Metal-Centered Photoluminescence as a Tool for Detecting Phase Transitions in EuIII- and TbIIIContaining Metallomesogens

The non-mesogenic ligand L1, derived from a 1,7-diaza-18-crown ether, forms mesogenic complexes with EuIII and TbIII nitrates, Eu(NO3)3L1·0.25 H2O (EuL1) and Tb(NO3)3L1·1 THF (TbL1). Thermal analyses and polarized light microscopy data indicate the formation of a liquid-crystalline phase at 86 (Eu) and 81 C (Tb), while isotropization occurs around 198-200 C, immediately followed by decomposition. In the case of EuIII, the mesogenic phase is unambiguously identified as being a hexagonal columnar mesophase by SAXD. According to a detailed luminescence study, the crystalline form of EuL1 features two different metal ion environments: (i) a C2-symmetrical site without coordinated water molecule (population: 79 (plus_minus) 6%) and (ii) a low-symmetry site with one water molecule coordinated onto the metal ion (population: 21 (plus_minus) 6%). For EuL1, the variation of both the relative intensity of the 5D07F2 transition, ln(IT/I295), and of the Eu(5D0) lifetime, ln(T/295), over the Cr-Colh transition have a S-shape dependence versus 1/T, allowing a precise determination of the transition temperature. In the case of the luminescence intensity, corrections for the expected decrease versus increasing temperature (determined on a reference compound EuL2 with L2 devoid of mesogenic side chains) and for the refractive index change over the transition still leave a sigmoid dependence, pointing to a genuine effect generated by the phase transition. Consequently, luminescence intensity and lifetime switching can be used to signal Cr-LC transitions in Eu-containing mesogenic compounds. A similar effect is observed for TbL1, as far as the intensity of the 5D47F5 transition is concerned.