Orientation-control of liquid crystal molecules by photochromism of azobenzene

Article abstract of DOI:10.1080/15421400490506720

Langmuir-Blodgett films of amphiphilic azobenzene were prepared on a glass plate as a substrate of liquid crystal (LC) cell. Photochemical regulation of LC alignment was dependent on adsorption structure of azobenzene on a glass plate. When the amphiphilic LC molecules were sandwiched with two glass plates covered with hydrophobic surface of azobenzene, the, transmittance change of the cell did nol occur by UV and for visible light irradiation. On the other hand, the relative transmittance of the cell in which the amphiphilic LC molecules were sandwiched with hydrophobic and hydrophilic surfaces of azobenzenec film was changed reversibly by UV and visible light irradiation. Similar behavior was observed in the cell constructed with hydrophobic azobenzene surface/hydrophobic LC/hydrophobic azobenzene surface. Copyright Taylor & Francis Inc.

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Orientation-Control of
Liquid Crystal Molecules by
Photochromism of Azobenzene
Naofumi Aoki a & Yasukiyo Ueda a
a Graduate School of Science and Technology, Kobe
University, Rokko, Nada, Kobe, Japan
Published online: 18 Oct 2010.
To cite this article: Naofumi Aoki & Yasukiyo Ueda (2004) Orientation-Control of
Liquid Crystal Molecules by Photochromism of Azobenzene, Molecular Crystals and
Liquid Crystals, 425:1, 159-166, DOI: 10.1080/15421400490506720
To link to this article: http://dx.doi.org/10.1080/15421400490506720
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Mol. Cryst. Liq. Cryst., Vol. 425, pp. 159=[437]–166=[444], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490506720
ORIENTATION-CONTROL OF LIQUID CRYSTAL
MOLECULES BY PHOTOCHROMISM
OF AZOBENZENE
Naofumi Aoki and Yasukiyo Ueda
Graduate School of Science and Technology, Kobe University,
Rokko, Nada, Kobe, 657-8501, Japan
Langmuir-Blodgett films of amphiphilic azobenzene were prepared on a glass
plate as a substrate of liquid crystal (LC) cell. Photochemical regulation of LC
alignment was dependent on adsorption structure of azobenzene on a glass
plate. When the amphiphilic LC molecules were sandwiched with two glass
plates covered with hydrophobic surface of azobenzene, the transmittance
change of the cell did not occur by UV and=or visible light irradiation. On
the other hand, the relative transmittance of the cell in which the amphiphilic
LC molecules were sandwiched with hydrophobic and hydrophilic surfaces of
azobenzene film was changed reversibly by UV and visible light irradiation.
Similar behavior was observed in the cell constructed with hydrophobic
azobenzene surface=hydrophobic LC=hydrophobic azobenzene surface.
Keywords: azobenzene LB film; nematic liquid crystal; orientation control
INTRODUCTION
The studies of induced mechanical effects in photochromic molecules had
been an area of great interest. Azobenzenes are one of the most extensively
studied groups of photochromic materials due to their potential use as
photoactive media for information storage and processing [1,2]. The
Langmuire-Blodgett (LB) technique has been shown to be a powerful
and convenient method for the preparation of ultra thin, uniform, and con-
trollable films. There are many reports on structures [3,4] and properties
Received 1 October 2003; accepted 10 February 2004
This work was partially supported by the Photonics Materials Laboratory Project of the
VBL of the Graduate School of Science and Technology, Kobe University, and by the Ministry
of Education (13555020).
Address correspondence to Yasukiyo Ueda, Graduate School of Science and Technology,
Kobe University, Rokko, Nada, Kobe, 657-8501, Japan. Tel.: þ 81 78 803 6182, Fax: þ 81 78
803 6205, Email: yueda@kobe.u.ac.jp
159=[437]

160=[438]
N. Aoki and Y. Ueda
such as aggregate formation [5], molecular switching of amphiphilic azo-
benzene derivatives in LB films [6]. Ichimura et al have reported that the
orientation of liquid crystal (LC) was changed reversibly by conformational
changes in the trans-cis isomerization process of azobenzene LB film [7].
However, detailed relationship between the adsorption structure of azo-
benzene against to the substrate and the orientation of LC isn’t clear.
In this study, LB films of azobenzene derivatives taking various adsorp-
tion structures were prepared. Orientation change of LC molecules induced
by photochromism of azobenzene in azobenzene=LC=azobenzene cell is
discussed based on the adsorption structure of azobenzene molecules
against to the substrate.
EXPERIMENTAL
Glass plates and fused silica plates were used as a substrate for preparation
of LB film. They were cleaned ultrasonically with detergent for 15 minutes,
with distilled water for 15 minutes, and with methanol for 15 minutes.
The surface of cleaned substrate was sufficiently hydrophilic after dipping
into distearyldimethylammonium chloride (0.5 wt%) aqueous solution.
Hydrophobilized substrates were obtained by dipping into sodiumdialkyl-
sulfosuccinate aqueous solution.
Three amphiphilic azobenzene derivatives used here were 4-octhyl-40
-(3-corboxy-trimetyleneoxy)azobenzene (8AzO3), 4-octhl-40-(5-corboxy-
penta-metyleneoxy)azobenzene (8AzO5), and 4-dodecyloxy-40-(3-corboxy-
trimetyleneoxy)azobenzene (12OAzO3). Their molecular structures are
shown in Figure 1. These samples were purchased from Dojindo laboratory.
LB films of azobenzene derivatives were prepared on air-water interface of
a LB trough (Kyowa kaimen kagaku). A chloroform solution of azobenzene
derivatives (3 ꢀ 10ꢁ4mol=l) was spread on deionized and distilled water.
The molecules were compressed with a moving barrier at the speed at
15 cm2=min. A single monolayer was transferred onto hydrophililized and
FIGURE 1 Molecular structures of azobenzene derivatives and liquid crystal
molecules.

Orientation-control of LC by Photochromism of Azobenzene
161=[439]
FIGURE 2 Schematic drawing of optical setup for transmittance measurement of
LC cell.
hydorophobilized glass plates by vertical dipping method and horizontal
lifting one, respectively.
Two kinds of rod-like nematic LCs were used for LC cells. 5CB is amphi-
philic and DON-103 has hydrophobic groups at both ends of molecule
(Fig. 1). The LCs were sandwiched between two glass plates covered with
azobenzene monolayer. The LC cell was set between two crossed polarizers
and exposed alternately to UV(365 nm) and visible(436 nm) light as shown
in Figure 2. Transmittance change of LC cell with UV and visible light
irradiation was defined by following equation.
Transmittance change ¼ Iphoto=Idark
Here, Idark is the light intensity passed through two crossed polarizers with
a LC cell and Iphoto is the light intensity after UV or visible light irradiation.
UV-visible spectra and transmittance change were recorded on a
Shimadzu UV-2200 spectrophotometer at room temperature (20–23ꢂC).
RESULTS AND DISCUSSION
Figure 3(a) shows the UV-vis absorption spectra of the 8AzO3 chloroform
solution. The band region from 400 nm to 500 nm is the feature of n-pꢃ tran-
sition of azobenzene compound. The p-pꢃ transition along the long axis of
trans-azobenzene chromophore is located at 355 nm. The short wavelength
absorption found at 245 nm is the electronic transition with the transition
moment roughly parallel to the short axis of the trans-azobenzene chromo-
phore. After UV irradiation, the absorption intensity of n-pꢃ transition
increased slightly, as shown in Figure 3(b). Moreover, it comes into notice

162=[440]
N. Aoki and Y. Ueda
FIGURE 3 UV-vis absorption spectra of the 8AzO3 in chloroform solution before
(a) and after UV (365 nm) light irradiation (b).
that the absorption due to the transition of the long axis blue-shifted to
310 nm and that of the short axis red-shifted to 255 nm, respectively. The
absorption spectrum after UV irradiation was assigned to cis-azobenzene.
By visible light irradiation, the photoisomerization from cis-azobenzene
to trans-azobenzene occurred quantitatively. 8AzO5 and 12OAzO3 mole-
cules in chloroform also indicated similar reversible photoisomerization.
Figure 4 shows the UV-vis absorption spectra of the 8AzO3 monolayer film
transferred on both sides of the fused silica plates by vertical dipping
method. The absorption peaks due to the p-pꢃ transition of trans- azo-
benzene form appear at 330 nm and 250 nm. The absorption intensity at
330 nm decreased and the absorption peak shifted slightly to the short
wavelength region by UV irradiation. Subsequent visible light exposure
regenerated the trans-form. This phenomenon indicates that 8AzO3 mole-
cules photoisomerize reversibly even in solid state. On the other hand, such
photoisomerization was not observed in the 8AzO3 five-layers film trans-
ferred on the fused silica plate by horizontal lifting method (Absorbance
intensity of monolayer on one side of substrate was too small to confirm
whether photoisomerization was occurred). The LB technique by vertical
dipping method possesses, besides its utility in molecular tailing, a superior
feature that in-plane structural anisotropy may be imprinted during the
dipping process. This leads frequently the molecular orientation along
the dipping direction. It seems that azobenzene molecules transferred by
vertical dipping method adsorb obliquely to the substrate surface. There-
fore, it is reasonable to consider the occupied area per one molecule in

Orientation-control of LC by Photochromism of Azobenzene
163=[441]
FIGURE 4 UV-vis absorption spectra of monolayer of 8AzO3 deposited on a fused
silica plate by vertical dipping method before (a) and after UV (365 nm) light
irradiation (b).
solid state is larger than that on the water. In-plane structural anisotropy,
on the other hand, dose not expect with horizontal lifting method. The
molecular packing of 8AzO3 in the film transferred by horizontal lifting
method seems to be denser than in the film transferred by vertical dipping
method. At present, we are not able to elucidate the detailed.
The 8AzO5 monolayer film transferred by vertical dipping method and
the five-layers film transferred by horizontal lifting method showed good
photoisomerization. In the case of 12OAzO3, LB films transferred by verti-
cal dipping and horizontal lifting methods did not show the photoisomeriza-
tion. Figure 5 shows the UV-vis absorption spectra of the 12OAzO3 in
chloroform solution and its monolayer film prepared by vertical dipping
method. The absorption peak due to the p-pꢃ transition of trans-azoben-
zene chromophore appears at 360 nm in solution and at 305 nm in the film.
Generally, the p-pꢃ transition peak of azobenzene chromophore is used to
research the aggregation of the molecule for which its transition dipole is
directed along the long axis of the azobenzene molecule. The peak position
of the film is blue-shifted to 55 nm in comparison with that in solution. The
blue shift occurs when the dipole moments of molecules in aggregates are
parallel. The parallel aggregates are known as H-aggregates. Although the
p-pꢃ transition peaks of 8AzO3 film and 8AzO5 film are blue shifted

164=[442]
N. Aoki and Y. Ueda
FIGURE 5 UV-vis absorption spectra of 12OAzO3 chloroform solution (dash line)
and of monolayer deposited on a fused silica substrate by vertical dipping method
(solid line).
approximately 20 nm in comparison with the peak position observed in sol-
ution, the blue shift of 12OAzO3 is bigger than those of 8AzO3 and 8AzO5.
This result is attributed to strong molecular interaction of 12OAzO3
because 12OAzO3 has strong hydrophobic group and ether linkage. There-
fore, 12OAzO3 didn’t show photoisomerization because it was densely
packed with the long axis parallel to each other and formed already rigid
H-aggregate on water surface.
The 8AzO5-coated glass plate was used as a substrate for LC cell
because 8AzO5 films transferred by vertical dipping method and horizontal
lifting method showed photochromism. Hydrophobic surface of 8AzO5 on
the glass plate was obtained by vertical dipping method, and hydrophilic
one was obtained by horizontal lifting method. When 5CB was sandwiched
with two glass plates covered with hydrophobic surface of 8AzO5, the trans-
mittance of the cell did not change by UV and=or visible light irradiation
(Figure 6(a)). On the other hand, the transmittance of the cell changed
reversibly by UV and visible light irradiation when 5CB was sandwiched with
hydrophobic and hydrophilic surface of 8AzO5, as shown in Figure 6(b). In
the case of DON-103, no transmittance changes revealed in the cells
constructed with hydrophilic 8AzO5 surface=DON-103=hydrophilic 8AzO5

Orientation-control of LC by Photochromism of Azobenzene
165=[443]
FIGURE 6 Transmittance changes at 633 nm by alternate exposure of UV and
visible lights in LC cell constructed with hydrophobic 8AzO5 surface=5CB=
hydrophobic 8AzO5 surface (a), and with hydrophilic 8AzO5 surface=5CB=
hydrophobic 8AzO5 surface (b).
surface and with hydrophilic 8AzO5 surface=DON-103=hydrophobic 8AzO5
surface. The transmittance change was observed only in the cell in which
DON-103 was sandwiched with two glass plates coated with hydrophobic
8AzO5 surface.
The mechanism of transmittance change of LC cell is shown schemati-
cally in Figure 7. When LC molecules were sandwiched with galss plates
modified with azobenzene LB film, they aligne by the molecular interaction
depending on the most surface structure of azobenzene film. In the cell
constructed with hydrophilic 8AzO5 surface=5CB=hydrophobic 8AzO5 sur-
face, 5CB molecules alinge by hydrophilic interaction between hydrophilic
8AzO5 surface and hydrophilic group of 5CB, and hydrophobic interaction
between hydrophobic 8AzO5 surface and hydrophobic group of 5CB as
shown left side in Figure 7(a). DON-103 molecules in the cell constructed
FIGURE 7 Schematic drawing of LC alignment change induced by photoisomeri-
zation of azobenzene in 5CB cell constructed with hydrophilic 8AzO5
surface=5CB=hydrophobic 8AzO5 surface (a), and in DON-103 cell constructed
with hydrophobic 8AzO5 surface=DON-103=hydrophobic 8AzO5 surface (b).

166=[444]
N. Aoki and Y. Ueda
with hydrophobic 8AzO5 surface=DON-103=hydrophobic 8AzO5 surface
also alinge by hydrophobic interaction between hydrophobic 8AzO5 sur-
face and hydrophobic group of DON-103 as shown left side in Figure
7(b). When the LC cell is irradiated by UV light, the trans=cis photochemi-
cal isomerization of azobenzen occurs. At that time, alignment change of
LC molecules is induced by conformational change of azobenzene, as
shown right in Figure 7(a) and (b)). After visible light irradiation, LC
molecules are reoriented in initial state and the transmittance of LC cell
is recovered.
CONCLUSION
This report provides the detailed results of the preparation of photoactive
azobenzene LB film and LC alignment on the substrate coated with azoben-
zene LB film taking various adsorption structures. The photochromic
activity of azobenzene aniphiphiles in LB film is dependent on the methyl-
ene chain length introduced as a spacer and the deposition condition. The
alignment of rodlike nematic LCs can be controlled by using the hydrophi-
lilized and=or hydrophobilized substrates modified azobenzene LB films
prepared with vertical dipping and=or horizontal lifting methods. When
amphiphilic LC (5CB) was sandwiched with hydrophililized glass plate
and hydrophobilized glass plate, the reversible transmittance change of
the LC cell was observed. In the case of DON-103 which has hydrophobic
groups at both ends of molecule, on the other hand, the transmittance
change was obserbed only in hydrophobilized glass plate=DON-103= hydro-
phobilized glass plate cell.
Photoinduced phase transitions and alignment changes of LCs are con-
sidered to have technological importance for photon-mode optical record-
ing and transducers of optical information.
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