CHEMICAL & ENGINEERING
NEWS OF THE WEEK JULY 1 4, 2003
EDITED BY WILLIAM Q. SCHULZ & MELISSA BRADDOCK
ADVANCED
MATERIALS
TWO-FACED LIQUID CRYSTALS Supramolecular materials found to self-organize like proteins
A
JANUS Liquid-crystal architecture incorporates cyanobiphenyl (blue) and chiral phenylbenzoate (yellow) liquid-crystalforming moieties. 10
C & E N / JULY U ,
2003
N E W CLASS OF
LIQUID
crystals with two chemically different halves could lead to the development of selfordering nano- and mesoscopic engineered materials with novel functional properties, suggest chemists at the University of Hull, in England. The compounds from which the new materials form are dendritic hexamers consisting of two Hquid-crystal-forming (mesogenic) units separated by a central scaffold. They were designed, synthesized, and characterized by senior research fellow Isabel M. Saez and John W Goodby professor of organic chemistry and head of the Hull liquid-crystal group [Chem. Commun., 2003,1726]. "Our new compounds selectively reflect light and could be used in filters and coatings," Goodby tells C&EN. "However, more importantly they open the door to the development of materials with proteinlike properties in that they self-organize and can have smart built-in functionality They are on the way toward becoming molecular machines." The liquid crystals have Janus structures, a term that refers to materials with two faces, such as hydrophobic and hydrophilic faces or fluorocarbon and hydrocarbon faces. "The system we chose has chiral and achiral faces," Goodby explains. "One part carries three cyanobiphenyl mesogenic moieties that form smectic [layered}
liquid crystals, and the other carries three chiral phenylbenzoate mesogenic moieties that form chiral nematic [nonlayered] phases." T h e central scaffold t h a t links the two parts is constructed from subunits based on pentaerythritol and tris(hydroxymethyl)aminomethane. It was designed to incorporate different end groups—olefin and tertbutyl ester—that can be fiinctionalized independently of one another with different mesogenic moieties. Saez and Goodby used differential scanning calorimetry and thermal polarized-light optical microscopy to examine the mesomorphic properties of their supramolecular materials. The investigations reveal that the materials exhibit a chiral nematic phase at temperatures above 36.1 °Cand a chiral smectic phase when cooled below this temperature. 'Although our giant material has a molecular weight in excess of 4 , 0 0 0 daltons, it has a low melting point and a wider liquidcrystal temperature range than pentyl-4-cyanobiphenyl—a revolutionary liquid-crystal material that was originally used in digital watches and changed the world of displays," Saez observes. Saez and Goodby point out that dendritic liquid crystals combine the ability of discrete lowmolecular-weight liquid crystals to self-organize with the ability of polymers to form secondary and tertiary structures.
"Originally we were researching in the area of liquid crystals with large discrete molecular structures," Goodby says. (cWe worked mosdy with dendrimers, with the objective ofcreating optical niters and selective reflectors. "Even though we made materials of over 14,000 Da, we still got mesophases existing from room temperature to about 80 °C," Goodby continues. "In their chiral nematic states, the materials have a helical macrostructure that acts like a diffraction grating and selectively reflects
SELF-ORGANIZED Goodby (left) and Saez examined a new class of liquid-crystal compounds. light just like a compact disc does." The group incorporated octasilsesquioxane units into the compounds in order to make hybrid inorganic-organic systems that could be developed for photonic band-gap materials. The development of supramolecular liquid crystals that self-organize into materials with desirable chemical functionality and physical properties at nano- and mesoscopic length scales is an exciting "bottom-up" approach to the design and synthesis of advanced functional materials, Saez and Goodby conclude.—MICHAEL FREEMANTLE HTTP://WWW.CEN-ONLINE.ORG
