Optical nonlinearity of J-aggregates in vapor-deposited films of a bisazomethine dye
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Transcript of Optical nonlinearity of J-aggregates in vapor-deposited films of a bisazomethine dye
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Thin Solid Films 509 (
Optical nonlinearity of J-aggregates in vapor-deposited
films of a bisazomethine dye
Takashi Kobayashi a,*, Shinya Matsumoto b, Tetsuya Aoyama c, Tatsuo Wada c
a Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japanb Department of Environmental Sciences, Yokohama National University, 79-2 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
c Supramolecular Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
Available online 25 October 2005
Abstract
The third-order optical nonlinearity of the J-aggregates of nonionic dye in vapor-deposited films has been investigated in terms of
electroabsorption measurement. Compared with that of a monomer film of the dye, the nonlinearity was enhanced by a factor of 4000. However,
the number of aggregates in the J-aggregate film was estimated to be only a few. The obtained enhancement is mainly caused by the concentration
of oscillator strength on the J-band and not by the development of the coherence size of excitons. Therefore, a much larger optical nonlinearity can
be expected in highly ordered J-aggregates of the nonionic dye, and it is demonstrated that J-aggregate formation is an effective strategy for
nonionic dye to improve its optical nonlinearity.
D 2005 Elsevier B.V. All rights reserved.
Keywords: J-aggregate; Third-order optical nonlinearity; Electroabsorption measurement; Vapor-deposited film; Functional dye
1. Introduction
The J-aggregates of organic molecules have attracted
considerable attention because in these systems, the concen-
tration of transition dipole moment in a narrow spectral region
induces strong coupling between photons and materials [1,2].
Although the concentration of transition dipole moment is
useful for various types of optical devices, the enhancement of
optical nonlinearity due to J-aggregate formation is significant
[3–5]. In fact, the enhancement of the third-order optical
nonlinearity v(3) by a factor of 1000 was reported in a J-
aggregate Langmuir–Blodgett film made of a cyanine deriv-
ative [5]. Therefore, J-aggregate formation has been considered
as an effective strategy to increase the optical nonlinearity of
small organic molecules.
Thus far, most of the research of J-aggregates was conducted
on ionic molecules, such as cyanine and merocyanine dyes. On
the other hand, Matsumoto et al. found J-aggregate formation in
vapor-deposited films of nonionic dye, N,NV-bis[4-(N,N-diethy-lamino)benzylidene]diaminomaleonitrile (Dye 1), whose chem-
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.09.134
* Corresponding author. Tel.: +81 72 254 9269; fax: +81 72 254 9908.
E-mail address: [email protected] (T. Kobayashi).
ical structure is shown in the inset of Fig. 1 [6–8]. The nonionic
J-aggregates have advantages of good stability against heat [7]
and the possibility of making a large homogenous film. In this
study, we measured the electroabsorption (EA) spectrum of
vapor-deposited films of dye 1 and its monomer film in order to
reveal the relation between J-aggregate formation and optical
nonlinearity in this dye. We also estimated the third-order
optical susceptibility of these films and found that the J-
aggregate film has an optical nonlinearity more than 4000 times
larger than that of the monomer film.
2. Experiment
Dye 1 was synthesized according to Ref. [9]. Dye 1 was
evaporated onto a glass substrate at room temperature from a
quartz crucible heated by a tungsten filament in a vacuum of
3�10�4 Pa. The deposition rate was 30–50 A/s. As its
monomer film, we prepared poly(methyl methacrylate) film
doped with 1 wt.% dye 1 using a cast technique. As shown in
Fig. 1, the 1500-A-thick film has a sharp absorption band (J-
band) at approximately 1.9 eV, whereas the 600-A-thick film
does not show such a sharp absorption band. In a 600-A-thick
film, the crystalline phase is dominant. However, if the film
2006) 145 – 148
ww
1.5 2.0 2.5 3.0 3.50.0
0.5
1.0
1.5
2.0
2.5
800 700 600 500 400A
bsor
banc
e (a
rb. u
nits
)
Photon Energy (eV)
(nm)
N
N
NEt2
Et2N
CN
NC
Fig. 1. Absorption spectra of monomer film (dotted line) of dye 1 and its vapor-
deposited films with thicknesses of 1500 A (solid line) and 600 A (broken line).
The chemical structure of dye 1 is shown in the inset.
-4
-2
0
2
4
-2
-1
0
1
2
3
1.5 2.0 2.5 3.0
-5
0
5
dA/d
E
(c)
(b)
(a)F = 24 kV/cm
Δ α (1
0-5 O
.D.)
|χ(3)|
Re Imχ(3
) (10
-10 e
su)
Photon Energy (eV)
Fig. 2. (a) EA spectrum, (b) first derivative of corresponding absorption
spectrum, and (c) v(3) spectrum of 1500-A-thick film. In (c), solid, broken, and
dotted lines indicate its absolute value, real and imaginary parts, respectively.
-5
0
5
10
-1
0
1
2
1.5 2.0 2.5 3.0-2
-1
0
1
2
dA/d
E
F = 20 kV/cm
Δα (
10-6 O
.D.)
(c)
(b)
(a)
|χ (3)|
Re Imχ(3
) (10
-13 e
su)
Photon Energy (eV)
Fig. 3. (a) EA spectrum, (b) first derivative of corresponding absorption
spectrum, and (c) v(3) spectrum of monomer film.
T. Kobayashi et al. / Thin Solid Films 509 (2006) 145–148146
thickness is more than 1000 A, J-band appears. Thus, the J-
aggregate films of dye 1 are considered to consist of J-
aggregate and crystalline phases [6–8].
The mechanism for the J-aggregate formation of dye 1 is
still not clear and is under further investigation. Similar
behavior, i.e., the coexistence of two types of aggregate, is
also observed in merocyanine derivatives [10,11]. On the other
hand, the absorption spectrum of the monomer film of dye 1 is
almost identical to that in solution [6], and thus, molecular
interaction can be negligible in the monomer film.
For EA measurement, Al electrodes with a 500-Am gap
were deposited on the films. Furthermore, we also prepared a
‘‘sandwich sample’’ for EA measurement: the 1500-A-thick
film of dye 1 was deposited onto an ITO substrate, and then
transparent Al with a thickness of 100–200 A was deposited
on the film. EA measurement was performed using a
homemade EA spectrometer [6]. All EA spectra shown in this
study were measured under the condition that the signal is
proportional to the square of the strength of applied electric
field (F).
3. Results
Fig. 2(a) and (b) shows the EA spectrum of the 1500-A-
thick film with the gap Al electrodes and the first derivative of
the corresponding absorption spectrum. These spectra agree
with each other at around the J-band, which indicates that this
signal can be attributed to the Stalk shift of the Frenkel exciton
band. In the ionic J-aggregates, second derivative-like EA
spectra are usually observed. Its origin is considered to change
in the relative geometrical configuration between cationic dyes
and counteranions. However, the origin of the optical
nonlinearity of the J-aggregates of dye 1 is purely electronic,
and thus, an ultrafast response can be expected. Following the
method described in Ref. [5], we calculated the v(3) spectrumof this film and show the spectrum in Fig. 2(c). The maximum
value of |v(3)| was estimated to be 7�10�10 esu in this film.
Fig. 3(a) and (b) shows the EA spectrum of the monomer
film and the first derivative of the corresponding absorption
spectrum. These shapes agree with each other at the absorption
band (2.4 eV). We show the v(3) spectrum in Fig. 3(c), whose
maximum value is 1.5�10�13 esu. Therefore, we conclude
that J-aggregate formation of the dye results in the enhance-
ment of v(3) by a factor of 4000. Even in the 1500-A-thick film,
the aggregation number of the dye is not so large (as discussed
later). Thus, dye 1 has a great potential to show a much larger
v(3) in its highly ordered J-aggregate film.
-1
0
1
2
3
-2
0
2
4
1.5 2.0 2.5 3.0-2
-1
0
1
2
dA/d
EF = 24 kV/cm
Δα (1
0-6 O
.D.)
(c)
(b)
(a)
|χ (3)|
Re Imχ(3
) (10
-10 e
su)
Photon Energy (eV)
Fig. 4. (a) EA spectrum, (b) first derivative of corresponding absorption
spectrum, and (c) v(3) spectrum of 600-A-thick film.
-40
-20
0
20
40
-2
-1
0
1
2
1.5 2.0 2.5 3.0-10
-5
0
5
10 (c)
(b)
(a)
x3
dA/d
E
F = 60 kV/cm
Δα (1
0-4 O
.D.)
|χ(3)|
Re Imχ (3
) (10
-10 e
su)
Photon Energy (eV)
Fig. 5. (a) EA spectrum, (b) first derivative of corresponding absorption
spectrum, and (c) v(3) spectrum of sandwich sample with thickness of 1500 A.
T. Kobayashi et al. / Thin Solid Films 509 (2006) 145–148 147
We also show EA spectrum, the first derivative of the
absorption spectrum, and the v(3) spectrum of the 600-A-thick
film in Fig. 4. Its maximum value of |v(3)| is approximately
1.5�10�10 esu, and its |v(3)| is three orders of magnitude larger
than that of the monomer film. Since the absorption band of
this film is red-shifted with respect to that of the monomer film,
it is reasonable to consider that dye 1 formed the crystalline
phase in the 600-A-thick film. This result indicates that the
enhancement of v(3) can be obtained in ordered aggregates
(crystalline phase) even if it is not J-aggregate.
4. Discussion
EA spectrum is generally analyzed in terms of the first and
second derivatives of the absorption spectrum and then the
linear polarizability difference (Da) and the permanent dipole
moment difference (Dl) between the excited and ground states
are estimated. The first derivative-like component results from
the Stark shift of the exciton band, whereas the second
derivative-like component is attributed to a charge-transfer
state. Since dye 1 is a nonionic molecule, all the EA spectra
shown here can be well reproduced by the first derivatives of
the corresponding absorption spectra (see Figs. 2–4). From
these fittings, we estimated Da for all the films; Da =60 A3 for
the 1500-A-thick film, Da =27 A3 for the monomer film, and
Da =200 A3 for the 600-A-thick film.
Under the assumption that Da is proportional to the
aggregate number of the dye [4,5], the numbers of aggregates
in the J-aggregate (1500 A) film and in the crystalline (600 A)
film are estimated to be ¨2 molecules and ¨7 molecules,
respectively. This estimation suggests that the 600-A-thick film
has a higher ordered structure and a larger coherence size than
the 1500-A-thick film. In the vapor-deposited films of dye 1,
the crystalline phase is more stable than the J-aggregate phase
and probably has less structural disorder. In the single crystal of
dye 1, each unit cell has three molecules and all of them are not
parallel to each other [12]. Thus, its oscillator strength is
distributed among many Davydov components, which results
in a relatively broad absorption band. On the other hand,
although the order of molecular arrangement is not good in the
1500-A-thick film, most of the oscillator strength of three
molecules is concentrated on only the J-band and then a sharp
absorption and a large optical nonlinearity emerge.
In Fig. 5, we show the EA and v(3) spectra of the sandwich
sample with a thickness of 1500 A. The sandwich sample shows
a larger optical nonlinearity than the sample with a gap
electrode; its maximum |v(3)| was estimated to be 9�10�10
esu. In the sandwich sample, the direction of the optical electric
field is perpendicular to the direction of the applied electric
field. If one-(or two-) dimensional J-aggregates are formed
parallel to the substrate, the EA signal should not be observed in
the sandwich sample. However, almost the same nonlinearities
were obtained in these samples, which means that the direction
of the J-aggregates is neither parallel nor perpendicular to the
substrate. In the single crystal of dye 1, the molecules form two-
dimensional sheets and are stacked up with a gradual side shift
[12]. If we assume that the two-dimensional sheets are formed
parallel to the substrate, the nearest neighbor dye always
appears obliquely upward, and similar optical nonlinearity is
expected in the two types of samples. Therefore, as one of
possible structures of the J-aggregates, we consider that the J-
aggregates take over such geometrical relationship of the
nearest neighbor dyes in the single crystal and grow with a
tilt angle with respect to the substrate.
T. Kobayashi et al. / Thin Solid Films 509 (2006) 145–148148
5. Conclusions
In this study, we investigated the optical nonlinearity of the
vapor-deposited films of dye 1 using EA measurement.
Compared to a monomer film of the dye 1, the nonlinearities
in a J-aggregate film and a crystalline film were enhanced by
factors of 4000 and 1000, respectively. However, the number
of aggregates in the J-aggregate film was estimated to be only a
few. The obtained enhancement in the J-aggregate film is
mainly caused by the concentration of the oscillator strength on
its J-band and not by the development of the coherence size of
excitons; the latter rather contributes to the enhancement of the
crystalline film. Therefore, J-aggregate formation was con-
firmed to be an effective strategy also for nonionic dye to
improve its optical nonlinearity. In addition, EA measurement
suggests that the direction of the J-aggregates is not parallel
and perpendicular to the substrate, and thus, lager nonlinearity
is obtained in the sandwich-type sample of dye 1.
Acknowledgments
This work was supported by a Grant-in-Aid from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, and Industrial Technology Research Grant Program in ’02
from New Energy and Industrial Technology Development
Organization (NEDO) of Japan.
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