New Sm(III) complexes as electronic-excitation donors of the Seta-632 squaraine dye

We have found optimal formation conditions of new Sm(III) chelate complexes with derivatives of oxoquinolinecarboxylic acid (L₁ and L₂) and determined their spectral-luminescent characteristics (the luminescence and luminescence excitation wavelength maxima and the luminescence lifetimes). We have revealed that the Seta-632 squaraine dye (a fluorescent label of proteins and other biological molecules) quenches the luminescence of complexes Sm(III)-L₁ and Sm(III)-L₂. The quenching of chelate complexes is caused by the Förster resonant electronic-excitation energy transfer (FRET) from the donor (Sm(III)-L₁ or Sm(III)-L₂) to the acceptor (Seta-632). In this case, the luminescence intensity of the Seta-632 dye in the presence of Sm(III)-L₁ and Sm(III)-L₂ increases by factors of 64 and 27, respectively. The values of the Förster radii (R_0(Sm-L1) = 38 Å, R_0(Sm-L2) = 35 Å) and the overlap integrals of the luminescence spectra of the two energy donors with the absorption spectrum of the acceptor (J_Sm-L1 = 1.22 × 10¹² M⁻¹ cm⁻¹ nm⁴ and J_Sm-L2 = 1.06 × 10¹² M⁻¹ cm⁻¹ nm⁴), which have been calculated from the luminescence quantum intensity of the donors and from the absorption spectrum of the acceptor and its molar absorption coefficient, have made it possible to characterize the Seta-632 dye as an efficient quencher of the luminescence of Sm(III) ions. We are the first to propose Sm(III)-L₁ and Sm(III)-L₂ chelate complexes as FRET donors.