Biphenyl-based disc- vs. rod-shaped phenylacetylenes: Mesomorphism and electronic properties

Article abstract of DOI:10.1002/ejoc.200800568

A standard Sonogashira coupling protocol has been applied for the preparation of a C₃-symmetric phenylacetylene (2) and the corresponding linear phenylacetylene (3) incorporating terminal biphenyl substituents. The study of the mesomorphic properties of the target compounds, as well as mixtures of both, reveals that only the calamitic biphenylylacetylene 3 forms smectic liquid crystals. Upon investigating the electronic situation of both biphenylylacetylenes by spectroscopic methods and computational calculations, it becomes evident that 2 can be considered the trimeric analogue of 3 which however does not form liquid crystalline phases. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800568

FULL PAPER
DOI: 10.1002/ejoc.200800568
Biphenyl-Based Disc- vs. Rod-Shaped Phenylacetylenes: Mesomorphism and
Electronic Properties
Gunther Hennrich,*[a] Pedro D. Ortiz,[a][‡] Emma Cavero,[b] Robert E. Hanes,[c] and
José Luis Serrano[b]
Keywords: Liquid crystals / Structure-activity relationships / Alkynes / Cross-coupling / Biphenyls
A standard Sonogashira coupling protocol has been applied
for the preparation of a C3-symmetric phenylacetylene (2)
electronic situation of both biphenylylacetylenes by spectro-
scopic methods and computational calculations, it becomes
and the corresponding linear phenylacetylene (3) incorporat- evident that 2 can be considered the trimeric analogue of 3
ing terminal biphenyl substituents. The study of the meso-
morphic properties of the target compounds, as well as mix-
tures of both, reveals that only the calamitic biphenylylace-
tylene 3 forms smectic liquid crystals. Upon investigating the
which however does not form liquid crystalline phases.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
macrocyclic compounds,[5] but also more simple π-extended
(hetero)benzenes and self-assembled systems.[6] The factors
controlling the self-assembly of columnar LCs are governed
by weak interactions such as π-stacking, van der Waals and
dispersion forces, dipolar interactions and others.[7] A re-
cent study by Williams et al. has stressed the importance of
electrostatic contributions in usually highly polar or polar-
izable systems.[8] We have recently demonstrated the pos-
sibility to obtain functional, bulk-phase devices from pre-
functionalized benzene-based, low-molecular-weight build-
ing units forming liquid crystals, directed towards nonlinear
optical and opto-electronic applications.[9] As an intermedi-
ate between the discotic and calamitic mesogens, disc–rod-
shaped mesogens have raised considerable interest over the
last few years, and the resulting LC phases have been
studied extensively. Mesogenic structures of this type are
commonly designed by connecting rod-like subunits to a
discotic central platform, for example benzene or triphen-
ylene.[10] The search for a (theoretically predicted) biaxial
nematic phase, as well as the uncommon miscibility of rod-
like with disc-shaped molecules in the LC phase has pushed
the limits in this recent field of LC research.[11]
Introduction
The enormous advance which the field of liquid crystals
(LCs) has experienced during the past three decades is a
direct consequence of the application of predominantly ne-
matic LCs in display technology. The underlying physical
phenomena consist in the ability of liquid-crystalline mate-
rials to rotate plane-polarized light (birefringence), which
can be electrically switched by reorienting the calamitic me-
sogens in an electric field.[1] Smectic LCs are typically rod-
like molecules composed of an elongated rigid core with
flexible side chains.[2] Aside from smectic LCs, formed by
rigid-rod mesogens such as biphenyl derivatives as a prime
example, the construction of columnar LCs based on disk-
shaped mesogens has attracted increasing attention, lately.
Again, their potential technical application in electro-op-
tical devices has been the driving force to press forward
this relatively young research area.[3] Here, one of the most
attractive features is the one-dimensional electronic conduc-
tance along the axis perpendicular to the columnar stacks,
typically formed by discotic molecules with extended π-sys-
tems.[4] Examples of discotic mesogens with electro-optical
activity include aromatic and heteroaromatic polycyclic and
[a] Departamento de Química Orgánica, Universidad Autónoma
de Madrid,
Results and Discussion
28049, Madrid, Spain
In the present contribution we wish to report the synthe-
sis of biphenylylacetylene-based linear and disc–rod-shaped
molecules and establish a relationship between the molecu-
lar and electronic structure, and their mesomorphic behav-
ior. Acetylenic scaffolding has gained increasing importance
for the construction of functional opto-electronic materi-
als.[12] The conjugation pathway in phenylacetylene systems
is governed by the symmetry and directionality of the car-
Fax: +34-91-497-3966
E-mail: gunther.hennrich@uam.es
[b] Departamento de Química Orgánica ICMA-INA, Universidad
de Zaragoza,
50009 Zaragoza, Spain
[c] Beacon Sciences LLC,
78738, Austin, TX, USA
[‡] Present address: Departamento de Química Inorgánica, Uni-
versidad de La Habana,
10400 La Habana, Cuba
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bon–carbon triple bond, thus it can be considered as strictly employed (Scheme 2). In all coupling steps, the Pd0/CuI cat-
one-dimensional (linear) and free from conformational alyst system is utilized in a 2.5 mol-% quantity per aryl ha-
complications arising from the presence of energetically rel- lide. In both cases, the reaction mixtures were heated at
evant (cis/trans or rotational) isomers.[13]
70 °C for 18 h in a toluene/triethylamine mixture (1:1).
While 3 is obtained in 67% yield without optimizing the
reaction conditions, a low yield of 19% for the threefold
coupling product 2 reflects the unfavorable electronic and
steric conditions of the triiodo starting material brought
about by the bulky, electron-rich alkoxy side-chains.
Synthesis
We have chosen the Sonogashira cross-coupling reaction
as the method of choice to prepare the respective phenyle-
thynylene compounds in a straight forward, target-oriented
approach.[14] The biphenyl-substituted acetylene 1 is ob-
tained in two steps from commercially available 4Ј-(4-bro-
mophenyl)acetophenone in good yields, and is used as the
free acetylene compound in the following coupling reac-
tions (Scheme 1).
Electronic Structure
The optical transitions can be tuned conveniently by ad-
justing the substitution pattern of the phenyl subunits. Both
2 and 3 are conjugated donor-π-acceptor systems with an
octopolar or dipolar character, respectively, where the alk-
oxy substituents act as donor groups and the acetyl moiety
as electron-withdrawing function.[15] The type of the LC
phase formed should primarily depend on the molecular
structure of the constituting mesogens: Compound 3 repre-
sents the typical elongated rigid-rod like mesogen, while 2
is a π-extended, disc-shaped molecule with C3-symmetry.
Formally, 2 can also be considered as a trimer of 3, being
composed of three linear biphenylylacetylene subunits con-
nected to a central benzene core. In order to assess the elec-
tronic nature of the target compounds, the UV/Vis absorp-
tion and fluorescence spectra of 2 and 3 have been recorded
in solution. Spectroscopically, compound 2 behaves as a tri-
meric assembly of 3 in which the three biphenylylacetylene
fragments are electronically isolated and the electronic tran-
sitions are located in the linear fragments.[16] The long-
wavelength charge-transfer absorption band is situated
Scheme 1. Synthesis of the biphenylylacetylene 1.
In the syntheses of the 1,3,5-tris(alkynyl)benzene 2, start- roughly at the same position for both 2 [334 nm, log(ε334) =
ing from 1,3,5-triiodo-2,4,6-tris(dodecyloxy)benzene,[9] and 5.084] and 3 [329 nm, log(ε329) = 4.533]; the bathochromic
the linear reference compound 3, obtained from 1-iodo-4- displacement of 5 nm is negligible. The molecular absorp-
dodecyloxybenzene, a 1.5-fold excess of 1 with respect to tivity for the trigonal 2 amounts approximately to the three-
the iodo functions in the starting aryl compound has been fold value (3.52) compared to the linear biphenylylacetylene
Scheme 2. Synthesis of the 1,3,5-tris(alkynyl)benzene 2 and the linear phenylacetylene 3.
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Biphenylylacetylene-Based Mesogens
3. Further evidence for the distinctive electronic situation
Compound 3 shows two crystalline phases and a high-
in both push-pull systems is obtained from computational temperature SmA mesophase which is identified from the
calculations which reveal a substantial dipole moment of homeotropic texture (Figure 2, a). To confirm the type of
3.47 D for 3.[17] In contrast, 2 exhibits a pronounced octo- mesophase, X-ray diffraction experiments were performed
polar character as manifested by the tensor elements in the and the patterns obtained present a diffuse halo corre-
molecule plain (i.e. the x- and y-direction) contributing to sponding to a mean distance of 4.4 Å. This feature is char-
the overall polarization of 2 (Table 1).[18]
acteristic of the liquid-like order of the aliphatic chains in
a liquid-crystal phase. A set of sharp maxima in the low-
angle region with a reciprocal spacing ratio of 1:2:3 is also
observed (Figure 2, b). This maxima can be assigned to the
(001), (002) and (003) reflections of a lamellar arrangement.
The maxima provide a layer spacing of 36.5 Å for this me-
sophase which corresponds to the estimated molecular
length.
Table 1. Calculated polarization (field-independent basis Deye-
Ang**2).
β-Components
XXX
XXY 808.79
XXZ –486.09
5.44
YYY –324.51
YYZ 237.37
YYX 14.95
ZZZ
YZZ
XZZ
1.68
2.69
57.94 XYZ 383.56
Both compounds are well fluorescent with quantum
yields ΦFl(2) = 0.26, and ΦFl(3) = 0.12, respectively.[19]
Again, the fluorescence emission maxima are located in the
same wavelength region [2: λmax (em) = 446 nm; 3: λmax
(em) = 435 nm]. The large Stokes shifts Ͼ 100 nm are at-
tributed to the structural changes (planarization) occurring
in the biphenyl subunits upon excitation (Figure 1).[20]
Figure 1. UV/Vis absorption and fluorescence spectra of 2 (___) and
3 (···) in CH2Cl2 solution, c = 10–6 molL–1, normalized to the ab-
sorption and emission maximum of 2.
Figure 2. a) Microphotograph of 3 observed by POM of SmA
phase at 202 °C. b) XRD pattern of SmA phase.
Liquid Crystal Properties
In contrast to the mesomorphic behavior of the rod-
shaped derivative 3, the discotic biphenyl-substituted acetyl-
ene 2 does not display LC properties. It directly melts from
the crystal at 168 °C. In order to probe the miscibility of
both compounds, and in order to induce liquid crystallinity
in 2, different mixtures of 2 and the calamitic 3 have been
prepared at mol ratios of 1:1, 1:2, and 1:3. However, in all
three cases the mixtures behave as two separate compounds
with the crystalline 2 simply acting as “impurity” in the
smectic LC phase formed by 3.
The mesomorphic properties of 2 and 3 have been
studied by polarized optical microscopy (POM) and
differential scanning calorimetry (DSC). Transition tem-
peratures and their associated enthalpy changes in the sec-
ond heating cycle recorded at 10 °C/min are gathered in
Table 2.
Table 2. Optical and thermal properties of 2 and 3.
Conclusions
In conclusion, we have described the synthesis of a rigid,
extended trigonal biphenyl-substituted acetylene 2 and its
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FULL PAPER
vacuo, the remaining solid was suspended in water (20 mL) and
extracted with EtOAc (3ϫ20 mL). The combined organic phases
were dried (MgSO4) and concentrated to dryness to leave a solid
which was purified by column chromatography (hexane/EtOAc,
10:1) and final recrystallization from EtOH, yielding pure 1a as
colorless plates. Yield 621 mg, 85%; m.p. 138 °C. 1H NMR
(300 MHz, CDCl3): δ = 8.02 (dAB, J = 8.4 Hz, 2 H), 7.67 (dAB, J
= 8.6 Hz, 2 H), 7.56 (s, 4 H), 2.63 (s, 3 H), 0.27 (s, 9 H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 197.5, 144.6, 139.6, 136.0, 132.4,
128.9, 127.0, 126.9, 123.0, 104.6, 95.6, 26.6, –0.1 ppm. C19H20OSi
linear analogue 3 by Sonogashira coupling of the respective
aryl iodides with the biphenyl-substituted acetylene 1,
which itself constitutes a versatile building block for the
preparation of extended π-systems. Both target compounds
display attractive spectroscopic properties as a consequence
of the donor-acceptor substitution pattern, being of an oc-
topolar nature in the case of 2 and dipolar for the linear 3.
While the calamitic alkyne 3 displays interesting mesomor-
phic properties, the disc–rod-shaped 3 does not form a LC
phase; neither do mixtures of both 2 and 3. However, it is (292): calcd. C 78.08, H 6.85; found C 77.63, H 6.86.
reasonable to assume that the preparation of analogues of
2 with advanced LC properties can be achieved in a
straightforward manner by simply replacing the linear C12
alkoxy side chains by longer, branched, or fluorinated
ones,[21] and thereby counteract the non-covalent (stacking)
4Ј-(4-Ethynylphenyl)acetophenone (1): Compound 1a (584 mg,
2.0 mmol) was stirred with K2CO3 (560 mg, 4.0 mmol) in MeOH
(20 mL) at room temp. for 5 h. The solvent was removed in vacuo,
the remaining solid was suspended in water (20 mL) and extracted
with EtOAc (3ϫ10 mL). The combined organic phases were dried
interactions with favor crystallization of the extended π-sys- (MgSO4) and concentrated to dryness to leave a colorless solid
which was recrystallized from EtOH to give pure 1 as colourless
crystals. Yield 357 mg, 81%; m.p. 164 °C. 1H NMR (300 MHz,
CDCl3): δ = 8.03 (dAB, J = 8.4 Hz, 2 H), 7.67 (dAB, J = 8.6 Hz, 2
H), 7.59 (s, 4 H) 3.16 (s, 1 H), 2.64 (s, 3 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 197.6, 144.7, 140.1, 136.2, 132.7, 128.9,
127.1, 127.0, 122.0, 83.2, 78.4, 26.6 ppm. C16H12O (220): calcd. C
87.25, H 5.49; found C 87.85, H 5.63.
tem.
Experimental Section
General: All solvents and reagents were purchased from Aldrich
and used without further purification. Melting points were deter-
mined with a Gallenkamp melting point apparatus and are uncor-
rected. 1H and 13C NMR spectra were recorded with a Bruker AC-
300 and spectrometer in deuterated chloroform (deuteration grade
Ͼ99.80%) with the solvent signal serving as internal standard.
Mass spectra (MALDI, EI) were recorded with a HP1100MSD
spectrometer. Elemental analyses were performed with a LECO
CHNS 932 micro-analyzer. Spectroscopic measurements were per-
formed using HPLC-quality solvents, and are solvent corrected.
UV/Vis spectra were measured with a HP 8453 (Hewlett–Packard)
spectrophotometer. A Perkin–Elmer LS50B luminescence spec-
trometer was employed for the fluorescence studies, in a four-sided
quartz cell at room temperature in a right-angle geometry and are
corrected for the spectral response for the detection system. Meso-
phase analysis was performed using a Linkam THMS600 hot stage
and an Olympus polarizing microscope equipped with an Olympus
DP12 digital camera. Transition temperatures and enthalpies were
obtained by differential scanning calorimetry with a DSC-MDSC
TA Instruments Q-1000 apparatus at heating and cooling rates of
10 °C/min. The apparatus was previously calibrated with indium
(156.6 °C, 28.44 J/g). Powder X-ray diffraction patterns were ob-
tained using a pinhole camera (Anton Paar) operating with a point-
focused Ni-filtered Cu-Kα beam. The sample was held in Lindem-
ann glass capillaries (0.9 and 1 mm diameter) and heated, when
necessary, with a variable-temperature attachment. The diffraction
patterns were collected on flat photographic film.
1,3,5-Tris[4Ј-(4-acetylphenyl)phenylethynyl]-2,4,6-tris(dodecyloxy)-
benzene (2): 1,3,5-Triiodo-2,4,6-tris(dodecyloxy)benzene (502 mg,
0.5 mmol) and 1 (495 mg, 2.25 mmol) were heated together with
Pd(PPh)2Cl2 (27 mg, 0.038 mmol), CuI (7 mg, 0.056 mmol), in tol-
uene/NEt3 (1:1, 15 mL) at 70 °C for 18 h. After aqueous and chro-
matographic (hexane/EtOAc, 5:1) workup and final recrystalli-
zation from diethylketone, 2 was obtained as colourless solid in
1
19% yield (122 mg). H NMR (300 MHz, CDCl3): δ = 8.06 (dAB
,
J = 8.2 Hz, 6 H), 7.71 (dAB, J = 8.2 Hz, 6 H), 7.64 (s, 12 H), 4.40
(t, J = 6.1 Hz, 6 H), 2.65 (s, 9 H), 1.91 (q, J = 7.0 Hz, 6 H), 1.62
(q, J = 7.8 Hz, 6 H), 1.56–1.12 (m, 48 H) 0.84 (t, J = 6.5 Hz, 9 H)
ppm. 13C NMR (125 MHz, CDCl3): δ = 197.5, 163.1, 144.7, 139.5,
136.0, 131.8, 129.0, 127.2, 127.0, 123.5, 108.2, 96.7, 82.9, 74.9, 31.9,
30.6, 29.7 (4), 29.6 (9), 29.4 (4), 29.3 (5), 26.6, 26.4, 22.6, 14.1 (one
alkyl 13C signal missing) ppm. C90H108O6·H2O (1286): calcd. C
82.91, H 8.50; found C 82.87, H 8.43.
[4Ј-(4-Acetylphenyl)phenylethynyl]-4-dodecyloxybenzene (3): 4-Iodo-
(dodecyloxy)benzene[22] (280 mg, 0.5 mmol) and
1 (165 mg,
0.75 mmol) were treated and purified under the same conditions as
for 2 to give pure 3 as pale yellow solid after final recrystallization
1
from EtOH in 67% yield (161 mg). H NMR (300 MHz, CDCl3):
δ = 8.04 (dAB, J = 8.2 Hz, 2 H), 7.70 (dAB, J = 8.2 Hz, 2 H), 7.60
(s, 4 H), 7.48 (dAB, J = 8.8 Hz, 2 H), 6.88 (dAB, J = 8.8 Hz, 2 H),
3.98 (t, J = 6.5 Hz, 2 H), 2.64 (s, 3 H), 1.79 (q, J = 5.5 Hz, 2 H),
1.48–1.27 (m, 18 H) 0.88 (t, J = 6.8 Hz, 3 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 197.7, 159.4, 144.9, 139.1, 136.0, 133.1,
132.0, 129.0, 127.3, 127.1, 127.0, 123.7, 114.6, 90.9, 87.6, 68.1, 31.9,
29.6 (4), 29.6 (2), 29.5 (8), 29.4, 29.3, 29.2, 26.7, 26.0, 22.7, 14.1
ppm. C34H40O2 (480): calcd. C 84.96, H 8.39; found C 84.79, H
8.30.
Computational Methods: Compounds 2 and 3 were evaluated with
Gaussain 03 and its suite of algorithms. Both compounds geome-
tries were optimized using HF 32–1 G, followed by a frequency
calculation to determine the zero point energy. For the purposes of
understanding the behavior of the octopolar compound, 2, this was
deemed a sufficient level of theory for understanding its electro-
optical properties.
Materials
Acknowledgments
4Ј-[4-(Trimethylsilylethynyl)phenyl]acetophenone (1a): 4Ј-(4-Bro-
mophenyl)acetophenone (690 mg, 2.5 mmol) was heated together
This work was supported by the Spanish Ministerio de Ciencia y
with Pd(PPh)2Cl2 (44 mg, 0.063 mmol), CuI (12 mg, 0.063 mmol), Tecnología (MCYT) (grants CTQ2007-65683, MAT 2006-13571-
and trimethylsilylacetylene (0.7 mL, 5.0 mmol) in NEt3 (15 mL) at
CO2-01), and the Comunidad Autónoma de Madrid / UAM (grant
70 °C for 18 h in a pressure tube. The solvent was removed in
CCG06-UAM/MAT-0449).
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Biphenylylacetylene-Based Mesogens
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Published Online: August 14, 2008
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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