Monolayer alignment on azobenzene surfaces during UV light irradiation: Analysis of optical polarized absorption measurement results and theoretical treatment

Article abstract of DOI:10.1063/1.2141956

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl- 4′ -cyanobiphenyl (5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2 (t) using the optical polarized absorption measurement is also investigated.

Full text of DOI:10.1063/1.2141956

Monolayer alignment on azobenzene surfaces during UV light irradiation: Analysis of
optical polarized absorption measurement results and theoretical treatment
A. V. Zakharov, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamoto
Citation: The Journal of Chemical Physics 124, 024701 (2006); doi: 10.1063/1.2141956
View online: http://dx.doi.org/10.1063/1.2141956
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THE JOURNAL OF CHEMICAL PHYSICS 124, 024701 ͑2006͒
Monolayer alignment on azobenzene surfaces during UV light irradiation:
Analysis of optical polarized absorption measurement results
and theoretical treatment
A. V. Zakharova͒
Saint Petersburg Institute for Machine Sciences, the Russian Academy of Sciences,
Saint Petersburg 199178, Russia
Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamotob͒
Department of Physical Electronics, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku,
Tokyo 152-8552, Japan
͑Received 5 July 2005; accepted 3 November 2005; published online 9 January 2006͒
The influence of the charge separation during the trans-cis conformational change on the surface
of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as
4-n-pentyl-4 -cyanobiphenyl͑5CB͒, is investigated. The effective anchoring energy ͑in the
Ј
Rapini-Papolar form͒ is phenomenologically described in the framework of the molecular model,
which takes into account the interaction between the surface polarization and surface electric field,
for number of conformational states of the boundary surface. It is shown, using the experimental
data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique,
that the charge separation during the conformational changing, caused by the UV irradiation, may
lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the
photoisomerization process on the orientational order parameter S2͑t͒ using the optical polarized
absorption measurement is also investigated. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2141956͔
One of the most attractive methods for controlling the
surface anchoring is the photoalignment technique which
uses light-induced anisotropy in mono-͑multi͒ layer͑s͒ Lang-
muir films on the photosensitive substrates. The problem of
trans-cis photoisomerization in mono- and multilayer sys-
tems remains one of the fundamental problem in Langmuir
monolayer physics.1–3 Some organic materials such as
azobenzene, which may exhibit the liquid-crystalline proper-
ties ͓azobenzene exhibits a nematic liquid-crystal ͑LC͒ phase
in the temperature range ϳ300−341 K͔, undergo a trans-cis
isomerization during a laser beam transmittance. The photoi-
somerization process, however, may result in changes of the
surface charge density caused by the charge separation, tak-
ing place during the trans-cis isomerization due to the verti-
cal component of dipole moment of surface azobenzene
monolayer. Recently, the Maxwell-displacement-current
͑MDC͒ and surface-potential measurements have been use-
fully suggested to observe the changing of the surface charge
density during the trans-cis conformational transition be-
tween the surface azobenzene monolayer and LC film.4 The
surface-potential signal allows us to determine the changing
of the surface charge density in azobenzene monolayer de-
posited on the metal electrodes ͑see Fig. 1͒.
͑632 nm in wavelength͒ undergoes the trans-cis
isomerization.5 By putting, for instance, the 6Az10PVA
monolayer in contact with the polar LC monolayer film, such
as 4-n-pentyl-4 -cyanobiphenyl͑5CB͒, one can expect the
Ј
changing, during the laser beam transmittance, of the surface
alignment of the LC molecules. Such orientational transition,
in turn, can be induced by changing of the surface charge
density caused by the charge separation, taking place during
the conformational change on the surface azobenzene mono-
layer. In order to examine the magnitude of the changing
surface charge density ⌬␴=␴trans−␴cis, we consider the data
for changing of the voltage V͑z͒ across both the 6Az10PVA
and 6Az10PVA+5CB films on the metal electrodes. Using
the potential which is built up across these films, the spatial
charge density ␳ can be written in the following form:6
␴
⑀w⑀0 dV͑z͒
␳ =
=
,
͑1͒
L
L
dz
where L is the film thickness, ␳=␴/L is the spatial charge
density, Lϳl5CB+lAz or LϳlAz is the film thickness both for
the 6Az10PVA+5CB and 6Az10PVA films on the metal
electrodes, ⑀0 is the dielectric permittivity of free space, and
z is the distance away from a metal electrodes. In the follow-
ing we use the experimental data for the dielectric constant
of 6Az10PVA ⑀wϳ3.0,7 and the values of the lengths of the
5CB and 6Az10PVA molecules to be equal to l5CB
ϳ2.0 nm and lAzϳ4.5 nm, respectively. The sequences of
the surface potential values V and the conformational states
changes both in azobenzene 6Az10PVA+5CB ͓upper lines
both in Figs. 2͑a͒ and 2͑b͔͒ and azobenzene 6Az10PVA
Specifically, it has been observed that the monolayer of
6Az5PVA, which in the initial state exhibits the trans con-
formation, with the transmittance of a He–Ne laser beam
a͒
Author to whom correspondence should be addressed. Electronic mail:
alexandre.zakharov@fys.kuleuven.ac.be
b͒
Electronic mail: iwamoto@pe.titech.ac.jp
0021-9606/2006/124͑2͒/024701/5/$23.00
124, 024701-1
© 2006 American Institute of Physics
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024701-2
Zakharov et al.
J. Chem. Phys. 124, 024701 ͑2006͒
FIG. 1. Sample structure for the surface-potential measurements of the sur-
face voltage across the 6 Az 10 PVA+5CB film on Al electrodes.
͓lower lines both in Figs. 2͑a͒ and 2͑b͔͒ films, caused by the
charge separation, taking place during trans-cis and cis-trans
isomerizations are measured using the surface-potential sig-
nals and shown in Figs. 2͑a͒ ͑case 1͒ and 2͑b͒ ͑case 2͒,
respectively.
The values of the surface charge densities ␴ and the
voltages V across both 6Az10PVA+5CB and 6Az10PVA
films were measured by the surface-potential technique, and
these data are collected in Table I.
FIG. 2. The relationship between the surface potential V and the conforma-
tional state both the 6 Az 10 PVA+5CB ͓lines ͑1͒ and ͑3͔͒ and 6Az10PVA
͓lines ͑2͒ and ͑4͔͒ films on metal electrodes, during the trans-cis and cis-
trans conformational transitions in the 6Az10PVA monolayer caused by UV
illumination, at T=298 K. ͑a͒ 6Az10PVA is in trans form at sequence 1, 3,
and 5, and in cis form at sequence 2, 4, and 6, respectively. ͑b͒ The same as
͑a͒, but for another sequence of the cis-trans conformational changes.
6Az10PVA is in cis form at sequence 1, 3, and 5, and trans form at sequence
2, 4, and 6, respectively.
produced by irradiating the spread layer with ultraviolet ra-
diation. The two molecular configurations produce qualita-
tively different isotherms. In the case of the trans conforma-
tional state of 6Az10PVA monolayer, the surface pressure
shows a steep increase at a molecular area Aϳ0.4 nm2 ͑solid
line͒, and saturates at ␲͑trans͒ϳ43 mN/m, whereas the cis
conformational state occurred at Aϳ1.2 nm2 ͑dashed line͒,
and saturates at ␲͑cis͒ϳ33 mN/m, respectively. So, the sur-
face area per molecule is markedly higher for the cis con-
figuration, which reflects the more efficient packing possible
with the trans state. The existence of higher collapse pres-
sure for the trans isomer compared to the cis isomer suggests
that intermolecular interactions are stronger in trans mono-
layer. In order to elucidate the role of the charge separation
during the trans-cis or cis-trans conformational changes on
the surface azobenzene monolayer, sandwiched between
liquid-crystal film and Al substrate, we use the molecular
model which takes into account the interactions between the
surface polarization and surface electric field. The interaction
of the LC film with the solid surface leads to changing of the
molecular orientation in the film. Our two-dimensional ͑2D͒
LC monolayer system is composed of axially symmetric
molecules, the hydrophilic heads of which are uniformly dis-
tributed on the azobenzene surface and the hydrophobic tails
are directed away from the interface. It allows us to consider
the model of axially symmetric molecular rods, the hydro-
philic heads of which are uniformly distributed on the sur-
face and the hydrophobic tails are directed away from the
water surface and tilted at the average equilibrium angle
ˆ
The isomerization involves a decrease of the distance
between the pair carbon atoms in azobenzene from about 0.9
nm in the trans form to 0.55 nm in the cis form. The trans
planar isomer, which has a rod-shaped configuration, is
transformed to the bent-shaped cis conformations in which
the −NwN− group is in a plane perpendicular to the phe-
nylene groups. Likewise, trans-azobenzene has very small
dipole moment ͑0–0.5 D͒, while the dipole moment of the cis
compound is 3.0–3.5 D.8 This large conformational change
induced by isomerization is reversible. In case 1, it was ob-
served that the initial state of surface azobenzene monolayer,
which exhibits the trans conformation with the molecular
area Aϳ0.4 nm2, with the transmittance of a He–Ne laser
beam undergoes the trans-cis, cis-trans, trans-cis, and cis-
trans sequences of the isomerizations, at the different values
of the voltages V across both 6Az10PVA+5CB and
6Az10PVA films. In case 2, the initial state of surface
azobenzene monolayer exhibits the cis conformation ͑A
ϳ1.2 nm2͒, and with the transmittance of the laser beam
undergoes the inverse, to case 1, sequence of the photoi-
somerization changes.
It should be pointed out that the voltage V across the
5CB film on 6Az10PVA monolayer, caused by UV illumina-
tion, generates a positive charge density range in the interval
of 1.66ϫ10−3 C/m2ഛ␴ഛ11.25ϫ10−3 C/m2, whereas the
voltage V across the 6Az10PVA film on Al electrode gener-
ates a negative charge density range in the interval of
−7.74ϫ10−3 C/m2ഛ␴ഛ−0.77ϫ10−3 C/m2, respectively.
The surface charge density in ␴ϳ10−3 C/m2 corresponds to
a surface ion concentration nsurf=␴/e=6.25ϫ10+15 m−2,
which is in agreement with the experimental data nsurf
Ϸ1015−1016 m−2.9 Here e is the proton charge.
␪ ͑A͒ with respect to the unit vector k directed perpendicu-
eq
ˆ
lar to the interface. The unit vector i is directed to be parallel
to the interface with the distance x, whereas z is the distance
away from the air-azobenzene interface ͑see Fig. 1͒. We use
the molecular model which takes into account the interaction
between the surface polarization ͑SP͒ and the surface electric
field originating from surface charges density ␴. In the sim-
plest case that electric field can be written in the form
The surface pressure ␲−A isothermal diagram, both for
trans and cis conformations of 6Az10PVA monolayer, is
shown in Fig. 3.
To produce the trans form, the monolayer of 6Az10PVA
was spread in the absence of light and the cis form was
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024701-3
Monolayer alignment on azobenzene surfaces
J. Chem. Phys. 124, 024701 ͑2006͒
TABLE I. The values of the surface charge density ␴ and the voltage V
across both 6 Az 10 PVA+5CB and 6Az10PVA films on Al electrode, dur-
ing the trans-cis and cis-trans conformational transitions in the 6Az10PVA
monolayer caused by UV illumination, at T=298 K.
trans
cis
trans
cis
6 Az 10 PVA+5CB monolayer ͑trans form͒
␴͑ϫ10−3 C/m2͒
V͑mV͒
9.75
87.5
4.94
58.0
6.0
1.66
19.5
70.5
6Az10PVA monolayer ͑trans form͒
␴͑ϫ10−3 C/m2͒
V͑mV͒
−6.3
−1.49
−7.74
−1.09
−18.5
−106
−25
−131
trans
cis
trans
cis
6 Az 10 PVA+5CB monolayer ͑cis form͒
FIG. 3. ␲−A isotherm of the 6Az10PVA monolayer during the lateral com-
pression, for two trans ͑solid line͒ and cis ͑dashed line͒ conformational
states of azobenzene monolayer.
␴͑ϫ10−3 C/m2͒
V͑mV͒
11.25
6.37
7.48
2.15
31
131.5
75
88
6Az10PVA monolayer ͑cis form͒
␴͑ϫ10−3 C/m2͒
V͑mV͒
−6.7
−0.77
−5.9
−1.49
25
as 11.0342. Hence, the following expression for the ⌬͑A͒
−113
−13
−100
is
obtained,4
⌬͑A͒=␮zF͑A͒cos ␪s.
Here
F͑A͒=1
−͑10.27␣/A3/2͓͒1+͑⑀w−1/⑀w+1͔͒, ␮z is the z component of
the molecular dipole moment corresponding to the tail of the
5CB molecule on the azobenzene surface, ⑀w is the dielectric
ˆ
ˆ
¯
¯
E=E0k=␴/⑀0⑀k, where ⑀=͑⑀ʈ +2⑀Ќ͒/3 is the average di-
electric permittivity, and ⑀ʈ and ⑀Ќ are the dielectric con-
stants parallel and perpendicular to the director n, respec-
tively. The electric field E has, therefore, an orienting effect
on the LC monolayer, and the related dielectric energy per
unit area can be written in the form
constant of 6Az10PVA, a=͑2A/ 3͒1/2, A is the molecular
ͱ
ˆ
area in Å2, and ␣ is the electronic polarization given by9
3
ˆ
͉4␲⑀0͉͑Å ͒. By putting the easy axis n0 normal to the sub-
strate ͑␪0=0͒, and by integrating over the z component of the
d
2
ˆ
¯
−Ps·kE0, one has −͐0Ps·Edz=−͑␴⌬͑A͒/⑀⑀0A͒cos ␪s. So,
the total energy per unit area, playing the role of the effective
anchoring energy, is given by
d
d
1
2
ˆ
f =
el
F ͑z͒dz = − ⑀ ⑀
͑E · n͒ dz
͵
͵
el
0
a
2
0
0
1
1
1
feff = − weff cos2 ␪s = − ͑␣1␴2 + ␣2␴͒cos2 ␪s,
͑4͒
= − wel cos2͑␪s − ␪ ͒,
͑2͒
0
2
2
2
d
2
2
2
where weff=␣1␴2+␣2␴ is the effective anchoring energy
¯
where wel=⑀0⑀a͐0E ͑z͒dz=␴ ⑀ad/⑀ ⑀0,d is the monolayer
strength in the Rapini-Popular form,13 ␣1=͑⑀ad͒/͑⑀ ⑀0͒, ␣2
2
−1
ˆ
¯
ˆ
thickness, ␪s=␪eq=cos ͑n·k͒ is the angle between the sur-
face director and the substrate normal, ␪0=cos ͑n0·k͒ is the
polar angle of the easy axis n0, and ⑀a=⑀ʈ −⑀Ќ is the dielec-
¯
=͑2⌬͑A͒͒/͑⑀⑀0A͒, and ␴ is the surface charge density corre-
−1
ˆ
ˆ
sponding to the trans or cis conformational state.
ˆ
It is important to stress here that the coefficient ␣1 is the
constant, and has dimension in meters, whereas coefficient
␣2=␣2͑A͒ is the function of the molecular area A, and has
dimension in D/m2. In order to elucidate the role of the
charge separation during the conformational trans-cis or cis-
trans changing on the surface azobenzene monolayer or in
azobenzene+5CB monolayer film, caused by the ultraviolet
͑or visible light͒ irradiation, one needs the data ⑀ʈ, ⑀Ќ, A, ␮,
and ␴. In the following, we use the calculated ⑀ʈ =18, ⑀Ќ
tric anisotropy of the LC monolayer. The additional contri-
bution to the effective anchoring energy should be accounted
d
ˆ
for by adding the term −͐0Ps·Edz, where Ps=ns⌬͑A͒n
ˆ
ˆ
=ns⌬͑A͒͑sin ␪si+cos ␪sk͒, ns is the film surface density
given by ns=1/Ad, ⌬͑A͒=␮ cos ␪s+4␲⑀0␣Ek is the total
z
magnitude of the depolarized dipole moment of the 5CB
molecules on the azobenzene surface, ␮ is the z component
z
of the molecular dipole moment corresponding to the tail of
the 5CB molecule on the azobenzene surface, ␣ is the elec-
tronic polaralizability, and Ek is the depolarizing field.10 The
depolarizing field due to a single-point dipole of moment ␮
¯
=8, ⑀=11.3, and ⑀a=10 from molecular-dynamics simulation
of 5CB at T=300 K,14 using the conventional potential-
energy function composed of intra- and intermolecular con-
tributions. Taking into account that the electronic polarizabil-
ity of a C-H bond is ϳ0.65ϫ4␲⑀0 ͑Å3͒,9 the upper bonds
for ␣ can be taken as ϳ1.95ϫ4␲⑀0 ͑Å3͒.11 Figure 4 shows
the dependence of weff͑␴͒ on the surface charge density ␴,
both for trans ͑Aϳ0.4 nm2͒ and cis ͑Aϳ1.2 nm2͒ confor-
mational states in 6Az10PVA+5CB monolayer film. In the
framework of the SP mechanism, such changing of the an-
choring energy can be explained by changing of the surface
in the plane z=0 is given by11 E =−␮/͑4␲⑀0a ͒k, where a is
s
3
ˆ
k
the distance between the nearest neighbors. Summing for the
11
+ϱ
infinite array, the total field Ek=͚i=−ϱEik is given by
+ϱ +ϱ
␮
2
2
3
ˆ
ͱ1/͑u + ␷ − u␷͒ k
Ek = −
͚ ͚
4␲⑀0a3
u=−ϱ ␷=−ϱ
11.0342␮
4␲⑀0a3
ˆ
= −
k.
͑3͒
charge density, for instance, from ␴
taking place during UV irradiation.
to ␴ ͑see Fig. 4͒,
trans
sic
Here the double summation has been evaluated by Topping12
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024701-4
Zakharov et al.
J. Chem. Phys. 124, 024701 ͑2006͒
FIG. 5. In situ observation of the second orientational order parameter S2͑t͒
during 5CB deposition onto the trans ͑a͒ and cis ͑b͒ conformation azobenze
monolayers. The solid lines represent that azobenzene was in trans form
͑curves 1 and 3͒ when Langmuir-Blodgett ͑LB͒ deposition was done and the
dashed lines represent that azobenzene was in cis form ͑curves 2 and 4͒
when LB deposition was done. Surface pressure of 6Az10PVA was kept at
5 mN/m during deposition.
FIG. 4. The surface charge density ␴ dependences of weff͑␴͒ calculated
using Eq. ͑4͒ at T=300 K for cis ͑circles͒ and trans ͑squares͒ conforma-
tional states, respectively.
cis
In that case, wterfafns=w , and one deals with changing of
eff
the surface charge density from ␴trans=2.15ϫ10−3 C/m2 to
␴
cis=1.66ϫ10−3 C/m2, respectively. Such changing of the
ecule on the azobenzene monolayer in the trans conforma-
tional state is different from the value of ⌬z͑Acis͒ correspond-
ing to cis conformational state. So, by using of equation
surface charge density in ⌬␴=␴trans−␴cisϳ4.9ϫ10−4 C/m2
can be caused by the charge separation taking place during
the conformational change on the surface azobenzene
6Az10PVA+5CB monolayer. In order to examine the mag-
nitude of the changing surface charge density ⌬␴, one should
be considered data for changing of the voltage V across the
6Az10PVA+5CB film on metal electrodes. The surface
charge density ␴transϳ2.15ϫ10−3 C/m2 ͑see Fig. 4͒, corre-
sponding to the trans conformational state of the surface
azobenzene monolayer, may realized at Vϳ31 mV ͓see Fig.
⌬ = ␮zF͑A͒cos ␪s,
one can calculate the tilt angle ␪s=cos−1͓⌬͑A͒/␮zF͑A͔͒,
which is composed of director with respect to the air-solid
interface. At the molecular area Atransϳ0.4 nm2, the z com-
ponent of the total dipole moment ⌬z͑Atrans͒, greater than
⌬z͑Acis͒, at the molecular area Acisϳ1.2 nm2. It means that
ˆ
the director ns in the 5CB film both on trans and cis confor-
2 ͑upper curve, N=5͔͒, whereas the surface change ␴
cis
ϳ1.66ϫ10−3 C/m2, corresponding to the cis conformational
state of the surface azobenzene monolayer, may realized at
Vϳ19.5 mV ͓see Fig. 2 ͑upper curve, N=5͔͒, respectively.
So, our analysis shows that the conformational changes
trans-cis, during the photoisomerization in azobenzene
monolayer, caused by the UV ͑or visible light͒ irradiation
may lead to changing of the surface alignment of liquid-
crystalline molecules, such as 5CB, having contact with that
solid substrate.
mational states of 6Az10PVA monolayer aligns to be tilted to
the interface, and that tilting is higher in the case of cis
conformational state ͓␪ ͑Acis͒Ͼ␪ ͑Atrans͔͒ than in the case of
s
s
trans conformational state of 6Az10PVA monolayer.
Our optical polarized absorption measurement, basically
based on the linearly polarized absorption measurement,16,17
using 6Az10PVA azobenzene film deposited in trans-form,
has shown that the time-dependent order parameter ͑OP͒
S2͑t͒= 3 ͗cos2 ␪ ͑t͒͘− 1 , ͓␪ ͑t͒ is the polar angle between the
i
i
2
2
ˆ
molecular axis i and the director n, and the angular bracket
denotes the statistical-mechanical average͔ during the depo-
sition of 5CB molecules by evaporation onto the film, after
changing into cis-form azobenzene ͑lower lines in Fig. 5͒,
In the wide region of the molecular area, the MDC sig-
nals allows us to determine the dipole moment ⌬͑A͒ of the
cyanobiphenyl molecules at the air-solid interface. It has
been found from MDC measurement of 5CB molecules at
the air-water interface15 that with decreasing of the molecu-
results in the final trans state, with limt→t S2͑t͒ϳ−0.2,
1
whereas the behavior of the S2͑t͒, during the deposition of
5CB molecules onto the same film, corresponding to the
trans-form 6Az10PVA azobenzene film ͑upper lines in Fig.
5͒, results in the final cis state, with limt→t S2͑t͒ϳ0.1, re-
spectively, with t1ϳ60 min. Experimentally, 1this means that
the orientational order in the monolayer film of 5CB mol-
ecules on 6Az10PVA azobenzene monolayer reflects the con-
formational changes of azobenzene surface layer, and the
inequality ␪ ͑Acis͒Ͼ␪ ͑Atrans͒ is valid over the entire region
lar area the z component of the molecular dipole moment ␮
z
corresponding to the tail of the 5CB molecule on the water
surface increases from zero, at the molecular area of
0.6 nm2ഛAഛ1.2 nm2, up to 0.5 D per molecule, at the mo-
lecular area of 0.2 nm2ഛAϽ0.6 nm2. Physically, this means
that the initial dipole moment of the 5CB molecule ͑␦
ϳ5D͒ is the fully compensated, due to interaction with the
water molecules, at the molecular area of 0.6 nm2ഛA
ഛ1.2 nm2, and that compensation is decrease up to 0.5 D per
molecule, with increasing of the molecular area. When the
same tendency is true for the case of 5CB+azobenzene sys-
tem, the value of the total molecular dipole moment
⌬z͑Atrans͒ corresponding to the z component of the 5CB mol-
s
s
of the deposition. Indeed, it is expected that in the final stage,
the average tilting angle ␪s of 5CB molecules on the cis-form
film is ␪ ͑cis͒ϳ63°, whereas in the case of 5CB molecules
s
on the trans-form film that angle is ␪ ͑trans͒ϳ50.8°. So,
s
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024701-5
Monolayer alignment on azobenzene surfaces
J. Chem. Phys. 124, 024701 ͑2006͒
according to our optical polarized measurements, both the
trans and cis molecular configurations of 6Az10PVA pro-
duce qualitatively different alignments of the 5CB film on
is shown, using the experimental data for the voltage across
the 6Az10PVA+5CB film, provided by the surface-potential
technique, that the charge separation during the conforma-
tional changing, caused by the UV irradiation, may lead to
changing of the surface alignment of liquid-crystalline mol-
ecules. We believe that the present study not only shows
some useful routes for understanding the nature of the sur-
face charge-density changes due to photoisomerization in
Langmuir films, but also for analyzing the surface potential
at the metal-organic interface.
azobenzene substrate, with the average tilting angle ␪ ͑cis͒
s
ϳ63°, corresponding to the cis form of the azobenzene film,
and ␪ ͑trans͒ϳ50.8°, corresponding to the trans form of the
s
azobenzene film, respectively.
The azobenzene in the trans phase, which has a rod-
shaped configuration, produce a higher degree of orienta-
tional order in two-dimensional 5CB monolayer ͓S2͑trans͒
ϳ0.1͔ than the azobenzene in the cis phase ͓S2͑cis͒ϳ−0.2͔,
with a bent-shaped configuration. This suggests that there are
additional molecular interactions between azobenzene mol-
ecules in cis monolayer and 5CB molecules in LC film com-
pared to the trans+5CB system. Actually, our optical polar-
ized absorption measurement using the 6Az10PVA
azobenzene film deposited in cis form, but changed into
trans form during the deposition of 5CB molecules by
evaporation onto the azobenzene film, was somewhat smaller
and results in the final state with limt→t S2͑t͒ϳ0. Taking into
account that the trans-azobenzene has 1almost no dipole mo-
ment, while the dipole moment of the cis compound is high,
the dipolar interaction between the hydrophilic head of the
5CB and cis-azobenzene molecules decrease the orienta-
tional ordering in the LC film.
One of the authors ͑A.V.Z.͒ gratefully acknowledges the
receipt of a Senior Research Fellowship of the Research
Council of the K. U. Leuven.
1 K. Ichimura, Y. Suzaki, T. Seki, A. Hosoki, and K. Aoki, Langmuir 4,
1214 ͑1988͒.
2 K. Ichimura, in Photochromism: Molecules and Systems, edited by H.
Durr and H. Bouas-Laurent ͑Elsevier, Amsterdam, 1990͒.
3 T. Ikeda, T. Sasaki, and K. Ichimura, Nature ͑London͒ 361, 428 ͑1993͒.
4 M. Iwamoto and W. Chen-Xu, The Physical Properties of Organic Mono-
layers ͑World Scientific, Singapore, 2001͒.
5 A. V. Zakharov and M. Iwamoto, J. Chem. Phys. 118, 10758 ͑2003͒.
6 M. Iwamoto, A. Fukuda, and E. Itoh, J. Appl. Phys. 75, 1607 ͑1994͒.
7 W. Y. Kim, M. Iwamoto, and K. Ichimura, Jpn. J. Appl. Phys., Part 1 35,
5395 ͑1996͒.
8 K. S. Yin and G. G. Fuller, Phys. Rev. E 67, 041601 ͑2003͒.
9 J. N. Israelachvili, Intermolecular and Surface Forces, 2nd ed. ͑Aca-
demic, London, 1992͒.
In this paper we investigate the effect of the charge sepa-
ration during the trans-cis conformational change on the
surface of azobenzene 6Az10PVA monolayer on
the polar liquid-crystal monolayer film, such as 4-n-pentyl
10 H. Fröhlich, Theory of Dielectrics ͑Clarendon, Oxford, 1958͒.
11 D. M. Taylor and G. F. Bayes, Phys. Rev. E 49, 1439 ͑1994͒.
12 J. Topping, Proc. R. Soc. London, Ser. A 114, 67 ͑1927͒.
13 A. Rapini and M. Papoular, J. Phys. Colloq. 30, C4-54 ͑1969͒.
14 A. V. Zakharov and A. Maliniak, Eur. Phys. J. E 4, 435 ͑2001͒.
15 A. Tojima, T. Manaka, and M. Iwamoto, J. Chem. Phys. 115, 9010
͑2001͒.
-4 -cyanobiphenyl͑5CB͒, both experimentally, using the
Ј
surface-potential measurements, and theoretically, in the
framework of the molecular model, which takes into account
interactions between the surface polarization and electric
field build up across the LC monolayer film, for number of
conformational states of the boundary azobenzene surface. It
16 D. Taguchi, T. Manaka, and M. Iwamoto, Jpn. J. Appl. Phys., Part 1 44,
1037 ͑2005͒.
17 D. Taguchi, M. Hamatsu, K. Kitazawa, T. Manaka, and M. Iwamoto,
Thin Solid Films ͑in press͒.
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