Anal. Chem. 1997, 69, 240-248
Synthesis, Spectroscopic Characterization, and
Electro-Optical Properties of Noncentrosymmetric
Azobenzene/Zirconium Phosphonate Multilayer
Films
Dennis G. Hanken, Roberta R. Naujok, Jesse M. Gray, and Robert M. Corn*
Department of Chemistry, University of WisconsinsMadison, 1101 University Avenue, Madison, Wisconsin 53706
noncentrosymmetric material and is described by the electro-optic
coefficient, r.7 Current electro-optical devices are typically based
on the inorganic crystal lithium niobate, LiNbO3, which has an
electro-optic coefficient, r33, of 31 pm/ V.8 Most of the research
on molecular alternatives to LiNbO3 have focused on the incor-
poration of NLO organic chromophores into polymer networks.
These NLO polymers are poled in a strong electric field at elevated
temperatures in order to create a noncentrosymmetric film. The
electro-optic coefficients generated by such films vary from 1 to
55 pm/ V in the presence of the poling fields but can decay
significantly with time.9-14
An alternative to poled polymer films are ultrathin noncen-
trosymmetric organic films that have been created one monolayer
at a time with a self-assembly process. Marks et al. have shown
that noncentrosymmetric NLO films of bifunctional stilbazole
chromophores can be formed by covalent coupling reactions
utilizing an organosilane-based chemistry.15-17 A second method
for creating noncentrosymmetric self-assembled multilayers uses
a zirconium phosphonate (ZP) ligand coupling chemistry first
reported by Mallouk and co-workers for centrosymmetric films18-20
and subsequently modified by Katz et al. to create noncentrosym-
metric multilayers.21-24 Ultrathin noncentrosymmetric films formed
from ZP multilayers with NLO chromophores are more chemically
and thermally stable than most NLO polymer films,22 but spec-
Ultrathin noncentrosymmetric nonlinear optical films
based on zirconium phosphonate (ZP ) self-assembled
multilayers that incorporate the asymmetric azobenzene
chromophore [5-[4-[[4-[(6-hydroxyhexyl)sulfonyl]phenyl]-
azo]phenyl]pentoxy]phosphonic acid (HAP A) are synthe-
sized and constructed. The ZP film structure and multi-
layer deposition chemistry are characterized by a com-
bination of polarization/ modulation Fourier transform
infrared reflection absorption spectroscopy, surface plas-
mon resonance (SP R) measurements, and optical second
harmonic generation (SHG). SP R measurements on the
HAP A multilayer films yield an average monolayer thick-
ness of 2 7 ( 0 .5 Å. The resonant SHG at 3 6 5 nm from
ultrathin HAP A ZP films on silica surfaces increases
quadratically with the number of self-assembled HAP A
monolayers, and an analysis of the polarization depen-
dence of the surface SHG yields an orientation parameter
D ) 0 .7 9 ( 0 .0 3 corresponding to an average tilt angle
of 2 7 ( 2 ° for the azobenzene chromophores with respect
to the surface normal. An electro-optic coefficient r 3 3 for
the HAP A monolayers of 1 1 pm/ V at 6 3 2 .8 nm is
obtained from SP R modulation experiments of ZP films
on gold substrates that have been incorporated into air-
gap capacitors. SP R modulation experiments are then
performed on a HAP A monolayer in an in situ electro-
chemical environment in order to determine that a modu-
lation of (2 5 mV corresponds to a change in the electric
field strength of 1 × 1 0 4 V/ cm within the ultrathin organic
film at the electrode surface.
(7) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984.
(8) Yariv, A.; Yeh, P. Optical Waves in Crystals; Wiley: New York, 1984.
(9) Ahlheim, M.; Barzoukas, M.; Bedworth, P. V.; Blanchard-Desce, M.; Fort,
A.; Hu, Z.-Y.; Marder, S. R.; Perry, J. W.; Runser, C.; Staehelin, M.; Zysset,
B. Science 1 9 9 6 , 271, 335.
(10) Chen, T.-A.; Jen, A. K.-Y.; Cai, Y. J. Am. Chem. Soc. 1 9 9 5 , 117, 7295.
(11) Robello, D. R.; Dao, P. T.; Schildkraut, J. S.; Scozzafava, M.; Urankar, E. J.;
Willand, C. S. Chem. Mater. 1 9 9 5 , 7, 284.
(12) Verbiest, T.; Burland, D. M.; Jurich, M. C.; Lee, V. Y.; Miller, R. D.; Volksen,
W. Science 1 9 9 5 , 268, 1604.
(13) Sekkat, Z.; Kang, C.-S.; Aust, E. F.; Wegner, G.; Knoll, W. Chem. Mater.
1 9 9 5 , 7, 142.
(14) Singer, K. D.; Kuzyk, M. G.; Sohn, J. E. J. Opt. Soc. Am. B 1 9 8 7 , 4, 968.
(15) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am.
Chem. Soc. 1 9 9 0 , 112, 7389.
(16) Yitzchaik, S.; Roscoe, S. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, Z.;
Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1 9 9 3 , 97, 6958.
(17) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.; Marks,
T. J.; Lin, W.; Wong, G. K. Langmuir 1 9 9 3 , 9, 388.
A wide variety of novel organic materials have been designed
and synthesized for use in nonlinear optics.1-3 One important
application of these nonlinear optical (NLO) materials is the
creation of noncentrosymmetric thin organic films for electro-
optical devices.4-6 The electro-optical or Pockel’s effect results
from a change in the index of refraction of a material upon
application of an external electric field; this response requires a
(1) Prasad, P.; Williams, D. J. Introduction to Nonlinear Optical Effects in
Molecules and Polymers; Wiley: New York, 1991.
(2) Marder, S. R., Stucky, G. D., Sohn, J. E., Eds. Materials for Nonlinear Optics:
Chemical Perspectives; ACS Symposium Series 455; American Chemical
Society: Washington, DC, 1991.
(3) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1 9 9 4 , 94, 195.
(4) Zyss, J. Molecular Nonlinear Optics; Academic Press: San Diego, CA, 1994.
(5) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. Rev. 1 9 9 4 , 94, 31.
(6) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steier, W. H.; Ziari, M.; Fetterman,
H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.-Y.; Shea, K. J. Chem. Mater. 1 9 9 5 ,
7, 1060.
(18) Lee, H.; Hong, H. G.; Mallouk, T. E.; Kepley, L. J. J. Am. Chem. Soc. 1 9 8 8 ,
110, 618.
(19) Lee, H.; Mallouk, T. E.; Kepley, L. J.; Hong, H. G.; Akhter, S. J. Phys. Chem.
1 9 8 8 , 92, 2597.
(20) Hong, H. G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1 9 9 1 , 3, 521.
(21) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A.
M.; Emerson, A. B. Langmuir 1 9 9 0 , 6, 1567.
(22) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.;
Chidsey, C. E. D. Science 1 9 9 1 , 254, 1485.
240 Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
S0003-2700(96)00756-1 CCC: $14.00 © 1997 American Chemical Society