Supramolecular organisation of oligo(p-phenylenevinylene) at the air-water interface and in water

Article abstract of DOI:10.1039/b103966k

Two novel chiral (bola)amphiphilic oligo(p-phenylenevinylene)s (OPVs) have been synthesised and fully characterised. Decoration of the hydrophobic OPV backbone with a hydrophilic tris[tetra(ethylene oxide)]benzene wedge on one side and a hydrophobic tris(

Full text of DOI:10.1039/b103966k

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Supramolecular organisation of oligo(p-phenylenevinylene) at the
air–water interface and in water
Pascal Jonkheijm, Michel Fransen, Albertus P. H. J. Schenning and E. W. Meijer*
2
Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology,
P.O. Box 513, 5600 MB, Eindhoven, The Netherlands. E-mail: E.W.Meijer@tue.nl; Fax: 31
(0)40 2451036; Tel: 31 (0)40 2472655
Received (in Cambridge, UK) 3rd May 2001, Accepted 13th June 2001
First published as an Advance Article on the web 11th July 2001
Two novel chiral (bola)amphiphilic oligo(p-phenylenevinylene)s (OPVs) have been synthesised and fully
characterised. Decoration of the hydrophobic OPV backbone with a hydrophilic tris[tetra(ethylene oxide)]benzene
wedge on one side and a hydrophobic tris(alkoxy)benzene wedge on the other side, resulted in amphiphilic OPV1.
In bolaamphiphile OPV2, two hydrophilic tris[tetra(ethylene oxide)]benzene wedges are connected at both ends of
the OPV backbone. The organisation of the amphiphiles has been investigated at the air–water interface and in
water. Langmuir monolayers of OPV1 showed that these amphiphiles are perpendicularly oriented at the air–water
interface. In the case of OPV2, the OPV units are lying flat on the subphase with the hydrophilic ethylene glycol
wedges pointing into the water phase. In chloroform, the OPV derivatives are present as molecularly dissolved
species. In water, the amphiphilic OPV derivatives aggregate in chiral stacks, as can be concluded from UV–vis,
fluorescence and CD spectroscopy. Temperature dependent measurements showed for OPV1 a transition at 50 ЊC
from a chiral aggregated state to disordered aggregates. In the case of bolaamphiphilic OPV2, the transition at
55 ЊC between those states is a less cooperative process. The chiral order in the assemblies of the bolaamphiphiles
can be influenced by the addition of salt.
interface.16 The Langmuir–Blodgett technique can be used
Introduction
to prepare light-emitting diodes (LED) that show linear
π-Conjugated oligomers are regarded as the ideal model com-
pounds for understanding the electro-optical properties of π-
conjugated polymers. Impressive advances in research have
been achieved on π-conjugated materials1 and have led towards
a better understanding of the relationship between the desired
functional properties and the enhanced supramolecular struc-
ture, by making use of its well-defined chemical structure
and conjugation length.2 Highly ordered local structures are
the prerequisite for obtaining electronic properties such as
high carrier mobility. Controlled organisation of π-conjugated
oligomers has been achieved by using a suitable solvent–
nonsolvent mixture,3 thermotropic and lyotropic liquid crystal-
linity4,5 hydrogen bonding motifs,6,7 and by block copolymers.8
Efficient macromolecular assembly of oligomers will become
critically important to achieve new applications, molecular
electronics providing the most outstanding prospect.9
polarisation of the emitted light.17
Here, we report the synthesis of two novel chiral (bola)amphi-
philic OPV’s (Scheme 1, OPV1 and OPV2) as an extension
of our work on the fully aliphatic OPV oligomers of different
lengths.18 Amphiphile OPV1 consists of a hydrophobic OPV
tetrameric core with, at one end a hydrophobic trialkoxy-
benzene wedge and, at the other end, a hydrophilic tris[tetra-
(ethylene oxide)]benzene wedge. Bolaamphiphile OPV2 con-
tains a hydrophobic pentameric OPV core, substituted with two
hydrophilic tris[tetra(ethylene oxide)]benzene wedges at both
ends. We have chosen chiral amphiphiles because a stereocenter
in the side-chain has been proven to be a very informative probe
in studying the expression of chirality at the supramolecular
level.19 The organisation of the OPV amphiphiles has been
studied at the air–water interface and in water.
Surfactant interactions are often used in nature to assemble
molecules into large systems, such as membranes. This method-
ology has inspired chemists to synthesise a variety of artificial
surfactants, e.g. amphiphiles, to construct well-defined
assemblies in water and at the air–water interface.10 Remark-
ably, lipophilic interactions have rarely been applied in the
supramolecular organisation of π-conjugated oligomers in con-
trast to amphiphilic π-conjugated polymers.11 Recently, the self-
assembly of α,ω-disubstituted sexithiophenes in butanol and
water has been reported.12 Alternating α-oligothiophene blocks
with poly(ethylene oxide) blocks have been synthesised and the
oligothiophenes aggregate in a 83% water–dioxane mixture in a
face-to-face manner.13 Oligo(p-phenylenevinylene) (OPV) has
been functionalised with ethylene glycol chains14 and pendant
ethylene glycol dendrons15 showing superstructures in THF–
water14a and water14b and at the air–water interface, respec-
tively. Monolayers have also recently been constructed from
amphiphilic OPVs containing a pyridine or diaminotriazine
headgroup to obtain controllable organisation at the air–water
Experimental
General methods: 1H NMR and 13C NMR spectra were
recorded at room temperature on a Varian Gemini 300 or a
Varian Mercury 400. Chemical shifts are given in ppm (δ) rela-
tive to tetramethylsilane. Abbreviations used are s = singlet,
d = doublet, dd = double doublet, t = triplet and m = multiplet.
Infrared spectra were run on a Perkin Elmer 1600 FT-IR
spectrometer. MALDI-TOF MS spectra were measured on a
Perspective DE Voyager spectrometer utilizing an α-cyano-4-
hydroxycinnamic acid matrix. Elemental analyses were carried
out on a Perkin Elmer 2400. Differential scanning calorimetry
measurements were performed on a Perkin Elmer DSC Pyris 1
at a heating rate of 40 ЊC minϪ1. UV–vis and fluorescence
spectra were performed on a Perkin Elmer Lambda 40 spectro-
photometer and a Perkin Elmer LS-50 B. CD spectra were
recorded on a JASCO J-600 spectropolarimeter. A description
of the CPL apparatus can be found in the literature.20
1280
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103966k

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The aqueous solutions for optical measurements were pre-
pared via a methanol injection method. The amphiphiles were
dissolved in 50 µL methanol and this solution was injected into
5 mL demineralised water. After evaporation of the methanol,
the temperature dependent optical spectra were recorded. The
time between two measurements was 30 min.
Monolayer experiments were performed with a KSV-5000
LB instrument placed on an anti-vibration table in a controlled
atmosphere room. 10 µL of a chloroform solution (0.48 mM
OPV1 and 0.27 mM OPV2) was spread onto the subphase
(micro-filtered de-ionised water, resistivity = 18.2 M Ω cm) at
20 ЊC. The Langmuir film compression (30 mm minϪ1) started
30 minutes after spreading. The surface pressure was measured
using the Wilhelmy plate method with plates made of filter
paper. Brewster angle microscope experiments were carried out
with a NFT BAM1 instrument, manufactured by Nano Film
Technology, Göttingen, which was equipped with a 10 mV He–
Ne laser (beam diameter of 0.68 mm, 632.8 nm). Materials:
3,4,5-Tris(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)-
benzyl alcohol (2)
Under an argon atmosphere, a solution of 1 (17.59 g, 23.30
mmol) in 100 mL dry THF was added dropwise to a suspension
of LiAlH4 (0.97 g, 25.60 mmol) in 20 mL dry THF at 0 ЊC.
After refluxing overnight, the solution was poured on 100 mL
crushed ice. Sulfuric acid (60 mL, 10%) was added and the
aqueous layer was extracted twice with dichloromethane.
The collected organic layers were washed with brine and dried
(Na2SO4), filtrated and evaporated to dryness, resulting in
16.09 g (95%) of 2. 1H NMR (300 MHz, CDCl3): δ 3.18
(s, 6H, OCH3), 3.19 (s, 3H, OCH3), 3.35–3.70 (m, 42H, OCH2),
3.95 (m, 6H, ArOCH2), 4.38 (s, 2H, ArCH2OH), 6.42 (s, 2H,
ArH); 13C NMR (100 MHz, CDCl3): δ 25.0, 58.3, 63.9,
67.2, 68.2, 69.2, 69.8, 70.0, 70.0, 70.1, 71.3, 71.7, 105.6, 136.7,
136.7, 152.0; MALDI-TOF MS (M = 726.86) m/z = 749.57
[M ϩ Na]ϩ.
methyl
3,4,5-tris(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}-
3,4,5-Tris(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethoxy)-
benzyl chloride (3)
ethoxy)benzoate 1, (E,E)-4-{4-[3,4,5-tris(dodecyloxy)styryl]-
2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-
benzaldehyde7 5, diethyl {2,5-bis[(S)-2-methylbutoxy]-4-
bromobenzyl}phosphonate21 and 2,5-bis[(S)-2-methylbutoxy]-
l,4-dibromobenzene22 6 were synthesised according to literature
procedures. All solvents were of AR quality and chemicals were
used as received. BioBeads S-XI Beads were obtained from
Bio-Rad Laboratories.
To a stirred solution of 2 (10.0 g, 13.76 mmol) in 25 mL dry
dichloromethane, were added three drops of DMF and thionyl
chloride (4.9 g, 41.3 mmol). The progress of the reaction was
monitored by TLC. After 1 h, the reaction was completed and
the solvent was evaporated. The residue (10.0 g, 13.42 mmol)
1
was not further purified. H NMR (400 MHz, CDCl3): δ 3.25
Scheme 1 A) LiAlH4, 8 h, 0 ЊC. B) DCM, SOCl2, 1 h. C) PO(Et)3, 8 h, 160 ЊC. D and H) DMF–THF, 4, KOC(CH3)3, 8 h. E and G) DMF, n-BuLi,
Ϫ10 ЊC, 1 h. F) DMF, KOC(CH3)3, diethyl {2,5-bis[(S)-2-methylbutoxy]-4-bromobenzyl}phosphonate, 3 h, rt.
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286
1281

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(s, 6H, OCH3), 3.26 (s, 3H, OCH3), 3.43–3.75 (m, 42H, OCH2),
4.10 (m, 6H, ArOCH2), 4.40 (s, 2H, ArCH2Cl), 6.53 (s, 2H,
ArH); 13C NMR (100 MHz, CDCl3): δ 46.2, 58.5, 68.5, 69.3,
70.0, 70.1, 70.2, 70.4, 71.5, 71.9, 107.9, 132.3, 138.0, 152.2.
by column chromatography (silica gel, hexane–toluene 1:1).
The first fraction appeared to be the monoaldehyde and the
dialdehyde was removed from the column by eluting it with
chloroform. Recrystallisation from hexane yielded 7 (3.93 g,
52%). 1H NMR (300 MHz, CDCl3): δ 0.97 (t, 6H), 1.05 (d, 6H),
1.35 (m, 2H), 1.60 (m, 2H), 1.90 (m, 2H), 3.8 (m, 4H), 7.44
(s, 2H, ArH), 10.54 (s, 2H); 13C NMR (100 MHz, CDCl3):
δ 11.24, 16.53, 26.08, 34.68, 73.88, 111.55, 129. 29, 155.32,
189.30.
Diethyl (3,4,5-tris(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}-
ethoxy)benzyl)phosphonate (4)
A mixture of triethyl phosphite (13.85 g, 83.4 mmol) and 3
(10.00 g, 13.42 mmol) was stirred at 160 ЊC for 8 h. During this
time, ethyl chloride was distilled from the reaction mixture.
Subsequently, the mixture was cooled to 70 ЊC and the excess of
triethyl phosphite was distilled under reduced pressure. The
(E,E )-1,4-Bis{4-bromo-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-
bis[(S)-2-methylbutoxy]benzene (8)
1
product was used without further purification: H NMR (300
Dialdehyde 7 (4.80 g, 15.69 mmol) was dissolved in 100 mL dry
DMF and slowly added under an argon atmosphere to a
solution of diethyl {2,5-bis[(S)-2-methylbutoxy]-4-bromo-
benzyl}phosphonate (16.94 g, 35.29 mmol) and potassium
tert-butoxide (3.96 g, 35.68 mmol) in 220 mL dry DMF and
THF (1:1 mixture). After stirring for 3 h at room temperature,
the mixture was poured into ice and 250 ml HCl (6 M) was
added and extracted with chloroform. The organic layer was
washed several times with 250 mL HCl (3 M) and again with
200 mL water and subsequently dried over MgSO4, filtered
and evaporated to dryness. The crude mixture was purified
by column chromatography (silica gel, DCM–pentane 1:1).
After recrystallisation from ethanol, 8.00 g (53%) of 8 was
obtained. 1H NMR (300 MHz, CDCl3): δ 1.00 (m, 18H, CH3),
1.10 (m, 18H, CH3), 1.36 (m, 6H, CH2), 1.65 (m, 6H, CH), 1.95
(m, 6H, CH2), 3.78–3.94 (m, 12H, OCH2), 7.11 (s, 2H, ArH),
7.17 (s, 2H, ArH), 7.18 (s, 2H, ArH), 7.44 (d, 2H, J = 16.7 Hz,
ArCH᎐CH), 7.52 (d, 2H, J = 16.7 Hz, ArCH᎐CH); 13C NMR
(100 MHz, CDCl3): δ 11.33, 11.43, 16.59, 16.73, 16.79,
26.12, 26.29, 26.36, 34.84, 34.95, 35.01, 74.21, 74.47, 74.74,
110.10, 110.70, 111.47, 118.01, 122.38, 123.39, 127.26, 149.98,
150.97, 151.09; MALDI-TOF MS (M = 956.6) m/z = 956.6 [M];
IR (KBr): ν (cmϪ1) = 663, 717, 737, 772, 815, 855, 876,
916, 971, 982, 1027, 1048, 1200, 1236, 1289, 1325, 1344, 1388,
1419, 1462, 1500, 1563, 2050, 2874, 2920, 2959, 3060; Anal.
Calcd. for C52H76O6Br2: C 65.33, H 8.00. Found: C 65.03,
H 7.75%.
MHz, CDCl3) δ 1.21 (t, 6H, OCH2CH3), 3.02 (d, 2H, ArCH2),
3.34–3.80 (m, 51H, OCH2, OCH3), 3.98 (m, 4H, POCH2), 4.10
(m, 6H, ArOCH2), 6.53 (s, 2H, ArH); 39P NMR (400 MHz,
CDCl3) δ 27.37.
(E,E,E )-1-{4-[3,4,5-Tris(dodecyloxy)styryl]-2,5-bis[(S)-2-
methylbutoxy]phenyl}-2-{4-[3,4,5-tris(2-{2-[2-(2-methoxyeth-
oxy)ethoxy]ethoxy}ethoxy)styryl]-2,5-bis[(S)-2-methylbutoxy]-
phenyl}ethene (OPV1)
Phosphonate 4 (77 mg, 0.09 mmol) was dissolved in 10 mL
anhydrous DMF. Potassium tert-butoxide (60 mg, 0.54 mmol)
was added to the solution under an argon atmosphere. After
15 minutes, a solution of aldehyde 5 (100 mg, 0.08 mmol) in
15 mL THF was added dropwise to the reaction mixture. The
solution was stirred overnight at room temperature. 100 mL
HCl (6 M) was added and the aqueous layer was extracted three
times with chloroform. The collected organic layers were
washed with HCl (3 M) and dried over MgSO4, filtrated and
evaporated to dryness. The crude mixture was purified by
column chromatography (silica gel, 3% methanol in CH2Cl2)
and dried thorougly over P2O5 to afford 111 mg (73%) of OPV1
᎐
᎐
1
as a red wax. Mp 51 ЊC; H NMR (400 MHz, CDCl3): δ 0.90
(t, 9H, (CH2)9CH3), 1.02 (m, 12H, CH3), 1.13 (m, 12H, CH3),
1.3 (m, 54H, OCH2CH2(CH2)9CH3), 1.5 (m, 8H, CH2), 1.7
(m, 2H, CH2), 1.85 (m, 4H, CH), 1.96 (m, 4H, CH2), 3.38 (s,
6H, OCH3), 3.39 (s, 3H, OCH3), 3.53–3.77 (m, 42H, OCH2),
3.82–3.93 (m, 6H, OCH2(CH2)10CH3), 4.01 (m, 8H, OCH2-
CH(CH3)CH2CH3), 4.21 (m, 6H, ArOCH2), 6.76 (s, 2H, ArH),
(E,E )-1,4-Bis{4-formyl-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-
bis[(S)-2-methylbutoxy]benzene (9)
6.78 (s, 2H, ArH), 7.05 (d, 2H, J = 16.2 Hz, ArCH᎐CH), 7.11
᎐
(d, 2H, J = 5.1 Hz, ArCH᎐CH), 7.21 (s, 2H, ArH), 7.36 (d,
᎐
Dibromide 8 (2.00 g, 2.09 mmol) was dissolved in 20 mL THF.
The solution was cooled to Ϫ10 ЊC and 2.3 mL of a 2.5 M
n-butyllithium solution in hexane were added slowly. After
stirring for 5 min the cooling bath was removed and 1 mL dry
DMF was added dropwise. The reaction mixture was stirred for
another hour at room temperature. After additon of 100 mL
HCl (6 M), the organic layer was washed with water (2 times
150 mL), a saturated NaHCO3 solution (150 mL) and again
with water (150 mL). The organic layer was dried over MgSO4
and the solvent was evaporated. The residue was purified by
column chromatography (silica, CH2Cl2–pentane 1:1) and
recrystallization from heptane yielded 1.23 g (69%) 8. 1H NMR
(300 MHz, CDCl3): δ 1.00 (m, 18H, CH3), 1.10 (m, 18H, CH3),
1.36 (m, 6H, CH2), 1.65 (m, 6H, CH), 1.95 (m, 6H, CH2), 3.81–
4.03 (m, 12H, OCH2), 7.24 (s, 2H, ArH), 7.26 (s, 2H, ArH), 7.35
J = 12.4 Hz, 1H, ArCH᎐CH), 7.41 (d, 1H, J = 12.4 Hz,
᎐
ArCH᎐CH), 7.53 (s, 2H, ArH); 13C NMR (100 MHz, CDCl3)
᎐
δ 11.69, 11.71, 11.76, 14.34, 17.02, 17.06, 17.09, 22.92, 26.36,
26.44, 26.62, 29.60, 29.63, 29.66, 29.85, 29.89, 29.94, 29.97,
29.99, 30.57, 32.15, 32.17, 35.22, 35.31, 35.38, 59.24, 68.86,
69.03, 69.31, 69.84, 70.00, 70.73, 70.74, 70.82, 70.90, 71.06,
72.15, 72.65, 73.78, 74.37, 74.41, 74.65, 105.29, 106.33, 109.99,
110.12, 110.65, 111.00, 122.73, 122.79, 122.9, 123.35, 126.75,
127.02, 127.64, 127.79, 128.61, 128.79, 133.46, 133.93, 138.36,
151.25, 151.30, 151.38, 152.97, 153.47; MALDI-TOF MS
(M = 1900.73) m/z = 1900.43 [M], 1923.46 [M ϩ Na]ϩ, 1941.58
[M ϩ K]ϩ. Anal. Calcd. for C113H190O22: C 71.41, H 10.08.
Found: C 71.62, H 9.96%.
2,5-Bis[(S)-2-methylbutoxy]-1,4-diformylbenzene (7)
(s, 2H, ArH), 7.54 (d, 2H, J = 16.8 Hz, ArCH᎐CH), 7.68 (d,
᎐
Under an argon atmosphere 36.75 mL of 1.6 M n-butyllithium
in hexane were added dropwise to a solution of dibromide 6
(10 g, 24.5 mmol) in 200 mL dry ether at Ϫ10 ЊC. During the
first 5 min 4.55 mL of dry DMF were added and a gel was
formed. After stirring for 2.5 h the reaction mixture was poured
into 200 mL HCl (6 M) and extracted with ether (150 mL). The
organic layer was washed 3 times with 200 mL HCl (1 M), then
with 300 mL water, followed by a saturated NaHCO3 solution
(300 mL). The organic layer was dried over MgSO4, filtrated
and evaporated to dryness. The crude mixture was purified
2H, J = 16.8 Hz, ArCH᎐CH), 10.5 (s, 2H, CH᎐O); 13C NMR
᎐
᎐
(100 MHz, CDCl3): δ 11.33, 11.43, 16.59, 16.73, 16.79, 26.12,
26.29, 26.36, 34.84, 34.95, 35.01, 73.70, 73.83, 74.12, 109.73,
110.19, 122.50, 123.11, 126.25, 127.42, 134.95, 150.67, 151.33,
156.38, 189.04; MALDI-TOF MS (M = 855.21) m/z = 854.49
[M]ϩ; IR (KBr): ν /cmϪ1 = 672, 723, 774, 814, 851, 874, 915,
970, 1007, 1041, 1124, 1152, 1200, 1263, 1343, 1390, 1423,
1463, 1504, 1595, 1668 (C᎐O), 2040, 2858, 2928, 2959, 3064.
᎐
Anal. Calcd. for C54H78O8: C 75.94, H 9.21. Found: C 75.62,
H 9.46%.
1282
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286

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(E,E,E,E )-1,4-Bis{4-[3,4,5-tris(2-{2-[2-(2-methoxyethoxy)-
ethoxy]ethoxy}ethoxy)styryl]-2,5-bis[(S)-2-methylbutoxy]-
styryl}-2,5-bis[(S)-2-methylbutoxy]benzene (OPV2)
Under an argon atmosphere, phosphonate 4 (925 mg, 1.08
mmol) was dissolved in 10 ml anhydrous DMF and potassium
tert-butoxide (310 mg, 2.77 mmol) was added. After 15 minutes,
a solution of dialdehyde 8 (310 mg, 0.36 mmol) in 7.5 mL THF
was added dropwise to the reaction mixture. The solution was
stirred overnight at room temperature. 100 mL HCl (6 M) was
added and the aqueous layer was extracted three times with
chloroform. The collected organic layers were washed with
HCl (3 M) and dried over MgSO4, filtrated and evaporated to
dryness. The crude mixture was purified by column chroma-
tography (silica gel) with eluents hexane–ethyl acetate (2:1) and
CH2Cl2–ethanol (97:3) and, subsequently, Bio-Beads column
chromatography (CH2Cl2) was performed. Thorough drying
over P2O5 afforded 435 mg (54%) of OPV2 as an orange wax.
Mp 82 ЊC; 1H-NMR (400 MHz, CDCl3): δ 1.00 (m, 18H, CH3),
1.10 (m, 18H, CH3), 1.36 (m, 6H, CH2), 1.65 (m, 6H, CH), 1.95
(m, 6H, CH2), 3.36 (s, 12H, OCH3), 3.39 (s, 6H, OCH3), 3.5–3.8
(m, 84H, OCH2), 3.8–3.9 (m, 12H, OCH2CH(CH3)CH2CH3),
4.21 (m, 12H, OCH2), 6.76 (s, 4H, ArH), 7.03 (d, 2H, J = 16.4
Hz, ArCH᎐CH), 7.08 (s, 2H, ArH), 7.19 (s, 4H, ArH), 7.34
᎐
(d, 2H, J = 16.4 Hz, ArCH᎐CH), 7.52 (s, 4H, ArCH᎐CH); 13C
᎐
᎐
NMR (400 MHz, CDCl3): δ 11.33, 11.43, 16.81, 16.85, 26.38,
34.95, 35.01, 35.14, 59.00, 68.80, 69.77, 70.49, 70.53, 70.58,
70.59, 70.64, 70.65, 70.82, 71.91, 72.38, 74.15, 74.23, 74.43,
106.12, 109.83, 109.95, 110.75, 122.56, 122.75, 123.12, 126.50,
127.40, 127.59, 128.36, 133.69, 138.22, 151.03, 151.10, 151.20,
152.73; MALDI-TOF MS (M = 2240.89) m/z = 2239.94 [M]ϩ,
2262.92 [M ϩ Na]ϩ, 2278.91 [M ϩ K]ϩ; IR (KBr): ν /cmϪ1
=
663, 692, 731, 773, 810, 852, 965, 1044, 1103, 1201, 1247, 1289,
1349, 1387, 1421, 1462, 1505, 1579, 2050, 2873, 2920, 2958,
3059. Anal. Calcd. for C122H198O36: C 65.39, H 8.91. Found:
C 65.27, H 9.05%.
Results and discussion
Synthesis and characterisation
Optically active (bola)amphiphilic π-conjugated oligomers
OPV1 and OPV2 (Scheme 1) were synthesised from the pre-
cursor diethyl [3,4,5-tris(2-{2-[2-(2-methoxyethoxy)ethoxy]-
ethoxy}ethoxy)benzyl]phosphonate, 4. The synthesis of the
precursor required three steps starting from methyl 3,4,5-
tris[tetra(ethylene oxide)]benzoate, 1. Reduction by LiAlH4 of 1
afforded the corresponding alcohol derivative 2. Chlorination
of 2 in refluxing thionyl chloride with a catalytic amount of
DMF yielded chloride derivative 3. The obtained product was
subsequently converted to 4 by performing a Michaelis–
Arbuzov reaction with triethyl phosphite. Pure OPV1 was
obtained via a Wittig–Horner coupling reaction of aldehyde
derivative 5 with 4. Compound OPV2 was synthesised starting
from the dibromide derivative 6. Substitution of the bromines
by formyl groups, by reaction with n-BuLi in DMF according
to the Bouveault method, resulted in pure 7. Wittig–Horner
coupling of 7 with diethyl {2,5-bis[(S)-2-methylbutoxy]-4-
bromobenzyl}phosphonate yielded dibromide OPV derivative
8 which was subsequently converted to the dialdehyde 9
according to the same procedure as applied for 7. After the
coupling of 9 with 4, bolaamphiphile OPV2 was isolated. Both
amphiphiles, OPV1 and OPV2, were fully characterised by
1H NMR, 13C NMR, IR spectrometry, MALDI-TOF mass
spectrometry and elemental analysis.
Fig. 1 Normalized UV-Vis and fluorescence spectra at 20 ЊC in
chloroform (solid lines) and water (dashed lines) for a) OPV1, b) OPV2
and c) circular polarisation in absorption (gabs) and luminescence (glum
)
in water for OPV2.
(Fig. 1). The fluorescence maxima in chloroform are positioned
at λmax = 495 for OPV1 and 525 nm for OPV2 (Fig. 1).
The absorption and fluorescence maxima are typical for a
molecularly dissolved OPV tetramer and pentamer, respec-
tively.18 The lack of a Cotton effect in chloroform supports the
supposition that both amphiphiles are molecularly dissolved.3
Organisation at the air–water interface
In order to organise our amphiphiles into monolayers,
OPV1 and OPV2 were spread on the air–water interface. The
pressure–area isotherms (Fig. 2) display in both cases a slow
increase in surface pressure with an onset of 375 and 750 Å2
moleculeϪ1 for OPV1 and OPV2, respectively. In the case
of OPV1 a collapse was found at a pressure of π = 45 mN mϪ1
and a mean molecular area of 84 Å2 moleculeϪ1 is obtained by
extrapolating the steep rise in the condensed region to zero
Compounds OPV1 and OPV2 melt at 51 and 82 ЊC, respec-
tively. Both amphiphiles are highly soluble in organic solvents
such as THF, chloroform, and methanol. The UV-Vis spectra
of OPV1 and OPV2 show broad absorption π–π* bands at
λmax = 434 and 450 nm, respectively in chloroform solutions
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286
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Fig. 2 Surface pressure–area (π–A) isotherms of OPV1 (solid line)
and OPV2 (dotted line) obtained at 20 ЊC.
pressure. This value is similar to related OPV amphiphiles16
indicating that OPV1 is perpendicularly oriented at the air–
water interface. A large hysteresis was found in the isotherm
suggesting monolayers with low stability (Fig 2). This
behaviour is remarkably different from that of the related
amphiphilic OPVs.16 The instability may be caused by the
intrinsic flexibility of the tris[tetra(ethylene oxide)]benzene
wedges. In the case of OPV2, Brewster angle microscopy
(BAM) showed a homogeneous film after spreading, and small
islands were formed at a pressure of π = 12 mN mϪ1. Unlike
OPV1, the monolayers formed from OPV2 show a flat rise in
pressure with large hysteresis. This enlarged instability can be
associated with the different orientation of this molecule at the
air–water interface. Presumably, the OPV units are lying flat
on the subphase with the hydrophilic ethylene glycol wedges
pointing into the water. In such an orientation, an earlier onset
of the pressure is expected while π–π stacking is unfavourable
resulting in the formation of less stable monolayers.
Organisation in water
In comparison with chloroform solutions, the UV-Vis
absorption spectra in water are blue shifted to λmax = 402 nm
(OPV1) and to λmax = 426 nm (OPV2) with the appearance of
a vibronic shoulder at 467 and 486 nm, respectively (Fig. 1).
The fluorescence is strongly quenched in water and red shifted
to λem,max = 548 nm (OPV1) and λem,max = 546 and 585 nm
(OPV2). The absorption and fluorescence data show that
the OPV oligomers are aggregated.3 A bisignate Cotton effect
(Fig. 3) was observed for OPV1 in water at the π–π* band with a
positive Cotton effect at λ = 384 nm (gabs = 6.3 × 10Ϫ4) and a
negative Cotton effect at λ = 424 nm (gabs = Ϫ1.1 × 10Ϫ3). The
zero-crossing of the CD signal (λ = 400 nm) lies close to the
absorption maximum of the chromophore. Compound OPV2
also gave a strong bisignate Cotton effect with a positive and
negative sign at λ = 405 nm (gabs = 5.6 × 10Ϫ3) and λ = 447 nm
(gabs = Ϫ6.5 × 10Ϫ3), respectively with the CD signal going
through zero at λ = 423 nm, near the absorption maximum
of the π–π* band. Additionally, circular polarization of the
luminescence (CPL) of OPV2 in water was measured (Fig. 1).
The CPL effect, expressed as the dissymmetry factor (glum), was
found to be negative and essentially constant over the emission
Fig. 3 Temperature dependent a) UV-Vis, b) fluorescence and c) CD
measurements for OPV1 at 1.6 × 10Ϫ5 mol LϪ1 in water.
trap sites.20 Since the gabs of the aggregates is negative in the
475–500 nm range, the aggregated molecules can cause an
artefact in the CPL measurement by preferentially absorbing
right circularly polarised light emitted by the non-aggregated
molecules and, hence, the emitted light shows a positive glum
below 500 nm.2
The aggregation process of both amphiphiles was further
investigated by temperature dependent UV-Vis, fluorescence
and CD spectroscopy (Fig. 3 and 4). By plotting the CD
intensity against the temperature, OPV1 shows a sharp phase
transition at 50 ЊC (Fig. 5) indicating a highly cooperative alter-
ation. The phase transition is fully reversible, however, with a
hysteresis of about 10 ЊC. Fluorescence measurements reveal
that the emission of OPV1 is still quenched24 at a temperature
above 50 ЊC, while the maximum is red shifted compared to that
in chloroform. This behaviour indicates that the amphiphiles of
band for OPV2 between 500 and 630 nm (glum = Ϫ1.4 × 10Ϫ3
)
and identical to the sign of gabs in the long wavelength tail of the
absorption. The observations in CD and CPL strongly indicate
that the exciton coupling for the amphiphilic OPVs originates
from aggregated OPV-oligomers assembled in chiral supra-
molecular stacks.23 The aggregates possess not only a chiral
ground state but a chiral excited state as well. The magnitude of
glum, however, is considerably lower than the maximum value of
gabs. This difference might be explained in terms of luminescent
1284
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Fig. 6 Change of the Cotton effect of OPV2 in water (2.5 × 10Ϫ5 mol
Ϫ1) as a function of the salt–ethylene oxide (EO) ratio.
L
(1)
Evaluation of the temperature dependency of these equi-
librium constants yielded the thermodynamic parameters
∆H = 165 kJ molϪ1 and ∆S = 570 J molϪ1 KϪ1 for OPV1 and
∆H = 35 kJ molϪ1 and ∆S = 104 J molϪ1 KϪ1 for OPV2. The
positive values of ∆S are in agreement with the proposed loss in
organisation when going from chiral aggregates to disordered
stacks.
Fig. 4 Temperature dependent a) UV-Vis and fluorescence and b) CD
measurements for OPV2 at 1.6 × 10Ϫ5 mol LϪ1 in water.
The influence of salts on the aggregates formed by our
amphiphiles was studied by adding LiClO4 to a solution of
OPV2 in water. Addition of the salt resulted in a non-linear
decrease of the Cotton effect as function of the salt concentra-
tion (Fig 6). UV-Vis spectra show a small blue shift (3 nm) upon
adding lithium perchlorate. The cations presumably interrupt
the packing of the ethylene glycol wedges, hence inducing flexi-
bility between the OPV-oligomers. This interruption results in a
decrease of helical order in the supramolecular aggregates.
Based on our optical studies in water, the OPV units are
arranged in helical aggregates and shielded from the aqueous
solution by the ethylene glycol wedges. In the disordered stacks,
the OPV moieties are not helically arranged and, presumably,
are free to rotate. The symmetric bolaamphiphile OPV2 is
probably organised in monolayers while OPV1 is assembled
in (interdigitated) bilayers due to its asymmetric nature. OPV1
behaves as a classical amphiphile in water and shows the typical
sharp phase transition found for bilayers. Interestingly, the
phase transition temperature of OPV1 is very close to the
melting temperature of this amphiphile in the solid state (vide
infra). Surprisingly, the properties of OPV2 are different, a
more gradual transition from a chiral aggregate to a disordered
stack is observed, which must be due to a modified hydrophilic–
hydrophobic balance.
Fig. 5 Change of the Cotton effect of OPV1 (squares) and OPV2
(diamonds) in water (1.6 × 10Ϫ5 mol LϪ1) as a function of temperature
and fitted curves.
OPV1 are still aggregated above this transition temperature.
The disappearance of the Cotton effect above 50 ЊC shows,
however, that the transition is due to the transformation of
chiral aggregates into non-chiral, less ordered aggregates. In the
UV-Vis spectra, a red shift from λmax = 400 to λmax = 417 nm is
observed upon heating, which could point to conformational
changes of the oligomers. Another plausible explanation is that
the distance between the oligomers increases in the disordered
aggregates resulting in a weaker exciton coupling. Compound
OPV2 displays similar features, but in this case without a
sharp phase transition. The Cotton effect, the fluorescence
and absorption maxima all show a nearly linear decrease with
increasing temperature (Fig. 5). Therefore, we assume that
the transition between the well-ordered chiral aggregates and
disordered aggregates is a less cooperative process.
For the preparation of Langmuir monolayers it turned out
that the ethylene glycol headgroups are not suitable for obtain-
ing stable layers, presumably due to the introduced intrinsic
flexibility by the chosen hydrophilic groups.
Preliminary experiments show that the π-conjugated oligo-
mers can be applied in FET-devices; however, the mobility is
low, e.g. at a gate voltage of Ϫ100 V an on-current of 0.5 µA
with a mobility of around 10Ϫ6 cm2 VϪ1 sϪ1 is found. Optimisa-
tion of the performance of the devices is underway.
Acknowledgements
The transition of OPV1 and OPV2 between the well-ordered
chiral aggregates and disordered stacks can be described by the
Van’t Hoff equation assuming that the number of molecules
remains the same in both phases. The equilibrium constants for
this transition is given by eqn. (1).
J. van Dongen is acknowledged for performing MALDI TOF
measurements, K. T. Hoekerd (TNO Institute of Industrial
Technology, Materials Technology Division) for assistance
with the FET devices and measurements and J. M. Hannink
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286
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and S. P. Williams, J. Am. Chem. Soc., 1993, 115, 11608; (d )
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(University of Nijmegen) for his help with the monolayer
experiments. The research of A. P. H. J. Schenning has been
made possible by a fellowship of the Royal Netherlands
Acadamy of Arts and Sciences.
12 A. F. M. Kilbinger, A. P. H. J. Schenning, F. Goldoni, W. J. Feast and
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1286
J. Chem. Soc., Perkin Trans. 2, 2001, 1280–1286