Luminescent palladium(II) metallomesogens based on cyclometalated Schiff bases and N-benzoyl thiourea derivatives as co-ligands

Abstract

Luminescent cyclometalated palladium(II) complexes based on Schiff bases and N-benzoylthiourea (BTU) ligands with different numbers of alkoxy groups at their periphery were designed and prepared. The BTU ligands coordinate through the O and S atoms in a deprotonated bidentate fashion as confirmed by IR and NMR spectroscopy data. Additionally, giving the asymmetrical structure of BTU ligands, the 1H-NMR showed the presence of only one of the two possible isomers. The liquid crystalline behavior of these palladium(II) complexes was investigated by a combination of polarized optical microscopy (POM) and differential scanning calorimetry (DSC) while their thermal stability was studied by thermogravimetric analysis (TGA). The mesomorphic behavior of palladium(II) complexes depends on the number and chain length of alkoxy groups grafted on BTU derivatives (monotropic smectic A and C phases). The palladium(II) complexes are thermally stable on a broad temperature range up to about 220 °C. Interestingly, when the number of alkoxy groups on the BTU ancillary ligands was increased, the corresponding palladium(II) complexes lost their mesogenicity and only melting to the isotropic state was observed. The solid-state emission spectra of palladium(II) complexes show two maxima around 580 and 650 nm, respectively, with a shoulder around 710 nm when the samples are irradiated in the 330–380 nm range.