Nano-Confinement Modulates the Photo-Oxidation of Water-Soluble Rubrene in Liquid and Solid Films

Photo-oxidation remains a challenge for achieving long-term stability in organic electronic devices. Rubrene (Rub), a small-molecule semiconductor known for its exceptional optoelectronic properties, illustrates this problem as it is highly sensitive to light and oxygen. In addition, its limited processability, requiring the use of halogen solvents or thermal evaporation for deposition, further restricts its practical use in large-area processing. This work shows how both problems can be addressed by a colloidal approach where Rub is encapsulated through a miniemulsion method, producing stable, water-dispersible nanoemulsions (NPs). Their emission and absorption properties in liquid and solid films show that nanoencapsulation preserves the intrinsic vibronic fingerprint of Rub, while inducing spectral shifts and emission narrowing, enhancing molecular ordering and stronger intermolecular coupling, quantified by calculating the Huang–Rhys factor. Systematic investigation of photo-oxidation kinetics reveals a modulation effect: in dispersion, Rub NPs oxidize faster than molecular Rub in chloroform due to enhanced oxygen access from their large surface area while maintaining colloidal stability. In contrast, NP-based Rub films exhibit significantly improved photostability, preserving over twice the residual absorbance of annealed neat Rub films under 40 W/m 2 irradiation. Atomic Force Microscopy and Fourier Transform Infrared Spectroscopy confirm the synergistic role of excess surfactant and nanoencapsulation in decreasing Rub oxidation, while post-thermal annealing fine-tunes film morphology, improving stability and reducing oxygen diffusion pathways. This work establishes nanoemulsion-based confinement as an effective strategy to modulate photo-oxidation in highly reactive organic semiconductors, offering new design rules for environmentally processed, photostable optoelectronic materials and oxygen-sensing platforms.