Induction of liquid crystallinity by host-guest interactions

Article abstract of DOI:10.1021/ja9603113

A molecular clip is described which binds aromatic guests by an induced fit mechanism. It contains twelve long aliphatic chains and can evoke liquid-crystalline properties in a variety of molecules, including polymers and porphyrins, by a process of molecular recognition.

Full text of DOI:10.1021/ja9603113

J. Am. Chem. Soc. 1997, 119, 283-291
283
Induction of Liquid Crystallinity by Host-Guest Interactions
Johanna L. M. van Nunen, Brigitte F. B. Folmer, and Roeland J. M. Nolte*
Contribution from the Department of Organic Chemistry, NSR Center, UniVersity of Nijmegen,
ToernooiVeld, 6525 ED Nijmegen, The Netherlands
ReceiVed January 29, 1996. ReVised Manuscript ReceiVed October 21, 1996X
Abstract: A molecular clip is described which binds aromatic guests by an induced fit mechanism. It contains
twelve long aliphatic chains and can evoke liquid-crystalline properties in a variety of molecules, including polymers
and porphyrins, by a process of molecular recognition.
Introduction
compound 2 exist in three different conformations (Figure 2):
anti-anti (aa), syn-anti (sa), and syn-syn (ss). The conformers
differ in the way the walls are oriented relative to the phenyl
groups of the diphenylglucoluril part. The molecules change
from one conformation to another by the flipping of one
naphthalene wall. Guest molecules can be clamped between
the walls and held by π-π stacking interactions and hydrogen
bonding to the urea carbonyl functions. Binding occurs by an
induced fit mechanism: one of the walls of 2 flips up to form
a cleft which accommodates the guest.4
In this paper we will demonstrate that the idea of making
polymers liquid crystalline by clipping molecules of type 1 to
them is viable. Furthermore, we will show that the concept is
general and can be extended to other host-guest combinations
(see Figure 3).8
Supramolecular chemistry involves the design, synthesis, and
study of molecular systems and assemblies of molecules held
together by relatively weak forces, e.g. hydrogen bonding,
electrostatic interactions, Van der Waals forces, etc. In the last
two decades attention in supramolecular chemistry has gradually
shifted from the host-guest binding of alkali metals in crown
ethers to the complexation of neutral molecules in different types
of synthetic receptors.1 Current efforts are directed toward the
design of more sophisticated host molecules which can bind
guests by an induced fit or allosteric mechanism.2-4 Much of
this research is inspired by the elegant molecular systems found
in nature, which operate by similar mechanisms, leading to
efficient catalysis or in a number of cases to the induction of
special (materials) properties. A similar incentive underlies the
work presented in this paper. In a number of recent studies the
binding of protein molecules to specific domains of nucleic acid
chains has been described.5 Some of these proteins are dimers
having the shape of a super clip or super tweezer, as in the
case of the Gene V protein encoded by the bacteriophage M13,
which binds to single stranded DNA chains and changes the
physical properties of these biomolecules (Figure 1).6 This
particular example from nature inspired us to design a synthetic
clip molecule (1) which can bind to a polymer chain by an
induced fit mechanism. The intention was to change the
properties of the polymer, i.e. to make it liquid crystalline.7
Experimental Section
General Methods. Acetonitrile and CH2Cl2 were distilled from
CaH2, diethyl ether and tetrahydrofuran (THF) from LiAlH4, and toluene
from sodium. Dimethyl sulfoxide (DMSO) was dried over CaH2.
Pyrrole was distilled before use. Methyl 3,5-dihydroxybenzoate (MDB)
was a commercial product used without purification. For column
chromatography Merck silicagel 60 and for flash column chromatog-
raphy Merck silicagel 60H were used. Melting points were measured
with a Jeneval polarizing microscope connected to a Linkam THMS
600 hot stage. 1H NMR spectra were recorded on Varian EM-390 and
Bruker AC-100 spectrometers. Chemical shift values are reported
relative to tetramethylsilane as an internal standard. IR spectra were
recorded on a Perkin-Elmer 298 infrared spectrophotometer. Mass
spectra were obtained with a double focusing VG 7070E spectrometer.
Elemental analyses were determined with a Carlo Erba Ea 1108
instrument. For the determination of the optical rotations a Perkin
Elmer 241 polarimeter was used. A Perkin Elmer λ-5 UV-vis
photospectrometer was used to obtain the UV-vis spectra. Thermo-
grams were recorded at a rate of 10 °C/min using a Perkin Elmer DSC
7 instrument. Samples were prepared in stainless-steel large volume
pans (75 µL). Transition temperatures and enthalpies were determined
from the second heating and first cooling scans. GPC measurements
were performed on a Waters 590 GPC instrument equipped with a PL-
GEL 352 column using tetrahydrofuran as the eluent and different
polystyrenes as standards.
The compound (2, Scheme 1) from which 1 is synthesized
has been described previously.3,4 Its most important structural
features are a concave framework composed of two urea units
and two methylene linked aromatic walls. The molecules of
17b,17c-Dihydro-1,6,10,15-tetrakis(3,4,5-tris(dodecyloxy)benz-
oyloxy)-17b,17c-diphenyl-7H,8H,9H,16H,17H,18H-7a,8a,16a,17a-
(7) Brienne, M. J.; Gabard, J.; Lehn, J.-M.; Stibor, I. J. Chem. Soc., Chem.
Commun. 1989, 1868. Liebman, A.; Mertesdorf, C.; Plesnivy, T.; Ringsdorf,
H., Wendorff, J. H. Angew. Chem. 1991, 103, 1358. Kumar, U.; Kato, T.;
Frechet, J. M. J. J. Am. Chem. Soc. 1992, 114, 6630. Percec, V.; Johansson,
G. J. Mater. Chem. 1993, 3, 83. Alexander, C.; Jariwala, C. P.; Lee C. M.;
Griffin A. C. Macromol. Symp. 1994, 77, 283. Kato, T.; Frechet, J. M. J.
Macromol. Symp. 1995, 98, 311. Paleos, C. M.; Tsiourvas, D. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1696
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
(1) Lehn, J.-M. Angew. Chem. 1988, 100, 91. Cram, D. J.; Cram, J. M.
Container Molecules and Their Guests; Monographs in Supramolecular
Chemistry, 1994.
(2) Schneider, H.-J.; Ruf, D. Angew. Chem. 1990, 102, 1192.
(3) Sijbesma, R. P.; Nolte, R. J. M. J. Am. Chem. Soc. 1991, 113, 6695.
(4) Sijbesma, R. P.; Nolte, R. J. M. J. Am. Chem. Soc. 1992, 114, 9807.
(5) Pabo, C. O.; Sauer, R. T. Annu. ReV. Biochem. 1992, 61, 1053.
(6) Folmer, R. H. A.; Nilges, M.; Folkers, P. J. M.; Konings, R. N. H.;
Hilbers, C. W. J. Mol. Biol. 1994, 240, 341.
(8) For a preliminary communication describing part of the work, see:
van Nunen, J. L. M.; Steven, R. S. A.; Picken, S. J.; Nolte, R. J. M. J. Am.
Chem. Soc. 1994, 116, 8825.
S0002-7863(96)00311-3 CCC: $14.00 © 1997 American Chemical Society

284 J. Am. Chem. Soc., Vol. 119, No. 2, 1997
Van Nunen et al.
Figure 1. (a) Structure of the Gene V protein. (b) Complex of twelve protein molecules and a DNA chain (see ref 6).
After recrystallization from diisopropyl ether the allyl-protected meth-
ylbenzoate 3a was obtained in 75% yield. Mp 33 °C.
1H NMR (CDCl3, ppm) δ 7.1 (s, 2H, ArH), 6.6 (s, 1H, ArH), 6.2-
5.7 (m, 2H, CHdCH2), 5.4-5.1 (m, 4H, CHdCH2), 4.5 (d, 4H, OCH2,
J ) 6 Hz), 3.9 (s, 3H, OCH3). IR (KBr, cm-1) ν 3150-3050 (arom
C-H), 2980-2880 (aliph C-H), 1730 (CdO), 1600 (arom CdC). MS
(EI, m/z): 248 (M)+, 233 (M - CH3)+, 189 (M - COOCH3)+, and 41
(allyl)+. Anal. (C14H16O4) C, H: calcd 67.73, 6.50; found 67.74, 6.44.
Methyl 3,5-Bis(benzyloxy)benzoate (3b). A mixture of methyl 3,5-
dihydroxybenzoate (5.0 g, 30 mmol), benzyl bromide (15.2 g, 89 mmol),
and K2CO3 (11.7 g) in acetone (50 mL) was refluxed overnight. The
inorganic salts were filtered off and washed with CHCl3. The solution
and the CHCl3 extracts were combined and evaporated to dryness. The
residue was crystallized from diisopropyl ether to give the methyl
benzoate 3b in 97% yield. Mp 70 °C.
1H NMR (CDCl3, ppm) δ 7.50-7.22 (m, 12H, ArH), 6.80 (t, 1H,
ArH, J ) 2 Hz), 5.06 (s, 4H, CH2Ph), 3.90 (s, 3H, OCH3). IR (KBr,
cm-1) ν 3120-2980 (arom C-H), 2980-2800 (aliph C-H), 1715
(CdO), 1600 (arom CdC). MS (EI, m/z): 348 (M)+, 317 (M -
OCH3)+, 181, and 91 (CH2Ph)+. Anal. (C22H20O4) C, H: calcd 75.85,
5.79; found 75.65, 5.75.
Figure 2. Structures of the three conformers of 2.
tetraazapentaleno[1′′,6′′:5,6,7;3′′,4′′:5′,6′,7′]dicycloocta[1,2,3-de:
1′,2′,3′-d′e′]dinaphthalene-8,17-dione (1). Receptor molecule 24 (n
mmol) and NaH (10n mmol) were refluxed in THF (10n mL) and after
1 h 3,4,5-tris(dodecyloxy)benzoyl chloride9 (4.4 equiv) in a mixture
of THF/CH2Cl2 (1:1 v/v, 5n mL) was added. The mixture was stirred
for 2-24 h and subsequently quenched with a few drops of water.
The solvent was evaporated under reduced pressure and the residue
was dissolved in CHCl3. The organic layer was extracted (2×) with
aqueous 1 N HCl, then with H2O, and dried (MgSO4). The crude
product was subjected to flash column chromatography (eluent ethyl
acetate-hexane 1:25 v/v). Yield 57%. K to I transition at 159-162
°C.
3,5-Bis(allyloxy)benzoic Acid (4a). A mixture of methyl benzoate
3a (1.0 g, 4.0 mmol) and powdered KOH (0.53 g) in ethanol (15 mL)
was refluxed for 2 h. The solvent was evaporated in Vacuo and ethyl
acetate was added. The organic layer was acidified with an aqueous
solution of 1 N HCl to pH 1. The mixture was washed with water
(2×) and dried (MgSO4), and the solvent was evaporated. Crystal-
lization from ethanol yielded 60% of the benzoic acid 4a. Mp 71 °C.
1H NMR (CDCl3, ppm) δ 7.3 (s, 2H, ArH), 6.8 (s, 1H, ArH), 6.4-
5.8 (m, 2H, CHdCH2), 5.6-5.2 (m, 4H, CHdCH2), 4.5 (d, 4H, OCH2,
J ) 6 Hz). IR (KBr, cm-1) ν 2900 (COOH), 1690 (CdO), 1600 (arom
CdC). MS (EI, m/z): 234 (M)+, 189 (M - COOH)+, and 41 (allyl)+.
Anal. (C13H14O4) C, H: calcd 66.66, 6.02; found 66.73, 6.04.
1H NMR (CDCl3, ppm) [see Table 1 for uncomplexed host, host-
guest complex]: δ host 7.67 (s, 8H, ArH(OR)3), 7.44 and 6.99 (2d,
8H, naph-H, J ) 8.9 Hz), 7.11-7.05 (m, 6H, ArH), 6.94 (d, 4H, ArH),
5.41 and 4.14 (2d, 8H, NCHHAr, J ) 16.8 Hz), 4.12-3.84 (m, 24H,
OCH2), 1.84-1.08 (m, 240H, OCH2(CH2)10CH3), 1.04-0.80 (m, 36H,
CH3), guest (resorcinol) 6.90 (t), 6.21 (dd), 5.75 (br s), 5.49 (s). Anal.
(C212H334N4O22) C, H, N: calcd 77.37, 10.23, 1.70; found 77.13, 10.56,
1.67.
Phenyl 3,5-Dihydroxybenzoate. This compound was synthesized
by an esterification reaction from 3,5-bis(benzyloxy)benzoic acid and
phenol. Deprotection of the product was carried out according to a
literature procedure.10
Methyl 3,5-Bis(allyloxy)benzoate (3a). A mixture of methyl 3,5-
dihydroxybenzoate (1.0 g, 6.0 mmol), allyl bromide (1.0 mL, 12 mmol),
and K2CO3 (1.8 g) in acetone (10 mL) was refluxed for 4 h. The solvent
was removed under reduced pressure, water was added, and the product
was extracted with CH2Cl2. The organic layer was washed with water
(2×), dried (MgSO4), and evaporated to dryness under reduced pressure.
3,5-Bis(benzyloxy)benzoic Acid (4b). For the synthesis of this
compound the same procedure was followed as described for 4a, using
3b (1.6 g, 4.6 mmol) and powdered KOH (0.8 g) in ethanol (60 mL).
Yield 94%. Mp 213-218 °C.
1H NMR (CDCl3 + CD3OD, ppm) δ 7.5-7.1 (m, 12H, ArH), 6.7
(s, 1H, ArH), 5.0 (s, 4H, CH2Ph). IR (KBr, cm-1) ν 2880 (COOH),
1690 (CdO), 1600 (arom CdC). MS (EI, m/z): 334 (M)+, 181, and
91 (CH2Ph)+. Anal. (C22H20O2) C, H: calcd 75.43, 5.43; found 75.24,
5.44.
(9) Percec, V.; Lee, M.; Heck, J.; Blackwell, H. E.; Ungar, G.; Alvarez-
Castillo, A. J. Mater. Chem. 1992, 2, 931.
(10) Hawker, C. J.; Lee, R.; Frechet, J. M. J. J. Am. Chem. Soc. 1991,
113, 4583.
1,4-Bis[3,5-bis(allyloxy)benzoyloxy]benzene (5). For the synthesis
of this compound a procedure described by Hashimoto et al. was

Induction of Liquid Crystallinity by Host-Guest Interactions
J. Am. Chem. Soc., Vol. 119, No. 2, 1997 285
Figure 3. Schematic representation of the liquid crystalline complexes that can be obtained from clip molecules 1 and different types of guest
molecules.
followed.11 To a solution of 3,5-bis(benzyloxy)benzoic acid 4a (1.18
g, 5.0 mmol) and hydroquinone (0.25 g, 2.3 mmol) in acetonitrile (3
mL) were subsequently added under an argon atmosphere CCl4 (0.52
mL, 5.4 mmol), Et3N (0.69 mL, 5.0 mmol), and PPh3 (1.39 g, 5.0
mmol). The solution was stirred for 2 days and in addition refluxed
for 4 h. The reaction mixture was evaporated to dryness under reduced
pressure and the product was dissolved in CHCl3. The organic layer
was extracted with aqueous 1 N HCl (2×), aqueous 1 N NaOH (2×),
and water (1×) and dried (MgSO4). After evaporation of the solvent,
the residue was subjected to column chromatography (eluent: ethyl
acetate-hexane 1:9 and subsequently 1:3 v/v). Crystallization from
diisopropyl ether yielded 61% of the protected hydroquinone dimer 5.
Mp 115 °C.
1H NMR (CDCl3, ppm) δ 7.36 (d, 4H, (AllylO)2ArH, J ) 2 Hz),
7.27 (s, 4H, ArH), 6.78 (t, 2H, (AllylO)2ArH, J ) 2 Hz), 6.22-5.89
(m, 4H, CHdCH2), 5.52-5.27 (m, 8H, CHdCH2), 4.60 (d, 8H, OCH2,
J ) 6 Hz). IR (KBr, cm-1) ν 3150-3050 (arom C-H), 2950-2850
(aliph C-H), 1740 (CdO), 1600 (arom CdC). MS (EI, m/z): 542
(M)+, 217 (COAr(OAllyl)2)+, and 41 (allyl)+. Anal. (C32H30O8) C,
H: calcd 70.84, 5.57; found 70.58, 5.53.
OCH2(CH2)6), 0.88 (t, 6H, CH2CH3). IR (KBr, cm-1) ν 3300 (OH),
3020-2800 (aliph C-H), 1750 and 1720 (CdO). MS (CI, m/z): 375
(M + 1)+, 263 (M + 1 - C8H16)+, 151 (M + 1 - 2*C8H16)+. Anal.
20
(C20H38O6) C, H: calcd 64.14, 10.23; found 64.33, 10.10. [R]D
+8.3° (c 0.5, EtOH).
)
Dioctyl (2R,3R)-O-Bis[3,5-bis(benzyloxy)benzoyl]tartrate (7). This
compound was synthesized according to a literature procedure.13
A
solution of 4b (110 mg, 0.3 mmol) and N,N-carbonyldiimidazole (67
mg, 0.5 mmol) in dry toluene was stirred overnight at room temperature.
Subsequently dioctyl tartrate (60 mg, 0.16 mmol) was added to this
solution. After 6 days the solvent was evaporated and the residue was
dissolved in CH2Cl2. The organic layer was washed with water (2×),
dried (MgSO4), and concentrated in Vacuo. The residue mixture was
subjected to flash column chromatography (eluent: ethyl acetate-
hexane 1:7 v/v) and the product was obtained as a colorless oil in 81%
yield.
1H NMR (CDCl3, ppm) δ 7.3 (s, 20H, ArH), 7.1 (s, 4H, Ar(CdO)-
H), 6.7 (s, 2H, Ar(OBz)2H), 5.9 (s, 2H, CH(OCOAr)), 5.0 (s, 8H, CH2-
Ph), 4.1 (m, 4H, OCH2CH2), 1.8-1.0 (m, 24H, C6H12), 0.8 (t, 6H,
OCH2CH3). FAB-MS (m-nitrobenzyl alcohol, m/z): 1006 (M + H)+,
20
915 (M - CH2Ph)+. [R]D -26.6° (c 0.6, CHCl3).
1,4-Bis[3,5-dihydroxybenzoyloxy]benzene (6). To a suspension
of compound 5 (0.53 g, 0.97 mmol) in acetonitrile/water (4:1, v/v, 50
mL) were added triethylammonium formate (0.45 g, 3.0 mmol), PPh3
(52 mg, 0.19 mmol), and Pd(OAc)2 (8.0 mg, 3.6 × 10-2 mmol),12 and
the mixture was refluxed for 2.5 h. Workup was accomplished by
evaporation of the solvent under reduced pressure. The residue was
dissolved in ethyl acetate and washed with water. The organic layer
was dried (MgSO4) and evaporated in Vacuo. After purification by
flash column chromatography (eluent ethyl acetate-hexane 2:1 v/v)
and precipitation of an acetone solution of 6 in water, the product was
obtained in 90% yield. Mp >290 °C dec.
Dioctyl (2R,3R)-O-Bis(3,5-dihydroxybenzoyl)tartrate (8). Com-
pound 7 (33 mg, 3.3 × 10-5 mol) was suspended in ethanol (20 mL)
and subjected overnight to hydrogenation using palladium on carbon
as a catalyst. The catalyst was filtered off and the solvent was
evaporated under reduced pressure. The residue was extracted with
ethyl acetate, and the organic layer was washed with water, dried
(MgSO4), and concentrated to afford 8 as a light yellow oil in 86%
yield.
1H NMR (acetone-d6, ppm) δ 6.94 (s, 4H, ArH), 6.51 (s, 2H, ArH),
5.81 (s, 2H, CHCOOC8H17), 3.6 (br s, 4H, OH), 4.23-3.75 (m, 4H,
OCH2CH2), 1.6-0.8 (m, 24H, C6H12), 0.71 (t, 6H, CH2CH3). FAB-
MS (m-nitrobenzyl alcohol, m/z): 647 (M + H)+, 493 (M - OCOAr-
1H NMR (acetone-d6, ppm) δ 7.23 (s, 4H, ArH), 7.04 (d, 4H, (HO)2-
ArH, J ) 2 Hz), 6.55 (t, 2H, (HO)2ArH, J ) 2 Hz). IR (KBr, cm-1
)
20
(OH)2)+. [R]D -32.7° (c 0.7, CHCl3).
ν 3440 (OH), 1720 (CdO), 1620 (arom CdC). FAB-MS (m-
nitrobenzyl alcohol, m/z): 383 (M + 1)+, and 137 (COAr(OH)2)+. Anal.
(C20H14O8) C, H: calcd 62.83, 3.69; found 62.58, 3.89.
5,10,15,20-Tetrakis(3,5-dimethoxyphenyl)porphyrin, H2(T3,5di-
MeOPP) (9a). This compound was synthesized according to a
literature procedure14 from 3,5-dimethoxybenzaldehyde (2.48 g, 14.9
mmol) and pyrrole (1.0 mL, 14.4 mmol) in propionic acid (150 mL)
as the solvent. The product was purified by column chromatography
(eluent: ethyl acetate-hexane 1:1 v/v) and obtained in 53% yield. Mp
>300 °C dec.
L-(+)-Dioctyl Tartrate. A solution of L-(+)-tartaric acid (7.69 g,
50.6 mmol) and concentrated aqueous HCl (1.5 mL) in octanol (100
mL) was heated at 80 °C overnight. The excess of octanol was distilled
off under high vacuum and the crude product was purified by column
chromatography (eluent: ethyl acetate-hexane 1:3 v/v), which yielded
83% of L-(+)-dioctyl tartrate as white needles. Mp 41 °C.
1H NMR (CDCl3, ppm) δ 9.0 (s, 8H, pyrrole-H), 7.4 (d, 8H, ArH,
J ) 2 Hz), 6.9 (t, 4H, ArH, J ) 2 Hz), 4.0 (s, 24H, OCH3), -2.8 (br
1H NMR (CDCl3, ppm) δ 4.54 (d, 2H, CHOH, J ) 7 Hz), 4.26 (t,
4H, OCH2CH2), 3.21 (d, 2H, CHOH, J ) 7 Hz), 1.90-1.10 (m, 24H,
(13) Bos, W. F.; Kelly, C. J.; Landsberger, F. R. Anal. Biochem. 1975,
64, 289.
(14) Tsuchida, E.; Komatsu, T.; Hasegawa, E.; Nishide, H. J. Chem.
Soc., Dalton Trans. 1990, 2713.
(11) Hashimoto, S.; Furukawa, J. Bull. Chem. Soc. Jpn. 1981, 54, 2227.
(12) Yamada, T.; Goto, K.; Mitsuda, Y.; Tsuji, J. Tetrahedron Lett. 1987,
28, 4557.

286 J. Am. Chem. Soc., Vol. 119, No. 2, 1997
Van Nunen et al.
s, 2H, NH). IR (KBr, cm-1) ν 2950 (aliph C-H), 1600 (arom CdC),
1220 (C-O). FAB-MS (m-nitrobenzyl alcohol, m/z): 855 (M + H)+,
824 (M + 1 - OCH3)+. UV-vis (CH2Cl2, λ/nm, log(ꢀ/L‚mol-1‚cm-1):
419 (3.1), 514 (2.6), 549 (2.0), 588 (2.0), 645 (1.8).
5,10,15,20-Tetrakis(3,5-dihydroxyphenyl)porphyrin, H2(T3,5di-
HOPP) (9b). This compound was synthesized according to a literature
procedure14 from 9a (0.85 g, 1.0 mmol) and BBr3 (0.8 mL, 8.5 mmol)
in CH2Cl2 (80 mL) as the solvent. Porphyrin 9b was obtained in 94%
yield. Mp >300 °C dec.
crystallization from methanol yielded 78% of the protected benzalde-
hyde. Mp 72 °C (lit.16 mp 72 °C).
1H NMR (CDCl3, ppm) δ 9.9 (s, 1H, CHO), 7.6 and 7.0 (2*d, 4H,
Ar(CHO)H, J ) 9 Hz), 7.3 (s, 5H, ArH), 5.1 (s, 2H, OCH2). IR (KBr,
cm-1) ν 3120-3000 (arom C-H), 2960-2730 (aliph C-H), 1690
(CHdO), 1595 (arom CdC). MS (EI, m/z): 212 (M)+, 91 (CH2Ph)+.
Anal. (C14H12O2) C, H: calcd 79.23, 5.70; found 79.17, 5.72.
1-Benzyloxy-4-vinylbenzene (14). This compound was synthesized
as described for compound 12, using methyl triphenylphosphonium
bromide (2.84 g, 7.8 mmol), a solution of butyllithium in hexane (4.9
mL of a 1.6 M solution, 7.8 mmol), and 4-benzyloxybenzaldehyde (1.43
g, 7.1 mmol) in THF (40 mL) as the solvent. Purification by column
chromatography (eluent: ethyl acetate-hexane 1:19 v/v) yielded 88%
of compound 14. Mp 67 °C (lit.17 mp 68 °C).
1H NMR (CDCl3, ppm) δ 7.4-7.1 (m, 7H, ArH), 7.0-6.8 (m, 2H,
ArH), 6.8-6.5 (m, 1H, CHdCH2), 5.7-5.0 (m, 2H, CHdCH2), 5.0
(s, 2H, OCH2). IR (KBr, cm-1) ν 3100-3000 (arom C-H), 2950-
2780 (aliph C-H), 1625 (conj CdC), 1600 (arom CdC). MS (EI,
m/z): 210 (M)+, 91 (CH2Ph)+. Anal. (C15H14O) C, H: calcd 85.68,
6.71; found 85.34, 6.69.
Copolymer 15. 1,3-Bis(benzyloxy)-5-vinylbenzene (0.63 g, 2.0
mmol), styrene (0.18 g, 1.73 mmol), and AIBN (2.4 mg, 1.46 × 10-5
mol) were suspended in butanone (3 mL). The reaction mixture was
degassed and heated overnight in an argon atmosphere at 80 °C. The
solvent was evaporated and the residue was dissolved in CHCl3 and
precipitated in methanol. This afforded copolymer 15 as a white
powder in 85% yield.
1H NMR (CDCl3, ppm) δ 7.2-5.8 (br s, ArH), 4.6 (br s, OCH2Ph),
2.1-0.8 (br s, CHPh-CH2). IR (KBr, cm-1) ν 3020 (arom C-H),
2910 (aliph C-H), 1590 (arom CdC), 1150 (C-O ether). GPC
(THF): Mw 35500, Mw/Mn ) 2.26. Calculation of the composition of
the polymer was performed by elemental analysis, i.e. from the oxygen
content, which was assumed to be O ) 100 - C - H - N. Elemental
analysis C, H, N: found 84.83, 6.73, 0.64. From these data the ratio
of 3,5-bis(benzyloxy)-5-vinylbenzene and styrene units was calculated
to be 1.2.
1H NMR (acetone-d6, ppm) δ 8.9 (s, 8H, pyrrole-H), 7.11 (d, 8H,
ArH, J ) 2 Hz), 6.71 (t, 4H, ArH, J ) 2 Hz), -3.0 (br s, 2H, NH). IR
(KBr, cm-1) ν 3300 (OH), 1600 (arom CdC). FAB-MS (m-nitrobenzyl
alcohol, m/z): 743 (M + H)+. UV-vis (MeOH, λ/nm, log-
(ꢀ/l‚mol-1‚cm-1): 417 (6.1), 512 (5.4), 547 (4.9), 587 (4.9), 644 (4.6).
[3,5-Bis(benzyloxy)phenyl]methanol (10). Under an argon atmo-
sphere a solution of 3b (2.8 g, 8.0 mmol) in diethyl ether (20 mL) was
carefully added to a suspension of LiAlH4 (780 mg) in diethyl ether
(100 mL). The reaction mixture was refluxed for 24 h and afterwards
cooled down to -78 °C. After the addition of water (20 mL) the
mixture was allowed to warm up and was acidified with concentrated
aqueous HCl. The organic layer was washed with aqueous 1 N HCl
(2×) an aqueous saturated NaHCO3 solution (1×), dried (MgSO4), and
concentrated until the crystallization started. In this way white needles
were obtained in 81% yield. Mp 77 °C.
1H NMR (CDCl3, ppm) δ 7.4 (s, 10H, ArH), 6.8-6.5 (m, 3H, ArH),
5.0 (s, 4H, CH2Ph), 4.6 (s, 2H, CH2OH), 1.9 (br s, 1H, OH). IR (KBr,
cm-1) ν 3300 (OH), 3100-2980 (arom C-H), 2960-2800 (aliph
C-H), 1590 (arom CdC). MS (EI, m/z): 320 (M)+, 181, and 91
(CH2Ph)+. Anal. (C21H20O3) C, H: calcd 78.73, 6.29; found 78.47,
6.27.
3,5-Bis(benzyloxy)benzaldehyde (11). This compound was pre-
pared according to a literature procedure.15 To a cooled solution (-78
°C) of oxalyl chloride (715 mg, 5.7 mmol) in dry CH2Cl2 (14 mL) was
added, under an argon atmosphere and over a period of 5 min, a solution
of dry DMSO (940 mg, 15.7 mmol) in CH2Cl2 (3 mL). After 10 min
a solution of 10 (1.7 g, 5.2 mmol) in CH2Cl2 (5 mL) was added
dropwise over a period of 5 min, and after 15 min Et3N (3.7 mL) was
added, also over a period of 5 min. The reaction mixture was allowed
to warm up to room temperature and water (16 mL) was added. The
organic layer was washed with water (2×), dried (MgSO4), and
evaporated to dryness in Vacuo. Crystallization from hexane yielded
74% of the benzaldehyde 11. Mp 79 °C.
1H NMR (CDCl3, ppm) δ 9.8 (s, 1H, CHO), 7.5-7.0 (m, 13H, ArH),
5.1 (s, 4H, CH2Ph). IR (KBr, cm-1) ν 3150-3050 (arom C-H), 1690
(CHdO), 1600 (arom CdC). MS (EI, m/z): 318 (M)+, 181, and 91
(CH2Ph)+. Anal. (C21H18O3) C, H: calcd 79.23, 5.70; found 79.10,
5.65.
Copolymer 16. Copolymer 15 (0.53 g) was stirred overnight at
room temperature in a solution of hydrobromic acid in acetic acid (3
mL). The reaction mixture was concentrated and extracted with ethyl
acetate, aqueous 1 N NaOH (2×), saturated aqueous NaHCO3 (1×),
and water (2×). The organic layer was dried (MgSO4) and concentrated
in Vacuo. The residue was dissolved in methanol and precipitated in
water. After filtration copolymer 16 was obtained as a beige solid in
93% yield.
1H NMR (acetone-d6, ppm) δ 7.4-5.9 (br s, ArH), 2.1-1.0 (br s,
CHPh-CH2). IR (KBr, cm-1) ν 3400 (OH), 3030 (arom C-H), 2920
(aliph C-H), 1600 (arom CdC).
1,3-Bis(benzyloxy)-5-vinylbenzene (12). Under an argon atmo-
sphere at -10 °C a solution of buthyllithium in hexane (0.5 mL of a
1.6 M solution, 0.8 mmol) was added to a solution of methyltri-
phenylphosphonium bromide (289 mg, 0.78 mmol) in dry THF (5 mL).
After the mixture was stirred for 15 min a solution of 3,5-bis-
(benzyloxy)benzaldehyde (196 mg, 0.63 mmol) in THF (2 mL) was
added. After another 2 h at room temperature water (3 mL) was added.
The solvent was evaporated in Vacuo and the residue was dissolved in
CH2Cl2. The organic layer was washed with water (2×), dried
(MgSO4), and evaporated under reduced pressure. The product was
purified by column chromatography (eluent: ethyl acetate-hexane 1:5
v/v) and obtained in 87% yield. Mp 46 °C.
1H NMR (CDCl3, ppm) δ 7.5-7.2 (m, 10H, ArH), 6.9-6.6 (m, 3H,
ArH), 6.6-6.5 (m, 1H, CHdCH2), 5.8-5.1 (m, 2H, CHdCH2), 5.0
(s, 4H, CH2Ph). IR (KBr, cm-1) ν 3020 (arom C-H), 2940 (aliph
C-H), 1600 (conj CdC), 1590 (arom CdC). MS (EI, m/z): 316 (M)+,
225 (M - CH2Ph)+, 181, and 91 (CH2Ph)+. Anal. (C22H20O2) C, H:
calcd 83.52, 6.37; found 83.42, 6.37.
Copolymer 17. For the synthesis of this copolymer the same
procedure was followed as described for copolymer 15 using 1-ben-
zyloxy-4-vinylbenzene (0.55 g, 2.6 mmol), styrene (0.20 g, 1.9 mmol),
and AIBN (4.4 mg, 2.68 × 10-5 mol) in butanone (3 mL) as the solvent.
Precipitation yielded 90% of copolymer 17 as a white powder.
1H NMR (CDCl3, ppm) δ 7.4-6.3 (br s, ArH), 4.9 (br s, OCH2Ph),
2.0-1.0 (br s, CHPh-CH2). IR (KBr, cm-1) ν 3040 (arom C-H),
2920 (aliph C-H), 1610 (arom CdC), 1250 (C-O ether). GPC
(THF): Mw ) 22 000, Mw/Mn ) 2.46. Elemental analysis C, H, N:
found 86.67, 6.94, <0.1. From these data the ratio of 1-benzyloxy-
4-vinylbenzene and styrene units was calculated to be 2.6.
Copolymer 18. Copolymer 17 (0.30 g) was deprotected with a
solution of hydrobromic acid in acetic acid (3 mL) as described for
copolymer 15. Precipitation afforded copolymer 18 as a beige powder
in 100% yield.
1H NMR (acetone-d6, ppm) δ 7.5-6.3 (br s, ArH), 2.0-1.0 (br s,
CHPh-CH2). IR (KBr, cm-1) ν 3440 (OH), 3020 (arom C-H), 2920
(aliph C-H), 1600 (arom CdC).
4-Benzyloxybenzaldehyde (13). This compound was synthesized
as described for 3b, using 4-hydroxybenzaldehyde (1.11 g, 9.1 mmol),
benzyl bromide (1.98 g, 11.6 mmol), and K2CO3 (1.52 g, 11.0 mmol)
in acetone (100 mL) as the solvent. Purification by column chroma-
tography (eluent: ethyl acetate-hexane 1:10 v/v) and subsequent
(16) Bergmann, E. D.; Sulzbacher, M. J. Org. Chem. 1951, 16, 84.
(17) Oda, U. J. Soc. Chem. Ind. Jpn. 1945, 48, 57. (C. A. 1952, 8044).
(18) Bifunctional, tetrafunctional, etc. refers to the number of 1,3-
dihydroxybenzene functions in the guest molecule that can be captured by
clip molecule 1.
(15) Mancuso, J.; Swern, D. Synthesis 1981, 165.
(19) Nakahama, S.; Hirao, A. Polym. Sci. 1990, 15, 299.

Induction of Liquid Crystallinity by Host-Guest Interactions
J. Am. Chem. Soc., Vol. 119, No. 2, 1997 287
1
Table 1. Assignments of H NMR Resonances to Conformations
Scheme 1
of 1a
conformer NCHHAr
NCHHAr napht-H(3,6) napht-H(4,5)
5.06 (s); 7.38 (s); 7.91 (s);
4.31 (a) 7.13 (a) 7.38 (a)
sa
5.89 (s);
5.50 (a)
ss
aa
5.89
5.42
5.23
4.16
7.44
7.23
b
7.73
conformer
Ph-H(2,6)
6.50 (s);
Ph-H(3,5)
6.29 (s);
Ph-H(4)
6.37 (s);
(RO)3Ar-H
7.69;
sa
6.83 (a)
7.01 (a)
6.23
7.03
7.01 (a)
7.53
7.56
7.66
Copolymer 19. For the synthesis of this polymer the same
esterification method was used as described for compound 5. Copoly-
mer 18 (120 mg), 4b (200 mg, 0.60 mmol), CCl4 (0.16 mL, 1.7 mmol),
triethylamine (0.05 mL, 0.36 mmol), and PPh3 (190 mg, 0.68 mmol)
were mixed in acetonitrile (1.5 mL). This mixture was stirred for 3
days at room temperature. The workup procedure was similar to that
described for compound 5, except that at the end the residue was
dissolved in CHCl3 and precipitated in methanol. In this way 68% of
the esterified copolymer was obtained.
ss
aa
∼7.0
∼7.0
6.91
6.92
a In CDCl3 with [1] ) 5 mM. Chemical shifts are in ppm relative
to tetramethylsilane. The designation (s) and (a) are used for syn and
anti, see text. b Due to the low abundance of the conformer and the
complexity of the signals, no assignments could be made.
Scheme 2
1H NMR (CDCl3, ppm) δ 7.4-6.2 (br s, ArH), 5.1 (br s, OCH2Ph),
2.2-1.0 (br s, CHPh-CH2). IR (KBr, cm-1) ν 3490 (OH), 3020 (arom
C-H), 2920 (aliph C-H), 1760 and 1730 (CdO), 1590 (arom CdC),
1150 (C-O ether). GPC (THF): Mw ) 24000, Mw/Mn ) 2.38.
Elemental analysis C, H, N: found 79.60, 6.2, 0.72.
Copolymer 20. Deprotection of copolymer 19 (0.07 g) was
performed using a solution of hydrobromic acid in acetic acid (2 mL).
The residue was dissolved in acetone and precipitated in water. The
copolymer was dried over P2O5 under high vacuum. In this way
copolymer 20 was obtained as a beige powder in 96% yield.
1H NMR (acetone-d6, ppm) δ 7.8-6.5 (br s, ArH), 2.5-1.3 (br s,
CHPh-CH2). IR (KBr, cm-1) υ 3440 (OH), 3020 (arom C-H), 2920
(aliph C-H), 1730 (CdO), 1600 (arom CdC).
Complex Formation. The 1:1 complexes were prepared by mixing
ca. 20 mg of clip molecule (5 × 10-6 to 10 × 10-6 mol) and an
equimolar amount (between 0.7 and 1.5 mg) of guest (5 × 10-6 to 10
× 10-6 mol) in 0.2 mL of CHCl3. If necessary 1-3 drops of acetone
or MeOH was added.20 The solvent was slowly evaporated overnight
at 40 °C, and the complexes were dried at room temperature using
high vacuum. For the DSC-measurements ca. 10 mg of complex was
weighed out in a stainless-steel large volume pan (75 µL). Thermo-
grams were recorded at 10 °C/min. and repeated heating and cooling
runs were recorded to study the stability of the complex and all the
reproducibility of the measurements. Polarizing microscopy was carried
out using the same heating and cooling rates.
Discussion
Synthesis: Receptor Molecule. Clip molecule 1 was
synthesized from 2 by an esterification reaction in 57% yield
(Scheme 1). Interpretation of the 1H-NMR spectrum of clip 1
was complicated by the fact that this molecule has three
conformations (aa, sa, and ss), which interconvert slowly on
the NMR time scale. Earlier work performed in our laboratory
has shown that the binding of a suitable guest molecule in
receptor molecules of type 2 increases the relative amount of
the aa conformer, which is the dominant binding conformer.3,4
We found that upon addition of an excess of resorcinol to a
solution of receptor molecule 1, the equilibrium of the conform-
ers was almost completely shifted to the aa conformer. The
resonances could be assigned by taking the following points
into consideration: (i) the presence of a guest molecule in the
cleft will significantly shift the signals, (ii) placing a naphthyl
group into the syn orientation will cause a considerable upfield
shift of the naphthyl and phenyl signals, due to the ring current
effects of these moieties, and (iii) the methylene protons of the
sa conformer must give rise to two AX systems with equal
intensity.3,4 The assignments of the most important signals of
compound 1 are given in Table 1. These NMR data showed
that at 25 °C, 61% of the molecules are in the as, 33% in the
aa, and 6% in the ss conformations. From a 1H NMR titration
the association constant for the complex with resorcinol was
calculated to be Ka ) 400 M-1 and for the complex with methyl
3,5-dihydroxybenzoate (MDB) Ka > 2500 M-1 8
.
Bi- and Tetrafunctional Guest Molecules.18 Hydroquinone
was used as a spacer in the bifunctional molecule 6 (Scheme
2). This compound was synthesized from 4a and hydroquinone
by an esterification reaction using carbon tetrachloride, triphen-
ylposphine, and triethylamine11 in 61% yield. Removal of the
allyl groups yielded 90% of pure guest 6.
Bifunctional guest 8 was synthesized from tartaric acid
(Scheme 2). Reaction of 4b, N,N-carbonyldiimidazole, and
L-(+)-dioctyl tartrate in toluene afforded the protected guest 7
(20) A 1:1 complex of 1 and resorcinol, prepared using CHCl3-methanol
as the solvent mixture, showed a smectic phase with a clearing temperature
of 110 °C. After repeated heating and cooling, phase separation took place,
indicating that the complex was unstable when prepared from this solvent
mixture. However, if this complex was prepared by evaporation of a
CHCl3-acetone solution a stable liquid-crystalline phase was observed.
During the course of our study we found out that complex formation in
general proceeds much better with the latter method than with the method
we published previously (see ref 8).

288 J. Am. Chem. Soc., Vol. 119, No. 2, 1997
Van Nunen et al.
Scheme 3
Scheme 5
Scheme 4
to afford benzaldehyde 11 (74% yield). This benzaldehyde
could be transformed into styrene 12 by a Wittig reaction with
methyltriphenylphosphonium bromide. Compound 14 was
synthesized in two steps, Viz. benzyl protection of p-hydroxy-
benzaldehyde and conversion of the latter into styrene 14 by
the Wittig reaction described above with an overall yield of
69% (Scheme 3).
Copolymerization reactions were carried out using styrene
(S) and the benzyloxystyrene derivatives 12 and 14 as monomers
and azobis(isobutyronitrile) (AIBN) as an initiator in butanone
at 80 °C (Scheme 4). The results are presented in Table 2.
The reaction of the disubstituted styrene 12 and styrene in a
1.2 to 1 ratio produced copolymer 15. The composition of 15,
which corresponded to the monomer feed, was determined by
elemental analysis. Copolymerization of monosubstituted sty-
rene 14 with styrene (ratio 14/S ) 1.4) resulted in the formation
of 17. As can be seen in Table 2, copolymer 17 contains 2.6
repeat units of 14 for every styrene unit. Apparently, styrene
derivative 14 is more reactive in the copolymerization reaction
than styrene itself. The molecular weight (Mw) and molecular
weight distribution (Mw/Mn) were determined by gel permeation
chromatography (GPC). The results are also given in Table 2.
Deprotection of the benzyl groups of the polymers appeared to
be very difficult and only starting material was recovered after
catalytic hydrogenolysis or reaction with trimethylsilyl iodide.
We succeeded in achieving quantitative deprotection by treating
copolymers 15 and 17 with HBr in acetic acid. Copolymers
16 and 18 were isolated by precipitation.
in 81% yield.13 Deprotection of the benzyl groups was achieved
by catalytic hydrogenolysis in ethanol to give 8 in 86% yield.
The synthesis of 2,6-disubstituted tetraphenylporphyrins has
been described in the literature.14 The same synthetic route was
used for the preparation of the 3,5-disubstituted derivative 9b.
Reaction of 3,5-dimethoxybenzaldehyde and pyrrole in propi-
onic acid gave the protected porphyrin 9a in 53% yield.
Deprotection of the methoxy groups with boron tribromide in
CH2Cl2 yielded the tetrafunctional guest 9b in 94%.
Polyfunctional Guest Molecules. The polyfunctional guest
molecules were synthesized by radical polymerization reactions
of styrene derivatives. Since phenol and related hydroxy
substituted aromatic compounds are known to be inhibitors or
retarders of radical polymerizations, protected styrene derivatives
were used.19 Starting from 3b, three steps were necessary to
synthesize the disubstituted styrene derivative 12 (Scheme 3).
Unfortunately, a selective reduction of 3b with diisobutylalu-
minum hydride in CH2Cl2 did not result in benzaldehyde 11
but in a mixture of benzyl alcohol 10 and starting material.
Compound 3b was, therefore, first reduced to benzyl alcohol
10 with lithium aluminum hydride (81% yield) and the latter
compound was subsequently oxidized by a Swern oxidation16
Copolymer 18 was esterified with 3,5-bis(benzyloxy)benzoic
acid derivative 4b using the PPh3/CCl4 method described above
for compound 5 (Scheme 5). The yield of the esterification
reaction was calculated from the elemental analysis of the
resulting polymer 19. It was estimated that 40% of the hydroxyl
functions had reacted. Quantitative removal of the benzyl
groups of 19 with hydrobromic acid in acetic acid was possible
without noticeable hydrolysis of the ester functions in copolymer
20.
Table 2. Results of Copolymerization Reactions of Styrene and Styrene Derivatives. Composition and Physical Properties of the
Copolymersa
comonomer
(BS)
comonomer ratio
(BS:S)
polymer
polymer
yield (%)
composition of
c
c
polymerb (X:1 - X)
Mw
Mw/Mn
12
14
0.55:0.45
0.58:0.42
15
17
19
85
90
0.55:0.45
0.72:0.28
35500
22000
24000
2.26
2.46
2.38
a S ) styrene, BS ) benzyl protected styrene derivative. b Determined by elemental analysis, X ) BS. c Determined by GPC.

Induction of Liquid Crystallinity by Host-Guest Interactions
J. Am. Chem. Soc., Vol. 119, No. 2, 1997 289
Figure 4. Liquid-crystalline behavior of clip 1 in the presence of
different amounts of methyl 3,5-dihydroxybenzoate.
Table 3. Phase Transition Temperatures and Enthalpy Changes of
Liquid-Crystalline Complexes of 1 and Different Guest Moleculesa
guest
host-guest ratio transition temp (°C) ∆H (kJ/mol)
MDB
10:1
10:2
10:3
K f S
S f S′
S′ f I
K f S
S f S′
S′ f I
K f S
S f S′
S′ f I
K f N
N f I
K f S
S f S′
S′ f I
K f S
S f S′
S′ f I
K f S′
S′ f I
K f D
D f D′
D′ f I
K f D′
D′ f I
0 [-8]
95 [86]
144 [136]
-1 [-8]
84 [78]
141 [133]
-3 [-13]
82 [74]
131 [127]
-4 [-12]
110 [94]
49
58.8
1.0
5.9
50.6
1.5
5.3
45.6
2.1
3.8
29.5
1.1
123.7
12.4
24.8
135.5
3.9
MDB
MDB
MDB
6
1:1
1:1
90
157
18 [4]
8
2:1
111 [102]
133 [121]
12 [-2]
114 [99]
21 [10]
86 [75]
131 [118]
30 [19]
141 [129]
9.9
66.2
6.8
8
1:1
1:2
16
73.7c
0.7c
4.8c
75.3c
7.0c
20
1:1b
Figure 5. (A) Textures of the smectic mesophases of the 2:1 host-
guest complex of 1 and 8 observed at 95 °C (top) and at 110 °C
(bottom), as viewed under the polarizing microscopy during a cooling
run from the isotropic phase. (B) Influence of the host-guest ratio on
the liquid-crystalline behavior of the complex.
a Determined by DSC. K, crystalline phase; S, smectic phase, N,
nematic phase, D, discotic phase; I, isotropic phase. The values in
square brackets are obtained from cooling runs. Structural assignments
are based on polarizing microscopy measurements. b Ratio 1/modified
and unmodified 4-HS ) 1:1. c Per mole repeating unit (mru) of the
polymer that contains one receptor molecule.
point is observed. Apparently, complexation of guest molecules
in the cleft of 1 influences to an appreciable extent the stacking
of the rigid central frameworks of the molecules, but has almost
no effect on the packing of the alkyl chains, and consequently
has almost no influence on the melting point.
Induction of Liquid Crystallinity by Complexation of
Guest Molecules. Compound 1 displayed, in the first and
subsequent heating and cooling runs, a reversible K f I
transition at 159-164 °C. Utilizing fast cooling to prevent
crystallization, a monotropic liquid crystalline phase with a
clearing temperature of 161 °C was observed. The binding of
a dihydroxybenzene derivative in the cleft of 1 was found to
change this behavior and to induce reversible liquid-crystalline
behavior.20 In the following we will describe the influence of
the complexation of mono-, bi-, tetra-, and polyfunctional guest
molecules on the mesogenic properties of receptor molecule 1.
Monofunctional Guest Molecules. Variation of the host-
guest ratio in complexes of clip 1 and MDB gave rise to the
appearance of different mesophases covering a wide temperature
range as shown in Figure 4 and Table 3. At low concentrations
of guest two mesophases were present, which were interpreted,
on the basis of the polarizing microscopy pictures, as being
smectic phases. At higher concentrations of guest only a
nematic phase was visible. Figure 4 shows that an increase in
the amount of guest molecule is accompanied by a slight change
of the melting point, whereas a large decrease of the clearing
Bifunctional Guest Molecules. The bifunctional guest 6
contains two resorcinol groups, which are linked by a rigid
spacer. A 1:1 complex of this molecule with receptor 1
displayed in the first heating run two smectic phases between
49 and 157 °C (Table 3). During the subsequent cooling and
heating runs the complex decomposed and only crystalline
material of the uncomplexed host and guest was left. Appar-
ently, the hydroquinone spacer in 6 has an unfavorable influence
on the stability of the liquid-crystalline complex. To prove that
this instability originates from the rigidity of the spacer rather
than from substitution of the phenyl ring itself, a 1:1 complex
of 1 and phenyl 3,5-dihydroxybenzoate, the monomeric analogue
of 6, was prepared. Two reversible smectic-like phases (bire-
fringent mosaic type textures, I142M1103M2) were observed
for this complex by polarizing microscopy.
An enhancement of the stability of the complex could be
achieved by introducing a more flexible connection, a chiral
spacer derived from L-(+)-dioctyl tartrate, between the resorcinol

290 J. Am. Chem. Soc., Vol. 119, No. 2, 1997
Van Nunen et al.
Table 4. Phase Transition Temperatures and Enthalpy Changes of Different Host-Guest Complexes of 1 and Porphyrin 9ba
host:guest
ratio
∆H
∆H
∆H
∆H
run
T (°C)b
(kJ/mol)
T (°C)c
(kJ/mol)
T (°C)d
(kJ/mol)
T (°C)e
157
(kJ/mol)
4:1
3:1
1:1
1
2
3
1
2
3
1
2
3
4
74.2
17 [5]
17 [5]
157.4
166.5
127 [115]
126 [114]
2.50
2.78
149 [132]
148 [131]
27.0
26.0
206
155.9
15 [3]
15 [3]
189.9
183.7
130 [118]
124 [112]
2.76
1.91
150 [132]
146 [131]
17.3
17.9
202
199 [138]
197 [136]
197
55.3
14.12
8.51
2.98
13 [2]
13 [2]
14
66.3
73.9
53.8
152 [121]
150 [121]
148
5.97
5.81
5.85
128
0.61
a Determined by DSC. The values in square brackets are obtained from cooling runs. b K f S transition. c S f S′ transition. d S′ f I transition.
e K f I transition.
functions in the guest (compound 8). Addition of receptor
molecule 1 to 8 in a 2:1 host-guest ratio led to the formation
of a complex, which displayed two reversible liquid-crystalline
phases: a smectic phase between 18 and 111 °C and another
smectic phase of lower order between 111 and 133 °C (Table
3). The objective of using the chiral dioctyl tartrate spacer was
to induce chirality at the macroscopic level, but unfortunately,
no chiral textures were observed (Figure 5). For this guest
molecule we also studied the influence of the host-guest ratio
on the liquid-crystalline properties of the complex. The 1:1
complex exhibited only one liquid-crystalline phase between
12 and 114 °C, which corresponds with the lower ordered
smectic phase of the 2:1 complex. As can be seen from Table
3 and Figure 5 the complexation of more than one molecule of
1 to 8 hardly influences the melting point, whereas the clearing
point is strongly affected.
Tetrafunctional Guest Molecule. The tetrafunctional guest
molecule was a porphyrin modified at the meso positions with
3,5-dihydroxyphenyl groups (9b). When four receptor mol-
ecules 1 were bound to porphyrin 9b, two almost identical
smectic-like phases were generated, after the first heating and
cooling run. These phases were separated by a transition with
a small enthalpy change at ca. 127 °C (Table 4). To show that
binding in the cleft is necessary for inducing the mesogenic
properties we also used a porphyrin modified with 2,6-
dihydroxyphenyl groups.14 Because of steric constraints this
porphyrin is not able to form hydrogen bonds with the carbonyl
functions in the cleft of molecular clip 2. With the 2,6-
disubstituted porphyrin no liquid-crystalline behavior could be
observed.
Figure 6. Thermograms of (a) the second heating run of the 4:1
complex and (b) the second and (c) the fourth heating runs of the 1:1
complex of 1 and 9b.
We also studied complexes of porphyrin 9b with different
equivalents of receptor molecule 1 (Table 4). The 3:1 host-
guest complex as well as the 1:1 complex initially displayed a
phase transition to the isotropic liquid at ca. 200 °C (∆H > 50
kJ/mol). In the subsequent cooling and heating runs of the 3:1
complex a similar behavior was observed as for the 4:1 complex
(Table 4). In the case of the 1:1 complex more heating runs
were required than in the case of the 3:1 complex to achieve
this behavior (see Figure 6a-c). The polarizing microscopy
results in combination with the large enthalpy changes (Table
4) in DSC suggest that the transitions at 157 and ca. 200 °C
are crystal to isotropic liquid transitions. Apparently, crystalline
complexes are initially formed, which after repetitive heating
and cooling cycles disappear at the expense of the formation
of the mesophases of the 4:1 complex and free porphyrin. This
suggests that the porphyrin, which is completely surrounded
by clip molecules, possesses an enhanced stability. A computer
generated drawing of the supramolecular complex is presented
in Figure 7.
Figure 7. Computer generated structure of a 4:1 complex of molecular
clip 1 and porphyrin 9b.
investigated the possibilities of inducing liquid crystallinity in
polymers. The complex of copolymer 20 (21 mol% of 1,3-
dihydroxybenzene functions) with molecular clip 1 (ratio
1/modified and unmodified 4-HS ) 1:1) was found to give a
very stable liquid-crystalline phase from 30 to 141 °C (Table
3) with a discotic-like (fan-shaped) texture (Figure 8), which
was clearly different from the smectic-like (mosaic type) textures
of the complexes of 1 with the low molecular weight guests.
Complexation to the polymer chains probably induces a change
Polyfunctional Guest Molecules. Stimulated by the results
obtained with the low molecular weight guest molecules we

Induction of Liquid Crystallinity by Host-Guest Interactions
J. Am. Chem. Soc., Vol. 119, No. 2, 1997 291
molecules. The taper-shaped clip molecules may surround the
polystyrene chains in a cylindrical fashion.21 X-ray studies are
underway to further investigate the structure of the complex.
In copolymer 16 the dihydroxybenzene (DHB) moieties are
directly attached to the polymer backbone. When receptor
molecule 1 was added to polymer 16 in a 1:1 ratio (based on
the number of 3,5-dihydroxybenzene units in 16) the DSC
thermogram showed the presence of free receptor molecules 1.
A 1:2 complex (clip 1: DHB units) was, therefore, prepared,
which resulted in the induction of reversible liquid-crystalline
phases, Viz. from 21 to 86 °C and from 86 to 131 °C. The
former phase was a more highly ordered one. Its texture
resembled a crystalline phase, but the value of the enthalpy
change measured by DSC was more in line with a liquid-
crystalline phase. The high temperature liquid-crystalline phase
displayed a discotic-like texture, similar to the polymer complex
described above.
Conclusions
In summary we have shown that molecular recognition can
be used as a tool to provide molecules with interesting materials
properties. Similar results have been obtained recently by other
goups.7 The binding of a guest to receptor molecule 1 leads to
the induction of liquid crystallinity. Compound 1 consists of a
mixture of three conformers. The guest changes the equilibrium
of the conformers and, in this way, probably tunes the properties
of the material. Experiments with complexes having different
ratios of host and guest show that complexation of the guest
molecule affects the clearing, but not the melting temperature
of the material. This suggests that guest molecules have an
effect on the packing of the rigid central frameworks of the
host molecules and not on the packing of the alkyl chains. The
results described here indicate that the concept of introducing
liquid crystalline properties by host-guest interactions is very
general and can be applied to both low molecular weight and
polymeric guest molecules.
Figure 8. The discotic-like (fan-shaped) texture observed at 120 °C,
viewed under the polarizing microscope during a cooling run from the
isotropic liquid (top) and a thermogram of the complex of polymeric
guest molecule 20 with clip 1 (bottom), showing the K f D′ and the
D′ f I transitions in the heating (top) trace and the reversed events in
the cooling (bottom) trace.
JA9603113
in the conformation of the clip molecules in such a way that
their trialkoxybenzoyl groups adopt a more spread out arrange-
ment, resulting in an overall taper-shaped structure for these
(21) Percec, V.; Heck, J.; Johansson, G.; Tomazos, D.; Kawasumi, M.;
Chu, P.; Ungar, G. Pure Appl. Chem. 1994, A31, 1719.