Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (4), 594 - 596
Electric Nudge toward Self-organization
In search of materials
with nonlinear optical properties
Window panes and eyeglass lenses that get darker as it gets brighter are especially great in the summer. Their secret lies in optical properties that change as the light intensity changes - nonlinear optical properties. Such materials are also of interest as optoelectronic components. One prerequisite for these unusual optical characteristics is a very orderly microstructure, which can often only be forced upon the materials through expensive techniques. Materials that organize themselves into such microstuctures are therefore quite interesting.
Polymeric organic-inorganic hybrid materials, synthesized by Robert Corriu’s research team, seem to hold a lot of promise. One of the current test subjects in the French laboratory consists of rod-shaped organic units that are highly cross-linked through their inorganic tips of silicon or oxygen. The starting solution of precursor molecules turns into a solid gel within about three days. The chemists also add to the mixture a chromium compound, which attaches itself to the "rods". This sensitizes the "rods" to an electric field; if an electric voltage is applied to the reaction chamber, the gel forms within 24 hours. What is unusual about this gel is that the individual "rods" are all lined up. Even after the voltage is shut off, this order remains intact, a fact discovered by the researchers when they looked at gels that had been stored for two years. Corriu hopes to get interesting nonlinear optical properties out of this new class of materials.
Also striking was the observation that if the voltage is applied to only half of the reaction chamber, an ordered gel forms within a day but only in that half of the chamber. At this point the voltage is turned off. Two days later a gel, which has the same ordered microstructure, has formed in the other half of the chamber. "After cross-linking, the gel restructures as it ages. This step leads to the order," explains Corriu. "The electric field leads to ordered microdomains during the cross-linking reaction. When these drift from one half of the chamber to the other, they act as nucleation points for the self-organization process. For the self-organization to continue, the electric field is no longer necessary."