Optical spectroscopy study of the interaction between curcumin and acrylic polymers

This paper presents the results of a study conducted on the interaction between curcumin, a compound with several biomedical applications in traditional and modern medicine, and the acrylic polymers poly(methyl methacrylate), poly(ethyl methacrylate), and poly(n-butyl methacrylate), through photophysical experiments in curcumin/acrylic polymers casting films. Optical absorption intensity at ~340 nm increases relatively to its maximum at ~417 nm when the amount of curcumin in the polymeric film decreases, due to a significant change in the concentration of the isomers cis- or trans-form of curcumin, regardless of the acrylic polymer. Fluorescence (FL) spectra of the films depend on the curcumin concentration in the matrix with well-resolved line shape. They show two distinct bands, one at ~525 nm, for higher curcumin concentration (5.00 mmol.L^-1), related to the aggregated curcumin species, and another at ~465 nm, for lower concentration of curcumin (0.10 mmol.L^-1), related to the effects of the solvent on the conformational structure of the curcumin molecule and the presence of the trans-form of curcumin. The parameter K_agg, related to the contribution of the aggregated curcumin, shows the influence of the polymeric lateral chain length of the matrix in the de-aggregation of the curcumin. The Huang-Rhys factor indicates that curcumin aggregated species are conformationally more stable, and that the isolate species depends on the chemical environment and the matrix/curcumin interaction, decreasing its conformational degrees of freedom. Arrhenius plots, obtained via FL experiment in function of the sample temperature, show that, for higher curcumin concentration, the value for the relaxation energy process is not well defined, due the decrease in the interaction between the matrix and the curcumin molecules. With these results, it is possible to infer that the interaction matrix/curcumin must occur via lateral chemical alkyl groups.

Energy transfer mechanisms in pH-tuned aggregates of curcumin

Curcumin ((1,7-bis-(4-hidroxi-3-metoxifenil)-1,6-heptadieno-3,5-diona)), is a natural yellow-orange polyphenol dye, obtained from Curcuma longa rhizomes, with vast potential in the biotechnology field, such as anti - inflammatory and cancer preventive properties, and a very environmental-sensible structure, mostly resulting in aggregate formations, with low solubility in aqueous solvents and remarkable pH dependence. Hence, it is essential to comprehend its aggregation mechanisms, optical properties and conformational equilibrium to develop curcumin-based drugs and biomaterials for several applications. With the purpose of understand such concepts and its properties, the results presented in this work, obtained through spectroscopic measurements and the Hill's modeling, show a conformational equilibrium, driven by pH, between the enol-enolate and aggregated forms in curcumin, and the respective energy transfer processes between these species. Additionally, the photophysical mechanisms for the energy transfer between aggregated forms, observed with the ellipsometry measurements, showed an "all-or-nothing" character, pointing to a very complex conformation for curcumin aggregates.

Two-Photon Absorption Dependence on the Temperature for Azoaromatic Compounds: Effect of Molecular Conformation

This work reports on the effect of temperature on the two-photon absorption cross section of azoaromatic chromophores. A linear decrease in the two-photon absorption cross section with the temperature was observed for several azochromophores. This process was characterized by introducing a two-photon absorption thermal coefficient (ddelta/dT), whose typical values are approximately 2GM/degrees C for all the azochromophores studied here. Such an effect was attributed to thermal induced molecular conformation changes, described by the sum-over-states model and semiempirical calculations, which affect the molecular dipole moments. The characterization of the phenomenon reported here for other nonlinear materials can help in the design of specific applications using two-photon absorption.