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

The non-mesogenic ligand L 1, derived from a 1,7-diaza-18-crown ether, forms mesogenic complexes with Eu III and Tb III nitrates, Eu(NO 3 ) 3 L 1 ·0.25 H 2 O ( EuL 1 ) and Tb(NO 3 ) 3 L 1 ·1 THF ( TbL 1 ). 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 Eu III, the mesogenic phase is unambiguously identified as being a hexagonal columnar mesophase by SAXD. According to a detailed luminescence study, the crystalline form of EuL 1 features two different metal ion environments: (i) a C 2 -symmetrical site without coordinated water molecule (population: 79 ± 6%) and (ii) a low-symmetry site with one water molecule coordinated onto the metal ion (population: 21 ± 6%). For EuL 1, the variation of both the relative intensity of the 5 D 0 → 7 F 2 transition, ln( I T / I 295 ), and of the Eu( 5 D 0 ) lifetime, ln(τ T /τ 295 ), over the Cr−Col h 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 EuL 2 with L 2 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 TbL 1, as far as the intensity of the 5 D 4 → 7 F 5 transition is concerned.