title: Excited-state dynamics of the medicinal pigment curcumin in a hydrogel.
abstract: Curcumin is a yellow polyphenol with multiple medicinal effects. These effects, however, are limited due to its poor aqueous stability and solubility. A hydrogel of 3% octadecyl randomly substituted polyacrylate (PAAC18) has been shown to provide high aqueous stability for curcumin under physiological conditions, offering a route for photodynamic therapy. In this study, the excited-state photophysics of curcumin in the PAAC18 hydrogel is investigated using a combination of femtosecond transient absorption and fluorescence upconversion spectroscopy. The transient absorption results reveal a multiexponential decay in the excited-state kinetics with fast (1 ps & 15 ps) and slow (110 ps & ≈5 ns) components. The fast decay component exhibits a deuterium isotope effect with D2O in the hydrogel, indicating that the 15 ps decay component is attributable to excited-state intramolecular hydrogen atom transfer of curcumin in the PAAC18 hydrogel. In addition, solvent reorganisation of excited-state curcumin is investigated using multiwavelength femtosecond fluorescence upconversion spectroscopy. The results show that the dominant solvation response (τ = 0.08 ps) is a fast inertial motion owing to the presence of bulk-like water in the vicinity of the hydrophobic octadecyl substituents of the PAAC18 hydrogel. The results also show an additional response with longer time constants of 1 and 6 ps, which is attributable to translational diffusion of confined water molecules in the three-dimensional, cross-linking network of the octadecyl substituents of PAAC18. Overall, we show that excited-state intramolecular hydrogen atom transfer and solvent reorganisation are major photophysical events for curcumin in the PAAC18 hydrogel.
authors.affiliation: Department of Chemistry, The University of Adelaide, Adelaide, South Australia 5005, Australia. tak.kee@adelaide.edu.au.
authors.forename: Takaaki
authors.initials: T
authors.lastname: Harada
authors.affiliation: Department of Chemistry, The University of Adelaide, Adelaide, South Australia 5005, Australia. tak.kee@adelaide.edu.au.
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Lincoln
authors.affiliation: Department of Chemistry, The University of Adelaide, Adelaide, South Australia 5005, Australia. tak.kee@adelaide.edu.au.
authors.forename: Tak W
authors.initials: TW
authors.lastname: Kee
mesh_terms: D003474:Curcumin / Q000737:chemistry*; D020100:Hydrogels / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D012995:Solubility; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence
chemical_list: D020100:Hydrogels; D012997:Solvents; D003474:Curcumin