Synthesis and phase behavior of liquid crystalline diphenylacetylene derivatives possessing high clearing temperatures

Article abstract of DOI:10.1080/15421400490425784

A series of diphenylocetylene derivatives were synthesized to serve as ligands for metal alkyne complexes. The target compounds, alkyl 4′-(4-phenylethy-nylbenzoyloxy)-biphenyl-4-carboxylates, were prepared using a three-step synthetic procedure. Both the

Full text of DOI:10.1080/15421400490425784

Mol. Cryst. Liq. Cryst., Vol. 408, pp. 21–31, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400490425784
SYNTHESIS AND PHASE BEHAVIOR OF LIQUID
CRYSTALLINE DIPHENYLACETYLENE
DERIVATIVES POSSESSING HIGH CLEARING
TEMPERATURES
Douglas D. Young and Eric Scharrer*
Department of Chemistry, University of Puget Sound, 1500 N.
Warner, Tacoma, WA 98416, USA
Mark V. Yoa
Department of Chemistry, Franklin & Marshall College,
Lancaster, PA 17604, USA
A series of diphenylacetylene derivatives were synthesized to serve as ligands
for metal alkyne complexes. The target compounds, alkyl 40-(4-phenylethy-
nylbenzoyloxy)-biphenyl-4-carboxylates, were prepared using a three-step syn-
thetic procedure. Both the diphenylacetylene derivatives as well as their
immediate precursors, alkyl 40-(4-iodobenzoyloxy)-biphenyl-4-carboxylates,
exhibit liquid crystalline behavior. The diphenylacetylene derivatives possess
large liquid crystalline phase ranges and high clearing temperatures. Synthetic
details and the phase behavior of these compounds are discussed.
Keywords: crystalline polymorphism; diphenylacetylene; high melting and clearing temperatures;
smectic A and nematic phases
INTRODUCTION
The incorporation of diphenylacetylene or a related moiety into liquid
crystalline compounds allows for a number of favorable characteristics. For
example, such compounds have been investigated for their enhanced
optical anisotropy [1], nonlinear optical properties [2], and ferroelectric [3]
or antiferroelectric behavior [4]. The diphenylacetylene moiety also
We wish to thank the Donors of the American Chemical Society Petroleum Research Fund
and the National Science Foundation (CCLI-A&I program) for partial support of this research.
Additional support, in the form of startup funds and summer student stipends, was provided
by the University of Puget Sound and Franklin & Marshall College.
*Corresponding author. E-mail: escharrer@ups.edu
21

22
D. D. Young et al.
provides several sites for metal coordination. It is this feature of these
molecules that interests us because we wish to prepare new metal-con-
taining liquid crystals. There has been a great deal of interest in metallo-
mesogens over the past fifteen years due to their potential to exhibit
unique physical properties [5]. Most of the metallomesogens reported
possess a metal center that adopts a square planar or linear geometry.
There are far fewer examples of metallomesogens in which the geometry
about the metal is octahedral. The incorporation of an octahedral metal
center significantly broadens the molecule. In many [6a–6e], but not all [6f],
cases this lowers the phase transition temperatures of the complex relative
to the organic precursor; liquid crystallinity is sometimes destroyed
entirely. Nevertheless, we are greatly interested in preparing metallome-
sogens possessing an octahedral metal center because it may allow for the
possibility of obtaining liquid crystallinity at or near room temperature. In
particular, we wish to study the phase behavior of Co2(CO)6 complexes of
diphenylacetylene derivatives because alkyne complexes of this cobalt
fragment are relatively stable [7]. However, the Co2(CO)6 moiety will
introduce a large molecular kink, which may inhibit mesomorphism. Bruce
and Deschenaux have carried out research on some complexes in which the
octahedral metal center creates a large disruption to packing [6a–6e]. In
these cases, they demonstrated that if one starts with a sufficiently ani-
sotropic organic derivative (that possesses a high clearing point), the
resulting octahedral metal complex will also maintain liquid crystallinity.
Our inital goal, therefore, was to prepare a homologous series of meso-
morphic diphenylacetylene derivatives that possessed high clearing tem-
peratures, and we report here the synthesis and phase behavior of these
compounds.
RESULTS AND DISCUSSION
Synthesis
The synthesis of a homologous series of the target compounds was
achieved using a three-step procedure (Scheme 1). Compound 1, alkyl 40-
hydroxybiphenyl-4-carboxylate, was prepared by selective alkylation of 40-
hydroxybiphenyl-4-carboxylic acid in the presence of 1 equivalent of
potassium hydroxide and 1.5 equivalents of the appropriate alkyl halide.
Average yields for this step were 56% after purification. An alternate
procedure to prepare these derivatives (via Fischer esterification) has been
previously reported [8]. While the reported yields were comparable to ours,
our method represents an improvement because it does not require a
vacuum distillation as part of the purification procedure. Upon treatment of
compound 1 with 4-iodobenzoic acid, dicyclohexyl carbodiimide (DCC)

Liquid Crystalline Diphenylacetylene Derivatives
23
SCHEME 1 Synthesis of diphenylacetylene derivatives.
and catalytic dimethylaminopyridine (DMAP), alkyl 40-(4-iodobenzoyloxy)-
biphenyl-4-carboxylate, 2, was obtained with an average yield of 52%. This
was then reacted with phenylacetylene in the presence of catalytic
(PPh3)2PdCl2 and CuI to yield the diphenylacetylene target compound
(alkyl 40-(4-phenylethynylbenzoyloxy)-biphenyl-4-carboxylate), 3, in an
average yield of 54%.
Phase Behavior
Both the diphenylacetylene compounds, 3, as well as the intermediate iodo
derivatives, 2, exhibit liquid crystallinity. The identities of the liquid crys-
talline phases were established by examining their optical textures using
polarizing microscopy. As can be seen in Table I, at least two crystalline
phases are observed for all of the iodo derivatives, 2. However, the even
chain derivatives, as well as the derivative in which R ¼ C13H27, possess a
third crystalline phase; in these cases, the first crystal-to-crystal transition
occurs at relatively low temperatures ( ꢀ 90ꢁC) and generally has a low
transition enthalpy. Upon further heating, all of these derivatives exhibit
one liquid crystal phase, a smectic A phase, before clearing to the isotropic
liquid. The range of the Smectic A phase varies from 45ꢁC when R ¼ C5H11,

24

Liquid Crystalline Diphenylacetylene Derivatives
25
2a, to 36ꢁC when R ¼ C14H29, 2j. The observation of an exclusively
orthogonal smectic phase is expected because these derivatives possess
only one alkyl chain [9].
The phase behavior of the iodo derivatives, 2, can be compared to that of
the 400-iodophenyl-40-alkoxy-biphenyl-4-carboxylate derivatives, 4, pre-
pared by Bruce et al [10] (Figure 1). Bruce’s compounds possess a reversed
ester linkage relative to our derivatives, and they exhibit significantly
higher clearing points and larger mesophase ranges. These differences can
be ascribed to the higher dipole moment in Bruce’s compound; Spartan
calculations show that compound 4 has a dipole moment of 4.68 debye,
while compound 2c possesses a dipole moment of 1.41 debye.
Phase transition temperatures and enthalpies for the diphenylacetylene
derivatives, 3, are presented in Table II. The mesophase range for these
compounds varies from 84ꢁC to 134ꢁC. Two crystalline phases are observed
along with a smectic A phase, and in some cases a nematic phase. As is
typically observed in homologous series of liquid crystals, the range of the
nematic phase and the clearing temperature increase as the length of the
alkyl chain is shortened [11]. For example, the derivative in which
R ¼ C5H11, 3a, clears to an isotropic liquid at 278ꢁC, and the nematic phase
shows a range of 44ꢁC. The derivative in which R ¼ C12H25, 3h, clears to the
isotropic state at 224ꢁC, and the nematic phase range is only 1ꢁC. The
derivatives in which R ¼ C13H27, 3i, and C14H29, 3j, do not exhibit a nematic
phase and show clearing points of 222ꢁC and 218ꢁC, respectively. These
trends are illustrated in Figure 2.
The homologues with higher clearing temperatures decompose slightly
upon entering the isotropic state. Interestingly, this affects the texture of
the smectic phase upon cooling. If the derivatives are heated to the iso-
tropic liquid and then cooled, the focal conic texture appears broken and ill
defined. However, if the compounds are heated only to the nematic phase
and then cooled, the focal conic texture is much better defined and is
clearly representative of the smectic A phase.
Given the high clearing points and large liquid crystal phase ranges of
the diphenylacetylene derivatives, it is possible that the Co2(CO)6
FIGURE 1 Comparison of phase behavior for derivatives with a reversed linkage.

26

Liquid Crystalline Diphenylacetylene Derivatives
27
FIGURE 2 Phase transition temperatures of diphenylacetylene derivatives, 3.
complexes of these ligands may exhibit liquid crystallinity. We are currently
in the process of preparing such complexes and examining their phase
behavior.
EXPERIMENTAL
General
NMR spectra were obtained using a Varian Gemini 300 MHz FT NMR
spectrometer and analyzed using a Sun Microsystems Workstation. Ther-
mal analysis was conducted with a TA Instruments DSC 2920 and a Nikon
Labophot-2-Pol microscope equipped with a Mettler Toledo Hot Stage
FP82HT and FP 90 Central Processor. Unless otherwise noted, transition
temperatures were determined by DSC using a heating rate of 10ꢁC/min.
Spartan ’02 Version 1.0 was used to calculate dipole moments. Davisil Silica
˚
Gel, 100–200 mesh, 150 A and Aluminum Oxide-Activated (Brockman I)
Neutral, 150 mesh were used for column chromatography. Unless otherwise
noted, reactions were carried out under standard atmosphere and at room
temperature. Elemental analysis was conducted by Quantitative Technol-
ogies Inc. in Whitehouse, New Jersey. Dry THF was obtained from Aldrich.
˚
Dichloromethane was dried by stirring over 4 A molecular sieves for 24 h.
40-hydroxy-biphenyl-4-carboxylic acid was purchased from TCI (Portland,
Oregon). All other chemicals were purchased from Aldrich.
Synthesis of pentyl-40-hydroxybiphenyl-4-carboxylate, (1a)
(The synthesis (via a different method) and characterization of these
derivatives has been reported elsewhere) [8].

28
D. D. Young et al.
A 500 mL round-bottom flask was charged with 40-hydroxybiphenyl-4-
carboxylic acid (2.02 g, 9.43 mmol), DMSO (30 mL) and pulverized KOH
(0.58 g, 10.37 mmol). After stirring this mixture for 20 min, bromopentane
(2.20 g, 14.5 mmol) was added. The reaction was stirred for 48 h and then
quenched by the addition of 30 mL of water. Ether (40 mL) was added to
the reaction mixture, and the organic layer was washed three times with
water. The aqueous layer was back-extracted once with ether, and the
organic layers were combined, dried over MgSO4, filtered, and solvent was
removed by rotary evaporation. The solid product was dissolved in hexane
and loaded onto a short column of activated alumina. Hexane (100 mL) was
rinsed through the column to remove residual alkyl halide. The eluent was
switched to ether, and this solution was collected and concentrated by
rotary evaporation. The crude product was recrystallized from 3:2 CH2Cl2/
EtOH to yield a white powder, 1.62 g (5.70 mmol, 60.5% yield).1H NMR
(300 MHz CDCl3) d 0.95 ppm(t, 3H, -CH3, J ¼ 7. Hz), 1.42 (m, 4H, -C2H4),
1.78 (p, 2H, -OCH2CH2, J ¼ 6.9 Hz), 4.34 (t, 2H, -OCH2, J ¼ 6.6 Hz), 5.16
(s, 1H, -OH), 6.94 (d, 2H, aromatic, J ¼ 8.8 Hz), 7.54 (d, 2H, aromatic,
J ¼ 8.8 Hz), 7.62 (d, 2H, aromatic, J ¼ 8.5 Hz), 8.08 (d, 2H, aromatic, J ¼ 8.5
Hz).13C NMR (75.5 MHz CDCl3) d 13.99 ppm, 22.36, 28.20, 28.43, 65.27
(aliphatic); 115.87, 126.44, 128.45, 128.57, 130.07, 132.47, 145.21, 156.09
(aromatic); 167.00 (C¼O).
All other derivatives were prepared in a similar manner. Yields are shown
in Table III.
Synthesis of pentyl 40-(4-iodobenzoyloxy)-biphenyl-
4-carboxylate, (2a)
A 250 mL flask equipped with condenser was charged with compound 1a
(1.00 g, 3.53 mmol), 4-iodobenzoic acid (0.88 g, 3.53 mmol), 4-(dimethyl-
amino)-pyridine (DMAP) (0.05 g, 0.36 mmol) and 150 mL dry CH2Cl2.
TABLE III Yields for Compound 1
Compound
Percent yield
1a, R ¼ C5H11
1b, R ¼ C6H13
1c, R ¼ C7H15
1d, R ¼ C8H17
1e, R ¼ C9H19
1f, R ¼ C10H21
1g, R ¼ C11H23
1h, R ¼ C12H25
1i, R ¼ C13H27
1j, R ¼ C14H29
60.5
70.5
95.3
53.9
37.4
32.6
43.5
55.7
57.8
55.8

Liquid Crystalline Diphenylacetylene Derivatives
29
Dicyclohexylcarbodiimide (DCC) (0.733 g, 5.55 mmol) was added to the
reaction mixture, and this was stirred for 24 h. The solution was then
refluxed for an additional 4 h. After cooling to room temperature, the
solution was gravity filtered to remove dicyclohexylurea, and the solvent
was removed by rotary evaporation. The solid was then redissolved in
CH2Cl2 and washed through a short plug of alumina. Solvent was removed
by rotary evaporation to yield a white powder. This was recrystallized with
3:2 CH2Cl2/EtOH to give 1.42 g of a white solid (2.75 mmol, 78% yield).
1H NMR (300 MHz, CDCl3) d 0.95 ppm(t, 3H, -CH3, J ¼ 7.1 Hz), 1.42
(m, 4H, -C2H4), 1.80 (p, 2H, -OCH2CH2, J ¼ 7.1 Hz), 4.35 (t, 2H, -OCH2,
J ¼ 6.9 Hz), 7.31 (d, 2H, aromatic, J ¼ 8.5 Hz), 7.66 (d, 2H, aromatic,
J ¼ 8.3 Hz), 7.68 (d, 2H, aromatic, J ¼ 8.7 Hz), 7.89, 7.94 (ABq, 4H, aro-
matic, J ¼ 8.9 Hz), 8.12 (d, 2H, aromatic, J ¼ 8.5 Hz). 13C NMR (75.5 MHz,
CDCl3) d 14.03 ppm, 22.40, 28.23, 28.46, 65.23 (aliphatic); 101.80, 122.13,
127.02, 128.46, 128.86, 129.44, 130.12, 131.57, 138.04 (2 overlapping
peaks), 144.55, 150.78 (aromatic); 164.75 (C¼O), 166.54 (C¼O). Anal.
Calcd for C25H23O4I: C 58.38; H 4.51. Found: C 58.31; H 4.59.
All other derivatives were prepared in an analogous manner. Yields and
results of elemental analysis are provided in Table IV.
Synthesis of pentyl 40-(4-phenylethynylbenzoyloxy)-
biphenyl-4-carboxylate, (3a)
A three-neck round-bottom flask equipped with a condenser and a gas inlet
adapter was charged with compound 2a (1.00 g, 1.89 mmol), phenylacety-
lene (0.19 g, 1.89 mmol), and (PPh3)2PdCl2 (0.03 g, 0.05 mmol) under a
nitrogen atmosphere. Dry THF (40 mL), and diisopropyl amine (1.32 mL,
9.45 mmol) were added via syringe. The solution was stirred and degassed
by a nitrogen purge for approximately 15 min. CuI (0.05 g, 0.25 mmol) was
TABLE IV Elemental Analysis and Yields for Compound 2
Compound
C% calc. (found)
H% calc. (found)
Percent yield
2a, R ¼ C5H11
2b, R ¼ C6H13
2c, R ¼ C7H15
2d, R ¼ C8H17
2e, R ¼ C9H19
2f, R ¼ C10H21
2g, R ¼ C11H23
2h, R ¼ C12H25
2i, R ¼ C13H27
2j, R ¼ C14H29
58.38 (58.31)
59.10 (58.88)
59.79 (59.68)
60.44 (60.49)
61.06 (60.87)
61.74 (61.70)
62.61 (62.25)
62.74 (62.66)
63.26 (62.85)
63.75 (63.78)
4.51 (4.59)
4.77 (4.57)
5.02 (4.89)
5.25 (5.19)
5.48 (5.45)
5.70 (5.57)
5.89 (5.86)
6.10 (6.07)
6.27 (6.26)
6.45 (6.48)
77.9
60.7
60.5
57.5
73.0
64.7
44.2
45.8
17.5
10.7

30
D. D. Young et al.
TABLE V Elemental Analysis and Yields for Compound 3
Compound
C% calc. (found)
H% calc. (found)
Percent yield
3a, R ¼ C5H11
3b, R ¼ C6H13
3c, R ¼ C7H15
3d, R ¼ C8H17
3e, R ¼ C9H19
3f, R ¼ C10H21
3g, R ¼ C11H23
3h, R ¼ C12H25
3i, R ¼ C13H27
3j, R ¼ C14H29
81.12 (81.11)
81.25 (80.85)
81.37 (81.18)
81.48 (81.26)
81.59 (81.41)
81.69 (81.55)
81.78 (81.52)
81.88 (81.60)
81.96 (81.56)
82.05 (81.76)
5.78 (5.78)
6.02 (6.01)
6.24 (6.20)
6.46 (6.44)
6.66 (6.60)
6.85 (6.81)
7.04 (7.01)
7.21 (7.15)
7.38 (7.47)
7.54 (7.51)
47.6
35.4
54.6
55.5
65.2
85.9
42.6
45.0
49.5
61.5
then added to the reaction mixture, which caused the solution to imme-
diately darken. The solution was allowed to stir for 48 h under nitrogen.
The reaction mixture was rinsed through an activated alumina column with
150 mL THF, and the filtrate was collected and solvent removed by rotary
evaporation. The product was extracted three times with CH2Cl2. The
combined organic layers were washed with dilute HCl, dried over MgSO4,
and solvent was removed in vacuo. The solid product was recrystallized
from 1:1 CH2Cl2/EtOH. This material was further purified by flash chro-
matography (eluent: 3:2 CH2Cl2/Hexane, Rf ¼ 0.45). A final recrystallization
with 1:1 CH2Cl2/EtOH was necessary to obtain reproducible phase behavior
(0.44 g (0.90 mmol, 47.6% yield).1H NMR (300 MHz, CDCl3) d 0.87 ppm
(t, 3H, -CH3, J ¼ 7.1 Hz), 1.35 (m, 4H, -C2H4), 1.72 (p, 2H, -OCH2CH2,
J ¼ 7.1 Hz), 4.27 (t, 2H, -OCH2, J ¼ 6.6 Hz), 7.28 (m, 5H, aromatic), 7.50
(m, 2H, aromatic), 7.61 (m, 6H, aromatic), 8.04 (d, 2H, aromatic,
J ¼ 8.2 Hz), 8.13 (d, 2H, aromatic, J ¼ 8.2 Hz). 13C NMR (75.5 MHz, CDCl3)
d 14.03 ppm, 22.41, 28.25, 28.50, 65.17 (aliphatic); 88.50 (alkyne), 92.97
(alkyne); 122.09, 122.54, 126.92, 128.34, 128.39, 128.59, 128.83 (2 over-
lapping peaks), 129.38, 130.06 (2 overlapping peaks), 131.63, 131.71,
137.85, 144.52, 150.86 (aromatic); 164.48 (C¼O), 166.38 (C¼O). Anal.
Calcd. for C33H28O4: C 81.12; H 5.78. Found: C 81.11; H 5.78.
All other derivatives were prepared in a similar manner. Yields and
results of elemental analysis are provided in Table V.
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