of the order of 10^-3in all cases. Measurements were done relative to Ru(bpy)₃²⁺as standard [3].

Therefore, forHYPs decomposition is the path with the highest quantum yield as singlet oxygenformation is the prevailing path for IPNs. In relation to the photoproductsformed, for IPN derivatives dechlorination and decyanation took place withsmall quantum yields, while for HYPs the loss of the -SO₂-CF₃moiety as well as re-aromatization processes occurred with high quantum yields.