Photochemical behavior of Safranine-Riboflavin complex in the degradation of folic acid

•The low value of ΔG of complexation between safranine and riboflavin indicates an energetically favorable process.•Photo-induced intermolecular electron transfer was studied within the scope of sensitizer design.•Semioxidized radical of riboflavin and semireduced radical of safranine were generated following safranine sensitization.•Folic acid is degraded by safranine sensitization through a radical pathway.

The photochemistry of safranine complexed with riboflavin and the photoinduced electron transfer from folic acid to the complex were investigated by steady-state and time resolved absorption spectroscopy, and the photoproducts were further analyzed by LC-orbitrap–FT–MS. UV–vis spectra of the complex showed a decrease in the safranine band at 520 nm with increasing concentration of riboflavin, which was attributed to complex formation. A 1:1 (safranine:riboflavin) stoichiometry was determined using Job's method. Following safranine photoexcitation, there is formation of a radical pair in the complex by donation of an electron from ground-state riboflavin to excited-state safranine. The cation radical of riboflavin within this complex reacts with folic acid to yield FA+•, which may accept an electron from the anion radical of safranine, avoiding the photobleaching of the safranine. In contrast, the reaction of the anion radical of non-complexed safranine with folic acid leads to safranine photobleaching.

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