Temperature control of the two-photon brightness and excited state properties of diimide dibenzene sulfonic-acid perylene monomers/aggregates in a binary water-dimethyl sulfoxide solvent mixture

Abstract

Perylene Diimides (PDIs) are known for their high emission quantum yields and have been used in organic semiconductor and fluorescent probe applications. However, their insolubility in polar solvents, such as water, limits their biological by causing fluorescence-quenching aggregation. To address this, we explored modifications to the PDI structure to enhance solubility. This study focuses on the photophysical properties of Perylene Diimide Dibenzene Sulfonic Acid (PDI BzSA) in different solvent mixtures of Dimethyl Sulfoxide (DMSO) and water. We observed changes from isolated molecules in DMSO to predominantly aggregated forms in water. Using nonlinear optical and ultrafast time-resolved methods, we explored the effects of temperature and solvent composition on the photophysical parameters. Key findings include the solvent environment effect on the excited state and linear optical properties of the monomer, modeled phenomenologically, as well as the aggregation process. We found that the fluorescence quantum yield increased in a 50 % water-DMSO mixture, with quenching at greater water content due to aggregation. Despite the changes in fluorescence yield, two-photon absorption (2PA) cross-sections were minimally impacted by aggregation. Additionally, we demonstrated that temperature can be used to control the two-photon brightness significantly. Ultrafast transient absorption (TA) studies revealed distinct relaxation pathways for monomers and aggregates, with aggregates exhibiting charge transfer features in their excited state absorption spectra. TA anisotropy indicated that aggregates stack in a slightly twisted configuration due to the non-planar sulfonic acid groups.