Mechanistic elucidation of superoxide anion radical-induced photobleaching in prodigiosin: An integrated experimental and DFT study

Py-Pro undergoes Type I photobleaching via superoxide anion radical generation. • NBT assays and EPR spectra confirmed light-induced ROS formation in the Py-Pro system. • Replacing pyrrole donors with thiophene or phenyl groups improved photostability. • DFT calculations correlated donor strength with ROS generation and photodegradation. Prodigiosin, a promising microbial pigment for sustainable textile dyeing, suffers from poor lightfastness, the mechanistic origin of which remains poorly understood. Herein, a synthetic analogue (Py-Pro) was employed to elucidate its photodegradation behavior. Under simulated illumination, Py-Pro undergoes self-sensitized photobleaching via a Type I pathway, generating superoxide anion radicals ( O 2 − · ${\mathrm{O}}_2^{-·}$ ). HPLC-MS analysis identified the major degradation products, supporting oxidative cleavage of the conjugated chromophore. Replacing the electron-rich pyrrole donor with thiophene and phenyl groups yields two analogues (Thio-Pro and Ph-Pro), which exhibit enhanced photostability, reducing degradation from 57.48% (Py-Pro) to 17.57% and 11.41%, respectively. Density functional theory (DFT) calculations reveal that the strong donor character promotes intramolecular charge transfer, reduces the S 1 -T 2 energy gap, and increases spin-orbit coupling, thereby facilitating intersystem crossing and enhancing O 2 − · ${\mathrm{O}}_2^{-·}$ generation. These findings provide mechanistic insight and a rational strategy for improving the photostability of natural dyes. The possible photobleaching mechanism and structure-photostability relationship of Py-Pro . a. Energy level diagram of Py-Pro. Py-Pro undergoes photobleaching degradation via electron transfer, generating superoxide anion radicals. b. Effects of different heterocyclic substitutions on the left ring of Py-Pro on photobleaching. Download: Download high-res image (278KB) Download: Download full-size image