Photophysics of 3-Carboxy-Salicylideneaniline in Rare Earth Metal Complexes with Silicon-Containing Schiff Base Ligands

The investigations reported in this work concern the capabilities of the 3-carboxy-salicylidene unit in three dicompartmental Schiff bases with a flexible spacer bearing silane or siloxane units to efficiently coordinate rare earth and transition-metal ions as well as the efficiency to act as antenna or organometallic antenna for Ln(III) sensitization. The Ln(III) complexation studies of the H4L1, H4L2, and H4L3 ligands were performed by spectrophotometric titration. The selectivity toward seven Ln(III) ions and the trend of the stability constants are analyzed in connection with the structure of the ligands. In order to find the energetic factors that impact the efficiency of Ln(III) sensitization, the corresponding Gd(III) complexes were prepared and were in-depth analyzed from the point of view of the photophysical properties. The investigations were carried out using time-resolved emission and transient absorption spectroscopy. Phosphorescence data prove the unhindered path of the ligands to the triplet state and the probability to transfer the excitation energy to the Ln(III) ion. Applicability of two selected Gd(III) complexes in detection and recognition of transition and heavy metal ions has been studied with a double purpose: to evaluate their sensitivity and recognition ability so that they could be applied in pollutants detection and to estimate the capability of the M(II)-Schiff base unit as an organometallic antenna for Visible emission sensitization. The Gd(III) complex with a long π-conjugation showed multiple fluorescence responses to various kinds of 3d ions, which gave it pattern recognition ability. The discrimination power was finally analyzed by a principal component analysis.