Technologically promising, room temperature luminescent columnar liquid crystals derived from s-triazine core: Molecular design, synthesis and characterization

Article abstract of DOI:10.1016/j.tetlet.2011.11.010

Three fold Horner-Wadsworth-Emmons (HWS) reaction of triphosphonate with achiral/chiral 3,4,5-tris(alkoxy)benzaldehydes yield novel achiral/chiral star-shaped liquid crystals (LCs), in which three fluorophore arms such as 1,2,3-tris(alkoxy)-5-styrylbenzen

Full text of DOI:10.1016/j.tetlet.2011.11.010

Tetrahedron Letters 53 (2012) 186–190
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Technologically promising, room temperature luminescent columnar liquid
crystals derived from s-triazine core: molecular design, synthesis
and characterization
⇑
Hashambi K. Dambal, C. V. Yelamaggad
Centre for Soft Matter Research, PO Box No. 1329, Professor U. R. Rao Road, Jalahalli, Bangalore 560013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Three fold Horner-Wadsworth-Emmons (HWS) reaction of triphosphonate with achiral/chiral 3,4,5-
tris(alkoxy)benzaldehydes yield novel achiral/chiral star-shaped liquid crystals (LCs), in which three fluo-
rophore arms such as 1,2,3-tris(alkoxy)-5-styrylbenzenes are tethered to a central s-triazine core. These
LCs, the first of their kind, display a columnar liquid crystalline phase over a wide temperature range
existing between well below and above the room temperature, which is evidenced by optical and calo-
rimetric studies. Besides, they show photoluminescence both in solution and mesomorphic states. Thus,
given their self-organization into fluid one-dimensional columnar array coupled with light generating
capability, these organic ordered-fluids can be regarded as novel media for advanced technological
applications.
Received 16 September 2011
Revised 31 October 2011
Accepted 3 November 2011
Available online 9 November 2011
Keywords:
s-Triazine
Styrene
Discotics
Columnar phase
Photoluminescence
Ó 2011 Elsevier Ltd. All rights reserved.
Since 1977 when Chandasekhar et al.1 discovered columnar
(Col) liquid crystal (LC) phases in benzene-hexa-n-alkanoates,
there has been a phenomenal growth in research activity relating
to the rational molecular design and synthesis of structurally di-
verse organic molecules capable of exhibiting such mesophase(s).2
This is because Col phases, generally formed by the spontaneous
the Col phases to serve as emissive layers as well in OLEDs. Thus,
in recent years there has been a growing interest in developing
luminescent organic molecules exhibiting columnar behavior
where a large variety of shape-anisometric motifs bearing fluoro-
phores have been reported.7 Among these, 1,3,5-triazine-based
LCs8 appear to be suitable substances in view of the fact that
1,3,5-triazine core possesses strong electron affinity when com-
pared to other aromatic rings.9 Although a large number of tri-
azine-based compounds have been prepared,8 the efforts in
design and synthesis of such functional materials capable of exhib-
iting luminescent Col phase existing between well below, and
above the ambient temperature, a vital feature required for device
performance, are absolutely necessary.
In order to achieve these goals, C3-symmetric three-armed star-
shaped compounds Ia and IIa–b have been rationally designed,
synthesized, and probed for their physical properties. Scheme 1 de-
picts the general molecular structure of these systems. As can be
seen, they contain three 1,2,3-tris(alkoxy)-5-styrylbenzenes, as
fluorophore arms, linked covalently to a central electron-deficient
1,3,5-triazine core. Here, styryl arms not only accounted for the
effective conjugation but also serve as electron-rich cores as they
possess electron-donating branched alkoxy tails. These branched
alkoxy chains have been especially chosen given the experimental
observation that they reduce melting (crystal-to-Col phase) transi-
tion temperatures and thus, widen the thermal range of the
required LC phase. They are also known to lower the clearing
(Col-to-isotropic liquid phase) transition temperatures.2,10 In fact,
self-assembly of shape-anisotropic
fluid columnar stacks, are highly anisotropic and ordered struc-
tures with extensive -orbital overlap leading to a high degree of
p-conjugated molecules into
p
uniaxial charge-carrier mobilities. Besides, they can be readily
and repeatedly processed from the isotropic melt of mesogens to
eliminate structural/electronic defects when compared to single-
crystals or inorganic semiconductors used in technological applica-
tions.2,3 They have been recognized as potential media for
hole- and electron-transports given the fact that the measured
charge-carrier mobility value in some Col LCs exceeds 1 cm2 V
À1 sÀ1 4
. Thus, Col phases hold immense promise in electronic de-
vices such as photovoltaic cells,3 field effect transistors5 and organ-
ic light emitting devices (OLEDs).6
Their usage in fabricating OLEDs is especially significant if an
additional parameter such as the transport of excitation energy,
the luminescence behavior, is incorporated in the constituent mes-
ogens; in fact, the intrinsic photoluminescence property enables
⇑
Corresponding author. Tel.: +91 80 2838 8199; fax: +91 80 2838 2044.
E-mail
addresses:
yelamaggad@gmail.com,
yelamaggad@csmr.res.in
(C.V. Yelamaggad).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.010

H. K. Dambal, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 186–190
187
CHO
Br
CHO
(iiib)
R
O
R
HO
OH
R
OH
CN
(89 %)
(i)
3b : R =
(iiia)
(iv)
Br
CHO
R'
R
R
R
R
N
N
R
R
N
O
O
Br
Br
4 : R' = CH2OH, R =
3c : R' = CHO, R =
(79 %)
(81 %)
3a : R =
O
(v)
1 (68 %)
(88 %)
(ii)
R
R
R
PO(OCH3)2
(vi)
N
N
N
(H3CO)2OP
PO(OCH3)2
N
N
2 (82 %)
N
R
R
R
R
R
R
(86%)
I : R =
O
IIb : R=
IIa : R =
O
O
(84 %)
(82%)
Scheme 1. Reagents and conditions: (i) CF3SO3H, CH2Cl2, 0 oC, 12 h. (ii) Trimethyl phosphite, reflux, 5 h. (iiia) 1-Bromo-3,7-dimethyloctane, anhyd. K2CO3, DMF 80 oC, 12 h.
(iiib) (S)-(+)-Citronellyl bromide, anhyd. K2CO3, DMF 80 oC, 12h. (iv) H2/Pd–C, abs. EtOH, rt, 8h. (v) PCC, CH2Cl2, rt, 1 h. (vi) n-BuLi, THF, 12 h.
we opted to employ branched achiral, (3,7-dimethyloctan-1-oxy)
and chiral [(S)-3,7-dimethyloct-6-en-1-oxyand (S)-3,7-dimethy-
loctan-1-oxy] to realize both achiral and chiral star-shaped com-
pounds and to understand the structure-property correlation. In
essence, the molecular design, which represents the first of its
kind, consists of 1,3,5-triazine core and styryl arms substituted
with branched achiral/chiral terminal tails.
3a–c furnish the necessary star-shaped target compounds I and
IIa–b in good yields (82-86%). These compounds have been fully
characterized with the help of spectroscopic viz., UV–vis, FT-IR,
1H and 13C NMR, MS, techniques, and microanalytical data.
As expected, they show nearly similar spectral (UV–vis, IR, 1H
and 13C NMR) patterns. The 1H and 13C NMR spectra are especially
found to contain the anticipated spectral patterns that assist in elu-
cidating the proposed structures of the compounds unambigu-
ously. For example, olefinic protons of the styryl moiety resonate
as a quartet at d 7.17–7.22 with the vicinal coupling constant (J)
of 16–16.4 Hz, indicating the trans configuration of the double
bond; the 18 aromatic protons appear at d 8.77–8.78 (d, 6H,
J = 8.4 Hz), 7.7–7.71- (d, 6H, J = 8.4-8.48 Hz) and 6.79 (s, 6H). How-
ever, the mesogen IIa shows an additional multiplet pattern in the
region d 5.11–5.14 arising due to olefinic proton of the peripheral
alkoxy tails. The 13C spectra (downfield from the CDCl3 peaks; a
1:1:1 triplet at d = 77) of mesogens I and IIb show the expected
eleven signals; while the spectrum of IIa possesses four peaks
additionally. Notably, the signal at d 171.15 commonly found in
all the three spectra arises due to the carbons of s-triazine core.
The synthesized compounds I and IIa–b have been examined
for their LC properties with the help of multiple, complementary
experimental techniques. First, the occurrence of LC behavior in
these compounds at room temperature (rt) was immediately
apparent as they were isolated as a sticky yellow mass, which
Specifically designed target C3 symmetric compounds I (achiral)
and IIa–b (chiral) have been prepared by the route outlined in
Scheme 1. Cyclotrimerization of 4-(bromomethyl)-benzonitrile
upon treating with trifluoromethanesulfonic acid yields 2,4,6-
tris(4-(bromomethyl)phenyl)-1,3,5-triazine (1).11a The Arbuzov
reaction of 1 and trimethyl phosphite under reflux for 12 h affords
the key intermediate, hexamethyl (4,4’,4’’-(1,3,5-triazine-2,4,6-
triyl)tris-(4,1-phenylene))tris(methylene)triphosph-onate (2), in
an 82% yield.11b Williamsons reaction, the O-alkylation, of 3,4,5-
trihydroxybenzaldehyde with 1-bromo-3,7-dimethyloctane and
(S)-(+)-citronellyl bromide furnish 3,4,5-tris(3,7-dimethyloctyl-
oxy)benzaldehyde (3a)8 and 3,4,5-tris((S)-3,7-dimethyloct-6-enyl-
oxy)benzaldehyde (3b), respectively. Subjecting aldehyde 3b to Pd/
C-catalyzed hydrogenation gives (3,4,5-tris((S)-3,7-dimethyloctyl-
oxy)phenyl)methanol (4) which oxidizes in the presence of PCC
yielding 3,4,5-tris((S)-3,7-dimethyloctyloxy)benzaldehyde (3c).12
In the key step of synthesis, the reaction of triphosphonate 2 under
Horner-Wadsworth-Emmons conditions (HWS) with aldehydes

188
H. K. Dambal, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 186–190
could be readily spread with the aid of a glass rod/spatula. To cor-
roborate this observation both polarizing optical microscope
(POM) and differential scanning calorimeter (DSC) investigations
were performed on the samples. The results of these studies of
compounds I and IIa–b have been summarized in Table 1 and dis-
cussed in more detail in the following paragraphs.
The textural patterns of the samples confined between un-
treated, clean glass substrates were observed by POM. The sheared
(mechanically stressed) samples at rt exhibit nonspecific, highly
birefringent pattern. However, on heating the samples till their
clearing temperature followed by cooling gradually, they show a
mesophase having specific, informative textural pattern emanating
strikingly from the dark background of isotropic liquid. For exam-
ple, when a thin film of compound I is gradually cooled from the
isotropic phase, the LC phase sets in at 71.9 °C manifesting opti-
cally, as shown in Figure 1, in the form of a pattern featuring the
combination of mosaics with elongated birefringent defects and
homeotropic domains. This pattern is well-known to be an impor-
tant signature and conclusive evidence for the occurrence of Col
phase.2 On lowering the sample temperature further, neither any
other textural change nor crystallization of the sample were seen
till room temperature implying that the Col phase once formed
upon cooling from the isotropic phase, crystallizes by no means in-
stead it remains unaltered. The chiral compounds IIa and IIb exhi-
bit analogous behavior with the exception that mosaic and
homeotropic domains in the optical texture of Col phase appear
relatively tiny.
In all the cases, the extent of occurrence of the mesophase be-
low the ambient temperature could not be determined in the opti-
cal study due to limitations of the experimental setup. However,
this information could be obtained with the help of calorimetric
study. In the DSC traces, obtained during the first heating-cooling
cycles, as shown in Figure 2, no exothermic (from just below the
Iso-Col to À40 °C) or endothermic (from À40 °C to near the Col-
Iso transition) peaks exist for these three samples. Most impor-
tantly, these features of DSC traces found to be highly reproducible
for any number of heating-cooling cycles indicate clearly that these
compounds stabilize Col phase below and above the room temper-
ature. Further, the chemical stability of the samples has been ascer-
tained by taking DSC scans after keeping the samples very close to
their clearing temperatures for 6–7 h and noted that the transition
temperatures agree well with the previous runs. It is immediately
apparent from the results accumulated in Table 1 that the phase
transition temperatures and thus, the thermal width of Col phase
of the three compounds vary significantly although their periphe-
ral branched-alkoxy tails possess an identical carbon skeleton. That
is, compound I possessing achiral branched (3,7-dimethyloctan-1-
oxy) tail shows higher clearing temperature and thus, wider
Figure 1. Optical texture of the Col phase shown by I at 70 °C.
0.4
0.2
Col
IIb: Heating
IIa: Heating
I : Heating
I
I
Col
Col
I
I : Cooling
Col
Col
I
0.0
IIa : Cooling
I
IIb : Cooling
I
Col
I : Isotropic phase
Col : Columnar Phase
-0.2
-40
-20
0
20
40
60
80
Temperature (oC)
Figure 2. DSC thermograms of mesogens I and IIa–b.
thermal range of Col phase when compared to that of chiral ana-
logues IIa and IIb. Among the chiral compounds IIa and IIb, the for-
mer (with olefinic-bond in the chiral tail) stabilizes the Col phase
over a slightly higher thermal range than the latter (without ole-
finic-bond in the chiral tail). These findings clearly suggest that
the presence or absence of molecular chirality and double bond
greatly affect the phase behavior of the star-shaped mesogens.
The photophysical characteristics of the synthesized star-
shaped mesogens have been studied both in solution and fluid
Col states at room temperature by UV–vis absorption and
Table 1
Phase transition temperatures (°C)a and associated enthalpies (J/g), and photophysical (absorption and emission) properties of I and IIa–b. Col = Columnar phase; Iso = Isotropic
phase, rt = room temperature
c
Compound Phase sequence
Heating; cooling
Dichloromethane solution
Columnar LC phase at rt
d
d
Absorption kmax
Emission kmax
Stokes shift
(nm)
Absorption kmax
(nm)
Emission kmax
Stokes shift
(nm)
(nm)
(nm)
(nm)
I
Col 76.6 (0.5) I; I 71.9 (0.55)
Col
290 e, 383
289 e, 381
292 e, 381
543
160
161
161
291 e, 376
293 e, 376
289 e, 378
509
133
132
130
b
IIa
IIb
Col 55.2 (0.49) I; I 49.9 (0.75)
542
542
508
508
b
Col
Col 44.4 (0.42) I; I 33.3 (0.83)
b
Col
a
b
c
Peak temperatures in the DSC traces recorded during the first heating-cooling cycles at 5 °C/min.
No other phase transition was observed till À40 °C.
Micro molar concentration.
Excitation wavelength (kex) = 380 nm.
Shoulder adjoining the main (intense) peak.
d
e

H. K. Dambal, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 186–190
189
photoluminescence. The results have been accumulated in Table 1.
The absorption and luminescence spectra of solutions in CH2Cl2 of
compounds I and IIa–b were found to be nearly identical, as shown
in the top panels of Figure 3. In fact, the spectral pattern and the
absorption and emission intensity appear to be independent of
the concentration of compounds in solution within the working
concentration ranging from 5.5 Â 10À6 to 6.5 Â 10À6 mol LÀ1. As
can be seen in the top panel of Figure 3, all the three mesogens
show two absorption maxima centered around 290 nm and
sample. Specifically, such a blue-shift observed earlier in the Col
phase has been assumed to be associated with the lowering of
the magnitude of disorder along the column and a decrease in
the ability of the molecules to form self-quenching aggregates.13
Seemingly, the large Stokes shift values of these films are of the
same magnitude as in solution. These results point toward the high
charge transfer efficiency and conformation relaxation in the ex-
cited state of the molecules. On the whole, Stokes shifts of the
absorption and emission wavelengths are almost identical imply-
ing that photophysical properties of the three mesogens are the
same. It may also be pointed out here that the Col phase of the chi-
ral compounds IIa–b was found to be CD inactive implying that
molecular chirality has not facilitated molecular organization in
helical fashion in the LC (Col) phase.
In summary, the first examples of photoluminescent, room tem-
perature Col LCs derived from a combination of s-triazine and sty-
rylbenzene cores have been prepared by three fold HWS reaction of
triphosphonate with achiral/chiral aldehydes, and characterized.
The thermal width of the Col phase, which is remarkably high, de-
pends on the nature of the peripheral alkoxy tails. Whereas the
photophysical properties found to be independent of such a struc-
tural parameter as all the three mesogens behave analogously in
this respect with a large Stokes shifts both in solution and meso-
morphic states. These mesogens and /or their future variants might
thus turn out to be appropriate candidates for various technologi-
cal applications.
380 nm corresponding to the p–
p⁄ and n–p⁄ transitions originating
due to high absorption coefficients; this means that these meso-
gens readily absorb photons. By irradiating these solutions with
380 nm light, fluorescence maximum occurs at around 540 nm.
This is in agreement with the general observation that the emis-
sion occurs at a longer wavelength than the absorption. The visu-
ally perceivable green light in the emissive state has been shown
in the inset of Figure 3. In all the cases, the Stokes shift, the differ-
ence between positions of the band maxima of the absorption, and
emission spectra, were found to be nearly 160 nm (Table 1). The
absorption and emission spectra of fluid Col phase at rt have been
recorded for the mesogens held between two quartz plates. They
show two absorption bands analogous to the solution state (see
the lower left-portion of Fig. 3 and Table 1). As shown in the lower
right-portion of Figure 3, upon excitation with 380 nm light, they
exhibit an intense broad signal with photoluminescence maximum
at 508 nm; the inset of Figure 3 shows visually perceived green
light in the emissive state evidencing the preservation of optical
properties in fluid columnar assembly. It can be seen that the emis-
sion band of the fluid phase appears at a relatively lower wave-
length when compared to that of solution state. This blue-shift
can be attributed to the degree of intermolecular interactions with-
in the Col structure (local order) and the thickness (viscosity) of the
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