Stereochemical Effects on Monolayer Formation of (2+) (dpp = 4,7-Diphenyl-1,10-phenanthroline) at an Air-Water Interface

Article abstract of DOI:10.1021/jp952181a

Stereochemical effects on monolayer formations have been studied at an air-water interface for the racemic mixture and enantiomer of (ClO4)2 (dpp = 4,7-diphenyl-1,10-phenanthroline).Stearic acid (SA) was added to reduce the cohesive interaction of the Ru(II) complexes.The ration of SA to Ru(II) complex, /, was varied from 0 to 4.At / = 0 and 1, the lift-off area per molecule at which the surface pressure increased from zero was smaller for the racemic mixture than for the enantiomer.The results indicated that the racemic mixture of the Ru(II) complex formed a more compact monolayer than the enantiomer.There was no difference observed in the surface pressure-molecular area (?-A) curves at / larger than 3.The monolayer was transferred onto a hydrophobic glass or silicon wafer substrate as a Langmuir-Blodgett (LB) film of Y-type.At / = 1, the d(001) spacing of the film was determined by X-ray diffraction measurements, leading to d(001) = 50.4 and 37.8 Angstroem for the racemic mixture and the enantiomer, respectively.The IR reflection-adsorption spectroscopy showed that the alkyl chains of SA molecules took different orientations for the racemic and enantiomeric films.It is postulated that SA molecules which intervened between the Ru(II) complexes are more perpendicular to the substrate surface due to the closer packing of the racemic Ru(II) complexes than the enantiomers.

Full text of DOI:10.1021/jp952181a

J. Phys. Chem. 1996, 100, 1827-1832
1827
Stereochemical Effects on Monolayer Formation of [Ru(dpp)3]2+ (dpp )
4,7-Diphenyl-1,10-phenanthroline) at an Air-Water Interface
Akihiko Yamagishi,* Yasutomo Goto, and Masahiro Taniguchi
DiVision of Biological Sciences, Graduate School of Science, Hokkaido UniVersity, Sapporo 060, Japan
X
ReceiVed: July 31, 1995; In Final Form: October 19, 1995
Stereochemical effects on monolayer formations have been studied at an air-water interface for the racemic
mixture and enantiomer of [Ru(dpp) ](ClO (dpp ) 4.7-diphenyl-1,10-phenanthroline). Stearic acid (SA)
was added to reduce the cohesive interaction of the Ru(II) complexes. The ratio of SA to Ru(II) complex,
SA]/[Ru], was varied from 0 to 4. At [SA]/[Ru] ) 0 and 1, the “lift-off” area per molecule at which the
3
4 2
)
[
surface pressure increased from zero was smaller for the racemic mixture than for the enantiomer. The
results indicated that the racemic mixture of the Ru(II) complex formed a more compact monolayer than the
enantiomer. There was no difference observed in the surface pressure-molecular area (π-A) curves at [SA]/
[Ru] larger than 3. The monolayer was transferred onto a hydrophobic glass or silicon wafer substrate as a
Langmuir-Blodgett (LB) film of Y-type. At [SA]/[Ru(II)] ) 1, the d(001) spacing of the film was determined
by X-ray diffraction measurements, leading to d(001) ) 50.4 and 37.8 Å for the racemic mixture and the
enantiomer, respectively. The IR reflection-absorption spectroscopy showed that the alkyl chains of SA
molecules took different orientations for the racemic and enantiomeric films. It is postulated that SA molecules
which intervened between the Ru(II) complexes are more perpendicular to the substrate surface due to the
closer packing of the racemic Ru(II) complexes than the enantiomers.
Introduction
Recently the Langmuir-Blodgett (LB) films of metal com-
plexes have been investigated extensively with the purpose of
developing functional materials for sensors, electrode-modifiers,
nonlinear optical devices, and pyroelectric materials.1-6 The
electronic and magnetic properties of metal complexes are
intended to be utilized to achieve these functions. In these
attempts, it is of vital importance to clarify the packing structures
of surfactant metal complexes on a two-dimensional surface. It
is of value to establish the relationship between the molecular
arrangements and the stereochemical properties of the compo-
nent molecules.
Figure 1. Structure of [Ru(dpp)3]2+.
When optical isomers form a monolayer at an air-water
interface, they are expected to be mixed uniformly on a
molecular scale because their steric and electronic properties
are the same except for the absolute configurations. In some
cases, however, the isomers are separated into two homochiral
consequence, the optical isomers are expected to interact with
each other in a highly stereoselective way.
7
phases. Such systems provide an ideal binary system to
Experimental Section
investigate the mixing and packing structures of two different
kinds of molecules. There have been a number of investigations
reported on the monolayer and LB film formations of chiral
Materials. [Ru(dpp)3](ClO4)2 (dpp ) 4,7-diphenyl-1,10-
phenanthroline) was prepared by refluxing K2[Ru(H2O)Cl5] (0.5
g) with dpp (2.0 g) in 20 mL of a 1:1 water-ethanol (v/v)
solution for 12 h. The reactant was reduced by adding 1.2 mL
of hypophosphorous acid, and then the mixture was refluxed
further for 2 h. An orange precipitate was formed by adding
8
-12
organic molecules.
As far as we know, however, no
attempts have been reported on the chirality effects of metal
complexes on monolayer formation.
In the present work, we report the stereochemical effects on
the monolayer formation of an optically active metal complex,
2
+
NaClO4 to the reactant mixture. Resolution of [Ru(dpp)3]
was performed by adding 0.1 g of sodium antimonyl tartrate to
2
+
[
Ru(dpp)3] (dpp ) 4.7-diphenyl-1,10-phenanthroline), at an
2
0 mL of a 1:1 water-ethanol (v/v) solution of the racemic
air-water interface. We compare the monolayer and LB film
properties between the racemic mixture and the pure enantiomer
of the metal complex. The monolayer behaviors of racemic
2
+
mixture. ∆-[Ru(dpp)3] was filtered off as an insoluble
antimonyl tartrate salt. The anion of the salt was converted to
the perchlorate by use of an anion-exchange resin. Λ-[Ru(dpp) ]-
3
2+
13
[
Ru(dpp)3] were investigated previously. This complex has
(ClO4)2 was obtained by adding an excess amount of sodium
been chosen because it has a remarkable geometrical structure
in the periphery region (Figure 1). Two phenyl rings in each
ligand project out from the central metal ion, and as a
perchlorate to the filtrate. The optical purity of each isomer
1
was checked in the H-NMR spectrum by use of tris[3-
(
(
(heptafluoropropyl)hydroxymethylene)-(+)-caphorato]europium-
III) derivative (Aldrich Chemical Co.) as a chiral shift reagent.
The optical purities of both enantiomers were estimated to be
*
To whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, January 1, 1996.
X
0022-3654/96/20100-1827$12.00/0 © 1996 American Chemical Society

1
828 J. Phys. Chem., Vol. 100, No. 5, 1996
Yamagishi et al.
Figure 3. π-A curves of [Ru(dpp)
3
](ClO
4
)
2
at 20 °C. The conditions
Figure 2. π-A curves of [Ru(dpp)
3 4 2
](ClO ) at 20 °C. The subphase
is 0.1 M NaClO . Solid and dotted curves are the enantiomer and
were the same as in Figure 1 except that stearic acid (SA) was
incorporated at the ratio [Ru]/[SA] ) 1. Curves a are the initial (s)
and second (30 min later) (- ‚ -) compression results for the
enantiomeric ruthenium(II) complexes, respectively. Curves b are the
initial (- - -) and second (30 minute later) (‚ ‚ ‚) compression results
for the racemic mixture of the ruthenium(II) complexes, respectively.
4
racemic mixture, respectively. Curves a are the results at the initial
compression experiments; curves b, the results when the initially
compressed layers were expanded and compressed again.
higher than 95%. The CD spectra of δ and λ enantiomers gave
3
24 and -330 as ∆ꢀ at 281 nm, respectively. The solubility
of racemic or enantiomeric [Ru(dpp)3](ClO4)2 was estimated
to be less than 10- M in water. Stearic acid (SA) was purified
by recrystallization from a 1:1 (v/v) chloroform-hexane solu-
tion. A hydrophobic glass substrate was prepared by treating
a quartz glass plate with a toluene solution of trimethyl-
monochlorosilane. Other reagents were used as purchased.
Water was deionized with a Milli-Q SP reagent water system
7
(Millipore) to a specific resistivity of 18.4 MΩcm).
Instruments. A surface pressure versus area per molecule
π-A) curve was obtained with a Langmuir trough and a
(
Joyce-Loeble monolayer coating unit with a Whilhelmy
balance. A chloroform solution of the Ru(II) complex and SA
was spread over an aqueous 0.1 M NaClO4 solution at 20 °C.
3 4 2
Figure 4. π-A curves of [Ru(dpp) ](ClO ) at 20 °C at the ratio [Ru]/
[
SA] ) 2. The other conditions were the same as in Figure 3. Solid
2
The rate of compression was 90 cm /min. A LB film was
and dotted curves are the enantiomer and racemic mixture, respectively.
prepared by a vertical dipping method by transferring a
monolayer to a hydrophobic glass or silicon wafer substrate at
the dipping rate of 4 mm/min. The transfer ratio was close to
unity for both downward and upward directions, confirming the
formation of a Y-type film. An X-ray diffraction pattern was
recorded with an X-ray diffractometer (Rigaku Co., Japan) at
the wavelength of CuK (1.540 56 Å) under the conditions of
Curves b in Figure 2 are the results when compression was
repeated 1 h after the surface was compressed up to 60 mN/m
and expanded again to its initial area. Solid and dotted curves
2+
b are for ∆ and racemic [Ru(dpp)3] samples, respectively. As
shown in Figure 2, π remained at zero until the surface was
2
compressed to 120 and 80 Å /molecule for the δ enantiomer
4
0 kV and 30 mA. The reflection-absorption IR spectrum of
and racemic mixture, respectively. [Ru(dpp)3](ClO4)2 was
insoluble in the present subphase (0.1 M NaClO4). Thus, the
decrease of A1 at the second compression experiment was
ascribed to the irreversible formation of microcrystallites during
a LB film deposited on a silicon wafer was measured with a
FT-IR spectrometer, JIR-7000 (JEOL, Japan) at the incident
angle of 85°. An electronic absorption spectrum was obtained
with a spectrophotometer, UVIDEC (JASCO, Japan).
1
3
the initial compression. Once the crystallites were formed at
higher surface pressure, they did not disperse to isolated
molecules even after the surface was again expanded.
Results
A. Monolayer Behavior. The results of the surface pres-
sure-molecular area curves are shown in Figure 2 when a
chloroform solution of pure [Ru(dpp)3](ClO4)2 was spread on
an aqueous solution of 0.1 M NaClO4. For ∆-[Ru(dpp)3](ClO4)2
With the goal of obtaining a monolayer of [Ru(dpp)3](ClO4)2
with reversible compressibility, stearic acid (SA) was added to
1
4
a chloroform solution of [Ru(dpp)3](ClO4)2. Curves a and b
in Figure 3 denote the results when the chloroform solution was
spread at the ratio of SA to the Ru(II) complex (denoted by
[SA]/[Ru]) of 1 for the ∆ and racemic complexes, respectively.
Nearly reproducible π-A curves were recorded for both systems
when the curves were measured 30 and 60 min after the initial
(
solid curve a), surface pressure (π) increased from zero at 300
2
Å /molecule (denoted by “lift-off area” of A1) to a detectable
value and reached a maximum of 45 mN/m at 100 Å /molecule.
2
When another compression experiment was repeated again on
a fresh sample, the same curve was obtained until the surface
pressure reached 40 mN/m. The curve was not reproducible,
however, above a surface pressure of 40 mN/m. For racemic
2
experiments. A1 was found to be 150 ( 5 and 170 ( 10 Å /
molecule for the racemic mixture and the ∆ enantiomer,
respectively. The results indicated that the racemic mixture
occupied a smaller area per molecule than the enantiomer even
in the presence of an equal molar amount of stearic acid.
Figures 3, 4, and 5 are the results for [SA]/[Ru] ) 2, 3, and 4,
respectively. Differences in the π-A curves between the
enantiomer and racemic mixture were still observed for [SA]/
[Ru] ) 2, but they disappeared for [SA]/[Ru] ) 3 and 4.
[
Ru(dpp)3](ClO4)2 (dotted curve a), π increased from zero at
2
A1 ) 210 Å /molecule, a value appreciably smaller than for
the enantiomer. It reached a maximum of 58 mN/m at 70 Å /
2
molecule after passing though a region of a small plateau around
2
1
00 Å /molecule. The reproducibility of a curve was also
limited in the region of surface pressure less than 40 mN/m.

Metal Complex Monolayers and LB Films
J. Phys. Chem., Vol. 100, No. 5, 1996 1829
Figure 5. π-A curves of [Ru(dpp)
SA] ) 3. The other conditions were the same as in Figure 3. Solid
and dotted curves are the enantiomer and racemic mixture, respectively.
3 4 2
](ClO ) at 20 °C at the ratio [Ru]/
[
3 4 2
Figure 6. π-A curves of [Ru(dpp) ](ClO ) at 20 °C at the ratio [Ru]/
[
SA] ) 4. The other conditions were the same as in Figure 3. Solid
and dotted curves are the enantiomer and racemic mixture, respectively.
One interesting aspect was that a small reflection existed in
the π-A curve of the racemic sample at [SA]/[Ru] ) 2 around
the surface pressure of 10 mN/m (indicated by an arrow in
Figure 4). No such reflection was observed for the enantiomeric
sample. This indicates that the racemic monolayer underwent
some phase transition during the compression process. Because
the enantiomeric monolayer did not show such a transition, the
transition must be related to the stereochemical packing modes
of the metal complex.
The effects of temperature were investigated at [SA]/[Ru] )
from 10 to 35 °C. It was concluded that temperature changes
1
had little influence on the monolayer behaviors of the present
samples.
3 4 2
Figure 7. X-ray diffraction patterns of the films of [Ru(dpp) ](ClO )
and stearic acid at [Ru]/[SA] ) 1. The film was deposited on a
hydrophobic glass substrate to 50 layers: (upper) racemic mixture and
(lower) enantiomer.
B. X-ray Diffraction Analyses of the LB Films. X-ray
diffraction paterns were obtained for the LB films (50 layers)
transferred onto a hydrophobic quartz at 25 mN/m under the
conditions of Figures 3-6. The upper and lower parts of Figure
TABLE 1: Basal Spacings of LB Films of Racemic and
2+
Enantiomeric [Ru(dpp)
3
]
(Ru) and Stearic Acid (SA)
7
show the results for the racemic mixture and the enantiomer
ratio of [SA] to [Ru]
racemic mixture (Å)
enantiomer (Å)
at [SA]/[Ru] ) 1, respectively. The LB film of the racemic
mixture gave sharper diffraction peaks than that of the ∆
enantiomer, suggesting a more regular spacing of the layers in
the racemic mixture. The peaks at 2θ ) 1.75° and 5.46° for
the racemic sample were assigned to the d(001) and d(003)
diffractions, respectively. The peaks at 2θ ) 2.34° and 4.70°
for the enantiomeric sample were assigned to the d(001) and
d(002) diffractions, respectively. From the d(001) values, the
interlayer distance was calculated to be 50.4 and 37.8 Å for the
racemic and enantiomeric metal complexes, respectively. These
values are compared to the interlayer distance of a LB film of
stearic acid alone of 39.7 Å.15 Table 1 summarizes the basal
spacings of the LB films for [SA]/[Ru] ) 1-3. The basal
spacing for the racemic mixture was smaller than that for the
enantiomer at [SA]/[Ru] ) 1. The former decreased with the
1
2
3
:1
:1
:1
50.4
48.7
38.4
37.8
38.9
39.4
increase of [SA]/[Ru], while the latter increased with an increase
in [SA]/[Ru] until they were almost identical at [SA]/[Ru] ) 3.
C. UV and IR Spectral Analyses of the LB Films. The
electronic absorption spectrum was measured for 50 layers of
a LB film deposited on a quartz substrate. In the measuremts,
unpolarized light was perpendicularly incident on the samples.
Figure 8 shows the spectra of the racemic and ∆ complexes at
[SA]/[Ru] ) 1. It is seen that the absorbance peak was observed
at 480 nm due to the metal-to-ligand charge transfer band of
2
+
[Ru(dpp)3] . The position was nearly the same as observed
in a chloroform solution (482 nm). The intensity of the peak

1
830 J. Phys. Chem., Vol. 100, No. 5, 1996
Yamagishi et al.
Discussion
The effects of homo- and heterochirality on monolayer
8
-12
formation have been investigated extensively.
The works
are concerned with organic surfactant molecules such as the
amino acid molecules derivatized by long alkyl chains. The
chirality of a molecule is due to the asymmetric carbon atoms.
In these cases, the stereochemical effects on molecular packing
are caused by the interactions among the functional groups such
as carbonyl, amine, and hydroxyl groups. The orientations of
these functional groups play an essential role in determining
the molecular packing in a monolayer state.
Figure 8. Electronic absorption spectra of the films of [Ru(dpp)
3
]-
4 2
) and stearic acid at [Ru]/[SA] ) 1. The film was deposited on
In contrast to the earlier studies, the present work is focused
on the effects of the helical coordination structure in a tris-
chelated complex. A complex of this type has notable asym-
(ClO
a glass substrate to 50 layers: (solid) racemic mixture and (dotted)
enantiomer.
1
7
metry especially when it possesses bulky planar ligands. In
the packed states, the steric interactions between the ligands of
neighboring molecules affect the efficiency of molecular packing
remarkably. In fact, the crystal structure of a chiral tris-chealted
is about 2 times higher for the racemic film than for the
enantiomeric film, leading to the conclusion that the surface
density of the Ru(II) complex was higher for the racemic sample
than for the enantiomer sample when the monolayers were
deposited at the same surface pressure (20 mN/m). This was
coincident with the higher packing of the former as deduced
from the π-A curves (Figure 3).
1
8-20
complex is dependent on the homo- and heterochilarity.
2
+
The complex studied in the present work, [Ru(dpp)3] , has
six phenyl groups at the periphery region of the phenanthroline
ligands (Figure 1). These groups are expected to cause a
stereochemical effect when the complex interacts sterically with
its neighboring molecules. From the curves of the surface
pressure-molecular surface area, the monolayers collapse at a
surface pressure less than 40 mN/m irrespective of the presence
of stearic acid (SA). This indicates that the monolayers are
stable only when they form a liquid-like packing structure. When
the ratio of incorporated SA is high enough, the monolayer is
expected to approach a solid-like state in which the long alkyl
IR reflection-absorption spectra were measured for 100
layers LB films deposited on a silicon wafer at [SA]/[Ru] ) 1.
Parts a and b of Figure 9 are the spectra of the ∆ and racemic
films, respectively. It is noted that the intensity of the doublet
-1
at 2800-2900 cm was much higher than that of the singlet
at 1100 cm- for the enantiomeric film, whereas the former
was lower than the latter for the racemic film. The doublet
1
-
1
peaks at 2923 and 2854 cm were assigned to the asymmetric
and symmetric stretching vibrations of the methylene groups
1
4
chains are packed to form a crystalline-like ordered array.
1
6
-1
of SA, respectively. The singlet at 1106 cm was assigned
to the out-of-plane bending vibration of the CH group of the
We note that there exists a definite difference in the surface
pressure-area curves between the enantiomeric and racemic
2+
2+
dpp ligands of [Ru(dpp)3] . Because the transfer ratios were
close to unity in both samples, the enantiomeric and racemic
films contained equal amounts of SA and the metal complex.
Thus, the intensity difference in the above spectra implied that
the CH2 groups in SA and the CH groups in the metal complex
take up different orientations in the enantiomeric and racemic
films. As a comparison, the transmission spectra were recorded
on the cast samples of the chloroform solutions (Figure 10a,b).
mixture of [Ru(dpp)3] when the ratio of SA to the complex,
[SA]/[Ru], is varied from 0 to 2. One result is that the lift-off
area, A1, at which the molecules start to interact sterically is
smaller for the racemic mixture than for the enantiomer at [SA]/
[Ru] ) 1. It is concluded that the monolayer of the racemic
mixture attains closer packing than that of the pure enantiomer
even in a liquid-like state. The difference between the racemic
mixture and the enantiomer is largest in the pure metal complex
and decreases with an increase in the fraction of the incorporated
SA molecules. The pure metal complexes, however, do not
form a reversible monolayer film. It is previously reported that
-1
In both of the samples, the doublet at 2800-2900 cm had
higher intensity than the singlet at 1106 cm . The interpretation
of the results will be given in the Discussion section.
-1
3 4 2
Figure 9. Reflection-absorption IR spectra of the films of [Ru(dpp) ](ClO ) and stearic acid at [Ru]/[SA] ) 1. The film was deposited on a
silicon substrate to 100 layers: (a) racemic mixture and (b) enantiomer. The light was incident on the film at an angle of 85°.

Metal Complex Monolayers and LB Films
J. Phys. Chem., Vol. 100, No. 5, 1996 1831
Figure 10. Absorption IR spectra of the cast films of [Ru(dpp)
as a chloroform solution under the same conditions as in Figure 9: (a) racemic mixture and (b) enantiomer.
3
](ClO
4
)
2
and stearic acid at [Ru]/[SA] ) 1. The film was cast on a silicon substrate
+
[
Ru(dpp)3]2 irreversibly forms a microcrystallite at an air-
were in agreement with the monolayer behaviors, as stated
above. The results of the IR spectra indicate that the orientation
of the alkyl group of SA differs between the racemic and
enantiomeric films. According to theory, the reflection-
absorption absorbance, AR, is given by
13
water interface when the monolayer is compressed.
According to a space-filling molecular model, [Ru(dpp)3]
occupies an area of about 300 Å (denoted by the theoretical
limiting area) when it rotates freely on a water surface. In the
absence of SA, A1 for the enantiomer almost coincides with
the theoretical limiting area, whereas A1 for the racemic mixture
2
+
2
16
2
2
A ) 2m cos R + m sin R
R
z
x
2
is much smaller than the model value (300 Å ). Thus, the
enantiomers repel each other at the distance at which the
molecules are in contact with each other, whereas the racemic
mixture forms a more dense monolayer, probably with the
phenyl groups of the facing ligands stacked closely together.
Interestingly enough, as the SA molecules are incorporated at
the ratio of [SA]/[Ru] ) 1, A1 is smaller than the above
theoretical limitting area for both enantiomeric and racemic
complexes. This decrease is understood through the assumption
that the association of SA molecules reduces the steric interac-
tion between the metal complexes, leading to closer stacking.
Most probably the SA molecules are located in the narrow
spaces between the phenyl groups. It has been reported that
alkyl chains enhance attractive interactions with the aromatic
where R is the angle between the transition moment and the
normal direction of a surface and mz and mx are the intensity
enhancement factors of the normal and surface compoments of
the extinction coefficients due to the substrate surface. In the
present studies, an accurate angle, R, was not calculated because
mz and mx were not reported for a silicon wafer. If the surface
of the silicon wafer acts as a semiconductor substrate, mz is
much larger than mx, leading to the well-used approximation
2
A ) 2m cos R
R
z
According to this equation, the intensity increases as the
transition moment takes a more perpendicular direction from
the substrate surface. As for the orientation of the CH groups
in the Ru(II) complex, the groups are assumed to be oriented
uniformly in all directions since 18 CH groups are located on
the periphery region of the spherically shaped Ru(II) complex
21
groups. Even in the presence of the SA molecules, the racemic
mixture still attains higher packing states than the enantiomer.
In a crystalline state, a tris(acetylacetonato)complex, [Cr-
(
acac)3] or [Ru(acac)3], has its planar ligands facing closely the
19,20
-1
ligands of its neighboring molecules.
In a racemic mixture,
in different directions. Thus, the peaks at 1106 cm would
a homochiral column, in which the molecules are stacked with
not show any orientational effect in both the racemic and
enantiomeric films. On the basis of these situations, we assume
that the intensity of the peak at 1106 cm- may be taken as a
normalizing factor in estimating the relative intensities of other
peaks. On the basis of this assumption, the ratios of the peak
2
1
8
their C3 axes on the same straight line, is formed.
The
1
columns composed of the ∆ and Λ complexes are arranged in
an alternative way. In contrast to this, no such close packing
is observed in the enantiomeric crystal,1
9,20
in which the
complexes are associated along the C2 axes. Although we did
not observe the molecular arrangements of the monolayers
directly, we expect that similar stacking is achieved in the
monolayers of the racemic and enantiomeric complexes. The
recent theoretical calculations on the phase diagram of the
monolayers of chiral tripodal-shaped molecules predict that the
molecules attain higher stacking when they interact mainly
through a van der Waals interaction.22
intensities of the CH groups of SA molecules (2700-2800
-1
-1
cm ) to that of CH groups in the dpp ligands (1106 cm ) are
calculated and given in Table 2. The results indicate that the
transition moments of the symmetric and asymmetric stretching
vibrations of the CH2 groups in the LB films are more
perpendicular for the enantiomeric film than for the racemic
film. Because these transition moments are perpendicular to
the alkyl chain, it is deduced that the alkyl chain of SA has a
more upright direction for the racemic mixture than for the
enantiomer. In contrast, the difference of the relative intensities
between the racemic and enantiomeric cast films was much
The X-ray diffraction results show that the spacing of the
LB film of the racemic mixture is larger than that of the
enantiomer at [SA]/[Ru] ) 1 and 2 (Table 1). These results

1
832 J. Phys. Chem., Vol. 100, No. 5, 1996
Yamagishi et al.
TABLE 2: Relative Intensities of the IR Peaks Due to the
molecules form a two-dimensional crystal, the racemic mixture
is expected to have a larger number of packing states than the
CH Stretching of the Alkyl Chains of Stearic Acid
Incorporated in the LB and Cast Films of 1:1 [Ru(dpp)
and Stearic Acid According to the Results in Figures 9 and
2
]2
+
3
7
enantiomer. The situation is most remarkable when the crystal
7
a
takes an oblique lattice. In this case, the racemic mixture has
1
0
two diastereomeric lattices, whereas the enantiomer has a single
lattice. The phase transition observed in Figure 4 may be related
to the transition of the racemic mixture between these phases.
The stereochemical effects observed herein may provide a
way to control the packing of amphiphilic molecules in a
peak
position
racemic
mixture enantiomer
-
1
film (cm
)
assignment
LB
2923
asym. stretching of CH
2
0.589
0.333
1.00
0.704
0.481
1.00
3.00
2.00
1.00
0.860
0.672
1.00
2
1
854
106
sym. stretching of CH
out-of-plane bending of CH
asym. stretching of CH
sym. stretching of CH
out-of-plane bending of CH
2
1
6,23
molecular layer by the use of molecular chilarity.
cast
2917
2
2
1
848
118
2
References and Notes
(
1) Roberts, G. G. AdV. Phys. 1985, 34, 475-512 and references
therein.
(2) Richardson, T.; Roberts, G. G. Thin Solid Films 1988, 160, 231-
39.
a
The intensity of the peak due to the out-of-plane bending vibration
of CH groups is taken to be unity in each film.
2
(
3) Richardson, T.; Roberts, G. G.; Polywka, M. E. C.; Davies, S. G.
Thin Solid Films 1989, 179, 405-411.
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(
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(
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(
(
Figure 11. Proposed structures of the monolayers of [Ru(dpp)
and stearic acid at [Ru]/[SA] ) 1: (a) racemic mixture and (b)
enantiomer.
3
](ClO
4
)
2
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2
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1
(
We rationalize the above results in terms of the following
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intervene in metal complexes, whereas in the racemic film,
stearate anions take a more upright direction because the racemic
metal complexes are so closely packed that there is less room
left for stearate anions. Our packing model is schematically
shown in Figure 11.
(
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1
(
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The presence of a phase transition was possible for the
racemic mixture at [SA]/[Ru] ) 2 (Figure 4). When chiral
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