Femtosecond transient absorption spectroscopy of copper(II)–curcumin complexes

Abstract

Ligand–metal interaction between curcumin and Cu(II) in methanol and sodium dodecyl sulfate (SDS) micelles was investigated using fluorescence spectroscopy and transient absorption spectroscopy. The Cu(II) ion exhibits a high efficiency in quenching the fluorescence of curcumin. By quantifying fluorescence quenching as a function of Cu(II) concentration, the complexation constants, K₁ and K₂, for the formation of the 1 : 1 and 1 : 2 Cu(II)–curcumin complexes, [CuII–Cur]⁺ and [CuII–Cur₂], have been determined. In methanol, K₁ and K₂ are (1.33 ± 0.47) × 10⁸ M⁻¹ and (6.79 ± 1.77) × 105 M−1, respectively, whereas those in SDS micelles are (9.90 ± 1.68) × 105 M−1 and (1.70 ± 0.48) × 106 M−1, respectively. The transient absorption spectra of curcumin and the Cu(II)–curcumin complexes from 520 nm to 700 nm show a combination of stimulated emission and excited state absorption (ESA). However, the transient absorption signal at 500 nm corresponds to ESA exclusively. For curcumin, the ESA kinetics exhibit two rising components with time constants of 0.9 ps and 8.2 ps in methanol, and 0.5 ps and 2.5 ps in SDS micelles, which are consistent with solvation dynamics of excited state curcumin in these media. In addition, the ESA kinetics show a decay component with a time constant of 125 ps in methanol and 64 ps in SDS micelles, reflecting the excited state intramolecular hydrogen atom transfer of curcumin in these media. The ESA kinetics of the Cu(II)–curcumin complexes exhibit a sharp rise and a fast decay with a time constant of approximately 1 ps in both media due to the strong interaction between Cu(II) and curcumin.