Bent Tridentate Receptors in Calamitic Mesophases with Predetermined Photophysical Properties:  New Luminescent Lanthanide-Containing Materials

Abstract

IntroductionThe recent recognition of the importance of metallomesogens (i.e., liquid crystals containing metal ions) for the development of advanced materials 1 with new electronic, optical, and magnetic properties 2 has encouraged coordination chemists to design new lipophilic rodlike or disklike complexes exhibiting mesomorphic behavior. 3 Liquid crystalline (or mesomorphic) properties result from phases in which the molecular order is intermediate between that of an ordered solid crystal and a disordered liquid. A thermotropic mesophase is formed by the influence of temperature, and the resulting thermotropic liquid crystals are divided in two main categories depending on their molecular axial anisotropies: calamitic (rodlike) and discotic (disklike). 3 A large variety of bidentate chelating units has been successfully used for the design of metallomesogens, 3 but it has been shown that metal complexes with thermotropic calamitic 5 and discotic 6 mesogenic ligands containing linear bidentate 2,2′-bipyridine units 4 display only poor 7 or no 8 mesogenic behavior. The origin of these failures has been tentatively attributed (i) to the large increase of the electric dipole moment which occurs upon complexation 8 and (ii) to the limited anisometry of the rigid core, although this has been recently overcome in extended systems. 9 The closely related tridentate receptor 2,2′:6′,2′′-terpyridine is essentially ignored in this field, 3 as are other tridentate binding units. 10 To the best of our knowledge, a single . Particular attention has been focused on structure-properties relationships, which can be modulated by the size of the lanthanide metal ions.

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

“…The synthon 2,6-bis-(1-ethyl-5-methoxymethyl-benzimidazol-2-yl)pyridine (1) was prepared according to a literature procedure. [6] The nitrate salts Ln(NO 3 ) 3 ·x H 2 O (Ln = LaLu, Y, x = 1-4) were prepared from the corresponding oxides (Rhodia, 99.99 %). [32] The Ln content of solid salts was determined by complexometric titrations with Titriplex III (Merck) in the presence of urotropine and xylene orange.…”

“…[5] For instance, the mesogenic ligand L 1 !I) lose their mesomorphism upon complexation of the central tridentate 2,6-bis(benzimidazol-2-yl)pyridine to LnA C H T U N G T R E N N U N G (NO 3 ) 3 . [6] Among the rare successes obtained in designing lanthanidomesogens, that is, lanthanide-containing mesogens, [2c] three main strategies can be recognized.…”

“…Cn (n=0, 1, 12): A mild and selective KMnO 4 oxidation performed under catalytic phase-transfer conditions, [15] followed by alkaline hydrolysis transform the starting tridentate unit 2,6-bis(1-ethyl-5-methoxymethylbenzimidazol-2-yl)pyridine (1) [6] into 2,6-bis(1-ethyl-5-carboxybenzimidazol-2-yl)pyridine (3) (Scheme 2). The preparation of the air-sensitive electron-rich substituted tetraphenol partner 6 is more delicate and requires a careful Grignard reduction of the gallic ester 4, [9d] followed by a Baeyer-Villiger type oxidative rearrangement, eventually leading to the target tetraphenol 6.…”

“…[9d] We ascribe the dramatic decrease of the enthalpic driving force of the dime- 6 ], to the coplanar orientation of the carbonyl groups of the ester spacers with the benzimidazole rings ( Figure 4b and Figure S5c, Supporting Information), which hinders the approach of the second complex when forming the double nitrate bridge in the dimer.…”

“…These characteristics dominate the experimental electronic emission spectra of both ligands with E( 1 pp*) % 25 000 cm À1 (0-0 phonon band) and E( 3 pp*) % 21 000 cm À1 (0-0 phonon band) for the lowest emitting excited states, which are mainly centered on the tridentate 2,6-bis(benzimidazol-2-yl)pyridine unit (Table 5, Figure 7b,c). [6,26] [30] [c] 3 pp* luminescence quenched by transfer to Ln ion. [d] The weak luminescence intensity prevented determination of reliable luminescence decay times.…”