Effect of the Aggregation of 4-[5-(4-Hydroxyphenyl)-3-oxopenta- 1,4-dienyl]benzoic Acid and Its Potassium Salt in a Polymeric Matrix on the Third-Order Nonlinear Optical Susceptibility of Polymer-Chromophore Composites

Article abstract of DOI:10.1134/S1070427212090194

Aggregation of 4-[5-(4-hydroxyphenyl)-3-oxopenta-1,4-dienyl]benzoic acid (chromophore) and its potassium salt in optically neutral polymeric matrices was studied. The dependence of the third-order nonlinear optical properties of the chromophore-polymer composites on the aggregate size was determined.

Full text of DOI:10.1134/S1070427212090194

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 9, pp. 1422−1427. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.O. Savitsky, L.V. Vinogradova, V.A. Lukoshkin, A.E. Bursian, A.V. Ten’kovtsev, 2012, published in Zhurnal Prikladnoi Khimii, 2012,
Vol. 85, No. 9, pp. 1511−1516.
MACROMOLECULAR COMPOUNDS
AND POLYMERIC MATERIALS
Effect of the Aggregation of 4-[5-(4-Hydroxyphenyl)-3-oxopenta-
1,4-dienyl]benzoic Acid and Its Potassium Salt
in a Polymeric Matrix on the Third-Order Nonlinear Optical
Susceptibility of Polymer–Chromophore Composites
A. O. Savitskya, L. V. Vinogradovaa, V. A. Lukoshkinb, A. E. Bursiana,
and A. V. Ten’kovtseva
a Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
e-mail: vinogradovaLV@rambler.ru
b Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, Russia
Received June 14, 2012
Abstract—Aggregation of 4-[5-(4-hydroxyphenyl)-3-oxopenta-1,4-dienyl]benzoic acid (chromophore) and its
potassium salt in optically neutral polymeric matrices was studied. The dependence of the third-order nonlinear
optical properties of the chromophore–polymer composites on the aggregate size was determined.
DOI: 10.1134/S1070427212090194
The development and improvement of telecommuni-
cation methods and the development of optical systems
for processing and control of light beams require new
materials with high values of the third-order optical
susceptibility, low optical loss, and short response time.
Among the developing fields of science and engineer-
ing that use light (photons) for information processing,
electronics and photonics have particular demand for
optical switches. Therefore, search for and study of new
organic, in particular, polymeric materials capable for
nonlinear interaction with electromagnetic radiation is
of large interest.
supramolecular structures (J-aggregates) [3–6], but such
materials do not have sufficient stability of properties in
the course of prolonged storage [7–9].
This study deals with the effect of the chromophore
component aggregation on the third-order nonlinear
optical properties of chromophore–polymer compounds
under the conditions ensuring predominant interactions
between the NLO-active molecules. For this purpose,
we prepared systems with 4-[5-(4-hydroxyphenyl)-3-
oxopenta-1,4-dienyl]benzoic acid as chromophore and
optically neutral polymers [polyethylene glycol (PEG),
polymethyl methacrylate (PMMA), poly-N-vinylpyrrol-
idone (PVP)] as polymeric matrices.
The most widely used method for preparing nonlinear-
optical (NLO) materials is dispersing chromophores in a
polymer matrix. In this approach to preparing NLO com-
posites, the role of intermolecular interactions and order-
ing practically was not taken into account until recently,
despite the fact that these factors form the basis for the use
of nonlinear optical methods for studying the structure of
biological and polymeric objects [1, 2]. The effect of ag-
gregation on variation of third-order NLO properties was
considered only for a small group of cyanine dyes forming
EXPERIMENTAL
The chromophore was synthesized by two-step aldol
condensation. In the first step, by the reaction of 4-hy-
droxybenzaldehyde with acetone, we prepared 4-(4-hy-
droxyphenyl)but-3-en-2-one [10], which in the second
step reacted with the excess of 4-carboxybenzaldehyde to
form the chromophore. The potassium salt of the chromo-
1422

EFFECT OF THE AGGREGATION
1423
Scheme.
CHO
OH
O
OH _
O
+
50oC, 30 min
HO
O
O
CHO
_
EtOH, OH
+
50oC, 30 min
HO
COOH
HO
Chromophore
O
COOH
O
KOH
EtOH
_ O
COO _ K+
K+
HO
COOH
Chromophore potassium salt
phore was prepared by the reaction of equimolar amounts
of the chromophore and KОН in ethanol, followed by
removal of the solvent (see the scheme).
of spatial π–π interaction of the adjacent molecules.
The intermolecular π–π interactions of the chromo-
phore in PMMAand PEG matrices (in films) were studied
by UV spectroscopy, which is sensitive to changes in the
electronic states of molecules. The spectra were recorded
in the wavelength range λ = 290–650 nm. The electronic
spectrum of the chromophore in the PMMA matrix con-
tained two overlapping bands with the maxima at λ = 310
and 360 nm (Fig. 1). With PEG of different molecular
weights (М = 2 × 103, 12 × 103 and 35 × 103) used as
the matrix, the pattern did not change essentially, but an
The film samples were prepared by casting from solu-
tions of the components in a common solvent onto glass
supports [cover glass, 18 × 18, GOST (State Standard)
6672–75], followed by solvent evaporation. Films of
composites made of chromophore dispersed in PMMA
matrix were cast from a tetrahydrofuran solution, and
those of chromophore in PEG matrix and chromophore
potassium salt in PVP matrix were cast from an ethanol
solution. Prior to tests, the films were dried to remove re-
sidual solvent in a vacuum (5 mm Hg) to constant weight.
The microstructure of the films was analyzed with a
Biolam microscope (Leningrad Optical and Mechanical
Association) using MI-90-1.25 and 40-0.65 objectives
lenses in transmitted polarized light. The crystallite di-
mensions were determined with ImageTool 3.0 program
(University of Texas). From the data on size distribution
of crystallites, we estimated their mean radius <R> (μm).
It is known that the most important characteristics of
the chemical structure of organic molecules, determining
their NLO properties, are the length of the conjugated
fragment and the electronic effects of substituents in
this fragment [11, 12]. The molecule of the chromophore
under investigation contains polar functional groups fa-
voring aggregation of molecules via hydrogen bonding
[13]. It has also an extended π-electron system and the
geometry close to planar, which increases the probability
Fig. 1. (1) UV spectrum of the chromophore (1.4 wt %) in
PMMA film and (2) result of the band deconvolution. (D) Op-
tical density and (λ) wavelength; the same for Figs. 2 and 3.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 9 2012

1424
SAVITSKY et al.
(a)
(b)
Fig. 3. UV spectra of the chromophore in the PEG matrix (М =
2 × 103), recorded in the course of crystallization. Crystalliza-
tion time: (1) 10 min and (2) 1 h; the intermediate curves were
obtained after the first measurement at 10-min intervals.
(c)
absorption bands.
A number of methods are used for estimating third-
order nonlinear optical coefficients χ(3): degenerate four-
wave mixing [14], Z-scan technique [15], third-harmonic
generation (THG) [16], etc. In this study, to examine the
NLO properties of the objects under consideration, we
used the THG method, which allows estimation of only
the electronic contribution to χ(3) and is very efficient and
simple in implementation. The harmonic was excited in
the samples by a YAG : Nd3+ pulse laser with the emis-
sion wavelength λ = 1064 nm.
Fig. 2. UV spectra of the chromophore in PEG matrix (М = 2 ×
103). Chromophore concentration, wt %: (a) 1.4, (b) 6, and (c) 8.
From concentrated (10 wt %) solutions of the com-
ponents in a common solvent, we prepared samples in
the form of thin (0.5–5 μm) films on a glass support.
The examined polymeric matrices themselves (PMMA,
PEG, PVP) exhibited no capability for THG under the
experimental conditions and showed no absorption in the
region of two- and three-photon resonance of the laser
radiation (532 and 355 nm, respectively). These allowed
the matrices to be considered as optically inert media.
increase in the chromophore concentration caused redis-
tribution of the relative intensities of these bands (Fig. 2).
Presumably, the concentration dependence reflects the
equilibrium of, at least, two types of supramolecular
formations of the chromophore with higher (λ = 310 nm)
and lower (λ = 360 nm) degrees of aggregation.
Similar conclusion can be made from the kinetics
of variation of the spectral characteristics of the chro-
mophore incorporated in PEG matrices (Fig. 3). Rapid
evaporation of the solvent in the course of film preparation
ensured “freezing” of the polymer chains occurring in the
amorphous state. This was followed by crystallization of
the PEG matrix, accompanied by diffusion of chromo-
phore molecules from crystalline to amorphous domains,
due to thermodynamic incompatibility of high- and low-
molecular-weight compounds. An increase in the local
concentration of the chromophore led to enhancement
of its aggregation, which was manifested as a change
in the relative intensities of the long- and short-wave
Because of crystallization of the PEG matrix, the poly-
mer–chromophore composite films were insufficiently
transparent and did not meet the requirements of THG
measurements.
In PMMA films containing the chromophore in
amounts of 1–16 wt %, the coefficient χ(3) was of the
order of 10–14 CGS units, which is close to that of the
standard, fused silica (3.11 × 10–14 CGS units [17]), but is
considerably lower than that of cyanine dyes immobilized
in a polymeric matrix (~10–11 CGS units [5]). In the films
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 9 2012

EFFECT OF THE AGGREGATION
1425
under consideration, the chromophore was in the neutral
form. It should be noted that the chromophore molecule
has a cross-conjugated structure in which both aromatic
fragments, phenolic and acid moieties, are conjugated
with the ketone С=О bond. There is no direct conjuga-
tion between the aromatic fragments, because, because
they are separated by two σ bonds at the keto group.
Delocalization of the negative charge of the phenolate
anion upon chromophore ionization leads to the forma-
tion of the quinoid form with classical conjugation. The
structure becomes more planar relative to the un-ionized
form, and the conjugated π-electron system covers the
whole molecule:
R, μm
O
Fig. 4. Distribution of crystallites of the chromophore potassium
salt (12 wt % in PVP film) with respect to size R.
O
O
3.2 μm depending on the chromophore salt concentration,
at relatively narrow size distribution (see table).
O
Presumably, due to the planar structure of the quinoid
form of the chromophore and its strong tendency to ag-
gregation the formation of supramolecular structures via
π–π interactions should give rise to cooperative effects
that depend on the aggregate size. To find such depen-
dence, by varying the concentration of the chromophore
component and the film thickness, we obtained samples
with the same optical density, D ~0.5 at λ = 460 nm, in
the maximum of the absorption band of the chromophore
ionic form in the solid phase (see table). For the films
studied, the Beer law was observed:
O
O
O
O
It is known that the conjugation length in chromophore
molecules strongly affects third-order NLO effects [12].
Taking these data into account, we used the ionic form
of the chromophore in the further studies.
D = kcl = knl/V,
In films containing the chromophore in the anionic
form (chromophore potassium salt in the PVP matrix),
we observed a sharp increase (by almost three orders of
magnitude) in the efficiency of generation of the third
harmonic of the laser radiation, upto χ(3) ~10–11 CGS
units, at the weight content of the chromophore compo-
nent similar to that of the un-ionized chromophore in the
PMMA matrix.
where k is the absorption coefficient of the NLO com-
ponent, с is the concentration of the NLO component, l
is the film thickness, n is the number of particles, and V
is the volume.
The equality of the optical densities D of the films
means that the samples contain approximately equal
amounts of the absorbing chromophore species, despite
differences in the chromophore concentration and film
thickness. As seen from the table, “equally absorbing”
films differed in the THG efficiency χ(3), which may be
due not only to concentration effects, but also to intermo-
lecular interactions of NLO-active species (formation of
associates) and to effects of cooperative interactions of
NLO-active species in association. This is indicated by
data obtained when comparing the series of films with
Aggregation of the potassium salt of the chromophore
in the PVP matrix (chromophore salt concentration
1–16 wt %) was studied by optical microscopy. At salt
concentrations exceeding 4 wt %, the films exhibited
well-defined microstructure. Statistical analysis of the
distribution of the effective radii of chromophore salt
crystallites (Fig. 4) showed that the mean radius <R> of
the crystallites in the examined samples varied from 1.2 to
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 9 2012

1426
SAVITSKY et al.
Characteristics of “equally absorbing” films of chromophore salt in PVP matrix
Standard
deviation, σ<R>
μm
Chromophore salt con-
χ(3) × 1012,
CGS units
<R>,
μm
Sample
1a
Film thickness, μm
D at λ = 460 nm
,
centration, wt %
4.0
1.8–2.0
0.45–0.50
5.7–7.0
1.20
0.3
2
3
4
5
4.0
8.1
1.7–1.9
1.0–1.1
0.7–0.9
0.5–0.8
0.42–0.49
0.40–0.49
0.42–0.51
0.46–0.58
8.4–9.2
9.4–11.5
18–19
1.50
2.45
2.80
3.20
0.4
0.7
0.6
0.7
11.9
15.4
20–28
a Films were obtained by applying the solution onto a rotating support.
the chromophore concentration of 4.0 wt % (see table,
sample nos. 1 and 2).As seen from the table, when prepar-
ing samples by rapid evaporation of the solvent (sample
CONCLUSIONS
(1)Theeffectofaggregationof4-[5-(4-hydroxyphenyl)-
no. 1), the aggregates formed were smaller than in the case
of slow evaporation of the solvent (sample no. 2). The
presence of bigger aggregates led to higher values of χ(3).
3-oxopenta-1,4-dienyl]benzoic acid (chromophore) and
its potassium salt in optically neutral polymeric matrices
on third-order nonlinear-optical properties of chromo-
phore–polymer composites was examined.
THG measurement in the films show that the depen-
dence of log χ(3) on the size <R> of chromophore salt
crystallites is linear and corresponds to the equation:
χ(3) ≅ K<R>1.6 (Fig.5). Because χ(3) depends on the
amount of NLO-active species in the film and the ag-
gregate radius is determined by the number of molecules
that it incorporates, at the same concentration of the NLO
component in films the quantity χ(3) can be a measure of
the degree of aggregation of the chromophore component.
The latter fact, in combination with variation of the NLO
properties according to the degree of ionization of the
chromophore, is of particular interest, because it opens
possibilities for using nonlinear optical methods, namely,
THG, for studying interactions and structuring in forma-
tion of polymer–chromophore complexes.
(2) The cubic susceptibility χ(3) of poly-N-vinylpyr-
rolidone films incorporating the chromophore potassium
salt increases with an increase in the size of chromophore
component aggregates, following the power law.
(3) The THG efficiency for the anionic form of the
chromophore component is higher by three orders of
magnitude than that for the neutral form, owing to a
considerable increase in the conjugation system length
upon ionization.
REFERENCES
1. Schaller, R.D., Snee, P.T., Johnson, J.C., et al., J. Chem.
Phys., 2002, vol. 117, no. 14, pp. 6688–6698.
2. US Patent 6922279.
3. Bogdanov, V.L., Viktorova, E.N., Kulya, S.V., and Spiro,
A.S., Pis’ma Zh. Eksp. Teor. Fiz., 1991, vol. 53, no. 2,
pp. 100–103.
4. Markov, R.V., Ivanova, Z.M., Plekhanov,A.I., et al., Kvant.
Elektron., 2001, vol. 31, no. 12, pp. 1063–1066.
5. Kuznetsov, K.A., Laptinskaya, T.V., Mamaeva, Yu.B., et
al., Kvant. Elektron., 2004, vol. 34, no. 10, pp. 927–929.
6. Markov, R.V., Plekhanov, A.I., Shelkovnikov, V.V., and
Knoester, J., Microelectronic Eng., 2003, vol. 69, nos. 2–4,
pp. 528–531.
<R>, μm
Fig. 5. Plot of log χ(3) vs. mean radius <R> of crystallites in
films of the chromophore potassium salt in the PVP matrix.
7. Shelkovnikov, V.V., Zhuravlev, F.A., Orlova, N.A., et al.,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 9 2012

EFFECT OF THE AGGREGATION
1427
13. Savitsky, A.O., Andreeva, O.A., and Tenkovtsev, A.V.,
Abstracts of Papers, 2nd Int. Symp. “Frontiers in Polymer
Science,” Lyon (France), May 29–31, 2011, paper 3.095.
J. Mater. Chem., 1995, vol. 5, no. 9, pp. 1331–1334.
8. Horiuchi, H., Ishida, S., Matsuzaki, K.-ichi, et al., J. Phys.
Chem., Part C, 2011, vol. 115, no. 14, pp. 6902–6909.
14. Hattori, T. and Kobayashi, T., Chem. Phys. Lett., 1987,
9. Zakharova, G.V. and Chibisov, A.K., Khim. Vys. Energ.,
vol. 133, no. 3, pp. 230–234.
2009, vol. 43, no. 4, pp. 346–349.
15. Sheik-bahae, M., Said, A.A., and Stryland, E.W.V., Opt.
Lett., 1989, vol. 14, no. 17, pp. 955–957.
10. Buck, J.S. and Heilbron, I.M., J. Chem. Soc., 1922,
pp. 1095–1101.
16. Kubodera, K. and Kobayashi, H., Mol. Cryst. Liq. Cryst.,
11. Bredas, J.L., Adant, C., Tackx, P., et al., Chem. Rev., 1994,
1990, vol. 182, no. 1, pp. 103–113.
vol. 94, no. 1, pp. 243–278.
17. Wang, X.H., West, D.P., Mckeown, N.B., and King, T.A.,
J. Opt. Soc. Am., Part B, 1998, vol. 15, no. 7, pp. 1895–
1903.
12. .Nonlinear Optical Properties of Organic Molecules and
Crystals, Chemla, D.S. and Zyss, J., Eds., New York:
Academic, 1987, vol. 2.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 9 2012