Tuning the Polarization Along Linear Polyaromatic Strands for Rationally Inducing Mesomorphism in Lanthanide Nitrate Complexes

Abstract

The opposite orientation of the ester spacers in the rodlike ligands L 4C12 (benzimidazole‐OOC‐phenyl) and L 5C12 (benzimidazole‐COO‐phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, LnIII, and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4C12)(NO3)3] (Ln=La–Lu) vanishes in [Ln(L 5C12)(NO3)3], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time‐dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid‐crystalline phases. As a rule of thumb, an alternation of electron‐poor and electron‐rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4C12, L 5C12, [Ln(L 4C12)(NO3)3], but not for [Ln(L 5C12)(NO3)3]. The intercalation of an additional electron‐rich diphenol ring on going from [Ln(L 5C12)(NO3)3] to [Ln(L 6C12)(NO3)3] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model.

Rational Tuning of Melting Entropies for Designing Luminescent Lanthanide‐Containing Thermotropic Liquid Crystals at Room Temperature

“…The latter parameters reported in this work systematically support our simple thermodynamic model based on the explicit consideration of two successive phase transitions, respectively mainly affecting: a) the intermolecular cohesion between the flexible part (melting) and; b) the rigid cores of the molecules (clearing). [10] We therefore conclude that, after DH c which can be rationally influenced by a judicious choice of rigid aromatic cores possessing specific alternating polarization, [14] DS m is a second thermodynamic parameter which can be controlled by chemical synthesis for designing molecular objects with accessible mesomorphic properties. We do believe that the enthalpically unbalanced increase of DS m in divergent polycatenar systems is a general phenomenon, which is not limited to lanthanidomesogens, but also applies to the general preparation of room-temperature metallomesogens, in which specific magnetic, electronic, or photophysical properties can be introduced.…”

“…Taking Cs 3 [Ln(dipicolinate) 3 ] in aqueous solution as external reference (Ln = Eu, Tb) [37] and neglecting the faint emission arising from directly excited noncomplexed EuA C H T U N G T R E N N U N G (NO 3 ) 3 , we obtain absolute quantum yields ) back transfer operating between these two almost resonant levels. [24] The case of Eu ), [14] a characteristic which is known to improve: 1) intersystem crossing (ISC) thanks to the increase in the ligand-centered singlet-triplet energy gap, [38] and; 2) 3 pp*!Eu( 5 D J , J=1, 2) energy transfer processes thanks to the energy matching of the donor and acceptor levels. [39] Finally, the 1 Figure 10).…”

“…[10] In this context, we have recently shown how some judicious alternation of the polarization of the rigid aromatic rings within the rigid cores may affect DH c , and induce T c to be large enough to ensure the existence of a liquid crystalline state. [14] Interestingly, our rough qualitative approach had some successes in fairly explaining; 1) the high-temperature mesomorphism of the bicatenar ligand L9 (Cr…”

“…[10] Indeed, the family of ligands and complexes derived from L9 and L10 systematically displayed glassy or second-order phase transitions for the melting processes and decomposed quickly at high temperature when entering the isotropic liquid. [14][15][16][17][18][19] These characteristics severely limit any reliable determination of the thermodynamic parameters controlling the phase transitions. Herein, we address this limiting point by designing a novel dodecacatenar ligand L11 and its lanthanide complexes, which are able: 1) to produce entropically-driven low-temperature melting processes, in complete agreement with the predicting model, and; 2) to systematically exhibit first-order phase transitions with easily accessible thermodynamic parameters.…”

Thermotropic Ionic Liquid Crystals

“…In the last decade, significant efforts have been directed toward synthesis of lanthanide mesogenic complexes, which combine luminescence with anisotropy in physico-chemical properties, induced by liquid crystalline molecular order. For most of so far synthesized lanthanide mesogenic complexes, counteranions (monodentate Cl − or bidentate NO 3 − anions) are found in the inner coordination sphere connected with the metal center [113,114]. Consequently, these compounds cannot be considered as true ionic mesogens with the complete separation of positive and negative charged species.…”