Metallo-Responsive Liquid Crystalline Monomers and Polymers

Small 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. 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. 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, catalysis, and a host of biological applications highlight themany practical properties of organic metal-coordinating species, while some specific lanthanide complexes also exhibit interesting luminescent and paramagnetic properties. 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 ametal ion, lose their original liquid crystalline order and thus offer a useful method of amplification for detection schemes and other applications. Second, ligands that bindmetal ions and retain, change, or acquire liquid crystalline order (metallomesogens) can also be envisaged. The 2,6-bisbenzimidazolylpyridine (Bip) ligand offers a versatile scaffold fromwhich highly functionalized derivatives can be accessed (e.g., 1, Figure 1). 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. Noting that the incorporation of a large metal center disrupts the organization of simpler Bip mesogens, they decorated the Bip core with a highly alkylated liquid crystallinity-enhancing moiety in the 50 positions, which allowed access to a range of metallomesogens. Upon complexation with a variety of Ln(NO3)3 salts, this Bip derivative displayed columnar or cubic mesomorphism at elevated temperatures. Further functionalization with the highly alkylated moiety at the N 0 positions increased the geometric screening of the bulky metal complex, facilitating access to room-temperature columnar metallomesogens. Thus, they have presented a room