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

Feasibility of Hydroxy-Chalcone Linker in Dendrimer
Photocrosslinking: A Study of Chromophore Stability under
High-intensity Ultraviolet Light
Genette I. McGrew, University of Southern California
Andrew Akelaitis, Phil Sullivan, and Larry Dalton
Dalton Lab, Dept. of Chemistry, University of Washington
Introduction
Recently, there has been a great deal of
interest in organic molecules with large
hyperpolarizabilities for their versatility, low
cost, and ultra-high bandwidth potential as
components in optical devices,
telecommunication, and phased array radar, to
name a few. These Non Linear Optical materials
(NLOs) are constantly being optimized for high
molecular first hyperpolarizability (β) and bulk
electro-optic activity (expressed as r 33 ).
One promising design for NLOs is the
dendrimer, in which multiple arms of NLO
color-compounds (called chromophores) are
insulated with bulky functional groups and
attached to a central core. These large and
generally spheroid molecules tend to have better
r 33 s when doped into a polymer because the
chromophore component of the molecule (a
strong dipole) does not have the opportunity to
pack as tightly and interact with others in the
same manner as a free chromophore. When
aligned with an electric field (poled), dendrimers
also relax from their aligned state slower than
free chromophores.
However, the dendrimers do not stay aligned
indefinitely; to preserve orientation after poling,
the arms of the dendrimer must somehow be
locked into place, which is where crosslinking
comes into play. The bulky functional groups on
the arms can be structures which link together
under either high temperature or light. Compared
with thermal crosslinking, photo-crosslinking is
a much less efficient process. However, thermal
crosslinking can compromise the integrity of the
molecule by causing decomposition or polymer
shrinkage. 1 Some members of the Dalton lab are
exploring the feasibility of using photocrosslinking
in dendrimers.
However, in initial compatibility tests of
different photo-crosslinking agents by secondyear
graduate students, Andrew Akelaitis and
Philip Sullivan, the absorbance of a spin-coated
film of free crosslinker and EZ-FTC (a popular
chromophore for its convenience , ease of
synthesis, and stability) decreased immensely
after exposure to UV light. Especially beside a
hydroxychalcone linker, the characteristic
chromophore absorbance peak almost
disappeared. This implied that the chromophore
was decomposing under UV, but still left open
the question of whether it was chiefly due to
inherent instability in the chromophore itself,
radical oxygen, or the photo-crosslinker, and if it
was the crosslinker, what part of the
chromophore was being attacked. One theory
was that the photo-crosslinker attacked the cyano
groups in the tricyanofuran (TCF) acceptor, in
which case the damage caused by photocrosslinking
would be hard to overcome.
Sullivan and Akelaitis theorized, however, that it
was the trans double bonds connecting the rings
(ethylene linkages) which were being attacked
by the radicals. If that was the case, a
chromophore lacking ethylene linkers would
hold up to photo-crosslinking processes
significantly better than chromophores with
ethylene linkages, and dendrimers made of these
ethylene-lacking chromophores could be further
pursued.
Hydroxy-chalcone was chosen as the test
photo-crosslinker because it was the ‘harshest’ of
the crosslinkers, causing the most degradation in
EZ-FTC. Stability despite the hydroxyl-chalcone
would imply a greater stability under other
photo-crosslinkers. Each chalcone contains an
α,β-unsaturated carbonyl group, which dimerizes
under UV light in a 2π + 2π cyclo-addition 2
(Figure 1).
Figure 1.
HO
HO
C
H
H
C
C C
H
C
H
O
H
C H O
C C
C C
H
C
H
O
O
C
OH
OH
CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004 43