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

Hyper-Rayleigh Scattering Studies of Three Chromophores
Containing a Novel Pyrroline-Based Acceptor
Field Cady, Stanford University
David Lao, Kim Firestone, Yi Liao, Bruce Eichinger, Bruce Robinson, Phil Reid and Larry Dalton
Dalton and Robinson Labs, Dept. of Chemistry, University of Washington
Introduction
In recent years, materials with large nonlinear
polarization responses have shown
increasing promise as the material basis for a
new generation of information technologies.
Lithium niobate, currently the leading such
material, suffers from several defects, including
slow switching time. But charge-transfer
chromophores have the potential to be
significantly faster, as well as more durable and
cheaper. An important part of such
chromophores is the “acceptor,” a region of the
molecule capable of accepting extra electron
density.
Recently a new acceptor complex, a
derivative of 3-cyano-2-(dicyanomethylene)-4,5-
dihydrocarbyl-5-hydroxy-3-pyrrolines known as
TCHP, has been synthesized by Dr. Yi Liao and
incorporated into a series of three chromophores
analogous to three other well-understood
chromophores containing a TCF acceptor.
Hyper-Rayleigh scattering was used to evaluate
the strength of the new acceptor relative to TCF
by comparing the first hyperpolarizability (β)
values of the new chromophores both to
theoretical calculations and to K. Firestone’s
experimental β values for the analogous
chromophores. With the exception of a
chromophore in which significant levels of
impurities are believed to have been present, the
tests indicated that TCHP was slightly stronger
than TCF. Coupled with its other potential
advantages, including ease of synthesis and blueshifting,
these tests indicate that TCHP could be
a useful acceptor in future photonics studies.
Experiment
One of the most successful methods of
measuring β is hyper-Rayleigh scattering (HRS).
In HRS, two photons of an incident frequency ω
are destroyed and a single photon of half the
wavelength is created, moving in a direction
independent of the original photons (scattering).
The intensity of this hyper-Rayleigh scattering is
proportional to the square of the β value of the
sample. Theory has provided us with the
following relation for a solvated chromophore:
By
determining the intensity of the HRS signal from
both a solvated chromophore and the pure
solvent (we used chloroform, which is the
standard solvent.), one can calculate
In light of
disagreement within the literature on the exact
value of β chloroform only β rel is reported in this
abstract. The tests were performed with a 500
mW Ti-Sapphire laser producing 880 nm light
which was sent through a flow cell containing
the flowing solvated chromophore. As will be
seen in the discussion of absorbance, 880nm is
sufficiently far from the frequency band of the
chromophore that resonance enhancement of the
β values would be minimized, though not trivial.
For further details of the experimental
apparatus, see Firestone, K.A.; Bale, D.H.;
Westphal, J.B.; Scott, D.C.; Reid, P.J. & Dalton,
L.R. “Frequency Agile Hyper-Rayleigh
Scattering Studies of Non-Linear Optical
Chromophores.” Polymeric Materials: Science
and Engineering, 2003, 88, 294-295, which used
the apparatus on which the experiments in this
paper were performed.
The Chromophores
The series of new chromophores is
analogous to three benchmark chromophores.
Figure 1 shows the structures of the new
chromophores alongside their counterparts. As
can be seen, YLII-37 and YLII-18 they are
structurally identical to their analogs (EZ-FTC
and SHJ-1, respectively), except that the YL
chromophores have TCHP in place of TCF. In
the case of YLII-39 the benchmark chromophore
(CLD-5) has hydroxyl groups on the donor in
place of a metal-oxide group and has an extra
alkyl chain coming off the bridge, but theory
22 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004