Different substituents at the sydnone ring can drive the photoreactivity towards different products. In particular, phenyl-substitution favours the formation of benzodiazepine derivatives Display omitted •Different substituents at the sydnone ring can drive the photoreactivity towards different products.•Phenyl-substitution at the sydnone ring favours the formation of benzodiazepine derivatives interesting for biomedical applications.•Ns flash photolysis gave insight in the transient precursors responsible for the different reactivity of the substituted sydnones.•Oxygen effect and energy transfer from triplet donors measurements added information about the mechanism of the photoreactions.•QM calculations evidenced low-lying forbidden states with CT character or completely localized on the sydnone, the most photoreactive moiety. This work follows an earlier one where some sydnone derivatives, interesting for their biological and biomedical applications, were synthesized and the effect of substituents on their photoreactivity was studied in irradiation conditions of high intensity and high substrate concentrations. The aim of the present work is to investigate the deactivation pathways of the excited states of three of these compounds, bearing a stilbene moiety, and to determine the quantum yield of the competitive fluorescence/photoreactions in soft irradiation conditions. Flash photolysis measurements with nanosecond resolution were also carried out to obtain information on the transient intermediates responsible for the photoproducts found in the previous work. The oxygen effect on the photoproducts and the energy transfer from triplet donors were also investigated. The experimental results, supported by quantum chemical calculations, showed that different substituents at the sydnone ring can drive the photoreactivity towards different products, thus confirming the results of the previous work. In particular, phenyl-substitution favours the formation of benzodiazepine derivatives.