Angewandte Chemie International Edition 2007, 46, 8804–8807
Tailored for Optical Applications
Coordination polymers as materials with very high birefringence
Contact: Daniel B. Leznoff, Simon Fraser University (Canada)
Registered journalists may download the original article here:
Highly Birefringent Materials Designed Using Coordination-Polymer Synthetic Methodology
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