Tetrahydrocurcumin as a bioinspired ligand for lanthanide complexes: synthesis, characterization, and photophysical properties

Besides health-related applications, curcumin has been investigated as a ligand in coordination chemistry, with complexes of Cu²⁺, Mn²⁺ and lanthanides reported. However, its highly conjugated structure yields a low-lying triplet state (~14000 cm⁻¹) that is below the emitting levels of visible-range lanthanides (Eu³⁺, Tb³⁺, Dy³⁺), making energy transfer inefficient — though NIR-emitting ions such as Nd³⁺ and Yb³⁺ can be effectively sensitized. To overcome this limitation, we propose the reduced form of curcumin, tetrahydrocurcumin (h4-Cur) as a bioinspired ligand that efficiently sensitize lanthanide ions. The reduction breaks the conjugation between the diketonate and aromatic rings, blue-shifting the excited states by ~11000 cm⁻¹ (absorption maximum: 285 nm for h4-Cur vs. 418 nm for curcumin), changing the energy gap and resulting in efficient energy transfer to lanthanide ions.