Novel photoluminescent lanthanidomesogens forming bilayer smectic phase derived from blue light emitting liquid crystalline, one ring O-donor Schiff-base ligands

Abstract

A series of new mononuclear lanthanide(III)-salicylaldimine complexes of the type [Ln(LH) 3(NO 3) 3] (Ln = La, Pr, Sm and Gd; LH = N-(2-hydroxyethyl)-4 n-alkoxysalicylaldimine, n = 14, 18) have been synthesized and characterized by FT-IR, 1H NMR, 13C NMR, UV–Vis, FAB-mass and magnetic susceptibility measurements. The ligand (LH) coordinate to lanthanide ions in zwitterionic form via the phenolic-oxygen with the proton shifted to the imine–nitrogen. The nitrato groups occurring in chelated bidentate fashion complete a nine-coordinate geometry. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) show that the ligands are monotropic and their complexes exhibit enantiotropic highly viscous smectic A (SmA) mesophase in the temperature range 60–185 °C. A bilayer self organized assembly of the molecules in the mesophase are proposed on the basis of the small angle XRD study. The ligands are blue light emitters with a broad emission maxima at ∼447 nm while the lanthanide complexes show intense emission in the visible range (∼465–679 nm) at 350 nm excitation. The samarium(III) complex, [Sm(LH) 3(NO 3) 3] is distinct from the rest in emitting bright orange light (∼660 nm, Φ = 48%). The S o–S 1 excitation band being stronger than the direct f–f excitation in the samarium complex clearly suggests that the Schiff-base ligands efficiently sensitize the luminescence of Sm 3+. DFT calculations have been performed using DMol3 program at BLYP/DNP level to obtain the stable electronic structure of the ligand and complex.