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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: tkobaya@pe.osakafu-u.ac.jp (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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