Coordination polymers as materials with very high
birefringence.
When a calcite crystal is placed onto a printed page, the letters appear
doubled. This is the result of a property called birefringence.
Scientists at the Simon Fraser University in Canada have now developed a
material that is among the most birefringent solids ever observed. As
described in the journal Angewandte Chemie, this material is not a
mineral, but rather a coordination polymer.
Refraction is the change in direction of a wave of light when it
passes from air into water or a crystal. It is caused by a local
change in the speed of propagation. In the case of birefringence, the
light is divided into two perpendicularly polarized beams, which move
at different speeds and exit the material shifted relative to each
other. The source of this effect is a crystal lattice that has
different optical properties along its various axes (anisotropy).
Birefringent optical components are usually made of calcite. The
critical value for these applications is the difference in the
refractive index of light in two directions in the crystal, the
birefringence, which is 0.17 for calcite.
The team led by Daniel B. Leznoff and Zuo-Guang Ye has now produced a
highly birefringent coordination polymer. Coordination polymers are
one-, two-, or three-dimensional bridged metal complexes. The
advantage to this type of compound is the limitless number of design
possibilities: The individual components - metal center, chelating
ligands, and bridging ligands - can be selected
and combined almost at will to get the desired material properties.
Leznoff�s team, spearheaded in the lab by Michael J. Katz, decided to
use a �terpy� ligand, a flat ring system consisting of three pyridine
units (six-membered aromatic rings with one nitrogen atom), and lead
as the metal center. The complexes are linked by linear bridging
ligands made of a central silver or gold ion and two cyanide groups to
form two-dimensional layers. If the central lead atom is replaced with
manganese, one-dimensional ladder-like structures are formed. Within
their crystals, however, the lead and manganese polymers have
analogous arrangements: the terpy molecules are piled up
plane-to-plane, perpendicular to the axis of crystal growth. This is
clearly the crucial factor leading to the high birefringence, which
reaches values from 0.43 to just under 0.4, significantly higher than
those of the numerous inorganic birefringent materials.
Improved optical data storage and data transfer in communications
technology are possible applications for such highly birefringent
materials.
Further Information and Source:
-
Michael J. Katz, Harini Kaluarachchi, Raymond J. Batchelor, Dr.,
Alexei A. Bokov, Dr., Zuo-Guang Ye, Dr., Daniel B. Leznoff, Dr.: Highly Birefringent Materials Designed Using Coordination Polymer
Synthetic Methodology.
In: Angewandte Chemie International Edition 2007, 46,
8804�8807.
doi: 10.1002/anie.200702885.
