Synthesis, transition temperatures, and optical properties of compounds with simple phenyl units linked by double bond, triple bond, ester or propiolate linkages

Article abstract of DOI:10.1039/b001164i

A series of compounds has been prepared with 4-butylsulfanylphenyl and 4-cyano- or 4-isothiocyanato-phenyl units connected by -CH=CH-, -COO-, -C≡C- , or -C≡C-COO- linking groups. The synthesis of the novel compounds is presented and the transition temperatures and optical parameters of the compounds are discussed and compared with those for related biphenyl reference systems. The ester linking group reduces optical and polarisability anisotropy, but the other linking groups give increased optical anisotropy (up to Δn = 0.50) and polarisability anisotropy [up to Δα = 45.2 A³ (10^-30 m³)].

Full text of DOI:10.1039/b001164i

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J O U R N A L O F
Synthesis, transition temperatures, and optical properties of
compounds with simple phenyl units linked by double bond, triple
bond, ester or propiolate linkages{
Gregory J. Cross, Alexander J. Seed,{ Kenneth J. Toyne, John W. Goodby, Michael Hird and
M. Carmen Artal
C H E M I S T R Y
Liquid Crystals and Advanced Organic Materials Research Group, The Department of
Chemistry, The University, Hull, UK HU6 7RX. E-mail: aseed@kent.edu;
E-mail: k.j.toyne@chem.hull.ac.uk
Received 11th February 2000, Accepted 4th April 2000
Published on the Web 5th June 2000
A series of compounds has been prepared with 4-butylsulfanylphenyl and 4-cyano- or 4-isothiocyanato-phenyl
units connected by -CHLCH-, -COO-, -CMC-, or -CMC-COO- linking groups. The synthesis of the novel
compounds is presented and the transition temperatures and optical parameters of the compounds are
discussed and compared with those for related biphenyl reference systems. The ester linking group reduces
optical and polarisability anisotropy, but the other linking groups give increased optical anisotropy (up to
Ê 3
Dn~0.50) and polarisability anisotropy [up to Da~45.2 A (10230 m3)].
Many applications of liquid crystals require systems of as high
a birefringence as possible and in particular compounds of high
optical anisotropy are essential for fast third-order non-linear
switching using the optical Kerr effect;1,2 some examples of our
previous work to meet these objectives are given in refs. 3±6. In
addition to the speci®c use of compounds of high optical
anisotropy in optical Kerr effect devices, such compounds are
useful generally in mixtures designed for twisted nematic and
supertwisted nematic displays which require a precise combi-
nation of cell thickness and optical anisotropy values; in order
to maximise the transmission of light, thinner cells are needed
and therefore compounds of higher optical anisotropy have to
be used. As is always the case with the formulation of liquid
crystal mixtures for any application, a compromise has to be
reached since no single component will ever meet the extensive
list of requirements of physical properties. Speci®cations of
mesophase type, mesophase range, mesophase sequence,
melting point, clearing point, viscosity, dielectric, elastic and
optical properties etc. provide some targets that have to be
We have undertaken an extensive programme of work to try
to assess how those groups and molecular units which are
known to confer high optical anisotropy can be combined to
give even better additives. Some common groups and rings
which have been identi®ed as giving rise to high optical
anisotropy are RS-, -CN, -NCS, -CMC-, wCLCv, -CSO-,
-COS-, -CSS-, phenyl, naphthyl, thiophenyl etc. as reported in
earlier work.3±6 We have found that the butylsulfanyl unit is a
good terminal group in the design of compounds of high
optical anisotropies because it is suf®ciently long to permit a
reasonable mesogenic character, but it is short enough so as not
to dilute the polarisability of the core units, and the sulfur atom
gives enhanced polarisability compared to the oxygen atom in
an alkoxy group. The presence of cyano or isothiocyanato
groups also gives rise to good mesogenicity and because of their
unsaturated p-system structure they provide an extended
region of polarisability in the molecule. In this part of the
programme we consider wCLCv, -CMC- and -COO- linking
groups between phenyl rings with -CN, -NCS, C4H9S- or
C5H11- terminal groups. The fundamental parent core system
used is biphenyl (compounds 395,6 and 403) and carboxylate
ester or ethynyl groups are inserted separately (compounds 41,4
42,4 14 and 16) and in combination (compounds 9 and 10)
between the rings. The compounds 27 and 28 (and their
precursors 25 and 26) are included to show the effect of an
ethenyl link and the values for 37 (this work, and from ref. 7 for
the neat material) and 383 are given to show the comparison
approached to provide an acceptable formulation in
a
particular case. Usually, a broadly acceptable composition is
formulated from a small number of components and then an
attempt is made to optimise the formulation by using additives
which enhance a required physical property without diminish-
ing the performance of the basic mixture. High birefringence
nematic materials is one example of additives which are
particularly valuable and the dilema that has to be faced in
designing such materials is that increased molecular polarisa-
bility generally leads to increased melting points and reduced
miscibility whereas acceptable melting points and mesophase
ranges arise from relatively small compounds of only moderate
optical anisotropy. The challenge is to design molecules which
contain as many advantageous features for enhancing optical
anisotropy as possible, whilst still retaining an ability to mix
with a host system without altering its other physical properties
too seriously.
with the pentyl parent compounds; one example of
a
pyrimidine compound (32) is reported to show the extent to
which heterocyclic nitrogen atoms decrease optical anisotropy
in these systems.
Experimental
The synthetic pathways to compounds 9, 10 (Scheme 1), 14, 16
(Scheme 2), 25±28 (Scheme 3) and 32 (Scheme 4) are shown
below. Compound
2 was converted into the alkyne 4
{Experimental details for compounds 3, 5, 9±11, 14±16, 18, 19, 21±28,
30±32, 35 and 36 are available as supplementart data. For direct
electronic access see http://www.rsc.org/suppdata/jm/b0/b001164i/
{Present address: Department of Chemistry, Williams Hall, Kent State
University, Kent, Ohio 44242-0001, USA.
(Scheme 1) via compound 3 by using the 2-methylbut-3-yn-2-
ol method;8±10 the alternative preparation of compound 4
(Scheme 2) used the more expensive reagent trimethylsilylace-
tylene.11 The lithium salt of 5 was acidi®ed more ef®ciently with
DOI: 10.1039/b001164i
J. Mater. Chem., 2000, 10, 1555±1563
This journal is # The Royal Society of Chemistry 2000
1555

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Scheme 2 Reagents: 2a, Me3SiCMCH, Et3N, Pd(PPh3)4, CuI; 2b, KOH,
MeOH; 2c, (i) n-BuLi, ZnCl2, (ii) Pd(PPh3)4; 2d, CSCl2, CaCO3.
Melting points of intermediates were determined using a
Gallenkamp melting-point apparatus (model MFB 59501CM).
Transition temperatures were determined using a Mettler FP52
heating stage and FP5 temperature control unit in conjunction
with an Olympus BH-2 polarising microscope, and the
transitions were con®rmed by using Differential Scanning
Calorimetry (Perkin-Elmer DSC7 and IBM data station).
Virtual TN-I values were determined from four binary mixtures
(from 5±30% m/m) in E7 (Merck Ltd., Poole, Dorset, UK; TN-
I~60 ³C); the values obtained for the mixtures were extra-
polated to 100% for the compound being examined using a
linear regression computer program to give a straight line of
best ®t and are accurate to ¡5 ³C, assuming ideal mixing
behaviour.
The optical anisotropies of the compounds (accurate to
0.001) were measured using an Abbe refractometer (model 60/
HR) at 589 nm (D1 sodium line) in conjunction with a Haake Q
silicone oil (Dow-Corning 200/10 CS) bath and Haake F3
temperature control unit.6 Three mixtures of each compound
were made up between 5 and 25% m/m (depending on the
solubility of the material) using an I eutectic mixture as the host
material. The refractive indices (n, and n^) of each mixture were
measured between 5 and 65 ³C at 10 ³C intervals and for each
measurement a 20 min period was allowed for the temperature
of the prisms of the refractometer to stabilise and for the
refractive index to become constant. The optical anisotropy
(Dn) values (~n,2n^) were plotted against temperature and
gave a straight line, which in all cases passed through the
individual values. The value for the optical anisotropies at
25 ³C for each mixture was taken and extrapolated to 100% of
the compound. The values of the optical anisotropies at 25 ³C
were measured in order to provide a direct indication of the
optical properties of the compounds at room temperature. To
gain an insight into the value of the different core groups and
differing chain lengths, it is necessary to compare compounds
at a ®xed reduced absolute temperature in order to take into
account the variation in TN-I transition temperatures for the
different compounds. An arbitrary reduced temperature (T/TN-
I) value of 0.7815 was chosen for this purpose (which
corresponds to a temperature of 21.6 ³C for the I compounds,
Scheme 1 Reagents: 1a, NaOEt, n-BuBr, EtOH (80 ³C); 1b,
HO(Me)2CCMCH, Pd(PPh3)4, CuI, i-Pr2NH; 1c, KOH, toluene; 1d,
(i) n-BuLi, hexanes, THF, (210 ³C), (ii) CO2(s), Et2O, (iii) glacial
AcOH; 1e, CS2, DCC, C5H5N, (210 ³C); 1f, DCC, DMAP, DCM.
glacial acetic acid than with mineral acid; the preparation of
compound 7 employed the method of Jochims12 and DCC
esteri®cation of the phenols13 gave compounds 9 and 10.
Attempts to couple compound 4 to haloaryl isothiocyanates to
give 16 and 32 were unsuccessful (we believe that the palladium
or copper catalysts are poisoned by coordination with the
sulfur atom in the isothiocyanate), but reactions with
haloarylamines (e.g. 13 and 30) were satisfactory. Compounds
25 and 27 were synthesised following the method reported for
compounds 26 and 28.14
1H NMR spectra were obtained using a JEOL GX
NM270FT NMR spectrometer with tetramethylsilane as
internal standard. IR spectra were recorded using a Perkin-
Elmer 783 spectrometer, and mass spectra using a Finnigan-
MAT 1020 GC-MS spectrometer. UV spectra were recorded
using a Philips PU8720 UV-VIS spectrometer with cyclohexane
as solvent (only the major absorption bands are presented).
Thin-layer chromatographic analyses were performed using
aluminium-backed silica gel plates (Merck 60 F254) and were
examined under UV light. Column chromatography was
performed under gravity using May and Baker Sorbsil C60
40±60H mm silica gel. The purity of all ®nal products was
determined by HPLC using a Merck-Hitachi HPLC chroma-
togram incorporating a D6000 interface, a D4000 UV detector
and an L6200A intelligent pump in conjunction with a
Commodore 286 data station. Gas chromatography was
carried out using a Perkin-Elmer 8320 capillary gas chromato-
graph with a QC2/BP1-1.0 SGE (12 m) capillary column. All
®nal compounds are w99.5% pure by HPLC and gave
satisfactory C, H and N analyses.
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Scheme 3 Reagents: 3a, (COCl)2, DMF; 3b, AlCl3; 3c, LiAlH4; 3d, HCO2H; 3e, CuCN.
which have a TN-I transition at 104 ³C, and is a reference value
used by DERA, Malvern, for many years).
(2H, d), 7.30 (2H, d). IR (®lm) nmax/cm21 820, 910, 960, 1090,
1160, 1270, 1490, 1590, 2220, 2860, 2920, 2940, 3360. MS m/z
248 (Mz), 233 (C14H17OS), 177 (C11H13S), 57 (C4H9), 43
(100%, C3H7).
4-Butylsulfanylphenylethyne 4
Method A. Prepared by a procedure similar to that reported
for compound 10 in ref. 18.
Yield 78%, bp 120±125 ³C at 0.2 mmHg. 1H NMR (CDCl3) d
0.97 (3H, t), 1.50 (2H, sext), 1.71 (2H, quint), 2.97 (2H, t), 3.13
(1H, s), 7.26 (2H, d), 7.42 (2H, d). IR (®lm) nmax/cm21 820,
1090, 1380, 1400, 1460, 1480, 1590, 2100, 2880, 2940, 3300. MS
m/z 190 (Mz), 134 (100%, C8H6S), 89 (C7H5), 74 (C6H2).
The polarisabilities (expressed in units of 10230 m3) and
order parameters were calculated as described in previous
papers4,6 (the error bars are ¡1.14 and ¡0.10, respectively)
using a combination of Vuks equation, the Haller plot,15 the
Lorentz±Lorenz equation and the Maier±Saupe expression16
for the polarisability along the principal molecular axes. The
density is assumed to be 1 g cm23 and the error in Da is
estimated to be ¡0.34 for the variations in molecular weight.
Compounds 2 and 7 were prepared as described in ref. 4, and
compounds 33 and 34 as described in ref. 17; compounds 6, 8,
12, 13, 17, 20 and 29 were obtained from Aldrich.
Method B. Prepared by a procedure similar to that reported
for compound 32 in ref. 17.
Yield 100%.
3-(4-Butylsulfanylphenyl)propiolic acid 5
Prepared by a procedure similar to that reported for compound
12 in ref. 18.
Yield 70%, mp 112±114 ³C. 1H NMR (CDCl3) d 0.59 (3H, t),
1.47 (2H, sext), 1.67 (2H, quint), 2.96 (2H, t), 7.24 (2H, d), 7.50
(2H, d), 10.54 (1H, s). IR (KBr) nmax/cm21 760, 820, 920, 1010,
1090, 1130, 1210, 1300, 1400, 1490, 1660, 2180, 2220, 2860,
2920, 2960. MS m/z 234 (Mz), 190 (C12H14S), 178 (100%,
C11H14S), 89 (C7H5).
4-Cyanophenyl 3-(4-butylsulfanylphenyl)propiolate 9
Prepared by a procedure similar to that reported for compound
7 in ref. 4.
Yield 65%. Transitions (³C) Cryst 78.4 (N 28) Iso liq. 1H
NMR (CDCl3) d 0.59 (3H, t), 1.50 (2H, sext), 1.54 (2H, quint),
2.59 (2H, t), 7.27 (2H, d), 7.34 (2H, d), 7.53 (2H, d), 7.72 (2H,
d). IR (KBr) nmax/cm21 840, 860, 930, 1080, 1140, 1170, 1200,
1290, 1490, 1500, 1580, 1600, 1720, 2200, 2220, 2860, 2920,
2960. lmax/nm (e/l mol21 cm21; cyclohexane) 197 (44 260), 230
(13 878), 332 (24 573). MS m/z 335 (Mz), 291 (C17H9NO2S),
217 (100%, C13H13OS), 165 (C10H13S). Calc. for C20H17NO2S:
C, 71.62; H, 5.11; N, 4.18. Found: C, 71.56; H, 4.95; N, 4.17%.
Scheme 4 Reagents: 4a, Br2, H2O; 4b, (i) n-BuLi, hexanes, ZnCl2, THF,
(210 ³C), (ii) Pd(PPh3)4, (re¯ux); 4c, CSCl2, CaCO3.
Optical properties and calculated parameters. At T/TN-
I~0.7815: n,~1.94, n^~1.57, Dn~0.37, Da~27.07, S~0.79.
At 25 ³C: n,~1.83, n^~1.54, Dn ~0.29, Da~25.32, S~0.67.
1-Butylsulfanyl-4-(3-hydroxy-3-methylbut-1-ynyl)benzene 3
Prepared by a procedure similar to that reported for compound
8 in ref. 18.
Yield 98%. 1H NMR (CDCl3) d 0.97 (3H, t), 1.44 (2H, sext),
1.62 (6H, s), 1.63 (2H, quint), 2.34 (1H, s), 2.91 (2H, t), 7.19
4-Isothiocyanatophenyl 3-(4-butylsulfanylphenyl)propiolate 10
Prepared by a procedure similar to that reported for compound
7 in ref. 4.
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Yield 64%. Transitions (³C) Cryst 75.7 (N 34) Iso liq. 1H
NMR (CDCl3) d 0.59 (3H, t), 1.50 (2H, sext), 1.54 (2H, quint),
2.59 (2H, t), 7.04 (2H, d), 7.05 (2H, d), 7.05 (2H, d), 7.51 (2H,
d). IR (KBr) nmax/cm21 820, 850, 930, 1080, 1140, 1160, 1190,
1290, 1490, 1580, 1710, 2020, 2200, 2220, 2860, 2920, 2960.
next step without puri®cation. Yield 23.16 g (quantitative). 1H
NMR (CDCl3) d 4.10 (2H, s), 7.15 (2H, d), 7.52 (2H, d). IR
(®lm) nmax/cm21 800, 850, 960, 1010, 1405, 1485, 1795, 2340,
2920. MS m/z 238,236 (Mz), 171,169 (100%), 91,89.
l
max/nm (e/l mol21 cm21
; cyclohexane) 204 (66 570), 222
Butylsulfanylbenzene 19
(38 961), 292 (21 814), 317 (28 421), 329 (33 036). MS m/z 367
(Mz), 217 (100%, C13H13OS), 161 (C9H5OS). Calc. for
C20H17NO2S2: C, 65.36; H, 4.66; N, 3.81. Found: C, 65.90;
H, 4.71; N, 3.78%.
Compound 19 was prepared in a similar way to that described
for the preparation of compound 2.
Yield 98%, bp 100±110 ³C at 12 mmHg. 1H NMR (CDCl3) d
0.94 (3H, t), 1.47 (2H, sext), 1.67 (2H, quint), 2.96 (2H, t), 7.36
(2H, dd), 7.47 (1H, tt), 7.61 (2H, dd).
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~2.01, n^~1.58, Dn~0.43, Da~38.59, S~0.69.
~
At 25 ³C: n,~1.90, n^~1.56, Dn~0.34, Da~36.37, S~0.60.
4-Butylsulfanyl-a-(4'-bromophenyl)acetophenone 21
Compound 18 (10.0 g, 0.042 mol) was added dropwise to a
stirred, cooled (0 ³C) mixture of compound 19 (7.60 g,
0.046 mol) and aluminium chloride (6.10 g, 0.046 mol) at
0 ³C. The reaction mixture was maintained under these
conditions for 1 h and was heated at 70 to 80 ³C for 2 h and
added to hydrochloric acid (18%, 50 cm3). The product was
extracted into dichloromethane (2675 cm3), washed with
water (75 cm3) and dried (MgSO4). The solvent was removed
in vacuo and the crude product was recrystallised from ethanol
to afford colourless needles which were dried in vacuo (P2O5).
1-Butylsulfanyl-4-(trimethylsilylethynyl)benzene 11
Prepared by a procedure similar to that reported for compound
31 in ref. 17.
1
Yield 69%. H NMR (CDCl3) d 0.17 (9H, s), 0.85 (3H, t),
1.37 (2H, sext), 1.56 (2H, quint), 2.84 (2H, t), 7.13 (2H, d), 7.30
(2H, d). IR (®lm) nmax/cm21 680, 760, 820, 840, 1010, 1090,
1220, 1250, 1490, 1595, 2160, 2930, 2970. MS m/z 262 (Mz),
247 (100%), 190, 115, 73.
1
Yield 9.74 g (64%), mp 136±137 ³C. H NMR (CDCl3) d 0.95
1-(4-Butylsulfanylphenyl)-2-(4-cyanophenyl)ethyne 14
(3H, t), 1.50 (2H, sext), 1.69 (2H, quint), 3.00 (2H, t), 4.18 (2H,
s), 7.14 (2H, d), 7.26 (2H, d), 7.45 (2H, d), 7.89 (2H, d). IR
(KBr) nmax/cm21 805, 815, 995, 1010, 1090, 1200, 1210, 1330,
1585, 1740, 2920, 2960. MS m/z 364,362 (Mz), 193 (100%),
137, 109.
Prepared by a procedure similar to that reported for compound
11 in ref. 19.
Yield 31%. Transitions (³C) Cryst 80.3 (N 53.3) Iso liq. H
1
NMR (CDC13) d 0.94 (3H, t), 1.47 (2H, sext), 1.67 (2H, quint),
2.96 (2H, t), 7.26 (2H, d), 7.43 (2H, d), 7.58 (2H, d), 7.64 (2H,
d). IR (KBr) nmax/cm21 825, 840, 1015, 1090, 1400, 1505, 1590,
2220, 2230, 2870, 2940, 2960. UV lmax/nm (e/l mol21 cm21
;
cyclohexane) 208 (16 600), 238 (16 800), 326 (22 900). MS m/z
4-Pentyl-a-(4'-bromophenyl)acetophenone 22
Compound 22 was prepared in a similar way to that described
for the preparation of compound 21.
Yield 65%, mp 125±126 ³C. 1H NMR (CDCl3) d 0.90 (3H, t),
1.34 (4H, m), 1.63 (2H, quint), 2.64 (2H, t), 4.23 (2H, s), 7.13
291 (Mz, 100%), 235, 190, 57, 45.
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~1.90, n^~1.56, Dn~0.34, Da~27.14, S~0.62.
~
(2H, d), 7.30 (2H, d), 7.44 (2H, d), 7.91 (2H, d). IR (KBr) nmax
/
cm21 805, 1220, 1330, 1410, 1485, 1605, 1675, 2860, 2930, 2950.
At 25 ³C: n,~1.83, n^~1.54, Dn~0.29, Da~26.10, S~0.56.
MS m/z 346,344 (Mz), 175,177 (100%), 120,118.
1-(4-Aminophenyl)-2-(4-butylsulfanylphenyl)ethyne 15
2-(4-Bromophenyl)-1-(4-butylsulfanylphenyl)ethanol 23
Compound 15 was prepared in a similar way to that described
for the preparation of compound 14.
Yield 67%.
A solution of compound 21 (6.0 g, 0.017 mol) in dry THF
(75 cm3) was added dropwise to a stirred mixture of lithium
aluminium hydride (0.54 g, 0.014 mol) in dry THF (100 cm3)
under dry nitrogen at room temperature. The reaction mixture
was stirred at 55 ³C for 3 h and the excess of the reducing agent
was destroyed by the careful addition of water (18 cm3)
followed by concentrated hydrochloric acid (11 cm3). The
product was extracted into ether (26200 cm3), washed with
water (400 cm3) and dried (MgSO4). The solvent was removed
in vacuo to afford a white solid which was used in the next step
without puri®cation. Yield 6.70 g (quantitative), mp 58±
1-(4-Butylsulfanylphenyl)-2-(4-isothiocyanatophenyl)ethyne 16
Prepared by a procedure similar to that reported for compound
10 in ref. 3.
Yield 42%. Transitions (³C) Cryst 84.8 B 85.4 [N 65] Iso liq.
1H NMR (CDCl3) d 0.94 (3H, t), 1.47 (2H, sext), 1.66 (2H,
quint), 2.95 (2H, t), 7.20 (2H, d), 7.25 (2H, d), 7.41 (2H, d), 7.48
(2H, d). IR (KBr) nmax/cm21 820, 840, 1090, 1400, 1500, 1600,
2110, 2190, 2920, 2960. UV lmax/nm (e/l mol21 cm21; cyclo-
hexane) 202 (46 000), 219 (30 320), 329 (49 860), 349 (37 780).
MS m/z 323 (Mz, 100%), 266, 228, 207, 165.
1
59 ³C. H NMR (CDCl3) d 0.92 (3H, t), 1.46 (2H, sext), 1.62
(2H, quint), 1.98 (1H, s), 2.92 (2H, t), 2.93 (2H, d), 4.81 (1H, t),
7.02 (2H, d), 7.23 (2H, d), 7.28 (2H, d), 7.40 (2H, d). IR (KBr)
max/cm21 800, 830, 1010, 1050, 1070, 1400, 1480, 2920, 2950,
n
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~2.08, n^~1.58, Dn~0.50, Da~45.16, S~0.61.
~
3100±3600. MS m/z 366,364 (Mz), 195,193 (100%), 139.
At 25 ³C: n,~2.05, n^~1.56, Dn~0.49, Da~44.18, S~0.61.
2-(4-Bromophenyl)-1-(4-pentylphenyl)ethanol 24
4-Bromophenylacetyl chloride 18
Compound 24 was prepared in a similar way to that described
for the preparation of compound 23.
Yield quantitative, mp 59±60 ³C. H NMR (CDC13) d 0.91
(3H, t), 1.35 (4H, m), 1.63 (2H, quint), 1.94 (1H, s), 2.61 (2H, t),
2.97 (2H, d), 4.84 (1H, t), 7.06 (2H, d), 7.16 (2H, d), 7.25 (2H,
d), 7.41 (2H, d). IR (KBr) nmax/cm21 815, 1010, 1070, 1400,
1485, 1510, 2920, 2950, 3120±3600. MS m/z 348, 346 (Mz),
177, 175 (100%), 91.
A solution of oxalyl chloride (23.62 g, 0.186 mol) in dry
benzene (50 cm3) was added dropwise to a stirred solution of 4-
bromophenylacetic acid (17) (20.0 g, 0.093 mol) and dry
dimethylformamide (35 drops) in dry benzene (500 cm3) at
room temperature. The mixture was stirred at room tempera-
ture overnight and the excess of oxalyl chloride and benzene
was removed in vacuo to give a red liquid which was used in the
1
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(E)-4-Bromo-4'-butylsulfanylstilbene 25
2-Amino-5-bromopyrimidine 30
A solution of compound 23 (6.48 g, 0.018 mol) in formic acid
(98%, 100 cm3) was heated under re¯ux with stirring for 5 h
(GLC and TLC analysis revealed a complete reaction). The
cooled reaction mixture was added to water (200 cm3) and the
product was ®ltered off and washed well with water. The
product was recrystallised from ethanol±ethyl acctate 1 : 1 to
afford colourless crystals which were dried in vacuo (P2O5).
Yield 4.02 g (64%). Transitions (³C) Cryst 149.5 [N 134] Iso liq.
1H NMR (CDC13) d 0.93 (3H, t), 1.46 (2H, sext), 1.65 (2H,
quint), 2.94 (2H, t), 6.98 (1H, d), 7.06 (1H, d), 7.29 (2H, d), 7.36
(2H, d), 7.41 (2H, d), 7.47 (2H, d). IR (KBr) nmax/cm21 525,
820, 970, l005, 1070, 1405, 1490, l590, 2920, 2960. lmax/nm (e/
l mol21 cm21; cyclohexane) 206 (19 930), 235 (11 760), 338
(36 400). MS m/z 348,346 (Mz,100%), 292,290, 177.
Bromine (18.6 g, 0.116 mol) was added dropwise to a stirred,
cooled solution of compound 29 (10.0 g, 0.105 mol) in water
(100 cm3) at room temperature. The mixture was stirred at
room temperature for 2 h; the resulting precipitate was ®ltered
off and recrystallised (ethanol) to give yellow/grey plates that
were dried in vacuo (P2O5). Yield 7.6 g (42%), mp 246±248 ³C
1
(lit.21 242±244 ³C). H NMR (DMSO-d6) d 7.55 (2H, s), 8.60
(2H, s). IR (KBr) nmax/cm21 680, 880, 1360, 1685, 3100. MS m/z
175,173 (Mz), 94 (100%, C4H4N3).
1-(2-Aminopyrimidin-5-yl)-2-(4-butylsulfanylphenyl)ethyne 31
Compound 31 was prepared in a similar way to that described
for the preparation of compound 14.
Yield 59%.
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~1.87, n^~1.53, Dn~0.34, Da~36.27, S~0.58.
At 25 ³C: n,~1.93, n^~1.54, Dn~0.40, Da~35.85, S~0.66.
~
1-(4-Butylsulfanylphenyl)-2-(2-isothiocyanatopyrimidin-5-
yl)ethyne 32
Prepared by a procedure similar to that reported for compound
10 in ref. 3.
(E)-4-Bromo-4'-pentylstilbene 26
Yield 43%. Transitions (³C) Cryst 107.3 [N 3] Iso liq. 1H
NMR (CDCl3) d 0.95 (3H, t), 1.48 (2H, sext), 1.68 (2H, quint),
2.98 (2H, t), 7.28 (2H, d), 7.45 (2H, d), 8.74 (2H, s). IR (KBr)
Compound 26 was prepared in a similar way to that described
for the preparation of compound 25.
max/cm21 760, 810, 1090, 1170, 1400, 1480, 1540, 1580, 2200,
Yield 68%. Transitions (³C) Cryst 149.2 B 151.1 [N 110] Iso
liq. H NMR (CDC13) d 0.89 (3H, t), 1.32 (4H, m), 1.62 (2H,
n
2920, 2960.
1
l ; cyclohexane) 194
max/nm (e/l mol21 cm21
quint), 2.61 (2H, t), 6.98 (1H, d), 7.08 (1H, d), 7.17 (2H, d), 7.36
(2H, d), 7.42 (2H, d), 7.47 (2H, d). IR (KBr) nmax/cm21 530,
820, 970, 1070, 1505, 1630, 1740, 2850, 2920. lmax/nm (e/
l mol21 cm21; cyclohexane) 207 (19 180), 232 (14 685), 306
(34 040), 320 (34 310). MS m/z 330,328 (Mz,100%), 272,270,
191,189.
(32 597), 270 (19 757), 327 (29 635), 349 (26 728). MS m/z 325
(Mz,100%), 268 (C13H6N3S2), 236 (C13H6N3S), 57 (C4H9).
Calc. for C17H15N3S2: C, 62.74; H, 4.65; N, 12.91. Found: C,
62.68; H, 4.67; N, 12.86%.
Optical properties and calculated parameters. At T/TN-
I~0.7815: n,~1.90, n^~1.62, Dn~0.28, Da~13.01, S~0.59.
At 25 ³C: n,~1.76, n^~1.53, Dn~0.23, Da~11.35, S~0.97.
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~1.83, n^~1.53, Dn~0.30, Da~25.90, S~0.68.
~
At 25 ³C: n,~1.82, n^~1.53, Dn~0.29, Da~24.75, S~0.67.
3-(4-Butoxyphenyl)propiolic acid 35
Prepared by a procedure similar to that reported for compound
5.
(E)-4-Butylsulfanyl-4'-cyanostilbene 27
Yield 70%, mp 130±131 ³C. 1H NMR (CDCl3) d 0.98 (3H, t),
1.42±1.57 (2H, m), 1.72±1.84 (2H, m), 4.00 (2H, t), 6.89 (2H, d),
Prepared by a procedure similar to that reported for compound
19 in ref. 19.
Yield 36%. Transitions (³C) Cryst 96.7 (N 90.7) Iso liq. H
NMR (CDC13) d 0.94 (3H, t), 1.47 (2H, sext), 1.67 (2H, quint),
2.96 (2H, t), 7.04 (1H, d), 7.17 (1H, d), 7.30 (2H, d), 7.44 (2H,
d), 7.56 (2H, d), 7.63 (2H, d). IR (KBr) nmax/cm21 825, 970,
1090, 1485, 1635, 1650, 2220, 2330, 2920, 2960. lmax/nm (e/
l mol21 cm21; cyclohexane) 206 (21 660), 243 (11 120), 346
(35 280). MS m/z 293 (Mz,100%), 237, 204, 190, 116.
1
7.55 (2H, d), the acid proton was not detected. IR (KBr) nmax
/
cm21 830, 1170, 1210, 1255, 1600, 1675, 2195, 2400±3400. MS
m/z 218 (Mz), 162, 134, 118, 58 (100%).
4-Cyanophenyl 3-(4-butoxyphenyl)propiolate 36
Compound 36 was prepared in a similar way to that described
for the preparation of compound 9.
1
Yield 38%. Transitions (³C) Cryst 80.4 (N 69.6) Iso liq. H
Optical properties and calculated parameters. At T/TN-I
0.7815: n,~2.00, n^~1.56, Dn~0.44, Da~35.72, S~0.64.
At 25 ³C: n,~1.97, n^~1.55, Dn~0.42, Da~33.53, S~0.64.
~
NMR (CDCl3) d 0.99 (3H, t), 1.42±1.58 (2H, m), 1.73±1.85
(2H, m), 4.01 (2H, t), 6.91 (2H, d), 7.34 (2H, d), 7.58 (2H, d),
7.73 (2H, d). IR (KBr) nmax/cm21 830, 1140, 1210, 1260, 1510,
1607, 1722, 2203 cm21. MS m/z 319 (Mz), 201 (100%), 145, 58.
(E)-4-Cyano-4'-pentylstilbene 28
Compound 28 was prepared in a similar way to that described
for the preparation of compound 27.
Results and discussion
Yield 93%. Transitions (³C) Cryst 57.1 N 102.8 Iso liq (lit.20
Cryst 55.1 N 101.0 Iso liq). H NMR (CDC13) d 0.90 (3H, t),
Transition temperatures
1
The transition temperatures of the new compounds and related
compounds are given in Table 1. Only two of the compounds in
Table 1 (compounds 28 and 37) show enantiotropic nematic
phases and, of the others, seven show monotropic phases and
six of the values are from virtual determinations (see
Experimental section). Compounds 37±40 are the parent
compounds which provide the reference values on which to
judge the effect of using various linking units. For the
compounds with C4H9S- and -NCS terminal groups relative
to the parent biphenyl compound 40 (TN-I 44 ³C), the -COO-
(compound 42) and -CMC- (compound 16) linking groups give
1.32 (4H, m), 1.62 (2H, quint), 2.62 (2H, t), 7.04 (1H, d), 7.20
(2H, d), 7.21 (1H, d), 7.56 (2H, d), 7.63 (2H, d), 7.45 (2H, d). IR
(KBr) nmax/cm21 830, 970, 1070, 1410, 1505, 1595, 2220, 2860,
2920. lmax/nm (e/l mol21 cm21; cyclohexane) 208 (13 910), 236
(13 550), 292 (15 550). MS m/z 275 (Mz), 218 (100%), 203, 189,
91.
Optical properties and calculated parameters. At T/TN-
I~0.7815: n,~1.93, n^~1.54, Dn~0.39, Da~24.02, S~0.79.
At 25 ³C: n,~1.87, n^~1.52, Dn~0.35, Da~22.13, S~0.75.
J. Mater. Chem., 2000, 10, 1555±1563
1559

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Table 1 Transition temperatures (³C) of compounds with 4-butylsulfanylphenyl and 4-cyano- or 4-isothiocyanato-phenyl units connected by
various linking groups, and related reference compoundsa
Transition temperature/³C
Compound
Structure
Cryst
E
B
N
Iso liq
9
.
78.4
75.7
80.3
84.8
149.5
149.2
96.7
57.1
107.3
24.0
53.5
64.8
78.4
82.2
75.2
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
74.5
Ð
Ð
Ð
Ð
Ð
Ð
(.
28)
.
10
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
Ð
.
Ð
Ð
.
Ð
(.
(.
[.
[.
[.
(.
.
34)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
14
Ð
53.3)
65]
16
85.4
Ð
25
Ð
.
134]
110]
90.7)
102.8
3]
26
151.1
Ð
27
Ð
Ð
Ð
Ð
Ð
Ð
.
28
Ð
32
Ð
[.
.
3724
383
395,6
403
414
424
Ð
35.3
43]
Ð
[.
(.
[.
(.
(.
Ð
Ð
Ð
Ð
Ð
35.5)
44]
78.6
Ð
Ð
Ð
56.3)
54.1)
Ð
a( ) denotes a monotropic transition. [ ] denotes a virtual transition determined from mixtures in E7 (see text).
higher TN-I values (54.1 and 65 ³C respectively), but the
combined -CMC-COO- link (compound 10) gives a lower value
(34 ³C). The parallel result is seen for the C4H9S- and -CN
compounds, where the parent biphenyl (39) value of 35.5 ³C is
increased to 56.3 and 53.3 ³C by the -COO- (41) and -CMC- (14)
links respectively, but the -CMC-COO- link (9) causes a lower
value of 28 ³C. The -CHLCH- link is clearly the most effective
unit with respect to mesogenicity and raises TN-I values to 90.7
(27) from 35.5 ³C (39) and to 102.8 (28) from 35.3 ³C (37).
One would expect that the presence of the ethynyl and ester
linking groups together would give a longer and more
polarisable region in the molecule, and that this should lead
to enhanced TN-I values compared to those for the compounds
with either unit used separately or for the compounds with a
single bond as the link. It is therefore surprising to ®nd that the
propiolates 9 and 10 have lower TN-I values than the
compounds which have the two linking groups present
independently (i.e. the benzoates 41 and 42, and tolanes 14
and 16 respectively) and the values are even lower than for the
parent compounds without any linking group (39 and 40
respectively). However, a butylsulfanyl terminal group is not
commonly used in the design of nematic materials and
examples for this terminal unit and other linking groups are
not easy to ®nd. In order to check the ranking of various
linking groups in their ability to promote nematogenicity in
more usual systems, we therefore prepared the butoxy
compound 36; the route used is shown in Scheme 5 and the
method of preparation illustrates an additional way of
obtaining ethynyl compounds to those shown in Schemes 1
and 2. The preparation of compound 36 completes a more
extensive range of linking groups and the TN-I values for this
series of 4-butoxyphenyl/4-cyanophenyl compounds are given
1560
J. Mater. Chem., 2000, 10, 1555±1563

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in Table 2. The value for compound 43 provides the reference
point and all the single linking units of ethenyl (44), ethynyl
(45) and carbonyloxy (46) give increases in TN-I values of 64.5,
35.5 and 11.6 ³C respectively. With two ethenyl (47) or two
ethynyl (48) groups the TN-I values are signi®cantly enhanced
from the values for compounds with only one unit present; in
fact the presence of the second ethynyl group gives a bigger
increase (82.9 ³C) than when the ®rst unit is introduced
(35.5 ³C), but the second ethenyl group gives a smaller increase
(46 ³C) than for the ®rst double bond (64.5 ³C). Comparing the
values for the compound with the ethenylcarboxy unit (49) and
compound 44 shows that adding the carboxy group to the
double bond gives a further small increase of 4.5 ³C, but for the
comparison of the ethynylcarboxy compound (36) and
compound 45 there is a decrease in TN-I value of 41.4 ³C. The
clear conclusions to be drawn from these comparisons are that
the carbonyloxy linking group is inferior to the ethenyl and
ethynyl groups in promoting mesogenicity and the combina-
tion of ethynyl and carbonyloxy units is inferior to either group
used separately and inferior to the parent system without any
linking group; these observations con®rm the comments made
above for the butylsulfanyl compounds in Table 1.
At ®rst sight it appears strange that propiolate systems have
lower clearing points than the parent systems with a single-
bond linkage. This reduced level of mesophase stability can be
attributed to the fact that there is not a free ¯ow of electrons
through the alkynic unit, as might be expected, and it can be
justi®ed in the following way. In aryl systems, the most ef®cient
simple linking group for promoting mesogenicity is the
-CHLCH- unit, with the -CMC- and -CO2- units having similar
but inferior ef®ciencies (see the transition temperatures for
compounds 44, 45, 46 and 27, 14, 41). This ef®ciency order can
be rationalised on the following basis: (a) the -CHLCH- unit is
composed of p-orbitals on sp2-hybridised carbon atoms, which
are similar in size and nature to the neighbouring carbon atoms
in the aryl rings, and so allow good conjugation and
polarisability, (b) the -CMC- unit is composed of smaller p-
orbitals on sp-hybridised carbon atoms which retain the
electron density more effectively and reduce the extent of
conjugation and polarisability, (c) the -CO2- unit allows
ef®cient delocalisation in the aryl-carbonyl and the oxygen-
aryl regions but also has an opposing effect from the phenol
oxygen into the carbonyl group. For the -CHLCH-CO2-
linkage, the high ef®ciency of the -CHLCH- unit is only
improved slightly by the low ef®ciency of the ester group which
is still linked to sp2-hybridised carbon atoms (compounds 49
and 44). This situation contrasts with that for -CMC-CO2- in
which the ester group is now acting on a triple bond composed
of sp-hybridised carbon atoms; the more compact orbitals of
the triple bond resist electron withdrawal by the carbonyl
group which therefore remains distinctly localised as C(dz)±
Table 2 Values of TN-I for compounds with various linking groups (X)
in the following structurea
Compound
Linking group X
TN-I values/³C
4325
4426
4527
4628
4729
4830
4931
36
Single bond
(75.5)
140
111
87.1
186
193.9
144.5
(69.6)
trans -CHLCH-
-CMC-
-CO2-
trans,trans -CHLCH-CHLCH-
-CMC-CMC-
trans -CHLCH-CO2-
-CMC-CO2-
a( ) denotes a monotropic transition.
O(d2). The consequences are that this situation would
encourage a greater back-¯ow of electron density from the
phenol oxygen atom (diminishing its delocalisation into the
aryl ring) and would give a hard-core dipole in the ester group
leading to greater lateral repulsion; both of these effects
diminish the ef®ciency of the propiolate linking group.
The comparison of pentyl and butylsulfanyl groups reveals
erratic differences in TN-I values depending on the nature of the
rest of the molecule. For the simple cyano and isothiocyanato
biphenyls (compare compounds 37, 38 with 39, 40 respec-
tively), the pentyl and butylsulfanyl groups give very similar
TN-I values which suggests that the effect of the increased
polarisability of the butylsulfanyl group may be fortuitously
balanced by its increase in size. However, the pentyl compound
gives higher TN-I values than butylsulfanyl for the ethenyl
linked cyano compounds (28 and 27) and lower values for the
ethenyl linked bromo compounds (26 and 25). Two other
notable points on the values of TN-I are that those of the
terminal bromo compounds (25 and 26) are higher than those
of the terminal cyano counterparts (27 and 28 respectively),
and the pyrimidine compound 32 has a much lower TN-I value
than the phenyl analogue 16.
With few exceptions, any modi®cation to molecular
structure by introducing a linking group leads to a higher
melting point than that of the parent compound. For example,
for the C5H11-/-CN terminal groups, the melting point of the
parent compound (37) increases from 24.0 to 57.1 ³C (28), for
the C4H9S-/-CN terminal groups the increases are from 64.8
(compound 39) to 78.4 (9), 80.3 (14), 82.2 (41), and 96.7 ³C (27),
and for the C4H9S-/-NCS terminal groups the changes are
relatively small, from 78.4 (compound 40) to 84.8 (16), 75.2 (42)
and 75.7 ³C (10). A plausible contributory factor to the changes
in melting points is that the parent biphenyl systems are
signi®cantly non-planar because of inter-annular twisting, but
the twisting is diminished by the presence of linking groups
which allow a greater planarity, better packing and
a
consequential higher melting point. The overall result of
melting point and clearing point changes is that all the
compounds with linking groups suffer from relatively high
melting points without offering the bene®ts of particularly
good nematogenicity.
Refractive indices, optical anisotropies, polarisabilities and order
parameters
The optical properties and related parameters for the novel
compounds and known reference compounds are given in
Table 3. For a fair comparison of structural changes on the
optical properties, the values at a constant reduced temperature
should be used (in this case T/TN-I~0.7815 is the value chosen;
see Experimental section), but the values at 25 ³C are also
reported because in practical terms it is important to have the
Scheme 5 Reagents: 5a, (i) HCMCLi?H2NCH2CH2NH2, ZnCl2, THF,
(ii) compound 33, Pd(PPh3)4, THF; 5b, (i) n-BuLi, hexanes, THF,
(210 ³C), (ii) CO2(s), Et2O, (iii) glacial AcOH; 5c, DCC, DMAP,
DCM, 4-hydroxybenzonitrile (8).
J. Mater. Chem., 2000, 10, 1555±1563
1561

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Table 3 Refractive indices, optical anisotropies, molecular polarisabilities and order parameters for compounds with 4-butylsulfanylphenyl and 4-
cyano- or 4-isothiocyanato-phenyl units connected by various linking groups, and related reference compounds, at a reduced temperature (RedT) of
T/TN-I~0.7815 (®rst line) and at 25 ³C (second line)
Compound
Structure
n,
n^
Dn
Da/10230 m3
S
9
RedT
25 ³C
1.94
1.83
1.57
1.54
0.37
0.29
27.07
25.32
0.79
0.67
10
14
16
25
26
27
28
32
3724
RedT
25 ³C
2.01
1.90
1.58
1.56
0.43
0.34
38.59
36.37
0.69
0.60
RedT
25 ³C
1.90
1.83
1.56
1.54
0.34
0.29
27.14
26.10
0.62
0.56
RedT
25 ³C
2.08
2.05
1.58
1.56
0.50
0.49
45.16
44.18
0.61
0.61
RedT
25 ³C
1.87
1.93
1.53
1.54
0.34
0.40
36.27
35.85
0.58
0.66
RedT
25 ³C
1.83
1.82
1.53
1.53
0.30
0.29
25.90
24.75
0.68
0.67
RedT
25 ³C
2.00
1.97
1.56
1.55
0.44
0.42
35.72
33.53
0.64
0.64
RedT
25 ³C
1.93
1.87
1.54
1.52
0.39
0.35
24.02
22.13
0.79
0.75
RedT
25 ³C
1.90
1.76
1.62
1.53
0.28
0.23
13.01
11.35
0.59
0.57
RedT
25 ³C
1.77
1.74
1.72
1.52
1.50
1.53
0.25
0.24
0.19
15.37
14.52
23.30
0.71
0.67
0.55
25 ³C7,a
383
395,6
403
414
424
RedT
25 ³C
1.96
1.86
1.56
1.53
0.41
0.33
29.06
27.29
0.68
0.57
RedT
25 ³C
1.86
1.83
1.56
1.55
0.30
0.28
20.83
19.50
0.67
0.63
RedT
25 ³C
1.96
1.91
1.56
1.56
0.40
0.36
32.06
30.23
0.63
0.59
RedT
25 ³C
1.80
1.77
1.54
1.52
0.26
0.25
20.05
19.58
0.69
0.68
RedT
25 ³C
1.87
1.84
1.55
1.54
0.32
0.30
28.39
27.08
0.67
0.63
aNeat material.
information for a temperature at which the materials may be
used; unless otherwise stated, the discussion below concentrates
on the reduced temperature values.
line. One possible effect of heterocyclic nitrogen atom
substitution is neatly illustrated by comparison of the refractive
indices for compounds 16 and 32. It appears that the lone pairs
of electrons on the nitrogen atoms of 32 give a signi®cant
increase in n^ but greatly reduce n, probably because the more
electronegative nitrogen atoms reduce the polarisability of the
molecule by localizing the electron density, and the value of
13.01 is by far the lowest value for Da given in Table 3.
Heterocyclic nitrogen centres at the 2-position in a biphenyl, as
in 2-phenylpyridine or 2-phenylpyrimidine systems, increase Dn
values22 and it may be that this type of nitrogen substitution
leads to less interannular twisting and so allows the existence of
a more planar core unit,23 with consequentially increased
polarisability. The type of nitrogen substitution in compound
32 does not affect the planarity of the core in this way and the
factors referred to above dominate in this situation.
The molecular short axes for all the compounds presented
have similar dimensions and a general analysis of the results in
Table 3 shows that for all the benzene-based systems with a
butylsulfanyl terminal group, as a consequence of this similar
breadth, the values of n^ are substantially constant (1.53±1.58).
However, there is a much greater variation in the n, values,
which range from 1.80±2.08 in response to alterations to the
nature of the p-system along the molecular axis, and the
changes in n, values are therefore the major factor in
determining Dn values; a plot of Dn values against n, shows a
close correlation between the two and the only point clearly off
the line is for the pyrimidine compound 32 (see Fig. 1). There is
also a close relationship between the anisotropy of molecular
polarisability (Da) and n,, but in this case compounds 32
(heterocyclic) and 25 (terminal bromo) lie off the correlation
Using 39 as the reference point for terminal cyano
compounds, the incorporation of a linking unit between the
1562
J. Mater. Chem., 2000, 10, 1555±1563

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non-heterocyclic cyano and isothiocyanato compounds, with
the values for -NCS terminal groups being consistently slightly
lower than those for the -CN counterparts.
Acknowledgements
The work reported here was supported by the Defence
Evaluation Research Agency (Malvern) and is published by
permission of the Director, HMSO. We thank Mrs
B. Worthington, Dr D. F. Ewing, Mr R. Knight and Mr
A. D. Roberts for spectroscopic measurements. #British
Crown Copyright 1999/DERA.
References
1
L. Domash, P. Levin, J. Ahn, J. Kumar and S. Tripathy, in Lower-
dimensional Systems and Molecular Electronics, ed. R. M. Metzger,
P. Day and G. C. Papavassiliou, Plenum Press, New York, 1991,
p. 579.
Fig. 1 A plot of Dn values against n, values (at T/TN-I~0.7815) for the
compounds listed in Table 2.
phenyl rings affects the optical anisotropies in the following
way; the reference value of 0.30 for compound 39 falls for the
ester link (0.26 for 41), increases slightly for the triple bond
(0.34 for 14) and increases further for the propiolate linking
group (0.37 for 9). None of these changes are large but the
highest value of all arises for the compound with an ethenyl
link (27) which gives a Dn value of 0.44. A similar, but not
identical, sequence of changes occurs for the terminal
isothiocyanato compounds; in this case the reference parent
compound is 40 (Dn~0.40) and the ester link gives a reduction
to 0.32 (42), the triple bond gives the greatest increase to 0.50
(16) and the propiolate link gives a more modest increase to
0.43 (10). The ethenyl compound (27) has a higher polarisa-
bility and higher birefringence than the ethynyl compound (14),
probably because the sp-hybridised carbon atoms of the alkyne
more effectively localise the p-electrons in the linking group
than do the sp2-hybridised carbon atoms of the alkene.
The values given in Table 3 for compound 37 from
measurements on the neat material at 25 ³C7 differ slightly
from the values we report which are obtained by extrapolation
from mixtures in a nematic host. The major differences are seen
for the Da and S values and it seems likely that extrapolation of
measurements from an `ordering' nematic host lead to higher
values of order parameters than those from the neat material
although the values of S obtained in this work are internally
consistent.
2
P. N. Pasad, in Lower-dimensional Systems and Molecular
Electronics, ed. R. M. Metzger, P. Day and G. C. Papavassiliou,
Plenum Press, New York, 1991, p. 563.
M. Hird, A. J. Seed, K. J. Toyne, J. W. Goodby, G. W. Gray and
D. G. McDonnell, J. Mater. Chem., 1993, 3, 851.
A. J. Seed, K. J. Toyne, J. W. Goodby and D. G. McDonnell,
J. Mater. Chem., 1995, 5, 1.
A. J. Seed, K. J. Toyne and J. W. Goodby, J. Mater. Chem., 1995,
5, 653.
A. J. Seed, K. J. Toyne and J. W. Goodby, J. Mater. Chem., 1995,
5, 2201.
R. G. Horn, J. Phys., 1978, 39, 105.
3
4
5
6
7
8
S. J. Havens and P. M. Hergenrother, J. Org. Chem., 1985, 50,
1763.
C. Huynh and G. Linstrumelle, Tetrahedron, 1988, 44, 6337.
9
10 K. Seto, H. Shimojitosho, H. Imazaki, H. Matsubara and
S. Takahashi, Bull. Chem. Soc. Jpn., 1990, 63, 1020.
11 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara,
Synthesis, 1980, 627.
12 J. C. Jochims, Chem. Ber., 1968, 101, 1746.
13 A. Hassner and V. Alexanian, Tetrahedron Lett., 1978, 19, 4475.
14 G. W. Gray and A. Mosley, Mol. Cryst. Liq. Cryst., 1976, 37, 213.
15 I. Haller, Prog. Solid-State Chem., 1975, 10, 103.
16 W. Maier and A. Saupe, Z. Naturforsch., 1959, 14a, 882.
17 M. Hird, K. J. Toyne and G. W. Gray, Liq. Cryst., 1993, 14, 741.
18 C. J. Booth, D. A. Dunmur, J. W. Goodby, J. S. Kang and
K. J. Toyne, J. Mater. Chem., 1994, 4, 747.
19 M. Hird, K. J. Toyne, G. W. Gray, S. E. Day and
D. G. McDonnell, Liq. Cryst., 1993, 15, 123.
20 R. J. Cox and N. J. Clecak, Mol. Cryst. Liq. Cryst., 1976, 37, 263.
21 J. P. English, J. H. Clark, J. W. Clapp, D. Seeger and R. H. Ebel,
J. Am. Chem. Soc., 1946, 68, 453.
22 D. Coates, J. Mater. Chem., 1995, 5, 2063.
23 V. Barone, L. Commisso, F. Lelj and N. Russo, Tetrahedron, 1985,
41, 1915.
24 A. J. Seed, PhD Thesis, The University of Hull, Hull, UK, 1994.
25 G. W. Gray, K. J. Harrison and J. A. Nash, Pramana Suppl. 1,
1975, 381.
26 J.-C. Dubois, A. Zann and H. T. Nguyen, C. R. Seances Acad. Sci.
Ser. C, 1977, 284, 137.
27 P. Adomenas, V. Butkus, J. Daugvila, J. Dienyte and
D. Girdziunaite, Adv. Liq. Cryst. Res. Appl., 1980, 1029.
28 I. H. Ibrahim, H. Paulus, M. Mokhles and W. Haase, Mol. Cryst.
Liq. Cryst., 1995, 258, 185.
This systematic study of the effect of various common
linking groups on mesogenicity and optical parameters allows
some general observations to be made, as follows:
(a) modest increases in TN-I values are achieved by
introducing -COO- or -CMC- linking groups, but the -CMC-
COO- unit gives a reduction in TN-I
;
(b) any of the three linking groups between phenyl rings
generally increases melting point;
(c) the values of n^ change only slightly as the linking unit is
altered but the n, values, and consequently the Dn values, are
affected more markedly;
(d) the ester linkage decreases, but the ethynyl, ethenyl or
propiolate links increase, the Dn values (the most effective unit
is the ethenyl link);
(e) heterocyclic nitrogen atoms decrease n, and increase n^ so
that Dn is much reduced;
(f) order parameter values lie in the range 0.61±0.79 for the
29 R. Brettle, D. A. Dunmur, N. J. Hindley and C. M. Marson,
J. Chem. Soc., Chem. Commun., 1992, 410.
30 B. Grant, N. J. Clecak and R. J. Cox, Mol. Cryst. Liq. Cryst.,
1979, 51, 209.
31 V. V. Titov, E. I. Kovshev, A. I. Pavluchenko, V. T. Lazareva and
M. F. Grebenkin, J. Phys. (Paris), 1975, Suppl. 36 C1, 387.
J. Mater. Chem., 2000, 10, 1555±1563
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