A new disc-shaped mesogenic compound with olefinic linkage derived from triphenylamine: synthesis, mesogenic behavior and fluorescence properties

Article abstract of DOI:10.1016/j.tetlet.2009.09.140

A new disc-like triphenylamine containing mesogenic compound has been synthesized by the implementation of the Heck and ring-closing metathesis-based reactions in good yield. The designed and synthesized compound showed rectangular columnar mesophase and this is the first report of liquid crystalline phase of the triphenylamine-based compound with an olefinic linkage. The disc-shaped compound exhibited excellent fluorescence properties.

Full text of DOI:10.1016/j.tetlet.2009.09.140

Tetrahedron Letters 50 (2009) 6901–6905
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new disc-shaped mesogenic compound with olefinic linkage derived
from triphenylamine: synthesis, mesogenic behavior and fluorescence
properties
*
Krishna C. Majumdar , Buddhadeb Chattopadhyay, Pranab Kumar Shyam, Nilasish Pal
Department of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2009
Revised 22 September 2009
Accepted 24 September 2009
Available online 27 September 2009
A new disc-like triphenylamine containing mesogenic compound has been synthesized by the implemen-
tation of the Heck and ring-closing metathesis-based reactions in good yield. The designed and synthe-
sized compound showed rectangular columnar mesophase and this is the first report of liquid crystalline
phase of the triphenylamine-based compound with an olefinic linkage. The disc-shaped compound
exhibited excellent fluorescence properties.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Heck reaction
Triphenylamine
Discotic liquid crystal
Rectangular columnar phase
Liquid crystalline (LC) phases represent a fascinating state of
soft matter, combining order and mobility on a molecular and
supramolecular level. Molecular shape determines to a large extent
the type of mesophase observed for thermotropic liquid crystals.
For example, in general, disc-like molecules self-organize to give
columnar mesophases. The first well-defined examples of disc-
shaped (discotic) liquid crystals, hexa-substituted benzenes, were
reported in 1977 by Chadrasekhar.1,2 The mesophases of these
compounds usually have a columnar structure in which the discs
are stacked one on top of the other to form columns. Attractive
intermolecular interactions between the aromatic cores are pri-
marily responsible for the stacking of cores. Columnar phases are
also observed when a flat core is replaced by a pyramid- or cone-
shaped unit3 or even when the central core is absent.4 These types
of materials are promising for potential applications3 such as one-
dimensional conductors,4 photoconductors,5 molecular wires and
fibers,6 light emitting diodes7 and photovoltaic cells.8 Triphenyl-
amine moiety has been widely exploited for its physical and elec-
tronic properties in display applications.9 Oligomeric and
polymeric materials based on triphenylamine have attracted much
attention because of their stability and useful properties such as
electrical conductivity and electroluminescence.10–12 Triphenyl-
amine is characterized by a high hole-transporting ability and a
bulky volume.
Recently, we have reported13 a columnar mesophase from a
disc-like molecule derived from triphenylamine. To the best of
our knowledge, there is only one more report14 of triphenyl-
amine-bearing discotic liquid crystal. The aforesaid liquid crystal-
line materials possess triphenylamine core, and are Schiff bases.
In order to study the structure–property relationship we have syn-
thesized a triphenylamine-based compound with ethylenic link-
age. Herein we report the results in Scheme 1.
The synthetic protocols for the compound 13 are summarized in
Schemes 1–3. The disc-shaped compound 13 was synthesized by
three different methodologies. The starting material, triphenyl-
amine trialdehyde 2 was prepared according to our earlier pub-
lished procedure.13 Compound 2 was then subjected to Wittig
olefination reaction to give the corresponding trivinyl derivative
3 in excellent yield (ꢀ100%).
The second partner (12) of this coupling reaction was prepared
from ethyl gallate (5). Alkylation of (5) with dodecyl bromide in
dry ethyl methyl ketone–K2CO3 under reflux afforded the corre-
sponding ester derivative (6). The ester group was hydrolyzed to
give (7) which was esterified with p-iodo phenol in the presence
of triethylamine and DMAP (cat.) to give compound (12). The final
product 13 was synthesized by the utilization of the intermolecular
threefold Heck reaction. Initially, we attempted the Heck reaction
by the application of Jeffery’s two-phase protocol15 and obtained
the final compound 13 in only 11% yield. Therefore, the protocol
was modified16 with a view to increase the yield of the desired
compound. When the cross-coupling reaction was conducted using
tri-o-tolyl phosphine as a ligand in conjunction with the previous
* Corresponding author. Tel.: +91 33 2582 7521; fax: +91 33 2582 8282.
E-mail address: kcm_ku@yahoo.co.in (K.C. Majumdar).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.140

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K. C. Majumdar et al. / Tetrahedron Letters 50 (2009) 6901–6905
CHO
ii
i
N
N
N
OHC
CHO
3
1
2
C
12H25
12H25
12H25
O
HO
HO
C12H25
O
iv
iii
v
HO
HO
COOH
C
O
COOH
COCl
HO
HO
COOEt
C12H25
O
COOEt
5
C12H25
O
C
O
7
6
4
vi
C
12H25
O
O
viii
ix
vii
C
12H25
O
I
OH
H2N
OH
O2N
OH
9
10
11
8
C12H25
C
12H25
12H25
C12H25
O
C
12H25
C12H25
C12H25
O
O
O
I
+
C
O
COCl
I
OH
O
O
11
O
12
8
O
O
O
O
O
N
x
3
12
+
O
O
O
O
O
13
O
O
O
O
O
Scheme 1. Reagents and conditions: (i) POCl3, DMF, 0 °C for 1 h and then heated at 100 °C for 7 h; (ii) Ph3PMeI, nBuLi, THF, 0 °C for 15 min, then 100 °C for 1 h; (iii) EtOH,
H2SO4 (cat.), reflux, 15 h; (iv) 1-bromododecane, EMK, K2CO3, NaI, reflux, 6 day; (v) EtOH, KOH, reflux, 4 h; (vi) SOCl2, reflux, 1 h; (vii) EtOAc, Pd–C (10%), H2, 4 h; (viii) NaNO2–
HCl, NaI, 0 °C, 2 h; (ix) DCM–Et3N, DMAP, 0 °C for 30 min, then rt overnight; (x) Heck reaction, 30 mol % Pd(OAc)2, KOAc (2.75 equiv), TBAB (6 equiv), DMF (20 ml), 100 °C,
72 h./ 30 mol % Pd(OAc)2, KOAc (2.75 equiv), TBAB (6 equiv), P(o-tol)3, DMF (10 ml), 100 °C, 72 h.
C12H25
O
O
C12H25
12H25
12H25
O
O
O
CHO
ii
+
C
12H25
O
COCl
OHC
I
OH
C
O
C12H25
C
O
14
8
15
iii
i
C
12H25
C12H25
C12H25
O
O
O
N
N
O
I
I
1
O
17
16
iv
17
13
16
+
Scheme 2. Reagents and conditions: (i) HgO, I2, EtOH, overnight; (ii) DCM–Et3N, DMAP, 0 °C for 30 min, then rt overnight; (iii) Ph3PMeI, nBuLi, THF, 0 °C for 15 min, then
100 °C for 1 h; (iv) Heck reaction.

K. C. Majumdar et al. / Tetrahedron Letters 50 (2009) 6901–6905
6903
RCM
16
3
13
+
Scheme 3. Reagents and conditions: Grubbs’ 1st-generation catalyst, yield 7%;
Grubbs’ 2nd-generation catalyst, yield 16%.
protocol we obtained the desired product 13 in an improved yield
(ꢀ28%).
The Heck cross-coupling strategy was next altered by replacing
the coupling partners with 16 and 17. Compound 17 was prepared
from triphenylamine by treatment with HgO and molecular iodine
in almost quantitative yield. The other partner 16 was synthesized
by Wittig olefination of compound 15. Compound 15 was in turn
prepared by the esterification of p-hydroxy benzaldehyde 14 with
3,4,5-tridodecyloxy benzoyl chloride. When these two partners are
allowed to react by the phosphine-free and phosphine-assisted
intermolecular Heck cross-coupling, the corresponding cross-Heck
product 13 was obtained in 23% yield. The lower yield of product
13 in this type of reactions is due to the possibility of formation
of competitive side products along with the desired tri-Heck prod-
uct (13) (Scheme 2).
Figure 3. Fluorescence spectra of compound 13 in benzene solution at different
concentrations (excitation: 282 nm).
Next, we attempted the synthesis of a disc-like molecule by the
implementation of the cross-ring-closing metathesis approach.17
To this end, when the cross metathesis was conducted with the
two partners 3 and 16 in dry dichloromethane in the presence of
Grubbs’ 1st-generation catalyst (Fig. 1) under nitrogen atmosphere
for three days, the desired product 13 was obtained in only 7%
yield. However, when the reaction was carried out with Grubbs’
2nd-generation catalyst (Fig. 2) the yield of the product 1318 im-
proved to 16% (Scheme 3).
The fluorescence property of the discotic molecule 13 was mea-
sured in different solvent systems at different concentrations to get
the information regarding absorption and emission maxima,
Stokes shift of fluorescence and the ‘on–off’ fluorescence proper-
ties. Two peaks at 380 nm and 282 nm were found in the UV–vis
at 437 nm with a Stokes shift of the order of 57 nm was attributed
to energy transfer at the excited state.
We have studied the fluorescence spectra of this designed mes-
ogenic compound in different solvents such as CHCl3, THF, CH3CN
and H2O at the same concentration and the corresponding plot is
presented in Figure 4. From the plot it is seen that on gradual
change from less polar solvent to more polar solvent, the emission
peak is drastically shifted to longer wavelength resulting in a sig-
nificant quenching in fluorescence intensity. The shifted emission
peak is shown in Table 1.
absorption spectra. Usually, the highest peak at 380 nm is due to
*
the
p–p transition of the highly conjugated system with the tri-
phenylamine moiety at the core system. Moreover, we studied
the fluorescence spectra of the compound 13 to examine the ex-
cited state characteristics of the compound.
When we studied the fluorescence spectrum in a less polar sol-
vent (benzene) (Fig. 3), emissions at 321 and 437 nm (concentra-
tion) were observed on excitation at 282 nm. The emission peak
PCy3
Cl
Ru
Cl
Ph
PCy3
Grubbs' 1st generation
catalyst (Grubbs' Cat. I)
Figure 1.
Figure 4. Fluorescence spectra of compound 13 in different solvents at the same
concentration, [2 Â 10À6] (excitation: 380 nm).
N
N
Mes
Mes
Ph
Table 1
Cl
Wavelength at different solvents of 13
Ru
Entry
Solvent
kmax
concn
Cl
PCy3
1
2
3
4
PhH
CHCl3
THF
435
446
450
474
10À5
10À5
10À5
10À5
Grubbs' 2nd generation
catalyst (Grubbs' Cat. II)
CH3CN
Figure 2.

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K. C. Majumdar et al. / Tetrahedron Letters 50 (2009) 6901–6905
Figure 5. (a) POM texture at 79.5 °C; (b) POM texture at 74 °C.
The thermotropic properties of the compound 13 were investi-
gated using polarizing optical microscopy (POM), differential
scanning colorimeter (DSC) and X-ray diffraction (XRD). DSC
analysis of the compound 13 shows monotropic mesophase. Com-
pound 13 shows a crystal to crystal transition at 64.6 °C with
D
H = 28.96 kJ/mol and crystal to mesophase transition at 72.3 °C
with small but noticeable change in enthalpy. Isotropic transition
occurred at 90.5 °C (0.03 kJ/mol) in the heating cycle. In the cooling
cycle, an isotropic to liquid crystalline phase transition occurred at
about 91.5 °C with small enthalpy change (0.06 kJ/mol) but sur-
prisingly, transition to crystal phase was not observed. The ob-
served enthalpy changes for the transition liquid crystal-isotropic
are low. Examples of low-enthalpy transition are available in the
literature.19,20 On slow cooling from the isotopic phase, pseudo fo-
cal-conic texture or fan-like texture (Fig. 5) of typical columnar
phase appeared with large amount of homeotropy which grows
slowly and fills up all field of view. No other phase was observed
during the cooling cycle.
The mesophase structure was established by X-ray diffraction
studies. Figure 6 shows the intensity versus 2h plot derived from
diffraction pattern of compound 13 at 80 °C. In the small angle
region, three peaks were observed, taken in the ascending order
of diffraction angle, the d-spacing of the first reflection (lowest
angle and highest intensity) to the other two was in the ratio of
1:(1/2)1/2:1/2. These values correspond to those expected from a
rectangular columnar phase. In the wide angle region there are
two diffuse peaks: a broad one at d = 4.5 Å and another relatively
Figure 6. X-ray profile of compound 13.
It should be mentioned here that the emission peak shifted
maximum with maximum fluorescence quenching in the case of
acetonitrile (474 nm). With CH3CN/H2O = 1:1 ratio the fluores-
cence quenching is maximum, that is, in the presence of water
there is no fluorescence property (totally off).
N
N
N
N
N
Figure 7. The schematic drawings of the molecules packed in a columnar phase formed by compound 13.

K. C. Majumdar et al. / Tetrahedron Letters 50 (2009) 6901–6905
6905
3. (a) Zimmermann, H.; Poupko, R.; Luz, Z.; Billard, J. Z. Naturforsch., A. 1985, A40,
149–160; (b) Levelut, A. M.; Malthête, J.; Collet, A. J. Phys. (Les Ulis. Fr.) 1986, 47,
351–357.
4. Malthête, J.; Nguyen, N. T.; Levelut, A. M. J. Chem. Soc., Chem. Commun. 1985,
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5. Adam, D.; Schuhamcher, P.; Simmerer, J.; Hayssling, L.; Siemensmeyer, K.;
Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141–143.
6. Osburn, E. J.; Schmidt, A.; Chau, L. K.; Chen, S. Y.; Smolenyak, P.; Armstrong, N.
R.; O’Brian, D. F. Adv. Mater. 1996, 8, 926–928.
7. Christ, T.; Glusen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stumpflen, V.; Tsukruk,
V.; Wendorff, J. H. Adv. Mater. 1997, 9, 48–52.
8. Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.;
MacKenzie, J. D. Science 2001, 293, 1119–1122.
9. Bellmann, E.; Shaheen, S. E.; Thayumanavan, S.; Barlow, S.; Grubbs, R. H.;
Marder, S. R.; Kippelen, B.; Peyghambarian, N. Chem. Mater. 1998, 10, 1668–
1676. and references cited therein.
10. Kuwayama, Y.; Ogawa, H.; Inada, H.; Noma, N.; Shirota, Y. Adv. Mater. 1994, 6,
677–679.
11. Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989, 1145–1148.
12. Tanaka, H.; Tokito, S.; Taga, Y.; Okada, A. Chem. Commun. 1996, 2175–2176.
13. Majumdar, K. C.; Pal, N.; Debnath, P.; Rao, N. V. S. Tetrahedron Lett. 2007, 48,
6330–6333.
narrow peak at d = 3.3–3.5 Å. (We also observed a diffuse peak in
the low angle region in between 2h = 5–7.5 due to fluctuation of
the holder, which was confirmed by taking the X-ray at the same
temperature in the absence of any samples.) The broad peak with
d-spacing of 4.5 Å was due to liquid-like packing of the aliphatic
chains. The relatively narrow peak at higher angle region is due
to core to core (intracolumnar separation) interaction.
The liquid crystalline behavior may be explained by considering
the driving force for the self-organization of two types of segments
viz., the promesogenic part of triphenylamine group and the non-
polar hydrophobic terminal alkyl chain. In the self-assembly, the
polar and non-polar parts may segregate into separate micro-do-
mains to promote the organization of individual molecules into
columns with a polar column which are assembled in the centre.
Each molecule is surrounded by nine hydrophobic dodecyl alkyl
units which are large enough to efficiently assemble to surround
the polar parts and allow separation of the polar and non-polar
parts, generating a weak force which makes them to stack into a
column. The molecular stacking is shown in Figure 7.
14. Wang, Y. J.; Sheu, H. S.; Lai, C. K. Tetrahedron 2007, 63, 1695–1705.
15. Jeffery, T. Chem. Commun. 1984, 1287–1289.
16. Procedure for the preparation of compound 13:
Procedure 1: To a mixture of the substrates 3 (42 mg, 0.13 mmol) and 16
(300 mg, 0.386 mmol) in dry degassed dichloromethane (40 ml) under
nitrogen atmosphere, either Grubbs 1st-generation catalyst (10 mol %,
32 mg) or Grubbs 2nd-generation catalyst (10 mol %, 33 mg) was added and
the reaction was stirred at rt for 3 days. The solvent was distilled off and the
residue was purified by flash column chromatography on silica gel using 3%
ethyl acetate–petroleum ether to afford the compound 13.
Procedure 2: A mixture of 3 (30 mg, 0.09 mmol), 12 (322 mg, 0.37 mmol),
anhydrous potassium acetate (25 mg, 0.26 mmol), tetrabutylammonium
bromide (180 mg, 0.56 mmol) and tri-o-tolyl phosphine (17 mg, 60 mol %)
was taken in dry N,N-dimethylformamide (10 ml) under nitrogen atmosphere.
Pd(OAc)2 (6.2 mg, 30 mol %) was added and the mixture was stirred on an
oil bath at 100 °C for 72 h. The reaction mixture was cooled, water (10 ml)
was added and extracted with DCM (3 Â 30 ml) and washed with water
(2 Â 40 ml), followed by brine (30 ml) and then dried (Na2SO4). The solvent
was distilled off to furnish the viscous mass, which was purified by column
chromatography over silica gel. Elution of the column with 2% ethyl acetate–
petroleum ether afforded the product 13.
In conclusion, we have designed and synthesized the first exam-
ple of discotic liquid crystalline material based on triphenylamine
as the core moiety possessing a rigid sp2-hybridized olefinic link-
age with a permanent dipole moment. The mesogenic behavior
of the compound was studied by the POM and DSC study. The rect-
angular columnar structure of the mesophase was established
from X-ray diffraction studies. The compound exhibited excellent
solvent dependent fluorescence properties. Our result is significant
in view of a recent report by Wang et al. in which it was claimed
that a triphenylamine-based compound possessing ethylenic link-
ages did not show any mesomorphic behaviour. Interestingly, we
have successfully demonstrated that triphenylamine possessing
an olefinic linkage can exhibit mesogenic behaviour, a rectangular
columnar mesophase.
17. (a) Grubbs, R. H.; Burk, P. L.; Carr, D. D. J. Am. Chem. Soc. 1975, 97, 3265–3267;
(b) Grubbs, R. H.; Carr, D. D.; Hoppin, C.; Burk, P. L. J. Am. Chem. Soc. 1976, 98,
3478–3483.
Acknowledgements
18. Compound 13: Yield: 28%; solid, IR (KBr, cmÀ1): 2922, 2852, 1735, 1587; 1H
NMR (CDCl3, 400 MHz): dH = 0.88–1.83 (m, 207H, aliphatic hydrogen’s of alkyl
chains), 4.03–4.07 (m, 18H, –OCH2), 7.05 (s, 6H, CH@CH), 7.11 (d, 6H,
J = 8.8 Hz), 7.17 (d, 6H, J = 8.8 Hz, ArH), 7.41 (s, 6H, ArH), 7.42 (d, 6H,
J = 8.4 Hz, ArH), 7.54 (d, 6H, J = 8.8 Hz, ArH) ppm; 13C NMR (CDCl3, 75 MHz):
dC = 14.1, 22.7, 26.0, 29.3, 29.3, 29.4, 29.6, 29.6, 29.7, 29.7, 30.3, 31.9, 69.2, 73.6,
108.5, 122.0, 122.1, 123.8, 124.1, 127.2, 128.2, 153.3, 136.1, 138.1, 143.0, 146.8,
150.5, 152.9, 165.0 ppm; MALDI MS (m/z): 2570.676. Anal. Calcd for
C171H261NO15: C, 79.89; H, 10.23; N, 0.54. Found: C, 78.85; H, 10.56, N, 0.58.
19. Yelamaggad, C. V.; Achalkumar, A. S.; Rao, D. S. S.; Prasad, S. K. J. Am. Chem. Soc.
2004, 126, 6506–6507.
We thank DST (New Delhi) for financial assistance and CSMCRI,
Bhavnagar, for providing XRD facility. Two of us (BC, PKS) are
grateful to the CSIR (New Delhi) for their fellowships.
References and notes
1. Chandrasekhar, S. Liq. Cryst. 1993, 14, 3–14.
2. Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 9, 471–480.
20. Boden, N.; Bushby, R. J.; Cooke, G.; Lozman, O. R.; Lu, Z. J. Am. Chem. Soc. 2001,
123, 7915–7916.