Metallo-Responsive Liquid Crystalline Monomers and Polymers

Abstract

b S Supporting Information ' INTRODUCTIONSmall molecule and polymeric liquid crystalline materials have applications in a multitude of areas, ranging from displays, optical/ electronic imaging, data storage, and stress/temperature sensing to chemical/fire resistance and artificial muscle actuation. 1 The design of typical molecular architectures that facilitate thermotropic liquid crystalline behavior is well documented. Molecules with structural anisotropy, i.e., a rigid moiety with high aspect ratio are frequently utilized to impart the order inherent in a liquid crystalline phase. Functionalization of these cores with flexible alkane chains of varying length then provides the structural complexity necessary to hinder crystallization while preserving liquid crystalline order. As such, thermotropic liquid crystals are inherently stimuli-responsive, as they can undergo a range of thermally induced transitions between glassy/crystalline, liquid crystalline, and isotropic states. Examples of thermotropic liquid crystalline small molecules or polymers that are also designed to respond to specific chemical stimuli are less prevalent. 2 One approach to designing a chemo-responsive liquid crystal involves incorporating a metal-binding ligand into the mesogenic core. In addition to introducing additional, nonthermal, stimuliresponsive behavior incorporation of metal ions into liquid crystalline materials offers the opportunity to impart a number of additional properties upon these systems. Chemical sensing, 3 catalysis, 4 and a host of biological applications 5 highlight the many practical properties of organic metal-coordinating species, while some specific lanthanide complexes also exhibit interesting luminescent and paramagnetic properties. 6 Additionally, the vast number of transition and lanthanide metal ions offers a multitude of potential geometries and binding motifs, which can be utilized to design the specific shapes necessary to retain, induce, or otherwise impact liquid crystallinity. This approach enhances the traditional thermotropic liquid crystal in two ways. First, ligands can be envisioned that, upon binding a metal ion, lose their original liquid crystalline order and thus offer a useful method of amplification for detection schemes and other applications. 7 Second, ligands that bind metal ions and retain, change, or acquire liquid crystalline order (metallomesogens) can also be envisaged. 8 The 2,6-bisbenzimidazolylpyridine (Bip) ligand offers a versatile scaffold from which highly functionalized derivatives can be accessed (e.g., 1 Et , Figure 1). 9,10 Piguet and co-workers have prepared a number of low molecular weight mesogenic Bip derivatives and investigated the effects of metal binding on their liquid crystalline properties. 11 Noting that the incorporation of a large metal center disrupts the organization of simpler Bip mesogens, 12 they decorated the Bip core with a highly alkylated liquid crystallinity-enhancing moiety in the 5 0 positions, which allowed access to a range of metallomeso...

Perfluorinated Aromatic Spacers for Sensitizing Europium(III) Centers in Dinuclear Oligomers: Better than the Best by Chemical Design?

“…[2] A statistical mechanics (Ising model) analysis suggests that standard repulsive nearest neighbor intermetallic interactions operating in linear polymers with regularly spaced binding sites should provide the targeted ordered …-Ln 1 -Ln 2 -Ln 1 -Ln 2 -… microstates. [3,4] Pioneering work in this field has relied on the bulk electropolymerization of didentate 1,10-phenanthroline with thienyl spacers [5] and the acyclic diene metathesis of tridentate 2,6-bis(benzimidazol-2-yl)pyridine, [6] followed by reaction with Eu(β-diketonate) 3 or Eu(NO 3 ) 3 to yield red-emitting metallopolymers. A reliable exploitation of this concept for the development of luminescent materials, however, requires the efficient sensitization of the luminophore via the rational optimization of each photophysical step by using chemical tools.…”

Reversible cross-linking polymer-based self-healing materials

“…Nowadays, many research teams concentrate their attention on exploiting metal-ligand coordination to construct metallosupramolecular polymers [48][49][50][51] with reversibility in response to external stimulus, such as redox or light.…”

Perfluorinated Aromatic Spacers for Sensitizing Europium(III) Centers in Dinuclear Oligomers: Better than the Best by Chemical Design?

“…[2] A statistical mechanics (Ising model) analysis suggests that standard repulsive nearest neighbor intermetallic interactions operating in linear polymers with regularly spaced binding sites should provide the targeted ordered …-Ln 1 -Ln 2 -Ln 1 -Ln 2 -… microstates. [3,4] Pioneering work in this field has relied on the bulk electropolymerization of didentate 1,10-phenanthroline with thienyl spacers, [5] and the acyclic diene metathesis of tridentate 2,6-bis(benzimidazol-2-yl)pyridine, [6] followed by reaction with [Eu(b-diketonate) 3 ] or Eu(NO 3 ) 3 to yield red-emitting metallopolymers. A reliable exploitation of this concept for the development of luminescent materials, however, requires the efficient sensitization of the luminophore through the rational optimization of each photophysical step by using chemical tools.As a first step toward this goal, the rigid segmental ligand strands L1-L3, made of two tridentate binding units separated by a rigid and electronically tunable aromatic spacer, have been treated with trivalent europium to give the dinuclear complexes [Eu 2 (L)(hfac) 6 ] (Scheme 1; hfac = hexafluoroacetonato).…”

“…As a first step toward this goal, the rigid segmental ligand strands L1-L3, made of two tridentate binding units separated by a rigid and electronically tunable aromatic spacer, have been treated with trivalent europium to give the dinuclear complexes [Eu 2 (L)(hfac) 6 ] (Scheme 1; hfac = hexafluoroacetonato). [7] The use of a simple method for deciphering the various contributions to the sensitization mechanism clearly showed that [Eu 2 (L3)(hfac) 6 ] had the largest global emission quantum yield [F L Eu = 0.206 (7), Eqs.…”

“…[8] Correspondingly, the expected decrease in the k F nr and k Eu nr values, and of the so-called p-conjugation length A p [Eq. (5)] [9] in [Eu 2 (L4)(hfac) 6 ], [7] should optimize both the intersystem crossing efficiency [h ISC ¼0.6(1), Eq. (2)] and the intrinsic Eu-centered quantum yield [F Eu Eu ¼0.76(2), Eq.…”