Electro-functional octupolar π-conjugated columnar liquid crystals

Article abstract of DOI:10.1021/ja2035255

A series of propeller-shaped π-conjugated molecules based on 2,4,6-tris(thiophene-2-yl)-1,3,5-triazines has been designed and synthesized to obtain ambipolar charge-transporting liquid-crystalline materials. The 3-fold electron-donating aromatic units are attached to the electron-accepting triazine core, which forms electro-functional octupolar π-conjugated structures. These octupolar molecules self-organize into one-dimensional columnar nanostructures and exhibit ambipolar carrier transport behavior, which has been revealed by time-of-flight measurements. In this approach, electron-donor and acceptor electro-active segments are assembled individually in each column to give one-dimensional nanostructured materials with precisely tuned electronic properties. Their desirable electronic structures responsible for both hole and electron conductions have also been examined by cyclic voltammetry and theoretical calculations. The present results provide a new guideline and versatile approach to the design of ambipolar conductive nanostructured liquid-crystalline materials.

Full text of DOI:10.1021/ja2035255

Article
pubs.acs.org/JPCB
Trigonal 1,3,4-Oxadiazole-Based Blue Emitting Liquid Crystals and
Gels
Deepak D. Prabhu, N. S. Saleesh Kumar, Aneesh P. Sivadas, Shinto Varghese, and Suresh Das*
Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science
and Technology (NIIST) and Network of Institutes for Solar Energy, CSIR, Trivandrum 695 019, India
ABSTRACT: Star-shaped molecules consisting of a 1,3,4-oxadiazole core derivatized
with alkoxy-substituted phenyl ethynylenes, FD12 (dodecyl) and FD16 (hexadecyl)
were synthesized. These molecules exhibited enantiotropic columnar mesophases over
a wide temperature range, with the liquid crystalline phases exhibiting strong blue
fluorescence. On cooling, FD12 transformed into a transparent glass at room
temperature wherein the liquid crystalline texture was retained. The glassy film
remained stable over a period of one year and exhibited blue luminescence with an
absolute quantum yield of 26%. The oxadiazole derivatives formed stable luminescent
gels in decane and study of their morphology by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) indicated formation of interlocked
network of self-assembled fibers. X-ray diffraction (XRD) analysis of the xerogel of
these derivatives indicated oblique columnar ordering of the molecules within the
fibers. The length of the alkyl substituent was observed to have a significant effect on
the absorption and fluorescence properties of the gels, which was attributable to the role of the alkyl substituents in controlling
the nature of the molecular packing within the self -assembled fibers of the gels.
We had earlier reported on a novel class of star shaped
oxadiazole derivatives exhibiting columnar mesophases.2a 1,3,4-
Oxadiazoles were incorporated in the core of the discotic
molecules in view of their excellent electron transporting and
luminescent properties as well as their ability to undergo
efficient π-stacking.8 Although in solutions the molecules
inherently exhibited strong blue fluorescence, the columnar
liquid crystals derived from these materials exhibited green
fluorescence as a result of strong π-stacking interaction between
neighboring molecules leading to the formation of fluorescent
aggregates. In view of the significant interest in development of
blue light emitting materials for applications in electro-optic
and electroluminescent devices,9 we have attempted to fine-
tune the structure and self-assembly of this class of materials to
obtain the desired emission. Here we report on the design and
study of two new oxadiazole-based columnar liquid crystals
(FD12 and FD16) wherein the conjugation between the
alkoxyphenyl donor substituent and the oxadiazole acceptor
substituent was diminished by changing the linker group from
vinyl to ethynyl (Chart 1).
INTRODUCTION
■
Columnar liquid crystals are a class of soft materials which have
attracted special attention because of their ability to form highly
ordered superstructures.1 The arrangement of π-conjugated
materials into columnar stacks in such materials imparts them
with unique functional properties such as the ability to
efficiently conduct charge and energy in a highly anisotropic
manner, making them useful materials for a variety of organic
electronic and electro-optic devices.2 In recent years there has
been an increasing focus on the design of columnar liquid
crystals exhibiting high degree of luminescence in their solid
and liquid crystalline states because of their utility in display
devices and organic light emitting diodes.3 Design of such
materials provides an interesting challenge, since the very
intermolecular interactions which are essential for bringing
about the self-assembly of molecules into columnar stacks can
be detrimental to their luminescence efficiency. For example,
π−π-interactions between neighboring molecules which can
help in stabilizing columnar mesophases can lead to drastic
reduction in the fluorescence efficiency of the molecules.4,5 As a
result most columnar liquid crystals exhibit weak fluorescence.
Design of luminescent columnar liquid crystals will therefore
require both proper choice of the chromophore as well as fine
control of the self-assembly of the molecules. Geert’s and co-
workers have recently reported on highly fluorescent pyrene-
based columnar liquid crystals, wherein the high luminescence
efficiency in the solid state of these materials was ascribed to
rotated chromophores, leading to minimal π−π interaction
between the neighboring molecules.6 Suppression of π-stacking
resulting in luminescent columnar liquid crystals was also
reported in triazolotriazine derived molecules.7
These materials exhibited blue emission with high efficiency
both in solution as well as in their liquid crystalline phase. Both
these molecules exhibited enantiotropic columnar mesophases
over a very wide range of temperature, among which FD12
transformed into a transparent glass at room temperature
wherein the liquid crystalline order was retained. The glassy
films remained stable over a period of one year and exhibited
Received: June 1, 2012
Revised: October 6, 2012
© XXXX American Chemical Society
A
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(FMAS) Software]. The fluorescence lifetime values were
obtained using DAS6 decay analysis software. Absolute
quantum yield in the liquid crystalline glassy film as well as
in the gel state was measured using a calibrated integrating
sphere in a SPEX fluorolog FL-1039 fluorimeter. For scanning
electron microscopic (SEM) measurements, samples were drop
cast and air-dried on flat surface of cylindrical brass stubs and
subjected to thin gold coating using JEOL JFC-1200 fine
coater. The probe was inserted into JEOL JSM-5600 LV
scanning electron microscope for taking photographs. TEM
measurements were carried out in JEOL 100 kV HRTEM. The
Chart 1
samples were prepared by drop casting 25 μL of 5 × 10−5
M
onto a carbon coated copper grid and solvent was allowed to
evaporate under vacuum. Liquid crystalline phase transitions
were observed using a Leica DFC 490 polarized light optical
microscope, equipped with a Mettler TOLEDO FP82HT
(Temperature programmer) heating and freezing stage. Differ-
ential scanning calorimetry was performed using a Perkin-Elmer
Pyris 6 DSC instrument in sealed aluminum pans under
nitrogen flow, at a heat/cooling rate of 5 °C/min. X-ray
diffraction studies of xerogel was carried out on samples coated
in glass slide and measurements were recorded on a Phillips
diffractometer using Ni-filtered Cu Kα radiation (λ = 1.5418
Å). Details of the setup for the XRD studies carried out at
CSMR, Bangalore, using samples filled using Lindemann
capillaries, are given elsewhere.10 The apparatus essentially
involved a high-resolution X-ray powder diffractometer
(PANalytical X’Pert PRO) equipped with a high-resolution
fast detector, PIXCEL.
Synthesis. General Procedure for the Synthesis of 1,2-
Bis(alkoxy)benzene 1a and 1b. Catechol (1 equiv), alkyl
bromide (4 equiv), and K2CO3 (7 equiv) were dissolved in 30
mL of degassed DMF and were stirred at 80 °C for 24 h. The
reaction mixture was then poured into ice cold water. The
precipitate formed was filtered, washed with water, and dried.
The crude product was further purified by column chromatog-
raphy using silica gel (100−200 mesh) as the stationary phase
and hexane as the mobile phase.
blue luminescence with an absolute efficiency of 26%. The
ability of these molecules to gel nonpolar solvents such as
decane has also been investigated. The gels thus formed
exhibited blue to bluish-green luminescence. Gelation of the
solvents was observed to be brought about by formation of a
self-assembled fibrillar network wherein the molecules retained
their columnar arrangement. These aspects as well as the
detailed study on the effect of molecular structure and self-
assembly on the photophysical properties of the molecules in
their solution, liquid crystalline phase, gels, and aggregated state
are described herein.
EXPERIMENTAL SECTION
■
Instrumentation. The solvents and the reagents were dried
and purified by standard methods prior to use. Melting points
were determined with a Mel-Temp-II melting point apparatus
and are uncorrected. FT-IR spectra were recorded on a
Shimadzu IR Prestige-21 Fourier Transform Infrared Spec-
1
trophotometer. H (300 and 500 MHz) and 13C NMR (125
1,2-Bis(dodecyloxy)benzene (1a). This was obtained in
MHz) spectral analysis were performed on a Bruker Avance
DPX spectrometer with TMS as internal standard. MALDI−
TOF mass spectrometry was conducted on a Perspective
Biosystems Voyager DE PRO MALDI−TOF mass spectrom-
eter using α-cyano-4-hydroxy cinnamic acid as the matrix.
Electronic absorption spectra were recorded on a Shimadzu
UV-3101 PC NIR scanning spectrophotometer and emission
spectra were recorded on a SPEX-Fluorolog FL-1039
spectrofluorimeter. Optical absorption measurements were
carried out using 1 mm or 10 mm cuvettes with a thermistor
directly attached to the wall of the cuvette holder for
controlling the temperature. Fluorescence measurements were
carried out using 1 mm cuvette in a front face setup.
Fluorescence quantum yields in solution were measured using
quinine sulfate in 0.1 M H2SO4 (ϕF = 0.54) as reference.
Fluorescence lifetimes were measured using IBH (FluoroCube)
time-correlated picoseconds single photon counting (TCSPC)
system. Solutions were excited with a pulsed diode laser (<100
ps pulse duration) at a wavelength of 375 nm (NanoLED-11)
with a repetition rate of 1 MHz. The detection system
consisted of a microchannel plate photomultiplier (5000U-09B,
Hamamatsu) with a 38.6 ps response time coupled to a
monochromator (5000 M) and TCSPC electronics [Data
station Hub including Hub-NL, NanoLED controller and
preinstalled fluorescence Measurement and Analysis Studio
1
85% yield as a colorless amorphous solid; mp: 43−44 °C. H
NMR (300 MHz, CDCl3): δ 6.88 (s, 4H), 3.96−4.00 (t, 4H),
1.78−1.85 (m, 4H), 1.26−1.47 (m, 36H), 0.85−0.90 (t, 6H)
ppm. 13C NMR (125 MHz, CDCl3): 149.25, 121.0, 114.14,
69.29, 31.93, 29.72, 29.65, 29.45, 29.37, 26.06, 22.69, 14.12
ppm. IR (KBr) νmax: 2951, 2914, 2850, 1587, 1508, 1461, 1455,
1390, 1257, 1220, 1122, 997, 732 cm−1.
1,2-Bis(hexadecyloxy)benzene (1b). This was obtained in
1
84% yield as a colorless amorphous solid; mp: 55−56 °C. H
NMR (300 MHz, CDCl3): δ 6.88 (s, 4H), 3.96−4.00 (t, 4H),
1.78−1.85 (m, 4H), 1.26−1.47 (m, 52H), 0.85−0.90 (t, 6H)
ppm. 13C NMR (125 MHz, CDCl3): 149.25, 121.0, 114.14,
69.29, 31.93, 29.72, 29.65, 29.45, 29.37, 26.06, 22.69, 14.12
ppm. IR (KBr) νmax: 2953, 2916, 2846, 1593, 1508, 1465, 1452,
1390, 1257, 1220, 1122, 997, 732 cm−1.
General Procedure for the Synthesis of 1,2-Bis(alkoxy)-4-
iodobenzene, 2a and 2b. 1,2-Bis(alkoxy)benzene (1 equiv)
and N-iodosuccinimide (1.1 equiv) were dissolved in 20 mL of
acetonitrile (ACN), to which 5 drops of trifluoroacetic acid was
added, and the mixture was stirred for 6 h at room temperature.
The reaction mixture was then extracted with dichloromethane
and the organic layer was washed thoroughly with sodium
thiosulphate solution and dried over anhydrous sodium sulfate.
Excess solvent was removed under reduced pressure, and the
B
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crude product was further purified by column chromatography
using silica gel (100−200 mesh) as the stationary phase and 5%
EtOAc: hexane as the mobile phase.
equiv), and NH4Cl (2 equiv), and the reaction was stirred at
100 °C for 12 h. The reaction mixture was then poured into ice
cold water and neutralized with 50 mL of 10% HCl. The
precipitate formed was then filtered, washed with water and
dried. The crude product was then purified by column
chromatography using silica gel (100−200 mesh) as the
stationary phase and THF as the mobile phase.
1,2-Bis(dodecyloxy)-4-iodobenzene (2a). This was obtained
1
in 87% as a colorless amorphous solid; mp: 55−56 °C. H
NMR (300 MHz, CDCl3): δ 7.16−7.19 (d, J = 8.4 Hz, 1H),
7.12 (s, 1H), 6.60−6.62 (d, J = 8.4 Hz, 1H), 3.92−3.96 (t, 4H),
1.78−1.85 (m, 4H), 1.26−1.47 (m, 36H), 0.85−0.90 (t, 6H)
ppm. 13C NMR (125 MHz, CDCl3): 150.14, 149.78, 149.25,
129.80, 123.79, 122.68, 115.73, 95.96, 82.51, 69.43, 69.36,
31.94, 29.72, 29.62, 29.38, 29.18, 29.05, 25.98, 22.70, 14.12
ppm. IR (KBr νmax: 2950, 2922, 2841, 1590, 1508, 1461, 1391,
1254, 1131, 1070, 993, 840, 800, 721 cm−1.
5-(4-((3,4-Bis(dodecyloxy)phenyl)ethynyl)phenyl)-2H-tet-
razole (4a). This was obtained in 85% yield as a colorless
1
amorphous solid; mp: 121−123 °C. H NMR (500 MHz, d-
DMSO): δ 8.07−8.09 (d, J = 8 Hz, 2H), 7.64−7.66 (d, J = 8.0
Hz, 2H), 7.09−7.11 (d, J = 8.5 Hz, 1H), 7.05 (s, 1H), 6.89−
6.90 (d, J = 8.0 Hz, 1H), 3.96−4.00 (t, 4H), 1.78−1.85 (m,
4H), 1.26−1.47 (m, 36H), 0.85−0.90 (t, 6H) ppm. 13C NMR
(125 MHz, d-DMSO): 154.95, 153.35, 137.19, 132.49, 130.59,
130.23, 121.45, 118.95, 118.62, 97.44, 92.29, 73.66, 73.48,
1,2-Bis(hexadecyloxy)-4-iodobenzene (2b). This was ob-
tained in 90% yield as a colorless amorphous solid; mp: 60−61
°C. 1H NMR (300 MHz, CDCl3): δ 7.16−7.19 (d, J = 8.4 Hz,
1H), 7.12 (s, 1H), 6.60−6.62 (d, J = 8.4 Hz, 1H), 3.92−3.96 (t,
4H), 1.78−1.85 (m, 4H), 1.26−1.47 (m, 52H), 0.85−0.90 (t,
6H) ppm. 13C NMR (125 MHz, CDCl3): 150.14, 149.78,
149.25, 129.80, 123.79, 122.68, 115.73, 95.96, 82.51, 69.43,
69.36, 31.94, 29.72, 29.62, 29.38, 29.18, 29.05, 25.98, 22.70,
14.12 ppm. IR (KBr) νmax: 2954, 2918, 2846, 1581, 1502, 1465,
1394, 1257, 1136, 1070, 993, 840, 800, 721 cm−1.
General Procedure for the Synthesis of 4-((3,4-Bis(alkoxy)-
phenyl)ethynyl)benzonitrile, 3a and 3b. To a 100 mL two
necked round-bottom flask kept under argon atmosphere were
added 1,2-Bis(alkoxy)-4-iodobenzene (1.1 equiv), 4-ethynyl-
benzonitrile (1 equiv), and a catalytic amount of palladium(II)
chloride, CuI, and PPh3 to 15 mL of degassed THF. To this
was added 10 mL of degassed diisopropyl amine (DIPA) and
the mixture stirred at room temperature for 12 h. The reaction
mixture was then passed through a Celite column and the crude
product was further purified by column chromatography using
silica gel (100−200 mesh) as the stationary phase and 5%
EtOAc: hexane as the mobile phase.
39.23, 36.44, 33.95, 30.76, 27.31, 19.12 ppm. IR (KBr) νmax
:
2954, 2916, 2848, 2200, 1591, 1519, 1467, 1334, 1259, 1226,
1120, 846, 800, 570 cm−1.
5-(4-((3,4-Bis(hexadecyloxy)phenyl)ethynyl)phenyl)-2H-
tetrazole (4b). This was obtained in 90% yield as a colorless
1
amorphous solid; mp: 115−116 °C. H NMR (500 MHz, d-
DMSO): δ 8.07−8.09 (d, J = 8 Hz, 2H), 7.64−7.66 (d, J = 8.0
Hz, 2H), 7.094−7.111 (d, J = 8.5 Hz, 1H), 7.05 (s, 1H), 6.89−
6.90 (d, J = 8.0 Hz, 1H), 3.96−4.00 (t, 4H), 1.78−1.85 (m,
4H), 1.26−1.47 (m, 52H), 0.85−0.90 (t, 6H) ppm. 13C NMR
(125 MHz, d-DMSO): 154.95, 153.35, 137.19, 132.49, 130.59,
130.23, 121.45, 118.95, 118.62, 97.44, 92.29, 73.66, 73.48,
39.23, 36.44, 33.95, 30.76, 27.31, 19.12 ppm. IR (KBr) νmax
:
2954, 2916, 2848, 2200, 1591, 1519, 1467, 1334, 1259, 1226,
1120, 846, 800, 570 cm−1.
General Procedure for the Synthesis of FD12 and FD16.
To a 250 mL two necked round-bottom flask kept under argon
atmosphere was added 1,3,5-benzene tricarbonyl trichloride (1
equiv) dissolved in 10 mL of degassed pyridine. To this was
added 5-(4-((3,4-Bis(alkoxy)phenyl)ethynyl)phenyl)-2H-tetra-
zole (3.6 equiv) dissolved in 30 mL of degassed pyridine
dropwise using a pressure equalizer with constant stirring at
room temperature for 15 min. The reaction mixture was then
heated at 120 °C for 12 h. The reaction mixture was then
poured into ice cold water, and the precipitate formed was then
filtered, washed with water, and dried. The crude product was
then purified by column chromatography by using silica gel
(100−200 mesh) as the stationary phase and 30% EtOAc:hex-
ane as the mobile phase.
1,3,5-Tris(5-(4-((3,4-bis(dodecyloxy)phenyl)ethynyl)-
phenyl)-1,3,4-oxadiazol-2-yl)benzene (FD12). This was ob-
tained in 75% yield; mp: 177 °C .1H NMR (300 MHz, CDCl3):
δ 9.08 (s, 3H), 8.21−8.23 (d, J = 7.8 Hz, 6H), 7.73−7.71 (d, J =
7.8 Hz, 6H), 7.14−7.16 (d, J = 8.1 Hz), 7.08 (s, 3H), 6.84−6.87
(d, J = 8.1 Hz), 3.96−4.00 (t,4H), 1.78−1.85 (m, 4H), 1.26−
1.47 (m, 36H), 0.85−0.90 (t, 6H) ppm. 13C NMR (125 MHz,
CDCl3): 165.16, 162.79, 150.24, 148.79, 132.08, 127.86,
127.11, 126.19, 125.30, 122.15, 116.65, 114.50, 113.14, 93.48,
87.08, 69.30, 69.13, 31.94, 29.71, 29.65, 29.44, 29.38, 29.24,
29.19, 26.02, 22.70, 14.12 ppm. IR (KBr) νmax: 2928, 2856,
2200, 1589, 1518, 1474, 1330, 1249, 1124, 1012, 844 cm−1. MS
(MALDI−TOF): calcd for C126H174N6O9 (M + H+), 1917.77;
found, 1917.64
4-((3,4-bis(dodecyloxy)phenyl)ethynyl)benzonitrile (3a).
1
This was obtained in 82% yield; mp: 85−86 °C. H NMR
(300 MHz, CDCl3): δ 7.61−7.63 (d, J = 7.8 Hz, 2H), 7.56−
7.59 (d, J = 8.1 Hz, 2H), 7.11−7.14 (d, J = 8.1 Hz, 2H), 7.05 (s,
1H), 6.84−6.87 (d, J = 8.1 Hz, 1H), 3.96−4.00 (t, 4H), 1.78−
1.85 (m, 4H), 1.26−1.47 (m, 36H), 0.85−0.90 (t, 6H) ppm.
13C NMR (125 MHz, CDCl3): 150.46, 148.81, 132.01, 131.84,
125.40, 118.63, 116.69, 114.11, 113.13, 111.01, 94.46, 86.39,
69.33, 69.13, 31.93, 29.71, 29.63, 29.40, 29.37, 29.20, 29.15,
26.01, 22.69, 14.11 ppm. IR (KBr) νmax: 2953, 2916, 2848,
2227, 2196, 1589, 1517, 1467, 1417, 1392, 1330, 1257, 1274,
1226, 1118, 1143, 1068, 995, 835, 810, 721, 557 cm−1.
4-((3, 4- bis(hexadecyloxy)phenyl)ethynyl)benzonitrile
1
(3b). This was obtained in 84% yield; mp: 91−92 °C. H
NMR (300 MHz, CDCl3): δ 7.61−7.63 (d, J = 7.8 Hz, 2H),
7.56−7.59 (d, J = 8.1 Hz, 2H), 7.11−7.14 (d, J = 8.1 Hz, 2H),
7.05 (s, 1H), 6.84−6.87 (d, J = 8.1 Hz, 1H), 3.96−4.00 (t, 4H),
1.78−1.85 (m, 4H), 1.26−1.47 (m, 52H), 0.85−0.90 (t, 6H)
ppm. 13C NMR (125 MHz, CDCl3): 150.46, 148.81, 132.01,
131.84, 125.40, 118.63, 116.69, 114.11, 113.13, 111.01, 94.46,
86.39, 69.33, 69.13, 31.93, 29.71, 29.63, 29.40, 29.37, 29.20,
29.15, 26.01, 22.69, 14.11 ppm. IR (KBr) νmax: 2953, 2916,
2848, 2227, 2196, 1589, 1517, 1467, 1417, 1392, 1330, 1257,
1274, 1226, 1118, 1143, 1068, 995, 835, 810, 721, 557 cm−1.
General Procedure for the Synthesis of 5-(4-((3,4-Bis-
(alkoxy)phenyl)ethynyl)phenyl)-2H-tetrazole, 4a and 4b. In
25 mL of dry degassed DMF were dissolved 4-((3,4-
bis(alkoxy)phenyl)ethynyl)benzonitrile (1 equiv), NaN3 (2
1,3,5-Tris(5-(4-((3,4-bis(hexadecyloxy)phenyl)ethynyl)-
phenyl)-1,3,4-oxadiazol-2-yl)benzene (FD16). This was ob-
tained in 70% yiled; mp: 191 °C. 1H NMR (300 MHz, CDCl3):
δ 9.08 (s, 3H), 8.21−8.23 (d, J = 7.8 Hz, 6H), 7.73−7.71 (d, J =
C
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a
Scheme 1
a
Reagents and conditions: (i) R−Br, K2CO3, DMF, 80 °C, 24 h; (ii) N-iodosuccinimide, trifluoroacetic acid, ACN, room temperature, 6 h; (iii) 4-
ethynylbenzonitrile, PdIICl2, CuI, PPh3, THF, DIPA, room temperature, 12 h; (iv) NaN3, NH4Cl, DMF, 100 °C, 12 h; (v) 1,3,5-benzene tricarbonyl
trichloride, pyridine, 120 °C, 12 h.
Figure 1. Polarized light optical microscopic (POM) images of FD16 (top panel a, b, c) and FD12 (bottom panel d, e, f).
7.8 Hz, 6H), 7.14−7.16 (d, J = 8.1 Hz), 7.08 (s, 3H), 6.84−6.87
(d, J = 8.1 Hz), 3.96−4.00 (t, 4H), 1.78−1.85 (m, 4H), 1.26−
1.47 (m, 52H), 0.85−0.90 (t, 6H) ppm. 13C NMR (125 MHz,
CDCl3): 165.16, 162.79, 150.24, 148.79, 132.08, 127.86,
127.11, 126.19, 125.30, 122.15, 116.65, 114.50, 113.14, 93.48,
87.08, 69.30, 69.13, 31.94, 29.71, 29.65, 29.44, 29.38, 29.24,
29.19, 26.02, 22.70, 14.12 ppm. IR (KBr) νmax: 2922, 2851,
2204, 1595, 1512, 1467, 1330, 1249, 1124, 1012, 844 cm−1. MS
(MALDI−TOF): calcd for C150H222N6O9 (M + H+), 2254.41;
found, 2254.36
RESULTS AND DISCUSSION
Figure 2. DSC thermogram of FD16 and FD12.
■
Synthesis. Molecules FD12 and FD16 possessing C12 and
C16 alkyl chain substituents, respectively, were synthesized
according to Scheme 1.
ized in Table 1. The first heating cycle of DSC was neglected to
exclude the influences of thermal history of the samples. Both
the derivatives were found to exhibit enantiotropic liquid
crystalline mesophases over a wide range of temperature.
In the first cooling cycle, FD16 underwent an isotropic to
liquid crystalline phase transition at 188 °C and the mesophase
The final reaction involved the conversion of tetrazole
derivatives into oxadiazole derivatives by the Huisgen reaction
mechanism.11 All the intermediates as well as the final products
1
were characterized using H, 13C NMR and IR spectroscopy.
Mesomorphic Properties. The mesomorphic properties
of FD12 and FD16 were investigated by polarized light optical
microscopy (POM) and differential scanning calorimetry
(DSC). The DSC thermogram showed peaks at temperatures
corresponding to those noted in the microscopic studies,
confirming the phase transitions. POM images at various
temperatures and the DSC thermogram of FD12 and FD16 are
shown in Figures 1 and 2, respectively, and the phase transition
temperatures and corresponding enthalpy values are summar-
Table 1. Phase Transition Temperature and Corresponding
Thermodynamic Parameters of FD16 and FD12
heating cycle [ΔH(kJ/mol)]
cooling cycle [ΔH(kJ/mol)]
FD16 Cr 49.4 °C (25.66) Colob 135 °C Iso 188 °C (2.24) Colh 125 °C
(16.18) Colh190 °C (2.35) Iso (17) Colob33.2 °C (26.32) Cr
FD12 Colob 129.2 °C (8.05) Colh 182 Iso 179 °C (1.7) Colh 114.1 °C
°C (1.87) Iso (10.37) Colob
D
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Figure 3. X-ray diffraction pattern of (a) FD16 at 180 (red line) and 90 °C (blue line) and (b) FD12 at 170 (red line) and 70 °C (blue line). Inset
shows the corresponding XRD pattern obtained at higher 2θ region.
was assigned to hexagonal columnar mesophase based on the
characteristic focal conic texture observed.12,2a Subsequent
cooling resulted in the formation of striations on the arms of
the focal conic texture at 125 °C. The appearance of striations
on the arms of the focal conic texture are indicative of the
formation of oblique columnar phase (Figure 1b).12,2a Further
cooling resulted in the crystallization of FD16 at 33 °C which is
also clear from the appearance of an exothermic peak in the
cooling cycle of DSC trace (Figure 2).
For FD12 the first cooling cycle indicated an isotropic to
hexagonal columnar mesophase transition at 179 °C. Similar to
that observed for FD16 further cooling resulted in the
formation of striations on the arms of the focal conic texture
(Figure 1e) indicating the formation of oblique columnar
phase. Unlike for FD16 however, the striated texture observed
at higher temperatures was retained on cooling down to room
temperature by which time the sample had solidified into a
glass. POM image indicated that the liquid crystalline texture
was retained in the glass (Figure 1f).13 The formation of the
glass was further confirmed from the absence of a clear
crystallization peak in the cooling cycle in the DSC analysis
(Figure 2).
Table 2. d-Spacing Values and Lattice Parameters of FD12
and FD16
dmeas
(Å)
dcalc
(Å)
lattice
parameters
mesophase
assigned
sample
(hk)
FD12
40.76
4.73
40.76
(10)
T = 170 °C
a = 47.06
Colh
44.75
41.29
31.15
22.66
20.15
18.86
4.58
44.88
41.24
31.08
22.44
20.10
19.07
(10)
(01)
(11)
(20)
(21)
(12)
ColOb
T = 70 °C
a = 44.92 (Å)
b = 41.28 (Å)
γ = 2.6
3.47
FD16
43.76
4.76
T = 180 °C
a = 50.53
Colh
48.53
46.83
34.65
24.28
21.54
4.65
48.48
46.87
34.46
24.24
21.92
(10)
(01)
(11)
(20)
(21)
ColOb
T = 90 °C
a = 48.53 (Å)
b = 46.92 (Å)
γ = 2.5
The liquid crystalline mesophases were further confirmed
using variable temperature X-ray diffraction (XRD) analysis
(Figure 3).
3.51
The X-ray diffraction pattern of FD16 at 180 °C showed a
strong sharp peak at low angle (with a d-spacing of 43.76 Å)
and broad and diffuse maxima in the wide angle region (with a
spacing of 4.8 Å). Similarly for FD12, a strong sharp peak at
low angle (with a d-spacing of 40.76 Å) and broad and diffuse
maxima in the wide angle region (with a spacing of 4.7 Å) was
observed at 170 °C. Owing to the presence of only one peak in
the low angle region, the support of the POM texture is taken
to label it as a hexagonal columnar phase, with a lattice spacing
of 50.53 and 47.06 Å for FD16 and FD12 respectively. On
further cooling the formation of striation on the arms of the
focal conic texture was observed under POM at ∼125 °C for
FD16 (at ∼114 °C for FD12). The XRD measurements in this
mesophase showed, for both the compounds, multiple peaks in
the low angle region. The indexing of these peaks to a tilted
columnar lattice was statistically slightly better than that to a
rectangular columnar lattice; the tilt angle turns out to be ∼3
degrees. The corresponding lattice parameters are tabulated in
the Table 2.
even in the micro molar concentration range. On increasing the
concentration beyond a threshold value (∼10−2 M) formation
of stable gels was observed. The gels were prepared by
dissolving small amounts of the oxadiazole derivative in decane
at higher temperature and allowing them to cool to room
temperature in a closed vial. Gel formation was confirmed by
the failure of the soft mass to flow by inverting the glass vial.14
The critical gelation concentrations were found to be 15.6 mM
and 13.3 mM for FD12 and FD16, respectively. It must be
mentioned here that the derivative FD12 formed gel very
slowly and it took several days (1−2 weeks) for complete
immobilization of the solvent whereas FD16 formed a gel
within few minutes. Since the discotic molecules FD12 and
FD16 do not possess any specific hydrogen bonding moieties,
the self-assembly of these molecules would most likely occur
through the combined effect of π−π and dipole−dipole
interactions. The microscopic structures of the self-assembled
materials were investigated by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) of these
derivatives at 5 × 10−5 M concentration in decane. Figure 4
shows the SEM and TEM images observed for FD16 (Figure
Gel Formation. The compounds FD12 and FD16 were
found to aggregate strongly in nonpolar solvents such as decane
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Figure 4. (a, b) SEM and TEM images of FD16; (c, d) SEM and TEM images of FD12 (concentration 5 ×10−5 M in decane).
4a,b) and FD12 (Figure 4c,d), which indicated a typical
interlocked network structure of fibers.
assemblies.15 The same molecular arrangement was also
observed for FD12 xerogel from their XRD analysis.
In order to elucidate the molecular arrangements in the fibers
we carried out X-ray diffraction studies (XRD) of both
derivatives in their xerogel state. In the case of FD16, the
XRD pattern showed intense peaks compared to its lower
homologue in the higher 2θ regime (Figure 5), clearly
Photophysical Properties. The photophysical properties
of FD12 and FD16 were observed to be very sensitive to their
state of aggregation. In solution the absorption spectra of these
compounds were insensitive to solvent polarities. Their
absorption spectra were however significantly broadened and
red-shifted in the liquid crystalline and gel state compared to
that in solution. The emission spectra showed significant
broadening and red shift with increasing polarity of the solvent.
In the liquid crystalline and gel state also the emission spectra
were significantly broad and red-shifted comparable to that
observed in the most polar solvent studied, namely
benzonitrile. These aspects as well as detailed photophysical
studies in the liquid crystalline and gel state have been
investigated in detail.
(a). Solution State. Figure 6 shows the absorption and
emission spectra of FD12 in various solvents and the
Figure 5. XRD pattern of FD12 (blue line) and FD16 (green line) in
the xerogel state.
indicating the presence of much stronger π−π interaction in
the gel assemblies of FD16 compared to that of FD12. Xerogel
from FD16 showed reflection peaks at 38.21, 23.76, 19.29,
11.98, and 4.15 Å. The reciprocal d-spacing ratio of these
reflection peaks were identical to that obtained for oblique
columnar arrangement (Colob) in liquid crystalline phase from
variable temperature XRD analysis (Table 2). This clearly
reveals the presence of oblique columnar arrangement in gel
Figure 6. (a) Absorption and (b) emission spectra of FD12 in
different solvents.
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absorption and emission data for both FD12 and FD16 are
summarized in Table 3. The absorption spectra of both these
Table 3. Absorption, Emission, and Fluorescence Lifetime
Characteristics of FD12 and FD16
absorption
λmax (nm)
349
emission
compound
solvent
decane
λmax (nm)
ΦF
τ (ns)
FD12
382, 403,
430
0.66
0.83
Figure 7. Temperature dependent changes on (a) emission spectra
(λexc.= 350 nm) and (b) excitation spectra (emission collected at 475
nm) of FD16.
toluene
dichloromethane
tetrahydrofuran
benzonitrile
decane
352
350
350
353
350
407, 421
477
0.75
0.77
0.76
0.58
0.68
0.99
2.33
2.18
2.68
0.87
468
490
FD16
382, 405,
432
toluene
350
350
347
343
410, 419
477
0.77
0.75
0.78
0.56
1.01
2.31
2.20
2.70
dichloromethane
tetrahydrofuran
benzonitrile
468
488
Figure 8. Temperature dependent changes on (a) emission spectra
(λexc.= 350 nm) and (b) excitation spectra (emission collected at 475
nm) of FD12.
derivatives were found to be nearly identical, signifying that at
the molecular level their optical properties were not affected by
the length of the alkoxy substituent. Their absorption spectra
were found to be fairly insensitive to solvent polarity (Figure
6a), which clearly indicates that the dipole moments of their
ground and the corresponding Franck−Condon (FC) excited
states are nearly the same. Their fluorescence spectra, however,
showed a significant bathochromic shift with increasing polarity
of the solvent (Figure 6b). In a weakly polar solvent like
decane, the emission spectra exhibited sharp and structured
emission bands whereas in polar solvents the emission spectra
were broad and red-shifted.
The bathochromic shift in emission maximum and broad-
ening of the band with solvent polarity point to the emissions
arising from a charge transfer state, i.e., a state in which the
dipole moments of the emissive state is significantly higher
compared to that of their ground state. This can occur when
excitation of these molecules leads to a nonemissive locally
excited state (LE) of nearly same dipole moment as the ground
state, which then subsequently transforms into a more polar
emissive state via intramolecular charge transfer (ICT)
process.16 The existence of dual excited states has been
reported in several donor−acceptor molecules17 including
stilbene18 and butadiene derivatives.19
(b). Liquid Crystalline State. The excitation and emission
properties of the liquid crystalline phases of FD12 and FD16
were investigated in the reflection mode using thin films
maintained at controlled temperatures, by initially heating the
materials to their isotropic phase and slowly cooling them to
the desired temperatures. The emission and excitation spectra
for FD16 and FD12 films at the various temperatures are
shown in Figures 7 and 8, respectively.
The emission spectra were broad for both the derivatives,
and an increase in temperatures led to a reduction in the
emission intensity without any significant change in the spectral
shape. Interestingly, both the materials exhibit appreciable
fluorescence even at the higher temperatures required to
generate the isotropic form. Cooling of the isotropic phase to
the columnar liquid crystalline phase resulted in an increase in
the fluorescence intensity and a further increase in intensity was
observed on solidification. The glass formed by FD12 on
cooling, was highly transparent as shown in the Figure 9a and
Figure 9. Photograph of FD12 in the liquid crystalline glassy state
under (a) ordinary light and (b) 365 nm UV illuminations.
was observed to be stable over a period of 1 year. The glassy
film of FD12 exhibited blue emission (λmax = 475 nm; Figure
9b) with an absolute quantum yield of 26%. The crystalline film
formed by cooling FD16 also exhibited blue luminescence (λmax
= 472 nm) although with a significantly lower absolute
quantum yield of 9%. The broad emission band combined
with the significant red shift in the maxima of the excitation
spectra of these films compared to their absorption spectra in
solution are indicative of the emission arising from the
aggregated state of these molecules in the solid, liquid
crystalline as well as isotropic phases.
(c). Aggregated and Gel State. As discussed earlier, FD12
and FD16 had a strong tendency to form aggregates in
nonpolar solvents, forming gels in decane at concentrations
greater than 15.6 mM and 13.3 mM, respectively. Since the
aggregation process for these materials in solvents was a slow
process, and since aggregation was also observed to lead to
significant differences in their photophysical properties, we have
investigated the time dependent changes in the UV−vis
absorption of these derivatives. With time, a reduction in the
intensity of the peak observed at the short wavelength with a
maxima centered on 350 nm, attributable to the monomer peak
by comparison with the absorption spectra obtained in
solutions (Figure 6a), was observed and this change was
accompanied by an increase in intensity in the longer
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wavelength region was observed. For FD16, the increase in
intensity in the long wavelength was much clearer and a distinct
shoulder band at 400 nm could also be observed (Figure 10a).
Figure 12. Effect of temperature on absorption spectra of (a) FD16
(1.5 × 10−5 M; 1 cm ×1 cm) and (b) FD12 (1.5 × 10−5 M) in decane.
temperatures (Figure 12a). This clearly indicates that
aggregates formed in FD16 were much stronger.
The fraction of aggregate at each temperature (αagg(T)) was
estimated using the following equation:
Figure 10. Time dependent changes in absorption spectra of (a)
FD16 (10−5 M; 1 cm ×1 cm) and (b) FD12 (10−5 M) in decane.
This change was completed by about 10 min. For FD12
however, the decrease in intensity of monomer peak with time
was much slower, taking about 48 h for completion, and only a
broadening of the band could be observed (Figure 10b). These
results clearly show that the tendency to form closely packed
exciton coupled aggregates is much stronger for FD16 than for
FD12.
The time dependent formation of aggregates can be clearly
visualized by subtracting the contribution of the monomer band
(obtained from the absorption spectrum measured at zero time
where molecule exists mainly in the molecularly dissolved state)
from the absorption spectrum obtained at the end of
measurements (10 min for FD16 and 48 h for FD12)
following normalization at 347 nm (Figure 11).
A(T) − Amono
αagg(T) ≈
Aagg − Amono
Here αagg(T) is the fraction of aggregate at temperature T, and
mono, A(T), and Aagg are the absorbance at 347 nm for the
monomer, the solution at temperature T, and the pure
aggregate solutions, respectively.20 The αagg(T) values were
then plotted as a function of temperature and from such a plot
the α50(T) (temperature at which αagg = 0.5) was calculated
(Figure 13).
A
Figure 11. Difference spectra (blueline) of (a) FD12 and (b) FD16
obtained by subtracting the absorption spectrum of monomer (black
line) from absorption spectrum of completely aggregated solution (red
line).
Figure 13. Plot of fraction of aggregate (αagg) as a function of
temperature (1.5 × 10−5 M) for FD12 (blue line) and FD16 (green
line).
The melting of the stacks in FD12 takes place in a relatively
narrow temperature range (∼10 °C) indicating that the thermal
stability of the stacks is relatively weak compared to FD16
(temperature range ∼20 °C). The α50(T) values obtained for
FD16 and FD12 are 57 and 48.5 °C respectively. The lower
α50(T) value for FD12 compared to FD16 indicates that the
latter derivative forms stronger aggregates.
The breakup of the aggregates at higher temperatures into
their monomer form was also indicated by the changes in their
fluorescence behavior. For FD16, with increase in temperature
the intensity of aggregate band at 480 nm in the emission
spectrum decreased and that at 382 nm attributable to the
monomer peak increased (Figure 14a). For FD12 however
only a general increase in intensity of main emission band with
increasing temperature was observed (Figure 14b). The
temperature dependent excitation spectra of both derivatives
in their aggregated solutions (2 × 10−5 M) also showed
significant differences. In the case of FD16, the solution at
The difference spectra obtained for both derivatives showed
two peaks, one blue-shifted and the other red-shifted compared
to the monomer absorption band. For both derivatives the
intensity of blue-shifted band are comparable whereas the
contribution of red-shifted aggregate is significantly higher for
FD16 compared to FD12, indicating an increased propensity
for FD16 to form red-shifted aggregates.
Aggregate formation in these molecules was observed to be
sensitive to the temperature of the solution. Figure 12 shows
the absorption spectrum of both the derivatives as a function of
temperature from 20 to 70 °C with an increment of 5 °C. An
increase in temperature leads to a reduction in the intensity of
absorption in the long wavelength region and a concomitant
increase in the intensity of absorption in the lower wavelength
region attributable to the monomer absorption was observed.
Unlike in the case of FD12, the breakup of aggregates to
monomer species was not complete in FD16 even at higher
H
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On the basis of these studies, it is clear that FD16 molecules
has higher tendency for formation of strongly exciton coupled
aggregates compared to FD12. The large red-shift in absorption
and emission spectra especially for FD16 in its condensed
phases as well as in solutions at higher concentrations as well
their strong dependence on temperature clearly shows that
these effects arise due to aggregation of the molecules.
Aggregation of molecules in the ground state can result in
exciton coupling as described by Kasha et al., in which the
excited state of aggregates splits into two energy levels
(Davydov splitting).21 The transition to the upper state is
allowed in the case of molecules with cofacial arrangements (H-
aggregates), and this is characterized by a hypsochromically
shifted absorption band compared to that of the monomer,
whereas transition to the lower state is allowed for aggregates
arranged in a slipped stack or a head-to-tail fashion (J-
aggregates) resulting in a bathochromically shifted absorption
band compared to the isolated monomer. Although the
derivatives investigated in the present study form both blue
and red-shifted aggregates indicating the formation of both H-
type and J-type aggregates, exciton coupling was much stronger
for FD16 indicating that its molecules are much more closely
packed in the aggregated state.
Figure 14. Temperature dependent changes in emission spectra (λex
=
350 nm) of (a) FD16 (1.5 × 10−5 M; 1 cm × 1 cm) and (b) FD12
(1.5 × 10−5 M) in decane. Effect of temperature on excitation spectra
of (c) FD16 (2 × 10−5 M; λem = 470 nm) and (d) FD12 (2 × 10−5 M;
λem = 470 nm) in decane.
lower temperature showed an excitation maximum at 400 nm
whereas monomer had an excitation maximum centered at 360
nm, indicating the nature of the excited species to be clearly
different, namely the aggregate (Figure 14c). The band at 400
nm coincides with the absorption spectrum of aggregated
solution (Figure 10a). Here also the excitation spectrum at
higher temperature shows contributions from the aggregate
which is evident from the shoulder at 400 nm, indicating the
aggregate formed is much stronger for FD16. However, in the
case of FD12, there was no considerable change in the
excitation maximum with temperature (Figure 14d).
The differences in photophysical properties between the
aggregated state of FD12 and FD16 were even more
prominent in their gels. The FD12 gel absorption peak
obtained from its reflectance spectra was significantly blue-
shifted compared that of the FD16 gel and this difference could
be visually perceived, with the FD12 gel being nearly colorless,
and the FD16 gel exhibiting a pale green color (Figure 15a).
CONCLUSION
■
Two discotic molecules FD16 and FD12, based on the 1,3,4-
oxadiazole moiety have been synthesized and characterized.
Both derivatives exhibited enantiotropic columnar mesophases
over a wide range of temperature. FD12 with C12 alkyl chain
transforms upon cooling from isotropic state to a glassy film at
room temperature wherein the liquid crystalline order is
maintained. The glassy film formed was highly transparent and
stable and was found to show blue luminescence with an
absolute quantum yield of 26%. The columnar liquid crystalline
ordering along with a fairly good blue emission in the glassy
state of FD12 could make this material applicable for nondoped
blue emitting diodes and display devices. Both the derivatives
form stable luminescent gels in decane, wherein the molecules
are arranged in an oblique columnar fashion as evident from
their XRD measurements. The optical properties of these
derivatives in gel state were found to be alkyl chain dependent.
The gel derived from FD12 exhibited blue emission whereas
FD16 exhibited bluish-green emission in decane. The self-
assembling processes were studied in detail using time and
temperature dependent UV−vis absorption and emission
measurements. The extent of aggregation was found to be
higher in FD16 which is evident from their large red shift
observed in absorption and emission spectra compared to the
other derivative. These observations clearly demonstrate that
perturbation of molecular structure has remarkable influence on
their bulk macroscopic properties. These effects exemplify the
future possibilities of fine-tuning the optical properties of
molecules in soft organic materials.
Figure 15. (a) Absorption and (b) emission spectra of FD12 (blue
line), FD16 (green line) in gel (λex = 350 nm; 2 × 10−2 M in decane; 1
mm cuvette). Inset shows the corresponding photographs of gel under
white light and 365 nm UV illumination.
AUTHOR INFORMATION
■
The gels derived from FD12 exhibited blue emission with the
(λmax= 450 nm) whereas the gel from FD16 exhibited bluish
green emission (λmax= 480 nm, Figure 15b). The absolute
quantum yields of these gels were determined to be 11% and
6% for FD12 and FD16 respectively.
Corresponding Author
*Fax: (+91) 471-2490186. E-mail: sureshdas@niist.res.in.
Notes
The authors declare no competing financial interest.
I
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ACKNOWLEDGMENTS
■
This work is supported by Council of Scientific and Industrial
Research (CSIR) under the Project NWP 55. D.D.P, N.S.S.K,
A.P.S., and S.V. are grateful to CSIR for fellowships. This is
Contribution No. NIIST-PPG-329. The authors thank Dr. S.
Krishna Prasad and Dr. D. S. Shankar Rao, CSMR Bangalore,
for carrying out variable temperature XRD measurements and
discussions.
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