Reversible Light-Driven Structural Changes between the Mono- and Bilayer Stacking Alignments in 4-Octadecyloxystilbazolium Arylcarboxylate Films

Article abstract of DOI:10.1021/ja030310s

The present investigation deals with the light-driven morphological changes in multilamella films of N-methyl-4-octadecyloxystilbazolium arylcarboxylates (C₁₈OStz⁺X^- cast on glass slides. The results of XRD analysis show a

Full text of DOI:10.1021/ja030310s

Published on Web 10/31/2003
Reversible Light-Driven Structural Changes between the
Mono- and Bilayer Stacking Alignments in
4-Octadecyloxystilbazolium Arylcarboxylate Films
Osamu Ohtani,† Hiroki Kato,† Tatsuto Yui,†,‡ and Katsuhiko Takagi*,†
Contribution from the Department of Crystalline Materials Science, School of Engineering,
Nagoya UniVersity, Chikusa, Nagoya 464-8603, Japan and CREST (JST)
Received May 21, 2003; E-mail: ktakagi@apchem.nagoya-u.ac.jp
Abstract: The present investigation deals with the light-driven morphological changes in multilamella films
of N-methyl-4-octadecyloxystilbazolium arylcarboxylates (C18OStz+X-) cast on glass slides. The results of
XRD analysis show a photostimulated layer expansion and shrinkage of the stacked thin films along the
c-axis under alternative illumination at >350 and 254 nm, respectively. It was revealed that such lamellar
changes could be switched either way by a reversible transformation between the mono- and bilayer units
in these stacked multilamella films. Moreover, such controlled structural adjustments in the alignment could
be initiated by the photocyclodimerization of the stilbazolium moieties of the arylcarboxylate salts; i.e., a
monolayer-to-bilayer transformation could be induced at a stage of only 10% cyclodimer formation. The
photoinduced patterning on the surface of the films was also analyzed by SEM and fluorescence microscopic
investigations.
Introduction
methylammonium bromide and cinnamic acid take place not
only in dispersion but also when cast on glass slides, and that
Much interest has recently been focused on photochemically
functional devices composed of light-responsive materials in
organic supramolecular matrixes.1 Along these lines, Langmuir-
Blodgett (LB) membranes as well as micelles and vesicles have
been used as the potent hosts able to organize ionic organic
materials in self-assembling multilayer films on glass slides.2
such changes can be induced by a photochemical E-Z photo-
isomerization and/or a [2+2] photocycloaddition of the cin-
namate regions.9-15 These self-assembling structures have been
revealed to be variable with the photochemical transformations
of the component olefins.
Although we have briefly communicated an example of the
controlled conformational reversibility in cast films of N-methyl-
There have been a number of works dealing with the
morphological changes which can be induced in self-assembling
materials by their surrounding physicochemical driving forces.3-6
Many studies of the E-Z photochemical isomerization of
azobenzenes have been carried out to investigate the morpho-
logical changes seen in organic solid matrixes. The polymer
matrixes covalently bonded by the azobenzene units have been
noted to undergo morphological changes,4 and a structural
reversibility has also been observed between the bilayer and
amorphous layers in such self-assembling hydrogen-bonded
azobenzene under UV illumination.7,8
4-octadecyloxystilbazolium isophthalate (C18OStz+X-; X-
)
1,3-benzenedicarboxylate),14 the present paper is a comprehen-
sive summary of our further detailed investigations into the
photoinduced reversible multilayer conformational changes
induced in the cast film of C18OStz+X- (Chart 1) by photo-
chemical [2+2] cyclodimerization and cycloreversion upon UV
light irradiation. Such a control over the properties of the
stacking alignment in these thin films can be considered
significant for its potential in the design of photofunctional
materials.
We have previously reported that structural changes in 1:1
ion-pair composite bilayer films consisting of dioctadecyldi-
Experimental Section
Materials. A. 4-Octadecyloxy-N-methylstilbazolium Salts (C18-
OStz+X-). n-Octadecyl bromide, 4-hydroxybenzaldehyde, methyl
iodide, 4-methylpyridine, and 1,2-, 1,3-, and 1,4-benenedicarboxylic
† Nagoya University.
‡ CREST.
(1) Fukuda, K.; Nakahara, H.; Shibasaki, Y. The Chemistry of Supra Thin
Molecular Organized Film; Kodan-sha: Tokyo, 1993.
(2) Shimomura, M. Immobiled Bilayer Membrane; Bunshin-sha: Tokyo, 1990;
pp 96-136.
(3) Sommerdijk, N. A. J. M.; Booy, K. J.; Pistorius, A. M. A.; Feiters, M. C.;
Nolteand, R. J. M.; Zwanenburg, B. Langmuir 1999, 15, 7008.
(4) Jonas, U.; Shah, K.; Norvez, S.; Charrych, D. H. J. Am. Chem. Soc. 1999,
121, 4580
(9) Takagi, K.; Itoh, M.; Usami, H.; Imae, T.; Sawaki, Y. J. Chem. Soc., Perkin
Trans. 1994, 1003.
(10) Takagi, K.; Sawaki, Y. Chem. Lett. 1993, 2103.
(11) Nakamura, T.; Takagi, K.; Fujita, K.; Katsu, H.; Itoh, M.; Imae, T.; Sawaki,
Y. J. Chem. Soc., Perkin Trans. 1997, 2, 2751.
(12) Takagi, K.; Nakamura, T.; Katsu, H.; Itoh, M.; Sawaki, Y.; Imae, T. Mol.
Cryst. Liq. Cryst. 1996, 277, 135.
(5) Aoki, K.; Nakagawa, M.; Ichimura, K. J. Am. Chem. Soc. 2000, 122, 10997.
(6) Seki, T.; Fukuda, K.; Ichimura, K. Langmur 1999, 15, 5098.
(7) Seki, T.; Ichimura, K. Thin Solid Films 1989, 179, 77.
(8) Tamaoki, N. Angew. Chem. Int. Ed. 2001, 40, 1135.
(13) Takagi, K.; Aoshima, K.; Sawaki, Y. J. Am. Chem. Soc. 1985, 107, 47.
(14) Ohtani, O.; Kato, H.; Takagi, K. Chem. Lett. 2002, 49.
(15) Ohtani, O.; Sasai, R.; Adachi, T.; Takagi, K.; Hatta, I. Langmuir 2002, 16,
1165.
9
10.1021/ja030310s CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 14465-14472
14465

A R T I C L E S
Ohtani et al.
Chart 1. Ion Pairs of C18OStz+X- Including the Varying X-
J ) 6.3 Hz), 4.54 (3H, s, N-Me), 6.95 (2H, d, aromatic protons, J )
8.6 Hz), 7.00 (1H, d, olefin, J ) 16.0 Hz), 7.69 (2H, d, aromatic protons,
J ) 9.0 Hz), 7.65 (1H, d, olefin, J ) 16.0 Hz), 7.94 (2H, d, pyridinium
protons, J ) 7.0 Hz), 8.90 (2H, d, pyridinium protons, J ) 7.2 Hz).
UV-vis (MeOH): λmax 385 nm (ꢀ ) 25000).
2. Perchlorate (X- ) ClO4-). The iodide salt C18OStz+I- (11.4
mg) dissolved in methanol (20 mL) was added to excess amounts of a
saturated aqueous solution of sodium perchlorate to give a yellow
precipitate by filtration after condensation of the solvent. Yield: 75%.
Mp: 150-158 °C. ESI-MS: m/e ) 464 (rel intens 100, M - ClO4-).
3. Carboxylates. In addition to the iodide and perchlorate salts,
aromatic carboxylate salts of C18OStz were obtained by substituting
C18OStz+I- with the corresponding sodium aromatic carboxylates in
aqueous MeOH.
a. Benzoate. Yield: quantitative. Mp: 96 °C dec.
1
b. 2-Hydroxybenzoate. Yield: 76%. Mp: 113-115 °C. H NMR
(δ, Varian INOVA 500 MHz spectrometer, CDCl3): 0.88 (3H, t, Me,
J ) 7.5 Hz), 6.83 (2H, d, aromatic proton, J ) 8.0 Hz), 6.93 (1H, d,
olefin, J ) 15.5 Hz), 6.95 (1H, d, aromatic proton, J ) 9.0 Hz), 7.21
(1H, d, olefin, J ) 7.0 Hz), 7.55 (1H, d, aromatic proton, J ) 7.5 Hz),
7.57 (1H, d, olefin, J ) 16.5 Hz), 7.81 (2H, d, aromatic protons, J )
6.5 Hz), 7.94 (2H, d, pyridinium protons, J ) 7.5 Hz), 9.89 (2H, d,
pyridinium protons, J ) 7.5 Hz). ESI-MS: m/e ) 464 (rel intens 100,
M - C7H5O3-).
c. 3-Hydroxybenzoate. Yield: 92%. Mp: 93 °C dec.
d. 4-Hydroxybenzoate. Yield: 75%. Mp: 92 °C dec.
e. 1-Hydroxy-2-naphthoate. Yield: 80%. Mp: 122-124 °C.
f. 1,2-Benzenedicarboxylate. Yield: 95%. Mp: 105-107 °C.
1
g. 1,3-Benzenedicarboxylate. Yield: 96%. Mp: 129-132 °C. H
NMR (δ, Varian INOVA 500 MHz spectrometer, CD3OD): 0.89 (6H,
t, Me, J ) 7.0 Hz), 1.28 (68H, m, CH2), 4.04 (4H, t, OCH2, J ) 6.5
Hz), 4.27 (6H, s, N-Me), 7.00 (4H, d, aromatic protons, J ) 9.0 Hz),
7.24 (2H, d, olefin, J ) 16.0 Hz), 7.33 (1H, t, protons at the 5-position
of isophthalic acid, J ) 7.5 Hz), 7.69 (4H, d, aromatic protons, J )
8.5 Hz), 7.87 (2H, d, olefin, J ) 16.5 Hz), 7.97 (2H, q, protons at the
4- and 6-positions of isophthalic acid, J ) 7.5, 2.0 Hz), 8.07 (4H, d,
pyridinium protons, J ) 7.0 Hz), 8.61 (1H, t, proton at the 2-position
of isophthalic acid, J ) 2.0 Hz), 8.63 (4H, d, pyridinium protons, J )
7.0 Hz). ESI-MS: m/e ) 464 (rel intens 100, M - C8H5O42-).
Table 1. Product Distributions of the C18OStz+X- Films after
Irradiation at >350 nm
selectivity/mol %
irradiatn
X-
time/h
trans
cis
syn HH
anti HH
perchlorate
2
10
2
2
6
2
15
2
2
1
85
81
53
82
69
78
63
73
71
53
40
62
15
19
32
13
19
22
37
13
29
21
32
21
0
0
0
0
0
0
0
0
0
0
0
15
5
12
0
0
14
0
13
15
9
2-hydroxybenzoate
3-hydroxybenzoate
h. 1,4-Benzenedicarboxylate. Yield: quantitative. Mp: 124-125
°C.
4-hydroxybenzoate
B. syn and anti Head-to-Head Cyclodimers of MeOStz+X-
Salts. syn and anti head-to-head cyclodimers (HH dimers) as well as
the syn head-to-tail cyclodimer (HT dimer) of 4-methoxy-N-methyl-
stilbazolium ions (MeOStz+) were prepared by irradiation of the AOT
(sodium bis(2-ethylhexyl)sulfosuccinate) salt of MeOStz. Irradiation
of a 1 L hexane solution of the AOT salt of MeOStz was carried out
by a high-pressure Hg lamp for 16 h through a 10% KNO3 solution
(>350 nm). Here, the AOT salt of MeOStz was obtained by extraction
with hexane from a mixture of MeOStz+Cl- (488.5 mg) in H2O with
AOT (2.06 g) in CH2Cl2. After hydrolysis with 6 N HCl (70 mL)
followed by neutralization by 1 N NaOH, the photolyzed mixture was
separated on a SiO2 column with acetone-ethyl acetate (1:1 v/v) as
the eluent to yield the starting monomers (Rf ) 0.41), syn HT dimer
(Rf ) 0.28), anti HH dimer (Rf ) 0.41), and syn HH dimers (Rf )
0.08) as free bases in this order. The obtained cyclodimers were then
methylated under reflux with excess amounts of methyl iodide in
benzene to obtain yellow precipitates. The characterization data are as
follows. 1H NMR (δ, 200 MHz, CDCl3): (syn HH) 3.70 (6H, s, MeO),
4.29 (6H, s, N-Me), 4.66 (2H, d, cyclobutane protons, J ) 7.0 Hz),
5.06 (2H, d, cyclobutane protons, J ) 7.2 Hz), 6.75 (4H, d, aromatic
benzoate
1-hydroxy-2-naphthoate
1,2-benzenedicarboxylate
1,3-benzenedicarboxylate
1,4-benzenedicarboxylate
13
13
8
1
1
acids of extra pure grade were purchased from Tokyo Kasei Co. Ltd.
4-n-Octadecyloxybenzaldehyde was obtained by mixing n-octadecyl
bromide (40 mmol) with 4-hydroxybenzaldehyde (40 mmol) and
anhydrous potassium carbonate (42 mmol) in 300 mL of dry acetone.
After the mixture was heated under reflux for 36 h, the product was
obtained by extraction with hexane and purification by recrystallization
in hexane. 4-[2-(4-n-Octadecyloxy)phenylethenyl]pyridine (C18OStz)
was obtained by heating a mixture of 4-n-octadecyloxybenzaldehyde
(6.34 mmol) with 4-methylpyridine (19 mmol) in acetic anhydride (20
mmol) under reflux for 16 h. A number of quaternary salts (Table 1)
were synthesized as follows.
1. Iodide. The iodide was obtained by mixing C18OStz with methyl
iodide (20 mmol) in 30 mL of benzene. The resulting pale yellow solid
powder was purified by silica gel column chromatography using hexane
and benzene (1:3 v/v), and recrystallized with hexane. Yield: 99%.
1
Mp: 209-212 °C (lit.16 >200 °C). H NMR (δ, 200 MHz, CDCl3):
(16) Lupo, D.; Scheunemann, U.; Ringsdorfs, H.; Ledoux, I. J. Opt. Soc. Am.
B 1988, 5, 300.
0.87 (3H, t, Me, J ) 6.3 Hz), 1.25 (34H, m, CH2), 4.01 (2H, t, OCH2,
9
14466 J. AM. CHEM. SOC. VOL. 125, NO. 47, 2003

Structural Changes in C OStz+X- Films
A R T I C L E S
18
protons, J ) 8.8 Hz), 7.14 (4H, d, aromatic protons, J ) 8.8 Hz), 8.00
(4H, d, pyridinium protons, J ) 7.0 Hz), 8.69 (4H, d, pyridinium
protons, J ) 6.8 Hz); (anti HH) 3.90 (6H, s, MeO), 4.34 (6H, s, N-Me),
4.01 (2H, d, cyclobutane protons, J ) 9.2 Hz), 4.14 (2H, d, cyclobutane
protons, J ) 9.0 Hz), 6.96 (4H, d, aromatic protons, J ) 8.6 Hz), 7.47
(4H, d, aromatic protons, J ) 8.6 Hz), 8.00 (4H, d, pyridinium protons,
J ) 6.8 Hz), 8.77 (4H, d, pyridinium protons, J ) 7.0 Hz); (syn HT)
3.70 (6H, s, MeO), 4.26 (6H, s, N-Me), 4.98 (4H, t, cyclobutane
protons), 6.79 (4H, d, aromatic protons, J ) 8.6 Hz), 7.39 (4H, d,
aromatic protons, J ) 8.8 Hz), 8.00 (4H, d, pyridinium protons, J )
6.8 Hz), 8.65 (4H, d, pyridinium protons, J ) 6.6 Hz).
C. syn Head-to-Head Cyclodimers of C18OStz+X- Salts. The syn
HH cyclodimers of N-hydrogen 4-n-octadecyloxystilbazolium chloride
were prepared by irradiating a 60 mL aqueous solution of this chloride
obtained by treating the free base C18OStz (50.6 mg) with 10 mL of
0.6 N HCl. Irradiation was carried out by a high-pressure Hg lamp for
16 h through a 10% KNO3 solution (>350 nm) under an Ar atmosphere.
The photolyzed mixture was neutralized by 1 N NaOH, extracted with
CHCl3, and then separated on a SiO2 column eluted with acetone-
benzene (1:1 v/v) to yield the starting monomers (Rf ) 0.58) and syn
HH dimers (Rf ) 0.33). The resulting HH dimers were then methylated
with excess amounts of methyl iodide under reflux for 2 h to yield the
Figure 1. Changes in the absorption spectra of the cast film of C18OStz+X-
(X- ) C6H5CO2-) under UV irradiation for 2 h.
1
corresponding iodide. Detailed data are as follows. H NMR (δ, 200
glass plate of 5 mm thickness and irradiated from the film surface at
an ambient temperature of around 20 °C, in turns, with light of λ >
350 nm through a filter of aqueous 10% potassium nitrate with a 300
W medium-pressure Hg lamp and by λ ) 250.5 nm light with a xenon
lamp of a FP-750 fluorescence spectrophotometer equipped with a
monochromator.
MHz, CDCl3): 0.90 (6H, t, Me), 1.30 (68H, m, CH2), 3.89 (4H, t,
OCH2), 4.34 (3H, s, N-Me), 5.10 (2H, d, cyclobutane protons), 6.74
(4H, d, aromatic protons, J ) 8.6 Hz), 7.12 (4H, d, aromatic protons,
J ) 8.6 Hz), 8.04 (4H, d, pyridinium protons, J ) 7.0 Hz), 8.77 (4H,
d, pyridinium protons, J ) 6.8 Hz). ESI-MS: m/e ) 464 (rel intens100,
dication), 928 (rel intens <20, monocation).
D. (Z)-C18OStz Salts. The Z-isomer (cis-isomer) of 4-n-octadecyl-
oxy-N-methylstilbazolium bromide ((Z)-C18OStz+Br-) was prepared in
a diluted chloroform solution (ca. 10-4 M) by >310 nm light irradiation
for 24 h. 1H NMR (δ, Varian INOVA 500 MHz spectrometer,
CDCl3): 0.87 (3H, t, Me, J ) 6.3 Hz), 1.25 (34H, m, CH2), 3.97 (2H,
t, OCH2, J ) 6.5), 4.62 (3H, s, N-Me), 6.46 (1H, d, olefin, J ) 12.0
Hz), 6.85 (2H, d, aromatic protons, J ) 9.0 Hz), 7.06 (1H, d, olefin,
J ) 12.0 Hz), 7.18 (2H, d, aromatic protons, J ) 9.0 Hz), 7.74 (2H, d,
pyridinium protons, J ) 6.5 Hz), 9.00 (2H, d, pyridinium protons, J )
7.0 Hz). ESI-MS: m/e ) 464 (rel intens 100, M - Br-).
Measurements. X-ray diffraction analysis was carried out with a
RINT 2000 diffractometer (Rigaku) using Cu KR radiation, operating
at 40 kV and 40 mA as the applied voltage and current, respectively.
The ultraviolet-visible (UV-vis) absorption spectra were recorded on
JASCO (type V-550) and Shimadzu (type UV-265) spectrophotometers.
The polarized infrared (IR) and ultraviolet (UV) absorption spectra were
recorded on a Hitachi FT-IR spectrophotometer with a polarized unit
attachment and a JASCO V-500 spectrophotometer with a JASCO
polarizer unit attachment, respectively. The 1HNMR spectra were
obtained in CDCl3 using TMS as an internal standard reference sample
by a Varian Gemini 200 MHz FT spectrometer (unless otherwise
specified).
Preparation of the Films. A. Cast Films. A 0.3 mM solution of
C18OStz+X- in 10% aqueous methanol was cast on a silica glass slide
to give a transparent thin film by heating in a water bath at 60 °C for
a few hours and subsequently cooling to 20 °C.
B. LB Membranes. A sample solution was prepared by dissolving
several milligrams of C18OStz+X- in 0.5 mL of ethanol, which was
then diluted to 5 mL by adding chloroform. The resulting chloroform
solution was gently developed using a 50 µL microsyringe on the
surface of water placed in a Teflon-coated trough of an LB membrane
apparatus from the Nippon Laser Electronics Corp. (type NL-BIO20).
The pressure-area (π-A) isotherm profiles were taken with the
evaporation of the solvents after the solutions stood for 30 min under
a ventilated atmosphere.
Results and Discussion
Photolysis of the Thin Films of C18OStz+X-. Irradiation
of the C18OStz+X- thin films with light of >350 nm was
continued until the disappearance of its absorbance intensities
at 350-400 nm. Figure 1 shows the spectral changes observed
in the C18OStz+X- thin films during light irradiation, clearly
indicating the consumption of the olefins with the passing of
the irradiation time. After the reaction, the products were
extracted from the photolyzed thin films by immersion in a 1:1
(v/v) methanol-acetone mixed solvent, while the reaction
1
mixture was analyzed by NMR after condensation in vacuo.
Table 1 summarizes the dependence of the product distribu-
tion on the type of counterion, X-, in the photolysis of the
C18OStz+X- thin films. Photochemical [2+2] cyclodimeriza-
tions in matrixes such as micelles, membranes, or crystals have
been reported to take place stereospecifically through an excited
singlet state.9-12 In such cases, a strong excimer fluorescence
could be observed. In a similar manner, the C18OStz+X- films
emit a strong excimer emission for X- ) benzoate and 2- and
3-hydroxybenzoates in which considerable amounts of cy-
clodimers were formed, while little or nil for X- ) perchlorate,
1-hydroxy-2-naphthoate, and 4-hydroxybenzoate. It can, thus,
be assumed that an excimer is preferably formed in the
aggregated alignment of stilbazolium ions in these thin films.
In fact, C18OStz+X- with 1- and 2-hydroxybenzoates as X-
have been reported to form an antiparallel packing structure,17-19
as shown in Scheme 1. Moreover, the molecular packing of
C18OStz+ sandwiched by the X- anions shows a preferable
formation of HH dimers, which is in good correspondence with
the stereochemistry shown in Table 1. Table 1 also shows that
(17) Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379.
(18) Bijima, K.; Engberts, J. B. F. N. Langmuir 1997, 13, 4846.
(19) Bijima, K.; Engberts, J. B. F. N. Langmuir 1998, 14, 79.
Irradiation of the Films. The films (20 mm × 20 mm × 1 µm in
length, width, and thickness, respectively) were developed on a Pyrex
9
J. AM. CHEM. SOC. VOL. 125, NO. 47, 2003 14467

A R T I C L E S
Ohtani et al.
Table 2. (001) Diffraction Angles and the Lamella Distances of the Octadecyloxystilbazolium Salts C18OStz+X-
X-
2θ/deg
lamella unit distance/nm
X-
2θ/deg
lamella unit distance/nm
perchlorate
3.26
2.6
2.22
2.25
2.65
2.7
3.4
3.9
3.9
3.3
1-hydroxy-2-naphthoate
1,2-benzenedicarboxylate
1,3-benzenedicarboxylate
1,4-benzenedicarboxylate
2.83
2.24
2.57
2.15
3.1
3.9
3.3
4.1
2-hydroxybenzoate
3-hydroxybenzoate
4-hydroxybenzoate
benzoate
Scheme 1. A Model of the Aggregated Structure of C18OStz+X-
Leading to the Formation of syn HH and anti HH Dimers
the syn HH dimers as well as the anti HH dimers are almost
equally formed with 1,2-, 1,3-, and 1,4- benzenedicarboxylates
as the counterions (X-), which indicates that the packing
conformation is sensitive to the structure of the X- anions. In
contrast, C18OStz+X- with perchlorate as X- may be loosely
dissociated due to the rather poor hydrophobicity in which the
only observable photochemical reaction is the E-Z isomeriza-
tion.
Characterization of the Packing Structure of the C18-
OStz+X- Film. The XRD diffraction patterns are shown in
Figure 2, and the (001) diffraction angles are summarized in
Table 2 along with the lamella unit distances (d). These values
indicate the high regularity of the lamella stacking, judging from
the fact that higher diffraction peaks of up to the fifth order
could be observed.
The lamella unit spaces of 2.7-4.1 nm, which depend on
the type of X-, could be seen to correspond to the distance
between the mono- and bilayers for C18OStz+X-. It is, thus,
important to determine the tilt angles of the packed ion pairs in
the films to gain a clear understanding of the lamella structure.
Polarized spectroscopies20-23 were carried out to estimate the
tilt angles, γ, of the ion pairs which were calculated by eq 1.
Figure 2. X-ray diffraction profiles of C18OStz+X-: (a) perchlorate
(X- ) ClO4-), (b) benzoate (X- ) C6H5CO2-), (c) 1,4-benzenedicarbox-
ylate [X- ) 1,4-C6H4(CO2-)2].
The ion pairs C18OStz+X- possess both a π-π* transition
of the stilbazolium chromophore at 350-390 nm in the UV
region and C-H stretching vibrations of the all-trans configu-
rational alkyl chain at 2917 cm-1 in the IR region. These
characteristic UV and IR absorption peaks were utilized for an
estimation of the tilt angles for each chromophore by means of
polarized UV and IR spectroscopies, and are compiled in Table
3. The tilt angles of the stilbazolium and alkyl chain moieties
arrive at their corresponding values depending on the bulkiness
or structure of X-.
On the basis of the estimation that the stilbazolium ions and
alkyl chain of C18OStz+X- tilt at around 58° and 32° against
the normal line, respectively, C18OStz+X- was found to form
an antiparallel interdigitated monolayer with the alkyl chains
regularly overlapping with those of the adjacent molecules, thus
(20) Akutsu, H.; Kyogoku, Y.; Nakahara, H.; Fukuda, K. Chem. Phys. Lipids
Ryx ) Ay/Ax )
1975, 15, 222.
(21) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VHC: New
York, 2000.
2[sin2 θ + (sin2 R)(3 cos2(θ - 1))] - [3 sin2(R - 1)][3 cos2(θ - 1)] sin2 γ
(22) Sonobe, K.; Kikuta, K.; Takagi, K. Chem. Mater. 1999, 11, 4.
(23) Sasai, R.; Ogiso, H.; Shindachi, I.; Shichi, T.; Takagi, K. Tetrahedron 2000,
56, 6979.
2 sin2 θ + (2 - 3 sin2 θ)] sin2 γ
(1)
9
14468 J. AM. CHEM. SOC. VOL. 125, NO. 47, 2003

Structural Changes in C OStz+X- Films
A R T I C L E S
18
Table 3. Tilt Angles of the Stilbazolium and Polymethylene Parts of C18OStz+X- Normal to the Silica Substrate of the Cast Film
X-
γstilbazolium
γpolymethylene
X-
γstilbazolium
γpolymethylene
perchlorate
68
58
52
54
58
42
38
29
30
34
1-hydroxy-2-naphthoate
1,2-benzenedicarboxylate
1,3-benzenedicarboxylate
1,4-benzenedicarboxylate
58
54
58
51
38
29
32
30
2-hydroxybenzoate
3-hydroxybenzoate
4-hydroxybenzoate
benzoate
indicating the parallel aggregation of the stilbazolium moieties
to those of the surrounding ion pairs.
π-A Isothermal Profile Measurements. To gauge the effect
of the counterion of the ion pair on the correlation between the
photochemical behavior and morphological characteristics,
observations of the π-A curve were carried out to provide useful
information on the solubilization site. Hydrophobic counterions
such as salicylate or isophthalate ions can be solubilized by the
stilbazolium ions within the adjacent molecules due to favorable
hydrophobic interactions, whereas hydrophilic counterions such
as perchlorate ions are not able to penetrate into the aggregated
stilbazolium molecules. The π-A isothermal profile was
measured by developing monolayer membranes of a series of
C18OStz+X- on the surface of by using an LB membrane
apparatus, as shown in Figure 3. The cross-section of the
C18OStz+X- molecules was estimated to be ca. 0.33 nm2 at a
surface pressure of 0 mN/m for X- ) perchlorate, which is
comparable to the value of 0.35 nm2 for X- ) iodide.18 On the
other hand, C18OStz+X- molecules with hydrophobic counter-
ions such as arylcarboxylates had cross-sections of 0.4-0.6 nm2
for their polar headgroups.
The perchlorate ion-pair is assumed to be perpendicularly
oriented on the substrate on the basis of Chem 3D calculations
which showed the cross-section of the ion pair to be 0.35 nm2.
On the other hand, C18OStz+X- molecules with hydrophobic
counterions, such as arylcarboxylates, possess larger cross-
sections of around 0.4-0.6 nm2 for their polar headgroups due
to the intimate interaction of the counterions with the cationic
headgroups. This is understood by the assumption that the
hydrophilic perchlorate ion pair is easily hydrated and dissociates
at the stage of compression during π-A measurements, whereas
the arylcarboxylate ion pairs are easily sandwiched between the
hydrophobic stilbazolium aggregates.
Indeed, the pressure from the collapse of the arylcarboxylate
ion pair monolayers tends to be lower than that for the
perchlorate ion pair monolayers. This is explicable by the fact
that the headgroups of the arylcarboxylate ion pair are bulkier
than those of the perchlorate ion pair, and as a result, the
monolayer films consisting of arylcarboxylate are considered
to be more fragile than those of perchlorate.
Photoinduced Morphological Changes in the Cast Films.
The photoinduced morphological changes which can be induced
in these self-assembling cast films may be developed for
applications in the design of photofunctional materials capable
of reversible memory storage.3-6 Ogawa et al. have reported
the changes observed in the basal spacings triggered by the E-Z
photoisomerization of the azobenzene chromophores in orga-
nophilic clay-azobenzene intercalation compounds.24
Figure 3. π-A isotherm of C18OStz+X-: (1) perchlorate (X- ) ClO4-),
(2) 2-hydroxybenzoate [X- ) 2-C6H5(OH)CO2-], (3) 3-hydroxybenzoate
[X- ) 3-C6H5(OH)CO2-], (4) 4-hydroxybenzoate [X- ) 4-C6H5(OH)-
CO2-], (5) benzoate (X- ) C6H5CO2-), (6) 1-hydroxy-2-naphthoate
[X- ) 1,2-C10H6(OH)CO2-], (7) 1,2-benzenedicarboxylate [X- ) 1,2-C6-
H4(CO2-)2], (8) 1,3-benzenedicarboxylate [X- ) 1,3-C6H4(CO2-)2], (9) 1,4-
benzenedicarboxylate [X- ) 1,4-C6H4(CO2-)2].
To verify the origin of the structural changes observed in
these cast films upon light irradiation, the correlation between
the efficiency of the photochemistry and the structural changes
has been studied in films of such stilbazolium salts, i.e., for
perchlorate and isophthalate (C18OStz+isoph-). And, in fact,
(24) Ogawa, M.; Fujii, K.; Kuroda, K.; Kato, C. Mater. Res. Soc. Symp. Proc.
1991, 233, 89.
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A R T I C L E S
Ohtani et al.
Figure 4. Changes in the X-ray diffraction patterns of perchlorate
-
C18OStz+ClO4 (a) before and (b) after irradiation at >350 nm.
Figure 6. X-ray diffraction changes in the isophthalate C18OStz+X-
[X- ) 1,3-C6H4(CO2-)2] (a) before and (b) after irradiation at >250 nm.
Table 4. Change of the Tilt Angles of the Stilbazolium and
Polymethylene Parts of C18OStz+X- in the Cast Films before and
after Irradiation at >350 nm
X-
γstilbazolium
γpolymethylene
before irradiation
after irradiation
58
55
32
47
2θ ) 2°, and only 10% HH dimers was confirmed to be
sufficient in inducing a complete transformation into a bilayer
lamellar structure, which can be observed by the change into
completely new peaks, as can be seen in XRD pattern e of
Figure 5. These new peaks showed a good correspondence with
those for the photolyzed films.
Figure 5. X-ray diffraction changes in the isophthalate C18OStz+X-
[X- ) 1,3-C6H4(CO2-)2] upon irradiation at >350 nm: (a) 0 min, (b) 15
min, (c) 30 min, (d) 1 h. Pattern (e) shows a mixture of syn HH and
C18OStz+isoph- in a molar ratio of 0.02.
Photocycloreversion. Stilbazolium ion dimers are capable
of undergoing photochemical cycloreversions. In fact, photolysis
of N-methylstilbazolium methyl sulfate dimers at 254 nm in
water reverted these dimers to the original stilbazolium salts
with an efficiency in the order syn HH < syn HT < anti HH
dimer ions.25 Similar photochemical cycloreversions of C18-
OStz+isoph- may take place in the thin films containing the
syn HH dimers formed by photodimerization upon irradiation
of 254 nm light to generate the starting C18OStz+isoph-. The
olefins were formed with a yield of 15% after 2 days of
irradiation at 254 nm. Along with the cycloreversion, the
resulting thin film was observed to lose its characteristic bilayer
structure and to regenerate the peak at 3.3 nm, as shown in
Figure 6. Moreover, around 50% recovery of the photoinduced
morphological change could be observed by direct irradiation
at 254 nm.
Changes in the Molecular Angle against the SiO2 Glass
Surface. To determine the tilt angles of polymethylene in the
C18OStz+isoph- film, the Ryx ratio was calculated from the
absorbance intensity at 2916 cm-1 and plotted against the angle
R. Both of the Ryx values of the films before and after 2 h of
irradiation were plotted against the angle R. The values of the
angle γ, calculated from eq 1, are compiled in Table 4. The
values of the angle γ of the component polymethylene chains
against the normal line on the substrate in the irradiated films
were found to be larger than those of the nonirradiated samples,
indicating the greater tilt of the polymethylene chain in the
irradiated, as-prepared samples than for the nonirradiated
samples.
two divergent reaction pathways, i.e., E-Z isomerization and/
or [2+2] cyclodimerization, could be observed upon light
irradiation of these stilbazolium salts. Moreover, our studies
showed that only the Z-isomer was formed in the case of
perchlorate, while both the Z-isomer and anti HH dimers could
be seen for isophthalate.
Figure 4 shows the X-ray profiles for the perchlorate before
and after irradiation at >350 nm. As the isomerization pro-
gressed to form a Z-isomer, little change in the intensity of the
X-ray diffraction peak of the starting perchlorate (E-isomer)
could be observed. The present E-to-Z isomerization could not
induce lamella structural changes along the c-axis, most probably
due to the fact that the molecular length of the Z-isomer of
perchlorate is essentially the same as that for the E-isomer.
In contrast, the X-ray profiles measured for the isophthalate
(C18OStz+isoph-) film before and after irradiation at >350 nm
show that the structural changes observed for C18OStz+isoph-
were different from those of the perchlorate film, as shown in
Figure 5. That is, in the case of photodimerization, the reflection
peaks (001) of the starting film decreased with an increase in
the new peaks (001) at a lower angle of about 2θ ) 2°, in stark
contrast to the case of only E-Z photoisomerization.
Here, a control experiment was undertaken to estimate the
amount of dimer content necessary to induce a structural change
from mono- to bilayer in the lamella of the films. An estimated
content percentage of the HH dimers, from 0 to 10%, was
dissolved in aqueous C18OStz+isoph- solution, cast onto the
glass slide, and then analyzed by XRD.
With an increase in the percentage of the HH dimers in the
mixture, new peaks (001) appeared at a lower angle of around
(25) Nakamura, T.; Takagi, K.; Sawaki, Y. Bull. Chem. Soc. Jpn. 1998, 71,
419.
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Structural Changes in C OStz+X- Films
A R T I C L E S
18
Scheme 2. Reversible Changes Seen in the Self-Assembling Structure of C18OStz+isoph- upon Irradiation: (a) Interdigitated Monolayer,
(b) Bilayer Structure
Cyclodimerization, and not the E-Z photoisomerization, is
assumed to be the crucial driving force behind the origin of the
morphological changes in the ion pair film of the stilbazolium
ions during UV light irradiation. This is verified by an
experiment showing no difference in the lamella unit distances
before and after E-to-Z photoisomerization of the perchlorate
ion pair. Additionally, XRD analysis showed no changes in the
lamella unit distance of the ion pair film of the Z-isomer,
whereas the stilbazolium ion pair film including 10% syn head-
to-head cyclodimers was observed to exhibit a lamella distance
Figure 7. Changes observed in the fluorescence intensities of C18OStz+isoph-
1.5 times larger than that of the original stilbazolium ion pair
upon UV irradiation for 1 h.
film. It is understood that a [2+2] cycloaddition of the adjacent
two stilbazolium ions worked to drive out the alkyl chains of
the oppositely oriented stilbazolium ion pairs to induce mor-
phological changes from an interdigitated monolayer to a bilayer
packing morphology, as is described in the following section.
Structural Elucidation by X-ray Diffraction and Polarized
Spectroscopies. The lamella structure of the C18OStz+isoph-
film was clearly characterized to be tilted against the normal
line on the substrate, as shown from estimations of the lamella
unit length and electron density distribution of the ion pair films,
as depicted in Scheme 2. XRD analysis indicated that the lamella
unit distance of 3.3 nm corresponds exactly to the value (3.3
nm) of the interdigitated monolayer of the C18OStz+isoph-
molecules in the nonirradiated film (Scheme 2a), when the
lengths of the stilbazolium and octadecyloxy moieties are 1.0
and 2.6 nm, respectively. Hydrophobic counterions, such as
these aromatic carboxylates, can hardly be dissolved or diffused
in the water bulk and, hence, are easily and preferably
solubilized with the hydrophobic stilbazolium ions. Here, a
bilayer structure could be depicted so as to fit the lamella units
having a distance of 4.5 nm, as shown in Scheme 2b. Moreover,
Figure 8. Fluorescence images of the C18OStz+isoph- patterns.
the stilbazolium groups were seen to be bulkier and more tilted
than the octadecyloxy moieties.
The electron density distribution of the two morphologies,
i.e., interdigitated monolayer and bilayer, was estimated by using
higher reflection signals for the XRD analysis. The interdigitated
monolayer structure could be clearly confirmed by electron
density distribution analysis using a series of XRD peaks up to
the fifth-order reflection; however, the bilayer structure exhibited
no peaks higher than the second-order reflection, so it could
not be estimated.
SEM Images. The surface of the C18OStz+isoph- thin films
on the SiO2 glass plate was analyzed by means of a scanning
electron microscope. UV irradiation of the C18OStz+isoph- thin
films at >350 nm through a photomask resulted in the
appearance of definite and clear patterns after 1 h of irradiation,
together with the formation of some cracks in the patterns. In
contrast, the perchlorate salt C18OStz+ClO4 exhibited no
-
patterns even after UV irradiation under similar conditions.
Patterning of the lamella unit can be explained by the generation
of cyclodimers which induced a larger expansion of the lamella
unit in the course of the photochemical [2+2] cycloaddition, a
crucial factor in clear patterning formation.
Fluorescence Microscopy. The images of the patterns can
also be observed using fluorescence microscopy excited at
around 350 nm. Figure 7 shows the successive changes of the
fluorescence intensities of the C18OStz+isoph- thin films while
undergoing photocyclodimerization upon UV irradiation at 350
nm.
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A R T I C L E S
Ohtani et al.
Figure 8 shows the black and white images excited by the
fluorescence intensities of the C18OStz+isoph- thin film at 350
nm. In the patterning, the bright and white zone was observed
to emit a strong fluorescence at around 520 nm.
photocyclodimerization shows great potential for applications
in the design of photofunctional and erasable recording ma-
terials.
Acknowledgment. This work was partly supported by a
Grant-in Aid for Scientific Research on Priority Areas (417) of
the Ministry of Education, Science, Culture, and Sports, Science
and Technology (MEXT), of Japan. Thanks are also extended
to CREST (Core Research for Evolutional Science and Technol-
ogy) of JST (Japan Science and Technology Corp.) for their
kind support.
Conclusions
Fluorescence and UV-vis spectroscopies, NMR analysis, and
π-A isotherm investigations have revealed a correlation between
the photochemical behavior and the packing structure of
stilbazolium salts C18OStz+X- in thin films cast on SiO2 glass
plates. Photocyclodimerization was found to be a crucial factor
in initiating structural changes in the morphology of the
C18OStz+X- films. Significantly, the alignment of the C18-
OStz+X- self-assembly in the cast films could be well controlled
by the kinds of counterions of the salts. Moreover, analyses of
the X-ray diffraction patterns and polarized IR/UV-vis spec-
troscopy showed that UV irradiation at >350 nm induces a
dramatic change from interdigitated monolayer units to partial
bilayer units in the lamella structure of the solid films. Such
controlled structural changes in organic compounds initiated by
Supporting Information Available: XRD profiles of a
mixture of HH dimers and C18OStz+isoph-, relationship be-
tween Ryx and R of C18OStz+isoph- films on SiO2 solid glass,
and SEM images of the C18OStz+isoph- patterns (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA030310S
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14472 J. AM. CHEM. SOC. VOL. 125, NO. 47, 2003