CMDITR Review of Undergraduate Research - Pluto - University of ...

Hybrid Iridium NLO Chromophore
Robert Snoeberger, University of Washington
Nick Buker, Sanchali Bhattacharjee, and Larry Dalton
Dalton Lab, Dept. of Chemistry, University of Washington
Dipolar chromophores achieve high hyperpolarizabilities but lack loading density and noncentrosymmetric
order while octapolar chromophores achieve high loading densities. These two
types of chromophores are the extreme of polar backbone engineering. Creating a hybrid
chromophore with the desired properties of both types of chromophores could be very desirable
for photonic applications.
Introduction
Organic non-linear optical (NLO)
chromophores are attractive for research due to
their use in electro-optic (EO) devices.
Currently, these devices use lithium niobate,
which is expensive and has a limited bandwidth
potential. 2 The NLO chromophores being
developed consist of a donor, conjugated bridge,
and acceptor. A donor is an electron rich
structure that is able to donate electron density.
The acceptor is an electron deficient structure
that is able to accept electron density. The
conjugated bridge facilitates the transfer of
electron density through the molecule. The
hyperpolarizability (β) describes the ease which
electron density is transferred in the molecule.
This electron transfer when viewed on the
macroscopic scale will alter the speed of light
traveling through a material, which is called the
EO effect. The EO effect can be used to change
the refractive index of the EO material, which
allows the control of incident light for
modulation of optical signals.
Achieving materials with a greater EO effect
requires the maximization of β, loading density,
and non-centrosymmetric order. A substantial
amount of EO effect is lost by centrosymmetric
order due to aggregation and strong
intermolecular forces. 3 One of the main
challenges faced by researchers is aligning the
chromophores in the host polymer to generate a
net dipole. The traditional method for alignment
uses a strong electric field that orients the
chromophores with the field lines. The problem
has been that electro-static interactions due to the
strong chromophore dipoles cause them to
arrange in a centrosymmetric arrangement which
eliminates a significant amount of overall dipole
for the EO material. Increasing loading density,
which is required to maximize device operation,
also exaggerates alignment problems by forcing
the chromophores in a closer proximity that
increases electro-static forces between
molecules.
A method for achieving alignment and
greater loading density is chromophore site
isolation. Preventing the chromophores from
interacting together eliminates both aggregation
and centrosymmetric order. Site isolation has
been performed using bulky side chains, which
are attached to the chromophore in such a way
that the planar property is disrupted which
interferes with π-stacking. The bulky groups also
minimize intermolecular dipole forces, which
cause the centrosymmetric order, by maximizing
the distance between chromophores in the
polymer. Another method has been the
engineering of different chromophore backbones.
This has been investigated using octupolar
chromophores which do not have a permanent
dipole and are symmetrically shaped which
eliminates unfavorable intermolecular ordering
and increases loading density. 1,4
Octupolar chromophores could be an
effective alignment solution but they suffer from
low β values and they require octupolar order. 1,4
Creating a hybrid octupolar/dipolar chromophore
could allow the chromophore to be ordered using
poling methods. Engineering a chromophore
with the backbone structure of an octupolar
chromophore with a small permanent dipole
allowing for electric field poling could be a
solution for obtaining higher poling efficiency
and loading density.
54 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004