Interfacial Enrichment and Demetallation of a Zinc Porphyrin in the Ionic Liquid [C₄C₁Im][PF₆]

Understanding and controlling the surface composition of metalloporphyrins in ionic liquids (ILs) is crucial for designing photoactive materials. Here, we investigate the interfacial behavior of zinc-didodecylporphyrin molecules (Zn-DDP) dissolved in the hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [C₄C₁Im][PF₆] using angle-resolved X-ray photoelectron spectroscopy (ARXPS). Upon mild heating, Zn-DDP undergoes spontaneous demetallation, evidenced by the appearance of aminic (-NH-) and iminic (=N-) nitrogen XPS signals characteristic of the free-base porphyrins. Partial hydrolysis of [PF₆]^- anions produces phosphate species that contribute to the demetallation process. Concentration-dependent measurements reveal extremely high surface enrichment of porphyrins at the IL/vacuum interface, reaching surface saturation already at about 0.50%_mol Zn-DDP content in the bulk. The dodecyl chains of porphyrins at the outer surface are oriented toward the vacuum in a buoy-like configuration. Temperature-dependent ARXP spectra reveal that for unsaturated interfaces, surface enrichment increases with increasing temperature. Our findings provide molecular-level insights to guide the design of porphyrin-based photoactive interfaces in ionic liquid systems.