J-aggregate formation of spiropyran derivatives in LB and vapor-deposited thin films

Article abstract of DOI:10.1080/15421400500366753

J-aggregate formation in the films of spiropyran derivatives (SP1822 and SP18) prepared with Langumuir-Blodegett (LB) and vapor-deposition methods was investigated. In the films of SP1822, J-aggregate formation occurred with UV light illumination and thermal treatment. In the case of SP18 film, on the other hand, only photoisomerization occurred. When the PMC-form of SP18 was transferred onto a glass plate, J-like aggregate formation occurred after UV light illumination. By UV light illumination during evaporation process, SP1822 formed J-aggregate. Finally, SP1822 J-aggregate film oriented one-dimensionally was fabricated by using a PTFE layer as a substrate.

Full text of DOI:10.1080/15421400500366753

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J-Aggregate Formation of
Spiropyran Derivatives in LB
and Vapor-Deposited Thin Films
Kouji Matsumoto a , Takashi Shinohara a , Yasuko
Koshiba a , Yasukiyo Ueda a & Zhenguo Ji b
a Graduate School of Science and Technology, Kobe
University , Rokko, Nada, Kobe, Japan
b Zhejiang University , China
Published online: 23 Aug 2006.
To cite this article: Kouji Matsumoto , Takashi Shinohara , Yasuko Koshiba , Yasukiyo
Ueda & Zhenguo Ji (2006) J-Aggregate Formation of Spiropyran Derivatives in LB
and Vapor-Deposited Thin Films, Molecular Crystals and Liquid Crystals, 445:1, 17/
[307]-26/[316], DOI: 10.1080/15421400500366753
To link to this article: http://dx.doi.org/10.1080/15421400500366753
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Mol. Cryst. Liq. Cryst., Vol. 445, pp. 17=[307]–26=[316], 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500366753
J-Aggregate Formation of Spiropyran Derivatives in LB
and Vapor-Deposited Thin Films
Kouji Matsumoto
Takashi Shinohara
Yasuko Koshiba
Yasukiyo Ueda
Graduate School of Science and Technology, Kobe University, Rokko,
Nada, Kobe, Japan
Zhenguo Ji
Zhejiang University, China
J-aggregate formation in the films of spiropyran derivatives (SP1822 and SP18)
prepared with Langumuir-Blodegett (LB) and vapor-deposition methods was
investigated. In the films of SP1822, J-aggregate formation occurred with UV light
illumination and thermal treatment. In the case of SP18 film, on the other hand,
only photoisomerization occurred. When the PMC-form of SP18 was transferred
onto a glass plate, J-like aggregate formation occurred after UV light illumination.
By UV light illumination during evaporation process, SP1822 formed J-aggregate.
Finally, SP1822 J-aggregate film oriented one-dimensionally was fabricated by
using a PTFE layer as a substrate.
Keywords: J-aggregate; J-like aggregate; LB method; photochromic behavior;
spiropyran; vapor-deposition method
This work was partially supported by a Grant-in-Aid for Scientific Research (B)
(No. 16360040) from the Japan Society for the Promotion of Science. This work was also
supported by the Tokyo Ohka Foundation for the Promotion of Science and Technology
and by the Photonics Materials Laboratory Project of the VBL of the Graduate School of
Science and Technology, Kobe University.
Address correspondence to Yasukiyo Ueda, Graduate School of Science and
Technology, Kobe University, Rokko, Nada, Kobe 657-8501, Japan. E-mail: yueda@
kobe-u.ac.jp
17=[307]

18=[308]
K. Matsumoto et al.
INTRODUCTION
J-aggregates are assemblies of organic dye molecules with an ordered
structure. The typical feature of J-aggregates is the red-shift of the
absorption band with a small Stokes shift. J-aggregate has been
attracted wide attention. Interests in J-aggregates originally stemmed
from the spectral sensitization. Recent development in the research on
J-aggregates derives from the expectations that J-aggregate will be
used as materials for optical storage and ultrafast optical switching.
Spiropyran is one of the dye molecules that form J-aggregates. Spiro-
pyran derivatives reversibly isomerize between spiropyran (SP)-form
and photomerocyanine (PMC)-form by UV light illumination. Some
PMC-form molecules assemble spontaneously in J-aggregate. There
have been many reports on photochromic behavior and J-aggregate
formation of spiropyran derivatives [1–5]. Previously, we reported
J-aggregate formation of SP1822 in LB and vapor-deposited film [6].
J-aggregate formation in SP1822 LB film occurred by UV light
illumination at 35ꢀC.
In this study, the LB films of SP18 and of SP1822=SP18 mixture
were fabricated and J-aggregate formation in the LB film was investi-
gated. SP1822 was also vapor-deposited on its J-aggregate LB film and
a friction-transferred polytetrafluoroethylene (PTFE) layer. The
effects of substrates and the vapor-deposition process on J-aggregate
formation were investigated.
EXPERIMENTAL
Glass plates were used as a substrate for preparation of LB films. They
were cleaned ultrasonically in water with detergent for 15 minutes,
in distilled water for 15 minutes, and in methanol for 15 minutes.
Two spiropyran derivatives used here were 1-octadecyl-3,3-dimethyl-
6-nitro-8-[docosanoyloxymethyl]-spiro[2H-1-benzopyran-2,2-indoline]
(SP1822) and 10,30-dihydro-30,30-dimethyl-6-nitro-10-octadecylspiro[2H-
benzopyran-2,20-2H] indole (SP18). Their molecular structures and
photochromic behavior are shown in Figure 1. These samples were
purchased from Dojindo Laboratory Co. Lt. LB films of spiropyran deri-
vatives were prepared on air-water interface of a LB through (Kyowa
Kaimen Kagaku Co. Lt). A chloroform solution of SP1822 or SP18
mixed with arachidic acid (AA) and of SP1822=SP18=AA mixture
(molar ratio ¼ 1:1 and 1:1:1, respectively) was spread on a distilled
water kept at 5ꢀC. The molecules were compressed with a moving
barrier at the speed of 14 cm2minꢁ1. Layers were transferred onto glass
plates by vertical dipping method. Vapor-deposition of SP1822 was

J-aggregate Formation of Spiropyran Derivatives
19=[309]
FIGURE 1 Molecular structure and photochromism of spiropyran derivatives.
performed as follows. SP1822 was evaporated on a substrate from fused
silica crucible heated by a tungsten filament in a vacuum of 10ꢁ3 Pa.
A glass plate, SP1822 LB film and a friction-transferred PTFE layer
were used as a substrate. The substrate temperature was kept at room
temperature (r.t.) and 35ꢀC and the deposition rate was controlled to
be about 1 nm minꢁ1 in thickness monitored by a quartz crystal
microbalance. The film thickness was about 20 nm. When the UV
light was illuminated during evaporation process, the light intensity
was controlled to be about 5 mW cmꢁ2 and 15 mW cmꢁ2. The optical
properties and structures of the films were investigated by visible
absorption and photoluminescence spectroscopy and transmission
electron microscopy (TEM).
RESULTS AND DISCUSSION
LB Film
SP18 and SP1822 LB films were consisted of SP-form molecules
because these films had no absorption peaks in the visible wavelength
range. Figure 2 shows the spectral changes of SP18 and SP1822 LB
films with UV light (365 nm) illumination. Upon UV light illumination
at room temperature, the broad absorption band appeared in the
wavelength range of 450–650 nm, as shown in Figure 2(a). This band
is assigned to PMC-form of SP1822 molecule. When the SP1822 LB
film was exposed to UV light at ambient temperature of 35ꢀC, the
red-shifted sharp absorption peak appeared at 618 nm in addition to
absorption peak of PMC-form, as shown in Figure 2(b). In fluorescence
spectrum of the film, a single emission peak was observed at 624 nm
(Fig. 2(c)). The Stokes shift was 6 nm. From these results, this band
is assigned to J-aggregate of PMC-form. In SP18 LB film, on the other
hand, only photoisomerization from SP-form to PMC-form occurred.

20=[310]
K. Matsumoto et al.
FIGURE 2 Visible absorption (solid lines) and PL (dotted lines) spectra of the
LB films of SP1822 (a,b,c) and SP18 (d,e) after UV light illumination at r.t. (a),
35ꢀC (b,c) and 50ꢀC (d,e).
Another process is necessary for J-aggregate formation of SP18. Prior
to spreading on water, SP18 was exposed to UV light in chloroform
solution. SP18 changed from SP-form into PMC-form. The solution
containing PMC-form was spread on water and the film was trans-
ferred onto a glass plate. After UV light illumination at ambient tem-
perature of 50ꢀC, the red-shifted sharp absorption peak appeared at
570 nm, as shown in Figure 2(d). Single emission peak was observed
at 605 nm (Fig. 2(e)). The Stokes shift between the absorption peak
and the emission peak was 35 nm. Generally, J-aggregate is charac-
terized with a small Stokes shift (less than 10 nm). The absorption
band at 570 nm is assigned not to J-aggregate of PMC-form but to
different aggregate. Therefore, this band is assigned as J-like aggre-
gate. Figure 3 shows the absorption spectra of the LB film of a
SP1822=SP18 mixture after UV light (365 nm) illumination. Upon
UV light illumination at ambient temperature of 30ꢀC, two absorption
peaks appeared at 570 nm and 618 nm in the absorption spectrum of
the film (Fig. 3(a)). The absorption peaks at 570 nm and 618 nm are
assigned to J-like aggregate of SP18 and J-aggregate of SP1822,
respectively. With longer UV light illumination, the intensity of
absorption peak at 618 nm became remarkably strong, but the inten-
sity change of absorption peak at 570 nm was not so large comparing
with that of absorption peak at 618 nm. Upon UV light illumination at
50ꢀC, on the other hand, the absorption peak appeared only at 570 nm,
as shown in Figure 3(b). These results indicate that J-aggregate and

J-aggregate Formation of Spiropyran Derivatives
21=[311]
FIGURE 3 Visible absorption spectra of the LB film of SP1822=SP18 mixture
after UV light illumination at 30ꢀC (a) and 50ꢀC (b) for 15 min.
J-like aggregate formation in the LB film of SP1822=SP18 mixture
occurred selectively depending on the ambient temperature during
UV light illumination.
Vapor-Deposited Film
SP1822 was vapor-deposited on a glass plate without any thermal
decomposition. The deposited film was composed of SP-form mole-
cules. After UV light illumination at room temperature, the broad
absorption band corresponding to PMC-form appeared in the wave-
length range of 500–600 nm. The photoisomerization from SP-form
to PMC-form occurred easily even in a vapor-deposited film. When
the film was exposed to UV light under the ambient temperature at
35ꢀC, J-band appeared at 618 nm. J-aggregation in the vapor-
deposited film occurred under the condition of UV light illumination
at slightly high temperature than r.t. (i.e., 35ꢀC). Figure 4 shows
the absorption spectra of SP1822 film vapor-deposited on SP1822
J-aggregate LB film before and after UV light illumination. The
absorption spectrum of the deposited film shows two peaks at
570 nm and 618 nm (Fig. 4(a)). These peaks originate from PMC-form
and J-aggregation in LB film used as a substrate, respectively. It is
noticed that J-aggregate is stable in a vacuum. After UV light illumi-
nation without special heating, the intensities of both absorption
peaks increase (Fig. 4(b)). It was found that J-aggregation is induced

22=[312]
K. Matsumoto et al.
FIGURE 4 Visible absorption spectra of the SP1822 film vapor-deposited on
SP1822 J-aggregate LB film before (a) and after (b) UV light illumination at
r.t.
only with UV light illumination in this case. It is noteworthy that the
intensity ratio of I570=I620 is same before and after UV light illumi-
nation. There are two domains in SP1822 LB film. One is consisted
of PMC-form and the other is J-aggregate of PMC-form, as shown in
Figure 5(a). Evaporant molecules were deposited as SP-form (Fig. 5(b)).
Upon UV exposure, SP-form molecules photoisomerize to PMC-form. At
that time, SP-form molecules on the domain consisted of PMC-form
only photoisomerize. SP-form molecules on the domain consisted of J-
aggregate of PMC-form, on the other hand, photoisomerize and assemble
spontaneously since the J-aggregate domain acts as a nucleus for
aggregate formation, as shown in Figure 5(c). Therefore, both domains
coexist in the film and the ratio of PMC-form=J-aggregate is same value
before and after UV light illumination. J-aggregate formation induced
only with UV light illumination (without special heating) in the vapor-
deposited film was observed for the first time. This result suggests that
J-aggregate formation is dependent strongly on the substrate structure.
Since pioneering research by Wittmann and Smith, the usefulness
of a friction-transferred PTFE layer as a substrate for achieving high
molecular alignment has been attracted wide attention. We have
investigated the structure and properties of organic molecules such
as phthalocyanine and distyrylbenzene and so on [7–11]. Such mole-
cules oriented uniaxially on a PTFE layer and their optical and electric

J-aggregate Formation of Spiropyran Derivatives
23=[313]
FIGURE 5 Schematic representation of J-aggregate formation of evaporant
SP1822 deposited on its LB film.
properties indicated remarkable anisotropy. In this study, SP1822 was
deposited on a PTFE layer with UV light illumination during evapor-
ation process. Figure 6 shows the experimental setup and absorption
spectra of SP1822 film vapor-deposited on a PTFE layer. The
SP1822 film deposited on a PTFE layer without UV light illumination
also was consisted of SP-form molecules. After UV exposure under the
ambient temperature at 35ꢀC, J-band appeared at 618 nm, as shown in
Figure 6(a). The full width at half maximum (FWHM) of absorption
peak is 52 nm. When SP1822 was deposited at 35ꢀC under UV light
illumination (365 nm, 15 mWcmꢁ2) during evaporation process, a sharp
absorption peak corresponding to J-aggregate of PMC-form appeared
just after vapor-deposition, as shown in Figure 5(b). This indicates
that the J-aggregate formation occurred during vapor-deposition
process. The FWHM of J-band peak was 28 nm. On the other hand,
the FWHM of J-band peak of the film deposited under UV light illumi-
nation with 5 mWcmꢁ2 power is 40 nm. Generally, the FWHM relates
closely with molecular ordering in the film. The more the regularity
of molecular packing in the film improves, the narrower the FWHM
becomes. Therefore it is found that the regularity of molecular packing
in J-aggregate also depends on the UV light power during evaporation.
Figure 7 shows a TEM image and a high resolution electron diffraction

24=[314]
K. Matsumoto et al.
FIGURE 6 Visible absorption spectra of the SP1822 film vapor deposited on a
PTFE layer (a) Exposed to UV light at 35ꢀC after vapor-deposition (b) With UV
light illumination at 35ꢀC during vapor-deposition. The inset indicates the
schematic diagram of experimental setup.
FIGURE 7 TEM image and HRED pattern of the SP1822 film vapor-
deposited on a PTFE layer with UV light illumination during evaporation
process.

J-aggregate Formation of Spiropyran Derivatives
25=[315]
FIGURE 8 Visible polarized absorption spectra of the SP1822 film vapor-
deposited on a PTFE layer with UV light illumination during evaporation
process.
(HRED) pattern of the film vapor-deposited on a PTFE layer at 35ꢀC
with UV light illumination during evaporation process. The film was
composed of thin plate-like crystals and rod-like ones. The HRED pat-
tern of the film shows the superposition of the reflection spots from the
PTFE layer used as a substrate and those from the deposited J-aggre-
gate indicated by white arrows in the figure. The reflection spots from
the deposited J-aggregate appear as arc. This suggests that J-aggre-
gate oriented one-dimensionally on a PTFE layer. However, further
information concerning the molecular orientation can not be obtained
because the crystal structure of J-aggregate of PMC-form has not been
determined yet. Figure 8 shows the polarized absorption spectra of the
film on a PTFE layer. The anisotropic feature in absoption can be
observed. The dichroic ratio (I =I ) obtained from absorption peak at
k
?
618 nm is 2.1. This is the first report on fabrication of one-dimension-
ally oriented J-aggregate originated from spiropyran derivatives.
CONCLUSIONS
This report provides the detailed results of J-aggregate formation in LB
and vapor-deposited films of spiropyran derivatives. In SP1822 films
prepared with both LB and Vapor-deposition methods, J-aggregate
formation occurred with UV light illumination at 35ꢀC. In the case of
SP18 film, only photoisomerization occurred. When the PMC-form of

26=[316]
K. Matsumoto et al.
SP18 was transferred onto a glass plate, J-like aggregate formation
occurred after UV light illumination at 50ꢀC. In the LB film of SP1822=
SP18 mixture, on the other hand, J-aggregate and J-like aggregate
formation occurred selectively depending on the ambient temperature
during UV light illumination. SP1822 vapor-deposited on its J-aggregate
LB film formed J-aggregate only with UV light illumination (without
special heating). This is the first experimental result that J-aggregate
formation in the vapor-deposited film was induced only with UV light
illumination.
By UV light illumination during evaporation process, SP1822
formed J-aggregate. The FWHM at J-band peak of the film was
much smaller than that of the film exposed to UV light at 35ꢀC after
vapor-deposition. Finally, SP1822 J-aggregate film oriented one-
dimensionally was fabricated using a PTFE layer as a substrate.
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