Assemblies of aziridinemethanols

Article abstract of DOI:10.1039/b006428i

Two novel long chain aziridinemethanols (1b, c) are described and their molecular organisation in the bulk and self-assembling properties in aqueous dispersion are reported. The orientation of the NH hydrogen of the aziridinealcohol moiety in 1b can be ch

Full text of DOI:10.1039/b006428i

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Assemblies of aziridinemethanols
Peter J. J. A. Buijnsters,a{ Martinus C. Feiters,a Rene de Gelder,b Peter ten Holte,a
Roeland J. M. Nolte,a,d Arthur M. A. Pistorius,c Nico A. J. M. Sommerdijk,*a,d Sandy
A. C. Verhaegena and Binne Zwanenburga
aDepartment of Organic Chemistry, NSR Center for Molecular Structure, Design and
Synthesis, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The
Netherlands. E-mail: N.Sommerdijk@tue.nl
bDepartment of Inorganic Chemistry, NSR Center for Molecular Structure, Design and
Synthesis, University of Nijmegen, Nijmegen, The Netherlands
cDepartment of Biochemistry, University of Nijmegen, Nijmegen, The Netherlands
dLaboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology,
Eindhoven, The Netherlands
Received 7th August 2000, Accepted 8th November 2000
First published as an Advance Article on the web 4th January 2001
Two novel long chain aziridinemethanols (1b, c) are described and their molecular organisation in the bulk and
self-assembling properties in aqueous dispersion are reported. The orientation of the NH hydrogen of the
aziridinealcohol moiety in 1b can be changed by introducing a methyl substituent into the rigid three-membered
ring (1c), leading to a change in the hydrogen bonding pattern interconnecting these molecules. This change in
con®guration leads to marked differences in the ordering of these molecules in the solid state. Although
compounds 1b and 1c both form highly organised structures in aqueous media and on the air±water interface,
noteworthy differences are observed. Compound 1c yields left-handed helical ribbons whereas no chiral
aggregates are found for 1b. However, the addition of 2-acetoxybenzoic acid (aspirin) to an aqueous dispersion
of 1b leads to the generation of both left- and right-handed helical structures. Under these conditions a reaction
had taken place that was speci®c for the ortho-isomer of acetoxybenzoic acid.
Introduction
One of the current interests in supramolecular chemistry is the
design of well-de®ned supramolecular architectures by pro-
grammed self-assembly of amphiphilic molecules. Surfactants
designed to have speci®c intermolecular interactions have been
reported to form highly organised aggregates including
ribbons, tubes, toroids, and helical structures.1±7 Assemblies
with well-de®ned shapes and surfaces have been used as
templates for the structuring of inorganic8±11 and polymer
composite materials12 and as catalysts for (enantio)selective
transformations in solution.13 It is well recognised that
hydrogen bonding is very important in the generation of
supramolecular aggregates.1±7,14
It has been demonstrated by Mathis et al.16 that the
con®guration at the nitrogen atom of substituted aziridines can
be controlled by introducing an additional substituent at the
C2 position. They showed that the NH hydrogen atoms in cis-
2,3-disubstituted aziridines have a trans-con®guration with
As part of our studies on aziridine ring systems15 we
observed that highly ordered structures can be generated from
aziridinemethanol moieties by the formation of hydrogen
bonds. This is evidenced by the crystal structure of compound
1a (Fig 1a),15b in which the molecules of 1a are organised into
linear arrays. Intermolecular hydrogen bonds are formed
between the OH groups and the nitrogen atoms of neighboring
molecules, while an intramolecular hydrogen bond between the
NH group and the oxygen atom locks the aziridine methanol
group into a rigid conformation in which the plane through N±
C1±C2 is orthogonal to the C2±C3±O plane. We anticipated
that the high degree of interconnectivity and rigidity of these
molecules would offer prospects for the application of
aziridinemethanols as building blocks in the generation of
assemblies with organised and potentially reactive surfaces.
Fig. 1 (a) PLUTO drawing of the crystal structure of 1a. (b) Newman
projections of the possible con®gurations of an unsubstituted and a
substituted aziridine ring.
{Present address: Department of Medicinal Chemistry, Janssen
Research Foundation, Beerse, Belgium.
DOI: 10.1039/b006428i
J. Mater. Chem., 2001, 11, 269±277
This journal is # The Royal Society of Chemistry 2001
269

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respect to the substituents. In contrast, both cis- and trans-
con®gurations are possible in monosubstituted aziridines
(Fig. 1b). Since the inversion of the nitrogen atom can be
used to modify the hydrogen bonding pattern and hence the
interconnectivity of these molecules, we decided to synthesize
two chiral, amphiphilic, long chain aziridinemethanols 1b, c
(Scheme 1) which may be expected to show thermotropic as
well as lyotropic liquid crystalline behaviour.17,18
In this paper we demonstrate that differences in the
organisation pattern of molecules of 1b, c in the solid state
are directly linked to the con®guration of the aziridine
nitrogen. The molecular organisation of 1b is dominated by
a hydrogen bonding pattern similar to the one present in the
crystal structure of 1a. In the case of 1c, a different, weaker
hydrogen bonding network is present as a result of an
additional substituent at the aziridine ring, and the organisa-
tion of the molecules is predominantly determined by
molecular packing. When dispersed in aqueous media 1b
forms micellar ®bers, whereas in dispersions of 1c helical
ribbons are observed. The addition of 2-acetoxybenzoic acid
(aspirin) to aggregates of 1b, however, induces the formation of
helical structures.
obtained after evaporation of the solvent. Flash column
chromatography (SiO2, hexane±ethyl acetate, 20 : 1 (v/v))
yielded a colorless oil in 67% yield, which solidi®ed on
standing. Mp 35±36 ³C. [a]2D0 233.7 (c 1.0, EtOH). IR(KBr)
3486 (OH), 3084±3033 (CHaromatic), 2950±2849 (CHalkyl) cm21
.
1H NMR (CDCl3) d 7.47±7.19 (m, C19H15, 15 H), 3.07 (s, OH,
1 H), 1.94 (d, J~3.2 Hz, 1 H, b-CH), 1.41 (dd, J~6.5, 3.3 Hz, 1
H, a-CH), 1.27±1.20 (m, 52 H, 26(CH2)13), 1.10 (d, J~6.5 Hz,
1 H, b-CH), 0.89 (m, 6 H, 26CH3). CI-MS (m/z) 707 [Mz], 630
[C44H72NOz], 464 [C31H62NOz], 446 [C30H60Nz], 243
[C19H15z]. Anal. calcd. for C50H77NO: C 84.80, H 10.96, N
1.98; found: C 85.06, H 11.44, N, 2.06%.
(z)-[(2S,3S)-1-Trityl-3-methylaziridin-2-yl](ditetradecyl)-
methanol (3c). Using the same procedure as for 3b, 3c was
obtained in 73% yield as a colourless oil starting from 2c. [a]D20
z22.4 (c 1, CHCl3). IR(KBr) 3472 (OH), 3085±3020-
1
(CHaromatic), 2950±2552 (CHalkyl) cm21. H NMR (CDCl3) d
7.49±7.20 (m, 15 H, C19H15), 3.43 (s, 1 H, OH), 1.49 (d,
J~5.8 Hz, 1 H, a-CH), 1.32 (d, J~6.8 Hz, 3 H, 16CH3), 1.4±
1.1 (m, 53 H, 26(CH2)13 and b-CH), 0.8 (m, 6 H, 26CH3). CI-
MS (m/z) 479 [C32H64NOz], 282 [C18H35NOz], 243 [C19H15z],
225 [C15H29Oz], 166 [C13H10z], 57 [C3H7Nz], 43 [C2H5Nz].
Anal. calcd. for C51H79NO: C 84.82, H 11.03, N 1.94; found: C,
84.86, H 11.00, N 1.91%.
Experimental
Synthesis
General. Flash column chromatography was performed
on Merck silica gel 60H (0.005±0.040 mm) using a pressure
of ca. 1.5 bar. Melting points were measured on a Reichert
thermopan microscope equipped with crossed polarizers.
Optical rotations were determined at 20 ³C using a Perkin-
Elmer automatic polarimeter, model 241. Routine FT-IR
spectra were recorded using a Biorad WIN-IR FTS-25 single
beam spectrometer. 1H NMR and 13C NMR spectra were
recorded on a Bruker AC 300 (300 and 75.1 MHz) spectro-
meter. Mass spectra were recorded with a double-focusing
VG 7070E spectrometer. Elemental analyses were deter-
mined with a Carlo Erba Instruments EA 1108 element
analyser. Diethyl ether was pre-dried over potassium
hydroxide, then distilled from sodium±benzophenone.
Ethyl acetate and hexane were distilled under reduced
pressure using a rotary evaporator.
Compound 1a. The synthesis of compound 1a has been
described previously.15b
(2)-[(2S)-Aziridin-2-yl](ditetradecyl)methanol
(1b). Com-
pound 3b (1.40 g, 1.98 mmol) was dissolved in a mixture of
MeOH, water and concentrated H2SO4 (60 : 8 : 3; 200 ml) by
sonication for 5 min. After stirring overnight and subsequent
addition of ice (50 g), the white precipitate was ®ltered off and
successively washed with hexane (20 ml) and water (20 ml). A
white solid was obtained in 67% yield. The white precipitate
was dispersed in hexane and triethylamine (5 : 1; v/v, 50 ml) and
the organic layer was washed with saturated ammonium sulfate
solution (3610 ml), dried over Na2SO4 and concentrated in
vacuo. The pure compound was obtained by ¯ash column
chromatography (SiO2, hexane±ethyl acetate, 10 : 1 (v/v)) as a
white solid in 89% yield. Mp 40±41 ³C. [a]2D0 26.2 (c 1, EtOH).
1H NMR (CDCl3) d 3.36 (s, 1 H, OH), 2.06 (dd, J~6.0, 3.7 Hz,
1 H, a-CH), 1.69 (d, J~6.1 Hz, 1 H, b-CH), 1.59 (d, J~3.7 Hz,
1 H, b-CH), 1.26 (m, 53 H, NH, 26(CH2)13), 0.88 (m, 6 H,
(2)-[(2S)-1-Tritylaziridin-2-yl](ditetradecyl)methanol (3b). A
solution of n-tetradecyl bromide (1.37 g, 5 mmol) in dry diethyl
ether (25 ml) was added to freshly ground magnesium turnings
(0.12 g, 5 mmol) in re¯uxing dry diethyl ether (75 ml) and this
mixture was vigorously stirred for 2 h. After cooling to room
temperature a solution of 2b (0.50 g, 1.46 mmol) in dry diethyl
ether (50 ml) was added dropwise. The reaction mixture was
stirred overnight, quenched with saturated ammonium sulfate
and subsequently the aqueous layer was extracted with ether
(3610 ml). After separation, the combined ether layers were
washed with brine and dried over MgSO4. A yellowish oil was
26CH3). 13C NMR (CDCl3)
(26(CH2)C13H27), 38.9 (26C11H22CH2C2H5),
d
70.8 (COH), 41.2
36.2
(CH2(NH)CHCOH), 32.6±30.1 (26C3H7C8H16C3H7), 24.4±
23.4 (26CH2CH2CH2C11H23, 26C12H24CH2CH3), 21.5
(CH2(NH)CH), 14.8 (26C13H26CH3). IR(KBr) 3600±3100
(OH, NHaromatic), 2950±2552 (CHalkyl) cm21. CI-MS (m/z) 465
[Mz], 423 [C29H59Oz], 268 [C17H34NOz], 71 [C3H5NOz], 42
[C2H4Nz]. Anal. calcd. for C31H63NO: C 79.93, H 13.63, N
3.01; found: C 79.82, H 13.65, N 3.09%.
(z)-[(2S,3S)-3-Methylaziridin-2-yl](ditetradecyl)methanol
(1c). Using the same procedure as for 1b, 1c was obtained as a
white solid in 35% yield, starting from 3c. [a]2D0 z3.2 (c 1,
CHCl3). 1H NMR (CDCl3) d 2.83 (s, 1 H, OH), 2.13 (m, 1 H, b-
CH), 1.98 (d, J~6.0 Hz, 1 H, a-CH), 1.50 (m, 4 H,
26COHCH2), 1.35 (d, J~6.0 Hz, 3 H, CH3), 1.38±1.26 (m,
49 H, NH, 26(CH2)12), 0.88 (m, 6 H, 26CH3). 13C NMR
(CDCl3)
d 71.5 (quat. C), 42.4 (26CH2C13H27), 40.7
((CH3)CH(NH)CH), 38.5 (26C11H22CH2C2H5), 32.6±30.1
(26C3H6(CH2)8C3H7), 30.8 ((CH3)CH(NH)CH), 24.4±23.4
(26CH2CH2CH2C11H23,
((CH3)CH(NH)CH), 14.8 (26C13H26CH3). IR(KBr) 3356
26C12H24CH2CH3),
15.2
Scheme 1 i: C14H29MgBr±Et2O; ii: MeOH±H2O±H2SO4 (60 : 8 : 3, v/v/
v); iii: Et3N±hexane.
270
J. Mater. Chem., 2001, 11, 269±277

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(OH), 3269 (NH), 2950±2552 (CHalkyl) cm21. FAB-MS (m/z)
480 [Mzz1], 465 [C31H62NOz], 82 [C18H36NOz], 58
[C3H7Nz]. Anal. calcd. for C32H65NO: C 80.09, H 13.65, N
2.92; found C 80.02, H 13.82, N 2.88%.
FT-IR spectroscopy
IR samples were prepared by depositing a small amount of
solid on an AgCl window and subsequently heating the sample
in an oven above the melting temperature. FT-IR spectra were
measured using a Mattson Cygnus 100 single beam spectro-
meter, equipped with a liquid nitrogen cooled narrow band
MCT detector, interfaced to a microcomputer. The optical
bench was continuously purged with dry nitrogen gas
(20 l min21). The following acquisition parameters were used:
resolution, 4 cm21; moving mirror speed, 2.53 cm s21; wave-
number range, 4000±750 cm21; number of co-added interfer-
ograms, 128. Signal to noise ratios (2000±2200 cm21) were
better than 46103. Data acquisition was performed using
EXPERT-IR software (Mattson). For data analysis PeakFit2
v4 (Jandel Scienti®c Software) was used. Baselines were
corrected and peak positions were determined using second
derivative spectra. Curve ®tting procedures were repeated
several times and the quality of the ®tted spectra was checked
by comparing the generated spectra before and after deconvo-
lution.
(z)-2{[(2R)-2-Acetamido-3-hydroxy-3-tetradecylheptadecyl]-
oxy}benzoic acid (4). An aged (24 h) aqueous dispersion of 1b
(100 mg, 2% w/w) and 1 equivalent of aspirin (38.7 mg,
0.21 mmol) was extracted with CHCl3 (3650 ml), acidi®ed
with HCl (0.1 M, 2650 ml) and again extracted with CHCl3
(3650 ml). After drying (Na2SO4) of the combined organic
layers, the solvent was removed in vacuo and the residue
(140 mg) was subjected to ¯ash column chromatography (SiO2,
ethyl acetate), which afforded 4 as a white powder in 30% yield.
Mp 59±60 ³C. [a]2D0 z25.4 (c 2.3, CHCl3). IR(KBr) 3600±3000
(C(O)OH), 3407 (OH), 3337 (NH), 2920, 2850 (CH alkyl), 1683
(CLO(OH)), 1645 (amide I), 1522 (amide II), 1254 (arom.
ether) cm21. 1H NMR (CDCl3) d 10.56 (s, 1 H, COOH), 7.79±
7.75 (m, 1 H, HPh,6), 7.47±7.42 (m, 1 H, HPh,4), 6.98±6.95 (m, 1
H, HPh,5), 6.90±8.85 (m, 1 H, HPh,3) 6.08 (d, J~9.2 Hz; 1 H,
CHNHC(O)), 4.58±4.43 (m, 2 H, OCH2CHNH), 4.35±4.30 (m,
1 H, OCH2CHNH), 2.09 (s, 1 H, OH), 2.01 (s, 3 H, C(O)CH3),
Powder diffraction experiments
1.57 (m,
4
H, 26CH(OH)CH2), 1.25 (m, 48 H,
26(CH2)12CH3), 0.87 (t, J~6.3 Hz, 6 H, 26(CH2)12CH3).
13C NMR (CDCl3) d 172.86 (NHC(O)CH3), 165.5 (COOH),
155.63 (CPh,2), 129.59 (CPh,4), 127.3 (CPh,6), 118.71 (CPh,5),
118.04 (HOOCCPh,1), 109.25 (CPh,3), 86.44 (COH), 68.07
Samples were prepared by placing a drop of an aggregate
dispersion on a silicon wafer. The instrument was a commercial
Philips X-ray powder diffractometer of the Bragg Brentano
type that was optimized for measurements at low angle. The X-
ray tube was ceramic with a long ®ne focus and gave Cu-Ka
radiation (generator 40 kV, 40 mA). The goniometer had a
variable divergence and antiscatter slits, with the receiving slits
set at 0.1 mm. The detector was of the Peltier-cooled Si/Li type.
During the measurements, the sample was mounted in a
chamber the relative humidity of which could be controlled by
a humidifying instrument ¯ushed with N2 gas.19
(OCH2CHNH),
(26CH2C13H27),
55.66
30.86±22.11
(OCH2CHNH),
(26C11H22CH3),
34.69
23.15
(NHC(O)CH3), 13.6 (26C13H26CH3). CI-MS (m/z) 645
[Mz], 627 [C40H69NO4z], 508 [C33H66NO2z], 423
[C29H59Oz], 310 [C22H46z], 121 [C7H5O2z], 85 [C4H7NOz].
Anal. calcd. for C40H71NO5: C 74.37, H 11.08, N 2.17; found:
C 73.93, H 10.96, N 2.27%.
Electron microscopy
Results and discussion
A 2% (w/v) methanolic solution (50 ml) of 1b, c was injected in
1.0 ml of water of 60 ³C adjusted to pH 3.0 with H2SO4. After
1 h the dispersion was cooled to room temperature and left for
2 h before EM samples were prepared. When organic counter
ions were used a 2% (w/v) methanolic solution (50 ml) of 1b (to
which 1 equivalent of the organic acid had been added) was
injected in 1.0 ml of water of 60 ³C. After 1 h the dispersion was
cooled to room temperature and left for 24 h before EM
samples were prepared. Pt-shadowed samples were prepared by
bringing a drop of the dispersion on to a Formvar-coated
microscope grid. The excess of the dispersion was blotted off
with a ®lter paper after 1 min and the sample was shadowed
under an angle of 45³ by evaporation of Pt. Negatively stained
samples were prepared in an analogous manner and stained
with a 1% (w/w) solution of uranyl acetate. All samples were
studied with a Philips TEM201 microscope (60 kV).
Synthesis
Compounds 1b, c were prepared starting from the methyl N-
tritylaziridinecarboxylic acid esters 2b, c (Scheme 1).15a,b The
compounds 2b, c were converted into the corresponding N-
tritylaziridinemethanols 3b, c using an excess of tetradecyl-
magnesium bromide in ether. Detritylation using 6 M sulfuric
acid in methanol with sonication, followed by treatment with
base and subsequent chromatography afforded the pure
aziridine-2-methanols.
Characterisation of the solid state structures of long chain
aziridinemethanols 1b, c
Differential scanning calorimetry (DSC) experiments showed
that compound 1b exhibits two minor phase transitions
between 28 and 40 ³C (not detected by polarisation microscopy
or variable temperature FT-IR, vide infra) and a melting
transition at 55 ³C (onset temperature) which was also observed
with polarisation microscopy. In contrast, upon heating,
compound 1c showed pronounced exo- and endothermic
solid state transitions between 30 and 36 ³C and a melting
point at 44 ³C (Fig. 2a). Between 30 and 36 ³C polarisation
microscopy revealed a spherulitic structure,20 transforming ®rst
into an isotropic state and subsequently into a needle-like
morphology (Fig. 2b, c). The reorganisation was accompanied
by a net decrease in entropy (DS~274¡1 kJ mol21) indicat-
ing that a more ordered structure was formed. Comparison of
the melting entropies (DSm) of the two compounds (Table 1)
suggested that in the solid state 1c possesses a higher degree of
organisation than 1b, both before and after the transition.
Although related long chain amino alcohols have been
described to form thermotropic liquid crystalline phases,17,18
Monolayer experiments
Monolayers were spread on a themostatted double barrier
Riegler & Kirstein trough of dimensions 6625 cm using a
chloroform solution of the surfactant (y10 ml, 1 mg ml21) and
compression was started after 10 min at
a
rate of
7.0 cm2 min21. The surface pressure was measured using
Wilhelmy plates and for calibration octadecanol was used.
The surface of compressed monolayers was studied with a
Brewster Angle Microscope (NFT BAM-1) mounted on a
home built trough of dimensions 14621 cm.
Differential scanning calorimetry (DSC)
Thermograms were recorded at 1 ³C min21 using a Perkin
Elmer DSC7 instrument and were baseline corrected. Samples
were prepared using stainless steel large volume pans.
J. Mater. Chem., 2001, 11, 269±277
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gauche conformations associated with the melting of the alkyl
chains.23 This suggests that in the case of 1b at the melting
point the disordering of the hydrocarbon chains precedes the
breaking of the hydrogen bonds.
FT-IR spectra of 1c, recorded between 20 and 30 ³C,
revealed two bands between 3600 and 3100 cm21 after
deconvolution (Fig. 3b, c). The broad band at 3356 cm21 can
be ascribed to a polymeric hydrogen bonded structure
involving the hydroxy groups,24 whereas the sharper amine
stretching vibration at 3269 cm21 corresponds to an inter-
molecular NH±OH hydrogen bond.25 From the position of the
CH2 deformation band (1468 cm21) it can be deduced that in
this temperature window the alkyl chains are not in a close-
packed arrangement (Fig. 3e). Based on these data, in
conjunction with the fact that the amine hydrogen atom is
oriented trans with respect to the substituents,16 we propose a
molecular organisation in which the alkyl chains again are
localised alternately above and below the plane of the hydrogen
bonds, as depicted schematically in Fig. 4b.
Variable temperature FT-IR revealed that upon heating at
23 ³C the alkyl chains of 1c gradually start to become
disordered (i.e. start to melt, Fig. 3e), until at 33 ³C a
rearrangement of the hydrogen bonding pattern occurred as
was evident from the disappearance of the O±H and N±H
vibrations at 3356 and 3269 cm21 and the concomitant
appearance of 4 new bands (Fig. 3b, d). The two new O±H
vibrations at 3539 and 3470 cm21 can be ascribed to an alcohol
dimer26 and an intramolecular hydrogen bond between the
hydroxy group and the nitrogen atom,22 respectively. The
appearance of two N±H stretching bands at 3264 and
3214 cm21 indicates that the amine hydrogen atom is involved
in both a weak intramolecular and a strong intermolecular H-
bond, respectively.22,25,27 This proposed reorganisation was
also supported by the fact that the packing of the alkyl chains
increased upon heating as was evidenced by the shift of the CH2
deformation band, going from 1468 to 1471 cm21 (Fig. 3e),
indicating that above 35 ³C the alkyl chains adopt an all-trans
conformation.23 Hence from the DSC and IR data we conclude
that the reordering of the hydrogen bonding network leads to a
higher degree of organisation of the molecules and imposes an
all-trans conformation upon the alkyl chains. We propose that
above the transition temperature the molecules of 1c have
intramolecular H-bonds between the OH and the nitrogen
atom and are organised in arrays held together by inter-
molecular hydrogen bonds between the NH hydrogen atom
and the neighboring OH group. In addition inter-array
hydrogen bonds (alcohol dimers and NH±N hydrogen
bonds, see Fig. 4c) further enhance the ordering of the
molecules, resulting in a tight packing that forces the alkyl
chains to adopt an all-trans conformation.28 It should be noted
that in this scenario while going through the isotropic state a
conversion takes place to a state in which the alkyl chains are
no longer interdigitating, but all become localised on the same
side of the plane through the hydrogen bonds.
Fig. 2 (a) Thermograms of 1b (lower trace) and 1c (upper trace) and
textures observed between crossed polarisers of the (b) spherulitic and
(c) needle-like morphologies of 1c.
no such behaviour was detected for 1b, c. Temperature-
dependent FT-IR experiments were performed on annealed
®lms of 1b, c, in order to obtain insight into the hydrogen
bonding pattern and the packing of the alkyl chains as a
function of temperature. The O±H stretching vibration of
tertiary alcohols generally gives rise to absorptions in the range
of 3620±3600 cm21 and the N±H stretching vibration of
aziridines appears in the 3360±3300 cm21 region.16,20 Hydro-
gen bonding is known to result in a downward frequency shift
and, in general, peaks become broader as the strength of the
hydrogen bonding increases.21 The position of the CH2
deformation band can be used to assess the organisation of
the hydrocarbon chains. gauche Conformations give rise to a
band at low wavenumbers (y1463 cm21) whereas a highly
packed all-trans state leads to a vibration at 1471 cm21 22
.
FT-IR spectra of 1b, recorded at different temperatures,
revealed a very broad band between 3600 and 3100 cm21
(Fig. 3a). This indicates a strong hydrogen bonded network22
which continued to exist until the melting point was reached at
59 ³C (peak temperature, Fig. 2a). The spectrum of 1b in this
region was nearly identical to that of 1a, suggesting that both
compounds have a similar hydrogen bonding pattern. In such a
pattern the alkyl chains are placed, alternately, above and
below the plane of the hydrogen bonds, giving rise to an
interdigitated solid state structure (Fig. 4a). The position of the
CH2 deformation band (1468 cm21) indicated the presence of
trans conformations in the alkyl chains, but excluded an all-
trans conformation.23 At 55 ³C (onset temperature, Fig. 2a)
this band started to shift to lower wavenumbers (1468±
1465 cm21, Fig. 3e), re¯ecting the increase in the number of
Based on the molecular requirements formulated by Jeffrey17
and Van Doren et al.,18 we expected compounds 1b, c to show
liquid crystalline behaviour. Jeffrey17 has suggested that in the
liquid crystalline phase the weaker van der Waals interactions
are broken while the stronger hydrogen bonds remain intact,
and that at the liquid crystalline-to-isotropic transition the
hydrogen bonded structure breaks down. In contrast to this
model, Van Doren et al.18 proposed that the melting point is
determined by the breaking up of the network of hydrogen
Table 1 Data derived from calorimetric experiments
K1AK2
DHK/kJ mol21
DSm/J mol21 K21
KAI
DHm/kJ mol21
DSm/J mol21 K21
1b
1c
Not observed
30±35 ³C
Ð
223.1
Ð
274
59.9
44.1 ³C
33.5
61.3
101
193
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Fig. 3 Variable temperature FT-IR spectra of annealed ®lms of (a) 1b and (b) 1c. Overlay of experimental and ®tted FT-IR spectra of 1c at (c) 20 ³C
and (d) 36 ³C. (e) Temperature dependence of the position of the CH2 deformation band in the FT-IR spectra of (&) 1b and ($) 1c.
bonds. Upon entering the liquid crystalline phase molecules are
held together by lateral cohesive forces, which break up when
at the clearing point an isotropic liquid is formed.
vesicles of 1c were transformed into ribbon-like bilayer
structures, which rolled up to form right-handed helical
ribbons with lengthsw10 mm (Fig. 5B).
In the case of 1b the hydrogen bonding network is strong
enough to maintain an ordered arrangement in which melting
of the alkyl chains is inhibited until a temperature of 55 ³C is
reached. At this temperature the hydrogen bonding arrange-
ment weakens and the system enters the isotropic state at
59 ³C. In contrast, the weaker hydrogen bonding pattern in
®lms of 1c apparently can not prevent the molecules from
rearranging already at 23 ³C. The consequent reordering of the
hydrogen bonds leads to a much more ef®cient packing of the
alkyl chains which subsequently prevents the formation of a
mesophase until the melting point is reached. However, in
contrast to 1b, for 1c the breaking of the hydrogen bonds and
the melting of the alkyl chains occur simultaneously.
Powder diffraction patterns of cast ®lms of aged 0.1% (w/w)
dispersions of compounds 1b and 1c revealed a repetitive
Ê
distance of 40.6 and 41.6¡0.2 A, respectively, indicating that
the aggregates consisted of intercalated bilayers. Different
levels of relative humidity (0, 50 and 90%) had no effect on the
bilayer periodicity of these ®lms, indicating that no water
molecules are bound to the head groups.
Monolayer experiments
In order to obtain information about the ordering of the
molecules in the aggregates, surface pressure±surface area (p±
A) isotherms of 1b, c were recorded on an aqueous sub-phase
adjusted to pH 3.0 (Fig. 6A). Both isotherms showed a plateau
representing the coexistence of two phases during the transition
of a liquid expanded (LE) to a liquid condensed (LC) phase.
Brewster angle microscopy (BAM)30 on the monolayers of 1b
revealed that in the LE±LC coexistence phase two-dimensional
dendritic solid-like domains were formed with diameters of
approximately 300 mm (Fig. 6A, inset), whereas no domains
were observed in the plateau region of 1c.31 The occupied area
per molecule before and after this transition suggested that the
head groups of both compounds rearrange going from a
parallel to a perpendicular orientation with respect to the
interface (Fig. 6B).
Aggregation behaviour of aziridinemethanol surfactants
Compounds 1b, c did not dissolve in water of neutral or basic
pH. Aqueous dispersions (0.1%; w/w), therefore, were prepared
by injecting a 2.0% (w/v) methanolic solution of 1 into water of
60 ³C adjusted to pH 3.0.29 Transmission electron microscopy
demonstrated that both 1b and 1c formed vesicles with
diameters between 75 and 300 nm (not shown). Upon standing,
dispersions of 1b were transformed into ®bers with diameters of
approximately 5 nm and lengths of several micrometres
(Fig. 5A). After ageing for several days at room temperature,
J. Mater. Chem., 2001, 11, 269±277
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Fig. 5 Transmission electron micrographs taken after 24 h from 0.1%
(w/w) dispersions of (A) 1b (negative staining) and (B) 1c (platinum
shadow) at pH 3.0, bars represent 50 nm.
formation of domains on the microscopic scale. Remarkably,
this also results in a higher molecular area per molecule for
surfactant 1b compared to 1c. Apparently, the methyl
substituent of 1c prevents such a highly organised structure
and enables a close packing of the molecules at the air±water
interface governed by van der Waals forces, but leads to ®lms
of low stability. The fact that compound 1c forms more densely
packed monolayers compared to those of 1b is in line with the
difference in ability of these compounds to form helical
aggregates in aqueous dispersions.
Reaction with aspirin
In the course of our efforts to ®ne-tune the molecular packing
and thereby the morphology of the aggregates,32 we attempted
to induce the formation of chiral aggregates through the
addition of salicylate ions following procedures described by
Hoffmann and Ebert,33 and by Saikaigudin et al.34 This
experiment, however, was not successful and only ill-de®ned
aggregates were observed.35 However, when a methanolic
solution of 1b and 2-acetoxybenzoic acid (aspirin) was injected
into water at 60 ³C, electron microscopy revealed the formation
of helical structures (Fig. 7a, b). Although enantiopure (S)-1b
was used in all experiments, remarkably, both left- and right-
handed helices were generated. Analysis of the dispersion
showed that approximately 30% of 1b had been converted into
4 (Scheme 2). This suggests that under the action of aspirin the
aziridine ring of 1b was activated by acylation of the aziridine
nitrogen atom,15c and subsequently opened in a regioselective
manner by nucleophilic attack of the salicylate.
Fig. 4 Schematic representation of the proposed solid state structure of
(a) 1b and (b) 1c at 20 ³C; (c) 1c at 36 ³C.
The high collapse pressure observed for monolayers of 1b
(p~70 mN m21) indicates that this compound forms a rigid
®lm. It appears that speci®c intermolecular interactions
between the head groups of 1b are responsible for the
1H NMR investigation revealed that reaction of 1b and
aspirin in CD3OD indeed leads to the acylation of the aziridine.
although at this stage no ring opening was observed. Only after
274
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Fig. 7 Electron micrographs taken from 0.1% dispersions of 1b and 2 in
water (platinum shadowing, bars represent 250 nm). (a) Left-handed
and (b) right-handed helices from 1b and aspirin; (c) rod-like structures
from 2.
4 (Fig. 7c), whereas in dispersions of mixtures containing
different ratios of 1b and 4 no distinct morphologies could be
detected. The role of (unreacted) aspirin was further investi-
gated by adding this compound to aqueous dispersions of
mixtures containing different amounts of 1b and by injecting
methanolic solutions of mixtures of 1b and 4 into aqueous
solutions of aspirin. Helix formation was not observed in any
of these cases, leaving the precise effect of aspirin as yet
unexplained.
Conclusions
The experiments described above demonstrate that molecules
containing an aziridinemethanol moiety can form highly
organised structures. The rigid, speci®c and strong hydrogen
bonding pattern of 1b, similar to that of 1a, dominates the
organisation of these molecules in the solid state and prevents
Fig. 6 (A) Langmuir isotherms of 1b and 1c recorded on an aqueous
sub-phase of pH 3.0 at 25 ³C. Inset: Brewster angle micrograph of a
surface monolayer of 1b (p~3.0 mN m21, spot diameter 600 mm), at
15 ³C and pH 3.0. (B) Proposed orientations of the head groups of 1b
(top) and 1c (bottom) at the air±water interface in the liquid expanded
(LE, left) and liquid condensed phase (LC, right).
transfer of the methanolic reaction mixture to an aqueous
medium is compound 4 formed, which suggests that aggrega-
tion plays an important role in the nucleophilic ring opening by
salicylic acid.
Remarkably, the reaction of 1b was speci®c for aspirin. No
reaction was observed upon addition of 3- or 4-acetoxybenzoic
acid to methanolic solutions of 1b and no distinct aggregate
morphologies could be detected in aqueous dispersions of these
mixtures. We propose that the reaction proceeds via acylation
of the alcohol which, due to the rigidity of the aziridinemetha-
nol head group and the preorganisation of the molecules in the
aggregate can only occur in the case of 2-acetoxybenzoic acid.
Subsequent acyl transfer from the hydroxy group to the
aziridine nitrogen then may lead to activation and opening of
the aziridine ring. In order to unravel the origin of helix
formation it was attempted to generate these structures by
dispersion of 4, or mixtures of 4 and different amounts of 1b, in
water. This did not, however, lead to the formation of helical
aggregates. Rod-like structures were observed instead for pure
Scheme 2
J. Mater. Chem., 2001, 11, 269±277
275

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(e) N. A. J. M. Sommerdijk, P. J. J. A. Buynsters, H. Akdemir,
D. G. Geurts, A. M. A. Pistorius, M. C. Feiters, R. J. M. Nolte
and B. Zwanenburg, Chem. Eur. J., 1998, 1, 127; (f) M. C. Feiters
and R. J. M. Nolte, Adv. Supramol. Chem., 2000, 6, 41.
G. A. Ozin, Acc. Chem. Res., 1997, 30, 17.
(a) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710; (b) J. S. Beck, J. C. Vartuli,
W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt,
C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen,
J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 1992, 114,
10834.
the molecules from displaying thermotropic liquid crystalline
behaviour.
The interconnectivity of these molecules can be disrupted by
the introduction of an additional methyl substituent at the
aziridine ring. This leads to a change in the con®guration of the
NH hydrogen atom (trans with respect to the substituents) and
consequently to a change in the hydrogen bonding pattern of
the aziridinemethanol moieties. The hydrogen bonding net-
work of 1c is not as rigid and strong as that of 1b and therefore
the molecules reorganise into a more stable structure when the
compound is heated to the point where the alkyl chains start to
melt. In this structure it is the ef®cient packing of the alkyl
chains rather than the hydrogen bonding pattern that keeps the
molecules in the solid state until the melting point is reached at
45 ³C.
8
9
10 (a) J. P. Spatz, S. MoÈûmer and M. MoÈller, Chem. Eur. J., 1996, 2,
1552; (b) M. MoÈller and J. P. Spatz, Curr. Opin. Colloid Interface
Sci., 1997, 2, 177.
11 (a) S. Rajam, B. R. Heywood, J. B. A. Walker, S. Mann,
R. J. Davey and J. D. Birchall, J. Chem. Soc., Faraday Trans.,
1991, 87, 727; (b) B. R. Heywood and S. Mann, Chem. Mater.,
1994, 6, 311.
We have demonstrated that the introduction of the methyl
substituent has a profound effect on the aggregation behaviour
of the molecules in water and at the air±water interface. The
speci®c intermolecular interactions of the head groups of 1b
lead to a highly ordered monolayer, i.e. the formation of two-
dimensional crystalline domains. However, this rigid orienta-
tion of the head groups prevents the close packing of the
molecules and hence the expression of molecular chirality in the
aggregates. In contrast to that of compound 1b, the organisa-
tion of 1c is predominantly determined by the close packing of
molecules, resulting in the expression of chirality at the
supramolecular level. Our experiments show that it is possible
to ®ne-tune the aggregate morphology of 1b by the addition of
aspirin. The speci®c nature of this reaction is most probably
related to the rigid conformation of the head group, although
the helix formation remains as yet unexplained.
12 (a) A. Wenzel and M. Antonietti, Adv. Mater., 1997, 9, 487;
(b) M. Antonietti and C. GoÈltner, Angew. Chem., Int. Ed. Engl.,
1997, 36, 910.
13 M. C. Feiters, in Comprehensive Supramolecular Chemistry, ed.
J. L. Atwood, J. E. D. Davies, D. D. Macnicol and F. VoÈgtle,
J. M. Lehn (Series editor), Supramolecular Catalysis, vol. 10,
Pergamon, London, 1996.
14 J. M. Boggs, Biochem. Biophys. Acta, 1987, 906, 353.
15 (a) J. G. H. Willems, M. C. Hersmis, R. de Gelder, J. M. M. Smits,
J. B. Hammink, F. J. Dommerholt, L. Thijs and B. Zwanenburg,
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J. B. Hammink, A. M. Vaarhorst, F. J. Dommerholt and
B. Zwanenburg, Tetrahedron Lett., 1995, 36, 603 (the crystal
structure determination and the crystallographic data will be
published elsewhere); (c) N. A. J. M. Sommerdijk,
P. J. J. A. Buynsters, H. Akdemir, D. G. Geurts, M. C. Feiters,
R. J. M. Nolte and B. Zwanenburg, J. Org. Chem., 1997, 62, 4955.
16 (a) R. Mathis, R. Martino and A. Lattes, Spectrochim. Acta, Part
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Acknowledgements
17 G. A. Jeffrey, Acc. Chem. Res., 1986, 19, 168.
The authors wish to thank F. J. Dommerholt for the kind
donation of compound 1a, H. P. M. Geurts for his assistance in
performing electron microscopy experiments, A. M. Roelofsen,
for performing the Langmuir ®lm balance studies, D. S. J. van
der Gaast (NIOZ, Netherlands Institute for Sea Research) for
assistance with the powder diffraction experiments and L.Thijs
and G. J. F Chittenden for fruitful discussions.
18 (a) H. A. Van Doren, T. J. Buma, R. M. Kellog and H. Wynberg,
J. Chem. Soc., Chem. Commun., 1988, 461; (b) H. A. Van Doren,
R. Van der Geest, C. A. Keuning, R. M Kellog and H. Wynberg,
Liq. Cryst., 1989, 5, 265.
19 R. A. KuÈhnel and S. J. van der Gaast, Adv. X-Ray Anal., 1993, 36,
439.
20 (a) The spherulites show a characteristic pattern (Maltese cross)
under cross polarization, which may be similar to the patterns
commonly observed in the smectic phase of liquid crystals see e.g.
G. W. Gray and J. W. Goodby, Smectic Liquid Crystals, Leonard
Hill, Philadelphia, 1984, pp. 9±17; (b) Recently a ``spherular
crystal'' was observed when an amphiphilic single chain azacrown
ether crystallized from its melt (see ref. 20c). This spherular crystal
or spherulite has hierarchical structures: it consists of solid
cylinders, and the cylinders are composed of the bilayers of the
amphiphilic molecules; (c) R. Tang and Z. Tai, Chem. Mater.,
1998, 10, 1638.
21 A. R. H. Cole and P. R. Jefferies, J. Chem. Soc., 1956, 4391.
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27 In general, the frequency shift of an intramolecular hydrogen bond
is less pronounced than for intermolecular hydrogen bonding, see
ref. 22.
28 The enhanced degree of order after reorganisation of the molecules
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However, a sub-cell structure has not yet been deduced from these
complicated diffractograms.
29 The pKÃavalues of 1b, c in methanol±water (95 : 5; v/v) were
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277